Constraining the onset of orogenic contraction in fold-and-thrust belts using sedimentary stylolite populations (Umbria-Marche Apennines, Italy)

Stylolites are pressure solution structures, the roughness of which yields quantitative information about the magnitude of the normal stress applied to the stylolite plane. The stress magnitude is calculated from the cross over length (L_c) determined using the signal analysis of a 2D track. For a sedimentary, bedding-parallel stylolite (BPS), L_c scales to the magnitude of the vertical maximum principal stress σ₁, hence to the burial depth at the time the roughness of the stylolite froze. We present a dataset of nearly 200 BPS from Meso-Cenozoic folded carbonates across the Umbria Marche Apennine Ridge. The BPS population is used to address the discrepancies between L_c values obtained by using different signal analysis methods. Assuming that in compressional settings burial-related pressure-solution more probably halts because of the switch of σ1 from a burial-related vertical attitude to a tectonic-related horizontal attitude, the vertical stress magnitudes derived from the inversion of the BPS roughness were combined to burial models at each investigated fold to reconstruct the absolute timing at which the orogenic stress started to prevail. The results indicate that the onset of horizontal compression started at ∼10 Ma and ∼8 Ma in the innermost and outermost folds, respectively, and predated fold development as indicated by the age of growth strata. These results establish Stylolite Roughness Inversion Technique as a reliable depth gauge that enables refining the timing of the onset of contraction in fold-and-thrust belts.