UNSTEADY FOLDING RATES RECOVERED AT INTERMEDIATE TIME SCALES, SPANISH PYRENEES

Foreland basins are ideal sites to determine deformation rates with cyclostratigraphy in growth strata as they have sufficiently high sediment accumulation rates to recover high frequency orbital signals. Three synsedimentary fault-related folds are analyzed to recover accumulation and deformation rates along the Paleogene wedge-top basin/ foreland basin boundary near the Spanish Pyrenean thrust front. Rock magnetic based cyclostratigraphy (ARM, magnetic susceptibility), calibrated with new magnetostratigraphies were used to recover folding rates at 10⁴-10⁵ yr timescales. Folding was unsteady at all 3 localities where it was associated with episodic and variable thrusting rates. In the western Pyrenees, the Peña flexure developed in front of a passive roof, antiformal stack during development of the mountain front. Folding rates calculated from the growth strata at the resolution of the polarity chrons varied from 4˚- 13°/myr for 65˚ of folding. Growth of the anticlinorium was recorded by terrestrial sediment that was locally derived and accumulated in structural accommodation space. Along the thrust front of the central Pyrenees, folding rates at Pico del Aguila anticline, a transverse décollement fold, varied between 0°- 91°/myr for 76˚ of folding recovered from deltaic facies at 100 kyr (short eccentricity) and polarity chron intervals. Monte Carlo simulations of folding rate uncertainties demonstrate statistically significant variations in folding rates including times of quiescence. Further east, near Sant Llorenç de Mournuys cyclostratigraphy based folding rates varied from 0 - 100 °/myr for 110˚ of fault-propagation folding determined at 20kyr intervals (precession) including static intervals. Polarity chron averaged rates were uniformly slower and artificially steady as compared to astrochronology determined rates indicating an inability of such records to recover the complete variance of intrinsic depositional or deformational behavior. Coupling between synorogenic sedimentation and fold deformation depended on source area proximity to each growing structure. The higher the temporal resolution of the chronologies, the greater was the variability in both the determined rates of growth strata deposition and the deformation rates.