MILANKOVITCH-TUNED GROWTH STRATIGRAPHY RESOLVES 100,000-YEAR FOLDING RATES AT SIERRA DEL AGUILA, SPAIN

Growth strata provide a direct record of deformation kinematics. At Sierra del Aguila along the South Pyrenean thrust front, growth geometries in the Eocene Arguis Fm. record a pinned anticline hinge during folding. 800m of Arguis Fm. were measured for lithostratigraphy and sampled for paleomagnetism, (143 new sites) and rock magnetic analyses (1374 samples). A rock magnetic cyclostratigraphy, based on anhysteretic remanent magnetization (ARM) variations, was developed. Multi-species biostratigraphy anchors the observed magnetic stratigraphy to the GPTS to identify Chrons 19r (Upper Lutetian) through16r (Lower Priabonian) in the Arguis Fm. The ARM data series was converted to a time by tying the data to the chron ages. Spectral analysis of the ARM time series reveals orbital eccentricity variations and data are adjusted to the eccentricity and precession bands of the Earth’s orbital model. Growth strata were mapped with precision GPS and orthophotographic analysis and tied to the Arguis Fm. measured section. Cross sections were constructed by down-plunge projection of growth horizons and bedding attitudes. In combination, these data allow 100,000-year resolution of deformation rates at Sierra del Aguila, limited by outcrop and the spatial uncertainty of orientation data more than bed age resolution. The Arguis Fm. records ~6myrs of folding at rates of up to 38˚/myr, punctuated by comparable length intervals of relative tectonic quiescence. Folding rates at Sierra del Aguila are attributed to episodic thrusting in the fold core and along the Guarga thrust sheet décollement. The Arguis section consists of 120m of middle-outer neritic glauconite sand deposited at a rate of 5 cm/kyr overlain by 385m of prodelta silt and clay deposited at a rate of 22cm/kyr and >300m delta front sand deposited at 42cm/kyr. Prograding deposition in the wedge-top Jaca Basin buried the active Sierra del Aguila anticline. Rock magnetic experiments show that magnetic is dominant in controlling the ARM cyclostratigraphy and primary detrital magnetite concentrations vary at astronomically driven frequencies. Varying amounts of terrigenous fluvial input diluting a constant input of marine carbonate is most likely responsible for the cyclicity of the ARM record.