TECTONIC INFLUENCES IN PIECEMEAL CALDERA COLLAPSE AT GLENCOE VOLCANO,SCOTLAND

Glencoe Volcano is renowned as the archetype for caldera collapse that results from piston-like subsidence of a coherent crustal block on a ring fault. However, the caldera-floor rocks, and the geometry of seve n intracaldera units that record large volume explosive eruptions, sho w that the collapse involved incremental and haphazard subsidence of n umerous crustal blocks before development of the ring fault. Some subs idence involved flexure (downsag) with associated development of exten sional crevasses hundreds of metres deep. A system of orthogonal fault s and related grabens records the influence of two intersecting baseme nt discontinuities, one of which is inferred to have linked with the a ctive Great Glen fault. Substantial extensional and/or transtensional faulting, with distinct related sedimentary responses, occurred withou t eruptions. The tectonic framework substantially controlled the magma tic plumbing and the locations of vents, caldera depocentres, and thro ugh-going rivers. The related sediments influenced contrasting eruptio n styles (e.g. transitions between phreatomagmatic and magmatic) and t he shallow-level emplacement of sills.