Publication Date:
2020-02-12
Description:
The over 7000m high peaks of the Trans-Alai and the on average over 4000m elevated Pamir plateau is in some way the mirrored equivalent of the Himalaya and Tibet plateau on the northwestern promontory of India-Eurasia collision. Current shortening across the Trans Alai of 13mm/a takes up about 1/3 of India–Eurasia convergence, only little less than across the Himalaya and the highest rate localized far away from a plate boundary. Accumulated Cenozoic shortening reaches also a magnitude similar to the adjacent Himalaya-Tibet system, yet was accommodated over about half the meridional width. There are other marked differences between the two systems. The Pamir is presumably thrusted over Eurasia rather than India and instead of a foreland basin another major orogen, the Tien Shan, stands between its frontal thrust and stable Eurasia. The Pamir and adjacent Hindu Kush feature vigorous intermediate depth (100–300km) mantle seismicity, an attribute absent beneath all other major continental orogens. We will report on a new earthquake data set collected during a field campaign between 2008 and 2010, when we operated a network of 40 seismic stations across the southern Tien Shan and Pamir mountain ranges in Kyrgyzstan and Tajikistan. This is the first modern, digital and dense seismic network in the region. From more than 6000 well located earthquakes approximately two third are crustal. To derive robust source mechanisms and additional constraint on event depths, we use full waveform inversion of our local temporary and regional permanent seismic station recordings for events with magnitudes 〉 3.5. We use relocated hypocenters and fault plane solutions to describe the deformation pattern in the crust. Seismicity clusters in several well defined known and unknown structures. The Main Pamir Thrust (MPT) on the northern perimeter of the Pamir is clearly outlined by a string of earthquakes with thrust mechanisms, with the eastern part of the Alai valley more active than the western part. The largest earthquake we recorded, an M6.7, occurred at MPT’s north-easternmost point, where the Alai valley closes and the Pamir collides with the Tien Shan. This is a region of significant structural complexity, where the MPT fans out in a series of northeast trending orographic features. The mainshock shows an almost pure thrust mechanism with one steeper (55°) and one more shallow (38°) nodal plane. The aftershock seismicity displays two lineaments forming a hockey stick like feature that tightly follows the orographic relief. The “blade” strikes approximately 85° in agreement with one nodal plane of the mainshock double couple. A cross section through the aftershocks reveals that the steeper, south-dipping nodal plane is the fault plane. The earthquake probably ruptured the very tip of the Pamir frontal thrust where it reaches the surface in the southern margin of the Alai valley. On the Pamir plateau deformation is mainly extensional with dextral components. One seismically active zone crosses the entire plateau from Lake Karakul to the Wakhan corridor of Afghanistan. High seismicity is also detected along the deeply incised valleys of the western Pamir.
Keywords:
550 - Earth sciences
Type:
info:eu-repo/semantics/conferenceObject
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