Tectono‐thermal Evolution of a Distal Rifted Margin: Constraints From the Calizzano Massif (Prepiedmont‐Briançonnais Domain, Ligurian Alps)

Abstract

The thermal evolution of distal domains along rifted margins is at present poorly constrained. In this study, we show that a thermal pulse, most likely triggered by lithospheric thinning and asthenospheric rise, is recorded at upper crustal levels and may also influence the diagenetic processes in the overlying sediments, thus representing a critical aspect for the evaluation of hydrocarbon systems. The thermal history of a distal sector of the Alpine Tethys rifted margin preserved in the Ligurian Alps (Case Tuberto‐Calizzano unit) is investigated with thermochronological methods and petrologic observations. The studied unit is composed of a polymetamorphic basement and a sedimentary cover, providing a complete section through the prerift, synrift, and postrift system. Zircon fission track analyses on basement rocks samples suggest that temperatures exceeding ~240 ± 25°C were reached before ~150–160 Ma (Upper Jurassic) at few kilometer depth. Neoformation of green biotite, stable at temperatures of ~350 to 450°C, was synkinematic with this event. The tectonic setting of the studied unit suggests that the heating‐cooling cycle took place during the formation of the distal rifted margin and terminated during Late Jurassic (150–160 Ma). Major crustal and lithospheric thinning likely promoted high geothermal gradients (~60–90°C/km) and triggered the circulation of hot, deep‐seated fluids along brittle faults, causing the observed thermal anomaly. Our results suggest that rifting can generate thermal perturbations at relatively high temperatures (between ~240 and 450°C) at less than 3 km depth in the distal domains during major crustal thinning preceding breakup and onset of seafloor spreading.

Progressive Deformation Patterns from an Accretionary Prism (Helminthoid Flysch, Ligurian Alps, Italy)

“…With regard to the metamorphic grade, each flyschoid unit shows upwards-decreasing conditions, even in the epizone to anchizone conditions [19]. Specifically, several paleothermal indicator analyses performed on the Sanremo unit indicate a peak temperature of up to 200 • C, while hotter conditions were attained in the cataclasites of the basal thrust [20].…”

“…The convergence between the Adriatic and European plates since the Late Cretaceous encompassed the closure of the interposed Piedmont-Ligurian Ocean and the subduction-collision process during Eocene-Oligocene times [5][6][7]. The oceanic basement, together with most of the Briançonnais terrane, experienced HP metamorphic conditions during subduction [13][14][15][16], whereas the Pre-Piedmont units and the oceanic sedimentary covers (i.e., the Helminthoid Flysch nappes) were obducted, recording only epi-to anchi-metamorphism [12,[17][18][19][20]. The present-day structure of the Ligurian Alps comprises a tectonic pile that mainly preserves a geometric order that reflects the paleogeographic provenance of the unit.…”

“…They have been interpreted to represent a part of the detached cover of the underlying Prepiedmont Arnasco-Castelbianco unit, which is the innermost nappe that features a complete Triassic-Eocene succession. The Arnasco-Castelbianco unit marks the transition between the oceanic domain (Piedmont-Ligurian) and the European distal margin [9,19,47].…”

The Atlas-East Variscan -Elbe shear system and its role in the formation of the pull-apart Late Palaeozoic basins

“…In the Ligurian Alps basement, temperatures exceeding ~ 240 ± 25 °C before ~ 160-150 Ma (Late Jurassic) have been measured by zircon fission-track analyses on basement rock samples at a few kilometres depth (Decarlis et al 2017). Such a heating-cooling cycle developed during the development of rifting at the Alpine Tethys margin and ended during the Late Jurassic (160-150 Ma).…”

Time‐Dependent Heat Budget of a Thrust from Geological Records and Numerical Experiments

“…Furthermore, the signature of shear heating can be dissipated or masked by several factors, including (i) conductive heat loss (Platt, ), (ii) a preexisting metamorphic history, (iii) efficient heat diffusion across the shear zone, (iv) advection of high‐temperature crustal blocks from depth, (v) the lack of thermal markers recording transient thermal conditions, and (vi) upwelling of hot fluids or injection of melts (e.g., Ault et al, ; Decarlis et al, ; Gottardi et al, ; Lacroix et al, ; Mamadou et al, ; Morton et al, ; O'Neil & Hanks, ; Scholz, ; Sutherland et al, ; Torgersen & Viola, ). For these reasons, only a few field‐based studies have quantified how much mechanical heat is produced by brittle and/or ductile deformation (Ault et al, ; England et al, ; Evans et al, ; Fulton et al, ; Maino et al, ; Mori et al, ; Nabelek & Liu, ).…”