Subduction and exhumation slip accommodation at depths of 10–80 km inferred from field geology of exhumed rocks: Evidence for temporal-spatial localization of slip

ABSTRACT Field relationships in the Franciscan Complex of California suggest localization of subduction slip in narrow zones (≤300 m thick) at the depths of ∼10–80 km. Accretionary and non-accretionary subduction slip over the ca. 150 Ma of Franciscan history was accommodated across the structural thickness of the complex (maximum of ∼30 km). During accretion of a specific unit (<5 Ma), subduction slip (accretionary subduction slip) deformed the full thickness of the accreting unit (≤5 km), primarily on discrete faults of <20 m in thickness, with the remainder accommodated by penetrative deformation. Some faults accommodating accretionary subduction slip formed anastomosing zones ≤200 m thick that resulted in block-in-matrix (tectonic mélange) relationships but did not emplace exotic blocks. Mélange horizons with exotic blocks range in thickness from 0.5 m to 1 km. These apparently formed by sedimentary processes as part of the trench fill prior to subsequent deformation during subduction-accretion. Accretionary subduction slip was localized within some of these mélanges in zones ≤300 m thick. Such deformation obscured primary sedimentary textures. Non-accretionary subduction faults separate units accreted at different times, but these <100-m-thick fault zones capture a small fraction of associated subduction slip because of footwall subduction and likely removal of hanging wall by subduction erosion. Most exhumation was accommodated by discrete faults ≤30 m thick. Structural, geochronologic, and plate motion data suggest that of the ∼13,000 km of subduction during the ca. 150 Ma assembly of the Franciscan Complex, ∼2000 km was associated with accretion.

Cryo-EM with sub–1 Å specimen movement

Most information loss in cryogenic electron microscopy (cryo-EM) stems from particle movement during imaging, which remains poorly understood. We show that this movement is caused by buckling and subsequent deformation of the suspended ice, with a threshold that depends directly on the shape of the frozen water layer set by the support foil. We describe a specimen support design that eliminates buckling and reduces electron beam–induced particle movement to less than 1 angstrom. The design allows precise foil tracking during imaging with high-speed detectors, thereby lessening demands on cryostage precision and stability. It includes a maximal density of holes, which increases throughput in automated cryo-EM without degrading data quality. Movement-free imaging allows extrapolation to a three-dimensional map of the specimen at zero electron exposure, before the onset of radiation damage.

REGULARITIES OF DISTRIBUTION OF WIRE DEFORMATION IN MULTILAYERED STRAND AT CIRCULAR CALIBRATION COMPRESSION

The mechanism of plastic crimping of the strand has been identified and justified, as the process of formation of arches: a strong arch of wires, the appearance of each leads to a change in stressed state of the strand at reduction stages. It was established that before appearance of the first arch, wires of the outer layer and the central wire are the most priority to deformation, with the initial absence of side contacts. After appearance of each arch, stresses in wires of the arch layer become predominantly compressive, which temporarily prevents the given layer from actively deforming, up to the formation of arches in all other layers of the strand. After formation of all arches, wires of the upper layer again become the most priority to deformation. Central wire of the strand is overstrained in relation to all other wire strands at all stages of compression. The developed technique allows analyzing the degree of working out of each wire of a lock at a certain amount of reduction. It reflects the features of a multilayered strand deformation: sharp increase in width of the newly appeared contact at almost constant reduction; arches formation; non-simultaneous occurrence of new contacts in layers of strands due to the geometry of the strand and direction of the wires displacement. Application of the proposed technique allows to make rational designs of strands and ropes subjected to small and medium circular plastic crimping, as well as to determine the necessary amount of compression of strands and ropes of a particular design, proceeding from the conditions for retaining the flexibility of the rope and forming the required contact geometry of the wires. It was found that for strands with a diameter of 7.68 mm in the construction of 1 + 5 + 5/5 + 10, the most uniform development of the strand and the contacts is ensured during the reduction in the range of 3.74 < Q < 7.06 %. Intensive filling of the gaps in the strand begins at Q = 7.06 %, which determines the subsequent deformation as the limiting for the ropes working on bending both for performance characteristics and for the conditions of operation of the round caliber of a roller die.

Structure of the Granitic Pegmatite Field of the Northern Coast of Portugal—Inner Pegmatite Structures and Mineralogical Fabrics

On the coastline of Northern Portugal, metamorphic formations and pegmatites were the subject of structural analysis with the main goal of understanding Variscan kinematics and related pegmatite intrusion. This study also aims to discriminate, select and characterize relevant aspects of the structure and the paragenesis of pegmatites, well exposed as a result of coastal erosion, justifying its inclusion in the geological heritage of the Northern coast of Portugal. The pegmatite bodies show distinctive internal and external structures that are attributable to different modes of emplacement and subsequent deformation. The pegmatitic implantation in the areas of Moledo and Afife occurs in an intragneissic and perigranitic environment, for the first area, and perigneissic and perigranitic environment, for the second. In Pedras Ruivas predominates the implantation into an exo-gneissic to exo-granitic domain. The Moledo veins show evidence of multiphase open/filling, revealing positions, shapes, attitudes, sizes and internal structures that change as a function of the host lithology and host structure, but mainly due to the dilation and the cycles number of local telescoping. The structural analysis of the pegmatite bodies allows the deduction of a local fulcrum of expansion that hypothetically overlaps a hidden stock of parental granite. In Afife and Pedras Ruivas, some pegmatitic lenses are specialized and mineralized in Li, Cs and Ta, with spodumene and tantalite ± cassiterite. Spodumene occurs as giant crystals, centimetric to pluri-decimetric in length, which mark very clearly the structures of in situ or in flow crystallization inside the pegmatites (primary structures) and also the secondary structures resulting from deformation. The geometric analysis of fabrics helps the individualization of well-defined stages of progressive evolution of the deformation of the pegmatites, allowing its correlation with major D2–D3 episodes of regional Variscan deformation.