TERRESTRIAL DILATIONAL FAULTS: ANALOGS FOR NORMAL FAULTS ON MARS AND POSSIBLY ICY MOONS

Extension in the solar system may be manifested by normal faults, both on rocky planetary surfaces (Mercury, Venus, Earth, Mars, the Moon) and on the outer solar system icy moons (Ganymede, Europa, Enceladus, Dione, Rhea, Miranda, Tethys, Ariel). On the icy moons, extension may be driven by tidal forcing, nonsynchronous rotation, true polar wander, obliquity, libration, orbital recession, despinning, or ice shell thickening. On the terrestrial planets, excluding plate tectonic driving forces (Earth), extension may be driven by magmatic processes at volcanic centers, commonly manifested as radial patterns of faults that form above underlying intrusive dikes (Earth, Mars, and Venus). Impact events and subsequent basin collapse may also result in extension and normal faulting (Mars, Mercury, and the Moon), as will flexural uplift (Mercury). The prevalence of basalt on the surfaces of the terrestrial planets and the Moon, and the common association of normal faults with volcanic centers, implies that terrestrial normal faults that form in basaltic lava flows or other volcanic products provide the best analog to those on other planets. Normal faults in lava flows are typically vertical at the surface in response to the utilization of cooling joints in basalt columns as faults propagate towards the surface from below. At depth, fault dips are more typical for normal faults (60-75°). As a result, motion at depth causes dilation along the steeper portions of faults near the surface, creating gaping fissures. Fault traces are commonly flanked by hanging wall monoclines, which form by flexing at the surface above the upward-propagating fault prior to breaching of the surface. These monoclines may accommodate a significant portion of the fault throw, although they become passive features after the fault breaches the surface and may ultimately collapse to form a pile of rubble along the fault trace. Examples of such fault geometries are described from basalts in Iceland and California, and may be useful analogs for Martian faults in particular (e.g., Tharsis, Elysium, Hellas), given the observation of columnar basalts on Mars. Although icy moons do not contain analogous columnar features, the prevalence of true tension at the surface, combined with low tensile strength of ice, may promote dilational fault development.