February 2024 LIP of the Month
The 3700 km long Great Dyke of Atla Regio, Venus
H. El Bilali, R.E. Ernst
Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada; hafidaelbilali@cunet.carleton.ca, RichardErnst@cunet.carleton.ca.
Extracted and Modified from El Bilali, H. and Ernst, R.E. (2024) Far-travelled 3700 km lateral magma propagation just below the surface of Venus. Nature Communications, 15, 1759, https://doi.org/10.1038/s41467-024-45603-6
Mafic Dyke Swarms
Dyke swarms are the dominant mode of transport of mafic magma on planetary bodies including Earth, and the largest swarms have giant radiating or circumferential patterns that can be linked to mantle plumes (Halls 1982; Fahrig 1987; Ernst and Baragar 1992; Grosfils and Head 1994a,b; Ernst et al. 1995; Baragar et al. 1996; Mège and Masson 1996; Head and Coffin 1997; Ernst et al. 2001; Wilson and Head 2002; Scott et al. 2002; Hansen and Olive 2010; Srivastava et al. 2019; Buchan and Ernst 2021; El Bilali et al. 2023). Giant radiating swarms are interpreted to be laterally emplaced from their plume centre regions whereas giant circumferential swarms are likely fed both vertically, and in part laterally. The longest radiating swarms on Earth have a maximum known radius of up at least 2000 km (for the 1270 Ma Mackenzie of northern Canada) and perhaps up to nearly 3000 km (for the 201 Ma CAMP LIP of the Atlantic bordering continents).
Given this lateral emplacement interpretation, it can be predicted that individual dykes of radiating dyke swarms must extend the length of the swarm (e.g. up to 3000 km). However, the longest distance that individual mafic dykes have actually been traced so far on Earth is much shorter: the 1140 Ma Great Abitibi dyke of the Canadian shield is traced for 700 km (Ernst 1991), the Red Sea dyke for about 1000 km (Eyal and Eyal 1987), and the Great Dyke of Zimbabwe for 550 km (Podmore and Wilson 1987). On Earth it is often difficult to track the full extent of individual dykes for the following reasons: 1) a dense dyke swarm makes it difficult to track single dykes, 2) continental breakup and collisional deformation can fragment swarms, and 3) younger sedimentary or volcanic units can obscure the portions of dyke swarms.
In El Bilali and Ernst (2024) and below, we identify the longest individual graben (interpreted as overlying a dyke), traced so far, on Venus (and indeed in the solar system) and discuss the implication of this discovery, and more generally, the importance of identifying other long individual dykes.
Mafic dyke swarms on Venus
Due to the current hot surface temperature (ca. 450° C), there is an absence of fluvial erosion (and wind erosion is relatively minor) on Venus (e.g. Solomon et al. 1992) extending back at least hundreds of myr (Way and Del Genio 2020). The relatively minor role of erosion means that intrusive units such as dykes are not exposed. However, the presence of mafic dykes on Venus can be inferred from their surface expression as narrow grabens (and also pit chains/troughs and chains of shield volcanoes) which are inferred to overlie blind dykes (dykes not reaching the surface) (McKenzie et al. 1992; Grosfils and Head 1994a,b; Ernst et al. 2003; Studd et al. 2011; Davey et al. 2013; Graff et al. 2018; El Bilali et al. 2023; Chaddha et al. 2023; El Bilali and Ernst 2024).
Great Dyke of Atla Regio (3700 km long)
Through detailed mapping using 75-150 m/pixel Synthetic Aperture Radar (SAR) images from the 1989-1994 NASA Magellan spacecraft (e.g. Pettengill et al. 1991) we have traced a single graben on Venus nearly continuously for 3700 km (Fig. 1), and we interpret this graben to overlie a single laterally-emplaced mafic dyke. [Small gaps are due to younger volcanic or tectonic cover, or locally complicated geology that prevents tracing of the graben.] This Great Dyke of Atla Regio (GDAR) belongs to a giant radiating dyke swarm associated with Ozza Mons (volcano) of the Atla Regio superplume. The GDAR was fed from a magma reservoir ~600 km south of the Ozza Mons centre (rotated white box in Fig. 1). A 50-degree counter-clockwise swing of the GDAR at 1200 km from the centre is consistent with a 1200 km radius for the underlying Ozza Mons plume head (El Bilali and Ernst 2024). The swing into parallelism with the 10,000 km long Parga Chasmata rift system indicates that emplacement of the GDAR was coeval with active extension along Parga Chasmata (El Bilali and Ernst, 2024).
Our discovery of the GDAR, should encourage the search for additional long continuous single dykes on Venus (and Earth), with implications for estimating the size of the mantle plume head, locating magma reservoirs, mapping regional stress variation at a geological moment, and revealing relative ages over regional-scale distances (through cross-cutting relationships). More details on the GDAR are available in El Bilali and Ernst (2024).
Fig. 1: Tracing of the Great Dyke of Atla Regio (GDAR) (modified after El Bilali and Ernst 2024). The GDAR (solid purple line) can be traced for more than 3700 km from a source magma reservoir marked by a rotated rectangular white box. The GDAR is part of a radiating system of grabens (orange lines) after (El Bilali et al. 2023) interpreted to represent a giant radiating dyke swarm focussed on Ozza Mons with centre marked by blue star. Blue circle indicates the transition distance (1200 km) from the purely radiating pattern. Beyond 1200 km the GDAR swings into parallelism with the Parga Chasmata (rift system) (see inset). OZM=Ozza Mons, MM=Maat Mons, UM=Unnamed Mons, OM=Ongwuti Mons, NF=Ningyo Fluctus, OC=Oduduwa Corona, ONC= Onenhste Corona. The yellow numbers, 1-3, along GDAR correspond to the three closeup images of the GDAR graben. Inset diagram (after El Bilali et al. 2023) shows location of BAT region with major plume generated magmatic centres: Atla Regio (A), Beta Regio (B) and Themis Regio (T) connected by major rift systems (Chasmata), with background image after Herrick (1999) showing major volcanic edifices (green), corona (yellow) and rift zones (red) on the geoid (grey scale, lighter is higher).
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