When Were Glaciers Present in Tharsis? Constraining Age Estimates for the Tharsis Montes Fan-shaped Deposits

Introduction: Over the past several years, we have presented new data and proposed a cold-based glacial hypothesis for the Tharsis Montes fan-shaped deposits [14], which were first identified and analyzed using Viking data [5-8]. Based on stratigraphic relationships with young lava flows in Tharsis, we know that the deposits are of midto late-Amazonian age [8,9], but we have little information about their absolute ages. Temporal information is important because these deposits provide a record of geologically recent climates on Mars that are very different from the present. Recent GCM simulations confirm that these glaciers should form during periods of high obliquity (>45°) when water ice at high latitudes is precipitated on the flanks of the Tharsis Montes [10,11]. With more accurate age estimates, we may be able to define periods in the recent past when the specific conditions necessary to form the fan-shaped deposits, or even individual facies within the deposits, were present. Here we use several techniques in an attempt to constrain age estimates and timescales for the fan-shaped deposit at Arsia Mons (Fig. 1) including morphological observations, HRSC crater counts, superposition relationships, terrestrial analogs [12], and ice-sheet modeling [13]. Phases of Glaciation: The Tharsis Montes fan-shaped deposits share three characteristic facies, a ridged, knobby and smooth facies [1-3,6-8]. Each suggests unique climate conditions during deposition. Ridged Facies. The ridged facies is interpreted as a series of drop moraines deposited around the margins of a retreating cold-based glacier [1-3]. Based on their total number, regular spacing, size, and slight variations, we believe that the ridges represent a climate signal. After considering several possibilities (e.g., annual, orbital) along with the conditions necessary for ridge deposition, we speculate that the ridges were produced by the 120 kyr obliquity cycle at a time when Mars was at a high (>45°) mean obliquity. In addition, there are several locations where collections of ridges display cross-cutting relationships, suggesting that smaller episodes of readvance punctuated the phase of overall retreat during ridged facies formation [12,15]. Knobby Facies. The knobby facies is superposed on the ridged facies and is interpreted to be a till produced by ice sheet collapse. This interpretation implies a significant change in climate conditions, favoring rapid ice loss and downwasting as opposed to the relatively stable climate inferred for drop moraine formation [12]. The continuation of the ridged facies for >35 km beneath the outer margins of the knobby facies (Fig. 1) also suggests that an additional phase of glacial advance occurred before deposition of the knobby facies [2]. Smooth Facies. The smooth facies (Fig. 1) is the youngest unit within the deposits and is interpreted as lobate, debris-covered glacier ice [1,2,4]. Based on observations of individual smooth facies lobes that appear to define multiple advances [14,15], we believe that the smooth facies represents smaller, late-stage glacial events that occurred after the ice sheet forming the ridged and knobby facies had largely disappeared. An alternative explanation is that the smooth facies simply represents the waning stages of a large ice sheet at Arsia. We interpret a transitional unit observed around the present margins of the smooth facies (TKF, Fig. 1) as evidence that the lobes are currently experiencing retreat. The smooth facies currently covers ~23,000 km 2 , while the areal extent of the transitional unit suggests that the smooth facies covered at least 33,000 km 2 in the past.