Paper No. 8
Presentation Time: 3:30 PM
PRECISION DEFORMATION RATES FROM MILANKOVITCH MODULATED GROWTH STRATA, SOUTH PYRENEAN THRUST FRONT, SPAIN
Seismic, well, and field data from the central Spanish Pyrenees illuminate a two tiered thrust network; a lower basement duplex with a roof thrust in Triassic evaporates, which is also the décollement for upper tier thrust sheets, which carry the shortened cover and synorogenic wedge-top basins southward. The foreland thrust sheets contain leading- edge imbricate fans and extensive halotectonic deformation (diapirs, salt withdraw synclines, welds, sedimentary breccias) exposed in the External Sierra. A foreland dipping molasse monocline developed over a buried imbricate stack with variable geometry along the emergent mountain front. Synsedimentary emplacement of the Guarga thrust sheet and transverse halotectonic folds are being reinvestigated at Pico del Aguila anticline. Growth strata record the convolution of deformation and deposition revealing long-term deformation rates and deformation kinematics in the Pyrenean foreland. Synorogenic flysch shows both lateral facies and reduced thickness variation between the crests of the anticlines and the surrounding synclines up-section, suggesting a reduction in folding rate through time. A study of basinal Arguis Fm. lithostratigraphy, magnetic susceptibility, and anhysteretic remanent magnetization (ARM), defines high frequency sedimentary cycles modulated by the climatic effects of Milankovitch orbital forcing, not tectonic loading. Time series analysis of magnetic susceptibility and lithofacies spectrum reveals a quasi-periodic signal in both series. Bandpassing the two series shows that both series evolved as basic cycles modulated by another cycle that was longer by ca. 5 times, the hallmark of the Earths precession index in which ~20-kyr cycles are modulated by ~100-kyr eccentricity cycles. The preservation of Milankovitch-modulated growth stratigraphy allows unparalleled long-term deformation rate resolution. Preliminary data suggests a narrow distribution of magnetic grain sizes and a single source for the magnetic particles. Magnetic susceptibility and ARM variations may result from climate modulated carbonate production or turbidites in the Eocene Jaca basin or more likely, variable detrital inputs (e.g. atmospheric dust, watershed erosion) rather than a diagenetic source (chemical remanent magnetization).