Deformation of Franciscan blueschists and eclogites at depth in a subduction zone

Samantha Carruthers, McGill University
Christie Rowe, McGill University

Subduction zones produce large earthquakes and tectonic tremor, and the dynamics of these plate boundaries control the rates of plate tectonics. However, the stress conditions at depth cannot be directly observed, so are poorly understood. Blueschist and eclogite form at depths of >30 km in the subduction zone, and if exhumed, can be used to study stress and temperature conditions close to the plate interface. Due to subduction depth it is impossible to study this in an active environment, so we rely on exhumed rocks from the Franciscan Complex. By developing a detailed deformation history tied to the metamorphic assemblages, I aim to identify when and where the subducting slab deformed and tie that to stress conditions on the deep subduction interface. These blocks have been well-characterized in metamorphic studies, but since they occur in mélange terranes, past researchers have not made use of structural information. To our knowledge, this will be the first study to use high grade blocks from the Franciscan to research the structural aspects of subduction dynamics.

I will conduct fieldwork in two locations: Tiburon Peninsula and Jenner Headlands, CA. The blocks are composed of blueschist and eclogite, some bearing garnet and lawsonite that have been dated using Lu-Hf geochronology (Mulcahy et al., 2009; 2014). The individual blocks cannot be connected spatially, however if they deformed under the same P-T conditions we will be able to determine that they show us equivalent parts of the subduction system. I am developing a novel mapping method using a unique internal reference frame for each individual boulder to reveal 3-D rotations in the strain ellipse within each reference frame. I will build 3D models from 100s of field photos using AGIsoft Photoscan Pro software. I will use georeferenced orthophotos projected from the 3D model to create a unique reference frame for each of the boulders to document the structural fabrics. In order to relate the measured strain to paleo-stress along the Franciscan subduction zone, I will measure yield strength of my samples in the rock mechanics lab at Lamont-Doherty Earth Observatory. If we are able to successfully study these samples with our new approach, it will create opportunities to obtain structural data and information from mélanges.


Development of tectonic microstructures