Investigation of some valley networks and their associated drainage basins in western Terra Sabaea

Branching valley networks provide compelling evidence for an early martian climate that supported liquid water on the surface. Many previous studies of valley networks have not considered their enclosing drainage basins, which may have formed prior to the main era of valley network incision. We examined eight valley networks and their associated drainage basins in far western Terra Sabaea. We mapped the eroded portion of each valley networks watershed using THEMIS and CTX imagery supplemented with MOLA-derived topographic data. Digitized points on areas interpreted to have undergone minimal denudation were interpolated to generate two topographic surfaces: one prior to the formation of the deeply incised valley network and another prior to the formation of the valley networks broader drainage basin. By subtracting the current topography from these pre-erosional surfaces, we estimate eroded volumes. We find that the eroded volumes of these drainage basins are up to an order of magnitude larger than their respective valley networks - evidence of extensive fluvial erosion prior to the main era of valley network formation. Using cross-cutting relationships to identify craters that pre- or post- date the valley networks, we derive ages for the formational cessation of both the drainage basins and the valley networks. Basin formation largely ceased by ~3.9 Ga, but valley network formation continued for another .25- .5 Gyr. For typical valley depths of several hundred meters this implies long-term average erosion rates of ~1m/Myr, similar to rates derived from crater degradation for the Middle to Late Noachian [3]. This suggests a long era of minimal fluvial activity between formation of the Middle to Late Noachian drainage basins and the later valley networks. Alternatively, valley networks formed over a prolonged period but erosive episodes were highly episodic, with brief periods of intense runoff and erosion punctuated by long eras of quiescence. Reference DOIs: [1] 10.1038/ncomms15766, [2] 10.1016/j.icarus.2018.07.017, [3] 10.1029/92JE02508