FAULT PROPAGATION FOLD KINEMATICS RECOVERED FROM TERRESTRIAL GROWTH STRATA WITH 20 KYR TIME RESOLUTION, SANT LLORENÇ DE MORUNYS, PYRENEES, SPAIN

The Eocene-Oligocene synorogenic Berga Conglomerates were used to recover folding rates along the Spanish Pyrenees mountain front. The Berga Group consists of coarse alluvial fan and fluvial growth deposits which record the evolution of the Paleogene mountain front. The unit was sampled for cyclostratigraphy, rock magnetics, and anisotropy of magnetic susceptibility (AMS). In addition a regional magnetostratigraphy was developed along a 1.8 km section and compared to the geomagnetic polarity time scale (GPTS2012) to provide an absolute time scale. Analysis of rock magnetic experiments indicate a mixed magnetic mineralogy where paramagnetic clays are dominant in controlling magnetic susceptibility (χ) variations. AMS data is consistent with a pinned synclinal hinge during fault-propagation folding. The syncline formed as a result of fault propagation folding driven by blind thrust faulting. The geometry of progressive and angular unconformities in the growth strata are reproduced during trishear forward modelling. AMS data also records compaction and N-S layer-parallel-shortening fabrics >10 km south of the deformation front. The regional magnetostratigraphy was used to calibrate the χ cyclostratigraphy to recover significant Milankovitch cyclicity (e.g. >90% significance at 41 kyr) using spectral analysis. Significant cycles were recovered at intervals which are attributed to Earth’s obliquity and precession indexes. Unsteadiness in deformation and sediment accumulation rates with 10⁴ yr time resolution ranges from folding rates of 0-100 °/Myr during sediment accumulation varying from 100-371 m/Myr. Changes in deformation and sediment accumulation rates are not correlated suggesting sediment loading of the foreland basin was not a driver of deformation.