BRUMALIA THOLUS: AN INDICATION OF MAGMATIC INTRUSION ON VESTA

There are three long pit crater chains observed on the surface of the Vestalia Terra plateau (VT). On other planetary bodies these features are hypothesized to form when dilational motion on buried normal faults cause overlying material to collapse into the opening portions of the buried fault. Consistent with this hypothesis, the merged pits of the VT pit crater chains show signs of collapse but distinct fault faces can also be observed. It has thus been suggested that the pit crater chains on VT are representative of subsurface faulting of the plateau.

The pit crater chain Albalonga Catena phases along strike from being a topographically low feature of merged pits into being the elongate hill Brumalia Tholus. If Albalonga Catena represents a buried normal fault, then the topographic high that emerges along its length most likely formed as a magmatic intrusion utilizing the subsurface fracture as a conduit to the surface, intruding into and deforming the rock above it. In this scenario, the core of Brumalia Tholus would be comprised of a more plutonic rock. Teia crater impacts the northern face of Brumalia Tholus and its ejecta likely samples Brumalia’s core material. Dawn’s Framing Camera indicates that distinctly textured ejecta from Teia also have a distinct composition. Analysis of visible and infrared spectrometer data has shown that while the background VT material is howarditic, these Teia ejecta are more diogenitic, 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 Vestalia Terra 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 Rheasilvia impact occured, reactivating (and perhaps reorienting) the Albalonga and other VT faults. The surface of Vestalia Terra was covered by loose regolith material 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.