Erosion of the submarine flanks of the Canary Islands

Abstract

[1] Surveying with multibeam echo sounders around old ()1 Ma) volcanic ocean islands reveals that their submarine flanks contain a strong downslope-oriented ridge-and-valley corrugation, which modifies the original volcanic morphology of lava terraces and cones. By analogy with canyons in other settings, this corrugation was probably caused by channel incision by erosive sedimentary mass flows such as turbidity currents and debris flows. We adapt a method first used in subaerial geomorphology to isolate the erosion depth (exhumation) and apply it to the eroded flanks of the 6-8 Ma Anaga massif of Tenerife. The channels formed around this massif divert around local topographic highs. These highs, which are probably original volcanic cones, are therefore preferentially preserved during erosion, so that their elevations can be used to construct an artificial reference surface. Terrain depth was calculated by subtracting this reference surface from measured bathymetry. Comparison of the terrain depth of the old, eroded submarine flank of Anaga with that of the young, mostly unaltered submarine flank of El Hierro allows us to infer the mean depth of Anaga's submarine erosion, which is $100 m. Volcanic terrains can be dated by radiometric methods, so they also provide a way of quantifying long-term denudation rates. We infer that submarine denudation of Anaga has occurred at comparable rates to that of subaerial lowlands and much slower than denudation of highlands, illustrated locally by the more extensive erosion of the subaerial Anaga edifice.

Morphological and structural analysis in the Anaga offshore massif, Canary Islands: fractures and debris avalanches relationships

“…The width of the channel is 7000 to 10,000 m, but its relief is difficult of evaluate, because it is very modified. We estimate relief between 25 and 115 m. Mitchell et al (2003) described how V-shaped channels can be seen formed around local highs with greater vertical relief in the flanks than lower down, which they consider consistent with the greater flow power of turbidity currents associated with the steeper gradient. These authors consider than the channels may have been active earlier in Anaga's history, and now the edifice base is an area of deposition rather than erosion.…”

“…Interest in knowing which the prefered structural directions are, guided us to carry out a structural analysis. Previous analysis of the topography of the region (Mitchell et al, 2003) did not find faulting to be an important process in the evolution of this area. However we find that erosion is often taking place along existing structural directions.…”

“…This is confirmed by low acoustic backscatter in sonar data . As the shelf grew by coastal abrasion and subaerial erosion, the submarine canyons would have been progressively disconnected from their sources of hyperpycnal flows (Mitchell et al, 2003). Mitchell et al (2002) studied the volcanic flanks of Tenerife, La Palma and El Hierro and found the bathymetric contours around Anaga more crenellated, its flank appearing more deeply gullied by erosion than the constructional flanks of El Hierro.…”

“…The pinnacles in the studied area are 1-3 km across and 100-200 m high. Other pinnacles, similar to these have been recognized and studied in the offshore continuation of volcanic rift zones on El Hierro (Gee et al, 2001;Mitchell et al, 2003) and Tenerife Island (Mitchell et al, 2003).…”

“…This is probably the result of wave erosion during Pleistocene glacially induced regressions (Teide Group, 1997). Mitchell et al (2003) pay attention to the asymmetric effect of subaerial and submarine erosion of the Anaga Massif, finding that the north subaerial slopes of the island are steeper than those to the south, implying that the drainage divide lies north of the massif's center. From the analysis of trade winds data Mitchell et al (2003) deduced that a persistently greater precipitation to the north and a more powerful sea leads to a greater width of the abrasion platform in the north of Anaga.…”