ABSTRACT

The North and South China Block collision in Triassic resulted in the Qinling orogeny with massif batholith intrusion. In this study, we investigated the deformation structures of the Foping domal zone, the Ningshan bedding-parallel shear zone, and the Yangtian-Tongchewan strike-slip shear zone in East Qinling, aiming at unravelling orogen-parallel mid-lower crustal ductile flow during this late-stage orogeny. Field measurements of foliated felsic migmatites in the Foping domal zone show a subhorizontal stretching lineation associated with top-to-east shearing, and the quartz c-axis fabrics of rock samples from leucosome display characteristic prism <c> slip system (>650°C), consistent with granulite-facies metamorphic condition. The ductile fabrics observed along the W-E extending Yangtian-Tongchewan mylonitic zone along the southern Foping domal zone indicate dextral strike-slip shearing. The Ningshan bedding-parallel shear zone was mapped in the Palaeozoic sedimentary strata along the southern rim of the Foping domal zone and it exhibits subhorizontal stretching lineation. Zircon U-Pb dating results of migmatite from the Foping domal zone, mylonite from the Yangtian-Tongchewan shear zone and syntectonic granitic dyke from the Ningshan bedding-parallel shear zone, together with mica ⁴⁰Ar/³⁹Ar dating results, allow to constrain the timing of coeval development of these structures in Late Triassic at 220–208 Ma, broadly consistent with the pluton intrusion. The overall pattern is interpreted to result from a local indentation of the Hannan Massif with a salient boundary geometry, which possibly controlled the lateral extrusion of the orogenic wedge, orogen-parallel crustal ductile flow as well as the Late Triassic plutonic emplacements in East Qinling.

摘要

三叠纪华北陆块和华南陆块碰撞导致秦岭造山带地块岩基侵入。本研究研究了东秦岭佛坪穹窿带、宁山顺层-平行剪切带和阳天-铜车湾走滑剪切带的变形结构，旨在揭示东秦岭与造山带平行的中下地壳韧性流动。这个后期造山运动。佛坪穹窿带叶状长英质混合岩的现场测量显示出与自上而下剪切相关的亚水平拉伸线，并且来自无色体的岩石样品的石英c轴织物显示出特征性棱柱<c>滑移系统（> 650°C） )，与麻粒岩相变质条件一致。沿南佛坪穹窿带的WE延伸扬天-铜车湾糜棱带观察到的韧性组构表明右旋走滑剪切。宁山顺层-平行剪切带在佛坪穹窿带南缘古生代沉积地层中测绘，呈亚水平伸展线状。佛坪穹窿混合岩、阳天-铜车湾剪切带糜棱岩、宁山顺层-平行剪切带同构造花岗岩脉以及云母的锆石U-Pb定年结果⁴⁰ Ar/ ³⁹ Ar 测年结果，可以限制这些构造在 220-208 Ma 晚三叠世同时发育的时间，与岩体侵入大体一致。整体格局被解释为汉南地块的局部压痕具有突出的边界几何形状，这可能控制了造山楔的横向挤压、平行造山带的地壳韧性流动以及东秦岭晚三叠世深成岩就位。