Tectonostratigraphy of the northern Monviso Meta-ophiolite Complex (Western Alps) (original) (raw)
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Rendiconti Online della Società Geologica Italiana, 2013
In the Western Alps the superposition of tectonic and/or tectonometamorphic events, and mineral transformations, strongly concur in obscuring the original prevailing processes of formation of block-inmatrix structures. We document in this study the occurrence and nature of different types of block-in-matrix structures (i.e. sedimentary and tectonic mélanges, and broken formations), formed at the plate interface between continental (i.e. the Dora Maira Unit) and oceanic (i.e. the Monviso Metaophiolite Complex) units, that experienced the overprint of different tectonic stages from subduction to collision. A Sedimentary Mélange characterizes the metasedimentary cover of the Dora Maira Unit and consists of exotic blocks (i.e., olistholits and blocks of marble) gravitationally collapsed from the margins of the Triassic-to Jurassic carbonate platform of the European continental margin. The Broken Formation is obtained thanks to the superposition of two deformation phases that, during exhumation stages, dismembered the original coherent stratigraphic succession. The Tectonic Mélange started to be formed during the subduction via offscraping of exotic blocks from the heterogeneous oceanic seafloor. The superposition of exhumation-related deformation contributed to form its final "structurally ordered" block-inmatrix fabric. Our findings may provide better constraints for the tectonometamorphic evolution of the northern sector of the Monviso Metaophiolite Complex and its pre-orogenic physiography.
Geological Magazine
The eclogite-facies Monviso meta-ophiolite Complex in the Western Alps represents a well-preserved fragment of oceanic lithosphere and related Upper Jurassic – Lower Cretaceous sedimentary covers. This meta-ophiolite sequence records the evolution of an oceanic core complex formed by mantle exhumation along an intra-oceanic detachment fault (the Baracun Shear Zone), related to the opening of the Ligurian–Piedmont oceanic basin (Alpine Tethys). On the basis of detailed geological mapping, and structural, stratigraphic and petrological observations, we propose a new interpretation for the tectonostratigraphic architecture of the Monviso meta-ophiolite Complex, and discuss the role played by structural inheritance in its formation. We document that subduction- and exhumation-related Alpine tectonics were strongly influenced by the inherited Jurassic intra-oceanic tectonosedimentary physiography. The latter, although strongly deformed during a major Alpine stage of non-cylindrical W-ver...
Episodes, 2015
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 ophiolites in the Western Alps; (2) it presents a first coherent documentation of the intraoceanic extensional tectonic history of the Jurassic oceanic lithosphere preserved in the Western Alps, demonstrating that it is possible "to see through" the subduction-collision induced metamorphic overprint in multiply deformed orogenic belts; and (3) it provides a regionally consistent tectonic framework for the rift-drift, seafloor spreading, and contractional episodes of the Ligurian-Piedmont ocean basin evolution. Our data and observations from the Monviso ophiolite complement the diverse datasets available from the modern oceanic core complexes, and provide further insights into the geometry, internal structure, and stratigraphy of supradetachment basin sequences, which are missing from in-situ oceanic core complexes. 2. Regional Geology of the Western Alps and its Tethyan connection The Western Alps (Fig. 1) developed due to the collision between Adria (upper plate) and Europe (lower plate) as the intervening oceanic lithosphere of the Jurassic Alpine Tethys Ocean was consumed (see e.g. Coward and Dietrich 1989; Laubscher 1991; Dilek, 2006). The collision zone (i.e. the axial section of the Alpine belt) involves an exhumed fossil subduction complex bounded by the Penninic front and the Insubric and Canavese lines (Fig. 1A). Tectonic units of this subduction complex are overthrust WNW onto the European foreland (Ricou and Siddans, 1986; Platt et al. 1989; Schmid and Kissling 2000; Butler et al. 2013). Different meta-ophiolite units (i.e. the Piedmont Zone; see e.g. Dal Piaz et al., 2003) are tectonically sandwiched between the European and Adriatic continental margin units (see Bigi et al., 1990), and display varying metamorphic facies conditions ranging from high-pressure (HP) to ultra high-pressure (UHP) (Frey et al., 1999 and reference therein). The Piedmont Zone is distinguished by eclogite-facies units (i.e. the Zermatt-Saas Zone auct.; Bearth, 1967) and blueschist-facies ones (i.e. the Combin Zone auct., Fig. 1A). The orogenic structural architecture of the Western Alps as seen in the field today (Fig. 1) was built up during three main phases of deformation-metamorphism events (Balestro et al., 2015): (1) Edipping subduction zone tectonics and eclogite-facies metamorphism in the Paleocene to middle Eocene, during which contractional deformation (D1) structures, mainly S1 foliation, were developed; (2) Continental collision tectonics in the late Eocene-early Oligocene that caused W-vergent folding and thrusting (D2). Blueschist-to greenschist-facies metamorphic re-equilibration took place during this event, producing S2 foliation; (3) Crustal exhumation (D3) and deep crust/mantle indentation in
Earth-Science Reviews, 2015
Tethyan ophiolites show an apparent poorly organized association of ultramafic and mafic rocks. By contrast to the complete mantle-crustal sections of Semail-type ophiolite sheets, Tethyan ophiolites are characterized by a smaller amount of mafic rocks (gabbros and basalts), by the absence of any sheeted dyke complex and by the frequent occurrence of oceanic sediments stratigraphically overlying mantle-derived peridotites and associated gabbroic intrusions. Therefore, they are considered as typical remnants of oceanic lithosphere formed in slowspreading environment or in ocean-continent transition at distal passive margins. In the very first models of formation of the Tethyan ophiolites, in the years 1980, the geodynamical processes leading to mantle unroofing were poorly understood due to the paucity of data and concepts available at that time from the present-day oceans. In particular, at that time, little work had focused on the distribution, origin and significance of mafic rocks with respect to the dominant surrounding ultramafics. Here, we reconsider the geology of some typical metaophiolites from the Western Alps and Corsica, and we show how results from the past decade obtained in the current oceans ask for reassessing the significance of the Tethyan ophiolites in general. Revisited examples include a set of representative metaophiolites from the blueschists units of the Western Alps (Queyras region) and from Alpine Corsica (Golo Valley). Field relationships between the ophiolitic basement and the metasedimentary/metavolcanic oceanic cover are described, outlining a typical character of the Tethyan ophiolite lithological associations. Jurassic marbles and polymictic ophiolite metabreccias are unconformably overlying the mantle-gabbo basement, in a way strictly similar to what is observed in the non-metamorphic Appennine ophiolites or Chenaillet massif. This confirms that very early tectonic juxtaposition of ultramafic and mafic rocks occurred in the oceanic domain before subduction. This juxtaposition resulted from tectonic activity that is now assigned to the development of detachment faults and to the formation of Oceanic Core Complexes (OCCs) at the axis of slow spreading ridges. This fundamental Plate Tectonics process is responsible for the exhumation and for the axial denudation of mantle rocks and gabbros at diverging plate boundaries. In addition, field relationships between the discontinuous basaltic formations and the ultramafic-mafic basement indicate that this tectonic stage is followed or not by a volcanic stage. We discuss this issue in the light of available field constraints.
Corsica ophiolites: Geochemistry and petrogenesis of basaltic and metabasaltic rocks
Ofioliti
This paper presents a systematic geochemical characterization of the basaltic and metabasaltic rocks from the Alpine Corsica ophiolites. The various ophiolitic units of Alpine Corsica can basically be subdivided into two main types: (1) high-pressure/low-temperature, metamorphic ophiolites belonging to the Lower Schistes Lustrés (LSL) and Upper Schistes Lustrés (USL) Complexes and (2) the non-metamorphic, upper ophiolitic units.
Eclogitization of the Monviso ophiolite (W. Alps) and implications on subduction dynamics
Journal of Metamorphic Geology, 2012
To constrain deep (40-100 km) subduction dynamics, extensive P-T data are provided on the eclogitic Monviso ophiolite derived from the subducted Liguro-Piemontese oceanic lithosphere (which was exhumed, together with associated continental units, before the Alpine collision). The Monviso ophiolite has so far been interpreted either as a fossilized subduction channel, with tectonic blocks detached from the slab at different depths and gathered in a weak serpentinized matrix, or as a more or less continuous portion of oceanic lithosphere. To evaluate potential heterogeneities within and between the various subunits, extensive sampling was undertaken on metasedimentary rocks and Fe-Ti metagabbros. The results indicate that the Monviso ophiolite comprises two main coherent tectonic subunits (the Monviso and Lago Superiore Units) detached during subduction at different depths and later juxtaposed at epidote-blueschist facies during exhumation along the subduction interface. Raman spectroscopy of carbonaceous material suggests (i) a difference in peak temperature of 50°C between these two subunits and (ii) a good temperature homogeneity within each subunit. Pseudosections and average P-T estimates using THERMOCALC THERMOCALC in the Lago Superiore Unit suggest for the first time homogeneous HP to UHP conditions (550°C, 26-27 kbar). Parageneses, peak conditions and tectonic setting are very similar to those of the Zermatt-Saas ophiolite, 200 km northwards, thus suggesting a common detachment mechanism for the whole Western Alpine belt.
A Jurassic Oceanic Core Complex in the High-P Monviso Ophiolite 24 ( Western Alps , NW Italy )
The eclogite-facies Monviso ophiolite (MO) in the Western Alps displays a complex record of Jurassic rift-drift, subduction zone and Cenozoic collision tectonics in its evolutionary history. Serpentinized lherzolites intruded by 163±2 Ma gabbros are exposed in the footwall of a thick shear zone (Baracun Shear Zone = BSZ), and are overlain by basaltic lava flows and syn-extensional sedimentary rocks in the hanging wall. Mylonitic serpentinites with sheared ophicarbonate veins and talc-, and-chloriteschist rocks within the BSZ represent a rock assemblage that formed from seawater-derived hydrothermal fluids percolating through it during intra-oceanic extensional exhumation. Lower Cretaceous calcschist, marble and quartz-schist metasedimentary assemblage unconformably overlies the footwall-hanging wall units, representing a post-extensional sequence. The MO, BSZ and the associated structures and mineral phases represent a core complex formation (OCC) in an embryonic ocean (i.e., the Ligurian-Piedmont Ocean). The heterogeneous lithostratigraphy and the structural architecture of the MO documented here are the products of rift-drift processes that were subsequently overprinted by subduction zone tectonics, and may also be recognized in other (ultra)high-P-belts worldwide.
Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps
Tectonics, 2019
A subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch‐like metasediments, meta‐ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite‐facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al2O3 and CaO and depleted in TiO2, MgO, and SiO2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution fr...
Lithos, 1999
. The Betic Ophiolitic Association, cropping out within the Mulhacen Complex Betic Cordilleras , is made up of numerous metre-to kilometre-sized lenses of mafic andror ultramafic and meta-sedimentary rocks. Pre-Alpine oceanic metasomatism and metamorphism caused the first stage of serpentinization in the ultramafic sequence of this association, Ž . which is characterized by local clinopyroxene Cpx breakdown and Ca-depletion, and complementary rodingitization of the basic dykes intruded in them. Subsequent eo-Alpine orogenic metamorphism developed eclogite facies assemblages in ultramafic and basic lithotypes, which were partly retrograded in Ab-Ep-amphibolite facies conditions during a meso-Alpine event. The heterogeneous development of the oceanic metasomatism in the ultramafic rock-types led to the patchy development of highly serpentinized Ca-depleted domains, without gradual transition to the host, and less serpentinized, Cpx-bearing ultramafites, mainly lherzolitic in composition. The high-pressure eo-Alpine recrystallization of these ultramafites in subduction conditions originated secondary harzburgites in the Ca-depleted domains, consisting of a spinifex-like textured olivineq orthopyroxene paragenesis, and a diopsideq Ti-clinohumite paragenesis in the enclosing lherzolitic rocks. During the meso-Alpine event, secondary harzburgites were partly transformed into talc q antigorite serpentinites, whereas the diopside and clinohumite-bearing residual meta-lherzolites were mainly transformed into Cpx-bearing serpentinites. Relics of mantle-derived colourless olivine may be present in the more or less serpentinized secondary harzburgites. These relics are overgrown by the eo-Alpine brown pseudo-spinifex olivine, which contains submicroscopic inclusions of chromite, ilmenite and occasional halite and sylvite, inherited from its parental oceanic serpentine. The same type of mantle-derived olivine relics is also preserved within the Cpx-bearing serpentinites, although it has been partly replaced by the eo-Alpine 0024-4937r99r$ -see front matter q 1999 Elsevier Science B.V. All rights reserved.
Tectonophysics, 1998
The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide-blocks of ophiolites and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain and the thinned continental margin of the Adria plate. The slide-blocks preserve a record of the long-lived history of rifting which led to opening of the Jurassic Western Tethys Basin. The External Liguride ophiolites consist of: (1) undepleted spinel-peridotites, partly re-equilibrated under plagioclase-facies conditions, which were interpreted as unroofed subcontinental mantle; (2) rare gabbroic rocks (mainly troctolite to olivine-bearing gabbro) probably crystallised from N-MORB magmas; and (3) basalts with N-to T-MORB affinity covered by late Callovian-early Oxfordian radiolarian cherts. Both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re-equilibration to plagioclase-facies conditions. The slide-blocks of lower continental crust are composed of gabbro-derived mafic granulites and felsic granulites. The latter include quartzo-feldspathic granulites and rare quartz-poor to quartz-free charnockitic rocks. In both mafic and felsic granulites, granulite-facies re-equilibration was followed by a retrograde metamorphic evolution to amphibolite-, greenschist-and subgreenschist-facies conditions. Retrogression is commonly accompanied by deformations progressively changing from plastic to brittle. The upper crustal rocks occurring as slide-blocks consist of Hercynian granitoids with orogenic affinity, mainly biotite-bearing granodiorites and peraluminous two-mica leucogranites. Locally, the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts. The granitoids underwent polyphase brittle deformations under subgreenschist-facies conditions which predated the basalt emplacement. The tectono-metamorphic evolution recorded by the slide-blocks of the External Liguride Units started in the Late Carboniferous-Early Permian (about 290 Ma), with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites. The associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro-derived granulites. Mafic and felsic granulites subsequently underwent tectonic exhumation in Permo-Triassic times, as testified by the development of granulite-to amphibolite-facies ductile shear zones. The granulites were finally exhumed to shallow levels, probably in association with the subcontinental mantle, in Late Triassic-Middle Jurassic times. The latter period was most likely characterized by extensive brittle faulting at shallow crustal levels, thus giving rise to extensional allochthons formed by stretched slices of granitoids. The Western Tethys opening is finally testified by the basalt intrusion and effusion in the Late Jurassic, followed by deep-sea pelagic sedimentation. The External Liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent-ocean boundary. This reconstruction fits well with the available data on the present-day continental margins derived from passive lithosphere stretching.