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

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。

The end of ductile shear deformation of Ningshan fault: Constraint from the activity time of Longbozi shear zone

Three groups of ESR dating ages were obtained from different vein materials in Longbozi shear zone, i.e., 125.6~88.7Ma, 56.7~32.9Ma and 19.8~14.6Ma. The first type of vein materials sampled from schistosities and A-type fold cores marked the end of left-lateral shear deformation. It has been confirmed that the ductile deformation of Ningshan fault began from the early Late Triassic, and might have lasted to Early-Middle Jurassic. However, the result of the first type of vein material ESR dating shows that Ningshan fault left-lateral ductile shear effect lasted to Early Cretaceous. So Early Cretaceous was the critical period when the ductile shear deformation transformed to brittleness in Ningshan shear zone. Furthermore, Ningshan fault experienced Late Triassic-Middle Jurassic rapid-cooling phase, Late Jurassic-Cretaceous slow-cooling phase, and Paleocene-Quaternary rapid-cooling phase. The strain translation that occurred in the late slow-cooling phase suggests that Early Cretaceous also was the critical period of intracontinental deformation mechanism changing.