Search

The tectonic mechanism responsible for detachment of the Sibumasu terrane in the eastern Cimmerian continent from the Australian Gondwana margin remains poorly understood and debated. Our study on the late Paleozoic Woniusi basaltic province in the northern Sibumasu terrane may provide a new perspective on this problem, and further, on the rifting dynamics of eastern Gondwana's northern margin. In this study, we revealed that the Woniusi basaltic province is spread over an area of ~12,000 km² with a thickness of ~300-500 m for the eruptive phase. Direct zircon U-Pb dating of mafic dikes indicates that the Woniusi basaltic province was emplaced between the latest Carboniferous and late Early Permian (301 Ma to 282 Ma), synchronous with basaltic rocks from the Panjal Traps, Tethyan Himalaya (Bhote Kosi, Selong, Abor), Lhasa, and southern Qiangtang. Geochemically, the Woniusi basaltic rocks show subalkaline tholeiitic affinity and relatively homogeneous features of enriched light rare earth elements (LREEs) and large ion lithophile elements (LILEs) but depleted Nb and Ta patterns. They were most likely derived from an enriched subcontinental lithospheric mantle source and triggered by the mantle plume-induced rifting process. The Woniusi basaltic province, together with other coeval basaltic provinces from the Panjal Traps, Tethyan Himalaya, Lhasa, and southern Qiangtang, belongs to a fragmented large igneous province that may have had an original area of over 2 x 106 km², which is comparable to that of the Ontong-Java, Dec-can, or Siberian Traps. These Tethyan basaltic provinces share a common mantle plume centered in northern Greater India. It was this mantle plume that may have triggered the initial rifting of the eastern Cimmerian terranes (e.g., Sibumasu, southern Qiangtang) from both the northern Indian and Australian Gondwana margins, as well as the opening of Meso-Tethys Ocean. This study, moreover, sheds light on the paleopositions of the Lhasa and Tengchong terranes before their breakups. [ABSTRACT FROM AUTHOR]

Drowned carbonate reefs on passive margins are puzzling because of their enormous growth potential compared to typical rates of passive margin subsidence and moderate sea-level fluctuations. A possible solution to this paradox is that slow processes acting over geologic time weaken reefs and contribute to their ultimate demise. The Australian northeastern marginal plateaus, known for their drowned reefs, underwent a period of accelerated tectonic subsidence during the late Miocene to Pliocene that, combined with a sequence of second-order global sea-level rises, outpaced reef growth and drowned the once-thriving Miocene carbonate platforms. However, the mechanism for the observed anomalous subsidence of this relatively mature passive margin 1000 km from the nearest plate boundary is uncertain. We use a coupled plate, kinematic mantle flow model to show that in the late Miocene northeastern Australia overrode subducted slabs from Eocene Melanesian subduction north of Papua New Guinea. We find that the rate of surface subsidence induced by the sinking slabs increases the likelihood that relative sea-level rises outpaced late Miocene reef growth. In addition to the well-known effects of long-term plate processes and short-term global sea-level and climate change, our results demonstrate that deep Earth processes can play a substantial role in driving the evolution of passive margins and coral reefs. [ABSTRACT FROM AUTHOR]

Intraplate orogeny remains an enigmatic process in terms of plate tectonics. Regional continental stress fields agree well with stress orientations predicted by the main forces driving and resisting plate motion. However, regional continental stress fields fail to explain major localized compressive plate deformation in continental interiors far away from plate boundaries. Global or plate-wide intraplate stress models typically assume a laterally homogeneous plate rheology and are confined to modeling the present, without addressing the antiquity of the current stress field. An understanding of intraplate stress and deformation requires the linking of spatial variations in continental rheology with time-dependent plate geometry and driving forces. Here we demonstrate how the complex interplay between juxtaposed weak and strong geological provinces and changes in far-field plate boundary forces has caused intraplate orogenesis and tectonic reactivation in southeastern Australia during the Tertiary. Our findings are contrary to the opinion that continental interiors are insensitive to compression at plate margins, and help to explain the mechanisms causing intraplate orogenesis. [ABSTRACT FROM AUTHOR]

Surface roughness is an important rock property that is measured for structural geology and engineering purposes. We have developed an automatic technique to detect anisotropic features on rock faces based on fractal analysis. The analysis method has been applied to synthetic surfaces, and to digitally mapped point clouds of natural rock surfaces shaped by weathering, fault wear, and mining. We illustrate the technique using field examples from Permian sandstones containing brittle shear zones in northeast England, the surface of a neotectonic fault in Turkey, Proterozoic quartzite from central Australia, and Devonian Quartzite in an aggregate quarry in Germany. Roughness analysis of these natural examples suggests that a significant change of roughness value, anisotropy, and anisotropy direction can exist across scale. Our analysis method represents a step toward developing a toolkit to automatically detect and interpret surface charac teristics from digitally acquired data sets. It has widespread potential for applications in rock engineering and the geosciences. [ABSTRACT FROM AUTHOR]

The Macquarie Ridge Complex portion of the Australia-Pacific plate boundary south of New Zealand provides a unique, complete record of a 60°-90° change in spreading direction since 40 Ma that resulted in the transition from a spreading center to a transform plate boundary. Marine geophysical data show that during reorientation, most ridge segments completely disappeared and all shortened. Additionally, modification of newly created crust caused differences in widths of correlative spreading segment corridors on the two plates. We propose two models for ridge reorientation that explain the observed spreading fabric and arcuate fracture zone relationships. Nonrigid plate deformation was accommodated by failing and propagating spreading ridge segments (rifts) and transfer of crust between plates during the gradual reorientation of the spreading axes. [ABSTRACT FROM AUTHOR]

We present a new, near-comprehensive survey of the variations in seismic structure across the West Australian craton at the scale of the main terrane groups. Analyzing data from distant earthquakes recorded at temporary and permanent stations located across the region, we found the best-fitting structure by modeling the conversions from P- to S-wave motion (the receiver function) that take place as the seismic energy travels upward through the lithosphere. Such methods can be used to delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited, and they provide an effective alternative to active-source seismic techniques for deep crustal targets. The seismic structure is consistent within several of the individual Archean terranes, most notably the Pilbara, Murchison, and Southern Cross. These terranes are underlain by lower crust of low seismic velocity and show a sharp seismic Moho. The structure shows significant contrasts between neighboring terranes; thus, major tectonic units have a velocity profile that is a signature of that terrane or terrane group. We infer that the seismic structure of the Archean crust and upper mantle was fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units that formed the West Australian craton. [ABSTRACT FROM AUTHOR]

The southern margin of the Pilbara Craton in northwestern Australia underwent regional heating, folding, and thrusting as well as extensive fluid flow during collision in the Paleoproterozoic. However, the precise tinting of this event is uncertain. Previous geochronologic studies of Archean basement rocks have yielded a wide range of younger radiometric dates (from ca. 2.4 Ga to ca. 2.0 Ga), interpreted to record multiple hydro- thermal alteration events. In situ U-Pb geochronology of authigenic monazite and xenotime in very low grade Archean metasedimentary rocks from across the craton suggests that the Pilbara was affected by at least two cryptic thermotectonic events, the first between ca. 2430 Ma and ca. 2400 Ma, and the second between ca. 2215 Ma and ca. 2145 Ma. The older event is restricted to three localities in the west, and its cause is unknown. The younger event affected most of the craton (>100,000 km²), spanning a 70 m.y. period from ca. 2215 Ma nearest the collisional margin in the south, to ca. 2145 Ma toward the craton interior in the north. The widespread geographic and stratigraphic distribution of ca 2.2 Ga phosphates suggests that fluid flow was pervasive. This event was probably driven by the northward-advancing Ophthalmian fold-and-thrust belt that developed during protracted collision. The associated low-grade metamorphic front migrated over ∼350 km at an average rate of ∼5.0 mm/yr, placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings. [ABSTRACT FROM AUTHOR]

Banded iron formations of the ca. 2770-2405 Ma Hamersley province of Western Australia were locally upgraded to high-grade hematite ores during the Early Paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups of the Ashburton province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small-spot in situ Pb/Pb dating of baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has resolved the ages of two key suites of mafic intrusions, constraining for the first time the tectonic evolution of the Ashburton province and the age and setting of iron-ore formation. Mafic sills dated as ca. 2208 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dike swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore is Ca. 2008 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period between ca. 2208 and 2031 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ca. 2008 Ma dikes present a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ca. 2050-2000 Ma period of continental extension similarly recorded by Paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca. 2320 Ma. [ABSTRACT FROM AUTHOR]

New in situ sensitive high-resolution ion-microprobe (SHRIMP) U-Pb analyses of hydrothermal phosphates associated with orogenic gold mineralization in the Paleoproterozoic Ashburton and Pine Creek gold provinces of northern Australia provide ages of ca. 1740 and ca. 1730 Ma, respectively. Argon-argon analyses of gold-related hydrothermal mica from the Tanami gold province of northern Australia provide ages ca. 1730 Ma. It is important to note that late orogenic events across the western half of Australia coincide with gold metallogenesis across this time interval, in several widely separated provinces. Thus, this orogenic gold episode is interpreted to relate to tectonic events during the amalgamation of various continental blocks to form Paleoproterozoic Australia. It is potentially Earth's best-preserved record of orogenic gold formation during a major early Precambrian continental assembly event. [ABSTRACT FROM AUTHOR]

Antarctica's Lambert graben, Australia's North West Shelf, and the eastern Indian Peninsula all host thick, fault-bounded Permian-Triassic successions. These terranes were adjacent to each other in Gondwana. The Lambert graben intersects the modern coastline, strikes oblique to shelf architecture, and has a geophysical signature that can be traced >1000 km inland. Vitrinite reflectance data from the graben margins record Permian-Triassic infill. Australia's North West Shelf is the relict of an intracontinental Carboniferous-Permian rift that was infilled during the Permian-Triassic then driven to oceanic completion during Jurassic-Cretaceous Gondwana breakup. This rift was compartmentalized over length scales of ∼650 km, corresponding to accommodation zones, margin-normal geophysical lineaments, and long-lived crustal weaknesses. In eastern India, similar compartmentalization is marked by extensive coal-bearing graben systems. Gondwana reconstructions indicate that the Lambert graben corresponds to the orientation and length scale of Carboniferous-Permian rift compartmentalization. The Lambert graben represents an accommodation zone of a wide intracontinental rift that extended from Australia's North West Shelf, between India and Antarctica, to southern Africa. This rift collected Gondwana's thick Permian-Triassic sedimentary blanket and rich alluvial coal deposits. [ABSTRACT FROM AUTHOR]

Upper Miocene to lower Pliocene sedimentary rocks on Macquarie Island are dominantly volcaniclastic breccia, sandstone, and siltstone produced by the physical disintegration and tectonic abrasion of oceanic crust in fault zones and mass wasting of these tectonic features. They represent small debris fans and small-scale turbidites deposited at the base of active fault scarps, related to Late Miocene to Early Pliocene seafloor spreading. Most of the sediment is derived from basalts, hut diabase and gabbro clasts in some sedimentary rocks indicate that middle and lower oceanic crust was exposed to erosion on the sea floor. A lack of exotic clasts and a low degree of clast roundness are consistent with a local source for the sediment and no input from continental rocks. Spatial relationships between sedimentary rocks and major faults associated with seafloor spreading on the island and correlation between sedimentary clast and adjacent up-thrown block compositions allow us to infer paleotectonic relief for Macquarie Island crust during deposition. Our data support a model involving the deposition of these rocks at the inside corner of a ridge-transform intersection. Furthermore, a tectonic reconstruction of the Australian-Pacific plate boundary for the approximate time that Macquarie Island crust formed (10.9 Ma) also shows that Macquarie Island crust most likely formed near a ridge-transform intersection. This paper describes sedimentation associated with active faulting at a ridge-transform intersection that has been uplifted in situ above sea level along with the surrounding oceanic crust, and demonstrates that high-angle faults have the most pronounced influence, compared with low-angle faults, on sedimentation in this tectonic environment. [ABSTRACT FROM AUTHOR]

Examines the role of passive margin tectonics in determining continental drainage divide in northeastern Australia. Stratigraphy and sedimentology of Jurassic and Tertiary sediments within the Laura and Carpenteria basins; Account on the highlands evolution models adjacent to passive continental margins; Hypothesis on continental drainage divide denudation.

Focuses on a study conducted on the uncertainties in supercontinental reconstructions, with reference to the enigmatic ordovician setting of the Lachlan orogen in Australia. What the recognition of an intranoceanic mafic island arc establishes; Distribution of ordovician volcanic and volcaniclastic rocks in the Lachlan orogen; Recognition of an Ordovician arc in the Lachlan orogen documents.

Analyzes the seismic geometry of southern continental margin of Australia. Recognition of biogenic mounds throughout most of the Cenozoic succession; Paleoenvironmental implications of an extensive middle Miocene barrier reef; Effect of late Miocene eustatic sea-level fall; Influence of water temperature on carbonate platform and reef growth throughout the Cenozoic.