Arthur Iemmolo’s research while affiliated with Université de Montpellier and other places

In orogenic settings, fault networks accommodate crustal deformation during the evolution of mountain ranges. Fault zones exhibit multiphase ductile and brittle activity, complicating their interpretation. Dating deformation in orogenic basement remains a challenge. In this study, we conducted structural and microstructural observations, coupled with 40Ar/39Ar dating on encapsulated mineral fractions and chlorite thermometry, on major fault zones in the Eastern Pyrenees. To overcome the presence of K-feldspar within fault gouge, a major issue for fault gouge argon dating, we present a method to estimate the contribution of two mixed K-rich phases from 40Ar/39Ar step heating. This method provides limit or maximum age for the deformation recorded by fault gouges in which illite polytypes are mixed with K-feldspar. Our results reveal a ductile-to-brittle transition between 40-35 Ma (Priabonian-Bartonian), characterized by 2M1 muscovite and evidence of strike-slip movement along the Py NE-SW fault and NW-SE secondary faults. The Py fault gouge contains muscovite formed at temperatures exceeding 200-250°C. In the Têt NE-SW fault gouge, the coexistence of 2M1 and 1M illite polytypes suggests late-stage crystallization of 1M illite at 22.1 ± 1.4 Ma, at temperatures between 100-150°C, as determined by chlorite thermometry. These findings confirm significant normal faulting activity on the Têt fault during the Oligo-Miocene, consistent with published low-temperature thermochronology data indicating early exhumation of the Canigou massif relative to the Carança massif, facilitated by normal displacement along the Py fault, and a later exhumation of both massifs in relation to the Têt normal fault activity.

Sedimentary provenance studies using detrital zircon U–Pb ages represent an important tool to investigate the evolution of orogenic basins and to suggest tectonic settings and paleogeography reconstructions when it comes to supercontinent modeling. Syn-orogenic basins worldwide are characterized by a large proportion of zircons with ages close to the maximum depositional period, reflecting the proximity of recently formed magmatic rocks. In this work, we combine field observations, detrital zircon U–Pb ages, whole-rock Sm–Nd and mica Ar–Ar isotopic analyses to constrain the final tectonic evolution of a restricted basin located in a poorly studied region at the easternmost limit of the Paraguay Belt in the Tocantins Province, central Brazil. The investigated area corresponds to the boundary sector of the Paraguay Belt and the Brasília Belt and is also transected by the strikeslip fault system associated with the Transbrasiliano Lineament. This sector was previously mapped as part of the Cuiaba Group, internal zone of the Paraguay Belt. We provide 465 new detrital zircon U–Pb data with ages distributed from the early Cambrian to the Archean, with approximately 60% of the analyzed grains derived from Ediacaran-Cryogenian sources. The maximum depositional age of the basin is defined at ca. 590 Ma, constrained by the youngest age peak and an evident provenance shift in detrital input in the Paraguay Belt was demonstrated. The syn-orogenic character of the basin is inferred based on the main peak of the detrital zircon population age distribution around 600 Ma, which is very close to the maximum depositional period and points to a short time between erosion, deposition, and burial processes. The muscovite 40Ar/39Ar age of ca. 536–546 Ma obtained for muscovite schist metamorphosed under greenschist facies conditions indicates that the regional thermal regime was maintained up to the early-Cambrian in the area. The minimum fast cooling rate of 25 ◦C/ Myr defined in the investigated area is constrained by the 40Ar/39Ar analysis of biotite (549.16 ± 1.30 Ma) from a syn-to late-kinematic granodiorite intrusion. The very rapid magma emplacement occurred into relatively shallow crustal levels through the associated strike-slip faults. The Transbrasiliano Lineament would have facilitated the rapid unroofing of the study area and therefore the syn-orogenic deposition in a foreland domain at the final phase of West Gondwana amalgamation. The studied metasedimentary rocks represent a transition phase from the passive margin sedimentation (Cuiab´a Group) to an orogenic phase, constraining an inversion event in the West Gondwana around ca. 590 Ma. The data provide evidence of a late orogenic basin that was formed contemporaneously with the evolution of an active margin between the Amazonian Craton and the eastern blocks/cratons (S˜ao Francisco-Congo and Rio de la Plata cratons and Paranapanema Block) close to Cambrian times. Our results support the existence of the younger Clymene Ocean and the subsequent final assembly of West Gondwana in the Cambrian.

In orogens, most fault zones have polyphase tectonic activity under ductile and/or brittle conditions, which renders challenging their analysis in terms of kinematics, metamorphism, fluid flow and time evolution and may lead to different interpretations of their role during the orogenic evolution. Therefore, 40Ar/39Ar dating of the deformation is an efficient tool to decipher tectonic events in fault zones. This is particularly true in the Axial Zone of the Eastern Pyrenees where few geochronological studies have focused on these fault zones. The Têt and Py faults are major NE-SW trending crustal faults that have accommodated the complex exhumation/cooling history of adjacent crustal blocks, Mont-Louis, Canigou and Carança massifs, as highlighted by thermochronological data (Milesi et al., 2022). These faults record deformation from slightly above the ductile-brittle transition to brittle conditions as witnessed by the presence of thick fault gouges that partially rework low temperature mylonites. In this work, we report 40Ar/39Ar illite/muscovite dating on fault zones showing different activity phases at the brittle-ductile transition characterized by the presence of gouges with variable mixture of polytypes and with different phyllosilicate generations. For these fine-grained materials, we used a sample encapsulation technique to evaluate the amount of 39Ar lost by recoil during irradiation before dating the phyllosilicates. The new dates provide a scenario of fault motion between the early Eocene to the early Miocene that allows a comparison with those predicted by low-temperature thermochronological data and thermal modeling and shows a close link between fault activity and exhumation of adjacent crustal blocks.

Dating specific pressure–temperature–time–deformation–fluid (P–T–t–d–f) events is a major petrological issue, particularly for polymetamorphic assemblages. In order to better assess such events using mica populations, this study uses a combined in situ Rb–Sr and Ar–Ar dating approach coupled with chemical mapping, with application to an exhumed subduction complex (Schistes Lustrés, Western Alps). Geochronological investigation of the most Si-rich (Tschermak substitution) mica population allows us to investigate the (near-)peak burial ages of the various tectonometamorphic units of this high-pressure/low-temperature subduction complex. In the blueschist-facies units, a wide range of ages between ∼36 (younger than previously obtained peak ages) and ~ 52 Ma suggests a diachronous slicing of units with different initial paleogeography and/or a long residence time at near-peak conditions. The combined use of Ar–Ar and Rb–Sr isochron ages reveals the existence of some excess argon in the studied metamorphic rocks, interpreted as the result of (i) heterogeneous contamination from an argon-rich fluid in the higher grade eclogite-facies units and (ii) partial removal, during pseudomorphic recrystallization, of argon inherited from earlier stages of metamorphism in the lower grade blueschist-facies units.

Crustal deformation is characterized by brittle and ductile faults that accommodate at different scales the strain imposed by plate tectonics. The aim of this contribution is to show with the help of different examples how the in situ 40Ar/39Ar dating of synkinematic neocrystallized minerals in ductile shear zones and the step-heating 40Ar/39Ar dating of synkinematic authigenic clays in fault gouges can bring information on the timing of fault activity. However, due to their complex evolution, interpretation of the argon signature in fault zones requires consideration of several effects among which re- or neocrystallization, inheritance and fluid interaction processes are dominant.

Eastern Pyrenees relief is inherited from both the Eocene collision between the European and Iberic plates and from the Neogene extension link to the Gulf of Lion opening. In this study area, the Têt and Py faults are two major regional faults which allow the exhumation of the Canigou and Carança massifs (Fig. a). To better understand the tectonic evolution of the eastern Pyrenees thermal evolution of different massifs have been modelled using thermochronological data (Maurel et al. 2003; Milesi et al., 2020). The new thermal modelling (Fig. b) reveals a Priabonian (35-32 Ma) cooling stage only record in the Canigou massif. During the upper Oligocene and lower Miocene (26-19 Ma) an important cooling event is recorded in both massifs and a late Serravalian-Tortonian (12-9 Ma) cooling stage occurred in the Carança massif. The outcrops of the Têt and Py faults show metric scale core zone, suggesting important displacement on the fault. XRD and SEM analyses performed on these gouges make in evidence the presence of newly formed illite < 0.2 µm. Different granulometric fractions have been dated with 40 Ar/ 39 Ar method to highlight period of fault activity under brittle regime associated to illite formation. Illite ages obtained are respectively of 35 ± 4 Ma for the Py fault and 21 ± 3.5 Ma for the Têt fault. These results are consistent with the regional main cooling phases recorded by the low temperature thermochronology. Results obtained open new questions about the extent of fault movement and the temperature conditions necessary to the generation of illite in the fault zone.