Thermochronological Constraints on the Spatial and Temporal Growth and Segmentation along 300 km of the Actively Deforming NW Himalayan Foreland

Detrital apatite U-Th/He ages reveal structure-specific thermal exhumation histories. These data allow for comparison of the timing and pattern of fold growth and thrust displacement both across and along strike on 10⁵ yr timescales and longer. Ages from individual structures between the Pakistan-India border and the Kangra reentrant provide new understanding of the evolution of the NW Himalayan foreland in space and time. Three structurally and stratigraphically distinct segments characterize the NW-striking, south-vergent, imbricate fold-and-thrust system of the sub-Himalaya. The three domains are: (1) The Riasi segment in the NW, which is a narrowly deformed belt defined by a fold at the deformation front, the Suruin-Mastgahr Anticline (SMA), a blind Himalayan Frontal Thrust (HFT), and a thick foreland basin section (~8-10 km); (2) The Kangra segment, which is marked by a region of widely spaced, fault-cored folds and thrust sheets, an emergent HFT, and a thinner foreland basin sequence (5-8 km); (3) A central domain that links the two segments where the SMA extends SE from the Riasi into the middle of the Kangra domain and where the frontal thrust sheets of the Kangra region lose displacement to the NW. Whereas the structural style and map pattern of the Kangra and Riasi segments are distinct, the thermochronology of the principal structures are comparable. Active growth of the deformation front had initiated by at least 4.4 Ma in the Riasi domain and by 5.0 Ma from a fold core NE of the Kangra frontal structure. (U-Th)/He ages of about 2.4 Ma from the SMA in the central segment are younger than those from the deformation front in either the Kangra or Riasi domains. Detailed mapping and field observations show that strike and dip of Siwalik bedding systematically change, and map-scale Quaternary alluvial fans are exclusively preserved in the central segment. From these observations, we hypothesize the presence of an oblique basement ramp connecting basins of different thicknesses along the orogenic decollement. This basin-controlled geometry promotes along-strike propagation and linkage of upper plate structures. Prior to 2.4 Ma, along-strike deformation of upper plate segments occurred independently with lateral linkages of discrete structures occurring only after 2.4 Ma.