ALBERT

Description: Anomalous volcanism and tectonics between near-ridge mantle plumes and mid-ocean ridges provide important insights into the mechanics of plume-lithosphere interaction. We present new observations and analysis of multibeam, side scan sonar, sub-bottom chirp, and total magnetic field data collected during the R/V Melville FLAMINGO cruise (MV1007; May–June, 2010) to the Northern Galápagos Volcanic Province (NGVP), the region between the Galápagos Archipelago and the Galápagos Spreading Center (GSC) on the Nazca Plate, and to the region east of the Galápagos Transform Fault (GTF) on the Cocos Plate. The NGVP exhibits pervasive off-axis volcanism related to the nearby Galápagos hot spot, which has dominated the tectonic evolution of the region. Observations indicate that ∼94% of the excess volcanism in our survey area occurs on the Nazca Plate in three volcanic lineaments. Identified faults in the NGVP are consistent with normal ridge spreading except for those within a ∼60 km wide swath of transform-oblique faults centered on the GTF. These transform-oblique faults are sub-parallel to the elongation direction of larger lineament volcanoes, suggesting that lineament formation is influenced by the lithospheric stress field. We evaluate current models for lineament formation using existing and new observations as well as numerical models of mantle upwelling and melting. The data support a model where the lithospheric stress field controls the location of volcanism along the lineaments while several processes likely supply melt to these eruptions. Synthetic magnetic models and an inversion for crustal magnetization are used to determine the tectonic history of the study area. Results are consistent with creation of the GTF by two southward ridge jumps, part of a series of jumps that have maintained a plume-ridge separation distance of 145 km to 215 km since ∼5 Ma.

Description: The origin of the Bermuda swell and volcanism remains enigmatic. The lack of an associated time-progressive hotspot track and absence of present-day volcanic activity make it difficult to reconcile with a deep mantle plume model. We analyze shear wave splitting measurements to estimate mantle flow direction and receiver function stacks to place constraints on the mantle transition zone thermal structure. *KS phases exhibit well-resolved null arrivals (no splitting) beneath the swell over a range of back azimuths. We find that the 410 and 660 km discontinuities are 49 ± 5 km and 19 ± 5 km deeper than the global average, respectively, leading to a transition zone thickness that is 27 ± 4 km thinner than average. Together, an apparently isotropic upper mantle and a thinned mantle transition zone suggest that mantle flow is primarily vertical beneath the swell, consistent with the presence of hot, buoyant mantle at depth.

Description: Sandstone injectites form by up or down-section flow of a mobilized sand slurry through fractures in overlying rock. They act as reservoirs and high-permeability conduits through lower permeability rock in hydrocarbon systems. The Yellow Bank Creek Complex, Santa Cruz County, California is the largest known exposure of a sandstone injectite in the world. The complex contains granular textures that record processes of sand slurry flow, multiple pore fluids, and dewatering after emplacement. The injection was initially mobilized from a source containing both water and hydrocarbons. The water-sand slurry reached emplacement depth first, due to lower fluid viscosity. As the sand slurry emplaced, the transition from slurry flow to pore water percolation occurred. This transition resulted in preferred flow channels ∼6 mm wide in which sand grains were weakly aligned (laminae). The hydrocarbon-sand slurry intruded the dewatering sands and locally deformed the laminae. Compaction of the injectite deposit and pore fluid escape caused spaced compaction bands and dewatering pipes which created convolutions of the laminae. The hydrocarbon-rich sand slurry is preserved today as dolomite-cemented sand with oil inclusions. The laminae in this injectite are easily detected due to preferential iron oxide-cementation of the well-aligned sand laminae, and lack of cement in the alternating laminae. Subtle textures like these may develop during sand flow and be present but difficult to detect in other settings. They may explain permeability anisotropy in other sand deposits.

Description: Magmatism strongly influences continental rift development, yet the mechanism, distribution, and timescales on which melt is emplaced and erupted through the shallow crust are not well characterized. The Main Ethiopian Rift (MER) has experienced significant volcanism, and the mantle beneath is characterized by high temperatures and partial melt. Despite its magma-rich geological record, only one eruption has been historically recorded, and no dedicated monitoring networks exist. Consequently, the present-day magmatic processes in the region remain poorly documented, and the associated hazards are neglected. We use satellite-based interferometric synthetic aperture radar observations to demonstrate that significant deformation has occurring at four volcanic edifices in the MER (Alutu, Corbetti, Bora, and Haledebi) from 1993 to 2010. This raises the number of volcanoes known to be deforming in East Africa beyond 12, comparable to many subduction arcs despite the smaller number of recorded eruptions. The largest displacements are at Alutu volcano, the site of a geothermal plant, which showed two pulses of rapid inflation (10–15 cm) in 2004 and 2008 separated by gradual subsidence. Our observations indicate a shallow (

Description: We present an efficient method for high-volume heavy mineral separation from clay-rich rocks using an ultrasonic probe. The ultrasonic clay separator (UCS) is an easily constructed device that allows for the recovery of high-density minerals, as small as 10 microns, with a minimum of sample preparation. Heavy mineral recovery from clay-rich material with the UCS yields a greater number of small (〈100 micron) grains and approximately double the amount of material from that of gravity settling and decanting. Despite development with heavy mineral recovery in mind, the UCS should suitable for recovering small grain size geologic materials from flocculating clay-rich material.

Description: In 2003 two wide-angle reflection/refraction seismic transects were acquired in the Variscan Belt of SW Iberia. The approximately 250 and 300 km long, dense trace spacing transects revealed clear S wave arrivals in the shot gathers recorded by vertical component sensors in both transects. First S wave arrivals (Sg) and Moho reflections (SmS) are the most prominent phases that can be correlated from shot to shot. Sg is observed up to relatively large offsets and constrains the upper and middle crust S wave velocities. The SmS is seen from offset 0 (18 s twtt) to 150 km offset, where it intercepts first S wave arrivals (Sg). The upper mantle refracted phase (Sn) is difficult to recognize, although PmS/SmP converted phases can be identified. Using a 2-D ray tracing approach, two S wave velocity models for the crust of SW Iberia were obtained. These S wave velocity models complement the previous P wave velocity models and provide us with relatively well resolved Poisson's ratio crustal sections for SW Iberia. The resulting Poisson's ratio models present differences between tectonic zones at upper and middle crustal depths, thus supporting the existence of different tectonic zones prior to the Variscan collision. The most noteworthy feature is the high Poisson's ratio value (over 0.28) coincident with high P wave velocity areas (over 6.8 km/s) at midcrustal depths. In order to constrain the possible crustal composition, P wave velocities and Poisson's ratios have been compared with published laboratory measurements on different crustal rock types. This comparison indicates that the high P wave velocity and Poisson's ratios are compatible with a mixture of mafic to ultramafic rock types alternating with felsic ones. This result is consistent with the existence of mafic layered bodies in the middle crust, in the same way that has been suggested by previous works in this area.

Description: To investigate the coupling between stress-driven melt segregation and reaction-driven melt infiltration, we performed static annealing and torsional deformation experiments on couples constructed from a melt-rich basaltic source and melt-poor olivine + orthopyroxene + chromite + basalt sink. In this study (the second of two companion papers), the melt source is almost completely molten with $\phi$ = 0.8–0.9, well above the rheologically critical melt fraction. We deformed two geometries of source/sink couples with the source either as the core or the outer ring of a cylinder. Our experiments demonstrate (1) that deformation significantly enhances the rate of melt infiltration from the source into the sink and (2) that combined reaction and deformation promote greater strain localization than does deformation alone. Deformation and melt infiltration enhance disaggregation at the interface of the source and sink. The implication of these coupled interactions is that deformation and melt-rock reaction may enhance the corrosion (or ‘stoping’) rates of the wall rocks of magma chambers and intrusions in the crust and upper mantle.

Description: We present results from experiments designed to investigate the interactions between stress-driven melt segregation and reaction-enhanced melt infiltration, two mechanisms that have previously been studied independently of each other. A melt source (with a melt fraction below the rheologically critical melt fraction) in which the basaltic melt is either orthopyroxene-saturated or orthopyroxene-undersaturated was coupled with a nominally melt-free olivine + orthopyroxene sink in two cylindrical configurations deformed in torsion. As melt migrates from the source to the sink in samples with an orthopyroxene-undersaturated melt source, the basalt dissolves orthopyroxene and precipitates olivine. The local increase in melt fraction during this process increases permeability and enhances melt infiltration. As melt migrates from the source to the sink in samples with an orthopyroxene-saturated melt source, the reaction described above does not occur. These samples display modest infiltration associated with combined surface tension-driven flow and mechanical segregation. Our experiments demonstrate (1) that combined reaction and deformation leads to greater infiltration of melt than does either mechanism alone, and (2) that melt segregation associated with deformation is an effective way to create perturbations in melt fraction along the source-sink interface, which act as nucleation points for reaction-enhanced infiltration in the experiments.