Long- and short-term deformation of convergent plate margins: Numerical modelling of seismic cycles at the South-Chilean oblique subduction zone
Final Report Abstract
This project analysed the long-term processes related to mass transfer by accretion and erosion along the subduction thrust interface and to permanent deformation in the upper plate along the Southern Andes. This shall serve future studies that will investigate the relationship between the formation of tectonic structures and morphology in the upper plate and the short-term seismogenic processes in many subduction zones. Blueschist belts, characteristic of rock metamorphism at high pressure and low temperature, are common features of active subduction zones. Their formation is explained by subduction of continent-derived material and/or oceanic rocks, basal accretion at about 25-40 km depths and later exhumation. However, the factors controlling basal underplating are poorly constrained. Previous analogue or numerical models could not reproduce the observed characteristics, e.g. depth of basal accretion > 60 km, absence of exhumation or viscous forearc deformation without faulting. Therefore, we have developed a new modelling set-up using the thermo-mechanical code LAPEX 2D and compared with a typical study case, i.e. the Western Series in South-Central Chile. We have reconstructed the dynamic evolution of an accretionary wedge in the first 100 Myr after begin of subduction. We could obtain good fit for the depth of metamorphism, the exhumation rate, the size and formation period of the accretionary prism, as well as various tectonic structures. Subduction of an oceanic plate is accompanied by the formation of splay faults of different sizes. In the initial stage, they accompany frontal accretion in the outer wedge and basal accretion in the inner wedge. In a second stage, frontal accretion is still undergoing while new megasplay faults accommodate opening of the subduction channel and basal erosion. There is no need for extrinsic variation such as climate change or sediment thickness to observe a change from basal accretion to basal erosion. Nevertheless, for the specific case of South-Chile, a later external change of surface conditions or plate boundary is required to explain a 10-fold lower exhumation rate and why these structures have been particularly well preserved since then. In this study, we were able to reconstruct 100 Ma of geodynamic history and to reproduce very well the geologic and seismic structures in South-Chile by means of numerical models. Therefore, a deep understanding of the processes in play and the factors controlling the long-term formation of a plate margin could be gained. This allows us to better constrain the forces acting on a subduction zone and the boundary conditions for future models that could investigate the short-term deformation and seismic activity in South-Chile and other settings.
Publications
- Subduction earthquake deformation associated with 14 November 2007, Mw 7.8 Tocopilla earthquake in Chile: Results from InSAR and aftershocks, Tectonophysics, 490(1-2), p. 66-68, 2010
Motagh M., Schurr B., Anderssohn J., Cailleau B., Walter T. R., Wang R. and Villotte J.P.
- Numerical models of basal and frontal accretion to South-Central Chile subduction, Geo Frankfurt, 2014
Cailleau B., Petrunin A.G., Sobolev S.V., Babeyko A.Y., Glodny J., Oncken O. and Krawczyk C.M.
- Numerical simulation of subduction and accretionary processes in South-Central Chile, 24e Réunion des Sciences de la Terre, Geosciences conference Pau, France, RST2014-649, 2014
Cailleau B., Petrunin A.G., Sobolev S.V., Babeyko A.Y., Glodny J., Oncken O. and Krawczyk C.M.
