Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5° S: Upwelling centers and variations in crustal thickness

To decipher the distribution of mass anomalies near the earth's surface and their relation to the major tectonic elements of a spreading plate boundary, we have analyzed shipboard gravity data in the vicinity of the southern Mid-Atlantic Ridge at 31–34.5° S. The area of study covers six ridge segments, two major transforms, the Cox and Meteor, and three small offsets or discordant zones. One of these small offsets is an elongate, deep basin at 33.5° S that strikes at about 45° to the adjoining ridge axes.By subtracting from the free-air anomaly the three-dimensional (3-D) effects of the seafloor topography and Moho relief, assuming constant densities of the crust and mantle and constant crustal thickness, we generate the mantle Bouguer anomaly. The mantle Bouguer anomaly is caused by variations in crustal thickness and the temperature and density structure of the mantle. By subtracting from the mantle Bouguer anomaly the effects of the density variations due to the 3-D thermal structure predicted by a simple model of passive flow in the mantle, we calculate the residual gravity anomalies. We interpret residual gravity anomalies in terms of anomalous crustal thickness variations and/or mantle thermal structures that are not considered in the forward model. As inferred from the residual map, the deep, major fracture zone valleys and the median, rift valleys are not isostatically compensated by thin crust. Thin crust may be associated with the broad, inactive segment of the Meteor fracture zone but is not clearly detected in the narrow, active transform zone. On the other hand, the presence of high residual anomalies along the relict trace of the oblique offset at 33.5° S suggests that thin crust may have been generated at an oblique spreading center which has experienced a restricted magma supply. The two smaller offsets at 31.3° S and 32.5° S also show residual anomalies suggesting thin crust but the anomalies are less pronounced than that at the 33.5° S oblique offset. There is a distinct, circular-shaped mantle Bouguer low centered on the shallowest portion of the ridge segment at about 33° S, which may represent upwelling in the form of a mantle plume beneath this ridge, or the progressive, along-axis crustal thinning caused by a centered, localized magma supply zone. Both mantle Bouguer and residual anomalies show a distinct, local low to the west of the ridge south of the 33.5° S oblique offset and relatively high values at and to the east of this ridge segment. We interpret this pattern as an indication that the upwelling center in the mantle for this ridge is off-axis to the west of the ridge.     

Dynamic adaptive pathways in downscaled climate change scenarios

The parallel scenario process enables characterization of climate-related risks and response options to climate change under different socio-economic futures and development prospects. The process is based on representative concentration pathways, shared socio-economic pathways, and shared policy assumptions. Although this scenario architecture is a powerful tool for evaluating the intersection of climate and society at the regional and global level, more specific context is needed to explore and understand risks, drivers, and enablers of change at the national and local level. We discuss the need for a stronger recognition of such national-scale characteristics to make climate change scenarios more relevant at the national and local scale, and propose ways to enrich the scenario architecture with locally relevant details that enhance salience, legitimacy, and credibility for stakeholders. Dynamic adaptive pathways are introduced as useful tools to draw out which elements of a potentially infinite scenario space connect with decision-relevant aspects of particular climate-related and non-climate-related risks and response options. Reviewing adaptation pathways for New Zealand case studies, we demonstrate how this approach could bring the global-scale scenario architecture within reach of local-scale decision-making. Such a process would enhance the utility of scenarios for mapping climate-related risks and adaptation options at the local scale, involving appropriate stakeholder involvement.     

Australian conservation organisations' perspectives on tourism in national parks: a critique

Australia's conservation and parks organisations have become actively involved in the debate over tourism use of national parks and their associated wilderness areas. To date, at least seven organisations have produced at least ten formal policy statements and submissions. This paper presents an analysis of these policy statements.The policies reviewed use a narrow definition of tourism, based on the presence or absence of commercial, private sector activity. If commercial activity exists, then tourism exists; if not, then it is considered to be recreation. This distinction reflects the widely held opinion among many environmentalists that non-commercial use of national parks is acceptable, but that commercial use is essentially unacceptable, even though the groups may be participating in the same activity. The author argues that while making a number of valuable suggestions, these policies suffer from major conceptual flaws that minimise their contribution to the debate on acceptable human uses of parks.The author concludes by making a number of recommendations to rectify the conceptual flaws inherent in the policies.     

Three-dimensional temperature structure of a ridge-transform-ridge system

We have solved the problem of three-dimensional heat transfer by conduction and material transport within a ridge-transform-ridge system. The boundary conditions are those of a finite plate with temperatures fixed at the top and bottom. We do not solve for the pattern of flow, so must arbitrarily describe the material transport. Within triangular regions beneath the spreading centers, flow is assumed to be vertical and at constant velocity, with the volume of upwelling matching the volume of horizontal transport by the plates. The steady state solution is approached by repeated finite steps in time, each involving two stages. First, to represent material transport, temperatures in the grid of points are shifted horizontally and/or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field. Second, a three-dimensional Fourier Transform technique is used to calculate the cooling which would occur in the static case during the same period.

We find that the geometry of flow in the upwelling region, here parameterized by velocity of upwelling or width of the upwelling zone, is more important than spreading rate in determining the temperature structure. The results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone. For a 10 m.y. offset on the transform fault, conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault, in contrast to the deepening of the median valley over several tens of kilometers observed on the Mid-Atlantic Ridge.     

Crustal structure and isostatic compensation near the Kane fracture zone from topography and gravity measurements—I. Spectral analysis approach

Summary. Six gravity and bathymetry profiles perpendicular to the Kane fracture zone, each more than 300 km long, were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages. A spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths. For remaining wavelengths the admittance, the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry, is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate, about 6 km in thickness. After subtracting the gravitational attraction of the bathymetry and its compensation, substantial isostatic anomalies remain. We interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography, except at very long wavelengths. The apparent crustal thickness varies by as much as a factor of 2, but there is no evidence indicating systematic thinning of the crust beneath the fracture zone. Our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface. This indicates that there are two different modes of crustal formation with different gravity and topographic signatures: effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies, and intrusive volcanism or underplating producing gravity anomalies but little topographic relief.     

Multi-unit building address geocoding: An approach without indoor location reference data

Accurately mapped locations within multi-unit properties are useful for several organizations in today's society. Published work on geocoding methods either require detailed location reference data or does not apply to multi-unit buildings. In this research, a generalizable method is realized to map apartment addresses to their explicit locations without access to indoor location reference data based on publicly available address- and geospatial-building information. The performance of this approach is measured by conducting a comparative study between a linear interpolation baseline and gradient-boosted decision trees model. The proposed method can successfully geocode addresses across different building shapes and sizes. Furthermore, the model significantly outperforms the baseline in terms of positional accuracy proving the feasibility of approximating apartment locations by their address- and geospatial-building information.     

Evolution of drainage patterns on Cape York Peninsula,northeast Queensland

Drainage patterns along passive continental margins are often hypothesised to be the result of drainage disruption following highland uplift and downwarping of the highland flank. Several studies of stream catchments throughout southeast Australia have demonstrated that the opposite tends to be the case in this region because the field evidence favours stream and continental drainage‐divide stability. While significant advances have been made towards understanding this phenomenon in the southeastern corner of the continent, little is known of the evolution of streams and highlands in northeast Australia. Our study examines palaeochannels and fluvial sedimentary units close to the continental drainage divide in six stream catchments along the length of Cape York Peninsula. The results show that four of the catchments (Barron‐Mitchell and Stewart‐Holroyd) have experienced continental divide and drainage stability, whereas the Pascoe‐Wenlock system appears to have experienced westward migration of the continental drainage divide and diversion of the Pascoe River. River diversion here is likely to be a result of the raising of base‐level and flooding of stream channels during the Cretaceous marine transgression and subsequent stream incision by the Pascoe River along structural weaknesses in the underlying strata, following cessation of marine conditions.     

A Multiple Criteria Decision Modelling approach to selection of estimation techniques for fitting extreme floods

The problem of fitting a probability distribution, here log-Pearson Type III distribution, to extreme floods is considered from the point of view of two numerical and three non-numerical criteria. The six techniques of fitting considered include classical techniques (maximum likelihood, moments of logarithms of flows) and new methods such as mixed moments and the generalized method of moments developed by two of the co-authors. The latter method consists of fitting the distribution using moments of different order, in particular the SAM method (Sundry Averages Method) uses the moments of order 0 (geometric mean), 1 (arithmetic mean), –1 (harmonic mean) and leads to a smaller variance of the parameters. The criteria used to select the method of parameter estimation are: