Fission-track constraints on timing of Alpine nappe emplacement and rates of cooling and exhumation, Torrox area, Betic Cordilleras, S. Spain

Fission-track analyses were performed on apatite and zircon from 8 samples of an exhumed gneiss body belonging to the Alpine Alpujárride nappe complex of the Internal Zone of the Betics. The eight FT apatite ages range from 13.0 to 20.4 Ma, with a central age for six of them of 16.6 ± 0.8 Ma (1σ); a single zircon FT age determination yielded a central age of 17.2 ± 2.5 Ma. Apatite fission-track length distributions are narrow with mean lengths between 14 and 14.7 μm. Such distributions are indicative for very high cooling rates, confirming the results of a previous thermochronological study using RbSr, KAr and ArAr techniques; cooling rates of 250–350°C/m.y. from above 600°C to below ∼350°C at 20-18 Ma were indicated by the tight clustering pattern of the mineral ages. This age bracket may represent the best current estimate for the termination of the Alpujárride nappe emplacement in this region and the very fast large-scale tectonic unroofing. Subsequent cooling may be characterised by lower rates of ∼150°C/m.y. in the period 19-17 Ma and down to ∼5°C/m.y. in the period 17-14 Ma as indicated by best-fit thermal history modelling based on spontaneous apatite fission-track length distributions. This late-stage cooling process may (in part) be related to restoring the isotherm pattern disturbed by the rapid uplift. Further cooling at rates of ∼2°C/m.y. is suggested for the period 14-5 Ma and this may be related to late-stage exhumation of the metamorphic rocks. The data indicate that the period of very fast cooling and coeval extensional tectonics in this area ended at ∼18–19 Ma.