Kurt M. Knesel

Rapid change in drift of the Australian plate records collision with Ontong Java plateau

The subduction of oceanic plateaux, which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood-basalt provinces, is an important factor in many current models of plate motion and provides a potential mechanism for triggering plate reorganization. To evaluate such models, it is essential to decipher the history of the collision between the largest and thickest of the world’s oceanic plateaux, the Ontong Java plateau, and the Australian plate, but this has been hindered by poor constraints for the arrival of the plateau at the Melanesian trench. Here we present 40Ar–39Ar geochronological data on hotspot volcanoes in eastern Australian that reveal a strong link between collision of the Greenland-sized Ontong Java plateau with the Melanesian arc and motion of the Australian plate. The new ages define a short-lived period of reduced northward plate motion between 26 and 23 Myr ago, coincident with an eastward offset in the contemporaneous tracks of seamount chains in the Tasman Sea east of Australia. These features record a brief westward deflection of the Australian plate as the plateau entered and choked the Melanesian trench 26 Myr ago. From 23 Myr ago, Australia returned to a rapid northerly trajectory at roughly the same time that southwest-directed subduction began along the Trobriand trough. The timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the Pacific plate, including the archetypal Hawaiian chain, and thus provide strong evidence that immense oceanic plateaux, like the Ontong Java, can contribute to initiating rapid change in plate boundaries and motions on a global scale.

Geochronology of the Australian Cenozoic: a history of tectonic and igneous activity, weathering, erosion, and sedimentation*

The development and application of geochronological tools suitable for dating Cenozoic rocks and processes have been instrumental to our understanding of the modern history of Australia. Geochronology reveals a dynamic continent that traced a long and rapid trajectory from a position adjacent to Antarctica in the early Cenozoic to its present position near the tropics. The average travel velocity along this path is revealed by the age of hotspot volcanoes, derived by the K–Ar method, and is complemented by measured geomagnetic pole positions on dated igneous rocks and sedimentary deposits. K–Ar dating of volcanic rocks also provided constraints on rates of landscape evolution before and after volcanism and the timing and pattern of dispersion of life—including human inhabitation. K–Ar geochronological results reveal a history of faunal and floral evolution suggestive of a continent undergoing progressive cooling and dehydration with a few brief warm and humid excursions. In contrast, 40Ar/39Ar, SHRIMP U–Pb, fission-track thermochronology, luminescence techniques, and cosmogenic-isotope methods have played relatively minor roles in reconstructing the chronology of Cenozoic volcanism in Australia. Integrated application of these techniques will be critical to providing more precise constraints on the volcanic history of the continent and its climatic and biological evolution. While Cenozoic volcanism, uplift, and denudation were active along the eastern and southeastern margins, a significant part of Australia west of the Tasman Line remained relatively quiescent. The history of this part of the continent is marked by slow and subdued uplift and subsidence, with subtle displacements along major continental structures, and occasional invasion by shallow seas. Despite the general absence of Cenozoic igneous rocks west of the Tasman Line, Australia (east and west) is blanketed by Cenozoic sedimentary covers and weathering profiles. If we consider weathering as a fourth rock-forming process (in addition to igneous, metamorphic and sedimentary), Australia has one of the most complete Cenozoic rock covers of any continent. Retrieving information recorded in these weathering profiles is essential for unravelling its Cenozoic history. Paleomagnetic studies, calibrated δ18O curves, and weathering geochronology by K–Ar, 40Ar/39Ar, and (U–Th)/He provide insights into the imprint of climatic events and tectonic processes and illustrate the importance of erosion and weathering to the formation or enrichment of ore and mineral deposits. Except for diamondiferous lamproites of Western Australia and sapphire-rich volcanic rocks in eastern Australia, all other Cenozoic ore and mineral deposits in Australia are related to weathering and erosion. The widespread weathered blanket in Australia suggests low Cenozoic erosion rates. Numerical constraints on chronology and erosion rates are derived from the cooling and denudation histories retrieved from apatite and zircon fission-track and, more recently, (U–Th–Sm)/He thermochronology and cosmogenic isotope studies. Geochronological studies of veneers of sediments, lake and cave deposits, marine carbonates, organic matter and groundwaters provide information on sediment provenance, subtle tectonic movements, and the Australian Cenozoic climatic history. These studies reveal a continent sensitive to global climatic cycles and subject to active, but subtle, tectonism and erosion. This record shows that Australia suffered periods of extreme aridity during cyclical glaciation at high latitudes and precise dating of carbonate sediments and speleothems reveals the exact timing and duration of these glacial and interglacial periods. Cosmogenic isotopes also provide constraints on the age and migration paths of Australias limited and finite groundwater resources. Lastly, age information extracted from surficial deposits reveals a protracted history of human occupation.

Isotopic fingerprinting may provide insights into evolution of magmatic systems

A simple method may provide greater insights into the sort of pathways and time-scales in which magmatic systems evolve. The method, employing isotopic fingerprinting of the mineral components that characterize most magmatic rocks, may lead to a more comprehensive understanding of these types of rocks. Such an understanding is critical to resolving fundamental problems in geoscience, ranging from the origin and differentiation of the crust and mantle to the prediction of volcanic hazards. Much progress has been made in the past 30 to 40 years, first with the widespread application of experimental studies on natural and synthetic systems, and more recently with the application of chemical and isotopic constraints on the origin of igneous rocks. Despite this progress, however, many questions remain, and that is where isotopic fingerprinting may help.

Isotopic disequilibrium during melting of granite and implications for crustal contamination of magmas

The assumption that the isotopic ratios of anatectic melts generated in response to crust-magma interaction are equivalent to the bulk source rock from which they are derived was tested experimentally via dehydration melting of a Proterozoic biotite granite. Heating of the coarse-grained granite at 1250 °C for durations of 1 to 48 h produced extreme disequilibrium among simultaneously generated mafic and felsic melts (quenched to glass) and whole-rock 87Sr/86Sr ratios. In-situ microdrill Sr isotopic analyses across gradational contacts between mafic (high-87Sr/86Sr) and felsic (low-87Sr/86Sr) glass register significant isotopic and chemical heterogeneity on the submillimetre scale. Our experiments suggest that during the early stages of wall-rock melting, when the rate of heating may exceed melting rates, isotopic equilibrium may not be maintained, thus producing contaminant liquids having isotopic compositions different from the bulk source rock. If isotopic equilibrium is not attained during partial melting in the crust, current geochemical models of open-system magmatic processes may require modification.

The origin and evolution of large-volume silicic magma systems: Long Valley caldera

Despite extensive study, the origin of large-volume silicic magma systems remains poorly constrained. We review the source regions and processes involved in the generation, differentiation, and eruption of caldera-related silicic magma with particular reference to the Bishop Tuff erupted from Long Valley Caldera, California. Nd-isotopic compositions of the earliest-erupted rhyolites (between 2,1 and 1.2 Ma) at Glass Mountain, which may be associated with the Bishop Tuff magma chamber, are consistent with extensive fractional crystallization of basaltic magmas derived from an enriched lithospheric mantle source. In contrast, Nd-isotopic compositions of late Glass Mountain lavas (1.2 to 0.8 Ma) and the Bishop Tuff (0.76 Ma) suggest significant incorporation of continental crust. Shallow-crustal residence histories inferred from Sr-isotopic studies at Long Valley suggest that large-volume silicic magmas reflect either: (1) continuous growth with episodic eruption from a single, large, long-lived magma chambe...

Pb-isotopic evidence for rapid trench-parallel mantle flow beneath Vanuatu

Abstract: Recent geophysical and geochemical studies suggest that some subduction zones have a significant component of trench-parallel flow that is difficult to reconcile with conventional 2D corner-flow models. Mapping of different mantle Pb-isotopic signatures provides an opportunity to assess the role of along-strike flow beneath the Vanuatu region. Young back-arc spreading centres in the southern segment of the subduction zone have high 208Pb/204Pb for a given 206Pb/204Pb, similar to the Indian-type mantle signatures of present-day volcanoes in the central segment of the arc, but distinct from adjacent southern-arc volcanoes with Pacific-like mantle traits. This configuration indicates that convection in the mantle wedge beneath Vanuatu is dominated by southward flow parallel to the trench, rather than pure corner flow induced by viscous drag along the upper surface of the subducting plate. We propose that this southward flow is related to pressure gradients developed during asymmetric rollback of the Australian plate. Trench-parallel flow rates in Vanuatu are estimated at 10–28 cm a−1, further highlighting that along-strike flow can exceed the convergence-parallel mantle motions predicted from conventional 2D corner-flow models.

40Ar/39Ar constraints on the timing and origin of Miocene leucitite volcanism in southeastern Australia

Laser incremental-heating 40Ar/39Ar geochronology of seven leucitites from southeastern Australia indicates that leucite-bearing lavas in individual geographic clusters were erupted in one million years or less. The eruption ages range from 17.9 ± 0.3 Ma (2σ) at El Capitan in northern-central New South Wales to 8.9 ± 0.2 Ma (2σ) at Cosgrove in northern Victoria. The 40Ar/39Ar results demonstrate that the southward migration of leucite-bearing lavas was near-contemporaneous with age-progressive central-volcano magmatism in southeastern Australia. As such, the 40Ar/39Ar results are consistent with a hotspot-related origin for the leucitites. However, the question of whether single or multiple hotspots are required to explain these volcanic chains, which are separated by a distance of about 300 km, awaits a more complete geochronological picture of the onset, duration and migration of leucitite and central-volcano magmatism in eastern Australia.

40Ar/39Ar constraints on the timing of Oligocene intraplate volcanism in southeast Queensland ∗

Laser 40Ar/39Ar analyses were undertaken on Oligocene bimodal intraplate volcanic rocks from five volcanic areas spanning 320 km along a north-northeast-trending belt in southeast Queensland. Sixty-one mineral grains and groundmass fragments from 21 samples were analysed by incremental heating, while 99 mineral grains from 10 samples were analysed by total fusion. Fourteen samples are from localities dated previously by the K – Ar method. The 40Ar/39Ar ages are reproducible among different aliquots from the same sample and different samples from the same location, and correlate with latitude, ranging from 30.6 ± 0.3 Ma on Fraser Island in the north (25°S) to 25.8 ± 0.2 Ma at Flinders Peak in the south (27.7°S). This age progression is not evident from K – Ar ages compiled for the same region. The decrease of 40Ar/39Ar ages with latitude is consistent with previous suggestions for a hotspot origin for east Australian central volcanoes and yields an Australian Plate velocity, relative to the postulated fixed hotspot reference frame, of 71 (+7, −4) mm/y at an azimuth of N10°E for the period of ca 31 – 26 Ma. This velocity agrees within error with the values of about 65 ± 3 mm/y obtained from previously reported K – Ar analyses of east Australian central volcano provinces active in the last 35 million years. These results demonstrate that the improved accuracy and precision of the 40Ar/39Ar method permit resolution of age versus latitude relationships for narrower time windows, which may potentially provide constraints on changing plate velocities with time. This improved temporal resolution may also contribute to resolving current debate over the existence and the stationary versus mutable position of postulated mantle plumes and hotspots.