Article

Heat flow and the structure of Precambrian lithosphere

September 1999
Developments in Geotectonics 24(1-4):81-91

September 1999
24(1-4):81-91

Authors:

Studies of heat flow from Precambrian terrians have demonstrated three empirical relationships; a temporal relationship between heat flow and tectonic age, a spatial pattern between heat flow and the proximity of Archean cratons, and a temporal relationship between heat flow and the age of lithosphere stabilization. In the first relationship, heat flow is inversely related to tectonic age. The second pattern is characterized by low heat flow from Archean cratons and Proterozoic terrains adjacent to cratonic margins (pericratonic terrains), and higher heat flow from Proterozoic terrains that are more than a few hundred kilometers from a craton. In the third pattern, heat flow decreases as the age of stabilization of the lithosphere increases. A number of interpretations of Precambrian heat flow have been offered to explain one or more of these relationships. The simple cooling of a thermal boundary layer predicts essentially no change in heat flow in terrains older than ≈1.5 Ga, and therefore does not likely provide a comprehensive framework for the interpretation of Precambrian heat flow. By contrast, two other interpretations, (1) thicker lithosphere beneath Archean terrains than beneath Proterozoic terrains, and (2) greater heat production in Proterozoic crust than in Archean crust, when combined with the special structural configuration of sutures, can both contribute to the spatial and temporal heat flow distributions. Xenolith thermobarometry constraints on lithospheric temperatures, however, limit the contribution of age-dependent crustal heat production, and therefore at least part of the heat flow distributions derive from variations in lithosphere thickness.

Sensitivity of Cenozoic Antartic ice sheet variations to geothermal heat flux

Article

Nov 2005
GLOBAL PLANET CHANGE

The sensitivity of long-term Cenozoic variations of the East Antarctic ice sheet to geothermal heat flux is investigated, using a coupled climate–ice sheet model with various prescribed values and patterns of geothermal heat flux. The sudden growth of major ice across the Eocene–Oligocene boundary (~34 Ma) is used as a test bed for this sensitivity. A suite of several million year-long simulations spanning the transition is performed, with various geothermal heat flux magnitudes and spatial distributions reflecting current uncertainty. The climate–ice sheet model simulates the Eocene–Oligocene transition realistically as a non-linear ice-sheet response to orbital perturbations and a long-term gradual decline of atmospheric CO 2 . It is found that reasonable variations of geothermal heat flux have very little effect on overall ice volumes and extents, and on the timing of major ice transitions. However, they cause large changes in basal areas at the pressure melting point at a given time, which could strongly influence other aspects of Cenozoic Antarctic evolution such as basal hydrology, sediment deformation and discharge, subglacial lakes, and basal erosional forms. D 2005 Elsevier B.V. All rights reserved.

The provenance of sub-cratonic mantle beneath the Limpopo Mobile Belt (South Africa)

Article

Jan 2013

The provenance of sub-cratonic mantle beneath the Limpopo Mobile Belt (South Africa)

Article

Full-text available

Jun 2013
LITHOS

Petrological, whole rock major element and mineral chemical analysis of mantle xenoliths from the Venetia kimberlite pipes (533 Ma) in South Africa reveals an apparently stratified cratonic mantle beneath the Central Zone of the Limpopo Mobile Belt (LMB) that separates the Kaapvaal and Zimbabwe Cratons. Combined pressure–temperature (P–T) data and petrographic observations indicate that the mantle consists of an upper layer of Low-T coarse-equant garnet + spinel lherzolite (~ 50 to ~ 130 km depth). This layer is underlain by a region of mixed garnet harzburgites and garnet lherzolites that are variably deformed (~ 130 to ~ 235 km depth). An equilibrated geotherm did not exist at the time of kimberlite eruption (533 Ma) and a localised heating event involving the introduction of asthenospheric material to the High-T lithosphere below 130 km is inferred.

Building and Destroying Continental Mantle

Article

Full-text available

Apr 2011

Continents, especially their Archean cores, are underlain by thick thermal boundary layers that have been largely isolated from the convecting mantle over billion-year timescales, far exceeding the life span of oceanic thermal boundary layers. This longevity is promoted by the fact that continents are underlain by highly melt-depleted peridotites, which result in a chemically distinct boundary layer that is intrinsically buoyant and strong (owing to dehydration). This chemical boundary layer counteracts the destabilizing effect of the cold thermal state of continents. The compositions of cratonic peridotites require formation at shallower depths than they currently reside, suggesting that the building blocks of continents formed in oceanic or arc environments and became “continental” after significant thickening or underthrusting. Continents are difficult to destroy, but refertilization and rehydration of continental mantle by the passage of melts can nullify the unique stabilizing composition of continents.

Physical, chemical, and chronological characteristics of continental mantle

Article

Full-text available

Mar 2005

1] Unlike in the ocean basins where the shallow mantle eventually contributes to the destruction of the overlying crust, the shallow mantle beneath continents serves as a stiff, buoyant ''root'' whose presence may be essential to the long-term survival of continental crust at Earth's surface. These distinct roles for subcrustal mantle come about because the subcontinental mantle consists of a thick section of material left behind after extensive partial melt extraction, possibly from the wedge of mantle overlying a subducting oceanic plate. Melt removal causes the continental mantle to be cold and strong but also buoyant compared to oceanic mantle. These characteristics allow thick sections of cold mantle to persist beneath continental crust in some cases for over 3 billion years. If the continental mantle becomes gravitationally unstable, however, its detachment from the overlying crust can cause major episodes of intracontinental deformation and volcanism.

Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis

Article

Full-text available

Jul 2010
EARTH-SCI REV

The Arabian Plate originated ∼ 25 Ma ago by rifting of NE Africa to form the Gulf of Aden and Red Sea. It is one of the smaller and younger of the Earth's lithospheric plates. The upper part of its crust consists of crystalline Precambrian basement, Phanerozoic sedimentary cover as much as 10 km thick, and Cenozoic flood basalt (harrat). The distribution of these rocks and variations in elevation across the Plate cause a pronounced geologic and topographic asymmetry, with extensive basement exposures (the Arabian Shield) and elevations of as much as 3000 m in the west, and a Phanerozoic succession (Arabian Platform) that thickens, and a surface that descends to sea level, eastward between the Shield and the northeastern margin of the Plate. This tilt in the Plate is partly the result of marginal uplift during rifting in the south and west, and loading during collision with, and subduction beneath, the Eurasian Plate in the northeast. But a variety of evidence suggests that the asymmetry also reflects a fundamental crustal and mantle heterogeneity in the Plate that dates from Neoproterozoic time when the crust formed.

Mapping Global Lithospheric Mantle Pressure‐Temperature Conditions by Machine‐Learning Thermobarometry

Article

Full-text available

Apr 2024
GEOPHYS RES LETT

Plain Language Summary The temperature of the lithospheric mantle is pivotal to processes such as the production of magmas and the structural stability of cratons. Researchers have traditionally employed mineral‐based thermobarometers to lithospheric mantle temperatures, but those conventional techniques have limited applications and are susceptible to inaccuracies. Here, we explored a new approach using machine learning, a powerful tool for identifying complex patterns in high‐dimensional data space. We trained machine‐learning models using data from high‐pressure and high‐temperature experiments, then compared our machine‐learning models to traditional methods. Our results show that the machine‐learning models better predict pressure and temperature for specific mineral combinations. We also used our optimized model to predict mantle temperatures and pressures based on a global data set of xenolith analyses, revealing insights about the temperature of the lithospheric mantle and the depth to the lithosphere‐asthenosphere boundary beneath continents. This research shows that machine learning can greatly improve our understanding of Earth's deep processes, providing more accurate insights into its dynamics and evolution.

Building Archean cratonic roots

Preprint

Jun 2022

Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth's continents to the presence of their Archean cratonic roots. These roots are likely composed of melt-depleted, low density residual peridotite with high Magnesium number (Mg#), while devolatilisation from the upper mantle during magmatic events might have made these roots more viscous and intrinsically stronger than the convecting mantle.Several conceptual dynamic and petrological models of craton formation have been proposed. Dynamic models invoke far-field shortening or mantle melting events, e.g., by mantle plumes, to create melt-depleted and thick cratons. Compositional buoyancy and rheological modifications have also been invoked to create long-lived stable cratonic lithosphere. However, these conceptual models have not been tested in a dynamically self-consistent model. In this study, we present global thermochemical models of craton formation with coupled core-mantle-crust evolution driven entirely by gravitational forces.Our results with melting and crustal production (both oceanic and continental) show that formation of cratonic roots can occur through naturally occurring lateral compression and thickening of the lithosphere in a self-consistent manner, without the need to invoke far-field tectonic forces. Plume impingements, and gravitational sliding creates thrusting of lithosphere to form thick, stable, and strong lithosphere that has a strong resemblance to the Archean cratons that we can still observe today at the Earth's surface. These models also suggest the recycling of denser eclogitic crust by delamination and dripping processes. Within our computed parameter space, a variety of tectonic regimes are observed which also transition with time. Based on these results, we propose that a ridge-only regime or a sluggish-stagnant-lid regime might have been active on Earth during the Archean Eon as they offer favourable dynamics and conditions for craton formation.

Crustal heat production and estimate of terrestrial heat flow in central East Antarctica, with implications for thermal input to the East Antarctic ice sheet

Article

Full-text available

Feb 2018
TC

John W. Goodge

Terrestrial heat flow is a critical first-order factor governing the thermal condition and, therefore, mechanical stability of Antarctic ice sheets, yet heat flow across Antarctica is poorly known. Previous estimates of terrestrial heat flow in East Antarctica come from inversion of seismic and magnetic geophysical data, by modeling temperature profiles in ice boreholes, and by calculation from heat production values reported for exposed bedrock. Although accurate estimates of surface heat flow are important as an input parameter for ice-sheet growth and stability models, there are no direct measurements of terrestrial heat flow in East Antarctica coupled to either subglacial sediment or bedrock. As has been done with bedrock exposed along coastal margins and in rare inland outcrops, valuable estimates of heat flow in central East Antarctica can be extrapolated from heat production determined by the geochemical composition of glacial rock clasts eroded from the continental interior. In this study, U, Th, and K concentrations in a suite of Proterozoic (1.2–2.0 Ga) granitoids sourced within the Byrd and Nimrod glacial drainages of central East Antarctica indicate average upper crustal heat production (Ho) of about 2.6 ± 1.9 µW m−3. Assuming typical mantle and lower crustal heat flux for stable continental shields, and a length scale for the distribution of heat production in the upper crust, the heat production values determined for individual samples yield estimates of surface heat flow (qo) ranging from 33 to 84 mW m−2 and an average of 48.0 ± 13.6 mW m−2. Estimates of heat production obtained for this suite of glacially sourced granitoids therefore indicate that the interior of the East Antarctic ice sheet is underlain in part by Proterozoic continental lithosphere with an average surface heat flow, providing constraints on both geodynamic history and ice-sheet stability. The ages and geothermal characteristics of the granites indicate that crust in central East Antarctica resembles that in the Proterozoic Arunta and Tennant Creek inliers of Australia but is dissimilar to other areas like the Central Australian Heat Flow Province that are characterized by anomalously high heat flow. Age variation within the sample suite indicates that central East Antarctic lithosphere is heterogeneous, yet the average heat production and heat flow of four age subgroups cluster around the group mean, indicating minor variation in the thermal contribution to the overlying ice sheet from upper crustal heat production. Despite these minor differences, ice-sheet models may favor a geologically realistic input of crustal heat flow represented by the distribution of ages and geothermal characteristics found in these glacial clasts.

Earth's Continental Lithosphere Through Time

Article

Jun 2017

The record of the continental lithosphere is patchy and incomplete; no known rock is older than 4.02 Ga, and less than 5% of the rocks preserved are older than 3 Ga. In addition, there is no recognizable mantle lithosphere from before 3 Ga. We infer that there was lithosphere before 3 Ga and that ∼3 Ga marks the stabilization of blocks of continental lithosphere that have since survived. This was linked to plate tectonics emerging as the dominant tectonic regime in response to thermal cooling, the development of a more rigid lithosphere, and the recycling of water, which may in turn have facilitated plate tectonics. A number of models, using different approaches, suggest that at 3 Ga the volume of continental crust was ∼70% of its present day volume and that this may be a minimum value. The continental crust before 3 Ga was on average more mafic than that generated subsequently, and this pre-3 Ga mafic new crust had fractionated Lu/Hf and Sm/Nd ratios as inferred for the sources of tonalite-trondhjemite-granodiorite and later granites. The more intermediate composition of new crust generated since 3 Ga is indicated by its higher Rb/Sr ratios. This change in composition was associated with an increase in crustal thickness, which resulted in more emergent crust available for weathering and erosion. This in turn led to an increase in the Sr isotope ratios of seawater and in the drawdown of CO2. Since 3 Ga, the preserved record of the continental crust is marked by global cycles of peaks and troughs of U-Pb crystallization ages, with the peaks of ages appearing to match periods of supercontinent assembly. There is increasing evidence that the peaks of ages represent enhanced preservation of magmatic rocks in periods leading up to and including continental collision in the assembly of supercontinents. These are times of increased crustal growth because more of the crust that is generated is retained within the crust. The rates of generation of continental crust and mantle lithosphere may have remained relatively constant at least since 3 Ga, yet the rates of destruction of continental crust have changed with time. Only relatively small volumes of rock are preserved from before 3 Ga, and so it remains difficult to establish which of these are representative of global processes and the extent to which the rock record before 3 Ga is distorted by particular biases. Expected final online publication date for the Annual Review of Earth and Planetary Sciences Volume 45 is May 30, 2017. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Continental growth seen through the sedimentary record

Article

Jun 2017
SEDIMENT GEOL

Sedimentary rocks and detrital minerals sample large areas of the continental crust, and they are increasingly seen as a reliable archive for its global evolution. This study presents two approaches to model the growth of the continental crust through the sedimentary archive. The first builds on the variations in U-Pb, Hf and O isotopes in global databases of detrital zircons. We show that uncertainty in the Hf isotope composition of the mantle reservoir from which new crust separated, in the ¹⁷⁶Lu/¹⁷⁷Hf ratio of that new crust, and in the contribution in the databases of zircons that experienced ancient Pb loss(es), adds some uncertainty to the individual Hf model ages, but not to the overall shape of the calculated continental growth curves. The second approach is based on the variation of Nd isotopes in 645 worldwide fine-grained continental sedimentary rocks with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter referred to as K. This dimensionless parameter relates the proportions of younger to older source rocks in the sediment, to the proportions of younger to older source rocks present in the crust from which the sediment derived. We suggest that a Hadean/Archean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4–6, may be reasonable for the global Earth system. Models built on the detrital zircon and the fine-grained sediment records independently suggest that at least 65% of the present volume of continental crust was established by 3 Ga. The continental crust has been generated continuously, but with a marked decrease in the growth rate at ~ 3 Ga. The period from > 4 Ga to ~ 3 Ga is characterised by relatively high net rates of continental growth (2.9–3.4 km³ yr− 1 on average), which are similar to the rates at which new crust is generated (and destroyed) at the present time. Net growth rates are much lower since 3 Ga (0.6–0.9 km³ yr− 1 on average), which can be attributed to higher rates of destruction of continental crust. The change in slope in the continental growth curve at ~ 3 Ga is taken to indicate a global change in the way bulk crust was generated and preserved, and this change has been linked to the onset of subduction-driven plate tectonics. At least 100% of the present volume of the continental crust has been destroyed and recycled back into the mantle since ~ 3 Ga, and this time marks a transition in the average composition of new continental crust. Continental crust generated before 3 Ga was on average mafic, dense, relatively thin (< 20 km) and therefore different from the calc-alkaline andesitic crust that dominates the continental record today. Continental crust that formed after 3 Ga gradually became more intermediate in composition, buoyant and thicker. The increase in crustal thickness is accompanied by increasing rates of crustal reworking and increasing input of sediment to the ocean. These changes may have been accommodated by a change in lithospheric strength at around 3 Ga, as it became strong enough to support high-relief crust. This time period therefore indicates when significant volumes of continental crust started to become emergent and were available for erosion and weathering, thus impacting on the composition of the atmosphere and the oceans.

Distribution of subglacial sediments across the Wilkes Subglacial Basin, East Antarctica: Wilkes Subglacial Sedimentary Basins

Article

Full-text available

Apr 2016

Topography, sediment distribution, and heat flux are all key boundary conditions governing the dynamics of the East Antarctic Ice Sheet (EAIS). EAIS stability is most at risk in Wilkes Land across vast expanses of marine-based catchments including the 1400km×600km expanse of the Wilkes Subglacial Basin (WSB) region. Data from a recent regional aerogeophysical survey (Investigating the Cryospheric Evolution of the Central Antarctic Plate (ICECAP)/IceBridge) are combined with two historical surveys (Wilkes basin/Transantarctic Mountains System Exploration-Ice-house Earth: Stability or DYNamism? (WISE-ISODYN) and Wilkes Land Transect (WLK)) to improve our understanding of the vast subglacial sedimentary basins impacting WSB ice flow and geomorphology across geologic time. Analyzing a combination of gravity, magnetic and ice-penetrating radar data, we present the first detailed subglacial sedimentary basin model for the WSB that defines distinct northern and southern subbasin isopachs with average sedimentary basin thicknesses of 1144m±179m and 1623m±254m, respectively. Notably, more substantial southern subbasin sedimentary deposition in the WSB interior supports a regional Wilkes Land hypothesis that basin-scale ice flow and associated glacial erosion is dictated by tectonic basement structure and the inherited geomorphology of preglacial fluvial networks. Orbital, temperate/polythermal glacial cycles emanating from adjacent alpine highlands during the early Miocene to late Oligocene likely preserved critical paleoclimatic data in subglacial sedimentary strata. Substantially thinner northern WSB subglacial sedimentary deposits are generally restricted to fault-controlled, channelized basins leading to prominent outlet glacier catchments suggesting a more dynamic EAIS during the Pliocene.

Foerster etal

Data

Full-text available

Oct 2015

Heat flux and seismicity in the Fennoscandian Shield

Data

Jan 2001
PHYS EARTH PLANET IN

The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheological structure is analysed from the Kola Orogen to the Sorgenfrei–Tornquist zone. Moho temperatures and the heat flux from the mantle are typical of cratonic areas. The deep thermal field shows a cold root in the north-eastern sector. Larger lateral variations of mantle heat flux and Moho temperatures occur in the southern area and at the edges of the shield. The thickness of the thermal lithosphere is maximum (200–220 km), where the mantle heat flux is minimum (about 15–20 mW m −2). Three lithospheric cross-sections illustrate the expected lateral variation in viscosity and failure mode throughout the lithosphere as a consequence of the different geothermal conditions and the rheological stratification. In the upper crust, the depth of the brittle–ductile transition varies on average from 30, in the north-east, to 15 km, in the south-west. In the subcrustal mantle, the models predict a mainly ductile behaviour. At a depth of 60 km, the upper mantle viscosity is maximum (2.5 × 10 23 to 25.0 × 10 23 Pa s) beneath the Archean province and minimum (2.5 × 10 21 to 10.0 × 10 21 Pa s) below the Svecofennian and Sveconorwegian units. The comparison of the rheological calculations with the seismic activity shows a general agreement with the brittle–ductile transition depth expected in the Archaean and Proterozoic provinces. There is a difference both in number of earthquakes and distribution versus depth between areas of different age. The occurrence of larger magnitude seismic events near the base of the seismogenic zone, in the region of inferred peak of shear resistance, suggests some causal relationship. The Proterozoic areas show a more regular decrease of shocks with depth and, compared to the Archean provinces, a tendency to shallowing.

The continental record and the generation of continental crust

Article

Full-text available

Jan 2013
GEOL SOC AM BULL, GSA Bulletin

Continental crust is the archive of Earth history. The spatial and temporal distribution of Earth's record of rock units and events is heterogeneous; for example, ages of igneous crystallization, metamorphism, continental margins, mineralization, and seawater and atmospheric proxies are distributed about a series of peaks and troughs. This distribution reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. The physio-chemical resilience of zircons and their derivation largely from felsic igneous rocks means that they are important indicators of the crustal record. Furthermore, detrital zircons, which sample a range of source rocks, provide a more representative record than direct analysis of grains in igneous rocks. Analysis of detrital zircons suggests that at least similar to 60%-70% of the present volume of the continental crust had been generated by 3 Ga. Such estimates seek to take account of the extent to which the old crustal material is underrepresented in the sedimentary record, and they imply that there were greater volumes of continental crust in the Archean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but there was a marked decrease in the rate of crustal growth at ca. 3 Ga, which may have been linked to the onset of significant crustal recycling, probably through subduction at convergent plate margins. The Hadean and Early Archean continental record is poorly preserved and characterized by a bimodal TTG (tonalites, trondhjemites, and granodiorites) and greenstone association that differs from the younger record that can be more directly related to a plate-tectonic regime. The paucity of this early record has led to competing and equivocal models invoking plate-tectonic- and mantle-plume-dominated processes. The 60%-70% of the present volume of the continental crust estimated to have been present at 3 Ga contrasts markedly with the <10% of crust of that age apparently still preserved and requires ongoing destruction (recycling) of crust and subcontinental mantle lithosphere back into the mantle through processes such as subduction and delamination.

Peridotites, Garnets, Trace Elements, and the Structure of Mantle Lithosphere Beneath the Archean Superior Province

Article

May 2004

A recent large data compilation of trace element analyses on over 1700 mantle peridotite whole rocks and 80 peridotitic garnets from the literature leads to simple correlations between garnets and whole rock data that can be applied to empirically estimate the degree of depletion, mineralogy and potentially seimic velocity and density in the peridotite sections represented by kimberlite-borne garnet xenocrysts. More than 70 percent of the Yb in whole rock analyses of garnet peridotites is contained in garnet. Studies of garnets and coexisting whole rocks indicate that Yb in garnet Gt(Yb) can serve as a simple proxy of Yb in a whole rock garnet peridotite WR(Yb). The correlations of Gt(Yb) with WR(Yb) can then be used to estimate the whole rock Al content of a peridotite WR(Al) from which a garnet xenocryst was derived. Al is a useful depletion index in peridotites, and furthermore correlates very well with modal garnet in well-characterized peridotite samples, and with bulk Mg/(Mg+Fe) (Mg#) in world peridotite datasets from ophiolites, ocean basins, off- and on-craton volcanic-hosted xenoliths. The above correlations are applied to internally consistent trace element datasets (n=800+) we have determined by LAICPMS on garnet suites from 17 kimberlites in North America. Using Ni thermometry in the garnets projected to xenolith-derived geotherms, we construct lithospheric sections of WR (Al), WR (Mg#), and modal garnet with depth in the subcontinental lithosphere beneath cratonic regions. In almost all regions sampled in the Superior Province, depleted peridotite the shallowest sections (

Geochemical/Petrologic Constraints on the Origin of Cratonic Mantle

Article

Full-text available

Jan 2006

Cin-Ty A. Lee

Cratons are underlain by thick, cold, and highly melt-depleted mantle roots, the latter imposing a chemical buoyancy that roughly offsets the craton's negative thermal buoyancy associated with its cooler thermal state. Petrologic/geochemical predictions of three endmember scenarios for the origin of cratonic mantle are discussed: (1) high-degree melting in a very hot plume head with a potential temperature >1650°C, (2) accretion of oceanic lithosphere, and (3) accretion of arc lithosphere. The hot plume scenario predicts that cratonic peridotites were formed by high degrees of melting at very high pressures (≥7 GPa), whereas the two accretion scenarios predict an origin by melting on average at lower pressures (

Variation in effective elastic plate thickness of the East Africa lithosphere

Article

Full-text available

May 2003

We use new and existing Bouguer gravity data to characterize the long-term flexural rigidity of the lithosphere beneath East Africa. Four tectonic provinces are characterized by distinct effective elastic plate thicknesses. These are the Tanzanian Craton, the Proterozoic Mobile Belts, the Paleogene rift basins in Sudan and Kenya, and the Cenozoic East African Rift System. Beneath the Tanzanian craton the elastic plate thickness is ∼75 km, while the Pan African Mobile Belts and the Ubendian Belts are ∼55 and ∼65 km, respectively. The Paleogene rift basins exhibit an elastic plate thickness varying between 40 and 45 km. The effective elastic plate thickness within the faulted East African Rift System ranges from 14 km in the north to 33 km in the south. The minimum is found beneath the active rift of the Afar depression. The north to south along-rift axis variation in effective elastic plate thickness can be attributed to contrasts in the mechanical strength of the lithosphere. This is consistent with the north-south decrease in extensional velocity and the northward decrease in the focal depths of the earthquakes, reflecting a northward thinning in the brittle layer of the crust. The maximum elastic plate thickness is found beneath the Tanzanian craton, while the surrounding Mobile Belts have relatively moderate elastic plate thicknesses. The high effective elastic plate thickness of the Tanzanian craton suggests that this part of the lithosphere is rheologically competent compared to the surrounding Mobile Belts. The craton effectively resists deformation, even though it is located within a broad zone of an east-west extensional tectonic regime. Consequently, it has altered the direction of the rift propagation into the warmer and weaker lithosphere of the surrounding Mobile Belts.

Effect of the chemical composition of the crust on the metamorphic evolution of orogenic wedges: EFFECT OF CRUSTAL COMPOSITION ON METAMORPHISM

Article

Feb 2003
J METAMORPH GEOL

Petrological data provide a good record of the thermal structure of deeply eroded orogens, and, in principle, might be used to relate the metamorphic structure of an orogen to its deformational history. In this paper, we present two-dimensional thermal modelling of various subduction models taking into account varying wedge geometry as well as variation of density and topography with metamorphic reactions. The models clearly show that rock type accreted in the wedge has important effects on the thermal regime of orogenic wedges. The thermal regime is dominated by radiogenic heat production. Material having high radioactive heat production, like the granodioritic upper crust, produces high temperature metamorphism (amphibolitic conditions). Material with low radioactive heat production results in low temperature metamorphism of greenschist or blueschist types depending on the thickness of the wedge. Application of this model to seemingly unrelated areas of the Central Alps (Lepontine Dome, Grisons) and Eastern Alps (Tauern Window) explains the coexistence and succession of distinct Barrovian and blueschist facies metamorphic conditions as the result of a single, continuous tectonic process in which the main difference is the composition of the incoming material in the orogenic wedge. Accretion of the European upper continental crust in the Lepontine and Tauern Domes produces Barrovian type metamorphism while accretion of oceanic sediments results in blueschist facies metamorphism in the Valaisan domain.

Radiogenic heat production variability of some common lithological groups and its significance to lithospheric thermal modeling

Article

Jul 2010
TECTONOPHYSICS

Determining the temperature distribution within the lithosphere requires the knowledge of the radiogenic heat production (RHP) distribution within the crust and the lithospheric mantle. RHP of crustal rocks varies considerably at different scales as a result of the petrogenetic processes responsible for their formation and therefore RHP depends on the considered lithologies. In this work we address RHP variability of some common lithological groups from a compilation of a total of 2188 representative U, Th and K concentrations of different worldwide rock types derived from 102 published studies. To optimize the use of the generated RHP database we have classified and renamed the rock-type denominations of the original works following a petrologic classification scheme with a hierarchical structure. The RHP data of each lithological group is presented in cumulative distribution plots, and we report a table with the mean, the standard deviation, the minimum and maximum values, and the significant percentiles of these lithological groups. We discuss the reported RHP distribution for the different igneous, sedimentary and metamorphic lithological groups from a petrogenetic viewpoint and give some useful guidelines to assign RHP values to lithospheric thermal modeling.

Mantle seismic structure beneath the Kaapvaal and Zimbabwe Cratons

Article

Full-text available

Jun 2004

We present results of seismic tomography for a broad region of southern Africa using data from the seismic component of the Kaapvaal Project, a multinational, multidisciplinary experiment conducted in the late 1990s. Seismic images provide clear evidence of mantle structures that mimic the surface geology across the region and provide important constraints on subcrustal structure associated with Archean cratons. Specifically, a thick (~250 to 300km) mantle keel exists beneath the Kaapvaal craton; a slightly thinner (~225 to 250km) keel exists beneath the Zimbabwe craton and parts of the Archean Limpopo mobile belt. Mantle velocities lower than surrounding regions are evident across a broad swath beneath the surface expression of the Bushveld Complex, a ~2.05 Ga layered mafic intrusion. These reduced velocities may be due to mantle refertilization during intrusion of Bushveld magmas, or they may be caused by a thermal perturbation of more recent origin, perhaps related to the ~183 Ma Karoo magmatic event.

The surface heat flow of the Arabian Shield in Jordan

Article

Full-text available

Apr 2007
J Southeast Asian Earth Sci

Surface heat flow in southern Jordan (western part of the Arabian Plate) was determined in a dense cluster of five, up to 900-m-deep boreholes that have encountered sedimentary rocks of Paleozoic (Ordovician and Silurian) age. These rocks are underlain by an igneous and metamorphic basement, which has been studied for its radiogenic heat production, along the eastern margin of the Dead Sea Transform (DST) fault system. The heat flow, calculated from continuous temperature logs and laboratory-measured thermal conductivity of drillcores and surface samples, averages to 60.3±3.4mWm−2 and contrasts the common view of the late Proterozoic-consolidated Arabian Shield constituting a low heat-flow province of ⩽45mWm−2. Although only characterizing an area of about 300km2, this average is unlikely representing a positive local anomaly caused by voluminous HHP granites/rhyolites at shallow depths. Instead, a heat flow of 60mWm−2 is considered a robust estimate of the Phanerozoic conductive surface heat flow not only for Jordan, but for the Arabian Shield in areas unaffected by younger reactivation. The large variation in conductive heat flow (36–88mWm−2) previously observed in Jordan, southern Syria, and Saudi Arabia is irreconcilable with their broad similarity in lithosphere structure and composition and rather reflects a combination of factors including low-quality temperature data and insufficient knowledge on thermal rock properties.

Thermal thickness and evolution of Precambrian lithosphere: A global study

Article

Full-text available

Aug 2001

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat-producing elements is estimated using regional models for 300 blocks with sizes varying from 1 1 to about 5 5 in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200 –220 km and 300 –350 km. In general, thin (220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from 200 km beneath Archean cratons to intermediate values of 200 50 km in early Proterozoic lithosphere, to about 140 50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by 0.6 – 0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: 2.5 Ga, 2.5–1.8 Ga, and 1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

Evidence of a hydrological connection between the ice divide and ice sheet margin in the Aurora Subglacial Basin, East Antarctica

Article

Full-text available

Mar 2012

Subglacial hydrology in East Antarctica is poorly understood, yet may be critical to the manner in which ice flows. Data from a new regional airborne geophysical survey (ICECAP) have transformed our understanding of the topography and glaciology associated with the 287,000 km2 Aurora Subglacial Basin in East Antarctica. Using these data, in conjunction with numerical ice sheet modeling, we present a suite of analyses that demonstrate the potential of the 1000 km-long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin. We present results from our analysis of basal topography, bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures. Although no clear-cut subglacial lakes are found within the Aurora Basin itself, dozens of lake-like reflectors are observed that, in conjunction with other results reported here, support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the Dome C ice divide to reach the coast via the Totten Glacier.

Microstructure, texture and seismic anisotropy of the lithospheric mantle above a mantle plume: Insights from the Labait volcano xenoliths (Tanzania)

Article

Full-text available

Apr 2005
EARTH PLANET SC LETT

The impact of a mantle plume at the base of the Tanzania craton has modified the composition and seismic velocity of the lithospheric mantle. Lavas erupted by the Labait volcano have sampled the perturbed mantle from the lithosphere–asthenosphere boundary (N140 km) to the spinel-peridotite domain (b70 km). We have studied the microstructure, texture and seismic anisotropy of a set of xenoliths spanning these depths to investigate the effects of plume activity on the fabric and seismic properties of the lithospheric mantle. The microstructure changes with depth: first the grain-size increases significantly, and then nucleation recrystallization occurs. The deepest samples display a recrystallized equidimensional matrix embedding relicts of deformed paleoclasts. The crystallographic preferred orientation (CPO) of olivine remains clear and even tends to increase with depth. In most samples, the observed CPO is consistent with dominant activation of the (010)[100] slip system. Samples from the base of the lithosphere display more unusual CPO, suggesting increasing activity on the (010)[001] slip system. Nucleation recrystallization does not appear to modify the pre-existing CPO, since neoblasts have a crystallographic orientation close to the parent grain orientation. Seismic properties remain similar over the whole section. In particular, no weakening of the seismic anisotropy is observed with depth, either for the P azimuthal or for the S polarization anisotropies. These results are consistent with previous seismological observations suggesting a coherent seismic anisotropy over the entire thickness of the Tanzania cratonic lithosphere. Our data thus provide new constraints for interpreting shear wave splitting measurements in East Africa, and support a model of perturbed lithosphere characterized by seismic signatures transitional between the bnormalQ lithosphere (for seismic anisotropy) and asthenosphere (for seismic velocities). Published by Elsevier B.V.

Complex cratonic seismic structure from thermal models of the lithosphere: Effects of variations in deep radiogenic heating

Article

Jan 2010
GEOPHYS J INT

Cratons are the long-term tectonically stable cores of the continents. Despite their thermal stability they display substantial seismic complexity with lateral and vertical lithospheric anomalies of up to several percent in both VS and VP. Although some of these anomalies have been correlated with compositional variations, others are too large to be explained with any common mantle lithosphere compositions ranging from fertile peridotites to highly melt-depleted dunites, under the assumption that thermal perturbations are negligible. To test whether temperature anomalies could contribute to seismic complexity, we performed a set of 2-D thermal calculations for a range of cratonic tectonic models and converted them into seismic structure, accounting for variations in phase and elastic and anelastic response to pressure and temperature. With the long thermal equilibration time in cratonic settings, even relatively mild variations in concentrations of radioactive elements can leave long-lasting lithospheric thermal anomalies of 100–300 °C. Concentrations of radioactive elements decrease with increasing melt depletion (or decreasing metasomatic refertilization), resulting in lower temperatures and increased seismic velocities. This thermal seismic effect enhances the intrinsic velocity-increasing compositional seismic signature of melt depletion. The joint thermochemical effects can leave cratonic seismic anomalies of up to 3–4.5 per cent in VS and up to 2.5–4 per cent in VP, with gradients sometimes as sharp as a few kilometre in width. Thus the variations in major and minor element mantle lithosphere composition commonly seen in mantle samples can account for much of the variability in imaged seismic structure of cratonic lithosphere.

The deep structure of the Australian continent from surface wave tomography

Article

Sep 1999
LITHOS

We present a new model of 3-D variations of shear wave speed in the Australian upper mantle, obtained from the dispersion of fundamental and higher-mode surface waves. We used nearly 1600 Rayleigh wave data from the portable arrays of the Skippy project and from permanent stations (from Agso, Iris and Geoscope). AgSo data have not been used before and provide better data coverage of the Archean cratons in western Australia. Compared to previous studies we improved the vertical parameterization, the weighting scheme that accounts for variations in data quality and reduced the influence of epicenter mislocation on velocity structure. The dense sampling by seismic waves provides for unprecedented resolution of continental structure, but the wave speed beneath westernmost Australia is not well constrained. Global compilations of geological and seismological data (using regionalization based on tectonic behavior or crustal age) suggest a correlation between crustal age and the thickness and composition of the continental lithosphere. However, the age and the tectonic history of crustal elements vary on wavelengths, much smaller than have been resolved with global seismological studies. Using our regional upper mantle model we investigate how the seismic signature of tectonic units changes with increasing depth. At large wavelengths, and to a depth of about 200 km, the inferred velocity anomalies corroborate the global pattern and display a progression of wave speed with crustal age: slow wave propagation prevails beneath the Paleozoic fold belts in eastern Australia and wave speeds increase westward across the Proterozoic and reach a maximum in the Archean cratons. The high wave speeds associated with Precambrian shields extend beyond the Tasman Line, which marks the eastern limit of Proterozoic outcrop. This suggests that parts of the Paleozoic fold belts are underlain by Proterozoic lithosphere. We also infer that the North Australia craton extends off-shore into Papua New Guinea and beneath the Indian Ocean. For depths in excess of 200 km a regionalization with smaller units reveals that some tectonic subregions of Proterozoic age are marked by pronounced velocity highs to depths exceeding 300 km, but others do not and, surprisingly, the Archean units do not seem to be marked by such a thick high wave speed structure either. The Precambrian cratons that lack a thick high wave speed “keel” are located near passive margins, suggesting that convective processes associated with continental break-up may have destroyed a once present tectosphere. Our study suggests that deep lithospheric structure varies as much within domains of similar-crustal age as between units of different ages, which hampers attempts to find a unifying relationship between seismic signature and lithospheric age.

The lithospheric mantle of the Archean Superior Province as imaged by garnet xenocryst geochemistry

Article

Jul 2004
CHEM GEOL

The major and trace element chemistry of over 500 garnet xenocrysts from 12 kimberlite and alkaline rock bodies are used to examine the mantle lithosphere below the Archean Superior Province. All of the Superior Province kimberlites contain garnets that were in equilibrium with clinopyroxene. The majority of the Superior province garnets have normal rare earth element (REE) patterns but LREE-enriched sinuous patterns are encountered in garnets from almost all the pipes. Ni-in-garnet geothermometry shows the garnets sampled mantle lithosphere over a temperature interval of 850 to 1225 °C, corresponding to a depth range of ∼110 to 210 km when projected to a xenolith-derived geotherm for the southeastern Superior Province.Continuous increases in Y, and decreases in Sc/V with temperature are typical of mantle sections sampled by garnet in Paleozoic and Mesozoic kimberlites. In almost all regions sampled in the Superior Province, depleted peridotite with less than 1.5 wt.% Al2O3 and high Sc/V constitutes the shallowest sections (<120 km depth) of the lithospheric mantle at TNi<950 °C. These shallow, infertile parts of the lithosphere support the compositional buoyancy inferred to support the Superior cratonic root against convective removal. A different trend with depth is observed for garnet suites from 1.1-Ga kimberlites and alkaline rocks, perhaps due to a different mantle structure at this time. Differences in the chemical signatures between garnet suites in kimberlites within only kilometers of one another across major fault grabens in the Lake Timiskaming region imply chemically modified mantle lithosphere along extremely narrow zones that are still observed there today by seismic tomography.

Subduction initiation and continental crust recycling: The roles of rheology and eclogitization

Article

Dec 2001
TECTONOPHYSICS

A mechanical model with viscous (ductile) and plastic (brittle) rheologies is used to investigate the effect of eclogitization on the dynamics of convergence. Density increases by 300–600 km/m3 during eclogitization of crustal rocks and continental lower crust and oceanic crust reach higher density than mantle. We explore cases of intracontinental deformation, subduction, and continental collision. We consider a wide range of parameters for friction, activation energy, and initial thermal state, and cases with or without eclogitization. The style of deformation appears to be primarily controlled by the presence or absence of weak zones. Simulations are run with a constant convergence velocity (1.5 or 4 cm/year) and the evolution of the compressive force through time is thus a critical test of the model viability. For intracontinental deformation, when the brittle crust is decoupled from the mantle, the mantle deforms by symmetrical bending or subduction, and a variable amount of lowermost crust is entrained with the sinking mantle. In these cases, the compressive force remains in the (1012–3×1013 N/m) acceptable range. Oceanic subduction only occurs if a low friction shear zone is specified in the brittle realm. During the transition from oceanic subduction to collision the oceanic crust in this shear zone remains trapped between colliding crusts. Eclogitization has no influence on the initial mode of deformation. Thus, it is probably not an important factor in the initiation of subduction. In all simulations, the influence of eclogitization increases progressively with time, in proportion of the amount of lower continental crust and oceanic crust entrained into the mantle. The structural evolution of the orogen model depends on eclogitization if initial decoupling occurs at mid-crustal level. The simulations thus indicate that eclogitization promotes convergence and also, in some cases, enables the recycling of the whole continental lower crust into the mantle. We also suggest that eclogitization could regulate the long-term convergence rate in orogens.

Thermal state of continental and oceanic lithosphere

Article

Full-text available

Jan 2010

Derrick Hasterok

The thermal state of the continental and oceanic lithosphere is reassessed on the basis of new databases for global heat flow and lithospheric heat production, recent advances in thermophysical properties measurements of minerals at high pressures and temperatures, and a better understanding of convective heat loss in young seafloor. The updated global heat flow database incorporates >60,000 records with >44,800 heat flow determinations. The update significantly increases the quantity and spatial coverage of global heat flow data since the last update in 1993. A new family of continental geotherms is proposed that is parametric in surface heat flow and takes advantage of thermophysical property data. The range of geotherms is constrained by xenolith P–T estimates; a cratonic geotherm consistent with a surface heat flow of 40 mW/m2 is particularly well constrained. Upper crustal heat production represents ∼26% of the total surface heat flow. Average heat production for the continental lower crust and mantle are 0.4 μW/m3 and 0.02 μW/m3, respectively.

Structure and evolution of the Australian continent : insights from seismic and mechanical heterogeneity and anisotropy

Thesis

Jan 2002

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002. Includes bibliographical references (p. 235-261). In this thesis, I explore the geophysical structure and evolution of the Australian continental lithosphere. I combine insights from isotropic and anisotropic seismic surface-wave tomography with an analysis of the anisotropy in the mechanical properties of the lithosphere, inferred from the coherence between gravity anomalies and topography. With a new high-resolution waveform tomographic model of Australia, I demonstrate that the depth of continental high wave speed anomalies does not universally increase with age, but is dependent on the scale and the tectonic history of the region under consideration. I construct an azimuthally anisotropic three-dimensional model of the Australian upper mantle from Rayleigh-wave waveforms. I compare Bayesian inverse methods with discretely parameterized regularization methods, and explore the use of regular, tectonic and resolution-dependent tomographic grids. I advocate the use of multitaper spectral estimation techniques for coherence analysis of gravity and topography, applied to Australian isostasy. I investigate the importance of internal loading, the directional anisotropy of the gravitational response to loading, and the estimation bias affecting the long wavelengths of the coherence function. I develop a method for non-stationary coherence analysis which enables a complete characterization of continental strength by the dependency of gravity-topography coherence on wavelength, direction and geologic age. Combining high-resolution, depth-dependent anisotropy measurements from surface-wave tomography with the mechanical anisotropy from gravity/topography coherence, I assess the validity of two competing theories regarding the cause of continental anisotropy (vertically coherent deformation or simple asthenospheric flow) quantitatively for the very first time. by Frederik Jozef Maurits Simons. Ph.D.

Geothermal Structure

Chapter

Jan 2021

A. G. Dessai

The shield comprises cratons, both with steady-state and anomalous thermal structures. The WDC and Bundelkhand craton display thermal structure expressed by perturbed geotherms. The difference between the two is stark, whereas the WDC shows intra-craton variation in thermal structure and exhibits a Proterozoic geotherm overprinted by the Cenozoic thermal anomaly, the Bundelkhand craton has a Cenozoic thermal imprint with contributions from crustal accretion. These changes are the result of advective heat from magmas ponded at the crust-mantle boundary. The distribution of granulites and the eclogitic rocks provides an indication of the rate of thermal equilibration and its spatial distribution.

Regional variations in geothermal gradient and heat flow across the African plate

Article

Full-text available

Jul 2020
J AFR EARTH SCI

Duncan Macgregor

1813 geothermal gradient measurements from deep wells have been accessed across the African plate. Thermal conductivities based largely on North African sampling and publications have been used to either directly estimate heat flow from these or to calibrate other author's figures to form a standardised dataset. Variations seen in geothermal gradient can be due to lateral changes in lithology and thermal conductivity. The thermal regime of West Africa is shown to be consistent and featureless, with the exception of high heat flows over some igneous-influenced areas such as Senegal/Mauritania. Southern Africa is consistently thermally cold. More tectonically active regions in eastern and northern Africa show far more variation and generally higher heat flows. Two main controls are identified, namely time elapsed from rifting/break-up and an igneous related input of heat from the mantle. Plots of heat flow against age of basin show support for McKenzie models of exponential thermal decline. The Red Sea is therefore a valid analogue for the past thermal regime of many current passive margins. Extinct magmatic margins are colder than their amagmatic counterparts. The regions of highest heat flow show a correlation with those of young mantle-derived igneous activity and those of anomalous upper mantle seismic velocities, indicative of partial melting. PAPER ACCESSIBLE UNTIL8 OCT AT https://authors.elsevier.com/a/1bbFP,XIvbfF-0

Crustal heat production and estimate of terrestrial heat flow in central East Antarctica, with implications for thermal input to the East Antarctic ice sheet

Article

Full-text available

Aug 2017
TCD

John W. Goodge

Terrestrial heat flow is a critical first-order factor governing the thermal condition and, therefore, mechanical stability of Antarctic ice sheets, yet heat flow across Antarctica is poorly known. Previous estimates of terrestrial heat flow come from inversion of seismic and magnetic geophysical data, by modeling temperature profiles in ice boreholes, and by calculation from heat production values reported for exposed bedrock. Although accurate estimates of surface heat flow are important as an input parameter for ice-sheet growth and stability models, there are no direct measurements of terrestrial heat flow in East Antarctica coupled to either subglacial sediment or bedrock. Bedrock outcrop is limited to coastal margins and rare inland exposures, yet valuable estimates of heat flow in central East Antarctica can be extrapolated from heat production determined by the geochemical composition of glacial rock clasts eroded from the continental interior. In this study, U, Th and K concentrations in a suite of Proterozoic (1.2–2.0 Ga) granitoids sourced within the Byrd and Nimrod glacial drainages of central East Antarctica indicate average upper crustal heat production (Ho) of about 2.6 ± 1.9 μW m⁻³. Assuming typical mantle and lower crustal heat flux for stable continental shields, and a length scale for the distribution of heat production in the upper crust, the heat production values determined for individual samples yield estimates of surface heat flow (qo) ranging from 33–84 mW m⁻² and an average of 48.0 ± 13.6 mW m⁻². Estimates of heat production obtained for this suite of glacially-sourced granitoids therefore indicate that the interior of the East Antarctic ice sheet is underlain in part by Proterozoic continental lithosphere with average surface heat flow, providing constraints on both geodynamic history and ice-sheet stability. The ages and geothermal characteristics of the granites indicate that crust in central East Antarctica resembles that in the Proterozoic Arunta and Tenant Creek inliers of Australia, but is dissimilar to other areas characterized by anomalously high heat flow in the Central Australian Heat Flow Province. Age variation within the sample suite indicates that central East Antarctic lithosphere is heterogeneous, yet the average heat production and heat flow of four age subgroups cluster around the group mean, indicating minor variation in thermal contribution to the overlying ice sheet from upper crustal heat production. Despite their minor differences, ice-sheet models may favor a geologically realistic model of crustal heat flow represented by such a distribution of ages and geothermal characteristics.

Paleoproterozoic collisional system and diamondiferous lithospheric keel of the Yakutian Kimberlite Province

Article

Jan 2005

The Paleoproterozoic collisional system of the northeastern Siberian craton is compared with the underlying lithosphere mantle. This system appeared at about 1.9-1.8 Ga, through accretion of microcontinents with an age of 3.1-2. 5 Ga. Evidence comes from isotope dating of the formation of ancient terranes, their thermal transformation, and melting of collisional granitoids. The crustal structure inferred from geological and geophysical data bears a relict signature of collisional systems, including deformations, up to 58 km thickened crust, and even slope of seismic surfaces along the predicted directions of collisional thrust. The crustal structures are underlain by thick, up to 260-300 km, diamondiferous lithosphere mantle with higher seismic velocities, which thins out to ≤ 200 km toward the margins of the region. This local bulge may be identified as a lithosphere keel (root). The spatial relationship between this mantle keel and crustal collisional system of Proterozoic age is geometrically evident, and magmatic events are obviously coeval. But proportions of relevant processes are not clear. The simplest supposition is that the keel formed as a result of the accretion of fragments of the Archean lithosphere mantle together with the crustal terranes attached on top. This supposition contradicts the existing ideas of the exceptionally crustal manifestation of continental collision, whereas the underlying mantle slips free and far away. It seems to be the subject of future studies.

The collisional system in the Northeastern Siberian Craton and a problem of diamond-bearing lithospheric keel

Article

Nov 2005
GEOTECTONICS+

The diamond-bearing kimberlite diatremes in the northeastern Siberian Craton allow the tectonic models of the formation of Earth's crust to be compared with the specific features of the underlying upper mantle. The Mesoarchean (3.2-2.8 Ga) stage in the evolution of the craton was characterized by the creation of sialic continental masses that amalgamated into the Pangea-0 supercontinent 2.8-2.6 Ga ago. Pangea-0 subsequently broke up into new microcontinents that amalgamated again 1.9-1.8 Ga ago into the Siberian Craton in its present-day configuration as a part of the Pangea-1 supercontinent. This sequence of events is supported by the isotopic ages of ancient terranes, their subsequent thermal transformation, and the generation of collision-related granitoids. Interpretation of seismic profiles indicates that the attributes inherent to the Phanerozoic collision prisms have been retained in the crust of the Paleoproterozoic fold system. These attributes include the Earth's crust as thick as 58 km, fragments of the lower, high-velocity layer (presumably, underplating basaltic rocks), and deep-seated blocks of high-velocity upper mantle raised to the level of the lower crust. These crustal units are underlain by the high-velocity diamond-bearing lithospheric mantle that has thickened to 260-300 km. The thickness of this mantle layer decreases to approximately 200 km toward the margins of the region, and a local swell has been identified as a lithospheric keel (root). The spatial relationships between the crustal and mantle structures are evident. It would be the simplest to suggest that this keel was formed as a result of accretion of the lithospheric mantle fragments together with the crustal terranes attached to their roofs. Such a suggestion comes into conflict with the commonly accepted views on the exclusively crustal nature of the continental collision and may become a subject of further investigations.

Redox - pressure - temperature conditions in the continental upper mantle in relation to C-O-H fluid speciation

Article

Mar 2012

Alexey Goncharov

The thesis is based on a petrologic and geochemical study of mantle xenoliths from the central Siberian craton and the Baikal-Mongolia region of central Asia. Its goal is to establish the redox regime of the lithospheric mantle in these two domains with distinct tectonic settings and age and relate it to thermal regime and the speciation of C-0-H fluids. Oxygen fugacity is calculated based on Fe2+/Fe3+ ratios in spinel and garnet of mantle peridotites obtained by Mössbauer spectroscopy. The study deals with the following topics: (i) microstructures, chemical and mineralogical composition of the xenoliths; (ii) Fe2+/Fe3+ ratios in minerals by Mössbauer spectroscopy; (iii) equilibration temperatures and pressures using mineral thermo-barometry; (iv) oxygen fugacity from mineral compositions using oxybarometry; (v) proportions of molecular components in C-0-H fluids coexisting with the studied rocks. The three main conclusions of this study are: (1) Oxygen fugacity in the lithospheric mantle in the central Siberian craton decreases from +1 to -4 ΔlogʄO2 (FMQ) at depths from 70 to 220 km and shows significant lateral variations. (2) The lithospheric mantle beneath the Baikal-Mongolia region shows important redox heterogeneities, with a sharp decrease in oxygen fugacity (from +0 to -3 AlogfO2 (FMQ)) during the transition from the spine! to garnet facies peridotites at 50 to 90 km. (3) The speciation of C-O-H fluids changes with depth from essentially H2O-CO2 in the shallow lithospheric mantle to H2O-CH4 at the lithosphere-asthenosphere boundary regardless of the thermal state and the thickness of the lithosphere

Dynamic role of the rheological contrast between cratonic and oceanic lithospheres in the longevity of cratonic lithosphere: A three-dimensional numerical study

Article

Apr 2012
TECTONOPHYSICS

Masaki Yoshida

In the numerical modeling of mantle convection, it is still difficult to find the conditions which allow both stable cratonic lithosphere and plate tectonics. The three-dimensional numerical model presented herein makes it possible to model the cratonic lithosphere that survives for a sufficiently long geological timescale. An important factor in the longevity of cratonic lithosphere is the localized rheological (viscosity) contrast between the cratonic and oceanic lithospheres, i.e., the presence of a weak (low-viscosity) continental margin (WCM), such as tectonically mobile (orogenic) belts, that surrounds the lateral side of cratonic lithosphere. The WCM protects the cratonic lithosphere from being stretched by the surrounding convection force. In addition to the presence of a WCM, the higher viscosity of the cratonic lithosphere itself effectively contributes to the stability of the cratonic lithosphere, as suggested by the previous numerical modeling. However, the results of the present study suggest that the WCM plays a primary role in the longevity of cratonic lithosphere, even if the viscosity contrast between the cratonic and oceanic lithospheres is quite high, 103, and the high-viscosity of cratonic lithosphere may play a secondary role in the longevity of cratonic lithosphere. The combination of the presence of a WCM and the high-viscosity of cratonic lithosphere may realize the longevity of cratonic lithosphere that survives for over two billion years.

Diamond mega-placers: Southern Africa and the Kaapvaal craton in a global context

Article

Full-text available

Oct 2005

Diamond mega-placers, defined as ≥ 50 million carats at ≥ 95% gem quality, are known only from along the coast of southwestern Africa, fringing the Kaapvaal craton, where two are recognized. One is associated with the Orange-Vaal dispersal, the other, to the south, has an uncertain origin. Placers are residual when left on the craton, transient when being eroded into the exit drainage, and terminal. Terminal placers, the final depositories of diamonds, have the greatest probability of being a mega-placer. There are four main groups of controls leading to the development of a mega-placer: the craton, the drainage, the nature of the environment at the terminus and the timing. Cratons, being buoyant, have a tendency to leak diamonds into surrounding basins; however, being incompressible they may have orogens converge onto them resulting in some lost sediment being returned as foreland basin fills. The craton size, its diamond-fertility and the retention of successive kimberlite intrusions that remain available to the final drainage, are significant to mega-placer development. Maximum potential recovery is achieved when the drainage delivering diamonds to the mega-placer is efficient, not preceded by older major drainages and focuses the supply to a limited area of the terminal placer. There should be sufficient energy in the terminal placer regime to ensure that sediment accompanying the diamonds is removed to areas away from the placer site. All conditions should be near contemporaneous and most were satisfied in the Orange-Vaal Rivers-Kaapvaal system and mega-placers were consequently generated.

Controls of stable continental lithospheric thickness: the role of basal drag

Article

Jun 2007
LITHOS

Zvi Garfunkel

The lithosphere is the earth's strong outer layer that remains a coherent unit for long times during plate motions. The motions lead to shearing of its hot and ductile lower part. The base of the lithosphere is placed where the shearing is large enough to effectively decouple the coherent plates from the underlying mantle. Here the depth and temperature where this occurs under stable continental areas were modeled by using the experimentally determined creep properties of olivine (which depend on temperature, pressure and fluid content) for different geotherms and different magnitudes of the basal drag. It is found that the shearing increases rapidly downward in a ca. 20–25 km thick decoupling zone where the bottom of the lithospherecan can be placed. This zone is deeper for cooler geotherms and for smaller a basal drag, while the presence of fluids makes it shallower. In dry rock this zone is at the mantle adiabat or cooler when the basal drag is >0.5 MPa. Under Archean cratons whose temperature is represented by xenolith geotherms the decoupling zone is 180–240 km deep. Higher crustal heat generation than in cratons (common in post-Archean terranes) by itself causes the decoupling zone to be a few tens of kilometers shallower, and it will be even shallower if the mantle heat flow is larger than under cratons. The base of the lithosphere need not follow an isotherm. As the lithospheric thickness depends on several factors it will change when any of these changes, which is expected to have happened many times in earth history. To persist the lithosphere should escape being sheared off laterally, regardless of its buoyancy.

Effect of lithosphere thickness heterogeneities in controlling rift localization: Numerical modeling of the Oslo Graben

Article

May 2002

We present 2-D thermo-mechanical and numerical modeling at lithospheric scale to assess the impact of pre-existing lithosphere thickness contrast on rift localization. The results of the modeling suggest that thickness heterogeneity was responsible for focussing rifting and passive asthenospheric upwelling in the Permo-Carboniferous Oslo Graben at the western margin of the Fennoscandian Shield. These findings are consistent with inferences from recent studies showing that both the crust-mantle boundary and the base of the lithosphere are and were deeper at the eastern margin of the Oslo Graben, resulting in contrasting crust and lithosphere thickness.

The Lithosphere: An Interdisciplinary Approach

Book

Full-text available

Oct 2011

Irina Artemieva

Modern Earth science suffers from fragmentation into a large number of sub-disciplines with limited dialog between them, and artificial distinctions between the results based on different approaches. This problem has been particularly acute in lithospheric research, where different geophysical techniques have given rise to a multitude of definitions of the lithosphere – seismic, thermal, electrical, mechanical, and petrological. This book presents a coherent synthesis of the state-of-the art in lithosphere studies based on a full set of geophysical methods (seismic reflection, refraction, and receiver function methods; elastic and anelastic seismic tomography; electromagnetic, magnetotelluric, thermal and gravity methods; and rheological modeling), complemented by petrologic data on mantle xenoliths and laboratory data on rock properties. It also provides a critical discussion of the uncertainties, assumptions, and resolution issues that are inherent in the different methods and models. Most importantly, it discusses the relationships between methods and presents directions for their integration to achieve a better understanding of the processes that affect the lithosphere and thereby shape the Earth on which we live. Multi-disciplinary in scope, global in geographical extent, and covering a wide variety of tectonic settings over 3.5 billion years of Earth history, this book presents a comprehensive overview of lithospheric structure and evolution. It is a core reference for researchers and advanced students in geophysics, geodynamics, tectonics, petrology, and geochemistry, and for petroleum and mining industry professionals.

Heat flow in the Trans-Hudson Orogen of the Canadian Shield-Implications for Proterozoic continental growth

Article

Full-text available

Dec 1999

Fourteen new heat flow and radiogenic heat production measurements have been obtained in the Paleo-Proterozoic Trans-Hudson Orogen of the Canadian Shield. This orogen, which consists of several distinctive belts, corresponds to a pulse of crustal growth through island arc magmatism between 1.9 and 1.8 Ga. The data now available include 17 previously published measurements. Heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen. The average and standard deviation of heat flow values for the entire orogen, 425:11 mW m -2, are identical to those of the older Superior Province and of the younger Grenville Province. For the orogen as a whole, heat flow is weakly correlated to the heat production of surface rocks. High heat flow values are found in the Thompson belt, consisting of metasedimentary rocks deposited on the ancient continental margin of the Superior craton. There the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of r • dioelements in large volumes of rocks. The heat flow is low in the belts that expose juvenile Proterozoic crust consisting mostly of arc-related volcanic rocks. In the Flin Flon-Snow Lake Belt, the average heat flow is the same as the average of the orogen. The low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed a,t the surface are thin and rest on a basement richer in radioelements. In the Lynn Lake belt, the heat flow is significantly lower than the average for the orogen although the surface heat production is not low. The heat flow data require a thin (<10 kin) surface layer overlying the mid and lower crust depleted in ra. dioelements. Around the town of Lynn Lake, heat flow is consistently low over a distance of m40 kin. The coincidence between this "cold spot" and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures. The evolution of the Trans-Hudson Orogen exemplifies the interplay between the processes generating rocks of evolved composition, which require crustal thickening, and those forming "normal" continental crust with average thickness, which require crustal flow and soft crustal rheology.

Heat production and geotherms for the continental lithosphere

Article

Full-text available

Jul 2011
EARTH PLANET SC LETT

We propose a continental lithosphere heat production model based on the petrology of crust and mantle, heat production measurements of surface and xenolith samples, and tectono-thermal constraints. Continental elevation considered within a thermal isostasy rubric is used to partition crustal heat production into upper crustal and lower crustal contributions. The best-fitting partition model using elevation data from 33 North American tectonic provinces suggests upper crustal heat production on average accounts for ~ 6% of observed surface heat flow. An average heat production for the lower crust of 0.4 μW/m3 is based on measurements from exposed granulite terranes while a lithospheric mantle heat production of 0.02 μW/m3 is based on chemical analyses of mantle xenoliths. Results are relatively insensitive to mantle composition and thickness of the upper crustal heat producing layer. Continental geotherms are computed using the generalized heat production model and incorporating thermal conductivity results from a number of recent laboratory studies. P–T conditions of xenoliths provide further constraints to ensure that our geotherms and hence the heat production model are reasonable. P–T conditions of 10 Precambrian regions are consistent with surface heat flow of 40 mW/m2 and a lithospheric thickness of 200 km. Our generalized model for heat production can serve as a reference model from which anomalies are identified.Research highlights► Estimate of lithospheric heat production and temperatures. ► Average upper crustal heat production is estimated at 26% of surface heat flow. ► Geotherm family for the continental lithosphere parameterized in surface heat flow. ► Cratonic regions with xenoliths have average surface heat flow of 40 mW/m2.

Determining the temperature of the Earth’s continental upper mantle from geochemical and seismic data

Article

Mar 2006

A method is proposed for determining the temperature of the Earth’s upper mantle from geochemical and seismic data. The data are made consistent by physicochemical simulations, which enable one to derive physical characteristics from geochemical compositional models (direct problem) and to convert seismic velocity profiles into model for the temperature distribution (inverse problem). The methods were used to simulate temperature distribution profiles in the “normal” and “cold” mantle on the basis of profiles for the velocities of P and S waves in the IASP91 model and regional models for the Kaapvaal craton. The constraints assumed for the chemical composition included the depleted material of garnet peridotites and the fertile primitive mantle. The conversion of seismic into thermal profiles was conducted by minimizing the Gibbs free energy with the use of equations of state for the mantle material with regard for anharmonicity and the effects of inelasticity. The sensitivity of the model to the chemical composition and its importance in application to the solution of inverse problems is demonstrated. Temperature profiles derived from the IASP91 and some regional models for depths of 200–210 km display an inflection on geotherms toward decreasing temperatures, which is physically senseless. This anomaly cannot be related to either the presence of volatiles or the occurrence of partial melting, because both of them should have resulted in a decrease, but not an increase, in the seismic velocities. Temperature inversion can be ruled out by the gradual fertilization of the mantle with depth. In this situation, the upper mantle material at depths of 200–300 km should be enriched in FeO, Al2O3, and CaO relative to garnet peridotites and be simultaneously depleted in these oxides relative to the pyrolite material of the primitive mantle. It can be generally concluded that both the lithosphere and sublithospheric mantle of the Kaapvaal craton, as well as the normal mantle, should be chemically stratified.

On the role of heat flow, lithosphere thickness and lithosphere density on gravitational potential stresses

Article

Oct 2006
TECTONOPHYSICS

Christophe Pascal

Gravitational potential stresses (GPSt) are known to play a first-order role in the state of stress of the Earth's lithosphere. Previous studies focussed mainly on crust elevation and structure and little attention has been paid to modelling GPSt using realistic lithospheric structures. The aim of the present contribution is to quantify gravitational potential energies and stresses associated with stable lithospheric domains. In order to model realistic lithosphere structures, a wide variety of data are considered: surface heat flow, chemical depletion of mantle lithosphere, crustal thickness and elevation. A numerical method is presented which involves classical steady-state heat equations to derive lithosphere thickness, geotherm and density distribution, but additionally requires the studied lithosphere to be isostatically compensated at its base. The impact of varying surface and crustal heat flow, topography, Moho depth and crust density on the signs and magnitudes of predicted GPSt is systematically explored. In clear contrast with what is assumed in most previous studies, modelling results show that the density structure of the mantle lithosphere has a significant impact on the value of the predicted GPSt, in particular in the case of thick lithospheres. Using independent information from the literature, the method was applied to get insights in the state of stress of continental domains with contrasting tectono-thermal ages. The modelling results suggest that in the absence of tectonic stresses Phanerozoic and Proterozoic lithospheres are spontaneously submitted to compression whereas Archean lithospheres are in a neutral to slightly tensile stress state. These findings are in general in good agreement with global stress measurements and observed geoid undulations.

Heat flux and seismicity in the Fennoscandian Shield

Article

Nov 2001
PHYS EARTH PLANET IN

The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheological structure is analysed from the Kola Orogen to the Sorgenfrei–Tornquist zone. Moho temperatures and the heat flux from the mantle are typical of cratonic areas. The deep thermal field shows a cold root in the north-eastern sector. Larger lateral variations of mantle heat flux and Moho temperatures occur in the southern area and at the edges of the shield. The thickness of the thermal lithosphere is maximum (200–220 km), where the mantle heat flux is minimum (about 15–20 mW m−2). Three lithospheric cross-sections illustrate the expected lateral variation in viscosity and failure mode throughout the lithosphere as a consequence of the different geothermal conditions and the rheological stratification. In the upper crust, the depth of the brittle–ductile transition varies on average from 30, in the north-east, to 15 km, in the south-west. In the subcrustal mantle, the models predict a mainly ductile behaviour. At a depth of 60 km, the upper mantle viscosity is maximum (2.5×1023 to 25.0×1023 Pa s) beneath the Archean province and minimum (2.5×1021 to 10.0×1021 Pa s) below the Svecofennian and Sveconorwegian units. The comparison of the rheological calculations with the seismic activity shows a general agreement with the brittle–ductile transition depth expected in the Archaean and Proterozoic provinces. There is a difference both in number of earthquakes and distribution versus depth between areas of different age. The occurrence of larger magnitude seismic events near the base of the seismogenic zone, in the region of inferred peak of shear resistance, suggests some causal relationship. The Proterozoic areas show a more regular decrease of shocks with depth and, compared to the Archean provinces, a tendency to shallowing.

Imaging global chemical and thermal heterogeneity in the subcontinental lithospheric mantle with garnets and xenoliths: Geophysical implications

Article

Apr 2006
TECTONOPHYSICS

Suites of mantle-derived xenoliths in volcanic rocks provide estimates of the geothermal gradient and composition of the subcontinental lithospheric mantle (SCLM) at the time of the volcanic eruption. The development of single-grain thermometry and barometry, applied to xenocryst minerals in volcanic rocks, has greatly expanded the number of localities for which such data can be obtained and made it feasible to map the geology of the SCLM on a broader scale, both vertically and laterally. From garnet xenocrysts, it is possible to derive profiles showing mean values of olivine composition, bulk-rock composition, density and seismic velocities, as well as geotherm parameters and constraints on the thickness of the SCLM. Geochemical profiles, coupled with Re–Os dating of peridotites and their enclosed sulfide minerals, show that Archean or Proterozoic SCLM is preserved at shallow levels beneath many areas of younger tectonothermal age; this implies rapid vertical variations in Vs and Vp with depth, which may affect seismic interpretations. Data from several hundred localities worldwide define a secular evolution in the composition of the SCLM, related to the tectonothermal age of the overlying crust. Archean SCLM is typically strongly depleted in basaltic components, highly magnesian and thick (160–250 km), and has low geotherms; Phanerozoic SCLM is typically fertile (rich in basaltic components), Fe-rich, thin (50–100 km) and has a range of high geotherms; Proterozoic SCLM (much of which may be reworked Archean mantle) tends to be intermediate in all respects. The correlated variations in SCLM fertility, lithospheric thickness and geotherm reinforce the effects of each on seismic velocity, and produce more rapid lateral variations in seismic response than would result from thermal effects alone. These correlations are the key to using seismic tomography images to map the lateral extent of different types of SCLM.

Preservation patterns of Late Cenozoic fluvial deposits and their implications: Results from IGCP 449

Article

Oct 2008
QUATERN INT

IGCP 449 (2000–2004) amassed data on fluvial systems world-wide, concentrating on key sequences, especially those benefiting from multiple lines of dating evidence. These archives are preserved either in terraced or stacked sequences, the latter confined to areas of subsidence. Terrace staircases record repeated incision (in response to progressive surface uplift), which, alternately with aggradation, is thought to have been climatically triggered. This triggering can be attributed to Quaternary (Milankovitch) climatic fluctuation, but the number of terraces produced in each Middle–Late Pleistocene 100 ka climatic cycle varies significantly between different systems. An unexpected result of this data collection has been the recognition of differing patterns of fluvial sedimentation and valley evolution over Neogene and Quaternary timescales, apparently related to different types of continental crust with different uplift/subsidence histories. These fall into three groups: (1) a typical uplifting pattern, with extensive terrace staircases of the type that dominate the global fluvial archive; (2) a subsiding pattern, with stacked fluvial sediments, usually coinciding with major depocentres; and (3) a stable pattern, with preservation of deposits related to channel diversion rather than significant incision. The third type, in which Early and pre-Quaternary deposits occur within a few metres of modern river level, is generally restricted to ancient cratonic or shield areas, which have apparently experienced minimal Late Cenozoic uplift, in marked contrast with most continental areas on more recently formed crust. There is also an intermediate situation, in Early Proterozoic crust, involving alternations of uplift and subsidence, with little resultant net vertical motion.

Evidence for an upper mantle plume beneath the Tanzanian Craton from Rayleigh wave tomography

Article

Sep 2003
J GEOPHYS RES

The Archean Tanzanian craton, nestled between the eastern and western branches of the East African Rift, presents a unique opportunity to study the interaction of active rifting with stable cratonic lithosphere. The high density of Rayleigh wave paths recorded in a regional seismic array yields unusually precise determinations of phase velocity within the Tanzanian craton. Shear velocities in the cratonic lithosphere are higher than a global average to a depth of 150 +- 20 km. Beginning at 140 km, shear velocity decreases sharply, reaching a minimum of 4.20 +- 0.05 km/s at depths of 200-250 km. The base of the lithosphere, identified by the depth to the center of the maximum negative velocity gradient, is similar to that found beneath other Archean lithospheres. Where Cenozoic rifting crosscuts the southern corner of the craton, velocities up to 130 km depth are reduced, indicating recent disruption of the lithosphere. The anomalously low velocities beneath the Tanzanian craton indicate high temperatur

The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains

Article

Full-text available

Jan 1981

Raymond A. Price

The thick (∼40 km) slab of Hudsonian (>1750 Ma) continental crust that extends under western Canada from the Canadian Shield can be followed westward, on the basis of its distinctive magnetic anomalies, its influence on the Bouguer gravity values, the results of deep seismic refraction experiments, and the results of geomagnetic depth sounding of the deep electrical conductivity structure, to the Kootenay Arc. The Kootenay Arc is basically a W-facing monocline of crustal dimensions, across which the change in structural level involves an aggregate stratigraphic thickness of up to 20 km. It marks the western edge of the continental craton over which the displaced supracrustal rocks have been draped. Balanced structure sections of the thrust and fold belt, which take into consideration the deep crustal structure, as constrained by the geophysical data, show that: (i) in early Campanian time the continental crust that now lies beneath the western Rocky Mountains and the Purcell anticlinorium was covered with the platformal Palaeozoic to Upper Jurassic rocks and the exogeoclinal Mesozoic rocks that now form the northeasterly verging imbricate thrust slices of the eastern Rocky Mountains; (ii) the Cordilleran miogeocline developed outboard from the edge of the continental craton, on tectonically attenuated continental crust, or oceanic crust; and (iii) tectonic shortening of about 200 km in the supracrustal rocks in the Rocky Mountains must be balanced at a deeper level, W of the Kootenay Arc, by the shortening of the oceanic or attenuated continental crust. The net convergence between the Cordilleran magmatic arc and the autochthonous cover on the continental craton is a type of intra-plate subduction that was antithetic to the SW verging subduction zone marking the boundary of the North American Plate. The basement of the back-arc or marginal basin, in which the miogeocline formed, was consumed; but the adjoining continental margin was not. The foreland thrust and fold belt is a shallow subduction complex that was tectonically prograded over the margin of the continental craton, as the supracrustal cover scraped off the down-going slab was piled up against the overriding mass, and spread laterally eastward under its own weight.

Structure and Formation of the Continental Tectosphere

Article

Full-text available

Jan 1988

Thomas Jordan

This paper reviews the evidence for deep continental structure and the arguments against simple cooling-plate models as explanations of this structure. High-resolution seismological studies of the upper mantle confirm the existence of a thick (> 300 km) thermal boundary layer (TBL) beneath the ancient cratonic nuclei. Petrological and gravimetric data suggest that the continental TBL is stabilized against convective disruption by a buoyant, viscous, chemical boundary layer (CBL) depleted in basaltic constituents and enriched in large-ion lithophile (LIL) elements relative to the source mantle of mid-ocean ridge vulcanism. Geothermal constraints indicate high heat production within the CBL and low heat flow through its base. It is inferred that very little of the internal heat being convectively transported out of the earth is escaping through the continental cratons. The most plausible mechanism for the formation of this continental tectosphere is by the advective thickening of a basalt-depleted, LIL-charged CBL during major episodes of compressive orogenesis, particularly those accompanying the assembly of supercontinents. The stability of the continental tectosphere and the discrepancy in the heat fluxing from the deep mantle beneath continents and oceans are evidence that the surficial CBL is strongly coupled to large-scale convective flow in the mantle. A model of the earth is proposed consisting of four major dynamical systems: the two convecting shells of the mantle and core, and the two CBLs at the free surface and core-mantle interface. Strong interactions among the low-wave number states of these four systems offer new possibilities for explaining the earth's large-scale, long-term behavior.

Fluxes and excess temperatures of mantle plumes inferred from their interaction with migrating mid-ocean ridges

Article

Full-text available

Jul 1991

Jean-Guy Schilling

The behavior of thermal plumes in the earth's upper mantle is strongly affected by their interaction with nearby mid-ocean ridges. The magnitude of the buoyant topography and the length of the geochemical anomaly induced by plumes at migrating ridge axes provide a way to estimate their excess temperature and discharge rate, and thereby constrain their depth of origin.

Heat Flow from the Earth's Interior: Analysis of the Global Data Set

Article

Full-text available

Aug 1993

We present a new estimate of the Earth's heat loss based on a new global compilation of heat flow measurements comprising 24,774 observations at 20,201 sites. On a 5° × 5° grid, the observations cover 62% of the Earth's surface. Empirical estimators, referenced to geological map units and derived from the observations, enable heat flow to be estimated in areas without measurements. Corrections for the effects of hydrothermal circulation in the oceanic crust compensate for the advected heat undetected in measurements of the conductive heat flux. The mean heat flows of continents and oceans are 65 and 101 mW m-2, respectively, which when areally weighted yield a global mean of 87 mW m-2 and a global heat loss of 44.2 × 1012 W, an increase of some 4-8% over earlier estimates. More than half of the Earth's heat loss comes from Cenozoic oceanic lithosphere. A spherical harmonic analysis of the global heat flow field reveals strong sectoral components and lesser zonal strength. The spectrum principally reflects the geographic distribution of the ocean ridge system. The rate at which the heat flow spectrum loses strength with increasing harmonic degree is similar to the decline in spectral strength exhibited by the Earth's topography. The spectra of the gravitational and magnetic fields fall off much more steeply, consistent with field sources in the lower mantle and core, respectively. Families of continental and oceanic conductive geotherms indicate the range of temperatures existing in the lithosphere under various surface heat flow conditions. The heat flow field is very well correlated with the seismic shear wave velocity distribution near the top of the upper mantle.

Structure and Evolution of the Continental Lithosphere

Article

Full-text available

Feb 1984
Phys Chem Earth

Paul Morgan

For thermal considerations, the lithosphere is defined as the outer layer of the Earth in which heat transfer is dominated by conduction. Low density continental crust caps the continental lithosphere preventing its subduction, and thus it has, in general, a long and complex thermal history. The thermal regime in the near surface zone of the continental lithosphere is commonly complex, and is responsible for much of the scatter in shallow heat flow data, the main data set used to examine the thermal structure and evolution of the continental lithosphere. A review of continental heat flow data shows that while heat flow is commonly high in areas of recent tectonic activity, there is no simple relationship between heat flow and age of tectogenesis. Low heat flow values are found in some areas of recent tectonic activity, and high heat flow values are found in some areas of older tectonism. An analysis of the parameters controlling the thermal structure of stable continental lithosphere shows that for the same heat flux into the base of the lithosphere, the main factors controlling temperatures within the lithosphere and surface heat flow are the quantity and distribution of heat producing elements within the lithosphere. In regions of recent tectonic and/or magmatic activity, perturbations in the thermal structure of the lithosphere are a function of the style and intensity of the tectonothermal disturbance. In hot spot and extensional settings, the lithosphere is thinned and its thermal gradient increased. The thermal effects at strike-slip boundaries are generally only of local importance, but can be significant when lithospheres with different thermal structures are juxtaposed. A variety of thermal phenomena are observed and predicted for zones of convergence, including low heat flow associated with subduction, high heat flow associated with magmatic activity, and thermal inversions associated with underthrusting. In seventeen continental areas, sufficient data are available to estimate the contribution of upper crustal heat production to the surface heat flow. This contribution is highly variable, and does not appear to correlate with crustal age for Proterozoic and Phanerozoic sites, but appears to be generally lower and less variable in Archean sites. In contrast, the component of heat flow from below the upper heat producing layer, the reduced heat flow, is relatively uniform, around 27 mW m−2 for sites which last experienced tectonism or magmatic activity in pre-Mesozoic times, but is highly variable for younger sites. From the uniform reduced heat flow for pre-Mesozoic sites, the stable thickness of the continental lithosphere is estimated to range from 90 to 220 km, thinner lithosphere being associated with higher heat production in the lithosphere. From this thickness, thermal perturbations in the lithosphere associated with tectonothermal activity are predicted to last no more than a few 100 m.y., a prediction consistent with available reduced heat flow data. Some prolongation of thermal relaxation is expected due to post-tectonic sedimentation and erosion, but these effects appear to be negligible at sites of Early Paleozoic age and older. The scatter in surface heat flow values at these sites is primarily the result of crustal heat production variations and near surface effects. Relatively low and uniform heat flow at Archean sites perhaps reflects relatively low and uniform crustal heat production at these sites.

Composition and development of the continental tectosphere

Article

Full-text available

Aug 1978

Thomas Jordan

Beneath the old continental nuclei are thick root zones which translate coherently during plate motions. These zones are apparently stabilised against convective disruption by the depletion of the continental upper mantle in a basalt-like component. Construction of this delicately balanced tectosphere is accomplished by the dynamic and magmatic processes of the Wilson cycle.

Evidence for a 150–200-km thick Archaean lithosphere from diamond inclusion thermobarometry

Article

Full-text available

May 1985

Garnet inclusions from diamonds that erupted in the Finsch kimberlite and kimberlites of the Kimberley group, situated in the southern part of the Kaapvaal craton, are noted to have Rb/Sr and Sm/Nd model ages of 3200-3300 Myr. Since the Kimberlites erupted about 100 Myr ago, the diamonds are xenocrysts. The present applications of thermobarometry to silicate inclusions in the diamonds suggest that ambient temperatures at those depths, 3000 Myr ago, were 900-1200 C, which is similar to the range predicted from present day heat flow.

Diamonds and the African Lithosphere

Article

Full-text available

May 1986

Data and inferences drawn from studies of diamond inclusions, xenocrysts, and xenoliths in the kimberlites of southern Africa are combined to characterize the structure of that portion of the Kaapvaal craton that lies within the mantle. The craton has a root composed in large part of peridotites that are strongly depleted in basaltic components. The asthenosphere boundary shelves from depths of 170 to 190 kilometers beneath the craton to approximately 140 kilometers beneath the mobile belts bordering the craton on the south and west. The root formed earlier than 3 billion years ago, and at that time ambient temperatures in it were 900° to 1200°C; these temperatures are near those estimated from data for xenoliths erupted in the Late Cretaceous or from present-day heat-flow measurements. Many of the diamonds in southern Africa are believed to have crystallized in this root in Archean time and were xenocrysts in the kimberlites that brought them to the surface.

Crustal radiogeneic heat production and the selective survival of continental crust

Article

Full-text available

Mar 1985

Paul Morgan

It is pointed out that the oldest terrestrial rocks have so far revealed no evidence of the impact phase of earth evolution. This observation suggests that processes other than impact were dominant at the time of stabilization of these units. However, a use of the oldest terrestrial rocks as a sample of the early terrestrial crust makes it necessary to consider the possibility that these rocks may represent a biased sample. In the present study, the global continental heat flow data set is used to provide further evidence that potassium, uranium, and thorium abundances are, on the average, low in surviving Archean crust relative to younger continental crust. An investigation is conducted of the implications of relatively low crustal radiogenic heat production to the stabilization of early continental crust, and possible Archean crustal stabilization models are discussed.

On the variation of continental heat flow with age and the thermal evolution of continents

Article

Feb 1980

The observed decrease of continental heat flow with tectonic age is interpreted in a three-component model. The first component is radiogenic heat from the zone of crustal isotopic enrichment, which yields about 40% of the observed heat flow in terrains of all tectonic ages. Because the surface heat flow varies with age between about 90 and 45 mW m-2, this radiogenic component also varies with age between about 36 and 18 mW m-2, a net decrease of some 18 mW m-2. The absolute decrease with time of this component is achieved by erosional removal of radioisotopes from the surface; the erosion, exponentially decreasing with age, has a characteristic time of some 300-400 Ma (m.y. BP). The second component of surface heat flow is residual heat from a transient thermal perturbation associated with tectogenesis. This transient yields c. 30% of the heat flow (27 mW m-2) in Cenozoic tectonic zones, diminishing effectively to zero in late Precambrian to mid-Precambrian terrains. The decay of this component has been fitted with temperature perturbation models that show a maximum perturbation to a cool shield geotherm of 750o -800oC at a depth of 80-100 km. From the analysis of this transient component we conclude that for residual heat to be observable up to and beyond 500 Ma, conductive cooling likely extends at least to depths as great as 300-350 km, implying that the continental crust and upper mantle beneath 500-Ma and older terrains has remained as a single conductive unit to such depths for at least the age of the terrain. For younger terrains the residual heat is yet sufficient to weaken them and make them vulnerable to recurrent tectonism. The remaining 12 mW m-2 of the background comes from deeper within the earth.-Authors English

Continents as a Chemical Boundary Layer

Article

May 1981

Thomas Jordan

The earth's mass density increases with depth to a value of about 13 Mg/cu m at its center. Some of the increase is due to gravitational self-compression, but well over half occurs, across two major compositional transitions, one at the planetary surface and one at the core-mantle boundary. The mass-transport mechanisms presumed to be operating within the mantle and core should tend to deposit chemically differentiated material at these transitions and thereby to form compositionally distinct boundary layers of intermediate density. As yet, the evidence for a chemical boundary layer (CBL) at the core-mantle interface is largely circumstantial. The model discussed is founded on the notion that the crust forms only the upper part of the surficial CBL. Below the crust is presumed to be a layer of refractory peridotite. The model accounts for certain perplexing seismological and petrological observations and offers new insights into the problem of continental evolution.

Thrusts and nappes in the North American Appalachian Orogen

Article

Jan 1981

R. D. Hatcher

The Appalachian Orogen in North America was subjected to three major deformational-thermal events: the Taconic (Ordovician-Silurian), Acadian (late Devonian), and Alleghanian (Permian). Each event involved large-scale horizontal transport of thrust-nappes ranging from tens to hundreds of kilometres in different parts of the orogen. Transport was dominantly westward although major Acadian-generated eastward transport occurred in southern New England. There is a direct relationship between chronological proximity to a thermal peak and numbers of thrusts. Thrusts were produced during the Taconic and Acadian events which pre- and post-dated the thermal peak, as well as being synchronous with it. Transport of the Bay of Islands ophiolites and other large masses along the western margin of the orogen occurred before Taconic metamorphism, but probably only immediately before. Many large thrusts of the Appalachians were active during two or even three of the deformational-thermal events, and more than once within a single event. This is particularly true for the Blue-Ridge-Inner Piedmont mega-nappe, which involves at least 225 km of horizontal transport. Compressional tectonics were probably the dominant process responsible for all thrusts in the Appalachians, except the Taconic klippes. The Alleghanian décollement Valley and Ridge thrusts are overridden by crystalline Alleghanian and older thrusts of the Blue Ridge in the southern Appalachians. The same mechanism must apply to the central Appalachians. Thrusting and formation of crystalline thrust-nappes in the Appalachians and other mountain chains may be an adiabatic process which functions to dissipate much of the thermal energy produced during subduction (both A and B types) and collision events.

Terrestrial heat flow in Botswana and Namibia

Article

Jun 1987

Terrestrial heat flow measurements from 25 new sites in Botswana and Namibia obtained in 1983-1984 are presented. The methods used to measure heat flow, temperature, and thermal conductivity are described. The effect of the upper crustal heat production on surface heat flow is investigated, and the relation between the heat flow pattern in southern Africa and tectonic setting is studied. The data reveal that the heat flow on the central Archean cratonic nucleus varies from less than 40 mW/sq m near it center to about 60 mW/sq m near it boundary with the neighboring mobile belts; in the Proterozoic and Pan African mobile belts that surround the craton, the heat flow increases from about 60 mW/sq m at the cratonic margin to over 70 mW/sq m several hundred kilometers away; for the mobile belt, the heat flow is estimated as 65 mW/sq m, as compared to about 40 mW/sq m near the center of the craton.

Generation of long-wavelength heterogeneity in the mantle by dynamic interaction between plates and convection

Article

Apr 1991

Lateral variations in seismic velocity (through its dependence on temperature) can easily be generated at the gravest harmonics, including degrees one and two, by the dynamic interaction between plates and convection. Models of thermal convection with a single non-subducting plate have been formulated in a cylindrical geometry. Plates of width one to four times times the thickness of the convecting region strongly modulate the flow by being pushed over cold downwellings and inhibiting cooling of the fluid beneath. During rapid motion off of hot regions, a large-scale pattern of shear is developed causing small uprising limbs to be swept into the largest upwellings. Both insulation and plume-plume collisions pump energy into the lower wavenumber harmonics.

Subduction and coeval thrust belts, with particular reference to North America

Article

Jan 1981

A. G. Smith

Although thrust belts at present-day subduction zones are sub-parallel to such zones, several ancient thrust belts dip in the opposite direction to the inferred dip of the contemporaneous subduction zone. Gravity spreading, gravity sliding or compressional stresses transmitted from the subduction zone are unlikely causes of such thrust belts. The driving force is attributed instead to the hydrostatic head of a fluid-like rock welt. If the welt is hot—as in a granitic/metamorphic belt—foreland thrusts may form. The emplacement stresses are similar in magnitude to those of gravity spreading, but the thrust wedge need not be weak. The essential requirements of a thrust belt created by a fluid welt are a downhill surface slope in the direction of thrust transport and weak decollement horizon(s). In the Mesozoic foreland thrust belts of western North America, the volume of rock pushed onto the foreland is comparable to the volume of the batholiths in the orogenic core and to the volume of new material added by subduction to the overriding American Plate. After collision with another continent or an island arc the recognition of thrust belts emplaced by fluid welts is more difficult, particularly when ophiolites are present.

Upper mantle S velocity structure of North America

Article

Oct 1997

We have inverted fundamental and higher-mode Rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over North America to obtain an image of the upper mantle S velocity structure down to 660 km. Among the well-resolved features of the new model are (1) a high-velocity root beneath the North American craton which extends no deeper than 250 km except near the Archean core of the craton where depths of 350 km are reached, (2) a weak band of low-velocity along the eastern margin of the North American craton, which reaches into the transition zone, (3) a low velocity slab window beneath the western United States down to a depth of 300 km, (4) areas of low uppermost mantle velocity beneath the Cascade volcanoes, the Yellowstone hotspot track, the Colorado plateau, the Sierra Madre Occidental, and the grabens bordering the Jalisco block, and (5) a pronounced band of high velocities in the transition zone, coinciding with the expected location of the subducted trailing fragments of the Farallon plate. We introduce several improvements to the method of partitioned waveform inversion, which was used to compute the new model: rather than to correct for an estimated depth to the Mohorovicic discontinuity, we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion; we also improve the windowing and filtering operator used to select uncontaminated waveforms.

On the heat flow variation from Archean craton to Proterozoic mobile belts

Article

Jan 1997

Adrian Lenardic

Heat flow data from a number of continental shield regions show a trend of relatively low to high values from Archean cratons to bordering Proterozoic mobile belts. Of the two end-member explanations for this trend, low heat production in Archean crust or a relatively thick cratonic lithosphere, the latter has come to be generally preferred. Such an explanation assumes a strong, one-to-one correspondence between the mantle component of surface heat flow and local lithospheric thickness. This assumption has been well validated below oceans and its applicability to continental lithosphere has gone largely unchallenged. It is, however, not fully valid. This is demonstrated through numerical models that allow continents to form over a convecting mantle. Model continents consist of a core region of thickened crust and mantle residuum and a peripheral region of thick crust, analogs to a craton and a mobile belt, respectively. Despite a thicker thermal lithosphere in the core relative to the periphery, the equilibrium surface heat flux across a continent shows little variation. The finite thermal conductivity of buoyant continental material is at the heart of this behavior as it allows continents to enforce a spatially near-constant heat flux condition on the mantle below. Such a condition is associated with a weak correspondence between mantle heat flow and lithospheric thickness defined thermally or mechanically. This general result, together with specific modeling results applied to heat flow data, suggests that variable lithospheric thickness is most likely not the primary cause of heat flow variations near Archean cratons, leaving differing degrees of crustal heat production as the more likely candidate.

A Global Analysis of Heat Flow From Precambrian Terrains: Implications for the Thermal Structure of Archean and Proterozoic Lithosphere

Article

Jul 1993

Previous studies of heat flow from Precambrian terrains have yielded two patterns: a heat flow-tectonic age relationship, which is a global pattern, and a spatial pattern between heat flow and the proximity of Archean cratons, which have been documented in only a few geographic regions. Data on heat flow from tectonically stable Precambrian terrains worldwide are analyzed here to address two questions raised by different features of the two heat flow patterns: (1) is the spatial relationship between heat flow and the proximity of Archean cratons a global pattern? and (2) do the two heat flow patterns have a common underpinning? The analysis revealed a spatial pattern between heat flow and the proximity of Archean cratons, which is characterized by low heat flow in Archean cratons and Proterozoic terrains adjacent to cratonic margins, and higher heat flow in Proterozoic terrains that are more than a few hundred kilometers from a cratonic margin.

Age dependence of continental heat flow--fantasy and facts

Article

Jul 1982
EARTH PLANET SC LETT

Extensive use of empirical heat flow/age relations in the field of thermal studies indicates that the basic concept of a relation between heat flow and age is deeply entrenched. The idea of a thermal time constant and thermal thickness for the lithosphere depends on the validity of a heat flow/age relation. Several thermal and thermo-mechanical lithospheric models are constrained by a heat flow/age relation, a corrected heat flow/age relation, or a reduced heat flow/age relation. The theory of plate tectonics provides a physical basis for a heat flow/age relation for the oceans. Reliable heat flow measurements from well-sedimented areas of the oceans are known to support it. On the contrary, the theory of plate tectonics does not support an age relation for the continents, which is not well realized. It is pointed out that according to the theory of plate tectonics, a distinct heat flow profile should characterize a Cenozoic/Mesozoic orogenic belt over the continents, and a variety of profiles could be expected on account of several possible interactions at convergent plate boundaries. A specific characteristic heat flow value corresponding to a particular age loses meaning, and therefore a heat flow/age relation (or curve) becomes conceptually invalid. Presently available continental heat flow data, statistically analysed in a proper manner, do not (as they should not) support an age relation. Attempts at correcting age-dependent heat flow means for crustal radiogenic heat to obtain an age relation for mantle heat flow do not yield meaningful results.

The heat flow through oceanic and continental crust and the heat loss of the Earth

Article

Jan 1980

Oceans and continents are now considered to be mobile and interconnected. The paper discusses heat flow through the ocean floor, continental heat flow, heat loss of the earth, thermal structure and thickness of the lithosphere, as well as convection in the mantle and the thermal structure of the lithosphere, within the framework of the theory of plate tectonics. It is concluded that the observed subsidence of the ocean floor and the measured decrease of heat flow with age are accounted for by the creation of lithospheric plate. Furthermore, the marginal basins exhibit the same relation between heat flow and age as the deep ocean floor. On the continents the heat flow is generally high in the younger regions, decreasing to a constant value after 800 Ma. However, the nonradiogenic component reaches an equilibrium value after only 200-300 Ma. Other details are also presented.

Thermal model of continental lithosphere

Article

Sep 1976

The predictions of a simple model based on the concept that the lithosphere is a thermal boundary layer, somewhat analogous to a layer of ice on a pond, are in agreement with the relevant data from North America. The mean surface heat flow of tectonic provinces is approximately inversely proportional to the thickness of the lithosphere. If crustal thickness remains constant, a 40-km increase in lithospheric thickness results in a 1-km decrease in surface elevation, because unlike the ice analogy the lithosphere is denser than its underlying asthenosphere. It is suggested that part of the observed decrease of continental heat flow in the time following a thermal event is caused by cooling and thickening of the lithosphere.

Convection in the Earth's mantle: towards a numerical simulation

Article

Feb 1974
J FLUID MECH

Plate tectonics provides a remarkably accurate kinematic description of the motion of the earth's crust but a fully dynamical theory requires an understanding of convection in the mantle. Thus the properties of plates and of the mantle must be related to a systematic study of convection. This paper reviews both the geophysical information and the fluid dynamics of convection in a Boussinesq fluid of infinite Prandtl number. Numerical experiments have been carried out on several simple two-dimensional models, in which convection is driven by imposed horizontal temperature gradients or else by heating either internally or from below. The results are presented and analysed in terms of simple physical models. Although the computations are highly idealized and omit variation of viscosity and other major features of mantle convection, they can be related to geophysical measurements. In particular, the external gravity field depends on changes in surface elevation; this suggests an observational means of investigating convection in the upper mantle.

A comparative study of parametrized and full thermal convection models in the interpretation of heat flow from cratons to mobile belts

Article

Jun 1993
GEOPHYS J INT

Heat flow from Archean cratons worldwide is typically lower than from younger mobile belts surrounding them. The contrast in heat flow between cratons and mobile belts has been attributed in previous studies to the greater thermal resistance of thicker lithosphere beneath the cratons which impedes the flow of mantle heat through the cratons and forces more mantle heat to escape through thinner mobile belt lithosphere. This interpretation is based on thermal models which employ a parameterized convection algorithm to calculate heat transfer in the sublithospheric mantle. We test this interpretation by comparing thermal models constructed using the parameterized convection scheme with models developed using an algorithm for full thermal convection. We show that thermal models constructed using the two different convection algorithms yield similar surface heat flow and thermal structure to moderate depths within the lithosphere. Therefore, we conclude that the interpretation of the heat-flow observations in terms of thicker lithosphere under Archean cratons than under mobile belts is robust in the sense that surface heat flow is not sensitive to the details of heat transfer within the convecting mantle and how deep mantle heat is delivered to the base of the lithosphere.

Cratonization and thermal evolution of the mantle

Article

Oct 1986
EARTH PLANET SC LETT

Henry N. Pollack

The stabilization of continental lithosphere to form cratons is accomplished by volatile loss from the upper mantle during magmatic events associated with the formation of continental crust. Volatile depletion elevates the solidus and increases the stiffness of the mantle residuum, thereby imparting a resistance to subsequent melting and deformation. Freeboard is maintained in part by the buoyancy associated with an increased Mg/(Mg + Fe) ratio in the mantle residuum following extraction of crustal material. Augmented subcratonic seismic velocities derive from the same shift in this ratio. The higher effective viscosity of the stabilized subcratonic upper mantle inhibits its entrainment in mantle convection, and locally thickens the conductive boundary layer. Heat approaching from greater depths is diverted away from the stiff craton to other areas that continue to transfer heat by convection, thus yielding a low surface heat flow within cratons.Cratonization by devolatilization and petrologic depletion was most effective in the Archean and has diminished in effectiveness over geologic time as the mantle temperature has fallen because of the declining store of internal heat. From the Archean to the present that ascending mantle material which has undergone partial melting has encountered the solidus at progressively shallower depth, has remained supersolidus over a smaller depth range, has temperatures which have exceeded the solidus by lesser amounts, has undergone diminishing degrees of partial melting, and has experienced less thorough devolatilization. At a given time the rate of production of continental crust is likely to be proportional to the depth extent and fraction of partial melting. Integration of the partial melt zone over time yields a growth curve that is similar to some continental crustal growth curves inferred from isotopic evolution.

Diversion of heat by Archean cratons: A model for southern Africa

Article

Sep 1987
EARTH PLANET SC LETT

The surface heat flow in the interior of Archean cratons is typically about 40 mW m−2 while that in Proterozoic and younger terrains surrounding them is generally considerably higher. The eighty-four heat flow observations from southern Africa provide an excellent example of this contrast in surface heat flow, showing a difference of some 25 mW m−2 between the Archean craton and younger peripheral units. We investigate two possible contributions to this contrast: (1) a shallow mechanism, essentially geochemical, comprising a difference in crustal heat production between the two terrains, and (2) a deeper mechanism, essentially geodynamical, arising from the existence of a lithospheric root beneath the Archean craton which diverts heat away from the craton into the thinner surrounding lithosphere. A finite element numerical model which explores the interplay between these two mechanisms suggests that a range of combinations of differences in crustal heat production and lithospheric thickness can lead to the contrast in surface heat flow observed in southern Africa. Additional constraints derived from seismological observations of cratonic roots, the correlation of surface heat flow and surface heat production, petrological estimates of the mean heat production in continental crust and constraints on upper mantle temperatures help narrow the range of acceptable models. Successful models suggest that a cratonic root beneath southern Africa extends to depths of 200–400 km. A root in this thickness range can divert enough heat to account for 50–100% of the observed contrast in surface heat flow, the remainder being due to a difference in crustal heat production between the craton and the surrounding mobile belts in the range of zero to 0.35 μW m−3.

Hotspot tracks and the early rifting of the Atlantic

Article

May 1983
TECTONOPHYSICS

W. Jason Morgan

Many hotspot tracks appear to become the locus of later rifting, as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines. Examples are the west coast of Greenland-east coast of Labrador (Madeira hotspot), the south coast of Mexico-north coast of Honduras (Guyana hotspot), and the south coast of West Africa-north coast of Brazil (St. Helena hotspot). A modern day analog of a possible future rift is the Snake River Plain, where the North American continent is being “pre-weakened” by the Yellowstone hotspot track.This conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years. The relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots. However, once the motion of this one plate is chosen, the motions of all the other plates are prescribed by the relative motion constraints.In addition to the correlation between the predicted tracks and sites of later continental breakup, exposed continental shields correlate with the tracks. Their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover.

Large scale mantle convection an the aggregation and dispersal of supercontinents

Article

Apr 1988

Michael Gurnis

The first time-dependent numerical simulations of continental aggregation and dispersal demonstrate a dynamic feedback between the motion of continental plates and mantle convection. Plate velocity is intrinsically episodic. Continental plates aggregate over cold downwellings and inhibit subduction and mantle cooling; the mantle overheats and fragments the continent under tension. Overall, the models are in agreement with the present geophysical state of the mantle and the geological record over the last 200 million years.

Thermal regime of the continental lithosphere

Article

Aug 1984
J GEODYN

From studies of the global heat flow data set, it has been generalized, with respect to the continental lithosphere, that there is a negative correlation between heat flow and the lithosphere's tectonic edge, and that the lithosphere's thermal evolution is similar to that of the ocean basins, resulting in a 'stable geotherm' in both environments. It is presently noted that a regional study perspective for heat flow data leads to doubts concerning the general applicability of either statement. Rao et al. (1982) have demonstrated that the data are not normally distributed, and that it is not possible to establish a negative correlation between heat flow and age in a rigorous statistical fashion. While some sites of stable continental blocks may have a geotherm that is by chance similar to that for old ocean basins, this need not hold true generally, and many stable continental terranes will be characterized by geotherms very different from those for old ocean basins.

Heat flow and the structure of Precambrian lithosphere

Abstract

No full-text available

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