TSK 11 Göttingen 2006 Kenkmann The fate of sandstone during impact cratering: shock com- paction, cataclastic flow, and granular fluidization Vortrag Thomas Kenkmann1 Impact of solid bodies is the most funda- mental of all processes that have taken place on the terrestrial planets in our Solar system (Shoemaker 1977). On Earth, impact cratering was the dom- inant geologic process during the pe- riod of the early heavy bombardment until 3.8Ga. A constant asteroid im- pact flux exists since that time. Al- though deformation of the crust by me- teorite impacts is now subordinate with respect to tectonics, it represents an important, but often underestimated fraction of the bulk crustal deforma- tion. Short-term deformation during hypervelocity impact events differs in many respects from standard tecton- ics: Unique conditions exist at pres- sures above the so-called Hugoniot elas- tic limit (HEL) of a particular mineral or rock. This state of compression is reached in a shock wave that propa- gates from the point of impact. Shock waves travel at supersonic velocity, heat and irreversibly deform the rock, and cause a residual motion of the mate- rial they have passed, which ultimately leads to the formation of parabolically shaped crater cavity of much larger ex- tent than the projectile diameter. At pressure above the HEL minerals are subjected to shock metamorphism. For instance, the HEL of quartz is in the range of 3–5GPa, pending on crystal- lographic orientation. With increasing 1 Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität Berlin, Invalidenstrasse 43, 10115 Berlin shock pressure, quartz first displays re- duction in refraction indices and bire- fringence (10–35GPa). Localized amor- phization occurs in this pressure inter- val along certain crystallographic di- rections and leads to the formation of so-called Planar Deformation Features (PDF) in which small stishovite crystals can form. PDFs are among the most important diagnostic features to prove the existence of ancient impact struc- tures. Above 35GPa quartz is com- pletely transformed to amorphous glass while keeping its initial shape (diaplec- tic glass). Clusters of coesite can grow within this solid-state glass. Above 50GPa quartz melted upon pressure re- lease, as indicated by the formation of schlieren and vesicles. Vaporiza- tion of quartz is completed at about 100GPa upon pressure release (Stöf- fler & Langenhorst 1994). The vol- ume of rock affected by shock metamor- phism increases unproportionally strong with increasing crater size. Since pres- sure decays with increasing distance from the point of impact, the zone of shocked rock is surrounded by a zone in which deformation by strong pres- sure waves occurs without indications of shock metamorphism. Deformation products of this area comprise catacla- sites, monomictic and polymictic brec- cias, dikes, pseudotachylites, and vari- ously faulted and folded rocks. The pe- riod of deformation during initial com- pression, and the subsequent excavation flow and crater modification at ambient pressure is in the order of seconds to minutes, pending on crater size. How- ever, even in this short period the physi- cal boundary conditions for deformation change in time and space and lead to a complex deformation path in which reversals in motion direction are com- 1 Kenkmann TSK 11 Göttingen 2006 mon. A general characteristic of de- formation during excavation is a radial symmetric, divergent, outward and up- ward directed flow. This is followed by an inward directed, convergent flow that dominates the gravity-driven crater col- lapse (Kenkman 2002). Here, the de- formation history of porous sandstone during impact cratering is presented. These rocks were investigated experi- mentally (Kenkman 2006), and in the deeply eroded Upheaval Dome impact crater, Utah (Kenkman 2003, Kenkman et al. 2005). We can distinguish three stages of deformation: (i) deformation above the HEL of quartz, (ii) defor- mation at confining pressure below the HEL, (iii) deformation at fluctuating ambient pressure during the excavation and modifications stage of the cratering process: i) Porous rocks behave differently in shock waves with respect to non-porous, dense rocks. Before the rock can be pervasively com- pressed, the pore space must col- lapse. Thus, a large amount of shock wave energy is spent for com- paction prior to compression. This leads to reduced shock magnitudes, unusually high target heating, and more rapid shock decay. Since volume and size of the crater de- pend on shock magnitude and the decay in shock pressure with dis- tance, the resulting crater will be smaller than a crater in a dense rock with the same bulk density. In weakly-shocked porous sand- stone (<10GPa) pore closure is ac- complished by brittle fracturing of grains, in moderately and strongly shocked rocks, pore space collapse is accomplished by jetting, the ex- trusion of melted streams of hot SiO2 material into the pores (Kief- fer et al. 1976). PDF formation is relatively rare in porous sandstone. Most of the shock metamorphosed rock volume will be ejected from the crater. ii) At pressures below the HEL shock metamorphism of quartz does not occur. But the attenuated shock wave still provides a considerable pressure and initiates a cataclas- tic flow within porous sandstones. Distributed cataclastic flow is de- fined as a microscopically brittle process in which a material’s co- herence is reduced by pervasive mi- crocracking that affects the entire rock. The distributed cataclastic flow in the sandstones is initiated by grain crushing, collapse of pore space, and subsequent intergran- ular shear. An important result of the deformation at high confin- ing pressure is that the cohesive sandstone is transformed to a non- cohesive sand by pervasive, delocal- ized intergranular cracking. Hence, further deformation is controlled by frictional properties rather than the fracture toughness of the rock. iii) It is well-known that the strength properties of rocks are temporar- ily strongly reduced after the shock wave has passed through the rocks. Disturbances of the shock wave lead to strong residual seismic noise and vibrations behind a shock wave; a process which is called acoustic flu- idization (Melosh 1979). The basic idea of acoustic fluidization is that seismic vibrations of grains, frag- ments, or blocks within the target result in fluctuations of the over- burden pressure, which leads to slip 2 TSK 11 Göttingen 2006 Kenkmann events in periods of low pressures and reduced frictional strength. In terms of rheology the fluidized rock can be described with the prop- erties of a Bingham plastic mate- rial (Melosh & Ivanov 1999), which is characterized by a linear vis- cous behavior after a yield strength is exceeded. Deformation at such fluctuating ambient pressures al- lows dilatancy and a rearrangement of grains in periods of unloading. Hence, the cataclastically deformed sandstones behave like unconsol- idated sand and most likely de- form in a pervasive granular flow. A macroscopic result of granular flow of sandstones during impact crater collapse is the formation of large sandstone dike networks, which occur, e.g. in the center of the Upheaval Dome impact crater, Utah. These dikes show extreme thickness variations, blind termina- tions and frequent embranchments at nodular-like points and indicate an almost complete loss of inter- nal coherence during deformation. Fluidized sandstone accommodates space incompatibilities that arise from the deformation of more com- petent target rocks. References Shoemaker EM (1977) In: Roddy DJ et al. (eds) Impact and explosion cratering, Perg- amon Press, New York, 1–10 Stöffler D & Langenhorst F (1994) Meteoritics, 29, 155–181 Kenkmann T 2002, Geology, 30, 231–234 Kenkmann T, Thoma K & the MEMIN team (2006) LPSC, 37, #1587 Kenkmann T (2003) EPSL, 214, 43–58 Kenkmann T, Jahn A, Scherler D & Ivanov, BA (2005) GSA Spec. Pap, 384 Kieffer SW, Phakey, DP & Christie, JM (1976) Contrib Min Petrol 59, 41–93 Melosh HJ, (1979) J Geophys Res 84, 7513– 7520 Melosh HJ & Ivanov BA (1999) Annu. Rev. Earth Planet Sci 27, 385–415 3