Large‐Scale Troughs on Asteroid 4 Vesta Accommodate Opening‐Mode Displacement

Abstract

AbstractAsteroid 4 Vesta hosts two sets of enormous troughs, Divalia Fossae that encircle two‐thirds of the equator and Saturnalia Fossae located in the northern hemisphere. These troughs were interpreted as grabens, thus invoking faulting. The trough sizes, their linear arrangement, and overall morphology leave no doubt that their origin is tectonic, but structures other than faults have not been considered. To test if they are fault‐related or formed by accommodating opening‐mode displacement (i.e., jointing) without subsequent shear, we investigate the map patterns, cross‐sectional geometries, and variations of relief and width along the trough lengths. Relief and width could relate to the vertical displacement of faults and aperture of joints, respectively, and they therefore reveal differences in fracturing behavior. We analyzed six major troughs on Vesta, four belonging to Divalia Fossae, and two to Saturnalia Fossae. No map patterns are diagnostic of faulting or jointing. For each pair of trough‐bounding scarps, the maximum relief does not lie at the trough center, and the two maxima occur at different positions along the trough. In contrast, troughs are widest near the centers of the troughs. These characteristics are inconsistent with graben formation but are consistent with jointing. Furthermore, rock‐mechanical calculations that account for Vesta's low gravitational acceleration and degree of fracturing reveal that faulting is not favored to be initiated at depths above at least ∼3 but as much as 55 km within Vesta's lithosphere. Therefore, jointing or mixed‐mode fracturing, both involving opening‐mode displacements, are more plausible fracturing mechanisms for the Divalia Fossae.