NON-STEADY RATES OF FOLDING REVEALED BY GROWTH STRATA, SPANISH PYRENEES

Using growth strata and high-resolution age determinations constrained by magnetostratigraphy and cyclostratigraphy we show that folding at Pico del Aguila, a transverse halotectonic anticline in the Spanish Pyrenean foreland was non-steady over a 5 Myr long period. Folding along the External Sierra thrust front occurred synchronous with sedimentation in the Jaca wedge-top basin atop the active Guarga thrust sheet. Growth began at Pico del Aguila during Middle Eocene carbonate (Guara Fm.), deposition and continued as marine slope (Arguis Fm.), and Middle/Upper Eocene deltaic (Belsué-Atarés Fm.) deposits prograded across the region. Anhysteretic remanent magnetization (ARM) data from the Arguis Fm. show hierarchical cyclicity consistent with all predicted Milankovitch orbital frequencies. Differential lithologic decompaction was applied using empirical calibration of anisotropy of anhysteretic remanence (AAR) to volume loss from laboratory compaction experiments on comparable lithologies. The ARM depth domain is converted to time by scaling the data to a revised magnetostratigraphy within the growth section. We refine the magnetostratigraphic time scale by tuning bandpassed ARM data to the precession index according to the new La2004 model providing 10,000 yr age resolution for 760 m of flysch which records 47° of limb rotation. Ages for the initial 5° and final 3° of folding are based on magnetostratigraphic data and estimated sediment accumulation rates. Growth strata were mapped using precision GPS with post kinematic processing. Thus far, we divide 55° of westward limb tilting into 9 increments of ~180 to ~730 kyrs duration. Limb rotation increased from 8°/Myr at 41.6 Ma to 28°/Myr, then decreased to 5°/Myr at 38.5 Ma. Subsequently, limb rotation increased to 26°/Myr, and stopped for 300 kyr at 37.8Ma before slowly accumulating the last 3° of folding, which ended at 36.6 Ma. Episodic folding is attributed to varying rates of thrusting and synorogenic sedimentation along the Pyrenean thrust front.