Abstract

The distribution of particular shock metamorphic effects has been determined in the central uplift of the Charlevoix impact structure. Planar deformation features in quartz occur as much as 10 km from the central peak (Mont des Eboulements), whereas equivalent shock features in K-feldspar are restricted to within 2 km of the crater center. Weak planar features in K-feldspar are narrow (≤ 1 µm) but can become broad (4 to 8 µm) deformation twins in_more highly shocked samples. Common orientations are (24 (see PDF for formula)), ( (see PDF for formula) 41), and (110) in orthoclase, and (13 (see PDF for formula)) and (110) in microcline, and the relative abundance of specific orientations does not change with shock level. Film perthite lamellae in K-feldspar break down to spindle microperthite within 6 km of the center, either as a result of shock or as a function of original depth of burial.

Shock pressures were estimated for Charlevoix samples by equating observed planar feature development with experimental data. For example, type A shocked quartz develops above 7.5 GNm−2, type B above 10 GNm−2, type C above 14 GNm−2, and type D above 16 GNm−2. Maximum shock levels preserved on the central peak resulted from an estimated 22.5 GNm−2. Shock level contours at 5-GNm−2 intervals are broadly concentric with Mont des Eboulements.

The transient cavity of the Charlevoix impact was reconstructed with a radius of 13.5 ± 2 km and a depth of 9.5 ± 1.5 km (r√2). The distribution of peak shock pressure levels in basement rocks beneath the transient cavity was calculated from theoretical pressure attenuation rates. Using a model for central uplift formation, these rocks were elevated to positions underlying the central uplift. The resulting configuration of shock pressure zones agrees with the surface expression of shock zones mapped from planar features and confirms the validity of current theories of cratering and shock pressure attenuation.

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