Evidence of Late-stage Fluvial Outflow in Echus

Introduction: New high-resolution datasets have prompted a mapping-based study of the Echus Chasma and Kasei Valles system. Some of the highlights of our new findings from the Amazonian (<1.8 Ga) period in this area include (1) a new widespread platy-flow surface material (unit Apf) that is interpreted to be 2,100-km-runout flood lavas sourced from Echus Chasma; and (2) a fracture in Echus Chasma, identified to have sourced at least one late-stage flood, that may have been the origin for the platy-flow material and young north-trending Kasei floods. This abstract discusses details of the Echus Chasma fracture and putative outflow (Fig. 1). Supporting Evidence: In north Echus Chasma and also directly northeast of the chasma, there are floor fractures bounding downdropped blocks of plateau that have been subsequently covered by young lava units Apf and Tharsis Montes member 5 (At5; Fig. 1). One of these features in north Echus Chasma appears to have sourced outflow of water. Supporting geomorphic relations can be observed within the southernmost young lava lobe of Tharsis Montes Formation member At5 that flowed into the Apf-covered floor of Echus Chasma (Figs. 1 and 2). This lava lobe is eroded and dissected by shallow channels, leaving a ring of rubble marking the former lobate borders of the flow. Similar appearing terrestrial analogs of this eroded morphology can be observed in Iceland, where lava flows have been " washed " by catastrophic flood waters to leave behind blocky terminus areas and piles of internal rubble surrounded by mud-covered terrain (best observed at sites A,B on Fig. 2). The shallow channeling can be traced back to a fracture (fossa) on the floor of Echus Chasma (Fig. 3). Presumably, water forced upward from a buried aqui-fer would behave like magma dikes. Dikes utilize pre-existing joints and zones of weakness in the crust, enlarging these fractures and also creating adjacent, additional, parallel pathways to the surface [1]. Zones of weakness on the floor of Echus apparently included the floor fracture that cut relatively thick deposits of lava unit Apf (Fig. 1) and a wallrock spur on the floor of Echus Chasma (D on Fig. 2). Flood water being emitted from a crack or fossa is a fairly common scenario on Mars, with prime examples being Athabasca and Mangala Valles outflow channels [2,3].