Abstract
The Tien Shan is the largest active intracontinental orogenic belt on Earth. To better understand the processes causing mountains to form at great distances from a plate boundary, we analyse passive source seismic data collected on 40 broad-band stations of the MANAS project (2005–2007) and 12 stations of the permanent KRNET seismic network to determine variations in crustal thickness and shear wave speed across the range. We jointly invert P- and S-wave receiver functions with surface wave observations from both earthquakes and ambient noise to reduce the ambiguity inherent in the images obtained from the techniques applied individually. Inclusion of ambient noise data improves constraints on the upper crust by allowing dispersion measurements to be made at shorter periods. Joint inversion can also reduce the ambiguity in interpretation by revealing the extent to which various features in the receiver functions are amplified or eliminated by interference from multiples. The resulting wave speed model shows a variation in crustal thickness across the range. We find that crustal velocities extend to \~75 km beneath the Kokshaal Range, which we attribute to underthrusting of the Tarim Basin beneath the southern Tien Shan. This result supports the plate model of intracontinental convergence. Crustal thickness elsewhere beneath the range is about 50 km, including beneath the Naryn Valley in the central Tien Shan where previous studies reported a shallow Moho. This difference apparently is the result of wave speed variations in the upper crust that were not previously taken into account. Finally, a high velocity lid appears in the upper mantle of the Central and Northern part of the Tien Shan, which we interpret as a remnant of material that may have delaminated elsewhere under the range
Highlights
The Tien Shan, situated some 1000–3000 km north of the collisional front between Eurasia and India, forms the largest active intracontinental orogenic belt on Earth
GPS measurements indicate that about 20 mm yr−1 of north–south shortening, or approximately 40 per cent of the total convergence between India and Eurasia, is being accommodated in the Tien Shan (Abdrakhmatov et al 1996; Reigber et al 2001; Wang et al 2001; Zubovich et al 2010), of which 12–13 mm yr−1 occurs within the Kyrgz Tien Shan (Abdrakhmatov et al 1996)
The longest delay times are located beneath the Kokshaal and Kyrgyz ranges, while the shortest are found beneath the Kazakh shield and the Naryn Valley
Summary
The Tien Shan, situated some 1000–3000 km north of the collisional front between Eurasia and India, forms the largest active intracontinental orogenic belt on Earth. The collision between India and Eurasia began around 50–55 Ma (Molnar & Tapponnier 1975; Searle et al 1987). Thermochronologic and stratigraphic studies from the southern and eastern Tien Shan suggest that current deformation in the range began 24–20 Ma (Sobel & Dumitru 1997; Yin et al 1998), while geodetic studies (Avouac et al 1993; Abdrakhmatov et al 1996) and thermochronologic studies from the northern Tien Shan (Bullen et al 2003) suggest that deformation initiated more recently, perhaps around 10 Ma. GPS measurements indicate that about 20 mm yr−1 of north–south shortening, or approximately 40 per cent of the total convergence between India and Eurasia, is being accommodated in the Tien Shan (Abdrakhmatov et al 1996; Reigber et al 2001; Wang et al 2001; Zubovich et al 2010), of which 12–13 mm yr−1 occurs within the Kyrgz Tien Shan (Abdrakhmatov et al 1996). Deformation is distributed along folds and dip-slip faults across the whole range (Thompson et al 2002), resulting in a basin and range type topography
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