Salt movement and its implications on vertical-axis rotation in the Potwar Plateau, northern Pakistan

Abstract

The Potwar Plateau is formed as a result of the Cenozoic India-Eurasia collision. The deformation front of the Himalayas reached the plateau in Neogene time, where the precambrian evaporite décollement provided a frictionless surface for the most recently created thrust, The Salt Range Thrust. The Plateau is filled with a thick molasse of Lower Miocene to Pleistocene age of the Siwalik Group known for its continuous deposition. Only the Holocene appears undeformed, indicating the continuity of the Siwaliks provide an excellent record for the mechanics of the region. In the last 10 million years, the Salt Range as well as the Potwar Plateau plateau exhibited larger- scale movements which are different from the 0-2° CW (clockwise) rotation of the Indian plate (Torsvik et al., 2012) in the past 10 million years. New and existing paleomagnetic data shows a coherent rotational pattern throughout the Potwar Plateau, with no rotation in the Western Plateau and up to 30° CCW rotation in the Eastern Plateau. The differential rotation is explained by the evaporite décollement, being present in the Western plateau resulting in a low-friction environment, forming the the dextral Kalabagh Fault, a lateral ramp on the Western margin. The Eastern part of the plateau is marked by 50°/230° trending strikes of fold axes and fault planes. The rotational difference together with the local structures are part of an orocline within the plateau, indicating a deformation phase after deposition and formation of the local structures. A magnetostratigraphic analysis on the Soan Formation of the Siwaliks dated the base of the formation at 3.7 Ma with the top at approximately 2.2 Ma, which was concluded to be the oldest possible age for the most recent deformation phase. This is assumed to be in the same timeframe as the onset of ramping of the Salt Range Thrust. Due to the influence of the evaporite décollement on the larger part of the area, it is suggested that the Potwar Plateau and the Salt Range are a local expression of the India-Eurasia convergence heavily influenced by salt tectonics.