Geological Society of London
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Timing of tectonic events in the Alpujárride Complex, Betic Cordillera, southern Spain

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posted on 2016-06-21, 12:00 authored by J.P. Platt, S.P. Kelley, A. Carter, M. Orozco

Ar–Ar and fission-track dating from four low-grade sections in the Alpujárride Complex sheds light on the timing of the early contractional history of these high-pressure metamorphic units, and their subsequent exhumation during the early Miocene extensional collapse of the Betic orogen. White mica grains from the lowest-grade rocks yield partly reset detrital Ar–Ar ages; where deformation and recrystallization are more intense, however, they yield reproducible ages as old as c. 48 Ma, which we interpret as the approximate timing of the main contractional event and associated high-pressure metamorphism in these units. Rocks from slightly higher-grade sections give reproducible Ar–Ar ages between 30 and 40 Ma, but these younger ages probably reflect the interplay of growth and cooling in fine-grained micas heated to between 300 °C and 400 °C during prograde metamorphism. In each of the four areas studied the low-grade rocks are separated by an extensional detachment or shear zone from medium-grade rocks below. In two areas these detachments have been folded around major overturned north-vergent folds. Ar–Ar data from structurally below the detachments, and fission-track ages from all structural levels, indicate the onset of significant cooling in the early Miocene, which corresponds to the age of the extensional event. The folds must therefore be early Miocene or younger.