hatton bank northwest u k continental margin structure summary the continent ocean transition near hatton bank was studied using a dense grid of single ship and two ship multichannel seismic mcs profiles extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin at the landward shallowest end the dipping reflectors overlie continental crust while at the seaward end they are formed above oceanic crust beneath the central and lower part of the margin is a mid crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence beneath this mid crustal layer and above a well defined seismic moho which rises from 27 km continental end to 15 km oceanic end across the margin the present lower crust comprises a 10 15 km thick lens of material with a seismic velocity of 7 3 to 7 4 kds we interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas a considerable quantity of new material has been added to the crust under the rifted margin the present moho is a new boundary formed during creation of the margin and cannot therefore be used to determine the amount of thinning

crustal architecture and tectonic evolution of the sergipe alagoas and jacuipe basins offshore northeastern brazil abstract the sergipe alagoas and the jacuipe basins are divergent margin basins located in the south atlantic ocean at the northeastern brazilian margin high quality 18 s two way travel time seismic reflection profiles which extend from the coastline towards the boundary with the oceanic crust were integrated with potential field data and calibrated with the results of several exploratory boreholes when combined these data provide evidence that the deep seismic reflectors in the deep water region are related to the crustal architecture of the basin rather than to sedimentary features arrays of high impedance reflectors previously interpreted as top of basement probably correspond to detachment planes near the base of the crust or to underplated igneous rocks above the moho the extensional processes that were active during rifting resulted in normal crustal faults that apparently detach near the seismic moho in the deep water region these crustal faults control and rotate syn rift sedimentary units along the margin the seaward portion of the rift seems to have been uplifted and highly eroded during a post break up tectonic event the deep seismic reflection profiles also indicate that the deep water province is characterized by wedges of reflectors that dip seawards these features probably correspond to seaward dipping reflectors sdr which are composed of volcanic rocks marking the transition to oceanic crust the sdr wedges are locally associated with volcanic intrusions several volcanic plugs in deep water region are aligned with oceanic fracture zones that apparently penetrate through the whole crust and reach the upper mantle on the other hand there are some diapiric structures located near the boundary between continental and oceanic crust that bear evidence for deep water salt tectonics

geochemistry geochronology and tectonic setting of early cretaceous volcanic rocks in the northern segment of the tan lu fault region northeast china abstract we present new geochemical and geochronological data for volcanic and related rocks in the regions of the jia yi and dun mi faults in order to constrain the late mesozoic tectonic evolution of the northern segment of the tan lu fault zircon u pb dating shows that rhyolite and intermediate mafic rocks along the southern part of the jia yi fault formed at 124 and 113 ma respectively whereas the volcanic rocks along the northern parts of the jia yi and dun mi faults formed at 100 ma the rhyolite has an a type granitoid affinity with high alkalis low mgo ti and p contents high rare earth element ree contents and ga al ratios enrichments in large ion lithophile liles e g rb th and u and high field strength element hfses e g nb ta zr and y and marked negative eu anomalies these features indicate that the rhyolites were derived from partial melting of crustal material in an extensional environment the basaltic rocks are enriched in light rees and liles e g rb k th and u and depleted in heavy rees hfses e g nb ta ti and p and sr these geochemical characteristics indicate that these rocks are calc alkaline basalts that formed in an intraplate extensional tectonic setting the dacite is a medium to high k calc alkaline i type granite that was derived from a mixed source involving both crustal and mantle components in a magmatic arc therefore the volcanic rocks along the jia yi and dun mi faults were formed in an extensional regime at 124 100 ma early cretaceous and these faults were extensional strike slip faults at this time

mesozoic collision extrusion tectonics in eastern asia abstract the eastern part of the asian continent was intensively deformed in early cretaceous time deformation has been interpreted to be related to subduction tectonics along the eastern continental margin we propose an alternative tectonic framework of collision extrusion tectonics resulting from collision of the indochina block with the eurasian continent this event controlled the jurassic to cretaceous tectonic history in eastern asia and the evolution of the subduction zone along the eastern margin of the continent

plate tectonic history basin development and petroleum source rock deposition onshore china abstract china comprises a mosaic of distinct continental fragments separated by fold belts these fold belts are suture zones resulting from the accretion of various fragments formerly separated by intervening areas of oceanic crust the major sedimentary basins onshore china can be classified into four groups those in western china are flexural developing as a result of north south compression in contrast those in the east are extensional and related to development of the pacific oceanic margin in central china basins have a more problematic origin those of north central china ordos sichuan are flexural basins controlled by eastward directed thrusting along their western margin in contrast basins further south chuxiong shiwandashan are predominantly extensional and related to major strike slip movements by synthesizing basin stratigraphies across china in tectonostratigraphic terms and in particular comparing the nature and timing of unconformities it is possible to formulate a coherent model for the palaeoreconstruction of china we identify five major tectonostratigraphic breaks which equate with the collision of the following continental fragments tarim north china carboniferous permian south china block permian triassic qiantang late triassic early jurassic lhasa block late jurassic early cretaceous and india early tertiary prior to permian times the southern margin of eurasia ran approximately along the northern border of modern china the late carboniferous collision of tarim north china with eurasia resulted in the development of a flexural basin junggar and deposition of non marine clastics to the south of the suture shallow marine deposition continued in the late permian early triassic the progressive collision from east to west of the south china block with the north china block resulted in a change to fluvial lacutrine sedimentation across the entire north china tarim block open marine carbonate deposition in the north of the south china block passed southward into a deeper marine clastic sequence deposited in a backarc basin further south a subduction zone existed along the southeastern margin of the south china block in western china northward subduction throughout the triassic resulted in the development of the songban ganzi accretionary prism with retroarc thrusting resulting in flexure and the first development of the tarim basin oblique collision of the qiantang block in the late triassic along the east of the south china block resulted in east west directed thrusting which initiated the suchuan and ordos basins continued strike slip deformation along the south western margin of the south china block resulted in the development of basins with a significant extensional component such as chuxiong the collision of the qiantang block with the southern edge of the tarim basin early jurassic resulted in a renewed clastic influx in both the tarim and junggar basins along the eastern pacific margin a compressional arc and retroarc basin in the south passed northwards into an extensional arc system subduction rollback of the extensional arc initiated rifting in the late jurassic in the eren and songliao basins the late jurassic early cretaceous collision of the lhasa block in the west rejuvenated the thrust systems bordering the western basin and resulted in a renewed clastic influx in the southeast the compressional arc phase culminated in widespread thrusting and folding of early cretaceous age in the northeast extension continued with the progressive migration of the rift system southward with time the arrival of the indian block in the early tertiary rejuvenated the bounding thrust belts of all the western basins in the east the change in convergence of the pacific plate to a more westerly direction is marked by extension and widespread rifting along the entire length of eastern china throughout most of china mesozoic and cenozoic deposition occurred in predominantly non marine environments source rocks in such settings comprise principally mudstones deposited in lakes organic rich mudstones these can accumulate in both deep and shallow lakes in order to accumulate substantial volumes the lake must be significant in space and time in china lacustrine orms occur in both rift and flexural basins lacustrine orms deposited under humid climatic conditions are restricted to the period of maximum tectonic subsidence in the flexural basins of western china source rock deposition follows basin initiation by 20 30 ma in the extensional basins of eastern china source rock deposition takes place 5 15 ma after basin initiation by contrast semi arid and arid climate lacustrine orms whilst being best developed during the period of maximum tectonic subsidence occur at all stages in the basin history

great jurassic thrust sheets in beishan north mountains gobi areas of china and southern mongolia abstract jurassic thrust sheets with minimum displacements of 120 180 km have been discovered within the hercynian indosinian orogenic belt of the beishan of china and south gobi area the thrusts strike e w extend over 1200 km in length and carried meso proterozoic massive dolomitic limestones over strata ranging from neoproterozoic cryogenian and terminal proterozoic to lower middle jurassic slip linear plots based on kinematic indicators such as slickenlines and groove lineations fiber lineations and drag folds adjacent to the fault surface vergence of folds and imbricated thrusts in the upper plate indicate northward movement in the beishan area to the west and southward movement in the south gobi area to the east the two major thrust faults the beishan thrust and south gobi thrust are presumably separated by a major tear fault the ruo shui fault the major thrust faults were later deformed into a series of e w antiforms and synforms and the sheets are separated due to erosion into a number of klippen mainly located on synforms of the major faults the yagan metamorphic core complex which is a result of an extensional event that postdates the thrust event yields an 40 ar 39 ar plateau age of 155 1 10 ma and rb sr isochron age of 153 6 2 ma the thrust sheets formed during the late middle jurassic long after the closure of any oceans in the study area previously reported for this region and are ascribed to a phase of intracontinental deformation the closing of the jurassic tethys or retroarc deformation behind an active continental margin at the southern edge of asia prior to the tethyan collision or and the closing of mongolo okhotsk oceans might be responsible for this event

structural and lithological characteristics of the bayankhongor ophiolite zone central mongolia the mechanism of continental growth of central asia is currently debated between models invoking continuous subduction accretion or punctuated accretion due to closure of multiple ocean basins ophiolites in central asia may represent offscraped fragments in an accretionary complex or true collisional sutures the bayankhongor ophiolite a nw se striking sublinear belt 300 km long and 20 km wide is the largest ophiolite in mongolia and possibly central asia we present results of the first detailed structural and lithological study of the ophiolite the study area is divided into four zones baidrag complex burd gol bayankhongor and dzag zones the archaean baidrag complex comprises tonalitic granulites and metasediments the burd gol zone is a metamorphosed sedimentary and igneous melange the bayankhongor zone contains the dismembered ophiolite forming a serpentinite melange the dzag zone consists of asymmetrically folded chlorite mica schists resembling meta turbidites the structure is dominated by steeply dipping ne directed thrusts and ne vergent folds we suggest the bayankhongor ophiolite marks the closure of an ocean separating two microcontinents the baidrag complex with the burd gol accretionary complex to the south and a northern continent that forms the basement for the hangai region subduction was towards the sw with ne directed ophiolite obduction onto a passive margin represented by the dzag zone

exhumation of the ultrahigh pressure continental crust in east central china cretaceous and cenozoic unroofing and the tan lu fault the orogenic architecture of the world s largest ultrahigh pressure exposure the hong an dabie mountains of the triassic qinling dabie orogenic belt is dominated by cretaceous and cenozoic structures that contributed to its exhumation from 30 km depth cretaceous magmatic crustal recycling 50 for the entire dabie and heating 250 to 700 c were most prominent in dabie and exhumation magmatism and cooling were all controlled by cretaceous transtension exhumation was accomplished principally by an asymmetric cordilleran type extensional complex in the northern dabie northern orthogneiss unit between 140 and 120 ma at rates as fast as 2 mm yr and average horizontal stretching rates of up to 6 mm yr cretaceous reactivation occurred within a regional transtensional strain field as a result of far field collisions and pacific subduction the onset of crustal extension was preceded and possibly facilitated by a reheating of the hong an dabie crust 140 ma coeval with the onset of voluminous magmatism in eastern china 145 ma which resulted from a change in pacific subduction from highly oblique to orthogonal the tan lu continental scale fault was a normal fault zone in the mid cretaceous 110 90 ma and underwent 5 4 km dip slip and 4 km throw in the cenozoic during the india asia collision the qinling dabie belt acted as the structural discontinuity between the strike slip dominated escape tectonics south of the qilian qinling dabie belt and the rifting dominated tectonism north of it the most prominent cretaceous and cenozoic structures of the hong an dabie the xiaotian mozitang and the jinzhai fault zones respectively reactivated major lithospheric structures of the triassic orogen i e the huwan detachment zone and the suture

apparent polar wander paths of the major chinese blocks since the late paleozoic toward restoring the amalgamation history of east eurasia abstract high quality paleomagnetic poles a k a paleopoles are essential for quantitative lithospheric plate reconstruction however the current paleomagnetic database of the major chinese blocks including the north china block ncb the south china block scb and the tarim block tb since the late paleozoic era contain outdated and low quality data here we update the database by adding recently published high quality paleopoles and rejecting low quality outdated ones the database includes 288 paleopoles published 1980 2014 90 of them published after 2000 following the van der voo selection criteria 75 paleopoles each with a quality factor q smaller than 4 were rejected in the first round of selection we then removed another 59 paleopoles that have been locally rotated or shallowed since the acquisition of stable remanence eventually 154 paleopoles were selected and adopted to calculate the new apparent polar wander paths apwps for the major chinese blocks we found comparable clastic and igneous paleopoles at intervals when a quantitative comparison was available as such comparison was not possible during most periods epochs our conclusions reflect an unclear extent of inclination errors in chinese clastic paleopoles new models of the chinese apwps with and without inclination error corrections were computed from carefully selected paleopoles version 1 running mean v1rm paths were calculated at the period epoch level version 2 running mean v2rm paths were computed in a set sliding time window of 20 or 30 myr spline paths were calculated with the same time windows along with a smoothing parameter of 50 using a recent global reconstruction model and up to date geological observations new models of the chinese apwps allowed us to re evaluate the coalescence history of east eurasia since the late paleozoic era four major tectonic events were confirmed 1 the tb accreted with the kazakhstan orocline during amalgamation of the west altaids during the middle late permian period ca 265 250 ma 2 the suturing of the ncb and the scb likely occurred in a scissor like pattern and had been accomplished no later than the middle jurassic period ca 180 160 ma 3 the amalgamation between the ncb and the tb along with the microblocks between the two might have been achieved during the late jurassic early cretaceous periods ca 160 140 ma 4 the mongol okhotsk ocean should have been closed no later than the early cretaceous period ca 140 120 ma

structure of the crust and uppermost mantle of the yanshan belt and adjacent regions at the northeastern boundary of the north china craton from rayleigh wave dispersion analysis abstract we have used rayleigh wave dispersion analysis and inversion to produce a high resolution s wave velocity imaging profile of the crust and uppermost mantle structure beneath the northeastern boundary regions of the north china craton ncc using waveform data from 45 broadband ncisp stations rayleigh wave phase velocities were measured at periods from 10 to 48 s and utilized in subsequent inversions to solve for the s wave velocity structure from 15 km down to 120 km depth the inverted lower crust and uppermost mantle velocities about 3 75 km s and 4 3 km s on average are low compared with the global average the moho was constrained in the depth range of 30 40 km indicating a typical crustal thickness along the profile however a thin lithosphere of no more than 100 km was imaged under a large part of the profile decreasing to only 60 km under the inner mongolian axis ima where an abnormally slow anomaly was observed below 60 km depth the overall structural features of the study region resemble those of typical continental rift zones and are probably associated with the lithospheric reactivation and tectonic extension widespread in the eastern ncc during mesozoic cenozoic time distinctly high velocities up to 4 6 km s were found immediately to the south of the ima beneath the northern yanshan belt ysb extending down to 100 km depth the anomalous velocities are interpreted as the cratonic lithospheric lid of the region which may have not been affected by the mesozoic cenozoic deformation process as strongly as other regions in the eastern ncc based on our s wave velocity structural image and other geophysical observations we propose a possible lithosphere asthenosphere interaction scenario at the northeastern boundary of the ncc we speculate that significant undulations of the base of the lithosphere which might have resulted from the uneven mesozoic cenozoic lithospheric thinning may induce mantle flows concentrating beneath the weak ima zone the relatively thick lithospheric lid in the northern ysb may serve as a tectonic barrier separating the on craton and off craton regions into different upper mantle convection systems at the present time

early cretaceous denudation related to convergent tectonics in the baikal region se siberia we present 24 new apatite fission track aft ages and 18 track length measurements from the baikal region se siberia most samples have aft ages between 140 and 100 ma with relatively high mean track lengths c 13 2 m the relationship between aft ages elevation and mean track lengths indicate that the samples record rapid cooling during the early cretaceous 140 120 ma as also shown by thermal history inversion of track length distributions cooling took place during a late jurassic early cretaceous orogenic phase related to closure of the mongol okhotsk ocean and reflected in the exhumation of metamorphic core complexes followed by thrusting and reverse faulting basin inversion and large vertical motions the variation in aft ages throughout the study area can be partly explained by differences in geothermal structure but differential denudation also played a role minimum amounts of early cretaceous denudation are estimated at 2 3 km

tectonic evolution of the tancheng lujiang tan lu fault via middle triassic to early cenozoic paleomagnetic data the north striking tancheng lujiang tan lu fault is a conspicuous and controversial feature of the eastern asian landscape near the southeast extremity of the fault in anhui province we collected paleomagnetic samples at 17 middle triassic t2 and 10 upper cretaceous k2 to lower cenozoic e1 sites t2 remanent magnetizations are interpreted as primary in two of three areas the three areas are rotated 37 to 137 counterclockwise with respect to the south china block scb reference direction k2 e1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas counterclockwise rotation of t2 strata therefore ended before k2 and is attributed to left lateral shear acting along tan lu during the north china block ncb scb collision in shandong province 700 km north of the anhui sites four areas containing 33 upper jurassic j3 and cretaceous sites have negligible declination differences except for one which has dispersed directions the fold test is inconclusive for this latter area and positive for the other three regional concordance of the j3 e1 paleomagnetic data including paleolatitudes together with observed deformation patterns suggest that an extensional regime prevailed in the late cretaceous and cenozoic euler pole positions that constrain the north south china collision and account for tan lu motion suggest at least 500 km of sinistral shear took place along the fault and either 1 subduction and related ultrahigh pressure uhp metamorphism occurred near the present location of the qinling dabieshan and sulu uhp belts while tan lu acted as a transform fault that connected the two subduction zones or 2 tan lu and sulu were parts of the same transform fault system and no uhp rocks formed in situ at sulu in either case uhp rocks originally exhumed near dabieshan could have been transported by plate capture toward sulu along tan lu after north and south china impacted near dabieshan the tan lu fault grew within the scb as the dabieshan corner indented the scb causing folds in scb cover rocks to conform to the ncb margin late cretaceous to cenozoic reactivation of tan lu with both right lateral strike slip and normal fault motion occurred as the scb extruded east relative to the ncb under the influence of the india asia collision

the role of palaeocene magmatism in the tectonic evolution of the sea of the hebrides basin implications for basin evolution on the nw seaboard

timing duration and role of magmatism in wide rift systems insights from the jiaodong peninsula china east asia in east asia widespread extensional sedimentary basins together with the close association of metamorphic core complexes and magmatism are prominent features of the large scale wide rift system developed during late mesozoic times this region thus appears as a proper place to study continental extension as well as questions about the link between continental extension and magmatism this paper primarily provides pioneer exhumation time constraints on the linglong mcc jiaodong peninsula including cooling and deformation ages mcc cooling at mid crustal levels occurred at ca 143 ma besides last ductile deformation occurred at ca 134 ma while final exhumation stages under brittle 2 conditions occurred as late as ca 128 ma continuous crustal stretching were then recorded by the emplacement of a synkinematic pluton in the upper crust at ca 128 ma that cooled fast below an intracrustal shear zone crossing the ductile brittle transition at ca 123 ma the ca 120 118 ma age cluster is ascribed to the fast cooling of undeformed plutons marking the end of extension that lasted in the area over a minimum period of ca 30 my combining other mcc exhumation constraints and the onset of subsidence in the sedimentary basins total duration of late mesozoic extension in east asia could be estimated at ca 60 my related with a rather long process for extension from 160 to at least 100 ma east asian continental extension is heterogeneously distributed in space and time as revealed by fundamental differences between two end member classes of migmatite cored mcc already described in other wide rift systems extension seems to have first favoured partial melting which subsequently in turn maintained continental extension

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

geochemistry and sr nd pb hf isotopes of early cretaceous basalts from the great xinggan range ne china implications for their origin and mantle source characteristics abstract late mesozoic volcanism is widespread throughout northeastern china mainly clustering in the great xinggan range where the major period of volcanic eruption was during the early cretaceous the spatial distribution of volcanic rocks shows a nne extending pattern the lithology of mesozoic basaltic rocks in the great xinggan range ranges from trachytic basalts basalts to basaltic andesites on the basis of lithological associations and spatial relationships we divide the basalts into four groups i e tahe genhe zalute and xiwu the early cretaceous volcanism in the great xinggan range is characterized by significant large ion lithophile elements lile and light rare earth elements lree enrichment and high field strength elements hfse depletion with a slightly enriched to depleted nd hf and weakly enriched sr isotopic compositions initial 87sr 86sr 0 7046 0 7079 end t 3 6 to 6 9 and ehf t 8 9 to 4 6 the pb isotopic compositions are also variable with 206pb 204pb 207pb 204pb and 208pb 204pb values of 17 791 18 530 15 477 15 597 and 37 761 38 497 respectively the sr nd pb hf isotopic data indicate that the magmatic sources are heterogeneous showing an enriched to depleted signature of continental lithospheric mantle the magmatic sources of the genhe and zalute groups are characterized by mixing among pm em ii and dm whereas the tahe and xiwu groups derive from enriched lithospheric mantle that had experienced metasomatism by fluids or melts from subducted paleo asian and mongol okhotsk ocean slabs and possible influence of precambrian blocks the late mesozoic volcanic rocks in the great xinggan range resemble those of the cenozoic calc alkaline magmatism in the basin and range province of the united states we therefore suggest that the widespread mesozoic volcanic rocks in the great xinggan range were generated in an extensional tectonic setting

crustal structure across the yanshan belt at the northern margin of the north china craton abstract we applied an integrated receiver function imaging technique to the teleseismic data from 54 ncisp iii broadband seismic stations to study the fine scale crustal structure beneath the eastern yanshan belt ysb and adjacent areas at the northern margin of the north china craton ncc the robustness and reliability of the resultant crustal image were noticeably improved by achieving imaging consistency between common conversion point stacking and waveform inversion of the receiver functions our imaging result revealed substantial lateral heterogeneities in the crust of the study region as displayed by the undulating intra crustal interfaces highly varying upper middle and lower crust thicknesses and interlayers of low and high velocities the crustal structure of the ysb is characterized by significant structural variations together with a sharp moho a thin to normal crust and a lower crust with lower velocity and normal thickness it differs markedly from the bohai bay basin and the taihang mountains within the interior of the ncc the distinct features and structural complexity of the ysb might be attributed to the complex tectonic history of the region especially the multiple phases of contraction and possible occurrence of crustal delamination which is in large contrast to other subregions of the ncc where tectonic extension likely dominated the crustal deformation since the late mesozoic

the enigmatic yinshan fold and thrust belt of northern china new views on its intraplate contractional styles the east to east northeast trending yinshan belt lies within north china extending westward at least 1100 km from china s eastern coast to inner mongolia this intraplate jurassic cretaceous belt underwent contractional and normal faulting folding and contemporaneous terrestrial sedimentation and magmatism current views on its contractional deformational style favor relatively limited thick skinned faulting of archean basement and cover units these views are challenged however by recent discoveries in the eastern part of the belt of south directed ductile nappe formation and large displacement 40 45 km thin skinned northward thrust faulting both involving archean and younger rock units collision of the siberian and north china plates upon closure of a jurassic and early cretaceous mongolo okhotsk ocean more than 800 1100 km to the north may have been responsible for yinshan north south contraction some patterns of contraction e g jurassic cretaceous ductile nappe formation appear to have been influenced by a superposed magmatic regime related to westward subduction of a pacific basin plate beneath the north china plate

the crustal assembly of southern mongolia new structural lithological and geochronological data from the nemegt and altan ranges abstract the gobi altai region is an ideal setting for studying processes of continental growth and subsequent intracontinental and intraplate deformation including terrane accretion and dispersal ophiolite obduction crustal reactivation and intraplate mountain building to assess the diverse tectonic evolutionary models of the gobi altai and the wider region more field data and geochronological data are required to constrain the tectonic evolution of individual terranes and the relationship of adjacent crustal domains to each other throughout time in this paper we present new lithological structural and 40ar 39ar age data which constrain the crustal evolution across a previously unreported late paleozoic terrane boundary in the gobi altai nemegt and altan nuruu are topographically linked mountain ranges that were formed by miocene recent uplift at a right stepping restraining bend along the left lateral gobi tien shan fault system in southern mongolia ordovician carboniferous arc rocks and an ophiolite are exposed in the mountain ranges and form a small part of the east west arcuate trans altai zone field observations of rock types and structures combined with petrographic data are used to distinguish metamorphosed volcano sedimentary arc rocks in altan nuruu and western nemegt nuruu from arc rocks in central and eastern nemegt nuruu these distinct sequences are correlated with the dzolen and edrengin terranes in the trans altai zone along strike to the west integration of field data 40ar 39ar age data and published studies are used to describe a polyphase deformation history that includes late carboniferous ophiolite obduction mid permian to late triassic shortening and lateral terrane redistribution cretaceous rifting and late cenozoic intraplate mountain building

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

anisotropic rayleigh wave tomography of northeast china using ambient seismic noise abstract the ambient noise data recorded by 249 seismic stations in the permanent and temporary networks in northeast china are used to invert for the isotropic phase velocity and azimuthal anisotropy of rayleigh waves in the period band 5 50 s the inversion results reflect the structure from the shallow crust to upper mantle up to approximately 120 km depth beneath the songliao basin both the fast direction in shallow crust and strike of a low velocity anomaly in the middle crust are nne ssw which is coincident with the main tectonic trend of the paleo pacific tectonic domain this indicates that the rifting of the songliao basin is influenced by the subduction of paleo pacific plate the upper mantle of songliao block except the central area of songliao basin to the west of mudanjiang fault and the east of the north south gravity lineament is characterized by high velocity and weak anisotropy up to approximately 120 km depth we infer that there is delamination of lithospheric mantle beneath the songliao block obvious n s ne sw and e w trending fast directions are found in the lithospheric mantles of the east west and south sides of songliao block respectively which coincide with the strikes of the paleozoic tectonic in these areas this suggests that the frozen in anisotropic fabric in the lithospheric mantle can be used to indicate the historical deformation of the lithosphere in the northern margin of the north china craton the spatial variations of phase velocity and azimuthal anisotropy are more dramatic than those in northeast china blocks which indicates that the lithosphere of the north china craton has experienced more complicated tectonic evolution than that of the northeast china blocks

influence of margin segmentation upon the break up of the hatton bank rifted margin ne atlantic the hatton bank margin flanking the iceland basin is an example of a volcanic rifted margin and has been studied to examine the along margin tectono magmatic variability integration of 5660 km of new seismic reflection profiles with 60 000 km2 of new multibeam bathymetry has allowed the margin to be divided into three segments each of which are flanked by oceanic crust the southernmost segment is characterised by a series of inner and outer seaward dipping reflector sdr packages which are separated by an outer high feature the outer sdrs are truncated by endymion spur a chain of steep sided volcanic cones connected by narrow septa or necks the central segment has no inner sdr package and is characterised by the presence of a continental block the hatton bank block hbb the northern segment is adjacent to lousy bank with a wider region of sdrs recognised than to the south and characterised by many volcanic cones the variations in the distribution of the sdrs along the margin the presence of the hbb and endymion spur all suggest that the break up process was not a uniform smooth process along strike structural segmentation controlled the variations along the margin with break up initiated in the south producing the sdr packages the hbb prompted the focus of break up to relocate outboard of the block the northern segment was closest to the iceland hot spot and regular seafloor spreading did not become established until chron 21 shortly after break up the eruption of endymion spur occurred and may have been triggered by the passage of a pulse of hot asthenospheric material along the margin the margin segmentation pattern we describe controlled the location of the enhanced volcanism along the endymion spur to the southern sector in addition the segmentation has influenced the break up style presence or absence of sdr and also the location and nature of post break up volcanism

the south siberia central mongolia transect abstract studies of geological and geophysical data on the south siberia central mongolia geoscience transect which runs approximately along 100 e show that the asian continent was formed in the phanerozoic by accretion of terranes most of which were microcontinents with precambrian basement suture zones separating the terranes are deformed and deeply eroded magmatic arcs of different widths likewise classified as specific terranes in these sutures magmatic arc rocks are locally associated with fore and back arc basin series most of the oceanic crust has been subducted the observed ophiolite belts are as a rule attributed to the base of oceanic island arcs the fragment of the asian continent traversed by the transect was formed through early middle and late paleozoic stages of accretion

petrogenesis of the linxi granitoids northern inner mongolia of china constraints on basaltic underplating five representative granitoid plutons in the linxi area northern inner mongolia of china divided into two suites are mainly granodiorite and syenogranite and correspond to i and a type granites respectively zircon u pb dating yields an age of 230 ma but most ages range from 149 ma to 111 ma for intrusion of the five plutons suite 1 granitoids show lower sio2 66 1 72 51 and k2o higher al2o3 cao mgo feo and tio2 higher sr ba and eu and lower sm gd nb y and yb than suite 2 granitoids 71 73 78 14 sio2 modeling of major and trace elements shows that fractional crystallization of hornblende and lesser amounts of feldspars for suite 1 and plagioclase and alkali feldspar for suite 2 with overprint of magmatic hydrothermal fluids can explain the geochemical variations the linxi granitoids have initial end t values varying between 5 60 and 2 89 and initial 87 sr 86 sr t ratios ranging from 0 701764 to 0 707654 geochronology of inherited zircons coupled with the nd sr isotopic comparison between granitoids late jurassic basalts and proterozoic gneisses indicates that the linxi granitoids derive from a mixed source of dominantly underplated material or mafic magma with various amounts of the proterozoic gneiss basaltic underplating provides both material and heat to produce the late jurassic to early cretaceous granitoids variation of end t and 87 sr 86 sr t with age suggests involvement in the melting of increasingly greater amounts of the old lower crustal rock our refined equation yields less scattered nd isotope depleted mantle model ages the least fractionated granitoids from each suite have low ca sr rb sr rb ba and k ba ratios and hence represent parental magmas suite 1 parent magma is produced by dehydration melting or derives principally from the fluid metasomatized mantle melting of the underplated tholeiites and their differentiated products generates suite 2 parent magma d 2005 elsevier b v all rights reserved

geometric and temporal evolution of an extensional detachment fault hohhot metamorphic core complex inner mongolia china the early cretaceous hohhot metamorphic core complex and its master hohhot detachment fault are 400 km west of beijing in the daqing shan mountains of inner mongolia the complex developed across the east trending yinshan fold and thrust belt within 4 m y following cessation of thrusting ca 125 ma see note added in proof in main text postcontractional extension was initiated within a mid crustal zone of mylonitic and ductile shear that was in part controlled by carboniferous strata sandwiched between its proterozoic and archean crystalline basement and an overlying thrust sheet of similar crystalline rocks the hohhot detachment fault appears to have rooted into deep kinematically active levels of the mid crustal shear zone higher inactive levels of the mylonitic section were transected by the fault and carried upward in its footwall geometries of the footwall mylonitic rocks indicate localized ramp flat geometries of the fault within and across them the crosscut top of the mylonitic sequence defines a mylonitic front that departs from the gently south dipping detachment fault and dips northward into its footwall early cretaceous extension was widespread elsewhere in northern china and was particularly pronounced in the yunmeng shan core complex north of beijing the gravitational collapse of orogenically thickened crust acting in concert with localized centers of deep seated plutonism appear to have led to the development of isolated metamorphic core complexes within a broad region of more distributed extensional deformation

recognition of the latest permian to early triassic cu mo mineralization on the northern margin of the north china block and its geological significance abstract in this paper we present newly obtained molybdenite re os ages for the chehugou copper molybdenum deposit and the yuanbaoshan molybdenum mineral locality from the chifeng area on the northern margin of the north china block combined with u pb zircon ages of the host granites the re os isochron age of six molybdenite samples from the chehugou deposit is 257 5 2 5 ma and one molybdenite sample from the yuanbaoshan locality yields a re os model age of 248 0 2 7 ma these ages clearly indicate the signature latest permian to early triassic cu mo or mo mineralization that has not been previously recognized zircon la icp ms u pb dating shows that the host granites of the chehugou deposit and yuanbaoshan locality formed in the late devonian 376 3 ma and late permian 269 3 ma respectively and is not of jurassic age as previously thought the latest permian to early triassic cu mo mineralization is likely related to extension due to final closure of the paleo asian ocean and amalgamation of the mongolian arc terranes with the northern north china block the establishment of a latest permian to early triassic age for mineralization should encourage renewed exploration aimed at the discovery of new mo or cu mo resources along the northern margin of the north china block

late jurassic to early cretaceous sedimentary tectonic development in the chengde basin yanshan fold thrust belt north china craton abstract the chengde basin is located in the central part of the yanshan fold thrust belt in the northern north china craton the sediments in the upper jurassic to lower cretaceous tuchengzi formation in the chengde basin provide a detrital record of basin dynamics and uplift of the basin margins during that time we analyzed the sedimentary facies paleocurrents and provenance of the tuchengzi formation in the chengde basin for the period of the late jurassic and early cretaceous shortening in the yanshan fold thrust belt four sedimentary facies associations have been identified in the tuchengzi formation corresponding to proximal fan mid fan distal alluvial fan and fluvial facies the transport and distribution of the upper jurassic to lower cretaceous sediments in the chengde basin was controlled by the faults bounding the basin paleocurrent indicators and provenance data of conglomerate clasts reveal that the sediments of the tuchengzi formation in the northern part of the chengde basin were delivered from source regions to the north of the basin the early sediments of the tuchengzi formation in the southern part of the basin comprise a suite of fluvial deposits similar to the fluvial sediments in the northern part of the basin and their paleocurrent data and the compositions of conglomerate clasts also suggest a northern source however the subsequent sedimentation in the tuchengzi formation in the southern part of the basin changed markedly to proximal fan facies with sediments being derived from the south of the basin according to the paleocurrent data and conglomerate clast lithology the sandaohe sheet which is located in the southeast limb of the chengde syncline is not a klippe formed as a result of long distance northward thrusting but an autochthonous pop up tectonic wedge generated by n s shortening during the early cretaceous sedimentation of the tuchengzi formation the sedimentation ended before the onset of the early cretaceous volcanic eruption recorded by the zhangjiakou formation which unconformably overlies the tuchengzi formation

the liaonan wanfu metamorphic core complexes in the liaodong peninsula two stages of exhumation and constraints on the destruction of the north china craton 1 the liaodong peninsula early cretaceous extension province lep located in the eastern part of the north china craton ncc was highly extended in early cretaceous the liaonan and the wanfu metamorphic core complexes mccs are two most important elements in the province both are typical cordilleran type core complexes that are composed of a general three layered structure but show various differences shearing along the jinzhou detachment fault zone in the liaonan mcc generated ductile to brittle tectonites various deformation microstructures and several types of quartz c axis fabrics implying that the mcc has a complicated progressive exhumation history however medium temperature microstructures and fabrics are superimposed by low temperature features along the wanfu detachment fault zone suggesting a relatively short and simple exhumation history of the wanfu mcc magmatic zircon u pb geochronological dating reveals that the early cretaceous extension of the lep began with the initiation of shearing along the jinzhou detachment fault zone and exhumation of the liaonan mcc before ca 134 ma subsequently relatively slow cooling and exhumation of the lower plate accompanied a giant magmatic event from 130 ma to 120 ma exhumation of a new mcc i e the wanfu mcc was triggered by progressive extension after ca 120 ma rapid cooling and exhumation of the metamorphic lower plate of the two mccs are attributed to coeval detachment faulting along both the jinzhou and wanfu faults from 120 ma to 113 ma the extension ended at ca 107 ma these data provide reliable evidence that the two mccs were exhumed progressively and sequentially during two stages of crustal extension of the lep our results indicates that heterogeneous lithosphere extension and upper middle crust upper mantle detachment possibly due to the interaction between the paleo pacific and eurasian plates are among the most important processes during the early cretaceous thinning of the lithosphere tectonic extension may have led to the detachment in east asia which contributed to the thinning of the lithosphere and the destruction of the ncc a phase of uniform and consistent wnw ese extension was responsible for the generation of the extensional structures in the eastern asia continent and the destruction of the ncc

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

mesozoic felsic arc magmatism and continental olivine tholeiites in zhejiang province and their relationship with the tectonic activity in southeastern china abstract the mesozoic geology of zhejiang province in southeastern china is characterized by two distinct geodynamic settings separated by a major tectonic event during the late jurassic early cretaceous a continental magmatic arc developed with a westward subduction of the kula or izanagi plate under the south china block during the development of this magmatic arc microsyenite porphyries and quartz bearing porphyries were emplaced in western zhejiang about 124 ma ago these porphyritic rocks intruded felsic tuffs that rest unconformably on carboniferous and permian folded strata geochemically they are enriched in lil and light rare earth elements lree which are features of acid shoshonitic suites isotopically they are characterized by negative nd t 124 5 3 to 4 7 values and high initial 87 sr 86 sr ratios 0 7079 0 7084 later in eastern zhejiang quartz phyric ignimbrites and subordinate acid andesites were emplaced and intruded by nearly contemporaneous microsyenitic and rhyolitic domes during the cretaceous about 97 ma ago all these rocks belong to the high k calc alkaline suites they show geochemical features of volcanic arc granites emplaced in an active continental margin and are characterized by negative nd t 97 8 8 to 7 1 and high initial sr isotopic ratios 0 7081 0 7089 the negative nd t 97 values of all the cretaceous calc alkaline and shoshonitic rocks of intermediate to acid composition suggest that these rocks were probably derived from a mantle wedge source contaminated by variable amounts of old crustal components likely represented by the proterozoic basement rocks of the south china block the contemporaneous igneous rocks in vietnam as well as in the fujian and guangdong provinces china represent the lateral counterparts of the zhejiang calc alkaline plutonic and volcanic rocks in the late early cretaceous an important tectonic event occurred which provoked the ending of the inagazi or kula plate subduction and affected the continental magmatic arc this tectonic event is likely related to the collision of the china indochina margin with the west philippine block after this tectonic event in the beginning of the late cretaceous continental stretching and crustal attenuation affected the south china block leading to the development of ne sw faulted basins in which continental olivine tholeiites and minor alkaline rhyolites were erupted both characterized by low nd t 97 ratios ca 4 meanwhile continental conglomerates and sandstones were also deposited this rifting process began likely before the india asia indentation and lasted at least until the late tertiary

pattern and kinematic polarity of late mesozoic extension in continental ne asia perspectives from metamorphic core complexes 1 late mesozoic extension in ne asia resulted in the development of a large extensional province metamorphic core complexes mccs are the major features in this province and have 40ar 39ar ages of 130 110 ma for the mylonites and u pb zircon ages of 150 110 ma for the integral granitic intrusions based on this and previous studies this paper summarizes major characteristics of these mccs and recognizes a regional kinematic shear sense most mccs in the transbaikalia region sino mongolia border tract and the northwest central portion of the north china craton ncc show a top to the southeast se shear whereas those in the eastern and southern ncc locally underwent top to the northwest nw shear the three largest basins songliao huabei and ordos in north china are located in the transitional zone between domains of opposing shear sense we interpret the extension in the transbaikalia sino mongolia tract and northwestern part of the ncc to reflect late orogenic collapse of thickened crust following middle late jurassic collision along the okhotsk suture the southeastward extension is probably controlled by crustal scale top to the se tangential shear the transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage ecc i e c in sub horizontal ductile shear zones late in orogenic crustal thickening the combined effect of gravitational loading and thermal uplifting is considered to be the origin of the late or post orogenic collapse the top to the nw extension in the ne of the ncc might reflect antithetic sub extensional zones or mesozoic back arc extension as a far field effect of cretaceous pacific plate subduction

evolution of the mongol okhotsk ocean as constrained by new palaeomagnetic data from the mongol okhotsk suture zone siberia summary this paper presents new data from palaeomagnetic investigations on the upper palaeozoic and mesozoic geological units from the siberian platform and the mongol okhotsk suture zone within the southern portion of the mongol okhotsk suture we collected palaeomagnetic samples from the late permian belektuy formation borzya region 50 7un 116 9ue and the middle late jurassic shadaron formation unda daya 51 5un 117 5ue we sampled the late permian alentuy formation khilok region 50 8un 107 2ue the early to middle jurassic irkutsk sedimentary basin isb 52 0un 104 0ue the late jurassic badin formation mogzon region 51 8un 112 0ue and the early cretaceous gusinoozesk formation gusinoe lake region 51 2un 106 5ue additionally in the northern region of the mongol okhotsk suture apart from the results of the isb and gusinoozersk formations which show very large ellipses of confidence and might be the present day geomagnetic field overprint our results allow us to constrain the evolution of the mongol okhotsk ocean palaeomagnetically from the late permian to the middle late jurassic they confirm that this large permian ocean closed during the jurassic ending up in the late jurassic or the beginning of the cretaceous in the eastern end of the suture zone as suspected on geological grounds however although geological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data show evidence of a still large palaeolatitude difference between the amuria and siberia blocks this is interpreted as a result of the quite fast closure of the ocean after the middle jurassic finally our new palaeomagnetic results exhibit very large tectonic rotations around local vertical axes which we interpret as probably arising both from collision processes and from a left lateral shear movement along the suture zone due to the eastward extrusion of mongolia under the effect of the collision of india into asia

high resolution 3 d crustal structure beneath ne china from joint inversion of ambient noise and receiver functions using necessarray data abstract we used ambient noise data recorded by the necessarray a portable seismic array deployed in northeast china to construct rayleigh wave phase velocity dispersion curves and obtain phase velocity maps for periods from 6 to 40 s s wave velocity structure beneath ne china is obtained by joint inversion of the phase velocities and receiver functions our results show good correlation between crustal velocity structure and geological features at shallow depths the songliao basin is separated into two small sub basins by a fast velocity anomaly corresponding to a southwest uplifted horst and the southern sub basin is associated with the kailu depression a low velocity anomaly extending from the lower crust to uppermost mantle beneath the changbaishan mountain range is consistent with p and s wave tomography models the mantle low velocity has been interpreted as a local mantle upwelling attributed to the subducting pacific slab induced return flow finally the s wave velocity in the mid and lower crust is lower beneath the xing an mongolia orogenic belt xmob than beneath the changbaishan mountain range this was not expected since the changbaishan mountain range has experienced more extensive volcanic activity in the cenozoic including the currently active changbaishan and jingpohu volcanoes than the xmob we infer that the mid and lower crust of the xmob is mainly felsic in composition and propose that the low shear wave velocity of the mid and lower crust beneath the xmob could be the consequence of removal of deep mafic crust through a continental accretion and subsequent delamination event in the paleozoic or via convection removal induced by the rollback of the flat subducted paleo pacific slab during the middle mesozoic

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

characteristics structural styles and tectonic implications of mesozoic cenozoic faults in the eastern heilongjiang basins ne china abstract the eastern heilongjiang basins ehbs are the assemblage of a series of meso cenozoic residual basins located in the northeastern corner of china the deformation pattern of the ehbs has significant implications for the history of the pacific plate subduction beneath the eurasia since the late mesozoic in this paper research on the characteristics and structural styles of the meso cenzoic faults in the ehbs has been conducted on the basis of a comprehensive analysis of field geology drilling data and seismic reflection profiles as a result five different stages of the meso cenozoic faults in the ehbs have been recognized these are in accordance with the time and relevant characteristics of fault movements i e the early stage of the early cretaceous normal fault the early stage of the late cretaceous thrust fault the late stage of the late cretaceous thrust fault the cenozoic synsedimentary normal fault and the late stage cenozoic shear fault a regional geological section has been generated across the ehbs by linking four local seismic profiles together a step by step reconstruction has been made to help better understand the mesozoic cenozoic tectonic evolution of the ehbs two phases of extension rifting in the early cretaceous period and the paleogene respectively are demonstrated to be interfered with two phases of regional uplift compression and erosion in the late cretaceous period the complicated development of multiple fault systems within the ehbs has reflected the evolution of a complex tectonic subduction of the pacific plate beneath the eurasia since the cretaceous period

primary uranium sources for sedimentary hosted uranium deposits in ne china insight from basement igneous rocks of the erlian basin carboniferous permian triassic and jurassic igneous basement rocks around the erlian basin in northeast china have been investigated through detailed mineralogical whole rock geochemistry geochronological data and sm nd isotope studies carboniferous permian biotite granites and volcanic rocks belong to a calc alkaline association and were emplaced during the late carboniferous early permian 313 1 286 2 ma these rocks are characterised by positive end t 3 3 5 3 and fairly young t dm model ages 485 726 ma suggesting a dominant derivation from partial melting of earlier emplaced juvenile source rocks triassic biotite granites belong to a high k calc alkaline association and were emplaced during the middle triassic 243 3 233 2 ma their negative end t 2 to 0 1 and higher t dm model ages 703 893 ma suggest a contribution from precambrian crust during the magma generation processes leading to a strong enrichment in k and incompatible elements such as th and u highly fractionated magmas crystallised in u rich biotite up to 21 ppm u and two mica granites in biotite granite the major u bearing minerals are uranothorite and allanite they are strongly metamict and the major part of their uranium 90 has been released from the mineral structure and was available for leaching mass balance calculations show that the triassic biotite granites may have at least liberated 14 000 t u km3 and thus correspond to a major primary uranium source for the u deposits hosted in the erlian basin

morphologies and emplacement mechanisms of the lava flows of the faroe islands basalt group faroe islands ne atlantic ocean the palaeogene faroe islands basalt group fibg comprises three eruptive sequences or formations all emplaced into a subaerial environment during the development of the extensive continental flood basalt province that stretches from east greenland through the faroe islands and into the faroe shetland basin the beinisvor formation having a tabular classic facies architecture is composed of a sequence of simple flows each comprising a single sheet lobe the beinisvor formation is overlain by the distinctly contrasting malinstindur formation that has a compound braided facies architecture the enni formation occurs at the top of the sequence and consists of a mixture of simple and compound flows with tabular classic and compound braided facies architectures respectively surface and internal characteristics of the sheet lobes of the beinisvor and enni formations indicate emplacement through inflation which is more obvious for the tube fed compound flows of the malinstindur and enni formations the difference between the simple and compound flow sequences of the fibg is most likely linked to the manner in which the lava was supplied during the eruption and the eruptive style of the volcanic system the sheet lobes were erupted over laterally extensive areas from fissure systems which had a continuous supply of lava which contrasts with the tube fed compound flows which were erupted in a gradual piecemeal manner from point sourced low shield volcanoes with limited areal extents

timing and processes of late mesozoic mid lower crustal extension in continental ne asia and implications for the tectonic setting of the destruction of the north china craton mainly constrained by zircon u pb ages from metamorphic core complexes abstract late mesozoic large scale crustal extension occurred across the northeastern section of continental asia and metamorphic core complexes mccs are the major features in this region this paper presents new zircon u pb and ar ar ages of biotite and hornblende for pre or syn mylonitized and post kinematic unmylonitized intrusions from these mccs and attempts to determine the timing of initiation duration and termination of the mid lower crustal ductile extension by integrating with geochronological data from previous studies these mccs have similar 40 ar 39 ar biotite and hornblende ages 130 to 110 ma mainly from 130 to 120 ma which are interpreted as the cooling ages and represent the timing of the late stage formation of the mccs zircon u pb ages for syn to post kinematic intrusions from some mccs suggest that the mid lower crustal extension probably initiated at ca 150 145 ma with a peak occurring 145 130 ma and continued to 130 120 ma which is older than that suggested by 40 ar 39 ar ages by 10 20 ma the early ductile extension most likely initiated in mongolia and northern china during the late jurassic 150 145 ma at a deeper level then propagated southwards 140 130 ma and finally developed throughout the entire continental ne asia 130 120 ma based on the timing and processes of the extension a geodynamic model and tectonic setting of the destruction of the north china craton ncc are proposed

moho depth model for the central asian orogenic belt from satellite gravity gradients the main purpose of this study is to construct a new 3 d model of the central asian orogenic belt caob crust which can be used as a starting point for future lithospheric studies the caob is a paleozoic accretionary orogen surrounded by the siberian craton to the north and the north china and tarim cratons to the south this area is of great interest due to its enigmatic and still not completely understood geodynamic evolution first we estimate an initial crustal thickness by inversion of the vertical gravity component of the gravity field and steady state ocean circulation explorer goce and dtu10 models second 3 d forward modeling of the goce gravity gradients is performed which determines the topography of the moho the geometry and the density distribution of the deeper parts of the caob and its surroundings taking into account the lateral and vertical density variations of the crust the model is constrained by seismic refraction reflection and receiver function studies and geological studies in addition we discuss the isostatic implications of the differences between the seismic moho and the resulting 3 d gravity moho complemented by the analysis of the lithostatic load distribution at the upper mantle level finally the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of paleozoic and mesozoic geodynamics particularly the magmatic provinces and the orocline which preserve their crustal features

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

noyon uul syncline southern mongolia lower mesozoic sedimentary record of the tectonic amalgamation of central asia to explore the hypothesis that a regionally extensive early mesozoic collisional foreland basin existed on strike to the east of documented mesozoic foreland basins in western china we conducted a reconnaissance study of a 5 km section of upper permian lower jurassic nonmarine strata exposed in the noyon uul syncline in southern mongolia this paper documents the depositional settings of these poorly known strata their provenance and their regional tectonic significance in the context of the mesozoic amalgamation of central asia we interpret noyon uul strata to be composed of 1 a coarse grained braided fluvial facies 2 a meandering fluvial facies and 3 a poorly oxygenated lacustrine facies that represent three principal depositional environments sandstone compositions are dominantly volcanic mean compositions qm 20 f 23 lt 57 qp 13 lv 82 ls lm 5 likely reflecting derivation from carboniferous and older volcanic arc sequences increased proportions of alkalic granite pebbles and cobbles above the first coarse grained braided fluvial unit and the first stratigraphic appearance of detrital k feldspar in basal coarse grained braided fluvial strata suggest unroofing of permian alkalic granites the noyon uul deposits likely are syntectonic as suggested by their thickness environments of deposition coarse grained character and accumulation rates overlap of the noyon uul syncline by relatively undeformed upper jurassic and lower cretaceous strata together with the presence of a regional unconformity between middle and upper jurassic rocks requires a phase of middle late jurassic deformation the erosional remnants of a middle upper jurassic north verging thrust belt have been documented just 100 km south of noyon uul suggesting that the noyon uul deposits may be foreland in origin specifically triassic thrusting has not been recognized in southern mongolia but permian jurassic contractile deformation associated with the collisional amalgamation of asia is documented or reasonably inferred for extensive areas of china to the south and west although sedimentary attributes of the noyon strata are most compatible with a foreland interpretation it is possible that the noyon deposits and their deformation may be related to strike slip faulting neither the attributes of the noyon deposits nor their ages are compatible with widespread late jurassic early cretaceous rifting that occurred across southeastern mongolia

palaeomagnetism and palaeogeography of mongolia from the carboniferous to the cretaceous final report abstract the present study summarizes the results of palaeomagnetic investigations of carboniferous to cretaceous rocks with respect to global tectonic interpretations of eurasia normal and reverse directions of stable remanence components were found for the carboniferous to cretaceous rocks within single outcrops sometimes even within individual samples triassic permian and carboniferous pole positions for mongolia are strikingly different from those of the siberian platform the conclusion about the different palaeogeographical development of mongolia with respect to the siberian platform is well seen from differences in palaeolatitudes up to 50 and palaeorotations up to 150 the apparent polar wander paths for mongolia and the north china block are almost identical since the carboniferous this way palaeomagnetic and palaeogeographical data are contributing to the global tectonics model of the regions under study during the carboniferous permian and triassic the regions of mongolia investigated were not part of eurasia together with the north china block they show a palaeogeographical affinity with the pacific plate

new evidence on the structure of the labrador sea greenland continental margin a 275 km section of a regional seismic reflection profile from the greenland side of the labrador sea has been reprocessed and interpreted three zones of different structural style have been identified the innermost of which is block faulted magnetic modelling based on the seismic line shows that sea floor spreading anomalies are confined to the outermost zone and the oldest identified anomaly is 27n all attempts to model the area landward of anomaly 27n as a series of remanent magnetizations of alternating polarity failed however a model in which the innermost zone is interpreted as extended and block faulted continental crust and the intermediate zone as continental crust intruded and overlain by reversely magnetized igneous material fits the magnetic and seismic data it is concluded that at least in the northern labrador sea sea floor spreading started in palaeocene times chron 27 and large areas formerly thought to be underlain by oceanic crust should now be considered to be continental one plate kinematic consequence may be a reduced need to postulate large strike slip movements along nares strait

geological control of the eastern great wall mountain basin relationships in the eastern north china craton abstract the e w trending yanshan belt an intraplate fold thrust belt located in the northern north china craton has experienced several episodes of mesozoic deformation which resulted in the widely distributed magmatism and mountain basin tectonics that completely re shaped the topography of the eastern north china craton the eastern part of the famous chinese great wall was built on the high range of the southeastern yanshan mountain belt juxta posed to the plain which directly relates to the bohai bay basin our study focuses on which tectonic processes created such mountain basin couple in mid late mesozoic times the u pb la icp ms dating of zircons yield ages of 114 to 201 ma for various granites and 115 to 116 ma for volcanic rocks from yixian and jiufotang fms the detrital zircons from the lower cretaceous sandstones yield four age groups of 2587 to 2460 ma 2222 to 1828 ma 297 to 190 ma and 187 to 100 ma which are all sourced from the qinglong and surrounding areas and indicating that the qinglong area started to uplift after the middle jurassic the qinglong area underwent multiple deformation by ne sw compression in the middle late jurassic times wnw ese compression in the late jurassic to early cretaceous ene wsw extension in the early cretaceous and nnw sse compression in the late cretaceous during the final stage of the yanshanian orogeny meanwhile widely distributed granite intrusions and emplacement of the upper jurassic lower cretaceous volcanic rocks indicate a large amount of magma input into the area the qinglong area with the great wall along its southern margin close to adjacent plain to the south was uplifted to form the qinglong highland and surrounding related basins by combination of the following three processes the multiple tectonic late jurassic and late cretaceous shortening processes related to yanshanian orogeny early cretaceous regional extension triggered by slab retreat of the paleo pacific ocean and inflation of large amount magma at depth during jurassic and early cretaceous

palaeogene peraluminous magmatism crustal melting and continental breakup the erlend complex faeroe shetland basin ne atlantic well 209 3 1 located close to the centre of the tertiary erlend complex north of the shetland isles contains an unusual volcanic sequence in which thick basalts of n to t type morb affinity overlie highly peraluminous cordierite bearing dacites the dacites from the erlend complex are almost identical in terms of their petrography and major element trace element and isotope geochemistry to those of dacites from two previously described localities in the north atlantic and indicate the formation of the dacites by anatexis of aluminous upper crustal sediments which were probably pelitic in character the discovery of graphite in the erlend dacites suggests that carbonaceous shales may be implicated as parent material the geological settings of the erlend volcanic sequence and the other examples in the rockall trough and on the voring plateau correspond to intra continental pre break up rift systems underlain by thinned continental crust occurring close to the continental margin a possible petrogenesis for the dacites involves the partial melting of upper crustal sediments as a result of the extensive contemporaneous basic magmatism documented to have occurred within a wide area of the north atlantic continental margins during the initial stages of continental break up this mechanism suggests that the observed volcanic sequence may be more common than is obvious from the present very limited drilling evidence

seismic structure of a seaward dipping reflector sequence southwest of rockall plateau summary a seismic refraction line was obtained with a sonobuoy and airgun source over a seaward dipping reflector sequence on the southwest flank of the rockall plateau the record section has been modelled using synthetic seismograms to obtain a well constrained velocity depth structure down to about 4 km below the top of the dipping reflector sequence the smooth increase of velocity in the upper 1 5 km probably is due to the downward closure of cracks and voids the base of the dipping reflector sequence beneath the profile is associated with the bottom of a relatively rapid increase in velocity from about 5 5 6 5 km s 1 a similar association of the base with a velocity increase from about 5 5 6 5 km s 1 off queen maud land and under the voring plateau suggests it may be a general feature of seaward dipping reflector sequences

controls of oil family distribution and composition in nonmarine petroleum systems a case study from inner mongolia erlian basin northern china abstract the erlian basin is a continental rift basin located in inner mongolia northern china it is a typical representative of cretaceous northeast asian rift system which includes many small petroliferous basins in mongolia republic and northern china although lower cretaceous source rocks are understood to be most important in the erlian petroleum systems the precise identification of these source rock intervals and their determination on oil families distribution and composition are poorly understood in this tectonically complicated nonmarine basin new bulk data have been gathered from source rock intervals oil sands and crude oil samples in eight main oil producing subbasins geochemical analyses indicate that lower cretaceous aershan formation k1ba and tengger 1 formation k1bt1 are two main source intervals in the erlian basin and their source rock facies vary from profundal lacustrine to marginal lacustrine according to biomarker and trace elements calibration the profundal lacustrine facies is characterised by brackish water and anoxic environment which is similar to their correlative oils family 1 oils the marginal lacustrine facies is characterised by freshwater and suboxic environment which sourced the most common family 2 oils meanwhile different maturation processes exercise the second control on oil groups and their compositions the profundal lacustrine source rocks characterised by their sulphur rich kerogens lead to two oil groups group 1 and group 2 oils whose maturity range from low to normal while the marginal lacustrine source rock only lead to normal maturity oils at last biodegradation affected the composition of a certain oils and formed group 4 heavy oils in addition short migration distance in small subbasins made the contamination or fractionation less notable in the erlian basin

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

sedimentological and geochronological constraints on the carboniferous evolution of central inner mongolia southeastern central asian orogenic belt inland sea deposition in a post orogenic setting handling editor s j liu keywords central asian orogenic belt central inner mongolia carboniferous sedimentary detrital zircon sedimentological and geochronological analyses were performed on carboniferous strata from central inner mongolia china to determine the tectonic setting of the southeastern central asian orogenic belt caob sedimentological analyses indicate that the widespread late carboniferous strata in central inner mongolia were dominated by shallow marine clastic carbonate deposition with basal conglomerate above the precambrian basement and early paleozoic orogenic belts based on lithological comparison and fossil similarity five sedimentary stages were used to represent the carboniferous deposition the depositional stages include from bottom to top 1 basal molassic 2 first carbonate platform 3 terrigenous with coeval intraplate volcanism 4 second carbonate platform and 5 post carbonate terrigenous these five stages provide evidence for an extensive transgression in central inner mongolia during the late carboniferous detrital zircon geochronological studies from five samples yielded five main age populations 310 ma 350 ma 400 450 ma 800 1200 ma and some meso proterozoic to neoarchean grains the detrital zircon geochronological studies indicate that the provenances for these late carboniferous strata were mainly local magmatic rocks early paleozoic arc magmatic rocks and carboniferous intrusions with subordinate input of precambrian basement combining our sedimentological and provenance analyses with previous fossil comparison and paleomagnetic reconstruction an inland sea was perceived to be the main paleogeographic feature for central inner mongolia during the late carboniferous the inland sea developed on a welded continent after the collision between north china craton and its northern blocks

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

occurrence age and implications of the yagan onch hayrhan metamorphic core complex southern mongolia mylonitic rocks associated with the south dipping detachment fault of the yagan onch hayrhan metamorphic core complex in southernmost mongolia indicate subhorizontal south southeast directed extension in the early cretaceous synkinematic biotites give 40 ar 39 ar ages of 129 to 126 ma the yagan onch hayrhan core complex demonstrates that late mesozoic localized high strain extension recently recognized in other parts of eastern asia also occurred in mongolia the presence of mesozoic metamorphism at onch hayrhan previously presumed to be precambrian brings into question the existence of the south gobi microcontinent

early jurassic extensional basin formation in the daqing shan segment of the yinshan belt northern north china block inner mongolia abstract structural and stratigraphic studies of the western daqing shan segment of the yinshan belt have recognized an early jurassic extensional episode supported by several lines of evidence first normal faults cut the lowermost jurassic sequence and are overlapped by younger lower jurassic rocks second lower jurassic rocks include growth strata in small scale graben at the base of the jurassic basin third rapid lateral facies changes are mapped from boulder conglomerates along the basin bounding faults to lacustrine and meandering fluvial rocks in the basin center fourth paleodrainage systems provided sediment input from three sides of the basin two transverse and one axial finally there is a strongly asymmetric distribution of coarse proximal and fine distal facies within the basin the early jurassic extensional episode was responsible for formation of an east trending half graben basin in the western daqing shan in which at least 1800 m of syn extensional nonmarine sediments accumulated and are preserved previous studies in the triassic and jurassic of other parts of northwest and north central china have concluded that the early mesozoic was a time of continental amalgamation and contractile deformation the recognition of an early jurassic extensional episode along the northern margin of the north china block is problematic in this context at least superficially we propose two possible explanations for the early jurassic extension transtension associated with strike slip tectonics or gravitational collapse of a pre existing late paleozoic early mesozoic contractile orogenic belt the first possibility transtensional deformation is problematic because specific strike slip faults have not been identified that could control extension in the western daqing shan however several lines of evidence allow the possibility of such a driving mechanism including documented early jurassic transtensional systems in the southwestern north china block several candidate strike slip faults along the china mongolia border region structural discontinuity between the daqing shan and southern mongolia and along strike changes in structural style within the yinshan belt the second possibility orogenic collapse is similarly difficult to establish because of the limited amount of data concerning the regional distribution and orientation of pre jurassic contractile structures and early jurassic extensional structures however documented late paleozoic triassic contractile deformation as well as the close temporal and spatial association and parallelism between early jurassic extensional structures and older contractile structures in the western daqing shan requires consideration of gravitational collapse as a driving mechanism for early jurassic extension in the yinshan belt

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

the geology of the uk hatton rockall margin abstract seismic potential field sea bed sample and shallow drilling data have been used to investigate the geology of the uk part of the rockall plateau most of the rockall plateau is underlain by early proterozoic rockall bank islay terrane metamorphic basement although george bligh bank and the north eastern end of hatton bank may be underlain at depth by lewisian rocks the basement crops out on rockall bank but occurs at a much deeper level beneath hatton bank and hatton basin the widespread palaeogene lavas of rockall plateau have been sourced from large central igneous complexes rockall swithin sandastre mammal lyonesse and sandarro local volcanoes and fissures some of the lavas may also have been derived from the incipient rift axis west of hatton bank at the time of continental break up differences in seismic facies allow the extent of the lavas from the large centres to be mapped where the lavas are absent beneath parts of hatton bank either due to non emplacement or subsequent erosion during the cenozoic seismic data image the underlying mesozoic and older rocks here bgs boreholes 99 1 and 99 2a have proved mid cretaceous albian shales and sandstones respectively beneath a thin cenozoic cover the mesozoic and older rocks appear to be quite extensive and are folded and faulted mesozoic strata are not observed on rockall bank in uk waters the post lava cenozoic succession of rockall plateau can be sub divided into three megasequences separated by two unconformities which by analogy with rockall basin have been designated c30 intra late eocene and c10 intra early pliocene the ages can be corroborated by dsdp odp boreholes in the hatton basin the cenozoic succession contains eocene prograding fan sequences contourite deposits on the flanks of the banks and shallow faulting some of which affects the present day sea bed structurally rockall high appears mainly as a large basement block whereas hatton high is more complex exhibiting half grabens inversion structures and reverse fault geometries due to cenozoic tectonism the northern end of hatton high is a large anticline with its axis aligned ene wsw the structural differences between the highs may be due to their locations with rockall high exhibiting fewer tectonic effects due to its more landward intraplate setting further away from plate margin deformation

what drove late mesozoic extension of the northern china mongolia tract the northern china mongolia tract exhibited a tectonic transition from contractional to extensional deformation in late mesozoic time late middle to early late jurassic crustal shortening is widely thought to have resulted from collision of an amalgamated north china mongolia block and the siberian plate but widespread late late jurassic early cretaceous extension has not been satisfactorily explained by existing models some prominent features of the extensional tectonics of the northern china mongolia tract are 1 late jurassic voluminous volcanism prior to early cretaceous large magnitude rapid extension 2 overlapping in time of contractional deformation in the yinshan yanshan belt with development of extensionrelated basins in the interior of the northern china mongolia tract and 3 widespread occurrence of alkali granitic plutonism extensional basins and metamorphic core complexes in the early cretaceous a new explanation is advanced in this study for this sequence of events the collision of amalgamated north china mongolia with siberia led to crustal overthickening of the northern china mongolia tract and formation of a high standing plateau subsequent breakoff at depth of the north dipping mongol okhotsk oceanic slab is suggested as the main trigger for late mesozoic lithospheric extension of that tract slab breakoff resulted in mantle lithospheric stretching of the adjacent northern china mongolia tract with subsequent ascent of hot asthenosphere and magmatic underplating at the base of the crust collectively these phenomena triggered gravitational collapse of the previously thickened crust leading to late late jurassic early cretaceous crustal extension and importantly coeval contraction along the southern margin of the plateau in the yinshan yanshan belt the proposed model provides a framework for interpreting the spatial and temporal relationships of distinct processes and reconciling some seemingly contradictory phenomena such as the synchronous extension of northerly terranes during major contraction in the neighboring yanshan yinshan belt d 2003 elsevier b v all rights reserved

dipping reflectors in the norwegian sea odp leg 104 drilling results leg 104 scientific party the following is an extended abstract of a paper presented at the marine studies group meeting structure of continent ocean boundaries 11 december 1985 ocean drilling program odp leg 104 successfully completed a number of deep drill holes on the outer v0ring plateau and the vering basin during july and august 1985 fig 1 eldholm thiede taylor et al 1986 one of the principal objectives of the leg was to drill and sample a thick oceanward dipping wedge of seismic reflectors known to characterize much of the multichannel seismic mcs profile data recorded across ocean continent transitions in the ne atlantic north of 55 n elsewhere these reflectors have previously been the target of deep drilling during legs 48 and 81 of the international phase of ocean drilling ipod montadert roberts et al 1979 roberts schnitker et al 1984 these efforts met with some success sampling only the uppermost part of the sequence and identifying a series of tholeiitic lava flows over the outer v ring plateau a number of mcs profiles reveal a distinct change in seismic character at the base of the well stratified dipping reflector sequence where an irregular surface characterized by a band of low frequency high amplitude reflectors occurs this surface is referred to as k prior to leg 104 therefore relatively little was known regarding the variation in petrographic character of the flows at depth their evolution and origin and particularly the character of the material below the reflector sequence a single deep drillhole 642e successfully recovered a section through the

cyclic variations in the chemical composition of eastern mediterranean pliocene sediments a key for understanding sapropel formation abstract two pliocene mediterranean sediment cores from ocean drilling program odp leg 160 sites 964 pisano plateau ionian basin and 967 lower northern slope of eratosthenes seamount south of cyprus have been analyzed for major and minor elements si ti al fe mg ca k ba cr ni v zn zr by x ray fluorescence analysis at a depth resolution of 3 to 4 cm a total of 430 samples core 964c 9h comprises five sapropels with up to 9 7 total organic carbon toc one 2 5 m thick turbidite and a 50 cm segment of a second turbidite whereas core 967b 9h contains eight sapropels with toc contents of up to 6 7 cyclic variations in carbonate content and detrital matter composition are evident at both sites at site 964 pisano plateau terrigenous detrital matter chemistry reflects periods of stronger saharan dust input higher si al and zr al ratios alternating with fluvial input from the northern borderlands higher mg al ratios cyclicity is not well expressed at this location because turbidites and winnowing occur site 967 eratosthenes seamount sediments are strongly influenced by discharge from the nile river during humid periods low k al and mg al ratios at site 967 sapropels occur only during periods of enhanced nile discharge with correspondingly lower si al ti al mg al k al and zr al ratios accumulation rates of carbonate and terrigenous detrital matter were 30 lower during such episodes because of the reduction in eolian input and carbonate production and or carbonate dissolution periodical ba enrichments which are not only seen in the sapropels but in every k al minimum document the association of enhanced bioproductivity and sapropel formation as calculated from excess ba concentrations productivity increased by up to a factor of five during intervals of intensified nile runoff fluviatile discharge seems to be responsible for an enhanced input of nutrients and suspended matter containing low mg al and k al ratios based on organic carbon concentrations episodes of sapropel formation lasted at least from 1000 to 4000 years at site 964 and from 2000 to 6000 years at site 967 periods of enhanced bio productivity as defined by ba enrichments lasted from 8000 to 12 000 years at site 967 episodes of high river discharge and corresponding nutrient input as well as conditions of sapropel formation seem to have lasted longer in the eastern part of the basin site 967 than at the pisano plateau site 964

sedimentary record and tectonic implications of mesozoic rifting in southeast mongolia the east gobi basin of mongolia is a poorly described late jurassic early cretaceous extensional province that holds great importance for reconstructions of mesozoic tectonics and paleogeography of eastern asia extension is especially well recorded in the structure and stratigraphy of the unegt and zuunbayan subbasins southwest of saynshand mongolia where outcrop and subsurface relationships permit recognition of prerift synrift and postrift mesozoic stratigraphic megasequences within the synrift megasequence three sequences developed in response to climatic and rift related structural controls on sedimentation where best exposed along the northern margin of the unegt subbasin each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone resedimented ashes and basalt flows punctuate the synrift megasequence rifting began in the unegt subbasin prior to 155 ma with coarse alluvial filling of local fault depressions subsidence generally outstripped sediment supply and fresh to saline lacustrine environments expanding southward with time dominated the unegt zuunbayan landscape for much of latest jurassic early cretaceous time episodic faulting and volcanism characterized the basin system for the balance of the early cretaceous a brief period of compressional and or transpressional basin inversion occurred at the end of the early cretaceous prior to deposition of a widespread upper cretaceous overlap sequence the driver s of late jurassic early cretaceous extension remain uncertain because southeast mongolia occupied an intraplate position by the beginning of the cretaceous extension in the east gobi basin was coeval with collapse and extension of early mesozoic contractional orogenic belts along the northern and southern borders of mongolia and probably was a linked phenomenon strike slip faulting associated with collisions on the southern asian and mongol okhotsk margins likely also played a role in late mesozoic deformation of the east gobi region perhaps partitioning the gobi from apparently coeval large magnitude contractional deformation in the yinshan yanshan orogenic belt south of the study area in inner mongolia

geometric and kinematic evolution of basement cored structures intraplate orogenesis within the yanshan orogen northern china abstract detailed surface mapping lithology facies analysis drillcore data and aeromagnetic interpretation reveal the geometries of the basement involved structures of laowopu nantan liudaohe xinglong and malanyu to have been generated by folding and thrusting the resulting basement cored anticlines are asymmetric with a shallowly dipping top and backlimb and a steep to overturned forelimb and or frontal fault linked with a gently dipping synclinal limb both the fold axes and frontal faults trend roughly east west suggesting north south shortening north to northeast trending faults behave as tear lateral or oblique ramp structures and formed coevally with their frontal faults their trends may have been inherited from proterozoic synsedimentary faults and archaean basement ductile shear zones foliations and mafic dikes thin skinned thrusts and their three dimensional geometry are locally controlled by detachment within mechanically incompetent layers four types of interrelationships between the thick and thin skinned structures occur including 1 lateral changes along the length of a fault 2 displacement transfer between thick and thin skinned structures 3 buttressing of thin skinned thrusts against the basement involved structures and 4 generation of an out of the syncline thrust under thick skinned shortening most of the deformation within the study area involved brittle behaviour but ductile shear zones developed where fluid access was possible both the basement involved structures and the thin skinned structures formed in the early mesozoic as constrained by cross cutting relationships isotopic ages of igneous rock and syntectonic basins the model of geometric and kinematic evolution of the basement involved structures and their relationship with the thin skinned structures defined in this paper provides an alternative to the current metamorphic core complex model used for the yanshan intraplate orogen

accommodating large scale intracontinental extension and compression in a single stress field a key example from the baikal rift system the baikal rift system in southern siberia is one of the main intracontinental extensional features on earth the rift system represents the northwestern boundary of the amuria plate and in that respect can be considered as an evolving plate boundary the baikal rift system has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution however the age of the initiation of deformation and the mechanism driving this deformation are still largely debated while major extension has occurred since the late miocene pliocene the onset of extension seems older than the india asia collision implying that several driving mechanisms may have acted together or in relay through time in this work we review the available data and models for deformation in an area encompassing the baikal rift system the sayan ranges to the west and the transbaikal to the east using a synthesis of this data and our own field and mapping observations we show that the baikal rift system along with transpressional deformation in the sayan ranges and transtension in the transbaikal area can be explained through major left lateral strike slip systems the deformation is strongly controlled by inherited crustal and lithospheric structures and is distributed over a wide area within the western amuria plate that consequently cannot be considered as a rigid block such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust

detrital zircon u pb geochronology and stratigraphy of the cretaceous sanjiang basin in ne china provenance record of an abrupt tectonic switch in the mode and nature of the ne asian continental margin evolution abstract the age spectra obtained from 505 spots of detrital zircon u pb ages of five representative sandstone samples from the sanjiang basin in ne china point to a significant change in its provenance during the coniacian santonian the predominant detrital source for the sanjiang basin during the early cretaceous was the zhangguangcai range magmatic belt and jiamusi block along its western and southern periphery whereas it changed in the late cretaceous to its eastern periphery the timing of these inferred changes in the detrital source regions and drainage patterns nearly coincide with the age of a regional unconformity in and across the basin the time interval of non deposition and unconformity development was coeval with a transitional period between an extensional tectonic regime in the early cretaceous and a contractional deformation episode in the late cretaceous the sanjiang basin evolved during this time window from a backarc to a foreland basin the migration of the coastal orogenic belt and the fold and thrust belt development farther inland during the late cretaceous marked the onset of regional scale shortening and surface uplift in the upper plate of a flat or very shallow dipping subduction zone the stratigraphic record the detrital source and geochronology of the basinal strata and the internal structure of the sanjiang basin present therefore an important record of a tectonic switch in the nature of continental margin evolution of northeast asia during the late mesozoic

the 40ar 39ar dating of the early jehol biota from fengning hebei province northern china the bird fossil bearing deposits at the jiecaigou section correlative to the dabeigou formation in fengning hebei province northern china is well known for yielding a fossil assemblage representing the earliest evolutionary stage of the jehol biota the precise age of the fossil bearing deposits however is unknown the ar 40 ar 39 age spectrum obtained on bulk k feldspars from the tuff layer about 2 m below the bird fossil bearing layer gave a plateau age of 129 0 1 3 ma 2 sigma full external error and an isochron age of 132 3 4 5 ma 2 sigma full external error seventeen total fusion ar 40 ar 39 ages on k feldspars from the interbedded tuff about 6 m below the fossil bearing layer resulted in a weighted mean of 130 7 1 4 ma 2 sigma full external error these dates suggest an age of similar to 131 ma for the early jehol biota and combined with previous dating indicate that this biota lasted at least from 131 ma to 120 ma late hauterivian to aptian these dates also represent the earliest absolute age for known enantiornithine birds in the world

tectonic and stratigraphic evolution of the mongolian people s republic and its influence on hydrocarbon geology and potential abstract the stratigraphy of mongolia can be divided into five megasequences each characterized by a common tectonic and stratigraphic history the earliest megasequence 1 precambrian silurian records the amalgamation of precambrian early palaeozoic passive and active margin terranes and the resulting growth of a caledonian fold belt in western mongolia megasequence 2 devonian permian appears to be the most complex with a variety of active margin collisions in southern and eastern mongolia and the continued deformation of the caledonian fold belt in the west a substantial fold belt underpinned by a variety of granitic plutons existed across mongolia by the triassic there was widespread erosion and limited deposition in small foreland and intermontane basins megasequence 3 triassic early jurassic megasequence 4 mid jurassic cretaceous records extension across the partially eroded fold belt with deposition confined to a series of alluvial and lacustrine rift basins these basins later experienced transpression inversion and erosion followed by onlap and overstep giving more widespread sedimentation from large post rift alluvial systems during the late cretaceous and tertiary megasequence 5 megasequences 1 3 are usually extensively folded megasequences 4 and 5 have been involved in transpressive inversion and mongolia is an area of present day compressive and transpressive tectonics erosion and general uplift the late jurassic early cretaceous sequences megasequence 4 are the most prospective for finding extractable hydrocarbons oil has been successfully produced from megasequence 4 alluvial and lacustrine rift sequences with rich oil prone lacustrine mudrocks which can form both source and seal facies interbedded sandstones siltstones and mudstones form various combinations of reservoirs and seals a large number of hydrocarbon trap types are yet to be tested past drilling has been concentrated around surface seeps and on simple structures that can be mapped at the surface with usually only shallow tests