Unsteadiness of Deformation in Growth Strata From Magnetostratigraphy and Cyclostratigraphy, SE Pyrenees, Spain

Abstract:

Magnetic reversal stratigraphy in terrigenous rocks has been used for a number of years to decipher deformational histories in orogenic fronts and associated basins. When applied to syntectonic sediments, magnetostratigraphy in combination with rockmagnetic cyclostratigraphy, offers a unique opportunity to address fundamental questions such as the unsteadiness of deformation with high resolution. Along the northern boundary of the Ebro Foreland Basin, the footwall of the Vallfogona thrust exposes a spectacular regional-scale growth fault-propagation fold in the syntectonic Berga Conglomerate Group. Across the structure, bedding dips change from 70 degrees overturned to upright and horizontal over a few kilometers of section. We used these growth strata to recover deformation rates using a new 1.5 km long magnetostratigraphy and cyclostratigraphy section. Samples for paleomagnetism were taken every 10 to 15 m and progressively thermally demagnetized to isolate the charateristic remanent magnetization (ChRM) directions. Correlation of the local magnetostratigraphy to the Global Polarity Time Scale (GPTS) suggests that the Berga conglomerates were deposited during chrons C12r to C15r with a total duration of about 4 million years. Sediment accumulation rates decreased upsection and are related to changes in depositional facies with 45 cm/ka rates in sub-aerial debris flows, 25 cm/ka in axial fluvial fans, and 17 cm/ka in fans longitudinal to the mountain front. Deformation rates were also unsteady with 113^o of folding occurring in short time intervals separated by longer periods of tectonic quiescence. To develop the cyclostratigraphy, we used an anhysteretic remanent magnetization (ARM) data series. ARM data reveals variations in magnetite concentration that have been shown to vary with climatic changes at Milankovitch frequencies. Samples for cyclostratigraphy were collected every 0.75 cm for ~800 m from chron C12r-C13r. Time-frequency analysis of stratigraphically referenced ARM measurements, which reflect variation in primary magnetite concentration, were calibrated to time using the magnetostratigraphy and revealed cyclicity with significant power at Milankovitch periodicities. Low frequency ARM cyclicity aligns with lithologic cyclicity at ~100 m, visible at the outcrop scale.