A geodynamic model of subduction evolution and slab detachment to explain Australian plate acceleration and deceleration during the latest Cretaceous–early Cenozoic W.P. Schellart1,* 1DEPARTMENT OF EARTH SCIENCES, VRIJE UNIVERSITEIT AMSTERDAM, DE BOELELAAN 1085, 1081 HV AMSTERDAM, NETHERLANDS ABSTRACT During the latest Cretaceous–early Cenozoic, the northern margin of the Australian plate was characterized by a large (4000 km wide) north- to northeast-dipping subduction zone (New Guinea–Pocklington subduction zone) consuming a marginal basin. Geological and geophysical data imply that the subduction zone was active ca. 71–50 Ma, and suggest that it was responsible for plate acceleration from ~1.0 to ~7.3 cm/yr ca. 64–59 Ma, and plate deceleration from ~7.3 to ~0.3 cm/yr at 52–49 Ma. This paper presents a numerical model of buoyancy-driven subduction to test if the rates of Australian plate acceleration and deceleration can be ascribed to the progressive evolution of a subducting slab. The geodynamic model reproduces the first-order plate velocity evolution of the Australian plate, with a transient ~5 m.y. period of acceleration from 2 to 8 cm/yr during upper mantle slab lengthening, an ~5 m.y. period of rapid plate motion (~5–8 cm/yr), and a short, 3.9 m.y., period of plate deceleration, starting with a 2 cm/yr velocity drop during 3.1 m.y. of continental subduction and followed by ~0.8 m.y. of rapid deceleration (4 cm/yr velocity drop) during slab detachment. The geodynamic model demonstrates that plate velocity increases or decreases of ~4–6 cm/yr can occur over a period lasting <1 m.y. to a few million years, comparable to what is observed for the latest Cretaceous–early Cenozoic evolution of the Australian plate. Such rates of plate acceleration and deceleration could be tested against plate kinematic data for other subduction settings on Earth. LITHOSPHERE; v. 9; no. 6; p. 976–986 | Published online 11 October 2017 https: // doi .org /10 .1130 /L675 .1 INTRODUCTION 1983; Goes et al., 2008), slab width (Schellart et al., 2010), plate circum- ference attached to a slab (Forsyth and Uyeda, 1975; Gripp and Gordon, The Earth’s lithosphere is segmented into numerous larger and smaller 2002), double subduction (Jagoutz et al., 2015), and the transition from plates (Bird, 2003) that move at a wide variety of plate tectonic speeds, normal (i.e., oceanic) subduction to collision and/or continental subduc- to as much as ~10 cm/yr. The velocities of these plates have traditionally tion (e.g., Patriat and Achache, 1984; Klootwijk et al., 1992; Molnar and been calculated in hotspot reference frames, such as the Pacific (Minster Stock, 2009; van Hinsbergen et al., 2011; White and Lister, 2012; Zahi- and Jordan, 1978; Gripp and Gordon, 2002; Wessel and Kroenke, 2008), rovic et al., 2012; Capitanio and Replumaz, 2013), the models generally Indo-Atlantic (e.g., O’Neill et al., 2005), or global (e.g., Gordon and Jurdy, have the common element of subduction to explain rapid plate motion 1986; Doubrovine et al., 2012) hotspot reference frames, as well as no- (e.g., Conrad and Lithgow-Bertelloni, 2002). An important exception is net-rotation reference frames (e.g., Argus and Gordon, 1991; Kreemer the so-called plume push force, which has been proposed to explain the et al., 2003; Argus et al., 2011). More recently, other geological and extreme plate velocity of the Indian plate in the latest Cretaceous–earliest geophysical features have been used to constrain the absolute velocities Cenozoic (Cande and Stegman, 2011; van Hinsbergen et al., 2011). The of the tectonics plates, such as subducted slabs in the upper and lower ridge push force is generally thought to be an order of magnitude smaller mantle (e.g., van der Meer et al., 2010; Schellart, 2011; Butterworth et than the slab pull force (Forsyth and Uyeda, 1975). al., 2014), upper mantle seismic anisotropy (e.g., Long and Silver, 2009; The Australian plate is home to the Australian continent, which is cur- Kreemer, 2009; Conrad and Behn, 2010), and spreading ridges at the rently the fastest moving continent on Earth (Keep and Schellart, 2012) Earth’s surface (e.g., Becker et al., 2015; Wessel and Müller, 2016). A with north- to northeast-directed velocities as high as 6.5 cm/yr (Fig. 1). number of conceptual and geodynamic models have been put forward to These high velocities can be ascribed to the wide subduction zones that explain the variations in plate velocities. Although such models vary as are present along the northern boundary of the Australian plate, namely to what might be the main physical mechanism that explains variation in the north- to northeast-dipping Sunda subduction zone in the northwest absolute plate velocities, such as subducting plate age (e.g., Carlson et al., and the north- to northeast-dipping Melanesian subduction zone in the northeast. Over the past ~75 m.y., the Australian plate has generally under- *[email protected] gone rapid plate velocities, except for 2 periods, at 73–63 Ma and 49–40 976 © 2017 Geological Society of America.www.gsapubs.org For permission | toVolume copy, contact9 | Number [email protected] 6 | LITHOSPHERE Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/6/976/3985681/976.pdf by guest on 18 September 2018 Australian plate motion change due to subduction evolution and slab detachment | RESEARCH o 10 Spakman (2015), a 4000-km-wide (trench-parallel extent) subduction Pacific plate zone was identified in the New Guinea region active ca. 71–50 Ma, and Sunda plate NewGui ne fossil a o the phases of plate acceleration, rapid plate motion, and plate deceleration su - 0 b Po Pacific of the Australian plate were ascribed to the geodynamic evolution of this duc c Ocean klin tion gton zo subduction zone. The subduction zone consumed a small oceanic back- ne o 10 arc basin, the Emo backarc basin, with a maximum north-south extent of Indian Ocean ~1100 km. The phase of plate acceleration was ascribed (in Schellart and Spakman, 2015) to progressive upper mantle slab lengthening during the Australian plate o 5 cm/yr 20 initial transient subduction phase and the phase of plate deceleration was Incipient subduction ascribed to slab detachment during the final stage of subduction. Subduction Tasman I present here a geodynamic numerical model of buoyancy-driven Collision/ Sea obduction o Fossil Basin 30 subduction to test if the rates of plate acceleration and deceleration, and subduction Southern Ocean the high rate of Australian plate motion during the intervening period, can be ascribed to the progressive evolution of the subduction zone in the New o 40 Guinea region, from transient slab lengthening to the final stage of slab Antarctic plate detachment. The geodynamic model provides new insight into the rates ooo o o o o 80 100 120 140 1602180 00 of plate acceleration and deceleration during subduction of the edge of a Figure 1. Plate tectonic map of the central and eastern part of the Aus- plate that contains a large continent. tralian plate, showing the present-day plate boundaries and velocities (in cm/yr), as well as the fossil New Guinea–Pocklington subduction zone. METHODS The velocities are based on the relative plate motion model of DeMets et al. (1994) using the Indo-Atlantic moving hotspot reference frame from O’Neill et al. (2005). Numerical models are presented that have specifically been designed to investigate the subduction evolution of a small oceanic backarc basin (Emo backarc basin), with a particular focus on the latest stage of subduc- Ma, when plate velocities reached minima of 0.8–2.9 cm/yr and 0.3–1.6 tion when all oceanic lithosphere is consumed and continental passive cm/yr, respectively, in the Indo-Atlantic moving hotspot reference frame margin lithosphere approaches the trench. The models use the code Under- (Fig. 2). During the intervening period the velocities were much higher world (Stegman et al., 2006; Moresi et al., 2007, 2014), in which plate with a maximum of 7.3 cm/yr at 59–52 Ma. During the latest Cretaceous motion, subduction and mantle flow are modeled in a Cartesian domain (Maastrichtian) to early Eocene, the Australian plate was bordered to using compositional buoyancy contrasts in an incompressible Boussinesq the east, south, and west by spreading ridges in the Tasman Sea, South- fluid at a very low Reynolds number. Distinct volumes (e.g., continental ern Ocean, and Indian Ocean (e.g., Royer and Sandwell, 1989; Yan and upper crust, lower crust and lithospheric mantle, sublithospheric upper Kroenke, 1993; Gaina et al., 1998; Hall, 2012; Seton et al., 2012); this mantle, lower mantle) are represented by sets of Lagrangian particles cannot explain the period of rapid plate motion and the preceding and that are embedded within a standard Eulerian finite element mesh, which following phases of plate acceleration and deceleration. In Schellart and discretizes the problem to solve the governing equations. For additional ] 10 ] 10 A B 9 IAHS-2005 9 IAHS-2005 GHS-2012 GHS-2012 8 8 7 7 6 6 5 5 Australian plate [cm/yr 4 4 3 3 2 2 1 1 0 Northward velocity component [cm/yr 0 locity of point on 80 70 60 50 40 30 20 10 0 80 70 60 50 40 30 20 10 0 Ve Time [Ma] Time [Ma] Figure 2. Absolute velocity of the Australian plate at 74–0 Ma for a point currently located in the Gulf of Carpentaria (i.e., relatively close to the center of the New Guinea–Pocklington fossil subduction zone, circle with cross in Fig.