基于Pecube模型重建青藏高原东南缘临沧花岗岩地区新生代阶段性地貌演化

Reconstruction of Cenozoic landscape evolution in Lincang granite belt,southeastern Tibetan Plateau based on the Pecube Model

基于三维热-动力学数值模拟(Pecube),本研究分别采用稳定地形、高原-稳定地形以及高原-动态地形3种模型重建了青藏高原东南缘临沧花岗岩地区晚始新世以及中中新世时期的地貌演化过程。反演结果揭示了在新生代早期(晚始新世),只有高原-稳定地形模拟的结果能够约束研究区冷却-剥露过程,并论证了该时期发生强烈构造抬升的事件,正是因为此次抬升事件,使得高原东南缘地区古高度接近或达到现今状态;中中新世时期,澜沧江侵蚀下切约1 km,对原有的高原地貌进行了进一步改造,从而形成现今的地貌格局。该模型结果验证了青藏高原东南部的构造隆升主要发生在新生代早期,这与侧向挤出模型的预测相一致。

The southeastern(SE)Tibetan Plateau is characterized by widespread high-elevation and low-relief surfaces,different from other boundary landscapes.However,great debate exists regarding when and how the SE Tibetan Plateau achieved its current low-relief topography,and both lateral extrusion and lower crustal flow have been proposed as the dominant mechanism.The former model predicts large-scale lateral extrusion along major strike-slip faults together with upper crustal shortening at the margins of the plateau in the Early Cenozoic.In contrast,the latter suggested that the SE Tibetan Plateau formed the current landscape during the Late Miocene.With the lower crustal flow,the continuous gentle topographic slope across SE Tibetan Plateau has been interpreted to result from the uplift of the low-elevation and low-relief relict surface that extended continuously over thousands of kilometers from the high-elevation areas toward the southeast.Therefore,it is necessary to select an ideal area to clarify the underlying forcing mechanism and landscape evolution of the SE Tibetan Plateau.The Lincang granite belt(LGB)is located to the west of the Mekong River(Lancang);together with the Chongshan shear zone.The LGB is a ca.350-km-long,ca.10~50-km-wide,NS trending body.It is bounded by the Sukhothai Terrane in Northern Thailand and Peninsular Thailand to the south and by the Baima Mountains to the north,forming a giant compound granite batholith along the Lancang River in Southwest China.During the Cenozoic,the LGB experienced multiple periods of intense tectonic activities,and a large number of tectonic characteristics are still recorded and preserved on the surface,making it an ideal place to study the mechanism of the regional uplift and landscape evolution.In this paper,we select LGB of the SE Tibetan Plateau as the research object(23.50°~23.75°N,100.00°~100.25°E).Basing on the three-dimensional thermo-kinematic modeling code Pecube,we define three models(e.g.,stable,plateau-stable,plateau-dynamic)which are constrained by previously reported apatite and zircon(U-Th)/He thermochronological data.The aim is to reconstruct the exhumation history and landscape evolution of the LGB at Late Eocene and Middle Miocene.The inversion results demonstrate that only the plateau-stable terrain model can accurately constrain the temporal and spatial evolution of the Late Eocene cooling event.This cooling event caused the elevation to reach its current height,consistent with previous paleo-elevation evidence.However,since the Middle Miocene,with the regulation of the Mekong River driven by intensified Asian monsoon precipitation,there was about 1 km of downward incision,which further modified the original plateau landform and formed the present geomorphic evolution pattern.The simulation results are consistent with the prediction of the block extrusion model,but not consistent with the prediction of the lower crustal flow model.It is worth noting that we have preliminarily explored the landscape evolution at Late Eocene and Middle Miocene in LGB and possible mechanism of the region using various thermo-kinematic modelings for the above events,we,however,cannot exclude the existence of other possibilities.As we gain a deeper understanding of these issues,our viewpoint may need to be revised accordingly,and our work just serves as a primer for a more systematic study in the next step.