An inversion model

The procedure described here to reconstruct past ice sheets from the glacial geomorphological record, combined with the strati-graphical record, builds on previous inversion models of Kleman & Borgstrom (1996) and Kleman et al. (1997). The inversion model comprises a classification system for glacial landform assemblages and a stepwise deciphering procedure. It should be noted that it has to be adapted to the particular area and time frame in question. The following assumptions are used in the model (see further discussion under the key considerations described above):

1 the basic control on landform creation, preservation and destruction is the location of the phase boundary between water and ice, separating frozen from thawed material, at or under the ice-sheet base (i.e. basal temperature);

2 basal sliding requires a thawed bed;

3 glacial lineations can form only if basal sliding occurs;

4 glacial lineations are created aligned parallel to local ice-flow directions and perpendicular to the ice-sheet surface contours at the time of creation;

5 frozen-bed conditions inhibit the reshaping of the subglacial landscape;

6 regional deglaciation is always accompanied by the creation of a spatially coherent but metachronous system of meltwa-ter features, such as meltwater channels, eskers and glacial lake traces;

7 eskers are formed in an inward-transgressive fashion inside a retreating ice front (Norman, 1938; De Geer, 1940; Hebrand & Amark, 1989; Bolduc, 1992);

8 meltwater channels will form the major landform record during frozen-bed deglaciations, whereas eskers are typically lacking under these conditions.

The main analytical components used in the inversion model are called swarms. These are temporary tools in the inversion model and represent glacial landform systems, or landform sets, with similar morphological characteristics. The use of swarm serves the purpose of data reduction, because each swarm is a simplified and spatially delineated map representation of many individual landforms. Therefore, they allow relative chronologies to be applied to a manageable number of cartographical units. Coherent swarms are defined on the basis of spatial continuity of landforms in a landform system, and the resemblance to a glaciologically plausible pattern, i.e. a minimum-complexity assumption.

0 0

Post a comment