Flank collapse triggered by intrusion: the Canarian and Cape Verde Archipelagoes

The potential to develop kilometer-scale instabilities on the flanks of int raplate volcanoes, typified by the Canary and Cape Verde Archipelagoes, is investigated. A primary triggering agent is forced injection of moderate-sc ale dikes, resulting in the concurrent development of mechanical and therma l fluid pressures along the basal decollement, and magmastatic pressures at the dike interface. These additive effects are shown capable of developing shallow-seated block instabilities for dike thicknesses of the order of 1 m, and horizontal lengths greater than about 1 km. For dikes that approach or penetrate the surface, and are greater in length than this threshold, th e destabilizing influence of the magmastatic column is significant, and exc ess pore fluid pressures may not be necessary to initiate failure. The pote ntially destabilized block geometry changes from a flank-surface-parallel s liver for short dikes, to a deeper and less stable decollement as dike hori zontal length builds and the effects of block lateral restraint diminish. F or intrusions longer than about 1 km, the critical basal decollement dives below the water table and utilizes the complementary destabilizing influenc es of pore fluid pressures and magma "push" at the rear block-scarp. In add ition to verifying the plausibility of suprahydrostatic pressures as capabl e of triggering failure on these volcanoes, timing of the onset of maximum instability may also be tracked. For events within the Cumbre Vieja (1949) and Fogo (1951, 1995) pre-effusive episodes, the observation of seismic act ivity within the first 1 week to 4 months is consistent with the prediction s of thermal and mechanical pressurization. (C) 1999 Elsevier Science B.V. All rights reserved.