Pre-Alpine extensional tectonics of a peridotitelocalized oceanic core complex in the Late Jurassic, high-pressure Monviso ophiolite (Western Alps)

Abstract

The Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed, eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere. The recent recognition of a pre-Alpine detachment fault in the Lower Tectonic Unit of this ophiolite has led to the discovery of an oceanic core complex, which developed during the initial stages of the tectonic evolution of the Alpine Tethys. The NNW-striking, 20-25-km-long shear zone (Baracun Shear Zone) contains ductilely to cataclastically deformed blocks and clasts of Fe-Ti and Mg-Al metagabbros in a matrix made of mylonitic serpentinite and talc-chlorite schist with high Ni-Cr concentrations and high Cl contents. Intensely sheared ophicarbonate rocks and brecciated serpentinite within this shear zone are deformed by the Alpine-phase S1 foliation and D2 folds, providing a critical age constraint for the timing of its formation. Metabasaltic-metasedimentary rocks in the hanging wall increase in thickness away from the shear zone, characteristic of syn-extensional rock sequences in supradetachment basins. A Lower Cretaceous post-extensional sedimentary sequence unconformably cover the synextensional strata, the detachment shear zone, and the ophiolitic footwall, establishing a strong structural evidence for the intraoceanic, seafloor spreading origin of the tectonic fabric of the Monviso ophiolite, prior to the onset of subduction zone tectonics in the Alpine Tethys. The Monviso ophiolite and the Baracun Shear Zone represent a peridotite-localized oceanic core complex, which survived both the subduction and continental collision tectonic stages of the Alpine orogeny. Intraoceanic detachment faults and oceanic core complexes may play a significant role in subduction initiation, and hence their recognition in orogenic belts is an important step in reconstructing the record of ocean basin collapse and closure. However, detailed, field-based structural, petrological and geochemical studies of the seafloor spreading and extensional tectonic history of these ophiolites have been scarce. This has been in part due to the strong overprint of the Alpine-stage subduction-collision related deformation-metamorphic events that obscures the previously developed rift-drift and seafloor spreading generated structures and mineral assemblages in these units. In this paper, we document through detailed geological mapping, systematic structural and stratigraphic observations, petrographic and geochemical analyses the internal structure, tectonic fabric and evolution history of the Monviso ophiolite, one of the best preserved ophiolites in the Western Alps. We show that this ophiolite is an on-land exposure of an oceanic core complex, which formed through simple-shear seafloor spreading kinematics during the opening of the Ligurian-Piedmont ocean basin within the Alpine Tethys. This inferred oceanic core complex origin of the Monviso ophiolite is significant in that: (1) it better explains the dismembered and highly attenuated crustal architecture of the o...

Continental Margin Ophiolites of Neotethys: Remnants of Ancient Ocean–Continent Transition Zone (OCTZ) Lithosphere and Their Geochemistry, Mantle Sources and Melt Evolution Patterns

“…Some of the best examples of CM ophiolites include the Jurassic ophiolites in the Ligurian units of the Northern Apennines (e.g., Montanini et al, 2008;Marroni and Pandolfi, 2007), Alpine Corsica (Durand-Delga, 1984;Malavieille et al, 1998;Saccani et al, 2008a), and the Western Alps (Lagabrielle and Cannat, 1990;Desmurs et al, 2002;Rampone et al, 2005;Manatschal and Müntener, 2009; Microcontinent. et al, 2015;Balestro et al 2015;), and the early Cretaceous Dongba-Purang ophiolites in southern Tibet (Liu et al, 2015;Yang and Dilek, 2015). However, many more CM ophiolites are yet to be documented in the Phanerozoic and Precambrian orogenic belts through systematic structural field and geochemical studies.…”

Distinguishing the Mélange-Forming Processes in Subduction-Accretion Complexes: Constraints from the Anisotropy of Magnetic Susceptibility (AMS)

“…At the mesoscale, the broken formation is characterized by pinch-and-swell structures, necking of the less competent, shaly, clayey, and marly layers, and a brittle and/or ductile bedding-parallel boudinage of centimeter-to meter-long elongated and mainly tabular bed fragments, which represent "native" (i.e., intra-formational) blocks ( Figure 3A). Elongated blocks display a high aspect ratio [88] and [73]); (B) location of Figure 1A; (C) geological cross section across the Northern Apennines (modified from [89]).…”

“…Sedimentary mélanges or heterogeneous mass-transport deposits occur from slope instability in the upper part of frontal wedges of subduction-accretion complexes and in the trench inner-slope (e.g., [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]). Diapiric mélanges, including shale and mud diapirs, occur as the result of the upward rise of overpressured fluids [88] and [73]); (B) location of Figure 1A; (C) geological cross section across the Northern Apennines (modified from [89]).…”

Variscan ophiolites in NW Iberia: Tracking lost Paleozoic oceans and the assembly of Pangea

“…The evolution proposed for the Upper Units implies that the Variscan suture ophiolites were generated within the Rheic Ocean realm. The Cambrian ophiolites, excepting the Bazar Ophiolite, would be linked to the initial stages of the opening of this ocean reminiscent of the Western Alps Tethys (Dilek and Furnes, 2014;Balestro et al 2015;Saccani et al 2015), whereas the Devonian ophiolites would have originated during the final stages of its closure analogous to many supra-subduction zone ophiolites (Dilek, 2003;Dilek and Furnes, 2011;Yang et al 2015). The Devonian ophiolites, the commonest in the Variscan suture, have been interpreted as suprasubduction zone ophiolites generated at the intraRheic subduction zone located near the Southern margin of Laurussia Sánchez Martínez et al, 2007a).…”