An analytical model of soil–structure interaction with swelling soils during droughts

Lightly loaded structures constructed on expansive soils may develop structural damage as a result of changes in the soil’s moisture content. This study investigated an analytical model of soil–structure interaction to assess the settlement of dwellings built on swelling soils when droughts occur. The building behavior was investigated with the Euler–Bernoulli beam theory, and the ground behavior was investigated with a Winkler-derived model based on the state surface approach. The analytical model results were compared to those of a finite element analysis using the Barcelona Expansive Model (BExM) performed with Code_Bright.The analytical model was then used to assess the settlement transmission ratio for a typology of clayey soils and different parameters of building. The results indicated that the final deflection of the building increased with the building length and soil suction. The building deflection due to the suction variations was inversely proportional to the load, the rigidity of the building and the embedding depth of the foundation. Increasing these parameters made the building less vulnerable to shrinkage and swelling action.     

A cave in Dolerite at Wayatinah,Tasmania

A cave (60 ft. x 30 ft. x 17 ft.) occurs within Jurassic dolerite 600 ft. from the surface. It was formed by zeolitization of a zone in the dolerite followed by solution of the secondary minerals to leave a cavity.     

Geochemical discrimination of tectonic setting for Devonian basalts of the Yarrol Province of the New England Orogen,central coastal Queensland: An empirical approach *

Fault blocks and inliers of uppermost Silurian to Middle Devonian strata in the Yarrol Province of central coastal Queensland have been interpreted either as island-arc deposits or as a continental-margin sequence. They can be grouped into four assemblages with different age ranges, stratigraphic successions, geophysical signatures, basalt geochemistry, and coral faunas. Basalt compositions from the Middle Devonian Capella Creek Group at Mt Morgan are remarkably similar to analyses from the modern Kermadec Arc, and are most consistent with an intra-oceanic arc associated with a backarc basin. They cannot be matched with basalts from any modern continental arc, including those with a thin crust (Southern Volcanic Zone of the Andes) or those built on recently accreted juvenile oceanic terranes (Eastern Volcanic Front of Kamchatka). Analyses from the other assemblages also suggest island-arc settings, although some backarc basin basalt compositions could be present. Arguments for a continental-margin setting based on structure, provenance, and palaeogeography are not conclusive, and none excludes an oceanic setting for the uppermost Silurian to Middle Devonian rocks. The Mt Morgan gold–copper orebody is associated with a felsic volcanic centre like those of the modern Izu–Bonin Arc, and may have formed within a submarine caldera. The data are most consistent with formation of the Capella Creek Group as an intra-oceanic arc related to an east-dipping subduction zone, with outboard assemblages to the east representing remnant arc or backarc basin sequences. Collision of these exotic terranes with the continent probably coincided with the Middle–Upper Devonian unconformity at Mt Morgan. An Upper Devonian overlap sequence indicates that all four assemblages had reached essentially their present relative positions early in Late Devonian time. Apart from a small number of samples with compositions typical of spreading backarc basins, Upper Devonian basalts and basaltic andesites of the Lochenbar and Mt Hoopbound Formations and the Three Moon Conglomerate are most like tholeiitic or transitional suites from evolved oceanic arcs such as the Lesser Antilles, Marianas, Vanuatu, and the Aleutians. However, they also match some samples from the Eastern Volcanic Front of Kamchatka. Their rare-earth and high field strength element patterns are also remarkably similar to Upper Devonian island arc tholeiites in the ophiolitic Marlborough terrane, supporting a subduction-related origin and a lack of involvement of continental crust in their genesis. Modern basalts from rifted backarc basins do not match the Yarrol Province rocks as well as those from evolved oceanic arcs, and commonly have consistently higher MgO contents at equivalent levels of rare-earth and high field strength elements. One of the most significant points for any tectonic model is that the Upper Devonian basalts become more arc-like from east to west, with all samples that can be matched most readily with backarc basin basalts located along the eastern edge of the outcrop belt. It is difficult to account for all geochemical variations in the Upper Devonian basalts of the Yarrol Province by any simplistic tectonic model using either a west-dipping or an east-dipping subduction zone. On a regional scale, the Upper Devonian rocks represent a transitional phase in the change from an intra-oceanic setting, epitomised by the Middle Devonian Capella Creek Group, to a continental margin setting in the northern New England Orogen in the Carboniferous, but the tectonic evolution must have been more complex than any of the models published to date. Certainly there are many similarities to the southern New England Orogen, where basalt geochemistry indicates rifting of an intra-oceanic arc in Middle to Late Devonian time.     

Geochemical fingerprint of hyaloclasts in glassy fragmental rocks of Macquarie Island (Southern Ocean): implications for volcanogenic sedimentary processes at a waning mid-ocean ridge

Two main types of glassy fragmental rocks formed along the Proto-Macquarie Spreading Ridge: (i) hyaloclastite breccia; and (ii) pillow-fragment breccia. Examples now exposed on Macquarie Island, Southern Ocean, were largely sourced from proximal pillow lavas. In each of seven samples examined, hyaloclasts (basaltic glass grains) have a narrow major- and trace-element geochemical range, consistent with derivation of each sample from a single volcanic eruption event. Moreover, every sample analysed within the one stratigraphic section (at three sites) displays distinctive major- and trace-element geochemistry compared with the other two sections. This suggests that hyaloclasts at each site represent discrete magma batches. A single source for these glassy fragmental rocks contrasts with the dominant fault-scarp-derived polymict sedimentary rocks on Macquarie Island. We suggest that the hyaloclasts analysed in this study were deposited in small basins between the slopes of growing pillow cones along the mid-ocean ridge. The geochemical analyses presented here encompass (weakly) fractionated (e.g. Bauer Bay) to near-primitive (e.g. Pyramid Peak) compositions. All samples presented here lie within the range of the enriched- to normal-MORB suites previously reported for the island that include the least fractionated MORB melts known globally. The interpretation of geochemically distinct magma batches over the small area of the island suggests very limited magma mixing consistent with an immature or waning magmatic system. We relate these geochemical characteristics to: (i) volcanism near a very long offset transform; and (ii) genesis of magmas during the waning stages of slow seafloor spreading within a very narrow (<50 km-wide) spreading corridor.     

Lake Boga Granite,northwestern Victoria: mineralogy,geochemistry and geochronology

The Devonian Lake Boga Granite in northern Victoria, while almost entirely under thin Murray Basin cover, is one of the largest plutons in the western Lachlan Fold Belt. Its only exposure is a quarry penetrating the Cenozoic sediments. In the quarry, prominent pod pegmatites and miarolitic cavities suggest a high level of emplacement. The granite, a non-magnetic, fractionated S-type, contains a large range of accessory minerals, including primary uranium- and REE-bearing phosphates and oxides, and primary copper sulfides. Monazite-series minerals show an exceptionally wide range of compositions, from normal monazite-(Ce) through cheralite (Ca – Th-rich) to rare huttonitic monazite (Th-rich) and brabantite; U contents in monazite also vary widely (0 – 7.9 wt%). Primary low-Ca uraninites are well preserved and are unusual in having low Th/U and LREE. Late-stage cavity fluorapatite crystals up to several centimetres across show intricate elemental zoning patterns with extreme U gradients (<10 – 6900 ppm) in some crystals. New ⁴⁰Ar – ³⁹Ar ages for magmatic biotite, muscovite and K-rich feldspar average 365 ± 3 Ma, which approximates the emplacement age of the granite. This is supported by a 377 ± 12 Ma U – Th – Pb (CHIME) age for primary uraninite. New whole-rock geochemical data support earlier observations: the granite is strongly fractionated (SiO₂ 70.7 – 76.0 wt%; 4.2 – 0.6 wt% FeO_t) and peraluminous (ASI = 1.23 – 1.45), and has slightly elevated Na₂O and P₂O₅ (0.30 wt%) contents compared with other fractionated S-type granites from the Lachlan Fold Belt. Trace-element abundances are typical of fractionated granites, although U and Cu concentrations vary strongly and reach >60 and ≈1400 ppm, respectively. REE patterns also vary strongly, from LREE-enriched with moderate Eu depletion, to flat with strong Eu depletion. The flattest of the REE patterns, in samples with FeO_total < 1%, are characterised by M-type tetrad effects. These and other samples also show low (subcrustal average) and variable Zr/Hf (35 – 16) and Nb/Ta (8 – 4) ratios; these and other unusual elemental fractionations are related to changes in elemental partitioning during the late magmatic stage, when felsic peraluminous magma and high-temperature magmatic fluid coexisted.     

Structural history of the Greenvale Province,north Queensland: Early Palaeozoic extension and convergence on the Pacific margin of Gondwana*

The southeastern Georgetown Inlier (Greenvale Province) consists of Early Palaeozoic metamorphic rocks in fault contact along the Lynd Mylonite Zone with the Palaeoproterozoic to Mesoproterozoic craton of northeastern Australia. It has a central assemblage of metamorphosed silicic volcanic and sedimentary rocks considered equivalent to the Late Cambrian to Early Ordovician Seventy Mile Range Group that developed in an extensional backarc in the Charters Towers Province to the southeast. In the western part of the Greenvale Province, the Oasis Metamorphics have a U – Pb zircon SHRIMP metamorphic age of 476 ± 5 Ma and are intruded by the granodioritic Lynwater Complex with U – Pb zircon ages of 486 ± 5 Ma and 477 ± 6 Ma. These ages are consistent with these rocks forming basement and intrusive equivalents to the extensional volcanic basin. Existing geochronological constraints on the Halls Reward domain, located at the eastern margin of the province, are consistent with it being basement to the extensional basin. Several domains are recognised in the Greenvale Province with either dominantly steep or low to moderate dips of the main foliation, and each experienced multiple deformation with locally up to four overprinting structural phases. Steepening of foliation in several of the domains is attributed to contractional deformation in the Early Silurian that is inferred to have overprinted low-angle foliation developed during extensional tectonics in the backarc setting. Contractional deformation related to the Early Silurian Benambran Orogeny is considered responsible for multiple deformation in the Greenvale Province and reactivation of domain-bounding faults.     

Passive-margin prolonged volcanism,East Australian Plate: outbursts,progressions, plate controls and suggested causes

Prolonged intraplate volcanism along the 4000 km-long East Australian margin for ca 100 Ma raises many genetic questions. Studies of the age-progressive pulses embedded in general basaltic activity have spawned a host of models. Zircon U–Pb dating of inland Queensland central volcanoes gives a stronger database to consider the structure and origin of Australian age-progressive volcanic chains. This assists appraisal of this volcanism in relation to plate motion and plate margin tectonic models. Inland Queensland central volcanoes progressed south-southeast from 34 to 31 Ma (～5.4 cm/yr) until a surge in activity led to irregular southerly progression 31 to 28 Ma. A new inland southeastern Queensland central volcano line (25 to 22 Ma), from Bunya Mountains to North Main Range, followed 3 Ma behind the adjacent coastal progression. The Australian and Tasman Sea age-progressive chains are compared against recent plate motion modelling (Indian Ocean hotspots). The chain lines differ from general vector traces owing to west-facing swells and cessations in activity. Tectonic processes on the eastern plate margin may regulate these irregularities. These include subduction, rapid roll-back and progressive detachment of the Loyalty slab (43 to 15 Ma). West-flowing Pacific-type asthenosphere, related to perturbed mantle convection, may explain the west-facing volcanic surges. Such westward Pacific flow for over 28 Ma is known at the Australian–Antarctic Discordance, southeast of the present Australian plume sites under Bass Strait–West Tasman Sea. Most basaltic activity along eastern Australia marks asthenospheric melt injections into Tasman rift zone mantle and not lithospheric plate speed. The young (post-10 Ma) fields (Queensland, Victoria–South Australia) reflect new plate couplings, which altered mantle convection and stress regimes. These areas receive asthenospheric inputs from deep thermal zones off northeast Queensland and under Bass Strait.     

U–Pb zircon age of the Walloon Coal Measures in the Surat Basin,southeast Queensland: implications for paleogeography and basin subsidence

The Jurassic Walloon Coal Measures of the Surat Basin were previously estimated to be of Middle Jurassic age, ranging from Aalenian to Callovian, based on an uncalibrated eastern Australian biostratigraphic framework. New U–Pb dates of 162.55 ± 0.05 Ma and 158.86 ± 0.04 Ma obtained from zircons in ash-fall volcanic tuffs now place the Walloon Coal Measures of the Surat Basin in the Upper Jurassic Oxfordian. The new dates have several implications for the interpretation of the Jurassic strata in the Surat Basin. First-order subsidence rates of 61 m/Myr for the Walloon Coal Measures are more akin to those of foreland basins than the previously assumed intracratonic setting. The dates also imply deposition of the Walloon coals in substantially higher latitudes than previously assumed and that they accumulated as peats in mires that experienced more than three months’ continual darkness each winter. Zircon dating of tuffs and associated geochemistry should assist with the correlation of the laterally impersistent coals, fluvial sandstone and mudstone of the Walloon Coal Measures, which are currently difficult to correlate over distances of more than a few kilometres. Dating of the palynostratigraphic zones APJ4.2 to APJ5 (Aequitriradites norrisii Association Zone to Murospora florida Association Zone) will also need to be recalibrated.     

40Ar/39Ar evidence for the timing of Paleoproterozoic gold mineralisation at the Sandpiper Deposit,Tanami region,northern Australia

At the Sandpiper gold deposit in the Tanami region of northern Australia sericite is intimately intergrown with arsenopyrite in gold-bearing quartz veins and breccias, suggesting sericite crystallisation synchronous with gold-bearing fluid flow. This ore-stage sericite yields a ⁴⁰Ar/³⁹Ar plateau age of 1785 ± 32 Ma (2σ including both analytical and systematic uncertainties). Recalculation using revised and more precise values for the ⁴⁰K decay constants and the age of the Fish Canyon Sanidine standard shifts the age to 1794 ±12 Ma (2σ including all known uncertainties). Given the possibility of post-mineralisation isotopic resetting this age can be conservatively interpreted as a minimum constraint on the timing of gold deposition although, given local geological relationships and estimates for the argon retentivity of white mica, we consider complete isotopic resetting to be unlikely. The preferred interpretation is, therefore, that the sericite ⁴⁰Ar/³⁹Ar age indicates the timing of gold mineralisation. Thesericite age accords with a limited dataset of ²⁰⁷Pb/²⁰⁶Pb xenotime ages of ca 1800 Ma from other gold deposits in the Tanami region, interpreted as mineralisation ages. The agreement between independently derived ages from several gold deposits lends support for a widespread gold-mineralising event at ca 1800 Ma in the Tanami region.     

Synorogenic alluvial-fan – fan-delta deposition in the Papuan foreland basin: Plio-Pleistocene Era Formation,Papua New Guinea

The Pliocene to possibly Pleistocene uppermost Orubadi and Era Formations, southwest margin of the Papuan Peninsula, are interpreted as having been deposited in alluvial-fan, fan-delta and shallow-marine environments. The alluvial-fan facies consists primarily of lenticular, coarse-grained conglomerate (up to 2 m boulders) and cross-bedded and horizontally laminated sandstone. Conglomerate and sandstone were deposited in shallow fluvial channels and by overbank sheetfloods. The facies also contains thick mudflow diamictite and minor tuff and terrestrial mudstone. The shallow-marine and fan-delta facies, in contrast, consists of heterogeneously interbedded marine and terrestrial mudstone, sandstone, diamictite, conglomerate and limestone. Marine mudstone is calcareous, sandy, bioturbated, and contains marine shells. Limestone is mostly packstone that has a varied, open-marine fauna. Rare coral boundstone is also present. Marine sandstone is burrowed to bioturbated and is hummocky cross-stratified in places. Some marine mudstone contains sandstone pillows formed by loading of unconsolidated sand by storm waves. Other sandstone in the fan-delta facies is cross-bedded, lacks shells and was probably deposited by fluvial processes. Several conglomerate beds in the fan-delta facies are well sorted and imbricated and were also deposited by stream floods. The synorogenic Orubadi and Era Formations were deposited in a foreland basin formed from loading of the Papuan–Aure Fold and Thrust Belt on the edge of the Australian craton. Deformation in the fold and thrust belt was probably related to docking and compression of the Finisterre Terrane–Bismarck Arc against the New Guinea Orogen. The Era Formation interfingers with the reefal Wedge Hill Limestone in which reef facies likely grew on a deforming anticline. Era Formation siliciclastics were sourced from volcanic, metamorphic and sedimentary rocks that were uplifted in the orogen to the northeast. Volcanic sediment was derived mostly from a then-active volcanic arc likely related to southward subduction at the Trobriand Trough.