HR: 10:20h
AN: P31C-01    [PDF]
TI: The Mechanics of Pseudotachylite Formation in Impacts
AU: * Melosh, H J
EM: jmelosh@lpl.arizona.edu
AF: Lunar and Planetary Lab, University of Arizona, Tucson, AZ 85721 United States
AB: Geologist James Shand first recognized pseudotachylites near the Vredefort structure in 1916. They appear to be black, glassy veins that often contain broken fragments of country rock. The veins range from millimeters thick to masses many meters in extent. Since this discovery they have puzzled several generations of geologists. Pseudotachylites are evidently due to rapid melting of rock in place and seem to be associated with environments, such as faulting, landslides and impacts, where rock is put into rapid motion. I examine the basic constraints controlling the formation of pseudotachylites in the rapidly sheared rocks in the vicinity of a large meteorite impact. The prevailing opinion among many geologists is that pseudotachylites are formed by friction melting of rock. The principal mystery of pseudotachylite formation is not that friction can cause melting, but that it seems to form thick masses of it. Yet such thick masses ought to preclude melting by reducing the friction between sliding rock masses. I propose that a solution to this conundrum is that the melt produced by sliding on narrow shear zones is extruded into the adjacent country rock, thus keeping the sliding surfaces narrow while thick masses of melt accumulate in pockets opened by slip on faults oriented at large angles to the sliding plane.
DE: 5104 Fracture and flow
DE: 5420 Impact phenomena (includes cratering)
DE: 8010 Fractures and faults
DE: 8159 Rheology--crust and lithosphere
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting