EVIDENCE OF VOLCANO-TECTONIC INTERACTIONS ON VESTA

On Vesta, the pit crater chain Albalonga Catena phases along-strike from a topographically low series of merged pits into the elongate hill Brumalia Tholus. Pit crater chains form when dilational motion on buried steep normal faults causes overlying material to collapse into the opening portions of the buried fault. Consistent with this hypothesis, the merged pits of Albalonga Catena show signs of collapse but distinct fault faces can also be observed.

If Albalonga Catena represents a buried normal fault, then the topographic high that emerges along its length likely formed as a magmatic intrusion utilizing the subsurface fracture as a conduit to the surface, intruding into and deforming the rock above it. This is consistent with several studies that have assessed the role of volcanic and/or magmatic activity (including igneous intrusion) on Vesta. Earth-based spectroscopic studies of Vesta show that it has similar reflectance spectral signatures to the igneous howardite-eucrite-diogenite (HED) meteorites, which thus are thought to be vestan fragments.

If a magmatic intrusion, the core of Brumalia Tholus would be comprised of a plutonic rock. This can be tested, as Teia crater impacts the hill’s northern face and its ejecta likely samples Brumalia’s core material. Distinctly textured ejecta from Teia have been shown to be enriched in diogenite (the plutonic HED) compared to the background materials, consistent with the hill being the surface representation of a magmatic intrusion.

We suggest that the following sequence of events may have occurred on Vesta. Ancient fracturing and faulting occured in the Vesta sub-surface. The Albalonga fault sampled a region of partial melt and served as a conduit for this material to move upward and deform the surface. Brumalia Tholus formed due to magmatic injection and laccolith doming. The core molten material cooled slowly at depth, forming diogenite. Sometime later the Albalonga faults was covered by loose regolith material (perhaps ejecta from a nearby impact) which collapsed into dilational openings along the steep sub-surface faults, forming the pit crater chains. Then the Teia impact event occured and incorporated the diogenitic Brumalia core material into its ejecta.