Mid-continental Slope Research Articles

Megathrust reflectivity reveals the updip limit of the 2014 Iquique earthquake rupture

The updip limit of seismic rupture during a megathrust earthquake exerts a major control on the size of the resulting tsunami. Offshore Northern Chile, the 2014 Mw 8.1 Iquique earthquake ruptured the plate boundary between 19.5° and 21°S. Rupture terminated under the mid-continental slope and did not propagate updip to the trench. Here, we use state-of-the-art seismic reflection data to investigate the tectonic setting associated with the apparent updip arrest of rupture propagation at 15 km depth during the Iquique earthquake. We document a spatial correspondence between the rupture area and the seismic reflectivity of the plate boundary. North and updip of the rupture area, a coherent, highly reflective plate boundary indicates excess fluid pressure, which may prevent the accumulation of elastic strain. In contrast, the rupture area is characterized by the absence of plate boundary reflectivity, which suggests low fluid pressure that results in stress accumulation and thus controls the extent of earthquake rupture. Generalizing these results, seismic reflection data can provide insights into the physical state of the shallow plate boundary and help to assess the potential for future shallow rupture in the absence of direct measurements of interplate deformation from most outermost forearc slopes.

Оценка точности общедоступных цифровых моделей рельефа дна океанов на примере участков покрытия многолучевой съемкой Норвежского и Баренцева морей

The accuracy of publicly available bathymetry grids such as GEBCO_08, SRTM30_Plus, and Predicted Topography is evaluated at six studied polygons of GIN RAS multibeam (MB) coverage. High-resolution and accuracy, independent GIN RAS MB grids are used as the ground truth. Comparison of GIN RAS MB grids with USCGC “Healy” and RV “Oden” gridded MB data revealed no systematic errors in the GIN RAS data. In order to assess the accuracy an algorithm was developed to calculate depth difference between grids built in different map projections with grid cells that do not match. The assessment revealed that GEBCO_08 is more accurate than S&S over three of the six polygons, which include mid oceanic ridge, shelf and continental slope areas (polygons 1, 5 and 6). The observed differences in the accuracy between GEBCO_08 and S&S are due to the source data accuracy at polygon 1, better source data coverage in GEBCO_08 for polygons 5 and 6 (MB data), and poor performance of the interpolation in the shelf areas for S&S at polygons 4, 5 and 6. These results show that morphology of the seafloor, source data accuracy and coverage as well, as interpolation method affect the accuracy of the final output bathymetry surface.

Late Quaternary sporadic development of Desmophyllum dianthus deep-coral populations in the southern Labrador Sea with specific attention to their 14C- and 230Th-dating

About 70 fossil and live-collected specimens of Desmophyllum dianthus were collected from steep rocky surfaces and time-averaged assemblages at their base (“coral graveyards”) in the southern Labrador Sea at depths ranging from ~1700 m (Orphan Knoll) to ~2200 m (Flemish Cap). Aside from stable isotope and mineralogical analysis, U-series and 14C age determinations were performed on the samples. Fossil corals from the coral graveyards display two principal age clusters corresponding respectively to the Holocene and Marine Isotope Stage (MIS) 5c intervals. Three samples lay outside these clusters, at ~13.5 ka (Bølling-Allerød), ~ 64 ka (MIS 3) and at ~181 ka (MIS 7a). The clusters are thought to record intervals with both i) high food availability, either through pelagic primary production in the overlying water column or more effectively in relation with particulate and dissolved organic carbon transport via an active Western Boundary Undercurrent (WBUC), and ii) high carbonate micrite advection by this current from more open North Atlantic coccolithophorid production areas, maintaining an aragonite saturation horizon (now at 2300 m) slightly deeper than the coral habitat (~ 2200 m). Attempts at estimating ventilation ages from 230Th vs 14C age offsets in Holocene samples failed. This is due to uncertainties about the initial 14C-apparent age of coral skeletons and to their high initial excesses in 230Th. This initial excess relates mostly to the scavenging of 230Th produced by the dissolved marine U by the coral-feeding organic matter carried along the WBUC trajectory. In addition, discrete diagenetic U-fluxes can be documented in fossil samples. They are well illustrated in specimens from MIS 5c age, which depict variable excesses in 234U vs 238U, but similar 230Th/238U activity ratios, pointing to a recent but fractionating redistribution of uranium in the samples. These are also documented by the development of FeMn coatings of specimens exposed at the sea floor, due to the winnowing of embedding slope sediments, with the mid- to late Holocene intensified WBUC. Such diagenetic processes are to be notably taken into consideration for the dating of pre-Holocene samples. Thus, both the initial isotopic properties of corals from this WBUC-influenced, mid-continental slope, and subsequent diagenetic effects impact their calculated ages, and blur estimates of “ventilation ages” based on their 14C vs 230Th-ages offsets.

Effects of oceanographic changes on controlling the stability of gas hydrates and the formation of authigenic carbonates at mud volcanoes and seepage sites on the Iberian margin of the Gulf of Cadiz

The Gulf of Cadiz is characterized by extensive active and inactive fluid escape structures which manifest as mud volcanoes, diapiric ridges, pockmarks and cold seeps. The high methane content and the presence of gas hydrates in the shallow sediments of the most active fluid escape structures indicate that these are areas of preferential migration and escape of fluids enriched in hydrocarbons and methane. Extensive fields of methane-derived authigenic carbonates are found at fluid escape structures along the upper and mid-continental slope, where the Mediterranean Outflow water is in direct contact with the seafloor, at water depths generally <1400 m. These methane-derived authigenic carbonates occur in much higher abundance at these shallower depths than in the fluid escape structures of the deeper (>1400 m) parts of the basin. The estimated U/Th ages of the authigenic carbonates, suggest that they were probably formed during discrete episodes of gas hydrate activity that may have occurred as a result of rapid climatic changes (such as the termination of the last glacial stage.) Numerical modelling indicates that seafloor temperature variations, associated with glacial/interglacial cycles and the changes of the Mediterranean Outflow pathway, could have restricted or eliminated the gas hydrate stability zone at the mud volcanoes shallower than 1400 m, resulting in intense fluxes of methane-rich fluids to the shallow sediments or even the seafloor. This fluid flux was recorded by the microbially mediated precipitation of authigenic carbonates.

The impacts of deep-sea fisheries on benthic communities: a review

AbstractDeep-sea fisheries operate globally throughout the world's oceans, chiefly targeting stocks on the upper and mid-continental slope and offshore seamounts. Major commercial fisheries occur, or have occurred, for species such as orange roughy, oreos, cardinalfish, grenadiers and alfonsino. Few deep fisheries have, however, been sustainable, with most deep-sea stocks having undergone rapid and substantial declines. Fishing in the deep sea not only harvests target species but can also cause unintended environmental harm, mostly from operating heavy bottom trawls and, to a lesser extent, bottom longlines. Bottom trawling over hard seabed (common on seamounts) routinely removes most of the benthic fauna, resulting in declines in faunal biodiversity, cover and abundance. Functionally, these impacts translate into loss of biogenic habitat from potentially large areas. Recent studies on longline fisheries show that their impact is much less than from trawl gear, but can still be significant. Benthic taxa, especially the dominant mega-faunal components of deep-sea systems such as corals and sponges, can be highly vulnerable to fishing impacts. Some taxa have natural resilience due to their size, shape, and structure, and some can survive in natural refuges inaccessible to trawls. However, many deep-sea invertebrates are exceptionally long-lived and grow extremely slowly: these biological attributes mean that the recovery capacity of the benthos is highly limited and prolonged, predicted to take decades to centuries after fishing has ceased. The low tolerance and protracted recovery of many deep-sea benthic communities has implications for managing environmental performance of deep-sea fisheries, including that (i) expectations for recovery and restoration of impacted areas may be unrealistic in acceptable time frames, (ii) the high vulnerability of deep-sea fauna makes spatial management—that includes strong and consistent conservation closures—an important priority, and (iii) biodiversity conservation should be &amp;gt; balanced with options for open areas that support sustainable fisheries.

Morphotectonic architecture of an India–Madagascar breakup related anomalous submarine terrace complex on the southwest continental margin of India

An anomalous lateral bathymetric protrusion [Alleppey–Trivandrum Terrace Complex (ATTC)] in the form of two contiguous terrace-like features exists in the mid-continental slope off southwest coast of India. This bathymetric protrusion and a bathymetric notch in the Northern Madagascar Ridge (NMR) have been postulated as conjugate features related to India–Madagascar separation in Late Cretaceous. However, geoscientific studies aimed to understand the crustal structure and genesis of the ATTC or its postulated conjugate region are meagre and ambiguous. In this context, the present study, based on recently acquired as well existing bathymetry, gravity, magnetic and multi-channel seismic reflection data, was carried out to understand the morphotectonic architecture of the ATTC. The bathymetric data suggest an escarpment [Chain-Kairali Escarpment (CKE)] to demarcate the westward limit of the ATTC and another escarpment [Quilon Escarpment (QE)] to demarcate the boundary between the southerly, larger ‘Trivandrum Terrace (TT)’ and northerly, smaller Alleppey Terrace (AT). The multichannel seismic reflection data suggests a block-faulted basement and presence of a nearly N–S trending wide basement high in the central part of the Trivandrum Terrace. Forward modelling of gravity data constrained by seismic reflection information suggests that the crustal structure of the ATTC region is comparable to thinned continental crust. Forward modelling of prominent high amplitude magnetic anomalies suggest that the thinned continental crust of the ATTC region also contains volcanic bodies as intrusives/extrusives. The CKE is inferred to be a sheared continental margin segment along which the SE coast of Madagascar glided past India and the volcanic emplacements in the ATTC region perhaps relates to the Marion Hotspot volcanism.

Biogeographical structure and affinities of the marine demersal ichthyofauna of Australia

Aim To investigate the biogeographical structure and affinities of the Australian marine demersal ichthyofauna at the scale of provinces and bathomes for the purposes of regional marine planning. Location Australia. Methods Patterns of distribution in the Australian fish fauna, at both intra-regional and global scales, were examined using a science-based, management framework dividing Australia’s marine biodiversity into 16 province-level biogeographical units. Occurrences of 3734 species in eight depth-stratified bathomes (from the coast to the mid-continental slope) within each province were analysed to determine the structure and local affinities of their assemblages and their association with faunas of nearby regions and oceans basins. Results Strong geographic and depth-related structure was evident. Fish assemblages in each province, and in each bathome of each province, were distinct, with the shelf-break bathome more similar to the adjacent continental shelf bathome than to the upper slope bathome. Data based only on endemic species performed well as a surrogate of the entire dataset, yielding comparable patterns of similarity between provinces and bathomes. Tropical and temperate elements were better discriminated than elements of the Pacific and Indian oceans, with the central western province more similar to the tropical provinces (including those in the east), and the eastern province closer to southern temperate provinces. The fauna shares the closest regional affinities with those of the adjacent south-west Pacific, western Pacific Rim, and elements of wide-ranging Indo-Pacific components. Elements unique to the Pacific and Indian oceans are poorly represented. Main conclusions The complex nature of Australia’s marine ichthyofauna is confirmed. A hierarchy of provinces and bathomes, used to ensure that Australia’s developing marine reserve network is both representative and comprehensive, is equally robust when based on all known Australian fish species or on only those species endemic to this continent. Latitude and depth are more important than oceanic influences on the composition of this fauna at these scales.

Composition and distribution of deep-sea benthic invertebrate megafauna on the Lord Howe Rise and Norfolk Ridge, southwest Pacific Ocean

The deep-sea biodiversity of the Lord Howe Rise and Norfolk Ridge – two complex submarine features that extend in a north-south direction either side of a deep basin within the northern Tasman Sea and southern Coral Sea – was sampled in 2003 for the first time on a broad regional scale. The total of 1313 megabenthic invertebrate species from 17 higher order taxa collected between 100 and 1800 m depths showed faunal diversity and novelty was high. Only 256 of these species were named, and 10% of these were described as a result of this survey; 78% are un-named and believed to be mostly new species. Of the 1253 species included in quantitative analyses, most appeared to be rare – 85% were only found once. This indicates intra-regional endemism may be high, but undersampling is also likely. Species accumulation curves confirm that many additional species remain to be collected. There was high regional-scale spatial heterogeneity in species distribution patterns which appeared to be influenced by hydrographic patterns and feature-scale topography, and to a lesser extent by seabed type. Depth and oxygen concentration (correlated with depth) had most influence on distribution patterns of fauna, with assemblages identified from three depth-zones: 100–400 m (deep continental shelf and shelf edge), 400–700 m (upper continental slope) and >700 m (mid-continental slope). In the shallowest depth zone, there were north-south (latitudinal) patterns in invertebrate assemblages that appeared to be influenced by water mass distribution. Species overlap was higher in the south than the north, probably due to the Tasman Front forming a hydrographic connection between the southern parts of the Rise and the Ridge at shallower depths. At depths >700 m, the absence of a latitudinal pattern in assemblage structure was attributed to the continuity of Antarctic Intermediate Water in the study area. Differentiation of two assemblages in sled samples from the >700 m depth zone, as well as some patterns of diversity of large sessile fauna between sub-regions within the study area, suggested a relationship with bottom type but this was not fully analysed. While providing a major increase in scientific knowledge of marine biodiversity in deep waters of the Coral and Tasman Seas, these results also highlighted the paucity of biogeographical knowledge that exists for the area. Some science advances needed to inform national and international conservation plans currently under development are identified. They include taxonomic standardisation at a regional-scale (Australia, New Zealand and New Caledonia) for informative higher level taxa, and some additional surveys of selected areas and seabed features, including off northeastern Australia.

Dispersal and transformation of organic carbon across an episodic, high sediment discharge continental margin, Waipaoa sedimentary system, New Zealand

The rivers that drain active, collisional margins deliver ∼ 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y − 1 ) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ 13C, 14C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate that terrestrial OC is being transferred across the continental margin, with plant sterols, long chain alcohols and long chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1–10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

Dispersal and transformation of organic carbon across an episodic, high sediment discharge continental margin, Waipaoa Sedimentary System, New Zealand

The rivers that drain active, collisional margins of the southwest Pacific deliver up to 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y − 1 ) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ 13C, 14C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate that terrestrial OC is being transferred across the continental margin, with plant sterols, long-chain alcohols and long-chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1–10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

FAMENNIAN CONODONTS FROM KAL-E-SARDAR SECTION, EASTERN TABAS, CENTRAL IRAN

Sixty-three conodont species and subspecies from the Kal-e-Sardar section indicate that the strata span early to late Famennian. Contrary to most of the sections measured in the Shishtu Formation, the icriodid-polygnathid biofacies is restricted to the lower part of Kal-e-Sardar Famennian succession. Palmatolepid and bispathodid-dominated biofacies in the middle and upper part of section represent a deep shelf to mid-continental slope marine environment. The currently accepted ranges of P. tichonovitchi, P. tenellus and P. inconcinnus are early Famennian, late Frasnian-early Famennian, and early to middle Famennian, respectively, together with middle to late Famennian for P. nodoundatus. Two new species, Polygnathus sardarensis and P. yazdii, are proposed.

The terrace like feature in the mid-continental slope region off Trivandrum and a plausible model for India–Madagascar juxtaposition in immediate pre-drift scenario

Bathymetry of the southwestern continental margin of India reveals the presence of an anomalous terrace like feature in the mid-continental slope region off Trivandrum. The genesis of this terrace of large areal extent (∼ 9000 sq. km.) is yet to be established. Based on exercises with several existing paleogeographic reconstruction models and updated compilation of identified offshore tectonic elements, this study attempts to identify a plausible model of India–Madagascar juxtaposition in immediate pre-drift scenario, which provides idea about genesis of this terrace. It is inferred that the terrace off Trivandrum and an anomalous bathymetric notch located in the northern Madagascar Ridge are conjugate features related to India–Madagascar separation and the rifted and sheared segments of the pre-drift plate boundary have shaped their outlines. The drifting of India from Madagascar is suggested to have commenced at about 86.5 Ma.

Internal tide spatial variability off western Portugal detected by current meter observations

Long‐term (10–12 months) current meter observations from the western Iberian continental margin are analyzed focusing on oscillations in the internal gravity wave band. Kinetic energy spectra are characterized by background falloff rates with frequency ω ranging from ω−1 to ω−2, superposed by energetic narrow bands at specific frequencies. Significant spectral peaks are found at some of the inertial‐tidal nonlinear interaction frequencies (M2 + f and M4 + f) and at higher semidiurnal harmonics (M4, M6 and M8). Semidiurnal internal tide kinetic energy reveals a spatial distribution with a maximum coherent contribution (44 cm2 s−2) at 1200‐m depth on the mid continental slope and a maximum relative incoherent signal (&gt;60% of the total semidiurnal tidal band energy) at intermediate levels (650 to 1200 m depth and 30 to 130 km from the shelf break).

Variation of turbidity and particle transport in the bottom layer of the East China Sea

Abstract Field surveys of the bottom turbid layer were undertaken from the Changjiang estuary to the mid-continental slope area in the East China Sea to study the bottom turbid layer formation process and ascertain the role of material transport. A dramatic development of bottom turbid layers in the continental shelf area of the East China Sea was observed from summer to fall. This seasonal variation is relatively consistent with seasonal changes in the discharge of suspended particles (SS) from the Changjiang. The SS forming the bottom turbid layer in the East China Sea are often fine-grained particles transported from the continental coastal area, which undergo continuous settling and resuspension during transport. Through the convectively mixed seawater season, the average current velocities at the bottom B+10 m layer at the edge of the continental shelf were approximately 5.2 cm/s in an east-northeast direction, but 2.9 cm/s almost-true north during the stratified season. With the 200 m-depth contour as the benchmark, the bottom current at the continental shelf edge flowed from inside the continental shelf outward during the mixing season, and from outside the continental shelf inward during the stratified seawater season. The average horizontal flux of the SS is 60 mg/m 2 /s during the mixing season and 24 mg/m 2 /s during the stratified season with almost the same direction as that of the bottom current. The direction of the SS horizontal flux in the bottom layers is dominated by the bottom current, which is driven by the strong south–north monsoon in the East China Sea.

New Zealand Demersal Fish Assemblages

Demersal fish assemblages in the New Zealand Exclusive Economic Zone were identified using presence–absence data from 19 215 bottom trawl tows made over a 37-year period. The dataset spanned latitudes 34–54°S and depths of 4–1500 m. A total of 123 taxa occurred in more than 1% of the tows (121 fish and 2 squid). Multivariate ordination and classification (correspondence analysis and Ward's cluster analysis) identified four primary species assemblages that were associated with the inner continental shelf, mid–outer continental shelf and shelf edge, upper continental slope and mid continental slope. The most frequently occurring species (> 40% of tows) in each assemblage were (in descending order): inshore – Chelidonichthys kumu, Pagrus auratus and Zeus faber; shelf – Nototodarus spp., Squalus acanthias and Thyrsites atun; upper slope – Macruronus novaezelandiae, Lepidorhynchus denticulatus, Genypterus blacodes and Hydrolagus sp.; mid slope – Hoplostethus atlanticus, Etmopterus baxteri, Halargyreus johnsonii, Coryphaenoides subserrulatus, Deania calcea, Coryphaenoides serrulatus, Pseudocyttus maculatus, Mora moro, Diastobranchus capensis and Centroscymnus crepidater. Further species associations were also identified within each primary assemblage. Canonical correspondence analysis revealed that most of the explainable variation in species composition was associated with depth and latitude; longitude and season explained little extra variance. The usefulness of our results is limited by the use of presence–absence rather than abundance data, and by the uneven spatial distribution of trawl tows. However, the present study provides a large-scale framework within which to interpret the results of studies using abundance data over smaller spatial and temporal scales.

Pelagic biomass and community structure over the mid-continental slope off southeastern Australia based upon acoustic and midwater trawl sampling

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 146:21-35 (1997) - doi:10.3354/meps146021 Pelagic biomass and community structure over the mid-continental slope off southeastern Australia based upon acoustic and midwater trawl sampling Koslow JA, Kloser RJ, Williams A We compare estimates of biomass and community structure based upon depth-stratified sampling from 0 to 900 m at a site on the continental slope south of Tasmania, Australia, using an EK500 acoustic system with vessel-mounted and deeply towed 38 kHz split-beam transducers and a midwater trawl with opening-closing codend system. Multivariate analyses of the target strength (TS) distributions and of dominant species in the tows provided consistent views of community structure: an epi-pelagic community that was differentiated and mid-depth and deep communities not differentiated on a diel basis. Peaks in the TS distributions and the estimated TS of key biological groups were congruent, although the in situ TS data indicated that fishes with swimbladders in the size range of myctophids have lower TS (up to 5 dB) than previously predicted for physoclists, possibly due to the smaller volume of gas-filled swimbladders among mesopelagic fishes. There was a progressive increase in modal TS distributions with depth, consistent with the larger mean size of both squids and fishes in the deeper strata. Biomass was lowest in the daytime near-surface stratum, but there were no other trends of biomass with depth. The acoustic estimate of biomass based upon the trawl-derived assessment of community composition was 7-fold higher than the trawl-based volume-swept biomass estimate. This acoustic biomass estimate was 3-fold less than one based upon an assessment of community composition derived from TS distributions. The higher acoustic biomass estimates are consistent with regional estimates of primary production and simple trophodynamic calculations. Midwater trawling · Acoustics · Target strength · Community composition · Biomass estimation · Sampling bias Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 146. Publication date: January 30, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

Geomagnetic field measurements have been made at three new seafloor locations off the coast of southern Australia, to extend a magnetometer array deployed by White and Polatayko (1978). Geomagnetic depth sounding (GDS) and vertical gradient sounding (VGS) estimates provide a measure of both the TE and TM mode response of the continent-ocean boundary, which is relatively two-dimensional (2D) over hundreds of kilometres. The seafloor data from the continental shelf, mid-continental slope and at the edge of the abyssal plain show a strong geomagnetic coast effect which can largely be accounted for by the sea-water and a thick wedge of sediments in a rifted marginal basin on the continental shelf. A 2D inversion of the GDS and VGS estimates to determine the sub-seafloor conductivity structure suggests that (a) the oceanic crust and continental crust have conductivities of 0.05 and 0.005 S·m-1 respectively to a depth of 10 km, (b) the upper mantle above 140 km has conductivity 0.01-0.003 S·m-1, (c) the upper mantle between 140 and 390 km has conductivity less than 0.003 S·m-1, and (d) the lower mantle below 390 km has conductivity less than 0.3 S·m-1. No distinct boundary between oceanic and continental lithosphere conductivity can be discerned. The conductivity structure beneath the coastline of southern Australia is somewhat less conductive than the preferred model of Kellett et al.(1991) for southeastern Australia.

Mass-emplaced siliciclastic-volcaniclastic-carbonate sediments in Middle Miocene shelf-to-slope environments at Waikawau, northern Taranaki, and some implications for Taranaki Basin development

Abstract The Middle Miocene (Altonian-Waiauan) sequence at Waikawau (Mokau-Mohakatino Groups), totalling c. 350 m thick, sits directly on Mesozoic basement rocks or rarely on local Oligocene Te Kuiti Group. It comprises four newly defined formations. Nukuhakari Formation is dominated by hemipelagic slope mudstones and some turbidites; Tirua Formation comprises redeposited calcareous sandstone/sandy limestone sheets, frequently thoroughly bioturbated, formed in broad, shallow channels on an upper to mid-continental slope carbonate apron; Ngarupupu Formation consists of variably volcaniclastic sandstones and mudstones deposited from sediment gravity flows on the mid- to outer regions of deep-marine, coalescing, volcaniclastic fan-aprons; and Waikaretu Formation is dominated by fine nonvolcaniclastic sandstones deposited by sand-rich, high-concentration turbidity currents at bathyal depths. Two petrofacies occur, suggesting two distinct sediment provenances: Nukuhakari and Waikaretu Formations have a Mesozoi...

The “insulating” effect of a steep continental slope

Geochemical evidence shows that the midcontinental slope off the Mid‐Atlantic Bight constitutes an important sink for substances attached to fine particles. It is reasonable to infer that near‐bottom currents in this region are generally much weaker than those over the adjacent continental shelf, allowing the settling out of fine particles and their incorporation into permanent sediment. A partial dynamical explanation of this phenomenon may be sought in the “insulating” effect of a steep slope. At levels below the main thermocline the bottom pressure perturbation associated with steady flow is governed by a parabolic equation which expresses vorticity tendency balance between bottom stress curl and vortex stretching. A heat conduction analogy is appropriate, in which the friction coefficient corresponds to conductivity and the bottom slope corresponds to heat capacity. Over the continental slope the latter is large, suppressing pressure gradients originating from the shelf. Thus steady, near‐bottom currents over the middle slope are generally weak. Exceptions to this rule are locations subject to intense winter cooling and deep convection, which may generate strong thermohaline near‐bottom flow. Oscillatory motions of importance (which may be taken to be coastally trapped waves and topographic waves propagating shoreward from deep water) are weak partly because an efficient local generation mechanism is lacking. Disturbances originating from the Gulf Stream decay as the depth reduces below 3 km, by a mechanism the details of which are not clear at present. The findings in the Mid‐Atlantic Bight are likely to apply to other continental slopes distant from western boundary currents and not subject to intense winter cooling.