Abstract
In the upper Tesso Valley the folded contact between Piemonte Zone ophiolites and Sesia-Lanzo Zone continental crust is exposed. Here serpentinites, metabasites, calcschists and fine-grained gneisses are deformed by four ductile superposed groups of structures, associated with different mineral assemblages. Different serpentinite lithologies have been recognized and studied in detail. Mylonitic D2 structures are pervasive and mineral assemblages point to re-equilibration at T of 450 ± 50 ∘C and P of 0.8 ± 0.3 GPa, under blueschist/epidote amphibolite-facies conditions. Pre-D2 structures and mineral assemblages are relics within S2 and indicate a re-equilibration under eclogite-facies conditions, at T of 570 ± 50 ∘C and P > 1.8 GPa. Post-D2 occurs under greenschist-facies conditions. Numerical modeling of a subduction zone allows exploration of the geodynamic context in which such PT path could have developed, and to make hypotheses about the possible timing of such a scenario, in agreement with the timing generally proposed for the Alpine subduction and collision. Model predictions indicate that pre-D2 mineral assemblages may have developed during Paleocene at 60–90 km depth and 115–145 km from the trench, or, alternatively, during lower Eocene at ca. 70–90 km depth, and 135–160 km from the trench.
Highlights
The Piemonte Zone of the Western Alps comprises the ophiolitic remnants of the Ligurian-Piedmont Ocean, dismembered by the polyphase tectono-metamorphic evolution recorded during the Alpine convergence e.g., [1,2,3,4,5,6,7,8]
In the Lanzo Valleys, rocks from the Piemonte Zone (PZ) are mixed with those of the Sesia-Lanzo Zone (SLZ) continental crust, and show variable lithostratigraphic settings and heterogeneous metamorphic evolutions, with climax conditions ranging from greenschist-facies in the Middle Orco Valley, where metasediments dominate [15], to eclogite-facies conditions in the Lanzo Valleys, where slices of eclogites, metagabbros, variably re-equilibrated metabasites and serpentinites are mingled with calcschists and quartzites, together with slices of SLZ gneisses [13]
The detailed structural and petrographic analysis of the serpentinites of Gias Vej allowed to identify the successive mineral assemblages constraining the PT conditions for the development of pre-D2, D2, post-D2 (D3) fabrics, adding a quantitative estimate of the Alpine peak conditions in a portion of the PZ still poorly explored for this aspect
Summary
The Piemonte Zone of the Western Alps comprises the ophiolitic remnants of the Ligurian-Piedmont Ocean, dismembered by the polyphase tectono-metamorphic evolution recorded during the Alpine convergence e.g., [1,2,3,4,5,6,7,8]. The Lago di Monastero-Punta Gias Vej area (Lanzo Valleys) offers the observation of the articulated boundary between the PZ and SLZ, a dense interlocking of ophiolitic and continental material, produced by the Alpine subduction-related structural evolution [13], in which slices of serpentinite are quite common (Figures 2 and 3a). Our goal is to unravel the PTdt evolution of Punta Gias Vej serpentinite from the Lanzo Valleys subduction complex, and to explore whether this evolution can be achieved during an ocean/continent subduction. For this purpose the inferred tectono-thermal peak conditions will be compared with the predictions of a 2D numerical simulation of an ocean/continent subduction system, at different time steps, in order to give clues about the time and the paleogeography of peak conditions. Traces of the cross sections (1-5) are reported in Figure 2. (b) Stereographic projections (Schmidt net, lower hemisphere) of successive foliations (S1, S2, S3 and S4) of Gias Vej area (density plots and poles)
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