Basement shear zones development and shortening kinematics in the Ecrins Massif, Western Alps

Abstract

In the Western Alps, Oligocene shortening affected a highly heterogeneous European crust with Liassic half-grabens inherited from the rifting stage and the finite deformation was strongly partitioned between the rigid basement and the weak Jurassic sediments. In the Ecrins Massif (Oisans, external crystalline massifs), where the half-grabens are best exposed and preserved, compressional structures within the basement have to date never been described in details. This massif was shortened under moderate metamorphic conditions (250-350°C and 0.1-0.5 GPa), and the rheological contrast between the basement and the cover is strong. While the sediments are intensely folded, the cover-basement interface presents apparent open folds underlined by the Lower Triassic layers. The basement itself shows a more localized deformation along several brittle-ductile shear zones. We here report new evidences of such brittle-ductile shear zones characterized by anastomosed phyllonitic shear bands rich in phengite and quartz, a low-strength material where strain has localized. New detailed maps of reverse shear zones, faults, schistosity, and stretching lineations in both the cover and the basement are provided. We show that the Oligocene crustal shortening was mainly E-W to ENE-WSW. Local N-S to NW-SE shortening occurred and was limited to the eastern border of the Ecrins Massif, around the Penninic Frontal Thrust, which likely was a sinistral transpressive structure in this area. Finally, new balanced cross sections show that these basement shear zones have accommodated more than 50% of the Oligocene crustal shortening.

Collision kinematics in the western external Alps

“…At the Eocene‐Oligocene boundary, a N‐S to NW‐SE shortening phase is identified: while Dumont et al [] proposed that it affected the entire western ECM, Bellanger et al . [] showed that it may be restricted to the cover in a narrow zone below the Penninic Frontal Thrust. This latter observation is consistent with field evidences further north [ Ceriani and Schmid , ].…”

“…The ECM at the Oisans latitude can be divided into several units from west to east: the south Belledonne, Grandes Rousses, and Emparis units (Figure ). Further southeast, two other units are present: the Meije and Combeynot units [ Bellanger et al , , and references therein]. The total amount of shortening at this latitude is about 28 km, including 11.5 km accommodated within the ECM [ Bellahsen et al , ], where the sedimentary cover is not significantly detached over the basement but is rather disharmonically deformed over it.…”

“…In the Oisans massif (Figure 3), the main shortening phase is E-W [Dumont et al, 2008[Dumont et al, , 2011[Dumont et al, , 2012Bellahsen et al, 2012;Boutoux et al, 2014;Bellanger et al, 2014], although there was oblique shortening at certain times and in certain places [Dumont et al, 2008;Bellanger et al, 2014]. Simon-Labric et al [2009] showed that the earliest E-W alpine shortening (witnessed by strike-slip shear zones) started around 34-31 Ma.…”

“…On the other hand, the along‐strike variations of the molassic basin, i.e., the decrease in both width and thickness from Central to Western Alps [e.g., Ford et al , ], may be due to the western end of the orogenic load according to Ford and Lickorish []. However, there was an orogenic load in Western Alps during Oligocene times (in the western ECM [ Dumont et al , ; Bellahsen et al , ; Bellanger et al , ]). Thus, the role of the orogenic load is still poorly understood.…”

“…This suggests that the crust was weakened because of inherited faults. However, there are no clear evidences in the field of the reactivation of such inherited faults and the weakening effect is mainly due to the soft synrift sedimentary rocks filling the faultbounded basins [Bellahsen et al, 2012] and fluid-rock interactions leading to the formation of phyllosilicates in shear zones with distinct orientations to the normal faults [e.g., Wibberley, 1999;Bellanger et al, 2014]. Is the size (width and thickness) of the foreland basin linked to the growth of the orogenic load or rather to a slab pull effect?…”