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 (this paper) and the Saraillé massif (paper 2). The peridotites have been exhumed 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. Based on detailed geological cross-sections and microprobe mineralogical analyses, we describe the lithology of the two major extensional fault zones that accommodated: (i) the progressive exhumation of the lherzolites along the Cretaceous basin axis; (ii) the lateral extraction of the continental crust beneath the rift shoulders 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 of the detached pre-rift cover. The Urdach peridotites were exposed to the seafloor during the Late Albian and underwent local pervasive carbonation and crystallization of calcite in a network of orthogonal veins (ophicalcites). The carbonated serpentinized peridotites were partly covered by debris-flows carrying fragments of both the ultramafics and Paleozoic crustal rocks now forming the polymictic Urdach breccia. The mantle rocks are involved in a Pyrenean overturned fold together with thin units of crustal mylonites. Continent-derived and mantle-derived fluids that circulated along the Urdach crust-mantle detachment led to the crystallization of abundant metasomatic rocks containing quartz, calcite, Cr-rich chlorites, Cr-rich white micas and pyrite. Two samples of metasomatized material from the crust-mantle detachment yielded in situ zircon U/Pb ages of 112.9 ± 1.6 Ma and 109.4 ± 1.2 Ma, thus confirming the Late Albian age of the metasomatic event. The cover décollement is a 30-m thick fault zone which also includes metasomatic rocks of greenschist facies, such as serpentine-calcite association and listvenites, indicating large-scale fluid-rock interactions implying both ultramafic and continental material. The lowermost pre-rift cover is generally missing along the cover décollement due to tectonic disruption during mantle exhumation and continental crust elision. Locally, metasomatized and strongly tectonized Triassic remnants are found as witnesses of the sole at the base of the detached pre-rift cover. We also report the discovery of a spherulitic alkaline lava flow emplaced over the exhumed mantle. 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
The architecture of magma-poor passive continental margins is classically viewed as a succession of three structural domains, namely from continent oceanward: the proximal, necking and distal domains (Péron-Pinvidic and Osmundsen, 2016, and references within)
These lenses are relatively well exposed on the western (Mer de Her), southern (Soum d’Unars) and eastern side of the mantle body and are labelled on Figure 4. The foliation of these crustal lenses always parallels the geological boundary of the adjacent mantle rocks, a major structural characteristic noticed in the Saraillé massif (Corre et al, 2016; Asti et al, 2019); – the Urdach breccia and interbedded flysch strata are continuously exposed from Col d’Urdach to Col d’Etche and represent the early sedimentary cover of the peridotites and associated sheared Paleozoic rocks; – ophicalcites occur in the walls of a quarry located at Col d’Urdach, at the opposite side of the Bilatre quarry. This implies that a large portion of the peridotites exposed in the Urdach massif derive from a former seafloor composed of unroofed serpentinized mantle rocks associated with exhumed Paleozoic basement material; – our compilation of dip measurements confirms previous measurements by Debroas et al (2010) and demonstrates that the Urdach breccias and the Albian-Cenomanian flysch strata are in a reverse position along the western side of the mantle body (Fig. 4)
This implies that the Mesozoic sediments are involved in a kilometer-scale recumbent fold with the Urdach ultramafic basement in its core; – the cover décollement can be traced between a thin unit of Jurassic marbles and Paleozoic schists along the southern border of the massif south of Soum d’Unars
Summary
The architecture of magma-poor passive continental margins is classically viewed as a succession of three structural domains, namely from continent oceanward: the proximal, necking and distal domains (Péron-Pinvidic and Osmundsen, 2016, and references within). During the last 30 years, considerable academic efforts have been carried out in order to collect samples from the conjugate Iberia-Newfoundland margins with the aim to unravel the tectonic and metasomatic evolution of the hyperthinned crust and exhumed subcontinental mantle during continental breakup (i.e. Pérez-Gussinyé et al, 2006; Reston, 2009a, 2009b; Pérez-Gussinyé, 2013; Sutra et al, 2013) Despite these efforts, our knowledge of the geological architecture of the exhumed mantle domain, and our understanding of the processes that enable subcontinental peridotites exhumation and their unroofing to the seafloor, remains relatively poor. Through these geological and hydrothermal reconstructions, we aim at providing a new set of geological data that may be used as field references for remote distal domains of current passive continental margins
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