Segmentation of the Nazca and South American plates along the Ecuador subduction zone from wide angle seismic profiles

Abstract

International audienceWe describe the deep structure of the south Colombian-northern Ecuador convergent margin using travel time inversion of wide-angle seismic data recently collected offshore. The margin appears segmented into three contrasting zones. In the North Zone, affected by four great subduction earthquakes during the 20th century, normal oceanic crust subducts beneath the oceanic Cretaceous substratum of the margin underlined by seismic velocities as high as 6.0-6.5 km/s. In the Central Zone the subducting oceanic crust is over-thickened beneath the Carnegie Ridge. A steeper slope and a well-developed, high velocity, Cretaceous oceanic basement characterizes the margin wedge. This area coincides with a gap in significant subduction earthquake activity. In the South Zone, the subducting oceanic crust is normal. The fore-arc is characterized by large sedimentary basins suggesting significant subsidence. Velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing. Even if the distance between the three profiles exceeds 150 km, the structural segmentation obtained along the Ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

Seismological study of the central Ecuadorian margin: Evidence of upper plate deformation

“…Details of this work were given in previous works Gailler, 2005;Gailler et al, 2007). One structural characteristic of this area is the thick oceanic crust (Sallares and Charvis, 2003) due to the presence of the Carnegie Ridge; other results are the oceanic crust-type velocity of the upper plate (6.1e6.4 km/s), due to the accretion of several oceanic blocks to the Andean continental margin, and finally the presence of a low-velocity zone in the subduction channel .…”

“…The Malpelo ridge is thought to be the former continuation of the Cocos ridge, drifted away by the dextral strike-slip motion along the Panama fracture zone (Longsdale and Klitgord, 1978). These ridges are characterized by irregular topography, with important bathymetric variations and a thickened oceanic crust which can reach 19 km (Sallares et al, 2005;Gailler et al, 2007). The NAB consists of oceanic terrains that were accreted to the Andean margin during compressive periods in Late PaleoceneeEarly Eocene times (Jaillard et al, 1997).…”

“…We also deduced the presence of the western limit of the sedimentary Manabi basin and a slab dipping about 10 between 4 and 15 km depth. We built a 3D grid based on the projection of the 2D velocity models and their onshore extrapolation on a main perpendicular profile, following the methodology described in Gailler et al (2007). In this computation, we also consider small lateral velocity variations highlighted by the profiles parallel to the margin (Gailler, 2005).…”

“…This combined landsea network recorded the shots produced by air guns. This network configuration was chosen for a 2D wide-angle study Gailler et al, 2007) and 3D modeling (Gailler, 2005).…”

Chronostratigraphy and tectonic deformation of the North Ecuadorian–South Colombian offshore Manglares forearc basin

“…and FK filters, velocity-dependent gain restitution, a minimum-phase conversion of the signal, a pre-stack predictive deconvolution, "dip move out" and "normal move out" corrections, multiple attenuation in the FK domain and by an internal mute, an external mute, a stack, a time-variant Kirchhoff migration, and a time-variant dynamic equalization. Depth values are given for specific reflectors on the basis of velocities derived from wide-angle seismic data collected during the SISTEUR cruise (Graindorge et al, 2003;Agudelo, 2005;Gailler et al, 2007).…”