蒙古—贝加尔地区上地幔小尺度对流及地球动力学意义

Small-scale upper mantle convection beneath the Mongolia-Baikal Rift Zone and its geodynamic significance

上地幔小尺度对流是控制区域地球动力学过程的主要机制之一,蒙古—贝加尔地区的一些区域动力学过程被认为与上地幔小尺度对流相关.本文目的在于利用重力资料研究蒙古—贝加尔地区的上地幔小尺度对流,并探讨其与构造动力学的关系.基于区域均衡重力异常与上地幔小尺度对流的相关方程,本文利用区域均衡重力异常资料反演了蒙古—贝加尔地区上地幔小尺度对流流场及作用于岩石层底部的应力场.结果显示,蒙古—贝加尔地区地幔流场及对流应力场呈现非常复杂的图像,流场及应力场分布与地表构造具有很好的相关性.西伯利亚地台和蒙古褶皱带下地幔流场和对流应力场均较弱,这与这些地区现今较弱的构造活动性是一致的.贝加尔裂谷区下存在地幔上升流,对流应力场呈拉张状态,但应力场的幅值较小(约8 MPa),表明地幔对流不是贝加尔裂谷开裂的主要控制因素.Hangay高原、阿尔泰和戈壁—阿尔泰下存在地幔上升流,对流应力场为拉张状态,这一方面可能构成Hangay高原隆升的深部动力机制,另一方面,也为Amurian板块西边界划分提供了动力背景.

Small-scale upper mantle convection is one of the main mechanisms to control regional geodynamie processes. Some regional geodynamic processes in the Mongolia-Baikal Rift Zone （MBRZ） are suggested to be related to the small-scale upper mantle convection beneath the area. The aim of the present work is to study the small-scale upper mantle convection beneath the MBRZ by gravity data, and to investigate its contribution to the regional dynamics. Based on the equations relating regional isostatic gravity anomalies and small-scale upper mantle convection, the convection flow field beneath the MBRZ and convection-generated stress field were investigated by using the regional isostatic gravity anomalies data. The numerical results exhibit avery complicated pattern of the mantle convection and the generated stress field, which correlates well with the regional tectonics. The mantle flow and the stress beneath the Siberia Platform and Mongolian fold belt are weak, being consistent with the current inactive tectonics in these regions. A mantle upwelling and extension stress regime is found beneath the Baikal Rift Zone. The magnitude of the extension stress is low （-8 MPa）, suggesting that the mantle convection is not the cause responsible for the rifting of the Baikal rift. Mantle upwellings are also found beneath the Hangay Plateau, Altai and Gobi-Altai, leading to extension stress in these regions. The mantle upwellings may be the mechanism responsible for the uplift of the Hangay Plateau, and the dynamic background for defining the western boundary of the Amurian Plate.