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Insight Into Collision Zone Dynamics from Topography: Numerical Modelling Results and Observations

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Insight Into Collision Zone Dynamics from Topography: Numerical Modelling Results and Observations Solid Earth, 3, 387–399, 2012 www.solid-earth.net/3/387/2012/ Solid Earth doi:10.5194/se-3-387-2012 © Author(s) 2012. CC Attribution 3.0 License. Insight into collision zone dynamics from topography: numerical modelling results and observations A. D. Bottrill, J. van Hunen, and M. B. Allen Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK Correspondence to: A. D. Bottrill ([email protected]) Received: 28 June 2012 – Published in Solid Earth Discuss.: 19 July 2012 Revised: 12 October 2012 – Accepted: 19 October 2012 – Published: 23 November 2012 Abstract. Dynamic models of subduction and continental 1 Introduction collision are used to predict dynamic topography changes on the overriding plate. The modelling results show a distinct In this study we aim to look at the evolution through time evolution of topography on the overriding plate, during sub- of topography on the overriding plate at a collision zone. duction, continental collision and slab break-off. A promi- 2-D numerical models of lithosphere-mantle interactions at nent topographic feature is a temporary (few Myrs) basin on subduction and continental collision zones are used. These the overriding plate after initial collision. This “collisional generic modelling results are compared to specific observa- mantle dynamic basin” (CMDB) is caused by slab steepen- tions from the Arabia-Eurasia collision zone. The study aims ing drawing, material away from the base of the overriding to illustrate that topography can be used as an indicator of plate. Also, during this initial collision phase, surface up- the dynamics associated with continental collision and slab lift is predicted on the overriding plate between the suture break-off. zone and the CMDB, due to the subduction of buoyant conti- To a first order, the topography of the Earth’s surface can nental material and its isostatic compensation. After slab de- be explained by isostasy causing regions of variable crustal tachment, redistribution of stresses and underplating of the thickness to equilibrate at different heights. These varia- overriding plate cause the uplift to spread further into the tion in crustal thickness are caused by a variety of local overriding plate. This topographic evolution fits the stratig- processes including tectonic thrusting and rifting. However raphy found on the overriding plate of the Arabia-Eurasia crustal thickness is just one contributor to the Earth’s topog- collision zone in Iran and south east Turkey. The sedimentary raphy and to fully account for topography we need to con- record from the overriding plate contains Upper Oligocene- sider all forces that act on the Earth’s surface. The forces Lower Miocene marine carbonates deposited between terres- that influence topography can be broadly categorised into trial clastic sedimentary rocks, in units such as the Qom For- isostatic forces, from thermal and crustal buoyancy, flexure mation and its lateral equivalents. This stratigraphy shows forces associated with the lithosphere, crustal strength as that during the Late Oligocene–Early Miocene the surface of well as stresses imposed at the base of lithosphere due to the overriding plate sank below sea level before rising back mantle dynamics. Topography driven by flow in the mantle is above sea level, without major compressional deformation referred to as dynamic topography (Lithgow-Bertelloni and recorded in the same area. Our modelled topography changes Silver, 1998). Many modelling studies have found a change fit well with this observed uplift and subsidence. in the dynamics of mantle flow during the transition between subduction, collision, and oceanic slab break-off (Gerya et al., 2004; Andrews and Billen, 2009; Duretz et al., 2011). These changes in flow in the mantle would be expected to af- fect the topography generated due to changes in the stresses at the base of the lithosphere (Faccenna and Becker, 2010). Published by Copernicus Publications on behalf of the European Geosciences Union. 388 A. D. Bottrill et al.: Insight into collision zone dynamics from topography Subduction zone topography is characterised by a long lin- ear oceanic trench flanked by an outer rise on the subduct- ing plate side and raised topography on the overriding plate (Melosh and Raefsky, 1980; Hager, 1984; Gephart, 1994). The origin of this topography is the result of the sum of dy- namic forces and isostatic forces (Forte et al., 2010; Husson et al., 2011). Linking of subduction dynamics to topography (Husson, 2006) has shown how features such as the back arc basin depth can be correlated to subduction velocity. Numeri- cal modelling (Husson, 2006) and analogue models (Husson et al., 2011) further show predictions of topography due to mantle dynamics. Crustal shortening and thickening at collision zones pro- duces topographic signals, via isostasy, in addition to those caused by mantle processes. The transformation of a sub- duction zone to a fully formed collision zone is thought to be categorised by two important events: initial collision of the continental material and later, break-off of the subduct- ing oceanic slab. Slab break-off has been proposed to oc- Fig. 1. Location map of the Arabia-Eurasia collision zone. The cur when the arrival of continental material at the subduc- dashed line shows the position of the suture between the overrid- tion zone slows and eventually stops subduction (Davies and ing Eurasian plate in the north and the subducted Arabian plate in Blanckenburg, 1995). This resistance to continued subduc- the south. tion by the buoyant continental crust could then cause a build up of stress where the oceanic material joins the continent. The slowdown of subduction is also thought to allow the sub- linked to observations. The study by Shephard et al. (2010) ducting slab to heat up, further weakening it. A combination focuses on the proposed drainage reversal in South America of the build up in stress and thermal weakening is thought during the opening of the south Atlantic and proposes that to cause the slab to neck and detach. Recent numerical mod- the subsidence and uplift patterns required can only be ex- elling work (Gerya et al., 2004; Andrews and Billen, 2009; plained by changes in subduction dynamics influencing the van Hunen and Allen, 2011) has shown that both these mech- topography. anisms play important roles in the ability of slabs to detach. The Arabia-Eurasia collision zone (Fig. 1) offers an area Estimates for the lag time of break-off after initial collision for the study of topography changes associated with colli- vary between 5–40 Myrs (Andrews and Billen, 2009; Gerya sion and slab break-off (Agard et al., 2011). Collision oc- et al., 2004; van Hunen and Allen, 2011) and have been found curred here after the closure of the Neo-Tethys ocean basin. to be dependent on the age and associated strength of the sub- Collision is still active in the region with GPS measure- ducted lithosphere. ments putting the current north–south convergence rate at Many studies have produced numerical models of the dy- ∼2.5 cm yr−1 (Sella et al., 2002). The time of initial colli- namics of slab break-off (Buiter et al., 2002; De Franco et al., sion is still debated, but estimates vary between >65–5 Ma 2008; Faccenda et al., 2009; Andrews and Billen, 2009; van (Berberian and King, 1981; Philip et al., 1989; McQuarrie, Hunen and Allen, 2011). Various estimates have been made 2003; Ghasemi and Talbot, 2006). A common estimate is late of the change in topography due to slab break-off, ranging Eocene ∼35 Ma (Vincent et al., 2007; Allen and Armstrong, from 1–10 km (Gerya et al., 2004; Andrews and Billen, 2009; 2008; Ballato et al., 2011; Mouthereau et al., 2012; Agard et Duretz et al., 2011) of uplift due to the loss of the slab. This al., 2005) based on deformation on both sides of the suture large variation in estimates clearly shows the need for future at that time, and a shutdown of arc magmatism. In this study work. we will use the estimate of 35 Ma for the initial collision in One of the challenges with topography is to separate the this region. contributions to topography from different sources. Collision Local slab break-off has been estimated to have occurred zones produce elevated topography due to the shortening and at 10 Ma (Ghasemi and Talbot, 2006; Omrani et al., 2008) for thickening of the crust as well as uplift associated with slab the Arabia-Eurasia collision giving a delay time of ∼25 Myrs break-off, plate flexure and dynamic effects from the mantle. since initial collision. This estimate for the timing of slab Recent work (Duretz et al., 2011) has also highlighted that break-off comes from observations of collisional magma- different break-off dynamics would produce different topog- tism (Omrani et al., 2008) which is used as an indicator of raphy changes. Numerical modelling results have also been slab break-off. In that study, collisional magmatism is pro- related to observation for South America (Shephard et al., posed to be produced by slab break-off due to the descent of 2010) showing that the modelled topography changes can be the slab into the mantle draws hot material into the mantle Solid Earth, 3, 387–399, 2012 www.solid-earth.net/3/387/2012/ A. D. Bottrill et al.: Insight into collision zone dynamics from topography 389 wedge region. There is also evidence from mantle tomogra- phy that slab break-off has occurred at the Arabia-Eurasia collision zone (Lei and Zhao, 2007), which shows low ve- locity regions where the slab would be expected. These low velocity regions can be interpreted as areas where the slab is no longer present and has been replaced by hot mantle material.
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