EXTENSIVE POLYGONAL FRACTURE NETWORK IN SICCAR POINT GROUP STRATA, GALE CRATER, MARS

Rock fractures are indicators of stress within a geologic system, and fracture morphologies can commonly be used to infer formation conditions. Polygonal fracture systems are common in isotropic, contractional stress regimes such as in rocks exposed at the surface of a planet undergoing thermal cycling, or in sedimentary substrates undergoing repeated wetting and drying. Such polygonal fracture systems, ranging from centimeter to decameter scales, have been widely documented on Mars. Utilizing a combination of orbital- and ground-based images, we describe a laterally extensive polygonal fracture network that occurs within the lowermost strata of the Siccar Point group in Gale crater, Mars. Siccar Point group strata is exposed over approximately 20 km² in northwest Gale crater, where it unconformably overlies eroded strata of Mount Sharp (Aeolis Mons), and records dominantly aeolian deposition along the lower flanks of Mount Sharp. Images reveal an extensive network of polygons whose edges are preserved in erosional relief. Polygons are approximately 7.5 meters across (i.e. measured as the diameter of a circumscribed circle; n=8700) and exhibit interior angles (i.e. fracture intersections) with modes at 90° and 120° (n=650).

We interpret this fracture system as resulting from desiccation of episodically wet regions along the basal flanks of Mount Sharp during deposition of the Siccar Point group. Distinct modes in the angle of fracture intersection is interpreted to record the timewise evolution of fractures from T-junctions to Y-junctions, wherein 90° intersections reflect fracturing during primary desiccation events, and 120° intersections reflect modification of fracture intersections during repeated wet-dry events. Multiple cycles of expansion and contraction are attributed to desiccation and fluid recharge, either from groundwater discharge or surficial run-off from Mount Sharp. Erosional resistance of preserved fractures is inferred to reflect later diagenetic fluid flow along the sub-Siccar Point group unconformity and preferential cementation along fracture margins. Such evidence for multiple fluid events in the relatively young strata of the Siccar Point group provides evidence supporting a protracted history of fluid stability in Gale crater.