Incremental Folding Rates Determined With 104-5 Year Time Resolutions Along The Pyrenean Thrust Front, Spain

Abstract:

High-resolution rock magnetic cyclostratigraphy in growth strata are used to reconstruct unsteady fault-related folding rates at the regional Pico del Aguila anticline. Published and new magnetobiostratigraphy was used to determine absolute time and to calibrate a cyclostratigraphy based on anhysteretic remanent magnetization (ARM) intensity variations. The ARM data series was tuned to the orbital eccentricity model to remove the effects of sedimentary rate changes between late Lutetian-middle Priabonian chron boundaries (C19-C15) during syntectonic deposition. Sediment accumulation rates increased up section to 1.15 m/kyr during delta progradation with large oscillations in sedimentation rate in phase with eccentricity cyclicity. The ARM data was a proxy for climate change by recording Milankovitch cyclicity in the detrital magnetite concentration of deposits resulting from runoff variability in the wedge-top basin. Incremental tilting rates were calculated between selected growth horizons over 5 myrs of fold growth. The Eocene age limb tilt began rapidly then decreased more slowly at variable rates that were punctuated by periods of tectonic quiescence. Calculated folding rates varied between 0˚ and 100 ˚/myrs and averaged 14 ˚/myr over 100 kyr time increments. Accuracy and precision in the rate calculations include spatial errors associated with outcrop reconstruction and down plunge projection (<10 m and 10⁴ yrs), bedding attitude (few ˚), absolute chron ages (10⁵ yrs), sample spacing (10³ yrs), sample size (10² yrs), and orbital tuning (10⁴ yrs). The absolute age resolution on deformation is a few 100 kyrs while the uncertainties in the relative time between growth horizons is less and estimated at ~20 kyrs. Variation in folding rates of the Pico del Aguila anticline is attributed to unsteady thrusting in the fold’s core.