秦岭造山带内宁陕断裂带构造演化及其意义

宁陕断裂是秦岭造山带内部发育的一条近东西向区域性断裂。研究表明,宁陕断裂运动学性质为左行走滑,变形早期为韧性变形,晚期叠加脆性变形。早期变形形成的同变形变质矿物的⁴⁰Ar-³⁹Ar定年结果显示,变形时代为169~162Ma左右,属于秦岭造山带碰撞后陆内变形阶段产物。宁陕左行走滑断裂的存在暗示着在中晚侏罗世之前,现今南秦岭构造带很可能分属于两个不同的构造单元。宁陕断裂北西侧具有古老变质基底,并有大量早中生代花岗岩体侵入;南东侧只发育中上元古宙浅变质火山-沉积组合,发育晚元古宙-早古生代基性侵入岩脉及一些碱性岩脉。中晚侏罗世-早白垩世期间,围绕着扬子地块西缘和北缘,发生过左行走滑变形,这可能与扬子地块在这个时期的顺时针旋转相关。

Tectonic evolution and implication of Ningshan Fault in the central part of Qinling Orogen.

Ningshan Fault is a large-scale WE-trending fault in Qinling Orogen. This research indicates that the left-lateral shear zone exhibit abundant ductile shear fabrics of earlier stage which were superimposed by a late brittle deformation. ⁴⁰Ar-³⁹Ar ages on syn-deformational and syn-metamorphic minerals from the Ningshan Fault confirmed that the early ductile deformation occurred during 169~162Ma and was formed in the intracontinental deformation in the period of post-collision of Qinling Orogen. Existing of this left-lateral shear zone indicates that the tectonic belt of South Qinling Orogen may be subdivided into two different unites before Middle-Late Jurassic. Some metamorphic rocks of old basement crops out on the NW side of the Ningshan Fault and were intruded by many Mesozoic granitic plutons; on the SE side of the fault, however, dominant Meso- and Neo-Proterozoic low-grade metamorphic volcanic rocks and metasediment was intruded by basic and alkalic dykes of Neo-Proterozoic to Early Paleozoic. It has been confirmed that large-scale left-lateral shear movement occurred in the western and northern margins of Yangtze block in the Late-Middle Jurassic to the Early Cenozoic, which may be connected with the clock-wised rotation of the block in that time.