Subsurface Gravity Signatures of Subduction on Venus

Abstract:

While Venus has no global plate tectonics, there are features that strongly resemble terrestrial subduction zones. They are proposed to be caused by plume-induced subduction. A hot mantle plume breaks the lithosphere, volcanism loads the lithosphere causing it to sink and initiating slab rollback. Proposed subduction sites tend to be on the edges of large coronae or along chasmata with associated coronae. Prior gravity studies found evidence for a subducted slab in the interiors of Artemis and Quetzalpetlatl Coronae. The low resolution of the gravity field precludes the search for subduction in many locations, especially at smaller features.

This study analyzes gravity signatures of larger proposed subduction zones. Using the 180 MGNP180U gravity field and topography, we calculated Bouguer anomalies and geoid/topography ratios (GTR) of both sides of the trench. Similarly, we used the EGM2008 gravity field to examine two terrestrial subduction zones, the Fiji and Sandwich trenches, as possible analog sites. We compare signatures on either side of the trench, examining gravity anomalies above the overriding plate, and above the outboard portion of the down-going plate.

Our preliminary results show that both on Earth and Venus, a high GTR and a high Bouguer anomaly occurs above the down-going plate. We interpret this as an indication of flexural compensation of a broad topographic bulge, supporting the interpretation of the locations on Venus as subduction zones. The low Bouguer anomaly above the overriding plate is consistent with the presence of lower density material in the subsurface, such as subducted crust and/or a hot plume.

By analyzing the gravity signatures of possible subduction features, we have found evidence in favor of subduction on Venus. With a future gravity dataset that has a higher global resolution we would be able to better determine if these features have additional characteristics of subduction zones, as well as study the smaller proposed subduction features.