magnetic polarity stratigraphy of siwalik group sediments of tinau khola section in west central nepal revisited summary remanent magnetization of sandstones sampled at 127 levels sites throughout a 1710 m thick siwaliks molasse sequence exposed along the tinau khola river in nepal has been studied using thermal demagnetization the magnetic remanence consists of a secondary low temperature component of normal polarity unblocked mostly below 400 500 c close to the present day field and a high temperature characteristic remanence chrm unblocked mostly between 600 and 685 c represented by both normal and reverse polarities demagnetization behaviour and isothermal remanence acquisition indicate that the secondary component resides on goethite and maghemite magnetite whereas specular haematite carries chrm anisotropy of magnetic susceptibility data reveals primary depositional magnetic fabric as judged by oblate ellipsoids and subvertical tilt corrected minimum susceptibility axes a magnetic polarity sequence established using tilt corrected chrm directions from 124 levels reveals more details not found in the polarity stratigraphy worked out by munthe et al 1983 comparison of the sequence with a standard polarity time scale harland et al 1989 suggests a depositional time range between c 5 9 ma older than chron 3r and 11 ma younger than chron 5r 2 for the section considered according to new data the horizon of sivapithecus punjabiecus fells close to the reversal boundary at c 8 54 ma the lower age limit of the normal polarity chron 4ar 1 hence the hominoid should be regarded as of c 8 5 ma rather than 9 0 9 5 ma as suggested by munthe et al 1983

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

new late cretaceous and paleogene paleomagnetic results from south china and their geodynamic implications 1 to better understand the tectonic evolution of south china block scb in response to the india asia collision we present new paleomagnetic results from late cretaceous and paleogene red bed formations of the hengyang basin of hunan province in the interior part of the scb stepwise thermal demagnetization of the rocks isolated a high temperature component the tilt corrected mean direction from the late cretaceous rocks is d 15 6 i 29 9 with a95 5 7 n 26 sites corresponding to a paleopole at 71 9 n 236 3 e with a95 4 7 which passes reversal tests the tilt corrected mean direction d 358 9 i 35 4 with a95 5 0 n 22 sites from the paleogene rock which passes the reversal test gives a pole at 82 6 n 300 6 e with a95 4 4 the low field anisotropy of magnetic susceptibility results suggest that the red beds have not experienced significant strain due to compaction or tectonic stress while the anisotropy of isothermal remanence results suggest that postdepositional compaction in these red beds produced no more than 3 4 of inclination shallowing these paleomagnetic and rock magnetic tests imply that the remanence is primary on the basis of paleomagnetic results obtained from coeval basalts in mongolia and siberia we suggest that no significant latitudinal motion has taken place between the scb and siberia since the late cretaceous the significant latitudinal discrepancy between the scb and european block could be due to nonrigid behavior of the eurasian plate since the late cretaceous a 16 7 5 0 clockwise rotation of the scb during the late cretaceous to the paleogene could be related to the collision of india and asia in the west and the subduction of the circum pacific plate to eastern asia in the east

pyrometamorphic processes at the magma hydrothermal system interface of active volcanoes evidence from buchite ejecta of stromboli aeolian islands italy pyrometamorphic ejecta erupted during recent paroxysmal explosions and hydrothermally altered volcanic and subvolcanic rocks from stromboli volcano aeolian islands italy were studied by combined polarizing microscopy electron microprobe scanning electron microscope x ray diffraction transmission electron microscope and whole rock inductively coupled plasma optical emission spectrometry mass spectrometry analysis the pyrometamorphic ejecta are compositionally heterogeneous at the macroand micro scale and are classified as buchites that is glass coupled with a high temperature low pressure modal mineralogy three distinct facies i e cordierite plagioclase mullite plagioclase palisade and tridymite clinopyroxene are recognized among the buchites whose bulk rock chemistry is compatible with aluminousto silica rich protoliths abundant mineral growth e g plagioclase palisades and clinopyroxene coronae occurs at the contact between the ejecta and the crystal rich host shoshonitic basalt which typically coats the pyrometamorphic blocks mineral constraints pseudoporphyritic microstructures plagioclase relict phenocrysts and major and trace element compositions strongly support the origin of the buchite ejecta as hydrothermally altered rocks from thevancori edifice a period of activity spanning 410 kyr which underwent high temperature low pressure contact metamorphism in the uppermost magmatic system of an active island arc volcano such as stromboli volcanic and subvolcanic rocks usually undergo alteration processes induced by acid sulphate hydrothermal systems the circulation of acidic fluids results in leaching of the stromboli rocks in the upper part of the volcano edifice and depending on the ph temperature and exposure time to the hydrothermal fluids production of different alteration facies the hydrothermally altered stromboli rocks show facies consistent with argillic to silicic alteration containing abundant hydrous sulphate and hydroxy sulphate minerals such as the alunite group minerals hexahydrite millosevichite pickeringite and kieserite the hydrothermally altered wall rocks heated by contact with the basaltic magma of the feeder dyke system of stromboli can easily undergo pyrometamorphic processes inducing partial melting and subsolidus crystallization of the volcanic protoliths this results in the development of buchite rocks at the magma hydrothermal system interface

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

unusual lapilli tuff ejecta erupted at stromboli during the 15 march 2007 explosion shed light on the nature and thermal state of rocks forming the crater system of the volcano abstract textural and mineralogical study of high temperature angular blocks erupted during the stromboli explosion of 15 march 2007 was used to make inferences on the nature and thermal state of rocks forming the subsurface of the volcano summit crater terrace the studied ejecta consist of lapilli tuff that formed as a result of the transformation and high temperature induration sintering of the basaltic scoriae lapilli and ash originally accumulated as loose tephra during the current activity of the volcano the main processes leading to the tephra transformation were investigated through microstructural observations mineral and glass analyses sem eds and emp analyses investigations revealed that subsolidus reactions and partial melting of the tephra occurred at temperatures higher than 600 c and under variable fo 2 conditions from qfm to hm buffering curves in some blocks evidence of high t reheating and partial melting at the expense of secondary hydrothermal minerals was also observed in order to track the subsolidus reheating history of the basaltic pyroclasts a detailed study of the pseudomorphic phases and reactions after olivine driven by iron oxidation under high t conditions was performed the observed mineralogical transformation suggests that the lapilli tuff material originating from the burial of tephra routinely accumulated by persistent strombolian explosions within the crater terrace were in some cases altered by the circulation of acidic fluids and were in any case reheated due to isotherm rise forced by high heat flux and gas streaming delivered by the underlying magma system it is worth noting that the ejection of these unusual volcanic lithotypes was possible because a few days before the 15 march 2007 event the craters were clogged with lapilli tuff material that slid into the crater bottom between 7 and 9 march findings of this study suggest that the scattered permanently active vents and shallow conduits of stromboli are surrounded sideways and underneath the crater terrace by a fairly large volume of high temperature rocks with variable degree of compaction sintering up to partially melted such a spectrum of rock types is in good agreement with the conceptual model of prominent thermal zoning all around sideway and upwards the active magmatic system we speculate that continuous migration upwards of isotherms led to transformation and partial melting of the normal strombolian tephra

paleomagnetic evidence for clockwise rotation of the simao region since the cretaceous a consequence of india asia collision samples were collected for paleomagnetic analysis from 36 sites from six formations west of the red river fault in southwestern yunnan province china on the simao block 23 4 n 100 8 e in an attempt to examine the effects of the india asia collision on the asian continent paleomagnetic results from rocks ranging from cretaceous to miocene in age indicate a large clockwise rotation of these sites with respect to stable eurasia since the late cretaceous the tectonic corrected data from cretaceous samples mean direction of two formations are d 112 6 i 34 7 95 7 5 k 9 1 n 45 for eocene samples the data are d 84 7 i 38 9 95 7 6 k 12 0 n 32 and for miocene samples they are d 21 1 i 35 5 95 7 1 k 15 3 n 29 the samples from the paleocene and pliocene are either remagnetized or too weak to provide stable directions the cretaceous sites were sampled from folded beds and these sites pass the fold test the amount of rotation seen in this study r 86 1 9 9 as a mean value for the cretaceous r 76 8 11 for the eocene and r 14 7 10 0 for the miocene is significantly larger than that documented in previous studies from the lower cretaceous of northwestern and central yunnan province and may indicate that differential rotation has taken place in the region this rotation can be explained by the northward motion and collision of india with asia in the paleocene which caused continuous deformation of southwestern china in the form of strike slip faulting and clockwise rotation it is concluded that the rotation and deformation resulting from this collision probably continued from the paleocene until at least the miocene

anomalously shallow inclination in middle northern part of the south china block palaeomagnetic study of late cretaceous red beds from yichang area summary we present new palaeomagnetic results from late cretaceous red beds of the paomagang formation collected at 42 sites in the yichang area 30 7 n 111 7 e middle northern part of the south china block a high unblocking temperature component around 680 c was isolated from 26 sites by stepwise thermal demagnetization fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the paomagang and wangdian areas respectively normal and reversed polarity sequence found in the wangdian area passed a reversal test at the 95 per cent confidence limit the tilt corrected mean direction for yichang area is d 3 8 i 23 3 95 5 1 n 26 corresponding to a palaeopole at 71 5 n 280 0 e with a95 4 3 comparison with the expected inclination from the 80 ma eurasian apwp pole indicates a inclination flattening of 26 3 6 4 middle cretaceous to cenozoic nw se extension tectonics within the scb contributes only 2 0 to the inclination flattening while the massive samples of the paomagang formation show a moderate degree of anisotropy 1 017 pj 1 086 and steeper inclination i 29 3 the laminated samples which form more than 75 per cent of the studied samples show a higher degree of anisotropy 1 052 pj 1 227 and shallower inclination value i 22 7 a good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the yichang area

magnetostratigraphy of the miocene pleistocene surai khola siwaliks in west nepal summary phic age dating has been carried out on sandstones of a wellexposed continuous 5500 m thick miocene pleistocene section in the surai khola area west nepal the section is situated in the siwalik range which consists of molasse sediments from the himalaya progressive demagnetization and isothermal remanence experiments prove that haematite is the carrier of the characteristic remanent magnetization chrm throughout the entire section the final polarity sequence is based on 436 samples of which 233 have normal and 203 reverse polarity antiparallel mean directions of the normally and reversely polarized samples indicate that the chrm is of primary origin comparison of the polarity sequence with a standard polarity time scale yields a time period for the section between chron lr or chron 2r matuyama and probably chron 5ab between 1 2 myr and about 13 myr

a cretaceous pole from south china and the mesozoic hairpin turn of the eurasian apparent polar wander path to contribute to the apparent polar wander path apwp of the south china block and eurasia in general we collected paleomagnetic samples from mesozoic red beds around the city of ya an 30 n 103 e in the western tip of the sichuan basin in this paper we present the results from 373 oriented cores taken from one section representing 3 km of sedimentary rocks the section is dated with continental ostracods and with a magnetostratigraphic correlation between a densely sampled 272 m sequence and the polarity time scale giving an upper jurassic to upper cretaceous or lower tertiary age the remanent direction is remarkably stable throughout the section d 2 0 i 34 2 k 63 1 95 3 6 n 26 28 sites while this fact might suggest that the section has been remagnetized paleomagnetic and rock magnetic tests indicate that the remanence is primary the pole position 78 6 n 273 4 e dp 2 4 dm 4 1 corresponds to a rather low paleolatitude 18 8 2 4 but is consistent with other cretaceous poles from china if one accepts the eurasian apwp of irving and irving 1982 this result would imply that more than 1000 km of shortening took place between south china and eurasia following the acquisition of the remanence however there is no geological evidence for this large shortening we propose that the remanence was acquired within the time corresponding to the tip of the hairpin turn 150 50 ma in the revised apwp of besse and courtillot this issue the local geology suggests that the syncline from which the samples were taken has been rotated by 15 5 counterclockwise which is reflected in a similar discrepancy between the measured paleodeclination and that predicted by the besse and courtillot this issue eurasian apwp after correcting for this rotation the pole position is 70 9 n 225 2 e dp 2 4 dm 6 5 we conclude that eurasia was fully assembled by the end of the jurassic and that the mesozoic eurasian hairpin turn is a real feature

paleomagnetic study of permian and mesozoic sedimentary rocks from northern thailand supports the extrusion model for indochina abstract the mode of deformation of southeast asia resulting from the indian collision is still controversial is convergence between stable india and siberia absorbed principally by crustal shortening and thickening and minor strike slip faulting or by a mechanism of extrusion involving crustal thickening and lateral escape of blocks along major strike slip zones in order to test these models we have carried out a paleomagnetic study of sedimentary rocks of jurassic to cretaceous age on the khorat plateau in thailand a large discrepancy between the poles of indochina presented in this paper and a compilation of south china block pole positions both at the triassic jurassic and the jurassic cretaceous boundary imply some 1500 800 km of post middle cretaceous left lateral slip along the red river and xian shui he fault zones and approximately 14 7 of clockwise rotation for the indochina block relative to the south china block such relative motions are in agreement with a model of lateral extrusion of indochina during the india asia collision additional data of permian upper triassic and lower jurassic age collected in indochina indicate a post late permian remagnetization for the upper permian limestones a comparison of these data with those from yunnan south china the north china block and the south china block shows that these blocks have probably been in contact at least since the late triassic

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

origin of high silica liquids at stromboli volcano aeolian islands italy inferred from crustal xenoliths high silica igneous xenoliths granophyre and obsidian fragments with sio2 ca 75 wt and high silica glass sio2 between ca 64 and 75 wt within glass bearing tonalitic and dioritic xenoliths have been discovered at stromboli they are well beyond the silica range of the volcanic rocks erupted during the subaerial activity of the volcano the granophyre and the obsidian fragments occur within the petrazza pyroclastics high k andesite pumiceous scoriae of the paleostromboli i period 61 ka whereas the glass bearing tonalites and diorites are hosted in the omo basaltic andesite lava flows of the paleostromboli ii period 64 55 ka the obsidian represents an extremely evolved liquid derived from low pressure fractional crystallization of high k calc alkaline magmas of stromboli coupled with minor assimilation of upper crust or terrigenous sediments the other studied high silica products have an anatectic origin the granophyre composition is compatible with a genesis by low degree melting of leucotonalites similar to some calabrian arc lithotypes partial melting of crustal rocks from the stromboli basement is also demonstrated by interstitial glass in the tonalite and diorite xenoliths textural evidence and the presence of variable glass compositions suggest that in these plutonic xenoliths the melt chemistry was controlled by dehydration melting of biotite and non modal melting of local mineralogy high degree undercooling of granophyre melt and of the partially melted xenoliths is consistent with rapid rise of the host magmas leading to eruption whether partial melting was initiated during stoping and transport of xenoliths within the rising magma or in situ during magma storage within the crust has not been unequivocally established

carbonate assimilation at merapi volcano java indonesia insights from crystal isotope stratigraphy recent basaltic andesite lavas from merapi volcano contain abundant complexly zoned plagioclase phenocrysts analysed here for their petrographic textures major element composition and sr isotope composition anorthite an content in individual crystals can vary by as much as 55 mol an

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

magnetostratigraphy of tertiary sediments from the hoh xil basin implications for the cenozoic tectonic history of the tibetan plateau summary we conducted an integrated palaeomagnetic and stratigraphic study on a 5452 8 m thick sedimentary sequence of the hoh xil basin in northern qinghai tibet plateau to obtain a chronostratigraphic framework for these sediments a total of 966 individual oriented palaeomagnetic samples spaced at stratigraphic intervals were collected from six measured sections in the hoh xil basin magnetic directions in these samples were obtained by progressive thermal mainly and alternating field demagnetization experiments most samples exhibit two components of magnetization the lower unblocking temperature component is an overprint resembling the present day geocentric axial dipole field direction at the sampling locality the most stable characteristic remanence chrm appears to be an early chemical remanent magnetization residing mainly in haematite the positive results of fold and reversal tests indicate that the chrm is a record of the palaeomagnetic field close to the time of formation of these sediments further evidence for the magnetization of these sediments acquired close to their time of deposition is the fact that patterns of magnetic reversals can be matched with the established polarity timescale on the basis of the distinct interval of magnetic reversal zones and biostratigraphic datums 13 magnetozones can be recognized at the hoh xil basin that range from chrons c11 to c23 30 1 51 0 ma the age of the fenghuoshan group is palaeomagnetically dated as 51 31 ma early eocene middle early oligocene and the age of the yaxicuo group is between 31 and 30 ma middle early oligocene late early oligocene the new palaeomagnetic data from the fenghuoshan group suggest that it has undergone no significant rotation since the oligocene in contrast declination data from the yaxicuo group in wudaoliang area imply a vertical axis clockwise rotation 29 1 8 5 since the late oligocene the tertiary palaeomagnetic pole position of the hoh xil basin implies a significant northward convergence of the hoh xil basin 1600 km with respect to eurasia siberia since early eocene late oligocene time our results are consistent with the pattern of disturbingly low palaeolatitudes derived from a large number of high quality palaeomagnetic studies of tertiary rocks from sites that reach all the way from eastern china to kyrgyzstan future work is needed to separate the influences of sedimentary inclination shallowing and tectonic shortening

timing of the north south china collision from new middle to late mesozoic paleomagnetic data from the north china block we collected paleomagnetic samples from 43 sites of middle and upper jurassic and cretaceous rocks along a 250 km wide east west trending belt located north of the suture between the north and south china blocks in each case laboratory treatment isolates dual polarity antipodal magnetizations that are demonstrably pre folding the paleomagnetic poles show very good coherence with coeval poles from other localities from the north china block ncb suggesting the apparent polar wander path apwp from the ncb is not distorted by post middle jurassic local vertical axis block rotations a comparison of the ncb apwp with that from the south china block suggests that collision continued after the early jurassic and ended no later than the late jurassic a cusp in the ncb apwp coincides with a marked angular unconformity that separates middle jurassic from upper jurassic and younger sediments together these observations indicate that the scissor like collision and associated deformation ceased at the middle to late jurassic boundary 159 ma the time of rapid 1 ma relative angular velocity between the two plates 225 to 190 ma coincides with a peak in u pb and ar ar dates obtained from metamorphic rocks in the suture the combined late jurassic and cretaceous paleomagnetic data from both blocks suggest that the united china plate traveled 2000 km north relative to eurasia during the late jurassic before docking in the cretaceous this younger collision event is also marked by a cusp on the cretaceous part of the eurasian apwp

growth after collapse the volcanic and magmatic history of the neostromboli lava cone island of stromboli italy this study focuses on the holocene neostromboli lava cone which is part of the stromboli volcano aeolian islands italy it represents an important example of the structure and behaviour of lava cone re growth shortly after a large scale volcano sector collapse neostromboli s eruptive style is dominated by extrusion of sheet like a a lava flows and associated autoclastic breccias the lava cone developed during five phases of activity phases a to e representing discrete episodes of lava effusion characterised by rapid changes in magma composition spatial development of the feeder system and physical properties of erupted lavas through time although they all belong to the potassic series ks with sio2 49 55 wt and mgo 2 50 6 25 wt the neostromboli lavas have distinct petrogenetic characteristics ranging from leucite free and low sr shoshonitic basalts phases a and b to leucite bearing shoshonitic basalts with high phase c and intermediate phase d sr contents and to leucite free biotite bearing shoshonites phase e three different batches of mantle derived relatively primitive basic magmas fed the activity of phases a c and d the evolution phases a to b is inferred to have occurred in a continuously replenished tapped and crystallizing rtf magma chamber whereas phase e products were dominantly generated by fractional crystallization from phase d magmas the neostromboli cone shows summit flank and satellite vents and sheet intrusions with different geometry and location through time reflecting the interplay of regional tectonics precursory instability processes and volcano load

large n s convergence at the northern edge of the tibetan plateau new early cretaceous paleomagnetic data from hexi corridor nw china abstract nine early cretaceous paleomagnetic sites have been collected in the yumen area of the hexi corridor nw china magnetic directions isolated at lower temperatures fail the fold test and lie close to the geocentric axial dipole field direction before tilt correction high temperature components are carried by magnetite and or hematite all with normal polarity and pass the fold test the average paleomagnetic pole from the nine sites is at 75 5 n 169 9 e a 95 7 7 these results are consistent with those from other areas of the north china block ncb but significantly different from those from the qaidam basin on the southern side of the qilian mountains they suggest that 1 the yumen region behaved as a rigid part of the ncb since at least the early cretaceous 2 740 500 km of north south directed convergence has taken place between the ncb and qaidam within the qilian mountains and 3 extrusion of qaidam was accompanied by a 23 5 relative rotation with respect to north china this is larger than implied by the maximum left lateral slip on the altyn tagh fault system the same data imply some 1000 800 km of cenozoic motion between the tarim and ncb blocks which were so far believed to have formed a rigid entity since at least the jurassic one interpretation could be that all tarim and qaidam cretaceous paleomagnetic samples from red beds but not those from yumen and the ncb suffered significant inclination shallowing as observed in cenozoic red beds from central asia so far we do not find support for this possibility possible tectonic interpretations include 1 the existence of a large as yet uncharted tectonic discontinuity between tarim and the ncb in the vicinity of the desert corridor near 95 100 e longitude 2 the occurrence of significant deformation within southwestern tarim to the north of yingjisha where paleomagnetic sites were obtained or 3 persistent clockwise rotation of tarim with respect to the ncb for at least 20 ma at the rate found for current block kinematics

the lower cretaceous reference pole for north china and its tectonic implications summary a palaeomagnetic study of lower cretaceous continental sediments from the south southwestern part of the ordos basin north china block ncb has been carried out stepwise thermal demagnetization was used to isolate magnetic components the low temperature component is generally removed between 200 c and 350 c and corresponds to the earth s present field direction a hightemperature component of magnetization with two polarities was defined from 10 sites with d 16 4 i 50 5 k 50 8 ugs 6 8 the corresponding pole position is located at 75 8 n 208 7 e d 6 lo d 9 1 this lower cretaceous pole compares well with the hairpin loop of the eurasian synthetic apw path besse courtillot 1991 that is inferred from the north american eurasian african and indian plates the available late mesozoic palaeomagnetic data from the ncb and adjacent south china block scb are reviewed comparison of these data suggests important intraplate deformation the area to the east of the tancheng lujiang fault may have undergone local clockwise post cretaceous rotations of 10 29 relative to the stable western craton of the ncb the data also indicate that no palaeomagnetically significant eastward movement of the scb with respect to the ncb and eurasia occurred after the cretaceous

the apparent polar wander path for the north china block since the jurassic summary a palaeomagnetic study was carried out on sedimentary rocks from datong shanxi province and on volcanic rocks from chifeng inner mongolia and zhangjiakou hebei province china all of these sites belong to the stable part of the north china block ncb after detailed demagnetization experiments reliable palaeomagnetic directions were obtained for middle jurassic late cretaceous and late tertiary these data define the apparent polar wander path for the ncb since the jurassic free from the possible effect of the movement along tanlu fault since the mesozoic comparisons with reliable palaeomagnetic poles obtained in siberia and south china indicate that the middle jurassic poles for north and south china are almost the same while the siberian pole is displaced from them to the west by about 18 13 on the other hand the cretaceous poles for the three areas are not significantly different from each other consequently it is suggested that the amalgamation of north china and south china was almost complete in the middle jurassic and that their accretion to siberia continued until the late cretaceous

rotational overthrusting of the northwestern himalaya further palaeomagnetic evidence from the riasi thrust sheet jammu foothills india abstract thermal demagnetization results 316 samples are presented for the tertiary succession of the riasi thrust sheet jammu foothills northwestern himalaya primary and secondary magnetization directions of murree group red beds miocene to upper eocene sampled northeast of jammu indicate for this part of the riasi thrust sheet a clockwise rotation over about 45 with respect to the indian shield since late eocene early miocene time this accords with clockwise rotations of similar magnitude observed in the panjal nappe and the krol belt and is interpreted as representative for the northwestern himalaya results from the western part of the kalakot inlier sampled northwest of jammu i e basal murree claystone middle eocene and carbonate from the subathu group lower middle to lower eocene indicate an aberrant 20 25 counterclockwise rotation which is of local importance only available observations on rotation of himalayan thrust sheets with respect to the indian shield indicate that the himalayan arc has formed through oroclinal bending this supports powell and conaghan s and veevers et al s model of greater india with large scale intracontinental underthrusting along the main central thrust beneath the tibetan plateau minimal magnitudes of underthrusting of 550 km in the krol belt and 650 km in the thakkhola region are concluded palaeolatitude observations herein and in 1 agree with absolute positioning of the indian plate based on india africa relative movement data fixed to a hotspot frame in the atlantic ocean and with palaeolatitude observations from dsdp cores on the indian plate collision related secondary magnetic components observed both to the north and to the south of the indus tsangpo suture zone show palaeolatitudes between the equator and 7 n comparison of both datasets indicates that initial contact between greater india and south central asia had been established in the hindu kush karakorum region by about 60 ma ago whereas eastwards progressive suturing had advanced to the lhasa block segment of the indus tsangpo suture zone before 50 ma ago

a palaeomagnetic study from the mongol okhotsk region rotated early cretaceous volcanics and remagnetized mesozoic sediments we collected 47 sites of upper triassic to middle jurassic siltstones and sandstones and two sites of lower cretaceous andesites in a large basin located south of the mongol okhotsk suture the suture separates the siberian craton to the north from the mongolian and chinese blocks to the south laboratory treatment and analyses identify the same post folding direction in all rocks the mean direction with a corresponding palaeopole at 76 8on 152 2oe a95d 4 2o of the overprint component nd 49 sites is significantly different at the 95 confidence level from the expected time averaged brunhes or present day field directions the lower cretaceous andesites possess a stable remanent direction at high temperatures that is distinct at 95 confidence limits from the overprint direction the corresponding pole 58 3on 51 0oe dp dm d 3 8o 4 6o based on only fourteen samples is significantly rotated 78 4o 5 3o counterclockwise and insignificantly displaced 2 7o 3 4o north with respect to the early cretaceous reference pole for siberia we argue that the rotation is likely tectonic in nature and not due to a chance reading of the palaeosecular variation of the earth s magnetic field both the palaeomagnetic data and the folding patterns we observed in the field suggest that deformation associated with the suture continued after the early cretaceous and involved sinistral shear 1998 elsevier science b v all rights reserved

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

evidence for late paleozoic and mesozoic non dipole fields provides an explanation for the pangea reconstruction problems abstract paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole but in recent years it has become a concern that this assumption is not always valid because of long term contributions from non dipole fields in particular octupole fields we tested for the presence of a zonal octupole field in the late carboniferous permian mesozoic and early tertiary by comparing the observed paleomagnetic paleolatitude distributions for the laurentian north america and greenland and european landmasses with those predicted from the mean paleopoles such a test is possible for a good sized continent not subsequently deformed when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes if the field were purely dipolar coeval results should form a coherent dataset such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1 if the slopes of regression lines are systematically different from 1 0 for successive time intervals and granted that the earth s radius stayed the same then it is likely that non dipole fields can be held responsible regression line slopes standard errors for three intervals analyzed are 0 78 0 04 200 300 ma 0 93 0 11 120 200 ma and 0 82 0 06 40 120 ma as might be expected with a non dipole contribution to the total geomagnetic field paleomagnetic results from the most northerly areas of laurentia and europe i e ellesmere greenland and svalbard show clearly anomalous paleolatitudes because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain forest occurrences d v kent p e olsen earth planet sci lett 179 2000 311 324 we argue that long term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies estimates of the magnitude of the octupole dipole field ratio are not very precise but center around 0 1 which could cause errors in conventional paleopole determinations using the dipole formula of 7 5 because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid northern and mid southern hemisphere locations could thus be off by as much as 15 the well known misfit between the paleomagnetic results from the laurentia european and gondwana continents in a classical pangea a configuration could be explained by such errors due to octupole fields this explanation would negate the need to seek tectonic pangea b type solutions for the misfit these solutions have generally remained unacceptable in terms of the geology of the gondwana laurussia borderzone

inclination flattening and the geocentric axial dipole hypothesis abstract william gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis the gad hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude for decades there have been calls for permanent non dipole contributions to the time averaged field recently these have demanded large contributions of the axial octupole which if valid would call into question the general utility of the gad hypothesis in the process of geological recording of the geomagnetic field earth filters distort the directions many processes for example sedimentary inclination flattening and random tilting can lead to a net shallowing of the observed direction therefore inclinations that are shallower than expected from gad can be explained by recording biases northward transport or non dipole geomagnetic fields using paleomagnetic data from the last 5 million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model such a model can predict not only the average expected direction for a given latitude but also the shape of the distribution of directions produced by secular variation the elongation of predicted directions varies as a function of latitude from significantly elongate in the up down direction at the equator to circularly symmetric at the poles sedimentary inclination flattening also works in a predictable manner producing elongations that are stretched side to side and the degree of flattening depending on the inclination of the applied field and a flattening factor f the twin tools of the predicted elongation inclination relationship characteristic of the geomagnetic field for the past 5 million years and the distortion of the directions predicted from sedimentary inclination flattening allows us to find the flattening factor that yields corrected directions with an elongation and average inclination consistent with the statistical field model the method can be tested using sediments deposited in a known field application of the elongation inclination correction method to two magnetostratigraphic data sets from red beds in asia and pakistan brings the inclinations into agreement with those predicted from modern gps measurements and from global paleomagnetic data there appears to be no compelling reason at this time to abandon the geocentric dipole hypothesis which has provided such an excellent working model for so long

magnetic polarity stratigraphy of siwalik group sediments of karnali river section in western nepal the remanent magnetization of siltstones and sandstones sampled at 476 levels sites throughout a 3560 m thick molasse sequence belonging to the siwalik group 0 2015 m lower 2015 3560 m middle has been studied by stepwise thermal demagnetization this section is exposed along the karnali river in nepal the natural remanent magnetization nrm usually consists of two components a viscous or thermoviscous component of recent field origin and an ancient characteristic component chrm the former component is of normal polarity and resides either in goethite unblocking temperature 150 c resistant to afd up to 150 mt or in maghemite unblocking temperature 150 400 c goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section maghemite content is significant in the grey mud free lithologies from the upper half of the section the main component unblocked in the high temperature range commonly 610 680 c and believed to reside in haematite presumably of mostly detrital origin represents a characteristic remanence chrm the tilt corrected chrm directions at individual sites show antipodal clusters ratio of normal to reverse polarity sites 0 62 and yield mean inclinations recording significant inclination shallowing a feature well recorded in the siwaliks this chrm is interpreted to represent a largely primary detrital remanence the chrm data from 430 sites yield the karnali river magnetic polarity sequence whose correlation with the geomagnetic polarity timescale cande kent 1995 suggests a depositional age of 16 ma younger than chron c5cn 1n to 5 2 ma around the top of chron c3r for the 3560 m section sampled hence the karnali river exposes the oldest part of the siwalik group in nepal estimates of the sediment accumulation rate sar average to 32 9 cm kyr 1 for the 10 8 myr time span of deposition

mongolian plateau evidence for a late cenozoic mantle plume under central asia the 2500 x 700 km mongolian plateau average elevation 2000 m is situated between the altai orogen and the siberian craton and occupies much of mongolia and transbaikalia in russia the plateau is characterized by 1 basin and range topography and two major domes hentai 600 x 300 km and hangai 800 x 550 km where altitudes reach 3905 m 2 lithosphere that is thinner than adjacent areas minimum 50 km 3 elevated heat flow up to 120 mw m2 4 dominantly alkaline basaltic volcanism in the form of cones lava fields and volcanic plateaus mostly of miocene quaternary age and 5 rifts including baikal main evolution in the pliocene quaternary tunka oligocene early miocene and hobsogol pliocene quaternary existing models explain these features in terms of diapiric upwelling of a mantle asthenolith below the main rifts and or as a long distance effect of the india asia collision we propose that the late cenozoic uplift of the whole mongolian plateau and associated rifting magmatism high heat flow and lithospherec thinning are not externally driven by the india asia collision but are the expression of the interaction of a mantle plume with overlying lithosphere some rifts link and interact with major strike slip faults such as the bolnai such faults may be the major expression of the india asia collision in this region

late cretaceous to early paleogene paleomagnetic results from sikhote alin far eastern russia implications for deformation of east asia abstract welded tuffs in the bogopol and sijanov groups were sampled at 27 sites from 12 caldera formations in the sikhote alin mountain range around kavalerovo 44 3 n 135 0 e for chronological and paleomagnetic studies k ar age dates show that the welded tuffs erupted between 66 ma and 46 ma all sites yield reliable paleomagnetic directions with unblocking temperatures higher than 560 c the high temperature component at 12 sites and the medium temperature component at 3 sites in the bogopol group show reversed polarity d 193 7 i 57 6 95 8 1 the high temperature component at 11 sites in the sijanov group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north d 2 9 i 59 6 95 11 2 the combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250 5 e 84 1 n a 95 8 8 which falls near cretaceous paleomagnetic poles from outer mongolia inner mongolia the north china block and the south china block the sikhote alin area appears not to have been subjected to detectable motion with respect to east asia since about 50 ma this implies that the sikhote alin area behaved as an integral part of east asia during the opening of the japan sea at about 15 ma however significant separation between the paleomagnetic poles of east asia and europe during the jurassic paleogene implies a major relative movement between these two blocks since the paleogene

cretaceous paleomagnetic results from western tibet and tectonic implications we present paleomagnetic results obtained from samples collected in the summer of 1989 along a 1000 km traverse between yecheng xinjiang and shiquanhe tibet in the western qinghai xizang plateau along 80 e longitude during this field trip 480 paleomagnetic cores in jurassic to tertiary limestones and sandstones were drilled at 49 sites 400 were measured isothermal remanent magnetization analysis indicates the existence of high coercivity minerals and thermal rather than alternating field demagnetization was found to be more efficient in isolating paleomagnetic directions the characteristic magnetization has rather low unblocking temperatures 250 560 c in most limestone samples and higher unblocking temperatures 400 c 680 c in the sandstone samples about half of the measured samples show the present geomagnetic field or aberrant directions the remainder display stable multicomponent magnetizations the stable characteristic magnetizations indicate 1 a probable jurassic remagnetization 2 a cretaceous 120 80 ma magnetization which passes a fold test yielding a mean pole at 66 2 n 245 0 e a95 5 1 n 14 sites and 3 a large scatter of tertiary directions which may be due to tectonic problems and or remagnetization in the sampling zone the cretaceous paleodirections from aksaichin and longmuco 11 sites on the qiangtang north tibet block are very similar to those from shiquanhe three sites on the lhasa south tibet block separated by 300 km this suggests that the north and south tibet blocks formed a single unit at least as early as the cretaceous comparing our results with previous paleomagnetic work on the tibetan plateau it is found that the paleolatitude of 10 6 5 1 obtained from this study for a reference at domar 33 75 n 80 4 e is close to that 9 6 2 8 obtained by achache et al 1984 and lin and watts 1988 from the takena formation 110 100 ma in the lhasa region 30 n 91 e in addition it appears that the southern margin of the tibetan blocks i e eurasia occupied an e w trending position at about 7 6 n latitude paleodeclination differences between the western and eastern tibetan blocks suggest internal deformations of the plateau at different scales from 1 to 1000 km according to differences in paleomagnetic declinations the arcuate shape of the blocks would have been partly acquired after the collision

paleomagnetism of tertiary rocks on the korean peninsula tectonic implications for the opening of the east sea sea of japan abstract two different distributions of declinations with dual polarities are found in the tertiary rocks of the yeonil basin and yangnam basin along the yangsan fault in the southeastern part of the korean peninsula these are northward declinations in the middle miocene yeonil group and strong eastward declinations in the early miocene yangbuk group and in the eocene volcanic rocks this is interpreted by invoking 1 the yangsan fault as a dextral wrench fault system by spreading in the uleung basin of the east sea sea of japan and 2 a clockwise vertical rotation caused by the movement of the fault at around 17 3 ma just prior to the sedimentation of the yeonil group the timing of the deflection found in korea is distinguished from that in southeastern and northwestern japan at around 15 ma two stages of spreading are thus constrained in the east sea area parallel at 23 15 ma and pivotal at around 15 ma crustal thinning also occurred a reliable pole position 84 9 n 292 6 e d p d m 3 3 4 8 for the miocene about 15 ma in korea is obtained from the yeonil group

comparison of the dynamic and parameterized models of mantle convection including core cooling the two dimensional cooling model of mantle convection including core cooling was studied the core temperature is assumed to be constant in space but it changes with time because of the convective retrieval of heat the viscosity is also constant in space but it is a function of the area averaged mean temperature in order to check the possible effects of temperature dependent viscosity on the cooling the rate of cooling is controlled by three factors the temperature change of the core by a unit change in the core heat flux the viscosity law governed by the mean temperature and the contribution of the internal heating several different types of temperature field were used as the initial conditions some results showed considerable fluctuations in the relation between the temporal rayleigh number and the nusselt numbers which are appropriately defined for both the upper and lower thermal boundary layers a parameterized model of the cooling history of the mean and bottom temperatures was calculated assuming the power law relation between the rayleigh and the nusselt numbers which is determined by the least square fit of the results of the dynamic calculations for the parameterized models we integrated backward in time the agreement between the dynamic and parameterized calculations is generally good except for the cases when the rate of cooling is large or the decay in internal heating is small when the rate of the cooling is large the lower thermal boundary layer disappears this results in unpredictability of the bottom temperature however this case is not applicable for the earth when the decay in internal heating is small we find that the agreement between the dynamic and parameterized models is sensitive to the change in the amount of internal heating assumed in parameterized models this is because the internal heat source directly affects the rate of cooling the value which is the power index of the nu ra relation varies from 0 3 to 0 4 when the bottom thermal boundary layer becomes weak because of the fast cooling of the core deviates from 0 3 considerably

refining gondwana and pangea palaeogeography estimates of phanerozoic non dipole octupole fields summary reliable phanerozoic paleopoles have been selected from the stable parts of the gondwana continents and upon appropriate reconstruction have been combined in an apparent polar wander apw path which can be compared with a previously compiled path for laurussia this comparison once again confirms that pangea a reconstructions for late palaeozoic and early mesozoic times cannot be reconciled with the available paleomagnetic data unless these data are corrected for latitudinal errors caused by non dipole octupole field contributions or by inclination shallowing because the discrepancies persist even when only paleopoles from igneous rocks are used inclination shallowing cannot be the sole cause of the problem there is an apparent decrease in the percentage of octupole field contributions needed as a function of time for mesozoic and younger time gondwana laurussia comparisons require on average lower ratios of octupole dipole fields than for palaeozoic time however the gondwana paleopoles for the palaeozoic include a much greater proportion of results derived from sedimentary rocks than do those for the mesozoic so that this apparently diminishing octupole field contribution may be an artefact we have also examined whether the clustering of coeval gondwana poles improves with optimal g3 contributions but found that while there are improvements they are not systematic and not statistically significant a combined apw path has been constructed for pangea for times since the mid carboniferous which accounts for octupole fields or equivalently inclination shallowing we argue that this global path is an improvement over previous constructions as it represents a self consistent plate tectonic model and does not violate widely accepted pangea a reconstructions

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

mafic and ultramafic xenoliths in san bartolo lava field new insights on the ascent and storage of stromboli magmas mafic and ultramafic xenoliths are well represented within a large basaltic lava field of stromboli these basalts known as san bartolo lavas show a high k calc alkaline hkca affinity and were erupted 5 ka bp xenoliths consist of olivin gabbro gabbronorite anorthosite dunite wehrlite and clinopyroxenite thermobarometric estimates for the crystallization of gabbroic materials show minima equilibration pressures of 0 17 0 24 gpa at temperatures ranging from 940 to 1 030 c these materials interacted with hydrous ascending hkca basaltic magmas with temperatures of 1 050 1 100 c at pressures of about 0 2 0 4 gpa these pressure regimes are nearly identical to those found for the crystallization of phenocrystic phases within hkca basaltic lavas gabbroic inclusions are regarded as cumulates and represent crystallized portions of earlier hkca strombolian basalts

a palaeomagnetic reconnaissance of northeastern baluchistan pakistan a total of 560 samples from three areas in northeastern baluchistan the southern sulaiman range the central loralai range and the northern kirthar range were analyzed using thermal demagnetization techniques thirteen formations of late palaeozoic to early tertiary age were studied palaeomagnetically inclinations of the obtained results show a general affinity with the indian apparent polar wander path deviating declinations from the loralai range indicate a clockwise rotation over 50 with respect to the indian shield secondary magnetization components probably of late palaeocene to early eocene age were consistently present in the samples from the kirthar range and the sulaiman range but were not observed in samples from the loralai range acquisition of these components is attributed to crustal upwarping during the palaeocene which is tentatively related to initial collision of continental indo pakistan with an island arc off south central asia the baluchistan data support recent palaeomagnetic results from the indus tsangpo suture zone in ladakh nw himalaya which indicate that such an initial collision took place at very low northern palaeolatitudes

new cretaceous and early tertiary paleomagnetic results from xining lanzhou basin kunlun and qiangtang blocks china implications on the geodynamic evolution of asia we present the results of a paleomagnetic study of 360 cores drilled at three different areas from the tibetan plateau 13 sites from lower cretaceous red beds in the xining lanzhou basin around xining and lanzhou cities southwest of the qilian mountains 36 2 n 103 5 e 13 sites from cretaceous red beds in the kunlun block near maqin 34 5 n 100 1 and 9 sites from paleocene lower eocene red beds in the qiangtang block near fenghuoshan 34 5 n 92 8 e thermal demagnetization of the samples allowed us to isolate a high temperature component which passes both positive reversal and fold tests for all formations the corresponding paleopoles lie at 50 3 n 195 5 e a95 4 6 for xining lanzhou 80 1 n 281 2 e dp 7 8 dm 12 7 for maqin and 62 6 n 210 5 e dp 3 9 dm 6 8 for fenghuoshan we discuss these in the frame of a new paleogeographic reconstruction of the cretaceous paleoposition of the blocks forming the asian mosaic we conclude that the xining lanzhou area could not be part of the north china block but rather was associated with the tarim qaidam assemblage paleomagnetic data argue in favor of a qaidam kunlun tarim junggar assemblage in the cretaceous significantly to the south of its current position with respect to the asian continent siberia mongolia and north china the large n s convergence 800 500 km implied since the cretaceous appears to be far larger than could be absorbed in the altay range to the north and qilian shan to the east of the order of 300 km part of this motion could have occurred along a large left lateral strike slip fault system which may connect with the mongol okhotsk suture to the northeast verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions

paleomagnetic evidence for cenozoic block rotations in the tadjik depression central asia this paper presents results of a paleomagnetic study of oligo miocene red beds of the tadjik depression in central asia we sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the pamirs samples were thermally demagnetized and high temperature components appear to predate folding of upper tertiary age throughout the depression paleomagnetic inclinations are consistent with those observed on the stable turan platform at the western margin of the depression however they are shallower by about 30 than the inclination predicted from the reference apparent polar wander path this appears to indicate a 23 difference in latitude which is incompatible with paleogeographic reconstructions for the tertiary a sound interpretation of this anomaly would require a better constrained tertiary paleomagnetic reference for asia inside the tadjik depression paleomagnetic declinations are all significantly rotated counterclockwise with respect to those measured on the turan platform the eastern part of the depression is a domain of large rotation 52 13 to 46 15 whereas smaller amounts of rotation have occurred in the western part 27 14 to 14 15 the similarity between tertiary and cretaceous data available for the area shows that rotations have occurred since the miocene little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression paleomagnetic and structural data suggest that block rotations in the tadjik depression are associated with indentation of the pamirs into stable asia at a larger scale observed rotations are compatible with a model of regional sinistral wrenching along a strip running from the gulf of oman to lake baikal

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

paleomagnetism of the middle siwalik formations of northern pakistan and rotation of the salt range decollement abstract sampling for paleomagnetic study has been carried out at seven localities in the potwar plateau region of pakistan in the vicinity of the salt range in all 253 sites consisting of three samples per site were studied the sites were taken from the chinji nagri dhok pathan and upper siwalik formations which span the last 14 m y of earth history the formations have been deformed in the ongoing himalayan orogeny and are exposed today in sections which dip at angles of from 10 to 90 in different directions a characteristic direction of magnetization was isolated at all sections by thermal demagnetization at temperatures of from 550 to 650 c the sediments have been folded within the last 2 m y and because of a positive fold test it can be shown that the characteristic magnetization of these sediments was acquired before folding four of the sections have directions of magnetization which have been rotated in a counter clockwise fashion up to 40 however sections taken along the indus river the north flank of the soan syncline and the south flank of the salt range show small counter clockwise rotations of under 10 these observations are in agreement with previous suggestions that strata in the salt range have been moved to the south and rotated in a counter clockwise fashion the observations reported here indicate that the paleomagnetic poles derived from cambrian and permian formations within the salt range have probably also suffered rotation and should be removed from consideration in gondwanaland reconstruction of the paleozoic

new early cretaceous paleomagnetic data from volcanic and red beds of the eastern qaidam block and its implications for tectonics of central asia abstract in order to better understand anomalously low paleomagnetic inclinations in the cretaceous rocks of central asia a combined geochronologic and paleomagnetic investigation has been performed on cretaceous basalt sequences intercalated with red beds from zeku area in the eastern qaidam block northeastern tibetan plateau potassium argon k ar dating indicates that the basalt sequences are of early cretaceous age 89 9 2 8 to 112 1 2 4 ma isothermal remanent magnetization irm thermal demagnetization of a three component irm and curie point experiment suggest that magnetite and hematite dominate in red beds magnetite in basalt flows stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization chrm from 7 lava flows and 12 red beds sites the chrms of red bed and basalt sites pass fold test and likely represent primary remanent magnetization the tilt corrected mean direction from red bed sites basalts plus baked red bed site is d s 18 4 i s 55 3 s 69 1 95 5 5 n 11 sites d s 16 1 i s 52 0 s 42 6 95 8 6 n 8 corresponding to a pole at 75 2 n 182 3 e with a 95 7 07 76 9 n 194 9 e with a 95 10 0 respectively the consistent inclination recorded in red beds and basalts implies no significant inclination shallowing caused by deposition compaction in red beds from the studied area compared with the early cretaceous poles from the north china block ncb or eurasia insignificant post early cretaceous northward motion may have occurred between the qaidam block and the ncb on the basis of the existing regional paleomagnetic data the qaidam block has not experienced wholescale vertical axis rotation since the early cretaceous with respect to ncb

the configuration of asia prior to the collision of india cretaceous paleomagnetic constraints paleomagnetic data from central asia show that 1700 610 km of shortening of southern asia since cretaceous time have been absorbed by distributed deformation between southern tibet and the siberia craton this result is based on a compilation of cretaceous poles from the junggar tarim tibet indochina south china north china and mongolia blocks complementing the recent compilation of enkin et al 1992a we propose a paleogeographic reconstruction of asia in the cretaceous in which the position of siberia is derived from the synthetic apparent polar wander path of besse and courtillot 1991 the resulting map which likely represents asia as it remained throughout the cretaceous until the collision with india began features an unbent tibet with an east west trending andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from sichuan to tarim through tibet the map allows one to estimate continental shortening and rotations between the blocks which are attributed to the collision despite large uncertainties these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of avouac 1991 appendix is available with entire paper on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b93 007 2 50 payment must accompany order

paleomagnetism of cretaceous to lower tertiary rocks from southwestern sichuan a revisit abstract a re study has been carried out on the cretaceous to lower tertiary rocks from southwestern sichuan province on the south china block the characteristics remanent magnetization chrm resolved from the paleocene eocene leidashu formation is of dual polarity and very shallow inclination the anomalouslylow paleolatitude 6 9 6 3 n indicated by the inclination is not supported by geological evidence from this region and is probably due to inclination error related to depositional and or post depositional processes a positive fold test as well as the prevalence of normal polarity suggests that the chrm of the mid upper cretaceous xiaoba formation might be acquired during the cretaceous long normal superchron clns the tilt corrected mean direction of the xiaoba formation d i 8 1 38 8 95 6 6 n 18 sites is not significantly different from a preliminary result d i 357 6 31 6 95 8 6 n 12 samples we reported earlier for the collective xiaoba and leidashu formations the resultant paleopole 81 5 n 220 9 e a95 7 1 is coincident with other cretaceous poles for the chinese blocks comparison with the eurasian apparent polar wander path apwp indicates neither significant rotation nor latitudinal translation of the study area relative to eurasia the newly obtained mean direction for the xiaoba formation is believed to be more reliable than our earlier result since it is based on a larger sample size and is apparently free from effects of the shallow inclination observed in the leidashu formation and should supercede our previous preliminary result the paleopole derived from sevenentes of the lower cretaceous feitianshan formation 69 0 n 204 6 e a95 4 3 deviates from the track from the late jurassic to late cretaceous poles the deviation might be reduced by further sampling

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

tertiary paleomagnetism of north and south china and a reappraisal of late mesozoic paleomagnetic data from eurasia implications for the cenozoic tectonic history of asia abstract to contribute to the understanding of the cenozoic tectonic history of china and eastern eurasia in general we collected paleomagnetic samples from tertiary rocks in both north and south china our miocene paleomagnetic data for northern china are from four widely separated basalt sections in the inner mongolia autonomous region and shandong province both normal and reversed polarities of the characteristic magnetization are present in the stratigraphie sequence and the mean normal and reversed directions are closely antiparallel to each other the fact that the same characteristic direction was carried by both magnetite and hematite suggests that the hematite was probably formed by auto oxidation processes during primary cooling of the basalts the paleomagnetic data from these four areas also pass a regional generalized fold test the corresponding mean miocene paleomagnetic pole for northern china is longitude 261 6 e latitude 88 3 n a 95 6 4 our early tertiary paleomagnetic results for south china are from two sandstone sections around the city of nanning 22 7 n 108 3 e in guangxi zhuang autonomous region high temperature characteristic directions isolated in 31 samples from five sites pass both tilt and reversal tests the paleomagnetic pole position determined from the five sites lies at longitude 228 0 e latitude 83 6 n a 95 8 5 the tertiary paleomagnetic poles for north and south china are statistically indistinguishable from each other and thus confirm that north and south china were fully assembled at least by the tertiary our new tertiary paleomagnetic data are consistent with the hypothesis that the major continental blocks of north and south china were in the same position as today during the tertiary a critical evaluation and selection of paleomagnetic results from the late jurassic onward for siberia europe and north and south china further corroborate this hypothesis most previously existing jurassic and cretaceous paleomagnetic poles from europe siberia are of uncertain reliability and should not be used for tectonic reconstruction using poles derived from transferring reliable data from other continents we show that the previous paleomagnetic inferences of large eastward displacements between these chinese blocks during the cenozoic are no longer warranted the available paleomagnetic data that meet our acceptance criteria are also of interest concerning the boundary conditions of the propagating extrusion model for asian tectonics additional mesozoic and cenozoic paleomagnetic data from siberia and north and south china are still needed to resolve the smaller second order displacements that appear to characterize the cenozoic tectonic history of this part of asia

evidence of crustal melting events below the island of salina aeolian arc southern italy quartz rich xenoliths hosted in the basaltic andesitic and andesitic lavas of monte dei porri in the island of salina contain fluid and melt inclusions whose study makes it possible to trace back the magma ascent path of this volcano moreover they are a source of information about the anatectic processes that occurred under this island the texture mineralogy and whole rock chemistry of these rocks exhibit the typical characteristics of a residual rock which went through several melt extraction events the chemistry of melt inclusions hosted inside quartz grains sio 2 72 7 al 2 o 3 9 0 13 5 cao 0 2 4 3 k 2 o 3 1 mgo feo t 1 5 supports the hypothesis that a progressive and continuous anatectic processes affected a peraluminous high grade metamorphic rock gneiss or micaschist that constituted the calabro peloritani basement and generated a quartz bearing residue the fluids trapped inside these inclusions are rarely composed of high density co 2 with a homogenization temperature to liquid phase th l within the range 26 2 31 0 c corresponding to densities of 520 to 690 kg m 3 more commonly they are made up of low density co 2 with a homogenization temperature to vapour phase th v within the range 13 2 31 0 c corresponding to density values of 150 to 460 kg m 3 it is proposed that during ascent batches of magma rising from the mantle sampled various fragments of a residual rock from a depleted metamorphic basal layer and carried them to the surface magmas probably stopped inside at least two distinct reservoirs located at different depths 12 7 km and between 1 7 and 4 7 km in which they underwent fractional crystallization the ascent speed of the xenolith bearing magmas from the deeper storage area to the shallower reservoir was fairly high as confirmed by the low number of high density fluid inclusions preserved that show re equilibration features and by the contemporaneous presence of denser cumulitic ol cpx opx xenoliths in the host lava

miocene paleomagnetic results from southeastern korea paleomagnetic samples were collected from 25 sites of late oligocene to middle miocene age from southeastern south korea the mean direction obtained is d 39 4 i 46 2 with 95 9 5 which is statistically distinguishable from the present axial geocentric dipole field direction when combined with the previous reported results of eight sites from this region we get a mean direction d 40 1 i 47 4 95 7 9 for 33 sites of late oligocene to middle miocene age this direction yields an early neogene virtual geomagnetic pole of 56 5 n 215 2 e with an 95 of 7 9 discordant from the cretaceous reference pole for korea the paleomagnetic data suggest that portions of southeastern korea underwent a large clockwise rotation of about 40 during the neogene two large ne trending right lateral faults the yangsan fault and the dongnae fault fig 1 cut the study areas both these faults show evidence for recent activity in addition minor faults with similar trends to these major faults occur the observed deflections in paleomagnetic declinations were probably caused by rigid block rotations of crustal blocks in a right lateral shear system during the neogene the paleomagnetic directions for miocene aged rocks from southeastern korea and southwestern japan are similar in direction and magnitude this suggests that the cause of the rotations in both areas may be related to a similar tectonic event in this area during the neogene

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

cretaceous and tertiary paleomagnetic results from southeast china and their tectonic implications abstract we report cretaceous and tertiary paleomagnetic data from fujian guangxi and guangdong provinces in south china in guangxi province we sampled early cretaceous redbeds of the xinlong formation at eight localities separated by up to 100 km the high temperature component obtained by thermal demagnetization is of dual polarity and passes the fold test and implies 18 8 counterclockwise rotation and insignificant poleward displacement with respect to cratonic localities in sichuan province a tertiary basin lying in a southeast trending fault zone is also rotated counterclockwise by 40 8 both results can be explained very well by a simple block rotation model for the cenozoic tectonic evolution of western guangxi province that is driven by left lateral shear on southeast trending faults the timing and sense of shear are the same as for india asia collision models involving the extrusion of indochina along the red river fault zone which lies several hundred kilometers to the southwest but the magnitude of fault displacement in guangxi is much less in the coastal provinces of fujian and guangdong we find late cretaceous directions that are rotated clockwise with respect to sichuan province combined with similar clockwise rotations found by two other studies of coeval rocks from the same region the mean rotation for the coastal provinces is 12 8 this could indicate a coherent block rotation of the region driven by extrusion tectonics however the limited number of sampling localities also allows the possibility that the directions reflect local rotations due to dextral shearing driven by circum pacific tectonics

paleomagnetism of miocene sediments from the turfan basin northwest china no significant vertical axis rotation during neotectonic compression within the tian shan range central asia abstract paleomagnetic results are reported from 377 samples of miocene red sediments in the turfan basin of east tian shan northwest china thermal demagnetization for most specimens isolates a low temperature component that groups around the present geomagnetic field a high temperature characteristic component is isolated from four localities and interpreted as primary remanent magnetization from apparent pre folding origin and positive reversal test as for previous results obtained from tertiary rocks in central asia the magnetic inclination is significantly shallower than the reference field computed from the eurasian apw path at 10 and 20 ma magnetic susceptibility measured in 324 specimens exhibits low anisotropy of less than 1 05 for 80 of the sample the low anisotropy degrees ams shape and orientations suggest that the studied rocks are unlikely to have undergone significant strain during compaction or tectonism we argue that the shallow inclination is likely to result from an initially shallow detrital remanent magnetization comparison of the paleomagnetic declination from eurasia north china and tarim suggests that the turfan basin has been subjected to little or no vertical axis rotation since the miocene together with paleomagnetic declinations on both sides of the talas ferghana fault we propose that the tian shan to the east of the talas ferghana fault has remained relatively stationary with respect to adjacent blocks since the miocene

a new global paleocene eocene apparent polar wandering path loop by stacking magnetostratigraphies correlations with high latitude climatic data a new apparent polar wander path apwp from the beginning of the paleocene 65 ma to the middle of the mid eocene 42 ma is shown to be correlated with polar climatic data of the same time period rather than applying the classical method based on analysis of site based poles we stacked the apwps obtained from magnetostratigraphies magnetostratigraphies have the advantage of displaying an unbroken record of local apwps through time and for a magnetozone defined as the a combination of normal and reversed polarity intervals the instantaneous poles are synchronous seven magnetostratigraphies located on 4 different plates covered sufficient time to be used in the analysis an average apwp was then determined with respect to age at the magnetozone level for the african plate which was arbitrarily chosen as a reference frame virtual geomagnetic poles were transferred onto the african plate using ocean kinematic euler rotations the calculated apwp is characterized by a loop with two main changes of direction at magnetozones 26 25 61 5 56 5 ma and 24 22 56 5 48 6 ma distinct at a 95 level of probability and indistinct poles related to magnetozones 29 27 65 5 61 5 ma and 21 19 48 6 40 6 ma we also show that the implied rapid shift of the lithosphere with respect to the geographic pole possibly an episode of true polar wander was coeval with the time evolution of vertebrate occurrence on ellesmere island canadian arctic and with the tree ring growth rate in western antarctica

late jurassic early cretaceous closure of the mongol okhotsk ocean demonstrated by new mesozoic palaeomagnetic results from the trans baikal area se siberia summary we present new palaeomagnetic results from the transba ikal area se siberia from the mongol okhotsk suture zone the boundary between the amuria and siberia blocks in order to better constrain the time of closure of the mongol okhotsk ocean in the mesozoic we col lected 532 rock samples at 68 sites in six localities of basalts trachy basalts and andesites from both sides of the mongol okhotsk suture at unda river j 3 51 7 n 117 4 e kremljevka peak k1 51 8 n 117 5 e and torey lakes k 1 50 1 n 115 9 e on the southern side of the suture and at monostoy river j 1 51 1 n 106 8 e ingoda river k 1 51 2 n 112 2 e and bichura town k 1 50 6 n 107 6 e on the northern side progressive thermal demagnetization enabled us to resolve low ltc and high htc temperature components of magnetization at most sites jurassic palaeopoles computed from the htcs show a large discrepancy with respect to the apparent polar wander path of eurasia which we interpret in terms of 1700 2700 km of post late jurassic northward movement of amuria with respect to siberia although ge ological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data give evidence of a large remaining palaeolatitude difference between the amuria and siberia blocks in contrast early cretaceous sites cluster remarkably well along a small circle which is centred on the average site location this implies the absence of post early cretaceous northward motion of amuria relative to siberia and demonstrates the pre early cretaceous closure of the mongol okhotsk ocean finally we interpret the very large tectonic rotations about local vertical axes evidenced by the small circle distribution of poles as arising both from collision processes and from left lateral shear movement along the suture zone due to the eastward extrusion of amuria under the effect of the collision of india into asia

late permian to early triassic magnetostratigraphy a late permian to early triassic magnetostratigraphic reference section is presented the lower triassic part is based on results from marine limestone sections in south china published earlier 1 2 reliable new permian data are added here which have been collected in the nammal gorge salt range northwest pakistan where marine sediments have been deposited quasi continuously with occasional minor hiatuses during the late palaeozoic to early mesozoic about 50 of the permian samples from the nammal section contain hidden beneath a strong recent or tertiary overprint a characteristic remanent magnetization chrm which is very likely of permian age this component which was imprinted on the southern hemisphere has normal as well as reversed polarity with a normal mean direction decl 289 3 incl 50 3 95 4 3 n 113 which is in close agreement with the palaeofield direction expected for a site belonging to the indian plate as part of gondwanaland during the permian in the lower upper permian several normal polarity zones are recognized this contradicts the current assumption that rocks of this age belong to the long reversely polarized kiaman hyperzone the kiaman interval must end and the illawarra hyperzone of mixed polarity must begin in or prior to the lowermost upper permian the permian triassic boundary at nammal as well as in the chinese sections is situated very close to a transition from a reversed to a normal polarity zone the upper permian at nammal together with the lower triassic south china sections is estimated to cover about 20 ma nearly 30 polarity changes are observed which result in an average reversal frequency very similar to that observed during the early tertiary the reversal rate after the end of the long lasting reversed kiaman hyperchron apparently increases in a manner similar to that after the end of the cretaceous long normal superchron only a few polarity zones are found in the lower upper permian while progressively more frequent reversals follow in the lower triassic

new mesozoic apparent polar wander path for south china tectonic consequences a new paleomagnetic study in the south sichuan basin south china block has been carried out on sediments of early triassic to middle jurassic age detailed stepwise thermal and alternating field demagnetizations allowed us to isolate two to three components a low temperature component component a falls close to the local present earth field direction a middle temperature component component b is found both in limestone and red beds of early triassic age in limestones of middle triassic age and finally in red beds of jurassic age the component b directions failed the fold test but show a significant increase of the ks kg ratio at 60 when a progressive stepwise unfolding is applied the directions obtained at 60 unfolding are grouped best and yield a pole position at 76 5 n 174 1 e a95 3 7 that is close to the late cretaceous pole of the south china block scb or the early tertiary pole of the eurasian plate characteristic directions are finally obtained component c for early triassic and early to middle jurassic sediments only both with reversal and fold tests they yield pole positions at 44 1 n and 217 2 e a95 4 9 and 79 n and 227 e a95 4 5 respectively the early triassic result well fits other data of the same age from the scb the revised late permian to cretaceous paleopole data provide a new mesozoic apparent polar wander path for south china the successive pole positions from late permian to early triassic indicate motion of the south china block as a whole rather than local rotations linked to important strain within the block the early to middle jurassic pole is statistically indistinguishable from the compiled north china block ncb poles and indicates that final suturation between the two blocks occurred by the end of the triassic which is coherent with the age of ultra high pressure metamorphism in the dabie sulu areas of eastern china the asian plate including the mongol central asian block ncb and scb however was probably separated from siberia by the mongol okhotsk ocean during jurassic time

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

an integrated stratigraphic record from the paleocene of the chijiang basin jiangxi province china implications for mammalian turnover and asian block rotations new paleomagnetic and isotopic results from a similar to 1000 meter thick paleocene stratigraphic section in the chijiang basin of china s jiangxi province provide chromostratigraphic constraints on the shanghuan nongshanian asian land mammal age boundary and allow for a more accurate determination of an early paleogene paleomagnetic pole for the south china block paleomagnetic analysis of 121 sites 326 samples reveals that the shanghuan nongshanian boundary lies close to a normal to reverse polarity change stable carbon isotope analysis of dispersed organic matter and paleosol carbonates indicate a secular increase of similar to 1 5 superimposed on higher frequency variations correlation of this magnetochemostratigraphic pattern to the global timescale suggests that the polarity reversal near the shanghuan nongshanian boundary likely represents the chron c27n c26r transition the torrejonian tiffanian north american land mammal age boundary is closely correlated to this same polarity transition indicating nearly synchronous global mammalian turnover at this time because both land mammal age boundaries are thought to record turnover of mostly endemic taxa it is unlikely that the synchroneity of faunal change is due to intercontinental dispersal as documented for other early paleogene faunal transitions e g paleocene eocene boundary instead these mammalian faunal transitions may represent independent ecological or evolutionary responses to environmental changes that have been interpreted from marine records at this time e g top chron 27n event these new paleomagnetic results from the chijiang basin are also used to augment other published data and to calculate a paleocene paleomagnetic pole for the south china block the new pole shows no significant vertical axis rotation compared to the paleocene reference pole for eurasia indicating that much of the clockwise rotation that has been documented for the south china block from cretaceous deposits cannot be the result of extrusion tectonics associated with the early paleogene india asia collision observed cretaceous rotations of the south china block may be the result of late cretaceous early paleocene rifting in the backarc of the kula pacific subduction zone c 2008 elsevier b v all rights reserved

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

paleomagnetism indicates no neogene rotation of the qaidam basin in northern tibet during indo asian collision paleomagnetic data were obtained from tertiary red sedimentary rocks at two locations separated by several hundred kilometers within the qaidam basin in the east central part of the basin 30 sites from the lower pliocene youshashan formation yielded characteristic remanent magnetization chrm directions with intermediate unblocking temperatures 100 600 8c chrm with high unblocking temperatures to 680 8c was isolated from 14 sites in the same area chrm directions were obtained from six sites within the oligocene lower gancaigou formation characteristic magnetization was also determined from 16 sites within the lower gancaigou formation exposed in the e bo liang range of the north central qaidam basin when compared with equivalentage expected directions for eurasia the mean paleomagnetic directions indicate no neogene vertical axis rotation of the qaidam basin or the altyn tagh fault the qaidam basin may act as an indentor translating without rotation toward the north china block and driving clockwise vertical axis rotations by differential shortening within the nan shan fold and thrust belt

late cretaceous palaeomagnetic results from sikhote alin far eastern russia tectonic implications for the eastern margin of the mongolia block summary wepresent palaeomagnetic results from late cretaceous welded tuffs in the kisin group monastirskaya suite primorskaya series collected at 27 sites from the sikhote alin mountain range a high unblocking temperature magnetization component 590 c was isolated after stepwise thermal demagnetization from 25 sites combined with previously reported data reliable characteristic remanence directions from 39 sites are distributed at eight areas ranging from 46 7 n 138 1 e to 43 4 n 134 8 e in the sikhote alin block the bedding tilt test is positive for two area mean directions and inconclusive for the remaining six areas the data set for all 39 sites reveals a positive bedding tilt test at the 99 per cent confidence level and their tilt corrected mean direction is d 335 6 i 54 4 95 8 5 corresponding to a palaeopole at 71 5 n 38 9 e with a95 9 9 this westerly direction is ascertained through the tilt corrected mean direction d 331 1 i 53 5 95 8 5 based on the 25 data selected from four areas kema river terney plastun and moryak rybolov where each data set passes the positive bedding tilt test or reveals an increase in the precision parameter after tilt correction palaeomagnetic declination indicates that the sikhote alin block has rotated counterclockwise by 41 16 with respect to the eurasian continent between late cretaceous times and 53 50 ma compared with palaeomagnetic data from the surrounding regions we find that the rotation recorded in sikhote alin extends westward into the interior of the mongolia block the eastern margin of the asian continent experienced both counterclockwise rotation of the eastern part of the mongolia block and clockwise rotation of the eastern part of the north china block over the cretaceous we interpret these data in terms of a strong net horizontal force towards the ocean side acting on the lithosphere at the eastern margin of the asian continent between the late cretaceous and 53 50 ma intermittently occurring upwellings of mantle and associated horizontal flows may have played an important role in producing the net horizontal force acting on the continental block during late cretaceous times

cenozoic normal faulting and regional doming in the southern hangay region central mongolia implications for the origin of the baikal rift province abstract the hangay dome in central mongolia is a mountainous region covering more than 200 000 km2 with numerous flat topped peaks over 3000 m that preserve a late cretaceous paleogene erosion surface doming began in the middle oligocene producing more than 2000 m of regional topographic uplift the range represents an important kinematic link between the baikal rift province to the north and altai transpressional ranges to the south and west structural field investigations of major faults visible on satellite imagery indicate that the southern hangay dome region is dominated by late cenozoic normal faults that bound small half graben most faults are ne striking and faults at the highest elevations are the most recently active most graben appear to be isolated systems that constitute small sediment sinks perched on the flanks and crests of the dome the first order feature is the dome itself and most sediment eroded off of the dome is deposited in the mongolian valley of lakes or is carried northwards by the selenga river and its extensive tributaries the basement of the dome is a precambrian craton although the shape and dimensions of the craton are poorly constrained to the north and east late cenozoic uplift of the southern dome region appears to be confined to the area underlain by cratonic basement whereas the altai region to the south and west is underlain by mechanically weaker palaeozoic arc and accretionary belts with respect to the regional northeast directed shmax the hangay craton appears to have acted as a rigid passive indentor focusing late cenozoic transpressional deformation around its west and southern margins models invoking a late cenozoic plume as the driving force for doming and widespread alkaline volcanism on the dome are not strongly supported by geochemical and isotopic data on neogene recent volcanics and the spatial correlation between areas that are domed and older cratonic crust appears too coincidental to be ignored convective removal of an overthickened lithospheric root leading to adiabatically decompressed asthenosphere could explain regional doming and volcanism however major crustal thickening last occurred in the permian in the southern hangay region and the time lag between thickening and postulated root removal and plateau uplift 200 myr appears too long an alternative model is explored based on speculated lithospheric mantle flow patterns driven by india s continued northeastward indentation it is suggested that lithospheric mantle flow diversion around the overthickened hangay craton crustal keel could cause lithospheric thinning beneath the craton and passive asthenospheric upwarp leading to regional topographic uplift and decompression melting alkali volcanism in general the angular relationship between precambrian craton boundaries and the prevailing northeasterly shmax appears to control the kinematics of late cenozoic deformation throughout the hangay altai sayan and baikal regions

comment on the combined analysis of remagnetization circles and direct observations in paleomagnetism by p l mcfadden and m w mcelhinny

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

paleomagnetism of upper cretaceous rocks from south china paleomagnetic study of upper cretaceous redbeds from two areas in south china was undertaken in an effort to constrain better the history of tectonic movements in southeastern asia from the nanjing area 32 n l19 e high temperature characteristic directions which pass a fold test were obtained from ten sites 43 samples in the upper cretaceous puko and yanzijing formations the paleomagnetic pole position 76 3 n 172 6 e a95 10 3 agrees well with a late cretaceous reference pole for eurasia and thus confirms that this part of south china was fully sutured to eurasia by the end of the cretaceous with no paleomagnetically resolvable subsequent relative rotation in western sichuan 26 5 n 102 3 e high temperature characteristic directions of normal and reverse polarity were obtained from twelve samples 48 specimens in the upper cretaceous to lower paleocene xiaoba and leidashu formations a provisional paleomagnetic pole position 80 9 n 296 8 e a95 7 7 differs significantly from a late cretaceous eurasia reference pole in terms of apparent tectonic movement the difference in paleopole positions would indicate a 13 10 3 northward translation and 22 11 2 counterclockwise rotation of western sichuan relative to eurasia confirmation of this reconnaisance result which would suggest that western sichuan was pushed ahead of an impinging india is needed

anomalous low palaeomagnetic inclinations from oligocene lower miocene red beds of the south west tien shan central asia summary we studied oligocene to lower miocene red beds from the western margins of the south west ghissar range the westernmost part of the tien shan fold belt central asia a total of 19 sites were sampled on both limbs of a gentle syncline after removal of an unstable component at 200 to 300 c the natural remanent magnetization in these red beds from 18 sites is accounted for by one characteristic component chrm which persists up to 680 c intermediate directions of the geomagnetic field were recorded at five sites after tilt correction the overall mean direction for the remaining 13 sites is d 3 i 30 k 19 95 8 9 both the reversal and the fold tests are positive haematite is found to be the main remanence carrier in these red beds the calculated mean declination fits well with the reference data besse courtillot 1991 and no rotation with respect to eurasia is detected in accord with geological data in contrast the mean inclination is about 30 shallower than expected we argue that this result is not due to large scale northward transport of the area mean inclinations showing a similar amount of shallowing were also reported from the tadzhik and fergana basins and kirghiz tien shan from various rock types thomas et al 1993 1994 and we are of the opinion that neither inclination error nor compaction could have led to such a consistent pattern the analysis of all reliable early tertiary palaeomagnetic results extracted from the global paleomagnetic data base for the northern part of the eurasian plate reveals that inclination anomalies are very common in this data set whatever the rock type the general tendency is a progressive increase in the anomaly amplitude from the atlantic ocean to central asia the size and localization of this tertiary field anomaly is comparable with the present day non dipole field anomaly this suggests that there may have been a long term standing non dipole anomaly in the tertiary earth magnetic field perhaps correlated to major features of the contemporaneous plate tectonics for example the india asia collision

too low magnetic inclinations in central asia an indication of a long term tertiary non dipole field observed tertiary palaeolatitudes in central asia are more southerly by about 1600 km on average than those predicted from the eurasian reference palaeopoles subsequent northward displacements of the central asian terranes are unlikely to have been this large in this study we analyse to what extent non dipole fields especially octupole fields can explain this phenomenon a global zonal octupole field manifests itself in two ways 1 because the reference apwp is based mostly on results from the uk and north america its palaeopoles will be far sided as seen from the north atlantic but near sided as seen from eastern asia giving predicted palaeolatitudes that are too high 2 an octupole field contribution produces observed palaeolatitudes as calculated with the dipole formula for central asia that are too low both effects therefore increase the palaeolatitude anomalies in asia we find that an octupole dipole field ratio g3 of 0 06 or greater will reduce the palaeolatitude discrepancies significantly and is of the same magnitude as the g3 estimate of a recent analysis of early tertiary european and north american data

eastern asia in the cretaceous new paleomagnetic data from south korea and a new look at chinese and japanese data lower cretaceous continental sediments and volcanic rocks have been sampled in the gyeongsang basin south korea a detailed analysis of 160 specimens reveals two stable components of magnetization a postfolding low temperature lt component with normal polarity and a stable high temperature ht component with both normal and reversed polarities the blocking temperature of the ht component ranges from 540 c up to the neel temperature of hematite this component predates the folding of the series and yields a pole at 68 n 205 e a95 6 the prefolding magnetization indicates that south korea stood at a latitude nearly identical to its present day latitude in the lower cretaceous comparison of korean data with revised paleomagnetic data from north and south china and japan shows that all these blocks of eastern asia occupied the same positions in term of latitude and were probably parts of a single craton since at least the lower cretaceous the paleomagnetic directions of the sino korean block show large discrepancies with respect to the expected directions of the stable eurasian siberian block an interpretation involving significant n s motion 1000 km between either north and south china or between north china and siberia appears to contradict geological data a preferred interpretation is that the siberian data are in serious error and that the combined china korea lower cretaceous pole provides a valid estimate for the eurasian plate as a whole

magnetic polarity stratigraphy of the neogene siwalik group at khutia khola far western nepal the middle miocene pliocene siwalik group was deposited in the himalayan foreland basin in response to uplift and erosion in the himalayan fold thrust belt results of thermal demagnetization experiments on samples from the siwalik group in central and western nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies sandstones and paleosols of the siwalik group however generally display highly erratic paleomagnetic behavior during thermal demagnetization on the basis of these observations siltstones from a well exposed 2423 m thick section of the siwalik group in khutia khola far western nepal were sampled for magnetic polarity stratigraphy the siwalik group is composed of informal lower middle and upper members correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13 30 to 7 65 ma the lower middle siwalik boundary occurs at 11 05 ma near the beginning of chron c5n the rate of sediment accumulation increases upsection similar to rate changes previously observed in the pakistan siwalik group and probably in response to increasing proximity of the himalayan thrust belt in the khutia khola section a discordant declination indicates that this region has rotated about a vertical axis 16 6 counterclockwise with respect to the indian subcontinent measurements of 13 c in paleosol carbonate indicate the predominance of c 3 plants until 7 65 ma and the clear presence of c 4 plants higher in the undated portion of the section

the stationary cretaceous paleomagnetic pole of sichuan south china block upper jurassic to lower tertiary sites have been sampled from three localities separated by up to 150 km in the sichuan basin 30 n 103 e of the south china block each locality has at least one stability test but none alone gives a fully reliable result when taken together however identical directions are found with a global average d 12 7 i 34 2 k 42 9 95 3 3 n 44 sites and corresponding pole 73 9 n 234 4 e dp 2 2 dm 3 8 paleolatitude 18 8 there is no significant apparent polar wander apw with age which is consistent with an acquisition of magnetization during the period of the stable tip of the eurasian apw loop 130 70 ma as defined with poles from other continents transferred onto eurasia this age range is compatible with the available paleontological and magnetostratigraphic age constraints but is shorter than that suggested by the assigned formation ages the paleolatitude of sichuan is significantly lower 8 0 4 0 at 90 ma than that predicted by the reference apw path either because of northward shortening during the tertiary or because of errors in the apw path cretaceous poles from other chinese blocks are consistent with the sichuan pole but there is evidence that these blocks have suffered local rotations up to 10 during the tertiary probably as a result of the india eurasia collision

paleomagnetism indicates no neogene vertical axis rotations of the northeastern tibetan plateau 1 paleomagnetic data were obtained from 108 paleomagnetic sites collected in cretaceous to tertiary red beds from seven localities distributed in three general regions adjacent to the altyn tagh fault at the northern edge of the tibetan plateau in the hexi corridor 12 sites in oligocene strata at yaoquanzi 39 97 n 97 68 e yield a mean paleomagnetic direction i 33 0 d 8 6 95 6 0 with concordant declination and 36 sites in early cretaceous mudstones of the longshou shan 39 09 n 100 50 e provide a concordant paleomagnetic direction i 53 4 d 6 8 95 3 8 across the nan shan fold thrust belt miocene paleomagnetic directions from 39 sites distributed among four localities have a concordant mean declination i 40 6 d 7 2 95 5 8 in the altyn tagh range 21 sites in oligocene strata at xorkoli 38 93 n 91 43 e yield a paleomagnetic direction with concordant declination i 49 7 d 5 0 95 5 4 these results combined with existing regional paleomagnetic data indicate that 1 the hexi corridor along with the north china block has not undergone tectonic vertical axis rotation since at least early cretaceous time and is separated from the xining lanzhou basin and eastern tibet by an important post early cretaceous tectonic boundary 2 the nan shan fold thrust belt and the qaidam basin have not experienced wholesale vertical axis rotation during neogene time and 3 absence of vertical axis rotations in areas adjacent to the altyn tagh fault indicates that sinistral shear strain between the tarim basin and the northern tibetan plateau is concentrated on the fault these results are consistent with asian tectonic models that combine distributed lithospheric deformation and thickening with narrow and weak shear zones

did a large departure from the geocentric axial dipole hypothesis occur during the eocene evidence from the magnetic polar wander path of eurasia abstract low paleolatitudes are very frequent in eocene to oligocene formations particularly in the tethyan mobile belt these anomalies and their possible sources are discussed they are neither due to unreliable results nor to inclination errors in sedimentary rocks neither can large tectonic movements explain these discrepancies the most probable causes having been rejected therefore the best explanation consists in an alternative magnetic pole position for eurasia located at 69 n 215 e it is proposed that the mean magnetic field of the earth was dipolar but that its axis has swung from one stable position to another during the middle tertiary and was therefore sometimes different from the rotation axis

resolving the problem of shallow magnetizations of tertiary age in asia insights from paleomagnetic data from the qiangtang kunlun and qaidam blocks tibet china and a new hypothesis we present new paleomagnetic results obtained at 39 sampling sites from five sections of tertiary red bed formations two eocene formations from the qiangtang block of tibet xialaxiu locality 32 8 n 96 6 e and the xining basin of qaidam xining locality 36 5 n 102 0 e and three neogene formations from the xining basin jungong locality 34 7 n 100 7 e and the kunlun block tuoluo lake and west yushu localities 35 3 n 98 6 e and 33 2 n 96 7 e respectively thermal demagnetization of the rocks isolated a high temperature component that we interpret as the primary magnetization in four localities the paleopoles lie at 52 6 n 352 e dp dm 6 0 10 7 for xialaxiu 61 6 n 211 3 e dp dm 9 7 16 1 for xining 66 0 n 228 6 e dp dm 3 6 6 9 for jungong and 53 9 n 205 4 e dp dm 5 6 10 0 for west yushu as in previous studies of tertiary formations from asia the inclinations we obtained are shallower by 18 to 26 than the magnetic field computed from the eurasian apparent polar wander path apwp at 10 and 20 ma for neogene rocks and at 40 and 60 ma for eocene rocks on the basis of a compilation of eocene data from the south china block tibet central asia and kyrgyzstan we conclude that this inclination anomaly reflects erroneous predictions of positions of the siberian craton when based on the apwp of eurasia the main reason for this discrepancy might be nonrigid behavior of the eurasian plate in the tertiary combination of this with intracontinental shortening of asia under the penetration of india provides a full explanation for the anomaly verification of this new interpretation of the inclination anomaly will require new geologic and paleomagnetic data from the northern parts of these remote regions in mongolia and siberia

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

late devonian palaeomagnetism of the north tien shan kyrgyzstan can secular variation vary on a short timescale summary we studied more than 80 lava flows from a 600 m thick pile of upper devonian frasnian basalts and andesites of the aral formation in the north tien shan kyrgyzstan central asia with the aid of stepwise thermal demagnetization a high temperature dual polarity component was reliably isolated from most flows the primary origin of the high temperature component is demonstrated by positive reversal conglomerate and fold tests the most prominent and intriguing feature of this late devonian data set involves a clear distinction in angular dispersion between the lower and upper parts of the studied section a rather low concentration parameter k 13 and several directional anomalies characterize the lower section in contrast a much better grouping k 46 and a lack of directional outliers is observed for the dual polarity vectors from the upper flow sequence of the formation we analysed different mechanisms to account for this directional pattern and found that it is possible in just two ways one is to assume that secular variation sv in the upper sequence is strongly underrepresented and it is a coincidence that the mean directions of both polarities are statistically antipodal and the corresponding concentration parameters are statistically equal the other explanation is to hypothesize that the magnitude of sv can vary several fold at the same palaeolatitude and over time intervals estimated as 10 5 10 6 yr this is in sharp contrast with other models of sv where this magnitude has been assumed to be rather time independent for a givenlatitude ourhypothesisaccountsfortheobservedirregularitiesinpalaeomagneticdata but makes attempts to establish a correlation between sv and other parameters geographic latitude reversal frequency age etc more difficult we are aware however that more data are needed to refute or confirm it

cretaceous paleomagnetism of the eastern south china block establishment of the stable body of scb abstract a paleomagnetic investigation was performed on the cretaceous red sandstones collected at the eastern side of the south china block scb china and attempted to establish the stable part of the scb since the cretaceous paleomagnetic specimens were collected at 39 25 and 14 sites from three independent parts the northern central and southern regions of eastern scb respectively characteristic directions of higher temperature components htcs with an unblocking temperature of 680 c were isolated from 69 78 sites the optimal concentrations of global mean htc directions calculated using the direction correction tilt test were achieved at 79 19 95 27 71 37 and 117 98 untilting for the early cretaceous sites from the northern part late cretaceous sites from the northern central and southern parts respectively this treatment gives positive tilt tests or brings the optimal concentration not far from being indistinguishable from positive tilt tests although this observation can be due to imperfect separation of a primary component htc from a secondary one lower temperature component we adopted completely 100 untilted directions of the htcs as the paleomagnetic field directions during the cretaceous because the mean directions after complete untilting were almost equal to each mean direction after incomplete untilting showing the optimal concentration the mean paleomagnetic poles for three independent parts were located at almost the same positions and were indistinguishable from that for sichuan the western side of scb at the 95 confidence level this observation indicates that there is no relative movement between the eastern and western sides of scb and implies that a large part of the scb excluding a 400 km wide swath along the red river fault has behaved as its coherent stable body since the cretaceous the cretaceous paleomagnetic pole for the stable body of the scb latitude 80 0 n longitude 206 7 e a 95 2 5 is worth using as a reference cretaceous paleomagnetic pole of the scb and agrees strongly with the reference eurasian paleomagnetic pole during the cretaceous period