VEINS AND FRACTURES IN GALE CRATER, MARS: UNDERSTANDING POST-DEPOSITIONAL FLUID FLOW EVENTS

Mineralized fractures provide unambiguous evidence for post-depositional fluid flow on Mars. In Gale crater, polygonal networks of cemented fractures have been observed throughout strata of Mount Sharp, including boxwork structures near the Mount Sharp unconformity (Siebach and Grotzinger, 2014) and polygonal fractures described in the Siccar Point Group (Miklusicak et al., 2018). Here we utilize a combination of orbital and rover-based observations of the Murray formation and its stratigraphic context to explore post-depositional fluid flow in Gale crater.

The Murray formation represents the basal geologic unit of Mount Sharp. Mineralized fractures occur throughout the unit and include both mm-scale, discrete calcium sulfate veins and complex networks of cm-scale, multigenerational veins. A hydrofracture origin for veins is indicated by (1) the variable behavior of veins within different lithologic units, (2) the dilation of veins beneath impermeable units, and (3) the arrest of veins within caprock units. Previous investigations have estimated a minimum of ~1.2 kilometers of burial necessary to form such vein networks in Gale (Caswell and Milliken, 2017). However, models that account for overburden pressure alone are insufficient to resolve the conditions under which hydrofracturing occurred; hydrofracturing can occur at any depth if fluid pressures exceed rock strength. Episodic events, such as heat input to pore fluids from impacts, may sufficiently increase fluid pressures to initiate fluid migration and hydrofracture.

Stratigraphic relationships between the Murray formation and overlying rock units (e.g. Stimson formation, mound skirting unit) indicate the potential for multiple hydrofracturing events. The first occurred prior to erosion of the Murray formation and deposition of the unconformably overlying Stimson formation. A second event post-dates erosion of the Murray and deposition of the unconformably overlying units (Stimson formation and mound skirting unit). A third fluid flow event, post-dating deposition of the Stimson, appears to have involved fluid migration, but without mineralization of fractures (Yen et al., 2017). Combined, these observations indicate a protracted history of post-depositional fluid flow in Gale crater, Mars.