Preliminary balanced palinspastic reconstruction of cenozoic deformation across the Himachal Himalaya (northwestern India)

Abstract

A line-length balanced palinspastic reconstruction across the Himachal Himalaya is presented, highlighting different phases of Himalayan tectonic development: Eocene shortening of the north Indian margin, Early-Middle Miocene emplacement of the crystalline core, and subsequent growth of the range by underplating. The total preserved shortening is 518 km (72%). The reconstruction demonstrates geometric feasibility of crystalline core emplacement via tectonic wedging, i.e., between a southdirected thrust (the Main Central thrust) and a north-directed backthrust (the South Tibet detachment). Crystalline core exposure between these faults occurs ca. 5Ma in the reconstruction; initial exposure of these crystalline rocks ca. 11 Ma probably occurred in the hinterland within core complexes accommodating east-west extension. After Early-Middle Miocene crystalline core emplacement, ongoing orogenic growth is dominated by underplating processes. Outof-sequence faulting accomplishes <5% of this shortening; frontal accretion of foreland rocks occurs, but the resulting imbricate fan is largely eroded away. Over the past 3-5 m.y., an antiformal stack of mid-crustal horses and a hinterland-dipping duplex of upper crustal horses develop simultaneously. Minimum total shortening across the western Himalaya (fromundeformed foreland to the India-Asia suture) is estimated by adding the new results and results from prior work to the north; an estimate of ~703-773 km of assessed shortening is calculated. However, because large portions of the regional deformation remain unassessed, estimates of ~900-1100 km may more accurately refl ect the minimum preserved shortening here. This range is comparable to the~1350 km of shortening estimated by plate circuit reconstructions for this region. Apparent mismatch between geologic and plate circuit shortening estimates has recently instigated the new Greater India Basin hypothesis for two Cenozoic collisions along Asia's southern margin, but the new results suggest that this mismatch may not exist. © 2013 Geological Society of America.