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The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Viséan/Namurian boundary (B320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D 1 ), all structural levels shared the same tectonic evolution: D 2 (extension and exhumation), D 3 (refolding) and post-D 3 dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M 2 ). M 2 occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D 2 effects defines a bulk prolate strain in a ''pinched pull-apart'' setting. Ductile D 2 deformation during M 2 dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D 1 is only represented by the inverted position of bedding (overturned limbs of recumbent D 1 folds) and by refolded D 1 folds. U-Pb monazite and zircon ages and K-Ar muscovite ages are in accord with Ar-Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at C330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shearzone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward

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The Montagne Noire in the southernmost French Massif Central is made of an ENE-elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low-grade petrology, K-Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faug eres units, which constitute part of the southern hangingwall of the dome. It is shown that southward-directed Variscan nappethrusting (D 1 ) and a related medium-P metamorphism (M 1 ) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono-metamorphic event D 2 /M 2 largely transposes D 1 structures, comprises a higher metamorphic thermal gradient than M 1 (transition low-P and medium-P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post-dating D 1 /M 1 by more than 20 Ma. D 2 -related fabrics are refolded by D 3 , which in its turn, is followed by dextral-normal shearing along the basal shear zone of both units at c. 297 Ma. In the western Mont Peyroux and Faug eres units, D 2 /M 2 is largely synchronous with shearing along the southern dome margin between c. 311 and 303 Ma, facilitating the emplacement of the gneiss dome into the upper crust. D 2 /M 2 also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306 Ma, and a related low-P/high-T metamorphism at c. 308 Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D 2 fabrics and the M 2 metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone-epizone boundary to the diagenetic zone. The related zonation of the pre-D 3 metamorphic field gradients paralleled the dome. These observations indicate that D 2 / M 2 is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D 2 /M 2 . Based on integration of these findings with regional geological constraints, a two-stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300 Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull-apart-like geometry. The second stage from 300 Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by northdipping detachments and hangingwall basin formation along the northeastern dome margin.

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Analysis of offshore seismic lines suggests that a strong relationship exists between tectonic structures and fluid migration in accretionary prisms. However, only few field analogues of plumbing systems and their tectonic frameworks have been investigated in detail until now. The uplifted accretionary prism of the Hikurangi Margin (North Island, New Zealand) exposes early to late Miocene mudrocks in coastal cliffs of Cape Turnagain and in the Akitio syncline, south-east of the Pongaroa city. These outcrops display tubular carbonate concretions corresponding to complex subsurface plumbing networks of paleo-seeps within Miocene trench slope basins. We present here, new results on the spatial distribution of these tubular carbonate concretions, with particular attention to their relation to tectonic structures. In the Pongaroa area, tubular carbonate concretions in lower Miocene mudrocks occur along a N-S trend, while in middle Miocene strata they occur along a NNE-SSW direction. The N-S trend parallels a major fault zone (i.e. the Breakdown fault zone), which separates two wide synclines, the Waihoki and the Akitio synclines. During the Early-Middle Miocene, the Breakdown fault zone controlled the evolution of the Akitio trench slope basin constituting its western edge. The NNE-SSW strike parallels the axis of the Akitio syncline and is also parallel to the present-day subduction front. Our results therefore show that tubular concretions are parallel to post-Middle Miocene second order folding and thrusting in the northeastern limb of the Akitio syncline. In the Cape Turnagain area, tubular concretions occur in the western limb of the Cape Turnagain syncline, in the footwall of the major seaward-verging Cape Turnagain fault. This suggests that fluid migrations may occur not only in the crests of anticlines, as observed offshore for present-day plumbing system of cold seeps, but also in the footwalls of thrust faults. All these observations show that the spatial distribution of tubular concretions is controlled by regional tectonic structures with paleo-fluid migrations related to major deformation episodes of the accretionary prism. Thus, we distinguish three episodes events that likely triggered fluid migration leading to the formation of the tubular concretions: (1) In the Early Miocene, shortly after the onset of development of the Akitio trench slope basin, on its inner (western) edge; (2) During the late Middle Miocene, during an extensional deformation episode on the western limb of the Akitio trench slope basin; (3) At the end of the Late Miocene, during a second major shortening period at the footwall of major thrust fault, such as in the Cape Turnagain area.

The illite spectral maturity (ISM) method uses short-wavelength infrared refl ectance spectroscopy (SWIR) to measure K-white mica (KWM) physicochemistry within very low-grade metamorphic pelites. The three ISM measures used in this study parameterize KWM absorption features at 1900 nm and 2200 nm in terms of area, depth, and asymmetry. Through comparison with the powder X-ray diffraction (XRD)−derived Kübler index, we demonstrate that ISM differentiates anchizonal and epizonal from diagenetic domains in very low-grade pelites. The wavelength (wvl) of the 2200 nm absorption feature (2200wvl) provides a measure of the celadonite substitution in KWM. It shows a linear correlation (R 2 = 0.85) with the KWM b cell dimension (as determined by powder XRD), and can be used to differentiate the metamorphic pressure facies and related metamorphic thermal gradients in pelites of greenschist facies and anchizonal metamorphic grade. The boundaries between low/medium-pressure facies and medium/high-pressure facies series can be defi ned at 2204 and 2220 nm, respectively. In addition to their use as laboratory-based techniques, both ISM and 2200wvl show potential for remote sensing studies.on April 5, 2015 geology.gsapubs.org Downloaded from