The strong competition between growth and erosive processes on the Juan Fernández Archipelago (SE Pacific, Chile)

Abstract

Ocean Island Volcanoes experience growth and dismantling throughout their entire geological history. When these volcanoes are in their post-shield or post-rejuvenated stage, erosion and subsidence dominate leading them towards a dramatic end, with their extinction as subaerial and transition to the guyot or razed island stage. To quantify the long-term growth and erosion rates affecting these volcanoes is complex but essential to understand the interplay between competing processes, some of them coeval. The emerged parts of the Juan Fernandez Ridge (Robinson Crusoe (RC) and Alejandro Selkirk (AS) Islands) in the Pacific south east (Chile), separated ca. 200 km and different in age (ca. 4 and 1 Ma, respectively), are nowadays experiencing a staunch decay. Here we present new data related to the construction and erosion rates experienced by these major islands, whose emerged parts represent a small fraction of their entire volcano edifices.Based on geomorphological analyses of combined topographical and bathymetrical maps, and the reconstruction of the paleo-topography, we obtained total average long-term growth rates (emerged and submarine sections) that are very similar for the two edifices, between ~1100 and 1500 km3/My, despite their different age and evolutionary stage. Considering only the emerged section of the paleo edifices, or the present island volumes, result in lower growth rates (45–30 km3/My for RC and AS respectively), which are in the same order of magnitude than other oceanic islands worldwide.Long-term average-basin erosion rates are rather different. RC basins present lower erosion rates (116.77 t/km2yr) than AS (465.75 t/km2yr), which suggest a time-dependent process with RC being closer to the equilibrium profile. These values are in the same order of magnitude than those from other ocean settings with higher rainfall regimes, which would even imply higher erosion rates in Juan Fernández archipelago and a possible role for solid Earth processes that control vertical movements, mass wasting and hence long-term bulk erosion.These results reveal the need to attempt a fully reconstruction of paleo-edifices, considering also the submerged sections, when emerged parts are intensely eroded, thus aiming for more representative growth and erosion rates estimations as a first step towards understanding processes controlling morphological evolution of oceanic islands.