Basin shape and sediment architecture in the Gun Supersequence: A strike‐slip model for Pb–Zn–Ag ore genesis at Mt Isa

Sequence‐stratigraphic correlations provide a better understanding of sediment architecture in the Mt Isa and lower McNamara Groups of northern Australia. Sediments record deposition in a marine environment on a broad southeast‐facing ramp that extended from the Murphy Inlier in the northwest to the Gorge Creek, Saint Paul and Rufous Fault Zones in the southeast. Depositional systems prograded in a southeasterly direction. Shoreline siliciclastic facies belts initially formed on the western and northern parts of the ramp, deeper water basinal facies occurred to the east and south. The general absence of shoreline facies throughout the Mt Isa Group suggests that depositional systems originally extended further to the east and probably crossed the Kalkadoon‐Leichhardt Block. Fourteen, regionally correlatable fourth‐order sequences, each with a duration of approximately one million years, are identified in the 1670–1655 Ma Gun Supersequence. Stratal correlations of fourth‐order sequences and attendant facies belts resolve a stratigraphic architecture dominated by times of paired subsidence and uplift. This architecture is most consistent with sinistral strike‐slip tectonism along north‐northeast‐oriented structures with dilational jogs along northwest structures as the primary driver for accommodation. Although reactivated during deformation, the ancestral northwest‐trending May Downs, Twenty Nine Mile, Painted Rocks, Transmitter, Redie Creek and Termite Range Fault Zones are interpreted as the principal synsedimentary growth structures. Sinistral strike‐slip resulted in a zone of long‐lived dilation to the north of the May Downs/Twenty Nine Mile and Gorge Creek Fault Zones and a major basin depocentre in the broad southeast‐facing ramp. Subordinate depocentres also developed on the northern side of the ancestral Redie Creek and Termite Range fault zones. Transfer of strike‐slip movement to the east produced restraining or compressive regions, localising areas of uplift and the generation of local unconformities. Northwest‐ and north‐northeast‐oriented magnetic anomalies to the south and west of Mt Isa, identify basement heterogeneities. Basement to the south and west of these anomalies is interpreted to mark intrabasin siliciclastic provenance areas in the Gun depositional system. Pb–Zn–Ag deposits of the Mt Isa valley are interpreted as occurring in a major basin depocentre in response to a renewed phase of paired uplift and subsidence in late Gun time (approximately 1656 Ma). This event is interpreted to have synchronously created accommodation for sediments that host the Mt Isa deposit, while focusing topographically and thermobarically driven basinal fluids into the zone of dilation.     

Soldiers Cap Group iron‐formations,Mt Isa Inlier,Australia, as windows into the hydrothermal evolution of a base‐metal‐bearing Proterozoic rift basin

The Proterozoic Soldiers Cap Group, a product of two major magmatic rift phases separated by clastic sediment deposition, hosts mineralised (e.g. Pegmont Broken Hill‐type deposit) and barren iron oxide‐rich units at three main stratigraphic levels. Evaluation of detailed geological and geochemical features was carried out for one lens of an apatite‐garnet‐rich, laterally extensive (1.9 km) example, the Weatherly Creek iron‐formation, and it was placed in the context of reconnaissance studies of other similar units in the area. Chemical similarities with iron‐formations associated with Broken Hill‐type Pb–Zn deposit iron‐formations are demonstrated here. Concordant contact relationships, mineralogy, geochemical patterns and pre‐deformational alteration all indicate that the Soldiers Cap Group iron‐formations are mainly hydrothermal chemical sediments. Chondrite normalised REE patterns display positive Eu and negative Ce anomalisms, are consistent with components of both high‐temperature, reduced, hydrothermal fluid (≥250°C) and cool oxidised seawater. Major element data suggest a largely mafic provenance for montmorillonitic clays and other detritus during chemical sedimentation, consistent with westward erosion of Cover Sequence 2 volcanic rocks, rather than local mafic sources. Ni enrichment is most consistent with hydrogenous uptake by Mn‐oxides or carbonates. Temperatures inferred from REE data indicate that although they are not strongly enriched, base metals such as Pb and Zn are likely to have been transported and deposited prior to or following iron‐formation deposition. Most chemical sedimentation pre‐dated emplacement of the major mafic igneous sill complexes present in the upper part of the basin. Heating of deep basinal brines in a regional‐scale aquifer by deep‐seated mafic magma chambers is inferred to have driven development of hydrothermal fluids. Three major episodes of extension exhausted this aquifer, but were succeeded by a final climactic extensional phase, which produced widespread voluminous mafic volcanism. The lateral extent of the iron‐formations requires a depositional setting such as a sea‐floor metalliferous sediment blanket or series of brine pools, with iron‐formation deposition likely confined to much smaller fault‐fed areas surrounded by Fe–Mn–P–anomalous sediments. These relationships indicate that in such settings, major sulfide deposits and their associated chemical sediment marker horizons need not overlie major igneous sequences. Rather, the timing of expulsion of hydrothermal fluid reflects the interplay between deep‐seated heating, extension and magmatism.     

Groundwater chemistry of strike slip faulted aquifers: the case study of Wadi Zerka Ma’in aquifers, north east of the Dead Sea

Wadi Zerka Ma’in catchment area is located to the north east of the Dead Sea. It has two types of aquifers: (a) an upper unconfined aquifer and (b) a lower confined aquifer. The two aquifers are separated by a marl aquiclude. A major strike slip fault passes perpendicularly through the two aquifers and the aquiclude layer with embedded normal faults. The aim of the study was to specify the effect of the major strike slip fault on the groundwater chemistry. The spatial variability of the hydrochemical compositions and physiochemical parameters of the groundwater were investigated. It was found that the embedded normal faults, of the strike slip fault, form conduits that allow groundwater to flow from the lower aquifer to the upper aquifer, resulting in mixed groundwater. The ratio of mixing was estimated to be 94 % groundwater from the upper aquifer and 6 % from the lower aquifer. Since groundwater in the lower aquifer is around three times more saline than the upper aquifer, water mixing into the upper water aquifer generates a salinity hazard.     

Age and origin of laterite and silcrete duricrusts and their relationship to episodic tectonism in the mid‐north of South Australia∗

Differential earth movements occurred during Eocene, Miocene, and late Caino‐zoic times. The faulting formed basins of sedimentation, led to dissection of land‐surfaces in some localities and burial in others, and faulted the Cainozoic sediments.

Laterite and silcrete cap remnants of relict landsurfaces of two different ages. Laterite formed before the Eocene; it was faulted and dissected during the Eocene in the north but continued to develop until the Miocene in the south. Silcrete formed from Eocene to Miocene times; its dissection was promoted by late Cainozoic tectonism.

Since laterite and silcrete formed on the same strata in warm, very moist environments, lithology and climate are not important genetic factors causing laterite to form at one time and silcrete at another. Only base levels of erosion differed. The silcrete surface was largely developed by streams flowing into mid‐Cainozoic lacustrine basins, whereas there is no evidence that these drainage conditions prevailed for laterite formation.     

Volcanism in the rift‐valley system that evolved into the western margin of Australia

The Mesozoic forerunner of the western margin of Australia has been regarded tectonically as an ancient analogue of the multiple rift‐valley system of East Africa, which comprises two arms: volcanic on the E, and virtually non‐volcanic on the W. The abundance of widespread volcanics recently dredged and cored along the outermost margin, which corresponds with the volcanic arm of the East African system, contrasts with the apparent scarcity of volcanics inshore, in the inner arm of the rift system. We tested the possibility that volcanogenic material has been overlooked inshore by a petrographic study of the Late Jurassic to earliest Cretaceous Yarragadee Formation of the Perth Basin; only rare possible pyroclasts of quartz and glass (probably emplaced by air‐fall from the volcanic outer arm) were found, confirming the contrast in volcanism between the arms. This petrological evidence, together with the appropriate range of composition of the volcanism, from silicic to mafic, including alkaline and peralkaline members, reinforces the analogy with East Africa.     

Evolution of three unconformity‐bounded sandy carbonate successions in the McArthur River region of northern Australia: The Lawn,Wide and Doom Supersequences in a proximal part of the Isa Superbasin

Gamma‐ray curves from surface outcrops together with U–Pb SHRIMP zircon dating are used to redefine the evolution of a Palaeoproterozoic sandy dolostone succession from northern Australia. This case history indicates that gamma‐ray logging of surface sections should accompany lithostratigraphic logging or an inadequate interpretation of stratigraphic evolution is a likely outcome. The 1200 m‐thick Nathan Group from the McArthur River area had previously been interpreted as a more‐or‐less continuous package of carbonates deposited in lacustrine and associated shallow‐water environments. Now it is seen to comprise the preserved remnants of three truncated, second‐order supersequences—the Lawn, Wide and Doom Supersequences—each a few hundred metres thick and each deposited over a time period of a few million years. These supersequences are separated by major stratigraphic breaks each approaching probably 10 million years duration. Each supersequence comprises several third‐order sequences which themselves contain higher‐order cycles. These were deposited in a series of continental, shoreline, and inner to outer carbonate platform environments. Transgressive, high‐energy, fluvial to marginal marine, mixed clastic‐carbonate facies dominate most of the sequences. The middle, Wide Supersequence, however, preserves deeper water (mostly sub‐storm‐wave‐base) stromatolitic facies in one sequence, and storm‐reworked clastics in another. These are interpreted as condensed intervals deposited around their respective maximum flooding surfaces and are succeeded by regressive facies that probably represent highstand systems tracts. New correlations between these 1615–1575 Ma sandy carbonate successions of the McArthur Basin (Amos, Balbirini and Dungaminnie Formations) and time‐equivalent largely clastic successions in the Lawn Hill area (Lawn Hill and Doomadgee Formations), some 400 km to the southeast, are proposed.     

Development of methods for constructing models for intra-year irregularity in the Earth’s rotation

The development of methods for the construction of stochastic, dynamical models for intra-year irregularity of the Earth’s rotation is considered. A correlational model based on harmonically additive and parametrically random, colored and broadband, gravitational-tidal perturbations from the Sun and Moon is developed. One-dimensional and multi-dimensional characteristic functions are found for the case of Gaussian and non-Gaussian colored and broadband fluctuations in the irregularity of the Earth’s rotation. Examples of computer modeling of the irregularity in the Earth’s rotation based on a priori and a posteriori IERS data are presented.     

Young,active conjugate strike–slip deformation in West Sichuan: evidence for the stress–strain pattern of the southeastern Tibetan Plateau

The active kinematics of the eastern Tibetan Plateau are characterized by the southeastward movement of a major tectonic unit, the Chuan-Dian crustal fragment, bounded by the left-lateral Xianshuihe–Xiaojiang fault in the northeast and the right-lateral Red River–Ailao Shan shear zone in the southwest. Our field structural and geomorphic observations define two sets of young, active strike–slip faults within the northern part of the fragment that lie within the SE Tibetan Plateau. One set trends NE–SW with right-lateral displacement and includes the Jiulong, Batang, and Derong faults. The second set trends NW–SE with left-lateral displacement and includes the Xianshuihe, Litang, Xiangcheng, Zhongdian, and Xuebo faults. Strike–slip displacements along these faults were established by the deflection and offset of streams and various lithologic units; these offsets yield an average magnitude of right- and left-lateral displacements of ～15–35 km. Using 5.7–3.5 Ma as the time of onset of the late-stage evolution of the Xianshuihe fault and the regional stream incision within this part of the plateau as a proxy for the initiation age of conjugate strike–slip faulting, we have determined an average slip rate of ～2.6–9.4 mm/year. These two sets of strike–slip faults intersect at an obtuse angle that ranges from 100° to 140° facing east and west; the fault sets define a conjugate strike–slip pattern that expresses internal E–W shortening in the northern part of the Chuan-Dian crustal fragment. These conjugate faults are interpreted to have experienced clockwise and counterclockwise rotations of up to 20°. The presence of this conjugate fault system demonstrates that this part of the Tibetan Plateau is undergoing not only southward movement, but also E–W shortening and N–S lengthening due to convergence between the Sichuan Basin and the eastern Himalayan syntaxis.     

Lowstand ramps,fans and deep‐water Palaeoproterozoic and Mesoproterozoic facies of the Lawn Hill Platform: The Term,Lawn, Wide and Doom Supersequences of the Isa Superbasin,northern Australia

The Term, Lawn, Wide and Doom Supersequences represent tectonically driven, second‐order sedimentary accommodation sequences in the Isa Superbasin. The four supersequences are stacked to form two major depositional wedges or packages extending south from the Murphy Inlier onto the central Lawn Hill Platform. A major intrabasin structure, the Elizabeth Creek Fault Zone separates the two depositional wedges. The Term and Lawn Supersequences each form a thick, crudely fining‐upward sedimentary succession. The basal part of each supersequence comprises sand‐dominated facies, deposited under lowstand conditions. The overlying transgressive deposits comprise thick successions of carbonaceous, shale‐prone sediment that represents times of increased accommodation. Synsedimentary fault activity along the northwest‐trending Termite Range Fault and major northeast‐trending faults including the Elizabeth Creek Fault Zone resulted in overthickened sections of parts of the Term and Lawn Supersequences in regional depocentres. A regional extensional event occurred during Wide Supersequence time, and resulted in strike‐slip deformation, uplift and tilting of fault blocks and erosion of underlying Lawn sequences. This tectonic event created small, fault‐bounded depocentres, where basal silty turbidites of the Wide Supersequence are locally thickened. Denudation of fault blocks in the hinterland provided increasing coarse clastic sediment‐supply forming thick, sand‐dominated, lowstand deposits of the upper Wide Supersequence. Overall, the Wide Supersequence exhibits a coarsening‐upwards facies trend. Tectonic quiescence resulted in the accumulation of siltstone‐dominated transgressive and highstand turbidite deposits in mid‐Wide time. The base of the Doom Supersequence comprises thick, feldspathic, debris‐flow sandstones signalling a new provenance. Decreasing accommodation is reflected by coarsening‐ and shallowing‐upwards facies trends in late Doom time. Declining accommodation and the end of sedimentation in the Isa Superbasin were most likely initiated by deformation at the start of the Isan Orogeny.     

Long‐term landscape evolution in the Darwin region and its implications for the origin of landsurfaces in the north of the Northern Territory

The development of landsurfaces in the north of the Northern Territory has traditionally been attributed to successive episodes of uplift, erosion and weathering. The lower and younger two of the four landsurfaces attributed to such development, the Wave Hill and Koolpinyah surfaces, dominate the landscape in the Darwin region. Investigations of the relationship between the Cretaceous stratigraphy and the nature of deep weathering in the Darwin region show that these surfaces are structurally controlled. A bioturbated bed in the Darwin Member of the Cretaceous Bathurst Island Formation has limited the depth of weathering in this region mainly to the level of the Koolpinyah surface. A silicified horizon in the deeply weathered Cretaceous strata has controlled the level of the Koolpinyah and Wave Hill surfaces elsewhere. Furthermore, the presence of detrital laterite profiles, being the main form of evidence used for the identification of both of these surfaces, is challenged; in many localities these detrital profiles are in fact in situ, discounting the idea that these surfaces developed as a result of successive episodes of pediplanation. Similarities are drawn between the origin of these surfaces and the development of equivalent surfaces in the Daly River Basin to the south of Darwin. The practice of extrapolation between surfaces across widely separated regions of northern Australia has been previously shown to be based on tenuous grounds. The same degree of tenuity must now be placed upon such extrapolations in the north of the Northern Territory.     

Comparison of numerical models of two debris flows in the Cortina d’ Ampezzo area,Dolomites, Italy

The accurate prediction of runout distances, velocities and the knowledge of flow rheology can reduce the casualties and property damage produced by debris flows, providing a means to delineate hazard areas, to estimate hazard intensities for input into risk studies and to provide parameters for the design of protective measures. The application of most of models that describe the propagation and deposition of debris flow requires detailed topography, rheological and hydrological data that are not always available for the debris-flow hazard delineation and estimation. In the Cortina d’Ampezzo area, Eastern Dolomites, Italy, most of the slope instabilities are represented by debris flows; 325 debris-flow prone watersheds have been mapped in the geomorphological hazard map of this area. We compared the results of simulations of two well-documented debris flows in the Cortina d’Ampezzo area, carried on with two different single-phase, non-Newtonian models, the one-dimensional DAN-W and the two-dimensional FLO-2D, to test the possibility to simulate the dynamic behaviour of a debris flow with a model using a limited range of input parameters. FLO-2D model creates a more accurate representation of the hazard area in terms of flooded area, but the results in terms of runout distances and deposits thickness are similar to DAN-W results. Using DAN-W, the most appropriate rheology to describe the debris-flow behaviour is the Voellmy model. When detailed topographical, rheological and hydrological data are not available, DAN-W, which requires less detailed data, is a valuable tool to predict debris-flow hazard. Parameters obtained through back-analysis with both models can be applied to predict hazard in other areas characterized by similar geology, morphology and climate.     

Application of modified digital terrain models for geomorphological demarcation of large blocks of the Earth’s crust

A method for processing of a digital terrain model using a gradient module and the Laplace operator of a Gaussian surface is described, and the results of applying it to small-scale geological-structural demarcation are presented. The studied area (52–61° N, 120–133° E) covers the southern portion of the Siberian Platform (the Aldan-Stanovoi Shield), the southeastern flank of the Early Paleozoic Selenga-Stanovoi orogenic belt, and the western portion of the Mesozoic Mongol-Okhotsk orogenic belt. It has been demonstrated that the interpretation of modified digital terrain models allows confident determinations of the sizes of and relationships between geological features and zones of different types and the identification of faults and tectonic blocks transformed to different extents.     

Experimental modeling of the origin of the Moon’s metallic core at partial melting

Geochemical criteria of the Moon’s composition as deficient in Fe and depleted in volatile components and the distribution of siderophile elements in the planet offer the possibility of correlating, under certain conditions, the origin of the Moon and its core from an initial material of composition close to CI carbonaceous chondrites. In order to verify the model of the percolation of liquid metallic Fe through a silicate matrix of chondritic composition at low degrees of melting, we have experimentally modeled Fe movement and deposition in the course of high-temperature centrifugation. The starting experimental mixture had the composition 85% Ol, 10% ferropicrite, and 5% Fe-S (95% Fe and 5% S); the experimental conditions were 4000 g “gravity”, T = 1440°C, Δ log fO₂(IW) ～ ?5.5. In our experiments, Fe was segregated in systems with Fe sulfide and silicate melts at partial melting under reduced conditions and the deformation of the silicate framework. Our results indicate that the Moon could be produced from a material of composition close to CI carbonaceous chondrite.     

Comparison of Different Estimates of the Accuracy of Forecasts of the Earth’s Rotation Parameters

Astronomy Reports - Improvement of the prediction accuracy of the Earth’s rotation parameters (ERP) is one of the main problems of applied astrometry. In order to solve this problem, various...     

Computation of Probabilistic Seismic Hazard for the GHSAP Test Area ‘Caucasus’

As a result of work carried out during the first two stages of the Global Seismic Hazard Assessment Program (GSHAP) for the Test Area Caucasus, a uniform earthquake catalogue was compiled and a Seismic Source Zones Model was designed. At the final stage of the program, the computation of seismic hazard was done by different methods.The results of a computation done using the Probabilistic Seismic Hazard Assessment methodology, as well as primary intermediate steps and preparatory work are given in the present paper. Peak horizontal ground acceleration is chosen as the parameter representing seismic hazard. Final computer calculations were done with the SEISRISK III program. The two final Seismic Hazard maps for different return periods are presented. The work was carried out at the National Survey for Seismic Protection of the Republic of Armenia.     

Contrasting definitions for the term ‘karst aquifer’

It is generally considered that karst aquifers have distinctly different properties from other bedrock aquifers. A search of the literature found five definitions that have been proposed to differentiate karst aquifers from non-karstic aquifers. The five definitions are based upon the presence of solution channel networks, hydraulic conductivities >10^?6 m/s, karst landscapes, channels with turbulent flow, and caves. The percentage of unconfined carbonate aquifers that would classify as ‘karst’ ranges from <1 to >50%.