What Locks Subduction Thrusts?

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Under this award, the PI will examine a well-documented Eocene- Paleocene rock complex from a paleo subduction thrust, now exposed in Alaska, in order to quantify and understand the differences, at a variety of length scales, between the upper aseismic parts of a subduction trust and the lower seismogenic parts. This study will test the hypothesis that deposition of minerals in cracks strengthens and locks subduction thrusts in the periods between major eatthquakes. During field work, the PI and a PhD student will document changes in structural fabric and mineral veins of various generations. Later lab studies will look at changes in physical properties, and mineralogy under similar PT conditions as presently exist in the SW Japan subduction zone. Quantification of shear zone paramenters, changes in mineralogy, intensity of pressure solution, density and porosity, and size/shape of blocks and matrix, will allow interpretations of the importance of simple sliding (aseismic) vs. stick-slip (seismic) behavior. Further, the results of the lab experiments can be used to interpret textures and structural features indicative of sliding, stick-slip, and heating. These observations and measurements, including fluid inclusion studies with Vrolijk, will be useful for testing ideas about velocity weakening in actural subduction zones, providing guidance for experimentalists on what types of materials are best for deformation studies, a preview of what will be seen in riser drilling at Nankai, and 10-100m scale constraints on interpreting 3D seismic studies in subduction complexes.