Very high geothermal gradient during mantle exhumation recorded in mylonitic marbles and carbonate breccias from a Mesozoic Pyrenean palaeomargin (Lherz area, North Pyrenean Zone, France)

Abstract

International audienceAlthough they are famous among Earth scientists, the Lherz peridotites are exposed within geological formations of the North Pyrenean Zone (NPZ) still lacking detailed investigations. Our study focuses on the metasediments of the Aulus basin hosting the Lherz peridotite body and associated ultramafic fragments of smaller size. The new data set comprises of structural analysis and detailed geological mapping of the massive Mesozoic marbles that form the prerift sequence typical of the NPZ and of the ultramafic-rich clastic breccia formations surrounding the peridotite bodies. The massive marbles display an evolution from hot and ductile to cold and brittle deformation, indicative of an exhumation process ending with the sedimentary reworking of both the deformed Mesozoic metasediments and the exhumed ultramafic rocks. Crystal Preferred Orientations (CPO) measured in the marbles support a deformation mechanism by dislocation creep of calcite, which is dominant between 400 8C and 600 8C; these deformation temperatures are within the range determined earlier by Clerc et al. (2015), using RSCM (Raman Spectroscopy of Carbonaceous Material) geothermometry. As a consequence, we better describe the transition from ductile to brittle deformation in the prerift marbles and clarify the origin of the syn-rift breccias. Due to continuous exhumation along detachments' faults, the brecciated metamorphic carbonates of the prerift NPZ sedimentary cover were passively uplifted towards shallower levels and progressively unroofed, while transported passively on the back of the exhumed ultramafic footwall. These results are consistent with the recent interpretations of the North Pyrenean peridotites as remnants of subcontinental mantle rocks exhumed within the pre-Pyrenean rift system. We emphasize the importance of tectonic decoupling between the Mesozoic sedimentary cover and the Palaeozoic basement, which leads to the juxtaposition of metamorphosed and deformed Mesozoic Please cite this article in press as: Lagabrielle Y, et al. Very high geothermal gradient during mantle exhumation recorded in mylonitic marbles and carbonate breccias from a Mesozoic Pyrenean palaeomargin (Lherz area, North Pyrenean Zone, France). C. R. Geoscience (2016), http://dx

The pull apart-type Tardets-Mauléon Basin, a key to understand the formation of the Pyrenees

“…The later Pyrenean compressive history does not allow any detailed reconstruction of the geometry of these Cretaceous basins. Nevertheless, contrary to the proposition made by the recent genetic models (Lagabrielle and Bodinier, 2008;Lagabrielle et al, 2010;Clerc et al, 2012;Lagabrielle et al, 2016), the Albian extension did not lead to mantle exhumation. This way, the breccias from the Aulus basin are not marine nor interbedded at the base of the Albian flysch infilling.…”

Rifted margins: Ductile deformation, boudinage, continentward-dipping normal faults and the role of the weak lower crust

“…e.g. : Along the Cretaceous passive margins inverted in the Pyrenean range, recent work revealed the existence upper crust boudinage (Clerc and Lagabrielle, 2014;Clerc et al, 2016;Corre et al, 2016;Teixell et al, 2016) associated to strong thermal anomaly recorded in the pre-and syn-rift deposits (Clerc et al, 2015b;Lagabrielle et al, 2016). Similarly, in the South China Sea, Savva et al (2013) described a rift related crustal deformation widely distributed over numerous lower crustal detachments indicating a rather delocalized and ductile deformation of the crust.…”

“…sediment, or "blanketing effect", are regarded as major issues for the thermal history of rifted margins (Levell, 2010;Clerc et al, 2015bClerc et al, , 2016Lagabrielle et al, 2016), and, as illustrated in Figure 2, it is obvious that this effect is highly variable from a margin to another, depending on the sediment thickness and nature. Numerical modeling (e.g.…”

Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data

“…12 imply that the domains where the crust was extremely thinned or totally removed were covered by a thick succession of Triassic to Cretaceous evaporite, carbonate and flysch deposits (e.g. the Mauléon, Chaînons Béarnais and Aulus basins; Jammes et al, 2009;Lagabrielle et al, 2010Lagabrielle et al, , 2016Boucheville;Clerc et al, 2015Clerc et al, , 2016. During the closure of the mantle domain by subduction (previous to the collision of the margins), this buoyant sedimentary lid was detached and thrust in a pop-up fashion onto both continental margins Teixell et al, 2016).…”

“…It is now widely accepted that the Cretaceous rifting in the Pyrenees produced extreme thinning of the continental crust, and the occurrence of lherzolite bodies all along the northern Pyrenees attest to a central belt between the Iberian and Eurasian margins where the continental crust was removed. The subcontinental mantle lithosphere was exhumed to the base of the Mesozoic sedimentary series or even to the sea floor (Lagabrielle et al, 2010;Clerc et al 2012;Lagabrielle et al, 2016;de Saint Blanquat et al, 2016). The new paleotectonic models accounting for mantle exhumation bear profound implications not only for the understanding of the extensional basin architecture but as well for the kinematic history of the Pyrenees, and have spurred renewed interest and intensive research in the region.…”