Abstract
AbstractImpact‐induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater. This paper describes a simple model of dilatancy—the creation of porosity in a shearing geological material—and its implementation in the iSALE shock physics code. The model is used to investigate impact‐induced dilatancy during simple and complex crater formation on Earth. Simulations of simple crater formation produce porosity distributions consistent with observations. Dilatancy model parameters appropriate for low‐quality rock masses give the best agreement with observation; more strongly dilatant behavior would require substantial postimpact porosity reduction. The tendency for rock to dilate less when shearing under high pressure is an important property of the model. Pressure suppresses impact‐induced dilatancy: in the shock wave, at depth beneath the crater floor, and in the convergent subcrater flow that forms the central uplift. Consequently, subsurface porosity distribution is a strong function of crater size, which is reflected in the inferred gravity anomaly. The Bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius; larger craters exhibit a central gravity high within a suppressed gravity low. Lower crustal pressures on the Moon relative to Earth imply that impact‐induced dilatancy is more effective on the Moon than Earth for the same size impact in an initially nonporous target. This difference may be mitigated by the presence of porosity in the lunar crust.
Highlights
One of the most important collateral effects of impact cratering on planetary surfaces is fracturing and fragmentation of the target rocks surrounding the crater
This paper describes a simple model of dilatancy—the creation of porosity in a shearing geological material—and its implementation in the iSALE shock physics code
This paper describes a simple, computationally efficient, semiempirical approach for including dilatancy in numerical impact simulations
Summary
One of the most important collateral effects of impact cratering on planetary surfaces is fracturing and fragmentation of the target rocks surrounding the crater. Impact-induced fracturing increases the porosity and permeability of the cratered target, which has important implications for postimpact hydrothermal activity [Kirsimäe and Osinski, 2012], fluid (e.g., hydrocarbon) migration [Grieve, 2005], and possible microbial colonization [Cockell et al, 2012]. Fracturing and brecciation are responsible for many aspects of the geophysical signature of an impact crater, including the most characteristic feature: a circular negative gravity anomaly centered over the crater [Pilkington and Grieve, 1992]. Most impact simulations do not predict correctly density changes beneath an impact crater, which has limited the scope for comparison of model results with geophysical data
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