藏南定结地区喜马拉雅造山带新生代的隆升剥蚀历史及其地质意义

Cenozoic exhumation of the Himalayan orogen in the Dinggye area, southern Tibet, and its geological significance

本文利用裂变径迹方法研究藏南定结地区喜马拉雅造山带新生代的隆升和剥露过程，并探讨内动力和外动力地质作用共同塑造地貌形态的耦合性。由于构造活动的阶段性，可能导致大喜马拉雅结晶岩系（GHC）的冷却剥蚀在构造强烈时期主要由构造活动控制，在构造活动平静期主要与外动力地质作用密切相关。在定结地区的GHC选取两个剖面进行磷灰石裂变径迹（AFT）研究，剖面高程跨度为3800～4500m，其年代学结果范围为11～2Ma，揭示了中新世以来经历了3个阶段的冷却剥蚀历史。不同的冷却剥蚀阶段具有不同的驱动力，中新世晚期11Ma左右的隆升剥蚀阶段，主要是由构造活动驱动；7～3Ma的缓慢冷却剥蚀阶段，构造活动趋于平静，主要与区域内的河流侵蚀作用密切相关；上新世晚期3Ma以来较快速的冷却剥蚀阶段，以河流侵蚀为主的外动力作用加强。定结GHC在3Ma以来冷却剥蚀速率迅速增强，并且驱动力主要为外动力地质作用，暗示了GHC经历了中新世强烈的构造隆升之后已经确定了现有地貌格局，在3Ma已达到显著高度。

In this paper, the fission track method is used to study the uplift and exhumation processes of the Himalayan orogenic belt in the Dinggye area, southern Tibet, and to explore the coupling of internal and external dynamic geological functions to shape the topography. Due to the episodic tectonic activities, the cooling and denudation of the greater Himalayan crystalline complex （GHC） may be mainly controlled by tectonic activities or the external dynamic geological processes. This study uses apatite fission track （AFT） dating of samples from two section crossing the GHC in the Dinggye area, to constrain the timing and rate of Late Cenozoic denudation and further related tectonic and topographic information. Fission track thermochronology yields AFT ages between 11 Ma and 2 Ma across two north-south GHC section from 3 800 ~ 4 500 m. Combined with regional thermochronologies, the AFT ages suggest three episodes of cooling and exhumation of the GHC. Different cooling and denudation stages have different driving forces. In the Late Miocene, to 11 Ma, regional uplift and denudation are mainly driven by tectonic activities. During 7 - 3 Ma, the tectonic activities tends to be calm, and the slow cooling and denudation are closely related to the erosion of the river in the region. The rapid cooling and denudation stage since the Late Miocene, the external dynamics have been enhanced since 3 Ma by accelerated fluvial incision. The regional transition from tectonic to climatic exhumation of the Dinggye Himalaya may probably imply that the Himalaya had accomplished a significant elevation before Late Miocene, and further drove a significant change in local and even global climate, which in turn established the base for climate exhumation of the Himalaya since 3 Ma.