zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

vector constraints on western u s deformation from space geodesy neotectonics and plate motions the rate of slip vector on the san andreas fault in central california estimated from geodetic and holocene geological data 34 3 mm yr n41 w 2 is inconsistent with the prediction of rigid plate models such as rm2 56 3 mm yr n35 w 2 this well known san andreas discrepancy is diagnostic of plate deformation distributed both east of the fault in the basin and range and west of the fault along the california continental margin we construct constraints on the integrated deformation rates across these two regions consistent with 1 the kinematical boundary conditions imposed by the rigid plate model 2 neotectonic and paleoseismic estimates of deformation rates 3 ground based geodetic measurements and 4 rates of change observed by very long baseline interferometry along seven baselines to western u s sites the space geodetic data on basin and range extension taken over a 4 year interval are compatible with geological observations averaged over the holocene the best estimate of its integrated deformation rate provided by the joint inversion of both data types is 9 7 2 1 mm yr n56 w 10 too small and in the wrong direction to account entirely for the san andreas discrepancy we therefore attribute much of it to deformation west of the sierra nevada great valley block accommodated for example on faults such as the san gregorio hosgri system of coastal california the integral of this deformation estimated by subtracting the basin and range contribution from the discrepancy vector requires significant right lateral shear parallel to the san andreas 13 5 mm yr and some compression perpendicular to it 9 3 mm yr

evolution of archaean crust in the dharwar craton the nd isotope record abstract a synthesis of the available nd isotope data from the dharwar craton puts important constrains on antiquity of crust extent of basement terranes and events marking juvenile crustal addition and crustal recycling the craton is divided into two blocks namely eastern dharwar craton edc and western dharwar craton wdc nd model ages trace crust extraction as far as 3 5 ga back in both the blocks although rock record of such antiquity is yet to be found in the craton in wdc 3 35 3 0 ga is the most significant period of juvenile crustal addition as well as crustal recycling significantly depleted mantle 1 5 to 6 4 existed below the wdc as early as 3 35 ga which was probably refertilized in a later event from the resultant chondritic to slightly enriched mantle 0 3 to 0 4 juvenile mafic crust was added to the wdc during 2 9 2 6 ga a widespread crustal recycling event at 2 6 ga marks the last major event in this block in edc 3 3 3 0 ga granitoids occur as vestiges and show nd model ages higher than crystallizations ages suggesting recycling of palaeoarchaean crust during 2 7 2 5 ga extensive juvenile magmatism took place in edc from variably depleted mantle 1 4 to 5 6 which is later than the global 2 7 ga peak of crustal growth attendant crustal recycling destroyed most of the earlier crust distribution of nd model ages suggests in contrast to general believe that palaeoarchaean crust was not only abundant in the wdc but also widespread in an area extending from the wdc edc boundary to further 60 150 km east up to the hungund ramagiri kolar hrk belt this belt appears to be a major crustal boundary along which terranes of different nd isotope signatures were amalgamated during the late neoarchaean by horizontal accretion major neoarchaean crustal recycling events documented in the dharwar craton are missing from nd isotope record of the eastern block eb of the north china craton this fact does not support correlation between the two cratons as suggested by some authors

two dimensional magnetotelluric inversion of blocky geoelectrical structures 1 this paper demonstrates that there are alternative approaches to the magnetotelluric mt inverse problem solution based on different types of geoelectrical models the traditional approach uses smooth models to describe the conductivity distribution in underground formations in this paper we present a new approach based on approximating the geology by models with blocky conductivity structures we can select one or another class of inverse models by choosing between different stabilizing functionals in the regularization method the final decision whose approach may be used for the specific mt data set is made on the basis of available geological information this paper describes a new way of stabilizing two dimensional mt inversion using a minimum support functional and shows the improvement that it provides over traditional methods for geoelectrical models with blocky structures the new method is applied to mt data collected for crustal imaging in the carrizo plain in california and to mt data collected for mining exploration by inco exploration

diapirism bulk homogeneous shortening and transcurrent shearing in the archaean dharwar craton the holenarsipur area southern india abstract qualitative strain data are spatially integrated in order to define archaean foliation trajectories and variations in strain intensity and ellipsoid type in the holenarsipur area dharwar craton southern india the study provides constraints on the bulk geometry the progressive deformation and the kinematic history of this region of archaean continental crust two major structural events have shaped the study area i a regional diapiric event contemporaneous with a magmatic event and ii a later sinistral transcurrent ductile shear event which has locally reoriented both foliations and stretching lineations the entire holenarsipur supracrustal belt underwent a common tectono metamorphic history that included two metamorphic events each event corresponds temporally to a distinct strain field the first a regional amphibolite facies event was contemporaneous with the main tectonic event and probably with the intrusion of halekote trondhjemites the second occurred under greenschist facies conditions and was restricted to a late transcurrent shear zone at the eastern boundary of the greenstone belt the strain pattern prior to transcurrent shearing cannot be explained by a thrusting event unless all trace of this event had been erased by the diapirism we interpret all structures older than the transcurrent shearing to be the direct result of vertical movements linked to body forces augmented by minor horizontal crustal shortening

lead isotopic compositions of the western dharwar craton southern india evidence for distinct middle archean terranes in a late archean craton abstract lead and sr isotopic compositions of whole rocks and feldspar separates from several fielddefined suites of rocks from the western dharwar craton of southern india elucidate a complex history of the area the evolution of the craton earlier than 3 0 ga is difficult to determine because of thermal and metasomatic overprinting at 3 0 ga many whole rocks were completely reset then but feldspar separates preserve some of the earlier history apparently a series of terranes evolved from the mantle over the period 3 4 to 3 0 ga to form the western dharwar craton in each case metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle the oldest gneisses 3 4 ga had radiogenic pb and sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at or even before 3 8 ga the old gneisses were greatly depleted in rb probably at 3 4 ga when the protoliths of younger magmatic rocks may have separated from the mantle younger gneisses apparently formed from these mafic intermediate protoliths in the age range of 3 2 to 3 0ga the 3 0 ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites formed by melting of older crust and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying u and other lithophilic elements the resultant deep crust was greatly depleted in heat producing elements and the craton has been inactive since 3 0 ga except for production of minor granites from depleted source regions at 2 5 2 6 ga the lack of activity at 2 5 ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at 3 0 ga lower crustal depletion also has caused present day reduced heat flow to be very low

reconciling patterns of interseismic strain accumulation with thermal observations across the carrizo segment of the san andreas fault article i nfo the carrizo segment of the san andreas fault separates rocks of the salinian block southwest of the fault characterized by high heat flow 75 95 mw m 2 and shallow seismicity b 10 km depth from rocks of the franciscancomplexandgreatvalleygroupnortheastofthefaultassociatedwithlowheat flow 50 60 mw m 2 and deeper seismicity b 20 km depth gps data from this region suggest that the northeast side of the fault accommodates more interseismic strain than the southwest side we show that by incorporating variations in depthtothebrittleductiletransitioninferredbasedonheat flowandseismicitydata we fitthegeodeticdatawell and achieve agreement with geologically accepted long term average slip rates with a 20 km wide zone with lowyoung smodulusneofthefault thiswidecompliantzoneisconsistentwithobservationssuggestingthatthe presence of large overpressures may be decreasing the elastic moduli in this region

strain patterns in archaean dome and basin structures the dharwar craton karnataka south india abstract models simulating the role of gravity in the earth s crust are the main source of criteria used for the field identification of diapirs after a short review of the distribution of finite strain in experimental diapir models we describe an example where differential erosion of a tilted craton has provided 3d geometrical information on patterns in a diapiric system at various depths from greenschist to granulitic facies conditions within the archaean continental crust of the dharwar craton south india diapirs and anti diapirs sagducted greenstones are observed at different levels structures and strain are consistent with all the field identification criteria inferred from experimental simulation but some peculiar structural features are also observed which are not predicted by the models these features such as strain trajectory triple junctions result from i the progressive deformation of diapiric domes or sagducted basins ii interferences between diapiric structures body forces and iii the superimposition of diapiric strain fields on regional horizontal shortening boundary forces

robust smooth magnetotelluric transfer functions jimmy c larsen randall l mackie 2 adele man e l l a adolfo fiordelisi4 and shirley rieven2 pacific marine environmental laboratory national oceanic and atmospheric administration 7600 sand point way ne seattle wa 981 15 0070 usa wassachusetts institute of technology 42 carleton street cambridge m a 02142 usa cnr international institute for geothermal research piazza soljerino 2 pisa 56126 italy enel esplorazione mineraria via andrea pisano 120 pisa 56122 italy

significant resistivity changes in the fault zone associated with the 1999 chi chi earthquake west central taiwan abstract the purpose of this paper is to demonstrate the possible influence of an earthquake on the resistivity distribution in a fault zone we collected resistivity image profiles across a proposed fault trace prior to the 7 3 magnitude 1999 shallow earthquake in the chi chi area of taiwan significant positive resistivity anomalies were observed in the hanging wall after the earthquake however there were negligible resistivity changes in the footwall from an examination of geoelectric phenomena and surface rupture in the fault zone it is believed that geoelectric anomalies are associated with abrupt displacement along the active chelungpu fault this result indicates a potential for resistivity methods to provide a basis for the monitoring of an active fault

carbon enhanced electrical conductivity during fracture of rocks changes in electrical resistance during rock fracture in the presence of a carbonaceous atmosphere have been investigated using nugget sandstone and westerly granite the experiments were performed in an internally heated gas pressure vessel with a load train that produced strain rates between 10 6 and 10 5 s 1 samples were deformed at temperatures of 354 to 502 c and pressures of 100 to 170 mpa in atmospheres of ar or mixtures of 95 co2 with 5 co or 5 ch4 compositions that are well within the field of graphite stability at the run conditions in experiments using nugget sandstone resistance reached a minimum value when the maximum temperature was achieved and good electrode contact was made the resistance then increased as the experiment continued probably due to dry out of the sample a change in the oxidation state of the fe oxide associated with the cement or destruction of current bearing pathways at approximately 200 mpa end load the rock sample failed plots of load and resistance versus time show several interesting features in one experiment for example as the end load reached about 175 mpa resistance stopped increasing and remained fairly constant for a period of approximately 0 5 hour during loading the end load displayed small decreases that were simultaneous with small decreases in resistance when the end load and the displacement indicated rock failure resistance decreased dramatically from 150 m to 100 m in a single experiment the westerly granite also showed a decrease in resistance during dilatancy the nature and distribution of carbon in the run products were studied by electron microprobe and time of flight secondary ion mass spectroscopy top sims carbon observed by mapping with the former is clearly observed on micro cracks that based on the microtexture are interpreted to have formed during the deformation the tof sims data confirm the electron probe observations that carbon is present on fracture surfaces these observations and experimental results lead to the hypothesis that as microfractures open in the time leading up to failure along a fracture carbon is deposited as a continuous film on the new reactive mineral surfaces and this produces a decrease in resistance subsequent changes in resistance occur as connectivity of the initial fracture network is altered by continued deformation such a process may explain some electromagnetic effects associated with earthquakes

pyrite chlorite and pyrite biotite relations in sandstones abstract authigenic euhedral pyrite in feldspathic sandstones of the visingso group upper proterozoic southern sweden is observed to be closely associated with authigenic fine crystalline interstitial chlorite and or sand sized chlorite or biotite microprobe analyses revealed that chlorites which are accompanied by relatively large amounts 3 vol of pyrite are characterized by a lower fe fe mg ratio than chlorites with lesser amounts 0 5 vol of associated pyrite the lower fe fe mg ratio of the former chlorites might have resulted from subsequent substitution of fe 2 by mg 2 in an initially fe rich chlorite the released fe 2 would then react with sulfide ions in an anoxic sulfidic environment to form pyrite another alternative is that the chlorites with lower fe fe mg ratio were crystallized from solutions characterized by a higher activity of mg 2 than chlorites with higher fe fe mg ratio iron from the biotite grains became available due to diagenetic illitization kaolinization and or chloritization

proterozoic mn oxide precipitation by planktonic plant protists acritarchs microfossils of plant protists called acritarchs occur embedded in mostly authigenic chlorite cement in sandstones of the visingso group upper proterozoic southern sweden these microfossils are commonly encrusted by a mn oxide layer similar mn oxide was not observed elsewhere in the sandstones thus suggesting that certain acritarchs have played a role in the transfer and precipitation of mn in ancient sedimentary environments in the light of available information on the role of extant protists in biomineralization possible mechanisms for the precipitation of mnoxide in the visingso group are discussed

electrical resistivity changes not observed with the 28 september 2004 m6 0 parkfield earthquake on the san andreas fault california 1 an 18 a long experiment to capture changes in electrical resistivity prior to during and after a m6 earthquake on the san andreas fault at parkfield california culminated with an event of that magnitude on 28 september 2004 resistivity was monitored with a telluric array giving precisions in measured parameters of 0 2 or less no coseismic resistivity changes greater than 5 could have occurred or they would have been detected by the array and any precursory changes would have been much smaller because maximum possible precursory strains were 4000 times smaller than the coseismic strain given that the earthquake ruptured directly beneath the telluric array this experiment places a bound on the amount resistivity might change at the hypocentral location changes measured with instruments farther from earthquakes can only be smaller it is still possible that a measurable change with a larger earthquake or other fault geometries are capable of generating larger signals but our results show clearly that future experiments should be designed with precisions of much better than 0 1 if they are to be successful

resistivity structure of a seismic gap along the atotsugawa fault japan abstract seismicity along the atotsugawa fault located in central japan shows a clear heterogeneity the central segment of the fault with low seismicity is recognized as a seismic gap although a lot of micro earthquakes occur along this fault in order to elucidate the cause of the heterogeneity in seismicity the electrical resistivity structure was investigated around the atotsugawa fault by using the magnetotelluric mt method the regional geoelectrical strikes are approximately parallel to the fault in a low frequency range we constructed two dimensional resistivity models across the fault using tm mode mt responses to minimize three dimensional effects on the modeling process a smooth inversion algorithm was used and the static shifts on the apparent resistivity were corrected in the inversion process a shallow low resistivity zone along the fault is found from the surface to a depth of 1 2 km in the best fit model across the high seismicity segment of the fault on the other hand the corresponding low resistivity zone along the low seismicity segment is limited to a shallower depth less than 1 km the low resistivity zone along the atotsugawa fault is possibly due to fluid in the fracture zone the segment with higher levels of seismicity may have higher fluid content in the fault zone compared with the lower seismicity segment on a view of the crustal structure a lateral resistivity variation in a depth range of 3 12 km is found below the fault trace in the high seismicity segment while a resistive layer of wide extent is found at a depth of about 5 km below the fault trace in the low seismicity segment the resistive layer is explained by less fluid condition and possibly characterized as high rigidity differences in the resistivity structures between low and high seismicity segments of the fault suggest that the seismic gap in the central part of the atotsugawa fault may be interpreted as a locked segment thus mt is an effective method in evaluating a cause and future activity of seismic gaps along active faults the lower crust appears as a conductive zone beneath the low seismicity segment less conductive beneath the high seismicity segment fluid is inferred as a preferable cause of the conductive zone in this study it is suggested that the conductive lower crust beneath the low seismicity segment is recognized where fluid is trapped by an impermeable layer in the upper crust on the other hand fluid in the lower crust may upwell to the surface along the high seismicity segment of the fault

structure of the cuyama valley caliente range and carrizo plain and its significance to the structural style of the southern coast ranges and western transverse ranges a concerted exploration program in the cuyama area by arco during the last 10 years employing surface mapping deep drilling high quality seismic profiles biostratigraphic analysis and balanced cross sections has revealed several significant structural characteristics 1 thrust faults such as the morales and white rock flatten under the caliente range and merge into a single system with up to 14 km of slip this system intersects the san andreas fault at a high angle and may detach the san andreas fault at depth from 7 to 8 km 2 integration of surface and subsurface mapping shows that these thrust faults have little or no component of strike slip and are younger than 3 ma 3 major surface folds such as the caliente mountain anticline are due to thrust faults ramping upsection and producing fault bend and fault propagation folds 4 the young thrust system has cut across a late oligocene to early miocene extensional fault controlled basin and both tectonically transported and overthrust portions of this older basin these structural characteristics strongly suggest that the structural style of the cuyama area and possibly adjacent areas in the southern coast ranges and western transverse ranges is that of a fold and thrust belt and is not relatedmore to wrench tectonics less

magnetotelluric evidence for crustal suture zones bounding the southern great valley california a geoelectric section inferred from a regional magnetotelluric study across the coast ranges the great valley and the sierra nevada reveals significant variations in electrical resistivity zones of lower resistivity interpreted at depths from 10 km to at least 30 km lie near mapped geologic boundaries between the coast ranges and the great valley and beneath the eastern side of the great valley the former boundary is inferred by others to separate the subduction complex of the coast ranges from the mafic basement of the great valley the lower resistivities are most likely associated with metasediments trapped between the coast ranges ophiolite and the former oceanic crust beneath the great valley the latter boundary is problematic but may be evidence for a deep metasedimentary section trapped between the ophiolites beneath the great valley and the granitic rocks of the sierra nevada the lack of change in the magnetotelluric phase across the great valley indicates that a suture zone marked by lower resistivities is unlikely to be present beneath the valley however this does not preclude the existence of a resistive suture zone

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

marine magnetotellurics for base of salt mapping gulf of mexico field test at the gemini structure a sea floor magnetotelluric mt survey was conducted over the gemini subsalt petroleum prospect in the gulf of mexico gom to demonstrate that the base of salt can be mapped using marine magnetotelluric mmt methods the high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for mmt the gemini salt body located at 28 46 n 88 36 w is a relatively complex shape buried 2 5 km below the sea floor in 1 km deep water its geometry has been previously determined using 3 d seismic prestack depth migration with well log control in order not to confuse limitations in interpretation technique with limitations in data acquisition numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2 d inversion even though the body was clearly 3 d the seismic imaging of the base of salt along the chosen profile is considered good thus providing a good control for testing the mt method in many other ar

smnd ages of archaean metavolcanics of the dharwar craton south india abstract well constrained sm nd arrays of whole rocks are reported for the first time from three important stratigraphic units of the dharwar volcano sedimentary sequence of south india if interpreted as isochrons these arrays will correspond to the following ages and initial nd ratios relative to chur 2 911 0 049 ga and 0 20 0 40 for the kalasapura mafic basal metavolcanics of the bababudan greenstone belt 2 848 0 070 ga and 0 88 0 82 for the santaveri formation comprising mafic to felsic metavolcanics overlying the kalasapura rocks and 2 747 0 015 ga and 1 72 0 10 for the ingaldhal mafic felsic metavolcanic suite of the chitradurga greenstone belt the possibility that these sm nd arrays are mixing lines between unrelated components depleted and enriched respectively in light rare earth elements has been discussed leading to the conclusion that the ages closely correspond to their time of eruption the older suites from the bababudan belt from a mantle source of chur like composition the younger suite from the chitradurga belt from one with a prior history of lree depletion

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

a magnetotelluric investigation of the san andreas fault at carrizo plain california high quality wide band magnetotelluric data were collected along two profiles crossing the san andreas fault at carrizo plain california for crustal imaging as part of the san andreas deep drilling project two dimensional inversions of the data indicate that the upper crustal part of the san andreas fault does not appear here as an anomalously conductive zone additionally a broad resistive zone under the temblor mountains east of the fault suggests resistive crystalline or metamorphic rocks may be present here as opposed to the more conducting franciscan assemblage contradicting the generally accepted geologic model

electrical structures of the source area of the 1999 chi chi taiwan earthquake spatial correlation between crustal conductors and aftershocks abstract wide band magnetotelluric mt data were collected on an east west profile approximately perpendicular to the local strike of the chelungpu thrust through the hypocentral area of the chi chi earthquake for imaging the seismogenic structure mt data were then inverted for two dimensional resistivity models plus best fitting static shift parameters using a nonlinear conjugate gradient algorithm that minimizes the sum of the normalized data misfits and the smoothness of the model as shown in the inverted 2d resistivity models presented in this paper an electrical conductor beside the hypocenter of the chi chi earthquake indicates that deep crustal fluids may participate in the rupture process of the chi chi earthquake a striking spatial correlation between the crustal conductor and occurrence of aftershocks beneath the chelungpu fault suggests a postseismic pore pressure adjustment ongoing after the mainshock additionally the hypocenter exhibits an electrical resistive zone consistent very well with a predicted compact zone from a crustal deformation and transient fluid flow modeling

sm nd isotopic data from archean metavolcanic rocks at holenarsipur south india results of a sm nd isotopic analysis of archean metavolcanics in the holenarsipur greenstone belt karnataka south india give a whole rock isochron age of 2 62 ga for lightly deformed metabasaltic amphibolites in the northern part of the belt this is within error of the age of high grade metamorphism and crustal thickening which affected areas further to the south during the late archean together with the geochemical affinities of these and other metavolcanics in karnataka and results of regional structural analysis this unexpected age supports a model relating volcanism and crustal thickening to northward subduction and crustal accretion during the late archean data from basic and ultrabasic metavolcanics from the more strongly deformed and higher grade southern arm of the holenarsipur belt do not permit an age greater than 3 0 ga previously these rocks were regarded as part of an older supracrustal sequence that predated the local 3 0 to 3 3 ga gneissic complex the new dates therefore consider

late quaternary alluviation and offset along the eastern big pine fault southern california abstract determining late quaternary offset rates on specific faults within active mountain belts is not only a key component of seismic hazard analysis but sheds light on regional tectonic development over geologic timescales here we report an estimate of dip slip rate on the eastern big pine oblique reverse fault in the upper cuyama valley within the western transverse ranges of southern california and its relation to local landscape development optically stimulated luminescence osl dating of sandy beds within coarse grained alluvial deposits indicates that deposition of alluvium shed from the pine mountain massif occurred near the southern margin of the cuyama structural basin at the elevation of the cuyama river between 25 and 14 ka this alluvial deposit has been offset 10 m vertically by the eastern big pine fault providing a latest quaternary dip slip rate estimate of 0 9 m ky based on a 50 fault dip incision of the adjacent cuyama river has exposed a section of older cuyama river sediments beneath the pine mountain alluvium that accumulated between 45 and 30 ka on the down thrown footwall block of the eastern big pine fault corroborative evidence for holocene reverse slip on the eastern big pine fault is 1 m of incised bedrock that is characteristically exposed beneath 2 3 5 ka fill terraces in tributaries south of the fault the eastern big pine fault in the cuyama valley area has no confirmed record of historic rupture however based on our results we suggest the likelihood of multiple reverse slip rupture events since 14 ka

high resolution electromagnetic imaging of the san andreas fault in central california although there is increasing evidence that fluids may play a significant role in the earthquake rupture process direct observation of fluids in active fault zones remains difficult since the presence of an electrically conducting fluid such as saline pore water strongly influences the overall conductivity of crustal rocks electrical and electromagnetic methods offer great potential for overcoming this difficulty here we present and compare results from high resolution magnetotelluric mt profiles across two segments of the san andreas fault saf which exhibit very different patterns of seismicity parkfield which has regular small earthquakes and creep events and in the carrizo plain where the fault is seismically quiescent and apparently locked in both surveys electric fields were sampled continuously with 100 m long dipoles laid end to end across the fault from 100 to 0 1 hz the data from both profiles are consistent with a two dimensional 2 d fault parallel resistivity model when both transverse electric and magnetic te and tm mode data are included in the interpretation narrow approximately 300 endash 600 m wide zones of low resistivity extending to depths of 2 endash 4 km in the core of the fault are required at both locations however at parkfield the conductance conductivity thickness product of the anomalous region ismore an order of magnitude larger than at carrizo plain suggesting much higher concentrations of fluids for the more seismically active parkfield segment we also image structural differences between the two segments at carrizo plain resistive presumably crystalline rocks are present on both sides of the fault at depths below 3 endash 4 km in particular we clearly image resistive basement extending approximately 10 km or more east of the saf beneath the elkhorn hills and temblor range at parkfield the situation is quite different with a resistive block of salinian granite west of the fault and an electrically conductive presumably fluid rich franciscan complex to the east it is possible that these structural differences control the difference in mechanical behavior of the fault either directly by affecting fault strength or indirectly by controlling fluid supply copyright 1999 american geophysical union less

regional aquifer mapping using combined ves tem amt emap methods in the semiarid eastern margin of parnaiba basin brazil as part of a program to evaluate the utility of integrated geoelectrical methods for subsurface structural mapping and groundwater resource investigation in the semiarid eastern margin of the parnaiba basin in brazil several vertical electrical soundings ves and transient electromagnetic tem and tensorial audio magnetotelluric amt measurements were carried out along a 250 km long east west transect passing through major towns and cities and a 127 km long north south profile passing through small farm settlements the various data sets have been jointly processed using a novel integration scheme and a constrained 1 d inversion technique to yield the resistivity structure underneath each observation station regularized 2 d inversion of static shift corrected dual mode amt data provided additional deep structural information and together with the joint 1 d results enabled an assessment of the distribution of aquifers and major structural controls in the region the east west regional geoelectrical model evokes a picture of a gently dipping succession of conductive and resistive units in good agreement with the alternating shaly and sandy formations evinced from preexisting borehole data and previous geological studies the geoelectric models also show the presence of the large scale transbrazilian lineament and other graben like structures previously inferred from aeromagnetic data which may have some control on groundwater distribution the agreement with geology and aeromagnetic interpretation emphasizes the importance of integrated geoelectrical surveying as a complementary or independent means of obtaining useful stratigraphic and structural information for hydrogeological studies in this region

electrical structure of the tectonically active kalabsha fault aswan egypt in this work we use the magnetotelluric mt method to detect geoelectrical conductivity anomalies in the earth s crust and link them to local seismic activity this application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity mt measurements were carried out in the period range 0 0046 420 s at nine sites along a 15 km long north south profile crossing the kalabsha fault on the western bank of lake aswan data were analysed by 2d simultaneous inversion of both polarisations the resulting model is compared with the local seismicity map and reveals the conductive signature of the fault as well as geological and tectonic stresses prevailing in the aswan area our mt investigations show the following features the measured mt strike aligns with the seismic epicentre axis corresponding to the kalabsha fault while crossing the fault enhanced conductivity is found down to depths of 5 km on a 1 2 km profile segment at mid crustal depths 20 km a very high conductive body is found to coincide with the main seismic cluster in the aswan area these observations indicate that seismic activity and high electrical conductivity are related the link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed their presence is also required to trigger the observed seismicity in addition we explain the lower conductivity of the local upper crust in terms of stress modulated rock porosity we believe that these results are of general significance as they could explain the mid crustal seismicity of tectonically active zones d 2005 elsevier b v all rights reserved

miocene phosphate accumulation in the cuyama basin southern california abstract middle and upper miocene phosphatic strata in the cuyama basin of southern california contain approximately 6 4 10 9 metric tons 2 1 10 14 mol of phosphorus p that accumulated in slope and shelf strata at rates between 0 9 and 3 8 mol p cm 2 year 1 from 15 5 to 10 8 ma an estimated 63 of all phosphorus lies in a single third order sequence deposited between 14 8 and 13 ma the main impetus for this brief phosphogenic event was a 200 m relative sea level rise that resulted from strike slip faulting immediately east of the basin minor element and petrologic analyses suggest the overall increase in phosphorus accumulation correlates with less reducing conditions in the shallowly buried marine sediments on the slope and the expansion of phosphogenesis to the outer shelf both events were probably related to landward shifts of the pacific oxygen minimum zone and coastal upwelling centers during the relative sea level rise whereas these tectonic events were apparently local the phosphatic deposits of the cuyama basin clearly demonstrate that tectonism can strongly influence phosphate accumulation rates in marginal marine environments

coarse grained deltaic sedimentation in the miocene cuyama strike slip basin california coast ranges abstract the cuyama basin located in the southern coast ranges of california southwest of the san andreas fault developed early in the history of the san andreas transform system the miocene marine basin formed in a transtensional setting along a dextral strike slip fault of the transform system following oligocene non marine basin formation in an extensional setting the lower and middle miocene vaqueros formation in the northwestern part of the basin which represents the first of two transgressive regressive cycles is described here in terms of nine facies in two broad facies groups the 400 m thick soda lake shale member of the vaqueros comprises deep basin and starved basin facies a thin transgressive facies occurs locally at the base of the formation the overlying painted rock sandstone member of the vaqueros which is more than 2200 m thick and consists mostly of coarse grained sandstone and pebbly sandstone constitutes a delta complex of prodelta slope channel delta front tide influenced distributary channel interdistributary bay and fluvial channel facies the basinal depositional system consisted of turbidite mud and sand and hemipelagic and pelagic sediments of the basinal facies deposited in a rapidly subsiding basin the delta depositional system consisted of the delta complex facies that prograded into the deep basin and had a steep prodelta slope that extended to bathyal depths the delta is inferred to be a mixed fluvial wave dominated fan delta analogous in its delta front morphology and processes to a fjord delta in which coarse sediment delivered to the delta front by braided streams was transported down the prodelta slope into deep water by sediment gravity flows transgression and rapid deepening of the basin in the early miocene coincided with rapid tectonic subsidence deepening culminated with deposition of a starved basin facies or condensed section at the time of maximum transgression which was followed by the beginning of a regression and basin shallowing the overall basin history and geometry of the northwestern cuyama basin are typical of strike slip basins the initial rapid subsidence to bathyal depths at rates of more than 500 m m y in the early miocene is interpreted to be a result of extension at the releasing bend of a dextral strike slip fault

late cenozoic fold and thrust belt of the southern coast ranges and santa maria basin california 1 balanced cross sections across the santa maria basin and southern coast ranges of california show the pliocene to quaternary convergence to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by compressive earthquakes geodetic measurements of present shortening and folded quaternary deposits the pliocene and quaternary structure of the southern coast ranges is dominated by a series of large anticlinal structures that include the lompoc purisima trend the casmalia orcutt trend the point san luis anticline the santa lucia range anticlinorium and the la panza range anticlinorium the anticlinal trends are interpreted to be fault bend and fault propagation folds resulting from thrust ramps off thrust flat and a regional detachment at 11 14 km depth most of the thrust faults do not reach the surface blind thrust the southwestern range front of the san rafael mountains is interpreted to be the result of uplift above a ramp in the point san luis thrust the cross section across the southern coast ranges shows 26 8 km of shortening from the edge of the continental margin to the san andreas fault region wide shortening is interpreted to have begun between 2 0 and 4 0 ma which yields an average regional convergence rate of 6 7 13 4 mm yr the late pliocene and quaternary convergence across the onshore santa maria basin is interpreted to be 9 2 km and the average convergence rate is 2 3 4 6 mm yr compressive focal mechanisms and our structural analysis suggest that the ramp parts of the thrust system are the most likely sources for earthquakes the historic seismic record and length of the anticlinal trends suggest that the thrust ramps probably are capable of generating moderate to large earthquakes 5 0 m w 7 5 major thrust ramps underlie the city of san luis obispo and adjacent coastal towns cross section restoration reveals early formed hydrocarbon traps and accounts for the abundant hydrocarbon accumulations along the casmalia orcutt anticlinal trend and under the santa maria valley the cross sections and restorations also illustrate the importance of miocene and early pliocene normal faults to oil maturation and trapping concealed normal faults along the flanks of the major anticlinal trends and the subthrust area along the north side of the casmalia orcutt anticlinal trend form two relatively untested trap styles

geophysical images of the dead sea transform in jordan reveal an impermeable barrier for fluid flow 1 high resolutionseismictomography and magneto telluric mt soundings of the shallow crust show strong changes in material properties across the dead sea transform fault dst in the arava valley in jordan 2d inversion results of the mt data indicate that the dst is associated with a strong lateral conductivity contrast of a highly conductive layer at a depth of approximately 1 5 km cut off at a position coinciding with the surface trace of the dst at the same location we observe a sharp increase of p wave velocities from 5 km s to the east the high velocities in the east probably reflect precambrian rocks while the high electrical conductivity west of the dst is attributed to saline fluids within the sedimentary filling in this sense the dst appears to act as an impermeable barrier between two different rock formations such a localized fluid barrier is consistent with models of fault zone evolution but has so far not been imaged by geophysical methods the situation at the dst is remarkably different from active segments of the san andreas fault which typically show a conductive fault core acting as a fluid conduit

surface seismic and electrical methods to detect fluids related to faulting in the absence of drilling surface based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths these in situ physical properties can then be used to infer the presence and distribution of fluids along faults although such observations are by nature indirect and become less exact with greater depth multiple observations of a range of such geophysical properties as compressional and shear seismic velocity vp and vs vp v5 ratio related to poisson s ratio resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry fluids in the fault zone and fluid reservoirs in the surrounding crust for simple geometries fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide to address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid rock ratios we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry large changes in material properties and numerous earthquakes within the fault zone a synthetic 0 5 km wide fault zone with 20 velocity reduction is well imaged using local earthquake tomography when sequential velocity inversions are done the true fault width is found even to 9 km depth although the calculated amplitude of the velocity reduction is lower than the actual amplitude vp vs is as well determined as vp magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km further a narrow 1 km very low resistivity 3 ohm m fault core can be imaged within a broad 5 km low resistivity 10 ohm m fault zone illustrating that regions of a fault containing large quantities of interconnected fluids within a broader conductive fault zone should be detectable thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade off with other factors such as intrinsic variations in porosity mineralogy and pore geometry the best approach is combined modeling of varied seismic and electrical data

the arsikere granite of southern india magmatism and metamorphism in a previously depleted crust abstract the arsikere granite is a small diapiric potassic pluton intruded into archean gneisses of the western part of the dharwar craton of southern india a rb sr age of 2563 ma indicates that the granite was formed during a period of granulite development at lower crustal levels at least to the south of the dharwar craton schist belts in the craton were metamorphosed to greenschist or amphibolite facies at this time but gneisses underwent no isotopic or other geochemical effects the arsikere granite contains quartz oligoclase highly perthitic alkali feldspar originally precipitated as a single phase f rich biotite plus primary epidote fluorite and f rich muscovite sphene forms large euhedrons which incorporate nearly all of the ti in the rock leaving pure magnetite as the only opaque phase equilibration parameters include temperatures of 700 from feldspars f o 2 10 12 bar biotite and opaques and f oh 10 3 biotite nearby schist belts were metamorphosed at 5 kbar pressure at the same time as granite emplacement if the granite was emplaced at 5 kbar the magma could not have been water saturated perthite an initial 87 sr 86 sr ratio of 0 7021 in the arsikere granite is too low for the magma to have formed by partial melting of rocks with the rb sr ratios of gneisses currently exposed in the craton apparently the magma formed in a lower crust that had been depleted in rb and probably other lithophilic elements several hundred million years earlier at this time the craton also became water poor and inert to further geochemical activity such as resetting isochrons during later metamorphism

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

petrography chemistry and isotopic ages of peninsular gneiss dharwar acid volcanic rocks and the chitradurga granite with special reference to the late archean evolution of the karnataka craton southern india abstract new petrographic major and trace element and isotopic age data are presented for four suites of rocks from the low to medium grade part of the south indian archaean craton in central karnataka two suites are from the peninsular gneiss basement to the late archaean dharwar supergroup the third is a suite of dharwar potassic rhyolites and the fourth is from the chitradurga granite which has intrusive contacts with the dharwar supergroup the chikmagalur granite granodiorite which with its host gneisses forms part of the basement peninsular gneiss to the dharwar supergroup in the bababudan basin yields rb sr and pb pb whole rock isochron dates of 3080 110 ma and 3175 45 ma respectively an initial 87 sr 86 sr ratio of 0 7013 0 0009 and a 1 value 238 u 204 pb of 7 98 host tonalitic gneisses to the chikmagalur granite give an rb sr whole rock isochron date of 3060 160 ma with an initial 87 sr 86 sr ratio of 0 7015 0 0004 and a pb pb whole rock isochron date of 3190 100 ma with a model 1 value of 8 00 a suite of granites and granodioritic tonalitic gneisses chitradurga granitic rocks and gneisses forming part of the peninsular gneiss basement to the dharwar supergroup west of the chitradurga belt yields rb sr and pb pb whole rock isochron dates of 2970 100 ma and 3028 28 ma respectively with an initial 87 sr 86 sr ratio of 0 7035 0 0013 and a model 1 value of 7 62 the suite of dharwar acid volcanic rocks from north of honnali defines a poorly fitted 207 pb 206 pb linear array mswd 15 8 which gives an apparent age of 2565 28 ma and a model 1 value of 7 46 the poor fit of this isochron shows that a closed system was not maintained a conclusion supported by high k abundances which are attributed to a syn or post magmatic ion exchange process the chitradurga granite which intrudes the older rocks of the dharwar supergroup in the chitradurga belt yields a well fitted pb pb whole rock isochron date of 2605 18 ma with a model 1 value of 7 68 the u pb relations in these suites from karnataka are in marked contrast with the commonly severe u depletion and consequently unradiogenic pb isotopic compositions observed in deeply eroded high grade archaean gneiss terranes such as those bordering the north atlantic this contrast indicates that the cratonic rocks in central karnataka represent a relatively high level in the original archaean continental crust

crustal structure of the north anatolian and east anatolian fault systems from magnetotelluric data abstract magnetotelluric mt studies can map subsurface resistivity structure and have located zones of low resistivity high conductivity within major strike slip fault zones worldwide which have been interpreted as regions of elevated fluid content this study describes mt data from the eastern part of the north anatolian and the east anatolian fault systems nafs and eafs and presents the results of the first mt studies of these faults the inversion of the mt data produced 2 d resistivity models which showed that both fault systems are underlain by a broad low resistivity zone that extended into the lower crust however the resistivity beneath the east anatolian fault system was much lower than beneath the eastern part of the north anatolian fault system these conductors begin at a depth of 10 km not at the surface as on the central san andreas fault safs this difference is interpreted as being due to the fact that the eafs and nafs are young fault systems characterized in the upper crust by multiple fault traces as opposed to the safs that has evolved into a single through going fault different stages of the seismic cycle may also influence the resistivity structure although this is difficult to constrain without knowledge of time variations in resistivity structure at each location

the petrogenesis and setting of archaean metavolcanics from karnataka state south india abstract major and trace element data for ultrabasic to silicic metavolcanics from five archaean volcanic sedimentary belts in south india are summarized their total alteration to greenschist and amphibolite facies assemblages poses problems for petrogenetic interpretation of these data however in the absence of unmetamorphosed lavas some interpretation is essential the data suggest that volcanism in each belt was petrogenetically similar the presence of both light rare earth element enriched and depleted lavas in individual volcanic sequences together with variations in zr y ratios is interpreted as a consequence of fractionation within ultramatic mantle diapirs induced by upward vapour migration other chemical variations probably resulted from fractional crystallization of mafic phases from parental high mgo high cr magmas the influence of plagioclase fractionation only being noticeable in late andesites and dacites the consistent depletion of all basaltic samples in the high field strength elements mb and ta relative to the low field strength element th and to la further emphasises the importance of a vapour phase in their petrogenesis in this regard they resemble modern orogenic basalts the favoured tectonic setting is that of ensialic multiple back arc basins above a thinner lithosphere than in modern back arc environments which rode over a shallow angled subduction zone

conductivity structure of the san andreas fault parkfield revisited 1 a previous interpretation of a high resolution magnetotelluric profile across the san andreas fault at parkfield attributed an anomalously conductive region just west of the fault and extending to depths of 3 km to fractured rock saturated with brines laboratory measured electrical resistivities of samples of sedimentary rocks known to border the fault to the east are comparable to resistivities of the anomalous zone we suggest that the anomalous region results from conductive sedimentary rocks in a plunging syncline that is adjacent to the fault rather than caused by fractured rock if correct the current surface trace of the san andreas fault at middle mountain does not form the boundary between the salinian block and the north american plate instead that boundary is 1 km west and collocated with current seismicity

a magnetotelluric mt study across the koyna seismic zone western india evidence for block structure abstract wide band magnetotelluric measurements were carried out at 16 sites along guhaghar sangole traverse across the koyna seismicity zone the results of the study besides providing details of deccan trap thickness variation along the traverse brought out several structural characteristics of the subsurface extending to deep crustal levels the koyna fault zone is characterized by a weak expression of a moderately conductive feature extending only up to a depth of 4 5 km two dimensional modeling results bring out distinct block structure in the koyna region and are compared with velocity and density structure of the region the deep electrical structure is interpreted in terms of lateral lithological heterogeneities

correlation of electrical conductivity and structural damage at a major strike slip fault in northern chile 1 large scale strike slip fault zones are often imaged as electrically conductive structures in the brittle crust however the relationship of conductivity and internal architecture of the fault zone remains largely unclear this paper presents results of a study designed to compare the record of structural deformation across a fault zone with its electrical conductivity image two high resolution magnetotelluric profiles trend perpendicularly across the west fault a branch of the precordilleran fault system in northern chile the magnetotelluric and geomagnetic response functions in the frequency range from 1000 hz to 0 1 hz clearly image a fault zone conductor fzc about 350 m wide and 1500 m deep trending along the surface trace of the fault the position of the fzc and its geometric properties width dip correlate with a region of intense fluid alteration and the orientation of fault related damage elements minor faults fractures according to estimates of fluid salinity and rock porosity the conductivity anomaly results from meteoric water penetrating into a permeable zone this suggests that the increase in electrical conductivity is causally related to a mesh of minor faults and fractures acting as a pathway for fluids the fzc reflects the central and most fractured part of the damage zone in view of similar published magnetotelluric studies we infer a dependency of a fault s state of activity and the characteristics of the fzc such as its conductance width and depth extent ongoing deformation is the controlling factor for maintaining a permeable and electrically conductive fracture network

bacteria as nucleation sites for authigenic minerals in a metal contaminated lake sediment abstract sediment samples contaminated with metals arising from mine tailings drainage were obtained from lower moose lake in the onaping region near sudbury ontario canada the samples were examined by electron microscopy selected area electron diffraction and energy dispersive x ray spectroscopy individual bacterial cells and their remains were prominent as nucleation sites for both metal sulfides and a complex polymorphic fe al silicate the principal metal sulfide species associated with the bacteria were amorphous mackinawite fes 1 x and microcrystalline millerite nis trace amounts of cu and zu were also detected in some of the sulfide precipitates at least two structural forms of the fe al silicate were present and energy dispersive x ray spectroscopy point analyses revealed corresponding differences in chemical composition poorly ordered limonitic clay type phases had a granular morphology and contained less fe than well developed crystalline material which generated hexagonal diffraction patterns with reflections d 4 60 and 2 55 characteristic of an interstratified chamositic clay

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

3 35ga komatiite volcanism in the western dharwar craton southern india constraints from nd isotopes and whole rock geochemistry abstract we present field petrographic sm nd whole rock isochron and whole rock geochemical data for komatiites from sargur group greenstone belts of the western dharwar craton field evidence such as pillow structure indicates their eruption in a marine environment petrographic data reveal that the igneous mineralogy has been altered during post magmatic hydrothermal alteration processes corresponding to greenschist to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene a 16 point sm nd whole rock isochron gives an age of 3352 110 ma for the timing of eruption of komatiite lavas about 60 of the studied komatiite samples show al depletion whilst the remaining are al undepleted the al depleted komatiites are characterised by high cao al2o3 ratios 1 01 1 34 and low al2o3 tio2 5 16 whereas al undepleted komatiites show lower cao al2o3 ratios 0 59 0 99 and higher al2o3 tio2 17 26 trace element distribution patterns of komatiites suggest that most of the primary geochemical and nd isotopic compositions are preserved with only minor influence of post magmatic alteration processes and negligible crustal contamination the chemical characteristics of al depleted komatiites such as high gd yb n together with lower hree y zr and hf imply their derivation from deeper upper mantle with garnet majorite involvement whereas lower gd yb n slightly higher hree y zr and hf suggest derivation from shallower upper mantle without garnet involvement the observed chemical characteristics cao al2o3 al2o3 tio2 mgo ni cr nb zr y hf ree indicate derivation of the komatiite magmas from different depths in a plume setting whereas sub contemporaneous felsic volcanism and ttg accretion can be attributed to an arc setting in order to explain the spatial association of komatiite volcanism with contemporaneous mafic felsic volcanism and ttg accretion we propose a combined plume arc setting nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early 4 53 ga global differentiation of the silicate earth as suggested by boyet and carlson boyet m carlson r w 2005 142nd evidence for early 4 53 ga global differentiation of silicate earth science 309 577 581 or extraction of continental crust during the early archaean

the dharwar craton and the assembly of peninsular india peninsular india can be separated into five discrete crustal areas the bhandara singhbhum and aravalli bundelkhand cratons the eastern ghats and a block in southern india containing the dharwar craton and adjoining granulite terrain these areas are separated by four major joins eastern ghats front godavari rift mahanadi rift sukinda thrust and narmada son lineament and associated thrusts all joins except the godavari rift are former orogenic belts where thrusts juxtapose rocks of different metamorphic grade these thrusts and the absence of clear correlation of suites among different cratons suggest that the indian shield formed by accretion of separate continental fragments epicontinental late archean early pro terozoic sediments throughout india however may indicate that the joins are intracontinental formed within a coherent block the term dravidian shield is used for the southernmost of the five crustal blocks including all of india south of the deccan basalts and godavari rift

electrical resistivity structure at the northern margin of the tibetan plateau and tectonic implications 1 the ene wsw striking altyn tagh fault atf and the wnw ese trending western qilian mountains define the northern margin of the tibetan plateau new magnetotelluric data were collected along three profiles crossing the eastern section of the atf and the southern qilian mountains the basic sounding bandwidth ranged from 0 003 to 7000 s the transverse electric and transverse magnetic mode data sets were inverted into resistivity sections using a conventional two dimensional 2d inversion code and data at periods of 0 3333 5464 s were inverted using a commonly employed three dimensional 3d code the 3d results constrain the interpretation of the 2d models and the final interpretative resistivity models show that lithospheric structures are spatially variable along the atf and across southern qilian in profiles across the atf the main fault is imaged as a vertical resistivity boundary and the high resistivity body in the western profile extends about 15 km deeper than the corresponding body in the eastern profile positive flower structures are apparent in the western profile but are only weakly visible in the eastern profile these observations suggest that the depth of the atf is spatially variable mantle resistivity images indicate a relatively cold and rigid tarim lithosphere which is consistent with a geodynamic model of oblique subduction of the tarim basin below the atf a high resistivity body in the crust of southernmost qilian indicates a huge thick skinned structure low resistivity bodies at the crust mantle boundary south of the main atf are presumed to form a weak layer beneath northern tibet

tattekere conglomerate quartzite association origin and stratigraphic position of a disputed archean formation in the supracrustals of karnataka india abstract the origin and stratigraphic interpretation of the conglomerate quartzite rocks occurring in and around tattekere and kumkumna hosuru villages in the holenarsipur supracrustal belt is an important aspect of the precambrian geology of karnataka india due to the controversy regarding its origin and stratigraphic status the geology of the holenarsipur supracrustal belt has been interpreted by previous workers in different ways in the present work attempts have been made to resolve the controversy based on recent observations and analytical results the petrographic characteristics of conglomerates and quartzites the occurrences of primary sedimentary structures and their similarity to proven sedimentary quartzites in the northern sector along with the chemical composition of the quartzites point to a sedimentary origin for this association the heavy mineral and chemical composition of the quartzites suggest a major source rock contribution from a granitic terrain with subordinate contributions from metamorphic and or metasedimentary rocks al 2 o 3 and cr enrichment in quartzites resulted from deposition of al 2 o 3 rich chromite grains derived from isolated exposures of ultramafic rocks possible k metasomatism has also been inferred from the chemical composition of the quartzites from field distributions petrographic characteristics chemical compositional data and inferred source rock contribution it may be inferred that the conglomerate quartzite association occurring in and around tattekere and kumkumna hosuru villages in the holenarsipur supracrustal belt represents a single horizon with a near basal position in the stratigraphic sequence this suggests a basal near basal unconformity in the stratigraphic succession of holenarsipur supracrustal belt

magnetotelluric response and geoelectric structure of the great slave lake shear zone the great slave lake shear zone gslsz is a northeast trending 25 km wide dextral continental transform fault from the foothills of the rocky mountains in northeast british columbia to the southeast side of great slave lake based on its magnetic expression the gslsz can be correlated for at least 1300 km mostly in the subsurface magnetotelluric mt soundings were made at 60 sites in the southwest part of the northwest territories canada along the lithoprobe slave northern cordillera lithospheric evolution snorcle transect corridors 1 and 1a in the summer and autumn of 1996 of these 15 sites were along corridor 1a which crossed the gslsz the great bear magmatic arc and hay river terrane to the northwest of the shear zone and the buffalo head terrane to the southeast the primary objective of the corridor 1a deployments was to image the structure of the gslsz analysis of mt data indicates that along the corridor 1a the resistivity structure is approximately 1d at shallow depth 6 1 km corresponding to phanerozoic sedimentary rocks 2d in the upper to mid crust with a strike vn30 e and approximately 2d in the lower crust to lithospheric mantle with a strike of vn60 e the direction in the upper crust is interpreted to represent the local scale 6 50 km horizontal strike of the gslsz whereas the direction in the mantle is parallel to the larger scale strike of the gslsz 2d inversions of the mt data reveal that the gslsz forms a crustal scale resistive zone s 5000 6m that is spatially correlated with a magnetic low the gslsz comprises greenschist to granulite facies mylonites its high resistivity is interpreted to be due to the resistive nature of the granitic protolith of the mylonites and that mylonites are dominated by rocks deformed in the ductile regime to the northwest of gslsz the mt profile reveals crustal conductors beneath the great bear magmatic arc and hay river terrane the enhanced conductivity occurring beneath the great bear magmatic arc is interpreted to be caused by electronic conduction within deformed and metamorphosed rocks of the hottah terrane or the coronation supergroup the mt results also reveal a mantle conductor beneath the margin of the hottah terrane and great bear magmatic arc that is interpreted to be associated with the subduction of oceanic lithosphere a second mantle conductor to the southeast is truncated at the gslsz suggesting an older source for the enhanced conductivity and that the gslsz includes significant strike slip motion of sub crustal lithosphere fl 2002 elsevier science b v all rights reserved

full 3 d tomography for crustal structure in southern california based on the scattering integral and the adjoint wavefield methods we have successfully applied full 3 d tomography f3dt based on a combination of the scattering integral method si f3dt and the adjoint wavefield method aw f3dt to iteratively improve a 3 d starting model the southern california earthquake center scec community velocity model version 4 0 cvm s4 in f3dt the sensitivity frechet kernels are computed using numerical solutions of the 3 d elastodynamic equation and the nonlinearity of the structural inversion problem is accounted for through an iterative tomographic navigation process more than half a million misfit measurements made on about 38 000 earthquake seismograms and 12 000 ambient noise correlagrams have been assimilated into our inversion after 26 f3dt iterations synthetic seismograms computed using our latest model cvm s4 26 show substantially better fit to observed seismograms at frequencies below 0 2 hz than those computed using our 3 d starting model cvm s4 and the other scec cvm cvm h11 9 which was improved through 16 iterations of aw f3dt cvm s4 26 has revealed strong crustal heterogeneities throughout southern california some of which are completely missing in cvm s4 and cvm h11 9 but exist in models obtained from previous crustal scale 2 d active source refraction tomography models at shallow depths our model shows strong correlation with sedimentary basins and reveals velocity contrasts across major mapped strike slip and dip slip faults at middle to lower crustal depths structural features in our model may provide new insights into regional tectonics when combined with physics based seismic hazard analysis tools we expect our model to provide more accurate estimates of seismic hazards in southern california

geodetic measurement of tectonic deformation in the santa maria fold and thrust belt california we have analyzed geodetic observations to resolve tectonic deformation across the santa maria fold and thrust belt northwest of santa barbara california the geodetic network forms a braced quadrilateral with 40 km sides whose southwest corner is the vandenberg very long baseline interferometry station the data include 1 historical triangulation from the 1880s 1920s and 1950s 2 electronic distance measurement trilateration from 1971 and 1985 and 3 global positioning system observations from 1986 and 1987 we combine the three different types of data to estimate two dimensional station positions and strain rate parameters simultaneously using a model which constrains the relative velocity field to be linear in space and constant in time we find significant strain rates the maximum compressive strain is oriented n17 e 5 e and the compressive strain rate in this direction is 0 13 0 03 strain yr under the assumption that the unresolved rotational component of the velocity field is zero we estimate that the integrated rate of deformation across the basin is 7 1 mm yr oriented at n03 e 13 this vector can be decomposed into 6 2 mm yr of crustal shortening on the general structural trend of n30 e and 3 1 mm yr of right lateral shear across this axis on the basis of these values and earthquake focal mechanisms in the area we infer that the deformation occurs on northwest trending folds and thrusts within the belt these results are consistent with the rate and direction of deformation across the central california coast ranges inferred by balancing pacific north america plate motion against san andreas slip and basin and range extension they imply that the santa maria fold and thrust belt is the primary active element in transforming motion from the coast ranges to the western transverse ranges and the santa barbara channel