ABSTRACT Airborne radiometric (gamma-ray) data provide estimates of the concentrations of potassium (K), thorium (Th), and uranium (U) in soil, regolith, and bedrock. Radiometric data constitute an important source of geochemical information, commonly used in mineral exploration and for geological mapping of Earth and other planets. Airborne radiometric data have rarely been applied to the exploration and analyses of impact structures, in contrast with other conventional geophysical tools (e.g., gravimetry, magnetism, and seismic reflection/refraction). This work represents the first systematic survey of the K, Th, and U radiometric signatures of Australian impact structures, based on the continent-wide airborne radiometric coverage of Australia. We first formulated several hypotheses regarding the possible causes of formation of circular radiometric patterns associated with impact structures. Then, the radiometric signatures of 17 exposed impact structures in Australia were documented. Our observations confirmed the supposition that impact structures are commonly associated with circular radiometric patterns. We then selected the five structures with the most prominent circular radiometric patterns (Gosses Bluff, Lawn Hill, Acraman, Spider, and Shoemaker), and we discuss the possible origin of these anomalies. Based on these five case studies, we argue that such patterns result from either crustal deformation induced by the impact event and/or from postimpact superficial processes controlled by the crater topography. This work also suggests that airborne radiometric data may be useful, in combination with other geophysical tools, in the search for new possible impact structures.

Abstract New zircon and apatite fission-track ages obtained on samples from all lithotectonic units exposed on Naxos Island are presented. Zircon ages of the exhumed metamorphic rocks range from 25.2 to 9.3 Ma and from 13.0 to 6.4 Ma for apatite. Zircon track-length analysis distinguishes partial overprinting of an earlier event (M 1 ) in the south. Northwards no overprint is seen and the ages there represent rapid exhumation since c. 12 Ma. Both zircon and apatite ages are slightly older toward the north of the island probably due to variation of the geotherm in the proximity of the fault. Zircon fission-track ages of the granodiorite range from 13.7 to 12.2 Ma are statistically identical to previously determined U–Pb ages. Apatite fission-track ages however, yield a younging trend from south to north from 12.9 to 9.0 Ma. This could be due to differential depth of emplacement and/or to differential exhumation during tectonic unroofing by a top-to-the north detachment. Fission-track ages on detrital grains in Lower Miocene sediments indicate a source not identified within the present outcropping rocks of Naxos. Ages on boulders and grains in the Middle to Upper Miocene sediments point to rapid erosion until about 8.5–7 Ma.

Gneiss domes are ubiquitous structures in all exhumed orogens, and their formation represents a first order thermal-tectonic process that has operated from the Archean to the present. The vertical flow of crust to create domal structures is a significant factor in the redistribution of heat and material in orogens and therefore in the evolution of continents. Worldwide, gneiss domes display many similarities in geometry (aspect ratio), petrology, and structure, and these similarities transcend differences in tectonic setting. Gneiss domes are cored by high-grade metamorphic rocks (including migmatite) ± granitoids, and the core rocks commonly record a component of isothermal decompression, in contrast to mantling schists, and may exhibit a late, low-pressure–high-temperature metamorphic assemblage. Rapid cooling typically follows isothermal decompression, as hot rocks are rapidly emplaced at higher structural levels. Most gneiss domes are elongate parallel to the strike of the orogen. Domes with long dimension ≤90 km have a ratio of long to short axes of ∼2:1–3:1. The elliptical shape of gneiss domes worldwide suggests that their morphology, and therefore genesis, is controlled by crustal flow dynamics, including the magnitude of vertical versus lateral crustal flow. The conditions and mechanisms involved in dome formation inform the relative rates of vertical and lateral crustal flow during orogeny.

This paper presents new structural data on the Naxos migmatite dome, exhumed in the central part of the Aegean Sea in Greece. The dome is cored by anatectic granites and migmatites that have preserved magmatic textures, and it is mantled by a dominantly metasedimentary sequence grading outward from amphibolite to greenschist facies. The elliptical shape of the dome is outlined by a composite transposition foliation in the mantling metasedimentary sequence. The lineation trends NNE-SSW and is associated with top-to-the-NNE shearing. Within the first order dome, kilometer-scale second order domes are evidenced by the orientation of the magmatic fabric, of the syn-migmatitic foliation trends, and by a concentration of enclaves along their margins. A network of granitic veins, structurally rooted in the migmatites, intrudes the mantling metasedimentary sequence. Subvertical granitic dikes, discordant to the foliation, are dominantly oriented parallel or perpendicular to the lineation. These dikes have preserved a magmatic texture and cross-cut partially to totally transposed veins. Kinematic analysis indicates that transposition is consistent with top-to-the-NNE shearing combined with outward rotation of the veins in the mantling metasedimentary sequence during upward migration of the migmatites in the core of the dome. Accordingly, the Naxos migmatite dome is interpreted as a diapir formed in response to a gravitational instability developed in the buoyant, partially molten rocks in a context of regional NNE-SSW extension during gravitational collapse of the Ae gean orogenic wedge.