Crust and upper mantle structure of the North China Craton and the NE Tibetan Plateau and its tectonic implications Xuewei Bao a , Xiaodong Song a,b,n , Mingjie Xu a , Liangshu Wang a , Xiaoxiao Sun a , Ning Mi a , Dayong Yu a , Hua Li a a Institute of Geophysics and Geodynamics, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210093, China b Department of Geology, University of Illinois at Urbana-Champaign, IL 61801, USA article info Article history: Received 11 December 2012 Received in revised form 15 March 2013 Accepted 15 March 2013 Editor: T.M. Harrison Available online 20 April 2013 Keywords: North China Craton Tibetan Plateau ambient noise tomography lithospheric structure abstract Understanding the Mesozoic–Cenozoic tectonic evolution of the North China Craton (NCC) and the NE Tibetan Plateau (TP) requires detailed knowledge of the lithospheric structure. Using dense regional networks and temporary deployments as well as updated reference models, we obtain the crust and upper mantle structure to 120 km depth. Our tomographic results show several major features, which have particular implications for the Weihe–Shanxi rift system (WSRS), deformation of the NE TP, and lithospheric evolution of the NCC. Beneath the WSRS, the crust gradually thickens from south to north, the lithospheric mantle gradually becomes slower, and the mid-lower crustal velocities are lower in the Weihe Rift, where rifting of the WSRS initiated. We suggest that along-strike variations of the lithospheric structures of the WSRS have played an important role in its multistage evolution. A low- velocity zone (LVZ) in the mid-crust beneath the Qilian Orogen is characterized by relatively higher velocities compared to LVZs in other parts of the TP. Thus, coherent lithospheric deformation may occur due to the high viscosity of the LVZ during early plateau growth, causing strong anisotropy to develop. The western NCC (including the Ordos Block and part of the Alashan Block) shows a high-velocity cratonic root extending to the base of our model. In contrast, the lithosphere of the eastern NCC appears to have been completely modified during the Mesozoic through Cenozoic and presents a thin lithosphere of relatively low velocities underlain by hot asthenosphere. We observed significant upper-mantle heterogeneities in the NCC, which may reflect its diachronous lithospheric modification. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The North China Craton (NCC), one of the world's oldest Archean continental nuclei, is the Chinese part of the Sino-Korea Craton (Liu et al., 1992). Based on petrologic, geochemical, geo- chronological, structural and metamorphic P–T path studies, the NCC can be divided into three parts (Fig. 1): the Archean eastern NCC, the Archean western NCC, and the Trans-North China Orogen (TNCO), an intervening orogenic belt along which the eastern and western NCC were amalgamated to form the NCC at ~1.85 Ga (Zhao et al., 2005). The NCC is unique among the world's cratons due to its complex and contrasting evolution during the Phaner- ozoic: the eastern NCC (mainly the North China Basin) experienced dramatic thermotectonic reactivation that led to significant litho- spheric modification and widespread rifting and volcanism during the Late Mesozoic–Cenozoic, while the western NCC (mainly the Ordos Block) was much less involved in this process and most likely preserves the old cratonic root (Griffin et al., 1998; Menzies et al., 2007). The Ordos Block is located in a transition from compression of the NE Tibetan Plateau (TP) to extension of the NCC (Fig. 1). The intra- cratonic Ordos basin developed during the middle-late Triassic and early Cretaceous, with subsidence in its western part and uplift in its eastern part for nearly 200 million years (Liu et al., 2006). Cenozoic rifting in central and western NCC is localized along two rift systems surrounding the Ordos Block, namely the Yinchuan–Hetao rift system (YHRS) to the north and west and the Weihe–Shanxi rift system (WSRS) to the south and east (Fig. 1). Magmatism is localized around Datong Volcano in the northernmost part of the WSRS. The WSRS extends from the southeast margin of the Ordos Block north-northeastward to the northern part of the TNCO over a distance of approximately 1200 km, making a roughly S-shaped curve. It is one of the largest Cenozoic intracontinental rift systems in the world, consisting of a series of fault-controlled asymmetrical half-grabens ranging from 40 to 100 km wide (Xu and Ma, 1992; Ye et al., 1987). The northeast- or north-northeast-trending major faults that bound the grabens are mostly normal faults with varying degrees of right-lateral strike-slip motion and dip angles Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/epsl Earth and Planetary Science Letters 0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.03.015 n Corresponding author. E-mail address: xiao.d.song@gmail.com (X. Song). Earth and Planetary Science Letters 369–370 (2013) 129–137