Quantitatively-deciphered-paleostrain

The knowledge of the strain/stress field evolution in time is important to seismic hazard assessment and risk mitigation, and is fundamental to the understanding of the earth dynamic system. Based on the principle that past tectonic stress should have left traces in the rocks, geologists have been trying to determine the paleostress history from evidence found in rocks for decades. Recent development of techniques for automatic extraction of fracture surfaces from digital outcrop models and estimation of historical shear deformation on rock fractures provide an efficient way of quantitatively acquiring large amount of high quality fracture/fault slip data (direction and sense of slip occurs on the fault plane) from outcrops. So unlike traditional paleostress inversion methods whose data is manually collected in the field, this high quality fracture/fault slip data provide an opportunity to develop fully automatic and quantitative methods for deciphering paleostrain. In this study, for slip on each fracture, the corresponding local strain tensor is calculated, then the local strain tensors are grouped into populations corresponding to far-field strain events and local strain events using a clustering analysis technique. The applications on outcrops in the eastern Tian Shan area give a clear picture of the paleostrain variation over space and time, and also throw light on the relationship between paleostrain, fracture development and the distribution of shear displacements in a thrusting environment.随时间演化的应变/应力场（strain/stress field）相关认知，对于地震危险性评估与风险减缓具有重要意义，同时也是理解地球动力系统的基础。基于“古构造应力应在岩石中留下痕迹”这一原理，数十年来地质学家一直尝试从岩石中发现的证据重建古应力（paleostress）演化历史。近年来，从数字露头模型（digital outcrop model）中自动提取破裂面、并估算岩石破裂面历史剪切变形的技术取得进展，为从露头中定量获取大量高质量的破裂/断层滑动数据（即断层面上发生的滑动方向与滑动性质）提供了高效途径。与传统需野外手动采集数据的古应力反演方法不同，此类高质量的破裂/断层滑动数据为开发全自动、定量化的古应变（paleostrain）解译方法提供了契机。本研究中，针对每条破裂面上的滑动，先计算其对应的局部应变张量（local strain tensor），随后通过聚类分析技术，将局部应变张量划分为分别对应远场应变事件与局部应变事件的多个集群。针对天山东部区域露头的应用，清晰展现了古应变随空间与时间的演化规律，同时也阐明了古应变、破裂发育与逆冲环境下剪切位移分布之间的关联。