Thermal control on the modes of crustal thinning leading to mantle exhumation: Insights from the Cretaceous Pyrenean hot paleomargins

Abstract

The prerift Mesozoic sequences of the Cretaceous passive margins fossilized in the North Pyrenean Zone (NPZ) are characterized by high-temperature deformation in relation with thinning of the continental basement. Our compilation of chronological and geological data confirms a clear correlation between the distribution of the highest paleotemperatures in the prerift sedimentary cover and the loci of extreme crustal stretching. Geological evidences such as the occurrence of peridotite bodies directly underlying metamorphic prerift sediments indicate an early attenuation of the rifted continental crust. This leads us to propose a mechanism of rifting involving boudinage of the continental crust. The lateral extraction of the Paleozoic basement occurred under the prerift cover that is decoupled on the Triassic clays and evaporites. The thermal conditions allowing coeval ductile deformation of the crust and of the prerift sediments leaded to the widening of basins devoid of large faulted blocks. We discuss the implications on the origin and significance of the granulites and the relations between flysch deposition and high-temperature metamorphism of the prerift sediments. In the NPZ, Albian-Cenomanian flysch sequences were deposited synchronously with the synmetamorphic ductile deformation of the prerift sequences. Since the base of the flysch deposits also recorded locally the high-temperature tectonic event, we propose an original mechanism for the evolution of the basins involving continuous deformation of the prerift metamorphic sediments. At the scale of the Pyrenean domain, our results suggest a strong lateral variability in the tectonic style of passive margins, in direct link with their thermic pattern.

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

“…This phenomenon is observed and described further east, e.g., on the southern flank of the Mouthoumet massif and on the northern flank of the Agly massif, where sedimentary formations of various ages, from the Triassic to the Aptian, are intensely truncated and/or scalped (Durand-Delga, 1964;Bessière et al, 1989;Clerc and Lagabrielle, 2014 No stratigraphic repetition of any member of the sedimentary series has been observed, and such a disposition can be interpreted as the result of a pre-midCenomanian extensional tectonics described in the literature as the pre-Cenomanian phase (Casteras, 1933;Mattauer and Proust, 1965;Durand-Delga, 1965). …”

“…The isograds are here widely disturbed by later faulting locally outlined by tectonic breccias. Clerc and Lagabrielle (2014) correlated this thermal zonation across the NPZ to a variable mode of crustal thinning across the pre-Pyrenean paleomargins, suggesting the existence of "hot" margins (eastern domain) in opposition to the cooler western domain. For the authors the three domains can be described as follows:…”

“…The pre-Pyrenean inverted margins hence offer suitable direct access to the thermal imprint of crustal thinning and subsequent continental breakup. Regional high-temperature, low-pressure (HT-LP) metamorphism is known along the northern rim of the Pyrenean belt where it developed coevally with a major Cretaceous crustal thinning event leading (Ravier, 1959;Bernus-Maury, 1984;Azambre and Rossy, 1976;Golberg and Leyreloup, 1990;Dauteuil and Ricou, 1989;Clerc and Lagabrielle, 2014). The HT metamorphic rocks, deriving mainly from pre-rift to syn-rift sediments, are distributed in a narrow WNW-ESE belt defined as the North Pyrenean Zone (NPZ), bounded by two major post-metamorphic thrusts, the North Pyrenean Fault (NPF) to the south and the North Pyrenean Frontal Thrust (NPFT) to the north (Fig.…”

“…The zone affected by the HT-LP metamorphism is characterized by an intense ductile deformation -S1 of Choukroune (1972); Choukroune (1976) -first attributed to compression and later reinterpreted as resulting from syn-extensional or transtensional deformation (Golberg 1987;Golberg and Leyreloup, 1990;Lagabrielle et al, 2010;Clerc and Lagabrielle, 2014).…”

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

“…The geometry of normal faults in this model, especially in the first stages, is influenced by the assumption that the lower crust is strong (intermediate or mafic) as in numerical models such as Lavier and Manatschal (2006), which leads to the formation of downward-concave normal faults for instance. Based on field observation in the pre-Pyrenean rifted margins, Clerc & Lagabrielle (2014), propose an alternative model implying boudinage of the upper crust allowed by a high geothermal regime and blanketing effect of the decoupled thick preand syn-rift sediments ( fig. Fig.…”