Craig O'Neill

The Lithospheric architecture of Africa : seismic tomography, mantle petrology and tectonic evolution

We present a new analysis of the lithospheric architecture of Africa, and its evolution from ca. 3.6 Ga to the present. Upper-lithosphere domains , generated or reworked in different time periods, have been delineated by integrating regional tectonics and geochronology with geophysical data (magnetic, gravity, and seismic). The origins and evolution of lower-lithosphere domains are interpreted from a high-resolution global shear-wave tomographic model, using thermal/compositional modeling and xenolith/xenocryst data from volcanic rocks. These data are integrated to map the distribution of ancient highly depleted subcontinental lithospheric mantle (SCLM), zones of younger or strongly modified SCLM and zones of active mantle upwelling, and to relate these to the evolution of the upper lithosphere domains. The lithospheric architecture of Africa consists of several Archean cratons and smaller cratonic fragments, stitched together and flanked by younger fold belts; the continental assembly as we see it has only existed since lower Paleozoic time. The larger cratons are underlain by geochemically depleted, rigid, and mechanically robust SCLM; these cratonic roots have steep sides, extending in some cases to ≥300-km depth. Beneath smaller cratons (e.g., Kaapvaal) extensive refertilization has reduced the lateral and vertical extent of strongly depleted SCLM. Some cratonic roots extend ≥300 km into the Atlantic Ocean, suggesting that the upper lithosphere may detach during continental breakup, leaving fragments of SCLM scattered in the ocean basin. The cratonic margins, and some intracratonic domain boundaries, have played a major role in the tectonics of Africa. They have repeatedly focused ascending magmas, leading to refertilization and weakening of the SCLM. These boundaries have localized successive cycles of extension, rifting, and renewed accretion; the ongoing development of the East Africa Rift and its branches is only the latest stage in this process. The less depleted SCLM that underlies some accretionary belts may have been generated in Archean time, and repeatedly refertilized by the passage of magmas during younger tectonic events. Our analysis indicates that originally Archean SCLM is far more extensive beneath Africa than previously recognized, and implies that post-Archean SCLM rarely survives the collision/accretion process. Where continental crust and SCLM have remained connected, there is a strong linkage between the tectonic evolution of the crust and the composition and modification of its underlying SCLM.

Continents, supercontinents, mantle thermal mixing, and mantle thermal isolation: Theory, numerical simulations, and laboratory experiments

Super-continental insulation refers to an increase in mantle temperature below a supercontinent due to the heat transfer inefficiency of thick, stagnant continental lithosphere relative to thinner, subducting oceanic lithosphere. We use thermal network theory, numerical simulations, and laboratory experiments to provide tighter physical insight into this process. We isolate two end-member dynamic regimes. In the thermally well mixed regime the insulating effect of continental lithosphere can not cause a localized increase in mantle temperature due to the efficiency of lateral mixing in the mantle. In this regime the potential temperature of the entire mantle is higher than it would be without continents, the magnitude depending on the relative thickness of continental and oceanic lithosphere (i.e., the insulating effects of continental lithosphere are communicated to the entire mantle). Thermal mixing can be short circuited if subduction zones surround a supercontinent or if the convective flow pattern of the mantle becomes spatially fixed relative to a stationary supercontinent. This causes a transition to the thermal isolation regime: The potential temperature increases below a supercontinent whereas the potential temperature below oceanic domains drops such that the average temperature of the whole mantle remains constant. Transition into this regime would thus involve an increase in the suboceanic viscosity, due to local cooling, and consequently a decrease in the rate of oceanic lithosphere overturn. Transition out of this regime can involve the unleashing of flow driven by a large lateral temperature gradient, which will enhance global convective motions. Our analysis highlights that transitions between the two states, in either direction, will effect not only the mantle below a supercontinent but also the mantle below oceanic regions. This provides a larger set of predictions that can be compared to the geologic record to help determine if a hypothesized super-continental thermal effect did or did not occur on our planet.

The Archean-Proterozoic boundary: 500 my of tectonic transition in Earth history

Changes in the solid Earth at the end of the Archean fall into two categories. First are those related to cooling of the mantle and include a decrease in both komatiite abundance and MgO content, a decrease in Ni/Fe ratio in banded iron formation, and increases in incompatible element ratios (such as Nb/Yb, La/Yb, Zr/Y, La/Sm and Gd/Yb) in non-arc type basalts. A second group of changes is related to the extraction of continental crust from the mantle and stabilization of major cratons at 2.7 to 2.5 Ga. These include an increase in Nb/Th ratio and εNd(T) of non-arc basalts; significant increases in large-ion lithophile and high-field strength elements and a decrease in Sr in continental crust, which reflect a shift in magma types from TTG (tonalite-trondhjemite-granodiorite) to calc-alkaline; a prominent increase in the maximum values of δ18O of zircons from granitoids after the end of the Archean; a major peak in gold reserves is found at or near 2.7 Ga; and a peak in Re/Os depletion ages from mantle xenoliths at 2.7 Ga consistent with widespread thickening of the continental lithosphere at this time. All of these changes may be related to the widespread propagation of plate tectonics at the end of the Archean. Subduction produces continental crust in numerous arcs, which rapidly collide to form supercratons. Oceanic slabs sinking into the deep mantle could increase the production rate of mantle plumes, as well as increase the heat flux from the core, which warms the newly arrived slabs. The cooling of the deep mantle would begin after 2.5 Ga and continue until about 2.4 Ga when a 200-My slowdown in plate tectonics begins. This may be the reason for the rapid drop in temperature of the mantle recorded by basalts and komatiites. When plate tectonics comes back on track at about 2.2 Ga, Archean supercratons break up and are dispersed.

Deep earth recycling in the Hadean and constraints on surface tectonics

The Hadean mantle was efficiently heated from high internal heat production, high rates of impact bombardment, and primordial heat from accretion. As a result a strong case is made for extremely high internal temperatures, low internal viscosities, and extremely vigorous mantle convection. Previous studies of mixing in such high-Rayleigh number convective environments indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100 Myr. However, 142Nd and 182W isotope studies indicate that heterogeneous mantle domains survived, without mixing, for over 2 Gyr—at odds with expected mixing rates. Similarly, concentrations of platinum group elements in Archean komatiites, purportedly due to the later veneer of meteoritic addition on the Earth, only achieve current levels at 2.7 Ga—indicating a time lag of almost 1 to 2 Gyr in mixing this material thoroughly into the mantle. Previous studies have sought to explain slow Archean mantle mixing via mantle layering due to endothermic phase changes, or anomalously viscous blobs of material, with limited efficacy. Here we pursue another explanation for inefficient mantle mixing in the Hadean: tectonic regime. A number of lines of evidence suggest that resurfacing in the Archean was episodic, and extending these models to Hadean times implies the Hadean was characterized by long periods of tectonic quiescence. We explore mixing times in 3D spherical-cap models of mantle convection, which incorporate vertically stratified and temperature-dependent viscosities. We show that mixing in stagnant-lid regimes is, at the extreme, over an order of magnitude less efficient than mobile-lid mixing, and for plausible Rayleigh numbers and internal heat production, the lag in Hadean convective recycling can be explained. The attractiveness of this model is that it not only explains the long-lived 142Nd and 182W anomalies, but also posits an explanation for the delay between accretion of the late veneer—between 4.5 to 3.8 Ga on a stagnant surface—and its fully mixed signature apparent in elevated PGEs in 2.7 Ga komatiites. It also provides an explanation for the 400 Myrs of immobility of the mafic protolith from which the Jack Hill zircons were sourced, and retards early heat loss from the mantle, providing a solution to the “Archean thermal catastrophe” of parameterized Earth evolution models.

Gunnedah Basin 3D architecture and upper crustal temperatures

The Gunnedah Basin in New South Wales has long been an important coal and gas resource, but limited information exists on the temperature structure or crustal architecture at depth to enable development of its geothermal potential. Here we combine gravity modelling, seismic-reflection surveys and borehole drilling results to develop a 3D depth to basement structural map and geological model of the basin. The 3D structure of the Gunnedah Basin is characteristic of a typical intracontinental rift basin. Gravity modelling of the Lachlan Fold Belt basement, using borehole and seismic-reflection controls, shows a 2–3.5 km-deep approximately north–south-oriented channel between the basement highs of the Rocky Glen Ridge in the west and Boggabri Ridge in the east. Extensional basal volcanics during the Late Carboniferous–Early Permian fill this channel. Borehole data and gravity modelling show up to 1 km of Permian to Jurassic sedimentary rocks overlying the rift volcanics. Preliminary thermal modelling, incorporating the geological model and limited deep borehole temperatures, indicates temperatures at the top of basement are in the range 105–165°C.

Deep 3D structure of the Sydney Basin using gravity modelling

A detailed deep 3D geological model is an important basis for many types of exploration and resource modelling. Renewed interest in the structure of the Sydney Basin, driven primarily by sequestration studies, geothermal studies and coal seam gas exploration, has highlighted the need for a model of deep basin geology, structure and thermal state. Here, we combine gravity modelling, seismic reflection surveys, borehole drilling results and other relevant information to develop a deep 3D geological model of the Sydney Basin. The structure of the Sydney Basin is characteristic of a typical intracontinental rift basin, with a deep north–south orientated channel in the Lachlan Fold Belt basement, filled with up to 4 km of rift volcanics, and overlain with Permo-Triassic sediments up to 4 km thick. The deep regional architecture presented in this study will form the framework for more detailed geological, hydrological and geothermal models.

Inferences on Australia's heat flow and thermal structure from mantle convection modelling results

Mantle convection is important in understanding the heat flow and thermal structure of the continental lithosphere, as it produces time variability in the surface heat flow, and allows for lateral advection of heat under a continent. Many of the fundamental questions in continental heat flow depend on the magnitude and variability of the mantle contribution to continental heat flow. We summarise the current understanding of the thermal state of the Australian continent, and discuss the application of mantle-convection modelling results to continental heat-flow problems. A particle-in-cell finite-element code is used to show how the continental thermal field is modulated through time, and how the calculated mantle heat flow decreases with both root thickness and crustal heat production. An increase in root thickness is shown to enhance the stability of the deep continental thermal field. These modelling results imply a modest variation in the mantle heat flow of Australia through time, and suggest that the variation in mantle heat flow over the stable Precambrian shield will most likely be indiscernible. The thickness and thermal structure of the Australian lithosphere is, to a degree, dependent on the history of mantle convection around this continent.

Geothermal state of the Sydney Basin: assessment of constraints and techniques

The thermal structure of sedimentary basins is largely dependent on complex three-dimensional effects encompassing architecture, geology and groundwater, making it difficult to describe in a one-dimensional model. New equilibrated down-hole temperature measurements in the Sydney Basin, in conjunction with regional scale thermal modelling using the geodynamics simulation software Underworld, can provide an accurate assessment of the thermal structure of the basin. When compared with extrapolation maps, these results highlight important limitations of utilising extrapolation maps as an unaccompanied geothermal exploration tool. The extrapolated temperature method creates a ‘temperature-at-depth’ map, which propagates and exaggerates near surface variations, and is limited by coverage and number of boreholes that have temperature measurements recorded. Numerical simulations of basin heat flow, using basic material properties, combined with a deep three-dimensional geological model and calibrated by measured equilibrated temperature data are not limited by the borehole coverage but rather the chosen resolution of the model. The Underworld thermal model provides a realistic estimation of temperature at depth within the Sydney Basin, a clearer understanding of thermal structure and allows a more comprehensive assessment of potential geothermal targets.

Reconstructing the Wolfe Creek meteorite impact: deep structure of the crater and effects on target rock

The Wolfe Creek Meteorite Crater is an impact structure 880 m in diameter, located in the Tanami Desert near Halls Creek, Western Australia. The crater formed < 300 000 years ago, and is the second largest crater from which fragments of the impacting meteorite (a medium octahedrite) have been recovered. We present the results of new ground-based geophysical (magnetics and gravity) surveys conducted over the structure in July – August 2003. The results highlight the simple structure of the crater under the infilling sediments, and forward modelling is consistent with the true crater floor being 120 m beneath the present surface. The variations in the dip of the foliations around the crater rim confirm that the meteorite approached from the east-northeast, as is also deduced from the ejecta distribution. Crater scaling arguments suggest a projectile diameter of > 12.0 m, a crater formation time of 3.34 s, and an energy of impact of ∼0.235 Mt of TNT. We also use the distribution of shocked quartz in the target rock (Devonian sandstones) to reconstruct the shock loading conditions of the impact. The estimated maximum pressures at the crater rim were between 5.59 and 5.81 GPa. We also use a Simplified Arbitrary Langrangian–Eulerian hydrocode (SALE 2) to simulate the propagation of shock waves through a material described by a Tillotson equation of state. Using the deformational and PT constraints of the Wolfe Creek crater, we estimate the maximum pressures, and the shock-wave attenuation, of this medium-sized impact.

Tectonothermal evolution of solid bodies: terrestrial planets, exoplanets and moons

A framework for understanding the tectonothermal evolution of solid planetary bodies has historically been lacking owing to sparse observational constraints. Developments in simulating the physical interiors and tectonic behaviour of terrestrial planets have allowed insights into the relevant physics and important factors governing planetary behaviour. This contribution summarises the critical factors in determining a planets tectonic regime, and the application of this framework to understanding terrestrial planet evolution. Advances in modelling have led to the identification of new, unmapped tectonic regimes, such as episodic convection, which has relevance to our understanding of the evolution of the early Earth, Venus and Saturns moon Enceladus. Coupling of tectonic and atmospheric models for planetary evolution has contributed to our knowledge of Martian and Venusian degassing histories, and recent debate on the tectonic regime of exosolar planets informs outstanding questions on their habitability. Ultimately, a framework for terrestrial planet evolution will couple available cosmochemical, geochemical and astrophysical constraints into an emerging generation of simulation tools, facilitating the mapping of terrestrial planet behaviour over a wide parameter space.