Fore arc tectonothermal evolution of the El Oro metamorphic province (Ecuador) during the Mesozoic

Abstract

The El Oro metamorphic province of SW Ecuador is a composite massif made of juxtaposed terranes of both continental and oceanic affinity that has been located in a fore-arc position since Late Paleozoic times. Various geochemical, geochronological, and metamorphic studies have been undertaken on the El Oro metamorphic province, providing an understanding of the origin and age of the distinct units. However, the internal structures and geodynamic evolution of this area remain poorly understood. Our structural analysis and thermal modeling in the El Oro metamorphic province show that this fore-arc zone underwent four main geological events. (1) During Triassic times (230-225 Ma), the emplacement of the Piedras gabbroic unit at crustal-root level (∼9 kbar) triggered partial melting of the metasedimentary sequence under an E-W extensional regime at pressure-temperature conditions ranging from 4.5 to 8.5 kbar and from 650 to 900C for the migmatitic unit. (2) At 226 Ma, the tectonic underplating of the Arenillas-Panupalí oceanic unit (9 kbar and 300C) thermally sealed the fore-arc region. (3) Around the Jurassic-Cretaceous boundary, the shift from trench-normal to trench-parallel subduction triggered the exhumation and underplating of the high-pressure, oceanic Raspas Ophiolitic Complex (18 kbar and 600C) beneath the El Oro Group (130-120 Ma). This was followed by the opening of a NE-SW pull-apart basin, which tilted the massif along an E-W subhorizontal axis (110 Ma). (4) In Late Cretaceous times, an N-S compressional event generated heterogeneous deformation due to the presence of the Cretaceous Celica volcanic arc, which acted as a buttress and predominantly affected the central and eastern part of the massif.