Abstract Although the present environmental conditions on Mars prohibit the generation of significant volumes of liquid water, observations of several very young landforms, such as gullies and recurrent slope lineae, have been interpreted as signals for aqueous processes. To explore the range of conditions under which such features can be formed on Earth, a field site in northern Victoria Land, East Antarctica, was geomorphologically investigated. Despite the small size of the ice-free area, the site displays gullies, water tracks and other traces of liquid water. The gullies show clear evidence of sediment transport by debris flows, and are typical of paraglacial processes on steep slopes in a recently deglaciated area. Water tracks appear in different forms, and seem to recur seasonally in the austral summer. Melting of snow and surface glacier ice is the major water source for both debris flows and water tracks. The observations presented here highlight the potential for hyperarid polar deserts to generate morphogenetically significant amounts of meltwater. The gullies are morphologically analogous to Martian gullies, and water tracks on steep slopes appear very similar to recurrent slope lineae. The observations suggest that even small ice-free sites in continental Antarctica may enable observations which can serve as a basis for working hypotheses in Mars analogue studies, and future field work should consider more areas in Antarctica in addition to the McMurdo Dry Valleys to search for Mars analogue landforms.

Abstract Volcanism and tectonism are the dominant endogenic means by which planetary surfaces change. This book, in general, and this overview, in particular, aim to encompass the broad range in character of volcanism, tectonism, faulting and associated interactions observed on planetary bodies across the inner solar system – a region that includes Mercury, Venus, Earth, the Moon, Mars and asteroids. The diversity and breadth of landforms produced by volcanic and tectonic processes are enormous, and vary across the inventory of inner solar system bodies. As a result, the selection of prevailing landforms and their underlying formational processes that are described and highlighted in this review are but a primer to the expansive field of planetary volcanism and tectonism. In addition to this extended introductory contribution, this Special Publication features 21 dedicated research articles about volcanic and tectonic processes manifest across the inner solar system. Those articles are summarized at the end of this review.

Abstract In this work, we combined multi-scale geological maps of Mercury to produce a new global map where geological units are classified based on albedo, crater density and morphological relationships with other units. To create this map, we used the 250 m/pixel mosaic of images acquired by the narrow- and wide-angle cameras onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during its orbital phase. The geological mapping is supported by digital terrain model data and surface mineralogical variation from the global mosaic of MESSENGER Mercury Atmospheric and Surface Composition Spectrometer observations. This map comprises the global-scale intercrater plains, smooth plains and Odin-type units as reported in previous studies, as well as units we term bright intercrater plains, Caloris rough ejecta and dark material deposits. We mapped a portion of the Raditladi quadrangle (19–35°N, 106–133°E) at a regional scale at a resolution of 166 m/pixel. We characterized the geological context of the area and evaluated the stratigraphic relationships between the units. To obtain a representative geological section, we analysed and corrected available topographical data. The geological cross-section derived from our regional mapping suggests that volcanic emplacement of Raditladi’s inner plains followed the topography of the basin after the deposition of impact-related units (i.e. melts, breccias and rim collapse) and was driven by low-viscosity flows. Hollows that appear on Raditladi’s peak ring were possibly formed from low-reflectance intercrater plains materials exposed through the peak ring unit. Supplementary material: Cleaner, larger version of the global-scale geological map and a local-scale map for comparison are available at http://www.geolsoc.org.uk/SUP18741 .

Abstract The volcanic and tectonic histories of Venusian coronae appear to be intricately linked. We explore that link through the construction of geological maps of three coronae using Magellan synthetic aperture radar (SAR) imagery and altimetry radar data. Each examined corona – Aramaiti, Bhumidevi and Zemire Coronae – is characterized by an annulus of concentric fractures, lava flows, tholi and clusters of small shields. Radial fractures occur at one corona (Bhumidevi Corona), whereas the other two coronae (Aramaiti and Zemire Coronae) occur within linear fracture belts. Based on observed timing relationships, we propose that the evolution of these three coronae is similar to large silicic caldera formation on Earth in that the formation of concentric fractures facilitated a mass evacuation of a magma reservoir residing underneath each corona resulting in subsequent corona collapse. Our model emphasizes the late-stage evolution of these coronae and predicts that all the erupted products could be basaltic. Such predictions are testable through continued geological mapping of coronae, as well as through in situ spectral analyses by Venusian landers.

Abstract Explosive volcanism on Venus is severely inhibited by its high atmospheric pressure and lack of water. This paper shows that a deposit located near 16°S, 145°E, here referred to as Scathach Fluctus, displays a number of morphological characteristics consistent with a pyroclastic flow deposit. These characteristics, particularly the lack of channelization and evidence for momentum- rather than cooling-limited flow length, contrast with fissure-fed lava flow deposits. The total erupted volume is estimated to have been between 225 and 875 km 3 but this may have been emplaced in more than one event. Interaction between Scathach Fluctus and a small volcanic cone constrains the flow velocity to 48 m s −1 , and plausible volatile concentrations to at least 1.8 wt% H 2 O, 4.3 wt% CO 2 or 6.1 wt% SO 2 , the latter two values implying that magma was sourced directly from the mantle. The deposit has radar characteristics, particularly an exponential backscatter function, that are similar to those of nearly half the planetary surface, implying that pyroclastic deposits may be much more common on Venus than has been recognized to date, and suggesting both a relatively volatile-rich mantle and a volcanic source for atmospheric SO 2 .

Abstract On Earth, most tectonic plates are regenerated and recycled through convection. However, the Nubian and Antarctic plates could be considered as poorly mobile surfaces of various thicknesses that are acting as conductive lids on top of Earth’s deeper convective system. Here, volcanoes do not show any linear age progression, at least not for the last 30 myr, but constitute the sites of persistent, focused, long-term magmatic activity rather than a chain of volcanoes, as observed in fast-moving plate plume environments. The melt products vertically accrete into huge accumulations. The residual depleted roots left behind by melting processes cannot be dragged away from the melting loci underlying the volcanoes, which may contribute to producing an unusually shallow depth of oceanic swells. The persistence of a stationary thick depleted lid slows down the efficiency of melting processes at shallow depths. Numerous characteristics of these volcanoes located on motionless plates may be shared by those of the giant volcanoes of the Tharsis province, as Mars is a one-plate planet. The aim of this chapter is to undertake a first inventory of these common features, in order to improve our knowledge of the construction processes of Martian volcanoes.

Abstract Lunar mare basalts are spatially unevenly distributed, and their abundances differ between the nearside and farside of the Moon. Although mare asymmetry has been attributed to thickness variations in the low-density anorthositic crust, the eruptive mechanism of lunar magma remains unknown. In this study, we investigate the relationship between mare distribution and crustal thickness using geological and geophysical data obtained by the SELENE (Kaguya) and the Gravity Recovery and Interior Laboratory spacecraft, and quantitatively re-evaluate the influence of the anorthositic crust on magma eruption. We identify a lateral heterogeneity in the upper limit of crustal thickness that allows magma extrusion to the surface. In the Procellarum KREEP Terrane, where the surface abundances of heat-producing elements are extremely high, magmas can erupt in regions of crustal thickness below about 30 km. In contrast, magma eruptions are limited to regions of crustal thickness below about 20 km in other nearside regions, around 10 km in the South Pole–Aitken Basin and approximately 5 km in the farside Felspathic Highland Terrane. Such heterogeneity may result from lateral variations in magma production in the lunar mantle and/or crustal density.

Abstract Visible and Near-Infrared (VNIR) reflectance spectroscopy is an important technique with which to map mineralogy and mineralogical variations across planetary surfaces using remotely sensed data. Absorption bands in this spectral range are due to electronic or molecular processes directly related to mineral families or specific compositions. Effusive igneous rocks are widely recognized materials distributed on the surfaces of terrestrial planets, and are formed by primary minerals that can be discriminated by electronic absorptions (e.g. crystal field absorption). In this paper, we review the current knowledge of effusive rock compositions obtained by crystal field absorption in VNIR reflectance spectroscopy, and consider how different petrographical characteristics influence the mineralogical interpretation of such rock compositions. We show that: (1) the dominant mineralogy can be clearly recognized for crystalline material, especially with relatively large crystal dimension groundmass or high porphyritic index; (2) both grain and crystal size are important factors that influence the spectra of effusive rocks where groundmass is generally characterized by microscopic crystals; and (3) glassy dark components in the groundmass reduce or hide the crystal field absorption of mafic minerals or plagioclase otherwise expected to be present.

Abstract The Rembrandt basin is the largest well-preserved impact feature in the southern hemisphere of Mercury. Its smooth volcanic infill hosts wrinkle ridges and graben, and the entire basin is cross-cut by the Enterprise Rupes scarp system. On the basis of the Model Production Function crater chronology, our analysis shows that the formation of the Rembrandt basin occurred at 3.8±0.1 Ga during the Late Heavy Bombardment, consistent with previous studies. We also find that the smooth plains interior to the basin were emplaced between 3.7 and 3.6±0.1 Ga, indicative of a resurfacing event within the Rembrandt basin that is consistent with the presence of partially buried craters. These youngest plains appear temporally unrelated to basin formation, and so we regard their origin as likely to be due to volcanism. We identify the same chronological relationship for the terrain cross-cut by Enterprise Rupes to the west of the basin. Therefore, volcanic activity affected both the basin and its surroundings, but ended prior to the majority of basin- and regional-scale tectonic deformation. If Enterprise Rupes formed prior to the Rembrandt basin, then regional-scale tectonic activity along this structure might have lasted at least 200 myr.

Abstract Deception Island is a small and volcanically active caldera volcano of Quaternary age, located in the marginal basin of Bransfield Strait, NW Antarctica. The distribution and orientation of fracture and fault systems that have affected the Deception volcanic edifice, and the elongated geometry of its volcanic caldera, are consistent with a model of Riedel deformation induced by a regional left-lateral simple shear zone. It is suggested that this caldera was formed above a magma chamber stretched under the influence of the regional transtensional regime with left-lateral simple shear. The collapse may have occurred in at least two phases: first, a small volume event occurred along the compressed flanks of the volcano edifice; and second, a large collapse event affected the stretched flanks of the volcano edifice.

Abstract Of the features that characterize large shield volcanoes on Mars, flank terraces remain the most enigmatic. Several competing mechanisms have been proposed for these laterally expansive, topographically subtle landforms. Here we test the hypothesis that horizontal contraction of a volcano in response to the down-flexing of its underlying basement leads to flank terracing. We performed a series of analogue models consisting of a conical sand–plaster load emplaced on a basement comprising a layer of brittle sand–plaster atop a reservoir of viscoelastic silicone. Our experiments consistently produced a suite of structures that included a zone of concentric extension distal to the conical load, a flexural trough adjacent to the load base and convexities (terraces) on the cone’s flanks. The effects of variations in the thickness of the brittle basal layer, as well as in the volume, slope and planform eccentricity of the cone, were also investigated. For a given cone geometry, we find that terrace formation is enhanced as the brittle basement thickness decreases, but that a sufficiently thick brittle layer can enhance the basement’s resistance to loading such that terracing of the cone is reduced or even inhibited altogether. For a given brittle basement thickness, terracing is reduced with decreasing cone slope and/or volume. Our experimental results compare well morphologically to observations of terraced edifices on Mars, and so provide a framework with which to understand the developmental history of large shield volcanoes on the Red Planet.

Abstract The occurrence and distribution of monogenic eruptive features in volcanic areas testify to the presence of deep-crustal or subcrustal magma reservoirs hydraulically connected to the surface via a fracture network. The spatial distribution of vents can be studied in terms of self-similar (fractal) clustering, described by a fractal exponent D and defined over a range of lengths ( l ) between a lower and upper cutoff, L co and U co , respectively. The computed U co values for several volcanic fields on Earth match the thickness of the crust between vents and magma reservoirs at depth. This analysis can thus be extended to other volcanic fields and volcanoes on rocky planets in the solar system where features such as vents and dykes occur, and for where complementary geophysical data are currently lacking. We applied this method to the Ascraeus Mons volcano on Mars, which presents hundreds of collapse pits similar to those observed on Earth volcanoes that are most likely related to feeder dykes. Based on structural mapping with High Resolution Stereo Camera data at 12 m/px and Context Camera data at 6 m/px mosaics, more than 2300 collapse pits and dyke traces were analysed, revealing two distinct fractal clustered populations. The obtained U co values reveal the presence and likely depth of both a deep magma reservoir ( c. 60 km deep) and a small shallower chamber ( c. 11 km deep). This analysis can help to better constrain the depth and time evolution of volcanic processes on Tharsis, and on terrestrial planets’ volcanoes in general.

Abstract Large volcanic edifices constitute enormous loads at the surfaces of planets. The lithosphere, the mechanically strong outer layer of a planet, responds to growing edifice loads by flexing. The shape of this lithospheric flexure and the resulting stress state exert critical influences on the structure of the evolving edifices, which in turn feed back into the flexural response. Flexural subsidence of the lithosphere forms topographical moats surrounding volcanoes that are partially to completely filled by landslide debris, volcaniclastic materials and sediments, or by relatively flat aprons of volcanic flows. Flexure creates a characteristic ‘dipole’ state of stress that influences subsequent magma ascent paths and chamber dynamics in the lithosphere. Compression in the upper lithosphere can inhibit magma ascent and favour the development of oblate magma chambers or sill complexes. This compression can be transferred into the edifice unless a décollement allows the volcano base to slip over the underlying lithosphere; generally, basal décollements are found to operate via high pore-fluid pressure in a clay sediment-based layer. Volcanoes lacking such a layer, regardless of the thickness of the basal sediments, lack basal décollements and, thus, tend to be limited in size by compressive stresses adverse to magma ascent.

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.

Abstract At a global scale, Mercury is dominated by contractional features manifested as lobate scarps, wrinkle ridges and high-relief ridges. Here, we show that some of these features are associated with strike-slip kinematic indicators, which we identified using flyby and orbital Mercury Dual Imaging System (MDIS) data and digital terrain models. We recognize oblique-shear kinematics along lobate scarps and high-relief ridges by means of (1) map geometries of fault patterns (frontal thrusts bordered by lateral ramps, strike-slip duplexes, restraining bends); (2) structural morphologies indicating lateral shearing (en echelon folding, pop-ups, pull-aparts); and (3) estimates of offsets based on displaced crater rims and differences in elevation between pop-up structures and pull-apart basins and their surroundings. Transpressional faults, documented across a wide range of latitudes, are found associated with reactivated rims of ancient buried basins and, in most cases, linked to frontal thrusts as lateral ramps hundreds of kilometres long. This latter observation suggests stable directions of tectonic transport over wide regions of Mercury’s surface. In contrast, global cooling would imply an overall isotropic contraction with limited processes of lateral shearing induced by pre-existent lithospheric heterogeneities. Mantle convection therefore may have played an important role during the tectonic evolution of Mercury.

Abstract The tectonic evolution of Mercury is dominated at a global scale by contractional features such as lobate scarps that are widely distributed across the planet. These structures are thought to be the consequence of the secular cooling of Mercury. Therefore, dating these features is essential to place constraints on the timing of planetary cooling, which is important for understanding the thermal evolution of Mercury. In this work, we date an extended thrust system, which we term the Blossom Thrust System, located between 80°E and 100°E, and 30°N and 15°S, and which consists of several individual lobate scarps exhibiting a north–south orientation and a westward vergence. The age of the system was determined using several different methods. Traditional stratigraphic analysis was accompanied by crater counting of units that overlap the thrust system and by using the buffered crater-counting technique, allowing us to determine an absolute model age for the tectonic feature. These complementary methods give consistent results, implying that activity on the thrust ended between 3.5 and 3.7 Ga, depending on the adopted absolute-age model. These data provide an important insight into this portion of Mercury’s crust, which may have implications for models of the thermal evolution of the planet as a whole.

Abstract Craters cross-cut by faults are used as markers to obtain fault geometric and kinematic properties. Assuming that the shape of these craters was originally circular, it is possible to measure the horizontal and vertical components of fault displacement as well as the slip trend. By applying trigonometric relations, slip plunge, displacement magnitude, fault true dip and fault rake can be derived from the observed values. An example application of this method on craters faulted by lobate scarps on Mercury shows that most of these inferred reverse faults have moderate oblique-slip trends. Moreover, the derived dips of thrusts vary over a wide range of angles. Some preliminary results in terms of fault rake compared with fault dip, strike and latitude are presented together with a pilot study to test and discriminate global tectonic models suggested for the evolution of Mercury. The possibility of estimating quantitative fault parameters through remotely sensed data provides significant assistance in the structural characterization of faults on planetary surfaces.

Abstract Regional shear zones are interpreted from Bouguer gravity data over northern polar to low southern latitudes of Venus. Offset and deflection of horizontal gravity gradient edges (‘worms’) and lineaments interpreted from displacement of Bouguer anomalies portray crustal structures, the geometry of which resembles both regional transcurrent shear zones bounding or cross-cutting cratons and fracture zones in oceanic crust on Earth. High Bouguer anomalies and thinned crust comparable to the Mid-Continent Rift in North America suggest underplating of denser, mantle-derived mafic material beneath extended crust in Sedna and Guinevere planitia on Venus. These rifts are partitioned by transfer faults and flank a zone of mantle upwelling (Eistla Regio) between colinear hot, upwelling mantle plumes. Data support the northward drift and indentation of Lakshmi Planum in western Ishtar Terra and >1000 km of transcurrent displacement between Ovda and Thetis regiones. Large displacements of areas of continent-like crust on Venus are interpreted to result from mantle tractions and pressure acting against their deep lithospheric mantle ‘keels’ commensurate with extension in adjacent rifts. Displacements of Lakshmi Planum and Ovda and Thetis regiones on Venus, a planet without plate tectonics, cannot be attributed to plate boundary forces (i.e. ridge push and slab pull). Results therefore suggest that a similar, subduction-free geodynamic model may explain deformation features in Archaean greenstone terrains on Earth. Continent-like ‘drift’ on Venus also resembles models for the late Cenozoic–Recent Earth, where westward translation of the Americas and northward displacement of India are interpreted as being driven by mantle flow tractions on the keels of their Precambrian cratons. Supplementary material: Bouguer gravity and topographic images over a segment of the Mid-Atlantic ridge and Ross Island and surrounds in Antarctica, principal horizontal stress trajectories about mantle plumes on Earth, map and interactive 3D representations of cratonic keels beneath North America from seismic tomography, and a centrifuge simulation for comparison with Venus in support of our tectonic model are available at http://www.geolsoc.org.uk/SUP18736 .