A unifying model for the Torridon Group (early Neoproterozoic), NW Scotland: Product of post-Grenvillian extensional collapse
Abstract The Proterozoic continental deposits of the Torridonian Supergroup in NW Scotland unconformably overlie the Lewisian Complex at the eastern margin of Laurentia and form an important component of British stratigraphy. After more than 100 years of investigations, however, disagreement persists concerning the provenance, palaeoenvironment and tectonic setting of the ~  980 Ma fluvial Applecross and Aultbea formations (3.5 and 2 km thick, respectively), the dominant units of the early Neoproterozoic Torridon Group. Some workers envisage a mostly pre-Grenvillian source terrain that extended from the Outer Hebrides region westwards onto what are now the continental margins of the North Atlantic, with deposition on a bajada and braidplain in a rift or extensional basin. Others argue that these formations were derived mainly from the Grenville orogen in Canadian Laurentia and deposited by a trunk river system in a thermal relaxation or orogen-parallel Grenvillian molasse-type foreland basin. The model presented here draws on different aspects of these opposing views. The immature arkoses and feldspathic sandstones typical of the Applecross and Aultbea formations imply the derivation of much material from a nearby source area with quartzo-feldspathic crystalline rocks, and the varied pebble suite of the Applecross indicates that the source area also contained a supracrustal series including clastic sediments. Geochronological data for Applecross pebbles and detrital zircons from the Applecross and Aultbea formations together indicate dominantly late Palaeoproterozoic sources, with subordinate contributions from Archaean and late Mesoproterozoic (Grenvillian) rocks. Sm–Nd model ages ( t DM ) for Applecross and Aultbea shales and sandstones range from 1.77 to 2.05 Ga, consistent with the sources inferred from the detrital geochronology. Features of the Applecross Formation that accord with an adjacent western source area and extensional basin setting include immature arkoses and megafan pebble- and cobble-conglomerates derived mainly from relatively old crystalline basement and supracrustal rocks, southeastward palaeocurrents orthogonal to the Torridon Group outcrop belt and the Outer Hebrides Fault Zone (OHFZ) and Minch Fault in the west, and NE–SW dilational dykes of sandstone in the lithified Torridon Group and Lewisian basement indicating syndepositional faulting under tensional stress perpendicular to the marginal faults. Some detritus could have come indirectly from the Grenville orogen in eastern Laurentia, the Rockall Plateau and adjacent continental margins by the recycling of clastic sediments that were derived from the orogen and deposited prior to 1 Ga within the future source area of the Torridon Group. Evidence of Grenvillian tectonic events occurs widely in the Lewisian of NW Scotland, including the Outer Hebrides, and numerous geologists have concluded that Torridon Group deposition followed Grenvillian movement on the OHFZ with uplift of an adjacent source area to the west. It is argued here that a western source area was uplifted by late Grenvillian ( ~  1 Ga) craton-directed thrusting on the OHFZ, with development of the Applecross basin following post-Grenvillian extensional collapse and reactivation of the OHFZ as an extensional fault along the line of the Minch Fault. This tectonic model helps reconcile opposing views on the genesis of the Torridon Group.