RECOVERING THE UNSTEADINESS OF DEFORMATION WITH 104 YR RESOLUTION , SE PYRENEES, SPAIN

Terrestrial growth strata are used to recover deformation rates from the archtypical progressive unconformity in the footwall of the Vallfogona thrust using a new 1.5 km long magnetostratigraphic and cyclostratigraphic section. Paleomagnetic sites were spaced every ~15m, with 3-4 cores and ≥ 5 samples per site. Oriented samples were progressively thermally demagnetized over ten steps from 150 - 600°C. Characteristic magnetizations were determined by PCA and correlated to GPTS. The Berga Fm. was deposited during chrons 15r, 15n, 13r, 13n, and 12r with a total duration of 4 myr. Sediment accumulation rates decreased upsection and are strongly related to changes in depositional facies with 45cm/kyr rates in subaerial debris flows, 25cm/kyr in axial fluvial fans, and 17cm/kyr in fans longitudinal to the mountain front. Deformation rates were also unsteady with 113° of folding occurring in short time intervals seperated by longer periods of tectonic quiescence. The onset of deformation has been identified to be at 33.4 Ma during chron 13n. Analysis of isothermal remanent magnetization of representative samples revealed goethite, hematite, and magnetite in all samples. To develop the cyclostratigraphy, we used an anhysteretic remanent magnetization (ARM) data series. ARM data recovers variations in magnetite concentration that have been shown to vary with climatic changes at Milankovitch frequencies. Samples for cyclostratigraphy were collected every 0.75 m for ~800 m from chron 13r-12r. Time-frequency analysis of stratigraphically referenced ARM measurements, which reflect variation in in primary magnetite concentration, were calibrated to time using the magnetostratigraphy and revealed cyclicitity with significant power at Milankovitch periodicities. Low frequency ARM cyclicity aligns with lithologic cyclicity at ~100 m, visible at the outcrop scale. Rock magnetic based cyclostratigraphy was successfully recovered from terrestrial foreland basin strata that accumulated without accommodation space constraints.