Michel de Saint Blanquat

Transpressional kinematics and magmatic arcs

Abstract Most continental magmatic arcs occur in obliquely convergent settings and display strike-slip movement within, or adjacent to, the magmatic arc, and contractional structures in the forearc and backarc regions. Thus, three-dimensional transpressional kinematics typifies many arc settings, both modern and ancient. Intrusions cause magma-facilitated strike-slip partitioning, even in cases where the relative angle of plate convergence is almost normal to the plate boundary. Transpressional systems are preferentially intruded by magmas because of the steep pressure gradients in vertical strike-slip shear zones and their ability to force magma upward. Both buoyancy and transpressional dynamics cause a component of magma overpressuring, which in turn expels granitic magma upward following the vertical pressure gradient. The tectonic and magmatic processes are linked in a positive feedback loop which facilitates the upward movement of magma. We propose a lithospheric-scale, three-dimensional model of transpressional arc settings. Strike-slip motion is partitioned into the magmatic arc settings because of the linear and margin-parallel trend of the vertical, lithospheric-scale weakness caused by ascending magma. The parallelism of contraction structures in the forearc and backarc regions is caused by mechanical coupling through the lower crust and upper lithospheric mantle. The displacement field of the basal layer of the arc system provides the boundary condition for the upper-crustal, strike-slip partitioned deformation.

Shape anisotropy versus magnetic interactions of magnetite grains: Experiments and application to AMS in granitic rocks

The magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains. Measurement of the low-field magnetic anisotropy of single multi-domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio. Interactions between two elongated grains were studied experimentally using two types of grain arrangement: an “aligned” configuration and a “side-by-side” configuration. For a distance between the grain centers equal to approximately twice the average grain size, the magnetic susceptibility and its anisotropy are enhanced in both configurations, and the direction of kmax, the easiest magnetization axis, is stable in the “aligned” configuration, whereas it rotates toward an orthogonal direction in the “side-by-side” configuration. Depending on the distribution of the interacting magnetite grains, magnetic interactions may therefore either increase the whole-rock anisotropy magnitude, or reduce it as in the given example of the granitic rocks from Madagascar.

Development of Magmatic to Solid-State Fabrics during Syntectonic Emplacement of the Mono Creek Granite, Sierra Nevada Batholith

Interpretation of rock fabric is a key objective in obtaining kinematic information about crustal dynamics. In order to constrain the connection between tectonism and magma emplacement, we have conducted a structural, microstructural, and magnetic fabric study of the Mono Creek Granite (MCG), one of the youngest plutons of the Sierra Nevada batholith, California (USA). In addition to field measurements, we have used the Anisotropy of Magnetic Susceptibility (AMS) to investigate granite fabrics. The magnetic susceptibility (K) of the MCG varies between 0.4 and 4×10−2, and the total anisotropy (P) reaches 1.6. Thermomagnetic measurements (not given), and reflected and optical microscopic observations indicate that Ti-poor multi-domain magnetite is the primary AMS carrier, accouting for the high K values. The AMS intensive parameters, which are difficult to interpret in ferromagnetic rocks, provide semi-quantitative information: K correlates with rock mineralogy, and P is a good indicator of the rock texture. The AMS directional data of the MCG are characterized by good clustering at each sampling station, and by a sigmoidal pattern of foliation and lineation. This pattern results from deformation in the Rosy Finch shear zone during syn-magmatic dextral shearing deformation event. The AMS data, combined with the microstructural study, indicate that dextral shearing occurred under magmatic, high-temperature solid-state, and low-temperature solid-state conditions. The geometrical, structural and temporal continuity between the magmatic and solid-state deformations strongly support a model of continuous shearing during emplacement of the MCG. Our study demonstrates that the fabric of the MCG retains a record of all the pluton construction, which is syntectonic. Therefore, within the MCG, the magmatic and solid-state fabrics are both “intrusion-induced” and “tectonic-induced”.

High-resolution imaging of the Pyrenees and Massif Central from the data of the PYROPE and IBERARRAY portable array deployments

The lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate.

Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth

Plutons have long been viewed as crystallized remnants of large magma reservoirs, a concept now challenged by high-precision geochronological data coupled with thermal models. Similarly, the classical view of silicic eruptions fed by long-lived magma reservoirs that slowly differentiate between mafic recharges is being questioned by petrological and geophysical studies. In both cases, a key and yet unresolved issue is the rate of magma transfer in the crust. Here, we use thermal analysis of magma transport to calculate the minimum rate of magma transfer through dikes. We find that unless the crust is exceptionally hot, the recharge of magma reservoirs requires a magma supply rate of at least ∼0.01 km 3 /yr, much higher than the long-term growth rate of plutons, which demonstrates unequivocally that igneous bodies must grow incrementally. This analysis argues also that magma reservoirs are short lived and erupt rapidly after being recharged by already-differentiated magma. These findings have significant implications for the monitoring of dormant volcanic systems and our ability to interpret geodetic surface signals related to incipient eruptions.

The normalised optimised anisotropic wavelet coefficient (NOAWC) Method: An Image processing tool for multi‐scale analysis of rock fabric

The 2D Anisotropic Wavelet Transform (2DAWT) is an image analysis tool which is able to decipher signals where information obtained from different scales are intermixed. Extended from the Optimised Anisotropic Wavelet Coefficient method (OAWC) of Ouillon et al. [1995], we present a method which discriminates the objects and groups of objects depending on their area, shape ratio, orientation and position. Illustrated in a synthetic example, we show that this method allows one to distinguish between different sub-populations of objects within a single phase, and quantify the anisotropies of shape, orientation and spatial distribution at different scales (objects, clusters of objects, alignments of objects or clusters). Applied to a natural rock sample (Sidobre granite, Montagne Noire, France), the 2DAWT has permitted us to detect and accurately characterise the different levels of mineral organisation, and thus, to contribute to the understanding of the physical processes, such as crystallisation, fluid migration, deformation, etc... responsible for such organisations.

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.

Succession of Permian and Mesozoic metasomatic events in the eastern Pyrenees with emphasis on the Trimouns talc–chlorite deposit

Abstract Recent studies proposing pre-orogenic mantle exhumation models have helped renew the interest of the geosciences community in the Pyrenees, which should be now interpreted as a hyper-extended passive margin before the convergence between Iberia and Eurasia occurred. Unresolved questions of the Pyrenean geology, as well as the understanding of the formation of hyper-extended passive margins, are how the crust was thinned, and when, where and how the crustal breakoff occurred. The study of the Variscan and pre-Variscan Pyrenean basement is thus critical to document and understand this Cretaceous crustal thinning. In order to specify the timing of Mesozoic metasomatism and the associated deformation in the pre-Mesozoic basement of the Pyrenees, we carried out a U–Th–Pb laser ablation ICP–MS study on a large panel of REE and titanium-rich minerals (titanite and rutile) from talc–chlorite ores from the eastern Pyrenees, with a special emphasis on the Trimouns deposit, the world’s largest talc quarry. Our results suggest that the Trimouns talc formation was restricted to the upper Aptian–Cenomanian time, while the talc and chlorite formation in the eastern Pyrenees occurred during several distinct Permian, Jurassic and Cretaceous episodes. These results give strong constraints on the tectonic setting of the Pyrenean domain during the transition between the Variscan and Alpine orogenic cycles, and particularly on when and how the upper crust was thinned before the crustal breakoff and the final mantle exhumation.

Homogeneity of Granite Fabrics at the Metre and Dekametre Scales

Magnetic fabrics of biotite-bearing granites were systematically determined, at the metre and dekametre scales, in three plutons previously studied for their overall magnetic structures, in order to characterize the spatial homogeneity and variability of the fabrics. These granites, with typically magmatic microstructures, have different mean magnetic anisotropies (P%): Sidobre (southwest Massif Central of France; P%=2.3), Bassies and Trois-Seigneurs (French Pyrenees; respectively P%= 3.3 and P%=5.6). In each site, two grids of 50 oriented specimens each, respectively one and ten metres apart from each other, have been studied in detail. The directional data, especially the lineations, strongly cluster around their means and have similar orientations on both scales. In map view, the fluctuations of these data are generally gradual and tend to form sigmoids but no clearly defined pattern, such as a C/S system was observed. The magnetic anisotropy and the bulk susceptibility are homogeneous as a whole, and display spatial organizations with no simple relationships with the structures. These results confirm, however, the validity of the homogeneous structural patterns obtained from entire plutons.

Structural inheritance in the Central Pyrenees: the Variscan to Alpine tectonometamorphic evolution of the Axial Zone

Estimating structural inheritance in orogens is critical to understanding the manner in which plate convergence is accommodated. The Pyrenean belt, which developed in Late Cretaceous to Paleogene times, was affected by Cretaceous rifting and Variscan orogeny. Here we combine a structural and petrological study of the Axial Zone in the Central Pyrenees to discuss structural inheritance. Low-grade Paleozoic metasedimentary rocks were affected by a Variscan transpressional event that produced successively: (1) regional-scale folds; (2) isoclinal folding, steep pervasive cleavage and vertical stretching, synchronous with peak metamorphism; (3) strain localization into ductile reverse shear zones. The persistence of a relatively flat envelope for the Paleozoic sedimentary pile and Variscan isograds, and the absence of Alpine crustal-scale faults in the core of the Axial Zone, suggests that the Axial Zone constitutes a large Variscan structural unit preserved during Pyrenean orogeny. This configuration seems to be inherited from Cretaceous rifting, which led to the individualization of a large continental block (future Axial Zone) against a hyper-extended domain along the North Pyrenean Fault zone. This study places the currently prevailing model of Pyrenean belt deformation in a new perspective and has important implications for crustal evolution and inheritance in mountain belts more generally. Supplementary materials: Raman spectroscopy of carbonaceous materials data and a figure illustrating peak-fitting of the Raman spectrum of carbonaceous material and Raman spectra from the various samples of the Pallaresa cross-section are available at https://doi.org/10.6084/m9.figshare.c.3906247