Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31g..07y&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31G-07
Other
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [6225] Planetary Sciences: Solar System Objects / Mars, [8135] Tectonophysics / Hydrothermal Systems, [8136] Tectonophysics / Impact Phenomena
Scientific paper
Impact cratering process is of primary importance in the evolution of solid bodies of the Solar System. But craters on basaltic rocks, which are the best analog for the surface of other planets and satellites, are rare on Earth. Most studies to date were done in the Lonar crater, a simple crater 1.8 km in diameter, formed on the basaltic flows of the Deccan Province (India). Recently, one medium-size complex crater was identified on volcanic rocks of the Paraná basin (south Brazil) and may provide additional analog to the craters of most rocky planets and satellites. The 12 km wide Vargeão is a very well-preserved impact structure formed on basaltic and subordinately rhyodacites flows of the Serra Geral Formation (about 133-131 Ma), which are locally intertrapped by eolian-sandstones of Botucatu Formation. The impact-related features are represented by shatter cones, breccia-veins and planar deformation features in quartz (few occurrence in the sandstones). This work is focused on the petrogenesis of the centimeter breccia-veins that are found in all lithologies. We conducted a detailed petrological study (petrography, microprobe, SEM, Raman spectroscopy, Spectroscopy of reflectance and XRD) on these veins and their host-rocks. Our results show that the veins were strongly affected by the post-impact hydrothermal fluids. The hydrothermal alteration varies geographically in the structure. On the rim area this alteration consists of total or partial substitution of the melt matrix by quartz, calcite, iron oxides and clay minerals. At the central area, the alteration mineral assembly is composed of quartz, iron oxides, zeolites, clay minerals and rarely calcite. Usually, the alteration shows a zoned setting, which also varies locally. The nature of occurrence of second mineral identified in the context of post-impact hydrothermal alteration of impact craters on basalt represent a critical interpretation to interpret alteration signature of impact craters and the old Noachian terrains of Mars. The interpretation of these signature remain controversial. These signature may result from the excavation of globally altered volcanic material during a warm and wet Mars, or alternatively may be more or less exclusively the results of impact-associated hydrothermal alteration. Our results at Vargeão may be used to assess these different hypotheses.
Baratoux David
Berger Guy
Nédélec Anne
Trindade R. I.
Yokoyama Etsuro
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