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The 1.85 Ga Sudbury impact structure is considered a remnant of a peak-ring or multi-ring basin with an estimated original diameter of 150 to 200 km. The Offset Dikes are radial and concentric dikes around the Sudbury Igneous Complex (SIC) and are composed of the so-called inclusion-rich Quartz Diorite (IQD) and inclusion-poor Quartz Diorite (QD), and in some Offset Dikes, Metabreccia (MTBX). We carried out a detailed field and analytical investigation of MTBX from the Parkin Offset Dike in the North Range of the Sudbury structure. Our observations suggest that MTBX represents impact breccia that originally formed underneath the Main Mass of the SIC and that was subsequently contact-metamorphosed and entrained during the emplacement of the Parkin Offset Dike. The MTBX bears no resemblance to the QD and IQD in which it is hosted, but it does share many similarities with Footwall Breccia (FWBX), suggesting that the two shared a similar initial origin. A genetic relationship between MTBX and FWBX is also supported by whole rock geochemical analyses. Key words: Sudbury, Metabreccia, Offset Dike, impact crater, recrystallization. La structure d'impact de Sudbury, vielle de 1,85 Ga, est considérée représenter le reste d'un bassin formé par l'anneau central ou plusieurs anneaux, de diamètre initial de 150 km à 200 km. Les apophyses sont des dykes radiaux ou concentriques autour du complexe igné de Sudbury (SIC) et sont composées de roches communément appelées diorite quartzifère riche en inclusions (IQD) et diorite quartzifère pauvre en inclusions (QD) et, dans quelques apophyses, une métabrèche (MTBX). Nous avons réalisé une étude analytique et de terrain détaillée de la MTBX de l'apophyse de Parkin dans la ceinture nord de la structure de Sudbury. Nos observations indiqueraient que la MTBX représente une brèche d'impact qui s'est formée initialement sous la masse principale du SIC et a par la suite subi un métamorphisme de contact et été entraînée durant la mise en place de l'apophyse de Parkin. La MTBX ne présente aucune similitude avec la QD et l'IQD qui l'encaissent, mais elle présente certaines similitudes avec la brèche de l'éponte inférieure (FWBX), ce qui donne à penser qu'elles auraient des origines similaires. Des analyses géochimiques sur roche totale appuient également l'interprétation d'une relation génétique entre la MTBX et la FWBX. [Traduit par la Rédaction] Mots-clés : Sudbury, métabrèche, apophyse, cratère d'impact, recristallisation., Introduction The 1.85 Ga (Krogh et al. 1984) Sudbury impact structure (Fig. 1), located in Ontario, Canada, is the remnant of a peak-ring or multi-ring basin (Stoffler et al. 1992; [...]

Schrodinger basin is a well-preserved peak-ring basin located on the lunar farside, along the rim of the much larger South Pole--Aitken (SPA) basin. The relatively young age (Lower Imbrian series, or 3.8 Ga) of this basin makes it an ideal site to study the geology of peak-ring basins in general, and the geological history of SPA specifically. Impact materials still recognizable include a well-defined crater rim, wall terraces, quasi-circular peak ring, and interior and exterior melt units. A small pyroclastic deposit fills a portion of the basin floor, along with several mare patches. This study uses Clementine multispectral ultraviolet-visible (UV-VIS) data, and a limited set of higher spectral resolution Chandrayaan-1 Moon Mineralogy Mapper ([M.sup.3]) data, as well as radar, camera, and topography data from the Lunar Reconnaissance Orbiter to better understand Schrodinger's geology. Sampled spectral profiles and linear unmixing models applied to the Clementine data indicate there is a heterogeneous distribution of both anorthositic and basaltic materials in the crater floor. [M.sup.3] data further validates this observation, and the high spectral resolution shows that most of the mafic content is dominated by pyroxene. These results challenge the traditional assumption that Schrodinger was formed in mostly highland terrain. Our assessment brings forth a new understanding regarding the placement of Schrodinger within SPA and the role SPA impact materials played in shaping the composition of Schrodinger. Le bassin de Schrodinger est un bassin d'impact d'anneau de crete bien preserve, situe sur la face cachee de la lune, le long de la bordure du bassin Pole Sud--Aitken (PSA), lequel est beaucoup plus grand. L'age relativement jeune (Imbrien inferieur ou 3,8 Ga) du bassin de Schrodinger en fait un site ideal pour etudier la geologie generale des bassins d'impact d'anneau de crete et plus specifiquement l'historique geologique du bassin PSA. Les materiaux d'impact encore reconnaissables comprennent une bordure de cratere bien definie, des terrasses murales, un anneau de crete quasi-circulaire et des unites interieures et exterieures de fusion. Un petit depot de roches pyroclastiques remplit une portion du plancher du bassin en plus de plusieurs parcelles de mers. La presente etude utilise des donnees multispectrales de la sonde UV-VIS Clementine et un ensemble limite de donnees a haute resolution spectrale de la sonde Chandrayaan-1 Moon Mineralogy Mapper ([M.sup.3]) ainsi que des donnees radar, de camera et de topographie de la sonde du Lunar Reconnaissance Orbiter afin de mieux comprendre la geologie de Schrodinger. Les profiles spectraux echantillonnes et les modeles d'ajustement lineaire appliques aux donnees Clementine indiquent que les materiaux anorthositiques et basaltiques du plancher du cratere sont distribues de maniere heterogene. Les donnees M3 valident cette observation et la haute resolution spectrale montre que le pyroxene constitue la plus grande partie du contenu mafique. Ces resultats remettent en question l'hypothese traditionnelle selon laquelle Schrodinger a ete forme surtout dans des hautes terres. Notre evaluation presente une nouvelle comprehension en ce qui concerne la place de Schrodinger dans le bassin PSA et le role que jouent les materiaux d'impact PSA dans la formation de la composition de Schrodinger. [Traduit par la Redaction], Introduction Data from the Lunar Reconnaissance Orbiter (LRO) and Chandrayaan-1 missions provide an opportunity to study lunar impact craters at previously unavailable resolutions and wavelengths (e.g., Bhandari 2004; Goswami et [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.04.018 Byline: R.J. Soare (a)(b), J.S. Kargel (c), G.R. Osinski (d), F. Costard (e) Keywords: Mars; surface; Mars; climate; Geological processes Abstract: We have identified a number of gullies that could be aqueous in origin near or at the rim of several impact craters in Utopia Planitia and western Elysium Planitia (30.0[degrees]-59.0[degrees] N; 241.0[degrees]-291.0[degrees] W). Based on the sharpness of their incisions and the general absence of superposed craters, we ascribe a relatively recent origin to the gullies. Scalloped depressions are commonplace throughout the region, as well as on the crater walls, rims and floors near the areas of gully issuance. Occasionally, the depressions cross-cut the gully debris-aprons, suggesting that the formation of some depressions is even more recent than that of the gullies. Previous research has proposed that the depressions are collapse basins formed by thermokarst processes. On Earth, thermokarst landforms occur in areas of low gradient topography where the permanently frozen ground (permafrost) is ice rich and has undergone a change in thermal equilibrium. This change can be triggered by long-term or episodic/cyclic climate change and accompanying rises in mean temperatures towards [approximately equal to]0a[degrees]C as well as by rises in seasonally sustained summer temperatures well above [approximately equal to]0a[degrees]C. In order to explain the origin of the rim or near-rim gullies we invoke high obliquity and the possibility that this region of Mars experienced obliquity-driven rises in temperature, atmospheric pressure and humidity sufficient to keep surface water and near-surface ground-ice stable for extended periods of time. We propose that gully formation is closely related to local freeze-thaw processes that, in turn, generate a thermokarst landscape (of which the gullies are a part). This geological and climatological scenario comprises the following steps: 1. An inundation of meltwater at high obliquity (due to the thawing of an atmospherically-deposited snowpack or ice-sheet) and the subsequent saturation of the underlying regolith to tens of metres of depth. 2. Loss of water on the surface, perhaps as obliquity decreases slightly, followed by the progressive freezing of the saturated regolith; this creates an aggrading mass of ice-rich regolith. 3. Obliquity-induced temperature rises that engender the thaw, drainage and partial evaporation of the near-surface, ice-rich regolith. 4. Localised formation of thermokarst collapse-basins (alases), as water is evacuated from these basins. 5. Formation of gullies near, or at, some impact-crater rims as the result of meltwater migration from nearby alases through the thawed regolith to the areas of gully issuance. Although the plains' materials in this region are in part very old (possibly Hesperian or even Noachian), the mantling deposits and their deformation by thermokarst processes appears to be relatively young. This suggests that recent climatic conditions could have been episodically warmer and wetter than had been previously thought. Author Affiliation: (a) Department of Geography, Planning and Environment, Concordia University, 1455 De Maisonneuve Blvd. W., Montreal, H3G 1M8, Canada (b) Department of Geography, Dawson College, 3040 Sherbrooke St. W., Montreal, H3Z 1A4, Canada (c) Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721, USA (d) Canadian Space Agency, 6767 Route de l'aeroport, St-Hubert, J3Y 8Y9, Canada (e) CNRS/UPS UMR 8148 IDES, Interactions et Dynamique des Environnements de Surface, Universite Paris-Sud, BAcentst. 509, 91405 Orsay Cedex, France Article History: Received 3 November 2006; Revised 9 April 2007