Province Boundaries Research Articles

Gravity inversion using seismically derived crustal density models and genetic algorithms: an application to the Caribbean-South American Plate boundary

SUMMARY Crustal density models derived from seismic velocity models by means of velocity–density conversions typically reproduce the main features of the observed gravity anomaly over the area but often show significant misfits. Given the uncertainty in the relationship between velocity and density, seismically derived density models should be regarded as an initial estimate of the true subsurface density structure. In this paper, we present a method for estimating the adjustments necessary to a seismically derived density model to improve the fit to gravity data. The method combines the Genetic Algorithm paradigm with linear inversion as a way to approach the non-linear and linear aspects of the problem. The models are divided into three layers representing the sedimentary column, the crystalline crust and the lithospheric mantle; the depths of these layers are determined from the seismic velocity model. Each of the layers is divided into a number of provinces and a density adjustment (Δρ) value is found for each province so that the residual gravity (difference between the observed gravity anomaly and the anomaly calculated for the seismically derived model) is minimized while keeping Δρ between predefined bounds. The preferred position of the province boundaries is found through the artificial evolution of a population of solutions. Given the stochastic nature of the algorithm and the non-uniqueness of the problem, different realizations can yield different solutions. By performing multiple realizations we can analyse a set of solutions by taking their mean and standard deviation, providing not only an estimate of the Δρ distribution in the subsurface but also an estimate of the associated uncertainty. Synthetic tests prove the ability of the algorithm to accurately recover the location of province boundaries and the Δρ values for a known model when using noise-free synthetic data. When noise is added to the data, the algorithm broadly recovers the features that define the known model despite greater standard deviations of the solutions and the occurrence of artefacts in the mean solutions. The algorithm was applied to four profiles across the Caribbean–South America Plate boundary. Some general patterns in the distribution of Δρ were observed consistently in the profiles and are correlated with the interpretations of the velocity models. Positive Δρ values in the sedimentary layer, negative Δρ values for island arc and extended island arc crust with an abrupt change to positive values in South American crust, and positive values in the mantle under the continent and island arc with a transition to negative values under the Caribbean oceanic crust.

Fiscal Decentralisation in the Netherlands: History, Current Practice and Economic Theory

Fiscal Decentralisation in the Netherlands: History, Current Practice and Economic Theory

Decadal changes in ecological provinces of the Northwest Atlantic Ocean revealed by satellite observations

Detecting the effect of climate change on ocean ecosystems is facilitated if the data are partitioned into ecological provinces. It is recognised that the boundaries of such provinces are dynamic, raising the imperative for frequent observations. Using satellite data, we have assembled a time series of ecosystem delineation for the Northwest Atlantic Ocean over a period of ten years. For each province identified in the study area, we have compiled data on sea‐surface temperature, and on the biomass and community composition of phytoplankton. We find significant trends in these properties but they vary according to the province concerned, which emphasizes how the physical and biological changes are correlated at large scale. Comparisons between the trends computed using the average position of the provinces and the trends derived when accounting for the movements of the boundaries underline the value of the new approach.

Biogeography of the livebearing fishPoecilia gilliiin Costa Rica: are phylogeographical breaks congruent with fish community boundaries?

One of the original goals of phylogeography was to use genetic data to identify historical events that might contribute to breaks among communities. In this study, we examine the phylogeography of a common livebearing fish (Poecilia gillii) from Costa Rica. Our goal was to determine if phylogeographical breaks in this species were congruent with previously defined boundaries among four fish community provinces. We hypothesized that if abiotic factors influence both community boundaries and genetic structuring in P. gillii then we might find four clades within our focal species that were geographically separated along community boundary lines. Similarly, we expected to find most of the genetic variation in P. gillii partitioned among these four geographical regions. We generated DNA sequence data (mitochondrial cytochrome b and nuclear S7 small ribosomal subunit) for 260 individuals from 42 populations distributed across Costa Rica. We analysed these data using phylogenetic (parsimony and likelihood) and coalescent approaches to estimate phylogenetic relationships among haplotypes, patterns of gene flow and effective population size. Contrary to our expectations, we did not find four monophyletic groups that mapped cleanly to our geographical community provinces. However, one of our clades was restricted to a single province, suggesting that common earth history events could be responsible for both genetic structuring in P. gillii and fish community composition in this area. However, our results show a complex pattern of gene flow throughout other regions in Costa Rica where genetic structuring is not predicted by community province boundaries.

EMScope: A Continental Scale Magnetotelluric Observatory and Data Discovery Resource

EMScope, a component of the NSF Earthscope project, is installing a 70 km-spaced grid of long-period magnetotelluric stations across the USA. Rapid data quality control and generation of derived data products provides an accessible resource openly available without charge or restriction through the IRIS Data Management Center and EMScope data portals via a set of data discovery tools. These data are available typically within two weeks of acquisition. Initial 3-d inversion of such EMScope data from the US Pacific Northwest shows substantial coherence between crust and mantle electrical conductivity variations, the boundaries of major physiographic provinces, and seismically delineated features.

Objective global ocean biogeographic provinces

Biogeographic provinces are categories used for comparing and contrasting biogeochemical processes and biodiversity between ocean regions. Provinces provide a framework for reasonable extrapolation of point or transect data to broader areas. However, their use is limited due to the non‐automatic, subjective nature of province classification. Furthermore, it is unknown how province boundaries respond to seasonal and climate forcing. These issues make province related hypotheses difficult to test with static provinces. To solve this problem, we use objective classification on global remote sensing data to automatically produce time and space resolved ocean provinces. Seasonal patterns in province geography reflect well‐known ocean processes. Our predictions of province boundaries are verified by in‐situ ship track data and province distributions in the equatorial Pacific correlate well with ENSO indexes. This objective classification system captures spatial and temporal province dynamics and provides objective categories for cross‐province biogeochemical hypotheses to be rigorously tested.

The Contamination Levels of Organochlorine Pesticides in Water and Sediment Samples in Uluabat Lake, Turkey

The Uluabat Lake is a reservoir of 125 km2 surface area, in the boundaries of Bursa province, Turkey. The results showed that Uluabat Lake was still contaminated by organochlorine pesticides and their residues despite the existence of bans over a long time. Considerable amounts of organochlorine pesticides (HCB, p.p'-DDT, p.p'-DDE, alpha-, beta- and gamma-BHC, Aldrin, Heptachlor epoxide, Endrin, Endosulfan I and II) were detected in water and sediment samples during one-year study in this lake. Samples of water and sediment were collected in different regions, (6 sampling point) of the lake between February 2002 and November 2002. The mean levels of organochlorine pesticides in water and sediments were determined. According to results, it was found that various environs in Uluabat Lake were contaminated by 11 different organochlorine pesticides. Residue analyses showed that higher organochlorine pesticides and their residues were generally more accumulated in the sediment samples.

An integrated multi-scale 3D seismic model of the Archaean Yilgarn Craton, Australia

The collection of a range of different seismic data types has greatly improved our understanding of the crustal architecture of Australia's Archaean Yilgarn Craton over the last few years. These seismic data include broadband seismic studies, seismic receiver functions, wide-angle recordings and mine-scale to deep seismic reflection transects. Each data set provides information on the three-dimensional (3D) tectonic model of the Yilgarn Craton from the craton scale through to the mine scale. This paper demonstrates that the integration and rationalisation of these different seismic data sets into a multi-scale 3D geological/seismic model, that can be visualised at once in a single software package, and incorporating all available data sets, significantly enhances this understanding. This enhanced understanding occurred because the integrated 3D model allowed easy and accurate comparison of one result against another, and facilitated the integrated questioning and interrogation across scales and seismic method. As a result, there are feedback questions regarding understanding of the individual seismic data sets themselves, as well as the Yilgarn Craton as a whole. The methodology used, including all the data sets in the model range, had to allow for the wide range of data sets, frequencies and seismic modes. At the craton scale, P-wave, S-wave and surface wave variations constrained the 3D lithospheric velocity model, revealing noticeable large-scale velocity variations within and across the craton. An interesting feature of the data, easily identified in 3D, is the presence of a fast S-wave velocity anomaly (> 4.8 km s − 1 ) within the upper mantle. This velocity anomaly dips east and has a series of step-down offsets that coincide approximately with province and terrane boundaries of the Yilgarn Craton. One-dimensional receiver function profiles show variations in their crustal velocity across the craton. These crustal velocity variations are consistent with the larger-scale geological subdivision of the craton, and provide characteristic profiles for provinces and terranes. The receiver function results and the deep seismic reflection data both agree on the depth to the Moho, and both indicate an increase in Moho depth to the east. The 2D seismic refraction results in the south-west of the craton provide crustal thickness information, an indication of middle and lower crustal compositions, and information regarding the broad-scale architectural framework. At the province- and terrane-scale, the deep seismic reflection data and the mine-scale seismic data provide geometric constraints on crustal architecture, in particular the orientation of the region's fault systems as well as variations in the thickness of the granite–greenstone succession. Integration of the results from wide-angle seismic refraction data coincident with the deep seismic reflection data provided additional constraints on likely upper crustal lithologies. The integrated 3D seismic model implies the dominant geodynamic process involved the development of an orogenic belt that developed with a series of contractional (folding and thrusting) events, separated by equally important extensional events. The seismic reflection data in particular suggests that extensional movement on many shear zones was more common than previously thought. The seismic reflection data suggest that the dominant mineral systems involved deeply sourced fluid flowing up crustal-penetrating shear zones. These deeply sourced fluids were further focussed into sites located above fault-breached domal regions in the upper crust.

Mapping seabed variability: Rapid surveying of coastal regions

Directional broadband reverberation can be exploited for rapidly probing seabed variability over large areas O (103–104)km2; the data need not be calibrated. Seabed variability thus mapped inherently combines spatial variability in geoacoustic properties that control reflection and those that control scattering. It is shown that in some cases, the variability can be separated. The method is applied to measurements in the Straits of Sicily where the data indicate the existence of two provinces where the statistics of the geoparameters are distinct. Three attributes of each province are quantified: the province boundaries, measures of spatial variability and/or uncertainty, and the location of discrete scattering features (that may lead to sonar clutter). One province is quite large and exhibits remarkably little spatial variability. The variability is probed on lateral scales ∼200×400m, yet the ratio of the seabed geoparameters varies less than 3 dB from 300−1600Hz over ∼1000km2. While this result seems contrary to prevailing notions of extreme seabed variability in shallow water, it indicates that certain kinds of geophysical (and concomitant geoacoustic) variability may be averaged in such a way that the seabed appears approximately homogeneous to long-range acoustic systems.

Investigating the deep structure of terranes and terrane boundaries: insights from earthquake seismic data

Abstract The crust and upper mantle structure beneath a single, three-component broad-band seismic station may be determined using energy from distant earthquakes. Features such as the depth of the seismic Moho, the abruptness of the change in seismic velocity across the Moho, the velocity profile through the crust and the seismic velocity of the upper mantle may be found using receiver function techniques. Waveforms are analysed which contain energy arising from the interaction of incoming signals with the Earth structure beneath the receiving station. Seismic characteristics of the deep crust are often continuous or slowly varying across a single terrane and, moreover, show sudden contrasts across terrane and other major tectonic boundaries. Such techniques are, therefore, appropriate tools in the exploration of deep structure and, hence, in the understanding of large-scale continental assembly. They complement geological and geochronological investigations of surface rocks by providing the third spatial dimension. In regions of no surface basement exposure, seismic and other geophysical methods provide the only means of mapping the lateral extent of a given terrane and/or tracing province boundaries. Receiver function methods are presented here in the context of terrane tectonics with illustrative examples from former Gondwanan provinces and locations on the Pacific margin.

The Seismic Structure of Wilkes Land/Terre Adelie, East Antarctica and Comparison with Australia: First Steps in Reconstructing the Deep Lithosphere of Gondwana

A determination of the seismic structure of the crust and uppermost mantle of East Antarctica, in the region of Casey station, Wilkes Land and Dumont DUrville station, Terre Adelie is presented. High-fidelity waveforms from teleseismic earthquakes recorded at stations CASY and DRV (1996-2001) are used to calculate the seismic receiver function, the signal produced as energy passes through layers in the seismic velocity structure under the receiving station. The receiver functions are stacked to improve the signal-to-noise ratio and then modelled using an inverse algorithm to find the structure that best fits the observed waveform at each station. Inferences are made regarding the tectonic structure, in particular, the crustal thickness and character of the seismic Moho. The crustal thickness under Casey Station is found to be 30 km (+/- 2 km) with a fairly sharp Moho, considerably less than Dumont D'Urville Station, where the crustal thickness is 42 km, and there IS a significant low velocity region the deep crust. The structure of the Wilkes Land lithosphere is comparable to that of the Albany-Fraser Orogen, Western Australia, part of its conjugate margin. This places a new constraint on the relative position of East Antarctica and Australia in the reconstruction of Gondwana, and earlier, supercontinents. A recent reinterpretation of Antarctic geology proposes tectonic province boundaries trending perpendicular to the coast with counterparts in southern Australia. Seismic techniques, determining structure beneath regions with no surface exposure, are vital tools in testing such tectonic hypotheses, towards the reconstruction of Gondwana to full lithospheric depth.

U–Pb and Sm–Nd evidence for 1.76–1.77 Ga magmatism in the Moriru region, Mato Grosso, Brazil: implications for province boundaries in the SW Amazon Craton

Abstract The Moriru region, situated in the SW Amazon Craton, Mato Grosso, Brazil, is composed of two main domains: (1) an area of undeformed bimodal volcanic rocks and (2) an area of deformed granitic–gneissic–migmatite rocks that also include a small occurrence of slightly deformed rhyolite. The undeformed domain is composed of felsic to mafic flows and volcanoclastic sediments; the felsic flows are essentially explosive deposits classified as ignimbrites, however flows lava occurs interlayed. The compositions of ignimbrites and associated felsic lava are rhyodacite to dacite. The mafic volcanics are phaneritic, fine-grained, and are often strongly hydrothermally altered. Unaltered samples have tholeiitic to transitional compositions and have within-plate petrologic and geochemical characteristics. The felsic volcanics are enriched in light REE, show a negative Eu anomaly, and are depleted in heavy REE. The trace elements indicate a post-collisional environment. U–Pb data on single zircon grains from the volcanic sequence yield ages between 1770±6 Ma and 1778±6 Ma. eNd( t ) values range from −1.4 to +1.5 ( t =1.75 Ga), and the T DM model ages range from 2.02 to 2.16 Ga. Rocks from the deformed domain are calc-alkaline to transitional and have metaluminous to peraluminous compositions. The metaluminous granites are hornblende-monzogranites and the peraluminous rocks are biotite-granites and granodiorite. Both groups plot in post-tectonic fields of trace element diagrams. U–Pb data on single zircon grains from the deformed granite–gneiss–migmatite domain range from 1662±13 Ma to 1803±6 Ma, but most cluster about a mean of 1763 Ma (1759±3 Ma to 1767±2 Ma). eNd( t ) values ( t =1.75 Ga) vary from −3.4 to +3.0, and the T DM model ages range from 1.94 to 2.28 Ga. The approximately 1775 Ma volcano-plutonic rocks in the area probably resulted from partial melting of slightly older continental lithosphere and may have formed during an extensional event about 1.78–1.76 Ga. The plutonic rocks seem to have a similar source and may be intrusive into the volcanic sequence, despite their more deformed nature. These ages are similar to those proposed for older parts of the 1.80–1.55 Ga Rio Negro-Juruena province and younger than 1.9–1.8 Ga, as proposed for the Ventuari-Tapajos province. Sm–Nd isotopic data are consistent with formation of these rocks from ca. 1.9–2.2 Ga continental lithosphere. It is not yet necessary to relocate the proposed boundary between the Rio Negro-Juruena and Ventuari-Tapajos provinces, since provinces should be defined by the ages of the main accretionary assemblages and not the ages of younger intrusive suites. More studies are needed to refine these province boundaries.

Petrochemistry of granitic rocks in the Mount Barcroft area—Implications for arc evolution, central White Mountains, easternmost California

The north-northwest–trending White-Inyo Range locally defines the western edge of the Great Basin. The northeast-trending Barcroft structural break lies astride the province boundary. Along this preintrusive, high-angle reverse fault, middle Mesozoic White Mountain Peak alkaline volcanic and intercalated volcaniclastic rocks on the north are separated from uppermost Proterozoic–Lower Cambrian miogeoclinal quartzite and carbonate strata on the south by the 165 ± 1 Ma (SHRIMP-RG, [sensitive, high-resolution ion microprobe–reverse geometry], U-Pb zircon) Barcroft pluton. Although locally faulted, the subparallel southeast and northwest borders of the body display intrusive contacts. Finer-grained comagmatic metadiorite occurs as early-stage dikes in the wall rocks. Eastward, the 100 Ma McAfee Creek granite intrudes the Barcroft pluton. Tertiary diabase dikes crosscut the section. The two granitic series, reflecting local evolution of the Mesozoic arc system, are described in this paper: (1) mafic granitoid rocks of the calc-alkaline Barcroft series, including chemically intergradational granodiorite, gabbro/diorite, metadiorite, and rare alaskite-aplite—all rich in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), and (2) granite of the felsic McAfee Creek series, which has even greater enrichment of LILEs and greater depletion in HFSEs than the Barcroft pluton. Rocks rich in hornblende ± clinopyroxene and belonging to the Ca-rich, metaluminous Barcroft series exhibit a broad range of chemical and mineral compositions and represent products of both mixing between high- and low-silica members of the series to generate intermediate compositions and fractional crystallization to generate the most mafic (cumulate) rocks. The younger, more homogeneous, K-rich McAfee Creek–type muscovite granite possesses mildly peraluminous, minimum-melt compositions. Field evidence for magma mixing and isotopic data for the Barcroft series (e Nd( t ) = −1.53 to −5.50, 87 Sr/ 86 Sr (i) = 0.7053−0.7063) and the McAfee Creek granite (e Nd( t ) = −5.64 to −9.76, 87 Sr/ 86 Sr (i) = 0.7062–0.7116) require open-system processes and involvement of preexisting crustal rocks in their genesis. The Barcroft series is one of several shallow-level magma systems now recognized in the Sierran arc that include significant amounts of reworked, preexisting crustal material. These magma series are distinct from those that represent demonstrably deeper levels of Mesozoic magma systems and lack isotopic evidence for significant involvement of older crust. Despite these differences in magma sources and evolution, both magma series evolved along indistinguishable petrologic and chemical trends. Igneous rocks in the White Mountains record post-Paleozoic growth of this part of the Californian margin: (1) Jurassic and younger subduction, partial fusion of the overlying mantle wedge, and/or deep-seated mafic crust, ascent of mildly alkaline and later calc-alkaline magmas attending Andean-style volcanic-plutonic arc production, with significant involvement of preexisting crustal rocks; (2) Late Cretaceous thickening, heating, and crustal contamination, followed by rise of peraluminous granite-minimum melts; and (3) Neogene Basin and Range lithospheric transtension, tapping of upper-mantle diabasic melt, and dike emplacement.

Fault structure and kinematics of the Long Valley Caldera region, California, revealed by high‐accuracy earthquake hypocenters and focal mechanism stress inversions

We have determined high‐resolution hypocenters for 45,000+ earthquakes that occurred between 1980 and 2000 in the Long Valley caldera area using a double‐difference earthquake location algorithm and routinely determined arrival times. The locations reveal numerous discrete fault planes in the southern caldera and adjacent Sierra Nevada block (SNB). Intracaldera faults include a series of east/west‐striking right‐lateral strike‐slip faults beneath the caldera's south moat and a series of more northerly striking strike‐slip/normal faults beneath the caldera's resurgent dome. Seismicity in the SNB south of the caldera is confined to a crustal block bounded on the west by an east‐dipping oblique normal fault and on the east by the Hilton Creek fault. Two NE‐striking left‐lateral strike‐slip faults are responsible for most seismicity within this block. To understand better the stresses driving seismicity, we performed stress inversions using focal mechanisms with 50 or more first motions. This analysis reveals that the least principal stress direction systematically rotates across the studied region, from NE to SW in the caldera's south moat to WNW–ESE in Round Valley, 25 km to the SE. Because WNW–ESE extension is characteristic of the western boundary of the Basin and Range province, caldera area stresses appear to be locally perturbed. This stress perturbation does not seem to result from magma chamber inflation but may be related to the significant (∼20 km) left step in the locus of extension along the Sierra Nevada/Basin and Range province boundary. This implies that regional‐scale tectonic processes are driving seismic deformation in the Long Valley caldera.

The Master and the Maharajas: The Sikh Princes and the East Punjab Massacres of 1947

EventDuring the spring, summer and autumn of 1947 India's richest province, the Punjab, played host to a massive human catastrophe. The trigger for the catastrophe was Britain's parting gift to its Indian subjects of partition. Confronted by a seemingly intractable demand by the All-India Muslim League for a separate Muslim homeland—Pakistan—a campaign which since 1946 had turned increasingly violent, the British government early in 1947 accepted viceroy Lord Mountbatten's advice that partition was necessary to arrest the country's descent into civil war. ‘Mahatma’ Gandhi notably excepted, the leadership of the Congress party came gradually and reluctantly to the same conclusion. Sardar Vallabhbhai Patel, Jawaharlal Nehru's deputy, likened it to the cutting off of a diseased limb. But in accepting the ‘logic’ of the League's ‘two-nation’ theory, the British applied it remorselessly. They insisted that partition would have to follow the lines of religious affiliation, not the boundaries of provinces. In 1947 League president Muhammad Ali Jinnah was forced to accept what he had contemptuously dismissed in 1944 as a ‘moth-eaten’ Pakistan, a Pakistan bereft of something like half of Bengal and the Punjab and most of Assam.

The application of Curie depth, effective elastic thickness, seismic tomography and seismicity in regional exploration area selection

When selecting geological provinces prospective for giant ore deposits knowledge of the location of deep lithospheric structures and craton margins, and an understanding of the temperature distribution of the Earth, are all important. Curie depth, effective elastic thickness, seismic tomography and seismicity are techniques with the potential to provide this information. Curie depth depends on the geothermal gradient and may, in principle, be derived from aeromagnetic data. However, it is concluded here that such Curie depth maps may have little value in many shield areas. Effective elastic thickness (TE) is a measure of the strength of the lithosphere. Methods for estimating TE are described and, as an example, a new TE map of Brazil is presented. TE gradients correlate well with province boundaries and seismicity, implying that they map major lithospheric structures. Seismic tomography uses the travel times of earthquake P- and S-waves to map velocity variations within the Earth. Many new tomographic velocity models are now becoming available, with resolution <100km in some cases. It appears that temperature is the major control on velocity, at least in the upper mantle. Global seismicity information may be analysed in terms of the distribution of earthquakes, their magnitudes and their focal depths. A correlation between seismicity and large mineral deposits has been observed in certain regions, which motivates the use of this data. Possible reasons for the correlation are discussed here. Since the different maps - TE, seismic velocity and seismicity - respond to different physical properties (or the same physical properties in different ways), it is most beneficial to interpret all three together.

Geomorphology of the Arun Valley, Eastern Nepal

The Arun Valley can be divided into the Upper Arun Periglacial Valley (UAPV) and Lower Arun Fluvial Valley (LAFV). The area separated by the linearly arranged ridges of Jaljale Danda, Harale Danda, and Sichok Danda. The UAPV lies in the north whereas the LAFV occupies the southern territories. The UAPV is further divided into four geomorphic provinces with fifteen distinct Holocene terrace levels, whereas the LAFV is divided into four geomorphic provinces with thirteen Pleistocene terrace levels.&#x0D; The concept of geomorphic province is introduced to denote distinct geomorphological features formed in a type locality within a given geological time span. The identified geomorphic surfaces are morpho-stratigraphically defined at respective villages and have a wide geographic distribution.&#x0D; This paper attempts to classify geomorphologically the whole Arun watershed and to compare the position of various terraces in terms of neo-tectonic activities. The main geomorphic features of the watershed and their relationship with the major geological structures are also discussed. The boundaries of the geomorphic province often coincide with the reported thrusts in the area.

Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden

The volcanic rocks of the Skellefte ore district (ca. 1.89–1.87 Ga), like other volcanic complexes of similar age in the Svecofennian orogenic province, have been regarded as juvenile arc material developed over a contemporaneous subduction zone, and subsequently accreted to the continent, during a period of very rapid crustal growth. A modelled geoelectric anomaly, dipping NE beneath the district from the southern province, has provided support for the subduction/ collision hypothesis. We have used recently acquired geological and geophysical evidence to suggest an alternative to this model. Regional aeromagnetic and gravity data indicate that the Skellefte district is the site of a major tectonic discontinuity. The province to the north of this discontinuity provides evidence of an Archaean basement to within 100 km of Skellefteå. The Skellefte Group, on the southern margin of this province, displays a narrow belt of strong upright folding, trending W to WNW. We correlate this folding with the second major deformation episode (D 2), of similar trend, in the metamorphic complex of the southern province (here informally called the Robertsfors Group). The family of D 2 structures, on both sides of the province boundary, defines the 80 km wide Skellefte Zone. D 2 in the Robertsfors Group consists of discrete ductile shear zones and associated upright folds. These zones are consistently steep, display southside up kinematics, and are superposed on earlier NE trending folding (D 1) and migmatisation in the Robertsfors Group. We infer from the style of D 2 that it could not be responsible for the accretion of an exotic volcanic arc to the continent. Upper crustal thickening was probably largely achieved during D 1. We conclude from the style and orientation of the D 1 structures in the Robertsfors Group, their absence from the Skellefte Group, and their relation to granitoid intrusions, that D 1 pre-dates deposition of the Skellefte Group. We infer that the Robertsfors Group forms part of the previously unknown basement of the Skellefte Group. The latter is therefore regarded as a supracrustal sequence, deposited in a rift setting, rather than a juvenile volcanic arc. It is also concluded that the basement stratigraphic package, which also occurs to the southwest of the Skellefte ore district, and has variable internal structures, is responsible for the modelled geoelectric anomaly. We therefore interpret the tectonic discontinuity as the boundary between pre- and post-1.9 Ga rock groups, both of which have been intruded by post-1.9 Ga granitoids and affected by the localised upright D 2 (post-1.87 Ga) deformation in the Skellefte Zone. The volcanism of the Skellefte ore district and the Ni bearing mafic intrusions of the southern province are attributed to an episode of extension, between D 1 and D 2, which was probably related to a contemporaneous active margin lying west of the present exposure of Svecofennian rocks. D 1 in the Robertsfors Group is attributed to the closure of an extensive marginal basin, possibly involving subduction, prior to Skellefte Group deposition (i.e. before 1.9 Ga). Geochronological data from the Robertsfors Group on the ages of D 1 and D 2, and the regional significance of pre-1.9 Ga metamorphic complexes, are discussed in a companion paper.

Clay mineral distribution in surface sediments between Indonesia and NW Australia — source and transport by ocean currents

The clay mineral distribution in sediments between Indonesia and NW Australia has been assessed on the basis of 166 core-top samples. Clay mineral assemblages are closely related to the geology and weathering regime of the adjacent hinterland and allow the distinction of four clay mineral provinces. Three provinces, Western, Central and Eastern Province are situated along the Indonesian Islands Arc, from Sumatra in the west to Timor in the east. Illite is the major clay mineral of the Western and eastern Province, whereas the Central Province abounds with smectite. The fourth province comprises the NW and West Australian shelf and slope, as well as offshore plateaus and is dominated by kaolinite. Transport of clays by surface and subsurface ocean currents can be observed within the provinces, e.g. with the Leeuwin and West Australian Current in the NW Australian Province and with the outflow of low-salinity water through the Sunda and Lombok straits in the Central Province. Transport of clays across province boundaries is inhibited by strong salinity fronts, with the exception of the boundary between the Central and Eastern Province.

Habitat Selection by the Texas Tortoise in a Managed Thornscrub Ecosystem

Brush encroachment on semiarid shrublands resulting from livestock grazing has created global concern. Southern Texas is dominated by Prosopis-Acacia mixed brush communities typical of the Tamaulipan Biotic Province, and the geographic range of the state-threatened Texas tortoise (Gopherus berlandieri) is nearly identical to the boundaries of this biotic province in Texas. In light of the perceived threat to Texas tortoises because of habitat change caused by brush encroachment, we monitored 36 Texas tortoises by radiotelemetry during 1994-1996 to assess habitat selection on a site containing grazed and ungrazed pastures. Tortoises did not exhibit habitat selection at the level of locations within home ranges. Differential habitat selection at the level of home ranges within study areas was not apparent for sex, but was evident for treatment (grazed or ungrazed). Analysis of pooled data indicated that tortoises exhibited broad-scale selection for home ranges within study areas. Selection was expressed as preferential avoidance of old-field and riparian habitats, which represented vegetational extremes of canopy cover. However, tortoises tolerated the broad continuum of other brush communities on the study site. Apparent treatment differences may be an artifact of our inability to adequately pair study areas given the scale of tortoise movement. Our data indicate that increases in the extent of woody canopy cover resulting from grazing-induced brush encroachment will not be detrimental to Texas tortoises. Furthermore, large-scale range improvement practices, such as root-plowing, create unsuitable habitats for this species.