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

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

insight into the cenozoic tectonic evolution of the qaidam basin northwest china from fracture information fractures can provide valuable information for tectonic evolution according to the data of outcrops cores thin sections and well logs the tectonic fractures in the qaidam basin can be divided into four types small faults including small normal fault and small reverse fault vertical open fracture bedding plane slip fracture and horizontal open fracture our fracture observations provide new constraints on the cenozoic tectonic evolution of the qaidam basin syn sedimentary small normal faults in the paleogene strata indicate the extension deformation during the paleogene small reverse faults vertical open fractures and bedding plane slip fractures occurred in the paleogene and neogene strata have genetic relationship according to the burial history and homogenization temperature of fluid inclusions of gypsum and calcite filled in the vertical open fractures it can be deduced that the vertical open fractures being formed mainly from the late miocene shangyoushashan formation with age of 5 1 ma to the end of pliocene shizigou formation with age of 2 6 ma indicating small reverse faults vertical open fractures and bedding plane slip fractures were simultaneously formed in the neogene these fractures were resulted from the compression deformation the horizontal open fractures occurred in the paleogene neogene and quaternary strata with apertures and intensities decreasing with depth were formed by the large scale quick uplift and denudation resulted from the strong compression deformation since the quaternary

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

the process and mechanism of the rise of the qinghai tibet plateau abstract during the period from the late cretaceous to eocene the qinghai tibet plateau started a new stage of continental collision crustal shortening and thickening and continuous rise the rising process of the plateau progressed through three stages rise due to subduction collision rise due to convergence compression and rise due to isostatic adjustment the crustal thickening and shortening of the plateau occurred through different modes of motion of the layers of the earth s crust the model of the plateau rise may be generalized as intracontinental convergence crustal thickening isostatic adjustment

temperature and pressure field in the tertiary succession of the western qaidam basin northeast qinghai tibet plateau china abstract based on pressure and temperature data a comprehensive analysis was carried out for the pressure and temperature regimes in four tertiary formations of the western qaidam basin northeast qinghai tibet plateau the pressure data were obtained from well pressure tests and the temperature data from systematic steady state temperature measurements in 10 wells temperature tests in over 200 wells and calculation for 60 theoretical calculated points in general the geothermal gradient in the four oligocene pliocene formations decreases with depth the pliocene upper youshashan formation has the highest gradient of 33 c km the miocene oligocene upper and lower ganchaigou formations have lower gradients of 25 27 5 c km in most areas but also several lower gradients down to 20 c km abnormally high pressures were found at depths of 2650 4350 m beneath the regional seal in the western qaidam basin the abnormal high pressure is mainly confined to the lower ganchaigou formation with a pressure coefficient up to 1 9 most parts of the pliocene upper and lower youshashan formations are in normal and low abnormal high pressure r 4 mg l indicates the formation was in a closed chemical environment of fluid flows over the majority of geological time the reservoirs in this formation all occur in an abnormally high pressure zone with a pressure coefficient of more than 1 5

cenozoic deformation history of the qaidam basin nw china results from cross section restoration and implications for qinghai tibet plateau tectonics abstract the deformation and uplifting history of the qinghai tibet plateau is a controversial issue and so far there has been insufficient evidence for interpretation investigations of the cenozoic deformation in the northeastern edge of this plateau may provide some relevant information in this paper the cenozoic deformation history of the qaidam basin has been studied based on results from the balanced cross section restoration of six geological sections they indicate that 1 the qaidam basin has been undergoing continuous shortening since the beginning of cenozoic with two relatively fast shortening phases the first during 42 8 40 5 ma and the second during 2 8 ma present 2 the shortening rate of the qaidam basin reached its maximum value since 2 8 ma and approximately 32 of the average total cenozoic shortening took place in this period and 3 the shortening of all sections reached approximately 50 of their respective total shortening by miocene the cenozoic deformation history of the qaidam basin shows good synchroneity with uplifting history of the qinghai tibet plateau it is therefore proposed that the deformation and uplifting in the south and north edges of the qinghai tibet plateau was almost synchronous with the altyn tagh fault formed at the beginning of india asia collision accordingly it is postulated that since the india asia collision at the beginning of cenozoic the indian plate moved northward continuously with two relatively fast phases the first during 42 8 40 5 ma and the second during 2 8 ma present approximately 50 of the total cenozoic northward displacement of the indian plate of the total amount of crustal shortening and uplifting of the qinghai tibet plateau and of the total cenozoic strike slip displacement of the altyn tagh fault were probably reached by the early miocene moreover another 30 40 might have taken place during 2 8 ma present the periods during 42 8 40 5 ma and during 2 8 ma present may also be the two fast phases for crustal shortening and uplifting of the qinghai tibet plateau and strike slip displacement of the altyn tagh fault

tectonic and sedimentary evolution of basins in the northeast of qinghai tibet plateau and their implication for the northward growth of the plateau the northeast of the qinghai tibet plateau from the hoh xil basin to the hexi corridor basin demonstrates a basin ridge geomorphy that is the result of the long term uplift of the qinghai tibet plateau the geological evolution of the qinghai tibet plateau is recorded in its associated basins from the cenozoic sedimentary filling pattern in the hoh xil qaidam and jiuquan basins we find that the evolution of these basins is similar they began as strike slip basins evolved into foreland basins and then to intermontane basins foreland basins are the direct result of orogenic activity in the northeast of qinghai tibet plateau and can be documented to have formed over the following time periods 49 23 ma for the hoh xil foreland basin 46 2 45 ma for the qaidam foreland basin and 29 5 0 ma for the jiuquan foreland basin this northward migration in the timing of foreland basin development suggests that the northeastern part of tibet grew at least in part by accretion of cenozoic sedimentary basins to its northern margin

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

cenozoic tectonic evolution of the qaidam basin and its surrounding regions part 3 structural geology sedimentation and regional tectonic reconstruction the qaidam basin is the largest topographic depression inside the tibetan plateau because of its central position understanding the tectonic origin of the qaidam basin has important implications for unraveling the formation mechanism and growth history of the tibetan plateau in order to achieve this goal we analyzed regional seismic reflection profiles across the basin and a series of thickness distribution patterns of cenozoic strata at different time slices the first order structure of the basin is a broad cenozoic synclinorium which has an amplitude ranging from 16 km in the west to 48 in the west to 15 s 1 to 1 3 10 17 s 1 the eastward decrease in upper crustal shortening requires a progressive shift in crustal thickening mechanisms across qaidam basin from dominantly upper crustal shortening in the west to dominantly lower crustal shortening in the east although sedimentation began synchronously at 65 50 ma across the entire basin the initiation ages of the southern and northern basin bounding structures are significantly different deformation started at 65 50 ma in the north and at 29 24 ma in the south this information and the existing inference that the uplift of the eastern kunlun range south of qaidam basin began after 30 20 ma imply that the paleogene 65 24 ma qaidam and hoh xil basins on both sides of the eastern kunlun range may have been parts of a single topographic depression 500 km wide in the north south direction between the qilian shan and fenghuo shan thrust belts in the north and south the development of this large paleogene basin in central tibet and its subsequent destruction and partitioning by the neogene uplift of the eastern kunlun range requires a highly irregular sequence of deformation possibly controlled by preexist ing weakness in the tibetan lithosphere

magellan initial analysis of venus surface modification initial magellan observations reveal a planet with high dielectric constant materials exposed preferentially in elevated regions with high slopes ejecta deposits extending up to 1000 kilometers to the west of several impact craters windblown deposits and features in areas where there are both obstacles and a source of particulate material and evidence for slow steady degradation by atmosphere surface interactions and mass movements

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

diagenesis in porosity evolution of opening mode fractures middle triassic to lower jurassic la boca formation ne mexico abstract opening mode fractures joints in middle triassic to lower jurassic la boca formation sandstones northeastern mexico have patterns of fracture porosity mineral fill structures and size distributions not previously described from outcrop patterns match those found in cores from many basins we used aperture measurements along lines of observation scanlines fracture trace maps petrography high resolution scanning electron microscope sem based cathodoluminescence and fluid inclusions to characterize fracture populations open fractures are lined by quartz that precipitated while fractures were opening whereas sealed fractures additionally contain calcite deposited after fractures ceased opening large fractures and arrays of contemporaneous microfractures have consistent power law aperture size scaling over approximately three orders of magnitude our results imply that open fractures and fracture sizes depend on diagenetic state the interplay of fracture mechanics and diagenetic history is a determinant on effective porosity within fractures and thus open fracture persistence connectivity and fluid flow

fractures in sandstone reservoirs with ultra low permeability a case study of the upper triassic yanchang formation in the ordos basin china the upper triassic yanchang formation in the ordos basin central china is a typical sandstone reservoir with an ultra low permeability high angle tectonic fractures and diagenetic fractures such as near horizontal bedding fractures intragranular fractures and grain boundary fractures are abundant fractures are major pathways and enhance fluid flow in sandstone reservoirs with ultra low permeability because of their weak lateral continuity and their small apertures under lithostatic pressure bedding fractures make a relatively small contribution to the overall permeability of reservoirs as they are both of small size and low permeability intragranular fractures and grain boundary fractures mainly improve the connectivity of reservoirs by connecting the matrix pores high angle tectonic fractures control the fluid movement in the ultra low permeability reservoirs under the effect of different successive tectonic stress fields four assemblages of high angle tectonic fractures developed in the sandstone reservoirs under the present day stress differently oriented fractures have different connectivities apertures and permeabilities the northeast southwest fractures parallel to the maximum principal stress have a good connectivity large apertures and a high permeability forming the dominant flow paths knowledge of these paths can be used for optimizing well placement because of their sensitivity to pressure fractures in different directions will show varying apertures as the formation pressure decreases therefore the permeability of the fractures of different orientations and its impact on reservoirs vary at different developmental stages

single layer burial joints vs single layer uplift joints in eocene chalk from the beer sheva syncline in israel abstract the single layer s l joints that occur in the lower eocene chalks near beer sheva israel developed during the burial history of the rock whereas the s l joints in adjacent middle eocene chalks developed during the uplift stage characteristically s l burial joints occur in orthogonal cross fold and strike sets and as conjugate sets they precede normal faults and multi layer joints and they do not exhibit strike rotation en echelon segmentation or fracture interaction with each other these joints are generally closed and during subsidence older beds fracture first on the other hand s l uplift joints do not occur in orthogonal or conjugate sets they are post strike slip faulting contemporaneous with multi layer joints and exhibit strike rotation en echelon segmentation and often interact with each other they are occasionally opened up to several millimetres and during uplift younger beds fracture first

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

eastward migration of the qaidam basin and its implications for cenozoic evolution of the altyn tagh fault and associated river systems the qaidam basin is the largest cenozoic intermontane basin within the tibetan plateau bounded by the qilian shan range to the north the east kunlun range to the south the altyn tagh range to the west and the ela shan range to the east its deposits consist of nonmarine sedimentary rocks with a maximum thickness exceeding 13 400 m modern sedimentation mainly takes place along the marginal areas of the basin where alluvial fans are formed along a series of rivers that originate from the surrounding mountains nevertheless drilling sedimentary and seismic data reveal that the depocenter of the basin during much of cenozoic time was not developed as foreland basins of the qilian shan belt to the north and the east kunlun belt to the south but instead the depocenter was in the center of the western part of the basin and had a northwest southeast trend that shifted eastward 380 km during oligocene to quaternary time this indicates that most of the basin sediments were not derived from the surrounding mountain ranges the results of the fi ssion track age dating reveal that the rapid uplift of the yousha shan anticline bounding the depocenter to the southwest initiated at ca 31 ma indicating that the southeastward shift of the depocenter began at that time yielding a migration rate for the depocenter of 10 2 mm yr the northwestern margin of the depocenter of the qaidam basin lies to the east of the westward prolongation of the qaidam basin the tula trough a 300 km long narrow trough fl anked by the altyn tagh range along the altyn tagh strikeslip fault on the north and the east kunlun thrust belt on the south the sedimentary and deformational evidence from this region indicates that the tula trough was formed as a syncline along which a paleoriver fl owed to the east into the qaidam basin to the southwest this river valley merges with the trace of the altyn tagh fault the latter and its westward continuation are interpreted to have extended to the northern margin of the pamir folded belt north of the western syntaxis of the himalaya we propose that most of the sediments within the depocenter of the qaidam basin were transported from the northern margin of the pamir folded belt along a progressively lengthening 2000 kmlong longitudinal river that developed along the left lateral altyn tagh fault and its western continuation as the eastward extrusion of the tibetan plateau took place in oligocene time both the crustal uplift and continuous southeastward aggradations of the sediments carried by the paleo kunlun river are interpreted to be the cause for southeastward migration of the depocenter of the qaidam basin the longitudinal river disappeared in late cenozoic time 2 4 ma due to stream capture and climate change

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

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

microfracturing in the upper triassic sichuan basin tight gas sandstones tectonic overpressure and diagenetic origins the western sichuan basin located at the front of the longmen mountain in the western sichuan province of china is a foreland basin formed in the late triassic the upper triassic xujiahe formation in the basin is an ultralow permeability and low porosity tight gas sandstone reservoir microfractures such as intragranular microfractures grain edge microfractures and transgranular microfractures are abundant in the tight sandstones microfractures improve storage and permeability and impact distribution of natural gas microfractures reflect tectonic overpressure and diagenetic origins using quartz and calcite fluid inclusions and burial history tectonic microfractures were determined to be formed at the end of the triassic the end of the cretaceous and the end of the neogene early pleistocene microfractures related to overpressure being tension microfractures are commonly filled with bitumen and were formed at the middle to late cretaceous when the maximum pressure coefficients were 1 6 to 2 1 in the middle to late neogene the pressure coefficient was reduced to less than 1 3 because of fault activity and tectonic uplift in the southwestern sichuan basin and these tension microfractures closed and were filled with calcite the transition of stress state from compression to tension by overpressure is the reason that tension microfractures were formed in the compression setting diagenetic fractures were formed at the end of the late triassic to jurassic under intense compaction grain crushing crackle fractures in quartz and cleavage fractures in feldspar formed intragranular microfractures some transgranular microfractures caused by diagenesis are along the bedding plane and parallel to the directional mineral grains

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

structure of the altyn tagh fault and daxue shan from magnetotelluric surveys implications for faulting associated with the rise of the tibetan plateau magnetotelluric measurements in the foreland of the daxue shan have imaged low angle 20 thrust faults that extend to a depth of 3 km the surface projections of the faults at depth coincide with the mapped traces of the chang ma m 7 6 1932 and north hills thrusts minimum cumulative offsets of 7 km on the chang ma thrust and 14 km on the north hills thrust are estimated from the horizontal extent of underthrust sediment assuming that regional thrusting began at 5 6 ma this corresponds to minimum convergence rates of 1 3 mm yr and 2 5 mm yr on the chang ma and north hills thrusts respectively these slip rates correspond to 3 mm yr of sinistral slip on the altyn tagh fault between 96 e and 97 e this is consistent with the 4 2 mm yr of slip estimated by geological studies assuming comparable slip rates and similar fault geometry in the neighboring ranges this requires a minimum of 11 mm yr of shortening parallel to the altyn tagh fault between the dang he nan shan and qilian shan both the style of thrusting and rate of shortening are in agreement with geologic studies that favor a relatively high rate of slip on the altyn tagh fault this in turn implies that lithospheric extrusion contributes significantly in accommodating the ongoing convergence between india and asia farther west the altyn tagh fault is imaged on four magnetotelluric profiles as a vertical resistivity contrast extending to a depth of at least 8 km two strands of the north altyn tagh fault are imaged east of the asperity near subei 39 30 n 95 e

cenozoic tectonic evolution of qaidam basin and its surrounding regions part 1 the southern qilian shan nan shan thrust belt and northern qaidam basin cenozoic qaidam basin the largest active intermountain basin inside tibet figures importantly in the debates on the history and mechanism of tibetan plateau formation during the cenozoic indo asian collision to determine when and how the basin was developed we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern qilian shan nan shan thrust belt and northern qaidam basin our geologic observations provide new constraints on the timing and magnitude of cenozoic crustal thickening in northern tibet specifically our work shows that the southernmost part of the qilian shan nan shan thrust belt and contractional structures along the northern margin of qaidam basin were initiated in the paleocene early eocene 65 50 ma during or immediately after the onset of the indo asian collision this finding implies that stress was transferred rapidly through tibetan lithosphere to northern tibet from the indo asian convergent front located 1000 km to the south the development of the thrust system in northern qaidam basin was driven by motion on the altyn tagh fault as indicated by its eastward propagation away from the altyn tagh fault the eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of qaidam basin suggesting thrust loading was the main control on the basin formation and evolution the dominant structure in northern qaidam basin is a southwest tapering triangle zone which started to develop since the paleocene and early eocene 65 50 ma and was associated with deposition of an overlying southwest thickening growth strata sequence recognition of the triangle zone and its longevity in northern qaidam basin explains a long puzzling observation that cenozoic depocenters have been located consistently along the central axis of the basin this basin configuration is opposite to the prediction of classic foreland basin models that require the thickest part of foreland sediments deposited along basin edges against basin bounding thrusts restoration of balanced cross sections across the southern qilian shan nan shan thrust belt and northern qaidam basin suggests that cenozoic shortening strain is highly inhomogeneous varying from 20 to 60 both vertically in a single section and from section to section across the thrust belt the spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of cenozoic rotations in different parts of qaidam basin the observed crustal shortening strain also implies that no lower crustal injection or thermal events in the mantle are needed to explain the current elevation 3000 3500 m and crustal thickness 45 50 km of northern qaidam basin and the southern qilian shan nan shan thrust belt instead thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern qaidam basin and the southern qilian shan nan shan region