THE BAT REGION, VENUS, PROPOSED ANALOGUE TO A TERRESTRIAL LLSVP, AND INSIGHTS FROM ATLA REGIO PLUME / LIP MAGMATISM

On Earth, two lower mantle regions called LLSVPs (Large Low Shear Velocity Provinces), underneath Africa and the Pacific, are thought to be the source of mantle plumes that generate Large Igneous Provinces (LIPs). Venus is also host to numerous plume-generated LIPs and the largest concentration of young plumes/LIPs is in the Beta-Atla Themis (BAT) and the approximately antipodal Eistla Reio region, which may each locate a Venusian LLSVP.

We consider several characteristics of terrestrial LLSVPs applied to these proposed Venusian analogues: 1) on Earth plumes/LIPs are preferentially located along the margin of the LLSVPs. We consider a boundary for the BAT- LLSVP mapped from the distribution of “young” volcanos and a similar approach is applied to Eistla Regio. 2) Some terrestrial plumes and their associated magmatism (both at the hotspot stage and the plume head stage) have bilateral compositional asymmetry which can be due to different mantle contributions (ambient deep mantle and LLSVP mantle) rising on opposite sides of the plume stalk. On Venus we are testing for the same process by searching for any differences in characteristics of plume head LIP magmatism between the sector facing away from the inferred BAT-LLSVP, and in the sector facing into the BAT-LLSVP. 3) Comparison of the distribution of these younger vs older plumes/LIPs, can test stability of these proposed Venusian LLSVPs through time.

Detailed mapping of 35,000 lineaments in the Atla Regio plume centre, one of the largest plumes/LIPs of the proposed BAT-LLSVP), reveals multiple giant radiating dyke swarms (up to >2000 km in radius) each identifying a magmatic centre. The large size of these radiating swarms indicates that none of the centres represent accumulation of plume tail magmatism (cf Iceland or the Emperor-Hawaiian hotspot chain, for which the maximum dyke swarm length is about 100 km). Therefore, it is inferred that Atla Regio is still at the plume head stage (<50 myr old), and did not yet reach the plume tail stage. Several of the Atla Regio magmatic centres are aligned and these can be related to either a zone of lithospheric weakness or potentially mark the local border of the proposed BAT-LLSVP