Tertiary igneous tectonics of Rhum (Inner Hebrides)

Abstract

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.

Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example from the Isle of Rum, NW Scotland

“…The varying igneous rock types observed and described from the Isle of Rum have greatly contributed to the development of modern views of the magmatic, as well as the temporal and structural evolution of the British Palaeogene Igneous Province (Bailey, 1945;Emeleus, 1997;Emeleus & Bell, 2005;Troll, Donaldson & Emeleus, 2004;Troll et al 2008). Rum is located on the Hebridean Terrane in NW Scotland (Fig.…”

Sedimentary and volcano-tectonic processes in the British Paleocene Igneous Province: a review

“…Much of this uplift (and later subsidence; see Section 7.a) was accommodated along the Main Ring Fault system, a set of steeply inclined arcuate faults, which delimit the Rum Central Complex (Bailey, 1945;Emeleus, 1997;Troll, Emeleus & Donaldson, 2000) (Fig. 2c).…”

Caldera formation in the Rum Central Igneous Complex, Scotland

“…Judd (1874) referred to the felsites as "lavas", whereas Harker (1908) suggested that a supposed Paleozoic shear zone marked by extensively brecciated horizons had been intruded by Tertiary felsite sheets. The shear zone was reinterpreted by Bailey (1945) as part of a major Tertiary ring fracture system termed the Main Ring Fault, within which major central uplift had occurred. Subsequent studies by Hughes (1960) and Dunham (1968) highlighted the close association of the breccias and felsites.…”