Abstract
In two companion papers we report the detailed geological and mineralogical study of two emblematic serpentinized ultramafic bodies of the western North Pyrenean Zone (NPZ), the Urdach massif (paper 1) and the Saraillé massif (this paper). The peridotites have been uplifted to lower crustal levels during the Cretaceous rifting period in the future NPZ. They are associated with Mesozoic pre-rift metamorphic sediments and small units of thinned Paleozoic basement that were deformed during the mantle exhumation event. In the Saraillé massif, both the pre-rift cover and the thin Paleozoic crustal lenses are involved in a Pyrenean recumbent fold having the serpentinized peridotites in its core. Based on detailed geological cross-sections microscopic observations and microprobe mineralogical analyses, we describe the lithology of the two major extensional fault zones that accommodated: (i) the progressive uplift of the lherzolites upward the Cretaceous basin axis, (ii) the lateral extraction of the continental crust beneath the rift margins and, (iii) the decoupling of the pre-rift cover along the Upper Triassic (Keuper) evaporites and clays, allowing its gliding and conservation in the basin center. These two fault zones are the (lower) crust-mantle detachment and the (upper) cover décollement located respectively at the crust-mantle boundary and at the base to the detached pre-rift cover. The Saraillé peridotites were never exposed to the seafloor of the Cretaceous NPZ basins and always remained under a thin layer of crustal mylonites. Field constraints allow to reconstruct the strain pattern of the mantle rocks in the crust-mantle detachment. A 20–50 m thick layer of serpentinized lherzolites tectonic lenses separated by anastomosed shear zones is capped by a thin upper damage zone made up of strongly sheared talc-chlorite schists invaded by pyrite crystallization. The cover décollement is a few decameter-thick fault zone resulting from the brecciation of Upper Triassic layers. It underwent strong metasomatic alteration in the greenschist facies, by multi-component fluids leading to the crystallization of quartz, dolomite, talc, Cr-rich chlorite, amphiboles, magnesite and pyrite. These data collectively allow to propose a reconstruction of the architecture and fluid-rock interaction history of the distal domain of the upper Cretaceous northern Iberia margin now inverted in the NPZ.
Highlights
Magma-poor hyper-extended rifted margins represent extensional environments with specific thermal, rheological and magmatic conditions
The E-W trending Pyrenean thrust-and-fold belt results from the collision between the margins of the northern Iberia and southern Eurasia plates during the Late CretaceousTertiary (Choukroune and ECORS team, 1989; Roure et al, 1989; Muñoz, 1992; Roure and Choukroune, 1998; Teixell, 1998; Mouthereau et al, 2014; Teixell et al, 2016, 2018). It consists of a core of Paleozoic rocks forming the elevated Axial Zone, bounded to the south by the South Pyrenean Zone (SPZ) mostly formed by detached Mesozoic thrustsheets comprising synorogenic Upper Cretaceous-Tertiary flysch and molasse sediments and to the north by the North Pyrenean Zone (NPZ), a narrow belt of Mesozoic sediments containing remnants of subcontinental mantle rocks (Monchoux, 1970; Debroas, 1978; Vielzeuf and Kornprobst, 1984; Fabriès et al, 1991, 1998)
In the Saraillé recumbent fold, the reconstructed crustmantle detachment appears composed of two layers from base to top: a 20–50 m thick deformed layer of anastomosed shear bands defining a lenticular fabric in the serpentinized lherzolites, followed by a few meter thick damage zone consisting of centimeter-sized symmetrical tectonic lenses where the serpentinized mantle rocks are intensively sheared and talcified
Summary
Magma-poor hyper-extended rifted margins represent extensional environments with specific thermal, rheological and magmatic conditions. Steep faults with up to a few kilometers of vertical displacement accommodate extension in the necking domain and lowerangle faults can accommodate tens of kilometers of horizontal motion in the distal domain The latter extensional detachment faults control the extreme thinning of the continental crust and, by place, the exhumation of the subcontinental mantle up to the seafloor. The southern European and northern Iberian passive margins collided to form the Pyrenees, but shortening did not exceed 150 km (Muñoz, 1992; Roure and Choukroune, 1998; Mouthereau et al, 2014) This was sufficient to allow the distal portions of the magma-poor northern Iberia margin to be uplifted and exposed all along the northern flank of the belt, in the North Pyrenean Zone (NPZ) (Jammes et al, 2009; Lagabrielle et al, 2010; Masini et al, 2014; Clerc et al, 2016; Corre et al, 2016; Teixell et al, 2016) (Fig. 1). We envision the evolution of the entire distal domain during the rifting of the northern Iberia margin, taking into account the geological constraints provided by the Saraillé massif (this study) and the nearby Urdach massif (see companion paper, Lagabrielle et al, 2019)
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