Tectonophysics 634 (2014) 198–207 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto The crustal thickness of NE Tibet and its implication for crustal shortening☆ Xiaobo Tian ⁎, Zhen Liu, Shaokun Si, Zhongjie Zhang State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China article info abstract Article history: The crustal deformation model for NE Tibet is key to understanding the outward growth of the plateau, especially Received 9 June 2013 along its northern front. This study describes receiver function images of the Moho beneath NE Tibet, as calculated Received in revised form 17 May 2014 from teleseismic data recorded by regional seismic networks. Moho depth from these images, coupled with results Accepted 2 July 2014 from previous wide-angle reflection/refraction studies (WARR), allowed crustal thickness estimates along several Available online 12 July 2014 profiles. Crustal shortening was estimated by restoring the present day thickened crust to its assumed initial crustal – Keywords: thickness. Our results show a relatively large amount of crustal shortening (250 350 km) at an orientation of N30°E – Receiver functions in the central part of NE Tibet. This indicates that prior to shortening, the southern edge of Asia lay 300 400 km Moho depth north of the location indicated by previous S-wave receiver function studies. This discrepancy may result from Crustal shortening southward subduction of a segment of Asia's lower-crust beneath central Tibet. It could also arise from crustal Tibetan plateau thickening predominantly caused by upper-crustal shortening, provided that delamination of eclogitized mafic lower-crust and erosion have not significantly reduced crustal volume. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Craton, NE Tibet has an average altitude of 3500 m, which is less than that of central Tibet (about 5000 m). NE Tibet experienced rapid sur- Continental collision between the Indian and Asian plates began face uplift at around 10 Ma (Lease et al., 2007; Zheng et al., 2006, around 50–60 Ma and created the Tibetan plateau (Molnar et al., 1993; 2010), an age interpreted as the inception of the youngest Tibetan Tapponnier et al., 2001; Yin and Harrison, 2000). GPS measurements plateau (Tapponnier et al., 2001). NE Tibet offers an excellent opportuni- have shown continuous deformation of the plateau interior characterized ty to further study how the plateau develops. Low S-wave velocity by NNE–SSW shortening and ESE–WNW extension (Zhang et al., 2004). (Owens and Zandt, 1997; Rapine et al., 2003; Tian et al., 2005), crustal Debate continues regarding the issue of whether the deformation evident seismic anisotropy (Chen et al., 2009, 2013; Shapiro et al., 2004), and in shallow layers extends into the lower-crust or upper mantle, thereby high conductivity (Kong et al., 1996; Unsworth et al., 2004; Wei et al., allowing for some degree of mechanical decoupling between upper- 2001) suggest widespread, lower-crustal flow beneath the plateau and lower-crust, or upper mantle. Several mechanisms for crustal defor- (Klemperer, 2006; Ozacar and Zandt, 2004), especially in eastern Tibet mation have been proposed, including uniform lithospheric shortening, where the crustal thickening is attributed to ductile lower-crustal flow lower-crustal injection and lateral flow, and dominant upper-crustal (Clark and Royden, 2000; Royden et al., 1997). A seismic experiment shortening (Bai et al., 2010; Meyer et al., 1998; Royden et al., 1997; (Karplus et al., 2011) and magnetotelluric study (Le Pape et al., 2012) Tapponnier et al., 2001; Tian and Zhang, 2013; Wang et al., 2011; Zhao also suggest penetration of ductile lower-crustal flow across the Kunlun and Morgan, 1987). Crustal shortening resulting from convergence is fault and into NE Tibet. Uncertainty persists however concerning the considered to be the main factor causing thickening and uplift of the relative importance of crustal shortening versus lower-crustal flow in plateau. The degree of shortening is thus an important parameter for determining crustal thickening in NE Tibet. constraining models of crustal deformation (Lease et al., 2012; Meyer To address this uncertainty, several studies have estimated the et al., 1998; Yin et al., 2008a, 2008b). amount of shortening in NE Tibet. Based on comparative studies of the With the initiation of the India–Asia continental collision, crustal amount of latitudinal shift of the plates, Li et al. (2002) showed that thickening and surface uplift likely began first in southern Tibet and only 3.5° of latitudinal shortening occurred between Qaidam and Siberia. later progressed northward, either in steps (e.g., Tapponnier et al., Assuming isostatic compensation, Meyer et al. (1998) reported that the 2001) or continuously (e.g., England and Houseman, 1986). As the minimum amounts of Late-Cenozoic crustal shortening between the intervening region between the Tibetan Plateau and the North China Kunlun fault and the Hexi corridor ranged from 100 to 200 km, based on surface topography observations. ☆ Article part of the Special Issue: Advances in seismic imaging of crust and mantle. ⁎ Corresponding author. Tel.: +86 1082998329. The development of broadband seismic networks and the increasing E-mail address: [email protected] (X. Tian). use of wide-angle reflection/refraction (WARR) profiles in recent years http://dx.doi.org/10.1016/j.tecto.2014.07.001 0040-1951/© 2014 Elsevier B.V. All rights reserved. X. Tian et al. / Tectonophysics 634 (2014) 198–207 199 have greatly enhanced our ability to image crustal thickness in areas like Tibet during the middle to late Miocene (Garzione et al., 2005; Lease NE Tibet. In this study, we image the Moho using receiver function anal- et al., 2007; Zheng et al., 2006, 2010). ysis, and then combine its depth estimates with previous WARR results to Based on GPS data, Chen et al. (2004) described the maximum con- infer crustal thickness and the extent of crustal shortening in NE Tibet. traction axis as having an azimuth of N32°E for NE Tibet. This orientation is consistent with the observed surface structure of NE Tibet, which is 2. Geological setting characterized by relatively large and regularly spaced mountain ranges separated by piggyback basins and south-dipping thrusts As the contiguous region between the Tibetan Plateau and the North faults (Metivier et al., 1998; Meyer et al., 1998). The mountain ranges China Craton, NE Tibet is surrounded by the Alashan block to the north, and active thrusts strike N110°E to N120°E (Metivier et al., 1998; the Tarim basin to the northwest, the Ordos block to the east and the Meyer et al., 1998; Tapponnier et al., 1990). Some studies indicate that Songpan–Ganzi terrane to the south (Fig. 1). NE Tibet is bound by crustal shortening contributes to internal deformation of NE Tibet. The three left lateral strike-slip faults. These include the easterly arm of lack of evidence for strike-slip faulting to the east of NE Tibet led Duvall the Kunlun fault (KF) to the south, the Altyn Tagh fault (ATF) to the and Clark (2010) to argue against the eastward escape or extrusion of north and west, and the Haiyuan fault (HF) to the north and east. Crustal tectonic blocks due to the advancing Indian plate (Tapponnier et al., deformation in NE Tibet began during the Cenozoic as a far-field 2001). Studies of Late Quaternary slip-rates along three primary bound- response to the India-Eurasia collision (Dayem et al., 2009). Strike-slip ary strike-slip faults indicated that any extrusion of Tibetan lithosphere fault offsets and basin sedimentation rates suggest that the deformation accomplished by slip along the faults must be absorbed by internal defor- began in NE Tibet shortly after collision (Fang et al., 2003; Ritts et al., mation of plateau surrounding the fault tip (Kirby et al., 2007; Zheng 2004; Tapponnier et al., 1981; Yin et al., 2002; Yue et al., 2005). Geolog- et al., 2013). Velocities along the N110°E direction for GPS stations posi- ical investigations (Chung et al., 2005; Xia et al., 2011) indicate that the tioned south of the Kunlun fault, between the Kunlun and Haiyuan faults, Tibetan plateau preserves evidence for widespread volcanism caused and stations north of the Haiyuan fault, indicate ~6 mm/year conver- by the Indian plate docking with Asia. Cenozoic volcanic rocks however gence in the general area along the eastern margin of the plateau. This have not been observed in NE Tibet. Although surface heat flow measure- observation resembles the average N110°E velocities observed through- ments are relatively scarce in the study area, the crust beneath NE Tibet out eastern China (Duvall and Clark, 2010). appears to be cool (Hu et al., 2000; Pollack et al., 1993). The presence of Quaternary folds and thrust faults (Tapponnier et al., 1990; Zhang et al., 3. Crustal thickness in NE Tibet 1991), thrust fault-plane solutions (Chen et al., 1996; Molnar and Lyoncaen, 1989), field mapping and cosmogenic 10Be exposure dating 3.1. Imaging the Moho with receiver functions (Palumbo et al., 2009) all indicate that mountain ranges in the study area are still experiencing uplift to accommodate crustal thickening. The teleseismic P waveform contains S waves generated by P-to-S Cross section restoration of the Qaidam basin indicates that the average conversion at velocity discontinuities in the crust and upper mantle shortening rate occurring from middle Miocene to present is several beneath seismic stations. Receiver functions (RFs) are radial waveforms times higher than rates from previous periods, even though NE Tibet created by deconvolving the waveform's vertical component from its apparently responded to the India–Asia collision shortly after its incep- radial component, to isolate the receiver site effects from other informa- tion (Wang et al., 2010).