ALBERT

Description: A rock salt-lamprophyre dyke contact zone (sub-vertical, NE-SW strike) was investigated for its petrographic, mechanic and physical properties by means of anisotropy of magnetic susceptibility (AMS) and rock magnetic properties, coupled with quantitative microstructural analysis and thermal mathematical modelling. The quantitative microstructural analysis of halite texture and solid inclusions revealed good spatial correlation with AMS and halite fabrics. The fabrics of both lamprophyre and rock salt record the magmatic intrusion, "plastic" flow and regional deformation (characterized by a NW-SE trending steep foliation). AMS and microstructural analysis revealed two deformation fabrics in the rock salt: (1) the deformation fabrics in rock salt on the NW side of the dyke are associated with high temperature and high fluid activity attributed to the dyke emplacement; (2) On the opposite side of the dyke, the emplacement-related fabric is reworked by localized tectonic deformation. The paleomagnetic results suggest significant rotation of the whole dyke, probably during the diapir ascent and/or the regional Tertiary to Quaternary deformation.

Description: This study revisits the palaeomagnetism of the Sines massif (~76 Ma) in the southwestern Iberian Margin (Portugal). The palaeomagnetic analysis was complemented by a comprehensive study of the magnetic mineralogy by means of rock magnetic measurements and petrographic observations. The overall dispersion of palaeomagnetic directions (declination ranging between ~N0° and ~N50°) and their migration observed during stepwise demagnetizations have revealed the superposition of remanence components. We interpret this complex palaeomagnetic behaviour as related to the regional hydrothermalism associated with the last stages of Late Cretaceous magmatic activity. This environment favoured mineralogical alteration and a partial chemical remagnetization, giving in most samples a composite magnetization, which has been erroneously interpreted as the primary one in a previous study, then leading to a questionable model for Cretaceous Iberia rotation. Nonetheless, for some samples a single component has been isolated. Interesting rock magnetic properties and microscopic observations point to a well-preserved magnetic mineralogy for these samples, with magnetite clearly of primary origin. The associated ChRM mean direction ( D / I  = 3.9°/46.5°, α 95  = 1.7°, N  = 31 samples) then represents the true primary magnetization of the Sines massif. This new palaeomagnetic direction and the corresponding palaeomagnetic pole (long = 332.0°, lat = –79.5°, A 95  = 1.7°) agrees with those from the other palaeomagnetic works for the same period and region (e.g. the Sintra and Monchique massifs), yielding a lack of significant rotation of Iberia relative to stable Europe since the uppermost Late Cretaceous (Campanian–Maastrichtian).

Description: We report evidence for ongoing lateral slump of part of the southeastern flank of the Pico volcanic ridge in the Azores. Data from a high-resolution digital elevation model, field work, GPS, and radar interferometry show that: (1) the slumping sector is several cubic kilometers in size; (2) the structure involves several curved scars with normal fault kinematics; (3) the central part is undergoing little horizontal displacement toward the southeast (1.6 ± 1.3 mm/yr), but significant downward movement (5–12 mm/yr); and (4) the outer part of the southeastern flank of Pico is subsiding faster than the inner parts; this likely reflects recent individualization of a steep seaward-dipping fault in the moving mass. The slump shares similarities with active slumps recognized elsewhere, although the studied area may represent only the proximal part of a much larger complex potentially affecting the deep submarine base of the island. Displacement of the subaerial part of the southeastern flank of Pico seems to be accommodated by the movement and rotation of large blocks along listric normal faults.

Description: Subduction initiation at straight passive margins can be investigated with two-dimensional (2-D) numerical models, because the geometry is purely cylindrical. However, on Earth, straight margins rarely occur. The construction of 3-D models is therefore critical in the modeling of spontaneous subduction initiation at realistic, curved passive margins. Here we report on the results obtained from gravitationally driven, 3-D thermomechanical numerical models using a visco-plastic rheology and a passive margin with a single curved section in the middle. The models show that the curvature angle β can control subduction initiation: the greater β is, the more difficult subduction initiation becomes. The 3-D thermomechanical models provide an in-depth physical understanding of the processes. Specifically, we find that pressure gradients, arising from density differences between oceanic and continental rocks, drive subduction initiation, and strongly influence the timing. The main difference between straight (cylindrical) and curved margins is that the orientation of the pressure gradient in 3-D is no longer constant, thus producing a horizontal, along-margin component of flow. We thus conclude that the reason for the impedance of subduction initiation is the result of partitioning of the vertical velocity component into a horizontal component, which therefore decreases the effective slab pull. We infer that, although favorable for subduction initiation in a 2-D model, because the estimated force balance is adequate, the pronounced curvature in the southeast Brazilian margin is a likely explanation why subduction initiation is hampered there.

Description: The concept of an ‘Azores mantle plume’ has been widely debated, and the existence of an Azores hotspot questioned. In an effort to shed new light on this controversy, we present He isotope and major, trace and volatile element compositions for basaltic scoriae from five monogenetic cones emplaced along the fissure zone of Pico Island, the youngest island of the Azores archipelago. The bulk scoriae and lavas are moderately alkaline basalts, and their He isotope ratios, determined on olivine crystals, vary between 10·2 and 11·1 ± 0·1 R a . In contrast, melt inclusions hosted in olivine (Fo 76–83·5 ) span a large range of compositions (K 2 O = 0·7–1·7 wt %; Ce = 32–65 ppm; Nb = 21–94 ppm), which extends the compositional field of lavas erupted along the Pico fissure zone. This chemical evolution is predominantly controlled by polybaric fractional crystallization. Most melt inclusions share similar enrichments in large ion lithophile and light rare earth elements, and trace element ratios (La/Sm, La/Yb, Sr/Nd, Ta/Th, Zr/Y) with their bulk-rocks. Only a few of them differ in their lower contents of incompatible elements and La/Sm, Li/Ta and Na/K ratios, a feature that is ascribed to distinct conditions of melting. As a whole, the melt inclusions preserve high and variable volatile contents, and contain up to 1·8–2·0 wt % of H 2 O and 0·4 wt % of CO 2 . The total fluid pressures, retrieved from the dissolved CO 2 and H 2 O concentrations, and the P CO2 from fluid inclusions, indicate magma ponding and crystallization at the crust–mantle boundary (ca. 18 km deep). The H 2 O/Cl and H 2 O/Ce ratios in the inferred parental undegassed basalts of the Pico fissure zone average 0·036 ± 0·006 and 259 ± 21, respectively. The latter value is significantly higher than that reported for typical mid-ocean ridge basalts from the southern Mid-Atlantic Ridge, but is similar to published ratios for submarine undegassed basalts from the Azores platform. Combining the calculated compositions of Pico primary magmas formed by low degrees of melting with recent geophysical data for the Azores, we propose a model for Azores magma generation involving the decompression melting of a water-enriched mantle domain (H 2 O = 680–570 ppm) with an estimated temperature excess of ≤120°C with respect to the Mid-Atlantic Ridge.