Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough

Doleritic sill complexes, which are an important component of volcanic continental margins, can be imaged using 3D seismic reflection data. This allows unprecedented access to the complete 3D geometry of the bodies and an opportunity to test classic sill emplacement models. The doleritic sills associated with basaltic volcanism in the North Rockall Trough occur in two forms. Radially symmetrical sill complexes consist of a saucer-like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined, commonly ragged, outer rim. Bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano. With an elongate, concave upwards, trough-like geometry bilaterally symmetrical sills climb away from the magma source from which they originate. Both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge, producing hybrid sill complexes. Radial sill complexes consist of a series of radiating primary flow units. With dimensions up to 3 km, each primary flow unit rises from the inner saucer and is fed by primary magma tube. Primary flow units contain secondary flow units with dimensions up to 2 km, each being fed by a secondary magma tube branching from the primary magma tube. Secondary flow units in turn are composed of 100-m scale tertiary flow units. A similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes, with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex. This branching flow pattern, as well as the interaction between flow units of varying orders, provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger. The data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer, grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature. These features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening. The branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex, suggesting that the general features of the laccolith emplacement model also apply.Editorial responsibility: J. Stix     

NMR Logging to Estimate Hydraulic Conductivity in Unconsolidated Aquifers

Nuclear magnetic resonance (NMR) logging provides a new means of estimating the hydraulic conductivity (K) of unconsolidated aquifers. The estimation of K from the measured NMR parameters can be performed using the Schlumberger‐Doll Research (SDR) equation, which is based on the Kozeny–Carman equation and initially developed for obtaining permeability from NMR logging in petroleum reservoirs. The SDR equation includes empirically determined constants. Decades of research for petroleum applications have resulted in standard values for these constants that can provide accurate estimates of permeability in consolidated formations. The question we asked: Can standard values for the constants be defined for hydrogeologic applications that would yield accurate estimates of K in unconsolidated aquifers? Working at 10 locations at three field sites in Kansas and Washington, USA, we acquired NMR and K data using direct‐push methods over a 10‐ to 20‐m depth interval in the shallow subsurface. Analysis of pairs of NMR and K data revealed that we could dramatically improve K estimates by replacing the standard petroleum constants with new constants, optimal for estimating K in the unconsolidated materials at the field sites. Most significant was the finding that there was little change in the SDR constants between sites. This suggests that we can define a new set of constants that can be used to obtain high resolution, cost‐effective estimates of K from NMR logging in unconsolidated aquifers. This significant result has the potential to change dramatically the approach to determining K for hydrogeologic applications.     

Origin and mechanism of Subantarctic Mode Water formation and transformation in the Southern Indian Ocean

The sources and pathways of mode waters and lower thermocline waters entering the subtropical gyre of the Indian Ocean are examined. A Lagrangian analysis is performed on an eddy-admitting simulation of the Global Ocean performed by the DRAKKAR Group (NEMO/OPA), which captures the main observed features. We trace the subducted mode water’s pathways, identify their formation regions and trace whether their source waters come from the Atlantic, Pacific or Indian sectors of the Southern Ocean. Three main sites for mode waters ventilation in the Indian sector are identified with different circulation pathways and source water masses: (a) just north of Kerguelen, where 4.2 Sv of lighter Subantarctic Mode Waters (SAMW); σ ₀ ∼ 26.5) are exported—originating in the Atlantic and Agulhas Retroflection regions; (b) SW of Australia, where 6.5 Sv of medium SAMW (σ ₀ ∼ 26.6) are ventilated—originating in the southern and denser Agulhas Retroflection region; (c) SW of Tasmania and along the South Australian coast, where 3 Sv of denser SAMW (σ ₀ ∼ 26.75) are ventilated—originating from three sources: Leeuwin Current waters, Tasman Sea (Pacific) waters and Antarctic Surface Waters. In all cases, modelled mode waters were last ventilated in the Indian Ocean just north of the deepest winter-mixed layers. For the waters subducted SW of Australia, the last ventilation site extends even further north. Waters ventilated in the deepest mixed layers north of the Subantarctic Front are then re-ventilated 5 years later southwest of Australia. The model results raise new hypotheses that revisit the classical picture of the SAMW formation and transformation, where a large homogeneous mixed layer is subducted and ‘slides’ equatorward, essentially maintaining the T/S characteristics acquired at the surface. Firstly, the last ventilation of the modelled mode waters is not in the region of the deepest mixed layers, as previously thought, but further north in regions of moderate meso-scale eddy activity. Secondly, the model shows for the first time a significant source region for Indian Ocean mode waters coming from deep winter-mixed layers along the south Australian coast. Finally, this analysis shows how the mode water characteristics are modified after subduction, due to internal eddy mixing. The simulation shows that resolved eddies have a strong impact on the mixed layer properties and that isopycnal eddy mixing also contributes to the generation of more homogeneous mode water characteristics in the interior.     

Community discourse and the emerging amenity landscapes of the rural American West

We explore the construction of community meaning on an emerging amenity landscape in the western United States through in-depth interviews and ethnographic fieldwork with residents of rural Fremont County, Colorado. The meanings ascribed to community center on different interpretations of home, social interaction, and personal identity. Although in some cases these differences correspond to the conventional division between new and long-term residents, the more significant finding was a discourse of “co-opetition” that captures a broader countywide understanding of community as simultaneous cooperation and competition among local stakeholders. Ultimately, this conceptualization of rural community is a way of negotiating the form and function of the amenity landscape. The production of meaningful social interaction points to opportunities for engaging residents in land management as they interact with one another and the landscape.     

The 2010 M w 7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics along the Mexican Pacific Margin

The El Mayor-Cucapah earthquake sequence started with a few foreshocks in March 2010, and a second sequence of 15 foreshocks of M?>?2 (up to M4.4) that occurred during the 24?h preceding the mainshock. The foreshocks occurred along a north?Csouth trend near the mainshock epicenter. The M _w 7.2 mainshock on April 4 exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15?s later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The aftershock zone extends for 120?km from the south end of the Elsinore fault zone north of the US?CMexico border almost to the northern tip of the Gulf of California. The waveform-relocated aftershocks form two abutting clusters, each about 50?km long, as well as a 10?km north?Csouth aftershock zone just north of the epicenter of the mainshock. Even though the Baja California data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border. The spatial distribution of large aftershocks is asymmetric with five M5+ aftershocks located to the south of the mainshock, and only one M5.7 aftershock, but numerous smaller aftershocks to the north. Further, the northwest aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus, the aftershocks also express a complex pattern of stress release along strike. The overall rate of decay of the aftershocks is similar to the rate of decay of a generic California aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north, but significant northward migration of aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley and the transfer of Pacific?CNorth America plate motion from the Gulf of California in the south into the southernmost San Andreas fault system to the north. We propose that the location of the 2010 El Mayor-Cucapah, as well as the 1992 Landers and 1999 Hector Mine earthquakes, may have been controlled by the bends in the plate boundary.     

Late Holocene dispersal and accumulation of terrigenous sediment on Poverty Shelf, New Zealand

We use coupled numerical models (HydroTrend and SedFlux) to investigate the dispersal and accumulation of sediment on Poverty Shelf, North Island, New Zealand, during the past 3 kyr. In this timeframe, we estimate that the Waipaoa River system delivered ∼10 Gt of sediment to Poverty Shelf,  5–10% of which was transported to the outer shelf and continental slope. The domain of the two-dimensional model (SedFlux) is representative of a 30 km traverse across the shelf. Comparing the model output with seismic reflection data and a core obtained from the middle shelf shows that, without extensively modifying the governing equations or imposing unrealistic conditions on the model domain, it is possible to replicate the geometry, grain size and accumulation rate of the late Holocene mud deposit. The replicate depositional record responds to naturally and anthropogenically induced vegetation disturbance, as well as to storms forced by long-period climatic events simulated entirely within the model domain. The model output also suggests that long-term fluctuations in the amount and caliber of river sediment discharge, promoted by wholesale changes in the catchment environment, may be translated directly to the shelf depositional record, whereas short-term fluctuations conditioned by event magnitude and frequency are not. Thus on Poverty Shelf, as well as in depocenters on other active continental margins which retain a much smaller proportion of the terrigeneous sediment delivered to them, flood-generated event beds are not commonplace features in the high-resolution sedimentary record. This is because the shelf sedimentary record is influenced more by the energy available to the coastal ocean which helps keep the sediment in suspension and facilitates its dispersal, than by basin hydrometeorology which determines the turbidity and velocity of the river plume.