Orogenic, ophiolitic, and abyssal peridotites represent subcontinental, suboceanic, and subarc mantle rocks that were exhumed to the surface in various tectonic settings. These rocks provide coverage of vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths. They notably allow the observation of a wide range of lithospheric mantle compositions, including cratonic roots and subduction mantle wedges (high-pressure orogenic garnet lherzolites), variably rejuvenated subcontinental lithosphere (lower-pressure orogenic spinel and plagioclase lherzolites), and newly accreted oceanic lithosphere (ophiolitic mantle and abyssal peridotites). It is shown here that most of geochemical variability recorded by these mantle rocks is attributable to melt processes associated with partial melting and asthenosphere–lithosphere interactions. Rather than remnants of pristine mantle, the fertile orogenic lherzolites are now widely considered as former refractory lithospheric mantle refertilized by upwelling partial melt.

Orogenic, ophiolitic and abyssal peridotites

http://www.gm.univ-montp2.fr/PERSO/tommasi/LeRoux-etal-EPSL2007.pdf

The Lherz spinel lherzolite: Refertilized rather than pristine mantle

[1] The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Bearn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009TC002588

Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies

The Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domains

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014TC003529

Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay‐Pyrenees

Estimating shortening in collision belts is critical to reconstruct past plate motions. Balanced cross-section techniques are efficient in external domains but lack resolution in the hinterland. The role and the original extent of the continental margins during the earliest stages of continental convergence are debated. Here we combine existing and new sequentially restored cross sections in the central Pyrenees, with Iberia/Europe (IB/EU) plate kinematic reconstructions and new apatite fission track, zircon (U-Th)/He, and U/Pb ages to discuss higher and lower bounds of crustal shortening and determine the amount of distal margin sutured during collision. We show that after extension in the Albian (~110 Ma), a 50 km wide extremely thinned crustal domain underwent subduction at 83 Ma. Low-temperature data and thermal modeling show that synorogenic cooling started at 75–70 Ma. This date marks the transition from suturing of the highly extended margin to collision of the more proximal margin and orogenic growth. We infer a relatively low crustal shortening of 90 km (30%) that reflects the dominant thick-skinned tectonic style of shortening in the Pyrenees, as expected for young (Mesozoic) and weak lithospheres. Our proposed reconstruction agrees with IB/EU kinematic models that consider initially rapid convergence of Iberia, reducing from circa 70 Ma onward. This study suggests that plate reconstructions are consistent with balanced cross sections if shortening predicted by age-dependent properties of the continental lithosphere is taken into account.

Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence

The Mesozoic sediments in the Eastern North Pyrenean Zone have suffered a high temperature-low pressure metamorphism which reached its climax before the major deformation event. The mineral associations in pelitic rocks are consistent with temperatures of 500°–600°C and a maximum pressure of 3–4 kb. Post-metamorphic brittle deformation has disturbed the initial thermal pattern. The Albo-Cenomanian (98–87 Ma) metamorphism is related to thermal anomalies contemporaneous with the crustal thinning in the North Pyrenean Zone. The distribution of paleotemperatures suggests that the intensity of metamorphism may have been related to the magnitude of crustal attenuation. High-grade rocks are associated with lherzolites and granulites, whereas low-grade rocks are associated with higher-level crustal material (gneisses and micaschists). Crustal thinning and metamorphism developed during sinistral transcurrent displacement of Iberia with respect to Europe.

A. F. Leyreloup

Papers

High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France)