prolonged magmatism juvenile nature and tectonic evolution of the chinese altai nw china evidence from zircon u pb and hf isotopic study of paleozoic granitoids abstract paleozoic granitoid magmatism played an important role in the tectonic evolution of the chinese altai and zircon u pb and hf isotopic compositions have been determined for samples from eleven granitic plutons batholiths the jiadengyu gneissic granitic pluton yielded a zircon u pb age of 479 ma and thus it does not represent precambrian basement as suggested previously our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30 area of the chinese altai during the period from 447 ma to 368 ma with a climax at ca 400 ma ages for zircon overgrowth rims demonstrate additional thermal events at ca 360 and 280 ma respectively positive e hf t values 0 to 9 of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources xenocrystic zircon cores are 543 421 ma old and also give positive e hf t values 2 5 to 12 suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly accreted meterials the strong magmatism at ca 400 ma significantly changed the hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period geophysical geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca 400 ma and the above mentioned change of hf isotopic composition in the magma source

the alashan terrane did not amalgamate with north china block by the late permian evidence from carboniferous and permian paleomagnetic results abstract rock magnetic and paleomagnetic studies have been carried out on the early carboniferous limestones and the late permian purple sandstones sampled in the eastern alashan terrane alt northwest of china two components were isolated from the early carboniferous limestone by thermal progressive demagnetisation a low unblocking temperature component ltc of recent origin a pre folding medium temperature component mtc the paleomagnetic pole is 13 1 n 11 0 e a 95 7 0 that is probably the result of the hydrothermal fluids from the qilian orgenic belt acquired during the late carboniferous early permian also two components were separated from the late permian purple sandstone by thermal progressive demagnetisation the ltc with the recent viscous remanent magnetisation and the higher temperature component htc revealed from three sections which has passed a regional fold test at the 95 probability level and reversal test suggesting a primary characteristic magnetisation the corresponding paleomagnetic pole is 27 2 n 18 8 e a 95 12 0 the apparent polar wander path including early carboniferous late carboniferous early permian late permian and early middle triassic poles of the alt is significantly different with those of the ncb comparison of the apwps between the alt and ncb shows a strong similarity if the apwp of hexi corridor alashan rotated counterclockwise around an euler pole at 44 n 84 e by 32 then the coeval apw path of the alt overlaps to that of the ncb this result indicates that the alt migrated to the ncb after the early middle triassic along a tectonic boundary located between helanshan mountain and zhuozishan mountain and finally amalgamated to the ncb before the early cretaceous

late paleozoic paleomagnetism of south mongolia exploring relationships between siberia mongolia and north china abstract the tectonic evolution of a large region that includes south siberia mongolia and north china is still a matter of fierce debate this is due at least in part to the contentious proposals detailing the kinematics of the major tectonic units in that area one solution to the problem is the acquisition of better defined paleomagnetic data for the various players in the system we carried out a paleomagnetic study of early carboniferous subduction related volcanic rocks of the trans altai zone in south mongolia and successfully isolated a pre folding and presumably primary component from 22 sites a critical analysis of late paleozoic paleomagnetic data from mongolia revealed several problematic data sets while the paleolatitudes derived from more reliable studies agreed well with the expected values for siberia and were consistently different from those for the north china block we also found that late paleozoic declinations from mongolia are rotated counterclockwise through various angles with respect to siberia in addition those rotations are widespread in both inner mongolia and the north china block in this paper we put forward some testable hypotheses and propose a preliminary paleogeographic scheme of the region 1 consistent difference in late paleozoic paleolatitudes between siberia and mongolia on the one hand and the north china block on the other is largely due to strong inclination shallowing in paleomagnetic data for the latter area where most data are from sedimentary rocks 2 widespread counterclockwise rotations in mongolia inner mongolia and north china may be accounted for by a wide zone of sinistral transpression that was active in the early mesozoic 3 in the early mesozoic the mongol okhotsk ocean was much narrower than is usually assumed 4 there was no large scale oroclinal bending between siberia and north china in the permian early mesozoic

late paleozoic anorogenic magmatism of the gobi altai sw mongolia tectonic position geochronology and correlation with igneous activity of the central asian orogenic belt abstract geological and geochronological u pb zircon id tims and 40ar 39ar amphibole data were obtained for six anorogenic magmatic associations in the gobi altai southwestern mongolia in order to reveal its relationships with late paleozoic tectonothermal events in the southern part of the central asian orogenic belt caob in the gobi altai massifs of alkaline granites and grabens filled with bimodal volcanic suites occur as a chain 450 km long east southeast direction the massifs and volcanic fields are controlled by contacts of pre ediacaran continental crustal blocks with late neoproterozoic cambrian juvenile ophiolite and arc type complexes of the lake zone gobi altai alkaline granites bear sodic rich mafic silicate minerals and possess typical properties of ferroan a type granites our geochronological data constrain emplacement time of the gobi altai alkaline granites and bimodal volcanic rocks at 293 1 279 1 ma this age interval coincides with timing of the emplacement of i type granites with a few s type granites and gabbroic intrusions in the mongolian and chinese altai as well as with numerous massifs of a type granites layered mafic ultramafic intrusions and eruption of bimodal volcanic rocks between the southern edge of the siberia craton and the northern margins of the tarim and north china cratons hence the gobi altai anorogenic magmatic complexes were produced by a large scale late paleozoic magmatic event or events that encompassed a wide spectrum of terranes in caob the most realistic models explaining these magmatic processes are i the effect of the tarim mantle plume ii slab or slabs break off after subduction blocking due to accretion of terranes of various types

a synthesis of zircon u pb ages and hf isotopic compositions of granitoids from southwest mongolia implications for crustal nature and tectonic evolution of the altai superterrane abstract granitoid magmatism is extensively developed in the altai superterrane but the timing of magmatic events is not so well determined in this study twelve granitoid intrusions from southwest mongolia were dated by the zircon u pb method and analyzed for zircon hf isotopic compositions the new age data indicated three phases of magmatic emplacement middle paleozoic 398 350 ma late paleozoic 317 289 ma and triassic ca 244 211 ma the first phase magmatism is mainly composed of hornblende bearing i type granitoids formed during a long lasting subduction process in the early to middle paleozoic the second phase magmatism was manifested by a few small intrusions primarily distributed along the erqis fault zone and probably formed in a transitional tectonic regime from subduction to post tectonic setting in the late paleozoic the third phase magmatism was represented by post tectonic granitoids emplaced in an intraplate extensional setting model ages that were calculated using the composition of new continental crust are predominantly neoproterozoic to early paleozoic indicating that the protoliths of granitoids were likely dominated by paleozoic juvenile crust with minor neoproterozoic materials the altai superterrane was a huge paleozoic tectonic collage produced by amalgamation of subduction accretion complexes in the margin of the siberian continent and the tuva mongol microcontinent

early mesozoic southward subduction history of the mongol okhotsk oceanic plate evidence from geochronology and geochemistry of early mesozoic intrusive rocks in the erguna massif ne china abstract in this paper we present new zircon u pb ages hf isotope data and whole rock major and trace element data for early mesozoic intrusive rocks in the erguna massif of ne china and we use these data to constrain the history of southward subduction of the mongol okhotsk oceanic plate and its influence on ne china as a whole the zircon u pb dating indicates that early mesozoic magmatic activity in the erguna massif can be subdivided into four stages at 246 ma 225 ma 205 ma and 185 ma the 246 ma intrusive rocks comprise a suite of high k calc alkaline diorites quartz diorites granodiorites monzogranites and syenogranites with i type affinities the 225 ma intrusive rocks consist of gabbro diorites and granitoids and they constitute a bimodal igneous association the 205 ma intrusive rocks are dominated by calc alkaline i type granitoids that are accompanied by subordinate intermediate mafic rocks the 185 ma intrusive rocks are dominated by i type granitoids accompanied by minor amounts of a types these early mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction related fluids the rock associations and their geochemical features indicate that the 246 ma 205 ma and 185 ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the mongol okhotsk oceanic plate the 225 ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the mongol okhotsk oceanic plate every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the mongol okhotsk suture belt taking all this into account we conclude the following 1 during the early mesozoic the mongol okhotsk oceanic plate was subducted towards the south beneath the erguna massif but with a pause in subduction at 225 ma and 2 the southward subduction of the mongol okhotsk oceanic plate not only caused the intense magmatic activity but was also favorable to the formation of porphyry deposits

defining the hyporheic zone in a large tidally influenced river summary an investigation was conducted to characterize the spatial and temporal distribution of the hyporheic zone of a large tidally influenced river the field site is located on the fraser river in british columbia canada approximately 30 km from its outlet to the ocean the physical attributes of the riverbed were mapped using geophysical techniques coupled with sediment sampling the spatial and temporal distribution of groundwater composition beneath the riverbed was determined through detailed profiling contaminated fresh groundwater discharges through a narrow band of the riverbed at a distance approximately 88 105 m from the shoreline coinciding with the termination of a massive silty unit saline groundwater as part of a regional flow system dominates the riverbed sediments from 105 m beyond the shoreline towards the centre of channel three water types occur within the upper 2 m of the riverbed sediments a result of both mixing of river water contaminated fresh groundwater and saline groundwater and modification by cation exchange reactions the interaction of these waters produced distinct zones of ca hco 3 na cl and ca cl type waters the distribution of groundwater solutes indicates that during a single tidal cycle river water penetrates the riverbed to a depth of approximately 15 cm but the long term effects of tidal pumping of river water into the riverbed is observed to a depth of approximately 1 m below the river bed

silurian and devonian paleomagnetic poles from north china and implications for gondwana abstract we have obtained paleomagnetic results from silurian and devonian rocks on the western margin of the north china block ncb ages which previously were thought to be unrepresented throughout north china high unblocking temperature components of magnetization isolated from both the silurian and devonian strata pass fold and reversal tests suggesting that they are primary remanence the corresponding paleopoles at 228 4 e 26 2 n silurian and 228 7 e 34 2 n devonian imply low paleolatitude positions of north china during the middle paleozoic probably adjacent to east gondwana

paleomagnetism of paleozoic and mesozoic sediments from the southern margin of mongol okhotsk ocean far eastern russia 1 this paper presents new data from paleomagnetic investigations of middle paleozoic early middle late devonian early carboniferous and middle jurassic geological units from the amuria block south of the mongol okhotsk suture zone with fold tests for all localities and two polarities for the devonian our new results constrain the evolution of the mongol okhotsk ocean from the devonian to the middle jurassic the corresponding paleopoles lie at 21 6 n 6 3 e dp dm 5 3 10 6 for early devonian 26 3 n 345 3 e dp dm 6 4 12 8 for early middle devonian 24 6 n 12 9 e dp dm 8 7 16 9 for middle late devonian 40 5 n 352 4 e dp dm 9 16 7 for late devonian 39 8 n 31 6 e dp dm 10 15 5 for early carboniferous and 46 n 37 9 e dp dm 9 4 13 4 for middle jurassic the poles confirm that the large paleozoic mongol okhotsk ocean closed during the jurassic ending up in the late jurassic or early cretaceous at the eastern end of the suture zone as originally proposed on geological grounds the new paleomagnetic results exhibit large tectonic rotations about local vertical axes which we interpret as probably arising from both the collision process and left lateral shear movement along the suture zone related to incipient extrusion of amuria due to indentation of india into asia

the late paleozoic to mesozoic evolution of the eastern margin of the central asian orogenic belt in china abstract the north east central asian orogenic belt caob in china records terminal closure of the paleo asian ocean in the mid to late permian along the solonker xar moron changchun suture this marks the end of the overall northward movement of the chinese blocks from a peri gondwana position toward siberia and a switch in tectonic processes to those dominated by activity associated with the paleo pacific plate to the east four distinct changes in tectonic regime can be recognised here in the late paleozoic to early mesozoic i north south compression resulted in orogenesis and gave way to post collisional extension at 260 250 ma with the emplacement of a type granites ii almost coeval with this there was an the onset of east west extension from 250 to 225 ma along the extreme eastern margin of the caob when a seaway opened between the songliao and jiamusi khanka blocks iii the onset of westerly directed compression associated with subduction of the paleo pacific plate from 210 ma resulted in re amalgamation of the jiamusi khanka block with the caob by 190 180 ma and the massive generation of jurassic i type granitoids throughout the region finally iv a change to east west extension from 140 ma onward resulted from roll back of the paleo pacific plate this latter event was accompanied by the emplacement of s type granitoids and the development of sedimentary basins and core complexes it also accompanied a more widespread thinning or delamination of the lithosphere across most of north east china during the cretaceous

paleomagnetic direction obtained by strain removal in the pyrenean permian redbeds at the col du somport france abstract in order to investigate the possibility and limitations of paleomagnetic works within strained regions a paleomagnetic study related with strain analysis has been conducted in the deformed pyrenean permian redbeds in the col du somport area paleomagnetic sampling together with strain estimates have been conducted in 6 sites through a fold the results obtained by measuring the orientation and axial ratios of elliptical reduction spots show that 1 the shale beds have undergone a penetrative strain 2 the sandy beds can be regarded as tectonically unstrained with reduction spots flattened in the bedding showing that they recorded the compaction it is shown that the total strain recorded in the slaty beds probably results from the superimposition of tectonic strain upon the compaction fabric the paleomagnetic study shows that the primary pretectonic magnetization is widely overprinted by a secondary syn or post tectonic magnetic component as both components appear to be carried by hematite pigment their separation using classical demagnetization procedures has been difficult a characteristic remanent magnetization chrm has however been determined when possible as the hardest component in demagnetization curves then the chrm direction distributions are represented in stereographic density plots although these chrm directions exhibit a clear tendency towards se declinations and shallow inclinations characteristic of permian paleomagnetic field direction for the iberian plate the tilt correction does not induce a clustering of these directions strain is inferred to be responsible for this situation assuming that both pretectonic magnetization directions and bedding planes closely follow the material plane and line strain response model of march 1 an attempt has been made to remove the effect of strain upon the remanent magnetization it is shown that when using a reconstructed tectonic strain tensor i e the total strain tensor as measured in the field corrected for an estimated compaction we obtain a significant clustering of chrm directions the computation of the relevant vgp gives a pole position 210 5 e 42 0 n compatible with the reference apwp for the iberian plate it is therefore inferred that the strain removal technique is a usable tool in order to obtain paleomagnetic results within such strained rocks

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

geochemistry and petrogenesis of early carboniferous volcanic rocks in east junggar north xinjiang implications for post collisional magmatism and geodynamic process abstract early carboniferous volcanic rocks in the batamayineishan formation overlie unconformably the molasse deposits and the ophiolitic melanges and are restricted in narrow zones along both sides of the kalamaili orogenic belt in north xinjiang southern central asian orogenic belt these rocks demonstrate the post collisional setting in east junggar commenced in tournaisian and also mark an important transitional period from the final amalgamation to late paleozoic voluminous juvenile granitoids in east junggar the volcanic rocks are composed of basalt basaltic andesite andesite trachyte and rhyolite both mafic and felsic rocks are characterized by enrichments in large ion lithophile elements light rare earth elements and depletion in nb and ta low initial 87sr 86sr and high positive nd t three groups of mafic rocks have been identified shoshonitic group 1 has the highest mgo cao ni and cr and the lowest na2o al2o3 la ba la yb and ba th with primary magma features group 2 calc alkaline and high k calc alkaline mafic rocks have the lowest k2o p2o5 th and th nb and the highest tio2 and group 3 shoshonitic to potassic alkaline has the highest k2o p2o5 la ba la yb and th nb and the lowest tio2 the a type like felsic rocks were derived from the differentiation of the mafic magma geological and geochemical evidences indicate that the batamayineishan formation was generated from the process of slab breakoff detachment group 1 samples are produced by decompressional melting of the upwelling asthenosphere mainly composed of spinel and garnet 50 50 lherzolite which has been enriched by overlying metasomatized lithosphere during ascent group 2 is derived from 5 10 partial melting of shallower spinel bearing lithospheric mantle induced by the hot rising asthenosphere where the contribution of slab derived fluid is predominant low partial melting 3 5 of the mantle wedge and or thickened lithospheric mantle enriched by slab derived components generates group 3 slab breakoff as an important geodynamic process accounts for the post collisional magmatism between 343 5 ma 330 ma providing a model for post collisional crust mantle interaction in the caob

petrogenesis of late paleozoic tholeiitic nb enriched calc alkaline and adakitic rocks in southwestern mongolia implications for intra oceanic arc evolution abstract intra oceanic subduction has been taken as one of the most important contributors to the crustal growth of the immense central asian orogenic belt caob however the detailed geological processes of the development and growth of numerous island arcs still remain unclear in order to explore the development history of representative ancient island arcs we have conducted geochronological and geochemical studies on an intrusive rock association in southwestern mongolia the studied rock association comprises a mafic intrusive complex consisting of norite and diabase porphyry diabase and diorite dykes new zircon geochronological data and whole rock geochemical compositions reveal sequential magmatic events of an island arc development the mafic intrusive complex has an emplacement age of 356 ma old geochemically its two components of norite and diabase porphyry have a common tholeiitic character but show striking differences in elemental contents the norites are characterized by relatively higher mgo 7 25 16 1 wt lower tio 2 0 54 0 93 wt and relatively lower nb contents 0 99 6 39 ppm than those of the nb enriched nb 9 ppm diabase porphyry these characters together with variable nb la ratios suggest that the parental magma of the norites was derived from a refractory mantle source metasomatized by slab derived fluids and or melts whereas the generation of the nb enriched diabase porphyry was associated with the mantle metasomatism by slab derived melts the 335 ma diabase dykes have negative nb ta ti anomalies and elevated lree and lile contents similar to calc alkaline subduction related magmas which together with high zircon e hf t values 10 6 to 15 8 indicate a derivation from a mantle wedge metasomatized by slab related fluids the 290 ma dioritic dykes show the highest sio 2 53 1 58 3 wt and total alkali k 2 o na 2 o 5 84 7 77 wt contents and their distinctively high sr y 56 1 97 9 ratios and low yb the timing and petrogenesis of the basic intermediate rock association may supply valuable information on tectonic evolution of an intra oceanic subduction accretion system of the caob the coexistence of the early carboniferous tholeiitic nb enriched mafic intrusive complex and boninite as well as picrite may represent an immature intra oceanic island arc the middle carboniferous calc alkaline diabase dykes were probably associated with the relatively mature island arc the permian adakitic rocks was likely produced by partial melting of a delaminated lower crust in an extensional regime when numerous subduction accretion complexes were likely amalgamated together and docked on the siberian active margin

new paleomagnetic data from the mongol okhotsk collision zone chita region south central russia implications for paleozoic paleogeography of the mongol okhotsk ocean we present a reconnaissance paleomagnetic study of rocks from several formations of the chita region of south central russia representative location 51 n 116 e within the mongol okhotsk collision zone sections of siltstone and fine grained sandstone dated using fossils as bashkirian to moscovian epochs ussr carboniferous system middle carboniferous early permian late permian and triassic in age were sampled the resulting collection from five stratigraphic sections is a total of 319 oriented samples from 51 sites generally 5 to 10 samples per site were collected sample orientation was determined using both magnetic and sun shadow azimuths all paleomagnetic measurements were completed in the paleomagnetic laboratory at the university of pittsburgh using a 3 component 2g superconducting rock magnetometer srm in a magnetically shielded room thermal demagnetization was completed using between 12 to 20 heating steps up to temperatures of 685 c principal component analysis of the demagnetization data was successful in isolating two characteristic remanent magnetizations the lower unblocking temperature component component a fails the fold test is always of downward directed magnetic inclination and may record the present day earth s pdf magnetic field pdf i 69 4 d 351 8 component a ig 65 1 dg 356 5 95 6 2 n sites 50 the higher unblocking temperature magnetic component b was observed in the triassic bt n 3 late permian blp n 14 early permian bep n 5 and bashkirian to moscovian epochs of the late carboniferous bmc n 8 sections the b component differs significantly from component a and is recorded by sites of both downward and upward directed magnetic inclinations in the late permian and bashkirian to moscovian epoch sections component bmc and blp may represent primary remanent magnetizations the bashkirian to moscovian epochs of the late carboniferous mean paleolatitude is mc 19 9 14 8 and the late permian mean paleolatitude is lp 19 6 14 5 both are similar to that expected from reference paleomagnetic poles from the north china block but significantly different from paleolatitudes calculated using reference poles from the siberian or european plates we interpret the results of this study to suggest that the sampled sections were located near or associated with the north china block during their deposition

paleozoic multiple accretionary and collisional processes of the beishan orogenic collage in the paper by xiao and others american journal of science v 310 p 1553 1594 1 please substitute figure 16 reproduced here for figure 16 that appears between pages 1573 and 1574 the original figure 16 2 was printed at an improper size figure 3 1 lookup volpage 310

late paleozoic magmatic record of east junggar nw china and its significance implication from zircon u pb dating and hf isotope abstract late paleozoic magmatic rocks including basic dykes basaltic andesite rhyolite keratophyre and syenite porphyry with minor tuff are widespread in the western margin of the east junggar terrane in situ zircon u pb dating and hf isotope analyses were carried out for these magmatic rocks from the baijiangou and zhangpenggou localities of east junggar integrating with geochemical data to investigate their tectonic evolution as well as crustal accretionary process of this region in the late paleozoic inherited zircons from basic dykes range in age from 435 ma to 300 ma the zhangpenggou rhyolite and keratophyre have typical arc like geochemical signatures and were formed in the early carboniferous 332 ma and 336 ma respectively suggesting they are products of subduction related magmatism the baijiangou rhyolites were formed in the late carboniferous 315 ma and 323 ma and their formation ages are similar to those of the syenite porphyries 307 ma and 312 ma the hf model ages and the formation ages of zircons from these magmatic rocks are alike with positive e hf t values vary from 0 7 to 16 6 implying that voluminous growth of juvenile crust happened in the east junggar terrane during the late paleozoic the absence of precambrian inherited zircons in basic dykes indicates the lack of precambrian basement beneath the east junggar terrane taking geochronological studies on regional ophiolites into account the east junggar terrane is considered as a devonian carboniferous oceanic island arc which has been continuously accreted to the southern active margin of the siberian craton since the early carboniferous

late devonian early permian a type granites in the southern altay range northwest china petrogenesis and implications for tectonic setting of a 2 type granites abstract the altay range is an important part of the central asian orogenic belt caob that contains a type granites that have been attributed to a post collisional setting during the late carboniferous permian however our new la icp ms zircon u pb age data demonstrates that there were two episodes of a type granite magmatism in the southern altay range a late carboniferous to early permian 308 291 ma suite does occur in the vicinity of qiakuerte but a distinct late devonian 382 367 ma suite is present in the kouan area the early kouan a type granites are mainly composed of alkali feldspar granites which are approximately synchronous with adakites boninites high ti basalts picrites ophiolitic rocks and high temperature low pressure metamorphic rocks whereas the late qiakuerte a type granites mainly consist of arfvedsonite and aegirine bearing granites which are approximately coeval with some mafic ultramafic rocks in the southern altay range a type granites are commonly classified as a1 and a2 sub types which are considered to be generated in anorogenic rift or plume related or post collisional settings respectively both suites of the southern altay range are geochemically similar to typical a2 type granites e g high k2o na2o feo mgo and ga al and y nb values and low cao ba sr and eu contents the kouan a type granites have relatively low la yb n 2 7 5 9 high end t 6 7 to 7 7 and nb la 0 59 1 67 and variable ehf t 5 0 to 14 1 values whereas the qiakuerte a type granites have comparatively high la yb n 4 2 9 2 low end t 5 3 to 6 0 and nb la 0 33 0 74 and variable ehf t 10 0 to 18 7 values the early kouan a type granites may have been generated by partial melting of a mafic source containing more depleted mantle derived components than the late qiakuerte a type granites we suggest that the late carboniferous to early permian qiakuerte granites were formed in a post collisional extensional setting as is typical of a2 type granites however the late devonian kouan a type granites were more plausibly generated in an extensional setting as a result of slab window caused by a ridge subduction in this case the upwelling of asthenosphere through the slab window provided the source of parental magmas or the heat for the generation of devonian magmas therefore our results suggest that a2 type granites can also form in a ridge subduction related extensional setting

a paleomagnetic study of tertiary formations from the kyrgyz tien shan and its tectonic implications as part of a general investigation of cenozoic deformation in central asia we studied the paleomagnetism of tertiary red beds and lava flows from intermontane basins in the tien shan region of kyrgyzstan we collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them thermally or with alternating fields for most sites there are magnetic components with unblocking temperatures higher than 600 c we infer that the magnetic carriers are mainly hematite and magnetite for most localities the high temperature component appears to predate tectonic folding for all localities mean inclinations are shallower than expected from apparent polar wander paths inclination anomalies range from 16 5 for the issyk kul basin to 26 7 for the fergana basin if due to changes in latitude these anomalies imply at least 2000 km of northward displacement of the tien shan during the tertiary for which there is no tectonic evidence we thus consider that the paleomagnetic reference directions cannot be directly compared with our tertiary data we explored other possible reasons for this anomaly which has also been reported from other parts of the alpine belt but we could not find a satisfactory explanation absolute rotations cannot be accurately determined because of problems with the reference direction nevertheless the mean declination for the fergana basin lies counterclockwise by 20 11 from the mean declination of the issyk kul basin this result is consistent with the counterclockwise rotation inferred for the fergana basin from the pattern of cenozoic faults and folds it suggests a cenozoic right lateral displacement of 110 60 km on the talas fergana fault

palaeomagnetic and structural constraints on 90 anticlockwise rotation in sw mongolia during the permo triassic implications for altaid oroclinal bending preliminary palaeomagnetic results abstract new and published paleomagnetic measurements from trans altai and south gobi zones in south mongolia document large tectonic motions in between late carboniferous and triassic magnetic inclinations confirm equatorial position of south mongolian terranes in late carboniferous permian times the evolution of magnetic declinations indicates 90 anticlockwise rotation in between latest carboniferous and early triassic of all studied tectonic units around the eulerian pole located close to axis of mongolian orocline the anticlockwise rotation continues in triassic being accompanied by a major drift to the north the structural and published geochronological data suggest carboniferous e w shortening of the whole region resulting in n s trend of all continental and oceanic geological units followed by orthogonal n s shortening during late permian to early jurassic both paleomagnetic and geological data converge in a tectonic model of oroclinal bending of mongolian ribbon continent westerly back arc oceanic domain and mongol okhotsk subduction zone to the east the oroclinal bending model is consistent with the coincidence of the eulerian pole of rotation with the structural axis of mongolian orocline in addition the mesozoic collisional tectonics is reflected by late remagnetizations due to formation of wide deformation fronts and hydrothermal activity

palaeomagnetism and palaeogeography of mongolia in the cretaceous permian and carboniferous preliminary data abstract the paper reports on the results of palaeomagnetic stability tests of oriented samples of lithologically different types of cretaceous permian and carboniferous rocks from northern and southern mongolia the permian andesites and tuffites from three localities barun buren tel tin gol and chulut tsagan del have yielded interesting results groups of normally and reversely polarized andesite tuffs were found even within a single outcrop by means of thermal field demagnetization and multicomponent analysis of the magnetization with a view to the number of samples 177 suitable for analysis and the number of localities 9 the reported palaeomagnetic data should be considered as preliminary the interpreted results indicate a different palaeogeographic evolution of northern and southern mongolia relative to the siberian platform and north china in the permian and carboniferous on the other hand similar apparent polar wander paths for north china lin et al 1985 and the studied regions of northern and southern mongolia indicate similar palaeogeographic evolution in the permian to carboniferous

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

the zhaheba ophiolite complex in eastern junggar nw china long lived supra subduction zone ocean crust formation and its implications for the tectonic evolution in southern altaids abstract the composite zhaheba ophiolite complex exposed in eastern junggar in the southern altaids records an unusually long record of oceanic crust and magmatic arc evolution the zhaheba ophiolite complex consists of ultramafic rocks gabbro diorite basalt and chert intruded by diabase dikes and diorite porphyry these rocks are overlain by a several km thick section of tuffaceous rocks volcaniclastic sedimentary rocks and intermediate volcanic rocks the igneous rocks of the ophiolite complex show negative nb and ta anomalies and lree enrichment relative to hree suggesting the influence of fluids derived from a subducting oceanic slab the la icpms u pb age of zircons from gabbro is 495 1 3 5 ma zircon ages from diorite and basalt are 458 3 7 2 ma and 446 6 6 0 ma respectively the basalt is locally overlain by bedded chert diabase dikes and diorite porphyry yield the u pb ages of 421 5 4 1 ma and 423 7 6 5 ma respectively the age of stratigraphically lower part of the overlying volcanic volcaniclastic section is constrained to be about 410 ma the maximum depositional age of the tuffaceous sandstone from u pb detrital zircon ages late rhyolite at the top of the stratigraphic section yielded a u pb zircon age of 280 3 3 7 ma the age and stratigraphic relationships for the zhaheba ophiolite complex and related rocks suggest that the period of 70 ma of initial supra subduction magmatism was followed by construction of a mature island arc that spanned an additional 140 ma many other ophiolites in the southern altaids appear to record similar relationships and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra subduction zone the occurrence of the zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic tethyan ssz ophiolites but similar to some ophiolites in north america although the zhaheba ophiolite belt is flanked by the dulate arc in the north and yemaquan arc in the south it cannot stand a suture between two arcs it is suggested that devonian carboniferous dulate arc was built on the late cambrian middle ordovician zhaheba supra subduction oceanic crust the late carboniferous rocks and early permian rocks in dulate arc are interpreted to form in the extensional process within zhaheba dulate arc composite system

conical folds and apparent rotations in paleomagnetism a case study in the southern pyrenees abstract the so called apparent rotation was defined as the angular deviation between a local paleomagnetic direction after the standard bedding correction and their corresponding paleomagnetic reference j geophys res 85 1980 3659 in this paper we make a theoretical exploration on this concept and we conclude that depending on the number sequence orientation and magnitude of the deformation axes that have affected to the rock volume the apparent rotation may be the addition of a vertical axis rotation plus a spurious rotation the later is an error whose origin is the inappropriate application of the bedding correction during the restoration which does not fit the reverse sequence of deformations then apparent rot s l spurious rot vertical axis rot conical folds are complex geometries that cannot be restored by using the bedding correction however appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts in this paper we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection the modelling has to be based on a good characterization of the geometry fold axis orientation and understanding of the kinematics of the fold general modelling has also allowed us to predict the apparent rotation in conical synclines its magnitude depends on the semiapical angle and on the degree of development of the fold the sense of the rotation clockwise or counter clockwise will depend on the sense of rotation the fold axis the western external sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall ebro foreland basin of the south pyrenean sole thrust in addition a vertical axis clockwise rotation up to 47 32 in average has been detected in the hagingwall an apparent rotation up to 28 20 in average is observed in the footwall of the structure when a simple bedding correction is used this deviation does not fit with the expected ebro basin direction reference and is caused by the effect of the riglos conical syncline developed by the flexure of the foot wall ramp of the south pyrenean sole thrust the forward modelling carried out considering the geometry and kinematics non significant rotations in the autochthonous foot wall of this structure predicts very well the paleomagnetic observations in the field geographic coordinates with angular departures of only 5 in average the only application of the bedding correction would introduce errors spurious rotations up to 21 12 in average related to the conical geometry that would not allow the differentiation of these distinct structural units

spatial temporal framework for the closure of the junggar ocean in central asia new sims zircon u pb ages of the ophiolitic melange and collisional igneous rocks in the zhifang area east junggar abstract the closure time of the junggar ocean is one of the hottest topics surrounding the tectonic evolution of the central asian orogenic belt caob this paper reports sims zircon u pb ages of the ophiolitic melange and collisional igneous rocks in the zhifang area east junggar our new results reveal the following evidence 1 the west hill ophiolitic melange in the zhifang area contains segments of the 371 ma morb type layered rocks and 363 ma oceanic islands which were intruded by the 348 ma syn collisional quartz diorites 2 the ophiolitic melange and 348 ma syn collisional quartz diorites are overlain by the 342 ma andesitic tuffs and 3 the 342 ma andesitic tuffs and successive 332 ma granodiorites gabbro were formed at late collisional setting whereas the 314 ma granitic porphyry at post collisional setting we suggest that the junggar ocean in the zhifang area was opened before 371 ma and 363 ma and possibly closed before 348 ma by combining the published u pb ages of the mayile tangbale darbute kalamaili zhifang daheishan mtdkzd ophiolite belt early carboniferous volcanic rocks unconformably overlying the mtdkzd ophiolite belt bimodal volcanic rocks and granitoids within and adjacent to the mtdkzd ophiolite belt it is suggested that the junggar ocean was possibly opened scissors like from the neoproterozoic ordovician mayile tangbale ocean eastwards through the silurian devonian darbute karamay ocean finally to the devonian early carboniferous kalamaili zhifang ocean and closed scissors like from the zhifang kalamaili area in the eastern segment westwards to the darbute karamay area in the western segment tectonic collage between the yemaquan xiemisitai arc and junggar block occurred at approximately 343 348 ma the junggar orogenic belt went through late collisional stage at 343 330 ma post collisional period after 330 ma and post collisional extensional regime after 320 ma with an intensive stage at around 303 ma

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

analogue models for the transpressional docking of volcanic arcs in the western pacific abstract many convergent zones involve oblique subduction of oceanic crust the subduction often ends with oblique collision of a microcontinent island arc or oceanic plateau against a larger continental area we have conducted a series of experiments in which scaled multilayered models of the lithosphere made of sand and silicone were subjected to horizontal shortening oblique to lithospheric boundaries the aim was to investigate tectonic phenomena occurring at such boundaries for example subduction reversal partitioning of dip slip and strike slip motions lateral translation of continental fragments block rotations and entrapment of oceanic material the physical models ranged in complexity from a simple uniform oceanic lithosphere to configurations involving a volcanic arc against a continental margin our models explore the importance of rheology and width of continental indenters rather than the obliquity of convergence the models confirm that intra oceanic deformation is likely prior to complete subduction beneath both the arc and the continental margin they further explain the presence of extensional features in a convergent setting subduction zones resulting from oblique convergence may be associated with large strike slip faults by partitioning of the convergence vector when a model volcanic arc collides against a model continent a subduction reversal occurs after suturing and the strike slip fault which accompanies the subduction reversal initiates with a horse tail pattern the resulting plate boundary is somewhat different from the one that existed prior to the docking of the allochthonous belt these experiments tested many of the mechanisms proposed for terrane formation and accretion the models have been used to interpret the late neogene geodynamic setting of the western pacific with emphasis on the southern philippines and the island of new guinea

did the paleo asian ocean between north china block and mongolia block exist during the late paleozoic first paleomagnetic evidence from central eastern inner mongolia china 1 the tectonic evolution of the paleo asian ocean between the north china block ncb and the mongolia block mob is a contentious issue and geodynamic models remain speculative in an effort to puzzle out this controversy a paleomagnetic study was carried out on the silurian to permian formations in central eastern inner mongolia china more than 680 sedimentary and volcanic samples were collected from 86 sites we have established titanium poor magnetite and hematite as the principal magnetic carriers anisotropy of the magnetic susceptibility measurements demonstrate negligible deformation of the majority of study rocks with sedimentary fabrics from primary magnetizations a late devonian and a permian pole are calculated for inner mongolia block imb at 46 8 n 349 1 e dp 14 6 dm 27 3 with n 3 and 48 7 n 3 7 e dp 5 2 dm 9 1 with n 6 respectively two stages of secondary magnetization are also identified probably due to early permian and early cretaceous magmatic events as preliminary results the comparison of our new paleomagnetic poles with available data from ncb mob and siberia indicates that 1 the paleolatitudes of imb ncb and mob are consistent between late devonian and permian suggesting pre late devonian closure of the paleo asian ocean and further evaluation of these three blocks as a single entity and 2 post permian intracontinental deformation was significant and characterized by block rotations which are due to strike slip faulting within the welded ncb imb mob block

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

re deciphering the tectonic jigsaw puzzle of northern eurasia abstract northern eurasia consists of the east european siberian north china and alai tarim cratons fragments of the supercontinent rodinia and the orogens of the baikalides timanides uralides altaids and mongolides these can be collectively classified as the central asian supercollage the baikalides and timanides host meso and neoproterozoic magmatic arc terranes that were sutured with the adjacent east european and siberian cratons in the end of the neoproterozoic the paleozoic part of the supercollage consists of three almost synchronous and subparallel neoproterosoic to paleozoic magmatic arc and turbidite superterranes as well as overlap assemblages bent into the world s largest oroclines analysis of their structural pattern supported by paleontological lithological and paleomagnetic data indicates that these superterranes might have been produced via formation of arc backarc systems at the margin of combined north china east european and siberian cratons and then deformed during paleozoic westward directed strike slip translation between the clockwise rotating siberian and eastward moving north china cratons it is proposed that this development took place against the respective breakup of the above mentioned cratons from the northern and southern margins of eastern europe in the neoproterozoic initially as a group of cratons called nena which reassembled in late paleozoic to early mesozoic times into laurasia part of the new supercontinent pangea in mesozoic cenozoic times the subduction related continental growth of northern eurasia continued in the nipponide kamchatka and kolyma alaska orogenic collages of the northern circum pacific which consist of paleozoic to cenozoic turbidite to island arc superterranes and overlap assemblages generally younging towards the pacific oceanic plate and also severely oroclinally bent it is proposed that terranes of the kolyma alaska and kamchatka collages were translated westward dextrally relative to siberia whereas nipponides were translated northward relative to north china similarly to the better constrained mesozoic cenozoic reconstructions of southeastern asia the two groups of collages started to collide along the mongol okhotsk suture zone in the south of the siberian craton in the end of the mesozoic and then continued to collide along the presently active plate boundary at the island of sakhalin the proposed scenario suggests similarities in paleozoic evolution of the central asian and mesozoic cenozoic evolution of the northern circum pacific supercollages both possibly formed in response to westward subduction and related strike slip translation of the paleo pacific oceanic plates the individual superterranes might have been consequently translated for as much as 4000 6000 km and oroclinally bent during such translation or and rotation of the adjacent cratons

strain removal applied to paleomagnetic directions in an orogenic belt the permian red slates of the alpes maritimes france abstract detailed investigations of relationships between strain and directions of characteristic remanent magnetization chrm have been conducted on the permian red series of the alpes maritimes france the analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations in most sites the mean direction of the strain shortening axis is close to the expected permian paleomagnetic direction in this case we observe a large scatter of the chrm directions within each site which is interpreted as an increase of an initial dispersion due to strain moreover this dispersion more or less increases with strain intensity in other cases where the angle between the shortening axis and the paleomagnetic direction is high the whole population is deflected towards the cleavage xy plane these observations lead us to assume a material line like behaviour of the chrm vectors and consequently an attempt has been made to recalculate these directions by strain removal these calculations give positive results at two levels 1 a statistically significant improvement in the clustering of the within site distributions and 2 after tilt correction using unstrained bedding planes a significant improvement in the clustering of the between site distribution the formation mean direction after strain removal is in good agreement with known permian results for stable europe

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

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

evolution of the solonker suture zone constraints from zircon u pb ages hf isotopic ratios and whole rock nd sr isotope compositions of subduction and collision related magmas and forearc sediments abstract the solonker zone in northern inner mongolia china is considered as the suture between the north china craton and the south mongolian microcontinent two magmatic belts are recognized along the suture zone a subduction related magmatic belt represented by the baolidao arc rocks and a younger collision related granite belt represented by the halatu granites we use zircon u pb ages zircon in situ hf isotopic analyses and whole rock nd sr isotopic data of the two magmatic belts and related forearc sediments the xilinhot metamorphic complex to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks a gabbroic diorite bld 1 of the baolidao arc was dated at 310 5 ma by shrimp this sample shows an e nd t value of 2 5 and i sr of 0 7052 hf isotopic analyses on 25 zircons from the same sample show e hf t 5 4 to 11 5 another diorite sample xh 2 of the same arc from south of xilinhot displays even more depleted isotopic compositions with e nd t 5 6 and i sr 0 7037 the main population of zircons from this sample have highly variable and depleted hf isotopic compositions e hf t 0 18 3 the large variation in hf isotopic composition of zircons with largely the same crystallization age from a single pluton is explained by a mixing process between depleted mantle derived magma and continental crust in an active continental arc setting the halatu granite hlt 2 was dated at 234 7 ma by shrimp zircons from the granite also show a large variation of e hf t values 9 1 to 26 despite most samples having whole rock e hf t 2 the large variation in e hf t values suggests that the granite formed probably by partial melting of two source regions a dominant juvenile crust and a subordinate old continental crust most zircons from the xilinhot metamorphic complex show ages comparable with those of the baolidao arc rocks suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism some zircons however show precambrian ages that fall into two groups one with ages of 780 900 ma resembling those from the south mongolian microcontinent and the other with ages of 1524 2900 ma similar to those of the north china craton thus the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents and metamorphosed subsequently during collision in the late paleozoic our zircon age data thus constrain timing of collision between the south mongolian microcontinent and the north china craton to have been between 296 and 234 ma

experimental insights into the partitioning of motion within zones of oblique subduction abstract in zones of oblique convergence the motion is often partitioned into subduction and an associated strike slip component fitch 1972 in this paper we first describe the geodynamic setting of the philippine region as a case study of deformation by strike slip faulting behind parallel subduction zones then we present six small scale laboratory experiments with analogue materials to investigate how partitioning may be controlled by internal boundaries between oceanic lithosphere and continental or island arc lithosphere during the early stages of a typical experiment deformation is taken up on oblique thrusts subduction zones which root into internal boundaries paired strike slip faults develop at later stages thereafter subduction zones and paired strike slip faults operate simultaneously the partitioning occurs without any need for complex constraints on external boundaries

magnetism of south mongolian middle paleozoic ophiolites abstract this paper presents new paleomagnetic data from south mongolia middle paleozoic rocks were studied from three different tectonic zones within the south mongolian fold belt from north to south these are the gobi altai zone in terms of paleotectonic position the north shelf of the paleobasin the trans altai zone ophiolite complexes and the south gobia zone the south shelf several paleomagnetic field tests including conglomerate baked contact and fold tests were carried out to determine magnetic stability in addition principal component analysis of the stepwise thermal demagnetization data was used the ophiolites are characterized by three ancient components one magnetic component is pre folding and directed to the south with a very gentle inclination declination d 180 210 inclination i 6 2 k 120 two post folding magnetic components were observed also the first post folding magnetic component has d 282 4 i 36 4 k 30 3 and the second has d 318 6 i 33 0 k 60 5 the final magnetic component is very close to the permian paleomagnetic direction expected from the north china platform the first post folding component of magnetization may have been acquired during the variscan accretion in early carboniferous time when the basin was closed the south mongolian paleo ocean was located in equatorial latitudes near the tarim block the south china platform and probably the north china platform in the devonian

mylonitic deformation of evaporites in decollements examples from the southern alps france abstract the ductile deformation of triassic evaporites is described in two decollements from the southern alps of france the arc de nice and arc de digne nappes we show that deformation is not chaotic as is commonly considered on the contrary it is commonly well organized and is consistent with progressive shearing typical shear structures include mylonitic foliation stretching lineations folds of various shapes and orientations a type folds and sheath folds asymmetrical pressure shadows composed of gypsum fibres shear bands and boudinage at two sites lineation directions and analysis of shear criteria allow the kinematics of nappe emplacement to be determined computer modelling of pressure shadows allows deformation mechanisms and strain measurements to be determined the results of the analysis can be placed in the context of the regional framework of the two decollements 1 deformation at the nice site corresponds to dextral wrenching on a lateral ramp of the decollement along the western limit of the arc de nice nappe deformation in the ductile fault zone is close to simple shear 2 deformation in the second site is associated with a ramp which affects the decollement in front of the arc de digne nappe the deformation corresponds to shearing towards the southwest including a pure shear component this result can be explained by taking into account the effects of the ramp geometry upon the deformation in the decollement this shows that deformation analysis of evaporites in decollements can be used to understand the mechanisms and kinematics of nappe emplacement

new tertiary paleomagnetic poles from mongolia and siberia at 40 30 20 and 13 ma clues on the inclination shallowing problem in central asia we report results of a paleomagnetic study of 490 cores from 59 sites corresponding to 52 distinct basaltic flows from mongolia and siberia khaton sudal 39 4 ma 44 5 n 101 4 e taatsyn gol 1 31 5 ma 45 4 n 101 3 e 2 28 0 ma 45 5 n 101 1 e ust bokson 19 9 ma 52 1 n 100 3 e and taatsyn gol 3 12 7 ma 45 5 n 101 0 e stepwise thermal and alternating field demagnetizations isolated a stable high temperature component htc of magnetization in most specimens which we interpret as the primary magnetization of these basaltic lava flows the four corresponding paleopoles appear consistent with coeval paleopoles from other asian effusive formations however except for the 12 7 ma paleopole the paleopoles are systematically far sided from the european apparent polar wander path apwp with respect to site locations corresponding to anomalously shallow inclinations in tertiary asian effusive formations in the hypothesis of a dipolar magnetic field in the tertiary this indicates a 1000 1500 km position of the siberia craton and amuria block farther south than expected at 40 and 30 ma tectonically this interpretation implies decoupling and relative rotations between the western and eastern parts of eurasia between the cretaceous and present we show that if siberia were located more to the south the 15 20 paleolatitude anomaly generally observed in sedimentary formations from central asia reduces to a more reasonable average value of 7 which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of asian blocks under the effect of ongoing penetration of india into eurasia in the tertiary

new cretaceous paleomagnetic poles from the tarim basin northwestern china most paleomagnetic studies of the tarim have been focused on the paleozoic with little data published for mesozoic rocks however the ancient configuration of this part of asia has been affected by the indian eurasian collision during the second french chinese karakorum expedition from kashkar to kunjerab in the autumn of 1990 we sampled about 500 cores from 53 sites of upper and lower cretaceous red sandstones from three areas near uytak yingjisha and wuqia 38 5 39 5 n 75 76 5 8 isothermal remanent magnetization acquisition indicates the presence of minerals with high magnetic coercivity stepwise thermal demagnetization confirms that hematite is the dominant carrier and allowed the separation of low temperature components ltc 600 c in most samples the mean direction of ltcs is aligned with that of the present earth field normal and reversed polarities have been found for the htcs of each age group htcs show positive fold tests for at least one sub study where the attitudes of bedding were sufficiently different to allow the test to be performed more important the four sub studies from yingjisha and wuqia give fully compatible results results from the uytak area show a wide dispersion in declinations as could be expected from the rather severe deformation and proximity to major thrust and strike slip faults data obtained in the kuche area from northern tarim by li and co workers and in southern junggar by chen and co workers are statistically identical to the new data from yingjisha and wuqia data from paicheng in northern tarim appear to have been subject to local rotation excluding those sites which appear from paleomagnetic study to have been significantly rotated we calculated a mean cretaceous pole for the tarim basin at 69 1 n 223 88 ass 3 5 n 5 this result indicates that the total post cretaceous rotation of tarim relative to junggar and to eurasia has been negligible 2 6 6 1 and 0 4 4 6 respectively continental shortening and rotation between tarim and junggar within the tien shan are not resolved by paleomagnetism although the values obtained 400 t 600 km 3 6 are compatible with tectonic models of the range a major discrepancy warranting further geological investigation is that between the tarim and north china block poles which would imply some 1400 1060 km of north south relative displacement

magnetic polarity stratigraphy and carboniferous paleopole positions from the joggins section cumberland structural basin nova scotia one hundred seventy seven samples were taken in stratigraphic sequence in redbeds from approximately 2100 m of namurian westphalian strata from the shepody enrage boss point and lower joggins formations between minudie and joggins nova scotia for the purpose of constructing a magnetic polarity stratigraphy in addition five samples from each of four sites were taken from north dipping strata of the cumberland group for the purpose of conducting a fold test for the joggins formation this yields a total coverage of 412 samples from approximately 2400 m of south dipping strata in the composite section when combined with previous results from the maringouin peninsula divenere and opdyke 1990 and 88 samples from north dipping strata collected for the purpose of conducting fold tests two components of magnetization are identified in these sediments a prefolding component termed c and a synfolding component termed b the present results extend the mid carboniferous magnetic polarity stratigraphy to include eight normal and eight reversed magnetozones in the namurian a through early westphalian a interval the results are dominantly to completely reversed in samples from the remainder of the westphalian a and the westphalian b but because of the complication of varying degrees of remagnetization and an 800 m sampling gap it is not possible to verify the base of the permo carboniferous reversed superchron in this time period northward plate motion is inferred from an observed shallowing of inclinations upsection a 5 7 cm yr average poleward drift rate is calculated for this time period from these results yielding a low latitude southern hemisphere paleogeographic position