Transient and Disruption Cavity Dimensions of Complex Terrestrial Impact Structures Derived from Magnetic Data

Introduction: Transient and disruption cavity sizes are fundamental parameters in the study of impact structures [1]. They are governed by the velocity, size and density of the impacting body, the target density and ambient gravitational acceleration. Hence, knowledge of transient and disruption cavity dimensions allows the calculation of energy release associated with impact. Since the recognition of the devastating environmental effects related to the Chicxulub crater it has been underscored that accurate estimates of transient and disruption cavity dimensions are crucial in evaluating potential environmental degradation from an impact. Cavity dimensions: For simple craters, the transient and disruption cavities are closely related to the final crater shape, both being well-described by paraboloids of revolution. The transient cavity is the melt-and breccia-lined cavity that collapses to form the final observed simple crater form. The final observed crater diameter in a simple crater corresponds to that of the disruption cavity; the limit of disruption of the impacted rocks by brecciation. The transient cavity diameter D t = 0.84D [2], where D is the final crater diameter (and equals disruption cavity diameter D d). For complex craters, the difference between D t, , D d , and D is greater because of the extensive collapse and slumping of the disruption cavity walls and rim. The resulting craterform is highly modified compared to smaller, simple craters and obscures the direct observation of disruption cavity size based on either geological or geophysical data. Nevertheless, bounds on the collapsed disruption cavity (CDC) can be established by determining the size of certain morphological elements of the complex crater form [3]. If the disruption cavity size can be established, the diameter of the transient cavity follows by calculation. The diameter of the central uplift, D CU , provides a lower bound on the diameter of the CDC. D CU gives only a weak bound on CDC because crater floor rebound does not extend outward as far as the CDC edge. Estimates of D CU are also strongly influenced by erosion level. A stronger bound comes from D M , the diameter of the thick coherent melt sheet or suevite deposits that occupy the crater floor out to the inner edge of the zone of slumped blocks. This definition of D M does not include melt/suevite that may be present