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）的运动学数据，用以解析该区域新近纪（Neogene）时期的构造演化历程。该高原西、东两侧分别被两条大型走滑断层夹持，二者共同协调其向南的滑移运动。基于古应力反演（paleostress inversion）与磁化率各向异性（Anisotropy of Magnetic Susceptibility, AMS）张量分析，本研究在高原内部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，以近垂直的最小主应力与南北向挤压为特征，指示逆冲构造背景；第二期变形则表现为近垂直的中间主应力，以及近水平的最大、最小主应力，自上新世晚期以来对应走滑构造环境。来自21个采样点的K_int矢量与最大主应力（σ₁）方向整体吻合度较高，且可划分为两个显著不同的构造域。这一分布特征可能源于：K_min方向与沉积物压实作用相关，而K_int方向始终与构造缩短方向一致，因此在走滑（上新世晚期之后）与逆冲（上新世晚期之前）两期变形过程中，均对应挤压应力方向。两期变形的核心差异为中间主应力（σ₂）与最小主应力（σ₃）的方向发生交换，而最大主应力（σ₁）的方向始终保持稳定。其中构造特征变化最为显著的区域位于波特瓦尔高原西部，该处最大主应力（σ₁）与K_int轴均呈西南向撒开的分布特征。东部构造域以NE-SW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。上述构造差异被认为与波特瓦尔高原基底的新元古代盐岭组（Neoproterozoic Salt Range Formation）沉积分布特征直接相关：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。该沉积差异导致波特瓦尔高原西部可沿盐层发生无约束的向南滑移，而东部区域则表现为摩擦滑动与显著的内部变形。