Abstract

Caldera collapse at Glencoe, Scotland, was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned. Intracaldera depocenters had the form of grabens or half grabens, locally with downsag and bounding monoclinal flexures, and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history. This piecemeal caldera collapse is in contrast to previous interpretations, in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault. Collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast. A dominant northwest-trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through Glencoe. The main graben was transected orthogonally by two cross grabens.

Each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity, which built tuff cones, followed by magmatic fountaining that produced lava-like silicic ignimbrites. The three lava-like ignimbrites total more than 300 m thick. Two later eruptions produced eutaxitic silicic ignimbrites, together more than 300 m thick, and contemporaneous progressive downsag was associated with the formation of extensional fractures (crevasses) hundreds of meters deep. Sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence. Unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and/or lacustrine deposition between each ignimbrite eruption. The changes in drainage and sedimentation, and the development of coarse debris-avalanche breccias, reflect tectonic faulting during the periods between major eruptions. The duration of the period including the five caldera-forming eruptions was probably ∼0.5 m.y., and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments (>0.5 km/m.y. normal displacement) is similar to that of the most actively subsiding sedimentary basins. Glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems, which, with knowledge that caldera volcanoes commonly overlie faults, suggests that piecemeal calderas may be more common than previously recognized. The research also shows that the emplacement of granites (s.l.) was episodic rather than synchronous throughout the magmatic province, and it suggests that numerous plutons in the province, e.g., in Donegal, Ireland, may have formed at sites of central volcanoes that are no longer preserved.

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