Three types of glass-indurated quartzose gouge are recognized (optically and with SEM) along the sliding surfaces of 29° to 45° precut specimens of dry Tennessee Sandstone, deformed at 0.14- to 5.0-kb confining pressure, shortening rates of 10 to 10−4/sec, 24° to 410°C, and with displacements <1.0 cm. “Welded clumps” form in tests at 24° and ≤1.0-kb confining pressure. The welded portions of each clump are composed of quartz fragments indurated by an optically isotropic matrix (index of refraction of 1.500 to 1.520) that supports brittle fracture and is probably glass. As the temperature of the experiments is increased, “fibrous patches” become widespread. These patches are vesicular and conspicuously striated, they stand out in optical relief on top of quartz grains, they contain ordered microfractures that suggest extension along the sliding direction, and their “lee” edges are marked by very fine fibers that emanate from tapered bodies rooting in the patches. At high pressures and low temperatures, “welded plates” cover much of the sliding surfaces. These are fractured and striated, but nonfibrous. The glass indicates that local silica–fusion temperatures occurred during frictional sliding. The change in the nature of the gouge is accompanied by a change in sliding mode from stick-slip (25°C) to episodic (150° to 250°C) to stable sliding (410°C), and by a progressive increase in the coefficient of sliding friction from 0.58 at 25°C, to 0.72 at 410°C.

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