Neogene Kinematics of the Potwar Plateau and the Salt Range, NW Himalayan Front: A Paleostress Inversion and AMS study

We provide new kinematic data from the Potwar Plateau (Pakistan) to evaluate the tectonic evolution of the region during the Neogene. The plateau is bound by two major strike-slip faults in the west and the east, accommodating its southwards translation. We have recognized two Neogene deformation phases in the plateau, based on paleostress inversion and Anisotropy of Magnetic Susceptibility (AMS) tensors. The first phase lasted until the early Pliocene and was characterized by vertical minor stress and N-S compression, implying thrust tectonics. The second deformation phase is characterized by a near-vertical intermediate principal stress and near-horizontal major and minor stresses, interpreted to be associated with strike-slip tectonics since the late Pliocene. Kint vectors from 21 sites are relatively compatible with the major principal stress orientations (σ1) and indicate two distinct domains. This is possibly because Kmin orientations are related to compaction, whereas Kint orientations were always parallel to tectonic shortening and hence compression direction during both strike-slip (post-late Pliocene) and thrusting (pre-late Pliocene) phases. These phases are characterized by swapping of (σ2) and (σ3) orientations while (σ1) maintained its orientation. The most prominent change occurs at the western part of the Potwar Plateau, where major principal stress directions (σ1) and Kint axes fan out south-westwards. The eastern domain is dominated by NE-SW trending folds and thrust faults, which are absent in the western domain. These structural features are interpreted to be the result of the distribution of deposits of the Neoproterozoic Salt Range Formation as a substratum below the Potwar Plateau. The Salt Range Formation is very thick and widespread in the west area and almost absent in the east. This factor led to unconstrained southwards gliding of the Potwar Plateau over the salt deposits in the west as opposed to frictional sliding and substantial internal deformation in the east.本研究提供采自巴基斯坦波特瓦尔高原（Potwar Plateau）的新增运动学数据，用以评估该区域新近纪（Neogene）时期的构造演化过程。该高原东西两侧分别受两条大型走滑断层约束，调控其向南的滑移运动。基于古应力反演与磁化率各向异性（Anisotropy of Magnetic Susceptibility, AMS）张量分析，本研究在该高原内识别出两期新近纪变形阶段：第一期变形持续至上新世早期，以垂直最小主应力与南北向挤压为特征，指示逆冲构造作用；第二期变形则以近垂直的中间主应力，以及近水平的最大与最小主应力为特征，自上新世晚期以来被认为与走滑构造作用相关。采自21个采样站位的Kint矢量与最大主应力（σ1）的方位整体吻合较好，并揭示出两个截然不同的构造域。这一现象可能源于：最小磁化率轴（Kmin）的方位与压实作用相关，而无论在走滑（上新世晚期之后）还是逆冲（上新世晚期之前）变形阶段，Kint矢量的方位始终与构造缩短方向（即挤压方向）保持平行。两期变形的核心差异在于中间主应力（σ2）与最小主应力（σ3）的方位发生了互换，而最大主应力（σ1）的方位始终保持稳定。波特瓦尔高原西部的变化最为显著：此处最大主应力（σ1）与Kint轴均呈西南向撒开的展布特征。东部构造域以北东-南西向展布的褶皱与逆冲断层为特征，而西部构造域并无此类构造。上述构造特征被认为是波特瓦尔高原下伏新元古代盐岭组（Neoproterozoic Salt Range Formation）沉积分布差异所导致的结果。盐岭组在西部区域厚度巨大且分布广泛，而在东部区域则几乎缺失。这一差异使得波特瓦尔高原西部可在盐层之上发生无约束的向南滑移，而东部则表现为摩擦滑动与显著的内部变形。