Comments on ?Epiclastic deposits and ?horseshoe-shaped? caldeiras in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago), French Polynesia? by Cl�ment et al. (2003)

Abstract

A quantitative geomorphological study has been made on 27 river basins in Tahiti-Nui volcanic island (French Polynesia) to reconstruct the erosional evolution of a young oceanic island subjected to heavy tropical rainfall. Tahiti-Nui is composed of a main shield volcano cut by two huge landslides on each side of a main E–W rift zone. The northern landslide depression was rapidly buried by the construction of a second shield, the late activity of which overflowed the crest and then filled the southern landslide depression. The island is now volcanically inactive and is deeply dissected by erosion. The present geometries of the river basins are first compared using dimensionless parameters derived from a digital elevation model. The original volcanic surfaces are then reconstructed to estimate the volumes removed by erosion and determine the average rates of long-term erosion. The basins developed on the flanks of the main shield are wider, shallower, and gentler than the basins incising the post-landslide second shield, indicating a higher degree of evolution. Rainfall concentration on the windward (eastern) side of the island also contributed to increase the vertical lowering of the volcanic relief and the enlargement of the valleys. The magnitude of erosion, however, is neither directly linked with the age of the units incised nor with the differential amounts of rainfall. Erosion rates determined over the last 1 Myr range between 10− 3 km3 kyr− 1 and 0.25 km3 kyr− 1. The highest values occur in the basins incising the main E–W rift zone and/or the lateral rims of the northern and southern landslide depressions. Long-term dissection has thus been enhanced along the geological discontinuities of the eruptive system. Deep erosion was first constrained along the axis of the main E–W rift zone, where numerous dykes compartmentalize the volcanic structure into large unstable blocks. Dykes most probably acted as mechanical discontinuities along which shallow gravitational landslides recurrently occurred. Such mass-wasting episodes produced significant amounts of debris, partly preserved as highly indurated sedimentary breccias of various ages exposed at various locations. Subsequent dissection of Tahiti-Nui was enhanced to the north and to the south, leading to the rapid evolution of the Papenoo and Taharuu drainage systems over the last 500 kyr. Long-term dissection on Tahiti-Nui has been responsible for the removal of at least 350 km3 of volcanic material from the surface, and for the partial exhumation of a shallow intrusive complex partly composed of coarse-grained plutonic rocks (gabbros and syenites) in the central part of the eruptive system. Structurally controlled erosion is thus a key component of landscape evolution on such high-relief oceanic tropical islands.