Strong amphibole fabric development during diffusion creep of hydrated mafic rocks
Abstract
Numerous observations of naturally deformed, hydrated mafic rocks show that amphibole is a common reaction product in the lower continental crust. Although plagioclase lattice preferred orientation (LPO) in these lithologies may be weak, amphibole LPO is usually strong, suggesting that dislocation creep was the active deformation mechanism. However, the lack of internally strained amphibole grains coupled with the lack of strong fabric in plagioclase suggests that diffusion creep is the dominant deformation mechanism. To examine the processes associated with synkinematic amphibole fabric development, we conducted general shear deformation experiments on powdered basalt with 1 wt% H2O added at pressure and temperature conditions similar to those at the base of the continental crust and within the amphibole stability field (800° C, 1 GPa). Similar to observations from natural shear zones, amphibole forms via the following hydration reaction: plagioclase + pyroxene + H2O = amphibole + less calcic plagioclase. Amphibole formed under hydrostatic conditions exhibits a random LPO, and amphibole growing during deformation exhibits both a strong shape preferred orientation (SPO) and LPO with a [001] maximum aligned sub-parallel to the shear direction. Plagioclase in both hydrostatic and deformed samples exhibits a very weak to random LPO. At low strain rates (10-6 to 10-7/s), strain rate stepping experiments indicate a stress exponent n = ~0.9 to ~1.4, consistent with diffusion creep. Because amphibole grains are elongate parallel to the [001] axis (i.e., parallel to the SPO), we hypothesize that the LPO results from rigid grain rotation and growth of amphibole during deformation. While this interpretation has previously been suggested as a way to produce amphibole LPO in natural shear zones, no experimental investigations have demonstrated this mechanism for producing strong fabric in syntectonically-generated amphibole. These experiments replicate not only the amphibole-forming hydration reactions but also the plagioclase and amphibole fabric observed in naturally deformed, mafic lithologies. Understanding the deformation mechanisms associated with lithologies dominated by amphibole/plagioclase rheology is important for the extrapolation of flow laws to the conditions of the lower continental crust. To first order, both our laboratory and field analyses indicate that plagioclase rheology provides a reasonable rheological constraint of the lower crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T51G..07G
- Keywords:
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- 8159 TECTONOPHYSICS Rheology: crust and lithosphere;
- 3902 MINERAL PHYSICS Creep and deformation;
- 8045 STRUCTURAL GEOLOGY Role of fluids