Neoproterozoic (Torridonian) alluvial fan succession, northwest Scotland, and its tectonic setting and provenance

The presence of alluvial fan deposits in the lower Neoproterozoic Torridon Group in northwest Scotland illuminates Torridonian basin development at th e eastern Laurentian margin. The 450 m thick Gape Wrath Member of the Apple cross Formation consists of alluvial fan conglomerate and arkose succeeded by more distal, braidplain feldspathic sandstone. Palaeocurrent data compri sing > 2650 measurements on trough cross-bedding are of low variability and show overall eastward Bow. The projection upcurrent of regionally divergen t flow directions for the lower part of the member indicates a fan of c. 50 km radius with its apex 30 km to the west near a basement (pre-Caledonian) normal fault with downthrow to the east beneath the north Minch Basin. Ext ensional tectonics controlled deposition of the Applecross Formation. Regio nal uplift, causing erosion of a youthful topography On the Lewisian gneiss , was followed by the development of the Applecross extensional basin in tw o main stages. Uplift of a western source area by movement on basin-boundin g normal faults occurred first in the north and caused pediplanation and al luvial fan deposition in the Cape Wrath area, with subsequent uplift of the source area for the main body of the Applecross Formation occurring furthe r to the west and south along the line of the Minch Fault. The bulk of the Applecross Formation was derived from a weathered terrain of felsic crystal line and related supracrustal rocks reaching from the Outer Hebrides region westward for up to c. 250 km onto what are now the continental margins of the North Atlantic. The tectonic events may mark an early phase in the crus tal extension that led ultimately to the opening of the lapetus ocean.