Geometric analysis of decameter scale fractures in the Peace Vallis fan lower Unit and Mt. Sharp Boxworks, Gale Crater Mars

The boxwork unit on Mount Sharp presents an interesting erosional geomorphic feature that will shortly be investigated by the Curiosity Rover. The boxworks have been interpreted as a decameter-scale rectilinear fracture pattern infiltrated by ground water subsequent to its formation [1]. It is hypothesized that a fluid-based fill cemented the area around the fractures leaving ridges of erosion-resistant material. Here we investigate the similarities between the Mt. Sharp boxwork unit, fracture patterns adjacent to the boxworks that do not exhibit a significant vertical expression, and a similar appearing decameter-scale fracture pattern on the lower unit of the Peace Vallis (PV) fan [2].

Unlike the raised topography of the Mt. Sharp boxworks, vertical expression along fractures in the PV lower fan unit is poorly developed and is only found in a few isolated locations. However, analysis of fracture patterns using the Symbolic Plane approach of Domokos et al. [3] shows that both areas have a similar underlying polygonal fracture pattern with resultant rectilinear forms. These forms average three junction angles at vertices and normally produce four sided blocks. This is similar to fracture patterns associated with Platonic attractors on both Earth and Mars [3]. In both areas, cells of a regular primary mosaic X-type nodes are sequentially bisected locally creating irregular T-type nodes in response to secondary fracturing. Both areas exhibit similar regularity measures (#T nodes/total # of X and Y nodes) that range from ~0.82 to 0.86.  The unit adjacent to the Mount Sharp boxworks that does not display raised ridges has a similar fracture pattern and regularity. The geometric similarity of all three areas suggests that they are likely to have formed from processes that produced similar stress fields.

We hypothesize for all three areas that aeolian erosion subsequent to fracture formation has removed some portion of the surface. However, in the case of boxworks, the contrast between the cemented and less resistant non-cemented material leads initially to the production of proto-boxwork rounded hollows. These hollows expand and eventually reach the more resistant rock of the infiltrated fractures resulting in high standing remnant ridges ~ 1-meter or less in height. If the boxwork groundwater hypothesis is correct, this suggests that limited groundwater may have been available on the Peace Vallis fan subsequent to its deposition sometime after ~3Ga [4]. The quantity of groundwater was insufficient to allow mineralization in all but a few isolated locations on the fan as well as in the non-boxworks fractured area adjacent to the boxworks unit.

 [1] Siebach and Grotzinger, JGR, 2014. [2] Oehler et al. Icarus, 2016. [3] Domokos et al., PNAS, 2020. [4] Scuderi et al., 2018 LPSC