Structural, tectonic and glaciological controls on the evolution of fjord landscapes

The fjord landscape of South America, stretching similar to 1500 km between Golfo Corcovado (similar to 43 degrees S) and Tierra del Fuego (similar to 56 degrees S), is the largest continuous fjord landscape on Earth. This paper presents the results of new structural geological and geomorphological mapping of this landscape using optical satellite images and digital elevation models. First-order geological structures are represented by strike-slip faults forming lineaments up to hundreds of kilometres long. The strike-slip faulting has been active since Late Cretaceous times and is responsible for the presence of a conspicuous structural cleavage visible as lineaments up to similar to 10 km long. A detailed analysis of these second-order lineaments from digital image data was carried out in three sectors. In Sector 1, located northwest of the North Patagonian Icefield, there are three distinct mean orientations, characterized by a main nearly orogen-parallel orientation (az. similar to 145 degrees) and two orogen-oblique secondary orientations (az. similar to 20 degrees and az. similar to 65 degrees). In Sector 2, located west of the South Patagonian Icefield, there are also three separate mean orientations, with most of the lineaments concentrated between azimuths 0 degrees and 80 degrees (mean at similar to 36 degrees); and two other orogen-oblique means at azimuth similar to 122 degrees and similar to 163 degrees. In Sector 3, around the Cordillera Darwin, there is a single main orogen-parallel mean at similar to 100-115 degrees. In all three sectors, mapped fjord orientations bear a striking similarity to the structural data, with fjords orientated preferentially in the same direction as structural lineaments. We infer that successive glaciations followed the same ice-discharge routes, widening and deepening pre-existing geological structures at the expense of the surrounding terrain to create the fjord landscape. This study has broader implications for ice sheet reconstructions and landscape evolution beneath ice sheets because we demonstrate that the primary control on fjord development in glaciated areas is geological and not glaciological. (C) 2008 Elsevier B.V. All rights reserved.