77-5 Constraints on Fracture Formation in the Boxwork Region Investigated by the Curiosity Rover in Gale Crater, on Mars

Session: Boxwork and Fracture Halos: Changes in mineralogy and erosion resistance around fracture features on Earth, Mars, and across the Solar System

Presenting Author:

Claire Mondro

Authors:

Mondro, Claire A.¹, Seeger, Christina², Siebach, Kirsten L.³, Gasda, Patrick⁴, Schwenzer, Susanne⁵, Thompson, Lucy M.⁶, O'Connell-Cooper, Catherine⁷

(1) Planetary Science Institute, , (2) Rice University, , (3) Rice University, , (4) Los Alamos National Laboratory, , (5) The Open University, Astrobiology, United Kingdom, (6) University of New Brunswick, , (7) University of New Brunswick, ,

Abstract:

The boxwork features within the stratigraphy of Aeolis Mons, in Gale crater, were originally identified in satellite image data as a horizon of rectilinear, positive-relief ridges forming polygons 3 - 40 meters across. The features extend more than 14 km along the northwest side of Aeolis Mons and appear to be contained within a distinct, continuous stratigraphic layer that changes in elevation by 100 meters along the 14 km span. Initial hypotheses for the formation of the boxwork features proposed a large-scale fracture network with associated fracture halos that created a differential erosional resistance in the host rock. Recent analyses by the Curiosity rover team while investigating the easternmost extent of the boxwork horizon revealed additional complexities in the hypothesized fracture history. We explored two possible environments of fracture formation: 1) subsurface fractures from over-pressured pore fluid and overburden pressure, and 2) surface fracture formation from cyclic volumetric changes (i.e. desiccation, salt fractures, or freeze-thaw). Subsurface fracturing is better supported by rover observations but additional questions remain about the fracture initiation environment. The fracture pattern observed by Curiosity is a network of interconnected, vertical, Mode I opening fractures with no indication of shear, forming rectilinear polygons. The fractures and polygons have no preferred lateral orientation. This pattern indicates a primary vertical stress (overburden) and isotropic confining stresses. Mechanical parameters of overburden pressure, porosity and permeability, pore fluid pressure, and layer thickness can be approximated to estimate the depth of burial at the time of fracture initiation. The spacing of the primary boxwork fractures is unusual in analogous terrestrial fracture networks and suggests that the mechanical layer in which the fractures are confined is thicker than individual bedding. Depth of burial, calculated from overburden pressure and fracture mechanics, helps constrain the fracture initiation within the sedimentary deposition sequence within Gale crater. The current understanding of the fracture initiation event depends on groundwater saturation of pore space within low-permeability sedimentary host rock. Constraining the timing of fracture initiation within the depositional history of Gale crater also places a temporal constraint on the presence of groundwater in the crater-fill sediment.

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