Evolution & Dynamics of the Western Pacific, IBM Subduction Initiation, and IBM-1 Mike Gurnis Caltech IBM-Margins Workshop, Nov, 2007

Outline Brief review of formation of arcs in the Western Pacific, especially IBM Regional geodynamic ideas & models for initiation of subduction Large-scale geodynamic ideas & models for the western Pacific Relevance of the proposed IBM-1 hole

Brief review of formation of arcs in the Western Pacific, especially IBM

Gurnis et al. 2004

An Earlier Evolutionary Model for the formation of the IBM Originally from Hilde et al. [1977] as modified by Stern & Bloomer [ 1992].

Stern [2004]

Taylor & Goodliffe [2004] emphasize that : The bulk of the fabric making up the WPB formed after the formation of the new arc at ~49 Ma. The approximate orthogonal orientation of the KPR cannot be used to argue that the subduction formed at an old transform fault. The new SZ formed at a high angle to existing structures. They suggest that the IBM did not nucleate on a pre-existing weak zone.

Regional geodynamic ideas & models for initiation of subduction

Stern [2004]

viscous resistance, F v fault friction, F f buoyancy, F b tectonic force, F t subduction occurs if F b + F t > F el + F f + F v (modified from McKenzie, 1977) Subduction Dynamics: Driving & Resisting Forces F el

Use an explicit finite difference method to solve the force balance equation Plastic strain C, Method akin to Fast Lagrangian Analysis of Continua (FLAC) [Poliakov and Buck, 1998; Lavier et al., 2000]. Explict method Visco elasto-plastic material Track plastic strain Frequent regridding Brittle crust ( Mohr-Coulomb) Non-linear, temperature dependent viscosity in crust, lithosphere and mantle  A. Poliakov, Y. Podladchikov & Talbot [ 1993] Benchmarked method against Rayleigh-Taylor problem

Homogeneous 30 Myr Plate

10 Ma – 40 Ma Fracture Zone

Hall et al., 2003

Evolution of topography for 10 Ma – 40 Ma Fracture Zone Model

Large-scale geodynamic ideas & models for the western Pacific Subduction initiation on the boundary of long-lived zone of downwelling Inability of models with radial variations in viscosity to generate rapid changes in plate motion Plate motions could be resisted significantly by ‘plate bending’ Idealized geodynamic models show that evolving deep buoyancy can be efficiently coupled to the surface and can lead to plate motions changes on ~10 Myr time- scales Incorporating these critical processes in numerical models of mantle convection in a spherical geometry is facilitated by the linkage with a new paleogeographic system

“…It is plausible that there is a relationship between the position of [the Izu, Mariana Trough, and Lau Basin]…which formed concurrently at Ma [at] the boundary of the asthenosheric domains… If the boundary is a major zone of downflow in the upper mantle….,one would predict a propensity for subduction zones to form there or to become stationary over time.” Hickey-Vargas et al. [1995] Philippine Sea Plate Formed within the ‘Indian Domain’

Simple viscous models with only radial variations in viscosity and long wavelength driving forces cannot predict rapid changes in plate motions (and presumably rapid changes in in-plane-stress) [Richards and Lithgow- Bertelloni, 1996] Conrad and Lithgow-Bertelloni [2004]

Plate motions could be resisted significantly by ‘plate bending’ A simple theory of a viscous plate bending atasubduction zone u o subduction velocity  plate viscosity H plate thickness Buffett [2006]

Plate bending slows the plate down and leads to a greater propensity for thinner lithosphere to subduction [Buffett, 2006]

Zhong and Gurnis [1995]

Limitations of plate motion models can be overcome with enhanced paleogeographic systems that are interfaced with dynamic models Continuously closed plate polygons Geometry of tectonic elements followed (such as the subduction polarity) Interfaced with paleo-age grids Ability of incorporate and explore alternative reconstructions Data passed to/from geodynamic models A geodynamists view of GPlates GPlates: A collaborative effort of Sydney Univ., Caltech & Norwegain Geological Survey

Müller, Sdrolias, Gaina, Age grids GPlates Reconstruction [Gurnis, et al. 2007]

Relevance of the IBM-1 hole (Proposal pending within IODP)

Higuchi et al. [2007]

Summary Points Periods of rapid back-arc extension follow subduction initiation. Numerical models and evolution of Western Pacific broadly consistent. Important regional factors for IBM still need to be considered: Initiation adjacent to Mesozoic ridges, without existing, coherent weak zones, and above coherent, long-lived mantle downwelling. Models of plate motions at large-scales must consider the strength of slabs as both a resisting force (plate bending) and an efficient coupling mechanisms of deep slabs with the plates. The proposed IBM-1 hole will facilitate discrimination between models of subduction initiation and if there was a period of uplift and potentially compression before the onset of boninitic magmatism.