基于断层滑动数据反演的燕山中西段晚中生代古构造应力场:对华北克拉通破坏峰期应力状态的启示

在燕山中西段晚中生代岩体中的116个观测点中共采集到2 321组断层和破裂面滑动数据,利用Tensor软件进行古构造应力状态反演。将反演结果与区域构造变形及构造演化过程研究结果相结合,发现研究区在侏罗纪末-白垩纪初以来存在5期可识别的构造应力场:(1)晚侏罗世末-早白垩世初近N-S向挤压应力场(D1,148~127 Ma);(2)早白垩世NW-SE向挤压应力场(D2,143~127 Ma);(3)垂向挤压主导的多向水平伸展应力场(D3,120~110 Ma);(4)NE-SW向挤压(D4);(5)E-W向挤压的应力场(D5)。D1和D2可能分别与蒙古-鄂霍茨克洋消减关闭作用的远程构造效应,以及古太平洋板块向欧亚大陆板块下的俯冲作用有关,且这两大构造系统在该时期可能同时作用于燕山地区。D3期应力场可能反映源自壳下深部地幔活动引起的垂向挤压动力作用。D4、D5期应力场相对较弱,其动力学背景尚待今后研究进一步分析。本区古构造应力场研究结果表明,华北克拉通破坏峰期期间,主要的伸展作用并不完全符合西太平洋板块后撤主导的被动伸展裂谷模型,而是表现为深部地幔活动引起的垂向挤压作用主导的水平方向多向伸展。

Late Mesozoic Paleotectonic Stress Fields Inferred from Fault-slip Data in Central and Western Yanshan Orogenic Belt:Implications for Destructive Geodynamics of North China Craton

A dataset of 2,321 fault-slip data were collected from 116 sites in Late Mesozoic plutons in the central and western Yanshan orogenic belt. Paleostress field inversion was carried out via Tensor from these data. A five-generation sequence of paleostress fields has been established from the inversion results combined with regional tectonic deformation analysis. We found that: (1) Latest Jurassic-Earliest Cretaceous (148-127 Ma) compression with N-S-trending maximum principle stress (σ₁) (D₁); (2) Early Cretaceous (143-127 Ma) compression with NW-trending σ₁ (D₂); (3) Multidirectional extension with a nearly vertical σ₁ (D₃, ~120-110 Ma); (4) Compression with NE-trending σ₁ (D₄), and (5) Compression with E-trending σ₁ (D₅). D₁ and D₂ are interpreted as the far-field effects correlated with the closing of Mongol-Okhotsk Ocean and west-dipping subduction of paleo-Pacific plate, respectively. These two tectonic systems have major impact on the deformation of the Yanshan orogenic belt. D₃ may have reflected the vertical compression derived from deep mantle activity. Tectonic events unambiguously related to D₄ and D₅ have not been reported so far, which implies relatively weak paleostress field, and their geodynamic setting remains unclear. We suggest that a model of multidirectional extension induced from vertical compression may be applicable to the peak destruction of the North China Craton, instead of a passive rifting model dominated by subduction roll-back of the paleo-Pacific plate as previously proposed.