Tertiary igneous tectonics of Rhum (Inner Hebrides)

Summary A great dislocation is associated with the Tertiary igneous complex of Rhum. In 1903 it was described by Harker, following closely upon Geikie, as an early Palaeozoic thrust. It is now claimed as a Tertiary ring-fault with central uplift. Harker was influenced by the curvature of its outcrop, a feature that at the beginning of the century was regarded as almost diagnostic of a low-angled thrust; today, of course, steep ring-faults * have become a commonplace. Harker was further misled by slump-bedding in the Torridonian, which he interpreted as tectonic; by a patch of Trias with basal cornstone, which he mistook for crushed Torridonian overlying transported Durness Limestone; and by superposition of Lower Torridonian shales on Torridonian sandstone, which latter he thought was post-shale Applecross Grit, instead of pre-shale Basal Grit. Harker misidentified these two grits, largely because he considered that certain associated gneisses were banded Tertiary intrusions, though Geikie had previously recognized their true nature as part of the Lewisian complex. Another correction may here be advanced. Harker took a set of Tertiary explosion-breccias which accompany the tilted Torridonian inside the ring-fault for crush-breccias developed by early Palaeozoic thrusting. Geikie had been hesitant; but Judd had been confident that these rocks were products of Tertiary explosions. The present account is in many respects an elaboration of some of Judd's ideas published in 1874. The outstanding tectonic feature of Rhum is a block uplift, with an elevation amounting locally to about 7000 feet. The uplift was accomplished at the start of Tertiary volcanicity, and was indirectly responsible for the famous inter-lava river gravels of the neighbouring island of Carina. The peridotites of Rhum were later than the block uplift, and they rose along ring-fissures about a centre well to the west of that surrounded by the great initial ring-fault.