Subduction Zone Blues: Laboratory and Field Constraints on the Rheology and Deformation Mechanisms of Mafic Blueschist at the Subduction Interface

Abstract

Subduction zones are key drivers of plate tectonics, facilitating crustal recycling and deformation. The associated geological hazards of these convergent margins, including megathrust earthquakes and tsunamis, pose significant risks to society. Although these hazards are generated by frictional, seismic slip along the shallow subduction interface, aseismic, ductile deformation plays a critical role by modulating the strength of the subducting slab and transferring stress up-dip to load the seismogenic zone. During subduction, progressive metamorphic transformations of mid-ocean ridge basalt to blueschist and, ultimately, to eclogite alters the chemical and mechanical properties of the subducting crust through changes in its mineralogy, volatile content, density, and rheological strength. Field observations and limited experimental work have offered some insights into the evolution of subducting oceanic crust—such as a strength hierarchy estimating blueschist to be weaker than both gabbro (basalt) and eclogite—but a substantial knowledge gap persists. This thesis addresses this knowledge gap by investigating key questions regarding subduction zone dynamics including: (1) Does blueschist generate an observable seismic signal that can improve seismic imaging of blueschist along the subduction interface? (2) What mechanism(s) accommodates blueschist deformation along the subduction interface down-dip of the seismogenic zone and what are the implications for the seismic-aseismic transition? (3) How do chemical and mechanical changes at the blueschist-eclogite transition influence interface subduction zone dynamics during the subduction and exhumation of HP/LT lithologies? Electron backscatter diffraction (EBSD)-based petrofabric analysis was applied to model the seismic anisotropy generated by a suite of mafic blueschists exhumed from a range of peak P-T conditions relevant to the ductilely deforming interface in active subduction zones. The blueschists displayed a broad range of P-wave anisotropies (AVp) up to 20%, correlating positively with the abundance and deformation-produced crystallographic preferred orientation (CPO) of the sodic amphibole mineral, glaucophane. Modeled AVp magnitudes were commonly ~10%, suggesting a common blueschist seismic anisotropy signal with potential to improve mapping of the extent and deformation of blueschists along the subduction interface. To investigate the underlying deformation that generates observed seismic anisotropies, EBSD techniques—including a novel technique coupling weighted Burgers vector and misorientation analyses—were applied to glaucophane in a lawsonite blueschist from the Catalina Schist that was exhumed from P-T conditions down-dip of the seismogenic zone. The results of this investigation reveal deformation of blueschists at these conditions to be primarily accommodated by the dislocation creep mechanism. A suite of deformation experiments were performed on glaucophane aggregates to derive a constitutive, power-law relationship (flow law) for dislocation creep in glaucophane and, by extension, blueschist. Extrapolation of flow laws for glaucophane dislocation creep and related subduction lithology flow laws to natural conditions predict deformation by dislocation creep in the subducting slab initiates at ~350°C, evolving to grain-size-sensitive mechanisms with increasing temperatures and pore fluid pressures or finer grain sizes. Detailed microstructural and petrological investigation were conducted on an exhumed blueschist-eclogite transition on Vroulidia Beach, Sifnos Island, Greece to investigate the complex interplay between deformation, metamorphism, and aqueous fluids in during the subduction and exhumation of blueschists and eclogites. This investigation demonstrates that fluid-deformation feedbacks promote zones of weakness that further localize deformation and fluid flow to enhance retrogression in such zones during exhumation. Together, these investigations advance our understanding of deformation and metamorphic processes in blueschists, eclogites, and at the blueschist-eclogite transition and subduction zone dynamics.