四川龙门山中段变形和变质历史(英文)

DEFORMATIONAL AND METAMORPHIC HISTORY OF THE CENTRAL LONGMEN MOUNTAINS,SICHUAN PROVINCE, CHINA

在龙门山的中段,四川盆地西缘的逆冲断层起源于紧靠汶川-茂汶断裂西侧的变质“根带”。汶川-茂汶断裂代表一条20～25km宽的剪切带的晚期脆性变形阶段。该剪切带活动于大约200Ma前的印支期。在汶川-茂汶剪切带北西侧的松潘-甘孜褶皱带中,印支期的NE-SW向挤压形成D_1逆冲断层,并被NW向F_2直立褶皱所叠加。当松潘-甘孜褶皱带受到D_1-D_2期缩短时,相邻的四川盆地并没有发生变形。两个地区的差异应变被发育于D_3的汶川-茂汶左行剪切带所容纳。松潘-甘孜褶皱带中持续的NE-SW向缩短导致了龙门山地区的SE向挤压。这种SE向挤压引起沿汶川-茂汶剪切带发生局部地壳加厚和巴罗型(Barrovian-type)变质作用。汶川-茂汶剪切带的运动学特点由D_3的左行剪切逐渐转变为D_4的SE向逆冲。这种逆冲作用引起了变质地区的初步隆起,以及龙门山地区第一期推覆体的就位。在印支期变形的后期(D_5),岩石发生褶皱并被花岗岩体侵入。现在的汶川-茂汶断裂位置是在更晚的变形阶段确立的。这个阶段的变形可能导致了彭灌基底杂岩沿着映秀-北川断裂发生隆起。在这一事件中,汶川-茂汶断裂是作为一条具有显著左行走滑分量的脆性正断层活动的。这一事件可能对应于龙门山地区的第二期推覆体运动,并且可能发生于侏罗纪—第三纪之间,或者是在喜马拉雅期。

In the central part of the Longmen Mountains, thrusts along the western edge of the SichuanBasin originated from a metamorphic" root zone "directly northwest of the Wenchuan-Mouwen Fault.This fault represents the last, brittle deformational stages of a 20～25 km wide shear zone. TheWenchuan-Mouwen Shear Zone, that was active during the Indosinian, about 200 Ma ago. In the Songpan-Ganzi Fold Belt NW of the Wenchuan-Mouwen Shear Zone, Indosinian, NE-SWcompression resulted in D_1 thrusts overprinted by NW-trending, F_2, upright folds. While the Songpan-Ganzi Fold Belt was subjected to D_1-D_2 shortening, the adjacent Sichuan Basin remained undeformed.Differential strain between both terrains was accommodated across the Wenchuan-Mouwen ShearZone, which developed during D_3 as a sinistral strike-slip zone. Continued NE-SW shortening of theSongpan-Ganzi Fold Belt led to SE-directed compression along the Longmen Mountains. This resultedin local crustal thickening and Barrovian-type metamorphism along the Wenchuan-Mouwen ShearZone which gradually changed its kinematic character from sinistral, strike-slip to D_4, SE-directedthrusting. This thrusting led to the initial uplift of the matamorphic terrains and the emplacement of afirst generation of nappes in the Longmen Mountains. The rocks were subsequently folded and intrudedby granite plutons during a late stage of Indosinian deformation, D_5. The current postion of the Wenchuan-Mouwen Fault was established during a later stage of de-formation which probably led to the rise of the Pengxian-Guanxian Basement Complex along theYingxiu-Beichuan Fault. During this event, the Wenchuan-Mouwen Fault behaved as a brittle normalfault with a considerable sinistral component. This event probably corresponds to the second stage ofnappe movement in the Longmen Mountains and may have occurred at the Jurassic-Tertiary boundaryor during the Himalayan movement.