Reading the landscape at volcano-tectonic locations within the Tharsis Montes, Mars Jóhann Helgason National Land Survey of Iceland EGU General Assembly - Vienna, 25 April, 2017.

The Noachian – Hesperian „transition“ – ca. 3.7 to 3.0 Gyr bp most water was buried and preserved as ice below a cover of dust or volcanic material ice accumulation may have been fast, extensive and permanent with regard to Tharsis volcanism may have been intermittent ice in Tharsis was cold-based I want to thank for the opportunity to give this presentation on the Tharsis Montes. I will have to make a number of assumptions in this short talk without providing detailed explanations. OK NASA/Goddard Space Flight Center Scientific Visualization Studio

THE THARSIS VOLCANIC PROVINCE Arsia, Pavonis and Ascraeus Mons The Thasis province is actually not that simple. All kinds of volcanoes have been active there over a very long time. Here the focus is on the Tharsis Montes, Pavonis, Ascraeus and especially Arsia, that each are equivalent in size to an intraplate Earhlike mantle plume. I will now lead you through six locations within these three volcanoes that are located on their SW and NE sides. From Michael Carr: The surface of Mars, 2006.

Valleys vs. depressions Arsia NE Valleys vs. depressions Down hill arrow This is Arsia NE side. Notice a pattern of elongated features. One end, the upper one, is oval and broad. The lower end is narrow. Overall these features do not form a straight line. Valley vs. depression. Valley has an opening and is long while depressions are near circular and closed. All stages between these two are noted. THEMIS: I01428001

Arsia - SW There are two well developed valleys on the Arsia SW-side. Each has a network or subsidiary valleys, in particular the main valley. It is rather clear that depressions occur as isolated in relation to concentric faults or lineaments and as more elongated on the NE-SE lineaments. We will focus more on this side later on. HRSC: H1893_0009_ND3

Pavonis SW Down hill THEMIS: V07245001 ESA/DLR/FU Berlin (G. Neukum) Pavonis SW-side. Here there are some examples of where the tail become narrower and extra long, perhaps because the flow left the main „tail“ and meandered on the furface – no collapse. Down hill THEMIS: V07245001 ESA/DLR/FU Berlin (G. Neukum)

Greatest erosion? Pavonis NE Here there is no „vally network“ left and the removal of material appears complete. Pavonis NE Pavonis Fossae Down hill Pavonis NE side. Broad valley, 23 km wide, „Pavonis Chasma“ and here the finer network presumably eroded. Unusual are the 4 or 5 concentric „graben-like“ features on the plain below. Pavonis Chasma HRSC: H3276_0000_ND3

Large „closed“ irregular depression area Ascraeus SW Ascraeus SW-side. Here we have a central valley network. A 2-6 km-broad and 400-m-deep depression occurs in the „apron“ area. Large „closed“ irregular depression area Apron area? Viking: 204A31

Ascraeus NE Down hill HRSC: H2054_0001_ND3 Ascraeus NE-side. Highly irregular pattern and minor „central“ clustering of depressions. HRSC: H2054_0001_ND3

Arsia - SW Arsia Mons Altitude 18 km Caldera Themis 20050613 Concentric faults Rim Isolated depressions Altitude Site 1 The drop in elevation from rim to apron site is from 17 to 11 km respectively. Suggested that primary mode of magma transport is injected through hydraulic fracturing rather than crustal extension and normal faulting. This is Arsia SW-side and I will mainly focus on this site later. Overall, it seems to me that we have a great deal of material transport here. I rule out thermal erosion as an alternative because of the immense material volume of involved. Valley 2 Valley 1 12 km Apron

Head and tail development Head and tail development system. The head is circular at the upper end of the system. Presuably, this is the site of magma injection into ice rich layer below the surface. Often there is just a head formation. If the melting is substantial, the water will „melt“ its way downward with corresponding surface collapse. If the release of meltwater is rapid then it may be argued that the collapse is narrow and more elongated in a downward direction.

Case at Site 1: Collapse in numbers Main crater Diameter: 3.3 km We could say that a common feature of each depression „system“ is a „head and tail“. By no means is this uniform but it is a way to approach a rather chaotic phenomenon. The „head“ (not to be confused with Jim Head) forms the top part and is often circular in form. Below that is often a row of smaller circles that may grade into a deep and long depression. When the „system“ has opened up at the lower end then is is common to see an advanced stage of mass vasting and removal of material. Arsia Mons visualized process: ice below a blanket of lavas and sediment melts, resulting in collapse equal to the thickness of the ice layer. Here the collapse is 300-700 m. C DEM (100 m pix): h0263_0000.da4.53 Hirise\ESP_026828_1690

Stage 2. Meltwater formation Magma injection Stage 2. Meltwater formation Stage 3. Jökulhlaup and collapse of surface Legend: Surface layer Ice and sediment Bedrock lavas A schemtatic model showing how magma may be injected into an ice rich layer (stage 1) with properties of cold-based glacier ice. Upon injection the uprise of magma is halted and meltwater is generated, perhaps more likely muddy meltwater (stage 2). Eventually, the meltwater finds a way to escape. This results in collapse of the surface and „tail“ formation further down slope (stage 3). Thickness of ice and sediment layer is probably at least proportional to subsidence.

Ásbyrgi canyon, N Iceland Ascraeus SW-apron Ásbyrgi canyon, N Iceland There are however gullies or canyons in Iceland that resemble gullies on Mars, such as the Ásbyrgi canyon in N Iceland. It is almost an exact replica of a gullie on the Ascraeus SW-apron. It took a jökulhlaup of 900.000 m3/sek to form the Ásbyrgi gullie. Jökulhlaup, 900.000 m3/sec (Ahlo et al., 2005). About 1 km wide, 100 m deep.

Water ice clouds hanging above Tharsis Thank you! Water ice clouds hanging above Tharsis PIA02653

Arsia Mons valley network formation - how it may have formed - Ice was precipitated early but continued volcanism caused Arsia Mons surface to be buried by a thick cover of lavas and pyroclastics Depth to the ice layer might be over 1 km Ice may be several hundred meters thick Ice is cold based Ice probably interfingers with various rock formations (lavas and sediment) Magma is injected into subsurface ice. Quenching of magma haults vertical flow Magma cooling leads to release of meltwater further below: jökulhlaup / mudflow Continued jökulhlaups cause erosion and form a valley network Each magma injection reduces the ice budget – drastic erosion comes to a hault