structural anatomy of the kushtagi schist belt dharwar craton south india an example of archaean transpression abstract the nw se trending kushtagi schist belt in the eastern dharwar craton records the tectonic consequences of a transpression regime it was formed due to late archaean oblique convergence in the dharwar craton this relatively unknown schist belt is characterized by two major episodes of deformation the dominant d 1 deformation forming tight to isoclinal non cylindrical folds and a regional axial planar schistosity is a consequence of transpression the transpression related sinistral shear regime is followed by development of a conjugate set of f 2 folds and s 2 cleavages that are superimposed on the d 1 structures the maximum shortening direction associated with the f 2 folds is nw se which suggests longitudinal compression subsequent to transpression riedel shear fractures were developed in the sinistral shear regime and dolerite dykes were emplaced along the conjugate riedel shear fractures intrusive granite bodies within and outside the schist belt were emplaced in the late phase of d 1 transpression

genesis of the late eocene bedded halite in the qaidam basin and its implication for paleoclimate in east asia abstract it has long been suggested that the eocene climate in east asia was characterized by three latitudinally distributed zones one middle arid zone and two humid zones to its north and south the middle arid zone was firstly proposed based on the widespread red beds and evaporites in a nww trending zone of china however this long standing model has recently been challenged by increasing doubts about the effectiveness of these arid climate indicators the qaidam basin as one of the eocene evaporites bearing basins in china can be studied to test whether the bedded evaporites can validly indicate arid climate over 300 m halite beds have been found in the late eocene upper xiaganchaigou uxg formation in a local saline center shizigou area of the qaidam basin its coexistence with source rocks and limited distribution together with the absence of red beds make it difficult to explain the halite genesis with the conventional climatic controlled playa model based on stratigraphic correlation lithological and mineralogical observation and oxygen carbon and chlorine isotopic analysis we identify several characteristics about the shizigou saline center 1 undulated lake floor 2 shallow water 3 limited hydrothermal impact 4 evaporation induced precipitation 5 high eolian influence and 6 fluctuated climate background according to these investigations we suggest a new model for the genesis of the bedded halite in the qaidam basin the tectonic climatic model the key point for this model is that the undulated topography caused by tectonic activity and the fluctuated climate together leaded to the hydrological isolation of the shizigou area the shizigou area was isolated from the larger brackish qaidam paleolake for independent evaporation during arid climate stage while connected with it during humid climate stage causing alternatively water influx and evaporation and inducing the occurrence of interbedded halite and source rocks thus the widespread bedded halite in the eocene strata over china may indicate fluctuated rather than arid climate in east asia

intensified aridity in the qaidam basin during the middle miocene constraints from ostracod stable isotope and weathering records the thick and continuous cenozoic successions in the qaidam basin provide an excellent paleoclimate archive here we focus on the ostracod fauna stable isotope records and paleoweathering indice

magnitude of post middle jurassic bajocian displacement on the central altyn tagh fault system northwest china the altyn tagh fault system a key structural feature in the tectonic collage of central asia is an active left lateral strike slip fault system however its age of initiation kinematic history and total magnitude of displacement are not well known middle jurassic nonmarine sedimentary rocks that crop out along the central segment of the altyn tagh fault were deposited in a contiguous tarim qaidam foreland style basin that was geodynamically linked to mesozoic contraction in the tian shan bei shan qilian shan and kunlun shan this study documents the best evidence of a piercing point to date consisting of an offset lacustrine shoreline demarcated by aalenian bajocian open lacustrine strata to the west and fluvial and alluvial strata of comparable age to the east detrital compositions palynology paleocurrents and lithostratigraphy support correlation of these sections restoration of this jurassic facies boundary indicates 400 60 km of post bajocian left lateral separation on the altyn tagh fault consistent with this result restoration of the jurassic shoreline aligns felsic plutons across the western segment of the altyn tagh fault system that may have been left laterally offset by 360 km these two pairs of offset features suggest that the current best estimate for net magnitude of post bajocian left lateral separation on the altyn tagh fault is about 360 km

paleomagnetic evidence for a mid miocene clockwise rotation of about 25 of the guide basin area in ne tibet abstract ten sections of neogene molasse type sediments were sampled in the guide basin of northeastern tibet for magnetostratigraphy x m fang m d yan r van der voo d r rea c song j m pares j gao j nie s dai late cenozoic deformation and uplift of the ne tibetan plateau evidence from high resolution magnetostratigraphy of the guide basin qinghai province china geol soc america bull 107 2005 1208 1225 1 but they also yield seven well dated formation mean directions that reveal changing declinations as rotations occurred in response to crustal deformation north of the india asia collision zone three formations are of early miocene and oligocene age as indicated by fossils and magnetic reversal records whereas four younger formations yield late miocene and pliocene ages the dual polarity magnetizations are typically antipodal but reveal inclinations that are too shallow most likely because of post depositional inclination flattening the late miocene and younger directions show formation mean declinations between 354 and 7 whereas three early miocene and late oligocene mean declinations range from 31 to 44 this indicates that a clockwise rotation of 25 1 4 6 took place during the middle part of the miocene best estimate 11 17 ma no rotations appear to have occurred during that time in the xining lanzhou linxia and jingning basins longzhong basin to the northeast and east of the guide basin however a rotation of similar magnitude has been documented by dupont nivet and colleagues for pre miocene 29 ma time in these areas collectively these results show that the basins in ne tibet have had independently evolving structural histories

flexural subsidence by 29 ma on the ne edge of tibet from the magnetostratigraphy of linxia basin china abstract this study provides a detailed magnetostratigraphic record of subsidence in the linxia basin documenting a 27 myr long sedimentary record from the northeastern edge of the tibetan plateau deposition in the linxia basin began at 29 ma and continued nearly uninterruptedly until 1 7 ma increasing rates of subsidence between 29 and 6 ma in the linxia basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to late oligocene late miocene time by late miocene early pliocene time a decrease in subsidence rates in the linxia basin associated with thrust faulting and a 10 clockwise rotation in the basin indicates that the deformation front of the tibetan plateau had propagated into the currently deforming region northeast of the plateau

mineralogical and geochemical discrimination of the occurrence and genesis of palygorskite in eocene sediments on the northeastern tibetan plateau palygorskite is a widely used indicator of semiarid to arid environments in paleoclimate studies in this study we present detailed mineralogical and geochemical investigations exploring the genesis of palygorskite found in eocene fluvial sediment in the northern qaidam basin on the northeastern tibetan plateau the presence of two types of palygorskite is revealed based on their crystallinity characteristics and distinctive rare earth element ree patterns in the coexisting clay fraction well crystallized palygorskite samples are characterized by remarkably negative ce anomalies and obvious middle rare earth element enrichment poorly crystallized palygorskite samples generally exhibit positive ce anomalies and less pronounced middle rare earth element enrichment which resemble those of nonpalygorskite bearing clay samples given the presence of an overall oxidized fluvial sedimentary environment we attribute the well crystallized palygorskite which has textures comprising long interwoven fibers to direct precipitation i e neoformation occurring within a reducing environment during early postdepositional processes while the poorly crystallized palygorskite which is characterized by short club shaped single crystals originates as catchment delivered detritus these poorly crystallized palygorskites occur mostly in 49 5 47 0 ma and are accompanied by decreasing kaolinite content increasing chlorite content and abundant xerophytic spore pollen from the qaidam basin and its neighboring xining basin collectively these evidences suggest that a less humid climate followed after the early eocene climate optimum

late pliocene quaternary evolution of redox conditions in the western qaidam paleolake ne tibetan plateau deduced from mn geochemistry in the drilling core sg 1 abstract manganese mn in lake sediments reacts strongly to changes of redox conditions this study analyzed mn concentrations in oxides carbonates and bulk phases of the calcareous lacustrine sediments of a 938 5 m long core sg 1 taken from the western qaidam basin well dated from 2 77 ma to 0 1 ma comparisons of extractions from diluted hydrochloric acid acetic acid and citrate bicarbonate dithionite demonstrate that variations of mn concentrations from acetic acid leaching mnhoac are mostly responsible for mn ii fluctuations in the carbonate phase taking into account the relevant processes during weathering transportation deposition and post deposition of mn bearing rocks we conclude that mn input from catchment weathering and paleolake redox condition provide the primary controls on variations in the mn records of carbonate and oxide phases we propose mnhoac as a new sensitive indicator of paleolake redox evolution and catchment scale climate change the mnhoac variations show a long term upward decreasing trend indicating a long term decrease of mn input from catchment weathering associated with increasing oxygen content in the paleolake bottom water the great similarities of the mnhoac record with other regional and global records suggest that paleolake redox changes and climatic drying in the qaidam basin may be largely related to global cooling

significant late neogene east west extension in northern tibet field mapping in northern tibet reveals that the normal slip along late cenozoic northsouth trending faults is comparable to that estimated for equivalent structures in southern tibet the orientation of fault striations in two north south trending rifts suggests an eastnortheast west northwest direction of extension in northern tibet which in turn implies that northeast striking active faults in northern tibet have significant left slip components initiation of rifting in northern tibet postdates the early oligocene and possibly occurred after 4 ma the broad similarities in the magnitude of slip and the direction of extension for normal faults in both northern and southern tibet imply that the entire plateau has been extending this precludes significant eastward extrusion of north tibet relative to south tibet and requires a regional boundary condition as the cause of east west extension for the entire tibet plateau

present day deformation of the qaidam basin with implications for intra continental tectonics abstract the qaidam basin is a plateau of low relief about 3 km in elevation and covering a large area of 1 2 105 km2 physiographically the basin marks a transitional region between the northeastern tibetan plateau 5 0 5 km in elevation and the surrounding low lying areas the northeastern and southwestern edges of the basin are bounded by the qilian shan nan shan and qimantag eastern kunlun shan mountain belts respectively at present the entire region is under northeast southwest compression based on a synthesis of recent geological and geophysical results we propose that each flanking mountain belt consists of a sequence of crustal nappes originating from ramp thrusts at mid crustal levels 15 km deep over a distance range of 700 km these two broad fold and thrust belts operating in tandem and with some uniform shortening in the intervening qaidam basin successively raised the surface elevation from below 2 km to 5 km at depth the crust is correspondingly thickened from 40 km beneath the foreland of the qilian shan to approximately 55 km under the interior of the qaidam basin and then to nearly 70 km beneath tibet in contrast to the himalayan front where earthquake faulting occurs down to a depth of 50 km in the foreland but the zone of active thrust faulting is less than 200 km in width plateau building in the qaidam region illustrates how a wide orogen is developing along northeastern tibet in the interior of eurasia

magnitude of late quaternary left lateral displacements along the north edge of tibet images taken by the earth observation satellite spot of the quaternary morphology at 18 sites on the 2000 kilometer long altyn tagh fault at the north edge of tibet demonstrate that it is outstandingly active long term left lateral strike slip offsets of stream channels alluvial terrace edges and glacial moraines along the fault cluster between 100 and 400 meters the high elevation of the sites mostly above 4000 meters in the periglacial zone suggests that most offsets resulted from slip on the fault since the beginning of the holocene these data imply that slip rates are 2 to 3 centimeters per year along much of the fault length and support the hypothesis that the continuing penetration of india into asia forces tibet rapidly toward the east

mesozoic and cenozoic tectonics of the northern edge of the tibetan plateau fission track constraints abstract fission track analysis on zircons and apatites yields new information about the timing of deformation of the northern tibetan plateau ages on zircons ranging from 221 22 to 96 4 ma are indicative of a general late triassic early jurassic cooling probably driven by the collision between the qiantang and kunlun blocks mid jurassic slow cooling is recorded also in the apatites in regions not affected by later cenozoic deformation this jurassic denudation was followed by a period of sedimentation during the cretaceous except along the altyn tagh fault atf zone and in some restricted areas of the western and eastern qilian shan this long and relatively quiet period ended at about 40 10 ma along the major altyn tagh and kunlun strike slip fault zones which were activated by the india asia collision this first movement along lithospheric faults resulted in the eastward extrusion of the tibet plateau which was followed in late oligocene miocene times by a major compression event initiating the formation of the high relief of north tibet a final compressional event took place at 9 5 ma and is well correlated with high sedimentation rates in the basins of this region this compression induced continental subduction in the kunlun ranges the altun shan belt and possibly the qilian shan belt

miocene pollen record of kc 1 core in the qaidam basin ne tibetan plateau and implications for evolution of the east asian monsoon abstract thick cenozoic deposits in the qaidam basin provide great potential for understanding the tectonic history paleoclimatic changes and evolution of the east asian monsoon this study examines the pollen record from the kc 1 core for the interval covering the later early to late miocene 18 5 ma thermophilic taxa percentages are high between 18 and 14 ma and decrease after this time a pattern which fits well with the middle miocene climatic optimum mmco between 18 and 14 ma and global climatic cooling after 14 ma during the same period xerophytic taxa percentages gradually increase and those of the conifers gradually decrease suggesting an aridification process in the qaidam region driven by the gradual strengthening of the east asian winter monsoon eawm and weakening of the east asian summer monsoon easm the global climate cooling process appears to have driven the climatic development of the qaidam basin region throughout the miocene but the uplift of the tibetan plateau also contributed

structural evolution of a major fault zone the cape ray fault zone southwestern newfoundland canada the cape ray fault zone is a major paleozoic structure in southwestern newfoundland and occurs at or close to the boundary between two major continental blocks laurentia and avalonia a detailed structural analysis demonstrates that the fault records early reverse sinistral thrusting of the grand bay complex at amphibolite grade d2 followed by a protracted event d3 characterized by reverse dextral thrusting of the grand bay complex rocks on top of the supracrustal rocks of the windsor point group and retrogression to greenschist facies as well as a pre 384 ma orogen parallel dextral transcurrent mylonite d4 during the later stages of the collision regional scale strain partitioning induced heterogeneity of strain both along and across the strike of the cape ray fault zone the east west oriented segment of the cape ray fault zone is a tear fault that accommodated differential displacement along the length of the fault later stages of the deformation include post 384 ma sinistral transcurrent r

late miocene pleistocene aridification of asian inland revealed by geochemical records of lacustrine fan delta sediments from the western tarim basin nw china abstract continuous cenozoic deposits have accumulated in the tarim basin northwestern china and are crucial to investigating the aridification history of the asian interior which has long been thought to be linked with the growth of the tibetan plateau retreat of the para tethys sea and global cooling however less and poor climatic records from the region hinder our understanding of the questions above here we report continuous lithologic sedimentary color and geochemical caco3 and salt ions records from precisely dated late miocene early pleistocene sedimentary sequence from western tarim basin nw china they indicate that arid climate had prevailed within the basin at latest at ca 5 7 ma and gradually increased in degree until ca 3 7 ma between ca 3 7 and 2 1 ma a hydrologic event occurred which supplied more water from the tien shan after ca 2 1 ma the climate in the basin turned to hyper arid we attribute the stepwise drying of the asian inland since ca 5 7 ma to the late miocene pliocene episodic uplift of the tibetan plateau pamir and the tien shan and the connection of the pamir with the tien shan in remote response to the collision of india with asia

long chain alkenones preserved in miocene lake sediments abstract over the last few decades the occurrence of long chain alkenones lcas in quaternary lacustrine settings has been reported the distributions being remarkably different from those of open ocean settings we report here the occurrence of lcas with modern lake type composition from middle miocene sedimentary strata in the dahonggou section qaidam basin northern tibetan plateau this is the first report of lcas with preserved lake compositions in pre quaternary lacustrine sediments the discovery implies that the producers of modern lacustrine lcas could originate from similar lake species living at least as far back as the miocene epoch it also sheds new light on the potential of alkenones as palaeotemperature indicators in lacustrine systems back to at least 10 5 ma

cenozoic environmental evolution of the qaidam basin and its implications for the uplift of the tibetan plateau and the drying of central asia abstract a complete cenozoic pollen sequence of the qaidam basin has been constructed with absolute age control the cenozoic environmental evolution of the qaidam basin and its implications for the uplift of the tibetan plateau and the drying of central asia during the cenozoic era is discussed based on the interpretation of the pollen sequence the commencement of eolian deposition in central china and the change of climatic distribution pattern of china the results suggest that in the qaidam basin relatively dry climates prevailed in the eocene oligocene and pliocene pleistocene these dry periods resulted from the effects of a subtropical high pressure zone and the uplift of the surrounding mountains and the tibetan plateau respectively about a 600 km northward drift of western china has probably occurred during the late cenozoic resulting in the eocene oligocene arid climate belt twisting about six degrees of latitude the tibetan plateau reached probably 1500 m elevation by the latest miocene 8 5 ma not as high as it was thought previously the cenozoic environmental evolution of northwest china does not support the idea that the retreat of the paratethys sea played an important role in the late cenozoic drying of central asia at least not in northwest china

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

distribution of active rock uplift along the eastern margin of the tibetan plateau inferences from bedrock channel longitudinal profiles 1 current models of long term river incision into bedrock suggest that the local rate of differential rock uplift should exert a primary control on the gradient of channel longitudinal profiles however discrimination of this effect from the influence of variations in substrate erodibility sediment flux precipitation and transient changes in profile shape has proved difficult in practice here we investigate the controls on the spatial distribution of bedrock channel gradients adjacent to the sichuan basin in an effort to assess the degree and nature of active deformation along this margin of the tibetan plateau analysis of river longitudinal profiles utilizing a channel steepness index a measure of profile gradient normalized for drainage area reveals a zone of anomalously steep channels adjacent to the topographic front of the plateau margin channel profiles are systematically less steep in their headwater reaches on the plateau and in their lower reaches east of the plateau margin comparison of steepness indices to mapped lithologic variations reveals that lithology has only a limited influence on channel gradient in this field area we observe no systematic relationship between steepness indices and upstream drainage area channels of all size are steeper near the plateau margin we argue that these systematic changes are not readily explained as a consequence of increased sediment flux or of orographic precipitation we are led to conclude that steep channel profiles along the topographic front of the plateau reflect active differential rock uplift between this region and the foreland

landscape and tectonic evolution of bayin river watershed northeastern qaidam basin northern tibetan plateau implications for the role of river morphology in source analysis and low temperature thermochronology

aridification recorded by lithofacies and grain size in a continuous pliocene quaternary lacustrine sediment record in the western qaidam basin ne tibetan plateau abstract the pliocene quaternary aridification of the asian interior is key to understanding the impacts of global cooling and tibetan plateau uplift and the potential linkage to north pacific ocean biochemical processes and global changes however there is a lack of detailed continuous pliocene quaternary paleoclimatic records from the asian interior because most climatic records have been obtained from discontinuous coarse sediment outcrops around the rims of inland basins here we provide a continuous high resolution 3 1 myr record of grain size from the well dated sg 3 borehole in the western qaidam basin on the northeastern tibetan plateau nw china the results reveal a long term upward coarsening trend in grain size that can be divided into three phases i 3 1 1 1 ma laminated fine sediments fine to medium silts containing scattered gypsum crystals and interbedded by thin beds of marl ii 1 1 0 15 ma massive medium to coarse silts interbedded by beds of mirabilite iii 0 15 0 ma coarse silts containing thin layers of mirabilite halite bloedite and polyhalite this pattern reflects the upward shallowing and shrinking of the lake from a brackish lake to a salt lake and then to a playa suggesting a long term stepwise drying of the asian interior since the late pliocene probably forced by global cooling and uplift of the tibetan plateau

magnetostratigraphy of cenozoic sediments from the xining basin tectonic implications for the northeastern tibetan plateau 1 the xining subbasin of the longzhong basin holds the longest continuous cenozoic stratigraphic record at the margin of the northeastern tibetan plateau despite a rich biostratigraphic content including the xiejia mammal fauna the tectonic evolution of the basin is largely unconstrained in this study we present stratigraphic biostratigraphic and magnetostratigraphic results that provide a basis for reconstructing the cenozoic tectonic evolution of the xining basin with respect to adjacent regions of the northeastern tibetan plateau magnetostratigraphic analysis from three red bed sections in the xining basin indicates continuous deposition at low and nearly constant accumulation rates average 2 2 cm kyr from 52 0 to 17 0 ma we interpret this result to indicate that no major regional tectonic event implying large sediment accumulation variations has affected the xining basin deposition during this considerable time window in detail accumulation rate variations outline a three stage evolution with 1 8 cm kyr from 52 0 ma to 34 5 ma 4 1 cm kyr from 34 5 to 31 0 ma and 2 3 cm kyr from 31 0 to 17 0 ma the second order increase between 34 5 and 31 0 ma can be interpreted to result from either a distal tectonic event or to be of climatic origin although the region was tectonically quiescent for much of the cenozoic tectonic activity occurred during basin initiation or reactivation at circa 55 0 52 5 ma and during intense basin deformation after 17 0 ma

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

magnetostratigraphy and sedimentologically derived geochronology of the quaternary lacustrine deposits of a 3000 m thick sequence in the central qaidam basin western china the qaidam basin is a large basin in western china where the thickness of quaternary deposits has a range of 2000 3000 m at its subsiding depocentre this study investigated the chronology of the quaternary deposits including palaeomagnetics biostratigraphic correlation rates of sediment accumulation seismic reflection surveys and electrical property correlations the results give a full interpretation of ages for the sequence from bottom to top the oldest sequence age was defined by the first appearance of microlimnocythere sinensis huang and was placed in the gauss epoch estimated at approximately 3 ma by extrapolation of accumulation rates the m g boundary occurs at the base of the third fossil zone of ostracoda the first appearance of qinghaicypris crassa huang or at the marker layer k 10 of the electrical log in the sebei anticline area considered here to be 2 48 ma the olduvai event in the fifth fossil zone spanning electrical layers k 4 and k 3 in the sebei area occurs at reflection layer t 0 1 of seismic stratigraphy which could be stratigraphically correlated in the whole basin the boundary of early middle pleistocene such as the brunhes and matsuyama b m boundary falls within the eighth fossil zone and coincides with electrical marker k 0 1 two boundaries corresponding to middle late pleistocene and pleistocene holocene occur at the bases of the 11th and 12th biozones respectively neither of which can be correlated with seismic stratigraphy and electrical logs in the basin due to the lack of widely distributed lacustrine deposits since the middle pleistocene the ages of the boundaries can be tested by deposition rates of different facies derived from the varves and synchronous deposits as a result geochronological sequences of subsurface stratigraphy could be established within a basin wide area on the synchronology of seismic reflectors and correlations of electrical logs as well as biozones c 1998 elsevier science b v all rights reserved

ground penetrating radar measurements of subsurface structures of lacustrine sediments in the qaidam basin nw china possible implications for future in situ radar experiments on mars abstract the stratigraphic profile of paleolakes on mars preserves a record of past climatic hydrological and sedimentary environment radar could be used to detect the structure and distribution of buried lacustrine sediments but orbiter radar sounders rarely detect the internal structure or the bottom of the ancient martian lake deposits future martian missions will employ ground penetrating radar gpr onboard rovers to investigate the shallow subsurface structure of paleolake basins in this case ground experiments are necessary to test the potential of gpr detection this study reports on gpr experiments operating at 100 mhz and 200 mhz carried out in the qaidam basin one of the analogue sites for martian paleolake environments to detect the subsurface structure of lacustrine sediments and measure the relative permittivity of sediments with both laboratory instrument and gpr data two gpr measurements were performed on top of yardangs representing layered lacustrine sedimentary outcrops another survey site is a polygonal patterned ground which is a common landform in the qaidam basin and on mars the observations revealed by gpr experiments show that 1 sediments of three sites have varied compositions density and porosity and have a wide range of relative permittivity values from 1 56 to 4 56 2 the detection depth of our measurements could reach 20 m 3 the interfaces between sedimentary layers can be seen in radargrams but reflectors in radargrams may not always match with exposed layers of the yardang outcrops due to the undetected change of permittivity values or thin thickness of layers 4 a distinct and continuous reflector is observed beneath the polygonal terrain which we interpret as a subsurface layer with low water brine content our ground experiments suggest that a similar gpr on martian rover could detect the near surface structure of lacustrine sediments e g deposition anomaly interfaces of drastic permittivity change such as buried ice brine considering that the design frequency is close to our equipment

early middle jurassic paleogeography reconstruction in the western qaidam basin insights from sedimentology and detrital zircon geochronology abstract the western qaidam basin wqb is situated at the northeastern margin of the tibetan plateau it is adjacent to the altyn tagh range to the west and is bordered at the south and north by the eastern kunlun range and southern qilian range respectively sporadic jurassic outcrops are exposed along the altyn tagh range although subsurface data have revealed that the jurassic rocks also developed in the basin interior little is known about their distribution and depositional characteristics hydrocarbon exploration has revealed that the lower middle jurassic strata are important source rocks and reservoirs in the wqb however due to post depositional uplifting exhumation or deep burial the sediments provenance paleogeography and the basin properties during the early middle jurassic in the whole wqb remain less understood in this study we examine the sedimentary records combined with detrital zircon u pb geochronology based on an analysis of post depositional modification to reveal the source to sink relationship between the wqb and surrounding potential source areas and to further reconstruct the early middle jurassic paleogeography of the wqb during the early jurassic there were a series of small scale separated rift basins which were characterized by proximal sedimentation and rapid facies shift in the wqb specifically the lower jurassic sediments in the southern wqb mainly originated from the eastern kunlun range with minor supply from the paleozoic intrusions that might have been exposed in the hinterland of the qaidam block the provenance in the northern wqb was dominated by the uplift area to the west there was a broader depression basin in the southern wqb the eastern kunlun range and the ancestral altyn mountains served as the two dominant source areas for the clastic material of the southern wqb during the middle jurassic integrated analysis on sedimentology and provenance reveals that the southeastern tarim basin and the wqb were likely to have been interconnected or adjacent during middle jurassic period we thus infer that no large scale strike slip displacement along the altyn tagh fault occurred during that period combining regional tectonic settings we tentatively suggest that the extensional setting following the orogenic movement in the eastern part of the paleo tethys ocean could have been responsible for the formation of the early jurassic rift basins by the early middle jurassic the bangonghu nujiang tethys ocean and the mongol okhotsk ocean were subducted northward beneath the qiangtang terrane and siberian continent respectively these led to the presence of an extensional stress field and the formation of a broader depression basin in the southern wqb during the middle jurassic paleogeography reconstruction results indicate that the northern wqb should have much potential for hydrocarbon exploration

does the arc accretion model adequately explain the paleoproterozoic evolution of southern laurentia an expanded interpretation comment and reply comment assembly of lithosphere in southern laurentia is best interpreted to have taken place along a long lived 1 8 1 0 ga active margin via subduction accretion processes broadly analogous to present day convergence between australia and indonesia karlstrom and bowring 1988 1 karlstrom et al

the crustal structure of the arizona transition zone and southern colorado plateau from multiobservable probabilistic inversion the arizona transition zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western us namely the southern colorado plateau and the southern basin and range provinces as such the internal crustal structure and physical state of this transitional zone hold clues for understanding i the amalgamation of these provinces ii the partitioning of deformation due to both past and present day stress fields and iii the role of thermal versus compositional effects in controlling surface observables here we employ and expand a novel multiobservable probabilistic inversion method and jointly invert fundamental mode rayleigh phase velocities receiver functions surface heat flow geoid height and absolute elevation to obtain an internally consistent 3 d model of the temperature density vs and vp of the arizona transition zone and the southern portions of the colorado plateau and basin and range our results confirm a significant crustal thickening from 28 km in the sw of the arizona transition zone and southern basin and range to 48 km beneath the southern colorado plateau inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere asthenosphere boundary is not deeper than 70 km in most of the region we find that major pre cambrian surface structures and or shear zones separate crustal domains with distinct bulk properties suggesting that the juxtaposed crustal blocks still retain at least in part their original characteristics however widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths locally overprinting their original compositions and creating highly heterogeneous crustal sections a dominant and large scale internal crustal pattern of sw dipping planes structures is evident in our models coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms while we cannot categorically corroborate the presence of melt or aqueous fluids within the crust our results are compatible with these scenarios beneath some parts of the basin and range the mogollon datil and springerville volcanic fields

refining the early history of the mojave yavapai boundary zone rifting versus arc accretion as mechanisms for paleoproterozoic crustal growth in southwestern laurentia abstractthree outstanding questions regarding paleoproterozoic crustal growth and assembly in southwestern laurentia are 1 the mechanisms of crustal growth back arc rifting and associated juvenile magmatism vs accretion of juvenile arcs 2 the nature and significance of pre yavapai orogeny 1 70 ga deformation and 3 how the isotopically mixed boundary zone between the mojave and yavapai paleoproterozoic crustal provinces formed supracrustal successions associated with the mojave yavapai boundary zone in northwestern arizona are dominated by 1730 ma bimodal metavolcanic sequences and compositionally siliciclastic metasedimentary rocks with minor ultramafic rocks chert and carbonate this lithologic assemblage requires juvenile mafic volcanism in a basin that also received abundant quartz rich i e continental detritus a pre d1 first deformational event back arc rift setting for northwestern arizona best explains the observed supracrustal assemblage plutonic rocks that intrude the s

high resolution magnetostratigraphy of the neogene huaitoutala section in the eastern qaidam basin on the ne tibetan plateau qinghai province china and its implication on tectonic uplift of the ne tibetan plateau abstract the closed inland qaidam basin in the ne tibetan plateau contains possibly the world s thickest 12 000 m continuous sequence of cenozoic fluviolacustrine sedimentary rocks this sequence contains considerable information on the history of tibetan uplift and associated climatic change however work within qaidam basin has been held back by a paucity of precise time constraints on this sequence here we report on a detailed paleomagnetic study of the well exposed 4570 m huaitoutala section along the keluke anticline in the northeastern qaidam basin where three distinct faunas were recovered and identified from the middle miocene through pliocene constrained by these faunal ages the observed thirty three pairs of normal and reversed polarity zones can be readily correlated with chrons 2n 5br of the gpts this study assigns the age of the section to the interval between ca 15 7 ma to 1 8 ma in addition the widely used stratigraphic units the xia youshashan shang youshashan the shizigou and qigequan formations were formed at 15 3 ma 15 3 8 1 ma 8 1 2 5 ma and

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

timing and magnitude of displacement on the altyn tagh fault constraints from stratigraphic correlation of adjoining tarim and qaidam basins nw china the altyn tagh fault plays an important role in growth of the north western tibetan plateau but the timing of its inception and magnitude of left lateral displacement are not well constrained this study provides new independent constraints by correlating stratigraphic development and spatial distribution of the neighbouring tarim and qaidam basins it is shown that the tarim and qaidam basins experienced quite similar stratigraphic and sedimentary evolution from late sinian to early palaeogene times this fact indicates that the tarim and qaidam basins were a single block at least until oligocene time which was then offset by the altyn tagh fault early palaeozoic and late triassic suture zones within the east and west kunlun and pinch outs of mesozoic strata are chosen as piercing lines restoration of these lines gives total offset estimates of 350 400 km across the south western and central segments of the altyn tagh fault

new early and late carboniferous paleomagnetic results from the qaidam block nw china implications for the paleogeography of central asia abstract there is an ongoing debate regarding the paleogeographic evolution of the qaidam block 37 3 n 96 4 e in northwest china during the late paleozoic to provide a reliable constraint on the paleogeographic reconstruction and to determine the relationship with the adjacent area we performed a paleomagnetic investigation of early and late carboniferous sediments from the qaidam block stepwise thermal demagnetization successfully isolated high unblocking temperature characteristic directions from the samples the tilt corrected mean direction of the early carboniferous sediments is ds is 298 6 46 4 with 95 4 7 and n 15 sites corresponding to a paleopole at 3 2 n 147 5 e with a95 4 8 the late carboniferous paleomagnetic direction is ds is 320 1 46 1 with 95 5 3 and n 8 sites corresponding to a paleopole at 15 1 n 132 2 e with a95 5 4 a positive fold test for the early carboniferous paleomagnetic directions and consistency with the kiaman reversed superchron for the late carboniferous paleomagnetic directions indicates the primary origin of the characteristic remanence of the studied section the new paleomagnetic results suggest that the qaidam block was located at about 27 n and did not undergo significant n s movement during the carboniferous however the qaidam block experienced a counter clockwise rotation of about 21 during this period comparisons of the paleomagnetic results from the major blocks of central asia suggest that the qaidam block may have experienced a relatively rapid northward movement and collided with the tarim block after the late permian based on the paleomagnetic poles of the qaidam block and adjacent blocks we present a tentative paleogeographic reconstruction for central asia during the early and late carboniferous

magnetostratigraphy of cenozoic deposits in the western qaidam basin and its implication for the surface uplift of the northeastern margin of the tibetan plateau abstract thick cenozoic deposits in the northwestern qaidam basin record erosion of the altyn tagh and high terrain west of that basin and presumably the concurrent growth of the northeastern tibetan plateau a detailed magnetostratigraphic study of the huatugou section northwestern qaidam basin reveals that this section spans the period from 30 to 11 ma magnetostratigraphic and sedimentological studies indicate that the accumulation rate abruptly increased near 15 ma the acceleration in sedimentation rate suggests enhanced tectonic deformation in the qaidam basin since 15 ma that may have begun simultaneously with accelerated deformation along the altyn tagh kunlun and haiyuan faults which contributed to the growth history of the qaidam basin and its surroundings since 15 ma

late cenozoic evolution of the eastern margin of the tibetan plateau inferences from 40ar 39ar and u th he thermochronology high topography in central asia is perhaps the most fundamental expression of the cenozoic indo asian collision yet an understanding of the timing and rates of development of the tibetan plateau remains elusive here we investigate the cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the sichuan basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed temperature time paths inferred from 40 ar 39 ar thermochronology of biotite multiple diffusion domain modeling of alkali feldspar 40 ar release spectra and u th he thermochronology of zircon and apatite imply that rocks at the present day topographic front of the plateau underwent slow cooling 30 50 c m y coincident with exhumation from inferred depths of 8 10 km at denudation rates of 1 2 mm yr samples from the interior of the plateau continued to cool relatively slowly during the same time period 3 c m y suggesting limited exhumation 1 2 km however these samples record a slight increase in cooling rate from 1 to 3 c m y at some time during the middle tertiary the tectonic significance of this change remains uncertain regardless late cenozoic denudation in this region appears to have been markedly heterogeneous with the highest rates of exhumation focused at the topographic front of the plateau margin we infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable sichuan basin and thus marks the onset of deformation related to the development of the tibetan plateau in this region the present margin of the plateau adjacent to and north of the sichuan basin is apparently no older than the late miocene or early pliocene 5 12 ma

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

late cenozoic deformation and uplift of the ne tibetan plateau evidence from high resolution magnetostratigraphy of the guide basin qinghai province china the cenozoic intramontane gonghe guide basin in qinghai province china is tectonically controlled by the sinistral strike slip framework of the kunlun and altyn tagh south qilian faults in the northeastern tibetan plateau the basin is filled with thick cenozoic clastic sedimentary formations which provide important evidence of the deformation of this part of the plateau although they have long lacked good age constraints detailed magnetostratigraphic and paleontologic investigations of five sections in the guide basin and their lithologic and sedimentary characteristics allow us to divide a formerly undifferentiated unit the guide group into six formations where ages are now magnetostratigraphically well established they are given in parentheses the amigang 1 8 2 6 ma ganjia 2 6 3 6 ma and herjia formations 3 6 to ca 7 0 7 8 ma and the older miocene ashigong garang and guidemen formations these rocks document a generally upward coarsening sequence characterized by increasing accumulation rates increasing gravel content and sizes of its components changes of bedding dips and source rock types and marginal growth faults collectively reflect accelerated deformation and uplift of the ne tibetan plateau after 8 ma punctuated by a sharp increase in sedimentation rate at ca 3 2 ma that reflects the boulder conglomerates of the ganjia formation interestingly much of the vergence of the compressional deformation in the basin is to the south accommodated by a sequence of six thrusts f1 f6 which become active one by one progressively later toward the south undoubtedly contributing to the uplift of this part of the plateau f1 likely initiated the guide basin due to crustal flexure in the oligocene f2 was active in the early miocene f4 and f5 at ca 3 6 ma and f6 was active in the early pleistocene the detailed late miocene and younger magnetostratigraphy allows us to place much improved time constraints on the deformation and hence uplift of northeastern tibet which when compared with ages for events on other parts of the plateau provides important boundary conditions for the geodynamical evolution of tibet

new paleomagnetic constraints on central asian kinematics displacement along the altyn tagh fault and rotation of the qaidam basin received 23 july 2001 revised 9 january 2002 accepted 26 february 2002 published 5 october 2002 1 in order to better understand the tectonic evolution of central asia under the influence of the india asia collision we carried out a paleomagnetic study of 1500 cores from 106 sites along the altyn tagh fault in the qaidam and tarim basins and on the tibetan plateau samples were mainly collected from jurassic to neogene siltstones and sandstones in most cases stepwise thermal demagnetization unblocks low and high temperature components carried by magnetite and hematite low temperature components are north and down directed and lie close to the recent geomagnetic field high temperature components from 10 of 13 age locality groups pass fold and or reversal tests and likely represent primary remanent magnetizations the ten overall mean directions display a complex pattern of vertical axis block rotations that are compatible with a tectonic model of clockwise rotation of the qaidam basin and concomitant left lateral slip on the altyn tagh fault two of the ten localities are rotated significantly counterclockwise they lie adjacent to the altyn tagh fault zone consistent with the idea that left lateral strike slip motion occurred along it the age of counterclockwise rotation near the eastern extremity of the fault was dated as younger than 19 ma three widely spread areas within the qaidam basin exhibit similar and significant clockwise rotations on the order of 20 with respect to the north china block tarim and eurasia the mean of the three values is thought to represent the total rotation of qaidam because the youngest rocks displaying clockwise rotations are oligocene the main phase of qaidam basin rotation and hence shear on the altyn tagh fault took place after or near the end of the oligocene 24 ma upper neogene strata located on the qaidam basin are not significantly rotated thus tectonic deformation acting since the upper neogene 5 ma is not resolvable by paleomagnetic methods given a 20 5 clockwise rotation of the qaidam basin with respect to the tarim basin the maximum left lateral displacement on the altyn tagh fault since 24 ma is 500 130 km index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 1527 geomagnetism and paleomagnetism paleomagnetism applied to geologic processes 8105 tectonophysics continental margins and sedimentary basins

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

sinistral transpression along the main boundary thrust in amritpur area southeastern kumaun himalaya india abstract outcrop mapping and structural analyses in a 100 m thick fault damage zone elucidate geometry and kinematics of the main boundary thrust in the study area the north dipping fault damage zone is sandwiched between the proterozoic amritpur granite in the hangingwall and the neogene siwalik sediments in the footwall the amritpur granite is overlain by the volcanic sedimentary sequence along a top to the south thrust the footwall sediments are deformed into gentle upright and steeply plunging folds by contrast the hangingwall granite is generally massive the km scale folds in the sub himalayan sediments are geometrically discordant to the folds in the lesser himalayan volcanic sedimentary sequence the damage zone contains both the sub himalayan sediments and the lesser himalayan amphibolite it consists of an imbricate structure tectonic wedges and pressure solution bands the imbricate structure is made up of seven fault bounded lithotectonic domains that record the imprints of coaxial refolding and subsequent modification of the folds into sheath like structures the geometry and kinematics of mesoscopic structures and the results of paleostress analysis reveal a sinistral wrench accompanied n s to nne ssw directed subhorizontal shortening in the damage zone the aforesaid evidences also help distinguish between pure contractional pure wrench contraction dominated and wrench dominated deformation in the different domains and the tectonic wedges the sinistral transpression in the damage zone is ascribed to the occurrence of an oblique lateral ramp along the main boundary thrust in the study area

preliminary magnetostratigraphy of dabusan lake qaidam basin central asia abstract sediment samples from a 528 m deep borehole located about 60 km north of golmud on the northern side of dabusan lake have been used to determine the change in magnetic inclination with depth normal polarity brunhes inclination values are present throughout the section but there are six departures from this value five with negative inclinations and one shallow inclination an attempt has been made to date the core using the magnetic inclination departures together with a limited number of 14c dates

a jackknife for magnetostratigraphy we investigate the problem of evaluating the reliability of a magnetostratigraphic section using a jackknife resampling scheme we define a parameter j as the slope relating the percentage of polarity zones of a sampled section remaining after random deletion of sites increasing in proportion to a total of 20 of the initial collection so defined j quantifies the dependence of a magnetostratigraphic record on the distribution of sampling sites based on simulated magnetostratigraphic sections j was found to be an indicator of the approximate percentage of polarity zones present in the original reference sections that were recovered in the sampled section j will perhaps be most helpful as a guide to investigators as to whether further sampling would provide useful information as well as to the non paleomagnetist user of black and white interpretive polarity logs as an estimate of the reliability of a given section

cenozoic qaidam basin china a stronger tectonic inversed extensional rifted basin qaidam basin is a composite sedimentary basin developed on typical continental crust it comprises a jurassic foreland basin and a cenozoic extensional basin the geologic history of the cenozoic qaidam basin can be divided into two megastages on the basis of stratigraphy and structure the first megastage from latest cretaceous to oligocene consisted of two periods of rifting due to upwelling of the hot upper mantle this is demonstrated by two tectono sequences formed by the rifting the synrifting magmatic thermal activity and the control of protobasin development by syndepositional faults the second megastage comprises three tectono sequences that developed in the miocene and pliocene and was a period of structural inversion that consisted of compressive downwarping and reverse faulting this tectonic inversion may be ascribed to the long distance propagation of compressive stress caused by the collisions of the indian and eurasian plates the thick sequence of primary oil source rocks was deposited in the major rifted depression and the restricted drainage graben of the rifted protobasin during the tectonic inversion megastage they were subject to deep burial and prolonged heating a major and a minor oil generating basin have developed the tectonic inversion processes produced several structural features that may contain potential hydrocarbon reservoirs and traps

new insights into the appalachian orogen from geology and geochronology along the cape ray fault zone southwest newfoundland reverse oblique overthrusting of high grade port aux basques gneiss over the lower grade supracrustal windsor point group along the cape ray fault zone occurred during late silurian to early devonian time and is attributed to major oblique collision between gondwanan and laurentian continental blocks new u pb zircon 40 ar 39 ar and k ar data limit the timing of this orogenic event the megacrystic granitic phase of the cape ray igneous complex and the cape ray tonalite have been dated at 488 3 ma and 469 2 ma respectively a black rhyolite from the windsor point group yielded an ordovician age 453 5 4 ma a pretectonic gabbro sill within the windsor point group dated at 424 4 3 ma is cut by the windowglass hill granite dated at 424 2 ma synkinematic hornblende and biotite from the grand bay complex mylonites of the port aux basques gneiss have 40 ar 39 ar ages of 407 4 ma and 403 4 ma respectively which is indicative of a late silurian early devonian age for the amphibolite grade deformation these ages are compatible with a previously determined 415 ma monazite age interpreted as the time of peak metamorphism in the port aux basques gneiss and suggests rapid exhumation the east west flexure of the cape ray fault zone represents a tear fault accommodating differential shortening between the cape ray fault zone and the correlative fault system in central newfoundland mid devonian granites 386 3 384 2 ma intervene between the thrusting events and a younger sinistral transcurrent movement this deformation and magmatic history can be correlated throughout the northern appalachians along other major fault boundaries

development and growth of basement involved structural wedges in the northwestern qaidam basin china structural wedges contain two connected fault segments a fore thrust and back thrust that bound a triangular or wedge shaped fault block coeval displacements on both faults drive the wedge into surrounding rock causing distinctive patterns of uplift and folding we identified a series of thick skinned structural wedges in northwestern qaidam basin china where syntectonic strata record deformation timing and kinematics the qaidam basin is located at the northern margin of the tibetan plateau and contains more than 100 large anticlinal structures many of which exhibit characteristics of structural wedges we interpreted and modeled these structures using seismic reflection data well logs surface geological exposures remote sensing images and digital elevation data these wedge structures are distinguished by having elevated and deformed strata in the footwalls of the back thrusts and synclinal folds that extend upward from the wedge tips with the constraints of fold shape growth strata structural relief and shallow fault geometry we developed a series of forward models that describe the geometry and kinematic evolution of these wedge structures the analysis suggests that some of the wedge structures reactivate preexisting normal faults our results provide a better understanding of how to identify thick skinned wedge systems and model their geometry and kinematic evolution which has implications for studying the tectonic history of the basin understanding the geometry and kinematics of the wedge structures is also crucial for petroleum exploration because both hanging wall and footwall traps constitute an important component of current exploration targets

a rb sr record of the weathering response to environmental changes in westerly winds across the tarim basin in the late miocene to the early pleistocene the long term record of lacustrine deposits in the tarim basin seems to provide a record of climate influenced by westerly winds to characterize the evolution of westerly winds we analyzed concentrations of rb v zn sc and sr for the drill core ls2 from lop nor in the eastern tarim basin the rb sr ratio in lacustrine deposits depends primarily on the sr element activity during weathering after the influence of source provenance has been removed we assume that with such a correction the rb sr ratio is controlled by precipitation which depends on moisture transport by westerly winds to the tarim basin the rb sr ratio displays a large amplitude oscillation that generally follows a pattern similar to that of pollen and ostracod assemblages which support the inference that rainfall amount affects the rb sr ratio and in turn suggests that it may serve as an index of precipitation that was transported by westerly wind all these climatic proxies obviously changed since the late miocene suggesting stronger aridification that may be attributed to a latitudinal shift in westerly winds since the late miocene a southward migration of westerly winds which then transported more moisture to the southern and eastern parts of the tibetan plateau agrees with modeling results and the geological record during that time this event may be causally related to the expansion of the northern hemisphere ice and or regional tectonic uplift of high mountains around the tarim basin

magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china abstract magnetostratigraphic and rock magnetic results from 1006 horizons in paleogene and neogene sediments between the upper kumugeliemu formation and the base of the kuche formation within the kuche depression of the tarim basin nw china are used to evaluate postcollisional uplift and deformation in the tian shan range characteristic remanences with both normal and reversed polarity were isolated by thermal demagnetization generally between 300 and 690 c and a composite magnetostratigraphy is constructed from data at 969 accepted levels correlating with polarity chrons c12n to c3r dated between 31 0 and 5 5 ma on the ck95 geomagnetic polarity time scale observed height dependent changes of rock magnetic parameters km and ams ellipsoid parameters indicate that these sediments were influenced by weak deformation with the succession accumulated before 15 ma recording the effects of compressive deformation this finding together with two substantial increases in accumulation rate at 16 17 and 7 ma define a framework for cenozoic uplift and deformation of the tian shan range we argue that thrusting in the south of the range was probably initiated at 20 ma and followed by rapid uplift at 16 17 ma on the assumption that sediment accumulation in western china was still predominately tectonically controlled at 7 ma another later pulse of rapid uplift with a much larger regional influence is also recognized in the tian shan the kuche depression was subjected to two postcollisional phases of rotational deformation with clockwise rotation prior to 12 ma 1 36 ma succeeded by counterclockwise rotation at 1 64 ma after 12 ma and possibly continuing to the present day this rotational deformation is not temporally related to the history of basin formation resolved from the magnetostratigraphy and may be linked to block interactions with adjoining terranes

plateau uplift forcing climate change around 8 6 ma on the northeastern tibetan plateau evidence from an integrated sedimentary sr record abstract the uplift of the northern tibetan plateau tp has long been regarded as one of major factors accounting for both the enhanced process of aridification recorded in central asia and the shifts in the asian monsoonal circulations that are known to have occurred at approximately 8 ma until now there have been few sedimentary records reported on which can directly and in an integrated fashion shed light on the region s tectonic and climate regimes this can be largely attributed to the fact that the influence exerted by the presence of source materials and subsequent sedimentary sorting in this tectonically active region would be likely to bias significantly any paleoclimatic interpretation of silicate derived proxies based on detailed investigations of strontium sr distributions revealed by multi step leaching in a fluvial sequence from the linxia basin which includes paleosols we reconstructed the long term late miocene regional climatic and environmental changes likely to have occurred on the northeastern tp from 12 1 to 5 2 ma we investigated sr related proxies in two typical paleosol profiles to constrain sr mobility in paleosol weathering and thus to further prompt the interpretation of sr related proxies along the whole section we would suggest that carbonate derived sr concentrations and sr ca ratios are suitable proxies for tracing regional climate change associated with weathering and pedogenesis together they exhibit weakened chemical weathering intensity and pedogenesis at 8 6 ma the concomitant appearance of immature sediments through poor sedimentary sorting revealed by silicate derived rb sr 87 sr 86 sr and eu anomaly eu eu alongside a sharp increase in sedimentation rates noted at this time suggests that it was indeed tectonic uplift that principally controlled the dramatic climatic and environmental changes this multi proxy record of concomitant tectonic uplift and climate change in the linxia basin provides direct geological evidence for uplift driven climate change in the northeastern tp region

late eocene clay boron derived paleosalinity in the qaidam basin and its implications for regional tectonics and climate abstract the qaidam basin located on the northeastern tibetan plateau and containing cenozoic sediments with a maximum thickness of 12 000 m is an ideal place to study the phased uplift of the ne tibetan plateau and regional climate change the estimation of the paleosalinity of sedimentary environments not only helps to evaluate the evolution of lakes in this region but offers insights into contemporaneous climate change we present detailed geochemical and mineralogical investigations from the lacustrine interval of the hongliugou section in the northern qaidam basin to reconstruct salinity fluctuations in the paleolake during the late eocene era 42 0 35 5 ma the clay mineral assemblages mainly contain smectite illite chlorite kaolinite and irregular illite smectite mixed layers clay boron derived paleosalinity estimates equivalent boron content couch s paleosalimeter and b ga ratios along with other proxies sensitive to salinity changes e g rb k ratios and ostracod assemblages collectively indicate an overall brackish sedimentary environment with a higher salinity period at approximately 40 0 39 2 ma this higher salinity period indicates a more arid environment and is probably related to global cooling however the global cooling in late eocene cannot explain the overall stable long term salinity pattern implying that other factors exist we propose that the migration of the yiliping depression depocenter in the northern qaidam and increased orographic rainfall induced by late eocene tectonic activity at the northern margin of the basin might have partly offset the increase in salinity driven by global cooling

maximum effective moment criterion and the origin of low angle normal faults abstract the origin of the low angle normal faults of metamorphic core complexes has been debated for over two decades proponents of andersonian fault mechanics have long argued that it is mechanically unfeasible for slip to occur along shallowly dipping normal faults a new theory named the maximum effective moment criterion is proposed here for their origin using an effective moment approach formulated as m eff fh where f is the force tangentially acting on the unit shear boundaries and h represents its arm the maximum value appears at angles of 54 7 with the 1 since extensional crenulation cleavages eccs and the contractional crenulation cleavages cccs occur in conjugate pairs with 110 angle between them it is suggested that they tend to be oriented in the directions of maximum effective moment the differential stress for formation of the ecc or ccc are less than that for fracturing the orientations of conjugate eccs depending on relative magnitudes of simple shear versus coaxial strain components the synthetic ecc set is much better developed than the antithetic set due to anisotropy upward propagation of the synthetic ecc set from mid crustal domains of mylonitization through strain localization and strain softening is considered an effective mechanism for the formation of the low angle normal faults of metamorphic core complexes

regional slaty cleavage formation and fold axis rotation by re use and reactivation of pre existing foliations the fiery creek slate belt north queensland abstract a deformation history comprised of six separate deformation events of differing intensity has affected the rocks of the south palmer river region of the the hodgkinson province north queensland within this region a zone of pervasive slaty cleavage herein termed the fiery creek slate belt has developed as a result of the superposition of fabrics formed during several of these events the most important processes in the formation of this composite cleavage were the re use and reactivation of the favourably oriented steep n s trending s2 foliation by the intense fourth deformation event d4 this produced micro meso and macroscopic folds in an originally shallow s3 foliation produced during the intervening d3 deformation with an axial planar s2 s4 foliation the d4 stretching lineation l44 plunges subvertically to steeply north and indicates that shear during d4 was oriented steeply north south in the fiery creek slate belt d2 fold axes are interpreted to have formed in much shallower orientations than their present moderately n s plunging to subvertical orientations we consider this to be a result of d4 shear which caused variable degrees of rotation of d2 fold axes toward the d4 stretching lineation due to subparallelism of the bulk shortening directions of the d2 and d4 events near total destruction of the pre d4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history there is no relationship between metamorphic grade and intensity of slaty cleavage development

formation and re orientation of the suerkuli basin within the altyn tagh in northeastern tibetan plateau since late miocene abstract the altyn tagh fault atf had formed in eocene time however the temporal evolution of slip along the fault remains elusive despite its important role in governing growth of the tibetan plateau tp and evaluating models of continental lithosphere deformation here we perform a high resolution magnetostratigraphic study on neogene sediments from the suerkuli basin a small basin located between two active strands of the atf results show that both depositional environment and the sedimentation rate changed at about 5 4 and 6 ma respectively lithofacies shift from conglomerates and sandstones with parallel and cross beddings to mudstones with horizontal bedding suggesting a sedimentary fluvial to lacustrine depositional environment shift even aridification increased over northern china from the tarim basin to the qaidam basin during late miocene to early pliocene times lacustrine deposition had started in the suerkuli basin still we argue that an increase in slip rate along the atf triggered concurrently forming of the intermontane basin the inferred timing of increased slip rate on the atf coincides with a variation in movement of the indian plate relative to eurasian plate since late miocene early pliocene time our work suggests that a re orientation of crustal shortening and deformation in the northern tp began since late miocene and supports the idea that the modern boundary of the tp orogen had rejuvenated to adjust deformation in plate boundary conditions

northward growth of the qimen tagh range a new model accounting for the late neogene strike slip deformation of the sw qaidam basin abstract situated along the western termination of the eastern kunlun mountains the qimen tagh range represents a key area to understand the cenozoic basin range interactions between the northeastern tibetan plateau and the qaidam basin within that region several huge bow like fault systems such as the kunbei and qimen tagh fault systems accommodate the transpressive deformation but their kinematic evolution is still highly debated newly acquired seismic profiles and isopach maps of the late eocene sediments strongly suggest that the kunbei fault system consisting of the kunbei arlar and hongliuquan faults in the southwestern qadaim basin was initially a left lateral strike slip fault system rather than a thrusting system growth strata indicate an early miocene onset age for this strike slip deformation however earthquake focal mechanisms show that the present day tectonic pattern of this fault system is dominated by ne sw transpression as for the qimen tagh fault system numerous linear geomorphic features and fault scarps indicate that it was again a strike slip fault system deformed sediments within the adatan valley prove that strike slip motion prevailed during the pleistocene yet the present day deformation is marked by ne sw transpression collectively the kunbei and qimen tagh fault systems were initially left lateral strike slip fault systems that formed during early miocene and pleistocene respectively colligating with these southward younging left lateral strike slip faulting ages and the fact that these convex northward structures converge to the center segment of active kunlun fault in the east we thus considered the kunbei and qimen tagh fault systems as former western segments of the kunlun fault once located further south in the present day location of that fault these faults gradually migrated northward since the early miocene while their kinematics changed from left lateral strike slip motion to ne sw transpression

magnetochronology of the upper cenozoic strata in the southwestern chinese tian shan rates of pleistocene folding and thrusting abstract the southwestern chinese tian shan of central asia is an actively deforming part of the indian asian collision system paleomagnetic investigations of two plio pleistocene terrestrial successions provide the first detailed magnetostratigraphy for the upper cenozoic strata in this region paleomagnetic samples were collected from 358 sites within the atushi formation and xiyu formation across the atushi talanghe anticline near atushi thermal demagnetization behavior reversal and fold tests of paleomagnetic stability indicate that the characteristic remanence directions were acquired before tilting and folding of the strata a composite magnetostratigraphic section for this sequence correlates with the mammoth subchron to the jaramillo subchron between 3 3 and 1 07 myr of the geomagnetic polarity timescale of cande and kent j geophys res 100 1995 6093 6095 the mean declination 349 3 in the boguzihe section indicates a counterclockwise vertical axis rotation 11 2 during the past 1 4 myr the conglomeratic xiyu formation is time transgressive along its progradational contact with the underlying atushi formation across a map distance of 6 km the basal contact ranges from less than 1 0 myr in the ganhangou section to 1 9 myr in the boguzihe section to the southwest and to 2 8 myr on the northwest limb of the anticline northeastward lateral propagation and growth of the atushi talanghe anticline initiated at 1 4 myr in the boguzihe and 1 2 myr in the ganhangou the cross section in the boguzihe provides a conservative maximum estimate of shortening rate of 3 3 4 4 mm yr and uplift rate of 3 mm yr this rate represents about half of the geodetically determined shortening rate between the northern tarim basin and the kyrgyz tian shan wang et al acta seismol sin 22 2000 263 270 reiger et al earth planet sci lett 191 2001 157 165 during fold growth over the past 1 4 myr the crest of the atushi talanghe anticline eroded at an average rate of 2 6 2 7 mm yr

the chaparral shear zone deformation partitioning and heterogeneous bulk crustal shortening during proterozoic orogeny in central arizona the proterozoic chaparral shear zone of central arizona is one of a network of subvertical northeast striking shear zones that divide the proterozoic orogenic belt of arizona into tectonic blocks the zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation stretching lineations plunge shallowly northeast and displacement across the zone was dominantly right lateral several lines of evidence constrain displacement across the zone to be greater than 5 km but probably less than tens of kilometers 1 stratigraphic and plutonic rocks can be correlated across the zone 2 structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar and 3 integration of shear strain estimated by deflection of earlier fabric suggests greater than 5 km of strike slip more important than scale of movement across the zone structural studies have clarified several aspects of the assembly history of the orogen early north to northwest striking s 1 foliation on both sides of the chaparral shear zone records one or more deformational events that took place between 1 75 and 1 7 ga early fabrics record northeast southwest or east west shortening and west verging thrusting that we interpret to have formed during development of a primitive arc complex in contrast intense deformation in the chaparral shear zone took place during regional northwest southeast shortening d 2 that produced the dominant subvertical northeast striking foliation and the present block architecture of the orogen d 2 is interpreted to have been in part synchronous with emplacement of 1 70 ga granitoids and to record assembly of volcanic belts to north america of general interest is the character of d 2 deformation partitioning in and across the shear zone two types of high strain domains were generated during shortening one type represented by rocks southeast of the shear zone involved extreme shortening and transposition by folding the other type represented by the shear zone involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones overprinting relationships suggest that the second type may have nucleated on the first implying an important component of deformation partitioning in time as well as space mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing conjugate shear bands suggest a shallow southeast plunging 1 late during strike slip deformation this is consistent with regional d 2 shortening but not with the incremental shortening direction expected during right lateral simple shear this supports a regional kinematic model in which the chaparral shear zone right lateral and a temporally related left lateral shear zone were regional scale conjugate shear zones that accommodated heterogeneous d 2 northwest southeast orogenic shortening via escape block tectonics thus orogen parallel strike slip displacements in the chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction

intermontane basin development in the past 4 myr in the north west himalaya the through the combined use of magnetic polarity stratigraphy with fission track dating of volcanic ashes a new chronology spanning 4 myr has been developed for the intermontane basin of kashmir in the northwestern himalaya

provenance of neogene eolian red clay in the altun region of western china insights from upb detrital zircon age data aridity of the asian interior plays an important role in the accumulation of eolian deposits in both eastern monsoon regime and western china westerly wind regime a better understanding of the provenance of those eolian deposits loess and red clay will shed light on the history and mechanisms of asian aridification in eastern china decrease in grain size from northwest to southeast shows that the neogene red clay of the chinese loess plateau clp was derived from the desert and arid lands of northwestern china by the east asian winter monsoon however in western china outcrops are limited and this is an obstacle to studies of the spatial variation of the provenance of eolian deposits we use upb geochronology of detrital zircons to determine the provenance of the altun red clay a recently discovered and continuous eolian deposit in western china our comparison of detrital zircon age spectra for the altun red clay with those of potential source regions and with results for the coeval red clay of the clp indicates that 1 the main zircon age components of the altun red clay are very different from those of the red clay on the clp suggesting that these deposits were sourced from different areas and 2 the altun red clay was likely sourced from the taklamakan desert and transported via westerly winds

structural styles and stratigraphic patterns of syndepositional faults in a contractional setting examples from quaidam basin northwestern china seismic and well studies have shown that thrust faults high angle reverse faults and transpressive faults are three typical styles of syndepositional faults that developed in the quaidam basin during its tertiary compressive and transpressive evolution these faults are characterized by intervals that are thicker within the footwall than within the hanging wall as well as by clear differences in lithology of contemporary sediments across the faults syndepositional thrust faults with listric geometry generally act as the leading front of the thrust belt and control the wedge shaped geometry of basin or compressive fault depression thickening toward the fault the hanging wall folds were formed by horizontal shortening after the main faulting period during which the upthrown side of the fault was tilted upslope due to the faulting along listric plane syndepositional reverse faults are planar having steeper 60 80 planes rooting into basement the strata on the upthrown and downthrown blocks generally are flat the syndepositional transpressive faults have either a steep dipping more planar fault plane or a plane steepening downward transpressive deformation after the faulting created the hanging wall folds wedge shaped sediment that thickens toward the fault flat sediment or wedge shaped sediment that thins toward the fault are three typical geometries of compressive fault depressions which are likely to be bordered by syndepositional thrust faults reverse faults and transpressive faults respectively lithologically the sides of the syndepositional faults of the inner quaidam basin have coarser sediments on the hanging wall than on the footwall the eroded hanging wall of the marginal fault supplied the sediments for the footwall these sediments change quickly basinward from coarse clastic deposits into finer grained deep water sediments in the quaidam basin the hanging wall anticlines have proven to be good hydrocarbon traps the syndepositional faults can act as a migrating path connecting the reservoirs on the hanging wall and the source rocks on the footwall

pliocene uplift of the northern tibetan plateau neogene redbeds passing upward into upward coarsening conglomerate and debris flow deposits at the foot of the kunlun mountains record the change in paleoslope related to uplift of the surface of the northern tibetan plateau detailed magnetostratigraphy of a 4 5 km section near yecheng in the western kunlun mountains shows that the change from deposition on distal alluvial plains to proximal alluvial fans occurred during the gilbert reversed chron 4 5 3 5 ma the change in depositional facies was accompanied by an increase in sedimentation rate from an average 0 15 mm yr between the earliest oligocene and the earliest pliocene to 1 4 mm yr in the gauss normal chron 3 6 2 6 ma we interpret the change in depositional facies and increase in sedimentation rate as indicating that the main uplift of the northwestern tibetan plateau began ca 4 5 ma

paleoproterozoic tectonic history of the cerbat mountains northwestern arizona implications for crustal assembly in the southwestern united states structural mapping metamorphic analysis and u pb geochronology from the cerbat mountains of northwestern arizona place new constraints on the tectonic significance and timing of the boundary between the mojave and yavapai proterozoic crustal provinces in the southwestern united states the cerbat mountains are the easternmost range that exhibits wholly mojave pb isotopic signatures yet their paleoproterozoic structural history is strikingly similar to that of the yavapai province two pervasive deformational events are recognized in the cerbat mountains d1 fabrics are well preserved in the northern cerbat mountains and are characterized by a north northwest striking moderately northeast dipping foliation s1 that is axial planar to macroscopic recumbent folds that we interpret to be west to southwest vergent similar structures have been documented locally in the yavapai province d1 is bracketed between 1740 and 1721 ma in the cerbat mountains on the basis of u pb zircon ages of the big wash granite 1737 4 3 ma and the diana and chloride granites 1719 1 2 and 1721 2 4 ma respectively d2 fabrics dominate the central and southern cerbat mountains and include a subvertical northeast striking foliation that truncates and transposes s1 d2 is bracketed between 1721 and 1682 ma on the basis of u pb ages of the diana and chloride granites and post d2 granite dikes in the eastern cerbat mountains 1682 5 2 4 ma the structural style orientation and timing of d1 and d2 fabrics in the cerbat mountains are identical to those of the yavapai province supracrustal rocks within the cerbat mountains were subjected to two periods of metamorphism that correspond temporally to the two periods of deformation the early phase occurred at 675 725 c and 5 6 kilobars the later phase occurred at 650 700 c and 3 5 4 5 kilobars we propose that the mojave and yavapai provinces were sutured together between ca 1740 and 1720 ma during regional d1 deformation collisional orogenesis in the eastern mojave province may have resulted in the tectonic burial of rocks in the cerbat mountains to depths of 20 km d2 between 1720 and 1682 ma corresponds to the widespread yavapai orogeny and is interpreted to record accretion of the amalgamated mojave yavapai terrane to proterozoic terranes that are now located in colorado

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

magnetostratigraphy of paleogene sediments from northern qaidam basin china implications for tectonic uplift and block rotation in northern tibetan plateau abstract we have carried out a detailed magnetostratigraphic study of tertiary sediments in the northern margin of the qaidam basin northern tibetan plateau a total of 400 paleomagnetic samples with an interval of about 2 m were collected from the shanggancaigou and xiagancaigou formations in the hongsanhan section stepwise thermal demagnetization was used to isolate the magnetic components in these samples a higher temperature component htc was separated between 300 and 680 c from most of samples positive fold and reversal tests suggest a primary origin for the htc a magnetic polarity sequence is established and then compared with the geomagnetic polarity time scale ostracoda assemblages and also a fission track age from an adjacent section suggest a tertiary age of the shanggancaigou and xiagancaigou formations in the hongsanhan area the magnetozones in the xiagancaigou formation may correspond to subchrons c16n c18 ranging from 40 to 35 5 ma mid eocene and the shanggancaikou formation to subchrons c8 2n c15 age from 35 5 to 26 5 ma late eocene to late mid oligocene the accumulation of sandy conglomerate and conglomerate layers on top of the shanggancaikou formation and an abrupt increase in sedimentation rates are probably resulted from the main uplift of the north qaidam at about 30 ma when compared with coeval directions expected from eurasia the mean declinations indicate a slight vertical axis rotation of the northern qaidam basin since the mid eocene sinistral shear strain may be concentrated on the altyn tagh fault atf

late cenozoic sporopollen records in the yangtze river delta east china and implications for east asian summer monsoon evolution abstract the late cenozoic east asian summer monsoon easm is generally linked with the tibetan plateau uplift and or global cooling in this study four cores zk004 zk005 sz03 and sz04 consisting of fluvial lacustrine deposits spanning the period of 8 0 0 ma were obtained from the yangtze river delta east china for sporopollen analysis to investigate trends in the easm evolution and its driving forces the results show that high percentages of arboreal taxa such as fagaceae ulmaceae juglandaceae taxaceae taxodiaceae liquidambar anacardiaceae rutaceae etc represent a relatively warm and humid climate between 8 0 and 2 6 ma the herb dominated vegetation poceae asteraceae cyperaceae etc is linked with a relatively cold and dry climate after 2 6 ma the compiled thermophilic percentages and ap arbors nap non arbors ratios of these four cores which show that the long term cooling and drying trends are similar to the global cooling since 8 0 ma imply that the easm experienced long term weakening coincident with the global cooling however over this super long time scale the thermophilic percentages and ap nap ratios have remained at roughly stable levels since 2 6 ma possibly due to the effects of the tibetan plateau uplifts