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

Abstract

S U M M A R YThe pre-and current collision of the Juan Fernández Ridge with the central Chilean margin at 31• -33• S is characterized by large-scale crustal thinning and long-term subsidence of the submarine forearc caused by subduction erosion processes. Here, we study the structure of the central Chilean margin in the ridge-trench collision zone by using wide-angle and multichannel seismic data. The transition from the upper to middle continental slope is defined by a trenchward dipping normal scarp with variable offsets of 500-2000 m height. Beneath the scarp, the 2-D velocity-depth models show a prominent lateral velocity contrast of >1 s −1 that propagates deep into the continental crust defining a major lateral seismic discontinuity. The discontinuity is interpreted as the lithological contact between the subsided/collapsed outermost forearc (composed of eroded and highly fractured volcanic rocks) and the seaward part of the uplifted Coastal Cordillera (made of less fractured metamorphic/igneous rocks). Extensional faults are abundant in the collapsed outermost forearc, however, landward of the continental slope scarp, both extensional and compressional structures are observed along the uplifted continental shelf that forms part of the Coastal Cordillera. Particularly, at the landward flank of the Valparaíso Forearc Basin (32• -33.5• S), shallow crustal seismicity has been recorded in 2008-2009 forming a dense cluster of thrust events of M w 4-5. The estimated hypocentres spatially correlate with the location of the fault scarp, and they highlight the upper part of the seismic crustal discontinuity.

Aftershock seismicity and tectonic setting of the 2015 September 16 Mw 8.3 Illapel earthquake, Central Chile

“…Profile locations are indicated in Fig. 1. striking, steeply inclined (30 • ), seaward dipping seafloor escarpment (800 m seafloor offset) located at around 2000 m water depth and constituting the surface expression of a large seaward dipping normal fault (Contreras-Reyes et al 2015 and Fig. 1).…”

“…4). At the lower continental slope, seaward of the maximum slip patch of the 2015 Illapel earthquake and at 10-20 km distance from the deformation front, a splay fault system separates the active accretionary prism to the west from the outermost forearc block to the east (Contreras-Reyes et al 2015).…”

“…Lange et al Furthermore, in order to assess along-strike and across-strike structural heterogeneities, we constructed quasi-3-D models by transposing the east-west oriented 2-D velocity models of Moscoso et al (2011) and Contreras-Reyes et al (2015) across the entire study region following the geometry of the trench. The determination of hypocentre locations using the quasi-3-D velocity models resulted in overall larger rms values than the location determination using the local 1-D velocity model together with station corrections.…”

The 2015 Illapel earthquake, central Chile: A type case for a characteristic earthquake?

“…A few normal‐faulting events occured below the forearc and close to the expected interface, making it difficult to decide whether they have ruptured in the continental or oceanic plate. In any case, shallow normal faulting aftershocks in the forearc, which were so prominent in the Maule and Tohoku aftershock sequences [ Ryder et al , ; Farías et al , ; Ruiz et al , ; Kato et al , ], are notably absent in spite of the presence of a prominent, apparently active normal fault in a seismic reflection profile at 31°S ∼25 km offshore [ Contreras‐Reyes et al , ].…”

Gravitational deformation and inherited structural control on slope morphology in the subduction zone of north‐central Chile (ca. 29–33°S)

“…This morphostructural segmentation is in agreement with a strong velocity contrast visible within the velocity depth model ( ca . 31°S) (Contreras‐Reyes et al ., , ) beneath the upper continental slope (6.0–6.5 km s −1 ), and beneath the lower and middle continental slope (3.5–5.0 km s −1 ). This seismic framework was interpreted as an inherited volcanic‐continental basement influenced by gravitational collapse as a result of frontal and basal erosion (Contreras‐Reyes et al ., ).…”