Crustal thickening and lateral extrusion during the Indo-Asian collision: A 3D viscous flow model Youqing Yang ⁎, Mian Liu Dept. of Geological Sciences, University of Missouri, Columbia, MO 65211, USA abstract article info Article history: Received 26 March 2008 Received in revised form 21 October 2008 Accepted 3 November 2008 Available online 12 November 2008 Keywords: Crustal thickening Lateral extrusion Indo-Asian collision Himalayan–Tibetan plateau Asian tectonics The ∼ 2000 km crustal shortening from the Indo-Asian collision in the past 40–70 million years has been accommodated mainly by crustal thickening in the Himalayan–Tibetan Plateau and lateral extrusion of blocks of Asian continents. However, the spatiotemporal evolution of crustal thickening and lateral extrusion, hence the far-field impact of the Indo-Asian collision on Asian tectonics, remains controversial. Here we present a 3D viscous flow model that simulates the partitioning of crustal material between thickening and lateral extrusion during the Indo-Asian collision. The model assumes conservation of crustal mass, and is constrained by the history of the Indo-Asian plate convergence and by the present-day topography of the Himalayan–Tibetan Plateau. The results show that much of the early collision was absorbed by crustal thickening within the Himalayan–Tibetan plateau. However, lateral extrusion of crustal material has gradually become dominant in the past ∼ 10–20 Myr, indicating increasing influence of the Indo-Asian collision on Asian tectonics. Published by Elsevier B.V. 1. Introduction The Indo-Asian collision in the past ∼ 40–70 Myr has caused about 2000 km crustal shortening (Yin and Harrison, 2000). Some collision was accommodated by crustal thickening in the Himalayan–Tibetan Plateau and surrounding regions (England and Houseman, 1986); the rest may be accommodated by lateral extrusion of blocks of Asian continent along numerous strike-slip fault systems (Tapponnier et al., 1982; Peltzer et al., 1989; Replumaz and Tapponnier, 2003) and by ductile lower crustal flow out of Tibet (Clark and Royden, 2000)(Fig. 1). How the shortened crust was partitioned between crustal thickening and lateral extrusion during the Indo-Asian has important implications for the diffuse Asian tectonics, yet the answers from previous studies are inconclusive. In a continuum mechanical model that approximates the Asian continent as a viscous thin sheet indented by a rigid Indian plate, England and Houseman (1986, 1988) have shown that the collision is accommodated mainly through crustal thickening in central Asia, mostly within the Himalayan–Tibetan plateau. Conversely, using an analogue experiment of indentation of a plasticene Asian continent, Tapponnier and co-workers show far-reaching influence of the collision through lateral extrusion of blocks of Asian continent (Tapponnier et al., 1982). Subsequent studies have narrowed the gap between these end member models (Houseman and England, 1993), and the perceptions of how crustal thickens and extrudes have evolved. The lateral extrusion could occur in forms of both discrete translation of crustal blocks and distributed extrusion of crustal material (Kong and Bird, 1996), or by lateral flow of lower crustal material out of the collision zone (Clark and Royden, 2000). On the other hand, the Himalayan–Tibetan Plateau may have risen in discrete steps (Tapponnier et al., 2001). Nonetheless, the partitioning of strain between crustal thickening and lateral extrusion, and more importantly, the spatiotemporal evolution of such strain partitioning, remain controversial (Tapponnier et al., 2001; Wright et al., 2004). The tectonic implications can be profound. When most of the Indo- Asian collision is absorbed by crustal thickening in the Tibetan Plateau, its impact would be limited to the plateau and surrounding regions (England and Molnar, 1997). Conversely, when the collision is accommodated mainly by lateral extrusion of Asian continent, its impact on Asian tectonics would be immediate and far reaching. Hence how the crustal shortening is partitioned between crustal thickening and lateral extrusion controls both the rise of the Himalayan–Tibetan Plateau and Cenozoic tectonics in much of the central and eastern Asia. In this study we first estimate the amount of crustal shortening and the portion accommodated by crustal thickening in the Himalayan– Tibetan plateau, based on the history of the Indo-Asian collision and the present crustal volume in the plateau. We then use a three-dimensional (3D) viscous flow model to simulate the spatiotemporal portioning of the shortened crust between crustal thickening and lateral extrusion. 2. The amount of crustal shortening and thickening The relatively well-defined plate convergence history and the present-day crustal volume of the Himalayan–Tibetan Plateau permit some estimation of the portioning of crustal mass between thickening Tectonophysics 465 (2009) 128–135 ⁎ Corresponding author. E-mail address: yangyo@missouri.edu (Y. Yang). 0040-1951/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.tecto.2008.11.002 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto