Abstract
AbstractLarge impact structures have complex morphologies, with zones of structural uplift that can be expressed topographically as central peaks and/or peak rings internal to the crater rim. The formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization. Here, samples of shock‐metamorphosed quartz‐bearing lithologies at the West Clearwater Lake impact structure, Canada, are used to estimate the maximum recorded shock pressures in three dimensions across the crater. These measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift. The distribution of peak shock pressures, together with apparent crater morphology and geological observations, is compared with numerical impact simulations to constrain parameters used in the block‐model implementation of acoustic fluidization. The numerical simulations produce craters that are consistent with morphological and geological observations. The results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse, and should be included in future implementations. Based on the comparison between observational data and impact simulations, we conclude that the West Clearwater Lake structure had an original rim (final crater) diameter of 35–40 km and has since experienced up to ~2 km of differential erosion.
Highlights
Complex crater formation requires a significant and transient reduction in the strength of target rocks compared to their quasistatic strengths (Melosh 1989)
Macroscopic Fieldwork in summer 2014 demonstrated that the stratigraphy preserved at West Clearwater Lake consists of (a) fractured basement, (b) monomict lithic breccia, (c) impact-melt-bearing lithic breccia, (d) clast-rich finegrained impact-melt rock, (e) clast-poor fine-grained impact-melt rock, and (f) clast-poor medium-grained impact-melt rock (Osinski et al 2015), similar to the identified stratigraphies of Bostock (1969) and Simonds et al (1978)
Shock pressure estimates from observations of planar deformation features (PDFs) in quartz-bearing lithologies at West Clearwater Lake show that peak shock pressures are fairly constant across the central uplift, with a maximum value in the center of the structure of at least 17.5 GPa
Summary
Complex crater formation requires a significant and transient reduction in the strength of target rocks compared to their quasistatic strengths (Melosh 1989). The effective strength required for the formation of central uplifts and crater collapse must be less than ~3 MPa and there must be little or no internal friction (Melosh 1977; McKinnon 1978; O’Keefe and Ahrens 1999; Wu€nnemann and Ivanov 2003). These properties are inconsistent with laboratory measurements of the properties of rocks and even those of damaged rocks. A release of overburden pressure due to vibrations in the material would be expected to cause
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