The structural setting and tectonic evolution of the Marguareis Massif (located at the boundary between the Maritime and Ligurian Alps) has long been matter of debate. After new meso-and micro-structural studies and field investigations, the tectonic unit interplay outlines a finite structural architecture more complex than that proposed in previous interpretations. We depict a stack of Briançonnais Units stemming from the Europe continental margin, and the Helminthoid Flysch Unit, detached from its original basement whose original palaeogeographic domain is still a matter of debate. Our results highlight for the first time that each unit recorded different prestacking folding events progressively developed at different structural levels. The events occurred during the stacking of the units produced shear zones with top-to-SW sense of movement. After the syn-stacking tectonics achieved during the Late Eocene-Early Oligocene collisional events, the whole stack recorded the same poststacking deformation history here represented by a fold system with sub-horizontal axial planes and faults. Similar tectonic evolution was documented in other sectors of the Western Alps by previous authors.
We present and discuss the results of a field-based approach including accurate geological mapping and micro-to map-scale structural analysis to highlight the finite strain pattern recorded in Marguareis Unit, a massif deformed at shallow crustal levels at the boundary between Maritime and Ligurian Alps. We describe superposed tectonic structures developed under low-grade metamorphic conditions during the Alpine collision and nowadays exceptionally well recorded in the area of interest. We demonstrate that the structural frame of the Marguareis Unit results from superposition of fourfold systems, later segmented, but without significant displacements, by brittle faults.
Along the Western Alps, the oceanic units showing blueschists to eclogite facies metamorphic imprint are classically regarded as fragments of the Ligurian-Piedmont Ocean. These units recorded a strongly deformation related to their subduction, accretion and subsequent exhumation into the Alpine wedge, developed during the convergence between the Europa and Adria Plates. However, some of these units, for example the Moglio-Testico Unit, are less pervasively deformed, providing evidence of their sedimentary evolution as well as the tectono-metamorphic history. Therefore, we present original stratigraphic, structural and thermo-barometric data to characterize the tectono-metamorphic history and the sedimentary evolution of the Moglio-Testico Unit, performing different techniques including fieldwork, structural analysis and chlorite-phengite multiequilibrium thermobarometry. Our dataset indicates that the Moglio-Testico Unit can be considered as a fragment of oceanic cover whose sedimentary evolution reflects that of a portion of oceanic lithosphere approaching to the subduction zone. Structural analysis combined with the thermobarometry indicate that this unit recorded a polyphase deformation history developed under High Pressure-Low Temperature metamorphic conditions (D1: 1.2–1.0 GPa and Tpeak: 330–260 °C; D2: 0.4–0.7 GPa and 230–170 °C) during its underthrusting, accretion into the Alpine wedge and subsequent exhumation up to the shallower crustal levels.
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