paleomagnetic versus gps determined tectonic rotation around eastern himalayan syntaxis in east asia abstract northward indentation of the indian plate has brought about significant tectonic deformation into east asia a record of long term tectonic deformation in this area for the past 50 m yr particularly the vertical axis rotation is available through paleomagnetic data in order to depict rotational deformation in this area with respect to eurasia we compiled reliable paleomagnetic data sets from 79 localities distributed around eastern himalayan syntaxis in east asia this record delineates that a zone affected by clockwise rotational deformation extends from the southern tip of the chuan dian fragment to as far as the northwestern part of the indochina peninsula a limited zone that experienced a significant amount of clockwise rotation after an initial india asia collision is now located at 23 5 n 101 e far away from an area 27 5 n 95 5 e where an intense rotational motion has been viewed by a snapshot of gps measurements this discrepancy in clockwise rotated positions is attributed to southeastward extrusion of the tectonic blocks within east asia as a result of ongoing indentation of the indian plate a quantitative comparison between the gps and paleomagnetically determined clockwise rotation further suggests that following an initial india asia collision the crust at 30 n 94 e paleoposition was subjected to southeastward displacement together with clockwise rotation which eventually reached to present day position of 23 5 n 101 e implying a crustal displacement of about 1000 km during the past 50 m yr

impact of india asia collision on se asia the record in borneo abstract borneo occupies a central position in the sundaland promontory of se asia it has a complex cenozoic geological history of sedimentation and deformation which began at about the same time that india is commonly suggested to have started to collide with asia some tectonic reconstructions of east and se asia interpret a large se asian block with borneo at its centre which has been rotated clockwise and displaced southwards along major strike slip faults during the cenozoic due to the indentation of asia by india however the geological history of borneo is not consistent with the island simply forming part of a large block extruded from asia the large clockwise rotations and displacements predicted by the indentor model for borneo are incompatible with palaeomagnetic evidence and there is no evidence that the major strike slip faults of the asian mainland reach borneo seismic tomography shows there is a deep high velocity anomaly in the lower mantle beneath se asia interpreted as subducted lithosphere but it can be explained just as well by alternative tectonic models as by the indentor model very great thicknesses of cenozoic sediments are present in borneo and circum borneo basins and large amounts of sediment were transported to the crocker turbidite fan of north borneo from the eocene to the early miocene but all evidence indicates that these sediments were derived from local sources and not from distant sources in asia elevated by india asia collision the cenozoic geological history of borneo records subduction of the proto south china sea and miocene collision after this ocean lithosphere was eliminated and a variety of effects resulting from long term subduction beneath se asia there is little to indicate that india asia collision has influenced the cenozoic geological record in borneo

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

miocene to present activity along the red river fault china in the context of continental extrusion upper crustal rotation and lower crustal flow detailed mapping of field relationships along the red river fault reveals information about the distribution and magnitude of slip along the fault about its interactions with other regional fault systems and the relationship between river incision and growth of the eastern margin of the tibetan plateau the red river fault is complex consisting of up to four strands and is dominated by right lateral strike slip displacement evidence for an extensional component of displacement is strongest along the northern part of the fault and decreases to the southeast to zero southeast of a major bend in the fault results of this study indicate dextral displacement on the red river fault is probably at least 40 km 15 16 km of which predates incision of the red river in pliocene time or later and probably also predates plateau growth and development of other regional fault systems long term average slip rate on the red river fault is a minimum of 5 mm yr however regional active tectonics are characterized by rotation of the upper crust around the eastern himalayan syntaxis bounded to the east by the xianshuihe xiaojiang fault system which deflects but does not cut the red river fault the distributed nature of the xianshuihe xiaojiang fault system as it approaches the red river fault is important for accommodating shearing across the strong crustal anisotropy formed by the red river fault and ailao shan shear zone the red river fault appears to terminate in the extending dali fault system in northwest yunnan pliocene surface uplift river incision and rotation around the eastern himalayan syntaxis are inconsistent with the pre pliocene displacement on the red river fault and lateral extrusion along the ailao shan shear zone advocated by the extrusion model this suggests a significant change in crustal conditions on the southeast margin of the tibetan plateau in pliocene time possibly the result of lower crustal flow

structural evolution of the yushu nangqian region and its relationship to syncollisional igneous activity east central tibet field mapping geochronological analyses and cross section construction reveal a protracted deformation history and a minimum of 61 km of cenozoic ne sw shortening in present coordinates across the yushu nangqian thrust belt in northern tibet cenozoic contraction started prior to 51 ma and was followed first by northwest striking right slip faulting and later by northwest striking left slip faulting renewed ne sw contraction is expressed by folding of neogene strata and thrusting which again was followed by northwest striking left slip faults late neogene deformation is expressed by local north striking normal faults shortening across the yushu nangqian belt appears to be accommodated by thin skinned thrusting which raises the question of how the deformation was accommodated in the lower crustal levels to resolve this problem we perform geochemical analysis of igneous rocks dated as 51 49 and 38 37 ma the rocks exhibit geochemical signatures characteristic of subduction which implies that coeval crustal thickening in northeastern tibet was most likely induced by continental subduction

two phase growth of high topography in eastern tibet during the cenozoic high topography in eastern tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the indo asian collision implying that crustal flow alone could not have created the high topography

magnetostratigraphy of the middle miocene continental sedimentary sequences of the mae moh basin in northern thailand evidence for counterclockwise block rotation abstract a magnetostratigraphic study has been performed on the mae moh basin well known for its lignite mining in the lampang province paleomagnetic samples were collected from 66 stratigraphic levels rock magnetic investigations indicate the presence of low coercivity minerals specimens subjected to progressive thermal or alternating field demagnetization procedures show that nearly all of them exhibit a low temperature magnetization component and a high temperature characteristic component chrm with either positive or negative virtual geomagnetic pole latitudes and opposite polarity considered as the characteristic miocene magnetization the overall mean paleomagnetic direction incl 22 2 decl 358 3 15 95 4 documents a counterclockwise vertical axis rotation of about 13 1 32 with respect to the expected miocene direction derived from the eurasian polar wander curve incl 42 decl 11 our paleomagnetic results are not consistent with the previously reported paleomagnetic data the rotation observed in the mae moh basin can be the response to local tectonics a section sampled for magnetostratigraphy reveals a polarity sequence of nine magnetozones that can reliably be correlated to the geomagnetic polarity time scale according to biochronological constraints the magnetostratigraphic results from the mammal bearing succession correlate with chron c5abn c5an2n between 13 5 and 12 1 ma according to the mean sedimentation rate of about 17 5 cm ka ages of 12 5 and 12 8 ma are proposed for the fossiliferous levels j5 and k1 k2 lignite zones were the mammal remains were found

persistently low asian paleolatitudes implications for the india asia collision history 1 paleomagnetism provides independent paleolatitude constraints on the india asia convergence however implied cenozoic latitudinal convergence within asia thousands of km largely exceeds geologic estimates of tectonic shortening hundreds of km this discrepancy may result from a notoriously low bias in paleomagnetically determined cenozoic paleolatitudes in asia we provide here new paleomagnetic data from cenozoic mongolian volcanic rocks and from chinese paleogene sediments corrected from the depositional bias of inclination shallowing these results combined with similar asian data sets confirm that paleolatitudes are still 5 10 lower than predicted by the paleomagnetic apparent polar wander path apwp for asia between 50 and 20 ma inclination shallowing being excluded from the selected data sets we investigate the likeliness of other proposed mechanisms for this discrepancy 1 more southerly positions of asia than expected by the apwp due to apwp inaccuracies rather than eurasian non rigidity or 2 non dipolar geomagnetic field contributions fully explaining this discrepancy by only one of these mechanisms would imply either unrealistically large 10 apwp inaccuracies or unrealistically large octupolar field contributions up to 16 a combination of these mechanisms is found more likely to have produced the observed latitudinal discrepancy but their respective contributions cannot be quantified given the still relatively low amount and poor quality of cenozoic paleomagnetic data from stable cratons of asia india and europe by allowing for reasonable time dependant non dipolar contributions and a slight 5 apwp bias the latitudinal discrepancy can be resolved and the excessive amounts of intra asian shortening decrease to values in line with tectonic shortening from structural studies

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

paleomagnetic study of cretaceous rocks from pu er western yunnan china evidence of internal deformation of the indochina block abstract cretaceous red sandstones and siltstones were collected at 31 sties for paleomagnetic investigation from the jingxing and nanxin formation around pu er 23 0 n 101 0 e western part of yunnan province china in order to examine quantitative amount of tectonic deformation of the simao terrane due to collision of india with asia after stepwise thermal demagnetization the high temperature component magnetization with unblocking temperature of around 680 c is isolated positive field tests show that the high temperature component magnetization from 25 sites is primary for the cretaceous formations a tilt corrected mean direction is d 59 9 i 45 2 with 95 5 1 corresponding to a paleopole at 35 8 n 173 1 e with a 95 5 6 comparison with cretaceous paleomagnetic pole of the whole china block indicates that amount of clockwise rotation in the pu er area is evaluated to be 46 9 with respect to the whole china block which is consistent with large amount of rotation in the mengla and jinggu areas 47 and 66 in the central part of the simao terrane clockwise rotation of the central part is larger by more than 20 compared to the yunlong and yongping areas of northern part of the simao terrane suggesting that the simao terrane experienced internal deformation the southward displacement of the pu er area expected from inclination data is 7 5 6 6 with respect to the chuan dian fragment which is a similar estimate to other areas in the simao terrane and the indochina block while the simao terrane displaced southward by 800 km together with clockwise rotation of 30 as a whole further clockwise rotation occurred in the central part

diachronous cooling along the mogok metamorphic belt shan scarp myanmar the trace of the northward migration of the indian syntaxis abstract we present new observations and new mineral ages eight by the 40 ar 39 ar and nine by the 40 k 40 ar methods from high grade metamorphic and foliated intrusive rocks collected along the mogok metamorphic belt of central myanmar that complete preliminary results we previously published all 30 ages presented here confirm the oligocene to middle miocene age of the mogok metamorphism moreover the ages are young from the gulf of martaban in the south to the china myanmar border in the north showing a clear diachronism these results confirm a northward migration of the tectonic event responsible for the cooling of our samples at a velocity similar to the india relative to indochina velocity at the same period in addition intensive field work along a 800 km long section of the mogok metamorphic belt has revealed the numerous ductile structures that resulted from a strong nnw sse trending extension from the analogy between the present day local extrusion around the eastern indian syntaxis and the extension observed along the mogok metamorphic belt and from the diachronism we document we suggest that the central part of myanmar has recorded the northward passage of the eastern indian syntaxis with respect to indochina from oligocene to middle miocene

post folding magnetization of the triassic rocks from western guizhou and southern yunnan provinces new evidence for large clockwise rotations in the simao terrane abstract thermal demagnetization of the redbed samples collected from the lower triassic feixianguan formation at two localities in western guizhou province reveals that the natural remanent magnetization was dominated by a postfolding component of probably recent origin and the prefolding possibly primary component was barely preserved this confirms that the feixianguan fm in western guizhou and northeastern yunnan provinces was severely remagnetized thermal demagnetization also resolves two postfolding components from the limestone samples of the mid triassic dashuijingshan formation from southern yunnan province a low temperature component acquired in recent times and a high temperature component probably acquired shortly after the india asia collision the mean in situ direction for the high temperature component based on eighteen sites is d 95 4 i 28 4 with 95 6 2 which indicates a clockwise rotation of 86 3 7 3 with respect to stable eurasia this result together with paleomagnetic results previously reported from the jurassic eocene rocks in the same area demonstrates beyond reasonable doubt that the central simao terrane has experienced large clockwise rotations due to the india s collision with and continuing penetration into asia

remagnetization by basinal fluids testing the hypothesis in the viola limestone southern oklahoma migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late paleozoic secondary magnetizations in the rocks of north america paleomagnetic and geochemical results from the ordovician viola limestone in southern oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations in the unit the viola limestone contains what we interpret to be a pervasive pennsylvanian synfolding magnetization residing in magnetite and a localized permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins both secondary magnetizations are interpreted as chemical remanent magnetizations crm based on low burial temperatures and the presence of authigenic magnetic phases the relative proportion of the permian crm in hematite gradually decreases whereas the magnetite crm increases with distance from the veins fluid inclusion and sr isotope studies indicate that the vein mineralization calcite with mississippi valley type oxides and sulfides precipitated from basinal fluids which were warm saline and radiogenic the radiogenic 87sr 86sr values of the limestones in the alteration zones and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones the coincidence of the geochemical and remagnetization trends suggests that the permian crm dates the migration of basinal fluids through the veins geochemical results from the viola limestone containing the pervasive crm indicate that it is relatively unaltered with no evidence for basinal fluids the lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive crm need to be tested the results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone

clockwise rotation of the red river fault inferred from paleomagnetic study of cretaceous rocks in the shan thai malay block of western yunnan china more than 150 samples were collected at 23 sites from the lower cretaceous jingxing formation near the city of yongping 25 5 n 99 5 e which is located on the west side of the red river fault sixteen sites have characteristic directions with a high temperature component above 500 c the high temperature component of magnetization from twelve sites is of pre fold origin and reveals clockwise deflection in declination d 42 0 i 51 1 and 95 15 7 the easterly declination of more than 40 at yongping is consistent with the cretaceous paleomagnetic direction d 45 of chuxiong 25 n 101 5 e which is located on the east side of the red river fault both areas were subjected to about 25 clockwise rotation with respect to the eastern part of the yangtze block since cretaceous time we conclude that the red river fault was rotated through25 16 with respect to the eastern part of the yangtze block associated with rotation of the yongping and chuxiong areas removal of the rotation from the red river fault indicates that the latter and the jinsha suture formed a straight line along n55 w s55 e within the asian continent from vietnam to the tibetan plateau in the present coordinate system during the cretaceous the linear feature of the red river fault and jinsha suture was probably deformed to the present curved shape as a result of collision of the indian continent with the asian continent

paleomagnetism of the middle late jurassic to cretaceous red beds from the peninsular thailand implications for collision tectonics abstract jurassic to cretaceous red sandstones were sampled at 33 sites from the khlong min and lam thap formations of the trang syncline 7 6 n 99 6 e the peninsular thailand rock magnetic experiments generally revealed hematite as a carrier of natural remanent magnetization stepwise thermal demagnetization isolates remanent components with unblocking temperatures of 620 690 c an easterly deflected declination d 31 1 i 12 2 95 13 9 n 9 in stratigraphic coordinates is observed as pre folding remanent magnetization from north trang syncline whereas westerly deflected declination d 342 8 i 22 3 95 12 7 n 13 in geographic coordinates appears in the post folding remanent magnetization from west trang syncline these observations suggest an occurrence of two opposite tectonic rotations in the trang area which as a part of thai malay peninsula received clockwise rotation after jurassic together with shan thai and indochina blocks between the late cretaceous and middle miocene this area as a part of southern sundaland block experienced up to 24 5 11 5 counter clockwise rotation with respect to south china block this post cretaceous tectonic rotation in trang area is considered as a part of large scale counter clockwise rotation experienced by the southern sundaland block including the peninsular malaysia borneo and south sulawesi areas as a result of australian plate collision with southeast asia within the framework of sundaland block the northern boundary of counter clockwise rotated zone lies between the trang area and the khorat basin

tectonic evaluation of the indochina block during jurassic cretaceous from palaeomagnetic results of mesozoic redbeds in central and southern lao pdr rock magnetic and palaeomagnetic studies were performed on mesozoic redbeds collected from the central and southern laos the northeastern and the eastern parts of the khorat plateau on the indochi

exhumation of the ailao shan shear zone recorded by cenozoic sedimentary rocks yunnan province china 1 the role of strike slip faults such as the ailao shan shear zone in southwestern china in accommodating the india eurasia collision remains controversial cenozoic sedimentary rocks preserved along the northeast side of the ailao shan shear zone however record the nature and timing of exhumation of shear zone rocks field mapping shows that the lower strata consist of early oligocene lacustrine red mudstone and gypsum and the upper of late oligocene to miocene fluvial conglomerate with rare fine grained fossil leaf bearing intervals sediment derived from shear zone gneiss is present only in the upper part of the section our observations indicate deformation began in oligocene time with the onset of rapid exhumation by early miocene time consistent with existing geochronologic results exhumation must have ended before the formation of a regional low relief erosion surface and the onset of brittle deformation along the red river fault probably in early pliocene time rapid exhumation of the ailao shan shear zone has been interpreted to have occurred in transtension accommodated by a normal fault on the northeast margin of the shear zone however we observe consistent and pervasive shortening structures within the cenozoic strata including overthrusting and folding of the basin along faults defined by thick gouge zones within weak cenozoic lithologies deformation is likely syndepositional these structures are distinct from subvertical deformational features associated with the active red river fault our field data thus support mid cenozoic transpressional exhumation of the ailao shan shear zone this is consistent with regional evidence for widespread left lateral and compressional deformation in the region to the east of the eastern himalayan syntaxis during this time focused erosion along the red river established by late oligocene time may have contributed to deeper exhumation of the northeast part of the shear zone

lower middle jurassic paleomagnetic data from the mae sot area thailand paleogeographic evolution and deformation history of southeastern asia we have carried out a paleomagnetic study 12 sites 85 samples of early middle jurassic limestones and sandstones from the mae sot area of western thailand this area is part of the shan thai malay stm block and its geological characteristics have led some authors to suggest a late jurassic accretion of this region against the rest of indochina along the changning menglian zone the latter sometimes being interpreted as a mesozoic suture the high temperature or high coercivity component isolated yields a paleodirection at d 359 8 i 31 4 95 5 0 the primary nature of the magnetization acquisition is ascertained at a site with reversed polarity and a positive fold test at the 95 confidence level comparison of the mae sot paleolatitude and another one from the stm with those recently published for the simao and khorat blocks show no significant difference at the 95 level showing that the stm was situated close to or had already accreted with the simao or khorat blocks in the early middle jurassic comparison of the latitudes from these blocks with those from china indicates a relative southward motion of 8 4 of indochina as a single entity relative to china most rotations of these regions relative to china are found to be clockwise between 14 and 75 these rotations and most prominently the 1200 500 km post cretaceous left lateral motion inferred for the red river fault provide quantitative estimates of the large amount of extrusion of indochina with respect to the rest of asia

new insights into regional tectonics of the indochina peninsula inferred from lower middle jurassic paleomagnetic data of the sibumasu terrane abstract the post jurassic occurrence of differential tectonic rotation between the indochina and south sundaland blocks remains an issue to be properly investigated new paleomagnetic study is used here to find the role of sibumasu terrane in this rotation which is located between a clockwise rotated indochina block and a counter clockwise rotated south sundaland block for this purpose lower to middle jurassic red sandstones of the umphang group in the sibumasu terrane were sampled at 21 sites in the ratchaburi area 13 6 e 99 6 e thailand stepwise thermal demagnetization by 680 c unblocked a pre folding characteristic remanent magnetization a mean direction of this component at 100 unfolding is ds 348 5 is 24 7 95 10 5 k 10 7 n 20 corresponding to an early middle jurassic pole of 78 6 n 10 6 e a95 9 3 comparison of this direction with those reported from other localities of the umphang group kalaw mae sot and north trang syncline localities reveal variable declinations between 348 5 and 44 7 for the sibumasu terrane we ascribe this variation to differential tectonic deformation in the sibumasu terrane as reflected from sinusoidal shaped structural features in the study area the presence of such features in the granitic rocks indicates the occurrences of deformational activities after their intrusion which took place in the period between 130 ma and 51 ma the sibumasu terrane behaved as an independent fragment at a time when indochina was undergoing a clockwise rotation and southward displacement as a result of extrusion tectonics after the gigantic india asia collision taking into consideration a westerly deflected declination d 342 8 from the west trang area in peninsular thailand a counterclockwise rotation of 15 is estimated for the sibumasu terrane as a result of continuous northward indentation of the australian plate into south sundaland block

magnetostratigraphy of the dali basin in yunnan and implications for late neogene rotation of the southeast margin of the tibetan plateau 1 the rotation pattern and fault activity in the southeast margin of the tibetan plateau semtp provide meaningful constraints on the geodynamic evolution of the plateau however the lack of cenozoic paleomagnetic studies and accurate age constraints on neogene sediments prevents a better understanding of the late cenozoic tectonic activity in this area to clarify the tectonic rotation pattern and deformation history of the semtp we report new magnetostratigraphic data from a late neogene sedimentary sequence in the dali basin northwestern yunnan province china rock magnetic analyses indicate that both magnetite and hematite are the main carriers of the characteristic remanent magnetizations chrms magnetostratigraphic results show that the sedimentary profile spans from chron c4n 1r to chron c2n the age of the sedimentary sequence in the dali basin can thus be paleomagnetically constrained to an interval from late miocene to early pleistocene the basal age of the sediments is 7 6 ma which indicates that the unroofing of diancang shan and activation of the dali fault system were initiated at this time the appearance of conglomerates and syntectonic sediments suggests the reactivation of the dali fault system at 2 5 ma moreover the overall mean chrm direction suggests that the dali basin experienced significant 4 4 2 5 but minor post late miocene rotation this indicates that most of the clockwise rotation demonstrated by previous paleomagnetic studies in the semtp occurred prior to late miocene and may be concentrated between eocene and miocene which is contemporaneous with the sinistral slip of the ailao shan red river fault

clockwise rotations recorded in redbeds from the jinggu basin of northwestern indochina paleomagnetic data have been used to infer clockwise rotations and significant southward motion of the indochina block during cenozoic extrusion from the india asia collision zone because the cenozoic of the indochina block has been sparsely sampled to date and is key to determining the timing of this tectonic motion we performed an extensive paleomagnetic study on paleocene to oligocene redbeds and middle miocene sandy silts from the jinggu basin 23 5 n 100 7 e in northern indochina paleomagnetic results from the redbeds pass fold tests but they show exclusively normal polarity there is controversy however on the age assignment to the lower part of the succession if the age of the redbeds is indeed paleogene this indicates a prefolding remagnetization associated with oligocene deformation of the indochina block if the age of the redbeds would be late cretaceous their magnetization may be primary and deposition could have taken place during the cretaceous normal superchron however the abundance of secondary hematite in the redbeds in combination with the biostratigraphy which indicates a paleogene age for at least the upper two formations of the redbeds in the jinggu basin implies a pervasive remagnetization the middle miocene sediments pass both the fold test and the reversals test and contain magnetite as well as hematite as carriers suggesting a primary magnetization our large data set from the redbeds 2000 paleomagnetic directions demonstrates an 30 35 clockwise rotation of the jinggu basin with respect to eurasia for both scenarios i e when compared to a ca 100 ma pole if the redbeds carry a primary natural remanent magnetization or to a ca 30 ma pole if the sequence is remagnetized the middle miocene results however indicate that the jinggu basin experienced no significant 2 5 6 rotation with respect to eurasia since no major deformation has occurred within northern indochina during late cretaceous to eocene times our results reflect a major clockwise rotation of the indochina block during its oligocene to early miocene extrusion from the india asia collision zone

paleomagnetism of the earliest cretaceous to early late cretaceous sandstones khorat group northeast thailand implications for tectonic plate movement of the indochina block abstract a total of 400 samples 33 sites were collected from the earliest cretaceous to early late cretaceous sandstones of the khorat group in the indochina block for paleomagnetic study to unravel the tectonic evolution of the region the sites were adopted from 3 traverses located in the northern edge of the khorat plateau northeastern thailand results indicate that almost all the sandstones exhibit similar magnetic values with an average declination d 31 7 inclination i 30 3 59 7 190 9 k 54 4 and a95 3 7 at reference point 17 30 n and 103 30 e the calculated paleolatitude points are inferred to deviate from the present latitude point by 1 2 2 3 only the lowermost part of the cretaceous sandstones can pass a positive fold test at 95 confidence level the relationship between the virtual geomagnetic poles vgps of cretaceous rocks of the indochina plate in thailand and those of the south china plate advocate that there is a major displacement of indochina along the northwest trending red river and associated faults by about 950 150 km with a 16 0 17 0 clockwise rotation relative to the south china plate during earliest cretaceous times paleomagnetic results of the early late cretaceous indochina plate point to a 20 25 clockwise rotation relative to the present occurring since very late cretaceous 65 myrs early neogene times which may be due to the collision between india and asia

cenozoic clockwise rotation of the tengchong block southeastern tibetan plateau a paleomagnetic and geochronologic study abstract paleomagnetic data from 50 35 ma likely 40 ma mafic dykes in yunnan southeastern tibetan plateau cutting 115 ma granitoids record the rotation of the tengchong lhasa block around the east himalayan syntaxis ehs ti rich titanomagnetite and magnetite carry a primary magnetic component group1 a magnetic overprint resides in magnetite group2 likely induced by low grade metamorphism between 30 and 10 ma the tilt corrected overall mean directions are d i 89 8 35 1 for group1 and d i 33 3 41 4 for group2 these data imply a clockwise rotation of 87 87 3 12 5 of the tengchong block since remanence acquisition at 40 ma with respect to stable eurasia the average rotation rate of 2 18 0 31 myr is at the upper limit of the present day rotation rates around the ehs obtained from gps velocities and quaternary strain rates the remagnetized group2 indicates a poorly defined rotation of 31 31 2 32 9 our key results the detection of 87 clockwise rotation with high rotation rates following the india asia collision and a decrease in rotation rates during the miocene suggest that first the tengchong block rotated rapidly around the ehs synchronous with eastward tectonic escape of lithospheric blocks the later slower clockwise rotation similar to those recorded geodetically may be related to viscous flow of tibetan crust

magnetostratigraphy of the xiaolongtan formation bearing lufengpithecus keiyuanensis in yunnan southwestern china constraint on the initiation time of the southern segment of the xianshuihe xiaojiang fault the late cenozoic extensional basins in yunnan province southwestern china which are kinematically linked with the regional strike slip faults can provide meaningful constraints on the fault activity history and tectonic evolution of the southeast margin of the tibetan plateau semtp and further on the geodynamic evolution of the tibetan plateau however this has been severely impeded by the lack of precise age constraints on the timing of fault activity to better constrain the timing of fault activity and the tectonic rotation of semtp we undertook a high resolution magnetostratigraphic study on the xiaolongtan formation in the xiaolongtan basin which is located at the southern tip of the xianshuihe xiaojiang fault and is well known by the presence of hominoid lufengpithecus keiyuanensis rock magnetic experiments indicate that magnetite is the main remanence carrier correlation to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic and biostratigraphic data our correlation suggests that the xiaolongtan formation sedimentary sequence spans from chron c5ar 1r to chron c5n 2n which indicates that the age of the xiaolongtan formation ranges from 10 ma to 12 7 ma and that the ages of the two sedimentary layers possibly bearing the hominoid l keiyuanensis are 11 6 ma or 12 5 ma the basal age of the sediments is 12 7 ma which indicates that the activation of the southern xianshuihe xiaojiang fault was initiated at this time the overall mean paleomagnetic direction d 353 2 i 34 2 95 2 1 n 166 documents a counter clockwise vertical axis rotation of 8 3 with respect to eurasia which is the response to the activity of the left lateral xianshuihe xiaojiang fault

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

paleomagnetism of the yuanmou basin near the southeastern margin of the tibetan plateau and its constraints on late neogene sedimentation and tectonic rotation abstract new paleomagnetic investigation was carried out on the late neogene fluviolacustrine sequence of the yuanmou basin located near the southeastern margin of the tibetan plateau magnetostratigraphic results indicate nine reverse magnetozones r1 to r9 and eight normal magnetozones n1 to n8 in the sedimentary profile which can be correlated to the geomagnetic polarity timescale from c3n 3r to c1r 1r the age of the sedimentary sequence of the yuanmou basin can thus be paleomagnetically constrained to an interval from early pliocene to pleistocene with sedimentation rates varying from 12 5 to 55 cm kyr in addition to its highly resolved magnetostratigraphic sequence the yuanmou basin provides a record of plio pleistocene tectono and climato sedimentary processes the mean declinations of the seventeen polarity units excluding samples with transitional directions can be grouped into three distinct directional intervals group i 2 58 1 37 ma group ii 4 29 2 58 ma and group iii 4 91 4 29 ma these directions indicate that the yuanmou basin has probably experienced vertical axis clockwise rotation of about 12 from 1 4 ma to 4 9 ma which may be related to slip activity of the red river fault to the southwest and the xianshuihe xiaojiang fault to the east

internal deformation of the shan thai block inferred from paleomagnetism of jurassic sedimentary rocks in northern thailand abstract paleomagnetic samples of red sandstones were collected at 16 sites from the lower to upper jurassic phu kradung formation and the upper jurassic phra wihan formation around nan city 19 2 n 101 0 e in the southern region of the shan thai block after stepwise thermal demagnetization a high temperature component with an unblocking temperature of about 680 c is isolated from 11 sites we interpret this to be a prefold characteristic remanent magnetization because of a positive fold test at the 95 confidence level the tilt corrected mean paleomagnetic direction from 11 sites is d 32 3 i 33 3 95 12 2 k 15 0 which gives a paleomagnetic pole for this area of 60 1 n 186 5 e with a95 11 7 compared with a jurassic paleopole for the stable part of the yangtze block we observe a clockwise tectonic rotation of 12 8 which is similar to the observed rotation 18 28 of the northern region of the shan thai block this amount contrasts with a large rotation of more than 45 that is estimated from the narrow zone extending from luxi to mengla in the central region of this block this declination aspect suggests that a coherent whole block rotation by about 20 of the shan thai block took place in an earlier phase during indentation of india into asia and that an internal deformation with additional local rotation between 25 and 77 followed within a limited zone in the central part the internal deformation of the shan thai block absorbed the stress on the asian continent induced by indentation of india as well as its coherent whole block motions of clockwise rotation and southward displacement

paleomagnetic study of middle cretaceous rocks from yunlong western yunnan china evidence of southward displacement of indochina middle cretaceous red sandstones and siltstones were collected at 20 sites for paleomagnetic study from the nanxin formation around yunlong 25 8 n 99 4 e western part of yunnan province china after stepwise thermal demagnetization high temperature component magnetization with unblocking temperature of about 675 c is isolated a positive fold test at the 99 confidence limit shows that the high temperature component magnetization is primary for the middle cretaceous nanxin formation a tilt corrected formation mean direction is d 40 2 i 49 9 with 95 3 9 corresponding to a paleopole at 54 6 n 171 8 e with a95 4 4 comparison with previously reported cretaceous paleomagnetic directions indicates that the yunlong area has been subjected to southward displacement by 11 9 7 5 corresponding to 1300 800 km with respect to the stable yangtze craton since cretaceous time as well as clockwise rotation through 36 3 13 6 this study elucidates that indochina was squeezed out of the asian continent due to collision of india accompanied with clockwise rotation

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

tectonic deformation of the southeastern tip of the indochina peninsula during its southward displacement in the cenozoic time abstract the terminal collision of india with asia brought about significant tectonic deformation into the asian continent in order to depict these deformational features in the southeastern margin of the indochina peninsula late cretaceous red beds of the dak rium formation were sampled from da lat area 11 7 n 108 3 e the kontum massif vietnam thermal demagnetization procedure revealed the presence of characteristic remanent magnetization with unblocking temperatures around 680 c in most samples primary nature of this magnetization is ascertained through positive fold test a tilt corrected mean direction calculated for the dak rium formation is dec inc 11 4 35 4 ks 331 7 95 1 7 n 21 with a corresponding paleopole at 76 5 n 161 2 e a95 1 7 compared with the european apwp a negative inclination flattening 15 2 4 9 is observed indicating southward displacement of the kontum massif by 9 2 4 9 since late cretaceous since comparable amount of southward displacement has been reported from 16 localities of the indochina and shan thai blocks it seems a reasonable assumption to predict that the indochina peninsula underwent a southward displacement of 5 1 2 4 570 km 270 km as a composite unit including the shan thai block indochina block and kontum massif on the contrary 27 0 9 8 counter clockwise rotation of the kontum massif with respect to the indochina block indicate a significant amount of internal deformation along the tip of south moving indochina peninsula this rotation of the kontum massif is attributed to 32 17 ma left lateral motion along the east vietnam boundary fault which makes up the 1000 km long seaward continuation of the ailao shan red river shear zone tectonic deformation of the indochina peninsula was thus partly accommodated by local block rotation e g counterclockwise rotation of the kontum massif and to some extent by internal tectonic deformation of the shan thai block

internal crustal deformation in the northern part of shan thai block new evidence from paleomagnetic results of cretaceous and paleogene redbeds in order to describe the internal deformation of the northeast part of the shan thai block resulting from the penetration of the indian plate into asia a paleomagnetic study was carried out on the cretaceous and paleogene strata in the southern part of the simao terrane high temperature magnetic components with unblocking temperatures around 680 degrees c were isolated and positive fold test shows its primary nature of magnetization the tilt corrected site mean directions of the lower cretaceous and upper cretaceous from the menglun area are d s 46 2 degrees i s 45 9 degrees k 11 0 alpha 95 10 6 degrees and d s 33 2 degrees i s 30 9 degrees k 68 4 alpha 95 8 2 degrees respectively the site mean directions of the upper cretaceous and lower paleogene for the mengban area are d s 50 5 degrees i s 31 0 degrees k 207 6 alpha 95 6 4 degrees and d s 43 5 degrees i s 23 0 degrees k 26 1 alpha 95 13 4 degrees respectively the results of the lower cretaceous and upper paleogene from the mengla area are d s 46 9 degrees i s 42 2 degrees k 27 3 alpha 95 7 7 degrees and d s 41 8 degrees i s 23 8 degrees k 38 9 degrees alpha 95 5 8 degrees respectively in comparison with previous paleomagnetic data of the simao terrane the rotational deformation can be divided into four stages the cretaceous and paleocene sediments of the simao terrane experienced intensive folding before the early oligocene during the early oligocene to early miocene the shan thai and indochina blocks experienced southeastward extrusion with similar to 20 degrees clockwise rotation relative to the south china block during the middle to late miocene the shan thai blocks experienced more 15 degrees clockwise rotation since the late miocene the activity of strike slip faults surrounding the simao terrane enhanced the internal differential rotations of the terrane these deformation processes finally resulted in a complex internal deformation of the simao terrane c 2013 elsevier b v all rights reserved

discrepant cretaceous paleomagnetic poles between eastern china and indochina a consequence of the extrusion of indochina abstract a paleomagnetic study of cretaceous and paleocene red beds has been carried out in the yunlong area yunnan province of china of the northern indochina block a high temperature component with dual polarities was isolated from both lower cretaceous and paleocene rocks and of solely polarity for upper early cretaceous and lower late cretaceous rocks thirteen magnetic polarity zones are found along the pijiang section the primary nature of cretaceous magnetic remanence is ascertained by positive and reversal fold tests these results combined with previous cretaceous results from the yunlong and yongping areas of the lanping basin indicate that these areas behaved as a relatively rigid block and rotated 37 2 17 clockwise relative to eurasia since the paleocene comparing available early cretaceous paleomagnetic results obtained both from indochina and eastern china a statistically significant paleolatitude difference of 9 6 5 7 is inferred this difference is consistent with an estimated 1000 400 km extrusion of indochina relative to south china through a counter clockwise rotation of 14 of indochina around a finite rotation pole inferred from the stage poles of rotation derived from the magnetic isochrons in the south china sea the reconstruction is fully consistent between the cretaceous pole obtained from the khorat basin of the stable core of the indochina block and the coeval pole of eastern china

miocene emplacement and deformation of the konga shan granite xianshui he fault zone west sichuan china geodynamic implications the presently active sinistral xianshui he strike slip fault xsh is a lithospheric scale strike slip fault in the eastern himalaya in the study area this fault affects the eastern edge of the konga shan granitic massif where it has caused both brittle and ductile deformation a rbsr isochron and nd and pb isotope study of three samples and a upb zircon study of a single sample were completed on the granite upb data indicate a granite emplacement age of 12 8 1 4 ma the rbsr isochrons show that the granite emplacement and the deformational event were synchronous at around 12 10 ma minimum age for deformation the nd and pb isotope compositions of whole rocks and k feldspars indicate the involvement of proterozoic continental crust which is confirmed by upb systematics indicating inherited zircons sinistral faulting along the xsh began at the latest at 12 ma and marks the extrusion toward the east of the west sichuan and south china blocks following their extrusion along the red river fault zone between 50 and 21 ma

paleomagnetic results from cretaceous and jurassic rocks of south and southwest yunnan evidence for large clockwise rotations in the indochina and shan thai malay terranes abstract cretaceous and jurassic redbeds have been sampled from south and southwest yunnan china for paleomagnetic study the paleomagnetic directions resolved from the cretaceous rocks indicate that the xiaguan area which is located on the red river fault has not been rotated with respect to stable eurasia since at least late cretaceous times and that the jinggu mengla area 200 400 km farther south has been rotated by 46 65 clockwise the jurassic results reveal the same rotation pattern at face value the differences between the observed cretaceous paleolatitudes and those predicted from the eurasian apparent polar wander path apwp indicate a southward extrusion of the region but the amount of extrusion is not large enough to be resolved by paleomagnetic methods comparison with mesozoic paleomagnetic data from central yunnan northern thailand and northwestern borneo suggests that the red river fault is not a demarcation between unrotated and significantly rotated regions and that sundaland did not respond to the india asia collision as a single coherent unit the existing paleomagnetic data from east asia appear to indicate that the crust in central tibet and tien shan north of tarim directly in front of the indian indentor has been substantially shortened and thickened whereas the crust in the syntaxial areas has probably been largely squeezed out to further evaluate the relative importance of crustal thickening and extrusion in the tectonic evolution of the tibetan plateau paleomagnetic investigation should be intensified in the bounding zones to the indian indentor and be expanded into the cenozoic sedimentary rocks in tibet and surrounding regions

a new late cretaceous to present apwp for asia and its implications for paleomagnetic shallow inclinations in central asia and cenozoic eurasian plate deformation based on a compilation of 533 cretaceous to present day palaeomagnetic poles obtained from both sedimentary and igneous rocks we present a new analysis of the so called asian inclination anomaly and demonstrated the anomaly to be twofold a 2nd order anomaly characterized by high palaeolatitudes in indochina and low palaeolatitudes over tibet and central asia is superimposed on a 1st order anomaly characterized by cenozoic low palaeolatitudes found all over northeastern asian stable blocks the analysis herein convincingly shows that the europe apparent polar wandering path apwp can no longer be used to interpret palaeomagnetic data east of the urals including interpretation of asian tertiary deformation related to the india asia collision we thus construct a new apwp for east asia based on palaeopoles from blocks assumed to be stable this new apwp is consistent with and reinforces previous analyses of asian tectonics such as the age 55 ma and locus 5 10 n of the indo asian collision the lateral extrusion of se asian continental blocks and the intracontinental shortening in central asia possible origins of the 1st order palaeolatitude anomaly are 1 a geomagnetic origin due to long lasting non dipolar contribution to the magnetic field and 2 a tectonic hypothesis in which a newly defined east asia plate was located 10 farther south than expected from the current europe apwp based on a set of six new reconstructions from 90 ma to present we show that our tectonic model reconciles geophysical geological and tectonic observations throughout eurasia from siberia to europe including kinematics in the arctic ocean up to northwestern arctic alaska beyond possible occurrences of non dipolar field contribution and or local inclination flattening in the sedimentary data our model leads us to conclude that cenozoic tectonics is the dominant contributor to the observed 1st order 10 low palaeolatitude anomaly over asia during the tertiary

new paleomagnetic constraints on the extrusion of indochina late cretaceous results from the song da terrane northern vietnam abstract samples were collected for paleomagnetic investigations from the upper cretaceous yen chau formation in the song da terrane 21 7 n 103 9 e bounded by the ailao shan red river fault system and song ma fault in an attempt to examine extrusion tectonics of east asia primary nature of the high temperature 600 690 c component of magnetization from 13 sites is supported by a positive fold test the tectonic corrected data provides the characteristic late cretaceous paleomagnetic direction for the song da terrane d 6 4 i 32 0 with 95 8 5 n 13 sites corresponding to a paleopole lying at 82 9 n 220 7 e a95 6 9 comparison with coeval paleomagnetic poles for the neighboring tectonic blocks indicates no latitudinal translation of the son da terrane with respect to the south china block whereas the shan thai block and the southern part of the indochina block revealed a southward displacement of 10 5 9 5 in latitude with respect to the song da terrane we conclude that the southeastern segment of the ailao shan red river fault system to the east of the dien bien phu fault is not a demarcation of the extruded indochina peninsula the indochina block extruded probably along some faults between the song da terrane and the khorat plateau

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

lipid taphonomy in the proterozoic and the effect of microbial mats on biomarker preservation the pre ediacaran biomarker record has several characteristic features that distinguish it from its phanerozoic counterpart including high concentrations of unresolved complex mixture ucm low concentrations or absence of eukaryotic steranes and a conspicuous carbon isotopic enrichment of n alkyl lipids relative to bulk sedimentary carbon we propose that these derive from a common non actualistic taphonomy based on the pervasive presence of microbial mats prior to the cambrian substrate revolution such mats would have formed a significant mechanical and biochemical barrier to in falling plankton leaving a biomarker record dominated by benthic microbial lipids most of which would have originated from the lower heterotrophically reworked layers of the mat most proterozoic biomarker assemblages studied in this work derive from shallow water sediments where cyanobacteria dominated upper mats represent a focused source of corrosive molecular oxygen this mat seal effect was broken with the onset of bioturbation in the ediacaran and early cambrian at which point the primary source of fossil biomarkers switched from the benthos to the plankton

tertiary paleomagnetic data from northwestern yunnan china further evidence for large clockwise rotation of the indochina block and its tectonic implications abstract eocene red sandstones were collected at 15 sites for paleomagnetic study from two areas lanping and yunlong of the western part of yunnan province china in the northern part of indochina the high temperature magnetization component with an unblocking temperature of around 680 c is isolated after stepwise thermal demagnetization characteristic directions from nine accepted sites reveal a positive fold test at the 99 confidence limit indicating that the high temperature magnetization component is possibly of eocene age a tilt corrected mean direction from nine accepted sites is d 266 1 i 39 8 with 95 11 2 corresponding to a paleopole at 14 5 n 169 7 e with a95 10 9 a comparison with previously reported tertiary paleomagnetic data indicates that the lanping basin of the northern part of the indochina block rotated clockwise through 91 15 with respect to the yangtze craton this large magnitude clockwise rotation is probably caused by the superimposition of regional and local rotations our previous middle cretaceous paleomagnetic results from the yunlong area indicate that about 40 of clockwise rotation out of 90 is explained by tectonic rotation due to a relatively rigid body rotation of the simao terrane during indentation of india the remaining component of rotation is probably ascribed to further eocene post eocene rotational motion of the tectonically independent small blocks within the lanping basin associated with fault movement