Sedimentary and tectonic evolution of the Pena flexure, southwest Pyrenean mountain front, Spain

The sedimentary and tectonic evolution of a section of Oligocene continental growth strata on the Pena flexure records growth of the thrust-related anticline that defines the southwest Spanish Pyrenean mountain front. A newly developed magnetostratigraphy for this section provides the time calibration necessary to calculate and document the relationship between the rates of deformation and sedimentation. The new magnetostratigraphy for the Campodarbe and Bernues Formations established in this study is further west than previous magnetostratigraphic studies and provides new age control for the timing of deformation along the Pyrenean thrust front. Analysis of more than 500 specimens from 164 horizons over the 3,400 m of section was used to develop the magnetostratigraphy. A progressive 10-step thermal demagnetization (250°--580° C) and principal component analysis were used to isolate the characteristic remanent magnetization (ChRM) directions for each sample. The ChRM is primarily carried by magnetite. Hematite and goethite are magnetic minerals also present in the samples, but they do not contribute to the ChRM. Sample ChRM directions and virtual geomagnetic poles (VGPs) were used to determine site polarities. The magnetic reversal stratigraphy that results is correlated to the Geomagnetic Polarity Time Scale (GPTS). Correlation to the GPTS shows that the stratigraphic section deposited on the Pena flexure extends from C12r (33.2 Ma) in the Middle Campodarbe Fm into C6C (∼23.4 Ma) in the Bernues Fm. The top of the section is defined by a slight angular unconformity (2° dip, 40° strike) between the Bernues and Uncastillo Formations. Depositional shallowing of the growth strata is related to the tilting of the mountain front during fault slip, so calculating the rate of sedimentation and depositional surface tilting provides new insights into thrust front kinematics. The first ∼1100 m of section deposited on the Pena flexure are pre-growth and the beginning of the progressive unconformity in the section records the onset of deformation at this position along the Pyrenean mountain front at 28.7--28.5 Ma. Tilt rates calculated with 0.15--1.12 myr chron resolution, constrained by the magnetostratigraphy, vary from 3°/myr to 35°/myr, and are interspersed with times of tectonic quiescence. This study documents an order of magnitude change in tilt rates over 100 kyr timescales and provides new age controls for deformation of the western Spanish Pyrenean foreland basin.