Development of texture and seismic anisotropy during the onset of subduction

Abstract

[1] How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strainhistory-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. (2012a). The setting lends itself as a case study, because it is fairly young and, therefore, early textures from the slab's descent from the near surface to the bottom of the mantle transition zone-which we simulate in our models-have not yet been overprinted by subsequent continuous steady state flow. Second, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel subslab mantle flow. We demonstrate that although a barrier amplifies trenchparallel subslab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se. In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle.

Geodynamics of divergent double subduction: 3‐D numerical modeling of a Cenozoic example in the Molucca Sea region, Indonesia

“…Previous DDS models were focused on the idealized, symmetrical subduction of a plate on both sides and revealed how DDS might lead to convergence of overriding plates and the basic pattern of mantle flow that allows escape of subslab mantle [e.g., Soesoo et al , ; Di Leo et al , ]. Our modeling results further reveal factors that have an influence on the morphology of DDS.…”

“…Cartoons illustrating several forms of double subduction. (a) Divergent double subduction (descripted in this study and by Soesoo et al [], Di Leo et al [], Li et al ., [], etc.). (b) Double subduction with opposite dipping directions (descripted by Maruyama et al [ ]).…”

“…It is estimated that the total length of the seismically active Molucca Sea plate is 1400–1600 km, 900 km of which was subducted under the Sangihe arc and ~450 km under the Halmahera arc [ Hall and Spakman , ] (Figures c and ). By means of shear wave splitting measurement, mantle flow pattern in response to subduction of the Sangihe and Halmahera slabs was inferred in this region, indicating lateral escape of mantle beneath the Molucca Sea plate [ Di Leo et al , , ].…”

“…From observations of ancient and Cenozoic examples, it is likely that DDS may have occurred many times during closure of oceanic basins in which there have been complex interactions between the subducting plates, overriding plates and mantle. However, despite its potential importance in plate tectonics, DDS was largely a conceptual model until recent 3‐D modeling study revealed an escape pattern of mantle flow induced by subduction of a narrow oceanic plate [ Di Leo et al , ]. The driving mechanism and factors that may influence the development of DDS remain poorly constrained.…”

Mid-mantle seismic anisotropy beneath southwestern Pacific subduction systems and implications for mid-mantle deformation

“…Trench migration has been shown to be important for the development of upper mantle anisotropy in subduction systems, both in the mantle wedge above the slab and in the sub-slab mantle (e.g., Long and Becker, 2010;Long, 2013). Migrating trenches create a three-dimensional flow field that is more complex than a simple two-dimensional model of entrained flow would predict; the resulting flow fields have a significant toroidal component and create complicated finite strain and anisotropy patterns (e.g., Druken et al, 2011;Faccenda, 2014;Leo et al, 2014;Paczkowski et al, 2014). Another reason trench migration is important is that it is likely connected to the geometry of slabs in the transition zone; one model for stagnant slabs in the mid-mantle is that slabs become anchored in the high-viscosity lower mantle and the slower sinking forces the trench to roll back (e.g., Billen, 2010).…”