T013-03
Integrating experimental and geologic observations to provide constraints on viscous rheology: assessing the role of calcic-amphibole in lithospheric strength

Tuesday, 8 December 2020: 05:43
Virtual
Cailey Brown Condit, University of Washington, Seattle, WA, United States, Matej Pec, Massachusetts Institute of Technology, EAPS, Cambridge, MA, United States, Kevin H Mahan, University of Colorado at Boulder, Geological Sciences, Boulder, CO, United States, Emily J Chin, UC San Diego, Scripps Institution of Oceanography, GRD, La Jolla, CA, United States, Teodora Mitroi, University of Colorado Boulder, Geological Sciences, Boulder, CO, United States and Cassandra Seltzer, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States
Recorded Presentation
Abstract:
Rheological constraints on viscous deformation often come by combining detailed experimental studies with observations of the naturally deformed rock record. We present a combination of experimental results and geologic observations of the strength of calcic-amphibole during viscous deformation. Amphibole, a common mineral group found in a wide variety of tectonic settings, can be a significant contributor to the viscous rheology of the deep lithosphere. Yet it’s behavior and strength remain poorly constrained.

Naturally deformed calcic-amphibole often exhibit strong crystallographic preferred orientations (CPOs) when acting as a strain localizing phase at deep crustal conditions. Often, CPO development is linked to deformation by dislocation creep. However, given the long Burgers vector of amphibole, dislocation creep likely only occurs at relatively high stresses. Calcic-amphibole with well-developed CPO typically lack obvious dislocation creep-related microstructures, revealing a marked discrepancy between CPO development, deformation mechanisms, and rheology.

We present observations from calcic-amphibole-rich deep crustal shear zones that indicate fluid-aided diffusion creep mechanisms and synkinematic metamorphic growth, rather than dislocation creep, produced a weak rheology and amphibole CPO and SPO. In contrast deformation experiments of fine grained (<10 μm) calcic-amphibole aggregates suggest amphibole is quite strong. Experiments were performed in a Griggs-type deformation apparatus at temperatures of 750-850 °C and pressures of 1.5 GPa. Our mechanical and microstructural results indicate dislocation related deformation at 750° C and high strain rates (10-3-4 s-1, stress exponent, n = 4-5), while at lower strain rates (<10-5 s-1), deformation is accommodated by diffusion mechanisms with a stress exponent ≤ 2. There is a marked reduction in strength with increasing temperature. Our experimental results suggest amphibole can be quite strong at moderate temperatures but weakens at high temperatures, small grain sizes and slow strain rates, when diffusion creep dominates. This comparative study demonstrates that in natural systems, temperature, fluid infiltration, and metamorphic reactions strongly influence viscous lithospheric rheology.

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