Damage Zones in Ultramafic Rocks: Grain Size Reduction through Hydration Over Cataclasis

Montserrat De Allende Silva, University of Southern California
Noah Phillips, University of Southern California
Alex Lusk, U.S. Geological Survey
Julie Newman, Texas A&M University
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Abstract

Many fault properties, including strength, fluid content, pore fluid pressure, and permeability structure, are controlled (or influenced) by the deformed and fractured rock surrounding the fault core, known as the damage zone. While damage zones in sedimentary and felsic igneous rocks have been described extensively, they are not well characterized in ultramafic rocks. Ultramafic rocks are found in a wide variety of tectonic settings, including subduction zones, exhumed ophiolites, hyper-extended rift systems, oceanic core complexes, and along major plate boundaries. Therefore, characterizing damage zone structure in ultramafic lithologies is crucial to understanding deformation and fluid transport in these environments. Because ultramafic rocks are especially susceptible to fluid-rock reactions and readily undergo serpentinization, we hypothesize that their damage zone structures differ significantly from those in more commonly studied lithologies. The Twin Sisters Complex in the North Cascades provides a unique natural laboratory for this work. This ~16 km-long, 5–6 km-wide mountain range is a large body of mostly unaltered dunnite and harzburgite, except along its margins and within thin cross-cutting faults which are extensively serpentinized to lizardite. We collected samples along a transect beginning at the western serpentinized margin and extending inward towards unaltered ultramafics (i.e., with increasing distance from the fault core). Our observations show that serpentinized ultramafic damage zones contain significant amounts of chemically-bound water (up to 11%), show increased grain rounding, and a stronger shape-preferred orientation with increasing proximity to the fault core. In addition, grain size distributions in the most damaged samples do not follow a power-law relationship and show decreasing log-linearity with proximity to the fault core, indicating that grain size reduction through chemical reactions may be more prevalent than grain size reduction through cataclasis. Chemical reactions hydrate ultramafic fault damage zones and may allow for more effective fluid transport during tectonic processes (e.g., transporting water into the mantle during subduction) than fault damage zones in other lithologies.

Session

Deformation in the upper crust