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¨CNd model ages (tDM) 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¨CSW 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.
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