Evolution of the Devonian Hornelen Basin, west Norway: new constraints from petrological studies of metamorphic clasts

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Abstract
The Middle Devonian Hornelen Basin of western Norway is a continental fault-basin which formed during the late stages of the Caledonian orogeny, closely following the Late Silurian crustal thickening and high pressure metamophism responsible for the formation of eclogites in the adjacent Western Gneiss Region. Clasts from alluvial fan conglomerates which fringe the basin provide important constraints on the type of basin tectonism and the uplift history of this part of the orogen. The clasts lack a petrographic linkage with the Western Gneiss Region (from which they are separated by a fault), but show a strong petrographic linkage with units of the allochthon (which lie unconformably below the basin fill). The clast sourceland had a similar metamorphic evolution to the allochthon, and both lack evidence for the high pressures recorded in the Western Gneiss Region. Both clasts and allochthon cooled through muscovite argon-retention temperatures about 40 Ma earlier than the Western Gneiss Region. Depth-time curves derived from isotopic and petrological results show that prior to basin initiation the Western Gneiss Region was uplifting faster than the clast source, but still lay at about 10 km depth during basin formation. A tectonic scenario is suggested in which much of the uplift of the Western Gneiss Region took place before the deposition of the Hornelen Basin sediments, aided by crustal thinning along a major extensional mylonitic shear-zone. However, final exposure of this high-pressure metamorphic terrain only took place after the Hornelen Basin sediments were deposited, aided by uplift in the footwall of the basin sole fault.