Eric B. Grosfils

Giant radiating dyke swarms on Earth and Venus

Abstract Concentrations of dykes of basic composition emplaced in the same igneous episode or along similar trends are known as mafic dyke swarms and they occur in a wide variety of environments and over a wide range of scales on Earth. Recent radar mapping of Venus has revealed families of linear features interpreted to be the surface expression of near-surface dyke swarms. The lack of significant erosion on Venus provides a view of the surface manifestation of dyke swarm emplacement, one which complements the terrestrial perspective of erosion to deeper levels. The goal of this review is to synthesize the information available on both planets in order to use the complementary and synergistic record of mafic dyke swarm emplacement to build toward a better understanding of this important phenomenon in planetary history. We focus on the formation and evolution of giant dyke swarms which cover tens to hundreds of thousands of square kilometres on both Earth and Venus. Mafic dyke swarms on Earth occur in a wide range of modes and are observed in environments ranging from volcanic edifices (e.g., Hawaii), to central complexes (e.g., Spanish Peaks Complex, USA; Ramon Swarm, Israel), spreading centres and ophiolite complexes, compressional plate boundaries in back-arc settings (Columbia River Basalts, USA) and in continent-continent collisions. One of the most impressive modes of occurrence is that linked to the formation and evolution of mantle plumes. Terrestrial examples include a giant radiating swarm covering 100° of azimuth (the Mackenzie swarm, Canada), a 360° giant radiating swarm (the Central Atlantic reconstructed swarm), deformed giant radiating swarms (the Matachewan swarm, Canada), rift-arm associated swarms (e.g., Grenville swarm, Canada; Yakutsk swarm, Siberia), and one consisting of widely separated dykes (e.g., the Abitibi swarm, Canada). We summarize the geometric, chemical and isotopic characteristics of terrestrial dyke swarms, including their size and geometry, ages, presence and absence of subswarms, and the relation between swarms of different ages. We also summarize the characteristics of individual dykes, examining dyke length and continuity, en echelon offsets, dyke bifurcation, dyke height, width and depth, dyke intrusion and cooling history, and evidence for flow directions. On Venus at least 163 large radiating lineament systems (radius generally > 100 km) composed of graben, fissure and fracture elements have been identified. On the basis of their structure, plan view geometry and volcanic associations, the radial elements of more than 70% of these are interpreted to have formed primarily through subsurface dyke swarm emplacement, with the remainder forming through uplift or some combination of these two mechanisms. These systems are essentially uneroded and provide a view of the surface characteristics of giant radial swarms prior to the erosion which commonly occurs on Earth. The individual graben, fissures and fractures of which the systems are composed are typically less than several kilometres in width and cluster near the centre, with fissures grading smoothly into fractures at greater distances to define the overall radial pattern. While the largest systems, like those on Earth, are thousands of kilometres in radius, the population average is about 325 km, and they generally do not extend to equal lengths in all directions. In their distal regions, however, the elements in 72% of the systems continue along a purely radial trend, while distal elements in the remaining 28% curve gradually into unidirectional, sub-parallel geometries, generally interpreted to be related to regional stress patterns. The radial systems have a strong association with volcanism; all but seven display some form of volcanic signature. A review of models of the emplacement of lateral dykes from magma chambers under constant (buffered) driving pressure conditions and declining (unbuffered) driving pressure conditions indicates that the two pressure scenarios lead to distinctly different styles of dyke emplacement. Emplacement of lateral dykes in the constant driving pressure (buffered) case, however, can produce dykes which have sizes and widths which are very large and independent of chamber size. On Earth, the characteristics of giant mafic dyke swarms such as the Mackenzie dyke swarm in Canada strongly suggest that they were emplaced in buffered conditions. On Earth, giant radiating dyke swarms are usually preserved as fan-shaped fragments which have been dismembered and distorted by subsequent plate tectonic rifting events. The abundant intact giant radiating swarms on Venus provide criteria by which fragmented terrestrial swarms can be reconstructed.

The global distribution of giant radiating dike swarms on Venus: Implications for the global stress state

Magellan radar data of Venus reveal 163 large radial lineament systems composed of graben, fissure, and fracture elements. On the basis of their structure, plan view geometry, and volcanic associations, at least 72% are interpreted to have formed primarily through subsurface dike swarm eraplacement, the remainder through uplift or a combination of these two mechanisms. The population of swarms is used to determine regional and global stress orientation. The stress configura- tion recorded from 330-210oE (Aphrodite Terra) is best ex- plained by isostatic compensation of existing long wave- length topography or coupling between mantle flow and the lithosphere. The rest are correlated with concentrations of rifting and volcanism in the Beta-Atla-Themis region. The global stress field on Venus is different than that of Earth, where plate boundary forces dominate. from their surroundings. Where domical topography occurs, the ra- dius of the associated lineament system is, on average, 2.5 times greater than that of the dome. Some 53% of the radiat- ing systems are associated with concentric structures, ranging from central, depression-bounding scarps 25 km wide to tec- tonic rings 575 km in diameter. In 51% of these cases, the ra- dial pattern originates within but also extends significantly beyond the annulus. Only 9% of the radiating systems are confined completely, leaving 40% located outside but focused upon a central annular structure. Finally, the radial geometry is usually quite pronounced near the center of each structure: 52% of the time radial lineaments occupy >270 o of azimuth, and 80% exceed 180 o (Figure 2c). In their distal regions, how- ever, only 72% of the radiating systems retain a purely radial geometry, as the remaining 28% gradually develop a non- radial, unidirectional lineament configuration. All but seven of the radiating systems exhibit volcanism (Figure 2d). Lobate flows emerging from individual linea- ments, or many such flows strongly correlated with multiple radial lineaments, occur for 45% of the systems. Clusters of small shields occur 72% of the time, and in 75% of these in- stances distinctive alignment of small shields and/or pits along radiating lineaments is seen. Finally, 65% are associ- ated with some other form of centralized volcanism, including multiple forms of edifice construction, limited extrusions, and sheet-like flows which extend into the surrounding terrain.

Graben-fissure systems in Guinevere Planitia and Beta Regio (264 ◦ -312 ◦ E, 24 ◦ -60 ◦ N), Venus, and implications for regional stratigraphy and mantle plumes

Detailed mapping in a 14,000,000 km 2 area of northwestern Guinevere Planitia and northern Beta Regio bounded by 264 ◦ –312 ◦ E, 24 ◦ – 60 ◦ N has revealed thousands of long extensional lineaments (graben, fissures and related fractures). These can be grouped into radiating, circumferential and linear systems. Thirty four radiating systems have been identified, of which 16 have radii greater than 300 km and eight have radii greater than 1000 km. Twenty six linear (straight) systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km. Linear systems are generally associated with rifts, although some may represent distal portions of radiating systems. In addition, 19 circumferential systems, some associated with coronae, have been identified. The distribution of each system is compared with the host geology in order to place the graben–fissure systems in a regional stratigraphic framework. The majority of systems are: (1) younger than tesserae, ridge belts and densely fractured plains, (2) coeval with, and in many cases, define fracture belts, (3) partially flooded by wrinkle-ridged plains units, and (4) older than smooth and lobate plains units and young rifts. The inventory of radiating graben–fissure systems that we catalogue represents a database of tectono-magmatic centers that complements the centers defined using other criteria, e.g., large volcanoes, coronae, and shield fields. We have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes. At least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms.

The timing of giant radiating dike swarm emplacement on Venus: Implications for resurfacing of the planet and its subsequent evolution

Stratigraphic study of a distributed population of 118 giant radiating dike swarms on Venus reveals that within each dike-intruded region, emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera, regional plains, and most wrinkle ridges. The density of impact craters superimposed on the swarm population (1.80±0.57 craters/10 6 km 2 ), when compared with the average global density (2.01±0.14 craters/10 6 km 2 ) and the densities reported for other geologic units, is consistent with the observed stratigraphy. On the basis of these data, we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing. The stresses recorded by the dike swarm population, when combined with its age and compared with the predictions of several proposed resurfacing models, best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer. Formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long-wavelength gravity and topography. In addition, the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long-wavelength characteristics. We interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long-wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago.

Radiating dike swarms on Venus: evidence for emplacement at zones of neutral buoyancy

Abstract Theoretical calculations of extrusive volcanic degassing on Venus yield atmospheric pressure-related rock density profiles consistent with the formation of magma neutral buoyancy zones and magma reservoirs at different depths as a function of altitude (Head and Wilson, J. geophys. Res. 97, 3877, 1992). Global analysis of radiating dike swarms interpreted to originate at magma reservoirs show that their distribution matches these predictions across approximately 90% of the planets surface; only those highland regions whose elevations exceed 6053 km appear anomalous. The distribution of the large volcano population (extrusive reservoir products) (Keddie and Head, Planet. Space Sci. 42, 455, 1994) has yielded similar results. Comparison between the dike swarm (intrusive) and large volcano (extrusive) populations suggests that neutral buoyancy plays an important role in governing volcanic processes near the venusian surface and that the depth to the level of neutral buoyancy increases systematically at altitudes above 6051 km.

Emplacement of a radiating dike swarm in western Vinmara Planitia, Venus: interpretation of the regional stress field orientation and subsurface magmatic configuration

Magellan radar data from western Vinmara Planitia on Venus reveal a system of radiating lineaments extending 450 km from a small central annulus. Spatial variations in lineament density, orientation, and morphology, as well as structural and volcanic correlations, provide strong evidence that formation of the lineaments was related to subsurface dike emplacement. We infer from the observed surface deformation that the dikes were emplaced laterally, at shallow depth, from a large central magma reservoir. This configuration is analogous to that of radiating dike swarms found on Earth. Because dikes inject normal to the least compressive stress direction, swarm plan view geometry will reveal the greatest horizontal compressive stress trajectories. We interpret strongly radial orientations near the swarm center to represent radial stresses linked to pressurization of the magma reservoir. Increasingly non-radial behavior dominating at greater distances is interpreted to reflect a N60E±20° regional maximum horizontal compressive stress. Contrary to previous inferences that a persistent E–W compressive stress dominated throughout, analysis of the arachnoid indicates that a N60E compressive stress must have existed across western Vinmara Planitia during a portion of its deformation. This and the absence of distributed shear within the adjacent deformation belts indicates that the regional maximum horizontal compression orientation has varied over time. Comparison between the regional stress orientations inferred from the arachnoid and several nearby ridge belts illustrates that stress orientations may potentially be useful for determining relative belt ages in areas where the timing of ridge belt formation is difficult to assess by more direct means. This demonstrates one way that identification and analysis of giant radiating dike swarms can provide new information critical for regional stress interpretations on Venus.

The protracted development of focused magmatic intrusion during continental rifting

The transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER—the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear- or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

Elastic models of magma reservoir mechanics: a key tool for investigating planetary volcanism

Abstract Understanding how shallow reservoirs store and redirect magma is critical for deciphering the relationship between surface and subsurface volcanic activity on the terrestrial planets. Complementing field, laboratory and remote sensing analyses, elastic models provide key insights into the mechanics of magma reservoir inflation and rupture, and hence into commonly observed volcanic phenomena including edifice growth, circumferential intrusion, radial dyke swarm emplacement and caldera formation. Based on finite element model results, the interplay between volcanic elements – such as magma reservoir geometry, host rock environment (with an emphasis on understanding how host rock pore pressure assumptions affect model predictions), mechanical layering, and edifice loading with and without flexure – dictates the overpressure required for rupture, the location and orientation of initial fracturing and intrusion, and the associated surface uplift. Model results are either insensitive to, or can readily incorporate, material and parameter variations characterizing different planetary environments, and they also compare favourably with predictions derived from rheologically complex, time-dependent formulations for a surprisingly diverse array of volcanic scenarios. These characteristics indicate that elastic models are a powerful and useful tool for exploring many fundamental questions in planetary volcanology.

Topographic and morphologic characteristics of Reull Vallis, Mars: Implications for the history of the Reull Vallis fluvial system

[1] The eastern rim region of Hellas basin is characterized by the four prominent and quite extensively researched (cf. Crown et al., 2005, and references therein) outflow channels. In this work we focus on the Reull Vallis. On the basis of observations from available data sets, we present a hypothesis for the evolution of Reull Vallis and its complimentary fluvial system. We suggest that this system consists of parts that were formed during several phases rather than being a single continuous channel. Our results show that the fluvial system of Reull Vallis consists of two main parts and likely had independent formation phases and different sources of water. Our results also show that the upper portion of the Reull Vallis was formed by outflow from beneath Hesperia Planum (as proposed already in earlier works), but the suggested segments 1 and 2 (Mest and Crown, 2001) of the Vallis are not directly linked. There seems to have been an on-surface source for the formation of the segment 2 in the form of a topographic depression that was filled before the subsequent draining and formation of segment 2. Our interpretation of the evolution and formation implies a complex history for the Reull Vallis system.

Effects of crustal-scale mechanical layering on magma chamber failure and magma propagation within the Venusian lithosphere

Understanding the connection between shallow subsurface magmatism and related surface expressions provides first-order insight into the volcanic and tectonic processes that shape a planets evolution. When assessing the role of flexure, previous investigations assumed homogeneous host rock, but planetary lithospheres typically include crust and mantle material, and the mechanical response of a layered lithosphere subjected to flexure may influence both shallow magma reservoir failure and intrusion propagation. To assess the formation of giant radial dike systems, such as those observed on Venus, we create axisymmetric elastic finite element models of a spherical reservoir centered at the contact between stiff, dense mantle overlain by softer, lighter crust. We analyze magma chamber stability, overpressure at rupture, and resulting intrusion types for three distinct environments: lithostatic, upward flexure, and downward flexure. In the lithostatic case, reservoir failure at the crust-mantle contact favors lateral sill injection. In the flexure cases, we observe that failure location depends upon the crust/lithosphere thickness ratio and, at times, will favor radial dike intrusion. Specifically, upward flexure can promote the formation of giant radiating dike swarms, a scenario consistent with a plume-derived origin. Our results present a mechanical explanation for giant radial dike swarm formation, showing that both the stability of magma chambers on Venus and the type of intrusions that form are influenced by lithospheric layering. Furthermore, where dike swarms occur, our approach provides a powerful new way to constrain local crust/mantle layering characteristics within the lithosphere at the time the swarm was forming.