Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada |
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| Authors: | Burkhard O Dressler Virgil L Sharpton Benjamin C Schuraytz |
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| Institution: | (1) Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas, 77058-1113, USA, US;(2) Planetary Science Branch, SN4, NASA Johnson Space Center, Houston, Texas 77058, USA, US |
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| Abstract: | The Slate Islands archipelago is believed to represent the central uplifted portion of a complex impact structure. Planar
microstructures in quartz and feldspars and shock vitrification of rocks are the most common shock metamorphic features encountered.
No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Shatter cones occur on all islands
of the archipelago suggesting minimum pressures of 4 ± 2 GPa. The relative frequency of low index planar microstructures of
specific, optically determined crystallographic orientations in quartz are correlated with results from shock barometric experiments
to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure
recorded in the target rocks from about 20–25 GPa in east-central Patterson Island to about 5–10 GPa at the western shore
of this island and on Mortimer Island. The shock attenuation gradient is ∼4.5 GPa/km across this section of the island group.
However, the shock attenuation has a roughly concentric plan only over the western part of the archipelago. There is no distinct
shock center and there are other deviations from circularity. This is probably the result of: (1) the shock wave not having
expanded from a point or spherical source because of the ∼1. 0 to 1.5 km size of the impactor; (2) differential movement of
large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of
planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure
exerts the primary control on microstructure development. Based on the results of XRD analyses, residual, post-impact temperatures
were high enough to cause annealing of submicroscopic damage in shocked quartz.
Received: 15 July 1997 / Accepted: 6 October 1997 |
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