Shoemaker impact structure, Western Australia


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The Shoemaker impact structure, on the southern margin of the Palaeoproterozoic Earaheedy Basin, with an outer diameter of ∼30 km, consists of two well-defined concentric ring structures surrounding a granitoid basement uplift. The concentric structures, including a ring syncline and a ring anticline, formed in sedimentary rocks of the Earaheedy Group. In addition, aeromagnetic and geological field observations suggest that Shoemaker is a deeply eroded structure. The central 12 km-diameter uplift consists of fractured Archaean basement granitoids of syenitic composition (Teague Granite). Shock-metamorphic features include shatter cones in sedimentary rocks and planar deformation features in quartz crystals of the Teague Granite. Universal-stage analysis of 51 sets of planar deformation features in 18 quartz grains indicate dominance of sets parallel to ω {1013}, but absence of sets parallel to π {1012}, implying peak shock pressures in the range of 10–20 GPa for the analysed sample. Geophysical characteristics of the structure include a −100 µs−2 gravity anomaly coincident with the central uplift and positive circular trends in both magnetic and gravity correlating with the inner ring syncline and outer ring anticline. The Teague Granite is dominated by albite–quartz–K-feldspar with subordinate amounts of alkali pyroxene. The alkali-rich syenitic composition suggests it could either represent a member of the Late Archaean plutonic suite or the product of alkali metasomatism related to impact-generated hydrothermal activity. In places, the Teague Granite exhibits partial to pervasive silicification and contains hydrothermal minerals, including amphibole, garnet, sericite and prehnite. Recent isotopic age studies of the Teague Granite suggest an older age limit of ca 1300 Ma (Ar–Ar on K-feldspar) and a younger age limit of ca 568 Ma (K–Ar on illite–smectite). The significance of the K–Ar age of 568 Ma is not clear, and it might represent either hydrothermal activity triggered by impact-related energy or a possible resetting by tectonothermal events in the region.