Camille Clerc

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

The pre-rift 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 pre-rift sedimentary cover and the loci of extreme crustal stretching. Geological evidences such as the occurrence of peridotite bodies directly underlying metamorphic pre-rift 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 pre-rift cover that is decoupled on the Triassic clays and evaporates. The thermal conditions allowing coeval ductile deformation of the crust and of the pre-rift 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 pre-rift sediments. In the NPZ, Albian-Cenomanian flysch sequences were deposited synchronously with the syn-metamorphic ductile deformation of the pre-rift 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 pre-rift 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

Ophicalcites from the northern Pyrenean belt: a field, petrographic and stable isotope study

Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and mid-oceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotopic characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter- to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and (3) pervasive substitution of serpentine minerals by calcite. Stable isotopic analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: (1) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; (2) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and (3) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the eastern and western Pyrenean ophicalcites belonging, respectively, to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characteristics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.

New Caledonia Obducted Peridotite Nappe: Offshore Extent and Implications for Obduction and Postobduction Processes

One of the largest ophiolitic peridotite masses in the world covers a quarter of the island of Grande Terre, New Caledonia. The Peridotite Nappe was obducted during the Eocene, is weakly deformed and corresponds to the highest of a structurally simple pile of thrust nappes. We present new marine seismic data that allows us to track the offshore continuation of the Peridotite Nappe along‐strike for a distance of more than 500 km south of New Caledonia, and to image its pre‐, syn‐ and post‐obduction sedimentary records. Offshore, the Peridotite Nappe underlies a c. 150‐km wide and 2 km‐deep basin. Flat‐topped horsts of peridotite are clearly bounded by major normal faults; in contrast faults are obscure onland. To the east, the Peridotite Nappe roots along the eastern margin of the Felicite Ridge (new name), a c. 300 x 25 km dome‐shaped ridge, which we interpret as being the southern extension of the HP/LT metamorphic core complex observed in New Caledonia. Two alternative tectonic models address the relative timing and relationships between Peridotite Nappe emplacement, uplift of a metamorphic core complex, and extensional tectonics. These models provide new ideas for the understanding the formation of the eastern margin of the Zealandia continent. Our results contribute to an understanding of how oceanic mantle is emplaced onto continental margins.

Deepwater fold-and-thrust belt along New Caledonia's western margin : relation to post-obduction vertical motions

Classically, deepwater fold-and-thrust belts are classified in two main types, depending if they result from near- or far-field stresses and the understanding of their driving and triggering mechanism is poorly known. We present a geophysical dataset off the western margin of New Caledonia (SW Pacific) that reveals deformed structures of a deepwater fold-and-thrust belt that we interpret as a near-field gravity-driven system, which is not located at a rifted passive margin. The main factor triggering deformation is inferred to be oversteepening of the margin slope by post-obduction isostatic rebound. Onshore erosion of abnormally-dense obducted material, combined with sediment loading in the adjacent basin, has induced vertical motions that have caused oversteepening of the margin. Detailed morpho-bathymetric, seismic stratigraphic and structural analysis reveals that the fold-and-thrust belt extends 200 km along the margin, and 50 km into the New Caledonia Trough. Deformation is rooted at depths greater than 5 km beneath the seafloor, affects an area of 3500 km2, and involves a sediment volume of approximately 13 000 km3. This deformed belt is organized into an imbricate fan system of faults, and one out-of-sequence thrust fault affects the seabed. The thrust faults are deeply rooted in the basin along a low-angle floor thrust and connected to New Caledonia Island along a major detachment. This study not only provides a better knowledge of the New Caledonia margin, but also provides new insight into the mechanisms that trigger deepwater fold-and-thrust belts.

Reply to comment by P. Olivier on “Thermal control on the modes of crustal thinning leading to mantle exhumation: Insight from the Cretaceous Pyrenean hot paleomargins”

We thank Philippe Olivier for the great attention he paid in reading our work and for the discussion he initiated. P. Olivier has been alarmed by our proposition that some Pyrenean granulites might have been (re)granulitized during the Cretaceous rifting event. In a rigorous scientific approach, this hypothesis has to be envisioned. However, in the paper, we proposed two alternative scenarios: one involving a Cretaceous (re)granulitization and another one implying a succession of metamorphic events with a first Paleozoic High Temperature (HT) metamorphism event responsible for the granulitization, followed by a second HT metamorphism event during the Cretaceous. Our main point here was to put in relation the strikingly similar fields of metamorphisms responsible for the granulitization of the crystalline basement and for the HT metamorphism of the prerift and synrift Mesozoic sedimentary cover. In other words: with temperatures up to 600°C in the metasedimentary cover, we would expect temperatures at least as high as 600°C in the basement. If not, it would mean that the continental crustal basement was already withdrawn from the metamorphic domain at that point. This scenario corresponds in fact to the concept of lateral extraction of the continental crust that we retained from our field constraints and that we defend here.