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

GLASSER, NEIL F.; GHIGLIONE, MATÍAS C.

Santiago de Chile

Conferencia; 4th Alexander von Humboldt International Conference - The Andes: Challenge for Geosciences; 2008

The fjord landscape of South America, stretching ˜1500 km between Golfo Corcovado(˜43oS) and Tierra del Fuego (˜56 oS), is the largest continuous fjord landscapeon Earth. Here we present the results of new structural geological and geomorphologicalmapping of this landscape using optical satellite images and digital elevationmodels. First-order geological structures are represented by strike-slip faults forminglineaments up to hundreds of kilometres long. The strike-slip faulting has been activesince Late Cretaceous time and is responsible for the presence of a conspicuousstructural cleavage visible as lineaments up to ˜10 km long. A detailed analysis ofthese 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 threedistinct mean orientations, characterized by a main nearly orogen-parallel orientation(az. ˜145) and two orogen-oblique secondary orientations (az. ˜20 and az. ˜65).In Sector 2, located west of the South Patagonian Icefield, there are also three separatemean orientations, with most of the lineaments concentrated between azimuths0ž and 80ž (mean at 36); and two other orogen-oblique means at azimuth122ž and 163ž. In Sector 3, around the Cordillera Darwin, there isa single main orogen-parallel mean at 100-115ž. In all three sectors, mappedfjord orientations bear a striking resemblance to the structural data, with fjords orientatedpreferentially in the same direction as structural lineaments. We infer that successiveglaciations followed the same ice-discharge routes, widening and deepeningpre-existing geological structures at the expense of the surrounding terrain to createthe fjord landscape. This study has broader implications for ice sheet reconstructionsand landscape evolution beneath ice sheets because we demonstrate that the primarycontrol on fjord development in glaciated areas is geological and not glaciological.