Internal and external modulation of folding rates with 104 to 105 year
time resolutions from growth strata, Pico del Aguila, Spain
Abstract
High-resolution cyclostratigraphy in growth strata are used to
reconstruct unsteady folding rates at the regional-scale Pico del Aguila
anticline, southern Pyrenees, to evaluate deformation modulation.
Magnetic polarity stratigraphy was used to determine absolute time and
to calibrate cyclostratigraphy-based anhysteretic remanent magnetization
intensity variations to establish precessional frequencies in the growth
strata record. Incremental tilting rates were calculated between
selected horizons over ~5.24 myr of fold growth. Careful
treatment of uncertainties enhances confidence that the results are
meaningful and results show significant variability in folding rates
over time. The acceleration phase of fold growth was variable,
punctuated by a prolonged period of tectonic quiescence, and correlated
to sedimentation changes in the wedge-top basin. Shallow-dipping bedding
intrinsically modulated the initial rates of folding for the first 25˚
of limb tilt until 38.9 Ma. Then, halotectonics in the Paleogene Jaca
Basin extrinsically modulated accelerating folding rates for the next
42˚ of folding, until ~37.5 Ma. Finally,
forelimb-steepening leading to geometric strain hardening and blunted
folding rates for the last 21˚ of fold tightening and causing a thrust
fault to cut the anticline’s core. Folding ended at Pico del Aguila
~35.9 Ma. Calculated folding rates varied between 0°±
5.5˚and 90˚± 19°/myr over 100s kyr time increments. Variations in the
folding rate of the Pico del Aguila décollement anticline are attributed
to both intrinsic modulation as a result of progressive bedding
steepening during folding and extrinsic modulation as a result of
variable deltaic sedimentation rates in the wedge-top basin.