Structural and rheological evolution of subduction interface shear zones : insights from exhumed rocks

Date

2019-12-04

Authors

Kotowski, Alissa Jeanne

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Abstract

The subduction interface is an inherently heterogeneous distributed shear zone occupying the boundary between a down-going (i.e., subducting) plate and overriding crust and mantle. The sinking of cold, dense oceanic lithosphere creates a slab pull force, which is a driving force for Plate Tectonics. Interface rheology (i.e., deformation or flow) exerts a first-order control on plate boundary strength, seismic style, and the propensity for rocks to exhume, or return to the surface of the Earth from mantle depths. When subducted rocks are exhumed, they provide rare snapshots of the rheological behavior of these complex plate boundary shear zones, and how rocks are subsequently modified by brittle and semi-brittle processes in the upper crust. In this dissertation, I combine a variety of observational and analytical techniques to investigate exhumed subduction-type rocks from Syros Island (Cyclades, Greece) and the sub-ophiolite metamorphic sole in Oman. This work provides constraints on the rheological behavior of subduction plate boundary shear zones, to better inform geodynamic models and to contextualize geophysical observations of active subduction zones. Rheological heterogeneities (i.e., outcrop- to regional-scale features producing strain gradients and/or significant differences in deformation mode or mechanism) are thought to be directly related to the seismic style that occurs along the subduction interface. At relevant depth and temperature conditions for Syros (~50-60 km, 500°C), the important seismic style is an enigmatic, coupled seismic-aseismic phenomena deemed Episodic Tremor and Slow Slip (ETS). ETS involves accelerated – but aseismic – slip over ~10’s-100’s km² of the interface, in conjunction with swarms of micro-seismicity, or tremor, and seems to occur nearly ubiquitously in subduction zones regardless of thermal structure, predicted depths of metamorphic dehydration reactions, or subducting rock type. In Chapter 2, I use exhumed blueschist- and greenschist-facies rocks on Syros to characterize the length scales, types, sources, and deformation mechanisms of rheological heterogeneities that occupy the deep subduction interface, and how they may contribute to enigmatic seismicity like ETS. Partial eclogitization of subducting rocks sets up stark rheological contrast across shear zones, which results in coupled brittle-viscous behavior assisted by near-lithostatic pore fluid pressures. Geologic observations are consistent with a mechanical model of ETS in which the deep interface comprises transiently brittle, potentially tremorgenic sub-patches, within a larger viscously creeping interface patch. These observations scale appropriately with geophysical constraints of tremor source areas and seismic moments. During a subduction-exhumation cycle, the length scales of mixing along the interface, maximum pressures (i.e., depths) that rocks reach, and mechanisms of rock exhumation depend in part on interface rheology. In Chapters 3 and 4, I combine structural and microstructural analysis, novel techniques in thermobarometry, and new interpretation of published metamorphic geochronology on Syros to understand bulk interface deformational style, progressive metamorphism, and rheology. The results are all consistent with a model of coherent subduction and underplating (i.e., transfer of subducting material to the overriding forearc), as opposed to large-scale, chaotic mixing in a mega-mélange. Exhumation of rocks on Syros occurred nearly entirely by buoyancy- and viscosity-driven subduction channel return flow, which accommodated vertical translations of ~40 km from peak depths (60 km) to the middle-lower crust. These inferences are consistent with calculated estimates of shear zone viscosity, and the balance between buoyancy forces and shear tractions at peak subduction depths. While exhumation mechanisms and mechanical behavior of thermally mature subduction zones are particularly important to an understanding of subduction tectonics, subduction initiation is even more enigmatic and poorly understood. The only geologic record of rocks deformed and metamorphosed in infant subduction zones are present as tectonic slivers beneath the world’s ophiolite sequences, deemed metamorphic soles. In Chapter 5, I investigate the structural and petrologic signatures of subduction, return flow, and ophiolite emplacement in a 100 m section of ”low-temperature” metamorphic sole acquired during Phase I of the Oman Drilling Project (Site BT-1B). The ”low-temperature” sole rocks acted as a mechanically coherent slice preserving various evidence for subduction and return flow prior to obduction. Intraoceanic subduction initiation is characterized by rapid cooling of the plate interface, which exerts primary control on metamorphic grade and changes in deformation mechanisms. Exhumation in the subduction channel accommodated at least 15 km of vertical translation, and occurred concurrently with early stages of ophiolite emplacement.

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