Litho-structural control on rock slope failures at Garmaksla, Billefjorden coastline, Svalbard.

The eastern slope of Garmaksla, a flat-topped mountain at the western margin of Billefjorden, Svalbard, is affected by mass movements of different types. Rotational rock slides, rock fall and a rock avalanche affecting the coastal cliff are shallow surface expressions covering a larger rock mass instability that is bordered to the west by the Balliolbreen Fault. This structural feature is part of the Billefjorden Fault Zone and accommodated multi-phase deformation since Devonian time. Based on a comprehensive morpho-structural analysis, the mapped surface features and rock slope failures are explained by a compound rock slide model that reveals a litho-structural control on the type and mechanism of slope instability. The Balliolbreen fault serves as an inherited zone of weakness that is re-activated as the rear rupture surface of the rock slide. In addition, favorably oriented bedding planes and pre-existing fault zones serve as prime conditioning factors for the compound rock slide. A postglacial age of at least 6 ka is derived from 14 C dated sediments of Garmaksla Lake, a perennial sag pond along the main scarp. While the current state of activity of the compound rock slide is unclear, an increase of shallow slope instabilities is expected due to climate warming.