One of India's great rivers, the Yamuna, leaves the Himalayas and comes rushing on to the plains through a great tear in the Himalayan frontal ranges of the Siwalik mountains.
That breach in the mountains is a dislocation along the Yamuna fault. The sense of movement is left lateral, which means if you are following a rock formation across the fault, you will have to turn left as you cross the fault to trace the same rock formation.
Miocene onwards a thick wedge of fluvial sediments filled up a foreland basin that formed in front of rising thrust sheets uplifted along the active Main Boundary Thrust (MBT). That phase ended about 0.5 to 1 mya.
This fluvial wedge over the last half a million years has been deformed into the Siwalik mountains. These mountains form broad synclines and tight anticlines cut by north dipping thrust faults, a result of the continuing compression of the sediment wedge. The southernmost of these thrusts which brings into tectonic contact the anticlinal Frontal Range of the Siwaliks over the alluvial plains in called the Himalayan Frontal Thrust (HFT).
The HFT is broken into segments and the amount of displacement along these segments or thrust blocks is unequal. For example the blocks west of the Yamuna and east of the Ganga have moved southwards with an opposite sense of movement relative to the central block known as the Dun block. To view this, turn on labels and pan southeastwards in embeddable map below until the town of Haridwar where the Ganga enters the plains.
Thus the Yamuna fault has a left lateral sense of movement while the Ganga fault has a right lateral sense of movement. These faults can be thought of as lateral ramps of the HFT accommodating the displacement caused by the southwards movement of the HFT blocks.
Structural considerations indicate that during the last 0.5 my there has been about 8 km of displacement along the Yamuna and Ganga faults, a slip rate of approx. 16 mm year.
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