Collaborative Research: Blueschist Rheology: Experimental Constraints On Glaucophane Strength And Deformation Mechanisms

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

Subduction zone faults host the largest earthquakes on earth and produce deadly tsunamis. At depths greater than about 30 kilometers, these faults deform continuously and build up stress on the locked shallower zones, ultimately causing major earthquakes. However, because of a lack of experimental constraints, the mechanisms by which deep portions of subduction zones behave is not well understood. This work investigates the mechanical strength of glaucophane, a ubiquitous subduction zone mineral, which largely controls the behavior of these ductile zones. A set of deformation experiments at MIT’s Rock Deformation Laboratory and the characterization of experimental products at the University of Washington’s Structural Petrology Laboratory provide new constraints on glaucophane strength. Results from this work directly inform our understanding of natural earthquake hazards by placing ductile deformation within the broader framework of subduction zone faults. This research funds two early career scientists, one of whom is a woman, and will support two graduate students, one of whom is an underrepresented minority in STEM. This work will also provide a summer research opportunity, mentorship, and funding for an underrepresented minority undergraduate student through the Research Experiences in Solid Earth Science program. Subduction zones are the loci of the world’s largest geologic hazards and constitute the main avenue for chemical recycling between surface material and the deep earth. The tempo and occurrence of these processes are directly influenced by the mechanical behavior and strength of the subduction interface, the thin tabular fault and shear zone that accommodates subduction deformation and motion. The rheological evolution of this interface is fundamental to understanding plate tectonics, subduction zone dynamics, convergence rates, and plate boundary slip behaviors. Observations of the rock record from between the brittle-ductile transition zone and sub-arc depths suggest that this rheology is controlled by viscous creep in blueschists and that deformation is accommodated in large part by glaucophane, a sodic amphibole. However, there is currently a lack of any rheological parameters for the viscous deformation of glaucophane, and thus blueschist facies rocks cannot be incorporated into geologic or geodynamic models of subduction zones. This knowledge gap is due to a dearth of experimental studies on creep in blueschist facies rocks and exists in part because of the challenges associated with accessing viscous deformation in hydrous minerals in the laboratory. A Griggs-type high pressure high temperature deformation apparatus at MIT’s Rock Deformation Laboratory is capable of producing viscous creep in glaucophane by utilizing predominantly single-phase aggregates. The mechanical results of these experiments provide key rheological parameters and a flow law for creep in blueschist facies rocks during subduction. Microstructural analyses of experimental products, done at the University of Washington’s Structural Petrology Laboratory, including high-resolution electron beam imaging and electron backscattered diffraction (EBSD), link these mechanical results to deformation mechanisms (e.g., dislocation creep or diffusion creep) and provide a bridge between the experimental results and observations from the rock record. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.