GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 142-1
Presentation Time: 1:30 PM


CONDIT, Cailey B., Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, MAHAN, Kevin H., Department of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309-0399, PEC, Matej, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave, 54-720, Cambridge, MA 02139 and CHIN, Emily J., Earth Science, Rice University, 6100 Main Street, MS 126, Houston, TX 77005

Minerals of the amphibole group are common in a wide variety of crustal tectonic settings from subduction and collision zones to magmatic arcs, transform faults, and rifts, making their deformation behavior a significant contributor to deep crustal rheology. Amphibole in shear zone tectonites often exhibit strong CPO and SPO that is kinematically compatible with shear zone development. In many minerals, CPOs are linked to dislocation creep. However, because of its long Burgers vector, amphibole should require relatively high stresses to deform by this mechanism. This would apparently preclude it from acting as a strain localizing phase if dislocation creep were dominant in the deep crust. Furthermore, naturally deformed amphibole, even with well-developed CPO and SPO, commonly does not exhibit other obvious dislocation creep-related microstructures, revealing a marked discrepancy between CPO, deformation mechanisms, and amphibole rheology.

We present observations from clinoamphibole-rich (hornblende) deep crustal shear zones suggesting that diffusion creep mechanisms and synkinematic metamorphic growth led to strain localization. It is these mechanisms, rather than dislocation creep, that produced a weak rheology and amphibole CPO and SPO. In these naturally deformed rocks, fluid infiltration was coeval with deformation and catalyzed metamorphic reactions from dry host into hydrated amphibole-rich tectonite. We combine these natural observations with deformation experiments of fine grained (<10 um) amphibole aggregates to quantify their rheology. Experiments were performed in a Griggs-type deformation apparatus at conditions of 750-850 °C and 1.5 GPa. Our results indicate dislocation related deformation at 750° C and high strain rates (10-3-4 S-1, 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. EBSD data show a moderate CPO with little inter-grain deformation consistent with passive rotation of elongated amphibole as the CPO-producing mechanism. Our experimental results suggest amphibole can be quite strong at moderate temperatures but weakens at high temperatures and slow strain rates, when diffusion creep dominates.