Active tectonics of the Pamirs and Karakorum

Abstract

The source parameters of moderate‐ to large‐sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three‐dimensional configuration of the subducted lithosphere in the Pamir‐Karakorum intracontinental convergence zone. The continental collision has resulted in intense deformation in the upper crust. The deformation is distributed over a broad area and is absorbed by thrust and strike‐slip faulting. Along the northern margin of the Pamirs, where the deformation due to continental collision is most intense, the Northern Pamir Thrust and several major fault zones mark the present boundary. Most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region. Along the Pamir Front, thrust faulting dominates, while on the western and eastern edges of the Pamirs the style of deformation is characterized by oblique thrusting with a component of strike‐slip motion. Thrust‐type events beneath the Tadjik Depression indicate that both the sedimentary rocks and the basement are involved in shortening. Right‐lateral strike‐slip motion is observed on the eastern edge of the Pamirs where they border the Tarim Basin and in the Talas‐Fergana‐Kun Lun fault zone. A southward dipping seismic zone beneath the Pamirs and Karakorum indicates that the Asian lithosphere has been subducted along the Pamir Front to a depth of at least 200 km. This interpretation is consistent with published geological and geophysical data. Focal mechanism solutions of some intermediate‐depth events beneath the Pamirs show strike‐slip faulting with approximately N‐S horizontally oriented P axes, indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation. A 90‐km‐deep event beneath the Karakorum is interpreted as occurring in the subducted Asian lithosphere; the fact that the P axis of this event is oriented parallel to the descending plate suggests downdip compression within the Asian lithosphere. The overall tectonics of the region is interpreted as a consequence of the underthrusting Asian lithosphere being impinged upon by the shallow northward underthrusting Indian lithosphere.

Geometry of the Pamir‐Hindu Kush intermediate‐depth earthquake zone from local seismic data

“…Further east, there is no continuation of Hindu Kush seismicity, and an unimpeded northward propagation of underthrusting India would not be compatible with the presence of a Pamir slab. Hence, the deeper part of the Indian slab must have detached, whereas a remnant stub could still impinge onto the Pamir slab today, possibly causing its steep dip angle [Fan et al, 1994]. A schematic representation of this concept is shown in Figure 8a.…”

“…There are essentially two prevailing lines of interpretation in the literature: (1) Both Pamir and Hindu Kush slabs belong to a single slab of Indian origin which is torn, contorted and overturned in its southdipping Pamir part [Billington et al, 1977;Pegler and Das, 1998;Pavlis and Das, 2000]. (2) Pamir and Hindu Kush are two distinct slabs subducting in opposite directions next to each other, where the Pamir slab is made up of Eurasian material and the Hindu Kush slab is of Indian provenance [Burtman and Molnar, 1993;Fan et al, 1994]. Alternatively, a two-slab model involving the subduction of two remnant oceanic basins (which would have to be embedded in Eurasian lithosphere) in opposing directions has been proposed by Chatelain et al [1980].…”

“…However, comparison of these models to nature is difficult because of a lack of observations, not least because of the generally aseismic nature of orogenic mantle deformation, with the apparent exception of the Pamir-Hindu Kush. A variety of models attempting to explain the Pamir-Hindu Kush zone's peculiar geometry (along-strike overturning of dip) and dynamics have been proposed, but fundamental questions, e.g., whether it is related to subduction [e.g., Billington et al, 1977] or exists due to a mantle drip [Koulakov, 2011], whether continental [Roecker, 1982;Burtman and Molnar, 1993] or oceanic material [Chatelain et al, 1980;Pegler and Das, 1998] hosts these earthquakes, or whether Pamir and Hindu Kush form one single [Billington et al, 1977;Pegler and Das, 1998;Pavlis and Das, 2000] or two separate structures [Chatelain et al, 1980;Burtman and Molnar, 1993;Fan et al, 1994] outlined by seismic activity, have not been conclusively solved.…”

Rates of river incision across the main tectonic units of the Pamir identified using optically stimulated luminescence dating of fluvial terraces

“…1, 2, own field observations and Stübner et al 2013). Away from the Pamir boundaries, seismic records indicate an intra-plateau neotectonic activity that relates to the east-west extension of the Karakul rift system, and scattered seismic activity in the Southern Pamir (e.g., Fan et al, 1994;Strecker et al, 1995;Sippl et al, 2013).…”

Source Parameters of Moderate and Strong Earthquakes in the Broader Area of Zakynthos Island (W. Greece) From Regional and Teleseismic Digital Recordings

“…Two kinds of regional waveform data are typically used for source estimation, surface waves (Thio & Kanamori 1995) and body waves (Fan et al, 1994;Dreger & Helmberger, 1993). In the present study body waves are used, which are generally less affected by shallow heterogeneities and are more stable than surface waves, although they have a lower signal-to-noise ratio due to their smaller energy.…”