Efficient Imaging Strategies For Next-Generation Radio Arrays

The performance goals of the Square Kilometre Array (SKA) are such that major departures from prior practice for imaging interferometer arrays are required. One class of solutions involves the construction of large numbers of stations, each composed of one or more small antennas. The advantages of such a “large-N” approach are already documented, but attention has recently been drawn to scaling relationships for SKA data processing that imply excessive computing costs associated with the use of small antennas. In this paper we examine the assumptions that lead to such scaling laws, and argue that in general they are unlikely to apply to the SKA situation. A variety of strategies for SKA imaging which exhibit better scaling behaviour are discussed. Particular attention is drawn to field of view issues, and the possibility of using weighting functions within an advanced correlator system to precisely control the field-of-view.     

Effects of zooplankton grazing on phytoplankton size-structure and biomass in the lower Hudson River estuary

The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64–210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various¹⁴C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as %=mg C ingested m^?2 h^?1/mg C produced m^?2 h^?1 (depth-integrated rates)×100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatomThalassiosira pseudonana over the larger diatomDitylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellateAmphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions—not grazing selectivity—may be the predominant factor explaining community size-structure.     

Segmentation and disruption of the East Pacific Rise in the mouth of the Gulf of California

Analysis of new multibeam bathymetry and all available magnetic data shows that the 340 km-long crest of the East Pacific Rise between Rivera and Tamayo transforms contains segments of both the Pacific-Rivera and the Pacific-North America plate boundaries. Another Pacific-North America spreading segment (Alarcon Rise) extends 60 km further north to the Mexican continental margin. The Pacific-North America-Rivera triple junction is now of the RRR type, located on the risecrest 60 km south of Tamayo transform. Slow North America-Rivera rifting has ruptured the young lithosphere accreted to the east flank of the rise, and extends across the adjacent turbidite plain to the vicinity of the North America-Rivera Euler pole, which is located on the plate boundary. The present absolute motion of the Rivera microplate is an anticlockwise spin at 4° m.y.^–1 around a pole located near its southeast corner; its motion has recently changed as the driving forces applied to its margins have changed, especially with the evolution of the southern margin from a broad shear zone between Rivera and Mathematician microplates to a long Pacific-Rivera transform. Pleistocene rotations in spreading direction, by as much as 15° on the Pacific-Rivera boundary, have segmented the East Pacific Rise into a staircase of en echelon spreading axes, which overlap at lengthening and migrating nontransform offsets. The spreading segments vary greatly in risecrest geomorphology, including the full range of structural types found on other rises with intermediate spreading rates: axial rift valleys, split shield volcanoes, and axial ridges. Most offsets between the segments have migrated southward, but within the past 1 m.y. the largest of them (with 14–27 km of lateral displacement) have shown dueling behavior, with short-lived reversals in migration direction. Migration involves propagation of a spreading axis into abyssal hill terrain, which is deformed and uplifted while it occupies the broad shear zones between overlapping spreading axes. Tectonic rotation of the deformed crust occurs by bookshelf faulting, which generates teleseismically recorded strike-slip earthquakes. When reversals of migration direction occur, plateaus of rotated crust are shed onto the rise flanks.     

A multibeam reconnaissance of the Tonga Trench axis and its intersection with the Louisville guyot chain

A Seabeam reconnaissance of 1200 km of the deep sediment-starved axis of Tonga Trench delineated the fine-scale relief at the outcrop of a subduction zone generally characterized by tectonic erosion rather than accretion. The commonest axial cross-section has a steep (12°) irregular inner slope intersecting the thinly sedimented surface of Mesozoic ocean crust, which dips under it at 5–6°. There is little or no intervening turbidite fill, but small lenses interpreted as debris deposits occur at the foot of parts of the inner slope that lack basins or benches which elsewhere obstruct downslope sediment transport. The oceanic slope is severely broken by parallel but slightly sinuous fractures induced by bending of the plate, and entry of outer-slope grabens into the subduction zone is confirmed to be a morphologically and tectonically important process. Arrival of oceanic seamounts and volcanic ridges at the trench outer slope and axis affects the fracture pattern of the oceanic plate, the depth of the temporarily plugged axis, and the relief of the lower inner slope. Subduction of the Louisville guyot chain, or of the extensive hotspot swell and thick sediment apron that surrounds it, has important regional effects as well, shoaling 400 km of trench axis and causing development of a small accretionary prism with trench-slope basins. Because the intersection point of the hot-spot chain has moved rapidly south along the trench, structural changes that occur in the wake of guyot-chain subduction can also be inferred: accretion at the inner slope is followed by rapid tectonic erosion, which unroofs a wider strip of downgoing lithosphere and thereby deepens the trench axis. The longitudinal profile of axial depths, made locally irregular by the collision of medium-scale volcanic and tectonic relief on the oceanic plate, also has a step near 18.5° S, where there is a regional depth difference in the oceanic crust entering the trench.     

Regional shape and tectonics of the equatorial East Pacific Rise

The geography of the East Pacific Rise (EPR) between 10°N and 6°S, redetermined by new surface ship surveys, is characterized by long spreading axes orthogonal to infrequent transform faults. Near 2°10N the EPR is intersected by the Cocos-Nazca spreading center at the Galapagos triple junction. The present pattern was established 27-5.5 m.y.b.p. by a complex sequence of rise-crest jumps and reorientations from a section of the Pacific-Farallon plate boundary. Transverse profiles of the rise flanks can be matched by thermal contraction curves for aging lithosphere, except between the triple junction and 4°S, where the east flank is anomalously shallow and almost horizontal. Most sections of spreading axis have the 10–30 km wide, 100–400 m high, axial ridge that is characteristic of fast spreading centers. However, within 60 km of the triple junction the rise crest structure is atypical, with an axial rift valley and elevated rift mountains, despite a spreading rate of 140 mm/yr. With the exception of this atypical section, the bathymetric profile along the spreading axis is remarkably even, with continuous, gentle slopes for hundreds of kilometers between major transform faults, where step-like offsets in axial depths occur. Most of the observations can be accommodated by a model in which the long spreading axes are underlain by continuous crustal magma chambers that allow easy longitudinal flow of magma, and whose size controls the style and dimensions of EPR crestal topography.Contribution of the Scripps Institution of Oceanography, new series.     

Fine-scale isotopic variation in Mariana Trough basalts: evidence for heterogeneity and a recycled component in backarc basin mantle

Fine-scale sampling with alvin and by dredging of the axial ridge in the Mariana Trough between 17°40′N and 18°30°N recovered basalts with isotopic compositions that span the range between N-type MORB and Mariana island arc basalts. There is a local tectonic-morphological control on basalt compositions; MORB-like basalts are found on the deeper ridge segment bounded by the Pagan transform and the ridge offset at 17°56′N, while basalts from the shallower ridge to the north are typical Mariana Trough basalts (MTB) having compositions intermediate between the two endmember rock types. Arc-like basalts were recovered from one site on the axial ridge.The discovery of basalts with such diverse isotopic characteristics from a short (100 km) section of this backarc spreading center constrains the chemical characteristics and distribution of mantle source variability in the Mariana Trough. SrNdPb isotopic variability suggests that the MTB source is heterogeneous on the scale of individual melt batches. The principal component in the MTB mantle source region is depleted peridotite similar to the source of MORB. The enriched component, most evident in the arc-like basalts and intimately mixed in MTB, has isotopic characteristics similar to those observed in the Mariana arc basalts. The isotopic data suggest that source variability for Mariana axial ridge basalts can be explained by mixed arc-like and MORB-like mantle. We hypothesize that there are fragments of old oceanic lithosphere in the backarc source region. This lithospheric component may reflect remnants of subducted seafloor or forearc-volcanic arc mantle that predate rifting in the backarc basin.     

A geomorphological reconnaissance of the submarine part of the East Rift Zone of Kilauea Volcano,Hawaii

More than half of the intensely active East Rift Zone of Kilauea Volcano crops out underwater along the crest of the submarine Puna Ridge. I present multibeam bathmetry of the entire ridge, near-bottom photographic and sonar observations of the plunging crest of its deeper distal half, and seismic profiles across the ridge tip and the adjacent structural moat. Analysis of large-scale relief, small-scale topography, and superficial rock types indicates that the rift zone is actively propagating across the moat but is probably a superficial structure that does not penetrate the underlying oceanic crust, that its tip is covered with large lava flows erupted at high rates and is surrounded with extensive debris flow deposits, and that the axial topography at depths of 2–4 km is dominated by gaping fissures and collapse pits, showing a preponderance of intrusive rather than extrusive events. Some aspects of this central-volcano rift zone, such as its geometry at small lateral offsets, resemble those at interplate rift zones along fast-spreading mid-ocean rises, but the great contrast in lithosphere thickness results in fundamental structural differences.     

Near bottom magnetic profile across the Red Sea

Results are presented from a high precision geophysical profile made at an altitude of about 100 m above the sea floor with the Deep Two instrument package, crossing the Red Sea at 17°30N. The emphasis is on the analysis and interpretation of the magnetic field, including an inversion which removes the distortions due to bathymetry and the orientation with respect to the earth's main field vector. The spreading rates are determined precisely and found to be highly asymmetric: only 5 mm yr^-1 to the east and up to 10 mm yr^-1 to the west. We conclude that the axis of spreading is located on a volcanic ridge, rather than on the axial graben, based on the presence of a zone of high magnetization. The magnetization high (40 Am^-1) is about twice as great as found on the Mid-Atlantic Ridge with the same instrument and analysis. The quality of the recording of the magnetic anomalies in the oceanic crust is much greater than expected for such a low spreading rate.     

Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3°25′S

A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25S, where the Pacific and Nazca plates are separating at 152 mm/yr. A single 90 km-long traverse of the rise crest extends near-bottom observations onto the rise flanks. A ridge at the spreading axis is defined by its steep regional slopes, probably caused by rapid crustal contraction as the spreading magma chamber freezes, rather than by outward-facing fault scarps. It can be divided into a marginal horst-and-graben zone with low (<50 m), symmetric fault blocks, and a 2 km-wide elongate axial shield volcano that is unfaulted except for a narrow crestal rift zone. This has a summit graben (10–35 m deep) probably formed by caldera collapse, and narrow pillow basalt walls built over vent fissures. Small, low (<50 m) volcanic peaks occur on the shield volcano and the horst-and-graben zone, and some may have been built away from the spreading axis. Major plate-building lava flows issue from the crestal rift zone and flow an average of 500 m down the sides of the volcano. The marginal horst-and-graben zone results from tensional faulting of a thin crust of lava, and evolves by progressive shearing on inclined fault planes. Crustal extension continues at least as far as 20 km from the axis, and the large, long fault blocks formed in thicker crust beyond the subaxial magma chamber develop into abyssal hills. Pelagic sedimentation, at a maximum rate of 22 m/10⁶ years, gradually infills open fissures and smooths the small-scale roughness of the fault blocks. Off-axis volcanism has also resulted in smoother crust, and built seamounts.Comparison of the EPR at 3°25S with the Famous Rift and Galapagos Rift reveals a similarity in the processes and small-scale landforms at fast, medium and slow-spreading ridges. There are significant differences in the medium-scale landforms, probably because plate-boundary volcanic and tectonic processes act on crust of very different strength, thickness, and age.Contribution of the Scripps Institution of Oceanography, new series.