Abstract
Viscous flow features (VFF) occur in the mid-latitudes of Mars and have characteristics consistent with being glaciers. Climate models suggest that martian glaciers are cold-based systems in which meltwater has never been widely produced. VFF are common in Phlegra Montes, a mountain range in the mid-latitudes of the northern hemisphere of Mars. However, in Phlegra Montes, the presence of an esker associated with an extant Amazonian Period VFF provides evidence that warm-based glacial processes did formerly operate. The problem at the centre of this paper is that the glacial meltwater responsible for this esker could have been produced as a consequence of its setting in a graben, with locally enhanced geothermal heating having been the driver of melt, not systemic heating associated with a regional warm-based regime. Given this uncertainty, this paper aims to determine if there are indicators of more widespread warm-based glacial processes in Phlegra Montes. The paper briefly describes the distribution and characteristics of VFF across the region, before focussing on the search for key landforms considered diagnostic of erosion by warm-based glaciers. From our observations, including discriminant morphometrics, we conclude that the landscape of Phlegra Montes is indicative of widespread warm-based glacial processes, including subglacial scour, linear abrasion and the incision of subglacial meltwater channels. Our findings have significance in constraining the contexts and process environments within which liquid water has been produced during the Amazonian Period on Mars and point to several lines of future research into martian glaciation, climate and landscape evolution.
Highlights
Phlegra Montes are a mountain complex on Mars extending NNESSW, over ~1000 km, between latitudes 30◦ - 52◦ N (Fig. 1)
viscous flow features (VFF) must be characterised by: (1) features suggestive of viscous flow over or around obstacles, in response to changes in underlying slope; (2) contextual topography indicative of modification by viscous flow over or around obstacles; (3) surface texture and morphology distinct from summital areas, inter-valleys and peripheral terrain; (4) longitudinal foliation, consisting of narrow ridges and furrows, where VFF are affected by local flow constriction; (5) extensively lineated surfaces, where VFF are laterally confined, and; (6) moraine-like ridges (MLR) that bound or partition VFF surfaces displaying foliation and/or lineation associated with viscous flow
(3) Some massifs or more compact uplands are fringed by lobate debris aprons (LDA) fed from the uplands through linear corridors by lineated valley fills (LVF); these compound VFF were mapped as COMP (Appendix S1). (4) Some craters, both upland and lowland, are partially occupied by viscous, concentri cally ridged material, mapped as concentric crater fill (CCF; Fig. 2b)
Summary
Phlegra Montes are a mountain complex on Mars extending NNESSW, over ~1000 km, between latitudes 30◦ - 52◦ N (Fig. 1). The uplands are characterised by rounded, dome-like mountains, with intervening valleys and basins, some of which are occupied by viscous flow features (VFF) of different types (e.g. Fig. 2, Appendix S1). Mid-latitude valleys within uplands on Mars are commonly occupied by lineated valley fills (LVF; Head et al, 2006), which closely resemble valley glaciers on Earth. The crater retention ages of martian mid-latitude VFF indicate that they formed over the past 1 Gy (Fassett et al, 2014; Baker and Head, 2015), most recently over the last ~0.6 Gy, in the Late Amazonian epoch (Milliken et al, 2003; Hubbard et al, 2011; Souness et al, 2012; Hubbard et al, 2014). Dating by Gallagher and Balme (2015) of the VFF referred to in this paper as LVF 393 (Fig. 1, Fig. 2a, Appendix S1) places it into the Amazonian, with a crater retention age of 150 ± 20 Ma, this is probably a very approximate age; the few larger craters on LVF 393 might follow an older isochron, still Amazonian, but on the order of 1–1.5 Ga
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