Circular troughs in tessera terrain on Venus: Morphometry, structural analysis and possible formation model

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Detailed observations on Venus tessera terrain reveal the presence of tectonic structures, characterized by a subcircular trough, limiting an internal area with the average elevation of the surrounding terrain. We named this new type of tectonic structures Circular Troughs. Here we show the results of a morphometric study and a structural analysis based on data from 609 structures obtained through a global survey on Venus. Circular Troughs have an average outer radius of 12 ± 4.4 km, an average inner radius of 9 ± 3.9 km and an average trough width of 3 ± 1.2 km. A comparison with other tectonovolcanic structures on Venus shows that they are smaller and their size-distribution is more bounded. Although Circular Troughs show morphological similarities with some corona types, their size distribution is significantly smaller than coronae indicating a shallower geodynamic formation process. A shape preferred orientation analysis shows that Circular Troughs in Western Ovda are elongated parallel to long-wavelength tessera folds indicating that they are at least partially deformed by them, thus Circular Trough formation predates or it is contemporary with folding. A detailed structural analysis suggests that they were generated by small scale diapirism exclusively developed in tessera terrain. Several hypotheses could explain this small scale diapirism inside tessera terrain including volcanically driven compositional diapirism, small scale thermal diapirism or compositional cold diapirism, considering that the probable dessication of an ancient ocean would have generated evaporitic deposits in the past later buried by younger geological units.
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