Sequence, duration, rate of deformation and paleostress evolution during fold development: Insights from fractures, calcite twins and U-Pb calcite geochronology in the Mirabeau anticline (SE France)

Abstract

The timing and duration of folding events have traditionally been constrained by the dating of syn-tectonic layers that record fold growth and limb rotation. However, recent advances demonstrate that absolute U-Pb geochronology on well-characterized distributed mesoscale structures such as fractures and stylolites allows the reconstruction of a detailed deformation timeline spanning the entire folding event even when growth strata are absent. In this study, we integrate structural analysis with calcite twin paleopiezometry, inversion of fault slip data, and U-Pb dating of syn-tectonic calcite mineralizations from faults, veins and stylolite-related jogs to refine the classical fold-fracture model and provide a more comprehensive view and precise timing of brittle deformation and stress evolution during folding using the example of the Mirabeau anticline (Provence, SE France). The onset of layer-parallel shortening (LPS) has been precisely constrained for the first time using absolute U-Pb dating of a calcite-filled jog associated with a bedding-parallel stylolite (BPS) and the earliest striated reverse faults associated with Pyrenean shortening. The brittle deformation associated with the folding event of the Mirabeau anticline is recorded continuously between ∼50 Ma and ∼37 Ma, with layer parallel shortening (LPS) lasting ∼5 My, fold growth lasting ∼3 My, and late-stage fold tightening (LSFT) lasting ∼5 My. Combining our shortening estimate derived from and balanced cross-section with absolute ages of fold growth, we estimate a Pyrenean shortening rate of approximately 0.6 mm/yr at Mirabeau. Our results further place important new time constraints on the sequence of structural development through the Provence fold-and-thrust belt, and support a forelandward propagation of Pyrenean deformation across Provence.

The integration of fracture sequence analysis, calcite twin inversion and striated fault inversion provides insights into the tectonic evolution of the Mirabeau area: (1) pre-Pyrenean alternating NNE-SSW and WNW-ESE extension between ∼112 and ∼90 Ma, likely related to the Durance uplift event (2) Eocene Pyrenean-Provençal shortening dominated by a N-S to NE-SW horizontal σ₁ under alternating compressional and strike-slip stress regimes, (3) Oligocene E-W extension associated with the West European rifting event, and (4) a minor compressional stress regime with NW-SE to NNW-SSE oriented σ1 attributed to Miocene Alpine shortening.