Abstract
ABSTRACT The Monviso metaophiolite complex (W. Alps) is an almost intact fragment of Tethyan oceanic lithosphere metamorphosed to eclogite-facies peak metamorphic conditions during Alpine subduction. This 1:20.000 scale geological map encompasses, in an area of ∼35 km2, the Monviso Unit (MU) and the Lago Superiore Unit (LSU). Major focus was given to the Lower Shear Zone sub-unit (LSZ), where in the strongly deformed serpentinite-rich matrix are embedded blocks of variably brecciated metagabbros. Here, the occurrence of eclogite-facies mylonitic foliation (paragenesis: omphacite + rutile + garnet ± ex-lawsonite ± quartz) cut by breccia planes (cemented by omphacite + garnet ± ex-lawsonite) indicates brecciation at pristine eclogitic conditions. This map (i) provides new lithological, structural and morphological insights regarding the stratigraphy of the Monviso metaophiolite complex and (ii) supplies an unprecedented detail on the distribution of eclogite-facies breccia blocks inside the Lower Shear Zone that crosscuts the Lago Superiore Unit.
Highlights
Since P-T conditions are undistinguishable on either side of the shear zones (Angiboust, Langdon, et al, 2012), vertical displacements along them are probably less than km-scale. In this contribution we focus on the Lower Shear Zone sub-unit (LSZ)
Coarse grained Fe-Ti metagabbro (Type3 blocks, with size decreasing towards the basal peridotite; Figure 7(c)) are restricted to the lower part of the LSZ (Figure 7(d)) and may correspond to sills or dykes intruded into the peridotite sole (e.g. Dick et al, 2008), progressively incorporated into the LSZ during shear zone widening (Locatelli et al, 2018)
These structures, advocating for the coeval eclogitic mylonitization of former clasts and surrounding matrix, could correspond to pre-alpine sedimentary or tectonic breccias, ascribed to detachment faulting (Balestro et al, 2015; Festa et al, 2015). This map provides new insights into a long-studied area of the Western Alps, the Monviso Massif. It supplies an unprecedented detail on the distribution of eclogite-facies metagabbro blocks inside the LSZ and ISZ crosscutting the Lago Superiore Unit (LSU)
Summary
The presented 1:20.000 scale geological map (Main Map) covers an area of ∼35 km in the Monviso meta-ophiolite complex (Western Alps, Italy) which belongs to the Liguro-Piemontese units.The Western Alps (Figure 1(a) and (b)) result from the convergence and subsequent collision between Adria and Europe plates, with the Liguro-Piemontese oceanic domain interposed, undergoing subduction (Coward & Dietrich, 1989; Laubscher, 1991; Ricou & Siddans, 1986; Schmid & Kissling, 2000).In detail, the Liguro-Piemontese domain is formed by exhumed eclogite-facies tectonic slices, such as the Monviso meta-ophiolite, juxtaposed against lower grade sedimentary-derived terrains of the fossil accretionary wedge, such as the Schistes Lustrés (e.g. Agard et al, 2009, 2002; Marthaler & Stampfli, 1989; Plunder, Agard, Chopin, & Okay, 2013; Figure 1(a) and (b)).In the Monviso area, the nappe stack shows, from West to East: (i) the Schistes Lustrés, (ii), the Monviso metaophiolite and the (iii) Dora Maira massif (Figure 1 (b)).In recent years, a series of metagabbro blocks with peculiar clast-in-matrix structures were identified within the shear zones crosscutting the Monviso metaophiolite. The Liguro-Piemontese domain is formed by exhumed eclogite-facies tectonic slices, such as the Monviso meta-ophiolite, juxtaposed against lower grade sedimentary-derived terrains of the fossil accretionary wedge, such as the Schistes Lustrés Some of them are breccias composed of 1 to 10 cm-long fragments of eclogite mylonite cemented by interstitial eclogite-facies matrix. They have been interpreted either as produced by a brittle event at eclogite-facies conditions (Angiboust, Langdon, Agard, Waters, & Chopin, 2012), potentially linked to intermediate-depth seismic events (Angiboust, Agard, Yamato, & Raimbourg, 2012) or as inherited pre-Alpine detachment fault rocks or sedimentary-derived breccias (Balestro, Festa, & Tartarotti, 2015; Festa, Balestro, Dilek, & Tartarotti, 2015)
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