Geological history of Theia Mons, Beta Regio plume, Venus: Recognition of two main magmatic centers for flows, radiating dyke swarms and triple junction rifting

Theia Mons (centered at 23.4° N, 79.4° W) is the main volcanic center for the Beta Regio plume, of the Beta-Atla-Themis (BAT) region, Venus. Synthetic aperture radar data (SAR) and altimetry data from the 1989–1994 NASA Magellan mission were used to produce a geological map and history of Theia Mons, revealing two distinct magmatic centers (200 km apart), each the focus of basaltic lava flows, extensional lineaments (representing the surface expression of dyke swarms), and associated rift zones. The study area spans 88°W to 72°W, 16°45′N to 29°45′N, and mapping was at 1:500,000 scale. Our detailed mapping makes this area a prime target for the future Venus missions (orbital and lander), also given that Beta Regio is a strong candidate for ongoing volcanic activity. Lava (basaltic) flow units (88 distinguished on the basis of variation in radar backscatter) belonging to Theia Mons volcano were combined into 19 Flow Groups and then into 3 Flow Packages. The lava flows appear to diverge from two distinct magmatic centers, labelled Center 1 (24.5°N, 78.1°W) and Center 2 (23.4° N, 79.4°W), with the older Center 1 being obscured by the volcanism of the younger Center 2. Center 1 consists of flows with low radar brightness and which extend to a maximum to 830 km from the center. Center 2 coincides with the currently preserved central caldera and consists of lava flows of low radar brightness with a maximum length of 620 km, followed by a second pulse of radar-bright flows that are less extensive and concentrated near the center. About 10,000 extensional lineaments (grabens, fissures, and fractures) were mapped and grouped into 19 systems, of which 15 are interpreted to overlie dyke swarms: 2 radiating systems are associated with Centers 1 and 2, and 13 other systems belong to other (older and likely unrelated) magmatic centers in the region. A partial circumferential swarm may also be present, associated with Center 2. Four other extensional lineaments sets are inferred to represent sets of normal faults associated with rift zones (Devana and Zverine, and additional rift zones). These rift zones exhibit two sets of ‘triple junction’ geometry, which are approximately also focussed on the same Centers 1 and 2, revealed by the lava flows and rift zones. An underlying mantle plume is interpreted to be responsible for the dykes, flows and triple junction rifting. The cause of the shift between Centers 1 and 2 (200 km to SW) is unknown, but plausible mechanisms include a shift of the lithospheric plate above a stationary single plume, or a bending of the mantle plume (e.g. in a mantle wind) between timing of Center 1 and Center 2 activity. • Detailed mapping of flows, graben systems & rift zones of Theia Mons, Venus. • Mapping of 88 lava flows, combined into 19 flow groups and 3 flow packages. • 10,000 graben systems mapped & grouped in 19 sets-at least 15 overlie dyke swarms. • Two centers, each the focus of radial flows, dyke swarms & triple junction rifting. • The 200 km between centers due to movement between lithosphere & underlying plume.