Regresar

Mass-transport deposits in the northern Ecuador subduction trench: Result of frontal erosion over multiple seismic cycles

Abstract:

Investigations of Mass-Transport Deposits (MTDs) and turbidite deposition in the confined North Ecuador subduction trench provide access to paleoseismic information and insights into long-term mechanisms for frontal tectonic erosion at a convergent margin. The studied trench has been the site of four great subduction earthquakes (7.7≤Mw≤8.8) during the 20th century. The trench is isolated from major continental sediment input, so that investigated MTDs and turbidites are considered of local origin. Swath bathymetry, seismic reflection and Chirp data, together with sedimentary cores and 14C dating revealed that seven MTDs were emplaced in distinct trench sub-basins since ~23kyr, and 27 turbidites deposited in the southernmost trench sub-basin since ~4.9kyr. Our analysis shows that six MTDs were derived from the margin, while a single one stemmed from the outer trench wall. Temporal correlations between MTDs emplaced within trench sub-basins separated by a structural saddle, indicate that the seven MTDs were emplaced during five main events. Three were triggered locally and tentatively dated 5.8, 1.6kyr and Recent, whereas four were emplaced in distinct trench sub-basins as a result of two regional events at 22.6 and 15.4kyr. None of the MTDs occurred during the fast stage of the last sea-level rise (~13 to 8kyr). However, dissociation of gas hydrates during the last 8kyr-stage of slow sea-level rise might have contributed to trigger the three youngest MTDs. The large 1.5-13.5kyr return time of the MTDs contrasts with that of 189yr of the turbidites. The later is consistent with the 73yr return time of two local Mw≥8.2 earthquakes, implying that turbidites might have been triggered by large earthquakes. The very large MTDs return time is attributed to long-term deformation processes and mechanical weakening of the margin outer wedge, in response to repeated variations in basal friction, pore pressure and margin extensional/contraction strain over multiple earthquake cycles. This process contributes to short-term frontal erosion, the rate of which is estimated to be 8.6 10-3km3/kyr/km, since at least 15.4kyr. © 2010 Elsevier B.V.