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 Cape 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 flow. The projection upcurrent of regionally diverge nt flow directions for the lower part of the member indicates a fan of c. 5 0 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. Ex tensional tectonics controlled deposition of the Applecross Formation. Regi onal uplift. causing erosion of a youthful topography on the Lewisian Gneis s, was followed by the development of the Applecross extensional basin in t wo main stages. Uplift of a western source area by movement on basin-boundi ng normal faults occurred first in the north and caused pediplanation and a lluvial fan deposition in the Cape Wrath area, with subsequent uplift of th e source area for the main body of the Applecross Formation occurring furth er 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 crysta lline and related supracrustal rocks reaching from the Outer Hebrides regio n westward for up to c. 250 kin onto what are now the continental margins o f the North Atlantic. The tectonic events may mark an early phase in the cr ustal extension that led ultimately to the opening of the Iapetus ocean.