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发表于 2010-12-16 14:50
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. w) C A) o* [4 mMy first research documented most representative eolian sediments in the Chinese Loess Plateau in order to assess regional as well as eastern Asia – paleomonsoon route and paleoenvironmental fluctuations during the last 130 kys. The AMS measurements evidently show that the maximum susceptibilities group in the SE quadrant and the minimum susceptibilities are clustered in the NW quadrant. To explain such result I postulated that the ellipsoild orientation of anisotropy of magnetic susceptibility is determined by the moist summer monsoon rather than the dry winter monsoon as previously hypothesized. The major stream of the paleowind was generally similar to the present day summer monsoon routes. The summer monsoon was stronger than the winter monsoon and played a major role in the process of formation and magnetic fabrics in the central and west Loess plateau although winter monsoon brings all the eolian sediments to Loess plateau. It is still a matter of the future study to verify if the AMS signal from the northern parts of the Plateau fits in our model. The winter monsoon wind in the north is much stronger than the summer monsoon wind and the magnetic particle could be probably oriented during the winter only. Such scenario may result in the AMS ellipsoid orientations toward NW.
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1 n3 [% t/ n: w! cWe conclude that the sedimentary particles including magnetite were rearranged, settled, and fixed during the windy and rainy summer time, and the measured AMS apparently reveal the orientation of the prevailing summer winds. Our new study demonstrates that the moist summer monsoon rather than dry winter monsoon as has been commonly accepted, plays a major role for the formation of the AMS ellipsoid orientation. We demonstrate that the summer monsoon has been always stronger than the winter monsoon in the central and western parts of the Chinese Loess Plateau and have been responsible for the magnetic grain imbrication although cold and dry winter monsoons brought all the dust from the northwestern desert areas.# |, ?0 j4 r- o" o# o; a" q* {* H
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My second study estimates the Eocene–Oligocene paleoclimate transition in central Asia and compared with the record in the Tibet and in the ocean. 8 {9 q( W# F( G& R
My paleomagnetic analysis yielded the reliable ChRM directions from 126 samples. I identified 11 normal polarity and 11 reversed polarity intervals. The detailed magnetostratigraphy was determined by the declination and inclination correlation to the geomagnetic polarity time scale (GPTS) of Cande and Kent (1995). Gradual change to the higher sedimentation rate, approximately at the EOB in our section, suggests that uplift of the Altay Mountains north of the Junggar Basin could have occurred due to shortening of the distance between India and northern Eurasia. Reorganization of faunal compositions of the Junggar Basin between 34.8 and 33.7 Ma fits with marine oxygen isotope data that demonstrate that the marine paleoclimate transition (34.1–33.6 Ma) occurred around the Eocene–Oligocene boundary (EOB, 33.9 Ma). The terrestrial paleoclimate transition in the Junggar Basin and central Asia occurred about the same time interval (34.8–33.7 Ma). It is possible that the central Asia experienced both cooling and aridification around the EOB, The uplift of Tibet and retreat of the Paratethys epicontinental sea triggered the aridification of the continental environment.
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In my third research theme I obtain the first paleomagnetic poles for the Junggar block for the time window from ~40 Ma to ~20 Ma and perform tectonic reconstructions of the paleopositions for the Asian blocks relatively to the European continent. The amount of the latitudinal displacements and relative to Europe rotations (12.2°±6.5° and –17.2°±9.6° at 40 Ma and 9.7°±4.1° and –11.8°±6.1° at 20 Ma) are consistent with with displacement and rotations of India, North China, South China and Tarim cratons, Amuria and Kazakhstan composite terranes as well as Eastern Sayan which could characterize the edge of the Siberian platform. The movement velocity of rotation and shortening is comparable with that of the present day GPS data and the left-slip fault system on the west of Altay Mountains. No significant vertical-axis rotation (5.4°±15.7°) and intracontinental shortening (3.5°±10.6°) between the Junggar block and stable Europe were observed from 40 Ma to 20 Ma. The major intracontinental shortening and rotation between the Junggar block and Europe occurred after 20 Ma. The CCW rotation (–11.8°±6.1°) and northward shortening (9.7°±4.1°) of Junggar block after 20 Ma are interpreted as the result of the enforced uplift of Altay Mountains and increase in tectonic activity in the Baikal Rift Zone. |
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