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Independent Project at the Department of Earth Sciences Självständigt arbete vid Institutionen för geovetenskaper

2015: 8

Pit Craters of Volcano, , and Their Relation to Regional Volcano-Tectonism Kollapskratrar på vulkanen Arsia Mons, Mars, och deras relation till regional vulkantektonism

Jesper Perälä

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Independent Project at the Department of Earth Sciences Självständigt arbete vid Institutionen för geovetenskaper

2015: 8

Pit Craters of Arsia Mons Volcano, Mars, and Their Relation to Regional Volcano-Tectonism Kollapskratrar på vulkanen Arsia Mons, Mars, och deras relation till regional vulkantektonism

Jesper Perälä

Copyright © Jesper Perälä and the Department of Earth Sciences, Uppsala University Published at Department of Earth Sciences, Uppsala University (www.geo.uu.se), Uppsala, 2015 Sammanfattning

Kollapskratrar på vulkanen Arsia Mons, Mars, och deras relation till regional vulkantektonism Jesper Perälä

Kollapskratrar och kraterkedjor relaterade till vulkanen Arsia Mons på Mars har karterats för att analysera deras spatiala mönster och för att komma till slutsatser för deras tillblivelse. Högupplösta satellitbilder tagna av Mars Express-sonden har använts för karteringen. Fördelningen av de karterade kraterkedjorna jämfördes med typiska fördelningar av magmatiska gångbergarter från vulkaner på jorden. Resultaten visar att fördelningen av kollapskratrar och kraterkedjor överensstämmer enligt förväntningarna och påvisar en relation mellan kollapskratrar och magmatiska gångbergarter på Mars.

Nyckelord: Strukturgeologi, kollapskratrar, kraterkedjor, Arsia Mons, Mars

Självständigt arbete i geovetenskap, 1GV029, 15 hp, 2015 Handledare: Michael Krumbholz och Steffi Burchardt Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se)

Hela publikationen finns tillgänglig på www.diva-portal.org

Abstract

Pit Craters of Arsia Mons Volcano, Mars, and Their Relation to Regional Volcano-Tectonism Jesper Perälä

Pit crater and pit crater chains associated to the volcano Arsia Mons on Mars have been mapped to analyse their spatial pattern and to conclude about their formation. For the mapping, high resolution satellite data gathered during the Mars Express mission were used. The spatial distribution of the pit craters was then compared with typical patterns of magmatic sheet intrusions within volcanoes as they are known from Earth. The results show that the pattern of the mapped pit craters and pit crater chains are in good agreement with these sheet intrusions and are therefore likely related to sheet intrusions.

Key words: Structural geology, pit craters, crater chains, Arsia Mons, Mars

Independent Project in Earth Science, 1GV029, 15 credits, 2015 Supervisors: Michael Krumbholz and Steffi Burchardt Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala (www.geo.uu.se)

The whole document is available at www.diva-portal.org Table of Contents Introduction 1 Geological background 1 The 1 Geological ages of Mars 2 Martian volcanism 3 Arsia Mons volcano 4 Pit craters on Mars 4 Sinuous rilles 5 Method 6 Results 7 Discussion 10 Patterns 10 Terrestrial analogues 12 Formation mechanisms 12 Acknowledgements 14 References 14 Online resources 17

Introduction

Among the planets of our solar-system, Mars has possibly caught the most attention over the years. The first flyby of the planet by 4 in 1965 scored the first close- up satellite images taken of the planet, and ever since, satellites and landers have provided pictures and data with increasing quality. Since few Martian meteorites have been found on Earth, most of the knowledge of Mars’s geology is based on remote sensing. The aim of this study is to improve the understanding of the formation of pit craters and pit crater chains on Mars. The volcano Arsia Mons, located in the region of Mars, was chosen as study region (Figure 1). The hypothesis is that pit crater chains are related to magmatic sheet intrusions (dykes). Dykes arrange in typical patterns within a given stress-field as e.g. regional dykes, cone sheets and radial dykes. If the typical pattern of dykes on Earth coincides with the patterns formed by the pit craters on Mars, this would give strong evidence for the pit craters to be dyke-related structures. So far several mechanisms are suggested for the formation of pit craters and pit crater chains on Mars. These can be categorized into four groups: carbonate dissolution related, related to opening mode fractures, lava tubes and related to dykes. Carbonate related mechanisms include dissolution of equatorial carbonates by water available during early greenhouse conditions on Mars, or by water released by volcanism (Spencer & Fanale, 1990). The opening mode fracture-related models suggest that the pit craters are related to graben formation in the Martian crust (Tanaka & Golombek, 1989). A different model based on opening mode fractures conclude that during faulting void space is created in the subsurface, which consequently lead to the observed pit craters (Ferrill & Morris, 2003). Another mechanism is based on roof collapse of evacuated lava tubes and subsequent pit crater formation (Cushing et al., 2007). Dyke-related formation of pit craters include four different models. Pit craters could be formed when dykes interact with the hydrosphere or cryosphere, resulting in a steam blast (Mège & Masson, 1996). Smaller pits are proposed to be formed when volatiles escape from a newly formed dyke, thus creating a void space, in which material collapses (Scott & Wilson, 2002). Larger pit structures are suggested to be formed as a result of Plinian-style eruptions (Scott & Wilson, 2002). Another explanation reported from Mège & Masson (1996) is that pits are formed above giant dyke swarms which are connected to a collapsing or deflating major magma chamber causing an evacuation of the associated dykes.

Geological background

The Martian dichotomy

One of the first impressions when investigating the Mars surface is the great dichotomy dividing the planet roughly by the hemispheres (Figure 1). The southern hemisphere of Mars is topographically higher, more cratered, and older compared to the northern hemisphere. There are still disagreements about the cause of these topographic differences. Neumann et al. (2004) suggest mantle convection as a mechanism for the formation of the duality of Martian topography. Wilhelms & Squyres (1984) and Marinova et al. (2008), in contrast, suggest that the northern plains are caused by an impact event. Another explanation for the formation of the northern basin is that of a paleo-ocean (Villanueva et al., 2015).

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Figure 1. Topographical map of Mars based on Mars Orbiter Laser Altimeter data. Arsia Mons shown in the inset. The three aligned are Arsia Mons (A), (B), (C). Other notable features include (D), (E) and the Hellas basin (F) (NASA, 2007).

Geological ages of Mars

The geological timescale of Mars is divided into four periods: Pre-, Noachian, and . Dating of these periods was carried out by a combination of meteorite dating and meteorite cratering chronology. Cratering chronology is based on comparing the size and frequency distribution of impact craters of different areas. Geologically older areas are more likely to have been impacted by larger meteorites and over longer periods of time. The periods have estimated ranges, based on the ages of Martian type localities (Hartmann & , 2001; Carr & Head, 2010). The Pre-Noachian age includes the formation and differentiation of the planet until the formation of the Hellas impact basin (Carr & Head, 2010). The Noachian period covers the time between 4.1-3.8 and 3.7 Ga, while the age of the Hesperian period is specified to range from 3.7 to 2.9-3.3 Ga. The Amazonian-Hesperian period boundary lies at around 1.4-2.1 Ga, and is the most uncertain due to limitations of the crater chronology. These limitations results from the fact that most small craters on Mars may be secondary craters (McEwen et al., 2005), formed from debris of larger impacts, thus decreasing the accuracy of the crater chronology for more recent periods. Another timescale is suggested by Bibring et al. (2006) based on mineral alterations. The Phyllocian era lies around the Noachian period and is defined by the phyllosilicates found, which indicate alteration of minerals by water interaction. The second era, called Theiikian, begins in the late Noachian to Hesperian. It is characterized by the presence of sulfate minerals deposited. The Siderikian era reaches until the present, defined by the presence of anhydrous ferric oxides formed without the presence of water.

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Martian volcanism

As early as 1973 it was suggested that Mars has a stationary crust, lacking plate tectonics and subduction as known from Earth (Carr, 1973). Recent studies suggest a weak form of plate tectonics for the Valles Marineris formation (Figure 1; Yin, 2012b). Volcanism on the other hand, is prominent and characteristic for many areas of Mars. Hodges & Moore (1994) list and describe 46 different volcanic features in their Atlas of volcanic landforms on Mars. The largest volcanoes on Mars can be found in the Tharsis region (Figure 2). The Tharsis rise is a topographical high- ground situated in the equatorial part of the western hemisphere. The Tharsis region hosts a number of gigantic volcanoes, among them Olympus Mons, the largest volcano of our solar system (Figure 1; Hodges & Moore, 1994). In the centre of the Tharsis region, the three volcanoes Arsia Mons, Pavonis Mons and Ascreus Mons form a NE-SW trending line (Carr, 1973). The alignment of the Tharsis Montes has given rise to theories of a common origin. A single source of magma for the later stages of eruptions is suggested by Bleacher et al. (2007). Mège and Masson (1996) provide a model where a mantle plume serves as the source for the early development of the Tharsis region. Yin (2012a) on the other hand proposes a model where episodic subduction explains the formation of the Tharsis rise. Based on spectroscopic evidence it was concluded that Martian rocks are mostly basaltic (Platz et al., 2015) with felsic rock being uncommon (Bandfield, 2002). In some places an andesitic signature was identified which is interpreted to origin from basaltic rocks weathered by interaction with water (Wyatt & McSween, 2002).

Figure 2. Oblique angle view of the eastern Tharsis rise with the three Tharsis Montes, Arsia, Pavonis, Ascraeus as seen from above Valles Marineris (NASA, 2000).

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Arsia Mons volcano

Arsia Mons is the southernmost volcano of the three Tharsis Montes and is expected to be the oldest of the trio. It has a basal diameter of 430 kilometres in NNE-SSW direction and a summit altitude of 17.7 kilometres. It is a shield volcano with convex flanks (Hodges & Moore, 1994). The centre of the volcano is dominated by a caldera, with a diameter of 130 kilometres and a depth of 1.3 kilometres (Plescia, 2004). Smaller shield-volcanoes have been found inside the caldera of Arsia Mons (Carr et al., 1977; Mouginis-Mark, 2003). Fan-shaped lava flows are found, oriented toward the NE and SW from vents high up on the volcano. Lava flows, lava channels, and lava tubes concentrate in the north and of Arsia Mons. Pit crater chains are common on the volcano’s flanks. The evolution of Arsia Mons volcano probably took place in three stages, as was suggested by Crumpler & Auberle (1978). The earliest stage includes the regional volcanism and shield-building phase. It was followed by caldera formation with possible parasitic volcanism from vents and concentric grabens on the NE and SW flanks. The end of the last succession is marked by fissure and vent volcanism in the NE and SW flanks with extensive lava flows. More recently another evolutionary sequence has been suggested for Ascraeus Mons by Byrne et al. (2012), which also could be applicable for Arsia Mons due to the similarities of the three Montes. This progression has also been divided into three stages. The first stage includes shield- building and caldera collapse with basement flexure and flank terrace formation. The second stage comprises the formation of rift aprons, enormous overlapping lava flows. Finally, summit volcanism with extensional tectonism and formation of more rift aprons, arcuate grabens, sinuous rilles and pit craters mark the end of the volcano- tectonic evolution. The former report (Crumpler & Auberle, 1978) also suggested that the Tharsis Montes could be represented by the three different phases of the volcano triplet. Ascraeus Mons as the youngest and least developed part, Pavonis Mons represents the second intermediate stage and Arsia Mons as the third and most developed part. Byrne et al. (2012) note that Ascraeus Mons shows well developed features in line with the ones seen on Arsia and Pavonis Mons and suggest that all three Montes have undergone a similar structural evolution.

Pit craters on Mars

Pit craters are typical features associated with Martian volcanism, but also found on other planetary bodies, including Earth (Okubo & Martel, 1998) and Venus (Bleamaster & Hansen, 2001). They are rimless craters with sizes of individual craters ranging from 1 to 4.5 kilometres in diameter (Wyrick et al., 2004). Pit crater shapes are circular to elliptical with conical, U-shaped or flat floors (Smart et al., 2011; Byrne et al., 2012). They can occur solitary, as crater chains (with at least 3 aligned craters) or troughs. Suggestions that pit craters can have a convex-upward morphology have also been made from comparing sun incidence to slope angle (Wyrick et al., 2004) and through numerical modelling (Smart et al., 2011). The idea that pit craters on Mars are formed by collapse of surface material into a subsurface void is generally supported, but the mechanism behind formation of the void has been widely discussed. Wyrick et al. (2004), categorized the proposed mechanisms into 8 groups. (1) Karst dissolution. The formation of pit craters in the Valles Marineris are proposed to be associated to carbonates in the region. After deposition of

4 carbonates, three mechanisms for carbonate dissolution were suggested by Spencer & Fanale (1990). Removal of carbonates by carbonic acid from atmospheric CO2, by acids of a magmatic source such as carbonic, sulphuric or chlorine acids or removal of carbonates by calc-silicate mineral formation. (2) Tension fracturing. Extensional deformation of basement rocks with resulting graben formation followed by pit crater formation. This occurs when material collapses into void spaces due to horizontal extension (Tanaka & Golombek, 1989). (3) Dilational faulting. Competent rock layers e.g. of magmatic origin are suggested to be interlayered with less competent layers consisting of volcanic ash or aeolian sediments. Extensional deformation would create dilational faults predominantly in the competent layers causing steep voids into which unconsolidated material collapses (Ferrill et al., 2003). (4) Lava tube sky lights. Lava tubes form when lava channels are emptied and ultimately form roofs made of encrusted lava. Lava tubes form from basaltic lavas on the slopes of volcanoes, and roof sky lights may form when a part of the lava tube roof collapses (Lockwood & Hazlett, 2010). Possible Martian sky lights have been discussed by Cushing et al. (2007) and related to the formation of pit craters. (5) Dykes interacting with the hydrosphere or cryosphere. Mège & Masson (1996) relate the formation of linear troughs and pit craters with dykes at different depths. Dykes at medium depths could increase the temperature of water in the hydrosphere or cryosphere enough to let the water escape laterally to form linear troughs. Dykes that come in direct contact with the hydrosphere or cryosphere could lead to phreatomagmatic eruptions and the creation of pit crater structures (Mège & Masson, 1996). (6) Dykes with exsolved volatiles. Smaller pit craters are proposed to be formed passively by dykes that do not reach the surface. When the magma degasses at the tip of the dykes, void space would be created with subsequent collapse and pit crater formation (Scott & Wilson, 2002). (7) Dykes with Plinian-style eruptions. Larger pit structures could be formed by very high initial gas escape from dykes. This would lead to an eruption of Plinian type which continues until the walls collapse into the magma conduit, thus creating a larger pit crater. The collapse is expected to remove evidence of pyroclastic deposits, which are not found in the vicinity of the pit craters (Scott & Wilson, 2002). (8) Deflation of magma chambers. The Tharsis region is suggested to be placed above a gigantic elongated magma body. Deflation or compaction of the magma body could induce magma flow-back within the dykes which then could collapse along the dykes. The collapse could then have created the grabens and pit craters associated with the Tharsis region (Mège & Masson, 1996; Mège et al., 2000).

Sinuous rilles

Sinuous rilles are rimless channels, which on Ascraeus Mons are found on the north- eastern embayments (Byrne et al., 2012). It is expected that these are found on Arsia Mons in a similar fashion. There are several suggested explanations for the formation of these channels, in which a fluid agent is key to all of them. Erosion by pyroclastic flows (Cameron, 1964), collapse of lava tubes (, 1971), incision by lava flows (Carr, 1974), mud flows or lahars (Murray et al., 2010). Pit craters have been shown

5 to crosscut sinuous rilles on Ascraeus Mons (Murray et al., 2010; Byrne et al., 2012). Byrne et al. (2012) suggest that the orientations of sinuous rilles are primarily influenced by gravity and secondarily by subsurface structures. They discuss tectonic influence to explain the sudden change of orientation in some of the rilles examined. Another explanation in which grabens, normal faults, pit craters and sinuous rilles all primarily are under structural control by local to regional stress fields is suggested by Pozzabon et al. (2010).

Figure 3. (A) White arrow indicate transition from single pit craters to coalesced trough. The red arrow points out a small with a raised rim, in comparison to the rimless pit craters. (B) White arrow showing pit crater formation in relation to a graben on the south- eastern flank of Arsia Mons. The red arrow indicates normal faulting associated with graben formation. (C) White arrow shows pit crater chain extending radially from Arsia Mons’ caldera, slightly west of the northern rift fissure. (D) Structure interpreted as a sinuous rille, showing similar morphological features to pit crater troughs.

Method

Pit crater chains and troughs were mapped in the proximity of Arsia Mons volcano (extending 300 kilometres north, 265 kilometres south, 175 kilometres east and 200 kilometres west from the caldera centre). Aerial photographs used in the mapping were photo-mosaics from the High Resolution Stereo Camera from the Mars Express mission by the European Space Agency. A total of eight images was used to produce a map covering most of Arsia Mons. The images have a resolution of 6 and 12 meters/pixel and use the Mars coordinate system with a sinusoidal projection. Mapping was carried out with ArcMap 10.3 GIS software.

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Guidelines to distinguishing pit craters from impact craters and sinuous rilles are adapted from Wyrick et al. (2004): “a pit crater does not have a raised rim (to distinguish from impact craters); a chain must consist of at least three separate pits in alignment, or two coalesced pits in alignment; pits that occur within a collapsed through are considered the same pit crater chain”. The mapping was done at a consistent view scale of 1:100 000. This scale was chosen due to difficulties distinguishing the raised rim of craters less than 300 metres in diameter. Polylines were created following the extent of crater-chains or troughs. These were then assigned to one of five groups. These five groups comprise pit craters or troughs which could correspond to the following three types of magmatic intrusions: cone sheets, regional dykes or radial dykes. The fourth group includes pit crater chains that show strong similarities to sinuous rilles and could consequently not be assigned to any certain type of magmatic sheets. The fifth group include crater chains that could not be assigned to any of the previous groups with sufficient certainty. Craters and troughs which were interpreted to belong to the same feature were grouped together. The length and azimuth of the polylines were calculated using Field calculator and EasyCalculate 10 (http://www.ian-ko.com/free/EC10/EC10_main.htm) add-in for ArcMap 10.3. Additional control measurements were made with the Measure/Angle tool add-in, to validate the azimuth calculations made by EasyCalculate 10. For further analysis the attribute tables were exported to Microsoft Excel 2013 to create tables and graphs. The azimuth data were analysed with Rose- diagrams using Stereo31 1.0.1 (http://www.ruhr-uni- bochum.de/hardrock/downloads.html).

Results

A total of 336 polylines were produced using ArcMap 10.3, following the extent of structures interpreted as pit crater chains or troughs (Table 1). There is a general absence of crater chains in the eastern and south-eastern flank of Arsia Mons, as well as on the western to north-western flank. A small portion of the volcanoes western flank is not included in the satellite photographs, as seen in Figure 5, A-F. The cone sheet group comprises 104 structures. The Rose-diagram shows that the azimuth varies greatly, with a mean azimuth of 65° (Figure 5-B). The maximum count in the Rose-diagram is 6. The crater chains are mostly 4000-10 000 metres in length (Figure 4). Regional dyke group includes 131 structures. The mean azimuth of the group is approximately 15°, displaying a trend in the NNE direction (Figure 5-C). The maximum count in the Rose-diagram is 15. The majority of the crater chains are 4000-8000 metres in length (Figure 4). 36 structures are classified as radial dykes. They populate an area of the north-western flank of Arsia Mons. The Rose-diagram show a trend toward NNW and a mean azimuth of 345° (Figure 5-D). The maximum count in the Rose-diagram is 4.5. The length of the crater chains are mainly in the 2000-6000 metres interval (Figure 4). There are 36 structures in the sinuous rille-like pit crater group. A trend is seen toward the NE in the Rose-diagram, with a mean azimuth of 35° (Figure 5-E). The maximum count in the Rose-diagram is 5. The majority of these crater chains are up to 8000 metres in length (Figure 4). 29 structures are included in the uncategorized group. The mean azimuth of the group is 10°, trending towards NNE as shown in the Rose-diagram

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(Figure 5-F). The maximum count in the Rose-diagram is 2.5. All crater chains in Group 4 are shorter than 10 000 metres (Figure 4).

Table 1. Summary of results. Group Quantity Mean azimuth Maximum count Mean length 1 104 65° 6 7240 m 2 131 15° 15 5696 m 3 36 345° 4.5 6970 m 4 36 35° 5 7079 m 5 29 10° 2.5 4931 m

Figure 4. Histograms showing the length distribution of the mapped pit crater chains.

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Figure 5. Continuing on the next page

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Figure 5. Mapping results including Rose-diagrams with the orientations of the mapped structures. (A) Measurements including all 5 groups, the maximum count in the Rose- diagram is 22.5. (B) Cone sheet-like structures, the maximum count in the Rose-diagram is 6. (C) Regional dyke-like structures, the maximum count in the rose diagram is 15. (D) Radial dyke-like structures, the maximum count in the Rose-diagram is 4.5. (E) Sinuous rille-like structures, the maximum count in the Rose-diagram is 5. (F) Unspecified pit crater group, the maximum count in the Rose-diagram is 2.5. 10° bin sizes in the Rose-diagrams. Mars sinusoidal coordinate system with sinusoidal projection.

Discussion

Patterns

Magmatic sheet intrusions can occur as dykes, sills and cone sheets (Figure 6). Sheet intrusions arrange within the stress field, in which they occur. Regional dykes are expected to follow the same orientation on a regional scale. Radial dykes would orient themselves radially from the magmatic centre. Cone sheets are oriented in a circumferential fashion around the magma reservoir.

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Figure 6. Schematic image of a shallow magma body feeding both dykes (blue) and cone sheets (red). Modified from Galland et al. (2014). Under copyright by John Wiley & Sons, Inc. Reproduced under permission number 3630200087527.

When analysing the results of the pit crater mapping of Arsia Mons typical trends can be distinguished, while the orientation of the total of all mapped pit crater chains is mainly NE-SW. The crater chains of the cone-sheet group were expected to show an even distribution toward all directions in the Rose-diagram. This is not the case, although the maximum amount in the Rose-diagram was merely 6. The lack of an even distribution of the cone sheet-like crater chains could be explained by the general absence of crater chains on the north-western and south-eastern flanks of Arsia Mons and should therefore not be taken as an argument against a cone-sheet like pattern. The visual appearance of the pattern of this group strongly correlates with that expected to be formed from cone-sheets. The regional dyke-like group show a mean azimuth of 15°. This group includes the greatest amount of pit crater chains. The pattern can be explained by the alignment of the pit craters within the regional stress field, in a similar fashion as observed in regional dykes found in the Mull swarm (Jolly & Sanderson, 1995) and the Cuillins Intrusive Complex, Scotland (Bistacchi et al., 2012) and coincide with the pit crater chain orientations on the other Tharsis Montes, which is strong evidence for a regional control on the orientation of the crater chains. The radial dyke-like group shows a mean azimuth of 345°, which is not in line with the overall trend of the other mapped pit crater groups. Instead, the strike of this group points towards the centre of the volcano, therefore they could be explained as a population of radial dykes. The sinuous rille-like group includes pit crater chains that generally follow the NE-SW trend. The crater chains in this group are close to sinuous rilles, and do in some cases change direction. Structural control by local changes in the stress field could be the cause of such alterations (Pozzabon et al., 2010).

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The unspecified group is aligned with a mean azimuth of 10°. The pit craters do not directly sort into any of the three groups above. The alignment of these pit crater chains could be influenced by both the stress field of the volcano and the regional stress field.

Terrestrial analogues

The Cuillins Intrusive Complex found on Isle of Skye, Scotland, offers an example where a proximal population of cone sheets is found as well as regional dykes. Bistacchi et al. (2012), showed that an oblate magma chamber can produce the pattern of dykes found on the Isle of Skye. An oblate magma chamber was used as a model by Pozzabon et al. (2010) to describe grabens, normal faults, crater chains and sinuous rilles of Ascraeus Mons. Such a model could be related to the model by Bleacher et al. (2007), suggesting a shared magma source for the Tharsis Montes. A shared source would most likely be elongated along the orientation of the three volcanoes. Eruptive fissure vents were mapped across the Fernandina and Isabela islands, Galapagos, by Chadwick Jr & Howard (1991), showing a pattern where circumferential fissures are located close to the summits of the volcanoes and radial to regional fissures are located at an increasing distances from the central calderas. The pattern is similar to the observed pit crater chains but contrasts as eruptional deposits are not found near pit craters (Byrne et al., 2012). Mège & Masson (1996) expect dyke swarms related to grabens and pit craters in the Tharsis region not to breach the surface. A model for the Mull swarm in Scotland postulated by Jolly & Sanderson (1995) may give insight into the distribution pattern. The dykes are more influenced by the regional stress field with increasing distance from the magma chamber. At closer range the opening angles of the dykes, in relation to the source, are highly variable whereas the dykes are more aligned farther away from the magma source. This could be reflected by the regional dyke-like pit craters, which are highly aligned, as shown by the Rose-diagram in Figure 5-C. The radial dyke-like and unspecified group pit craters could represent an intermediate form, where the stresses from Arsia Mons and the regional stresses are overlap, whereas cone sheet-like pit craters are highly influenced by Arsia Mons’ stress field.

Formation mechanisms

As described above, there are several hypotheses for the formation of pit craters on Mars. How well they fit the patterns observed on Arsia Mons is discussed in the following section. The mechanisms for carbonate dissolution and subsequent pit crater formation is mainly proposed for Martian equatorial basins (Spencer & Fanale, 1990). As Arsia Mons is a large shield volcano that consists of basaltic rocks, it is difficult to explain the occurrence of large amounts of carbonates. Karst dissolution thus seems to be an unlikely explanation for the pit craters of Arsia Mons. Tension fracturing mechanisms are not conflicting with the pit crater patterns seen on Arsia Mons. Magma chamber inflation for example could produce extensional fractures in agreement with the cone sheets and regional dykes. Dykes themselves can be considered tensional fractures (Rubin, 1993). The suggestion by

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Tanaka & Golombek (1989), that magma withdrawal produced grabens and pit craters, supports the explanation that pit craters can be related to dykes. Pit craters related to graben formation during dilational faulting is expected during local to regional extension (Wyrick et al., 2004). Regional scale extension is expected to require some form of tectonism as driving force. Such tectonism is described by Anderson et al. (2001). Byrne et al. (2012) expect pit crater formation late in the evolution of the Tharsis Montes during extension caused by magmatic upwelling in the Amazonian, explaining the circumferential patterns of the pit crater chains. Wyrick et al. (2004) suggest dyke intrusion into pre-existing faults. Dilational faulting cannot be fully excluded as explanation for the formation of pit craters on Arsia Mons. Lava tubes form when lava flows downslope in a channel which narrows to form the lava tubes (Lockwood & Hazlett, 2010). This downslope pattern does not explain the circumferential patterns in which pit crater chains are often found. Dykes interacting with the hydrosphere or cryosphere on Mars could explain the patterns, in which pit crater chains have been observed. Hydrovolcanic eruptions are explosive (Lockwood & Hazlett, 2010), which would be expected to leave a crater rim or around the pit craters, which is not supported by observations as pit craters are by definition rimless. However, wider dispersion of ash due to the lower Martian gravity and dust storm erosion are suggested to remove evidence of ejecta (Mège & Masson, 1996). Degassing of magma in dykes could be in agreement with some of the patterns observed, as they are suggested to form smaller pit craters. The non- explosive process would not produce ejecta debris (Scott & Wilson, 2002), but does not explain the larger pit craters observed. Plinian eruption is a suggested mechanism for larger pit craters. The underlying dykes could explain the pit crater patterns observed. Ejecta debris produced by the explosion is explained to be by the large collapse around the eruption. On Earth, Plinian eruptions are common for rhyolitic volcanoes and less typical for basaltic volcanoes. As Arsia Mons produced predominantly basaltic lavas, the probability of large enough degassing events can be debated. Scott & Wilson (2002) provide calculations which support this theory in which the Martian gravity and the, compared to Earth, lower density of the Martian crust is used. Deflation of a major magma chamber with subsequent pit crater chains forming over dykes could explain the dyke-like patterns of the pit craters. The deflation may be explained by the late Amazonian magmatic upwelling suggested by Bleacher et al. (2007) and the subsequent termination of the upwelling. Of the aforementioned formational mechanisms, some are more readily in agreement to the dyke related patterns. Lava tubes and carbonate dissolution do not fully explain all pit crater patterns. Extensional faulting could be in agreement with the observations and is not excluded. Mechanisms which rely on dykes in some fashion could provide explanations for the observations. Passive formation related to dykes, such as magma degassing or magma chamber deflation supports that positive structures are not found in relation to pit craters and are suggested as explanation for the results of this report.

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Acknowledgements

I would like to express my gratitude to Dr. Michael Krumbholz for his help and never- ending patience while guiding me through all stages of this study. I would also like to thank Dr. Steffi Burchardt for useful discussions and reviewing my work. Additional gratitude go to the DPS Geoscience Node of Washington University in St. Louis for hosting the data needed for this report and especially to Dr. Paul Byrne for providing links to all the data needed for this study.

References

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Online resources

ET Spatial Techniques, EasyCalculate 10 http://www.ian-ko.com/free/EC10/EC10_main.htm [2015-05-05] National Aeronautics and Space Administration (NASA): Photojournal, PIA02987: Photomosaic of the Tharsis Region (2000) http://photojournal.jpl.nasa.gov/catalog/PIA02987 [2015-05-16] National Aeronautics and Space Administration (NASA): The Mars Orbiter Laser Altimeter, Images (2007) http://mola.gsfc.nasa.gov/images.html [2015-05-16] Ruhr-Universität Bochum, Downloads http://www.ruhr-uni-bochum.de/hardrock/downloads.html [2015-05-05] Washington University in St. Louis: PDS Geoscience Node, Mars Express HRSC Data http://pds-geosciences.wustl.edu/mex/mex-m-hrsc-5-refdr-mapprojected- v2/mexhrsc_1001/data/ [2015-05-17]

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