| The Longmenshan (LMS) range which constitutes the eastern border of the Tibetanplateau is characterized by a steep topographic transition from the Sichuan Basin tothe plateau with about400km in length. The Cenozoic orogeny of this range wasresulted from the collision between the India plate and Eurasia since50Ma. The2008Wenchuan earthquake and the researches on the earthquake confirm the activity of theLMS since Late Pleistocene, but the Cenozoic evolution of the Longmenshan ThrustBelt (LTB) is still ambiguous, especially in the south segment. Furthermore,researchers have different views on the initial time and the mechanis of the uplift inthe eastern margin of the Tibetan plateau. For the initial time, there are two differentopinions: the first suggests that the eastern margin of the Tibet plateau uplift began20-30Ma, and expearenced several cooling event; the second contends that the theeastern margin of the Tibetan plateau remained stable from60-10Ma, and began touplift quickly from about10Ma to present. For the uplift mechanism, there are twoentirely different explanations: rigid lithospheric extrusion model and low-crustalchannel flow model.In this work, I use the zircon and apatite fission track method to research thethermal history of dozens of rocks at the LTB. Combining with previouslow-temperature thermochronology data, the fission track data reveal the Cenozoicexhumation process of the rocks and their relations with the activities of the faults atLTB. On that basis, I discuss the uplift mechanism of the eastern margin of theTibetan Plateau.In general, the fission track ages of the rocks at the LTB become older from southto north, which indicates that the average exhumation rate decreases from south tonorth of the LTB. In the southern segment, the Zircon Fission Track (ZFT) ages rangefrom11Ma to14Ma for the hanging wall of and close to the faults at south segmentof the LTB, and increase to about25-35Ma, following the increasing distance to thefault. In the middle segment, the youngest ZFT age of10.6Ma appeared at theXuelongbao Complex, the hanging wall of the Wenchuan-Maoxian fault, and the other ZFT ages are more than40Ma. In the northern segment, all the ZFT ages areover60Ma, which indicates all samples are not reset in the Cenozoic. Apatite FissionTrack (AFT) ages range from1.9Ma to5.0Ma in the southern segment. In the middlesegment, the youngest age also appears at same place with ZFT, and is in data of2.1Ma, while others are about8Ma. The AFT ages in the northern segment are obviousolder than that in the southern and middle segments, and range from30Ma to189Ma.The maximum average exhumation rates are2.1mm/yr from1.9Ma to present at thesouthern segment,1.9mm/yr from2.1Ma to present at the middle segment, and0.1mm/yr from33Ma to present at the northern segment, respectively. Moreover, thereare obvious differences in the ages cross faults, showing that the ages at the hangingwall are younger than the footwall, which indicates there are differential exhumationson the two sides of the faults, and all faults have thrust component in the Cenozoic.The differential exhumations are greater in the southern and middle segments thanthat in the northern segment, further indicating that the faults at the southern andmiddle segments have a larger thrust rate than that at the northern segment. Accordingto the width of the exhumation at the middle segment and the depth of the detachmentplane, I estimate the dip angles of the major faults, which are about30°, revealing thatthe faults at the middle segment are low dip angle faults. At the southern segment, theactivity of the faults migrate to the basin, and the present difference of movementbetween the Tibetan Plateau and the Sichuan Basin is mainly absorbed by front rangefaults and blind faults in the basin.The Gonggashan Granite located to the southwest of the LTB is cooling quicklyfrom late Cenozoic to present, and also shows the characteristics of a decreasing trendfrom south to north in the cooling rate. The biggest uplift rate from about1Ma topresent may equal or be more than3.3±0.8mm/a, while the uplift rate of the Triassicrocks is much smaller. The phenomena indicate that the partial area of the TibetanPlateau uplifted quickly when most of the plateau uplifted slowly. According to theresolution of vectors of the Sichuan-Yunnan Block’s horizontal movement, I think thatthe rapid uplift of the Gonggashan Granite is caused by the change of the strike of theXianshuihe fault, which absorbs and converts the eastward or southeastward horizontal movement of the Sichuan-Yunnan Block. The Gonggashan Granite may bethe product of this progress.The Danba Anticline to the west of the southern segment of the LTB is animportant region for research of Mesozoic-Cenozoic deformation, as it exposes arelated complete set of rocks, but the cooling history from30Ma to present is stillambiguous. In this work, I get16Apatite fission track and14Zircon fission trackages, and determine the cooling histories in different regions of the Danba Anticline.The result shows that the rocks in the Danba Anticline cooled quickly from ca.25Mato present, and the average cooling rates decreased from13.0±1.2℃/Ma at the core, tothe10.5±1.1℃/Ma at the subcenter, then to6.0±1.0℃/Ma at the periphery of theDanba Anticline. Supposing the paleo-geothermal gradient is25±5℃/km, thedenudation thicknesses from ca.25Ma to present decreased from13-15km at the coreto5-7km at the periphery of the Danba Anticline. According to the metamorphictemperature, the Cenozoic deformation takes up the1/4to1/2of the total deformationin the Danba Anticline. This research indicates that the Danba Anticline began to beaffected by the India-Asia collision at ca.25Ma and behaved as crustal shorteningtoward northeast.The Min Shan, located to the west of the north segment of the LTB, is also arange that uplifted quickly in late Cenozoic. I get12AFT ages and data of tracks’length, for thermal history modeling that shows that the Minshan range underwenttwo cooling events, which happened from50to30Ma and5Ma to present,respectively. The cooling amount in the later cooling event was about40℃, and theexhumation thickness and the exhumation rate were about1-2km and0.2-0.4mm/yr.Like the southern segment of the LTB, the activity of the faults in the Minshan regionalso moves east. The2-3mm/yr shortening revealed by GPS can cause the uplift witha rate of0.2-0.4mm/yr, and the low-crustal channel flow hypothesis is not needed forthe Minshan uplift.The thermal history research in different tectonic regions suggest that the easternmargin of the Tibetan plateau building, affected by the collision between the Indiaplate and the Eurasia plate, initialed at about30Ma, and the cooling rates decreased from south to north. The fault activity shows a tendency of moving to the Sichuanbasin. All of the phenomena revealed in this work, for example, the deformation styleof upper crust, and the initial time of the plateau building, don’t support thelow-crustal channel flow hypothesis. The reasons are mainly in the following fiveaspects:(1) The Wenchuan-Maoxian fault in the LTB is a thrust fault, while thelow-crustal channel flow hypothesis considers it as a normal fault. Themaximum exhumation regions are located on the hanging wall of theWenchuan-Maoxian fault, and not the Baoxing Complex and the PengguanComplex, which are thought to be the region with the biggest exhumation ratein the low-crustal channel flow explaination.(2) The Wenchuan-Maoxian fault, Beichuan-Yingxiu fault and Peng-Guan faultin the middle segment of the LTB control wide region uplift, which indicatesthat the ramps of those faults have slow slope. In other words, the faults havesmall dip angles, while the dip angles should be much bigger in thelow-crustal channel flow hypothesis.(3) The uplift of the Longmenshan and the other regions in the eastern margin ofthe Tibetan plateau initiated at about30Ma according to this research.However, the low-crustal channel flow hypothesis suggests that the easternmargin of Tibetan plateau uplifted quickly from about10Ma to present(Clark et al.,2005).(4) Cenozoic deformation of the Danba Anticline is controlled by Mesozoictectonics, which indicates the Tibetan plateau is not a homogeneous andcontinuous deformed material. The Danba Anticline behaves as upper crustalnortheast shortening, which is difficult to be explained by the low-crustalchannel flow hypothesis.(5) The uplift rates in most areas of the Min Shan are between0.2-0.4mm/yr, andthe2-3mm/yr shortening revealed by GPS can accomodate the uplift of theMin Shan, so the low-crustal channel flow hypothesis is not needed. |
PDF Full Text Request