Seafloor spreading in the Weddell Sea from magnetic and gravity data

A re-compilation of magnetic data in the Weddell Sea is presented and compared with the gravity field recently derived from retracked satellite altimetry. The previously informally named ‘Anomaly-T,’ an east–west trending linear positive magnetic and gravity anomaly lying at about 69°S, forms the southern boundary of the well-known Weddell Sea gravity herringbone. North of Anomaly-T, three major E–W linear magnetic lows are shown, and identified with anomalies c12r, c21–29(r) and c33r. On the basis of these, and following work by recent investigators, isochrons c13, c18, c20, c21, c30, c33 and c34 are identified and extended into the western Weddell Sea. Similarly, a linear magnetic low lying along the spine of the herringbone is shown and provisionally dated at 93–96 Ma. Anomaly-T is tentatively dated to be M5n, in agreement with recent tectonic models.Although current tectonic models are generally in good agreement to the north of T, to the south interpretations differ. Some plate tectonic models have only proposed essentially north–south spreading in the region, whilst others have suggested that a period of predominantly east–west motion (relative to present Antarctic geographic coordinates) occurred during the mid-Mesozoic spreading between East and West Gondwana. We identify an area immediately to the south of T which appears to be the southerly extent of N–S spreading in the herringbone. Following recent work, the extreme southerly extent of the N–S directed spreading of the herringbone is provisionally dated M9r/M10. In the oldest Weddell Sea, immediately to the north and east of the Antarctic shelf, we see subtle features in both the magnetic and gravity data that are consistent with predominantly N–S spreading in the Weddell Sea during the earliest opening of East and West Gondwana. In between, however, in a small region extending approximately from about 50 km south of T to about 70°S and from approximately 40° to 53°W, the magnetic and gravity data appear to suggest well-correlated linear marine magnetic anomalies (possible isochrons) perpendicular to T, bounded and offset by less well-defined steps and linear lows in the gravity (possible fracture zones). These magnetic and gravity data southwest of T suggest that the crust here may record an E–W spreading episode between the two-plate system of East and West Gondwana prior to the initiation of the three-plate spreading system of South America, Africa and Antarctica. The E–W spreading record to the east of about 35°W would then appear to have been cut off at about M10 time during the establishment of N–S three-plate spreading along the South American–Antarctic Ridge and then subducted under the Scotia Ridge.     

Multibeam bathymetry and the depositional environments of Kongsfjorden and Krossfjorden, western Spitsbergen, Svalbard

Kongsfjorden and Krossfjorden are two ice-proximal fjords on the western coast of Spitsbergen which have been surveyed using multibeam bathymetry, sub-bottom profiling and gravity coring. Central and outer Kongsfjorden is dominated by a 30 km² outcrop of bedrock, with a thin (<10m) sediment cover. The bedrock displays a relict sub-glacial, ice-scoured topography produced during the glacial re-advances of the Weichselian (20 Ky BP) and again during the last major Holocene re-advance of the Little Ice Age (550-200 yrs BP). Drumlins and glacial flutes are common across the floor of Kongsfjorden, with lengths of 1.5-2.5 km and widths of <100 m, rising up to 10 m in water depths of <100 m. This topography is smoothed by bottom currents from the wind-driven forcing of surface waters. The flow is counter-clockwise, matching boundary layer movement under the influence of Coriolis force. Both fjords are characterized by a variable acoustic character, based on sub-bottom profile data. The deepest basins are dominated by parallel, well-laminated reflectors and an irregular-transparent acoustic character indicating the presence of Holocene-age fine-grained sediments up to 30 m thick. A parallel, irregular-transparent acoustic character with waveform morphology in inner Kongsfjorden is interpreted as moraines, originating from the 1948 and 1869 surges of Kronebreen glacier. Mass-flows are common on the flanks of topographic highs as acoustically chaotic-transparent lensoid and wedge-shaped reflectors. The sediments of outer and central Kongsfjorden are characterized by bioturbated, gas-rich homogeneous muds interpreted as being the result of the settling of fine-grained sediment and particulate suspensions.     

Basement geology beneath the northeast Thelon Basin,Nunavut: insights from integrating new gravity,magnetic and geological data

Current models for unconformity‐associated uranium deposits predict fluid flow and ore deposition along reactivated faults in >1.76 Ga basement beneath Mesoproterozoic siliciclastic basins. In frontier regions such as the Thelon Basin in the Kivalliq region of Nunavut, little is known about the sub‐basin distribution of units and structures, making exploration targeting very tenuous. We constructed a geological map of the basement beneath the unconformity by extrapolating exposed features into the subsurface. The new map is constrained by detailed geological, geophysical, and rock property observations of outcrops adjacent to the basin and by aeromagnetic and gravity data over the geophysically transparent sedimentary basin. From rock property measurements, it is clear that the diverse magnetic and density characteristics of major rock packages provide quantitative three‐dimensional constraints. Gravity profiles forward modelled in four cross sections define broad synforms of the Amer Belt and Archean volcanic rocks that are consistent with the structural style outside the basin. Major lithotectonic entities beneath the unconformity include: supracrustal rocks of the Archean Woodburn Lake group and Marjorie Hills meta sedimentary gneiss and associated mixed granitoid and amphibolitic gneiss; the Amer Mylonite Zone and inferred mafic intrusions oriented parallel and sub‐parallel; other igneous intrusions of 2.6 Ga, 1.83 Ga, and 1.75 Ga vintage; and the <2.3 Ga to >1.84 Ga Amer Group. Four main brittle regional fault arrays (040°–060°, 075°–90°, 120°, and 150°) controlled development and preservation of the basin. The reactivated intersections of such faults along fertile basement units such as the Rumble assemblage, Marjorie Hills assemblage, Nueltin igneous rocks, and Pitz formation are the best targets for uranium exploration.     

Numerical investigation of alternative fracture stiffness measures and their respective scaling behaviours

We study the mechanical deformation of fractures under normal stress, via tangent and specific fracture stiffnesses, for different length scales using numerical simulations and analytical insights. First, we revisit an equivalent elastic layer model that leads to two expressions: the tangent stiffness is the sum of an “intrinsic” stiffness and the normal stress, and the specific stiffness is the tangent stiffness divided by the fracture aperture at current stress. Second, we simulate the deformation of rough fractures using a boundary element method where fracture surfaces represented by elastic asperities on an elastic half‐space follow a self‐affine distribution. A large number of statistically identical “parent” fractures are generated, from which sub‐fractures of smaller dimensions are extracted. The self‐affine distribution implies that the stress‐free fracture aperture increases with fracture length with a power law in agreement with the chosen Hurst exponent. All simulated fractures exhibit an increase in the specific stiffness with stress and an average decrease with increase in length consistent with field observations. The simulated specific and tangent stiffnesses are well described by the equivalent layer model provided the “intrinsic” stiffness slightly decreases with fracture length following a power law. By combining numerical simulations and the analytical model, the effect of scale and stress on fracture stiffness measures can be easily separated using the concept of “intrinsic” stiffness. We learn that the primary reason for the variability in specific stiffness with length comes from the fact that the typical aperture of the self‐affine fractures itself scales with the length of the fractures.     

Colonisation of South African kelp-bed canopies by the alien mussel Mytilus galloprovincialis: extent and implications of a novel bioinvasion

The Mediterranean mussel Mytilus galloprovincialis is the most significant invasive alien marine species in South Africa and, although not normally found subtidally, has recently been observed colonising heads and stipes of the kelp species Ecklonia maxima in False Bay. We quantified this invasion and explored its ecological implications. Transects laid across kelp beds revealed that 10.34% of kelp individuals surveyed bore canopy mussels, with kelp heads (10.07%) being far more commonly infected than stipes (0.27%). Twenty kelp individuals with infected heads and 20 with infected stipes were separately collected for more-detailed examination. Wet mass of mussels on these kelp heads ranged from 2.5 to 2 462 g (median 86.4 g, interquartile range 14.8–353.8 g) and that of mussels on the stipes from 7.6 to 3 492 g (median 595.5 g, interquartile range 194.0–955.0 g). Mussel clumps consisted mostly of individuals <40 mm in length. Mussel clumps supported a rich biota of 80 invertebrate and 13 algal species. Larger clumps supported more epibiotic species, and those on stipes more species than those of comparable mass on kelp heads. The mussels and their associated epibiotic species negatively affected kelp buoyancy, but rarely enough to overcome natural buoyancy. Some kelp individuals that had been toppled by the weight of mussels and their epibiotic species, however, were encountered in situ. Implications of this invasion include large increases in animal biomass and species richness in the kelp canopy, plus reductions in kelp buoyancy and increased hydrodynamic drag on infected kelps, increasing their probability of being uprooted. Uprooted kelp individuals can raft long distances, potentially transporting both native and alien species to distant sites.     

Analytical solutions of linear finite‐ and small‐strain one‐dimensional consolidation

Analytical solutions are presented for linear finite‐strain one‐dimensional consolidation of initially unconsolidated soil layers with surcharge loading for both one‐ and two‐way drainage. These solutions complement earlier solutions for initially unconsolidated soil layers without surcharge and initially normally consolidated soil layers with surcharge. Small‐strain solutions for the consolidation of initially unconsolidated soil layers with surcharge loading are also presented, and the relationship between the earlier solutions for initially unconsolidated soil without surcharge and the corresponding small‐strain solutions, which was not addressed in the earlier work, is clarified. The new solutions for initially unconsolidated soil with surcharge loading can be applied to the analysis of low stress consolidation tests and to the partial validation of numerical solutions of non‐linear finite‐strain consolidation. They also clarify a formerly perplexing aspect of finite‐strain solution charts first noted in numerical solutions. Copyright © 2004 John Wiley & Sons, Ltd.     

Fire intensity,slopewash and bio‐transfer of sediment in eucalypt forest,Australia

Sediment movement (slopewash and bio‐transfer), runoff, and organic matter movement (mainly leaf litter, ash and charcoal) were monitored on ten plots for a six‐month period following bushfires in eastern Australia in the 1990s. Plots were installed in areas which had experienced high, moderate and low intensity burns. Although between‐plot variability was high, slopewash and organic matter amounts were greatest on the intensely burnt areas, and progressively less on moderate and low intensity burns. In contrast, most leaf litter input from scorched leaf fall was derived from, and collected in, moderately burnt areas. Bio‐transfer of sediment (direct downslope surface movement produced by faunal activity) occurred mainly by ant mounding, and was more than ten times greater on moderately burnt areas than on plots having high or low intensity burns. Bio‐transfer accounted for approximately 36 per cent of total sediment collected, with this material being recorded least often and in smallest quantities on areas of high intensity burns. Bio‐transfer by ant mounding and animal scratchings contributed loose surface sediment for transport by overland flow, disproportionately increasing total sediment movement to plot aprons in areas of moderate intensity burns. Copyright © 2002 John Wiley & Sons, Ltd.