Forearc density structure of the overriding plate in the northern area of the giant 1960 Valdivia earthquake

Andrei Maksymowicz,Tomás Reyes,Daniel Díaz,Daniela Montecinos-Cuadros,María José Segovia

doi:10.5194/se-13-117-2022

Abstract

Abstract. The objective of this work is to analyse the density structure of the continental forearc in the northern segment of the 1960 Mw 9.6 Valdivia earthquake. Regional 2D and local 3D density models have been obtained from available gravity data in the area, complemented by new gravimetric stations. Models are constrained by independent geophysical and geological information and new TEM and MT soundings. The results show a segmentation of the continental wedge along and perpendicular to the margin, highlighting a high-density anomaly, below the onshore forearc basin, that limits the late Paleozoic–early Mesozoic metamorphic basement in the region where Chaitenia terrane has been proposed. A progressive landward shift of this anomaly correlates with the high slip patch of the giant 1960 Mw 9.6 Valdivia earthquake. Based on these results, we propose that the horizontal extension of the less rigid basement units conforming the marine wedge and Coastal Cordillera domain could modify the process of stress loading during the interseismic periods, and also that changes in position and extension of the late Paleozoic–early Mesozoic accretionary complex could be linked with the frictional properties of the interplate boundary. This analysis provides new evidence of the role of the overriding plate structure in the seismotectonic process in subduction zones.

Highlights

The physical structure of the oceanic and continental plates have had an important role in the long- and short-term deformation process of the subduction margins
In order to study the regional structure of the continental wedge and subduction zone, we modelled five profiles (P1_Toltén, P2_Unión, P3_Osorno, P4_Llanquihue and P5_Chepu; see Fig.2), which run perpendicular to the trench at latitudes of 39.25, 40.2, 40.5, 41 and 42◦ S, respectively
The general aspect of the Complete Bouguer gravity anomaly (CBA) is a sequence of bands with high and low gravity, roughly parallel to the margin

Summary

Introduction

The physical structure of the oceanic and continental plates have had an important role in the long- and short-term deformation process of the subduction margins. Bilek et al, 2003; Hackney et al, 2006; Hicks et al, 2014; Contreras-Reyes and Carrizo, 2011; Bassett and Watts, 2015; Poli et al, 2017) This geodynamical feedback is evinced by spatial correlations between the physical segmentation of the continental wedge, and ruptures of large megathrust earthquakes (i.e. ContrerasReyes et al, 2010; Li and Liu, 2017; Martínez-Loriente et al, 2019; Molina et al, 2021). Changes of the continental wedge geometry have been associated with variations of the interplate boundary friction at the maximum slip patches of the large 2011 Mw 9.0 Tohoku-Oki, 2010 Mw 8.8 Maule and 1960 Mw 9.6 Valdivia earthquakes (Cubas et al, 2013a, b; Maksymowicz, 2015; Contreras-Reyes et al, 2017; Molina et al, 2021).

Objectives

Methods

Results