2024 September LIP of the Month

Dyke swarms of Onenhste and adjacent coronae in Parga Chasmata, SE of Atla Regio, Venus: Detailed mapping, swarm interactions and geological history

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

¹Department of Earth Sciences, Carleton University, Ottawa ON K1S5B6, Canada; marzoug375@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:

Ben Marzoug, M., El Bilali, H., Ernst, R.E., Buchan, K.L., Head, J.W., Hannour, N. (2024) Dyke swarms of Onenhste and adjacent coronae in Parga Chasmata, SE of Atla Regio, Venus: Detailed mapping, swarm interactions and geological history. Icarus, 424, 116269, https://doi.org/10.1016/j.icarus.2024.116269

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, owing to the absence of any significant erosion due to the hyper-warm conditions (450°C), Venus provides a complimentary view to that available on Earth. In addition to preserving the surface expression of lava flows, the widespread extensional lineaments (grabens, fissures and fractures) are typically interpreted to be the surface expression of underlying mafic dyke swarms.

Parga Chasmata, a 10,000 km-long rift system on Venus, is notable for its association with numerous coronae. Our detailed geological mapping effort at a 1:500,000 scale focused on Onenhste Corona and its vicinity (Fig 1) contributes to the understanding of the relationship between corona and this rift zone. Over 46,000 extensional lineaments were mapped, which were grouped into 17 radiating, 28 circumferential, and 5 linear sets, thought to overlie mafic dyke swarms. These lineaments are associated with various coronae, including Onenhste (OC), Momu (MC), Ulgen-ekhe (UEC), Rzhanitsa (RzC), and five unnamed coronae (UC1-5), as well as Malibran Patera (MP) and Fedchenko Patera (FP). By analyzing crosscutting relationships between the dyke swarms and applying additional methods used to identify coeval centres by analyzing the swinging or deflection of dyke swarms to reveal stress interactions among them. The age sequence identified is UC2 > RzC > MC ≥ OC = UC1 = UEC ≥ MP > UC5, and FP > UC3. These findings enhance our understanding of the timing, evolution, and spatial relationships of coronae and rift zones, with several centres aligned along trends parallel to and perpendicular to the Parga Chasmata rift, providing insights into the stress interactions and developmental history of these features.

Introduction

Venus is dominated by volcanic activity, featuring thousands of volcanic centres, from small vents to large shields, primarily scattered across its extensive volcanic plains (Head et al., 1992; Ivanov and Head, 2022). These plains, which cover about 80% of the planet's surface, resemble Earth's flood basalts (Head and Coffin, 1997). Additionally, over 560 circular volcano-tectonic features known as coronae have been identified (Stofan et al., 2001). The remaining surface includes highly deformed highlands called tesserae and large topographic rises with dome-like shapes, similar to mantle plume features on Earth (Senske et al., 1992; Hansen, 2007). Volcanic rises are categorized into rift-dominated, volcano-dominated, and corona-dominated types (Stofan et al., 1995). Major rift zones, such as the Parga Chasmata, connect key plume centres and are associated with numerous coronae, thought to have formed during the Atlian Period of Venus's history (Smrekar et al., 2010; Graff et al., 2018). Coronae, typically circular or elongate, are believed to result from mantle upwelling and exhibit variations in morphology due to factors like lithospheric thickness and evolutionary stage (Basilevsky et al., 1986; Stofan et al., 1991); they typically have associated circumferential extensional lineaments which can be interpreted as circumferential dyke swarms (Buchan and Ernst, 2021).

Study area, geological setting and previous studies

Our study area is located along Parga Chasmata (Fig. 1). This area is about 2800 km SE of the centre of Atla Regio, and is located in the Taussing Quadrangle (V–39) on the SE margin of the Parga Chasmata rift system and in the northern part of the Wawalag Planitia, at 19°S, 139°W. In this area the grouping of rift segments and coronae into local triple junction centres is widespread (Graff et al., 2018). The focus of the current research reported here is Onenhste Corona and adjacent coronae (Fig. 1).

Methodology

High resolution (75-100 m/pixel) Magellan Synthetic Aperture Radar (SAR) (both Left and Right looking) and topographic data (Ford et al., 1993; Saunders et al., 1992) were used in this research, and were obtained from the USGS Astropedia website (https://astrogeology.usgs.gov/search?pmi-target=venus).

ArcGIS Pro v. 2.8 software was used to display and map key features on the raw Magellan SAR images for the study area. The JMARS (Java Mission-Planning and Analysis for Remote Sensing) software created by Arizona State University's Mars Space Flight Facility (https://jmars.asu.edu/; Christensen et al., 2009), was also used for generating topographic profiles from the Magellan topographic data and for regional reconnaissance interpretation of the SAR images.

Results and discussion

More than 46,000 grabens (grabens-fissures-fractures) were mapped at a scale of 1:500,000, primarily representing surface expressions of underlying dyke swarms (Fig. 2). These grabens are organized into 50 swarms, categorized as radiating (17), circumferential (28), and linear (5) (Fig. 3), each associated with a magmatic centre or rift zone.

These data are used to addresses three themes: (1) how radiating and circumferential swarms change direction under the stress influence of nearby magmatic centres, (2) the geological history of the area, and (3) implications for the rifting history related to Parga Chasmata.

Swinging pattern of some graben sets (dyke swarms)

Radiating graben systems on Venus, which represent dyke swarms, can swing into regional stress fields at distances where the central radial sigma 1 stress diminishes beyond the edge of a plume-caused domal uplift (e.g., Grosfils and Head, 1994a,b, 1996; Nagasawa et al., 1998; Ernst et al., 2003; Buchan and Ernst, 2019; El Bilali et al., 2023; El Bilali and Ernst, 2024). This behavior is also observed in terrestrial analogues (e.g., Baragar et al., 1996; Ernst and Buchan, 2001; Hou et al., 2010).

A key aspect relevant to this study is how graben sets (dyke swarms) from one magmatic centre can change their trend due to the stress influence of another magmatic centre, indicating their coeval nature. This concept of stress interactions between closely spaced centres was first explored by McKenzie et al. (1992), using a pressurized hole model. The present study further explores the full range of possible dyke interactions (Fig. 4), with examples from the study area illustrated in Figures 5 and 6, and we favour control by topographic changes rather than by the pressurized hole model (Ben Marzoug et al. 2024).

Radiating swarm influenced by circumferential stress of coronae

The radiating swarm gOC-3R of Onenhste Corona is influenced by the presence of Momu Coronae, causing it to diverge around the cluster (Fig. 5a) in a type D interaction pattern (Fig. 4d). The circumferential swarms of Momu Coronae are not affected by Onenhste Corona's radiating stress, suggesting they were emplaced before the radiating stress state of Onenhste developed. The stress state of Momu Coronae persisted into the period when the Onenhste radiating swarm (gOC-3R) formed.

In contrast, the older radiating swarm gOC-2R (Fig. 5b), which is missing within Momu Coronae, suggests that gOC-2R predates both Momu Coronae and gOC-3R. Additionally, the area southwest of MC-2 is avoided by gOC-3R but shows strong development of the earlier gOC-2R swarm, indicating that the presence of gOC-2R inhibited the emplacement of the later gOC-3R swarm.

Radiating swarm influenced by radial stress of other magmatic centres

In the example shown in Figure 5b, the earlier yellow radiating swarm gOC-2R of Onenhste Corona is influenced by the radiating stress states of other magmatic centres to the southeast and northwest. Each of these cases of deflected grabens (dykes) is illustrative of a Type C stress setting (Fig. 4c) in which multiple centres are coeval with respect to their radiating stress states. Furthermore, it should be noted that in each case there is an ambiguity in interpreting which specific grabens (dykes) belong to each centre.

Circumferential swarms whose stress stages are independent

In the scenario shown in Figure 6, a number of closely spaced or overlapping coronae have clear circular patterns indicating that there was enough time between the corona forming pulses to allow the circumferential stresses associated with one event to have dissipated before the next coronal stage began. The required timing is unknown and will require further modelling work to constrain. However, we speculate that the difference in timing could be short, based on the timing of stress changes from radiating to circumferential swarms in terrestrial LIPs (e.g. Buchan and Ernst, 2021). Thus, it remains possible that these nearby coronae could be different pulses within an overall single event.

Geological history

The development of Onenhste Corona occurred in three main stages (Figs. 7-10):

1. Stage 1 (OC-1) (Fig. 7: This stage features a central structure, OC-1, with a radiating substage (gOC-1R) and a circumferential substage (gOC-1C). These structures appear to be unaffected by nearby centres.

2. Stage 2 (OC-2) (Fig. 8): Similar to OC-1, OC-2 has a radiating substage (gOC-2R) followed by a circumferential substage (gOC-2C). The radiating substage is influenced by the stress fields of other magmatic centres (UC1-1, UEC-1, and UC3-1), indicating simultaneous activity.

3. Stage 3 (OC-3) (Fig. 9): This stage includes a central structure, OC-3, with radiating (gOC-3R) and circumferential (gOC-3C) substages. The radiating substage (gOC-3R) is influenced by the stress fields of some magmatic centres of Momu Coronae (MC-2 and MC-3) and Malibran Patera (MP-1).

Relationship to Parga Chasmata and P13 Linea

Several centres, RzC, UC2, MC, UC3 and FP, ordered from NW to SE, are aligned along a trend parallel to Parga Chasmata but offset about 900 km to the south from the main zone of rifting (Fig. 1). It is inferred that this alignment is related to a zone of weakness associated with the rift extension. Along this trend the centres do not show an age progression.

Coronae MC, OC, UC1 and UEC are aligned along the NNE trending P13 Linea, orthogonal to the main Parga Chasmata rift (Fig. 1). The main centre, Onenhste, is coeval with UC1, which in turn is coeval with UEC. Additional age relationships indicate that the late stage of OC was active at the same time as MP, MC and the UC2–9 centre of UC2. Linea P13 is older than Chondi Chasma (branch of Parga Chasmata rift zone). This study, and continuing, detailed geological mapping and analysis are increasing our understanding of the relationships between the formation of the main WNW trending Parga Chasmata rift zone, the orthogonal trends of rifting and the numerous magmatic centres (mainly coronae) distributed along both trends.

References