The Mechanics of Pseudotachylite Formation in Impact Events

Abstract

This paper presents a discussion of 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 one possible 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.3

Compactional deformation bands in Wingate Sandstone; additional evidence of an impact origin for Upheaval Dome, Utah

“…Indeed, pseudotachylites have not been reported at Upheaval Dome. Pseudotachylite can be expected to form along a 1 mm think shear zone that is longer than 3 cm to 30 cm and has a shear strain of 3% to 10% [56]. The compactional deformation bands within the Wingate are typically several tens of centimeters in length and 1 mm to 3 mm in width.…”

The Modification Stage of Crater Formation

“…Reimold and Gibson, 2005;Lieger et al, 2009). Moreover, although the total displacements and slip rates necessary to generate melt along a fault are easily achieved during crater formation, the volume of friction melt expected is very small (less than a few volume per cent) compared with the volume of collapsing material (Melosh, 2005). Hence, whether or not sufficient friction melt can be formed to lubricate crater collapse remains uncertain.…”

Structural characteristics of the Sudbury impact structure, Canada: Impact-induced versus orogenic deformation-A review

“…Also, the mechanism(s) that produced the large thickness of clast-rich pseudotachylite zones remains to be identified. Single-slip events are unlikely to account for the observed thicknesses of pseudotachylite, as shear-induced melting on a shear fracture would reduce its friction and thus inhibit further melting by this process (Melosh 2005). As high slip rates over a longer time interval seem necessary to produce thick pseudotachylite zones, it is conceivable that the localized generation of clastrich pseudotachylite is genetically related to acoustic fluidization.…”