宁陕断裂带韧性剪切变形终点 ——来自龙脖子剪切带活动时代的限定

The Longbozi shear zone on the northern edge of the Zhen'an section of the Ningshan fault in the South Qinling records the sinistral ductile shear deformation process of the Ningshan fault zone. The ESR ages of three types of quartz veins and calcite veins in the zone are 125.6 - 88.7 Ma, 56.7 - 32.9 Ma and 19.8 - 14.6 Ma respectively. Among them, the first type of quartz veins occurring in the tectonic schistosity and the core of A-type folds represents the era when the sinistral ductile shear deformation ended and the brittle structure was active. Studies have shown that the Ningshan sinistral strike-slip shear zone began to be active in the early Late Triassic and may have continued until the Early - Middle Jurassic. The determination of the age of the first type of veins indicates that the sinistral ductile shear deformation of the Ningshan fault zone could last until at least the Early Cretaceous; from the Late Cretaceous to the Eocene, the Ningshan fault zone was dominated by extensional - strike-slip brittle or ductile - brittle shear deformation. Therefore, the Early Cretaceous is a key period for the transformation from ductile shear deformation to brittle shear deformation in the Ningshan fault zone. The Ningshan fault zone has experienced a rapid cooling stage from the Late Triassic to the late Middle Jurassic, a slow cooling stage from the Late Jurassic to the Cretaceous, and a rapid cooling stage since the Paleogene. The transformation from ductile shear deformation to brittle shear deformation in the Ningshan fault zone in the late slow cooling stage (Early Cretaceous) indicates that the Early Cretaceous is also a key period for the transformation of the intracontinental deformation mechanism of the Qinling orogenic belt.

南秦岭宁陕断裂镇安段北缘的龙脖子剪切带记录了宁陕断裂带左行走滑韧性剪切变形过程。带内3 类石英脉体和方解石脉体的ESR 年龄分别为125.6~88.7Ma、56.7~32.9Ma 和19.8~14.6Ma。其中第一类产出于构造片理和A 型褶皱核部的石英脉体，代表左行走滑韧性剪切变形结束、脆性构造活跃的时代。研究表明，宁陕左行走滑剪切带在晚三叠世早期开始活动，且可能持续到早—中侏罗世。第一类脉体年龄的确定表明，宁陕断裂带左行走滑韧性剪切变形最晚可持续到早白垩世；晚白垩世—始新世，宁陕断裂带以伸展-走滑脆性或韧-脆性剪切变形为主。因此，早白垩世是宁陕断裂带韧性剪切变形向脆性剪切变形转换的关键时期。宁陕断裂带经历了晚三叠世—中侏罗世晚期快速冷却阶段、晚侏罗世—白垩纪缓慢冷却阶段和古近纪以来快速冷却阶段。宁陕断裂带在缓慢冷却晚期（早白垩世）实现韧性剪切变形向脆性剪切变形转换说明，早白垩世也是秦岭造山带陆内变形机制转变的关键时期。