Tesserae on Venus Feature Layered, Folded, and Eroded Rocks

Abstract:

Tesserae occupy ~7% of the Venus surface and locally are always stratigraphically older than surrounding plains units. These regions are characterized by pervasive tectonic deformation that includes normal faults, grabens, thrust faults, and folds. On the basis of morphology, gravity anomaly signature, and near-infrared surface emissivity, tesserae have been hypothesized to be Venus counterparts to continents on Earth. When resolved with Magellan synthetic aperture radar data, multiple tesserae show sets of curved, parallel lines of high radar backscatter that are cross-cut by extensional and shortening structures. Examples of this curvilinear outcrop pattern are seen in Alpha, Ovda, and Thetis Regiones, as well as in Tellus and Manatum Tesserae. These linear features bear a strong morphological resemblance to strata in layered sequences on Earth that have an arcuate or sinuous, terraced outcrop pattern from having been exposed by erosion on the flanks of ridges or valleys. Should this analogy hold for Venus, then these outcrop patterns imply some erosion of the tessera units in which these strata occur; radar-dark materials filling proximal lows might be deposits of that eroded material. The nature, attitude, and ages of these strata are unclear: they could be flood lavas or sedimentary sequences; they may be horizontal or gently folded; they may have formed after Venus experienced a runaway atmospheric greenhouse, or instead date from an earlier, more temperate climate. But the presence and map patterns of layering denotes a complex formational history for these enigmatic units that includes volcanic and/or sedimentary deposition, at least one phase of folding, and exhumation by some erosive action. These geological characteristics are reminiscent of both Jura-type folded frontal edges of marginal fold-and-thrust belts and Yakima-type folded areas in flood basalts on Earth, and must be considered when formulating interpretations for tessera formation. For now, however, a fuller understanding of the nature of these units awaits high-resolution radar and multi-spectral imaging, as well as in situ chemical analyses, by future orbiter and lander missions.