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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Does this mean that the Yamuna fault is a consequence of gradual process rather than a sudden event.
ReplyDeleteAlso that in remote history it may have flowed across Dehradun to join Ganga at Haridwar itself.
How do the thrust fault for Satlaj and Beas fare ?
Yes, the fault has a fairly long history of movement probably on the order of a few hundred thousand years ..although earthquakes and movements would be episodic
ReplyDeleteregarding the course of the Yamuna, the Siwaliks and post Siwalik sediments were deposited as alluvial fans by smaller streams dumping their sediment ahead of the rising Himalayas. there is no sedimentological signature of a large axial river (paleo Yamuna) with a flow to the east south east towards Haridwar in this region around Dehra Dun at least in the last several million years or so
Some people have proposed that the Yamuna flowed towards the Ghaggar (what is called as ancient Saraswati) and only recently turned eastwards around 4 thousand years ago.
Regarding the Sutlej, there too the Himalayan frontal thrust makes a bend accommodated by a lateral fault. Again there is a proposal that the Sutlej flowed towards the Ghaggar and turned westward recently.
Although these scenarios are possible, I have not come across convincing geological evidence for them.