GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 20-6
Presentation Time: 9:25 AM


WATKINS, Jessica A., Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, GROTZINGER, John P., Division of Geological and Planetary Sciences, California Institute of Technology, MC170-25, Pasadena, CA 91125 and AVOUAC, Jean-Philippe, Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, MC 100-23, Pasadena, CA 91125,

Gale crater’s 5-km-high central mound, Aeolis Mons (Mt. Sharp), has two leading hypotheses for its formation: buildup of windblown sediments, and exhumation of deeply buried strata. The deep burial hypothesis implies deformation by gravitational body forces and we evaluate that idea here. Ubiquitous fractures have been regionally mapped from orbit and observed by the Curiosity rover in sedimentary strata including the Murray formation (dominantly mudstone) and the unconformably overlying Stimson formation (sandstone). Large fractures which exhibit complex banding structures with distinct chemical trends (e.g. halos) are primarily found in the Stimson formation, but do extend into the Murray formation in one location. Smaller, sulfate-filled fractures are most prevalent in the Murray but are also associated with haloed fractures in the Stimson. We test a compaction-related burial origin for these features based on a mechanical model for mode I fracture formation in order to constrain the regional stress history. According to the Mohr-Coulomb failure criterion, extension fracturing requires that the minimum principal stress (σ3) exceed the elastic tensile strength in the plane normal to the opening. Given that tectonic driving processes are inoperative within Gale crater, non-tectonic mechanisms including overburden removal (maximum compressive stress; σ1 = ρgD) and pore fluid pressure release (pα D) during exhumation must account for this tensile stress. Significant compaction as a result of increased depth of burial is required for pf to exceed minimum compressive stress and cause fracturing. When applied to Gale crater, we find that the horizontal stress (σ3), as induced by crater topography and thermal contraction, requires a substantial burial depth to produce sufficient pf to cause hydrofracture. Rheology contrasts likely caused larger fractures to develop and propagate in the Stimson sandstone, which can support a smaller σ3, than in the Murray mudstone. In these permeable rocks, the sudden local decrease of pf at fractures likely caused pore water flow into the fracture, creating the observed alteration. These results imply that formation of these fractures requires at least one significant burial event in the evolution of Mt. Sharp, providing key insight into the geologic history of Gale crater.