Figure 1. The Tharsis Montes: Arsia, Pavonis and Ascraeus Mons. Contour interval: 400 m. The map, based on MOLA data, was prepared by Adrian Lark.

Method of approach: In the present study the focus is on relative age of different morphological features, such as ring- and depression-shaped collapse structures and erosional valley systems, and how these provide clues to recent interaction of volcanism and ice within the Tharsis Montes. Attension is given to the relationship between loci of such morphological features and sites where, based on volcano-tectonic relations, eruption of magma is most likely to have occurred. Hartmann et al., 1999 provided relative ages for different segments of Arsia Mons based on crater counts. In the present study, crater morphology or state of erosion, rather than crater counts or crater diameter, is used to distinguish age relationships. Basic assumption in this study are a) that lava surfaces should be visible in the neighbourhood of recent volcanism, b) impact craters are stable features and only in the presence of catastrophic erosional processes or extraordinary sediment deposition can young craters be obliterated.

Krafla volcano, northeast Iceland: The Krafla of northeast Iceland is located within a volcanically active rift zone on a plate boundary and is here regarded a analog for Tharsis Montes volcanism (Björnsson et al., 1977). Through the Krafla volcano center runs a fissure swarm made up of numerous near parallel fissures. The swarm extends up to 80 km north and south of the caldera. Beneath the caldera roof is a magma chamber, at some 3-7 km depth, into which magma accumulates from a greater depth. The magma accumulation exerts pressure on the caldera roof that is elevated. Monitoring of the chamber surface provides information on magma accumulation. Eventually, as the magma pressure exceeds the lithostatic pressure the fissure swarm fails and opens up. Then magma hydrofractures the crust and is fed laterally into the fissures, up to 80 km away from the caldera rim. Outside the caldera magma may or may not reach to the surface, depending on the supply and magma pressure. This volcano-tectonic situation is regarded comparable to volcanism within the Tharsis Montes on Mars. The valley systems on the northeast and southwest flanks of the Tharsis Montes are equivalent to the Krafla fissure swarm and are thus the locus of most intense volcanism. Paradoxically, no lavas are seen within the valleys, only sediments. On the other hand lava surfaces are common within the calderas and near the concentric caldera faults. What causes no lavas to be found in the valley systems of the Tharsis Montes?

Figure 2. Pavonis Mons. Contour interval is at 400 m, showing a deeply incised valley on the northeast side. This 3D map is based on MOLA data, generated by Adrian Lark.

Location of lava surfaces vs. most plausible eruptive sites: At the end of Viking era imaging had established the distribution of lava fields on Mars. This was achieved locally to the point of differentiating between new and old lavas, flow types and degree of sedimentary cover. With high resolution MOLA measurements detailed numerical data on morphological features became available. Within the Tharsis Montes lava flows were widely detected, such as within the caldera on Arsia Mons and on the concentric caldera rims. The high-resolution MOC images are presently contributing vital images from within the valley networks. Paradoxically, they show no lavas there although earlier studies had reached that conclusion.

Tharsis Montes erosional valley systems: Each of the Tharsis Montes has a caldera and a rift segment extending from it respectively on the southwest and northeast side (figure 2). These volcanoes have a total of six segments that represent a distinct evolutionary stage with regard to rifting, development of erosion and degree of volcanism. Although a high degree of variability exists between the six segments they all share three basic features, i.e. circular to elongated depressions, smooth valley floor without lava surfaces and an extensive fan area down-dip in front of the valley system. The valley systems have been regarded as lava channels. Thus, for the valley system on the southwest side of Ascraeus Mons, Crumpler et al., (1996) state: "pits, pit chanis, and flank channels interpreted as flank vents and lava channels." The present author is in agreement with the channels and ring-shaped features being the locus of volcanic extrusions. However, for the valleys to be lava channels surfaces of lavas need to be observed. This, however, is not the case and in the present paper an explanation is offered for this morphological discrepancy.

Arsia Mons erosional valley system (figures 3 and 4):

Northeast segment (figure 3). This segment differs considerably from the SW segment in having one advanced valley system and extending into the caldera. The upper segment is made up of a few but well developed elongated depressions. Some of the segments in between such depressions have the shape of "islands" or mountains. As on the SW side their height increases up-dip. Characteristic of the ring shaped depressions is the lack of an ash collar, as if the depressions formed in response to melting of a subsurface layer. There are clear differences between the northeast and southwest valley systems in that the northeast system is lower and extends morphologically into the caldera. The caldera opens up on the northeast side. Thus, it is plausible that melt-water generated by interaction of magma and ice within the caldera may preferably have been released or leaked through the northeast gate. This may explain the much higher degree of erosion there and more highly developed valley system.

Figure 3. Arsia Mons northeast flank. A network of heavily eroded valleys cuts into the volcano flank (Viking Orbiter Frame: 42b44).

Southwest segment (figure 4). In the southwest system large ring-shaped collapse structures are more abundant and the narrow elongated depressions are less well developed. Below the circular depressions the valley system opens up and is made up of two branches. The eastern branch, is more extensive and here the valley is fairly well developed but with a central threshold midway up dip. On the lower flanks of the eastern branch linear features are observed, probably volcanic fissures, extending up-dip into the volcano sides. The western branch is less well developed and can be defined as being at an intermediate stage between a valley and ring depression. Two notable features occur that are altitude dependant. The first is that up-dip the valley walls become higher. The second is that the ring-shaped depression have a greater diameter up-dip.

Figure 4. Arsia Mons southwest flank with deep erosional valleys and rather sharp valley walls (Viking Orbiter Frames: 90a03-5).

Pavonis Mons erosional valley system:

Southwest segment. The systems respectively on the southwest and northeast flanks of Pavonis Mons substantially (figures 5 and 6). Individual depressions within the southwest segment (figure 5) are characterized by a near-circular, upper terminus from which extends a graben like depression that becomes narrower down-dip. Down-dip the depression tails are intertwined forming a network or irregular relief. This segment is regarded at an early evolutionary stage.

Figure 5. Pavonis Mons southwest side (Viking Orbiter Frame: 49b57). Individual elongated depressions have each carved a valley but here the erosion appears to be at a relatively young evolutionary stage compared with the northeast side.

Northeast segment. Conversely, a nearly complete erosional valley is exposed on the northeast segment (e.g. Viking Orbiter Frame 643A54). Here a 25 km broad valley has formed with up to 2 km high horse shoe-shaped cliff walls. No "islands" or ridges are exposed on the valley floor. The erosion extends down into the concentric fissure system in front of the valley (see location on figures 5 and 6). At location A (figure 5) the ridges between the depressions have been eroded and the direction of erosive medium is clearly down-dip.

Figure 6. Pavonis Mons, northeast side. Sedimentary cover blankets the valley floor (Viking Orbiter Frame 643a54). The erosion has severely eroded the concentric fissures in front of the valley. At location a jokulhlaups have caused erosion of the ridge separating two concentric fissures.

Fortunately, the Mars Global Observer captured an image of the valley floor that extends into the valley walls. The location of this image within the valley is given on figure 7.

The image on figure 8 (MO401369) shows that the valley floor is riddled with small impact crater remnants. The larger of the crater remnants have "black eye" crater center. The image shows hundreds of still smaller remnants. Erosion has consistently acted on the remnants in a down-dip direction to the extent that there is an up-dip head and a down-dip tail. For the crater remnants direction of tail extension is perfectly parallel. A darker colored sediment, probably of air-borne origin, is draped over the valley floor. Its pattern runs obliquely with that of the erosional direction.

Figure 7. Pavonis Mons broad northeast valley (MO401370). The valley floor in the square is shown on figure 8.

On the valley floor there occur exceptional pristine impact craters. It is important to note that the ratio of young pristine craters to older crater remnants is at least 1/100. It is concluded that the crater remnants on the valley floor were deformed through extensive fluid flow originating up-dip. Hartmann et al., concluded that the youngest surfaces they detected on Arsia Mons were 40-100 million years and extrapolating their results for Pavonis Mons the age of the valley floor and thus the age of the latest erosive event there is very recent and must be well within 10 million years. This implies that Pavonis Mons is presently highly volcanically active.

Figure 8. Pavonis Mons northeast flank, part of MOC image MO401369, showing numerous impact crater remnants on the valley floor that have suffered severe erosion. The crater walls at location B have suffered erosion and the crater is nearly filled with sediment. At location C only the dark crater center is preserved. Note here the teardrop-shape of the remnant.

Within the fault-bounded terraces of the valley walls, however, no indication of crater erosion is noted. Sediment deposition (apparently wind blown) is, however, substantial.

The mechanism most likely responsible for the valley floor erosion is that of jokulhlaups. In this case the source of fluid would presumably be located within the Pavonis Mons caldera (Helgason, 2000, 2001). The erosional valley is here considered to be the locus or path through which all jokulhlaups must travel en route from the northeast side of the caldera or perhaps from the entire caldera. That would explain why the northeast valley system is so extensively eroded.

Ascraeus Mons erosional valley system (figures 9 and 10): Southwest segment. This segment (figure 9) is well evolved with a multitude of narrow depressions of irregular shape, width and length. Collapse features are mainly located on concentric fault lines. The elongated depressions extend down-dip across the concentric fault system. On Viking image F647A78 (figure 9) extensional faults cutting across the concentric faults are not observed. Erosion is advanced within the main valley system with a prominent "island" at the center. The valley walls are highly irregular. Outside the valley system, i.e. within the fan area the surface is predominantly smooth with local "second generation" depressions. There are striking similarities between the southwest segment of Ascraeus Mons and the northeast segment of Arsia Mons.

Figure 9. Ascraeus Mons southwest side (Viking Oriber Frame: 643a78). Here meltwater erosion has led to mountain building within the valley system.

Northeast segment. The northeast segment (figure 10) has a few elongated depressions. A central valley is not observed (Viking Orbiter Frame F648A30). Instead collapse features, both of the isolated ring-shaped type as well as elongated tail-like collapse kind, are scattered over a wide area.

Ring-shaped collapse structures: Some of the ring shaped structures on the southwest side of Arsia Mons are up to 6 km in diameter with a depth of 1 km. In Iceland ring-shaped collapse craters are common within basalt lava shields. The largest of such craters is Stora-Viti, 1 km in diameter and 100 m deep. Icelandic collapse craters have near vertical walls resembling a cylinder. Conversely, Martian collapse craters have gently inclined walls and the smaller of such craters have the shape of an overturned cone. Craters in excess of 5 km in diameter have a flat bottom. While collapse craters in Iceland are associated with lava extrusion the Martian craters here referred to lack such a relationship. This comparison suggests that a different mechanism is responsible for the formation of the Martian craters. The suggested mechanism for these Martian collapse craters is one of magma contact with a subsurface ice layer. The melting and eventual release of melt-water may have led to extensive subsurface melting and release of melt-water, resulting in the formation and growth of collapse craters at the surface.

Figure 10. Ascaeus Mons northeast side (Viking Orbiter Frame: 648a30). Individual elongated depressions have each carved a valley but here the erosion appears to be at an early evolutionary stage compared with the southwest side.

An attempt has been made to distinguish between two types of collapse structures, i.e. ring-shaped and elongated depressions. It appears that the elongated depressions are initially formed as ring-shaped structures that develop into elongated depressions. The elongated depressions are thought of as having initially been subsurface channels for melt-water. They are only visible on the surface because they have collapsed. The elongated depressions have invariably a ring-shaped upper terminus (head). If the volcano-ice interaction model is correct it follows that considerable subsurface melting is associated with the formation of the ring-shaped collapse structures. The melt-water generated during the ring-shaped must either have been released down dip or expelled to the atmosphere through evaporation.

Formation of the erosional valley systems: The large diameter of many ring-shaped collapse structures, e.g. at Arsia Mons southwest side, far exceeds what is normally encountered on earth. The formation of these structures in response to melting of a subsurface ice layer conforms well with the jokulhlaup mechanism. Thus, melting of an ice layer will eventually have led to the release of a major flood down-dip. Such floods would have led both to enormous erosion adjacent to the volcanic site and vast deposition of sediment within the fan area. Such jokulhlaups will initially have carved individual elongated depressions and collapse structures. Further erosion/volcanism will have carved deep valleys extending from the source area down-dip to the fan area. At this stage the roof may still have been intact or only partly collapsed. Eventually, a network of deeply incised valleys will have formed. The absence of lavas, despite abundant volcanism, is compatible with the jokulhlaup model due to erosion and blanketing effects. The distance to which units within the fan area extend, up to 600 km, is also in good agreement with the jokulhlaup model.

Age of the erosional valley systems: The sheer observation that lava flows are not observed within the erosional valley systems calls for an explanation because from a volcano-tectonic point of view this is presumably the locus of major volcanism. Let us first look at an alternative explanation for the formation and development of the erosional valleys, namely that they could have formed primarily through extensional faulting, i.e. are rift grabens. No doubt components of rifting and down-faulting of the valley center are present, although fresh faults are extremely rare. The valleys are smooth, of irregular morphology with regard to depth, wall relief and thus generally unlike grabens, that elsewhere on Mars are an extremely common feature . Within well developed valley segments, such as on the northeast side of Arsia Mons and southwest side of Ascraeus Mons, the valley systems have developed to the extent that "island" mountains rise within the valley system center. These mountains and associated valley network have irregularly bending walls that bear no resemblance to rift valleys. It is therefore concluded that the primary factor in their morphologic formation is extensive erosion.

Ground ice vs. glacier ice: In the discussion of ice within the Martian megaregolith the term ground ice is frequently used. By definition jokulhlaup refers to the melting ice when intruded by magma. As the jokulhlaups are best know from Iceland an integral part of the definition is that the ice involved is glacier ice, notably water ice. In the present study the term glacier ice has been used in discussing the potential ice beneath a subsurface layer at the Tharsis Montes. By definition ground ice refers to "bodies of more or less clear ice in permanently frozen ground. Deposits that are only temporary features are excluded under this definition, and the term is not applied to deposits that seem to be on top of the ground. Stagnant earth-covered glaciers appear to fall about in the dividing line on this definition. If their glacial origin is evident, they would be excluded."

Glacier ice by definition is: "a body of ice developed from snow which becomes large enough to move from its place of accumulation." The question thus arises: is there any flow of ice at the Tharsis Montes? Flow of glacier ice is not known to have occurred there although features resembling glacial moraines have been described for Arsia Mons (Williams, 1978). As the present model for jokulhlaups at the Tharsis Montes assumes substantial volumes of ice melting the occurrence of this ice under subsurface conditions needs to be discussed. Firstly, it is assumed that the ice volume involved by far exceeds the volume of lava pore voids or scoriaceous lava boundaries. Such void could locally be up to 30-40%. The volume of partly collapsed elongated structures emanating down-dip from collapse rings suggests that the ice volume is confined to massive ice deposits rather than pore voids. It is rather odd that the massive ice has not clearly moved down-dip. Ice that is intercalated on irregular flow boundaries with a high rock/ice ratio could be relatively stable. Furthermore, ice within the large bowl-shaped calderas may have been the principal source of melted ice. Although the term melt-water has been used throughout the text it is clear that the exact composition of the ice involved may only have a relatively small component of water ice.

Active erosional "pathway" system vs. local valley system: For each of the three Tharsis volcanoes there are two erosional valley systems that differ widely. One is far more advanced than the other with regard to degree of erosion. For Arsia Mons the advanced system is on the northeast side whereas the less developed is on the southwest side. Here the northeast system coincides with a gate in the caldera wall. For Pavonis Mons the advanced segment is on the northeast side while the less developed segment is on the southwest side. For Ascraeus Mons the advanced erosional valley system is on the southwest side whereas the northeast system is poorly developed.

It is speculated that, at least for Arsia Mons, the more advanced system may be connected with the caldera and act as a pathway for melt-water generated through volcanism within the caldera walls. The less developed system may be isolated and formed in response to local magma/ice interaction. Alternatively, the degree of erosion within the valley systems may simply express the intensity of volcanism or availability of subsurface ice.

Are the Tharsis Montes being neglected? A new era in Martian research has begun. The attention among scientist seems to be on the polar regions. The Viking era passed without any clear ideas as to what happened to the presumably lost water. Yet all the evidence were available for an entirely different interpretation, i.e. one that assumes an almost incredibly thick ice layer being present beneath a relatively thin surface layer. Based among other things on theoretical calculations of ice stability (e.g. Squyres et al., 1992) and the interpretation of impact crater diameter vs. latitude, it was concluded that water ice could not at present be stable except at higher latitudes than about ±40°. In this context there exist strong arguments for looking much closer at the Tharsis Montes. The young age of these volcanoes (less than 10 million years) predicts that they are for practical purposes active. Within this volcanic regime magma en route to the surface will likely have left a surface impression through interaction/melting of a subsurface ice layer. The impression may vary from collapse craters, elongated depressions, erosional valleys, substantial sediment deposition, formation of impact crater remnants, formation of near free impact crater terrains.

The importance of these volcanoes lies in their position that is high above the surrounding plateau. This results in the massive escape of melt-water through magma/wall rock (ice) interaction. For other volcanoes, e.g. Elysium Mons, graben structures have been observed and partly tributed to melting of ground ice through magmatic intrusion (e.g. Mouginis-Mark, 1985). Here, however, the low aspect ratio appears to cause the release of melt-water to be less efficient and erosion insignificant compared with the erosional valleys of the Tharsis Montes.

Why study the Tharsis Montes more closely? The apparent young age of the erosional valleys in the Tharsis Montes, i.e. much less than 10 million years, and their apparent mode of formation through volcano/ice interaction suggests that substantial ice is still present there. This region should therefore have priority over other region, such as Elysium which age is regarded 1 billion years. It is now pertinent to investigate further evidence for recent volcano/ice interactions within the Tharsis Montes that have stared us in the eye ever since Viking images of these volcanoes became available.

What to look for in MOC images? That the pattern of young impact craters at Pavonis Mons is seen to be heavily eroded raises the question what about other similar erosional valleys or depressions? High resolution MOC images could be used to study graben features associated with the various Martian volcanoes to specifically determine if recent erosion of impact craters can be detected there as well. The lava fields of many of these volcanoes have been regarded as 1 billion years old. It may just be that some of the erosional valleys of these volcanoes are considerably younger. Age dating through crater counts for such volcano sub-areas may therefore prove to be a worthwhile effort and provide clues or evidence for how recently these volcanoes possessed excessive ground ice.

Things we need to know about the Tharsis volcanoes: If, in accordance with the present model, there is indeed present volcanic activity in Pavonis Mons it follows that large quantities of ice reside at present beneath the surface layer, either on the volcano flanks or within the caldera of the Tharsis Montes. There is need to thoroughly investigate the morphology of these volcanoes in order to test whether morphological features on the surface can have formed in response to release of massive melt-water. We truly need to know more about the nature of the erosional valley floor. What type of sediment is covering the valley floor? Do they have an unusually high component of hyaloclastite, gravel, boulders, rock blocks or wind blown material? Can the aureole deposits, in particular the finely striated deposits on the lower flank of Arsia Mons, have formed through a gigantic jokulhlaup? How were the striated aureole deposits at Pavonis Mons formed? What would be the composition of the ice involved with the jokulhlaup model?

Summary and conclusions: On the northeast side of Pavonis Mons a deep and broad valley is carved into the volcano flank. Numerous small impact crater remnants on the valley floor have clearly been subjected to severe erosion. The erosive force has attacked the craters in a down-dip direction causing the crater remnants to have a broad upper terminus and a long narrow tail at the lower terminus. The erosive force was of fluid origin. The volcano-tectonic location of the erosional valley is where a volcano is most active volcanically and lavas should be most abundant. The absence of lavas in the erosional valley calls for an explanation. It is concluded that the close relationship between fluvial erosion and volcano-tectonic position of the valley lies in catastrophic floods referred to as jokulhlaups. Such floods form in response to melting of ice through the interaction of magma and ice deposits. The deposits of ice may either or both be located in the volcano flanks or within the caldera walls.

By way of extrapolation results of age dating of Arsia Mons, where lava fields as recent as 40-100 million year old were detected (Hartmann et al., 1999), it is concluded that the northeast erosional valley at Pavonis Mons must be well within 10 million years considering remnant denudation. The age of this volcanically induced event is so young that volcanism must therefore be considered as presently taking place within Pavonis Mons.

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