Amazonian syn-glacial tectonism in Bosporus Planum, Mars, evidenced by a tectonic fault cross-cutting a crater-filling glacier.

Introduction

The surface of Mars records a complex history of tectonic deformation evidenced by tectonic faults including extensional grabens and compressional faults termed ‘wrinkle ridges’. The majority are thought to have formed in Mars’ ancient past, >3.6 Ga, during the Noachian and early Hesperian periods [e.g., 1]. Morphological evidence for more recent tectonism, particularly during Amazonian period (<~3.4 Ga) is much rarer, and more spatially limited [e.g., 2-5].

We present new evidence that tectonic activity occurred in the Bosporus Planum region of southern mid-latitudes since the mid-to-late Amazonian (<1 Gyr). We observe a wrinkle ridge – the surface expression of a compressional fault zone - which cross-cuts a debris-covered glacier of mid-to-late Amazonian-age (Figure 1).

Figure 1: A wrinkle ridge (tectonic fault) dissecting a concentric crater fill (CCF, debris-covered glacier) in Bosporus Planum, Mars. (A) Location on a MOLA elevation map. (B) CTX image of the faulted CCF. White arrows indicate the fault. White dotted line is the crater ejecta. (C) Oblique CTX image with examples from HiRISE where the fault cuts the crater rim and CCF surface.

Observations

The faulted glacier infills a 7.5 km-diameter mid-latitude impact crater. It is morphologically consistent with features termed ‘Concentric Crater Fills’ (CCFs). CCFs, along with other types of putative debris-covered glacier in Mars’ mid latitudes (generally termed ‘viscous flow features’), are thought to have formed in the last few hundred Myr to 1 Gyr [e.g., 6-11].

We observe the fault trace crossing the CCF surface, and cross-cutting the walls, rim and ejecta blanket of the CCF host crater, as well as the surrounding plains. The fault appears to be associated with a regionally extensive population of SW-NE-oriented wrinkle ridges recording compressional tectonism in Bosporus Planum. The ejecta of the CCF-hosting crater embays NW-SE-oriented grabens to the south. Combined with the SW-NE-oriented wrinkle ridges, these indicate a complex history of both extensional and compressional stresses in this region.

The cross-cutting relationship we present between the wrinkle ridge and the CCF provides strong, direct evidence that this fault has been active since the CCF formed, and hence that syn-glacial, compressional tectonism has occurred in Bosporus Planum in the geologically recent past.

Estimating the timing of fault activity

To constrain the approximate timing of fault activity, we combine photogeologic mapping of the Bosporus Planum region with impact crater retention age estimation for various glacial and non-glacial units with different stratigraphic relationships to the tectonic faults in the region. We first generated a regional photogeologic map of the major geomorphic units and wrinkle ridges aligned with the CCF fault, using a basemap of 100 m/pixel Thermal Emission Imaging System (THEMIS) daytime and nighttime infrared images [12-13], and 6 m/pixel Context Camera (CTX) panchromatic images [14]. We are now performing targeted impact crater size-frequency analyses with a combination of CTX and 25 cm/pixel High Resolution Imaging Science Experiment (HiRISE) images [15] to estimate the impact crater retention ages of: (a) the regional population of CCFs; (b) the ejecta blanket of the impact crater hosting the faulted CCF; (c) the surrounding plains which are dissected by the fault; and (d) the ejecta blanket of a 33 km-wide crater 100 km NE of the faulted CCF, which also hosts evidence for faulting (though with a somewhat more ambiguous stratigraphic relationship between the ejecta and the associated fault).

To estimate the impact crater retention ages [e.g., 1] of the regional population of CCFs, we combined the CCF inventory of [16] with original observations and mapping to delimit crater counting areas for 114 CCFs across Bosporus Planum, avoiding portions of CCF surfaces which are significantly obscured by later deposits including aeolian bedforms and ice-rich mantling deposits. The combined area for CCF surface crater counting is 1140 km². Our CCF crater counts are based on CTX images, and impact craters with diameters >50 m. Below this diameter, it becomes challenging to distinguish impact craters from decametre-scale pits which commonly occur in ice-rich materials. We acknowledge the significant uncertainties which arise from impact crater counting on young, ice-rich surfaces, and we have classified the CCFs according to the level of apparent degradation, and additionally classified the impact craters in their surfaces into morphological categories which may reflect different degrees of CCF surface degradation or resurfacing. This allows us to scrutinise the influence of the different morphological subpopulations, and the potential CCF-surface modification processes they record, on the size-frequency distributions we observe.

Conclusions

Preliminary results indicate that CCFs in Bosporus Planum have a combined impact crater retention age of <1 Gyr. This confirms our hypothesis that the tectonic fault which cross-cuts a CCF in Bosporus Planum was active during the Amazonian period, and perhaps as recently as the mid-to-late Amazonian. At this meeting, we will present our full age estimation results, including for the ejecta blanket of the host crater and other units crossed by tectonic faults in Bosporus Planum.

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