Earth and Planetary Science Letters 431 (2015) 96–109

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Earth and Planetary Science Letters

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Eskers in a complete, wet-based glacial system in the Phlegra Montes region, ∗ Colman Gallagher a,b, , Matthew Balme c,d a UCD School of Geography, University College Dublin, Belfield, Dublin 4, Ireland b UCD Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland c Dept. of Physical Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK d Planetary Science Institute Tucson, 1700 E. Fort , Suite 106, Tucson, AZ 85719, United States a r t i c l e i n f o a b s t r a c t

Article history: Although glacial landsystems produced under warm/wet based conditions are common on Earth, Received 30 April 2015 even here, observations of subglacial landforms such as eskers emerging from extant glaciers are rare. Received in revised form 9 September 2015 This paper describes a system of sinuous ridges emerging from the in situ but now degraded piedmont Accepted 14 September 2015 terminus of a Late -aged (∼150 Ma) glacier-like form in the southern Phlegra Montes region Available online 29 September 2015 of Mars. We believe this to be the first identification of eskers that can be directly linked to their Editor: C. Sotin parent glacier. Together with their contextual landform assemblage, the eskers are indicative of significant Keywords: glacial meltwater production and subglacial routing. However, although the eskers are evidence of a Mars wet-based regime, the confinement of the glacial system to a well-defined, regionally significant , glacier and the absence of eskers elsewhere in the region, is interpreted as evidence of sub-glacial melting eskers as a response to locally enhanced geothermal heat flux rather than climate-induced warming. These wet base observations offer important new insights to the forcing of glacial dynamic and melting behaviour on geothermal control Mars by factors other than climate. © 2015 Elsevier B.V. All rights reserved.

1. Introduction the past ∼600 Ma (Kress et al., 2010; Fassett et al., 2014), most recently in the Late Amazonian (Milliken et al., 2003; Hubbard et The Phlegra Montes upland extends NNE–SSW over 1000 km, al., 2011, 2014; Souness et al., 2012). In common with other low between 30 N and 52 N on Mars (Fig. 1). Southern Phlegra Montes, plains bounding uplands, the Phlegra Montes piedmont is charac- 560 km east of Hecates on the Elysium rise, is 90 to 180 km terised by lobate debris aprons (LDA), accumulations of ice man- wide, overlooking the Elysium Rise to the west and sloping to- tled by lithic debris (Kochel and Peake, 1984; Holt et al., 2008; wards a flanking N–S trending piedmont basin to the east. The up- Parsons et al., 2011; Fastook et al., 2014). land relief is dominated by rounded peaks, intervening valleys and The majority of observational glaciological and landform evi- basins, some partially occupied by icy fills (Safaeinili et al., 2009; dence shows that extant martian glaciers/GLF are cold/dry based Dickson et al., 2010). The valley fills are longitudinally lineated (Hubbard et al., 2011, 2014) and (were) dynamic by virtue of (‘lineated valley fills’ or LVF; Squyres, 1979), like the basin fills ex- creep (Milliken et al., 2003; Parsons et al., 2011); the land- hibiting ridges, troughs and lobes. These morphologies, suggesting form evidence for relict glacial process-environments suggests this flow over or around obstacles in response to changes in underly- has been characteristic of Amazonian glaciation. Observations of ing slope, are typical of ‘viscous flow features’ (VFF; Milliken et landforms in contextually consistent landsystems diagnostic of al., 2003). These characteristics, together with associated erosional warm/wet based glacial regimes on Mars, especially eskers, are (mainly upland) and depositional (including piedmont) landforms rare in comparison to the widespread presence of glaciers. Kargel such as moraine-like ridges are regarded as evidence that VFF and and Strom (1991), however, were confident that many sinuous, of- LVF are glaciers, or glacier-like forms (GLF; Souness et al., 2012; ten branching, ridges on Mars are eskers that, as on Earth, display Hubbard et al., 2014), formerly thicker and more extensive. The a wide size range and ridge-network variety, ranging from single crater retention age of these landforms indicates they formed over to branching to arborescent and braided. More recently, several researchers have concluded that sinuous ridge systems in Dorsa * Corresponding author. Argentea (Head, 2000) and (Banks et al., 2009; E-mail address: [email protected] (C. Gallagher). Bernhardt et al., 2013)are eskers, the latter reflecting subglacial http://dx.doi.org/10.1016/j.epsl.2015.09.023 0012-821X/© 2015 Elsevier B.V. All rights reserved. C. Gallagher, M. Balme / Earth and Planetary Science Letters 431 (2015) 96–109 97

Fig. 1. Location of southern Phlegra Montes. Background image is THEMIS daytime mosaic overlain by colourised MOLA topography data (purple is low elevation, brown is high). Inset shows position of this figure in relation to a global MOLA hillshade map of Mars. Locations of other figures shown by white boxes. Image credit NASA/JPL/ASU/MOLA science team. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) routing of pressurised meltwater, generated both supraglacially 2.2. Determining the age of the system using impact crater and englacially, through Rothlisberger (R) channels cut upwards size-frequency statistics into extensive glacial ice. On Earth, landsystems pro- duced under warm/wet based conditions, including organised Planetary surfaces can be dated using impact crater size- englacial to subglacial meltwater routing and sediment flux, are frequency distribution data, although this becomes complicated very common, dominating the landscapes of many deglaciated ar- following resurfacing, surface modification or downwasting (Mi- eas. However, observations of subglacial landforms in a state of chael and , 2010). It is also difficult when considering emergence from degrading but extant glacial ice are rare, even on small areas in which insufficiently large populations of craters Earth, and previously un-reported on Mars. This paper describes have accumulated to provide statistically reliable ages (Warner et a system of sinuous ridges emerging from the degraded piedmont al., 2015). Both problems apply to the landsystem in Phlegra, as terminus of an LVF/GLF in the southern Phlegra Montes region. the LVF has probably downwasted over time through loss of ice, Based on analysis of the landsystem as a whole, the conclusion is and the extent of the LVF and associated landforms is only a few that these landforms are eskers emerging from a decayed glacial 100 km2 in areal extent. To estimate the formation and modifica- margin. Together with their contextual landform assemblage, the tion ages of the system, the size-frequency distribution of impact eskers are indicative of significant glacial meltwater production craters was measured for various sub-regions using the ArcGIS and subglacial routing – by definition evidence of a wet-based add-ons Cratertools (Kneissl et al., 2011) and plotted using the tool regime. Whether they are indicative of a climatically-determined CraterStats (Michael and Neukum, 2010). warm-based regime is discussed, and alternatives considered. The CraterTools 3-point method was used to digitise the rims of all visible impact craters. Crater discrimination was complicated by the many rimless circular features on the LVF – most are prob- 2. Approach ably degraded craters but the relationship between their present and original diameter is unclear. We counted all the crater-like cir- 2.1. Data cular features, recording their current diameter. Consequently, this crater-count errs towards overestimating the crater retention age Covering the study area, a mosaic of seven, 6m/pixel, geo- of the LVF. However, this is balanced to an unknown degree by the referenced ConTeXt camera (CTX, Malin et al., 2007) images (Ap- likely loss of many craters from the LVF surface due to sublima- pendix 1 and 2) was constructed using ArcGIS. Other data included tion and flow deformation. Hence, owing to the small count area, a High Resolution Stereo Camera (HRSC; Neukum and Jaumann, the low number of craters counted and surface modifications, the 2004) image and its associated (DEM), count data for the LVF especially give only a first order approxima- and gridded Mars Orbiter Laser Altimeter (MOLA; Zuber et al., tion of the age. 1992) topography data. The MOLA data have low spatial resolu- tion (∼500 m gridding) but high vertical precision (∼1m). The 3. Observations: landsystem components HRSC DEM has higher spatial resolution (75 m gridding) but verti- cal precision similar to the spatial resolution of the original image The landsystem components (Fig. 2a) are a parallel-sided, data (i.e. about 12 m). trough-like valley, striking WNW–ESE through the far-southern Download English Version: https://daneshyari.com/en/article/6427775

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