Orogen-Parallel Westward Oblique Uplift of the Qinling Basement Complex in the Core of the Qinling Orogen (China):An Example of Oblique Extrusion of Deep-Seated Metamorphic Rocks in a Collisional Orogen

摘要

Both vertical uplift and lateral extrusion of rocks in convergent orogens have been widely documented.This article,using the basement core of the Qinling orogen as an example,demonstrates orogen-parallel westward uplift with both vertical and horizontal displacement components.The Qinling complex of the Qinling orogen is a tectonic slice of a Precambrian crystalline basement.It is an elongate lens between younger terranes bounded by the Shangdan suture to the south and by the Zhuxia zone to the north.During the middle Paleozoic,the complex underwent large-scale orogen-parallel westward oblique ductile uplift.Supporting evidence for this includes the following:(1) opposite shear senses of the two bounding shear zones,i.e.,the thrust-sinistral shear on the south and thrust-dextral shear on the north;(2) well-developed eastward plunging (orogen-parallel) A-type structures in the complex,such as stretching lineation and sheath folds;(3) strong decompression metamorphism of the complex from 0.66 GPa to 0.45 GPa and eclogites decompressed from 1.2 GPa to 0.5 GPa;and (4) eastward migration in the emplacement of synkinematic granitic plutons.Integrated analysis of PT evolution paths suggests a vertical uplift of the basement complex relative to adjacent Paleozoic supracrustal rocks ranging from 15 to 40 km.Coeval west-directed horizontal displacement is then estimated to range from 32 to 86 km and total oblique displacement from 36 to 95 km,using an average 25° plunge of mineral stretching lineations in the boundary shear zones.This oblique uplift could be explained by soft oblique extrusion derived from a scissors-like (or zipped) subduction/collision between the North and South Qinling belts.Because plate boundaries are usually irregular and nonparallel both in the vertical and horizontal,orogen-parallel oblique uplift (soft oblique extrusion) may be widespread in collisional orogens.