Abstract
Abstract. Dating syntectonic sedimentary sequences is often seen as the unique way to constrain the initiation, duration, and rate of folding as well as the sequence of deformation in the shallow crust. Beyond fold growth, however, deformation mesostructures accommodate the internal strain of pre-folding strata before, during, and after strata tilting. Absolute dating of syn-folding mesostructures may help constrain the duration of fold growth in the absence of preserved growth strata. Absolute dating of mesostructures related to early-folding layer-parallel shortening and late fold tightening provides an access to the timing and duration of the entire folding event. We compile available ages from the literature and provide new U–Pb ages of calcite cements from veins and faults from four folds (Apennines, Pyrenees, Rocky Mountains). Our results not only better constrain the timing of fold growth but also reveal a contraction preceding and following folding, the duration of which might be a function of the tectonic style and regional sequence of deformation. This study paves the way for a better appraisal of folding lifetime and processes and stress evolution in folded domains.
Highlights
Quantifying the rates and duration of deformation processes is key to understanding how the continental crust deforms
The absolute dating of mesostructures definitely confirms the sequence of deformation usually deduced from orientation data and relative chronology with respect to bedding attitude, which includes LPS, fold growth, and late-stage fold tightening (LSFT) (Fig. 1)
This sequence is valid for the four folds studied, despite the San Vicino, Cingoli and Pico del Aguila anticlines developed above a decollement in a foldand-thrust belt, while the Sheep Mountain anticline formed as a basement-cored forced fold above a basement thrust
Summary
Quantifying the rates and duration of deformation processes is key to understanding how the continental crust deforms. We explore hereinafter the possibility to define the age and duration of folding by investigating how and for how long pre-folding strata have been accommodating shortening from the onset to the end of the horizontal contraction from which the fold originated, an event we define as the folding event (Fig. 1) This approach will help better constrain the duration of fold growth, by directly dating the syn-folding mesostructures and by bracketing the timing of fold growth through the dating of the mesostructures that immediately predate and postdate strata tilting. These two deformation stages have been overlooked since they accommodate much less shortening than folding itself They correspond to key periods of time for large-scale fluid flow and related ore deposition in fold-and-thrust belts and sedimentary basins This study paves the way to a better mechanical appraisal of contractional deformation and stress evolution in folded domains
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