VARIABILITY IN EXHUMATION ACROSS THE SOUTHERN FLANK OF THE SPANISH PYRENEES

Ebro Basin fill and synorogenic deposits preserved on the southern margin of the Pyrenean fold-thrust belt suggest that the mountain belt has experienced significant, but variable erosion. To date, most exhumation studies have focused within the axial zone at the core of the range. We address this deficiency with new apatite fission track (AFT) data along 4 across-strike transects to document the exhumation history of the central to eastern Spanish Pyrenees. We employ a new method that converts data collected using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) into the external detector method (EDM) format to allow for quantification of the sample grain-age distribution. This method uses the U and Th content of each grain to estimate an induced track density so that samples can be deconvolved in BinomFit software to identify component ages.

We present 17 new AFT samples of sedimentary, igneous, and meta-sedimentary bedrock with exhumation histories based on grain age distributions and thermal modeling. Pooled ages range from 20.1-76.8 Ma and most (14 of 17) samples fail the chi-squared test indicating a large spread in sample grain ages. Modeling results indicate the onset of rapid cooling from ~50-15 Ma across the western transect (WT), ~30-18 Ma across the central-western transect (CWT), ~30-24 Ma across the central-eastern transect (CET), and ~36-23 Ma across the eastern transect (ET).

These results show that exhumation began in the early Eocene on the WT, and in the late Eocene to early Oligocene on the CWT, CET, and ET. Along strike in the axial zone, exhumation is consistently late Oligocene. South of the axial zone, exhumation decreases in age across strike on both the CWT and ET. The exhumation pattern is more complicated on the southern thrust sheets of the WT and along the CET. However, timing of exhumation on the Montsec and Sierras Marginales thrust sheets of the WT is generally consistent with other geologic estimates for the chronology of thrust development. Our results also indicate that converting LA-ICP-MS data to EDM format is important for interpreting cooling histories of sedimentary rock samples and may be relevant for use in studies involving detrital minerals as well.