Abstract
An elongated, flat-floored basin, located in the northern part of Terra Sirenum on Mars, holds numerous enigmatic mounds (100 m wide) on the surface of its floor. We investigated their geological context, spatial distribution, morphological characteristics, and morphometric parameters by analyzing a variety of current remote sensing data sets of Mars. Over 700 mounds are identified; mapping of the mounds shows the spatial density of about 21 per 100 km2 and appearances of several clusters, coalescence, and/or alignment. Most of the mounds have smoother surface textures in contrast to the rugged surrounding terrain. Some of the mounds display depressions on their summits, meter-sized boulders on their flanks, and distinct lobate features. We also perform high-resolution topographic analysis on 50 isolated mounds, which reveals that their heights range from 6 to 43 m with a mean of 18 m and average flank slopes of most mounds are below 10°. These characteristics are consistent with the deposition and extension of mud slurries with mud breccia and gases extruded from subsurface, almost equivalent to terrestrial mud volcanism. If so, both abundance of groundwater and abrupt increase in pore fluid pressure are necessary for triggering mud eruption. Absolute crater retention ages suggest that the floor of the basin located among middle Noachian-aged highland terrains has been resurfaced during the Late Hesperian Epoch. Because of cross-cutting relationships with the basin and the mounds superposed on the basin floor, the faults and fissures (part of Memnonia Fossae) are thought to have formed during and/or after the period of mound formation. Compressional stress fields which likely formed Memnonia Fossae and Mangala Valles, expected from the dike emplacement model of Wilson and Head (JGR 107:1-1–1-24, 2002), may have facilitated undercompaction or overpressurization of subsurface fluids, focused pore fluids beneath the basin, and opened conduits along faults for upwelling voluminous sediments and fluids.
Highlights
Across the southern highlands of Mars, potential groundwater and/or hydrothermal circulation during the Noachian to Hesperian or possibly Amazonian ages has been repeatedly suggested, as evidenced by the morphology of valley networks, regional fissures and associated fluvial channels (e.g., Tanaka and Chapman 1990; Wilson and Head 2002), and distinct spectral signatures of phyllosilicates, chlorides, and sulfates (e.g., Glotch et al 2010; Wray et al 2011; Michalski et al 2017)
Geologic classification of the basin floor Based on the surface textures and stratigraphic relationships identifiable in High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) images, we classified the basin floor into three geological units (Fig. 2c): (1) upper rugged unit which has a rugged surface with superposed impact craters and the target mounds, (2) lower rugged unit which has a rugged surface with impact craters much more abundant than the overlying upper rugged unit, (3) lowermost basement unit which has rocky hillocks or degraded bedrock interbedded with strata of light-toned layered rocks (Wray et al 2011) in places
Distribution of the mounds Using the CTX and HiRISE images, we identified a total of 744 mounds from the upper rugged unit (Fig. 2c), giving the mean of their areal density of ~ 21 per 100 km2
Summary
Across the southern highlands of Mars, potential groundwater and/or hydrothermal circulation during the Noachian to Hesperian or possibly Amazonian ages has been repeatedly suggested, as evidenced by the morphology of valley networks, regional fissures and associated fluvial channels (e.g., Tanaka and Chapman 1990; Wilson and Head 2002), and distinct spectral signatures of phyllosilicates, chlorides, and sulfates (e.g., Glotch et al 2010; Wray et al 2011; Michalski et al 2017).
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