Seismic structure of the north‐central Chilean convergent margin: Subduction erosion of a paleomagmatic arc

Abstract

We study the erosive convergent margin of north-central Chile (at~31°S) by using high-resolution bathymetric, wide-angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2-D velocity-depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0-6.5 km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5-5.0 km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (~1 km offset), which overlies a strong lateral velocity contrast from~5.0 to~6.0 km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic-continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large-scale crustal thinning and long-term subsidence of the outermost fore arc.

Structure and tectonics of the central Chilean margin (31°–33°S): implications for subduction erosion and shallow crustal seismicity

“…Similar features have been observed along seismic reflection profile P3 (see Fig. 1 for map location) located approximately 100 km to the north (Contreras-Reyes et al 2014). Trenchward of this Harpoon structure, the forearc is characterized by the gravitational collapse of the outermost forearc block.…”

“…3). Along P3, the trenchward transition from the upper to middle slope is defined by an abrupt slope scarp (∼175 km profile) with an offset of >1000 m that is coincident with a Harpoon structure in the landward part of the upper slope (Contreras-Reyes et al 2014). The middle slope has a roughly constant slope (Fig.…”

“…Laursen et al 2002). The reduced seismic velocities found in the frontal prism suggest the presence of disrupted and fractured continental rock at high pore fluid pressure conditions (von Huene et al 2004;Contreras-Reyes et al 2014). At ∼30-40 km landward of the trench, the local uplift of the continental slope might be related to underplating of accreted sediments or seamount subduction (Fig.…”

“…2a). The upper and middle continental slope domain is defined by water depths ranging from 0 to 1000 m, and it forms part of the seaward extension of the uplifted Coastal Cordillera (Contreras-Reyes et al 2014). In this region, the inversion of several structures is observed.…”

“…Seismic investigations reveal that the frontal part of the margin off NC Chile (40-60 km) is composed of an outer forearc with velocities of 3.5-5.0 km s −1 , which are typical for volcanic rocks (Koulakov et al 2011;Kopp 2013;Contreras-Reyes et al 2014). However, these seismic studies reveal the presence of prominent lateral velocity gradients suggesting a change in rock type and strong lateral variation in the direction of the dip.…”