Late Cenozoic Stratigraphy and Paleomagnetic Chronology of the Zanda Basin,Tibet, and Records of the Uplift of the Qinghai-Tibet Plateau

The characteristics of Late Cenozoic tectonic uplift of the southern margin of the Qinghai- Tibet Plateau may be inferred from fluvio-lacustrine strata in the Zanda basin, Ngari, Tibet. Magnetostratigraphic study shows that the very thick fluvio-lacustrine strata in the basin are 5.89- 0.78 Ma old and that their deposition persisted for 5.11 Ma, i.e. starting at the end of the Miocene and ending at the end of the early Pleistocene, with the Quaternary glacial stage starting in the area no later than 1.58 Ma. Analysis of the sedimentary environment indicates that the Zanda basin on the southern Qinghai-Tibet Plateau began uplift at -5.89 Ma, later than the northern Qinghai-Tibet Plateau. Presence of gravel beds in the Guge and Qangze Formations reflects that strong uplift took place at -5.15 and -2.71 Ma, with the uplift peaking at -2.71 Ma.

Geochronological,Geochemical and Paleomagnetic Clues for Existence of a~2.1Ga Dyke Generation in the Mashishing Area,South Africa

Dyke swarms are widespread throughout the Kaapvaal Craton(KC)in South Africa and have been documented in some detail.Up to date,only the^2.9 Ga SE-trending and the^2.7 to^2.66 Ga radiating dyke arrays are

Paleomagnetic Data and Dyke Swarms Geometries – Important Tools for Precambrian Paleogeographic Reconstructions

There are only two quantitative tools for Precambrian paleogeographic reconstructions–paleomagnetic data and dyke swarms geometries.Paleomagnetic data provide information about paleolatitudes and orientation of rigid

Paleomagnetic Evidence for Tectonic Setting of Paleoproterozoic Dyke Swarms in the North China Craton, China

In order to investigate the tectonic setting of 1.77-1.78Ga dyke swarms emplaced into the central North China Craton(NCC),we carried out a paleomagnetic and magnetic fabric study on the well geochronologically

Late Paleoproterozoic Paleogeography of Baltica and Laurentia: New Paleomagnetic Data from 1.80–1.75 Ga Mafic Intrusions of Fennoscandia and Sarmatia

Tectonic evolution and paleogeography of the two major continental blocks Fennoscandia and VolgoSarmatia during their docking to form the East European Craton(Baltica)at 1.8–1.7 Ga represent important‘puzzle

Positioning the Southern Margin of Asia Prior to Its Collision with India: Paleomagnetic Constraints from Late Cretaceous Dykes in Gangdise Belt

The pre-collisional southern margin of Asia can be restored using paleomagnetic data from late Cretaceous rocks from the Lhasa terrane.However,the available data are based either on the red beds or on the intercalated thin layers of lava flows,both of which had been involved in strongly folding.Recent studies show clear evidence for the possibility of serious overprint hence the data could not be reliably used for tectonic interpretation.We report paleomagnetic data from diorite dykes and the grano-diorite country rock in the Gandise belt near the city of Lhasa.U–Pb isotopic dating indicates the intrusive rocks have an age of;2–86 Ma.Fifteen sites yield acceptable Ch RM directions which pass a reversal test.SEM and light microscope observations show primaryintergrowth relationship between magnetite and other minerals within the thin sections.AMS measurement defines a primary magma flow fabric for the intruded dykes and the country rocks.All the characteristics support that the Ch RMs are primary.The paleomagnetic pole calculated from the remanence of the dykes and the country rocks yields a paleolatitude of;4°N which provide a reliable constraint for the southern margin of Asia near Lhasa.Furthermore,the recorded declination shows significant counterclockwise rotation of;0°for the sampling location relative to the north.In consideration of the strike and tectonic setting of the dykes,the strike of the southern margin of Asia is restored which is compatible with the hypothesis of a quasi-linear margin of Eurasia prior to its collision with India.

Some Results of Paleozoic Paleomagnetic Research

This application note discusses the preliminary results of paleomagnetics studies of several incisions of the Paleozoic period the territory of Armenia and neighboring regions （Minor Caucasus）. It is supposed that before the Permian period the Armenian-lranian Mezocontinent was uniform with Afro-Arabia and only at the end of the late Paleozoic the formed Mesozoic ocean separated these areas from each other. A result of application of the complex laboratory investigations were identified from samples of primary magnetization the direction of which corresponds to the time of formation of the studied rocks. On the basis of paleomagnetic researches of Triassic, Permian, Devonian, Carbonian rocks a preliminary conclusion has been made about the dipole configuration of the geomagnetic field throughout Paleozoic. The comparison of the obtained data to paleomagnetic definitions of African, Arabian plates, Europe and the Siberian platform leads to the conclusion that the Minor Caucasus existed separately from the present general lithosphere blocks of Greater Caucasus and Europe.

Mesoproterozoic Nuna Supercontinent and the Geomagnetic Field in Light of Recent Paleomagnetic Data from Diabase Dykes of Finland

The number of good quality paleomagnetic data of the Mesoproterozoic supercontinent Nuna(e.g.Columbia,Hudsonland)has increased in recent years enabling more reliable global continental reconstructions(e.g Hoffman

Paleomagnetic results of Core NP93-2 from the Prydz Bay,Eastern Antarctica

Two reversal geomagnetic excursions are detected by systematically paleomagnetic measurements of the Core NP93-2 from the Prydz Bay, Eastern Antarctica.One is found in 60 to 67.5 cm from the tOP of the core, and the 14C dating age of layer 67.5 cm to 72.5 cm is 10315±800 a B.P.It is estimated that the geomagnetic excursion occurred at 9980-8880 a B.P.,and perhaps was the record of the Gothenburg geomagnetic excursion (about 12000 a B.P.).The other is found in 27.5-32.5 cm,and the 14C dating age is 5390±600 a B.P., with which no generally acknowledgedgeomagnetic excursion can be compared. But a few reports of geomagnetic excursions can be used for comparing with,Zhu et al.reported a geomagnetic excursion at 4980-4770 a B.P.from peat in Beijing, Wang et al.reported a geomagnetic excursion at5120±110 a B.P.from Core NS-89--76 in Nansha Waters and Zhou et al. reported ageomagnetic excursion at 6400-6000 a B.P.from Core QC2 in Yellow Sea. Whetherthe studied geomagnetic excursion does exist or whether those reported geomagnetic excursion are the same one is worth further study.

Petrographic, Radiometric and Paleomagnetic Studies for Some Alkaline Rocks, South Nusab El Balgum Mass Complex, South Western Desert, Egypt

Nusab El Balgum mass complex represents one of the alkaline igneous activities in the south Western Desert of Egypt. Petrographic investigations defined some different rock types in south of the complex represented by alkaline volcanics (pyroclastics [rhyolitic crystal tuffs], spherulitic rhyolites, alkaline rhyolite dykes), sub-volcanic peralkaline granites and structurally controlled mylonitic volcanoclastics. These rocks recorded significant concentrations in terms of the two radioactive elements Th and eU, which displayed considerable spatial variations, especially within the peralkaline granites. The abundance of Th and eU is mainly related to favorable combination of structural and pos-magmatic hydrothermal conditions. Paleomagnetic results give well-defined stable remanent magnetization directions of reliable VGP positions, which are presented and discussed in the context of the African APWP. Rock types, magnetization directions and VGP positions with the corresponding ages are as follows: 1) Rhyolitic crystal tuffs;D/I = 340.0°/—19.4°, α95 = 6.8°;VGP Lat./Long. = 51.4°N/240.5°E, A95 = 5.9° (Late Triassic). 2) Spherulitic rhyolites;D/I = 346.7°/—6.6°, α95 = 3.5°;VGP Lat./Long. = 60.4°N/237.0°E, A95 = 3.0° (Late Triassic/Early Jurassic). 3) Alkaline rhyolite dykes;D/I = 341.3°/16.7°, α95 = 5.4°;VGP Lat./Long. = 67.0°N/262.8°E, A95 = 4.2<span

The northern Qiangtang Block rapid drift during the Triassic Period:Paleomagnetic evidence

As one of the pivotal Gondwana-derived blocks,the kinematic history of the northern Qiangtang Block(in the Tibetan Plateau)remains unclear,mainly because quantitative paleomagnetic data to determine the paleoposition are sparse.Thus,for this study,we collected 226 samples(17 sites)from Triassic sedimentary rocks in the Raggyorcaka and Tuotuohe areas of the northern Qiangtang Block(NQB).Stepwise demagnetization isolated high temperature/field components from the samples.Both Early and Late Triassic datasets passed field tests at a 99%confidence level and were proved to be primary origins.Paleopoles were calculated to be at 24.9°N and 216.5°E with A95=8.2°(N=8)for the Early Triassic dataset,and at 68.1 N,179.9 E with A(95)=5.6°(N=37)for the Late Triassic,the latter being combined with a coeval volcanic dataset published previously.These paleopoles correspond to paleolatitudes of14.3°S±8.2°and 29.9 N15.6°,respectively.Combining previously published results,we reconstructed a three-stage northward drift process for the NQB.(1)The northern Qiangtang Block was located in the subtropical part of the southern hemisphere until the Early Triassic;(2)thereafter,the block rapidly drifted northward from southern to northern hemispheres during the Triassic;and(3)the block converged with the Eurasian continent in the Late Triassic.The^4800 km northward movement from the Early to Late Triassic corresponded to an average motion rate of^11.85 cm/yr.The rapid drift of the NQB after the Early Triassic led to a rapid transformation of the Tethys Ocean.

New Jurassic Paleomagnetic Results from Southeastern China and Their Geological Implication

A paleomagnetic study was carried out on late Jurassic sediments in the Nanjing area. Stepwise thermal demagnetization was used to isolate the characteristic higher temperature component （HTC）（D=354.0°,I=48.5°,a95=7.8°）,which passes the reversal test.A 24.7±8.7°counter-clockwise rotation relative to the Sichuan area is detected through comparing this new pole（84.4°N,7.0°E, A95=7.7°）with other coeval poles reported from South China.This rotation was conducted by sinistral action of slip faults.These cases presenting in several areas of eastern China indicate that deformation of eastern China is under the effect of subduction from the Pacific Ocean plate.The difference on the paleolatitude of several areas across the Tan-Lu fault zone is calculated and suggests about at most 250 km offset distance after the later Jurassic.

Paleomagnetic Excursions Recorded in the Yanchi Playa in Middle Hexi Corridor, NW China Since the Last Interglacial

Paleomagnetic determinations on lithological profiles of two paralleled[( )-275(long )]drilling cores covering the past 130 kyr B.P., GT40 and GT60, from the Yanchi Playa in the arid Northwestern China, indicate that a series of pronounced paleomagnetic excursions have been documented. By correlating our results with published regional and worldwide reports, 4 excursion events out of 10 apparent reversal signals (labeled from GT-1 to GT-10) were identified as excursion events coeval with the Mono Lake Event ([(28.4)( )]kyr~[(25.8)( )]kyr), Laschamp Event ([(43.3)( )]kyr^40.5 kyr), Gaotai Event (82.8 kyr~[(72.4)25( )]kyr) and the Blake Event (127.4 kyr^113.3 kyr), respectively. GT-9 correlates with the above-mentioned Gaotai Event, GT-7 and GT-6 correspond to two stages of the Laschamp Event and GT-5 to the Mono Lake Event. It is noteworthy that the so-called Gaotai Event has not been reported as a pronounced paleomagnetic excursion in the Northwestern China. Every magnetic excursion event corresponds to paleointensity minima, anteceding those established abrupt paleoclimatic change events, such as the Younger Drays and the Heinrich Events (H1-H6)[(. )-250( )]Here,[( )-250( )]we tentatively[( )-250( )]propose that these geomagnetic excursions/reversals can be viewed as precursors to climate abruptness. During the transitional stages when the earths magnetic field shifted between a temporal normal and a negative period, the earths magnetic paleointensity fell correspondingly to a pair of minima. Although more precise chronology and more convincing rock magnetic parameter determinations are essentially required for further interpretation of their intricate coupling mechanism, these results may have revealed, to some extent, that the earths incessantly changing magnetic field exerts an strong influence on the onset of saw-tooth shaped abrupt climate oscillations through certain feedback chains in arid Central Asia or even North Hemispheric high latitude regions.

PALEOMAGNETIC STUDY OF IGNEOUS ROCKS OF GUPIS—SHAMRAN AREA,KOHISTAN ARC, PAKISTAN,NW HIMALAYA

Kohistan Sequence has been considered as island arc formed during the subduction of oceanic lithosphere at the leading edge of northward moving Indian continent.. Sedimentary sequences indicate that formation of the intra\|oceanic Kohistan arc began in early Cretaceous time. The isotopic data demonstrate the involvement of enriched, DUPAL type mantle, suggesting that Kohistan arc was formed at or south of the present equator (Khan et al., 1997). The Intra oceanic phase of Kohistan lasted until sometime between 102 and 85 Ma, when Kohistan collided with Asia. From this time until collision with India about 50 Ma ago, Kohistan existed as Andean\|type margin. This paleomagnetic study is from the volcanic and plutonic rocks exposed in Gupis\|Shamran area (west of Gilgit) in northern part of the Kohistan arc. According to geochronological data these rocks were formed 61～55Ma ago (Treloar et al., 1989), when Kohistan was existing as Andean\|type margin. Seven to nine samples were collected from nine sites of Shamran volcanics (58±1)Ma and from five sites of Pingal, Gupis, and Yasin plutons (Ar\|Ar hornblende ages ranges from 61～52Ma). On the basis of one Rb\|Sr age of (59±2)Ma from these plutons, the above\|mentioned Ar/Ar ages may be regarded as reasonable intrusion ages of these plutons (Searle, 1991).

PALEOMAGNETIC ESTIMATE OF THE MESOZOIC—CENOZOIC LATITUDINAL DISPLACEMENT OF TERRENES IN THE QINGHAI—TIBET PLATEAU AND ITS SIGNIFICANCE

The Mesozoic—Cenozoic latitudinal displacement amounts of terranes (or blocks) in the Qinghai—Tibet plateau were calculated in paleomagnetism. These terranes (or blocks) include Tarim and Qaidam blocks, East Kunlun, Baryan Har, Qiangtang, Lhasa and Himalaya terranes. The calculated results are listed in table 1. These results show that:(1) There was the latitudinal displacement difference between central area and southwestern area in the Tarim southern margin since the lower Cretaceous. There was a southward latitudinal movement from the beginning of middle Jurassic or upper Jurassic (Zhou Qingjie, 1992). The northward movement amounts of the Tarim northern margin since the Paleocene are greater than that of the Tarim southern margin. Tarim southern margin has moved northward about 1100km since the Paleocene, Tarim northern margin has done about 1700km. Qaidam has moved northward about 3100km since lower\|middle Jurassic. The northward displacement amount of Qaidam since Paleocene is about 810km, near to that of the central area, Tarim southern margin.

New Paleomagnetic Study on Early Triasic Liujiagou Formation of NorthChina Block

This paper introduces the result of studying on paleomagnetism of 91 specimens from 13 sites from Liujiagou Formation in Dashu Village, Wu ’an city, Hebei Province. The result reveals that North China block (NCB) was not united with Yangtze block (YB) in the Early Triassic. From the Early Triassic to now, NCB has rotated 30.0° counterclockwise, but YB has rotated 45.1° clockwise.

On the Reliability (Remagnetization?) of Paleomagnetic Poles Obtained from Permo-Silurian Rocks from Oaxaca Mexico, Belize and Guatemala: Insights from Rock Magnetic Studies

In the reconstruction of past movements of tectonic plates, the determination of reliable paleomagnetic poles is of utmost importance. To achieve accurate results, a full knowledge of the rock magnetic properties of the samples is required particularly for Curie point, for grain-size analyses in addition to thermal and alternating field (a.f.) demagnetization experiments. We present the comparative results of 20 sites drilled at 3 different Paleozoic areas: The Permian rocks of the Juchatengo area in Oaxaca, Mexico;the Late Silurian (~418 Ma) Mountain Pine Ridge Granite, the Hummingbird Granite in Belize, and the Early Leonardian Chochal Limestone in Guatemala. The samples of all 20 sites were subjected to a.f. and thermal demagnetization in 16 steps from NRM to 100 mT, the thermally demagnetized samples were cleaned in 15 increasing temperature steps from NRM up to 675℃. Principal component analysis was applied to the samples in order to obtain their respective mean directions. Saturation Isothermal Remanent Magnetization (SIRM), hysteresis loops, and coercivity experiments performed indicate that about 90% of the samples were characterized by Multi-Domain (MD) grain sizes and the rest were in the Pseudo-Single Domain (PSD) range of the Day diagram. Curie point determinations results ranged from 190℃ to 660℃, indicating the presence of titanomagnetites as well as hematite. In the Juchatengo area reliable poles were obtained from 3 sites, in Belize 3 sites and only 2 sites of the Permian Chochal Formation, Guatemala yielded coherent and useful results. All the “reliable” paleopoles obtained do not agree with the APWP of North America.

New paleomagnetic evidences of paleogene tectonic rotation of the Qaidam Basin and adjacent region: mechanism and its tectonic implication

In order to better understand the tectonic evolution of the Qaidam Basin. The authors carried out a paleomagnetic study on 39 samples of 7 sites from Hongsanhan section, northwest Qaidam Basin. Stepwise thermo-demagnefizafion isolated a high-temperature component （HTC）, which passes the fold and reversal tests at 95 % confidence level. It suggests that the HTC should be primary. Unblocking temperature （about 685℃ ） and the experiment of isothermal remanence indicate that hematite is the mainly carrier of the remanence. A tilt-corrected mean direction is： Ds= 7.1°, Is= 38.5°, α95 = 7.4°. Corresponding to a palaeopole at, φ= 250. 1°E, λ= 72.0°N, A95 = 6.8°, Compared with the reference APWP of Eurasia, the sampling area occurred a non-significant rotation （3.4° ± 5.5°） relative to Eurasia since Eocene due to the affect of left-lateral Altyn fault, Based on the paleomagnetic results of early Cretaceous and Teritary within the Qaidam block, we can obtained the mean clockwise rotations from the samples deposited from 160 Ma to 45 Ma is 24.5° ± 9.0°, and from 38 Ma to present is - 0.5° ± 7.5° relative to present geomagnetism respectively. During the Mid Eocene （45 -38 Ma） rotation phase, there should existing an important early tectonic event in northern part of the present-day Tibetan Plateau, which probably represents one of large-scale strike-slip events of the Altyn strike-slip fault.

Paleomagnetic constraints on the tectonic history of the major blocks of China duing the Phanerozoic

Paleomagnetic study of China and its constraints on Asia tectonics has been a hot spot. Some new paleomagnetic data from three major blocks of China, North China Block （NCB）, Yangtze Block （YZB） and Tarim Block （TRM） are first reported, and then available published Phanerozoic paleomagnetic poles from these blocks with the goal of placing constraints on the drift history and paleocontinental reconstruction are critically reviewed. It was found that all three major blocks were located at the mid low latitude in the Southern Hemisphere during the Early Paleozoic. The NCB was probably independent in terms of dynamics, its drift history was dominant by latitudinal placement accompanying rotation in the Early Paleozoic. The YZB was close to Gondwanaland in Cambrian, and separated from Gondwanaland during the Late Middle Ordovician. The TRM was part of Gondwanaland, and might be close to the YZB and Australia in the Early Paleozoic. Paleomagnetic data show that the TRM was separated from Gondwanaland during the Late Middle Ordovician, and then drifted northward. The TRM was sutured to Siberia and Kazakstan blocks during the Permian, however, the composite Mongolia NCB block did not collide with Siberia till Late Jurassic. During Late Permian to Late Triassic, the NCB and YZB were characterized by northern latitudinal placement and rotation on the pivot in the Dabie area. The NCB and YZB collided first in the eastern part where they were located at northern latitude of about 6°\8°, and a triangular oceanic basin remained in the Late Permian. The suturing zone was located at northern latitude of 25° where the two blocks collided at the western part in the Late Triassic. The collision between the two blocks propagated westward after the YZB rotated about 70° relative to the NCB during the Late Permian to Middle Jurassic. Then two blocks were northward drifting （about 5°） together with relative rotating and crust shortening. It was such scissors like collision procedure that produced intensive compression in the eastern part of suturing zone between the NCB and YZB, in which continental crust subducted into the upper mantle in the Late Permian, and then the ultrahigh pressure rocks extruded in the Late Triassic. Paleomagnetic data also indicate that three major blocks have been together clockwise rotating about 20° relative to present day rotation axis since the Late Jurassic. It was proposed that Lahsa Block and India subcontinent successively northward subducted and collided with Eurasia or collision between Pacific/Philippines plates and Eurasia might be responsible for this clockwise rotating of Chinese continent.

Paleomagnetic results from Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane, Tibet, China, and their tectonic implications

Results of a systematic paleomagnetic study are reported based on Late Carboniferous to Early Permian sedimentary rocks on the north slope of the Tanggula Mountains, in the northern Qiangtang terrane （NQT）, Tibet, China. Data revealed that magnetic minerals in limestone samples from the Zarigen Formation （CP^z）are primarily composed of magnetite, while those in sandstone samples from the Nuoribagaribao Formation （Pnr） are dominated by hematite alone, or hematite and magnetite in combination. Progressive thermal, or alternating field, demagnetization allowed us to isolate a stable high temperature component （HTC） in 127 specimens from 16 sites which successfully passed the conglomerate test, consistent with primary remnance. The tilt-corrected mean direction for Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane is D°=30.2°, Is=-40.9°, ks=269.0, a95=2.3°, N=16, which yields a corresponding paleomagnetic pole at 25.7°N, 241.5°E （alp/rim=2.8°/1.7°）, and a paleolatitude of 23.4°S. Our results, together with previously reported paleomagnetic data, indicate that： （1） the NQT in Tibet, China, was located at a low latitude in the southern hemisphere, and may have belonged to the northern margin of Gondwana during the Late Carboniferous to Early Permian; （2） the Paleo-Tethys Ocean was large during the Late Carboniferous to Early Permian, and （3） the NQT subsequently moved rapidly northwards, perhaps related to the fact that the Paleo-Tethys Ocean was rapidly contracting from the Late Permian to Late Triassic while the Bangong Lake-Nujiang Ocean, the northern branch of the Neo-Tethys Ocean, expanded rapidly during this time.