FRACTURE PATTERNS IN MONT MERCOU RECORD CHANGING STRESS FIELDS DURING BURIAL AND EXHUMATION OF MOUNT SHARP, GALE CRATER, MARS

The sedimentary record of changing depositional environments within Gale Crater is exposed on the slopes of Mount Sharp in the ~5 km of stratigraphic section. The style of crater fill – uniform sediment accumulation versus mound-forming sediment – is still under debate, as is the timing of groundwater activity relative to the exhumation of Mount Sharp. In the absence of regional tectonics, fracture sets in Mount Sharp stratigraphy record changes in stress fields due to burial and exhumation events. Filled fractures constrain the timing of fracture initiation events to early Mars history when groundwater was still active. Mont Mercou is a 6 meter vertical outcrop in the Carolyn Shoemaker Formation which contains three distinct fracture sets, distinguished by their orientation: near vertical, high-angle, and low-angle. The dip angle of the low-angle fractures gradually shallows up-section, approaching sub-horizontal near the top of the outcrop. All three fracture sets are filled, indicating that groundwater was still active during or after the burial and exhumation events that created the fractures. The vertical fractures appear to be the earliest fracture set, resulting from overburden pressure during the initial burial of crater fill sediment. The high-angle fractures form in shear-band clusters in the lower part of the outcrop and occasionally exhibit small amounts of shear offset. These fractures transition to a more distributed pattern towards the top of the outcrop before fading out. Low-angle fractures appear near the top of the outcrop and become more numerous up-section, gradually overprinting the high-angle fractures. Ongoing investigations of stress fields and rock properties will help to determine whether the change in fracture orientation is a result of a change in either stress state or rock properties. Successive fracture sets mapped at Mont Mercou record a sequence of changing stress fields during burial and subsequent exhumation events that formed Mount Sharp. Long-lived fluid-filled fracture sets are a known habitat for subsurface microbes in terrestrial settings and further investigation of analogous features on Mars will contribute to our understanding of the habitability of Gale Crater.