Significant rotations related to cover–substratum decoupling: Example of the Dôme de Barrôt (Southwestern Alps, France)
Introduction
The propagation of décollement surfaces into fold-and-thrust belts often results in a significant decoupling of deformation patterns between shallower and deeper units (Costa and Vendeville, 2002, Dahlen et al., 1984, Malavieille, 2010). Such decoupling with differential shortening rates in fold-and-thrust belts with respect to the substratum rocks under a décollement is usually evoked for explaining the arcuate shape of the shallower units (Macedo and Marshak, 1999, Vidal-Royo et al., 2009). In the foreland of the Western Alps, the fold-and-thrust belts of Castellane and Nice in France are typical examples of arcuate fold-and-thrust belts (Fig. 1A). The main décollement surface is located in the Triassic gypsum layers (Graham, 1981), along which the Meso-Cenozoic cover overthrusted the underlying Permian substratum (Fig. 1C). These fold-and-thrust belts were first documented being of convex curvature in late 70s (Siddans, 1979). It was more explicitly interpreted as an inherited structure or a primary arc-shaped feature (Weil and Sussman, 2004), based on the evidence of no paleomagnetic rotation in the Permian substratum in the “Dôme de Barrôt”, the Argentera (Bogdanoff and Schott, 1977) and the Estérel massifs (Zjiderveld, 1975). Later on, although Aubourg and Chabert-Pelline (1999) observed a 40° anticlockwise rotation at level of the Digne nappe in the Neogene sediments, they did not question the primary arc-shaped nature because the rotations was interpreted to be controlled and bounded by the inherited structural pattern. Nevertheless such interpretations remained skewed by not taking into account (1) the strain decoupling between the substratum and the shallow cover, and (2) the structural pattern near the shear zone of the Rouaine–Daluis Fault Zone (RDFZ, Fig. 1B).
The Dôme de Barrôt is located in the inner part of the fold-and-thrust belt of Castellane, to the Southwest of the Argentera crystalline massif (Fig. 1A). To the southern edge of this Permian dome, the Meso-Cenozoic sedimentary cover presents a spectacular arcuate shape bounded farther to the West by the RDFZ. From West to East, the trend of the fold axes turn from NE–SW to E–W. This area provides a good opportunity to study a possible decoupling of deformation between the Paleozoic substratum and the Meso-Cenozoic cover through the analysis of magnetic fabric and paleomagnetism. The decoupling that occurred between the Permo-Werfenian substratum and the Rhetian to Chattian sedimentary cover is probably due to the existence of a décollement of gypsum layers, the Muschelkalk and Keuper (Triassic) in age. For simplification of terminology, we use in this paper the terms Permian substratum or Paleozoic substratum and Meso-Cenozoic cover or sedimentary cover to refer to the units below and above the décollement respectively.
Analyses on Anisotropy of Magnetic Susceptibility (AMS) in non-metamorphic rocks allow determining the Layer Parallel Shortening (LPS) trends (Averbuch et al., 1992, Borradaile, 1988, Borradaile and Jackson, 2010), which generally formed in the early stage of rock deformation, usually prior to folding (Aubourg et al., 2004, Parés et al., 1999, Weil and Yonkee, 2009). On the other hand, paleomagnetic data are able to document about possible block horizontal rotations. AMS is thus an excellent marker of deformation and can test the reliability of paleomagnetic rotations. Studies of paleomagnetism have been broadly conducted for determining horizontal block rotations in many orogens worldwide (e.g., Carey, 1955, Weil and Sussman, 2004). This paper intends to present recent results of AMS and paleomagnetic rotation within and around the Permian substratum at Dôme de Barrôt, in order to compare with the arcuate fold-and-thrust belt of the Meso-Cenozoic cover. The results also allow us to discuss the possible complex mechanisms including décollement overthrusting, thrust-stack crustal thickening (Ford et al., 1999, Labaume et al., 2008), gravitional sliding (Graham, 1981) and blind thrusting (Ford et al., 1999, Laurent et al., 2000), on the finite deformation of the sedimentary cover.
Section snippets
Geological setting
The Permian substratum, which constitutes the Dôme de Barrôt (Fig. 1B), is composed of a thick series (< 1 km) of continental deposit of red mudstones and slates issued from the erosion of the Variscan relief (Bourquin et al., 2011). It is unconformably overlain by Triassic quartzites and gypsum layers (Bordet, 1950, Vernet, 1958).
During the Alpine Tethys opening in Jurassic (deposition of Hettangian limestone to Oxfordian black shale), the Permian strata of Dôme de Barrôt were slightly exhumed
Sampling and methods
We sampled at 19 sites (Fig. 1B and Table 1) with a total of 526 oriented standard specimens for magnetic analyses within and around the Dôme de Barrôt for both the Permian substratum and the Meso-Cenozoic cover. The core-samples were collected by using an electric drill cooled by water and supplied by a generator. The orientation of the cores was measured in situ by a compass and an inclinometer. Five sites (DAL13, DAL15, CIA19, CIA63 and LEO01) are located in the Permian slate/mudstone
General characteristics
The majority of the samples, as well as from the Permian substratum and the Meso-Cenozoic cover, presents weak mean susceptibility Km (15 < Km < 300 μSI), and weak NRM intensity values (between 40 and 700 μA/m), revealing a low concentration of ferromagnetic minerals (Table 1). Nevertheless, some samples from the Jurassic and Cretaceous black shales (sites DEV, GIR and COU) are characterized by a stronger mean susceptibility of 600 < Km < 1000 μSI and an intensity of NRM about 7000 μA/m, exhibiting a
Discussion
The study of magnetic fabrics and paleomagnetic directions in the Meso-Cenozoic cover around the Paleozoic Dôme de Barrôt brings specific results. On the one hand, the AMS data highlight a similar discrepancy in the direction of magnetic lineation between a NW–SE trend in the northeastern side to E–W trends in the southern side for both the Permian substratum and the overlying Meso-Cenozoic cover. On the other hand, the paleomagnetic data from the cover in the southwest of the Dome reveal an
Conclusions
The structural and magnetic studies of the Permian substratum Dôme de Barrôt and its surrounding Meso-Cenozoic cover reveal contrasting results:
- •
The Meso-Cenozoic sedimentary cover in the southern flank of the Dôme de Barrôt, which show an arcuate shape of the stratigraphic and structural trends from SW to SE of the Dome, suggests an anticlockwise rotation along vertical axis of at least 34°, compared to Permian substratum of the Dome itself.
- •
The magnetic fabrics of the Meso-Cenozoic cover and
Acknowledgments
We thank the reviewers (Fabio Speranza and anonymous) and the editor for their comments that helped improving the manuscript. The first author also deeply thanks the Prof. Jacques Angelier who supervised him during his Ph.D thesis and supported him to study the sedimentary cover around the Dôme de Barrôt before passing away in January 2010. This study was part of the Doctorate thesis of the first author at the University of Nice Sophia-Antipolis. We are grateful to Academia Sinica for providing
References (74)
- et al.
Exhumation of oceanic blueschists and eclogites in subduction zones: timing and mechanisms
Earth Sci. Rev.
(2009) - et al.
Neogene remagnetization of normal polarity in the Late Jurassic black shales from the southern Subalpine Chains (French Alps). Evidence for late anticlockwise rotations
Tectonophysics
(1999) - et al.
Magnetic fabric as a structural indicator of the deformation path within a fold-thrust structure: a test case from the Corbières (NE Pyrenees, France)
J. Struct. Geol.
(1992) Magnetic susceptibility, petrofabrics and strain
Tectonophysics
(1988)- et al.
Tectonic applications of magnetic susceptibility and its anisotropy
Earth Sci. Rev.
(1997) - et al.
Structural geology, petrofabrics and magnetic fabrics (AMS, AARM, AIRM)
J. Struct. Geol.
(2010) - et al.
The Permian–Triassic transition and the onset of Mesozoic sedimentation at the northwestern peri-Tethyan domain scale: palaeogeographic maps and geodynamic implications
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2011) - et al.
Strain removal applied to paleomagnetic directions in an orogenic belt: the Permian red slates of the Alpes Maritimes, France
Earth Planet. Sci. Lett.
(1985) - et al.
Experimental insights on the geometry and kinematics of fold-and-thrust belts above weak, viscous evaporitic décollement
J. Struct. Geol.
(2002) Studies of microtectonics, anisotropy of magnetic susceptibility and paleomagnetism of the Permian Dôme de Barrôt (France): paleotectonic and paleosedimentological implications
Tectonophysics
(1973)
Collision and stress trajectories in Taiwan: a finite element model
Tectonophysics
Magnetic inference of in situ open microcracks in sandstone samples from the Taiwan Chelungpu fault drilling project (TCDP)
J. Asian Earth Sci.
Characterization of the magnetic fabric of rocks
Tectonophysics
Tertiary remagnetization of normal polarity in Mesozoic marly limestones from SE France
Tectonophysics
Magnetic susceptibility anisotropy, strain, and progressive deformation in Permian sediments from the Maritime Alps (France)
Earth Planet. Sci. Lett.
Secular variation in Permian red beds from Dôme de Barrôt, SE France
Earth Planet. Sci. Lett.
Evolution of magnetic fabrics during incipient deformation of mudrocks (Pyrenees, northern Spain)
Tectonophysics
Vertical axis rotations in fold and thrust belts: comparison of AMS and paleomagnetic data in the western external Sierras (southern Pyrenees)
Tectonophysics
Pre-folding magnetic fabrics in fold-and-thrust belts: why the apparent internal deformation of the sedimentary rocks from the Minervois basin (NE Pyrenees, France) is so high compared to the Potwar basin (SW Himalaya, Pakistan)?
Sediment. Geol.
Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera–Mercantour massif (SW Alps)
Lithos
Arcuate fold and thrust patterns in the subalpine chains of southeast France
J. Struct. Geol.
Finite strain patterns and their significance in Permian rocks of the Alpes Maritimes (France)
J. Struct. Geol.
Stratigraghy, paleoecology and foraminifers of the paleogene from the Alpes de Haute-Provence and the Alpes Maritimes
Rev. Micropaleontol.
Formation of orogen-perpendicular thrusts due to mechanical contrasts in the basal décollement in the central external Sierras (southern Pyrenees, Spain)
J. Struct. Geol.
The structural setting and palaeogeographical evolution of the Grès d'Annot Basin
Geol. Soc. Lond., Spec. Publ.
Post-Miocene shortening pictured by magnetic fabric across the Zagros–Makran syntaxis (Iran)
Geol. Soc. Am. Spec. Pap.
Burial, claystones remagnetization and some consequences for magnetostratigraphy
Geol. Soc. Lond., Spec. Publ.
Etude paléomagnétique et analyse tectonique dans les schistes rouges permiens du Sud de l'Argentera
Bull. Soc. Geol. Fr.
Le dôme permien de Barrôt (Alpes Maritimes) et son auréole de terrains secondaires
Bull. Serv. Carte Géol. Fr.
Les réaimantations chimiques et le filtrage des polarités du champ magnétique terrestre. Expériences pression–température et comparaison avec des réaimantations naturelles
Les formations paléogènes des Alpes Maritimes franco–italiennes
The orocline concept in geotectonics—Part I
Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt
Terra Nova
Paleomac: a macintosh™ application for treating paleomagnetic data and making plate reconstructions
Geochem. Geophys. Geosyst.
Counterclockwise rotation of the Western Alps since the Oligocene: new insights from paleomagnetic data
Tectonics
Le Crétacé inférieur de l'Arc subalpin de Castellane entre l'Asse et le Var, stratigraphie et sédimentologie
Cited by (9)
Orogenic architecture of the Mediterranean region and kinematic reconstruction of its tectonic evolution since the Triassic
2020, Gondwana ResearchCitation Excerpt :Paleomagnetic data from the Western Alps comprise a series of isolated sites in Mesozoic rocks of the External Massifs (light green datapoints in Fig. 25) and the Briançonnais in the southwestern Alps (pink datapoints in 22). In addition, there are sites with Oligocene and early Miocene (re)magnetizations from the External Massifs of the Western Alps (dark green datapoints in Fig. 25 (Crouzet et al., 1996)), and the Briançonnais of the southwestern Alps (orange datapoints in Fig. 25 (Sonnette et al., 2014)) and from the Briançonnais in the Ligurian Alps (red datapoints in Fig. 25 (Collombet et al., 2002)). When averaged, these data demonstrate that the External Massifs of the western Alps, that became incorporated in thrusting in the Miocene (e.g., Schmid et al., 2017) and references therein), did not significantly rotate.
Origin of a foreland-dipping seismogenic zone and its basal decollement in southwestern Taiwan
2018, Earth and Planetary Science LettersCitation Excerpt :Often, excess pore pressure may be responsible for the formation of a weak decollement (Suppe and Witteke, 1977). Dehydration is one of the processes that elevates pore pressure in a decollement, such as the transition from gypsum to anhydrite in the Alps (Ko et al., 1997; Sonnete et al., 2014), and from saponite to chlorite in the Naikai Trough (Kameda et al., 2017). While the decollement beneath a mountain belt is usually hinterland-dipping (e.g., Carena et al., 2002; Suppe, 2007), a number of studies have shown that opposed dipping decollement may occur close to or in front of the deformation front (e.g., Zapata and Allmendinger, 1996; Cristallin and Ramos, 2000; Barnes and Nicol, 2004).