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. K_int vectors from 21 sites are relatively compatible with the major principal stress orientations (σ₁) and indicate two distinct domains. This is possibly because K_min orientations are related to compaction, whereas K_int 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 K_int 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.

本数据集提供采自巴基斯坦波特瓦尔高原（Potwar Plateau）的全新运动学数据，用以解析该区域新近纪时期的构造演化历程。该高原东西两侧均被两条大型走滑断层夹持，二者协同调节高原的南向滑移运动。研究团队基于古应力反演与磁化率各向异性（Anisotropy of Magnetic Susceptibility, AMS）张量分析，在该高原内识别出两期新近纪变形事件。第一期变形持续至上新世早期，以垂直的最小主应力（σ₃）与南北向挤压为特征，指示逆冲构造活动。第二期变形则始于上新世晚期，以近垂直的中间主应力（σ₂）以及近水平的最大主应力（σ₁）与最小主应力（σ₃）为特征，对应走滑构造活动。来自21个采样点的中间主磁化率轴（K_int）矢量与最大主应力（σ₁）方位整体吻合度较高，同时指示研究区存在两个构造域。该现象的成因可能为：最小主磁化率轴（K_min）方位与沉积物压实作用相关，而中间主磁化率轴（K_int）方位始终与构造缩短方向（即挤压方向）一致，这一特征在走滑构造期（上新世晚期之后）与逆冲构造期（上新世晚期之前）均成立。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构造变化最为显著，该区域内最大主应力（σ₁）与中间主磁化率轴（K_int）均向南西方向发散。东部构造域以北东-南西向褶皱与逆冲断层为典型特征，而西部构造域并无此类构造发育。上述构造差异可归因于波特瓦尔高原下伏新元古代盐岭组（Salt Range Formation）的沉积分布特征。盐岭组在西部区域厚度巨大且分布广泛，而在东部区域几乎缺失。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南向滑移，而东部则表现为摩擦滑动与显著的内部构造变形。