We use the fact that geoid anomalies are directly related to the local dipole moment of the density-depth distribution to help constrain density variations within the lithosphere and the associated tectonic stresses. The main challenge with this approach is isolating the upper mantle geoid contribution from the full geoid (which is dominated by sources in the lower mantle). We address this issue by using a high-pass spherical harmonic filtering of the EGM2008–WGS 84 geoid to produce an “upper mantle” geoid. The tectonic implications of the upper mantle are discussed in terms of plate tectonics and intraplate stresses. We find that globally there is a ~9 m geoid step associated with the cooling oceanic lithosphere that imparts a net force of ~2.5 × 10 12 N/m in the form of “ridge push”—a magnitude that is consistent with one-dimensional models based on first-order density profiles. Furthermore, we find a consistent 6 m geoid step across passive continental margins which has the net effect of reducing the compressive stresses in the continents due to the ridge push force. Furthermore, we use the upper mantle geoid to reevaluate the tectonic reference state which previous studies estimated using an assumption of Airy-based isostasy. Our evaluation of the upper mantle geoid confirms the near-equivalence of the gravitational potential energy of continental lithosphere with an elevation of ~750 m and the mid-ocean ridges. This result substantiates early conclusions about the tectonic reference state and further supports the prediction that continental regions are expected to be in a slightly extensional state of stress.

Geomorphic analysis of the ~30-km-long Lake Edgar fault scarp in southwestern Tasmania suggests that three large surface-rupturing events with vertical displacements of 2.4 m to 3.1 m have occurred in late Quaternary time. Optically stimulated luminescence (OSL) age estimates from a sequence of three periglacial fluvial terraces associated with faulting constrain these events to ca. 18 ka, ca. 28 ka, and ca. 48–61 ka. A similar amount of vertical displacement during each faulting event suggests that surface-breaking earthquakes on this fault are characteristically of magnitude M W 6.8–7.0. Estimates for the average slip rate calculated over two complete seismic cycles range from 0.11 to 0.24 mm/yr, which is large for a stable continental region fault. This sequence represents the first recurrence data for surface-rupturing earthquakes on an eastern Australian Quaternary fault, and one of only a few for the entire Australian continent.

Abstract The Australian continent is actively deforming in response to far-field stresses generated by plate boundary interactions and buoyancy forces associated with mantle dynamics. On the largest scale (several 10 3 km), the submergence of the northern continental shelf is driven by dynamic topography caused by mantle downwelling along the Indo-Pacific subduction system and accentuated by a regionally elevated geoid. The emergence of the southern shelf is attributed to the progressive movement of Australia away from a dynamic topography low. On the intermediate scale (several 10 2 km), low-amplitude ( c . 100–200 m) long-wavelength ( c . 100–300 km) topographic undulations are driven by (1) anomalous, smaller-scale upper mantle convection, and/or (2) lithospheric-scale buckling associated with plate boundary tectonic forcing. On the smallest scale (10 1 km), fault-related deformation driven by partitioning of far-field stresses has modified surface topography at rates of up to c . 170 m Ma −1 , generated more than 30–50% of the contemporary topographic relief between some of Australia's highlands and adjacent piedmonts, and exerted a first-order control on long-term (10 4 –10 6 a) bedrock erosion. Although Australia is often regarded as tectonically and geomorphologically quiescent, Neogene to Recent tectonically induced landscape evolution has occurred across the continent, with geomorphological expressions ranging from mild to dramatic.

Abstract Neogene-to-Recent deformation is widespread on and adjacent to Australia’s ‘passive’ margins. Elevated historical seismic activity and relatively high levels of Neogene-to-Recent tectonic activity are recognized in the Flinders and Mount Lofty Ranges, the SE Australian Passive Margin, SW Western Australia and the North West Shelf. In all cases the orientation of palaeostresses inferred from Neogene-to-Recent structures is consistent with independent determinations of the orientation of the present-day stress field. Present-day stress orientations (and neotectonic palaeostress trends) vary across the Australian continent. Plate-scale stress modelling that incorporates the complex nature of the convergent plate boundary of the Indo-Australian Plate (with segments of continent–continent collision, continent–arc collision and subduction) indicates that present-day stress orientations in the Australian continent are consistent with a first-order control by plate-boundary forces. The consistency between the present-day, plate-boundary-sourced stress orientations and the record of deformation deduced from neotectonic structures implicates plate boundary forces in the ongoing intraplate deformation of the Australian continent. Deformation rates inferred from seismicity and neotectonics (as high as 10 −16 s −1 ) are faster than seismic strain rates in many other ‘stable’ intraplate regions, suggestive of unusually high stress levels imposed on the Australian intraplate environment from plate boundary interactions many thousands of kilometres distant. The spatial overlap of neotectonic structures and zones of concentrated historical seismicity with ancient fault zones and/or regions of enhanced crustal heat flow indicates that patterns of active deformation in Australia are in part, governed, by prior tectonic structuring and are also related to structural and thermal weakening of continental crust. Neogene-to-Recent intraplate deformation within the Australian continent has had profound and under-recognized effects on hydrocarbon occurrence, both by amplifying some hydrocarbon-hosting structures and by inducing leakage from pre-existing traps due to fault reactivation or tilting.

Abstract Although lithospheric modelling has provided extraordinary insights into the processes that shape the continental crust, considerable uncertainty surrounds the basic rheology that governs behaviour at geological timescales. In part, this is because it has proved difficult to identify the geological observations that might discriminate, or unify, models of lithospheric rheology. In particular, the relative strength of lower crust and upper mantle remains a contentious aspect of continental lithospheric rheology. We show that various models for lower crustal rheology may produce distinct patterns of inversion in extensional sedimentary basins, consistent with some of the observed natural variability of inversion styles. Inversion of basin interiors, as is common in European Mesozoic basins, is favoured by a lithospheric rheology more sensitive to lateral thermal structure than to changes in the depth of the Moho, consistent with there being little strength contrast between the lower crust and upper mantle in these settings. In contrast, inversion of basin margins, particularly involving basinward verging structures, is consistent with a rheological sensitivity to the depth of Moho as would apply for a lower crust much weaker than the upper mantle. We use an example from central Australia to demonstrate this latter response, together with thermochronologic data that suggests that a relatively weak lower crust in this setting may reflect abnormally high geothermal gradients.