Subglacial thermal organization of the northern part of the Patagonian Ice Sheet during the Last Glaciation

RUIZ, LUCAS.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

Until now, the Northern part of the Patagonian Ice Sheet (NPIS) (38°-44°S) was interpreted as a west-east asymmetrical ice sheet characterized by; a linear center ice divide (related to the highest summits), with an alpine style glaciation developed to the East, and an ice sheet style to the West (Glasser et al., 2008). These differences were associated with the direction of westerlies, the source of precipitation. The aim of this presentation is a first approach to the subglacial thermal organization (Kleman and Glasser, 2007) of the former NPIS during the last glaciation. Based on remotely sensed data and terrain control, different subglacial and supraglacial erosive landforms were identified and mapped in a zone former occupied by the NPIS (41° to 43°S). They range from mega scale landforms (cirques, troughs and rock basins), meso-scale glacial lineaments (rock drumlins, whalebacks and roches moutonnées) and meltwater channels to micro scale landforms such as striae and pforms. The subglacial landforms were grouped in different land-systems related to the subglacial thermal regime (ice domes, ice cold patches, ice tributaries, ice-streams, outlet glaciers, mountain glaciers and valley glaciers), the criteria used to distinguish them are; orientation, distribution, form and size of glacial lineaments in valley troughs, distribution, size and morphology of valley troughs, size and morphology of cirques, presence of low altitude pass with linear erosive forms, zones of no-glacial erosion and the relation between subglacial forms with marginal glacial landforms. In order to explain the spatial distribution of landforms, the land systems were temporally related in a complete glacial cycle model (onset, full glaciation and deglaciation). During full glaciation (continental ice sheet), the feeding zones were characterized by various ice domes centered in the highest mountains zones. Their locations were controlled by the structure and previous topography. They form a multiple-discontinuous ice divide, because of the presence of NW-SE and N-S structural valley troughs, which separate them. Ice domes were cold based and drained by ice tributaries that confluence in topography constrained or “Isbrae type” ice-streams (Evans, 1996; Truffer and Echelmeyer, 2003) present in the major valley troughs. Ice streams that flow to the East pass to outlet glaciers which confluence with valley glaciers in the marginal zone, the high relief of this zone produces the bifurcation of the ice mass in various terminal lobes. On the other side, ice streams that flew to the West found less restriction due to the general low relief and could form huge expanded piedmont lobes. At this stage the erosion was concentrated in the major valley troughs, and overdeepenings. During onset and deglaciation, the former zones of ice domes where occupied by mountain glaciers and ice fields, valley glaciers occupied the heads of the troughs, lakes formed in the rock basins and fluvial erosion generated deep gouges in the valley floors. At this stage the glacial erosion passed to the head of the troughs and the top of the mountains, remodeling the previous subglacial landforms and imprinting an alpine style to the landscape. It is concluded that zones of erosion (i.e. the source of sediment for the marginal ice land-system) and ice dynamics vary in concordance with the configuration/stage in the glacial cycle. This must be taken into account when the glacial records of the marginal zone are analyzed