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

• Published in: Geophysical research letters 2014-03, Vol.41 (5), p.1523-1529
• Main Authors: Contreras-Reyes, Eduardo, Becerra, Juan, Kopp, Heidrun, Reichert, Christian, Díaz-Naveas, Juan
• Format: Article
• Language: English
• Subjects: Basements; Chile; Coastal; Coastal environments; Continental crust; Continental slope; Depth; Dipping; Discontinuity; Erosion; Erosion mechanisms; Geophysics; Gravitation; Gravitational collapse; Gravity; High resolution; hydrofracturing; Igneous rocks; Magma; Plate tectonics; Plates (structural members); Reflection; Refraction; Seismic data; Seismic surveys; Seismological data; Seismology; Subduction; Subduction (geology); subduction erosion; Subsidence; Thinning; Velocity; Volcanic activity; Volcanic rocks
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2013GL058729

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. Key Points Our results have implications for subduction erosion A major velocity discontinuity is detected beneath the middle continental slope Possible gravitational collapse of the outermost fore‐arc block off north Chile