Michelle Salmon

Crustal architecture of the Capricorn Orogen, Western Australia and associated metallogeny

A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks, which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton. In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by the transport of fluid and energy direct from the mantle into the upper crust.

Instability of a lithospheric step beneath western North Island, New Zealand

Lithospheres of different thicknesses are often juxtaposed by movement on a continental-transform boundary. Such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle. Here we show, for plausible values of the lithospheric viscosity, a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere. As the mantle deforms it also progressively thickens and then thins the overlying crust, creating a topographic wave that migrates in concert with the removal of mantle lithosphere. Within western North Island, New Zealand, geophysical data define a sharp lithospheric step across the Taranaki-Ruapehu line, and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m.y. The rate of observed north to south migration of the wave (∼30 mm/yr), its wavelength (∼250 km), and amplitude (∼±1 km) are compatible with it being caused by progressive removal of mantle lithosphere, if the viscosity of the uppermost lithospheric mantle is ∼5 × 10 20 Pa⋅s, providing one of the clearest examples yet of this fundamental geological process.

AuSREM: Australian Seismological Reference Model

Although Australia has been the subject of a wide range of seismological studies, these have concentrated on specific features of the continent at crustal scales and on the broad-scale features in the mantle. The Australian Seismological Reference Model (AuSREM) is designed to bring together the existing information and provide a synthesis in the form of a 3-D model that can provide the basis for future refinement from more detailed studies. The model is grid based with a 0.5° sampling in latitude and longitude and is designed to be fully interpolable, so that properties can be extracted at any point. The crustal component makes use of prior compilations of sediment thicknesses, with cross-checks against recent reflection profiling, and provides P and S wavespeed distributions through the crust. The primary information for P wavespeed comes from refraction profiles, for S wavespeed from receiver function studies, and we are able to use the results of ambient noise tomography to link the point observations into national coverage. Density values are derived using results from gravity interpretations. AuSREM is able to build on a new map of depth to Moho, which has been created using all available information including Moho picks from over 10 000 km of full crustal profiling across the continent. In the upper mantle, the primary source of information comes from seismic surface wave tomography and a representative model has been developed to capture the features of a range of studies. Body-wave studies and regional tomography provide useful constraints on the relationship between P and S wavespeeds. The mantle model extends beyond the continent and so covers a larger area than for the crust. Below 350 km and in the surrounding area, AuSREM is linked to the S40RTS model. Potential applications of the AuSREM model come through the delineation of major structural features in the crust and mantle, and consequent improvements in, for example, earthquake location within the continent and at the nearby plate boundaries.

Stacking autocorrelograms to map Moho depth with high spatial resolution in southeastern Australia

Current estimates of Moho depth in southeastern Australia are based on sparse sampling. The results are augmented with 180 new Moho estimates constructed from spatial stacks of crustal P wave reflectivity derived from autocorrelograms at over 750 stations. The spatial stacks of reflectivity are constructed using a Gaussian with half width 0.5°. Picks of the base of crustal reflectivity are made with the aid of the previous Moho model, based on the sparser data, and knowledge of the variation in the character of the crust-mantle boundary across the region. Good ties can be made to previous results from deep reflection profiling. The new information fills in many holes in coverage and provides a Moho map with closer ties to geological provinces. The procedure exploits the continuous records at the stations and just the vertical components and so can be applied to older data for which receiver function techniques cannot be used.

Complex continental growth along the proto-Pacific margin of East Gondwana

We use ambient noise recordings from the largest transportable seismic array in the Southern Hemisphere to image azimuthal variations in Rayleigh wave phase anisotropy in the crust beneath southeast Australia. This region incorporates a transition from the Precambrian shield region of Australia in the west to younger Phanerozoic terranes in the east, which are thought to have been formed by subduction-accretion processes. Our results, which span the shallow to lower crust, show a strong and consistent pattern of anisotropy that is oriented north-south, approximately parallel to the former margin of East Gondwana. However, significant deviations from this trend persist through the period range 2.5 s to \textgreater10 s. One of the most notable deviations occurs along the edge of cratonic Australia, where the Curnamona Province forms a salient into the younger accretionary terrane, here, the fast axis of anisotropy follows the boundary almost exactly, and is virtually coincident with magnetic lineations extracted from aeromagnetic data. To the east of this boundary beneath the Lachlan orogen, a region masked by the Cenozoic Murray Basin, the fast axis of anisotropy becomes strongly curved and traces out a semicircular pattern with a radius of 200-250 km. Farther east, the fast axis of anisotropy returns to a dominantly north-south orientation. These new findings provide strong observational support to recent geodynamic modeling results that demonstrate how large-scale oroclinal structures can become embedded in accretionary mountain belts. © 2014 Geological Society of America.

Mechanical amorphization, flash heating, and frictional melting: Dramatic changes to fault surfaces during the first millisecond of earthquake slip

The evolution of fault strength and behavior during the initial stages of slip plays an important role in driving the onset of instability and fault weakening. Using small-displacement triaxial experiments on quartz sandstone, this study highlights the rapid onset of microstructural change on fault interfaces and identifies new evidence for an evolution in physical processes with increasing slip and velocity. Pre-ground fault surfaces have been slipped over a range of velocities (0.36 μm s –1 to 18 cm s –1 ) and at normal stresses comparable to upper- to mid-crustal conditions (92–287 MPa). Microstructural analysis of the fault interfaces reveals the formation of amorphous material at displacements <170 μm and slip durations < 1 ms. Mechanical and microstructural observations have been combined with numerical modeling to present the first documented examples of a transition from mechanical amorphization to flash heating, then frictional melting, with changes in slip conditions. The sequence of processes activated during the initial stages of fault movement may provide new insights into factors that influence the onset of slip in the seismogenic crust.

Origin of Lateral Heterogeneities in the Upper Mantle Beneath South-east Australia from Seismic Tomography

We use teleseismic body wave tomography to reveal anomalous P wave velocity variations in the upper mantle beneath south-east Australia. Data are sourced from the WOMBAT transportable seismic array, the largest of its kind in the southern hemisphere, which enables horizontal resolution of approximately 50 km to be achieved over a large region that includes Victoria, New South Wales and southern South Australia. In order to account for long-wavelength structure that is lost due to the use of multiple teleseismic datasets from adjacent arrays with non-overlapping recording periods, the AuSREM mantle model is included as prior information in the inversion. Furthermore, AuSREM crust and Moho structure is explicitly included in the initial model in order to account for the presence of shallow heterogeneity which is poorly constrained by the teleseismic dataset. The P wave velocity model obtained from the joint inversion of WOMBAT teleseismic data represents a vast new resource on the seismic structure of the upper mantle beneath south-east Australia. One of the most striking features of the model is the presence of a north-dipping low-velocity anomaly beneath the Newer Volcanics province, a Quaternary intraplate basaltic province in western Victoria. The anomaly appears to terminate at approximately 200 km depth and has a structure that is more suggestive of a source confined to the upper mantle rather than a deeply rooted mantle plume. Other features that can be observed include a high-velocity anomaly beneath the Curnamona province and a high-velocity salient beneath the New England Orogen . Of particular interest is an extensive high-velocity anomaly beneath the Lachlan Orogen, which coincides almost exactly with the surface expression of the Hay–Booligal Zone in the south, and extends northwards beneath the Macquarie Arc. The higher velocities beneath the Hay–Booligal Zone are consistent with the idea that it may be floored by a fragment of Proterozoic continental lithosphere that was once part of the east Gondwana margin, while the higher velocities beneath the Macquarie Arc may be related to its origin as an intra-oceanic arc.