Abstract:
Hrad Vallis is located in the transition zone between Elysium Mons and Utopia Planitia. Near its origin, at the northern edge of Elysium lavas, Hrad Vallis is characterized by a low-sinuousity channel within a north-northwest-trending, broad, flat-floored valley. A nearby flat-floored valley is parallel to the Hrad trend and parallel to elongate depressions, fissures, and faults in the region. An apparent hierarchy of landforms provides insight into the origin of the features associated with Hrad Vallis. The sequence leading to the development of Hrad Vallis consists of the following (1) formation of isolated depressions as either karst depressions or thermokarst valleys along faults and fissures in response to circulating ground water; (2) expansion of depressions along structural trends to coalesce as composite valleys, and (3) incision of a channel on the floor of Hrad valley by continued discharge of water from the subsurface after its initial formation by nonfluvial processes. Mud flows, polygonally fractured terrain, and chaotic terrain near the head of the major valleys suggest thixotropic behavior of saturated, clay-rich materials. An extended period of time is indicated during which freely circulating water existed on id beneath the surface of Mars. Karst and thermokarst processes imply very different climatic regimes and different host materials. The presence of karst topography implies extensive deposition of carbonates or other soluble rocks, whereas the presence of thermokarst basins implies the existence of porous, water/ice-saturated clastic or volcaniclastic materials