Arc migration due to variations in the Wadatti-Benioff zone: A new perpective

SPAGNUOLO, M. G; FOLGUERA A.; RAMOS V. A

Oslo, Noruega.

Congreso; 33rd International Geological Congress.; 2008

The Andean margin between 34º and 37º south latitude is considered a key area in order to constraint plate motion dynamics and especially the geometrical relation between the overriding and the subducting plate through time. Variations in the Wadatti-Benioff zone during the last 20 My strongly affected and controlled foreland deformation and emplacement of anomalously thick accumulations of arc-related rocks far away from the trench. At 36ºS the Late Miocene orogenic front extended for 430 km from the Pacific coast and was associated with arc-related products with mean ages around 11 Ma. Afterwards at 7 Ma the arc retreated towards the trench and was related to widespread extensional tectonics and probably incipient delamination phenomena. Various authors interpreted as a consequence of shallowing-steepenining slab mechanics. Moreover, expansion of the arc and related contraction in the foreland during Middle to Late Miocene were not only followed by Latest Miocene to Early Quaternary extensional tectonics and arc retreat but also by Late Quaternary transpression localized in several ancient basement-cored uplifts. Besides this general consensus, there is no clear knowledge if those processes were continuous through time, if they were somehow overlapped or even if the arc migration was by pulses and discontinuous. In this work through extensive structural fieldwork, new ages, geochemical analysis, and compilation of released volcanic ages into a GIS, we propose a new picture and long-lived history than previously assumed. Here a more comprehensive understanding of the arc migration explains many geological features such as irregular broken foreland basement uplifts as a function of latitude and an irregular orogenic collapse as a function of maximum and inhomogeneous arc foreland-propagation. Preliminary results show that the arc migration was anything but a simple homogeneous front. Instead, the arc migrated as a series of discrete branches in the order of a few tens of kilometers, controlling the emplacement of basement-cored structures at the foreland area, through creation of fragil-ductil discontinuities. Those transtensionally-reactivated structures were the conduits by which intraplate melts reached the surface even up to historical times.