川滇西部上新世以来构造地貌:断裂控制的盆地发育及对于远程陆内构造过程的约束

Morphotectonics of western Sichuan-Yunnan since Pliocene:The development of basins along fault zones and constraints on far-field intracontinental tectonic process

青藏高原隆升、周边地貌形成是新生代时期印度-欧亚板块碰撞后的重要响应。在滇西北地区发育了一系列由晚新生代(上新世以来)活动断裂所控制的盆地,例如宾川盆地、洱海盆地、鹤庆盆地、弥渡盆地等。宾川盆地是近南北向程海左行走滑断裂在走滑剪切作用下产生的北西向正断层和北东向走滑断层共同作用而形成的一个较大的拉分盆地。洱海盆地是由两组陡立的共轭张剪性(Transtensional)断层组限定的,为一伸展断陷盆地,总体上反映了近E-W向的区域伸展。滇西北地区发育的其它晚新生代盆地,如弥渡盆地、鹤庆盆地、剑川盆地等,也为区域走滑断裂及其分支断裂所控制,并且这些分支断裂在区域上为一组NE-SW和NW-SE向的共轭正断裂,反映了该区域近E-W向的伸展。将藏东南三江地区发育的活动断裂按照其走向分为三组:(1)NW-SE走向的断裂,如红河断裂、无量山-营盘山断裂等;(2)近N-S向断裂系,以程海断裂、小江断裂等为代表;(3)NE-SW走向的断裂,如丽江-剑川断裂、鹤庆-洱源断裂和南定河断裂等。这些断裂的震源机制解表明地震断裂活动性或者是走滑性质或者是伸展属性,它们的组合型式也揭示出藏东南三江地区在上新世以来表现为近E-W向的伸展。区域上,在藏东北部地区发育的断层构造组合普遍反映了以近E-W向挤压为主导的应力场。推测这一现象为上新世以来藏东地区上地壳围绕喜马拉雅东构造结做顺时针旋转所致,区域上受印度-欧亚会聚过程中印度板块顺时针旋转诱发的差异性应力场制约。

The uplift of Tibetan Plateau and the development of geomorphology in adjacent areas are the important response to the Cenozoic collision between Indian and Eurasian plates. There is a series of Late Cenozoic (since Pliocene) fault-bounded basins in northwestern Sichuan and Yunnan,e.g. Binchuan,Erhai,Heqing,Midu basins etc. Binchuan basin is of pull-apart origin bounded by normal faults in NW direction and strike-slip faults in NE direction in the N-S sinistral Chenghai strike-slip fault system. Erhai basin is an extensional rift basin delimited by two conjugate transtensional faults,suggesting an E-W extension. The other basins in northwestern Yunnan,such as Midu,Heqing,Jianchuan basins,were controlled by transtensional normal faults. Two branches of these faults in NE-SW and NW-SE direction form conjugate pairs,implying their origin by E-W extension. The active faults in the area are divided into three groups:(1) NW-SE trending faults,e.g. the Red River fault,Wuliangshan-Yingpanshan fault etc.; (2) N-S trending faults,e.g. Chenghai fault,Xiaojiang fault etc.; and (3) NE-SW trending faults,e.g. Lijiang-Jianchuan fault,Heqing-Eryuan fault,Nanting fault etc. From the focal mechanism solutions,these active faults possess strike-slipping or transextensional characteristics. Their association patterns reflect a regional extensional stress field in E-W direction in southeastern Tibet and Three River region since Pliocene. Regionally,the structural associations of active faults,in the northern part of the eastern Tibet,indicate the dominant role of an E-W compressional stress field. The overall stress field in eastern Tibet is possibly attributed to the clockwise rotation of the Tibetan crustal blocks around the eastern Himalayan syntaxis. The clockwise rotation of the Indian plate during its convergence with the Eurasian plate provide the plate scale driving force for the rotation of the Tibetan block.