Complex crater

Abstract: A major tool in the initial recognition and study of terrestrial impact craters, ∼20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl-shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of ∼20–30 mGal at diameters D of ∼20–30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures ( D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to 40 km) tend to exhibit central high-amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence.

Abstract: Impact-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.