Crustal and uppermost mantle velocity structure along a profile across the Pamir and southern Tien Shan as derived from project TIPAGE wide-angle seismic data

Abstract

SUMMARY Utilizing seismic refraction/wide‐angle reflection data from 11 approximately in‐line earthquakes, 2‐D P‐ and S‐velocity models and a Poisson's ratio model of the crust and uppermost mantle beneath the southern Tien Shan and the Pamir have been derived along the 400‐km long main profile of the TIPAGE (TIen shan—PAmir GEodynamic program) project. These models show that the crustal thickness varies from about 65.5 km close to the southern end of the profile beneath the South Pamir through about 73.6 km under Lake Karakul in the North Pamir, to about 57.7 km, 50 km south of the northern end of the profile in the southern Tien Shan. Average crustal P velocities are low with respect to the global average, varying from 6.26 to 6.30 km s−1. The average crustal S velocity varies from 3.54 to 3.70 km s−1 along the profile and thus average crustal Poisson's ratio (σ) varies from 0.23 beneath the central Pamir in the south central part of the profile to 0.265 towards the northern end of the profile beneath the southern Tien Shan. The main layer of the upper crust extending from about 2 km below the Earth's surface to 27 km depth below sea level (b.s.l.) has average P velocities of about 6.05–6.1 km s−1, except beneath the south central part of the profile where they decrease to around 5.95 km s−1. This is in contrast to the S velocities which range from 3.4 to 3.6 km s−1 and exhibit the highest values of 3.55–3.6 km s−1 where the P velocity is lowest. Thus, σ for the main layer of the upper crust is 0.26 beneath the profile except beneath the south central part of the profile where it decreases to 0.22. The low value of 0.22 for σ under the central Pamir, the along‐strike equivalent of the Qiangtang terrane in Tibet, is similar to that within the corresponding layer beneath the northern Lhasa and southern Qiangtang terranes in central Tibet and is indicative of felsic rocks rich in quartz in the α state. The lower crust below 27 km b.s.l. has P velocities ranging from 6.1 km s−1 at the top to 7.1 km s−1 at the base. Further, σ for this layer is 0.27–0.28 towards the northern end of the profile but is low at about 0.24 beneath the central and southern parts of the profile, which is similar to the situation found in the northeast Tibetan plateau. The low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite‐facies and possibly also eclogite‐facies metapelites in the lower part. Within the uppermost mantle, the average P velocity is about 8.10–8.15 km s−1 and σ is about 0.26. Assuming an isotropic situation, then a relatively cool (700–800°C) uppermost mantle beneath the profile is indicated. This would in turn indicate an intact mantle lid beneath the profile. An upper mantle reflector dipping from 104 km b.s.l., 120 km from the southern end of the profile to 86 km b.s.l., 155 km from the northern end of the profile has also been identified. The preferred model presented here for the crustal and lithospheric mantle structure beneath the Pamir calls for n...

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

“…It consists of roughly E-W trending structural belts that reflect a history of successive terrane accretion. These are commonly referred to as Northern (Paleozoic subduction-accretion complex), Central (Mesozoic platform rocks), and Southern (mostly Mesozoic metasediments) Pamir [Burtman and Molnar, 1993;Schwab et al, 2004;Mechie et al, 2012]. Correlation of the terranes and the sutures separating them from their counterparts in Tibet [e.g., Yin and Harrison, 2000] and Afghanistan [Burtman and Molnar, 1993] indicates a relative northward displacement of the entire Pamir by about 300 km.…”

“…For each event, a subset of picks Figure A4. Histograms of RMS travel time residuals obtained for the relocation of events after the S picking step, using the velocity model of Mechie et al [2012] and single event location (blue) and after relocation with the minimum 1-D model and station corrections (red). Figure A5.…”

“…In the south, the Shyok suture marks the transition to the Kohistan and Ladakh island arcs (Figure 1), which are, even further south, bounded by the continental suture between Eurasia and India (Indus-Yarlung suture or MMT), approximately 200 km south of our region of interest. Hence, the Pamir consists exclusively of material of Eurasian provenance [Mechie et al, 2012].…”

“…A declustered reference data set consisting of 1780 events, comprising 55,812 P and 30,699 S picks, was selected by subdividing the study area into subregions and selecting only the best located events respectively. The 1D velocity model of Mechie et al [2012] (their starting model for 2D inversion) was used as the initial model. A Wadati diagram for all available events yielded an average vp/vs ratio of approximately 1.74, which was used for the initial S model in the inversion procedure.…”

“…Slightly more than 4*10 6 trigger alerts were determined in this step, by far not all of them, however, related to actual earthquakes. The association of preliminary picks to preliminary events was performed using Binder (Rietbrock and Heath, personal communication, 2010), a program which uses a grid search approach on a rough orthorhombic grid of the region of interest, based on a preliminary 1-D velocity model [used by Mechie et al, 2012, as starting model for 2-D inversion in the eastern Pamir]. Traveltimes from each station to each lattice node are calculated, subsequently a time window is propagated over the stack of trigger alert times, and consistent origin times at one node are searched for looking for consistent travel times on the grid.…”