WIRGO in TIC's? [What (on Earth) is Really Going on in Terrestrial Impact Craters?]

Canada is well endowed with impact craters formed in crystalline rocks with relatively homogeneous physical properties. They exhibit all the main morphological-structural variations with crater size seen in craters on other rocky planets, from small simple bowl to large peak and ring forms. Lacking stratigraphy, analysis is based on the imprint of shock melting and metamorphism, the position of the GPL (limit of initial Grady-Kipp fracturing due to shock wave reverberations) relative to shock level, the geometry of late stage shears and breccias and the volume of shocked material beyond the GPL. Simple craters, exemplified by Brent (D = 3.7 km) allow direct comparison with models and experimental data. Results of interest include: 1. The central pool of impact melt and underlying breccia at the base of the crater fill is interpreted as the remnant of the transient crater lining; 2. The overlying main mass of breccias filling the final apparent crater results from latestage slumping of large slabs bounded by a primary shear surface that conforms to a sphere segment of radius, r_s approx. = 2d_tc, where d_tc is the transient crater depth; 3. The foot of the primary shear intersects above the GPL at the centre of the melt pool and the rapid emplacement of slumped slabs produces further brecciation while suppressing any tendency for the centre to rise. In the autochthonous breccias below the melt and in the underlying para-allochthone below the GPL, shock metamorphism weakens with depth. The apparent attenuation of the shock pulse can be compared with experimentally derived rates of attenuation to give a measure of displacements down axis and estimates of the size of a nominal bolide of given velocity, the volume of impact melt and the energy released on impact. In larger complex craters (e.g. Charlevoix, D = 52 km) apparent shock attenuation is low near the centre but is higher towards the margin. The inflection point marks the change from uplift of deep material in the centre to subsidence of near-surface material at the margins. From the observed general relationship P_GPL = 3.5 D^0.5, where P_GPL (in GPa) is the estimated level of shock metamorphism at the Grady-Kipp fracture limit, it is apparent that the differential stress due to shock wave reflections weakens at about twice the attenuation rate of the initial shock pulse. Thus, with increasing size, compression of the para-authochthone below the GPL plays an increasingly larger role in controlling the depth of the transient crater and hence the radius of the primary shear. It follows that, where the rate of relaxation of the para-authochthone is more rapid than the propagation of the primary shear from the rim towards the centre, the shear surface intersects below the GPL and central uplift occurs.