THE INFLUENCE OF ATTENUATION ON THE STRUCTURE OF HYPERVELOCITY IMPACT CRATERS (Invited Presentation)

The application of shock metamorphism and rates of shock wave attenuation to structural analysis of impact craters began in the mid-1960s. It was evident that, if actual attenuation is known, apparent attenuation rates can be used to estimate the displacement of melt and breccias at the transient cavity stage and of central uplifts in complex craters. A consensus later developed that the size and shape of transient cavities scale in proportion to energy, implying that, for a given target, attenuation is also constant. The shock level at the base of the cavity was accepted as about 30 GPa. The function d= 0.1D was adopted linking transient cavity depth (d), to the final diameter (D), based on observations at some complex craters formed in sedimentary rocks.

However, these observations are selective and are in conflict with many others. Simple craters in coherent media have d = 0.3D and have central shock levels at the top of the parautochthone of only about 5-10 GPa. In complex craters shock levels range from 10 GPa in smaller craters to about 30 GPa in larger. A revised general formula for transient cavity depth in crystalline rocks can be calculated on the basis that: (1) the depth of the transient cavity is determined by attenuation of rarefaction waves that fragment the target until the pressure decays to the dynamic tensile strength of the target (about 200 MPa for crystalline rocks); (2) rim slumping to form peripheral troughs and terraces is progressively more extensive as size increases; (3) the exponent for attenuation in the far field averages -2.5. The proposed formula, by which d = 0.45 D^0.75, violates scale proportionality but provides a smooth transition from simple craters to complex craters of intermediate size. It converges towards d = 0.1D at large sizes. An alternative solution would accept variation in the far field attenuation rates. The data suggest an increase in rate from about -2 for small craters towards -3 for the largest for both the initial shock compression and the subsequent rarefaction waves originating at the free surface. Calculations show the direction of such a change is in accord with impact velocity increasing with bolide mass though greater overburden pressure in large craters may be a contributing factor.