Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases

Lineated valley fill (LVF) in fretted valleys at the dichotomyboundary has been interpreted as glacial in origin. Unknownare (1) the original thickness of the glacier ice, (2) the amountof ice-surface lowering, through sublimation and retreat, toits presently observed level, and (3) whether there were multipleperiods of glaciation. We address these questions through analysisof an LVF glacial system. The elevation difference between theupper limit of a previous highstand and the current surfaceof the LVF at the study site is ~920 m. We interpret this differenceto be the minimum amount of ice-surface lowering of the glaciersystem. Consistent with a general lowering of the ice surfaceare multiple moraines and/or trimlines, and changes in LVF flowpatterns, as the ice retreated and decreased in thickness. Thesuperposition of several lobes onto the current surface of theLVF indicates that a phase of alpine glaciation followed thelowering of the valley glacial system. These data suggest thatthe Late Amazonian glaciation that produced LVF in this regioninvolved significantly larger amounts of ice than previouslythought, and that subsequent alpine glaciation followed.