REVERSAL OF PALEOCHANNEL SLOPES NEAR THE MARTIAN DICHOTOMY BOUNDARY CAUSED BY TECTONIC ACTIVITY: IMPLICATIONS FOR CRUSTAL PROCESSES

On Earth, tectonic activity may cause the deformation and reversal of fluvial channel slopes. Identifying fluvial slope reversal on Mars may therefore provide insight into tectonic processes. In the martian eastern hemisphere, compression and extension faults along the dichotomy boundary are evidence of tectonic activity, which may have resulted from either flow of highland lower-crustal material northward or flexure caused by loading (e.g., by deposition of material eroded from the highlands) of the northern lowlands (Watters, 2003; Nimmo, 2005). Each of these processes causes distinct direction and angle of slope reversal. Therefore, analyzing the slopes can provide a means for distinguishing between them, although other processes may have also affected the local topography and may complicate the interpretation.

In the Aeolis Dorsa region, ~275 km north of the dichotomy boundary, are located two fan-shaped networks of inverted channels, interpreted as either alluvial fans or deltas. A study of the longitudinal profiles of the channels shows that the current slope direction of these networks is opposite to the inferred flow direction. We test the hypotheses of lower crust flow and material erosion from the highlands and deposition in the lowlands by measuring the current average slope gradient of the inverted channels in the two networks and by comparing these measurements with predicted slope changes from geophysical models for each of the two processes. Taking the possibility of non-tectonic (i.e., collapse, compaction) processes into account, our results indicate that, between these two models, the slope reversal is more consistent with the predicted tectonic response to erosion and deposition of highland material, probably in conjunction with deposition of the Medusae Fossae Formation, than with the lower-crustal flow model. Based on these results, we construct a chronological sequence of the different geological and geomorphological processes in that area, which provides insight into the evolution of the dichotomy boundary in the Aeolis Dorsa region.