High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomargin

Abstract

Abstract. An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40 Ar-39 Ar (amphibole, micas) and U-Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.

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

“…Recent investigations confirmed that extreme thinning occurred under a pre-and synrift sedimentary cover that deformed ductilely, thus accompanying the duration of a thermal anomaly in this central belt (e.g. Clerc et al, 2015). This makes the Pyrenean rift a rare case of synextensional HT metamorphism worldwide.…”

“…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).…”

“…The mantle is exhumed in the footwall of a main low-angle detachment fault that cuts the mantle lithosphere, the entire system showing a dominantly brittle architecture in regional simple shear. On the other hand, Clerc and Lagabrielle (2014) and Clerc et al (2015Clerc et al ( , 2016 argue for a considerable amount of ductile stretching and thinning in the central and eastern Pyrenees in what they refer as a style of "hot" paleomargins ( Fig. 12B).…”

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.…”