fault generated mountain fronts in southeast spain geomorphologic assessment of tectonic and seismic activity abstract tectonic activity of se spain is evaluated through the application of a general morphometric analysis over 17 different mountain fronts the selected fronts are distributed along the two most prominent crustal scale structures of the mediterranean sector of spain the eastern betic shear zone ebsz and the valencia trough geomorphic indexes used in this study are mountain front sinuosity index smf and valley floor width ratio vf this work is largely based on the smf vf regression analysis of the studied fronts together with previous available data from sw usa the development of mountain fronts in both different lithologies and tectonic contexts enabled characteristic morphometric signatures for different activity scenarios tectonic activity classes to be obtained this study thus indicates that variations of mountain front geomorphology triggered by different styles of faulting can be detected by morphometric analysis it is demonstrated that discrete uplift rates of 0 15 0 08 m ka are sufficient to keep relatively low smf values down to 1 4 in active fronts class 1 taking into account the reported uplift rates and preliminary palaeoseismic data for some of the class 1 studied fronts recurrence periods for surface rupture events can be theoretically bracketed for se spain recurrence periods for class 1 fronts could theoretically range between 10 000 and 12 500 years for maximum slip conditions and 1750 and 2600 years for minimum slip conditions inactive fronts class 3 develop under uplift rates down to 0 03 m ka and tested recurrence intervals for rupture events range between 25 000 and 50 000 years class 2 fronts of intermediate characteristics and activity display uplift rates between 0 07 and 0 03 m ka

zircon u pb chronology of the jianping complex implications for the precambrian crustal evolution history of the northern margin of north china craton abstract the neoarchean to paleoproterozoic jianping complex in the western liaoning province is an important component of the precambrian basement of the north china craton this region connects the neoarchean fengning chengde complex in the west the neoarchean to paleoproterozoic north chaoyang complex in the northeast and the yixian fuxin archean greenstone belt in the east the precambrian jianping complex is dominantly composed of metamorphosed supracrustal sequence and dioritic to granitoid gneisses tonalite trondhjemite granodiorite ttg here we present results from la icpms zircon u pb isotope dating from the various lithological units in this complex which reveal that the magmatic precursors of the metavolcanic rocks associated with the supracrustal sequence were generated during 2555 2550 ma and up to 2615 ma a major magmatic pulse of dioritic to granitic suite occurred during 2538 to 2495 ma this was followed by ca 2485 ma granulite facies metamorphism and a retrograde event at ca 2450 2401 ma our data also reveal a major charnockite emplacement event in this region at ca 1694 ma integrating our new results with the available geologic and previous geochronological data we identify three major growth stages in the crustal evolution history of the northern margin of the eastern continental block of the north china craton at ca 2550 2495 ma ca 2403 2394 ma and ca 1700 ma the la icp ms zircon data provide new insights on the neoarchean to paleoproterozoic tectonothermal evolution history in the northern margin of the eastern continental block of the north china craton

paleoproterozoic high pressure high temperature pelitic granulites from datong in the north china craton and their geological implications constraints from petrology and phase equilibrium modeling abstract paleoproterozoic granulite facies metapelites are widespread in northern part of the north china craton ncc and their metamorphic conditions and p t paths provide important constraints for tectonic evolution of the ncc however it is still controversial on the metamorphic evolution of these rocks due to variations of retrograde modification in combination with detailed petrological studies geological mapping and geochronology we here present new evidence from gushan datong conjunction zone of the ji ning complex and huai an complex to constrain p t evolution of the metapelites field investigations reveal that the pelitic granulites are spatially associated with high pressure hp mafic granulites showing concordant characteristics of deformation and metamorphism fine mineralogical and microstructural studies reveal that kyanite bearing hp granulite facies indicators preserved in garnet porphyroblasts of the pelitic granulites it suggests the previously assumed moderate to low pressure mp lp and high to ultrahigh temperature ht uht pelitic granulites have experienced hp granulite facies metamorphism four metamorphic generation assemblages m1 m4 are recognized in the studied samples the pre pmax assemblages m1 are characterized by mineral inclusions within garnet core including bt qz rt ms liq representing the late stage of prograde metamorphism the pmax assemblages m2 hp granulite facies are represented by garnet mantle and its inclusion type minerals i e grt ky kfs bt rt liq qz ms zn spl it is followed by heating until the maximum of temperature stage m3 tmax featured by disappearance of biotite and transition from kyanite into sillimanite with mineral assemblages of grt sil kfs liq qz ilm zn spl the cooling stage m4 is featured by breakdown of garnet to bt qz pl sil ilm phase equilibrium modeling and titanium in biotite thermometer are combined to constrain the p t evolution of the studied samples the p t conditions for the m1 m4 are respectively estimated around 11 11 5 kbar 770 790 c 10 3 13 8 kbar 820 835 c 6 5 9 5 kbar 850 925 c 6 kbar 645 761 c which defined a clockwise p t path zircon u pb ages of two representative samples demonstrated comparable age clusters of 1950 ma and 1850 ma which are interpreted to represent the timing of hp and mp lp granulite facies stages respectively combining with other studies of the ncc we propose that the pelitic granulites and associated hp mafic granulites have experienced the common metamorphic processes from peak stage hp ht granulite facies to retrograde metamorphism due to intensive and pervasive overprint of sillimanite type granulite facies metamorphism the hp granulite facies mineral assemblages of the pelitic granulites are partially or completely reworked the metamorphic peak conditions and relatively high thermal gradients of these granulite facies rocks suggest that their formation is probably in response to a crustal scale subduction collision process during the late paleoproterozoic

deep crustal electromagnetic structure of central india tectonic zone and its implications magnetotelluric data at 45 locations along the mahan khajuria kalan profile in the central india tectonic zone are analysed this 290 km long profile yields data in the period range 0 001 1000 s across the tectonic elements of the study region bounded by purna fault gavligarh fault tapti fault narmada south fault and narmada north fault multi site multi frequency analysis suggests n70 e as the geoelectric strike direction data rotated into the n70 e strike direction are modelled using a non linear conjugate gradient scheme with error floors of 10 for both apparent resistivity and phase components two dimensional magnetotelluric model yields conductors that correlate with known faults in the study region and regional seismicity presence of a 30 mgal gravity high together with the observed conductive bodies less than 20 ohm m in the deep crust beneath the purna graben and tapti valley is explained by the process of magmatic underplating the conductive bodies beneath the mahakoshal rift belt and vindhyans accompanied by regional gravity lows of the order 70 mgal are attributed to the presence of deep crustal fluids following the re activation model proposed for the entire region the conductors 20 ohm m at various depth levels correspond to mafic magmatic and or fluid intrusions controlled by deep seated faults that seem to tap reservoirs beyond the crust mantle boundary the shallow depth localized faults also seem to have facilitated further upward movement of these underplated material and fluids release during this process

new evidence for neoarchean ca 2 7 ga crustal growth in the north china craton abstract the north china craton ncc is characterized by major ca 2 5 ga tectonothermal events which is markedly different from most other typical cratons worldwide whereas ca 2 7 ga rocks are sporadically exposed in the whole craton and only have been identified in several localities in this contribution we report new discovery of ca 2 7 ga rocks from the western huixian in the ncc zircon u pb dating reveals their protoliths were emplaced between 2714 and 2704 ma the rocks are tonalitic gneisses in composition and present similar geochemical characteristics to the archean ttg suites and phanerozoic high sio2 adakites worldwide zircons from the three tonalitic gneisses yield variable positive ehf t values of 2 3 6 7 4 2 8 7 and 3 3 6 3 with hf model average ages of ca 2848 ma ca 2838 ma and ca 2840 ma respectively the measured zircon 18o values range from 5 5 to 6 2 5 1 to 6 1 and 5 2 to 6 2 with weighted mean values of 5 7 5 7 and 5 8 respectively all of them overlap with the mantle range within two sigma uncertainty limits the hf o isotope compositions in zircons from the tonalitic gneisses indicate that their protoliths originated in a juvenile crust forming event at ca 2 7 ga available data suggest ca 2 7 ga is a major period of crustal growth in the ncc geochemical signatures of the ca 2 7 ga tonalitic gneiss reflect that their protoliths formed by partial melting of hydrated mafic rocks with variable proportions of the amphibole and garnet residues to be left behind in the residuum but absence of plagioclase and rutile in their source the rocks are classified as hp ttg their protolith formed under amphibolite facies conditions it is noted that the ca 2 7 ga tonalitic gneisses in huixian display low mgo

synchronous vertical and horizontal tectonism in the neoarchean kinematic evidence from a synclinal keel in the northwestern superior craton canada the long and narrow carrot river greenstone belt in the northwestern superior craton is a synclinal keel between two broad granitoid domes its unique structural geometry and kinematics place significant constraints on the timing relationship between vertical and horizontal tectonism a high strain zone spatially coincident with the belt records evidence for synchronous greenstone down granitoid up dip slip and dextral strike slip components kinematic and regional considerations lead to the conclusion that the dip slip component was a result of vertical tectonism and the strike slip component was a result of horizontal tectonism and therefore that the vertical and horizontal tectonism were synchronous it is emphasized that vertical and horizontal tectonism do not have to act exclusively of each other it is not necessary to downplay the significance of vertical tectonism to validate horizontal tectonism and vice versa evidence for synchronous vertical and horizontal tectonism reported here makes it necessary that both processes as opposed to either one or the other and their potential interactions are considered in interpretations of archean terrains it is suggested that synchronous vertical and horizontal tectonism was a common process in the neoarchean and might represent a transition from dominant vertical tectonism in the mesoarchean and paleoarchean to dominant horizontal tectonism in the proterozoic and phanerozoic it might be an important process for gold mineralization at the late stages of archean cratonization 2005 elsevier b v all rights reserved

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

strike slip faulting and block rotation in the lake mead fault system strike slip faults in the basin and range province have often been considered passive boundaries between differentially extended domains of tilted normal faults and are thus considered secondary in accommodating regional horizontal deformation paleomagnotic investigation of late miocene age volcanic rocks displaced by the left lateral fault system of lake mead nevada shows 1 that these rocks have not been affected by significant structural tilt the difference between observed and expected inclinations being only 0 6 14 9 and 2 a significant horizontal counterclockwise rotation of 29 4 8 5 about a vertical axis this rotation was accommodated by slip on northwest trending right lateral strike slip faults this implies significant west northwest elongation results of the investigation indicate that strike slip faulting is the primary process accommodating crustal deformation along the lake mead fault system and that tilting in response to normal faulting is secondary

the 1 75 1 68ga anorthosite mangerite alkali granitoid rapakivi granite suite from the northern north china craton magmatism related to a paleoproterozoic orogen abstract the amgrs is a suite of anorthosite mangerite alkali granitoid rapakivi granite in the northern north china craton ncc new zircon shrimp and laser ablation inductively coupled plasma mass spectrometry la icpms u pb dating of the damiao anorthosite changsaoying k feldspar granite lanying anorthosite and quartz syenite and the gubeikou k feldspar granite yielded emplacement ages of 1726 9 ma 1753 23 ma 1739 43 ma 1712 15 ma and 1692 19 ma respectively combined with previous geochronological data from the damiao norite and mangerite and shachang rapakivi granite it indicates that the whole amgrs was intruded between 1750 ma and 1680 ma the damiao anorthosites and norites exhibit a high mg and this decreases from mafic up to 63 to felsic t dm c ages ranging from 2 32 ga to 2 43 ga and 2 70 ga to 2 88 ga respectively this suggests that these rocks are the fractional crystallization products of one parental magma zircon hf and whole rock sr nd pb isotopic systematics combined with high la yb 10 and intermediate th ta ratios suggest that the parental magma of the amgrs could have been derived from em i lithosphere mantle enriched by recycled archean continent crustal materials accompanied by some assimilation of lower crustal components the mafic to felsic compositions of the amgrs are different from typical bimodal rocks formed in an anorogenic intra continent rift setting but resemble those emplaced during post collisional post orogenic extensional tectonic setting this is compatible with their age of

composite nature of the north china granulite facies belt tectonothermal and geochronological constraints granulite facies rocks are intermittently exposed in a roughly e w trending belt that extends for approximately 2000 km across the north china craton from the helanshan qianlishan wulashan daqingshan guyang and jining complexes in the western block through the huai an hengshan xuanhua and chengde complexes in the trans north china orogen to the jianping western liaoning eastern hebei northern liaoning and southern jilin complexes in the eastern block the belt is generally referred to as the north china granulite facies belt previously interpreted as the lowest part of an obliquely exposed crust of the north china craton recent data indicate that the north china granulite facies belt is not a single terrane instead it represents components of three separate terranes the eastern and western blocks and trans north china orogen each of these units records different metamorphic histories and reflect the complex tectonic evolution of the ncc during the late archean and paleoproterozoic mafic granulites in the eastern block and the yinshan terrane western block underwent medium pressure granulite facies metamorphism at about 2 5 ga with anticlockwise p t paths involving near isobaric cooling following peak metamorphism reflecting an origin related to intrusion and underplating of mantle derived magmas pelitic granulites in the khondalite belt western block underwent medium pressure granulite facies metamorphism at about 2 0 1 9 ga with clockwise p t paths which record the paleoproterozoic amalgamation of the yinshan and ordos terranes to form the western block mafic and pelitic granulites in the trans north china orogen experienced high to medium pressure granulite facies metamorphism at 1 85 ga with clockwise p t paths involving nearly isothermal decompression following peak metamorphism which are in accord with the final collision between the eastern and western blocks to form the north china craton at 1 8 ga the ncgb cannot therefore represent a separate unique terrane instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times d 2005 international association for gondwana research published by elsevier b v all rights reserved

morphotectonic segmentation and spatial variability of neotectonic activity along the narmada son fault western india remote sensing and gis analysis abstract morphotectonic analysis of landscape using remote sensing and gis is an effective way of deducing the pattern and spatial variation of neotectonic activity along poorly investigated active faults in this paper we evaluate the neotectonic activity along a part of the seismically active but poorly understood narmada son fault nsf a more than 1000 km long ene wsw trending fault transecting through the central part of the indian plate the nsf in the study area is geomorphologically expressed as an ene wsw trending line of north facing scarps that delimit the rugged mountainous topography of the uplands to the south and the alluvial basin to the north which prominently slope away from the scarps the scarps are developed in the south dipping basaltic flows belonging to the deccan trap formation of the late cretaceous eocene and continue westward into tertiary rocks and further as a paleobank of the narmada river field studies have revealed that the nsf in the study area is divisible into four morphotectonic segments i to iv which is attributed to the tilakwada karjan madhumati and rajpardi faults cutting across the nsf we characterize the neotectonic activity of the segments through dem analysis using geomorphic indices which include the mountain front sinuosity hypsometric curves hypsometric integral asymmetry factor stream length gradient index and channel sinuosity interpretations of morphometric data are well supported by field data both the entire length of the nsf and the cross faults are found to be neotectonically active we demonstrate that segment ii has undergone the highest intensity of neotectonic activity followed by segments iii i and iv this is corroborated by the highest elevation of scarps and the alluvial deposits the deepest incision 40 m depth and steep northward slope of the alluvial plain in segment ii we infer that the nsf is characterized by differential uplift in recent past which agrees with the high angle reverse faulting with oblique slip movements along the cross faults

high temperature s type granitoids charnockites in the jining complex north china craton restite entrainment and hybridization with mafic magma abstract the liangcheng garnet granitoids are parautochthonous s type granites that occur within granulite facies metasediments in the ultrahigh temperature uht jining complex eastern part of the khondalite belt north china craton these garnet granitoids contain mafic microgranular enclaves and are intimately associated with numerous small intrusions of the xuwujia gabbronorites showing magma mingling and mixing textures typical mineralogy in the garnet granitoids is plagioclase k feldspar quartz garnet biotite orthopyroxene u pb ion microprobe dating of zircon suggests that garnet granitoids and associated gabbronorites were coeval with uht metamorphism at 1 93 1 92 ga and also recorded 1 92 ga metamorphism during slow cooling garnet granitoids are characterized by the presence of orthopyroxene high modal proportion of garnet up to 25 and strongly peraluminous asi 1 11 1 55 along with lower sio2 57 8 70 1 wt and higher feot mgo 5 6 14 4 wt contents than typical s type granites mineral chemistry high ti biotite and high al orthopyroxene high al in orthopyroxene temperature 910 989 c and zr saturation temperature 825 901 c and low whole rock al2o3 tio2 ratios 11 28 indicate that garnet granitoids were derived from biotite dehydration melting of metasedimentary rocks under uht granulite facies conditions textural and compositional evidence suggests that a significant proportion of garnets in the liangcheng garnet granitoids are potentially a peritectic phase entrained from the source garnet aggregates may also represent entrained garnet rich restites the large hree variations of garnet granitoids also support the entrainment of garnet rich restites garnet granitoids display nearly linear trends with the mafic enclaves and gabbronorite for most major elements that demonstrates the involvement of mafic magma in the granitoid petrogenesis the ehf t values 1 0 5 6 and 18o values 9 2 11 6 of zircons in garnet granitoids partly overlap with those in gabbronorites ehf t 2 9 3 5 18o 7 7 10 9 suggesting the strong interaction and mixing process between them major element modelling suggests that the addition of 20 40 restites garnetites and gabbronorites to the anatectic melts was responsible for the large chemical variations therefore the liangcheng garnet granitoids represent high temperature 900 c h2o undersaturated strongly peraluminous granitoids and are produced by large scale melting of metasedimentary rocks and subsequent mixing between restite rich anatectic melt and mafic magma the voluminous high temperature granitoids coupled with uht granulites in the jining complex manifest the thermal response of a granulitic lower crust to the mafic underplating

geochemistry and u pb zircon ages of metamorphic volcanic rocks of the paleoproterozoic luliang complex and constraints on the evolution of the trans north china orogen north china craton abstract the paleoproterozoic luliang complex is situated in the central part of the western margin of the trans north china orogen and consists of volcanic rocks sedimentary rocks and paleoproterozoic granitoid intrusions the volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist to amphibolite facies these metamorphosed volcanic rocks are dominated by basalts to basaltic andesites the parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5 to 30 partial melting of spinel lherzolites to spinel garnet lherzolites which had been enriched by the subduction melts mafic magma experienced fractional crystallization and crustal assimilation u pb zircon dating on two metamorphosed volcanic rocks from the yejishan and luliang groups reveals that they formed at 2210 13 ma and 2213 47 ma respectively and were metamorphosed at 1832 ma this suggests that the metamorphosed volcanic rocks in the yejishan and luliang groups formed synchronously in the paleoproterozoic these new ages integrated with recently reported u pb zircon ages for the jiehekou group and paleoproterozoic granitoids suggest that all of the lithological assemblages of the luliang complex formed and were metamorphosed in the paleoproterozoic not in the neoarchean petrological geochronological and geochemical data suggest that the geodynamic evolution of the paleoproterozoic luliang complex was involved in the development of a magmatic arc system at an active continental margin generating widespread arc related magmatism at 2 2 ga the luliang complex then underwent intense deformation and metamorphism and was incorporated into the trans north china orogen during the 1 88 1 83 ga collisional event which was followed by post collision extension at 1 80 ga

ree geochemistry of early precambrian charnockites and tonalitic granodioritic gneisses of the qianan region eastern hebei north china abstract the charnockites and gneisses from the qianan area eastern hebei province china were investigated for their isotope and trace element abundances the majority of the charnockites are acid in composition only a few samples have a composition corresponding to intermediate to basic rocks a rb sr whole rock isochron of the charnockites gives an age of 2 65 0 05 ga the highly fractionated ree patterns and the variable ce yb n ratios in these rocks are interpreted as being a result of igneous processes it is believed that the charnockitic rocks are related to each other by fractional crystallization under high temperatures and low p h 2 o conditions the major precipitating phases are thought to be orthopyroxene and plagioclase by contrast the qianan gneisses are mainly of tonalitic and granodioritic composition however a few samples have composition corresponding to granite s s probably produced by metasomatic processes a rb sr whole rock isochron of the qianan gneisses yields an age of 2 13 0 06 ga and this age probably represents a remobilization event the qianan gneisses do not contain hypersthene they have strongly fractionated ree patterns similar to those found in many archaean tonalitic rocks of the world in addition the total ree abundances and the magnitudes of the eu anomaly show regular variation with the sio 2 content this may suggest that the qianan gneisses might have evolved under hydrous conditions with hornblende exerting a major control on their trace element chemistry

lithospheric electrical structure of south china imaged by magnetotelluric data and its tectonic implications abstract the region of south china mainly consists of the yangtze block in the northwest the cathaysia block in the southeast and the jiangnan orogen in between these two major precambrian continental blocks the yangtze block borders the north china craton in the north and the eastern margin of the tibetan plateau in the west the cathaysia block adjoins the pacific tectonic domain in the east the study of tectonics in this region is of great significance given its important role in understanding the formation of the asia continent under the auspices of sinoprobe project new magnetotelluric mt data were collected along a 1200 km long profile starting from central sichuan basin near suining extending southeastward passing through the yangtze block jiangnan orogen and terminating within the western cathaysia block near ganzhou based on data analysis results 2d inversions were conducted on the dataset resulting model shows that the lithospheric electrical structure of south china is generally resistive which is consistent with the basic feature of stable precambrian tectonic setting the resistive western yangtze block represents the stable archean aged cratonic region of the yangtze basement while the electrically conductive eastern yangtze block is characterized by lithospheric shearing of the strike slip fault system and extensional process that is probably caused by slab roll back of a flatly subducted plate the jiangshao fault performs as a northwestward dipping conductive layer which indicates the lithospheric underthrusting of cathaysia block beneath yangtze block with its frontal edge reaching the area of jishou in the upper mantle to the west of jiangshao fault eastern flank of the xuefengshan mountain marks the overthrusting frontier of the yangtze block as well as its southeastern boundary to the east of jiangshao fault the northwestern boundary of the cathaysia block displays the pattern of wedging tectonics which is characterized by a conductive layer wedging into the cathaysia lithosphere at the depth range of moho

zircon u pb ages of paleoproterozoic mafic granulites from the huai an terrane north china craton ncc implications for timing of cratonization and crustal evolution history abstract the north china craton ncc is considered to have been incorporated into the columbia supercontinent during late paleoproterozoic the huai an terrane located in north central part of the ncc is an important tectonic window to investigate the paleoproterozoic crustal evolution history of this craton particularly due to the wide occurrence of high grade metamorphic rocks including retrograded eclogites and high pressure granulites available data show two controversial peak metamorphic ages of these rocks as 1 95 1 90 or 1 85 1 80 ga thus determining the precise timing of metamorphism of the high pressure granulites is important in reconstructing the collisional history in this study we present zircon u pb ages bulk chemistry and mineral inclusions in zircons and matrix mineral assemblages from the high pressure granulites to evaluate the timing of metamorphism our results reveal ca 2 05 2 0 ga ages from magmatic zircons in these rocks representing the timing of emplacement of mafic magmas the metamorphic zircons yield two groups of ages at 1 95 1 90 and 1 85 1 80 ga clinopyroxene plagioclase titanite and apatite occur as common mineral inclusions within the 1 85 1 80 ga metamorphic zircons the chemical compositions of clinopyroxene and plagioclase occurring as inclusions are very similar to those of the rim compositions of those minerals in the matrix suggesting their formation during retrograde granulite metamorphism and are distinctly different from those of the core domains of the minerals in the matrix which represent high pressure granulite metamorphism these results indicate that the metamorphic zircons formed during retrograde granulite metamorphism dated as 1 85 1 80 ga and the age range of 1 95 1 90 ga corresponds to high pressure granulite metamorphism the dual nature of the geological and geochronological features suggest that the archean gray gneisses in the huai an terrane represent the basement rocks detached from the khondalite belt of the ncc after collision integrating with published data a double side collision with scissor like suturing model is proposed for the paleoproterozoic cratonization of the ncc

magnetotelluric studies in the epicenter zone of 2001 bhuj earthquake abstract some earthquakes to mw5 level are occurring along two faults viz kachchh mainland fault and south wagad fault near the epicenter of mw7 7 bhuj earthquake of 2001 these faults are not the causative fault of the 2001 earthquake and suspect for future large earthquake in absence of detail geophysical surveys the sub surface orientation of these faults was not known and mt profiles have proved useful in delineating the subsurface trends two mt profiles having lengths of 24 km and 15 km with an inter station spacing of 1 3 km are taken about 10 km east and west of the epicenter of mw7 7 2001 bhuj earthquake respectively the profiles are in nearly n s direction to detect the faults which trend e w in the area from 2 d inversion of mt data of 23 stations it is found that there are two distinctive conductive features matching with the south wagad fault and kachchh mainland fault south of it both being about 10 km apart the south wagad fault is seen extending to banni low lying area 30 km west of the present trace

magnetic fabric variations along the fault related anticlines of eastern kachchh western india abstract an investigation of the anisotropy of magnetic susceptibility ams along the kachchh mainland fault kmf and south wagad fault swf in the eastern kachchh has been carried out to determine the variation of magnetic fabric in different anticlines 105 cores drilled from 35 jurassic sandstone samples collected from both forelimbs and backlimbs of the anticlines are analyzed the various types of fabrics from the three sites indicate essentially three stages of generation of magnetic fabric the sedimentary fabric type i is affected a initially by layer parallel shortening resulting in the development of type iii fabric with oblique to parallel bedding and magnetic foliation b later by bedding parallel shear associated with the rotation of limbs resulting in the tectonic fabric type iv and c finally by bedding parallel thrusting dominantly in the backlimbs resulting in the imprint of higher tectonic fabric type v vi about 14 of the sampled sites belong to the sedimentary type i fabric 54 to the type iii and 32 show tectonic fabric type iv and v vi the different stages of fabric evolution are not accompanied by linear increase in the degree of anisotropy but show construction and destruction of magnetic fabric during the layer parallel shortening to late stage bedding parallel thrusting the tectonic magnetic lineation orientations are mostly not consistent with the bedding strikes or fold axes the intermediate fabric lineation direction of nne ssw is superimposed by later tectonic lineation with varying orientations of nw se in the western end to nne ssw to nnw sse along the eastern end of kmf we attribute the variations in shortening direction essentially to the transpressional deformation of the basin in general and individual anticlines in particular the variation in slip propagation ratio along individual segmented faults has resulted in the rotation of fold limbs

ultrahigh temperature mafic granulite in the huai an complex north china craton evidence from phase equilibria modelling and amphibole thermometers abstract the huai an complex in the north western trans north china orogen north china craton is recognized to have undergone high pressure metamorphism with controversial ages two mafic granulite samples 17ht13 and 16ht22 from huangtuyao of the huai an complex are documented for metamorphism based on petrography mineral chemistry phase equilibria modelling and zircon dating the rocks comprise garnet clinopyroxene orthopyroxene amphibole plagioclase and ilmenite they show clockwise p t paths involving the pre tmax decompressional heating the tmax and the post tmax cooling stages the pre tmax decompressional heating is revealed by the core rim mantle anorthite ascending zoning in coarse grained plagioclase the tmax stage is constrained to 8 5 10 5 kbar 980 1010 c 17ht13 and 7 5 9 5 kbar 1000 1030 c 16ht22 by the high ti content of amphibole and the anorthite content of coarse grained plagioclase rims mantles and fine grained plagioclase cores these temperatures are slightly higher than the results of amphibole thermometers 940 980 c the post tmax cooling is defined by the later growth of amphibole and some conventional geothermobarometers because it is dominated by temperature with ilmenite present the ti content of amphibole is proposed as a reliable thermometer and the ti rich amphibole with ti 0 28 p f u or tio2 2 4 2 6 wt is indicative of ultrahigh temperature 900 c uht conditions the anorthite content of plagioclase is also significant in confining peak temperatures for uht granulite but requires defining proper water content zircon dating for 17ht13 yields an upper intercept age of 1910 35 ma representing the cooling of uht granulite we conclude that the huai an complex may have at least locally undergone uht metamorphism at 1 92 1 91 ga after high pressure granulite metamorphism at 1 95 ga similar to the jining complex therefore the huai an and jining complexes share the same tectonic evolution involving 1 95 ga crustal thickening 1 92 1 91 ga extension with regional or local uht metamorphism and subsequent cooling and uplifting

the pb isotope geochemistry of granitoids from the himalaya tibet collision zone implications for crustal evolution abstract pb isotopic data on 80 k feldspars from six individual granitoid belts of the himalaya tibet collision zone are reported the isotopic trends observed in the trans himalayan complex which outcrops along the indus zangpo suture zone are best explained by a mixture of mantle derived pb component s with crustal derived pb these confirm that the batholith was built upon sialic continental crust as a result of the northward subduction of mesozoic oceanic crust beneath southern tibet the pb isotopic compositions of k feldspars from the three granitoid belts located within the lhasa block to the north of the indus zangpo suture zone are all radiogenic and indistinguishable from those of k feldspars extracted from basement rocks no pristine mantle derived component was identified and the granitoids are interpreted as being wholly derived from the partial melting of the continental crust furthermore the isotopic compositions of the granitoids are remarkably similar over the entire lhasa block implying that they were equilibrated by fluid remobilization during granulite facies metamorphism in the lower crust to the south of the indus zangpo suture zone and within the indian shield the lead isotopic composition of k feldspars from two granitoid belts although significantly more radiogenic than those of the lhasa block are identical to that of the basement gneisses indicating also that the granitic melts are the products of the anatexis of the continental crust in both the lhasa block and the indian shield the generation of granitic melts occurred as a consequence of large scale intracontinental thrusting events that created crustal repetition and induced the development of thermal anomalies at depths due to the burial of meta sedimentary rocks rich in water and radioactive heat producing elements the lhasa block and the indian shield define two distinct pb isotopic provinces which have undergone complex episodes of u th pb fractionation and are largely composed of precambrian material furthermore the correspondence between distinct pb isotopic provinces and discrete continental segments might indicate that the blocks originated from different sectors of the gondwanian continent

extensional collapse and uplift in a polymetamorphic granulite terrain in the archaean and palaeoproterozoic of north china abstract the archaean granulite terrain in the datong huai an area north china comprises a basement complex of felsic and mafic granulite overlain by a sedimentary sequence dominated by metapelite and metapsammite both lithological associations are separated by a tectonic contact and structural textural and stratigraphic data indicate that the terrain was affected by two prominent tectono thermal cycles c a and c b associated with two granulite facies events m1 and m2 which probably occurred 2500 ma and 1800 ma ago respectively of these m1 events were dominant and associated with all major structures in the area in the basement gneiss m1 reached peak metamorphic conditions of 12 14 kbar and 800 900 c during a foliation forming event d 2ba which transposed an earlier s 1ba foliation of unknown age and origin in the allochthonous cover sediments m1 peak conditions of about 8 kbar and 800 c were reached after a prograde thickening event d 1co associated with the formation of a bedding parallel foliation while large amounts of s type granites were emplaced the localised d 2ba and d 1co structures are interpreted as spatially distinct responses at different crustal levels to one metamorphic event juxtaposition of basement and allochthonous cover along a large low angle normal shear zone occurred during d 3 this detachment zone accommodated extension after crustal thickening and allowed unloading of the footwall resulting in a rise of the basement rocks through 4 6 kbar as deduced from syn d 3 decompression textures coeval cooling textures in the allochthonous cover sequence indicate that lateral rather than vertical displacements occurred in the hanging wall of the detachment during c b m2 peak conditions of about 4 6 kbar and 650 700 c were reached coeval with the development of discrete left lateral strike slip zones d 4 that transect both basement and allochthonous cover and accommodated some uplift the deduced c a and c b p t paths have equivalents in many precambrian granulites in general decompressional paths like the one recorded in the basement rocks are linked to processes involving crustal thickening followed by extensional collapse in the datong huai an area it can be shown that low angle detachments may have played an important role in the uplift and cooling history of granulite belts as long ago as the archaean proterozoic boundary

petrogenesis of ca 1 95 ga meta leucogranites from the jining complex in the khondalite belt north china craton water fluxed melting of metasedimentary rocks abstract anatexis occurs in response to changing p t fluid conditions during orogenesis is thus crucial for understanding the tectonic evolution of orogenic belts here we report an integrated petrological geochemical whole rock oxygen isotopic and zircon u pb hf o isotopic study of meta leucogranites from the jining complex north china craton to unravel its petrogenesis and constrain the tectonic evolution of the paleoproterozoic khondalite belt the meta leucogranites are weakly peraluminous with homogeneous minimum melt compositions and are characterized by low rb contents and rb sr ratios high ba and sr contents and sr y ratios and moderately fractionated ree patterns without obvious eu anomalies these trace element characteristics differ from that of leucogranites derived from fluid absent melting considering their low whole rock zr saturation temperatures 746 780 c and comparable compositions with experimentally generated melts produced by water fluxed melting we infer that the meta leucogranites are low temperature granites and were generated by water fluxed melting of metasedimentary rocks at 6 8 kbar sims zircon u pb analyses suggest that the meta leucogranites were emplaced at 1 95 ga or slightly earlier and were subsequently metamorphosed and cut by a pegmatite dike at 1 92 ga the abundant inherited zircons yield nearly concordant 207 pb 206 pb ages ranging from 2 24 to 1 98 ga which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the khondalite belt the hf isotopic compositions of inherited and magmatic zircons e hf t 0 2 to 7 7 closely overlap with the time evolved e hf range of detrital zircons from metasediments in the khondalite belt different types of zircons inherited magmatic and metamorphic zircons have similar 18 o values of 7 2 9 2 which is lower than that of metamorphic zircons 10 0 13 6 from surrounding metasediments thus we propose that the meta leucogranites were formed by water fluxed melting of metasedimentary rocks from the khondalite belt at 1 95 ga during the prograde upper amphibolite facies metamorphism

th u pb monazite geochronology of the luliang and wutai complexes constraints on the tectonothermal evolution of the trans north china orogen abstract recent lithological metamorphic and geochronological studies show that the basement of the north china craton ncc formed by collision of two discrete archean to paleoproterozoic blocks eastern and western blocks along a paleoproterozoic orogen named the trans north china orogen however the timing of the collision is controversial and post collisional exhumation has not been precisely dated this study applies the electron microprobe th u pb monazite dating technique to determine the ages of the peak metamorphic event and subsequent exhumation of the low to medium grade luliang and wutai complexes in the trans north china orogen electron microprobe th u pb monazite data for the luliang complex reveal five tho 2 pbo age ranges 1 1940 1938 ma 2 1880 1847 ma 3 1795 1755 ma 4 1720 1703 ma and 5 1648 ma of these age ranges the first four have also been recorded in the wutai complex which yields tho 2 pbo ages of 1 1930 ma 2 1838 1822 ma 3 1793 ma and 4 1719 ma the oldest age range of 1940 1930 ma is interpreted to record the widespread emplacement of mafic dikes in rocks of the luliang and wutai complexes the age range of 1882 1822 ma is in accord with the shrimp u pb ages of metamorphic zircons and mineral sm nd and 40 ar 39 ar ages obtained for other complexes in the orogen and is interpreted as the age of the major metamorphic event caused by amalgamation between the eastern and the western blocks the age range of 1795 1755 ma is consistent with emplacement of large scale unmetamorphosed mafic swarms at 1800 1765 ma interpreted as the time of post orogenic extension the 1720 1703 ma and 1648 ma ages are considered to date later multiple stages of hydrothermal alteration since monazites with such young ages only occur along fractures in older monazite grains and at the rims these new monazite tho 2 pbo ages support the tectonic model for the evolution of the north china craton that envisages discrete eastern and western blocks colliding along the trans north china orogen at 1 85 ga and then undergoing post collisional extension in the period 1795 1755 ma

luminescence chronometry and geomorphic evidence of active fold growth along the kachchh mainland fault kmf kachchh india seismotectonic implications abstract the kachchh region of western india is a pericratonic basin experiencing periodic high magnitude earthquakes events in 2001 a catastrophic seismic event occurred at bhuj measuring mw 7 7 the epicenters of both the 1956 and 2001 earthquakes were along the kachchh mainland fault kmf proximal to the eastern end of the northern hill range nhr the latter is a topographic expression of an active fault related fold on the hanging wall and is controlled by a south dipping blind thrust the present study deals with the eastern sector of nhr and uses optical dating to reconstruct the chronology of tectonically caused incisions along the backlimb of the nhr incision ages on channel fills and valley fill terraces progressively decrease from 12 ka to 4 3 ka this age progression along with geomorphic evidences decrease in topographic relief drainage capture and drainage migration across the fold nose suggests an active vertical and lateral fold growth along the kmf optical ages suggest that during the late holocene the average uplift rate along the eastern nhr was 10 1 mm a recent gps based estimates on crustal shortening are 12 mm a the kmf and the south wagad fault swf represent the bounding faults of a transtensional basin that formed during the initial rifting this basin is termed as the samakhiali basin the compressive stresses on account of structural inversion from normal to reverse phase resulted in lobate shaped anticlines along kmf and swf zone these anticlines subsequently coalesced and formed linked and overlap segments the present study suggests that eastward lateral deformation across the eastern portion of kmf has continued and has now resulted in its interaction with a left step over transfer fault called the south wagad master fault swmf this implies an increasing transpersional deformation of the samakhiali basin we therefore suggest that the eastward nhr ridge propagation along kmf resulted in the thrust faulting on the south dipping swmf resulting in the bhuj 2001 event the increasing strain on this basin may cause enhanced seismicity in the future along the eastern kmf and wagad region

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

magnetotelluric evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane chaco pampean plain argentina the extension of the transbrasiliano lineament abstract the transbrasiliano lineament represents a continental shear belt that transversely intersects the south american platform from nne to ssw much evidence of this lineament exists to the north but it remains uncharacterized in the distal andean foreland to the south where it is associated with the tectonic boundary between the rio de la plata craton and the pampean terrane this tectonic boundary is mostly unexposed in the chaco pampean plain argentina debate continues about the existence of an east or west dipping subduction zone and a later collisional event between the terranes here we report the results of a magnetotelluric survey along a w e profile at 27 s between 63 45 and 60 30 w to characterize the geoelectric structure the magnetotelluric data were processed the dimensionality and distortion of the geoelectrical medium were analyzed and a 2 d inversion model was developed the distribution of the resistivities at the lithospheric scale indicates a highly resistive crust 20 000 m on the east side of the profile that is correlated with the rio de la plata craton on the west side of the profile a less resistive but still highly resistive crust 12 000 m is correlated with the pampean terrane the highly resistive blocks are separated by an east dipping conductive anomaly 150 250 m that is correlated with the transbrasiliano lineament this conductive feature is consistent with the east dipping subduction model and can be explained by the presence of graphite in paleosutures in long stable geological terranes oblique convergence between the terranes may have developed the transpressional shear belt the results provide new geophysical evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane beneath the chaco pampean plain that extends southward into the transbrasiliano lineament the study also improves the knowledge of the amalgamation of western gondwana

deformation history of the hengshan wutai fuping complexes implications for the evolution of the trans north china orogen abstract the trans north china orogen separates the north china craton into two small continental blocks the eastern and western blocks as one of the largest exposure in the central part of the orogen the hengshan wutai fuping complexes consist of four lithotectonic units the wutai hengshan and fuping complexes and the hutuo group the hengshan complex contains high pressure mafic granulites and retrograded eclogites structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding d 1 to d 3 and two stages of ductile thrust shearing stz 1 between d 1 and d 2 and stz 2 after d 3 the d 1 deformation formed penetrative axial planar foliations s 1 mineral stretching lineations l 1 and rarely preserved small isoclinal folds f 1 in the hengshan and fuping complexes in the wutai complex however large scale f 1 recumbent folds with sw vergence are displayed by sedimentary compositional layers penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones stz 1 the kinematic indicators of stz 1 in the hengshan and wutai complexes show top to the s230 w thrusting likely related to northeastward oblique pre collisional subduction d 1 resulted in crustal thickening with resultant prograde peak metamorphism the hutuo group did not undergo the d 1 deformation either because sedimentation was coeval with the d 1 deformation or because it was at a high structural level and was not influenced directly by the early deformation the d 2 deformation produced nw verging asymmetric and recumbent folds the d 2 deformation is interpreted to have resulted from collision between the eastern and western blocks of the north china craton in the hutuo group and the fuping complex the development of ese verging asymmetric tight folds is associated with d 2 the structural pattern resulting from superimposition of d 1 and d 2 is a composite synform in the hengshan wutai fuping complexes all four lithotectonic units were superposed during the later d 3 deformation the d 3 deformation developed nw trending open upright folds ongoing collision led to development of transpressional ductile shearing stz 2 forming the transpressional zhujiafang dextral ductile shear zone between the northern hengshan complex and the southern hengshan complex and generating the sinistral longquanguan ductile shear zone between the fuping complex and the wutai complex respectively the stz 1 and d 2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites and retrograded eclogites the structural patterns and elucidation of the deformation history of the hengshan wutai fuping complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the eastern and western blocks underwent northeastward directed oblique subduction beneath the western margin of the eastern block and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the north china craton

estimation of sedimentary thickness in kachchh basin gujarat using sp converted phase an inexpensive method using natural earthquake data is utilized for determining the sedimentary thickness in kachchh the institute of seismological research isr is operating a network of broadband seismographs and strong motion accelerographs in gujarat we used data from 13 broadband seismographs and two strong motion accelerographs in the study the stations are within 5 to 80 km from the epicenters in this study the s to p converted phase sp is used this phase is generated due to large impedance contrast between sediments and basement this phase is clear in the vertical component the difference in the travel times of s and sp phases and velocities of p and s waves is used for determining the sedimentary layer thickness the thickness of sediments beneath each of these 15 stations was determined covering an area of 23 500 sq km

electrical conductance map for the kachchh rift basin constraint on tectonic evolution and seismotectonic implications geomagnetic field variations recorded by an array of magnetometers spread across the kachchh rift basin are reduced to a set of induction arrows as a diagnostic of lateral electrical conductivity variations a non uniform thin sheet electrical conductance model is developed to account for the salient induction patterns it indicates that the imaged conductivity anomalies can be related to the sediment filled structural lows in between the fault bounded uplifts it is suggested that sagging structural lows preserved the marine sediments deposited during the mesozoic sea transgression and later developed into first order embayment basins for the deposition of sediments in association with late eocene transgression depth integrated electrical conductance helped in mapping two depo centres along the ene wsw trending banni half graben bounded by the kachchh main fault on the south and second along the vinjan depression formed in response to the subsidence between the vigodi fault and westward extension of the katrol hill fault together with the westward bending of the median high presence of metamorphosed graphite schist clasts in shale dominated mesozoic sequence and or thin films of carbon resulting from the thermal influence of deccan activity on carbonate rich formations can account for the high electrical conductivity anomalies seen in the depo centres of thick mesozoic and tertiary sediments additionally two high conductivity zones are imaged encompassing a block defined by the 2001 bhuj earthquake and its aftershocks in agreement with gravity magnetic and seismic velocity signatures aqueous fluids released by recrystallizing magmatic bodies intruded in association with deccan trap activity account for mapped high conductivity zones high fluid pressure in such a fractured domain surrounding the intruded magmatic plugs perturb the regional stress concentrations to produce frequent and low magnitude aftershocks in the shallow section of the epicentral track of the 2001 bhuj earthquake

paleoproterozoic granulites from the xinghe graphite mine north china craton geology zircon u pb geochronology and implications for the timing of deformation mineralization and metamorphism abstract the xinghe graphite deposit is located in the north central part of the north china craton ncc where archean gray gneiss metasedimentary rocks khondalites and mafic high pressure granulites constitute the dominant rock types in this study we report results of detailed geological and structural mapping around the xinghe graphite mine we identify three stages of deformation d 1 d 3 and the graphite mineralization is correlated with d 1 or between d 1 and d 2 the first stage of deformation represented by rootless intrafolial folds whose axial surface cuts across the regional penetrative gneissosity s 2 of d 2 was constrained at ca 1 95 1 90 ga by u pb age of zircon grains from a mafic high pressure granulite the second stage of deformation is represented by tight folds with formation of regional penetrative foliation and strong ductile shearing when garnet broke down to plagioclase pyroxene this stage is dated as ca 1 85 ga from u pb analysis of zircons in leucosome from a migmatitic gneiss the third stage of deformation is represented by large scale folding which probably formed between 1 85 and 1 80 ga as constrained from the ca 1 80 ga emplacement age of an undeformed pegmatite that cuts across the host metasediments prior to the high pressure granulite metamorphism voluminous ca 2 15 2 0 ga granites were emplaced within the khondalites the graphitization postdates the emplacement of these granites as deduced from the presence of flaky graphites in the metagranites the protolith of the mafic high pressure granulite dike that intruded the khondalite at ca 2 0 ga was also metamorphosed at 1 95 1 90 ga and is part of the exploited graphite ore body we therefore constrain the timing of graphite mineralization to be between ca 1 95 and 1 85 ga our geological and geochronological data indicate that all the rock units in the xinghe graphite mine were subjected to high pressure granulite facies metamorphism at ca 1 95 1 90 ga

u pb dating of baddeleyite and zircon from the shizhaigou diorite in the southern margin of north china craton constrains on the timing and tectonic setting of the paleoproterozoic xiong er group abstract the xiong er volcanism marks an important thermal event in the southern margin of the north china craton during the paleoproterozoic precise u pb ages of zircon and baddeleyite from the shizhaigou diorite which intruded the majiahe formation of the upper part of the xiong er group were determined in this study by multi collector sims the mean 207 pb 206 pb ages are 1789 3 5 ma for baddeleyite and 1780 11 ma for zircon from shizhaigou diorite respectively the major and trace elements and sr nd isotopic compositions indicate an em i mantle type signature and crustal contamination features combining with previous chronological data it is proposed that the age of the xiong er volcanism is between 1770 ma and 1800 ma with a peak at 1780 ma our result supports the view that the xiong er volcanic rocks formed in an extensional rift environment

the bolivian altiplano conductivity anomaly 1 a long period magnetotelluric study was carried out in the central andes between latitudes 19 5 s and 21 s along two almost parallel profiles of 220 and 380 km length respectively the investigation area extends from the pacific coast to the southern altiplano plateau in the back arc of the south american subduction zone the main geoelectrical structure resolved is a broad and probably deep reaching highly conductive zone in the middle and deeper crust beneath the high plateau although the data show deviations from two dimensionality a two dimensional approach is justified for large parts of the profiles sensitivity studies were carried out in order to constrain the depth extent another electrically conductive structure was resolved in the middle crust of the chilean forearc thought to be connected with the precordillera fault system the andean continental research program ancorp seismic reflection profile carried out along the same line at 21 s revealed highly reflective zones below the altiplano in good correlation with the upper boundary of the altiplano conductor this highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp vs ratio and although not well resolved with low qp seismic quality factors taking into account the enhanced heat flow and a derived temperature model the most probable explanation lies in the assumption of granitic partial melts the good conductor below the volcanic arc which was found in regions farther south at 22 s gradually vanishes toward the north this is consistent with the results of seismic tomography concerning qp values and a gap of recent volcanism

sediment thicknesses and qs vs qp relations in the kachchh rift basin gujarat india using sp converted phases delineation of the top sedimentary structure and its qs vs qp relationship using the travel time difference of direct s and converted sp phase is key to understanding the seismic hazard of any sedimentary basin area we constructed filtered displacement waveforms from local etna episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 bhuj earthquake recorded by the kachchh seismological network of the national geophysical research institute ngri hyderabad india during 2001 2004 stations are within 15 70km of epicenters and the resulting displacement waveforms are generally simple displaying prominent p sp and s wave pulses particle motion of p and s waves suggest near vertical raypaths consistent with preliminary depth estimates the direct s wave on the horizontal component is characterized by lower frequency content than the converted sp phase on the vertical component this difference in frequency content between s and sp phases can be explained in terms of different attenuation effects for p and s waves in the unconsolidated sediments the sp phase is generated by s to p phase conversion at the base of mesozoic sediments of the kachchh basin travel time inversion velest of 2565 p and 2380 s arrivals from 658 well located aftershocks recorded at 8 14 three component local seismic stations led to 1 d velocity models indicated very slow sediments in the upper 0 2 km depth range vp 2 92 km s and vs 0 90 km s and an increasing trend of velocities with depth at 2 40 km depth the estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel time difference between s and converted sp phases reaches a maximum of 1 534 0 117 km beneath bandri near the location of 2001 bhuj mainshock and attains a minimum sediment thickness of 0 858 0 104 km beneath ramvav and burudia the spectral ratios between sp and s from 159 three component accelerograms have been used to study seismic wave attenuation in the kachchh rift basin the estimated qs vs qp relations for 12 accelerograph sites vary from qs 0 184 qp at chobari to qs 0 505 qp at dudhai for stations chobari chopdwa jahawarnagar vondh and tapar the spectral ratio slopes and hence the calculated qs vs qp relations are effectively the same and the correlation coefficients are quite high 0 91 0 93 stations adhoi manfara new dudhai dudhai and sikara have similar qs vs qp relationships to each other and also have high correlation coefficients 0 78 0 87 the spectral ratios for stations anjar and ramvav are small and poorly constrained resulting in less reliable qs vs qp relations this could be due to noisy data fewer available waveforms or scattering due to velocity heterogeneities and or interface irregularities

electrical resistivity cross section across the garhwal himalaya proxy to fluid seismicity linkage abstract magnetotelluric mt measurements along a profile cutting across the garhwal himalaya of india are inverted to obtain 2 d electrical resistivity structures of the himalayan wedge and of the underthrusting indian plate the imaged resistivity cross section is dominated by a low angle north east dipping intra crustal high conducting layer ic hcl with an average thickness of 5 km at transition from the lesser himalaya to the higher himalaya the ic hcl is marked by a ramp structure across which its top jumps from a depth of 8 km to 13 km high conductivity of the layer is caused by pounding of upward propagating metamorphic fluids trapped by tectonically induced neutral buoyancy in compression regime of the himalaya the mechanical weakening effects of the fluids counteract the fault normal stresses thereby facilitating thrust type earthquakes on a plane imaged as the top of the ic hcl it is suggested that in the himalaya collision belt like the active subduction zone the active seismic plane forming seat of large and great earthquakes is located a few kilometers above the top of the down going plate in this tectonic setting the high conductance ramp symbolizes a block of low shear strength and high strain which under the deviatoric stresses release accentuated stresses into the brittle crust thereby generating small but more frequent earthquakes in the narrow himalayan seismic belt in response to either the co seismic pumping or the stress transfer during inter seismic period the upward infiltration of fluid fluxes into the over pressurized zones sufficiently reduces the shear strength of local thrusts and shear zones turning these into locales of concentrated seismicity

deep electrical signatures across the achankovil shear zone southern granulite terrain inferred from magnetotellurics abstract two parallel magnetotelluric mt traverses across the achankovil shear zone aksz in southern india using a wide band 1 khz 1 ks data acquisition system reveal the regional geo electric average strike direction of n 40 w consistent with the geological strike direction of the aksz the derived model shows distinct high electrical resistivity 1000 m for the upper crust below the madurai granulite block mgb with a gentle dip towards south and a northerly dip below the kerala khondalite block kkb the lower crust is resistive below both mgb and kkb while it is moderately conductive

long lived transpression in the archean bird river greenstone belt western superior province southeastern manitoba abstract the archean bird river greenstone belt brgb is located on the southwestern edge of the superior province between the 3 2 ga old winnipeg river subprovince to the south and the metasedimentary belt of the english river subprovince ersp to the north this position between two major subprovinces makes the brgb a primary target for investigating the geodynamic and kinematic evolution of a major structural boundary new structural and geochronological data have allowed us to present an evolutionary framework for the southern boundary of the north caribou superterrane the brgb underwent 3 main deformation phases the d1 event took place ca 2698 ma and displays a north side up shearing the d2 event occurring at ca 2684 ma in a transpressive context presents a complex structural pattern mixing vertical tectonics in the brgb and strike slip tectonics along the boundaries of the greenstone belt with other subprovinces between the brgb and the ersp the 2832 2858 ma old maskwa batholith acted as a rigid passive block during the collision and marks the boundary between pure dextral strike slip tectonics along his northern boundary with the ersp and vertical south side up motion in the brgb the brgb can be considered as a pop up structure with anastomosed shear zones displaying different horizontal offset according to the orientation of the shear zones the southern boundary with the winnipeg river subprovince is represented by a sinistral south side up shear zone the same pattern is found at the regional scale where major shear zones acted as a conjugate set in the horizontal plane at ca 2640 ma the d3 event occurred in a general dextral transpressive tectonic regime coeval with the emplacement of rare elements pegmatitic plutons in a still hot 400 500 c country rock the geodynamical and mechanical significance of the partitioning between pure strike slip tectonics in the english river subprovince and vertical motion in the brgb can be explained by the rheological behaviour of a hot and weak lithosphere undergoing transpressive strain the structural framework of the brgb is the result of strong interactions between hot and weak domains coeval with widespread plutonism and a rigid older domain maskwa batholith during the d2 transpressive event

neoarchean tectonics insight from the baijiafen ductile shear zone eastern anshan liaoning province ne china abstract the north china craton is one of the major archean to paleoproterozoic cratons in the world and oldest craton in china which preserves a large amount of ancient basement and abundant structures showing the early earth tectonics the controversy over the archean tectonic regimes has lasted several decades centering around horizontal and vertical tectonics the two classical tectonic models for archean times thus more studies of the early crustal growth and tectonic evolution are requisite for better understanding geodynamic regimes in the early precambrian this study provides an example for revealing of archean tectonics the nwn trending ene dipping baijiafen ductile shear zone is located in the eastern anshan of the northeastern north china craton and mainly comprises two types of gneisses including the chentaigou porphyritic granitic gneiss and the baijiafen trondhjemitic gneiss in this study we have carried out a detailed study on the macrostructure microstructure and fabric characteristics of the two main types of deformed gneisses within the shear zone the ribbon structures formed by intensely elongated quartz grains are widespread in these gneisses well developed mineral stretching lineations and asymmetric fabrics indicate an ese directed downward shearing the quartz c axis fabric patterns obtained by electron backscatter diffraction technique imply low to middle temperature non coaxial deformation with active rhomb a slip and basal a slip deformation behaviors of minerals and quartz crystallographic preferred orientations demonstrate that the rocks underwent mylonitization at a temperature of 400 500 c under greenschist facies metamorphic conditions dislocation creep is the main rock deformation mechanism within the shear zone finite strain measurement results suggest that the strain types of the shear zone are generally related to elongate plane deformation and the tectonites change from l s to ls type across the shear zone the strain intensity increases obviously towards east and the baijiafen gneiss unit is located within the high strain zone kinematic vorticity values of gneisses indicate that the deformed rocks were produced by steady state simple shear dominated general shear based on abundant structural evidence and previous studies we infer that the deformation of the baijiafen ductile shear zone may have resulted from the downward adjoining rock flow during the sagduction of the banded iron formations and synchronous archean granite dome emplacement supporting a vertical tectonic regime in archean times

granulite facies metamorphism and crust melting in the huai an terrane at 1 95ga north china craton new constraints from geology zircon u pb lu hf isotope and metamorphic conditions of granulites abstract geological implications of 1 95 ga and 1 85 ga metamorphic ages for hp granulites in the huai an terrane are still controversial which have affected the crustal evolution models for the north china craton this study reports two geological sections in which distinct geological relationships of a quartz dioritic and a tonalitic dike cutting through mafic hp granulite and granulite facies meta sedimentary rocks are described in these two sections magmatic dikes mafic hp granulites and meta sedimentary rocks preserve conformable foliation to indicate their similar deformation history the u pb dating and lu hf isotopic analysis of zircons from metamorphic and meta igneous rocks provide new constraints on the timing and mechanism of thermal events associated with metamorphic and deformation events both magmatic dikes were formed around 1 95 ga and overprinted by later metamorphic event during 1 85 ga the s type granite at geological section gs 1 revealed its formation age was formed around 1959 34 ma n 3 mswd 1 5 and overprinted by later metamorphic event at 1840 8 ma n 10 mswd 1 2 additionally zircons from the mafic hp granulite in the gs2 are dominated by metamorphic ages of 1972 22 ma n 6 mswd 0 03 with few grains recording an age of 1856 29 ma n 3 mswd 0 17 in situ lu hf analyses on zircons from the quartz dioritic dike tonalitic dike mafic hp granulite and s type granite reveal that the magmatic dikes were derived from partial melting of 2 0 2 2 ga meta basic rocks and the s type granite was generated by melting of meta sediments beyond 840 50 c it suggested that those meta sediments were taken into the lower crust in a certain way at granulite facies conditions ca 1 95 ga ago a clockwise p t path acquired from mafic hp granulite revealed peak metamorphic conditions of 880 890 c and 11 13 kbar the temperature results recorded in the 1 95 ga zircons of mafic hp granulite similar to those of high pressure mineral assemblage in matrix suggest that the hp granulite metamorphism was approximately 1 95 ga ago according to the results above this contribution concludes that mafic and meta sedimentary rocks were metamorphosed together under high pressure granulite metamorphic conditions 1 95 ga ago followed by high temperature granulite facies melting event that gave rise to granite and s type granite at 1 95 ga the granulite facies metamorphic rocks and magmatic dikes in the huangtuyao graphite mine experienced the same evolution history after 1 95 ga the mafic and pelitic hp granulites in the huai an terrane are interpreted to be collisional products of the paleoproterozoic cratonization of the ncc

electrical imaging of deep crustal features of kutch india summary a regional magnetotelluric mt study was carried out with 55 mt soundings distributed along five traverses across the kutch mainland unit kmu on the west coast of india a region characterized by a series of successive uplifts and intervening depressions in the form of half graben bounded by master faults we obtain the deeper electrical structure of the crust beneath kutch from 2 d modelling of mt data along the 5 traverses in order to evaluate the geo electrical signatures if any of the known primary tectonic structures in this region the results show that the deeper electrical structure in the kutch region presents a mosaic of high resistive crustal blocks separated by deep rooted conductive features two such crustal conductive features spatially correlate with the known tectonic features viz the kutch mainland fault kmf and the katrol hill fault khf an impressive feature of the geo electrical sections is an additional well defined conductive feature running between jakhau and mundra located at the southern end of each of the five mt traverses and interpreted to be the electrical signature of yet another hidden fault at the southern margin of the kmu this new feature is named as jakhau mundra fault jmf it is inferred that the presence of jmf together with the kathiawar fault nkf further south located at the northern boundary of the saurashtra horst would enhance the possibility of occurrence of a thick sedimentary column in the gulf of kutch the region between the newly delineated fault jmf and the kathiawar fault nkf could thus be significant for hydrocarbon exploration

ultra high temperature overprinting of high pressure pelitic granulites in the huai an complex north china craton evidence from thermodynamic modeling and isotope geochronology abstract paleoproterozoic granulite facies rocks are widely distributed in the north china craton ncc the huai an terrane located within the northern segment of the trans north china orogen tnco a major paleoproterozoic collisional belt in the central ncc expose mafic and pelitic granulites as well as ttg tonalite trondhjemite granodiorite gneisses here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages the prograde metamorphism m1 is recorded by relict biotite and the compositional profile of xca grt isopleths the pmax m2 is distinguished by the xca grt isopleths which corresponds to the kyanite stable area with an inclusion mineral assemblage of grt c ky qz rt kfs liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 c however kyanite is absent in thin sections suggesting its consumption during later stages the tmax metamorphism m3 is characterized by the assemblage grt m qz pl rt kfs sil liq in the garnet mantle and also reflected in the compositional profile two feldspar geothermometry yields a p t range of 940 c 950 c and 9 5 10 5 kbar indicating ultra high temperature uht metamorphic overprinting the subsequent retrograde metamorphic stage m4 is characterized by grt r bt sil kfs pl qz rt ilm with symplectites of bt sil qz in the garnet rim suggesting garnet breakdown with p t conditions estimated as 770 c 840 c and 6 5 8 kbar the pelitic granulites show a clockwise path with p t estimates higher than those in estimated in previous studies using conventional techniques la icp ms u pb analysis of metamorphic zircon grains yield two groups of ages at 1972 9 8 1 ma and 1873 3 9 9 ma we suggest that the protoliths of the manjinggou hp uht granulites were deep subducted where they experienced hp metamorphism associated with the collision of the ordos and yinshan blocks at ca 1 97 ga subsequently the uht metamorphic overprint occurred during the assembly of the unified western and eastern blocks of the ncc along the tnco at ca 1 87 ga

zircon u pb geochronology and hf isotopic composition of the hongqiyingzi complex northern hebei province new evidence for paleoproterozoic and late paleozoic evolution of the northern margin of the north china craton abstract the hongqiyingzi complex located at the northern margin of the north china craton was previously defined as a paleoproterozoic unit it consists of volcanic and sedimentary rocks which underwent greenschist to amphibolite facies metamorphism this study presents zircon u pb ages and hf isotopic composition and whole rock nd isotopic composition of metamorphosed igneous and sedimentary rocks from the hongqiyingzi complex the age dating results show that different gneisses of this complex contain mainly late paleoproterozoic 1800 ma and late paleozoic 275 to 300 ma zircons and minor late archean zircons late paleozoic zircons either occur as magmatic grains or as metamorphic overgrowth and both types of zircon can be distinguished by their th u ratios and internal structures most zircon grains of a paleozoic magmatic rock sample hq14 have high e hf t values of 1 5 to 10 8 suggesting significant contribution of mantle or juvenile crustal material to the magma source s the results imply that the hongqiyingzi complex contains precambrian supracrustal sequences and paleozoic rocks of magmatic origin that underwent strong metamorphism and deformation during the late paleozoic related to the closure of the paleo asian ocean between siberia and the north china craton

low al and high al trondhjemites in the huai an complex north china craton geochemistry zircon u pb and hf isotopes and implications for neoarchean crustal growth and remelting abstract voluminous tonalite trondhjemite granodiorite ttg magmas were emplaced in the huai an complex of north china craton ncc during the neoarchean here we present the geochemical characteristics of these rocks and identify low al lal and high al hal types of trondhjemites within the ttg suite the lal group displays high silica sio2 77 80 wt and low al2o3 contents 11 06 11 89 wt a strongly peraluminous a cnk 1 16 and 1 19 composition and enrichment in hree lan ybn 4 and 5 gdn ybn 0 35 and 0 9 their low rb 13 and 28 ppm th 0 3 and 7 0 ppm relatively high na2o k2o values 3 6 and 4 1 and mgo 0 63 and 0 68 wt with moderately high mg 36 and 53 suggest crystallization from a primitive source the hal group is characterized by relatively low sio2 67 22 71 57 wt and high al2o3 contents 16 35 16 41 wt with trace element contents and distribution patterns closely comparable to those of archean high al ttgs our geochemical data do not favor a direct genetic link between the lal and hal trondhjemite groups the zircon u pb data reveal that the lal and hal groups represent coeval intrusions at ca 2 55 2 5 ga in situ analysis of hf isotopes in the zircons ehf 5 6 to 10 and tdm 2462 2595 ma together with the geochemical features of the low al trondhjemites suggest that they were generated under relatively low pressure

p t t paths and tectonic history of an early precambrian granulite facies terrane jining district south east inner mongolia china abstract the widespread khondalite series of south east inner mongolia consists largely of biotite sillimanite garnet gneiss and quartzo feldspathic gneiss with some marble and mafic granulite layers it has experienced two metamorphic events at c 2500 and 1900 2000 ma a pre peak stage of the first metamorphism at t 600 700 c and p 6 7 kbar is recognized by the relict amphibolite facies assemblage ky grt bt pl qtz and protected inclusions of biotite hornblende sodic plagioclase and quartz in garnet or orthopyroxene the peak stage with t c 800 50 c and p 8 10 kbar is characterized by the widespread granulite facies assemblages sil grt bt kfs pl qtz in gneiss and opx cpx pl hbl grt in granulite the p t t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during d1 d2 the beginning of the second metamorphism is characterized by further temperature rise to 700 c or more at lower pressure this stage is manifested by the appearance of cordierite after garnet fibrolite sil2 after biotite in gneiss and transformation of hbl1 into opx2 and cpx2 in granulite coronas of symplectitic opx2 pl2 surrounding grt1 and cpx1 in mafic granulite are interpreted as products of near isothermal decompression the p t t path may be related tectonically to waning extension of the crust by the end of the early proterozoic

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

a geochemical clogging model with carbonate precipitation rates under hydrothermal conditions abstract a step wise numerical calculation method was developed to provide predictions of when and where carbonate deposits might be found through reservoirs during co 2 sequestration flow experiments through porous media using a supersaturated carbonate fluid were also performed in order to observe flow rates in order to evaluate precipitation rates and permeability change in the formation calculated flow rates based on the proposed geochemical clogging model were compared with the experimentally observed data both high and low temperature cases were studied to understand how hydrothermal conditions can affect precipitation rates of carbonate according to chemical kinetics growth rates of minerals are generally proportional to the saturation index s i that depends on temperature thus a supersaturated fluid has the advantage of improving the filtration and the amount of c fixation however when the ratio of filtration coefficient to pore fluid velocity u increases the permeability around the injection point tends to be significantly reduced by carbonate accumulation and thus this might result in insufficient injection of co 2 therefore it is essential to understand how to control both and u so that the precipitation of carbonate can be located as far away from the inlet as possible

geomorphic indicators of active fold growth south mountain oak ridge anticline ventura basin southern california south mountain oak ridge near ventura california is an asymmetric anticlinal uplift forming at the present time above the active buried oak ridge reverse fault shortening along the oak ridge fault accumulated largely in quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0 5 m y tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth stream gradient indices are relatively high in the northern eroded fold scarp of the ridge a pattern consistent with the existence of active rapid slip on the oak ridge fault mountain front sinuosity along the northern slope of the anticlinal ridge roughly decreases from 2 to 1 toward the westernmost 10 km of observed surface folding valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from 1 5 to 0 5 values of the hypsometric integral along the northern flank increase significantly from 0 35 to 0 4 maximum 0 55 from east to west drainage density varies from 4 to 6 km km2 along both flanks of the south mountain oak ridge anticline entrenchment of streams into the southern backlimb of the fold along the westernmost 9 km of the structure decreases from 20 m to 1 m from east to west apparent backlimb rotation as measured by dip of strata along the westernmost 7 km of the fold decreases from east to west from 35 to 20 fold growth of the south mountain oak ridge anticline occurred during the past 0 5 ma following deposition of the saugus formation lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried oak ridge fault

electrical structure across the indus tsangpo suture and shyok suture zones in nw himalaya using magnetotelluric studies 1 magnetotelluric studies in the nw himalayan region have shown anomalously high conductance of about 20 000 siemens in the crust beneath the indus tsangpo suture its and the adjoining tso morari dome in the depth range of 1 20 km high heat flow and high attenuation of the seismic waves in the himalayan crust together with the observed high conductance indicate presence of wide spread partial melt generated from the subducted indian crust the ladakh batholith appears as a resistive block to the north of its a moderately conductive zone demarcates the ladakh batholith from karakoram batholith to the north the similarity in the resistive structure with the results reported from the tibetan region by wei et al 2001 about 1500 km to the east is rather significant indicative of a two dimensional nature of the himalayan collision belt at least to the first order

fluvial geomorphology and neotectonic activity based on field and gpr data katrol hill range kachchh western india abstract the largely rocky and rugged landscape of the katrol hill range composed of mesozoic rocks and structurally controlled occurrences of quaternary sediments is delimited to the north by north facing range front scarps of the seismically active e w trending katrol hill fault khf the landscape and drainage characteristics of the katrol hill range are documented together with ground penetrating radar gpr investigations along the khf to delineate its nature for understanding neotectonic activity in the contemporary tectonic setting the overall geomorphology is controlled by the south oriented tilt block structure of the range indicated by its pronounced influence on the morphology and drainage network the drainage comprises north flowing and south flowing rivers with the drainage divide located close to the northern edge of the range which also marks the highest topographic elevations the narrow zone between the crest line and the drainage divide has been identified as the zone of gorges where gorges and deeply incised fluvial valleys have been formed within quaternary sediments by the various north flowing streams the quaternary sediments consist of bouldery colluvial deposits in front of the range front scarps valley fill miliolites and alluvial deposits of late pleistocene age within the back valleys and scarp derived colluvium forming the youngest deposit based on the geomorphic and stratigraphic evidence three major phases of quaternary tectonic uplift in the katrol hill range are inferred the oldest pre miliolite phase middle pleistocene was followed by a prominent phase of fluvial incision with formation of gorges during early holocene and then by the last one during late holocene continuing at present uplift of the range occurred in well marked phases during the quaternary in response to differential uplift along the khf under an overall compressive stress regime gpr investigations at selected sites show that the khf is a steep south dipping reverse fault near the surface which becomes vertical at depth this suggests neotectonic reactivation of the khf under a compressive stress regime responsible for active southward tilting of the katrol hill range

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

two linear granite belts in the central western north china craton and their implication for late neoarchaean palaeoproterozoic continental evolution abstract precambrian granitoid rocks are widely distributed in the north china craton ncc the dominant granitic rocks in the central western ncc are trondhjemitic tonalitic and granodioritic ttg gneisses as well as some calc alkaline granitic gneisses and minor charnockites two linear granite belts in this area are different from other early precambrian granitic rocks in the ncc in terms of their occurrence and chemistry the liang tu lt garnet granite belt and the sanggan sg granite belt the lt belt occurs along two large scale ductile shear zones in metamorphosed al rich sedimentary country rocks khondalitic series in the central part of the lt belt are metagabbroic granulite lenses that were originally mafic dykes in the outer part of the belt garnet granites grade into khondalitic rocks lt belt granites have geochemical characteristics of s type granites the sg granite belt is 400 km long and composed of many deformed gneissic granitic bodies and granitic sills and veins with without gneissosity they contain various lenses including high pressure granulites and retrograded eclogites the sg granite belt trends east northeast parallel to the regional gneissosity of the country rocks major and trace element geochemistry of these granites shows characteristics of syn collisional and post collisional granite the country rocks beside the sanggan granite belt are different including the huai an fengzhen terrain hft to the northwest and the wutai fuping terrain wft to the southeast the lt and sg granite belts formed within an old continental craton and an old orogenic zone between two blocks respectively

geochronology and petrogenesis of neoarchean potassic meta granites from huai an complex implications for the evolution of the north china craton abstract neoarchean potassic meta granites occurring in the huai an complex within the north central part of the north china craton ncc comprise leucocratic syenogranites biotite monzogranites and garnet bearing syenogranites high pressure garnet granulites occur as lenses sheets or dismembered dikes with the granitoids here we present shrimp zircon u pb data from the leucocratic syenogranites which show an age of 2493 6 ma mswd 1 and la icp ms u pb data from the biotite monzogranites and garnet syenogranites revealing their formation ages of 2437 10 ma mswd 2 0 and 1977 2003 ma respectively we also report metamorphic ages of ca 1800 ma from zircons in the garnet syenogranites geochemical studies show low mgo 0 5 0 06 wt cr 6 19 0 53 ppm and ni 27 66 2 01 ppm with varying sio2 62 3 to 76 8 wt contents indicating their derivation from a crustal source the leucosyenogranites exhibit elevated positive eu anomalies with decreasing sr th u and bulk ree concentrations and th u ratio whereas rb and ba values increase with decreasing th u ratio indicating fluid melt interaction the leucosyenogranites have high sr and ba contents and sr y and la yb n ratios and low y and hrees with zircon ehf t values ranging from 1 7 to 5 8 and hf model ages of 2 60 2 73 ga reflecting their derivation by partial melting of overthickened juvenile lower crust of eclogitic composition or an eclogitic mineral assemblage in the residue geochemical compositions of the biotite monzogranites with positive end t values 2 3 to 2 5 and model ages

magnetotelluric study to characterize kachchh mainland fault kmf and katrol hill fault khf in the western part of kachchh region of gujarat india abstract the kachchh mainland fault kmf is a major e w trending fault in the kachchh region of gujarat extending 150 km from lakhpat village in the west to the bhachau town in the east the katrol hill fault khf is an e w trending intrabasinal fault located in the central region of kachchh basin and the south of kmf the western parts of both of the faults are characterized and the sediment thickness has been estimated in the region using a magnetotelluric mt survey at 17 sites along a 55 km long north south profile with a site spacing of 2 3 km the analysis reveals that the maximum sediment thickness is 2 3 km quaternary tertiary and mesozoic in the region out of which the mesozoic sediments feature a maximum thickness of 2 km the estimated sediment thickness is found consistent with the thickness suggested by a deep borehole depth approx 2 5 km drilled by oil and natural gas corporation ongc at nirona northern part of the study area from 2 d inversion of the mt data three conductive zones are identified from north to south the first conductive zone is dipping nearly vertical down to 7 8 km depth it becomes north dipping below 8 km depth and is inferred as kmf the second conductive zone is found steeply dipping into the southern limbs near manjal village 28 km south of nirona which is inferred as the khf a vertical dipping down to 20 km depth conductive zone has also been observed near ulat village located 16 km north of manjal village and 12 km south of nirona village this conductive zone becomes listric north dipping beyond 20 km depth it is reported first time by a geophysical survey in the region

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

geochemical evolution and metamorphic development of the early precambrian in eastern hebei china abstract within the archaean qianxi group in eastern hebei china two roughly coeval tectonic units can be recognized the qian an granulite complex and the zunhua high grade greenstone belt both belong to the high grade terranes and have recorded almost the same history of metamorphism the granulite facies metamorphism was generally followed by a retrograde metamorphism of amphibolite facies and multistage anatexis despite the similar metamorphic histories the geochemical features of the two units are different 1 1 the protoliths of the qian an granulite complex consist of a calc alkaline volcanic series dominated by andesitic composition and a shallow water sedimentary series whereas that of the zunhua greenstone belt clearly show the character of a bimodal magmatic series 2 2 most metabasalts of the qian an complex belong to the continental basalt type and grade into andesite whereas in the zunhua belt they are characterized by a suite of high magnesian series low k2o tholeiites and a large amount of ultramafic mafic layered complexes 3 3 in terms of regional geochemistry the qian an complex is rich in potassium and potassium charnockites of autochthonous anatectic and intrusive origin are widespread in contrast the zunhua belt is rich in sodium and tonalitic gneisses of deep seated mixed origin are widespread finally a model for archaean crustal evolution of high grade terranes is suggested

u pb isotopic evidence for the accretion of a continental microplate in the zalm region of the saudi arabian shield the area covered by this work includes three of the main tectonic units of the arabian shield the afif continental terrain the nabitah suture with its associated mobile belt and the asir ensimatic arc terrain the geology of the zalm area is well understood and this geochronologic and isotopic study confirms that the southern afif terrain was a continental microplate in the late proterozoic the study also provides a time frame for the crustal evolution of this part of the shield the kabid formation is the oldest in the region and comprises pelitic arkosic and felsic high grade paragneisses u pb zircon data from a pelitic garnet sillimanite gneiss show that this part of the continental basement in the southern afif terrain may be as old as 1770 ma isotopic nalyses indicate that lead from the kabid gneiss resided in the upper continental crust for a long period before 1770 ma and consequently archaean source rocks may be present within the southern afif terrain pb and rb sr isotopic data in the zalm region reveal a change in the nature of the underlying crust from continental basement in the northeast to less radiogenic marginal arc rocks in the southwest this change is coincident with both aeromagnetic data and a facies change within a pre collision marginal basin miogeosynclinal continental shelf facies of the siham group lie unconformably over the kabid formation and are in the area of continental lead signatures eugeosynclinal deep water sediments and volcanics in association with ultramafic rocks occur in the area of marginal arc signatures u pb zircon age determinations show that this 9andean9 continental margin developed before about 720 ma and emplacement of calc alkaline plutonic rocks continued until about 690 ma during the period 685 640 ma the continental afif microplate collided with the asir terrain as part of the nabitah orogeny at approximately 640 ma ago the najd strike slip faulting commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull apart grabens some of these grabens were floored by new oceanic crust and were filled with volcanosedimentary rocks of the bani ghayy group subsequently the najd fault system changed to sinistral strike slip motion at about 620 ma ago

crustal geoelectric structure of the sikkim himalaya and adjoining gangetic foreland basin abstract we present the results of a broadband magnetotelluric survey along a 200 km long profile across the sikkim himalaya the data were acquired at average station spacing of 5 6 km and transfer functions of 31 sites in 0 01 1000 s period range have been used for 2 d joint inversion of te and tm modes the composite model incorporating the effect of transverse strike reveals several features that correlate with the available seismic and kinematic models of the region a major result of the present study is that the main himalayan thrust forms the base of several resistive blocks within the wedge and that a ramp structure is present south of the main central thrust zone mctz another significant result is that the crust and mantle lithosphere beneath the mctz and the higher himalayan crystallines hhc seem to be compositionally geologically different from the lithosphere south of the mctz a steep crustal scale fault with the moho offset of 14 km is inferred to be separating these two blocks the deep crustal seismicity could be related to this fault whereas shallow seismicity can be linked to the deformation within the wedge the results also reveal the presence of some more conductors we relate the conductor within the hhc to the sedimentary rocks of the tethyan sequence exposed in a window about 40 km west of the profile and north of the south tibetan detachment system stds the conductor at 90 km profile location is linked to the gondwana rocks exposed in the rangit window a 4 6 km thick sedimentary layer overlies the basement in the gangetic foreland basin we also delineate a sub crustal conductor at 50 60 km depth beneath the foreland basin at the southern end of the profile the cause of which is not apparent and needs to be explored

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

application of the two feldspar geothermometer to ultrahigh temperature uht rocks in the khondalite belt north china craton and its implications the paleoproterozoic khondalite belt in the north china craton preserves evidence for ultrahigh temperature uht crustal metamorphism associated with the collision of the yinshan and ordos blocks here we apply two feldspar geothermometry to uht granulites from two localities newly reported in this study tuguishan and xuwujia and another two localities from previous studies dajing tuguiwula and dongpo in the khondalite belt the presence of abundant perthite mesoperthite in these rocks reflects post peak slow cooling the minimum estimated peak metamorphic temperatures are 832 998 819 952 844 1037 and 966 1019 c computed at 8 kbar for the dajing tuguiwula dongpo tuguishan and xuwujia areas respectively these results confirm the previous report of extreme metamorphism at dajing tuguiwula and dongpo and reveal similar conditions in the new localities reported here suggesting that uht metamorphism is widespread in the khondalite belt of the north china craton our study demonstrates that uht metamorphism can be recognized using the two feldspar geothermometry in rocks that do not possess other key uht assemblages

application of zr in rutile thermometry a case study from ultrahigh temperature granulites of the khondalite belt north china craton zr in rutile thermometry was applied to ultrahigh temperature uht granulites from three localities dongpo tuguishan and dajing tuguiwula of the khondalite belt north china craton zr concentrations of analyzed rutiles were detected by la icp ms and emp which display a mutative composition zoning a large inter grain variation a bimodal distribution at around 1 500 and 6 000 ppm and no relationship with the textural setting matrix vs inclusion these characteristics were likely caused by post peak diffusional resetting associated with slow cooling rates and the presence of a co2 rich fluid the grains with lower zr concentrations 500 to 3 000 ppm and temperature estimates 650 to 850 c occur close to or in contact with zircon which was easily affected by post peak processes for example diffusion dissolution reprecipitation the lowest temperatures 650 to 700 c we obtained represent the closure temperature of zr in rutile rutiles with higher zr concentrations 3 000 to 8 000 ppm and calculated temperatures 850 to 1 000 c were least affected by late resetting giving near peak metamorphic temperatures these temperature results higher than 900 c even in excess of 1 000 c from the three localities reconfirm the presence of uht metamorphism our results also suggest that zr in rutile thermometry is valid for ultrahigh temperature estimates in addition there are positive correlations between concentrations of zr and hf nb and ta of the investigated rutiles but the correlations weaken as the concentrations increase especially for nb and ta implying fractionation of nb and ta