Remotely India # 4 : Piggyback Basins Within The Siwaliks

Growing mountains involve a thickening of the crust and this thickened crustal mass weighs down the lithosphere causing a moat like depression in front of the mountain range.

These depressions are called foreland basins and they get filled up by sediments derived by the erosion of the growing mountain belt. Foreland basins are regions of compressional stresses and this causes the crust to deform through folding and breakage along thrust faults. During foreland basin evolution as compression squeezes the sediment pile and the basement, folds and thrust systems propagate further and further onto the foreland deforming earlier deposited sediment into folds and cannabilizing that newly uplifted sediment to fill the active depocentre forming ahead of this deformation front.

Structurally these depressions are categorized as two types. A foredeep or a foreland basin that develops in front of the active thrust system. And a piggy back basin that develops on top of the active thrust sheet.

Since early Cenozoic the Indian plate has been pushing into the Asian continental crust. The result has been the rising Himalayan mountains and a foreland basin south of the ranges. From Miocene onwards to recent, parts of the Himalayan foreland basin has undergone a transition from being a foredeep fill developed in front of the Main Boundary Thrust to a piggy back basin developed on top of the active Himalayan Frontal Thrust. See figure to left which shows the progressive deformation of the foredeep sediment wedge. Source:Thakur V C and Pandey A K 2004.

The Siwalk Group is the sediment pile which filled up the basin that developed in front of the Main Boundary Thrust beginning around the Miocene. This foredeep basin sedimentation continued until around 0.5 mya. Then another pulse of compression caused the Siwalik sediment wedge to be uplifted and deformed into two large wrinkles or folds. One just southwards of  the Main Boundary Thrust along the Santaurgarh thrust and another much further southwards along the Himalayan Frontal Thrust,  a new thrust that splayed or peeled off from the Main Boundary Thrust.

The Himalayan Frontal Range is the youngest new wrinkle to be added to the Cenozoic foreland basin sediments.

There is a depression.. an intermontane valley between the two large wrinkles or anticlines that form the Siwalik ranges, a piggy back basin developed on top of the Himalayan Frontal Thrust sheet. This basin has been receiving sediment during the late Quaternary, for about the last few hundred thousand years from the deformed Siwaliks and the lesser Himalayas.

The Main Boundary and Himalayan Frontal Thrust sheet systems are seismically active and pose a serious threat to society. Evidence of several large earthquakes over the last thousand years have been recognized using structural features like fault traces, sag ponds, pressure ridges and offset alluvial terraces. As recent as 1905 the Kangra earthquake (~ 7.8 magnitude) occurred in the region between the Main Boundary Thrust and the Frontal Thrust.  The deformation of the Himalayas continues southwards and will keep incorporating new sediment into a growing mountain chain.

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