龙门山后山震旦系—古生界变形变质作用:松潘-甘孜造山带中生代伸展垮塌下的中地壳韧性流壳层

中生代早期造山作用使松潘-甘孜地区地壳厚度加厚到约50~60km,因而随即经历了大规模区域性地壳伸展和减薄作用,然而迄今为止,对伸展和减薄事件的形成和发育机制还缺少深入了解。通过对龙门山前陆逆冲带腹陆地区,特别是其中发育的变质核杂岩及伸展变质穹隆体的详细构造解析,发现震旦系—古生界中普遍发育各种形式的顺层韧性流变构造,如韧性剪切带、透入性顺层面理及矿物拉伸线理、糜棱岩化及绿片岩相—低角闪岩相变质作用,并在龙门山北、中和南段造成大规模和不同程度的地层构造缺失或减薄;韧性流变构造流变方向在龙门山北段指向南或SSE,中、南段则指向SE;对志留系茂县群变质作用温压条件进行估算,其温度变化范围为265~405℃,压力变化范围为0.31~0.48GPa,代表了中地壳韧性流壳层(middle crustal ductile channel flow)的形成条件;前人用39Ar/40Ar和SHRIMP锆石U-Pb等方法对这一套区域动力变质岩石变质年龄的时代限定为190~150Ma,与中生代早期造山后板内伸展减薄事件相匹配。因此表明造山作用加厚地壳在中地壳层次以大规模韧性流变变形和变质作用对地壳厚度进行了调整,相对于上地壳层次变形和变质作用而言,中地壳韧性流壳层是松潘-甘孜造山带伸展和减薄的主要原因。在区域上如果消除新生代松潘-甘孜高原加厚和相对上扬子地块逆时针旋转的影响,中生代韧性流壳层流变方向总体为SSE或向南,因此代表南秦岭造山作用后的板内演化阶段,并且是造成松潘-甘孜造山带伸展垮塌的主要原因。

Collapse of the Songpan-Garze orogenic belt by a Mesozoic middle-crustal ductile channel flow: evidences from deformation and metamorphism within the Sinian-Paleozoic strata in the hinterland of Longmenshan foreland thrust belt

The early Mesozoic orogeny in the Songpan-Garze area resulted in a thickened crust (~50-60 km), which was followed by a regional subsequent extensional tectonics and crustal thinning. However, the process and mechanism of forming this subsequent tectonics are still unclear. This article describes the ductile deformation and metamorphism of the Sinian-Paleozoic strata in the hinterland, especially around the extensional domes within the hinterland of the Longmenshan overthrust belt, the easternmost of the Songpan-Garze orogenic belt. The structural styles include ductile shear zone, bedding penetrative foliation and mineral lineation, and mylonitic rocks formed at greenschist to low amphibolite facies. This extensional deformation removed or selectively thinned the strata. The kinematic senses indicate SSE or south-ward motion in the northern section, and SE-ward senses in the middle and southern sections of Longmenshan. The p-T conditions of the Silurian Maoxian Group are estimated to be 265-405 ℃, and 0.31-0.48 GPa, respectively, which are consistent with and thus define a middle crust ductile channel flow. Previous studies constrained the metamorphic ages of the ductile deformation at 190-150 Ma using 39Ar/40Ar and SHRIMP U-Pb methods, and thus the ductile deformation and metamorphism proved to be subsequent tectonic episodes of the early Mesozoic orogeny. It was thus concluded that the thickened crust was largely thinned by a middle crust ductile channel flow, which probably has major contribution to the collapse of the Mesozoic Songpan-Garze orogenic belt.