FOLIATION ORIENTATION AND STRAIN RATE CONTROLS ON STRENGTH OF PELITE DURING PARTIAL MELTING AND DEFORMATION

The rheological consequences of partial melting during deformation may result in either brittle or ductile behavior. This depends on such variables as strain rate, rate of pore pressure development, temperature, pressure and composition. Crustal rocks are commonly foliated by the alignment of mica which may also influence the nature of response to deformation. On an experimental scale, theses variables can be controlled and observed, using a Griggs rock deformation apparatus. Experiments using two-mica metapelite cores were performed to test strength and deformation mechanisms at conditions above and below the solidus of muscovite. In general, we observe a melt pore pressure increase above the muscovite-solidus, which causes embrittlement in melted areas and produces a zone of fractured grains and glass. Experiments 2M-D35 (26% strain) and 2M-D25 (19% strain) were performed above the muscovite solidus, at the same conditions (740^oC, 0.7GPa) but 2M-D35 was deformed at a slower strain rate, 10^-6. 2M-D35 has produced a greater melt fraction and cataclastic flow was observed. These experiments have been done in particular to observe if cataclasis was induced at the slower strain rate which would help confirm the large melt pore pressure effect during muscovite breakdown. The influence of the foliation orientation was expected to affect the sample by causing greater weakness when the foliation is oriented parallel to the direction of applied pressure (s₁). The parallel foliation allows micas to kink easily in response to applied pressure. Foliation control experiments 2M-D36, 2M-D37 (perpendicular) and 2M-D31, 2M-D23 (parallel) were conducted at 650^oC (below the solidus) and at 740^oC (supersolidus conditions) and 0.7 GPa. The subsolidus experiments' maximum differential stress ranged between 175 MPa and 225 MPa. The parallel experiment reached 225 MPa differential stress. With melt present, both experiments had a similar strength (<50 MPa maximum differential stress). This shows that mica orientation alone does not control the rock's strength.