structural and geochronological constraints on the early mesozoic north longmen shan thrust belt foreland fold thrust propagation of the sw qinling orogenic belt northeastern tibetan plateau

geomorphic constraints on middle yangtze river reversal in eastern sichuan basin china abstract the yangtze the longest river in asia was hypothesized to be assembled through a series of cenozoic capture events such as the reversal of middle yangtze river and the capture of upper yangtze river but the history remains largely unknown here we present new geomorphic observations in the structural context of the eastern sichuan basin namely the eastern sichuan fold belt and identify an important drainage divide along the midline of this arc shape fold belt based on longitudinal profile analysis we find that the river capture events more likely occurred in the syncline valleys of low relief landscape our results yield a new perspective on middle yangtze river reversal and we propose that the midline drainage divide rather than the three gorges was the starting site of middle yangtze river reversal in this manner the reversal could have been accomplished by a sequence of river reversal over range parallel segments in syncline valleys with less impact on the pre existing drainage system in eastern sichuan basin

structural and geochronological constraints on the tectonic evolution of the dulong song chay tectonic dome in yunnan province sw china abstract the dulong song chay tectonic dome lies on the border of china se yunnan province and northern vietnam and consists of two tectonic and lithologic units a core complex and a cover sequence separated by an extensional detachment fault these two units are overlain unconformably by late triassic strata the core complex is composed of gneiss schist and amphibolite shrimp zircon u pb dating results for the orthogneiss yield an age of 799 10 ma which is considered to be the crystallization age of its igneous protolith formed in an arc related environment a granitic intrusion within the core complex occurred with an age of 436 402 ma which probably formed during partial closure of paleotethys within the core complex metamorphic grades change sharply from upper greenschist low amphibolite facies in the core to low greenschist facies in the cover sequence there are two arrays of foliation within the core complex detachment fault and the cover sequence s 1 and s 2 the pervasive s 1 is the axial plane of intrafolial s 0 folds d 1 deformation related to this foliation is characterized by extensional structures the strata were structurally thinned or selectively removed along the detachment faults indicating exhumation of the dulong song chay tectonic dome the major extension occurred at 237 ma determined by shrimp zircon u pb and 39 ar 40 ar isotopic dating techniques regionally simultaneous tectonic extension was associated with pre indosinian collision between the south china and indochina blocks the s 2 foliation appears as the axial plane of nw striking s 1 buckling folds formed during a compressional regime of d 2 d 2 is associated with collision between the south china and indochina blocks along the jinshajiang ailao shan suture zone and represents the indosinian deformation the dulong granites intruded the dulong song chay dome at 144 2 140 2 and 116 10 ma based on 39 ar 40 ar measurement on muscovite and biotite the dome was later overprinted by a conjugate strike slip fault and related thrust fault which formed a vortex structure contemporaneously with late cenozoic sinistral movement on the ailao shan red river fault

zircon lu hf isotopic constraints on neoproterozoic subduction related crustal growth along the western margin of the yangtze block south china abstract voluminous neoproterozoic igneous rocks along the western margin of the yangtze block south china provide an ideal opportunity for examining the nature of the mantle source petrogenesis of the felsic plutons and processes and timing of crustal growth the dadukou tongde gaojiacun and shaba mafic plutons and datian and dajianshan adakitic plutons were selected for zircon lu hf isotope analysis zircons from the mafic plutons have hf t values ranging from 1 86 to 11 63 with average hf t values of the different samples ranging from 4 78 0 42 to 7 60 1 1 their single stage model ages range from 0 86 to 1 28 ga the two adakitic plutons have hf t values ranging from 2 26 to 11 68 and single stage model ages from 0 86 to 1 24 ga our data further confirm that the mafic plutons were extracted from depleted mantle sources whereas the datian and dajianshan adakitic plutons were products of melting of a subducted oceanic slab the arc geochemical affinity relatively high 143 nd 144 nd ratios and high zircon hf t values as well as the relatively young hf single stage model ages of these neoproterozoic plutons suggest that the continental crustal growth in the region was related to oceanic subduction rather than upwelling of a mantle plume in a rifting setting the neoproterozoic magmatism along the western margin of the yangtze block lasted more than 200 my not show episodic emplacement and contrasting zircon hf isotopes suggesting that continental crustal growth in south china was continuously resulted from subduction

geology of xanthe terra outflow channels and the mars pathfinder landing site the mars pathfinder landing site mpls was selected for its location at the mouths of ares and tiu valles outflow channels on mars where sediments from a variety of geologic units could have accumulated in a lacustrine environment recent geologic mapping at 1 2 m scale of the xanthe terra channels and chryse planitia near mpls reveals a progression of fluvial erosion and deposition throughout the hesperian period from the geology the watersheds of the channels were inferred and a sequence of paleogeologic maps was produced three outflow episodes are recognized 1 broad sheetwash during the early hesperian emanating from eastern valles marineris and equatorial chaos regions with initial erosion into highland plateau basement 2 early channel formation and 3 subsequent flooding which deepened the channels to their current morphologies the most recent flooding issued from simud and tiu valles during the late hesperian indicated by stratigraphic superposition striations along the floor of the channel and the orientation of streamlined islands in the flood direction this suggests that surface sediments deposited at mpls are most likely to have come from these channels and to a lesser extent from ares vallis a ranking scheme was developed to assess the potential origins for the sediments found at mpls the most abundant materials are inferred to have come from the noachian and hesperian highland plains and reworked sediments from previous floods underlying materials from earlier floods could also be found at mpls as ejecta from nearby impact craters

tertiary deformation and metamorphism se of tibet the folded tiger leap decollement of nw yunnan china the yulong haba xue shan range in the northwestern part of yunnan china is a large n s antiform that folds the paleozoic series of the yangzi platform the upper yangzi river jinsha jiang has cut a 3500 m deep valley hu tiao gorge across this antiform thus exposing folded bedding parallel ductile shear zones decollements with transport toward the ssw in the present geographical coordinates the large finite shear strain implies tens of kilometers of transport pointing to the regional significance of these decollements rb sr radiometric dating of phlogopites that crystallized in marbles within the foliation planes yields the age of the metamorphic and deformation event 35 9 0 3 2 ma the age derives from an internal rb sr isochron made on different size fractions of the same mineral which provides a novel demonstration of the feasibility of such plots transport on the decollement and related shortening occurred prior to or at the onset of extrusion of indochina along the ailao shan red river shear zone 80 km west of the yulong shan the 39ar 40ar age spectra of k feldspar from the core of the yulong shan suggest uplift by antiformal folding around 17 ma as indochina s extrusion came to an end we infer that other large scale cenozoic decollements such as that exhumed in the yulong shan underlie some of the vast folded areas that surround the eastern himalayan syntaxis transport on such decollements first toward the south and then toward the east and folding above them might have occurred during two principal shortening phases whose ages bracket indochina s escape toward the se

longevity of the permian emeishan mantle plume sw china 1 ma 8 ma or 18 ma after the formation of the 260 ma emeishan large igneous province there were two volumetrically minor magmatic pulses at 252 ma and 242 ma respectively alkaline mafic dykes intruding both 260 ma and 252 ma felsic plutons in the panxi region southwestern china have compositions similar to the emeishan flood basalts one dyke is dated using the shrimp zircon u pb technique at 242 2 ma 18 ma younger than the start of emeishan magmatism the dykes have enriched light rare earth element patterns la ybn 4 4 18 8 and trace element patterns similar to the emeishan flood basalts and average ocean island basalts some trace element ratios of the dykes zr nb 3 8 8 2 la nb 0 4 1 7 ba la 7 5 25 6 are somewhat similar to em1 source material however there are differences their end values end 2 6 and 2 7 and i sr i sr 0 704542 and 0 704554 ratios are indicative of a mantle source thus emeishan magmatism may have lasted for almost 20 ma after the initial eruption however geological evidence precludes the possibility that the post 260 ma magmatic events were directly related to emeishan magmatism which began at and ended shortly after 260 ma the 252 ma plutons and 242 ma dykes represent volumetrically minor melting of the fossil emeishan plume head beneath the yangtze crust the 252 ma magmatic event was likely caused by post flood basalt extension of the yangtze crust whereas the 242 ma event was caused by decompressional melting associated with the collision between the south china and north china blocks during the middle triassic

mesozoic cenozoic evolution of the danba dome songpan garze east tibet as inferred from la icpms u pb and fission track data abstract the eastern margin of the tibetan plateau is a key region to understand the formation and tectonic evolution of the plateau most studies concentrated on the longmen shan range that corresponds to the tectonically very active boundary between the tibetan plateau and the sichuan basin this work addresses the deformation and exhumation pattern in the poorly understood metamorphic danba dome situated at the conjunction between the southern longmen shan range and the major strike slip xian shui he fault the early mesozoic and cenozoic evolution of the dome are then compared to the regional evolution of eastern tibet the about 830 ma zircon u pb ages obtained on the gezong and gongcai gneiss as well as u pb ages on inherited zircons in the early mesozoic granites confirm the occurrence of neo proterozoic magmatic episodes in the danba basement at 800 900 ma and 650 750 ma following the closure of the palaeotethys ocean granitoids were emplaced in the danba metamorphic basement and triassic cover from about 230 ma to about 170 ma the western part of the lian he kau granitic massif emplaced between 210 and 200 ma within the triassic cover sequence of the dome was only slightly affected by the early mesozoic deformation however in the eastern deeper level compartment of the massif ductile deformation occurred between 200 and 190 ma contemporaneous with the onset of the barrovian type metamorphism dated at 200 180 ma that deformation is compatible with the early mesozoic deformation observed in the longmen shan range and with the development of the large scale decollement level in the songpan garze area apatite fission track data then indicate that following the oligocene middle eocene onset of eastward motion of the songpan garze terrane the danba dome developed as a crustal scale antiform during the late miocene early pliocene 10 4 ma the exhumation was controlled by the lian he kau fault a major transpressive structure parallel to the xian shui he fault and the associated manai thrust faults regionally while the oligocene middle miocene exhumation phase seems to have been of regional extent the late miocene to present day phase is localised along the longmen shan range thrusts as well as along the back thrusts formed by the lian he kau manai and associated faults

ni cu pge magmatic sulfide deposits in the yangliuping area permian emeishan igneous province sw china the ni cu pge sulfide deposits in the yangliuping area sw china are hosted in mafic ultramafic sills the four mineralized sills are located in the yangliuping tectonic dome and intrude devonian carbonaceous marble graphitic schist the sills are 200 300 m thick and 1 000 2 000 m in strike length and now consist chiefly of serpentinite talc schist tremolite schist and meta gabbro disseminated ni cu sulfide mineralisation occurs in the serpentinite in the lower parts of the sills massive sulfide mineralisation is located in the base of the sills and in the footwall along fractures beneath the mineralized serpentinite although the sulfide ores have been modified by hydrothermal activity there are relict cumulate textures in the disseminated sulfides indicating a magmatic origin for the ores the yangliuping intrusions and the dashibao formation have similar primitive mantle normalized trace element and platinum group element pge patterns indicating that they are derived from a common parental magma type the positive correlation between cu concentrations and cu zr ratios of the dashibao formation basalts indicates that the chalcophile elements were removed before eruption we propose that fractional crystallization of the yangliuping magma accompanied by the introduction of s and co2 from the wall rocks caused the magma to become s saturated leading to the segregation of magmatic sulfides that became enriched in ni cu pge the sills acted as conduits for the overlying dashibao formation basalts with the sulfide liquid along with early crystallizing olivine and pyroxene segregating from the magma as it passed through the conduits prior to eruption

comparison of mount pinatubo and el chichon volcanic events lidar observations at 10 6 and 0 69 μm analyses of a dense set of lidar observations of the volcanic clouds from mount pinatubo and el chichon primarily over boulder colorado and at a wavelength of 10 59 m reveal notable similarities and differences the importance of tropopause folding events for purging stratospheric debris into the troposphere is confirmed as well as the role of gravitational settling to bring debris into the grasp of the folding mechanism seasonal influences appear more important than time after eruption in both poleward transport and in purging stratospheric material downward we discuss the lengthening of the decay process for the mount pinatubo clouds by either an equatorial reservoir of debris or by eruptions of mount spurr simultaneous observations at 0 69 and 10 59 m help monitor aerosol effective radii versus altitude with good temporal and vertical resolution

constraints on the depth geometry and kinematics of blind detachment faults provided by fault propagation folds an example from the mesozoic fold belt of south china abstract the mesozoic multi layered thin skinned fault related fold belt in south china is characterized by propagation chevron folding and thrust faulting we examine the geometry and depth of blind detachment faults in this belt using faults and folds that crop out along a well defined cross section a combination of parameters such as backlimb dip angle 2 interlimb angle height of anticline h thickness of the referenced layer h0 width of the anticline ab and thickness change ratio of layering within the forelimb tf t are used to develop new equations to estimate the geometry and depth of the detachment faults in the cross section the huangdushan chevron anticline has parameters revealing a blind fault with a ramp angle of 42 and a flattened segment at a depth of 4 9 km based on existing seismic data we interpret this blind fault as a backthrust developed over an in sequence imbricated duplex of cambrian to ordovician strata a chevron anticline at qiyueshan also reveals a backthrust fault however a broad anticline in lengzhuba and a small cylindrical sinusoidal anticline in yupize reveal a flat ramp flat detachment fault with flat segments at depths of 7 4 and 3 4 km respectively suggesting that the sinian strata were thickened probably by an out of sequence imbricated duplex our results indicate that the flat ramp flat detachment fault along the bottom of the sinian strata produced the out of sequence imbricated duplex and cylindrical sinusoidal folds the in sequence imbricated duplex produced backthrusts and related chevron anticlines we consider that this fault related fold belt was produced by collision between the yangtze block and north china blocks

exhumation of the dabie shan ultra high pressure rocks and accumulation of the songpan ganzi flysch sequence central china the presence of coesite and diamond bearing ultra high pressure uhp metamorphic rocks in the dabie and sulu regions central china suggests that a 100 km thick crustal section 4 x 106 km6 in volume has been denuded this volume is comparable to that represented by the 10 15 km thick middle to upper triassic flysch rocks in the songpan ganzi region regional geology and radiometric dates are compatible with an interpretation that the majority of these sedimentary rocks were derived from denudation of the orogenic belt between north and south china blocks following their latest paleozoic to triassic collision this correlation implies a high denudation rate of 4 mm yr that may have been in part promoted by tropical precipitation and extreme topographic relief for as long as 25 m y

u pb dating on single detrital zircon grains from the triassic songpan ganze flysch central china provenance and tectonic correlations the songpan ganze flysch belt central china covers a huge triangular area of more than 200 000 km and is bounded by the continental blocks of south china north china and the tibetan plateau detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the u pb method two samples of middle triassic sandstones collected at different locations in the belt provide identical results which suggests similar source regions the detrital zircons yield a wide range of ages and indicate their principal derivation from mid proterozoic 1 8 2 0 ga source rocks with minor contribution from late archean ca 2 5 2 6 ga material the discordance and pb loss patterns from low u zircons indicate disturbances during a subsequent event which may be of caledonian age as suggested by concordant zircon grains at ca 420 and 450 ma one sample collected within the palang shan pass zone provides concordant zircon grains at around 230 ma 231 1 ma and 233 1 ma these triassic ages are synchronous to flysch deposition and suggest intense geological activity calc alkaline volcanism at that time in the area close to the basin the data support an origin of the clastic material mainly from a northeastern landmass corresponding to the southern margin of the sino korean craton to a lesser degree inputs from the yangtze craton and possibly from the northern margin of the basin kunlun arc are also detected the age spectrum from the upper triassic sandstone is significantly different and shows predominance of sinian ca 760 ma grains probably derived from the yangtze craton this change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late triassic collision between south china and north china in the middle triassic while subduction of the songpan sea northward beneath the north china plate was still taking place continental subduction of south china in the dabie region was responsible for uplift of the overriding plate i e the sino korean craton which supplied large volumes of sediments during the late triassic clockwise rotation of the south china block uplifted the indo sinian part of the qinling belt and closed the basin as the accretionary wedge was thickening along the southern margin of north china detritus derived from this source region were unable to reach the flysch basin the age spectrum presented by detrital zircons indicates

subduction related origin of the 750 ma xuelongbao adakitic complex sichuan province china implications for the tectonic setting of the giant neoproterozoic magmatic event in south china

petrogenesis and ore genesis of the lengshuiqing magmatic sulfide deposit in southwest china constraints from chalcophile elements pge se and sr nd os s isotopes the lengshuiqing magmatic ni cu ore deposit is hosted in four small mid neoproterozoic mafic ultramafic intrusions in the western margin of the yangtze craton in china the ore bearing intrusions were emplaced into early neoproterozoic metamorphosed volcanic sedimentary rocks granites and diorites the intrusions are dominated by lherzolite olivine websterite websterite and gabbro with sulfide mineralization associated with ultramafic rocks in the lower parts of the intrusions the forsterite contents of olivine crystals in the lengshuiqing ore bearing intrusions are up to 86 mol indicating that their parental magma experienced moderate degrees of fractional crystallization olivine crystals in the intrusions are depleted in ca

geochemistry petrogenesis and metallogenesis of the panzhihua gabbroic layered intrusion and associated fe ti v oxide deposits sichuan province sw china the panzhihua gabbroic layered intrusion is associated with the 260ma emeishan large igneous province in sw china this sill like body hosts a giant fe ti v oxide deposit with 1333 million ton ore reserves which makes china a major producer of these metals the intrusion has a marginal zone of fine grained hornblende bearing gabbro and olivine gabbro followed upward by lower middle and upper zones the lower and middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene plagioclase and olivine these zones also contain magnetite layers the upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine most rocks in the body show variable scale rhythmic modal layering in which dark minerals primarily clinopyroxene dominate in the lower parts of each layer and lighter minerals primarily plagioclase dominate in the upper parts the oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion ore textures and associated mineral assemblages indicate that the ore bodies formed by very late stage crystallization of v rich titanomagnetite from an immiscible oxide liquid in a fluidrich environment the rocks of the panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in fe ti and v they are enriched in light rare earth elements relative to heavy rare earth elements and exhibit positive nb ta and ti anomalies and negative zr and hf anomalies the silicate rocks and oxide ores of the panzhihua intrusion formed from highly evolved fe ti v rich ferrobasaltic or ferropicritic magmas the textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall rocks induced the separation of the immiscible oxide melts from which the fe ti v oxide ore bodies in the lower part of the intrusion crystallized

tectonic evolution of the mesozoic songpan ganze orogenic belt in the eastern margin of tibetan plateau constraints from structural analyses and 40ar 39ar dating

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

fault related fold styles and progressions in fold thrust belts insights from sandbox modeling fault related folds of variable structural styles and assemblages commonly coexist in orogenic belts with competent incompetent interlayered sequences despite their commonality the kinematic evolution of these structural styles and assemblages are often loosely constrained because multiple solutions exist in their structural progression during tectonic restoration we use a sandbox modeling instrument with a particle image velocimetry monitor to test four designed sandbox models with multilayer competent incompetent materials test results reveal that decollement folds initiate along selected incompetent layers with decreasing velocity difference and constant vorticity difference between the hanging wall and footwall of the initial fault tips the decollement folds are progressively converted to fault propagation folds and fault bend folds through development of fault ramps breaking across competent layers and are followed by propagation into fault flats within an upper incompetent layer thick skinned thrust is produced by initiating a decollement fault within the metamorphic basement progressive thrusting and uplifting of the thick skinned thrust trigger initiation of the uppermost incompetent decollement with formation of a decollement fold and subsequent converting to fault propagation and fault bend folds which combine together to form imbricate thrust breakouts at the base of the early formed fault ramps along the lowest incompetent layers which may correspond to basement cover contacts domes the upmost decollement and imbricate thrusts to form passive roof duplexes and constitute the thin skinned thrust belt structural styles and assemblages in each of tectonic stages are similar to that in the representative orogenic belts in the south china southern appalachians and alpine orogenic belts

subduction related origin of the 750 ma xuelongbao adakitic complex sichuan province china implications for the tectonic setting of the giant neoproterozoic magmatic event in south china abstract the xuelongbao plutonic complex in the eastern margin of the tibetan plateau is dated at 748 7 ma using the shrimp zircon u pb method and represents part of the neoproterozoic igneous assemblage of south china rocks in the complex include tonalite and granodiorite and have sio2 ranging from 62 0 to 74 8 wt and al2o3 from 14 3 to 20 9 wt their na2o contents range from 4 2 to 6 7 wt and k2o from 0 47 to 1 96 wt indicating that they belong to the na series these rocks show chondrite normalized ree patterns depleted in hree and variably enriched in lree they have positive sr and negative nb and ti anomalies in the primitive mantle normalized trace elemental spider diagram their sr contents range from 320 to 780 ppm and y contents are lower than 10 ppm resulting in high sr y ratios 52 320 characteristic of typical adakites their end t and initial sr isotopic compositions range from 0 36 to 2 88 and from 0 7033 to 0 7054 respectively the geochemical features of the xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab together with arc signatures of other granites and mafic intrusions in the region the 750 ma xuelongbao adakitic complex provides evidence for a major subduction related neoproterozoic magmatic event of south china

structural doming tectonics intracontinental deformation of the sw margin of the yangtze block china the southwest margin of the yangtze block underwent multiple tectonic events including the collision between the yangtze and the south china plate caledonian folding and uplift of south china subduction of the western pacific plate and cenozoic continental escape during the indo asia collision the structural framework and tectonic evolution of the area are currently unclear through field petrologic and structural studies microstructures within the deformed units and estimates of the temperature of deformation we constrain the conditions of deformation and further classify at least four stage deformation the first stage involved northeastward thrusting that produced folds mylonites and dynamically recrystallized quartz and calcite electron backscatter diffraction data indicate that deformation during this event occurred at 400 c corresponding to a depth of 10 km the second stage of deformation involved uplift extension and the generation of a domal structure during the third event high angle normal faults formed within the cover sequence that surrounds the dome the fourth stage of deformation occurred in the cenozoic and involved motion along sinistral and dextral strike slip faults which cut structures that formed during the previous stages of deformation the first deformation event was related to triassic n s collision with the indochina block the second and third stages resulted from far field compression related to westward subduction of the paleo pacific plate and the fourth event was related to displacement along the ailaoshan red river strike slip fault the dome in the study area represents a transitional structure between a metamorphic core complex and a gneissic dome which we refer to as structural doming

deformation partitioning and foliation reactivation during transpressional orogenesis an example from the central longmen shan china progressive localization of strain within the songpan garze fold belt at the eastern margin of the tibetan plateau has resulted in the formation of the wenchuan maowen shear zone and preservation of excellent overprinting relationships between three major phases of ductile and semi ductile deformation these relationships show that deformation in the eastern songpan garze fold belt evolved from crustal thickening during sw directed compression d1 to localized sinistral transcurrent shear d2 and finally to increasingly localized high temperature d3p and retrograde d3r reverse shear in response to se directed compression both d2 and d3 deformations were co planar and reactivation of the s2 foliation led to the formation of a composite s2 3 fabric however inclusion trails in garnet porphyroblasts preserve evidence of a transition between these strike slip and reverse shear end members indicating that d2 and d3 were part of an episode of progressive transpressional shear metamorphic grade increases from chlorite to kyanite zone within the wenchuan maowen shear zone reaching syn d3p peak conditions of 600 c and 10 kbar the d3p and d3r reverse shears were responsible for tectonic exhumation of the shear zone and resulted in the formation of retrograde d3r high strain zones along the margins of the xuelongbao granite and adjacent to the wenchuan maowen fault

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

emeishan basalt ar ar overprint ages define several tectonic events that affected the western yangtze platform in the mesozoic and cenozoic abstract ar ar whole rock dating was carried out as part of a detailed stratigraphical investigation of the emeishan basalt large igneous province lip in the stratotype area sichuan china thirteen from twenty one specimens from three sections yielded reliable reversed isochron and plateau ages maximum 1 errors of 2 1 and 1 6 m y respectively and form two clusters centered on the late jurassic early cretaceous and early late cretaceous with a tail spanning 82 40 ma however all are appreciably younger than the magnetobiostratigraphically constrained late middle permian 255 260 ma age of the basalts reflecting varying degrees of thermal resetting during the mesozoic and cenozoic recently middle jurassic early cretaceous ages reported from elsewhere in the lip have been used to infer a single 40 m y long tectonic episode that affected the western yangtze platform developing this idea the new information have been combined with data we hold for other parts of the terrain and results from three emeishan lip ar ar dating studies published during 2002 to give 32 reliable age dates three 10 12 m y events appear to be recorded in different parts of the province middle jurassic late jurassic early cretaceous and early late cretaceous with a fourth shorter middle eocene episode the 40 m y mesozoic episode appears to be a sampling artifact following a review of the regional deformation tectonic features it is argued that activity related to various phases of deformation in the longmen shan thrust belt is the most likely cause of resetting the final suturing of the north and south china blocks may have also been responsible for the middle jurassic event however the oligocene present indentation of india into asia appears not to have had an impact possibly due to the large scale related strike slip faults that have effectively shielded the lip sichuan basin

when do dragfolds not develop into sheath folds in shear zones abstract using a unified model for high strain zones rotation paths of material lines in different flow types are investigated and presented as stereographic projections a material fabric element such as a mineral lineation or a fold axis can be plotted on such stereograms and the rotation path revealed using this method we examine the rotation of folds and the development of sheath folds in general three dimensional zonal deformation we show that the evolution of dragfold geometry in a high strain zone depends on the flow type and the initial perturbation of the fold axis in simple shear zones and thickening zones monoclinic or triclinic dragfolds will evolve into sheath folds in monoclinic thinning zones with biaxially stretching boundaries if the shear direction is parallel to the maximum stretching direction of the boundary sheath folds will develop if the shear direction is parallel to the minimum stretching direction of the boundary dragfolds become more cylindrical as deformation advances in thinning zones with boundaries equally stretched in all directions the likelihood of sheath fold development depends on the ratio of the shear strain rate to the thinning rate in triclinic thinning zones fold evolution depends on the details of the flow and the value of initial fold axis perturbation

the longmenshan tectonic complex and adjacent tectonic units in the eastern margin of the tibetan plateau a review abstract the longmenshan tectonic complex lstc along the eastern margin of the tibetan plateau the site of devastating earthquakes such as the magnitude 8 0 wenchuan earthquake on 12 may 2008 preserves an exceptionally complete history of the tectonic evolution of the yangtze block and its relations to adjacent tectonic units due to sequential tectonic superposition and tectonic reactivation the tectonic nature of the lstc and in particularly the older history has been profoundly debated and many different tectonic models have been proposed herein we summarize the current understandings of the major tectonic events that have shaped this important tectonic complex highlighting problems left to be solved by future work including 1 the nature and constraints for at least 6 regional tectonic events i e building of the metamorphic basement art the columbia nuna supercontinent pt1t the rodinia supercontinent pt3t the paleozoic passive continental margin pzt the paleotethys orogeny mzt and the neotethys orogeny kzt 2 metamorphic basement exposures and their tectonic implications including rock types and geochronological constraints for the archean paleoproterozoic and neoproterozoic basements 3 nature of the present lstc and its affinity with adjacent tectonic units 4 consideration of the ne striking longmenshan thrust belt and arcuate shape yanyuan muli thrust belt as parts of a single tectonic feature 5 mountain basin coupled systems recording past tectonic eposides we draw the following conclusions and tectonic models based on published research combined with our own recent studies 1 the well preserved archean yudongzi gneiss group in the lstc has a genetic affinity with the kongling group and thus belongs to the yangtze block 2 the paleoproterozoic hejiayan group juxtaposed adjacent to the archean yudongzi group may represent a 2000 1800 ma orogenic belt which corresponds to the supercontinent nuna columbia amalgamation event 3 a neoproterozoic trench arc basin system which is reconstructed based on identification of a neoproterozoic ophiolite complex arc type magmatic rock assemblages and volcaniclastic basinal deposits along the western margin of the yangtze block and the lstc may represent the record of eastward subduction of the neoproterozoic mozambique oceanic lithosphere beneath the yangtze block during the assembly of the rodinia supercontinent 4 a complete bidirectional wilson cycle was reconstructed by the formation of the late permian to the middle late triassic back arc ganze litang rift and ocean following the early paleozoic mianlue continental rift and ocean and subsequent closure of the ocean basin by simultaneous bidirectional northward and southwestward subduction and later collision this relatively uncommon bidirectional wilson cycle might be attributed to the formation of the three armed rift system in the eastern paleotethys associated with the late permian eemeishan large igneous province in the lstc 5 a three stage tectonic sequence of in sequence imbricate thrust in the lstc during india eurasian collisional orogeny at 55 15 ma extrusion from 15 to 5 ma and the plateau uplift since 5 ma resulting from lower crustal channel flow is proposed for the formation of the present lstc

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

structural style and tectonic significance of the jianglang dome in the eastern margin of the tibetan plateau china abstract the jianglang tectonic dome jtd lies on the eastern margin of the tibetan plateau and consists of three tectono stratigraphic units separated by two extensional detachment faults a schist complex a middle ductile slab and an overlying sedimentary unit the schist complex is composed of quartz mica schists and amphibolites of possible mesoproterozoic age and exhibits two early foliations s 1 and s 2 overprinted by two later foliations s 3 and s 4 the ductilely deformed middle slab consists of paleozoic quartzite phyllite marble amphibolite and meta basalt it is characterized by pervasive intrafolial and intraformational recumbent folds with a penetrative axial foliation s 1 an equivalent of s 3 in the basement rocks the cover unit the triassic xikang group of flysch sedimentary rocks experienced shallow level buckling and flattening deformation with an axial cleavage defining s 1 an equivalent of s 4 in the middle slab and basement rocks the jtd appears to have experienced three stages of extension immediately after orogeny the earliest d 1 recorded by an axial surface s 1 of s 0 intrafolial folds and sub parallel to s 0 in the schist complex probably occurred prior to the collision between the south china and north china blocks this extensional event was followed by subduction of the yangtze block to the west towards the eastern margin of the tibetan plateau the two detachment faults are characterized by structures formed by ductile brittle to brittle shear deformation which are sub parallel to the layer of s 2 in the lower detachment and to s 0 in the upper detachment these faults mark the second stage of extension and may have caused thinning and selective removal of the sinian and paleozoic strata as well as exhumation of the schist complex and the middle slab in the middle jurassic 177 ma the final stage of extension d 5 shown by brittle deformation and thermo metamorphism from the lower detachment to the xikang group was associated with the indian eurasia collision

field study of a highly active fault zone the xianshuihe fault of southwestern china the xianshuihe fault of western sichuan province china is one of the world s most active faults having produced 4 earthquakes during this century of magnitude 7 along a 350 km length of the fault at least 8 such events have occurred since 1725 in the more limited 150 km long segment including luhuo and daofu major earthquakes in 1904 1923 1973 and 1981 m 7 7 7 6 6 9 were associated with overlapping surficial fault ruptures and with individual left lateral displacements as large as 3 6 m field studies indicate that this high degree of activity is typical of the fault s longer term history the holocene left lateral slip rate on the north western segment of the fault has been 15 5 mm yr decreasing to about 5 mm yr on its southeastern segment based on radiometrically dated offset stream channel and terrace deposits and on offset glacial moraines physiographic features of active faulting are fully as diagrammatic as those of california s san andreas fault mainly because of high altitude preservation and the absence of cultural modification on this eastern margin of the tibetan plateau detailed en echelon tensional and pushup features resulting from surface ruptures in 1973 1955 1923 and 1893 can still be recognized today and new data have been collected bearing on the offsets and fault rupture lengths during these and other events the locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the xianshuihe fault obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events whereas segmentation control for others remains obscure based on the historic record repeat times estimated from slip rates and current seismic gaps two segments are particularly likely sites for m 7 earthquakes in the near future the 65 km long segment between daofu and qianning and the 135 km long segment bracketing kangding continuing creep has been documented along some segments of the fault and this together with the high degree of activity and other unique attributes makes the xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard evaluation studies

miocene emplacement and deformation of the konga shan granite xianshui he fault zone west sichuan china geodynamic implications the presently active sinistral xianshui he strike slip fault xsh is a lithospheric scale strike slip fault in the eastern himalaya in the study area this fault affects the eastern edge of the konga shan granitic massif where it has caused both brittle and ductile deformation a rbsr isochron and nd and pb isotope study of three samples and a upb zircon study of a single sample were completed on the granite upb data indicate a granite emplacement age of 12 8 1 4 ma the rbsr isochrons show that the granite emplacement and the deformational event were synchronous at around 12 10 ma minimum age for deformation the nd and pb isotope compositions of whole rocks and k feldspars indicate the involvement of proterozoic continental crust which is confirmed by upb systematics indicating inherited zircons sinistral faulting along the xsh began at the latest at 12 ma and marks the extrusion toward the east of the west sichuan and south china blocks following their extrusion along the red river fault zone between 50 and 21 ma

discussion on mesozoic extensional structures of the fangshan tectonic dome and their subsequent reworking during collisional accretion of the north china blockjournal vol 163 2006 127 142 yu wang writes on the basis of descriptions of the stratigraphic sequences and structural features in the field and interpretations of published data yan et al 2006 a reach the surprising conclusion that early mesozoic early middle triassic ese oriented extension resulted in formation of the fangshan metamorphic core complex if correct the authors have made an important contribution to understanding of the tectonic evolution of north china however the way in which they cite previously published work to support their model and discrepancies between their findings in the field and those of other workers raise a number of problems that will be discussed below citing of published work yan et al 2006 a cited many references e g lei et al 1994 niu et al 1994 yu zhang 1996 chen 1999 fu 1999 meng et al 2003 in support of their conclusions about early triassic extension and formation of metamorphic core complexes as well as late cretaceous normal faulting in many cases however the cited references do not lend support to their interpretations for example the du shan granite and surrounding archaean and proterozoic metamorphic rocks yu zhang 1996 contain no evidence for extension or development of metamorphic core complex features but instead record ne trending strike slip ductile shearing not top to the se extension the malanyu dome is an old metamorphic sequence chen 1999 and no one has previously interpreted it as an extensional dome associated with se directed extensional shear zhang et al 1991 did not examine any aspect of the geology in the area of the so called yuerya metamorphic core complex furthermore no dome has been identified here by any previous geologists instead it is a c 175 174 ma u pb sensitive high resolution ion microprobe ages luo et al 2001 granitic sheet like

emplacement of the longmen shan thrust nappe belt along the eastern margin of the tibetan plateau abstract the topographic descent from the eastern margin of the tibetan plateau to the sichuan basin passes through the longmen shan thrust nappe belt which is sub divided by six nw dipping major listric thrusts with accompanying duplexes and imbricate fans into five large scale nappes each nappe has its own stratigraphic and deformational features in the inner longmen shan longmen mountains allochthonous nappe units have incorporated both mesoproterozoic basement and sinian neoproterozoic to triassic cover sequences as thick skinned horses whereas in the frontal longmen shan sinian cretaceous cover sediments have been stripped from the basement as thin skinned fold and thrust sheets including the extensively distributed klippen structures pre thrusting extension during devonian to middle late triassic times resulted in syn depositional normal faults structural inversion of these faults initiated the peng xian guan xian basement complex jiuding shan and tangwangzhai nappes during the early episode of the indosinian orogeny norian to rhaetian this was followed by episodic thrusting during latest triassic to early cretaceous times to develop the guan xian an xian and southeastern marginal nappes that have incorporated sediments from the neighbouring foreland basin into the frontal part of the thrust nappe belt during a late miocene reactivation of the pre existing thrusts differential thrusting occurred across the thrust nappe belt

extensional deformation assisted by mineralised fluids within the brittle ductile transition insights from the southwestern massif central france abstract extensional deformation assisted by mineralized fluids within the brittle ductile transition is inferred using structural microstructural and textural investigations applied to the sarlande shear zone of the southwestern massif central france the shear zone is a low angle normal fault that bounds the meuzac antiform to the south these structures were formed during the nw se trending synorogenic extension of middle carboniferous age pre existing lithologies involved in the shear zone led to partitioning of behaviour and hydrothermal alteration in response to crustal extension and channelling of mineralising fluids intense fluid rock interactions produce softening and weakening reactions induced by alteration of k feldspar remobilisation of graphite and rutile and growth of white micas chlorite and polymetallic sulphides extensional shear deformation and mineralization progressively and concomitantly developed within the brittle ductile transitional crust 9 13 km depth the derived crustal scale model emphasises the importance of collision which led to the production of large quantities of granite derived fluids and the creation of a crustal permeability that drove these fluids near to the brittle ductile transition late orogenic extension associated with fluid rock interactions led to growth of mineralizing low angle shear zones and footwall domes the role of mineralising fluids fluid pressure appears to be essential in promoting deformation within the brittle ductile transition of the extending crust

neoproterozoic adakitic plutons and arc magmatism along the western margin of the yangtze block south china abstract neoproterozoic adakitic plutons that crop out along the western margin of the yangtze block south china from kangding on the north to panxi on the south provide constraints on the origin of the giant jinningian magmatic event of south china representative plutons include the xuelongbao 750 ma datian 760 ma and dajianshan intrusions the latter two bodies consist mainly of granodiorite with relatively high sio2 51 0 73 4 wt and mg s 0 36 0 55 they have fractionated rare earth element patterns with la yb n ratios ranging from 2 6 to 101 8 and are characterized by high sr 344 1018 ppm and low y 4 3 17 9 ppm yielding sr y ratios ranging from 27 to 111 on primitive mantle normalized trace element diagrams these rocks show enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with positive zr hf and negative ti anomalies consistent with an arc related setting they have relatively constant initial whole rock 87sr 86sr ratios 0 70

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

late cenozoic to holocene deformation in southwestern sichuan and adjacent yunnan china and its role in formation of the southeastern part of the tibetan plateau from at least 2 4 ma to present crust in the southeastern part of the tibetan plateau west of the convex east xianshuihe xiaojiang fault system has deformed internally and rotated clockwise around the eastern himalayan syntaxis the northwest striking ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78 100 km of which 60 km is transferred to the xianshuihe fault zone across a diffuse transfer zone and 22 40 km is absorbed by bending of older structures and crustal shortening crustal shortening is expressed along and east of the eastern end of the ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the tibetan plateau a similar transfer of left slip into shortening occurs farther south across the xianshuihe fault in the high mountains around and east of gongga shan 7556 m the northwest striking convex east left lateral litang fault zone lies southwest of the ganzi xianshuihe xiaojiang fault zone and appears to be less well developed but otherwise similar to the ganzi fault zone the batang chenzhi and other northeast striking right lateral faults of small displacement occur within the rotating crustal fragment together with the left slip faults they accommodate east west shortening northeast of the eastern himalayan syntaxis south of this region of shortening the crust is extending to form grabens within the dali and southern xiaojiang fault systems and in the tengchong volcanic province the progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern himalayan syntaxis the assemblage of structures in southwestern sichuan geometrically resembles structures of eocene to miocene age in southern yunnan that were positioned northeast of the eastern himalayan syntaxis similar to present day southwestern sichuan at the time of their development the similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past 50 m y structures in sichuan are less fully developed than older structures in southwestern yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis deformation in these areas indicates that plateau formation has been complex inhomogeneous and diachronous at scales from 1000 km to less than 100 km

structural and geochronological constraints on the mesozoic cenozoic tectonic evolution of the longmen shan thrust belt eastern tibetan plateau 1 the longmen shan thrust belt in the eastern margin of the tibetan plateau underwent deformation of d1 associated with the late triassic collision between the north and south china blocks followed by deformation of d2 and d3 related to the eastward growth of the tibetan plateau the d1 is marked by moderately tight folds f1 spaced cleavage s1 mineral lineation l1 multistage reactivated faults and top to the south thrusting this deformation initiated before 237 ma and ended at 208 ma the 193 159 ma d2 deformation is top to the southeast directed and normal ductile in nature and was associated with the formation of half graben basins the mesozoic deformation of the longmen shan belt was related to the subduction of the paleo tethys oceanic lithosphere followed by the south china block underneath the songpan ganze terrane the d1 was produced in the pro side of the overriding songpan ganze plate most probably driven by slab corner flow above the retreating south china block the d2 and extensional songpan ganze turbidite basin were produced by roll back and migration of the overriding plate above the retreating subduction zone this model for the deformation from d1 to d2 is similar to a mediterranean style model the d3 is characterized by se ward thrusting with stepwise migration of rapid denudation and dextral strike slip faulting during the late cretaceous to present therefore the longmen shan thrust belt involved mesozoic compressional and extensional events that were overprinted by the late cretaceous to cenozoic compressional event