December 2023 LIP of the Month

Dyke swarm history in Belet-ili and Gaia (Gaya) Coronae region of Central Eistla Regio, Venus

Twinkle Chaddha ¹, Richard E. Ernst ², Hafida El Bilali ², Rajesh K. Srivastava ¹.

¹Department of Geology, Banaras Hindu University, Varanasi 221005, India

²Department of Earth Sciences, Carleton University, Ottawa, ON K1S5B6, Canada

This is extracted and modified from Chaddha, T., Ernst, R.E., El Bilali, H., Srivastava, R.K. (2024). Dyke swarm history in Belet-ili and Gaia (Gaya) Coronae Region (13◦–26◦E, 0◦–10◦N), Central Eistla Regio, Venus. Icarus, v. 410, 115893, https://doi.org/10.1016/j.icarus.2023.115893

Venus is a paradise for studying mafic dyke swarms, Large Igneous Provinces (LIPs) and mantle plumes (e.g. Ernst et al. 1995; Head and Coffin, 1997; Hansen 2007; Ernst, 2014; Buchan and Ernst, 2021; El Bilali et al. 2023). The absence of erosion due to the extreme surface conditions also provides the advantage of preserving the surface expression of LIPs. The prior November 2023 LIP of the Month http://www.largeigneousprovinces.org/23nov (based on El Bilali et al. 2023) profiled major radiating mafic dyke swarms associated with a large mantle plume centre, Atla Regio. In this December 2023 LIP of the Month, we look at mafic dyke swarms associated with a collection of smaller-size plumes in the central portion of Eistla Regio.

Venus is similar to the Earth in size and overall internal structure, but differs dramatically in having a high surface temperature (450 ◦C) and atmospheric pressure (90 times that of Earth), dominated by CO₂ (e.g. Solomon et al. 1992). Venus exhibits no plate tectonics and no fluvial erosion (although minor erosion through wind may occur). Owing to the absence of plate tectonics (at least for the past approximately billion years), all the volcanism is intraplate, which invites comparisons with terrestrial intraplate magmatism including that of the largest scale, LIPs (Ernst et al. 1995; Head and Coffin, 1997; Hansen 2007; Buchan and Ernst, 2021). Venus hosts volcanic rises (with LIP-scale magmatism) which are generally considered to form above large mantle plumes, analogous to terrestrial plumes originating from the deep mantle (Senske et al., 1992; Stofan et al., 1995; Ivanov and Head, 2013; Head and Coffin, 1997; Hansen et al., 1997; Smrekar et al., 1997; Ernst and Desnoyers, 2004; Stofan and Smrekar, 2005; Hansen, 2007; Hansen, 2018; El Bilali et al., 2023). The extent of these volcanic rises varies from 1000 to 2500 km in diameter and 1 to 2.5 km in height. They are recognized by their broad, gentle, dome-like topography and relatively deep apparent depths of compensation as suggested by the gravity signatures. The volcanic rises are classified into three categories on the basis of their morphology: rift-dominated (Atla and Beta Regio), volcano-dominated (Dione, Western Eistla, Bell and Imdr Regio) and corona-dominated (Themis, Eastern Eistla and Central Eistla Regio (Stofan et al., 1995; Smrekar and Stofan, 1999).

Coronae are quasi-circular/elliptical magma-tectonic features ranging in diameter from <100 km to 2600 km. They have several characteristic features, including the presence of a variably-shaped circular (to sub-circular to irregularly shaped) annulus and/or trough which is deformed by concentric faults and folds, and the association with volcanic flows and edifices (Stofan et al., 1992; Smrekar and

Stofan, 1997; Grindrod and Hoogenboom, 2006; Guseva and Ivanov, 2020; Gülcher et al., 2020). They are considered the surface manifestation of upwelling mantle plumes although other origins have been proposed. Recent studies have suggested that circumferential extensional lineaments (grabens, fissures and fractures) of coronae can be the surface expression of dykes, and analogous to terrestrial circumferential mafic dyke swarms (e.g. Buchan and Ernst, 2021).

Eistla Regio, locus of major plume-generated magmatism and tectonics. We focus on the central portion, termed Central Eistla Regio and focus on two corona Belet-ili and Gaia (Gaya) (Fig. 1). We mapped more than 19,000 extensional lineaments (grabens, fissures and fractures) (Fig. 2) in the study area (13◦-26◦E, 0◦-10◦N) at 1:500,000 scale using full resolution Magellan Synthetic Aperture Radar (SAR) images with resolution of 75 m per pixel. These structures are interpreted to represent the surface expression of underlying mafic dykes and have been grouped, on the basis of trend, into radiating, circumferential and linear dyke swarms. Thirty-two dyke swarms were identified and linked to 20 magmatic centres, most of which are located within or near two coronae, Belet-ili and Gaia (Gaya) coronae, while others are linked to magmatic centres outside the study area (Fig. 3).

These magmatic centres are interpreted to be linked to underlying mantle plumes, and the sizes of the underlying flattened plume heads are estimated from the interpreted dyke swarms (Fig. 3, 4): for radiating swarms, the radius of the transition from proximal radiating to more distal linear pattern, and for circumferential dykes, the radial distance to the outer boundary of the circumferential pattern (Baragar et al. 1996; Buchan and Ernst 2019, 2021; El Bilali et al. 2023). Plume heads for the largest centres, Belet-ili and Gaia coronae are estimated at about 230 km and 275 km in radius, respectively. These sizes are less than the 1200 km radii predicted for a plume arriving from the deep mantle, and would suggest an origin at a shallower level (likely a mid-mantle boundary, i.e. between upper and lower mantle). The NW alignment of Belet-ili and Gaia corona and other magmatic centres in our study area (Fig. 4) potentially represents structural control by a rift zone, which can be traced further north to Anala Mons, one of two main plume centres of Central Eistla Regio (Fig. 5).

References