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Title: Deformation and mantle flow in regions of complex subduction from seismic anisotropy : a case study of Indonesia
Author: Di Leo, Jeanette Frances
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Subduction, as part of the mantle convective system, is a key process in the Earth's dynamic and chemical evolution. However, many unknowns remain about the mantle's response to subducting slabs, which depends on the degree of slab-mantle coupling. Seismic anisotropy (the directional dependence of seismic wave speed) may provide insights into the dynamic nature of subduction systems, as it can result from the texturing of minerals by flow-induced deformation. Here, I investigate the anisotropic structure of the Indonesian region with shear wave splitting measurements. With several active and relict subduction zones in close proximity to one another, the region lends itself as an ideal natural laboratory to study subduction processes. I measure shear wave splilling in three phases: SKS (delay times, 01: 0.6-2.7 s), local S (01 : (U- I .I s), and source-side S (01: 0. 3- 1.7 ). Combining these phases prov ides improved vertical resolution of anisotropic fabrics, which allows me to distinguish various regions of mantle now within one subduction system. At both the Sangihe and the Banda subduction zone, 1 infer dip-parallel to dip-oblique shear layers above the slab (that possibly ex tend into the mantle transition zone) as well as trench-parallel sub-slab mantle fl ow. At the North Sulawesi subduction zone, toroidal flow is inferred around the slab edge. Additionally, evidence of fossil anisotropy in the overriding plate complements findings from other geophysical techniques (e.g. , Borneo's hi story of block rotation as detected by paleomagnetism). To test the veracity of inferring mantle flow direction from shear wave splilling fast polarisation, I simulate strain -hi story-dependent texture development and estimate the resulting shear wave splilling. In order to test the significance of a rear barrier to achieve trench-parallel sub-slab mantle flow, I compare a one-sided subduction model to one that approximates the doubles sided Molucca Sea subduction system and the shear wave splilling observations of that region. 1 show that, although such a rear barrier significantly amplifies trench-parallel sub-s lab ani sotropy due to mantle flow, it is not necessary to produce trench-parallel fa st directions per se. Such fabrics are achievable in a simple model of one-sided subduction due to a combination of simple and pure shear deformation in the sub-slab mantle. Evidence from shear wave splilling measurements and strain -hi story-dependent texture modelling suggests that the tectonic selling is a precondition for the resulting mantle flow, and that the slab and the surrounding mantle are at least partially coupled. I conclude that, given the right source-receiver geometry, careful observations of shear wave splilling using multiple shear phases are a reliable means of inferring the local mantle flow field , as well as large-scale deformational features in the lithosphere, even in tectonically complex regions such as Indonesia.
Supervisor: Wookey, James ; Kendall, J. Michael Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available