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Structure and evolution of the Carlsberg Ridge: Evidence for non-stationary spreading on old and modern spreading centres

August 2003
Current Science 85(3)

August 2003
85(3)

Authors:

St. Petersburg Filial Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of Russian Academy of Sciences

We present the results of magnetic and bathymetric data collected on board Russian vessels during the last decade. Our analysis shows that since late Cretaceous the proto-Carlsberg Ridge (CR) was spreading at a faster rate, prior to India's collision with Eurasia. Since Eocene, the present CR is characterized by slow sprea d- ing. Our results depict two discordant systems of li near magnetic anomalies. One corresponds with fast spread- ing with respect to latitude axis of the proto -ridge and the other with slow spreading with respect to modern axis of the CR, suggesting that the two spreading sy s- tems are asymmetric both relative to each other and relative to axial anomaly. We infer that during both these periods the structure and spreading on the CR was non-stationary.

Generalized map of the northwest Indian Ocean. The Central Indian and Madagascar basins are conjugate basins as flanks of CIR (Central Indian Ridge) and Arabia and Somalia basins as flanks of Carlsberg Ridge. The full dashed line encloses the area where marine magnetic anomaly profiles were obtained. Bathymetric contours at 3000 m and were taken from ETOPO5 map 23 .

…

Colour-scale image of magnetic anomalies in the NW Indian Ocean. Dashed lines correspond to pseudofaults of Carlsberg Proto Ridge during westward propagating rift (see also Figure 3).

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Mantle Bouguer anomaly over the Carlsberg Ridge. Gravity dataset is obtained from the global FAA deduced from satellite altimetry map 13 . Thick black line represents the axis of ridge segments.

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Spreading rate distribution for anomaly A5 along Carlsberg Ridge. 1, Spreading rate obtained for the west (a) and east (b) flanks of CR and half-full spreading rate (c), 2, Running average; 3, Theoretical half-full spreading rate calculated using A5-CR pole finite rotation (24.2 N; 28.3 E); 4, Carlsberg spreading axis; 5, Magnetic pick of anomaly 5.

…

Shaded bathymetric map of the NW Indian Ocean with magnetic lineation. Russian dataset grided at 2 min spacing is merged with predicted bathymetry 14 .

…

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SPECIAL SECTION: MID-OCEANIC RIDGES

CURRENT SCIENCE, VOL. 85, NO. 3, 10 AUGUST 2003 334

Structure and evolution of the Carlsberg Ridge:

Evidence for non-stationary spreading on old

and modern spreading centres

S. A. Merkouriev* and N. A. Sotchevanova

SPbFIZMIRAN, Muchnov per, 2, Box 188, St. Petersburg, 191023, Russia

We present the results of magnetic and bathymetric

data collected on board Russian vessels during the last

decade. Our analysis shows that since late Cretaceous

the proto-Carlsberg Ridge (CR) was spreading at a faster

rate, prior to India’s collision with Eurasia. Since

Eocene, the present CR is characterized by slow spread-

ing. Our results depict two discordant systems of linear

magnetic anomalies. One corresponds with fast spread-

ing with respect to latitude axis of the proto-ridge and

the other with slow spreading with respect to modern

axis of the CR, suggesting that the two spreading sys-

tems are asymmetric both relative to each other and

relative to axial anomaly. We infer that during both

these periods the structure and spreading on the CR

was non-stationary.

THE Carlsberg Ridge (CR) extends from 2°S to 10°N,

forming the NW–SE trending slow accreting plate boun-

dary between the African and Indian Plates and continues

as highly segmented Sheba Ridge in the Gulf of Aden to

Red Sea

1,2

(Figure 1). In the region north of 10°N on the

CR, the Arabia–India–Somalia triple junction has been

evolving since last 16 m.y. as Ridge–Ridge–Ridge triple

junction whose one arm trending N80°E is the ultra slow

divergent boundary between Arabian and Indian Plate

The evolution of the northern-western Indian Ocean (NWIO)

is the last event of the break-up of Gondwana, when sea-

floor spreading progressively stopped in the Mascarene

Basin

. India and Seychelles started separating along the

Proto-CR

5–7

. The spreading rate was not constant and

the phase of ultra-slow spreading (< 8 mm/a) between

about A18 (40 Ma) and A7 (24 Ma) was detected

. CR

on the modern phase is characterized by slow spreading

and segmented by a few transforms and non-transform

discontinuities

9–11

. The recent results

discussed the

presence of an axial discontinuity at 3°32′N in terms

of a propagating ridge head. The objective of the pre-

sent study was to know the evolutionary history, the

nature of spreading and structure of CR since the late

Cretaceous.

Data acquisition and processing

Detailed bathymetric and magnetic survey of CR between

9°N–58°E and 2°S–69°E (Figure 1) was carried out mainly

in the 1980s during successive Russian expeditions. The

digitization and compilation of these datasets were com-

pleted during 1990s. All magnetic anomaly and bathy-

metric profile data were digitized, combined, totally

reprocessed and loaded into the coherent magnetic and

bathymetric database. The close spacing (5–6 km) between

profiles allowed building accurate bathymetric and mag-

netic grids at a 2 min spacing for mapping in colour

shaded relief and contours maps. The corresponding gravity

dataset is obtained from the global FAA deduced from

satellite altimetry map

. This geophysical dataset allows

*For correspondence. (e-mail: SAM@ns1480.spb.edu)

Figure 1. Gene

ralized map of the northwest Indian Ocean. The

Cen

tral Indian and Madagascar basins are conjugate basins as flanks of

CIR (Central Indian Ridge) and Arabia and Somalia basins as flanks of

Carlsberg Ridge. The full dashed line encloses the area where marine

magnetic anomaly profiles were obtained. Bathymet

ric contours at

3000 m and were taken from ETOPO5 map

SPECIAL SECTION: MID-OCEANIC RIDGES

CURRENT SCIENCE, VOL. 85, NO. 3, 10 AUGUST 2003 335

studying of almost 1300 km along ridge section and more

than 10 Ma across the ridge. In addition to our dataset,

we have extracted the data from NGDC

files to study

the early evolution of the proto-CR during the Palaeo-

cene–Eocene period.

Results and discussion

The magnetic anomaly map of the CR and Eastern

Somali and Arabian basins (Figure 2) depicts two discor-

dant systems of linear magnetic anomalies. One corres-

ponds with fast spreading with respect to latitudinal axis

of the proto-ridge, and the other corresponds to slow

spreading with respect to modern axis of the CR. The main

feature of the two spreading systems is the asymmetry

relative to each other and relative to axial anomaly.

The analysis of magnetic anomaly and bathymetric

data (Figure 3) shows the clear limit between the rough-

ness of the bathymetry and the regularity of the abyssal

hills in the Eastern Somali basin (0.5°N, 60°E) to the

NW. The position of the similar limit in the Arabian

basin was obtained by reconstruction. These linear struc-

tures observed in the magnetic field and bathymetry sug-

gest that the spreading geometry of the Proto-CR has

changed in the form of propagating rift from Southeast to

Northwest during anomaly time 24–20.

The studied part of CR has a complex structure and is

characterized by an en-echelon system of spreading cen-

tres. The comprehensive analysis of the magnetic and

bathymetric data reveals transform/non-transform discon-

tinuities (Figure 4). The observed variation in spreading

direction of the ridge subdivides the region in two distinct

parts on either side of 2.5°N–66°E. The northwestern part

is characterized by a general trend almost orthogonal to

the present spreading direction, while the southeastern

one has about 45° oblique trend compared to the spreading

direction. We have defined segments as well-individualized

bathymetric high and MBA (Mantle Bouger Anomaly)

low. The axis of CR is segmented into 26 accretion

segments (10–85 km length) separated by two major

100–150 km long transforms and several 2nd–4th-order

discontinuities with offsets of 30 km or no offsets. Along

the northwestern ridge section, the axis is made up of 20

segments (mean length of 50 km). Most of the disconti-

nuities have small or zero offsets, only two of them are

transform faults. Along the southeastern section, the ridge

consists of six segments separated by larger offset (up to

100 km) discontinuities, among which some are not trans-

form faults but oblique relays.

The bathymetric and MBA variation along axis (Figures 4

and 5) also show two different sections, the limit of which

is at (3.5°N–64°E) more than 200 km west of the ‘geo-

metric’ limit, discussed above. The southeastern part is

characterized by along axis MBA variations of smaller

Figure 2. Colour-scale image of magnetic anomalies in the NW In

dian

Ocean. Dashed lines correspond to pseudofaults of Carls

berg Proto

Ridge during westward propagating rift (see also Figure 3).

Figure 3. Shaded bathymetric map of the NW Indian Ocean with mag-

netic lineation. Russian dataset grided at 2

min spacing is merged with

predicted bathymetry

SPECIAL SECTION: MID-OCEANIC RIDGES

CURRENT SCIENCE, VOL. 85, NO. 3, 10 AUGUST 2003 336

amplitude with a mean axial MBA value being more

negative than along the northwestern part.

The ridge axis geometry, which is inherited from

the opening of the oceanic basins, appears to control the

morphology of the ridge, but do not control the deep

structure, as the MBA values suggest a more continuous

magmatic source beneath these chaotic morphology. This

difference in deep structure could be due to a combina-

tion of hot-spot influence and increasing spreading rate

southward. The analysis of axial magnetic anomalies,

spreading rate and direction confirm that the ridge seg-

mentation is not the adaptation of inherited plate bound-

ary geometry to changes in spreading conditions but the

effect of the accreting processes, focused magma upwell-

ing along discrete spots of the slow spreading axis. The

segment scale morphological variations and associated

axial MBA suggest along-axis variations in the magmatic

and tectonic processes.

Full sequences of magnetic anomalies up to A5 (10 Ma)

have been identified on the CR by generating synthetics

for each profile (Figures 6 and 7). The mean half-spreading

rate increases from 1.2 cm/yr to 1.5 cm/yr from NW to

SE. The spreading rate for the west and east flanks of CR

and the spreading rate distribution for anomaly A5 along

the CR (Figure 6) have been computed using A5-CR

finite rotation pole (24.2°N, 28.3°E). The synthetic model

has been computed by Tisseau and Patriat method

, using

magnetic reversal scale of Candy and Kent

16,17

taking

magnetized layer thickness of 1 km at depth increasing

Figure 4. Shaded bathymetric map over Carlsberg Ridge with mag-

netic lineation, transform faults (grey plain line) and axis ridge seg-

ments (thick black line). Magnetic picks are plot

ted as points and

magnetic lineation are plotted as colour lines and num

bered; dashed

light lines are traces of ridge propagation.

Figure 5. Mantle Bouguer anomaly over the Carlsberg Ridge. Grav

ity

dataset is obtained from the global FAA deduced from satellite alti

metry map

. Thick black line represents the axis of ridge segments.

Figure 6. Spreading rate distribution for anomaly A5 along Carls

berg

Ridge. 1, Spreading rate obtained for the west (a) and east (b

) flanks of

CR and half-full spreading rate (c

), 2, Running average; 3, Theoretical

half-full spreading rate calculated using A5-

CR pole finite rotation

(24.2 N; 28.3 E); 4, Carlsberg spreading axis; 5, Mag

netic pick of

anomaly 5.

SPECIAL SECTION: MID-OCEANIC RIDGES

CURRENT SCIENCE, VOL. 85, NO. 3, 10 AUGUST 2003 337

with age

and the magnetization intensity adjusted for

matching the synthetic and observed amplitudes (Figure 7).

The asymmetrical model (Figure 7 c) is the best fitting

model with ridge jump at a distance of 30.4 km on west

flank of the CR, 2.5 m.y. ago, which matches with the

observed profile VG860242 and gives high correlation

(R = 0.78).

The intra- and inter-segmental variation in spreading

rate has been studied. The distribution of average spread-

ing rate for the last 10 Ma is not always symmetric. The

observed asymmetry is not attached to one flank or one

period but seems rather randomly distributed resulting

into the continuously changing offsets. This asymmetry is

the result of either inter-segment axis jump or intra-segment

propagation

19,20

We have used the principle of ordering (ranking)

for

building of the lithological correlation for magnetic ano-

maly identification and jumping ridge-axis location. This

principle consists in affirmation that a curve joining the

correlation points is a monotone function. The correlation

points are those where the highest coefficient of correla-

tion between an interval on one curve and all intervals

of the same length on other curve is obtained. The basic

idea of the algorithm used is the combination of the

ordering principle and similarity measure. Using this

technique of maximum cross-correlation of similar aligned

features, we have located the time and distance of the

ridge jump. The symmetrical model (Figure 8, top) shows

the low correlation between anomalies 2A and 3A, whereas

the asymmetrical model with axis ridge jump provided

good correlation. The best correlation (Figure 8, bottom)

was obtained after comparison between observed profile

and more than 200 models calculated by varying various

arguments of a spreading ridge jump. Thus, we find that

the method of maximum cross-correlation of similar

aligned fragments and resulting correlation figures deter-

mine the best time and distance of the ridge jump. The

identification of magnetic anomalies allows determining

isochrones on both flanks. However, the superposition

of two conjugate isochrones is not accurate. A variable

slope of the block limits coming from the 3-D ridge

structure could explain this misfit.

Conclusion

We find that prior to the Himalayan collision, seafloor

spreading on the Carlsberg proto-ridge was extremely

active. A major reorganization of the spreading plate

Figure 7. Possible identifications of the magnetic anomalies o

served on the CR (profile VG860242, see location on Figure 6) accor

ing to either a symmetrical (a) or an asymmetrical (b and c

) spreading

model. d, Sketch showing jumping spreading ridge.

Figure 8.

Objective of the magnetic anomalies identification by the

correlation figures method applied to the observed and calculated pr

files of Figure 5. Each solid circle with its associated number is related

to a short section of the observed profile and indica

tes the location

along the model where the best value of correlation is obtained.

SPECIAL SECTION: MID-OCEANIC RIDGES

CURRENT SCIENCE, VOL. 85, NO. 3, 10 AUGUST 2003 338

boundaries in the Indian Ocean during Eocene, as a con-

sequence of the hard collision of India with Eurasia, led

to the slow spreading on the present CR. Our results

show that during both these periods the structure and

spreading on the CR was non-stationary.

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ACKNOWLEDGEMENTS. We thank captain and crew of Russian

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MIRAN, St. Petersburg, Russia is gratefully acknowledged. The figures

were drafted using GMT software

Geochronological and genetic characterization of basaltic basement from western offshore basins in India

Article

Full-text available

May 2024

Western offshore basins Kutch offshore basin Mumbai offshore basin Kerala-Konkan offshore basin a b s t r a c t 40 Ar-39 Ar geochronology, geochemistry, and Sr-Nd isotopic compositions of 30 core samples from 24 offshore drill wells in the Western offshore basins have been used to characterize the genetics of the volcanic basement from the Kutch, Mumbai, and Kerala-Konkan offshore basins. Findings from the volcanic basement rocks demonstrate extremely varied isotopic and geochemical fingerprints, which are suggestive of significantly diverse parent magma compositions and emplacement processes. Basaltic tholeiitic basement from Kutch Offshore basin has Ar-Ar ages that range between 60 Ma and 62 Ma. This basement is characterised by a within-plate basalt signature, with depleted isotopic signatures similar to least contaminated Deccan Traps basalts, and a component of subducted crustal material. Basaltic basement from Mumbai Offshore Basin has eruption ages between 63 Ma and 65 Ma and show a strong within-plate OIB affinity. Geochemical and isotopic signatures are consistent with Renuion lavas and an enriched-end member of Deccan Traps basalts. However, Kerala-Konkan Offshore Basin shows significant E-MORB affinity and is characterized by primitive-mantle signatures and least contamination from the upper continental crust, with ages of eruption between 60 Ma and 61 Ma based on obtained 40 Ar-39 Ar ages. The study suggests that the Mumbai Offshore Basalts' mode of the eruption was comparable with onshore Deccan volcanism, whereas the basaltic basement in Kutch Offshore was formed after the main phase of Deccan eruption and may have been an offshoot of a ''tail" of the main Deccan volcanism. The parent magma for the volcanic E-MORB basement in the Kerala-Konkan Offshore Basin is thought to have come from mixing along the Carlsberg Mid-Oceanic Ridge and the material from the Réunion plume with the northward movement of the Indian Plate during the Early Paleocene. This is thought to have occurred concurrently with the formation of the Chagos-Laccadive Ridge. Ó 2024 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Dynamic evolution of competing same-dip double subduction: New perspectives of the Neo-Tethyan plate tectonics

Preprint

Full-text available

Jan 2024

Same-dip double-subduction (SDDS) systems are widely reported from present as well as past complex convergent plate tectonic configurations. However, the dynamics of their evolution is poorly understood, which is crucial to conceptualize anomalous subducting slab kinematics and associated observed geological phenomena, such as irregular trench migration rates, high convergence velocities, and slab break-off. To bridge this gap, we develop dynamic thermo-mechanical subduction models and investigate the initiation and evolution of SDDS systems, considering three different initial plate configurations: oceanic, oceanic-continental, and multiple continental settings, based on Neo-Tethyan paleo-reconstructions. Each model offers new insights into the complex tectonic history of the major Neo-Tethyan subduction zones, particularly the Indo-Eurasian and Andaman convergent systems. We evaluate the slab-slab interactions, trench and subduction kinematics, inter-plate reorganization, and temporally varying mantle flow patterns involved in the dynamic evolution of these SDDS systems. The oceanic SDDS model simulations reveal that a sizable oceanic plate can initiate two subduction zones synchronously, but will evolve unequally in a competing mode, leading to exceptionally high convergence rates (~16-17 cm/year) for a prolonged duration (~7-8 Myr). This finding explains the coeval activity of coupled subduction zones in the Indo-Eurasia convergence during the Cretaceous evolution of the Neo-Tethys. The ocean-continent SDDS model, on the other hand, localizes subduction preferentially at passive margins between the oceanic plate and the continental block, forming double subduction zones that grow almost equally to form a spreading centre between the two trenches. These model results allow us to reconstruct the Cenozoic evolution of the eastern Neo-Tethyan region, which ultimately led to the development of the Andaman subduction zone. We also show the post-Cretaceous evolution of the Indo-Eurasian collision zone as a consequence of the SDDS dynamics in presence of multiple continental blocks. These dynamics facilitated slab break-off, transforming the SDDS into a single subduction system in a relatively short time frame (~3 Myr). We finish with a synthesis of the paleo-reconstructions of the Neo-Tethys in the perspective of these SDDS models.

Iron isotope signature of magnetofossils and oceanic biogeochemical changes through the Middle Eocene Climatic Optimum

Article

Jul 2021
GEOCHIM COSMOCHIM AC

Magnetotactic bacteria (MTB) intracellularly precipitate magnetite (Fe3O4) crystals that can be preserved in the geological record. When MTB die, the so-called magnetofossils constitute valuable proxies for paleoenvironmental reconstructions and are suspected to represent some of the oldest traces of biomineralization on Earth. Yet, the biogenicity of putative magnetofossils found in ancient terrestrial and extra-terrestrial samples is still largely debated and their significance for past climate still holds uncertainties. Here we studied a sedimentary sequence from the Indian Ocean (ODP Hole 711A) recording the Middle Eocene Climatic Optimum (MECO) through which a magnetofossil-rich interval was deposited. We investigated for the first time the potential of Fe isotopes as a biosignature in magnetofossils and thoroughly describe MECO related paleoenvironmental disruptions based on major and trace element concentrations. Bulk sediment Fe isotopes showed limited variations, with δ⁵⁶Fe around −0.13 ± 0.04‰ (n = 24), linked to detrital iron rather than MTB activity. Hence, a sequential chemical extraction protocol was applied to determine the specific composition of magnetite. We discuss analytical biases related to this protocol (i.e. partial phyllosilicate and Mn-oxide leaching) and apply corrections to the data. Outside the magnetofossil-rich interval, Fe isotope compositions of oxides (mainly biotic and/or abiotic magnetites and possibly Fe coprecipitated with Mn-oxides) display a small range averaging −0.54 ± 0.05‰, and are interpreted as reflecting dominantly hydrothermal contribution, a conclusion also supported by prominent Eu anomaly. In contrast, the magnetofossil-rich interval shows larger δ⁵⁶Fe variability in oxides, from −0.12 to −0.94‰, decreasing upwards in the stratigraphic section. This interval likely records enhanced Fe supply from atmospheric fallout, increase in biological productivity (illustrated by increased Ba accumulation rate) and subsequent development of ferruginous conditions in the sediment porewater. Covariations of Fe isotope compositions and Mn/Fe ratios can be explained by a vertical migration of a redox front and associated diagenetic modifications. Precipitation of barite (BaSO4) in the sediments after organic matter decay probably favored the preservation of magnetofossils by decreasing SO4²⁻ concentration in porewaters and subsequent H2S production, which usually dissolve magnetite in the sulfidic zone. Finally, we model the evolution of porewater fluid and estimate Fe isotope fractionation between magnetofossils and fluid to Δ⁵⁶Femag-Fe(II)aq = 0.1–0.3‰, a value significantly different from abiotic magnetite fractionation (~1.5‰). Contrasting with recent results on MTB laboratory culture, no mass independent fractionation of Fe isotopes was observed in the present study. Nevertheless, the diverse geochemical proxies presented here provide important constraints on paleoclimate and magnetofossil biogenicity evaluation.

Detailed reconstructions of India–Somalia Plate motion, 60 Ma to present: implications for Somalia Plate absolute motion and India–Eurasia Plate motion

Article

Jul 2021

We estimate India-Somalia plate motion at 45 times since 60 Ma from ∼9,000 crossings of Carlsberg and northern Central Indian Ridge magnetic reversals C1 to C26 and numerous fracture zone crossings. The new rotations reveal at least seven significant spreading rate changes since ∼60 Ma, some previously unknown. The largest changes occurred before 46 Ma, when the forces acting on the India plate evolved rapidly due to the transition from subduction to continent-continent collision between India and Asia and the influence of the Reunion hotspot plume on the India plate. The new rotations reveal a gradual ∼50 percent decline in Carlsberg Ridge spreading rates from 57-52.7 Ma, but an end to the decline at ∼53 Ma, when spreading rates surged rapidly by up to 100 percent. From 52-46.7 Ma, Carlsberg Ridge spreading rates collapsed by ∼90 percent, possibly defining a protracted transition to continental collision between India and Asia. Significant kinematic events since 46.7 Ma have included a ∼25 percent spreading rate recovery from 42-40 Ma, ultraslow spreading from 38.6-33.2 Ma, a gradual rate doubling from 33-18 Ma, a ∼50 percent slowdown from 18-13 Ma, and apparently steady motion since 13 Ma. The new rotations successfully predict Carlsberg Ridge abyssal hill orientations for seafloor ages of 48-42 Ma and 20-0 Ma and Central Indian Ridge fracture zone traces for seafloor ages of 43 to 16 Ma, constituting useful tests of the rotation accuracies at these ages. When corrected for the movement of India relative to the Capricorn plate since 16 Ma, the new rotations also successfully restore magnetic lineations C13, C18, C20, and C21, and fracture zone segments from the Capricorn plate onto of their Somalia plate counterparts. This further confirms the accuracies of our new rotations back to C21n.1o (47.8 Ma), and validates a ∼16 Ma start date for India-Capricorn plate motion and published correction for India-Capricorn motion. Anticorrelated changes in India-Somalia and Antarctic-Somalia seafloor spreading rates from 37-18 Ma may be evidence that Somalia plate absolute motion changed during this period, possibly triggered by Somalia’s post-30-Ma detachment from the Arabian Peninsula or the kinematic effects of the Afar and/or Reunion mantle plumes on the Somalia plate. New India-Eurasia rotations that we estimate from an updated global plate circuit predict convergence rates from 53-47 Ma that are ∼30 percent faster than previous estimates and that decline ∼75 percent by ∼38 Ma. Changes in India-Somalia and India-Eurasia rates correspond closely with recently described Tibetan deformation pulses, consistent with linkages between all three. A joint inversion of Carlsberg and southern Central Indian ridge magnetic reversal and fracture zone data, including a correction for movement of the Capricorn plate relative to India, satisfactorily realigns the reconstructed magnetic lineations and fracture zones back to C23n.1n (50.7 Ma), but misfits some data by 100 km or more at earlier times. The misfits may be evidence for deformation within the IndoCapricorn and/or Somalia plates before 48 Ma or a misinterpretation of magnetic reversal and/or fracture zone data from times before 48 Ma.

Nature and composition of interbedded marine basaltic pumice in the ∼52–50 Ma Vastan lignite sequence, western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

Article

Full-text available

Mar 2017

The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels – one at ∼5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ∼52–50 Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated altered basaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO 2, Al 2O3, TiO 2, MgO, alkalies and REE, but very high in Fe 2O3, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO 2, Al 2O3, MgO and variable gain in Fe 2O3, TiO 2, Ni, V, Zr, Zn and REE. Bubble initiation to growth in the ascending basaltic magma (liquidus ∼1200–1250 ∘C) may have occured in ∼3 hr. Short-distance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.

Centrifugal radial drainages and their geological significance in southern Kerala, India

Article

Full-text available

Jan 2023

The drainage fabric is the reflection of the surface and sub-surface geological systems and processes of the Earth. In this context, the State of Kerala, has prolific network of drainages due to the unique physiographic conditions and it was studied to map the drainage fabric and the anomalies using IRS FCC wrapped DEM of SRTM data of the Southern parts of Kerala. The study revealed the occurrence of larger centrifugal radial drainages conspicuously in a number of places. Such drainages seem to indicate the phenomenon of doming due to the still prevalent northerly directed compressive force related to the post collision tectonics and the east north easterly compressive force due to the rising of the Carlsberg ridge in the Arabian Sea. These both forces in combination might have resulted in the phenomenon of simultaneous cross folding leading to the formation of the domes. The active tectonics have serious implications on the natural disasters in general and the tectonically vibrant Kerala State in particular, therefore the drainage anomalies of Kerala warrant demanding studies.

Positional Prediction of Undiscovered Seafloor Massive Sulfide Resources on Carlsberg Ridge, Northwest Indian Ocean

Article

Dec 2022

Increasingly more attention is being paid to the exploration of seafloor massive sulfide (SMS) resources because of the rich metal resources in them. Research has shown that the slow-spreading mid-ocean ridge is a hotspot for large-scale massive sulfide deposits, and should be prioritized in the future. Because of the complex technology and high cost of investigating the location of the mid-ocean ridge, it is necessary to narrow the prospecting space. In this study, a novel method is proposed that combines weights-of-evidence method and prospecting-information content method to quantitatively predict locations of undiscovered SMS resources on the typical slow-spreading Carlsberg Ridge. First, controlling factors were selected, which included topography, geophysics, and geology aspects. Then, the favorable value range of each controlling factor was determined to develop the prospecting prediction model. Finally, this study developed a mineral prospectivity map with four levels of likelihood for the occurrence of SMS deposits. Eight prospective targets were delineated, four of which were level A and four were level B (level A is better than level B). This research has great significance in guiding future research of SMS resources on the Carlsberg Ridge and similar ridges.

Preliminary Estimation of the Non-dipole Part of the Geomagnetic Field in the Quaternary Period Based on the Investigation of Marine Magnetic Anomalies on the Carlsberg Ridge

Chapter

Jan 2020

Preliminary Results of the Geohistorical and Paleomagnetic Analysis of Marine Magnetic Anomalies in the Northwestern Indian Ocean: International Conference on Geomagnetism, Paleomagnetism and Rock Magnetism (Kazan, Russia)

Chapter

Full-text available

Jan 2019

Blocking Temperature and Hysteresis Characteristics of Nanoparticles of Oxidated Magnetite: International Conference on Geomagnetism, Paleomagnetism and Rock Magnetism (Kazan, Russia)

Chapter

Full-text available

Jan 2019

An effect of a single-phase oxidation process on the hysteresis charac�teristics and blocking temperature of magnetite has been carried out within the framework of the model of core-shell nanoparticle. It has been shown that an increase of the degree of oxidation of magnetite grains results in a decrease of the spontaneous magnetization and slight change of coercive field and remanent sat�uration magnetization to spontaneous magnetization ratio. Increase of the portion of maghemite lead to decrease of the blocking temperature. All results are in agree�ment with an experimental data.

Evolution de la dorsale de Carlsberg: évidence pour une phase d'expansion très lente entre 40 et 25 Ma (A18 à A7)

Article

Full-text available

Jan 1996
Oceanol Acta

This paper compiles the data which led to alternative identifications of the Carlsberg Ridge (CAR) in order to arrive at a consistent interpretation and so decipher the CAR evolution since 50 Ma. The results are the following: (1) Whitmarsh's interpretation must be definitively changed, the anomalies 13, 18, 20 and 21, as shown on the maps of Karasik et al. (1986) or Mercuriev (1990), becoming respectively 20, 21, 22 and 23; and (2) both the interpretation of the anomalies younger than A20 and a comparison with the spreading history at the CIR before A5 time suggest a phase of ultra-slow spreading (<8 mm/a) between about A18 and A7 rather than the present spreading rate beginning at A11 and following a period of more than 10 Ma with no spreading at all. -from English summary

Marine gravity from Geosat and ERS-1 satellite altimetry

Article

Full-text available

Jan 1997

Closely spaced satellite altimeter profiles collected during the Geosat Geodetic Mission (∼6 km) and the ERS 1 Geodetic Phase (8 km) are easily converted to grids of vertical gravity gradient and gravity anomaly. The long-wavelength radial orbit error is suppressed below the noise level of the altimeter by taking the along-track derivative of each profile. Ascending and descending slope profiles aie then interpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using an iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivatives of the vertical deflection grids, while Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to a combination of high-density data from the dense mapping phases of Geosat and ERS 1 along with lower-density but higher-accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satellite-derived gravity anomaly is about 4-7 mGal for random ship tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some applications include inertial navigation, prediction of seafloor depth, planning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.

Basalts from the Carlsberg Ridge, Indian Ocean

Article

J.D.H. Wiseman

Geology of an area of the Carlsberg Ridge, Indian Ocean

Article

Jan 1965

An area of the crest of the Carlsberg Ridge, the mid-ocean ridge in the northwestern Indian Ocean, has been surveyed in detail; the area measures 50 by 38 nautical miles. Contour maps and profiles show the results obtained with a precision echo-sounder and a proton magnetometer. Within the surveyed area, a dextral tear fault displaces the median magnetic anomaly and the median valley through about ten miles. Fresh basalt lavas have been dredged from the area together with metamorphosed basaltic rocks. Brecciation and hydrothermal alteration of the crustal rocks in a comparatively wide fault zone may explain why the pattern of magnetic anomalies seems to have been expunged along the fault.

Revised calibration of the geomagnetic timescale for the Late Cretaceous and Cenozoic

Article

Jan 1995

Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geo-qtagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene anq Pliocene and with a new timescale for the Mesozoic.

An analysis of the v-ariation of Ocean floor bathymetry and heat flow with age

Article

Feb 1977

Two models, a simple cooling model and the plate model, have been advanced to account for the variation in depth and heat flow with increasing age of the ocean floor. The simple cooling model predicts a linear relation between depth and t1/2, and heat flow and 1/t1/2, where T is the age of the ocean floor. We show that the same T1/2 dependence is implicit in the solutions for the plate model for sufficiently young ocean floor. For larger ages these relations break down, and depth and heat flow decay exponentially to constant values. The two forms of the solution are developed to provide a simple method of inverting the data to give the model parameters. The empirical depth versus age relation for the North Pacific and North Atlantic has been extended out to 160 m.y. B.P. The depth initially increases as t1/2, but between 60 and 80 m.y. B.P. the variation of depth with age departs from this simple relation. For older ocean floor the depth decays exponentially with age toward a constant asymptotic value. Such characteristics would be produced by a thermal structure close to that of the plate model. Inverting the data gives a plate thickness of 125+/-10 km, a bottom boundary temperature of 1350°+/-275°C, and a thermal expansion coefficient of (3.2+/-1.1) ×10-5°C-1. Between 0 and 70 m.y. B.P. the depth can be represented by the relation d (t) =2500+350t1/2 m, with t in m.y. B.P., and for regions older than 20 m.y. B.P. by the relation d (t) =6400-3200 exp(-t/62.8) m. The heat flow data were treated in a similar, but less extensive manner. Although the data are compatible with the same model that accounts for the topography, their scatter prevents their use in the same quantitative fashion. Our analysis shows that the heat flow only reponds to the bottom boundary at approximately twice the age at which the depth does. Within the scatter of the data, from 0 to 120 m.y. B. P., the heat flow can be represented by the relation q (t) =11.3/T1/2 mucal cm-2 s-1. The previously accepted view that the heat flow observations approach a

Identification des anomalies magnétiques sur les dorsales à faible taux d'expansion: Méthode des taux fictifs

Article

Feb 1981
EARTH PLANET SC LETT

La similitude entre anomalies magnétiques observées et calculées est meilleure si l'on introduit dans le calcul des modèles une zone de transition entre deux blocs d'aimantation inverse. On montre que ces modèles sont très facilement calculés en prenant un taux d'expansion fictif plus faible que le taux réel et en transformant en conséquence l'échelle des distances. Du fait de ces faibles taux, les modèles contaminés sont très sensibles à la succession des inversions et apparaîssent ainsi comme un moyen de choisir entre différentes échelles chronologiques des inversions du champ magnétique terrestre. The fit between calculated and observed magnetic anomalies from slow-spreading centers is improved when allowing for a transition zone between two inversely magnetized blocks. In this paper it is shown that these models are very easily computed by choosing a fictitious spreading rate which is slower than the real spreading rate and by changing the distance scale appropriately. With these slow fictitious spreading rates, the models are very sensitive to the relative position of the successive inversions and could be used to adjust these positions in the magnetic time scales.

A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic

Article

Sep 1992

We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. The new time scale has several significant differences from previous time scales. For example, chron C5n is ~0.5 m.y. older and chrons C9 through C24 are 2-3 m.y. younger than in the chronologies of Berggren et al. (1985b) and Harland et al. (1990). Additional small-scale anomalies (tiny wiggles) that represent either very short polarity intervals or intensity fluctuations of the dipole field have been identified from several intervals in the Cenozoic. Spreading rates on several ridges were analyzed in order to evaluate the accuracy of the new time scale. Globally synchronous variations in spreading rate that were previously observed around anomalies 20, 6C, and in the late Neogene have been eliminated. The new time scale helps to resolve events at the times of major plate reorganizations. -from Authors

Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic

Article

Apr 1995

Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic.

Evolution of Indian Ocean since Late Cretaceous

Article

Apr 2007
GEOPHYS J INT

All available ship and aeroplane tracks across the Indian Ocean were searched for identifiable magnetic anomalies and transform faults, and hence the age and direction of motion at the time of formation of about two-thirds of the floor of the ocean established. The magnetic lineations show that India moved away from Antarctica at about 18 cm/y for 20 My in the Early Tertiary. This rapid motion ceased in the Eocene and was followed by a period in which little or no spreading took place west of the Ninety East Ridge. Australia separated from Antarctica during this period. The present spreading episode began about 36 My ago. This detailed study has permitted instantaneous poles of rotation to be obtained, and has established that Africa is now moving northward at 2cm/y relative to Antarctica in the South West Indian Ocean. The evolution of the triple junction between the South East, South West and Central Indian Ridges is clearly reflected in the topography and magnetic lineations. The depth of parts of the ocean formed since the Late Cretaceous increases with age in the manner expected from the temperature structure of a cooling plate, and supports the evolution determined from the magnetic lineations in a most remarkable way. Heat flow observations are more scattered but also consistent with the same thermal model. The proposed evolution agrees with the distribution of known continental fragments and with the Late Cretaceous palaeomagnetic poles from surrounding continents and one obtained from the shape of the magnetic lineations south of India. It is, however, not yet clear how to reconstruct Gondwanaland from the Late Cretaceous reconstructions.

Structure and evolution of the Carlsberg Ridge: Evidence for non-stationary spreading on old and modern spreading centres

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ХРЕБЕТ КАРЛСБЕРГ НА ДРЕВНЕМ И СОВРЕМЕННОМ ЭТАПАХ РАСКРЫТИЯ (The Carlsberg Ridge at the old and moder...

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