HYDRAULIC FRACTURING OF MARTIAN MUDSTONES
On Earth, hydraulically fractured shales show crosscutting mineralized fractures (quartz, carbonate, sulfate minerals etc.) that vary in width from sub-mm to decimeter, and deformation due to subsequent compaction. The latter can inform about depth of burial when fracturing occurred, and suggests that hydraulic fractures can form in low permeability sediments at burial depths as shallow as 500-1000 m. For vertical fractures to form, the fluid pressure has to exceed the local minimum principal stress and the tensile strength of the rock, and when horizontal and/or bedding plane parallel off-shoots of fractures occur the fluid pressure has to exceed the local lithostatic pressure.
Hydraulically fractured mudstones on Earth and presumed Martian counterparts compare well in fracture style, deformation (ptygmatic folding), and associated concretions. With Mars gravity 62% lower than on Earth, a mudstone on Mars would require substantially deeper burial to achieve the same level of consolidation as an Earth equivalent. Thus, the soft and tightly compacted mudstones of YB may have been buried to substantial depths (in excess of 1 km) by the time they were hydraulically fractured. To generate the substantial volumes of mineralizing fluids that must have passed through these rocks probably requires a thick section of compacting sediments beneath YB. In summary, hydraulic fractures in YB mudstones attest to substantial burial and vertical movement of evolved brines from underlying or laterally adjacent strata.