Retrogressive Thaw Slumps on Mars and Earth

Post by Dr. Colman Gallagher.

Mars’s Athabasca Vallis is a 10 km wide, 300 km long channel carved by floods originating in the Cerberus Fossae. Recent images acquired by the HiRISE instrument aboard the Mars Reconnaissance Orbiter provides strong evidence that the head reaches of Athabasca Vallis experienced repeated cycles of freezing, with the development of ground ice and polygonised terrain, and warming, marked by ground ice thaw.

Image 1: Retrogressive thaw slumps (RTSs) on the inclined margins (examples marked A) of thermokarst depressions (examples marked B). RTSs have steep, shallow headwalls fronted by inclined flow slumps. Thaw consolidation and ground lowering precedes retrogressive backwearing of the headwall (Czudek and Demek, 1970). RTS headwalls are often facetted due to retrogression exploiting exposed, thawing ice wedges spatially arranged in polygons. Thaw fluids transported through gullies and channels on the slump, from melting ground ice exposed in the headwall, are stored in the depressions fronting the RTSs. However, depressions frequently merge by the retrogressive erosion of inter-depressions. When this occurs, fluid in the higher depression may be tapped into the lower through breaches (examples arrowed), exposing the floor of the drained depression. So, as the depressions fronting these RTSs filled and later drained by tapping, residual taliks froze, epigenetic polygons formed on the exposed floor due to ice segregation and heave and pingos formed by the intrusion of pressurised liquid water into the frozen surface and/or by the freezing of enclosed taliks (e.g. at point of arrow marked C). The resulting “alas” form is a basin with an undulating floor pierced by conical pingo mounds and enclosed by gentle polygonised slopes. White boxes are approximate footprints of Images 2 and 3. All Mars images are sub-scenes from HiRISE image PSP_007843_1905. Image credit NASA/JPL/UofA.

This palaeoclimate interpretation comes from the widespread existence in the region of polygonised ground compartmented into separate surfaces and isolated depressions by scarps indented by cirque-shaped niches and gullies (Imagess 1, 2 and 3). These landforms have been interpreted as retrogressive thaw slumps (RTSs) characteristic of thaw-induced thermokarst degradation of ground ice (Balme and Gallagher, 2009). RTSs occur in the permafrost regions of Earth and are diagnostic of degrading permafrost (Harris, 1981). So, the assemblage of landforms near the head of Athabasca Vallis represents former conditions on Mars typical of terrestrial thermokarst environments.

Image 2: RTSs, each characterised by a perched, steep headscarp fronted by a gently sloping flow slide or tongue. On Earth, thaw of permafrost along RTS headscarps replenishes the lobes with both water and debris in the form of flows or slumps. The channels are cut by initially watery mudflows that lose water rapidly downslope and terminate as viscous flow lobes. Note the pingos (examples arrowed) that have formed at the foot of the RTSs on the exposed depression floor. All these forms occur only in the presence of permafrost (Harris, 1981).

Image 3: High centred epigenetic degradation polygons on retrogressive thaw slides form from the thaw lowering of polygon edges (circled). However, these polygons reflect polycylic growth and decay of secondary ground ice that developed due to the refreezing of thaw fluids on the slumps. They are significant forms in that they represent cyclical episodes of freezing and thawing. The toes of the thaw slides are bounded by solifluction lobes in places (examples arrowed) and terminate on an undulating depression floor. Note the relationship between polygon dimensions on the upper (northernmost) surface and the dimensional hierarchy of headwall alcove dimensions (example surrounding dotted arrow).

RTSs comprise three main elements (Lantuit and Pollard, 2008): a steep headwall incised into the active layer (the ice-rich layer that experiences seasonal melt), a headscarp indented by cirque-shaped niches that retreats by ablation of permafrost, and the slump floor, which consists of mudflow and debris flow deposits left as the permafrost ablates (Images 4 and 5). Terrestrial RTSs can be triggered by the formation of a depression induced by thaw consolidation and downwearing of the ground (Czudek and Demek, 1970) or by wave erosion of permafrost shorelines (Are, 1978; Lewkowicz, 1990b; Mackay, 1966), when thermoabrasion uncovers massive ground ice which then melts on exposure to the warmer atmosphere. In such cases, the slumped debris is often quickly washed away, leaving “clean” cirque-shaped niches. RTSs can also form as active layer detachments caused by deep seasonal ground thaw (Lewkowicz, 1987a; Lewkowicz, 1990a). The multiple surfaces and depressions separated by cuspate scarps in the head reaches of Athabasca Vallis indicate that the RTSs are polycyclic, the younger RTSs having formed within the margins of older ones from the epigenetic freezing and thawing of ice-rich material transported down-slope from the older flows. A terrestrial example of this behaviour is shown in Image 5 and described by Lantuit and Pollard (2008).

Image 4: Terrestrial analogues of landforms seen in the Athabasca head-region (after Balme and Gallagher, 2009). a) RTS in the Yukon, Canada (65.9° N, 134.92° W). The debris apron at the terminus of the RTS here is on the floodplain of a river rather than a thermokarst depression. North direction indicated by two-toned arrow. Image credit DigitalGlobe/GoogleEarth. b) Close-up view of the upper part of the RTS shown in Image 4a. Note the cuspate scarps, spurs, the tributary channels and blocky debris at the base of the head scarp. c) Subsidence slope failure of coastal permafrost terrain, Northwest Territories of Canada (69.825° N, 129.42° W). North direction indicated by two-toned arrow. Image credit DigitalGlobe/GoogleEarth. d) “Tuning-fork” shaped spurs in polygonally-patterned, permafrost coastal terrain, Northwest Territories of Canada (70.05° N, 129.65° W). The scale of the retrogressive niches is controlled by the polygonisation of the upper surface and displays the same dimensional hierarchy of retrogression seen on Mars. North direction indicated by two-toned arrow. Image credit DigitalGlobe/GoogleEarth. e) Diagram illustrating components of a RTS; the diagram shows a river as the trigger for back-wearing, but a slump can equally be triggered by thaw consolidation ground downwearing, a backwearing coastline or an active layer detachment. After Harris (1981).

The similarity of the landforms in the head reaches of Athabasca Vallis to terrestrial thermokarst landforms points to the former presence of flowing liquid water (or at least a water-based muddy fluid) and many bodies of standing water (Balme and Gallagher, 2009). This environment must have functioned after the enormous floods that carved the outflow channel had abated and the ground had frozen to considerable depth. So, it is probable that in the geologically recent past there was standing water, generated by the thaw of ground ice, in the shallow basins and water flowing through the channels that led away from thawing ground ice exposed in the scarp headwalls.

Image 5: Polycyclic Retrogressive Thaw Slumps, Thetis Bay, Herschel Island. Active (A), stabilized (S) and undisturbed (U) zones are marked. All active slumps are located within zones formerly affected by slump activity. The first slump from the right is the result of the coalescence of two smaller slumps. The pattern of cuspate scarps, tributary fluvial channels and multiple scarp surfaces is very similar to the examples seen on Mars. After Lantuit and Pollard (2008).

Further Reading:

Are, F. E., 1978. The reworking of shorelines in the permafrost zone. Second International Conference on Permafrost, Yakutz, U.S.S.R. U.S. National Academy of Sciences, Washington D.C., pp. 59-62.

Balme, M. R. and Gallagher, C., 2009. An equatorial periglacial landscape on Mars. Earth Planet. Sci. Lett 285, pp. 1-15. [Abstract]

Czudek, T., Demek, J., 1970. Thermokarst in Siberia and its influence on the development of lowland relief. Quat. Res. 1, 103-120. [Abstract]

Harris, C., 1981. Periglacial Mass-wasting: A review of Research. BGRG Research Monograph 4, GeoAbstracts, Norwich, 204pp

Lantuit, H., Pollard, W. H., 2008. Fifty years of coastal erosion and retrogressive thaw slump activity on Herschel Island, southern Beaufort Sea, Yukon Territory, Canada. Geomorphology. 95, 84-102. [Abstract]

Lewkowicz, A. G., 1987a. Headwall retreat of ground-ice failures, Banks Island, Northwest Territories. Can. J. Earth Sci. 24, 1077-1085.

Lewkowicz, A. G., 1990a. Morphology, Frequency and Magnitude of Active Layer Detachment Slides, Fosheim Peninsula, Ellesmere Island, North Western Territories. 5th Canadian Permafrost Conference. Université Laval, Quebec City, pp. 111-118.

Burn, C.R., Lewkowicz, A.G., 1990. Retrogressive thaw slumps. The Canadian Geographer 34, 273-276. [Abstract]

Mackay, J. R., 1966. Segregated epigenetic ice and failures in permafrost, Mackenzie Delta area, North Western Territories. Geogr. Bull. 8, 59-80.

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