喜马拉雅造山带南北向裂谷的冷缩成因模型

A cold shrink model for the NS-trending rifts in the Himalaya orogenic zone

长期以来,学者们普遍认为垂直于喜马拉雅造山带的南北向裂谷是东西向伸展的构造形迹。现代GPS观测数据却显示,喜马拉雅造山带东西位移(分)量很小,甚至为零。综合前人资料,喜马拉雅造山过程可划分为热造山(25~13Ma)及造山后(<13Ma)冷却两个时期,热造山期具有受热膨胀,物质向外运移的特点,高喜马拉雅热隆挤出并触发各主要断裂(MCT、STD、GCT)活动,印度板块向北汇聚速率下降。造山后则表现为冷却收缩,前期构造-热活动停止或减弱,印度板块向北汇聚加速。研究认为,南北向裂谷与高喜马拉雅等冷却过程的东西向收缩。且被局限于东、西两个构造结之间有关。并据此建立了裂谷的冷缩成因模型,模型估值与地质事实很吻合。

The NS-trending rifts perpendicular to the Himalaya orogenic zone have long been interpretated as the structural features of the E-W extension of the Qinghai-Xizang Plateau. Now it should be challenged by modern global positioning system （ GPS） data from recent studies, for that the eastward and westward movements of the crust are scarce or zero in this zone. Referenced to the previous data, the orogenic precesses may be discriminated two eposides ： hot mountain building （25 -13 Ma） and post-orogenic cooling （ 〈 13 Ma） . The Himalaya orogenic zone is characterized by thermal expansion and outward-growth of the crustal materials in the first eposide. The Great Himalaya Crystalline （ GHC ） was extruded and gave rise to the faulting of major faults such as the Main Central Thrust （ MCT） , South Tibet Detachment （ STD） and Great Counter Thrust （ GCT ）. The northward subduction rates of the Indian plate dropped down to the lowest levels, i. e. , about 45 mm/yr. After mountain building, the cold shrink happened in the second eposide. The pre-existing tectonic -thermal activities were gradually weakened or ceased, and the northward convergence rates of Indian plate speed up again to 50 mm/yr. It is suggested that the NS-trending rifts may be related to the post-orogenic cooling events, and result from the EW- trending cold shrink of the Great Himalaya Crystalline and leucogranites within the limit of eastern and western Himalaya syntaxis. A cold shrink model for the origin of the NS-trending rifts is introduced in this paper, by which the throw of normal faults in rift margins was calculated.