Dynamic fault weakening and the formation of large impact craters

摘要

Impact craters are the most common landform on planetary surfaces; however, the mechanics of the end stages of their formation are not fully understood. The final stage of crater formation involves the collapse of a hemispherical transient cavity. Around small craters, the limited amount of collapse preserves a bowl-shaped cavity. In contrast, the observed shallow depths and complex inner morphologies of large craters require very low shear strength in the collapsing material. Because the observed amount of collapse cannot be reproduced using quasi-static values for the frictional strength of fractured rock, a temporary weakening mechanism is necessary. Here, we investigate the hypothesis that craters collapse along a network of impact-generated faults that weaken during long displacements at high slip velocities via, for example, frictional melting. Using the CTH shock physics code, we simulate the formation of about 100-km diameter impact craters using a simple strain-rate weakening model with parameters constrained by fault friction experiments on crystalline rocks. The model reduces the coefficient of friction from a quasi-static value (0.6-0.85) to a weakened value (0.1-0.2) when a parcel of fractured material exceeds thresholds for cumulative plastic shear strain (a proxy for slip distance) and shear strain rate (a proxy for slip velocity). During crater formation, the strain-rate weakening model leads to strain localizations that are interpreted to be fault zones. Fault zones are spontaneously created and slip over discrete time intervals during collapse. The strain-rate weakening model reproduces the major geologic features observed around the largest terrestrial craters (Vredefort, Sudbury, and Chicxulub), including shallow depths, fault structures, frictional melt distributions, and deep-seated central uplifts. The good agreement between calculations and observations supports the hypothesis that small volumes of transiently weakened material in fault zones control the collapse of large impact craters.