Geophysical Journal International Geophys. J. Int. (2011) 187, 1151–1174 doi: 10.1111/j.1365-246X.2011.05236.x Switching between alternative responses of the lithosphere to continental collision Marzieh Baes, Rob Govers and Rinus Wortel Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands. E-mail: [email protected] Accepted 2011 September 19. Received 2011 September 19; in original form 2011 January 2 Downloaded from https://academic.oup.com/gji/article/187/3/1151/612017 by guest on 29 September 2021 SUMMARY We study possible responses to arc–continent or continent–continent collision using numerical models. Our short-term integration models show that the initial stage of deformation following continental collision is governed by the competition between three potential weakness zones: (1) mantle wedge, (2) plate interface and (3) lower continental crust. Depending on which of these is the weakest zone in the system, three different responses can be recognized: (1) subduction polarity reversal, (2) continuation of subduction and (3) delamination and back stepping. Subduction polarity reversal occurs if the mantle wedge is the weakest zone in GJI Geodynamics and tectonics the system. This happens only if the viscosity of the mantle wedge is at least one order of magnitude lower than the average viscosity of the lithosphere. In continent–continent collision, one additional condition needs to be satisfied for subduction polarity reversal to occur: for the subducting lithosphere the ratio of the viscosity of the lower continental crust to the viscosity of the upper lithospheric mantle must be equal to or higher than 0.006. The time required for polarity reversal depends on several parameters: the convergence rate, the sinking velocity of the detached slab and the relative strength of the mantle wedge, arc and backarc. The response to collision is continued subduction if the plate interface is the weakest zone, and is delamination and back stepping if the lower continental crust is the weakest area in the system. Our finding that a low-viscosity wedge is a prerequisite for a reversal of subduction polarity agrees with inferences about regions for which subduction polarity reversal has been proposed. Key words: Subduction zone processes; Continental margins: convergent; High strain de- formation zones; Rheology: crust and lithosphere. results in an oppositely dipping subduction zone, typically within 1 INTRODUCTION the arc or backarc. Subduction polarity reversal may presently be Arrival of continental lithosphere or buoyant oceanic plateaus (as occurring at the Wetar thrust belt in eastern Indonesia (Hamilton part of the subducting plate) at the trench of a convergent plate 1973; Curray et al. 1977). Along the Algerian margin, the presence boundary results in collisional tectonic settings. Understanding the of reverse faults has been proposed to be indicative of the earliest evolution of such settings is one of the great challenges in geody- stage of subduction polarity reversal (Deverchere et al. 2005). Sites namics. In exploring the possible scenarios for continuing conver- where a subduction polarity reversal may have occurred in the past gence following collision, not only the subducting plate but also include the San Cristobal trench in Solomon Islands (Cooper & the overriding plate should be considered. Referring to the part of Taylor 1985; Kroenke et al. 1986) and New Hebrides subduction the overriding plate adjacent to the trench as an arc, the nature of zone (Falvey 1975; Rodda & Kroenke 1984). the overriding plate behind an arc may be either (1) oceanic litho- Another possible response to continental collision is delamina- sphere, possibly an oceanic type backarc basin, or (2) continental tion. In this mechanism, the whole or part of the buoyant continental lithosphere. Previous studies have suggested different scenarios for crust separates from the rest of the lithosphere and is accreted to the lithospheric response following collision, including subduction the overriding plate (Bird 1978; Kerr & Mahoney 2007; De Franco polarity reversal and delamination. Subduction polarity reversal as a et al. 2008). Delamination has been proposed in several localities consequence of attempted continental subduction was first proposed including the Himalayas (Bird 1978; Mattauer 1986), the Aegean by McKenzie (1969). He suggested that the buoyancy of subducted region in Greece (Van Hinsbergen et al. 2005), the North American continental crust results in cessation of subduction. Subduction po- cordillera (Bird 1979; Ben-Avraham et al. 1981) and the collision larity reversal follows if continued convergence of the two plates zone of the North and South China blocks (Li 1994). C 2011 The Authors 1151 Geophysical Journal International C 2011 RAS 1152 M. Baes, R. Govers and R. Wortel We refer to arc–continent (or equivalently continent–arc) col- Another feature of subduction zones is the high temperature in lision for the collisional setting where the backarc is oceanic the shallow backarc mantle, further from the mantle wedge. The ob- lithosphere and to continent–continent collision when the backarc served surface heat flow and seismic velocities of the mantle reveal is continental. The two settings are jointly referred to as arc/ that high upper mantle temperature is not restricted to the arc but continent–continent collision or simply continental collision. We extends for several hundred kilometres across the backarc (Currie use numerical models to investigate the switches between different & Hyndman 2006). These observations indicate a thin lithosphere responses of the lithosphere to continental collision. Our particular (1200 ◦Cat∼60 km) over a backarc width of 250 km to more than focus is on subduction polarity reversal, which has received less 900 km. attention from a modelling point of view, compared to the other scenarios for collisional settings. Our study is composed of several parts. We first study the geological and geophysical evidence of 2.2 Sites of subduction polarity reversal subduction polarity reversal and of delamination. We then review previous studies of the mechanical response to collision and incip- 2.2.1 Wetar thrust ient subduction. Subsequently, we present our numerical models and their results. Finally, we end with a discussion of the results and Timor Island (eastern Indonesia) is the site of collision between the Downloaded from https://academic.oup.com/gji/article/187/3/1151/612017 by guest on 29 September 2021 their comparison with observations. Australian continental crust and the Banda arc. To the west, the Jurassic-aged Indian Ocean lithosphere is subducting northwards beneath the Sunda arc. GPS measurements reveal that most of the convergence between the Australian and Eurasian plates is now 2 OBSERVATIONS accommodated in the backarc along the Wetar thrust (e.g. Genrich et al. 1996; Kreemer & Holt 2000; Nugroho et al. 2009). 2.1 General observations: mantle wedge and backarc Prior to collision, the Indian Ocean had been subducting below characteristics of subduction zones the Banda arc since 15 Ma, when the Indo-Australian plate was Subduction carries a substantial amount of water into the Earth’s moving towards the Eurasian Plate at a rate of 7–8 cm yr−1 (Harris interior. This water is released by dehydration of the subducted 1991, and references therein). The continental crust arrived at the crust, serpentinized mantle and sediments. Hydration decreases the trench at about 3 Ma, leading to the formation of an arc–continent viscosity of the upper mantle (e.g. Chopra & Paterson 1984; Mei collision complex in the Timor region. Following collision, vol- & Kohlstedt 2000), and consequently changes wedge temperatures canic activity ceased in eastern Timor on the islands of Romang, (Van Keken 2003). This leads to a reduction of the viscosity of Wetar, Atauro and Alor. A south-dipping thrust, known as the We- the mantle wedge. Arcay et al. (2005) investigated the effect of tar thrust, was formed in the backarc at about 0.15 Ma (McCaffrey slab dehydration on the dynamics of the mantle wedge. In their 1996). Seismicity data shows high seismic activity and large earth- numerical experiments, the net convergence required to form a weak quakes in the backarc region (Silver et al. 1983; Kreemer & Holt mantle wedge varies from ∼550 to ∼900 km, which depends on 2000). Based on these geodetic, geologic and seismic observations, several parameters such as convergence rate and thermal structure. the deformation of the region can be explained by subduction po- Billen & Hirth (2005) indicated that a subduction rate of more than larity reversal, which was first proposed by Hamilton (1979). Using 2.5 cm yr−1 is required to prevent cooling of the low viscosity seismic data, McCaffrey et al. (1985) showed that continental crust mantle wedge. They also denoted that this subducting rate might has been subducted to a depth of about 150 km. They also proposed vary depending on parameters such as transition strain-rate, grain that the slab is detaching at a depth of ∼50–100 km in the east Savu size and water content. Sea. Observational constraints on the mantle wedge structure are heat flow, seismological data, composition of arc lavas, topogra- phy, gravity and geoid. The most direct observational constraint on 2.2.2 Reverse faults along the Algerian margin the thermal structure of the mantle wedge is surface heat flow. Surface heat flow generally decreases from the trench towards Algeria is located at the plate boundary between Africa and Eurasia. the arc, due to the cooling effect of the subducting slab. Further The Alpine orogen in this area formed as a consequence of closure into the fore-arc and into the backarc region, surface heat flow of the Ligurian ocean. Subduction of the Ligurian ocean beneath increases as a result of the mantle wedge flow (e.g. Wada et al. Iberia commenced about 30 Ma (Rosenbaum et al. 2002; Schettino 2008, and references therein). Seismological studies reveal a zone & Turco 2006), which was followed by southeastward retreat of the of low velocity and high attenuation (e.g.