The banded terrain on Mars – A viscous cufeve

Posted by Dr. Hannes Bernhardt, Arizona State University, School of Earth and Space Exploration.

An article on the banded terrain cannot be commenced by a traditional definition, as it appears to be a truly singular occurrence in the Solar System. In a competition for the most mysterious landscapes on Mars, the so called “banded terrain” (Image 1) would certainly be a hot contender – a fact illustrated by one of its other descriptive appellations: “Taffy pull terrain.” It is a strong reminder of the limitations that are intrinsic to remote sensing geology but also of the strengths of comparative geomorphology.

PlanGeomorph_Figure1

Image 1: CTX images of the banded terrain on the Hellas basin floor on Mars.

The banded terrain is a unique landscape occupying over 30,000 km² of northwestern Hellas Planitia, the floor of Mars’ largest and deepest impact basin located in the mid-latitudes of its southern hemisphere (Diot et al., 2015; Bernhardt et al., 2019). The banded terrain consists of a smooth surface (at meter-scale) dissected by up to ∼30 m deep, crevice-like, curvilinear troughs (“inter-bands”) at a median spacing of few 100 s of m (Image 1A). These troughs form various patterns, e.g., lobate/lamella-like or circular arrangements, but also confined zones of convolution with decameter-scale folding (Image 1B, white arrow). In addition to these apparent signs of widespread viscous deformation, smaller-scale incidents of brittle failure are evidenced by faults segmenting individual bands, often accommodating rotational movement akin to flexural slip, i.e., transverse displacement of a segment caused by larger-scale folding (Image 1A, black arrow). Moreover, in some locations, brittle failure was seemingly caused by one band segment breaking through another band (Image 1A, white arrow). The compressive stress implied by such movements is also evidenced by up-thrust parts of bands, indicative of buckling in a shallow thrust fault-like scenario (Image 1B).

But what do all these observations tell us? One decisive implication is that bands being segmented, rotated, and breaking through each other seem to contradict the banded terrain being the surface expression of a deep seating structure, e.g., truncated, folded m to dm-scale layers as they are often found on Earth around salt diapirs (so called “halokinetic sequences”) (Jackson et al., 1990). Furthermore, the apparent mobility of bands indicates that they were largely decoupled from underlying material, thereby enabling efficient movement both in groups but as well as individually. Lastly, folded bands at km- down to dm-scales, imply a low viscosity during deformation; congruent or subsequent faulting on the other hand indicate an increase in viscosity and/or changing strain rates, i.e., deformation speeds. As regional-scale band patterns are absent and local scale patterns do not correlate well with modern topography, neither large-scale tectonics nor downslope flow are adequate sources for the stress required for deformation.

The only terrestrial analog for all the observations listed above (viscous deformation, zones of convolution, block segmentation/rotation, buckling, decoupling) is an ice shelf scenario (Image 2), in which floating sea ice is affected by interacting stress fields by its parent glaciers on land (Crabtree and Doake, 1980; Rignot et al, 2011). This causes disintegration, often along curvilinear fissures, block (i.e., iceberg) segmentation and rotation (Image 2, white arrow), buckling (Image 2, black arrow), and even confined zones of convolution, i.e., small-scale folding.

PlanGeomorph_Figure2

Image 2: Landsat 8 image of the northern margin of the Ronne ice shelf off the coast of Antarctica.

However, the catch is that there is no morphologic evidence hinting at a past standing body of water, on which the banded terrain might have once floated (Bernhardt et al. 2016, 2019). Nevertheless, various adjacent landforms of likely glaciofluvial origin (potential eskers and sandur plains) might indicate that the banded terrain has formed in a glacial scenario, possibly as subglacial, wet sediments (i.e., viscous till) that were deformed by an interplay of ice overburden pressure and the local topography (e.g., Boulton et al., 1974). As no terrestrial analog exists for such a scenario at this scale, it remains a tentative hypothesis, though, meriting further investigation, ideally – one day – including in situ observations.

References:

Bernhardt, H., Hiesinger, H., Ivanov, M. A., Ruesch, O., Erkeling, G., & Reiss, D. (2016). Photogeologic mapping and the geologic history of the Hellas basin floor, Mars. Icarus, 264, 407–442. https://doi.org/10.1016/j.icarus.2015.09.031

Bernhardt, H., Reiss, D., Ivanov, M., Hauber, E., Hiesinger, H., Clark, J. D., & Orosei, R. (2019). The banded terrain on northwestern Hellas Planitia: New observations and insights into its possible formation. Icarus, 321(November 2018), 171–188. https://doi.org/10.1016/j.icarus.2018.11.007

Boulton, G. S., Dent, D. L., & Morris, E. M. (1974). Subglacial Shearing and Crushing, and the Role of Water Pressures in tills from South-East Iceland. Geografiska Annaler. Series A, Physical Geography, 56(3/4), 135. https://doi.org/10.2307/520703

Crabtree, R. D., & Doake, C. S. M. (1980). Flow lines on Antarctic ice shelves. Polar Record, 20(124), 31. https://doi.org/10.1017/S0032247400002898

Diot, X., El-Maarry, M. R., Guallini, L., Schlunegger, F., Norton, K. P., Thomas, N. H., … Grindrod, P. M. (2015). An ice-rich flow origin for the banded terrain in the Hellas basin, Mars. Journal of Geophysical Research: Planets, 120(12), 2258–2276. https://doi.org/10.1002/2015JE004956

Jackson, M. P. A., Cornelius, R. R., Craig, C. H., Gansser, A., Stöcklin, J., & Talbot, C. J. (1990). Salt Diapirs of the Great Kavir, Central Iran. The Geological Society of America. Retrieved from http://rock.geosociety.org/Store/detail.aspx?id=MWR177

Rignot, E., Mouginot, J., & Scheuchl, B. (2011). Ice Flow of the Antarctic Ice Sheet. Science, 333(6048), 1427–1430. https://doi.org/10.1126/science.1208336

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