May 2024 LIP of the Month

Heng-o Corona, Venus: Dyke Swarms Record Evolution of its Underlying Mantle Plume

Aya Tessier (1), Richard E. Ernst (2), Hafida El Bilali (2)

1 Department of Physics, Carleton University, Ottawa, Canada, 2 Department of Earth Sciences, Carleton University, Ottawa, Canada

Extracted and modified from:

Tessier, A., Ernst, R.E., El Bilali, H. (2024) Heng-o Corona, Venus: Dyke Swarms Record Evolution of its Underlying Mantle Plume. Icarus 417, 116090

Abstract

Detailed 1:500,000 scale mapping of 24,000 graben-fissure-fractures (interpreted to overlie dykes) of Heng-o, the second largest corona on Venus has provided new insights into its dyke swarm and plume history, and with implications for plume-generated Large Igneous Provinces (LIPs) on Earth. Three large radiating swarms are identified and are associated with arrival of the Heng-o mantle plume head (about 1000 km diameter) at the base of the Venusian lithosphere. This is followed by the formation of an annular ring system of similar diameter that has defined the size of Heng-o. This ring is characterized by an arcuate ridge and bounding troughs and is associated with the transition from plume generated uplift to spreading of the plume head along the base of the lithosphere. Formation of the annular ring is accompanied by three circumferential swarms (C1-C3). Continued spreading of the plume head (without development of further annular topographic rims) is associated with additional larger circumferential swarms (C4 and C5) which nearly double the diameter of Heng-o from 1000 km to 1970 km (interpreted to correspond to the maximum diameter of the underlying flattened plume head).

Introduction

The current surface of Venus has high temperatures, and high atmospheric pressure and the absence of erosion and the absence of plate tectonics, during which about 80% of the surface was covered by basaltic volcanism and cut by extensive graben systems thought to represent the surface expression (owing to the absence of erosion) of underlying mafic dyke swarms. As a contribution to ongoing studies to recognize and characterize Large Igneous Provinces (LIPs) analogues on Venus particularly through their flood basalts and regional mafic dyke swarms (Head and Coffin 1997; Ernst and Desnoyers 2007; Hanson 2007; Ernst 2014; Ernst et al. Braga et al. 2024; Chaddha et al. 2023; El Bilali et al 2023; El Bilali and Ernst, 2024) we here present a study of a Heng-o corona as a major plume-generated LIP (Tessier et al. 2024).

Coronae and graben-fissure systems

Coronae are common features on the surface of Venus with at least >500 recognized (Stofan et al. 2001; Glaze et al. 2002; Guseva and Ivanov, 2020). They are quasi-circular or quasi-elliptical tectono-magmatic features, but typically consisting of an annular rim and/or an annular trough, which often have circumscribing lineaments, typically extensional (grabens), but can also include compressional lineaments (wrinkle ridges) and a central region that can be depressed or elevated (Stofan et al. 1992; Smrekar et al. 2023; Grindrod and Hoogenboom, 2006; Guseva and Ivanov, 2020; Gülcher et al. 2020). Recent studies have suggested that circumferential extensional lineaments (grabens, fissures and fractures) of coronae can be analogues of terrestrial giant circumferential swarms (Buchan and Ernst, 2019, 2021).

Coronae are generally ascribed to mantle diapirs and plumes (with the larger coronae linked to larger plumes). Coronae are often spatially and genetically associated with rift zones (Stofan et al., 2001; Glaze et al., 2002; Ernst et al. 2007; Krassilnikov et al., 2012; Graff et al., 2018 Chaddha et al 2023). Radiating graben systems (interpreted as mafic dyke swarms) can also be associated with coronae, with the radiating system typically preceding the circumferential systems (Buchan and Ernst, 2021). The radiating systems of coronae can extend far outside the circumferential pattern. The largest corona, Artemis, has a radiating graben system, inferred to be underlain by a radiating dyke swarm 12,000 km in diameter (Hansen and Olive, 2010; Hansen and Lopez, 2018). It is also noted that radiating swarms can converge to multiple centers within corona (e.g. Ernst et al. 2003).

We undertook geological mapping at a much greater detail (1:500,000) than previous mapping at 1:5,000,000 (Bender et al. 2000; Copp and Guest 2007) and at 1:10,000,000 (Ivanov and Head 2011), in order to provide new insights into the origin of Heng-o corona, particularly the evolution of an underlying mantle plume as revealed by the dyke swarm patterns. Full details of our study are presented in Tessier et al. (2024).

Study Area and previous mapping

Heng-o Corona (Fig. 1) is located in the volcanic plains (Guinevere Planitia) south of two important regional volcanic rises (Western and Central Eistla Regio) and north of Alpha Regio (a major tesserae region). Heng-o Corona is defined by a prominent annulus with an overall radius of 1060 km (Fig. 2); this annulus is particularly well developed on the north, and also on the southeast and southwest sides. The prominent annulus (ridge and flanking troughs) on the north side has been termed Heng-o Chasma (Fig. 1).

Methodology

Geological mapping is being carried out using full-resolution (75 m/pixel) SAR (Synthetic Aperture Radar) images https://astrogeology.usgs.gov/search?pmi-target=venus ) from the 1989-1994 NASA Magellan mission Cycle 1 (left-look) and altimetry data, all provided by USGS. In this study we interpret radiating, circumferential and linear graben sets as overlying (mafic) dykes (based on criteria presented in Grosfils and Head, 1994a,b; Ernst et al. 2003; Davey et al. 2013; Patterson et al. 2016; El Bilali and Ernst, 2024).

Results

A total of 24,600 graben-fissure-fracture lineaments (“grabens” for short) were mapped (Fig. 3) and grouped on the basis of trend into radiating (Fig. 4), circumferential (Fig. 5) and linear dyke swarms (see Tessier et al. 2024).

Pre to Early Heng-o: The radiating and circumferential systems in Groups A, B and C (Fig. 6) either represent an event that entirely precedes the Heng-o event or represents an earlier stage of Heng-o.

Main Heng-o Event: The swarms of Group D (Fig. 6) represent the main radiating and circumferential systems of Heng-o. Cross-cutting relationships indicate that the three main radial swarms (R1, R2 and R3) preceded the development of the circumferential swarms. However, the relative ages of these three radiating swarms are not known. R2 is centrally located and for this reason might logically have been emplaced first, since the central part of a plume head would presumably reach the base of the lithosphere first.

The circumferential swarms, C1-5, are basically concentric and their relative ages are not determined. However, we suggest that swarm diameter increases with decreasing age (Fig. 5). This simple pattern would be consistent with a model of circumferential dyke emplacement from the outer edge of the expanding plume head as it flattens and spreads against the base of the lithosphere (see model in Buchan and Ernst, 2019). The circumferential grabens are most visible in the interior of the corona, on the elevated rim of the main annulus, and on topographic highs outside of that.

Post or late Heng-o event: The swarms in Group E (Fig. 6): R8 (350 km radius) and C8 (95 km radius) belong to a smaller coupled unnamed feature (center coordinates 3.97W, 2.80 N and 6.09W, 2.52 N, respectively), located to the northeast inside the main ring and which formed after the main activity of Heng-o. It is not clear whether it is a late pulse of the Heng-o event or entirely postdates the Heng-o event. Grabens associated with R12 and C7 have similar prominence to grabens from R1 and R2 in the center of the corona. They likely formed at a similar time as group D, or soon after, and are therefore plausibly linked to a late stage of Heng-o Corona.

DISCUSSION

Doubling the size of the Heng-o Corona through recognition of a larger outer circumferential ring: One of the most significant discoveries in this study is the existence of an outer circumferential swarm of grabens ranging between 1370 km and 1970 km in diameter (Figs. 5 and 7). This nearly doubles the diameter of Heng-o Corona, from 1060 km to 1970 km.

Plume history of Heng-o Corona: The plume history of Heng-o seems simple: initial uplift during arrival of the plume head and associated with local radiating swarms, followed by expansion of the plume head as it flattens against the base of the lithosphere (consistent with modelling in Griffiths and Campbell 1991) thus producing the set of multiple circumferential swarms that track the lateral spreading of the plume head (Buchan and Ernst, 2019). Our plume model for Heng-o is illustrated in Figure 8.

Uplift and the radiating swarms

Our mapping has revealed three main radiating swarms associated with Heng-o Corona. These are all large swarms with full radial extents of > 680 km (for R1), >980 km (for R2) and >995 km (for R3). We note that the hypothesized outer boundary of plume head uplift (collectively implied by the three radiating swarms, R1, R2 and R3 (Fig. 4) is largely within the overall outer boundary of the full circumferential swarm (cf, Figs. 5 and 7) consistent with the extent of domal uplift due to the ascending plume (cf models of Friedrich et al. 2018; Griffiths and Campbell, 1991). This would imply a broad regional uplift associated with plume ascent and would also be consistent with a plume head radius at this time that is ~500 km; measured from the geographic center of Heng-o and consistent with the distance of the R1 and R3 centers from the geographic center of Heng-o, 200 km and 450 km, respectively. This model would imply that flattening of the plume head against the lithosphere had already started by this time (Fig. 8b). Note that the focus of R2 is at the center of Heng-o and so could imply that the R2 radiating swarm was the first emplaced and then R1 and R3.

Spreading of the flattened plume head

There are arcuate segments of circumferential systems at all distances between the inner ring and the outermost rings and most of these are incomplete due to flooding by younger flows (Fig. 5 and 7). Apart from the innermost smaller circumferential systems (which are younger) the rest (in the interpreted age order oldest to youngest, C1, C2, C3, C4 and C5) have an increasing radius which would be consistent with a model of circumferential systems forming at the leading edge of the expanding plume head (see model in Buchan and Ernst, 2019). The relative age of C6 is uncertain. Each circumferential swarm has a width, and it can be inferred that each distinguish subswarm represents a pulse of lateral spreading of the plume head.

We note that all these circumferential systems have radii greater than the maximum radius of the inferred plume head size (~500 km) inferred from the radiating swarms. From this observation we conclude that the transition from radiating to circumferential swarms marks the transition from plume head arrival (and flattening upward against the base of the lithosphere) to active lateral spreading of the flattened plume head beneath the lithosphere.

Interpretation of the annulus

The origin of annular rings (of ridges and troughs) associated with corona have been discussed in many papers (e.g. Davaille et al. 2017; Gülcher et al. 2020) and modelling involves dynamics of the plume- lithosphere interaction. In the case of Heng-o Corona the main annular ring is located at a distance of about 500 km radius and in detail consists of three sub-stages (Fig. 2b). As mentioned above the earliest would seem to be the smaller one on the eastern side (grey line) and which is broadly correlated with circumferential swarm C1, while the main one (black line) encompasses both the prominent northern rim (termed Heng-o Chasma) and also the prominent southern and southwestern rim and is broadly correlated with C2 and C3. The final is the annular ring (white arcuate line in Fig. 2b) which marks the outer uplifted rim in the south, but is also within our C2. The annular ring can be interpreted to represent folding of the lithosphere at the edge of the plume, perhaps associated with proto-subduction (Davaille, 2017).

It is significant that that annulus is only developed during the transition from uplift associated with the radiating swarms, to some flattening of the plume head associated with the circumferential swarms (C1-3). Remarkably, the larger C4-C6 circumferential swarms are not associated with development of a topographic annulus (ridge and trough). Perhaps this indicates the absence of any proto-subduction associated with these later pulses of outward spreading (associated with C4-C6) of the plume head (Fig. 8d)

Future work

This study has provided a detailed analysis of the graben systems (interpreted to overlie dyke swarms) in Heng-o region. Future detailed mapping of the lava flows at the same scale (1:500,000) will utilize the identified radiating and circumferential swarms and the specific magmatic centers that they are associated with as a framework for interpreting the flows– as sources for flows. Also, the detailed mapping of the lava flows (building on the earlier published reconnaissance mapping discussed above, integrated with the distribution and age relationships of the grabens (dykes) characterized in this manuscript will allow a more comprehensive geological history to be determined.

The insights from the dyke swarm history for plume evolution and the distinguishing of plume head arrival and uplift associated with development of radiating dyke swarms and a second stage of circumferential swarms associated with lateral spreading of the flattened plume beneath the lithosphere- is a context that can potentially be applied to plume-generated LIPs on Earth.

References