What Goes Down Must Come Up: The Structural History of an Exhumed High Grade Block in Jenner, California
Samantha Carruthers, Department of Earth and Planetary Sciences, McGill University, Montreal, QC, Canada
Christie Rowe, Department of Earth and Planetary Sciences, McGill University, Montreal, QC, Canada
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Exhumed blueschist and eclogite rocks provide us with an opportunity to learn more about the conditions along the slab interface during Franciscan subduction. Since we are unable to directly observe the processes occurring at depth, we rely on data that the rocks record to understand their structural history. The deformation of lower plate metabasite during subduction and exhumation plays a poorly understood but potentially important role in accommodating plate convergence and controlling the strength of the subduction interface. I am analyzing a high-grade block composed of blueschist and eclogite from Jenner, CA using field and laboratory techniques to better understand the timing and manner in which this block deformed during subduction and subsequent exhumation. The block is predominantly composed of glaucophane (blueschist) which defines a foliation throughout and is compositionally banded with mica-rich eclogite. Cm-scale eclogite lenses are present in one unit and often display brittle tensile cracks that have later been filled with carbonate. Garnets are present in every unit and often have multiple rims with inclusions that preserve previous foliations which can be seen in BSE images. I have used a technique that includes micromapping these features onto photos of the block to better understand visible cross-cutting relationships in the field, as well as laboratory analyses to characterize equilibrium assemblages that correspond to deformational features. By linking P/T estimates from metamorphic modeling with changes in deformational environment recorded by the fabrics and foliations, I will test the potential for preservation of syn-subduction strain in the subducting slab, and potentially shed light on deformational regimes during exhumation that may be relevant to constraining the mechanism of exhumation of the Franciscan high-grade blocks.