Reconstruction of a Volcanic Edifice Using the Dike Swarm at Ocros, Peruvian Andes

Abstract

The original structure of an eroded stratovolcano has been reconstructed from dike-swarm data measured in the field at Ocros, Peruvian Andes. Flow lineations of magma within dikes were estimated from measurements of elongation of vesicles and orientated alignments of phenocrysts. 45 percent of dikes at Ocros area have a lineation plunging steeply eastward. Strikes of the dikes show slight radial variation pointing east along the measured route. Such evidence suggests that the magma of these dikes was supplied from a common central conduit located estward and close to the measured route. Remaining dikes have lineations plunging shallower and either westward (32%) or eastward (23%), but their strikes are nearly parallel. Thus the magma for these remaining dikes was supplied from two separate conduits located east and west and relatively farther from the measured route. None of systematic differences in chemistry or modal composition were detected among the three groups of dikes and associated extrusives. This evidence suggests that the original structure was a composite stratovolcano having three nearby magmatic conduits. Maximum horizontal compressional stress axis (MHC) determined from the dike orientation is N80°E. This direction is similar to the result obtained from most focal mechanism solutions and flank-crater alignments of several late Quaternary volcanoes in the Central Andes area.