The Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (<0·3 Ma) emplacement of over 95% of the preserved magmatic rocks. In particular, new and previously published data for the Lower to Upper unit samples yielded indistinguishable 40Ar/39Ar (mean age = 201·2 ± 0·8 Ma) and U–Pb ages (201·57 ± 0·04 Ma), suggesting emplacement coincident with the main phase of the end-Triassic biotic turnover (c.201·5 to 201·3 Ma). Eruptions are suggested to have been pulsed with rates in excess of 10 km3/year during five main volcanic pulses, each pulse possibly lasting only a few centuries. Such high eruption rates reinforce the likelihood that CAMP magmatism triggered the end-Triassic climate change and mass extinction. Only the Recurrent unit may have been younger but by no more than 1 Ma. Whole-rock and mineral geochemistry constrain the petrogenesis of the CAMP basalts. The Moroccan magmas evolved in mid-crustal reservoirs (7–20 km deep) where most of the differentiation occurred. However, a previous stage of crystallization probably occurred at even greater depths. The four units cannot be linked by closed-system fractional crystallization processes, but require distinct parental magmas and/or distinct crustal assimilation processes. EC-AFC modeling shows that limited crustal assimilation (maximum c.5–8% assimilation of e.g. Eburnean or Pan-African granites) could explain some, but not all the observed geochemical variations. Intermediate unit magmas are apparently the most contaminated and may have been derived from parental magmas similar to the Upper basalts (as attested by indistinguishable trace element contents in the augites analysed for these units). Chemical differences between Central High Atlas and Middle Atlas samples in the Intermediate unit could be explained by distinct crustal contaminants (lower crustal rocks or Pan-African granites for the former and Eburnean granites for the latter). The CAMP units in Morocco are likely derived from 5–10% melting of enriched peridotite sources. The differences observed in REE ratios for the four units are attributed to variations in both source mineralogy and melting degree. In particular, the Lower basalts require a garnet peridotite source, while the Upper basalts were probably formed from a shallower melting region straddling the garnet–spinel transition. Recurrent basalts instead are relatively shallow-level melts generated mainly from spinel peridotites. Sr–Nd–Pb–Os isotopic ratios in the CAMP units from Morocco are similar to those of other CAMP sub-provinces and suggest a significant enrichment of the mantle-source regions by subducted crustal components. The enriched signature is attributed to involvement of about 5–10% recycled crustal materials introduced into an ambient depleted or PREMA-type mantle, while involvement of mantle-plume components like those sampled by present-day Central Atlantic Ocean Island Basalts (OIB, e.g. Cape Verde and Canary Islands) is not supported by the observed compositions. Only Recurrent basalts may possibly reflect a Central Atlantic plume-like signature similar to the Common or FOZO components.

Radial and azimuthal anisotropy in seismic wave speeds have long been observed using surface waves and are believed to be controlled by deformation within the Earth's crust and uppermost mantle. Although radial and azimuthal anisotropy reflect important aspects of anisotropic media, few studies have tried to interpret them jointly. We describe a method of inversion that interprets simultaneous observations of radial and azimuthal anisotropy under the assumption of a hexagonally symmetric elastic tensor with a tilted symmetry axis defined by dip and strike angles. We show that observations of radial anisotropy and the 2ψ component of azimuthal anisotropy for Rayleigh waves obtained using USArray data in the western United States can be fit well under this assumption. Our inferences occur within the framework of a Bayesian Monte Carlo inversion, which yields a posterior distribution that reflects both variances of and covariances between all model variables, and divide into theoretical and observational results. Principal theoretical results include the following: (1) There are two distinct groups of models (Group 1, Group 2) in the posterior distribution in which the strike angle of anisotropy in the crust (defined by the intersection of the foliation plane with Earth's surface) is approximately orthogonal between the two sets. (2) The Rayleigh wave fast axis directions are orthogonal to the strike angle in the geologically preferred group of models in which anisotropy is strongly non-elliptical. (3) The estimated dip angle may be interpreted in two ways: as a measure of the actual dip of the foliation of anisotropic material within the crust, or as a proxy for another non-geometric variable, most likely a measure of the deviation from hexagonal symmetry of the medium. The principal observational results include the following: (1) Inherent S-wave anisotropy (γ ) is fairly homogeneous vertically across the crust, on average, and spatially across the western United States. (2) Averaging over the region of study and in depth, γ in the crust is approximately 4.1 ± 2 per cent. γ in the crust is approximately the same in the two groups of models. (3) Dip angles in the two groups of models show similar spatial variability and display geological coherence. (4) Tilting the symmetry axis of an anisotropic medium produces apparent radial and apparent azimuthal anisotropies that are both smaller in amplitude than the inherent anisotropy of the medium, which means that most previous studies have probably underestimated the strength of anisotropy.

Rocks in the continental crust are long lived and have the potential to record a wide span of tectonic history in rock fabric. Mapping rock fabric in situ at depth requires the application of seismic methods. Below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations influenced by deformation processes. Interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor. We compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements, including electron backscatter diffraction and ultrasound, on crustal rocks. The majority of samples developed fabric at pressures corresponding to middle to lower crustal condition. Tensor symmetry is a function of mineral modal composition, with mica‐rich samples trending toward hexagonal symmetry, amphibole‐rich samples trending toward an increased orthorhombic symmetry component, and quartz‐feldspar‐rich samples showing a larger component of lower symmetries. Seventy‐seven percent of samples have a best fit hexagonal tensor with slow‐axis symmetry, as opposed to mantle deformation fabric that usually has fast‐axis symmetry. The best fit hexagonal approximation for crustal tensors is not elliptical but deviates systematically from elliptical symmetry with increasing anisotropy, an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations. We present empirical linear relationships between anisotropy and ellipticity for crustal rocks. The maximum out‐of‐plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for nonelliptical symmetry. The elliptical assumption results in a bias of up to 1.4 times true anisotropy.

[1] New electron backscatter diffraction measurements show that the Papua New Guinea (PNG) ultrahigh-pressure (UHP) terrane is dominated by rocks with weakly oriented quartz and feldspar and less abundant strongly oriented hornblende, clinopyroxene, and mica. Velocities measured at high pressures (600 MPa) show that V P is 5.8-6.3 km/s for gneiss samples, 6.5-7.7 km/s for amphibolite, and 7.7-8.2 km/s for eclogite and V S is 3.4-3.9 km/s for gneiss, 4.0-4.4 km/s for amphibolite, and 4.5-4.6 km/s for eclogite. Velocities and anisotropies calculated from mineral crystal preferred orientations (CPOs) are equivalent to within 5% of the measured values. The highest seismic anisotropy for the PNG terrane is in amphibolite at 8% and 7% for V P and V S , respectively. Calculations of seismic velocities at depth based on predicted mineral assemblages indicate that the exhuming UHP terrane is of dominantly mafic composition below ∼20 km depth. Anisotropy in the PNG terrane is expected to be quite low and is controlled by the orientation of the foliation. If observable, changes in anisotropy across the exhuming body may be used to differentiate among the different proposed mechanisms of UHP exhumation.

[1] Lattice preferred orientations (LPO) of quartz in gneiss domes of the D'Entrecasteaux Islands, Woodlark Rift shed insight into exhumation of the world's youngest (~5-7 Ma) eclogite-bearing terrane at cm/yr rates. We focus on deformation that affected the terrane as it transited between lower crustal depths and the surface, including: (1) grain-scale deformation mechanisms; and (2) style of flow and mode of emplacement of the domes. Electron-backscatter diffraction was used to analyze microstructure and LPOs of 37 quartzofeldspathic gneiss samples that enclose meter-scale mafic blocks preserving original eclogite-facies assemblages. During exhumation of the ultrahigh-pressure (UHP) terrane, gneisses were retrogressed in the amphibolite facies at lower crustal depths. The LPOs change from dome cores to carapaces, consistent with decreasing deformational temperatures. In the relatively chilled outer carapaces of the domes, the quartz LPOs consist of mostly crossed-girdle [c]-axis patterns, with some cleft-girdle and small-circle LPOs, and record dislocation creep accommodated by mixed-< a > slip. In the cores of the migmatitic domes, a chessboard pattern of subgrains is common, and quartz LPOs primarily record prism-[c] slip, probably at >630 C. Other microstructures indicate recovery by high-temperature grain-boundary migration. Grain-boundary mobility was anisotropic, leading to strong grain-shape fabrics oblique to foliation, but not obviously relatable to shear sense. Evidence for melt-present deformation is abundant, and microstructures (including partially dissolved feldspar grains) indicate some deformation by fluid-assisted grain-boundary diffusion creep. LPOs in carapace rocks are symmetrical, recording flow that was dominantly coaxial. We interpret the gneiss domes to have been emplaced into the rift as partially molten diapirs.Components: 18,942 words, 13 figures, 1 table.

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[1] Paleomagnetic studies of the 91 Ma Ecstall pluton and other Cretaceous plutons of British Columbia imply large northward tectonic movements (>2000 km) may have occurred during the tectonic evolution of western North America. However, more recent studies have shown that the eastern edge of the Ecstall pluton experienced considerable mineralogical changes as younger Eocene plutons, such as the ∼58 Ma Quottoon Pluton, were emplaced along its margins. We investigated changes in the rock magnetic properties associated with this reheating event by examining isolated grains of intergrown ilmenite and hematite, the primary paleomagnetic recorder in the Ecstall pluton. Measurements of hysteresis properties, low-temperature remanence, and room temperature isothermal remanent magnetization acquisition and observations from magnetic force microscopy and off-axis electron holography indicate that samples fall into three groups. The groups are defined by the presence of mineral microstructures that are related to distance from the Quotoon plutonic complex. The two groups closest to the Quottoon Pluton contain magnetite within hematite and ilmenite lamellae. Reheating of the Ecstall pluton led to an increase in coercivity and magnetization, as well as to development of mixed phase hysteresis. These results indicate that shallow paleomagnetic directions from the western Ecstall pluton are not affected by reheating and are therefore likely to record original field conditions at the time of pluton emplacement. In the absence of structural deformation, these shallow inclinations are consistent with large-scale northward translation suggested by the Baja-British Columbia hypothesis.