2024 August LIP of the Month

Graben systems and geological history of Mbokomu Mons region, Parga Chasmata, Venus

Naima Hannour¹, Hafida El Bilali¹, Richard E. Ernst¹, Kenneth L. Buchan², James W. Head³, Mohamed Ben Marzoug¹

¹Department of Earth Sciences, Carleton University, Ottawa, ON, K1S5B6, Canada; hannournaima83@gmail.com

²273 Fifth Ave., Ottawa, Ontario, Canada

³Department of Earth, Environmental and Planetary Sciences Brown University, Providence, RI, USA

For full details see the full paper from which this summary has been extracted and modified:

Hannour, N., El Bilali, H., Ernst, R.E., Buchan, K.L., Head, J.W., Ben Marzoug, M. (2024) Graben systems and geological history of Mbokomu Mons region, Parga Chasmata, Venus. Icarus, 423, 116268, https://doi.org/10.1016/j.icarus.2024.116268

Abstract

Owing to coverage of 80% of the surface by basaltic magmatism and the absence of plate tectonics, Venus offers a remarkable opportunity to investigate intraplate magmatism including that of Large Igneous Province scale and also to investigate the role of mantle plumes and diapirs. Furthermore, because of the absence of any significant erosion (due to the hyper warm conditions of 450°C), Venus provides a complimentary view to that available on Earth. In addition to preserving the surface expression of lava flows, the widespread sets of extensional lineaments (grabens-fissures-fractures) are typically interpreted to be the surface expression of mafic dyke swarms.

The relationship between chasmata (rift zones) and spatially associated volcanism (mons and coronae) on Venus has been extensively discussed but remains enigmatic. One region where these features are prominently displayed is along the 10,000 km long, WNW trending, Parga Chasmata, which connects Atla Regio with Themis Regio. In this research, we have selected the Mbokomu Mons area (located about 2200 km SE of Atla Regio) for detailed study to provide insight into these relationships. More than 39,000 extensional lineaments (grabens, fissures and fractures) were mapped at 1:500,000 scale using full resolution Magellan Synthetic Aperture Radar (SAR) images and grouped into radiating, circumferential and linear systems. They are mainly interpreted to represent the surface expression of underlying mafic dyke swarms, on the basis of associated volcanic features and terrestrial analogues. Radiating and/or circumferential swarms are associated with Mbokomu Mons (which has both Corona and Mons stages) and the four coronae in the surrounding area, Among Corona (AC), Repa Corona (RC) and two unnamed coronae (UC1 and UC2). All four centres are aligned along a WNW-trend parallel to the Parga Chasmata (rift system). Mbokomu Mons is located at, and its emplacement may be linked to, the intersection of this WNW-trending zone of weakness and the orthogonal Jokwa Linea rift system. Mbokumo Mons is also younger than the nearby parallel Penthesilia Fossa (PF) (part of the Great Dyke of Atla Regio).

Introduction

Venus is a single-plate planet (e.g. Solomon and Head, 1982; Solomon et al., 1992; Phillips and Hansen, 1994) that is dominated by volcanism (Head et al., 1992; Ivanov and Head, 2011, 2013), with approximately 85,000 volcanic edifices, from kilometre-size vents to broad shield volcanoes hundreds of kilometres across (Hahn and Byrne, 2023) and at least 513 coronae (Stofan et al., 2001; Glaze et al., 2002). Despite the lack of plate tectonics, Venus has experienced major rifting (Guseva and Ivanov, 2020; Ivanov and Head, 2015; Hansen and DeShon, 2002; El Bilali et al., 2023). The youngest phase in the evolution of Venus involved the formation of major shield volcanoes, prominent rift zones and flow fields (e.g. Ivanov and Head, 2013, 2015). We focus our attention on Mbokomu Mons (Fig. 1), located along the 10,000 km long Parga Chasmata (Smrekar et al., 2010), which connects the Alta Regio and Themis Regio plume centres and marks the southern boundary of the Beta-Atla-Themis (BAT) region (Head et al., 1992; Graff et al., 2018). There are many coronae associated with Parga Chasmata, including in the Mbokomu Mons region (Fig. 1).

Study Area

We have selected the Mbokomu Mons region (square box in Fig. 1) for detailed mapping and analysis because of the opportunity to investigate the relationships between chasmata and spatially associated volcanism (mons and coronae). The study area is within Taussig Quadrangle (V-39). A Magellan Synthetic Aperture Radar (SAR) image and Magellan altimetry data for the study area are shown in Fig. 2.

Results

Graben systems (dyke swarms)

As noted above, the study area includes multiple magmatic centres: Mbokomu Mons (MM), Repa Corona (RC), Among Corona (AC), and two unnamed coronae (UC1, and UC2) recognized in this study (Fig. 1). The study area is also characterized by numerous graben systems with a range of trends (Fig. 3), as well as other extensional features that include normal faults (associated with zones of rifting), and contractional structures called wrinkle ridges. Finally, there are also numerous lava flows that are interpreted to be mafic (basaltic) in composition, and which will be the focus of a follow-up mapping study.

We have mapped 39,196 graben lineaments (Fig. 3) inclusive of fissures and fractures, and grouped and generalized them into different sets according to their geometric patterns (radiating, circumferential, linear and arcuate; Fig. 4). Many of the radiating and circumferential sets are associated with specific magmatic centres in the study area. Mbokomu Mons exhibits two sets of radiating and circumferential grabens, one associated with the central region of the mons and the other with the outer elevated annulus on the flanks of the mons. We interpret the outer radiating (R1) and circumferential (C1) graben sets as older than the inner radiating (R2) and circumferential (C2) graben sets.

Discussion

Mbokomu Mons dyke swarm history

Two distinct phases of magmatic activity are recognized for Mbokomu Mons, and are interpreted to be related to an underlying mantle plume. These are referred to as the earlier Corona Phase, and the younger Mons Phase.

Corona Phase: As shown in Fig. 5a, the arrival of a mantle plume in the Corona Phase causes domal uplift and emplacement of the radiating R1 graben set (interpreted as dykes) that is especially dense on the NE and SW sides of the unflooded annular uplift of the flanks of MbokomuMons. The C1 circumferential swarm of the Corona Phase was also emplaced on the uplifted annular rim, presumably during corona-style collapse of the central region and perhaps associated with formation of the annular rim. The central topographic low was then flooded by lava flows which covered R1 and C1 grabens in this area. Wrinkle ridges that circumscribe Mbokomu Mons (Fig. 5c) may have formed during the initial domal uplift of the Corona Phase, approximately coeval with the main R1 radiating swarm (perhaps associated with peripheral thrusting on the flanks of the domal uplift; cf. McKenzie, 1994; Mège and Ernst, 2001). If so, then the circumferential wrinkle ridges are older than the C1 circumferential grabens of Mbokomu Mons (which we interpret to have formed during the corona-stye collapse of the central region). This would imply the following age sequence for the Corona Phase: R1 radiating grabens and coeval wrinkle ridges, followed by C1 circumferential grabens. The circumferential grabens are interpreted to overlie a circumferential dyke swarm associated with spreading of the plume head and causing hoop-like extension in the overlying lithosphere (e.g. Buchan and Ernst, 2021; Tessier et al., 2024).

Mons Phase: This phase represents a second phase of magmatism which produced a volcanic edifice 1.1 km high (Fig. 5b). The initial stage of edifice building is associated with flooding of the topographic low produced during the Corona Phase, possibly in association with the R2 radiating swarm, indicating that the Mons Phase was associated with renewed uplift (probably a new phase of underlying plume uplift). The small C2 circumferential swarm is also emplaced in the Mons Phase. While this circumferential swarm could also represent spreading of the plume (e.g., Buchan and Ernst, 2021), its small radius of 38 km suggests that it may represent caldera collapse above a large magma reservoir.

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