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Patterns of Clay Mineral Transformations in Fault Gouges
Abstract
Neoformed clay minerals in fault rocks in the brittle crust are increasingly recognized as being key to the sealing behaviour of faults. Academic literature has recognized the importance of neoformation of clay in fault gouge for a number of years, but the concept has not reached most industry seal analysis workflows. Clay-rich gouges that form as a consequence of new clay mineral growth are distinct from clay smears or cataclastic fault rocks that form as a result of mechanical incorporation of wall-rock phyllosilicates, in that they form by chemical and not physical processes. We report a comprehensive field study of clay mineralogy on fault rocks from sedimentary basins and low-angle normal faults in the American Cordillera. We then synthesize the field study with a literature survey to identify controlling conditions for neo-formed clay in fault gouge. Neoformed mineral in gouges are illite, illite/smectite, smectite, and chlorite/smectite phases. Chlorite and kaolinite do not form as neoformed clays in fault gouges. Controlling conditions are wallrock chemistry, temperatures of ∼60-180 C and fluid availability.
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Segments of the modern San Andreas fault experience creep behavior, which is attributed to various factors, including (1) low values of effective normal stress, (2) elevated pore-fl uid pressure, and (3) low frictional strength. The San Andreas Fault Observatory at Depth (SAFOD) drill hole in Parkfi eld, California, provides new insights into frictional properties by recognizing the importance of smectitic clay minerals, as demonstrated by analysis of mudrock and fault gouge samples from zones between 3186 and 3199 m and 3295 and 3313 m measured depths. X-ray diffraction (XRD) results show illite, chlorite, and mixed-layered illite-smectite and chlorite-smectite minerals in the faulted mudrock, whereas serpentine, Mg-rich smectite, and chlorite-smectite minerals are concentrated in the southwest deformation zone and the central deformation zone of the two actively creeping sections in the San Andreas fault. These rocks are abundantly coated by shiny clay mineral layers in some cases, refl ecting mineral formation during creep. Secondary- and transmission-electron microscopy (SEM/TEM) and XRD studies of these slip surface coatings reveal thin fi lms of neoformed chlorite-smectite phases, similar to previously described illite-smectite microscale precipitations. The abundance of chlorite-smectite minerals within fault rock of the SAFOD borehole significantly extends the potential role of mineralogic processes to depths up to 10 km, with cataclasis and fl uid infi ltration creating nucleation sites for neomineralization on displacement surfaces. We propose that localization of illitic to chloritic smectite clay minerals on slip surfaces from near the surface to the brittle-ductile transition promotes creep behavior of faults.
Palygorskite fibers growing along fault planes in the outcrops of a large fault zone in SE Spain (Carboneras Fault Zone: CFZ; Serrata de Níjar) were studied by X-ray diffraction, scanning electron microscopy-energy dispersive X-ray analysis, and transmission electron microscopy-analytical electron microscopy. The structural formulae, calculated per half unit-cell, is: Si7.95Al0.05O20(Al1.93Fe0.08Mg1.92)(OH)2(OH2)4Na0.09K0.01Ca0.034(H2O). The samples have minor tetrahedral substitutions, with Mg/Al ratios close to one, and contain very small amounts of Fe3+. The number of octahedral cations per half unit-cell is 3.93. The fault-hosted palygorskite shows macroscopic ductile features including incipient foliation. Based on field and laboratory observations, as well as on regional geological evidence indicating the existence of widespread hydrothermal processes along the Serrata de Níjar and surrounding areas, we suggest that palygorskite may have formed during ongoing deformation in the CFZ, as a precipitate from Mg-rich hydrothermal fluids.
A belt of low-angle normal (or detachment) faults 250 km long extends from the northern end of the Salton Trough, California to southern Laguna Salada, Baja California, Mexico. The detachment system is divided into two principal segments. The northern segment, here termed the “west Salton detachment system,” comprises top-to-the-east detachment faults along the eastern Peninsular Ranges that root under the Salton Trough. The southern segment, here termed the Laguna Salada detachment system, comprises top-to-the-west detachment faults in northeastern Baja California and the Yuha Desert region of the southwesternmost Salton Trough. Detachments of that system root under Laguna Salada and the Peninsular Ranges of northern Baja California. Both of these systems experienced a major episode of activity in late Miocene to Pleistocene time, synchronous with deposition of the Imperial and Palm Spring formations, and the Laguna Salada detachment system may still be active. Thus, their activity temporally overlapped, partly or completely, with activity on dextral faults of the San Andreas boundary between the Pacific and North American plates, and with accretion of new transitional crust. Some of the detachment faults in the northern segment may have had mid-Miocene normal slip and/or Cretaceous thrust or normal slip as well, although compelling evidence for either is lacking. These detachment faults are distinctly younger than detachments east of the San Andreas fault, which generally ceased activity by middle or late Miocene time and are overlapped by marine or lacustrine rocks (Bouse Formation); these units are equivalent in age to the syntectonic strata of the Salton Trough but are much thinner and essentially undeformed.
Analysis of fault zone structure and composition of two intermediate-displacement faults in the Colorado Plateau reveal how fault structure varies as a function oflithology, and how faults impact fluid flow.The Little Grand Wash fault cuts Jurassic Summerville through Cretaceous Mancos Shale rocks, and consists of a complex set of interweaving fault strands. Fault relays are developed where sandstone and shale are cut by the fault in roughly equal amounts. Ancient and modern fluid flow is documented by the presence of travertine and tufa deposits, an oil seep, and CO2gas seeps. Analysis of the water, travertine, and gas composition indicate that the CO2emanates from a reservoir 1.5-2 km deep, and charges a shallow aquifer. Cross-fault flow is inhibited, and the gas and water flows in the footwall damage zone of the fault. Analysis of the Bighole fault in the San Rafael Swell shows how fault structure varies with displacement. The fault is exposed entirely in the aeolian Jurassic Navajo Sandstone, and consists of a dense fault core interpreted to be a densely packed set of deformation bands bounded by a narrow slip surface. The fault zone consists of conjugate deformation band sets in the hanging wall and footwall of the fault, and the fault core thickness does not vary significantly with net slip.
A detailed mineralogical and chemical investigation has been made of shale cuttings from a well (Case Western Reserve University Gulf Coast 6) in Oligocene-Miocene sediment of the Gulf Coast of the United States. The <0.1-, 0.1- to 0.5-, 0.5- to 2-, 2- to 10-, and >10-μm fractions from the 1,250- to 5,500-m stratigraphic interval were analyzed by x-ray diffraction. Major mineralogic changes with depth take place over the interval 2,000 to 3,700 m, after which no significant changes are detectable. The most abundant mineral, illite/smectite, undergoes a conversion from less than 20 percent to about 80 percent illite layers over this interval, after which the proportion of illite layers remains constant. Over the same interval, calcite decreases from about 20 percent of the rock to almost zero, disappearing from progressively larger size fractions with increasing depth; potassium feldspar (but not albite) decreases to zero; and chlorite appears to increase in amount. Variations in the bulk chemical composition of the shale with depth show only minor changes, except for a marked decrease in CaO concomitant with the decrease in calcite. By contrast, the <0.1-μm fraction (virtually pure illite/smectite) shows a large increase in K2O and Al2O3 and a decrease in SiO2 The atomic proportions closely approximate the reaction smectite + Al+3 + K+ = illite + Si+4. The potassium and aluminum appear to be derived from the decomposition of potassium feldspar (and mica?), and the excess silicon probably forms quartz. We interpret all the major mineralogical and chemical changes as the response of the shale to burial metamorphism and conclude that the shale acted as a closed system for all components except H2O, CaO, Na2O, and CO2. Compositional changes in the shale as a function of metamorphic grade closely parallel compositional changes in shale as a function of geologic age.
We report results of a new study of the San Jacinto fault zone
architecture in Horse Canyon, SW of Anza, California, where stream
incision has exposed a near-continuous outcrop of the fault zone at ~0.4
km depth. The fault zone at this location consists of a fault core,
transition zone, damage zone, and lithologically similar wall rocks. We
collected and analyzed samples for their bulk and grain density,
geochemical data, clay mineralogy, and textural and modal mineralogy.
Progressive deformation within the fault zone is characterized by mode I
cracking, subsequent shearing of already fractured rock, and cataclastic
flow. Grain comminution advances towards the strongly indurated
cataclasite fault core. Damage progression towards the core is
accompanied by a decrease in bulk and grain density, and an increase in
porosity and dilational volumetric strain. Palygorskite and mixed-layer
illite/smectite clay minerals are present in the damage and transition
zones and are the result of hydrolysis reactions. The estimated
percentage of illite in illite/smectite increases towards the fault core
where the illite/smectite to illite conversion is complete, suggesting
elevated temperatures that may have reached 150°C. Chemical
alteration and elemental mass changes are observed throughout the fault
zone and are most pronounced in the fault core. We conclude that the
observed chemical and mineralogical changes can only be produced by the
interaction of fractured wall rocks and chemically active fluids that
are mobilized through the fault zone by thermo-pressurization during and
after seismic events. Based on the high element mobility and absence of
illite/smectite in the fault core, we expect that greatest water/rock
ratios occur within the fault core. These results indicate that hot pore
fluids circulate upwards through the fractured fault core and into the
surrounding damage zone. Though difficult to constrain, the site studied
during this investigation may represent the top of a narrow, ephemeral
hydrothermal circulation cell that dissipates heat generated from
rupture events at deeper levels (> 4 km).
Detachment faults on the west flank of the Black Mountains (Nevada and California) dip 29–36 and cut subhorizontal layers of the 0.77 Ma Bishop ash. Steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0.64 Ma Lava Creek B tephra and the base of 0.12–0.18 Ma Lake Manly gravel. These faults sole into and do not cut the low-angle detachments. Therefore the detachments accrued any measurable slip across the kinematically linked hanging-wall faults. An analysis of the orientations of hundreds of the hanging-wall faults shows that extension occurred at modest slip rates (1 mm/yr) under a steep to vertically oriented maximum principal stress. The Black Mountain detachments are appropriately described as the basal detachments of near-critical Coulomb wedges. We infer that the formation of late Pleistocene and Holocene range-front fault scarps accompanied seismogenic slip on the detachments.
Metasomatized Tertiary lavas with anomalously high K2O and low Na2O content are distributed within the northwest-trending Miocene extensional terrane of southwestern Arizona. These rocks are common near core-complex related detachment faults at Picacho Peak and the Harcuvar Mountains and in listric-faulted terrane at the Vulture Mountains. In addition to systematic changes in K2O and Na2O, the rocks have been enriched in Zr and depleted in MgO. Secondary, introduced minerals include orthoclase, quartz, and calcite. Fine-grained, euhedral orthoclase (var. adularia), from 2 to 10 mum, is the dominant potassium mineral. Metasomatic changes at Picacho Peak are spatially associated with a major detachment fault. Thus, it is interpreted that detachment provided a conduit for hydrothermal fluids that altered the initial chemical composition of the Tertiary volcanics by potassium metasomatism and charged the upper-plate rocks with mineralizing fluids that carried Zr and Ba, along with Au, Ag, and Cu, during detachment 17 18 Ma.
Burial diagenetic reactions of di- and trioctahedral clay minerals were compared in Brazilian offshore, basinal sediment sequences of Cretaceous age. Originally dioctahedral smectite-rich shales of three basins—Potiguar,Ceara, and Ilha de Santana—exhibited the classical smectite-to-illite burial pattern. Trioctahedral clay-rich shales and trioctahedral clay-mineral cements in sandstones, however, showed a burial sequence of saponite to mixed-layer chlorite/saponite with progressive increase in the percentage of chlorite layers with increasing burial depth.
The change from disordered to ordered interstratifications of chlorite/saponite occurred in the temperature range 60°-70°C at a vitrinite reflectance value of about 0.65. These values are lower than those found for the ordering of illite/smectite clays. Increasing substitution of Al for Si in tetrahedral sites, followed by fixation of interlayer hydroxide sheets was found to be the major chemical change promoting transformation of saponite to chlorite via corrensite.
The Sierra Mazatán in northwestern Mexico is the southernmost metamorphic core complex in the North American Cordillera. Large-magnitude Tertiary extension at Sierra Mazatán involved both ductile and brittle slip along a major normal fault that presently dips 10°-15° west. Extension was polyphase and involved an early period of extension from 25 to 23 Ma followed by major slip from 21 to 16 Ma. Total slip was <20 km and occurred at rates of 3-4 mm/a. This extension predated the plate boundary change to transtension at ∼12 Ma and was largely decoupled from relative Pacific-North American plate motion. Numerous lines of evidence suggest that the presently low-angle normal fault initiated at a steep dip (50°-60°) and was rotated to lower angles during slip. When corrected for this tilting, fault corrugations at Sierra Mazatán had a similar geometry to the segmentation of many active normal faults, which is compatible with their origin as primary fault features. Many aspects of the Sierra Mazatán are comparable to large active normal faults, indicating that this core complex formed owing to prolonged extension on an otherwise typical high-angle normal fault. Therefore, core complexes need not represent a fundamentally unique mode of crustal extension.
Cambridge Core - Structural Geology, Tectonics and Geodynamics - The Mechanics of Earthquakes and Faulting - by Christopher H. Scholz
Palygorskite and sepiolite do not form sedimentary deposits of economic importance in Japan. Palygorskite occurs only in the Kuzuu district as fault- or fissure-fills in karst topography and appears to have formed by direct precipitation from solution. Sepiolite occurrences can be classified into one of four groups: 1) in serpentinite; 2) associated with metallic mineral deposits; 3) in karstic regions, 4) as an Fe-rich variety in Tertiary sediments. It formed by both hypogene-hydrothermal (<100oC) and supergene processes.-D.J.M.
Polytypes of illite from the Broadlands-Ohaaki geothermal system in New Zealand have been studied with transmission electron microscopy and electron diffraction. The illite is predominantly a one-layer polytype with 1.0-nm interlayer spacing. On the scale of a few millimeters, illite occurs as ordered crystals, disordered crystals, and crystals with regions of ordered and disordered stacking. Some crystals are composed entirely of either the one-layer or the two-layer polytype; others show regions of two-layer stacking in a one-layer host. Experimental data on the rate of transformation of 1M to 2M1 muscovite combined with estimates of the mole fraction of 2 M mica and temperatures of illite crystallization were used to place constraints on the duration of hydrothermal activity in the Broadlands-Ohaaki system. -from Authors
The metamorphic complex of the northern Ruby Mountains in northeastern Nevada exposes Paleozoic strata that are metamorphosed to sillimanite grade, migmatized, and recumbently folded. Nappes are variously overturned to the east, north, south, and west. The deeper part of this metamorphic infrastructure is a migmatitic zone pervaded by pegmatitic two-mica granite. A structurally higher transition zone underwent extreme tectonic flattening and some thrusting as the mobile infrastructure rose buoyantly against more rigid suprastructure. Relief by flow and stretching to the west-northwest and east-southeast resulted in a regionally constant lineation in this transition zone. The age of metamorphism is uncertain but may be Jurassic.
The Broadlands-Ohaaki geothermal system is a boiling hydrothermal system hosted by a sequence of Quaternary felsic volcanic rocks and Mesozoic metasediments. More than 50 wells have been drilled (400 to >2,600 m deep) to assess the geothermal potential for the production of electricity. Fluids and precipitates sampled from wells, along with descriptions of the alteration minerals in more than 500 drill cores, provide a three-dimensional picture of the distribution of fluid types and secondary minerals. Interpretation of these features and the distribution of gold and silver highlight the relationship between alteration and mineralization in an active, low-sulfidation epithermal environment. Quartz, illite, K feldspar (adularia), albite, chlorite, calcite, and pyrite are the main hydrothermal minerals that occur in the deep central upflow zone at ≥250°C and >600 m depth. These minerals form through recrystallization of the volcanic host rocks and incorporation of H 2O, CO 2, and H 2S in the presence of a deeply derived chloride water containing ~1,000 mg/kg Cl and ~26,400 mg/kg CO 2. At the same time, and on the periphery of the upflow zone, illite, smectite, calcite, and siderite form through hydrolitic alteration in the presence of CO 2-rich steam-heated waters that contain <30 mg/kg Cl and ~13,000 mg/kg CO 2. Upward and outward from the deep central upflow zone, mineral patterns reflect the shift from rock-dominated to fluid-dominated alteration and the prevailing influence of boiling, mixing, and cooling on fluid-mineral equilibria. Accordingly, the abundance of quartz and K feldspar increase toward the upflow zone, whereas clay abundance increases toward the margin of the upflow zone (with smectite dominating at <150°C and illite dominating at >200°C); the abundance of chlorite, pyrite, and calcite varies here, but albite is absent. Geothermal production wells with high fluid fluxes are the main sites of precious-metal mineralization. The deep chloride water (with or without minor amounts of vapor) enters the well at depths >500 m and undergoes a pressure drop that causes boiling. As a result, precious metals precipitate and accumulate as scales on back-pressure plates or as detritus in surface weir boxes; these deposits contain <10 to >1,000 mg/kg Au, <100 to >10,000 mg/kg Ag and ~10 to ~1,000 mg/kg As and Sb, each. Within production wells, platy calcite deposits as a scale at the site of first boiling near the fluid feed point, while crustiform-colloform-banded amorphous silica deposits in surface pipe work. By contrast, the hydrothermally altered host rocks contain low concentrations of gold, ranging from <0.01 to 1.0 mg/kg Au (68 analyses), and these correlate positively with arsenic (<100 to ~5,000 mg/kg) and antimony (<10 to ~500 mg/kg). Reaction path modeling using SOLVEQ and CHILLER shows that calcite, K feldspar, gold, and amorphous silica deposit in sequence from a chloride water that cools along an adiabatic boiling path (300°to 100°C), analogous to fluid flow in a production well. By contrast, calcite, quartz, K mica, and pyrite deposit from a chloride water that cools due to mixing with CO 2-rich steam-heated waters; dilution prevents precipitation of precious metals. Thus field observations and reaction path modeling demonstrate that boiling is the main process influencing the deposition of precious metals. The results of this study show how peripheral hydrolytic alteration by CO 2-rich steam-heated waters relate to propylitic and potassic alteration by chloride waters in the epithermal environment of a hydrothermal system. Both the distribution of alteration mineral assemblages associated with the different water types and the broad-scale distribution of temperature-sensitive smectite and illite reflect the location of the upflow zone. On a local scale, the occurrence of platy calcite, crustiform-colloform silica, and K feldspar in veins indicates the existence of boiling conditions conducive to precious-metal deposition.
Rocks of the upper plate range from Precambrian to Middle Miocene in age. Ten microprobe (not so designated) analyses of 'upper- plate crystalline units' are tabulated. All but one of these are averages of 2-9 analyses; Li, Rb, Sr, and Ba are reported in p.p.m. These rocks are dominantly tholeiitic, as shown on a modified Miyashiro diagram (M.A. 76-1022). Eleven microprobe analyses of 'lower-plate crystalline units' are tabulated with Rb, Sr, and Ba in p.p.m. Seven of the tabulated analyses are averages of from 2-5 individual rock analyses. These rocks are dominantly calc-alkaline, as shown on a modified Miyashiro diagram. Aside from this petrographic information, the emphasis is on the tectonics of the terrain.A.P.
Details chemical, mineralogical and textural changes during diagenesis. Variable temperatures for the transition may be due to several factors, including chemical heterogeneity of the original smectite, variation in water/rock ratio, porosity differences, diverse chemical character of available fluids, and the particular physical environment for diagenesis. Actual depths and temperatures for the transition are primarily a function of kinetic factors associated with a reaction for which at least one of the principal phases is metastable; the transition therefore is highly dependent on local geological factors. -from Authors
Quantification of fault-related illite neomineralization in clay gouge allows periods of fault activity to be directly dated, complementing indirect fault dating techniques such as dating synorogenic sedimentation. Detrital "contamination" of gouge is accounted for through the use of illite age analysis, where gouge samples are separated into at least three size fractions, and the proportions of detrital and authigenic illite are determined using illite polytypism (1Md = neoformed, 2M1 = detrital). Size fractions are dated using the 40Ar/39Ar method, representing a significant improvement over earlier methods that relied on K-Ar dating. The percentages of detrital illite are then plotted against the age of individual size fractions, and the age of fault- related neoformed material (i.e., 0% detrital/100% neoformed illite) is extrapolated. The sampled faults and their ages are the Absaroka thrust (47 ± 9 Ma), the Darby thrust (46 ± 10 Ma), and the Bear thrust (50 ± 12 Ma). Altered host rock along the frontal Prospect thrust gives an age of 85 ± 12 Ma, indicating that the 46-50 Ma ages are not related to a regional fluid-flow event. These ages indicate that the faults in the Snake River-Hoback River Canyon section of the Wyoming thrust belt were active at the same time, indicating that a significant segment of the thrust belt (100 km2+) was active and therefore critically stressed in Eocene time.
The thickness of the two-layer ethylene glycol complex of dioctahedral smectites varies under room conditions between 17.3 and 16.5 A because of such factors as layer charge density, type of interlayer cation, and relative humidity. Neglecting this variability can give up to 30% error in the X-ray powder diffraction estimation of the smectite:illite ratio of the mixed-layer structures. 3 methods have been developed for the interpretation of X-ray powder diffraction patterns of glycolated mixed-layer illite/smectite which take layer-spacing variability into account.- from Author
Mean thickness measurements and crystal-thickness distributions (CTDs) of illite particles vary systematically with changes in hydrothermal alteration type, fracture density, and attendant mineralization in a large acid-sulfate/Mo-porphyry hydrothermal system at Red Mountain, near Lake City, Colorado. The hydrothermal illites characterize an extensive zone of quartz-sericite-pyrite alteration beneath two deeply rooted bodies of magmatic-related, quartz-alunite altered rock. Nineteen illites from a 3000 ft vertical drill hole were analyzed by XRD using the PVP-10 intercalation method and the computer program MudMaster (Bertaut-Warren-Averbach technique). Mean crystallite thicknesses, as determined from 001 reflections, range from 5-7 nanometers (nm) at depths from 0-1700 ft, then sharply increase to 10-16 nm at depths between 1800-2100 ft, and decrease again to 4-5 nm below this level. The interval of largest particle thickness correlates strongly with the zone of most intense quartz-sericite-pyrite alteration (QSP) and attendant high-density stockwork fracturing, and with the highest concentrations of Mo within the drill core. CTD shapes for the illite particles fall into two main categories: asymptotic and lognormal. The shapes of the CTDs are dependent on conditions of illite formation. The asymptotic CTDs correspond to a nucleation and growth mechanism, whereas surface-controlled growth was the dominant mechanism for the lognormal CTDs. Lognormal CTDs coincide with major through-going fractures or stockwork zones, whereas asymptotic CTDs are present in wallrock distal to these intense fracture zones. The increase in illite particle size and the associated zone of intense QSP alteration and stockwork veining was related by proximity to the dacitic magma(s), which supplied both reactants and heat to the hydrothermal system. However, no changes in illite polytype, which in other studies reflect temperature transitions, were observed within this interval.
The Ismay-Desert Creek interval and Cane Creek cycle of the Alkali Gulch interval of the Middle Pennsylvanian Paradox Formation in the Paradox Basin of Utah and Colorado contain excellent organic-rich source rocks. The source rocks in both intervals contain types I, II, and III organic matter and are potential source rocks for both oil and gas. Thermal maturity increases from SW to NE for both the Ismay-Desert Creek interval and Cane Creek cycle, following structural and burial trends throughout the basin. Throughout most of the basin, the Ismay-Desert Creek interval is mature and in the petroleum-generation, window and both oil and gas are produced; in the south-central to southwestern part of the basin, however, the interval is marginally mature for petroleum generation, and mainly oil is produced. In contrast, the more mature Cane Creek cycle contains no marginally immature areas - it is mature in the central part of the basin and is overmature (past the petroleum-generation window. Burial and thermal-history models were constructed for six different areas of the Paradox Basin.
We examine the composition of small and medium-displacement faults in
the western San Bernardino Mountains, southern California, in order to
determine the nature and degree of alteration in faults, and to relate
this alteration to the heat generated by earthquakes. The development of
clay-rich fault gouge in the faults is clearly syntectonic, and the
reactions that produced the fault gouge are endothermic. We suggest and
test the hypothesis that a primary source of energy to drive these
reactions is the heat produced by earthquakes. We use standard
thermodynamic values for enthalpies of formation to estimate the energy
required to drive syntectonic alteration reactions. We use the Eastwood
fault, a ˜7 km long fault with a clay-rich fault core 3-10 cm
thick and a damage zone 2-40 m thick, as a model and show that these
reactions could consume approximately 20 % of the energy available from
the repeated earthquakes that slipped along the fault. Many of the
reactions that take place in and around faults to produce clay and mica
minerals are endothermic, suggesting that chemical reactions can be a
significant sink of energy produced in earthquake ruptures in faults in
the upper 10 km of the crust. Consumption of heat by chemical reactions
in and around faults would also result in a reduced heat flow anomaly
associated with the fault.
The Colorado River extensional corridor (CREC) accommodated up to 100%
crustal extension between ˜23 and 12 Ma. The southernmost
Sacramento Mountains core complex lies within this region of extreme
extension and exposes a footwall of Proterozoic, Mesozoic, and Miocene
crystalline rocks as well as Miocene volcanic and sedimentary rocks in
the hanging wall to the regionally developed Chemehuevi-Sacramento
detachment fault (CSDF) system. New structural, U-Pb-zircon, Ar-Ar, and
fission track geochronologic and paleomagnetic studies detail the
episodic character of both magmatic and tectonic extension in this
region. Extension in this part of the CREC was initiated with tectonic
slip along a detachment fault system at a depth between 10 and 15 km.
Magmatic extension at these crustal levels began at ˜20-19 Ma and
directly account for 5-18 km of extension (10-20% of total extension) in
the southern Sacramento Mountains. Three discrete magmatic episodes
record rotation of the least principal stress direction, in the
horizontal plane, from 55° to 15° over the following ˜3
Myr. The three intrusions bear brittle and semibrittle fabrics and show
no crystal-plastic fabric development. The final 3-4 Myr of stretching
were dominated by amagmatic or tectonic extension along a detachment
fault system, with extension directions rotating back toward 75°.
The data are consistent with extremely rapid cooling and uplift of
Miocene footwall rocks; the ˜19 Ma Sacram suite was emplaced at a
mean pressure of ˜3.0 kbars and uplifted rapidly to a level in the
crust where brittle deformation was manifested by movement on the
detachment fault at ˜16 Ma. By ˜14 Ma the footwall was
exposed at the surface, with detritus shed off and deposited in adjacent
hanging wall basins.
Czechoslovakia) are compared with talc and stevensite. Chemical analyses give a composition for kerolite near R3Si4010(OH)2.nH20 with R mainly Mg and n about 0.8-I.2. Infra-red data and dehydration-rehydration experiments suggest that the additional water is partly surface-held hydrogen-bonded molecular water, lost up to about 3oo ~ and easily recoverable, and 'water' held as surface hydroxyls, lost at temperatures up to about 65o ~ and less easily recoverable. The surface area of kerolite from North Carolina by nitrogen absorption measurements is 196 mZ/g. X-ray data show broad basal reflections, a basal spacing (after Lorentz-polarization correction) of about 9.6 J~ and a crystallite size of about five structural layers, which is consistent with the large surface area. Two-dimensional hk bands indicate a randomly stacked layer structure with b ~ 9"I 4 A and a crystallite 'diameter' of about r5o A. The basal spacing shows no expansion in water and no thermal contraction up to 50o ~ A partial and irregular swelling with ethylene glycol is obtained only after prolonged cxposure (several weeks). In both respects, kerolites differ from steven-site. Kerolites come close to talc in structure and composition but differ in having a highly random layer arrange-ment, a slightly enlarged basal spacing, which could be due to misfit of layers caused by random stacking, and weaker interlayer bonding. Kerolite is considered to be a useful varietal name for this talc-like mineral in agreement with the views of D'yakonov and of Maksimovic. It cannot be defined as serpentine + stevensite. THE name kerolite, suggested by Breithaupt (I823, p. 254) for a mineral of waxy appearance, was derived from the Greek KVpor (wax) and 2,~0o~ (stone). In English, two spellings have been commonly used, kerolite and cerolite; the former is preferred here because it appears to have priority and reflects more obviously the Greek origin of the word. The name has appeared in most of the major mineralogical texts but over the years has tended to become discredited owing to difficulties in obtaining an adequate definition and significant chemical formula; a useful survey has been given by Stoch (1974, pp. 286-8). Occurrence. Kerolite is commonly considered to be of low temperature origin; field evidence and the colloform texture support a deposition from colloidal suspension. It occurs generally in association with weathered ultramafic rocks as coatings and as veins filling cracks and often is intimately mixed with a poorly crystalline serpentine mineral. Maksimovic 0973) found that kerolite occurred at the bottom of a weathering profile in association with partially weathered and fresh rock. This description accords with our field observations, which indicate that kerolite is consistently found only where fresh or partially altered rock outcrops. Kerolite as a mineral species. Materials called kerolites are always extremely fine-grained and often occur as mixtures of minerals resembling talc or stevensite and a mineral of the serpentine group probably similar to lizardite. These components are recognized principally by their basal X-ray reflections from spacings of about 7"2 A and to.o/~ for the two components in question. The reflection profiles are broad and indicate extremely small crystal size or extreme disorder or both. All other diffraction features are two-dimensional hk bands, which overlap completely for the two components. The composite nature of the materials cannot be adequately determined without careful application of X-ray diffraction (XRD) methods, and studies prior to the use of XRD methods must be largely discounted. ~3 Copyright the Mineralogical Society.
We analyzed twelve fault gouge samples from the Bogd fault in south-western Mongolia to understand the origin and behavior of clay minerals. The investigation relies on x-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high energy synchrotron x-ray diffraction methods to investigate microstructure and preferred orientation. Smectite (montmorillonite), illite-smectite mixed layers, illite-mica and kaolinite are the major clay components, in addition to quartz and feldspars, which are present in all samples. The observations suggest that the protoliths and the fault rocks were highly altered by fluids. The fluid-rock interactions allow clay minerals to form, as well as alter feldspars to precipitate kaolinite and montmorillonite. Thus, newly formed clay minerals are heterogeneously distributed in the fault zone. The decrease of montmorillonite component of some of the highly deformed samples also suggests that dehydration processes during deformation were leading to illite precipitation. Based on synchrotron x-ray diffraction data, the degree of preferred orientation of constituent clay minerals is weak, with maxima for (001) ranging from 1.3 to 2.6 multiples of a random distribution (m.r.d). Co-existing quartz and feldspars have random orientation distributions. Microstructure and texture observations of the gouges from the foliated microscopic zone, alternating with micrometric isotropic clay-rich area, also indicate that the Bogd fault experienced brittle and ductile deformation episodes. The clay minerals may contribute to a slip weakening behavior of the fault.
The internal structure of the Median Tectonic Line (MTL) fault zone and the processes that prevailed at depths are described based on an analysis of a borehole. The fault plane which defines the boundary between the Ryoke- and Sanbagawa-derived rocks dips at 56° to the north. Immediately beneath the boundary, approximately 40 m thick fractured rocks form the major strand of the MTL fault zone. The hanging wall above the boundary comprises variably deformed Ryoke granitoids, including several mylonite zones and cataclasite zones. The fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200 °C. The mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300 °C. These mylonites and cataclasites represent, therefore, fault rocks that formed immediately below and above the brittle–plastic transition, respectively. Development of dissolution seams in these cataclasites suggests that the cataclasite has low strength. The presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle–plastic transition. On the other hand, the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle–plastic transition. It is therefore likely that the differential stresses immediately below the brittle–plastic transition are much higher than those immediately above the transition. Formation of laumontite in the major strand of the MTL fault zone occurred at temperatures of around 200 °C. The central slip zone of the major strand is about 30-cm thick, and is surrounded by thick gouge zones. This situation is favourable for thermal pressurisation during earthquake slips.
Cretaceous, Tertiary, and Quaternary sediments from Deep Sea Drilling Project Sites 164 and 196 (13°12′N, 161°31′W and 30°07′N, 148°34′E, respectively) were analyzed for major chemical elements and mineralogy. Sediments from these sites contain large proportions of authigenic minerals: mainly palygorskite, clinoptilolite and chert in the Cretaceous, and montmorillonite, phillipsite and chert in the Tertiary. The montmorillonite—phillipsite assemblage is thought to be derived from volcanic ash or glass, and the palygorskite—clinoptilolite assemblage is thought to be derived by reaction of biogenic silica with volcanic ash or glass or with montmorillonite and phillipsite. Both assemblages have generally moderate Ti/Al ratios, ranging from 0.026 to 0.047, so most of the palygorskite, clinoptilolite, montmorillonite and phillipsite could not be derived in situ from alteration of basaltic material. Plagioclase compositions suggest that the volcanic precursors were silicic or intermediate, but it is also possible that the sediments have been extensively fractionated by redistribution from nearby seamounts.Available data on other Late Cretaceous sediments in the Pacific were analyzed. Clinoptilolite and chert are present nearly everywhere where palygorskite is abundant; phillipsite is rare where palygorskite is abundant. It is suggested that increased water temperatures during the Cretaceous increased reaction rates and determined the alteration products.
The main Cenozoic extensional structure in the central Mojave Desert is the Waterman Hills detachment fault, which places brittlely deformed synorogenic Miocene rocks on ductilely and cataclastically deformed footwall rocks. New data are presented regarding the timing, distribution, magnitude, and significance of early Miocene extension in the area. The mylonitic fabric in the lower plate was formed at 23 Ma, based on a zircon U/Pb age from a synmylonitic intrusion. Upper plate strata consists of rhyolite flows overlain by sedimentary rocks that were apparently deposited during extensional faulting. These strata were tilted, folded, and intruded by synkinematic rhyolite plugs that are cut off at the detachment fault. Potassium metasomatism of the rhyolitic rocks is pervasive. Upper plate detrital sediment was derived from the rhyolitic rocks and from metamorphic and plutonic basement rocks not present in the area. The probable source of the exotic basement clasts is the Alvord Mountain area, presently located 35 km east-northeast of the Waterman Hills area. This source was probably much nearer to the Waterman Hills during deposition of the synorogenic deposits and has been subsequently shifted by extensional deformation. Distinctive Mesozoic plutonic rocks provide a possible tie between upper and lower plate rocks. Similar poikilitic gabbro bodies in the Goldstone area and the Iron Mountains suggest slip on the Waterman Hills detachment fault to be about 40-50 km. This is also consistent with other offset markers, such as the western edge of a Mesozoic dike swarm. When 15-20 km(?) of Tertiary extension is restored, Paleozoic eugeoclinal rocks are placed structurally above their miogeoclinal counterparts. Combined with the distribution of Triassic and Jurassic rocks, this implies miogeoclinal counterparts. Combined with the distribution of Triassic and Jurassic rocks, this implies post-Early Triassic and pre-Late Jurassic stacking of these lithologies.
Large displacement on a low-angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation. Simple models of the denudation process predict warping of the lower plate into a broad antiform, or antiform-synform pair, with axes perpendicular to the direction of extension. The amount of warping is strongly influenced by initial fault geometry, surface topography, the amount of extension, and the distribution of extension within the upper plate. Late-stage processes that augment antiform growth include one-sided denudation of the antiform, reverse faulting due to concave-upward flexure, and wholesale detachment and reverse-drag folding of the antiform. Antiformal uplifts, now exhumed by erosion, form mountain ranges in the southwestern United States where conditions were favorable for warping during mid-Tertiary crustal extension. Four major structural domains are apparent in transects across these uplifts and adjacent areas: (1) unextended area, (2) synformal upper plate, (3) antiformal uplift, and (4) wedge-shaped upper plate. Each domain is separated from adjacent domains by the traces of a master low-angle normal fault.
The formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension. Pulses of ductile deformation have taken place during short-lived thermal events initiated by the heat input from intruded plutons, sills, or dikes. Such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes. Fast cooling inferred from 40Ar/39Ar apparent ages may have taken place after periods of magma arrival, and need not be the result of rapid erosional or tectonic denudation. Heterogeneity of 40Ar/39Ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills (or sill swarms). Under such circumstances, apparently isothermal decompression paths may actually link pressure-temperature points set during periods of transient mineral growth during and after periods of igneous activity.
In this report, a geologist takes a close look at the question of whether it is now possible to confidently identify the record of seismic slip in exposures of fault zones, cores obtained by drilling, or thin sections of fault rocks. This question is addressed on the basis of the understanding of the seismological definition of an earthquake, and of what this definition implies about the physical character of the earthquake source.