February 2025 LIP of the Month

Theia Mons Large Igneous Province, Beta Regio mantle plume, Venus

A.S. Shimolina¹, R.E. Ernst², H. El Bilali²,

¹ Faculty of Geology and Geography, Tomsk State University, Tomsk, Russia; arina051299@gmail.com

² Department of Earth Sciences, Carleton University, Ottawa, Canada; RichardErnst@cunet.carleton.ca, HafifaElBilali@cunet.carleton.ca

Extracted and modified from:

Introduction

We learn a lot about terrestrial Large Igneous Provinces (LIPs) from studying analogues on Venus (Head and Coffin, 1997; Hansen 2007; Ernst, 2014; Buchan and Ernst, 2021). In contrast with Earth, Venus has essentially no surface erosion owing to high surface temperatures (450 °C) and a dry atmosphere. Thus, we see the surface expression of LIPs in contrast with the variable depth exposure of LIPs on Earth.

Also, Venus has no plate tectonics (at least back to the Great Climate Transition at about 1 Ga, e.g. Way et al. 2022), and hence all magmatism is intraplate, with the largest magmatic events of comparable to scale to plume-generated LIPs on Earth. For instance, the triangular Beta Atla – Themis region (Fig. 1) represents major plume related provinces connected by triple junction rifting (e.g. Airey et al. 2017).

The Atla Regio plume center is associated with triple junction rifting, at least 4 major magmatic centers, each with radiating mafic dyke swarms (with surface expression as grabens), the longest extending for at least 3700 km away (El Bilali et al. 2023; El Bilali and Ernst 2024; Mason et al. 2025).

Here we focus on detailed mapping in Beta Regio and report on detailed mapping of the main volcanic center Theia Mons. Mapping is undertaken with Synthetic Aperture Radar SAR (wavelength 1.26 cm) and altimetry data (4.6 km pixel size) of the 1989-1994 NASA Magellan mission, e.g. https://astrogeology.usgs.gov/astropedia.

Theia Mons

Theia Mons (centered at 23.4º N, 79.4º W) is the main volcanic center for the Beta Regio plume, of the Beta-Atla-Themis (BAT) region, Venus (Fig. 1).Beta Regio, along with Atla and Themis Regiones and their extensive connecting rift zones, define the BAT (Beta-Atla-Themis) region as a regional concentration of young volcanic and tectonic activity. Beta Regio (like Atla Regio and Themis/Phoebe Regiones) is thought to represent a currently active plume in that it is topographically elevated, has an associated geoid high, negative Bouguer gravity anomaly, and is also the locus of a triple junction rifting system defined by Zverine Chasma (a part of larger Hecate Chasmata) to the west and Devana Chasma to both the north and south (Figs. 1,2).

Basaltic lava flows of Theia Mons (volcano)

Theia Mons’ basaltic lava flows are grouped into three Flow Packages (Fig. 3). Flow Package 1 (fTMc1) includes 5 different flow groups (fTMc1-1 to fTMc1-5) and are defined based on their apparent divergence from a point named Center 1 which is later shown to be the same center exhibiting divergence of grabens and rift zones.

Flow Package 2 (fTMc2) is spatially associated with main center of Theia Mons marked by a large central caldera. There are two subdivisions of Flow Package 2: Flow Package 2a is defined by those Flow Groups (fTMc2_1 to fTMc2_6) which are on the flanks of Theia Mons and converge to the central caldera. Flow Package 2b is defined by those Flow Groups (fTMc2_7 to fTMc2_11) which are spatially associated with the summit region around the central caldera, and which also tend to be radar brighter (and therefore rougher at 12.6 cm wavelength than Flow Package 2a).

Radiating mafic dyke swarms

Two main radiating systems of graben-fissure-fracture systems (‘grabens’ for short) are connected to Theia Mons (gTMrad_1 and gTMrad_2).

Grabens of the gTMrad_1 (Fig. 4a) system are long (longest traced is 205 km), especially to the north and east of Theia Mons. This graben system spans an arc angle of about 280°. The convergence point of this radiating graben system does not coincide with the central caldera of the volcano, but instead is focused 200 km northeast of the caldera and this convergence point is not expressed in any way on the radar image. The location of this point coincides with the previously mentioned Center 1 as the focus of lavas of Flow Package 1. From here on, this point will be designated C1.

gTMrad_2 system (Fig. 4b) includes shorter individual grabens (longest traced is 124 km). The longest grabens are located on the north-east and southern sides of Theia Mons. This graben system spans an arc angle of approximately 360°. Gaps in the radiating pattern are due to obscuring by younger lava flooding and also by deformation (dense fracturing) associated with younger rifting (Section 3.3). The gTMrad_2 graben system converges to the central caldera of Theia Mons (С2).

Radiating (Triple Junction) Rift Systems

The previously recognized rift zones (Devana Chasma and Zverine Chasma) converge the location of the central caldera (Fig. 5), representing classic triple junction rifting associated with plume generated uplift.

Two additional systems of normal faults are also associated with topographic troughs representing rift zones, rBR_1 (blue) and rBR_2 (green) which approximately converge to the Center 1, and are inferred to represent a second center of triple junction rifting. The patterns of rifting can be very complex on Venus, and as shown for terrestrial rift zones, there than be local deviations and offsets likely reflecting heterogeneities in the host rock properties.

Geological history

The interpreted geological history of Theia Mons is summarized in Figure 6. The focus is on the two well-developed magmatic centers C1 and C2 (present caldera), recognized on the basis of lava Flow Packages, radiating graben sets and triple junction rifting.

The oldest stage started with the intrusion of a radiating swarm of dykes gTMrad_1 focused on Center 1 (Fig. 6a). Such large radiating dyke swarms are associated with domal uplift due to a mantle plume rising to the base of the lithosphere. After the introduction of the radiating swarm, lava eruptions began from Center 1. However, due to the overlapping of the Center 1 source area (caldera?) by younger flows, it is not possible to identify the earliest flows from Center 1.

The next event is a shift of magmatic activity 200 km to the southwest to Center 2, which is the present location of the major caldera center (Fig. 6b). A second radiating swarm of dykes gTMrad_2 was emplaced and focused on Center 2, which corresponds to the Theia Mons caldera. There are two stages of flows associated with Center 2: initial radar dark and long (Stage 2a), and younger radar bright and lobate (Stage 2b).

The volcanic history of Theia Mons is comparable with other mapped shield volcanoes on Venus: Maat Mons (Mouginis-Mark, 2016; El Bilali et al., 2021; Brossier et al., 2021), Atira Mons (Braga et al., 2024), Tuulikki Mons (Ivanov and Head, 2019), Sapas Mons (Keddie and Head, 1994, 1995), and NW area of Phoebe Regio (Antropova et al., 2024). A useful generalization is that the earlier flows are more voluminous and widespread (sheet flows) and are followed by more lobate flows localized near the summit. This process has been explained by the magma reservoir ascending into the edifice as the edifice grows (Head and Wilson, 1992).

In the case of Theia Mons, the volcanism of younger Center 2 has two Stages; the earlier Stage 2a is associated with earlier voluminous and widespread sheet flows, while the younger Stage 2b consists more of lobate flows restricted closer to the summit. In contrast, the inferred earlier Stage 1 volcanism linked to cryptic Center 1, consists of voluminous sheet flows. However, a younger lobate flow stage (like observed at Center 2) is missing. This can be explained by these being covered by the younger Stage 2 flows or that the shift from Center 1 to Center 2 occurred during the sheet flow emplacement at Center 1 and that the opportunity for a lobate flow stage at Center 1 was prevented by the shift from Center 1 to 2.

References