To what extent does U-Pb geochronology of calcite cements help to constrain the sequence of fracture development in folded strata? The Mirabeau Anticline (SE France) as a case study.

Folded sedimentary layers usually exhibit brittle mesostructures such as faults, joints, veins, and stylolites which accommodate the internal strain of strata during folding but also before strata started to be tilted, i.e. during Layer Parallel Shortening (LPS), and after tilting, i.e. during late stage fold tightening (LSFT) when shortening can no longer be accommodated by fold growth. We have established a fracture sequence in the Mirabeau anticline (SE France) using orientation data and relative chronology with respect to bedding attitude, which comprises the early-, syn- and late-folding fractures related to the folding event (1). This field-based approach is compared to 12 ages out of 32 samples analyzed by LA-ICP-MS U-Pb geochronology of selected syn-kinematic calcite, in order to provide an absolute time frame for the fracture development, along with a potential validation of the sequence of deformation. Along a section across the fold strike axis, the deformation related to the contraction lasted at least 12 Ma (or 26 Ma with uncertainties), bracketed between 52 Ma (± 8 Ma) and 40 Ma (± 6 Ma), indicating that the fold developed in response to the so-called Pyrenean-Provençal phase. In depth however, the sequence of deformation as inferred from the structural study of the fracture network is challenged by the absolute age of the syn-kinematic calcite: syn-folding flexural-slip surfaces are dated from 52 Ma (± 8 Ma), i.e. older than early-folding reverse faults associated to LPS (46 ± 4 Ma).  

 

In order to better constrain the onset of Layer-Parallel Shortening (LPS), we further analyzed the bedding parallel sedimentary stylolites through the inversion of their roughness to better define the burial depth range during which the Jurassic and Upper Cretaceous formations of interest were submitted to a vertical stress σ1. Once projected on the burial-time model of the investigated strata, the depth range, going up to 3 km, constrains the time at which σ1 presumably switched from vertical to horizontal in response to the onset of tectonic loading. Altogether, this work questions the limits of absolute dating in understanding fold-fracture relationships, and forewarn of potentially misleading interpretation of absolute ages when associated with deformation features. The example of the Mirabeau Anticline enables a discussion about the compatibility and complementarity of different approaches to date mesoscale and macroscale deformation features in fold-and-thrust belts where the sedimentary record is not well preserved.

 

Keywords : Absolute dating, fracturation, fold, carbonates

 

(1) Lacombe O., Beaudoin N., Hoareau G., Labeur A., Pecheyran C. & Callot J.P., 2021. Dating folding beyond folding, from layer-parallel shortening to fold tightening, using mesostructures : Lessons from the Apennines, Pyrenees and Rocky Mountains.  Solid Earth, 12, 10, 2145-2157