Reconstructing the syn-to-post tectonic exhumation history of the south-central Pyrenees with apatite fission track thermochronology

The topographic development of orogens is driven by deformation and erosion processes. Numerical and analogue models suggest these processes interact, causing the geometry of the orogen (taper) to adjust in response to continued crustal shortening and erosion. After mountain building ceases, erosion can continue to alter the landscape through changes in climate, base level, or drainage patterns. We examine the relationship between syn- and post-tectonic erosion along four N-S transects of the south Pyrenean flank by quantifying exhumation with low-temperature thermochronology. Previous thermochronometer studies in the Pyrenees have been mostly limited to the range center (Axial Zone), whereas we focus on the southern fold-thrust belt, and compare our results with the extensive, regional magnetostratigraphic record of deformation and sedimentation. We analyzed and applied inverse thermal modeling techniques to eighteen apatite fission track samples from sedimentary and metamorphic bedrock of the central-eastern Spanish Pyrenees. Our analysis shows that the samples record exhumation (1) associated with the source region and/or (2) post-dating deposition. For example, the Sierras Marginales thrust sheet and Graus Tremp Basin have an exhumation signal from the Axial Zone, indicating minor post-tectonic erosion (< a few kilometers). On the other hand, the Freser antiformal stack and Nogueres Zone indicate syn- and/or post-tectonic erosion exceeding ~4 km. In general, exhumation on the northern portions of all transects near or within the Axial Zone occurred 50-20 Ma, consistent with existing constraints on the timing of deformation and exhumation. Exhumation of some of the more southern structures, such as the Oliana anticline and Montsec thrust sheet, occurred 20-10 Ma, which post-dates known phases of deformation recorded by cross-cutting stratigraphic relationships. This rapid erosion is either (a) related to a previously unidentified late Miocene phase of orogenic activity, or (b) a post-tectonic response to external forcing, such as base level lowering following the capture of the Ebro Basin by the Mediterranean. We prefer the latter explanation due to the numerous stratigraphic constraints on the deformation of these structures. We conclude that in the Pyrenees and other inactive orogens, thermochronologically-recorded rapid exhumation may result from syn- and/or post-tectonic erosion. This complexity underscores both the necessity for interdisciplinary methodologies to quantify their relative contributions and highlights the complimentary use of low-temperature thermochronology and magnetostratigraphy in reconstructing mountain denudation histories.