Structural relationships between Helminthoid Flysch and Brianconnais Units in the Marguareis Massif: A key for deciphering the finite strain pattern in the external southwestern Alps

摘要

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 Brianconnais 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 pre-stacking 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 post-stacking 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.