1. 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.

2. 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

3. 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.

4. 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: I

5. 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

6. 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: V
ESA/DLR/FU Berlin (G. Neukum)

7. 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

8. 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

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

10. 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

11. 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.

12. 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 m. C
DEM (100 m pix): h0263_0000.da4.53
Hirise\ESP_026828_1690

13. 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.

14. 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 m3/sek to form the Ásbyrgi gullie.
Jökulhlaup, m3/sec (Ahlo et al.,
2005). About 1 km wide, 100 m deep.

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

16. 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