UNSTEADY FOLD GROWTH AT MILANKOVITCH TIME SCALES ENCODED IN MARINE GROWTH STRATA, SALSOMAGGIORE ANTICLINE, ITALY

Messinian-Early Pleistocene growth strata, including the Pliocene stratotype, are exposed in the Stirone River, northern Apennines, Italy, and record ~4.8 m.y. of unsteady growth of the Salsomaggiore fault-propagation fold. We measured 636 meters of stratigraphic section and collected samples every 1 meter, for which we measured magnetic susceptibility (MS), natural remanent magnetization (NRM), and anhysteretic remanent magnetization (ARM). Bedding dips exhibit a progressively shallowing trend with the exception of a sudden steepening of dips approximately halfway through the section. Growth strata in the Stirone section record 40° of forelimb rotation of the Salsomaggiore anticline; rates of folding are unsteady, with two periods (5.5-5.3 Ma and 3.1-1.8 Ma) of accelerated fold growth separated by ~2.2 m.y. of relative tectonic quiescence. We use rock magnetic cyclostratigraphy to investigate high resolution timing of fold growth unsteadiness between 3.1-1.8 Ma. The MS, NRM, and ARM stratigraphic series reveal frequencies associated with eccentricity (125 ky, 95 ky) and obliquity (41 ky) orbital cycles. The MS stratigraphic series exhibits the most distinct climate cyclicity. Comparison of the ARM stratigraphic series with the MS series suggests that paramagnetic silicates (clays) rather than ferromagnetic iron oxides control the MS. The MS climate cyclicity may reflect variations in the concentration of terrigenous sediments. We use obliquity and eccentricity cycles in the MS series to create a stratigraphic age model to investigate high-resolution fold unsteadiness during the most active period of fold growth from 3.1-1.8 Ma when the forelimb rotated 23°. Although rates of folding appear to be unsteady between 3.1-1.8 Ma, folding unsteadiness does not appear to respond in phase with variations in sedimentation rate.