Abstract

The Scafell caldera, an Ordovician ensialic andesite-rhyolite caldera in the English Lake District, is probably the best-exposed "piecemeal" caldera so far documented. Intra-caldera volcanotectonic faults indicate whole-sale fragmentation of the caldera floor and were reactivated repeatedly during a major subaerial caldera-collapse episode. Such piece-meal collapse is in contrast to calderas in which the floor has undergone coherent piston-like (en bloc), downsag, or trapdoor-like subsidence. Mesobreccias, large slide blocks, complex growth-fault geometries, and abrupt local thickness and facies variations in the caldera-fill ignimbrites show that the caldera floor repeatedly developed intricate and rapidly changing topographies of high relief, owing to formation of close-spaced volcanotectonic faults and their frequent reactivation, including movements of reversed sense. At the onset of caldera collapse, volcanotectonic faulting and associated seismicity caused widespread (>260 km2) and ubiquitous subaerial gravity sliding and spreading of an extensive, thick (>30 m), phreatoplinian ash layer. This is recorded as abundant low-angle soft-state slide surfaces, with compressional and extensional deformation off slipped material, including numerous close-spaced ramifying high-angle faults and zones of disaggregation. Particularly intense soft-state deformation is localized at steeply inclined faults that define numerous caldera-floor fault blocks, 0.1-2 km across, and at these faults, the style and geometry of the small-scale soft-state deformation show the sense of fault displacement. Intracaldera silicic ignimbrites overlie the disturbed ash layer and are cut by numerous steep zones of ductile (rheomorphic), brittle, and hybrid deformation structures that formed in response to faulting before the hot tuffs cooled. The deformation zones are characterized by rheomorphic folds, steep high-attenuation rheomorphic shear fabrics, and a facies termed "domain breccia" that is interpreted as fused rheomorphic breccia. The style of this hot-state deformation is variable and reflects differences in competence, strain rate, and timing of faulting relative to the emplacement and cooling history of each ignimbrite. The deformation zones separate relatively undeformed ignimbrite that underwent dip-slip displacements of as much as 400 m and angular rotations of as much as 20°. The zones can be traced downward into steeply dipping faults that cut caldera-floor rocks. Overlying lacustrine (caldera lake) sediments display slumps and a depositional architecture that record declining persistence of the fault activity. There is no evidence of largescale resurgent doming.

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