James P. Cull

Thermal regimes, anatexis, and orogenesis: Relations in the western Lachlan Fold Belt, southeastern Australia

Abstract Crustal evolution of the Lachlan Fold Belt of southeastern Australia has involved the mobilisation of significant melt volumes now preserved as granites, suggesting that large parts of the lower crust were at T > 800°C for part of the thermal evolution. This anomalous thermal activity has been variously related to continental extension, magmatic underplating and to thinning of the mantle lithosphere. Overriding considerations reflected in the surface geology must include the tectonic setting, the thermal regime and the state of stress in the crust at the time of anatexis. One-dimensional geothermal modelling suggests that no one mechanism can generate the required thermal regime necessary to produce the volume of granite observed in the crust. The observed geological relationships do not match those predicted for lithospheric thinning due to continental extension. Magmatic underplating can cause melting but must involve extensive source zones and allow continual feeding of the magma to higher crustal levels. The crustal thickening/lithospheric thinning mechanism involves upwelling of hot asthenospheric mantle to the base of the crust but the melt products are confined to a very thin layer at the crust-mantle boundary. Mechanisms for melt extraction and emplacement to higher crustal levels must be explained. In the western Lachlan Fold Belt the presence of a major mid-crustal detachment fault, ductile shear zones in the lower crust and marked internal shortening of the upper and possibly lower crustal blocks suggests that shear-heating may also provide a significant component to the heat budget of the evolving crust. Melting will not occur however, unless thermal equilibration of both the upper and lower plates takes place prior to shear-heating. Such equilibration will involve time scales in the order of 30–50 Ma, unless extra heat is added into the lower crust (e.g. advective input). Inferred flexural rigidity of the lithosphere, related to “basinal” subsidence during the period of orogenesis in central Victoria, suggests that thermal effects related to granitoid intrusion must be largely post-tectonic and that felsic intrusive activity is a consequence of the crustal thickening rather than the cause. Thermal regimes during orogenesis cannot be restricted to a single mechanism. Upwelling of hot asthenosphere causing significantly higher heat input at the base of the crust, coupled with shear-heating and internal strain-heating of the crust during the crustal thickening stage as well as thermal effects related to magma-intrusion into the lower crust result in a complex thermal history during collisional orogenesis.

Geothermal signatures and uranium ore deposits on the Stuart Shelf of South Australia

An analysis of temperature data from drill holes on the Stuart Shelf of South Australia demonstrates a major thermal anomaly associated with the Olympic Dam copper‐uranium‐gold deposit. The average heat flow on the Stuart Shelf (seven locations, excluding Olympic Dam) is 73mW/m2, but an additional heat flow of approximately 45mW/m2 is present in the sediments overlying the orebody. Although some of the anomalous heat flow appears to be generated in the mid‐Proterozoic basement at depths greater than 1 km, uranium assays indicate that approximately 30mW/m2 can be attributed to concentrations defining the orebody. Major anomalies in heat flow can be readily detected in the flat‐lying cover of Cambrian and late Proterozoic sediments. The Tregolana shale within this sequence is a widespread homogeneous unit, typically 100–200 m thick. It is easily identified on temperature logs by its high thermal gradient relative to other sections in the hole. The heat flow anomaly at Olympic Dam is clearly distinguished by...

Stress-field constraints from recent intraplate seismicity in southeastern Australia

Well-constrained fault-plane solutions are determined for five recent moderate-magnitude intraplate earthquakes recorded in southeastern Australia: Thomson Reservoir, Tatong, Corryong and Boolarra South in Victoria, and Appin in eastern New South Wales. The solutions, determined from analysis of vertical-component first-motions, provide an indication of the maximum horizontal stress influencing contemporary regional deformation. The fault-plane solution for the 1996 M L 5.0 Thomson Reservoir earthquake indicates reverse faulting with northwest – southeast-trending horizontal compression. The Yallourn Fault is the most likely source for this event given the orientation of the fault-plane solution and the hypocentral location of the earthquake. The 1997 M L 4.2 Tatong earthquake also indicates reverse faulting with approximate north – south compression. The 1998 M L 4.7 Corryong event indicated a strike-slip mechanism with a compressional axis trending northwest – southeast. Despite local complexities identified within the stress field of the Sydney Basin, the fault-plane solution determined for the 1999 M L 4.6 Appin earthquake indicated strike-slip faulting and is consistent with the proposed weak northeast – southwest compressional regime in the region. The 2000 M L 4.7 Boolarra South earthquake indicates a mechanism consistent with reverse faulting with horizontal compression oriented northwest – southeast. Further analysis of horizontal-component seismograms indicated that the earthquake was possibly a result of movement on the Yarragon Fault. A composite focal mechanism was constructed comprising data from the four Victorian earthquakes (which lie in a similar stress province) used in this study. The solution indicated horizontal compression, consistent with reverse faulting, with an overall maximum horizontal stress trending at approximately 325°.

Lower crust and upper mantle electrical anisotropy in southeastern Australia

The dominant north–south strike of the Palaeozoic outcrop of central Victoria has been well documented, but to the north, these rocks are covered by the Cainozoic sedimentary deposits of the Murray Basin. Two magnetotelluric surveys were completed to assist in extrapolation of the known structure and to identify possible new targets for mineral discovery. Supporting the results from previous seismic interpretations for the region, the 2D MT inversion models substantiate an intrazone thrust fault system of listric geometries in the Bendigo Zone connected in the mid-crust. With the zone boundary clearly defined the electrical resistivity structure is distinct between the major subdivisions, indicating a different tectonic evolution for the Bendigo and Melbourne Zones. However, the conductive overburden in the region poses complications for the generation of the 2D resistivity models. Static shifts and electrical anisotropy were identified as distortions in the dataset, with further processing needed to attain a complete picture of the underlying geology. The difficulties caused by galvanic distortion were allayed by using the phase tensor response in place of the distorted amplitude response. Phase tensor analysis of MT data has been completed subsequently, the results of which we present here, along with the original 2D inversion models, confirming that electrical anisotropy persists into the mantle. In regions where conductive overburden dominates the near-surface, static shifts and electrical anisotropy can present significant complications for the generation of 2D magnetotelluric resistivity models. Affecting results collected in southeastern Australia, such difficulties caused by these galvanic distortions have thus been allayed through phase tensor analysis of the data.

A new iterative method for computing the magnetic field at high magnetic susceptibilities

Failure to adequately correct for the effects of self-demagnetization can lead to misinterpretation of magnetic survey data, thereby reducing the success of mineral exploration programs. Numeric methods commonly used to correct for self-demagnetization of finite three-dimensional bodies are restricted to moderate magnetic susceptibilities (χ < 1 SI) because at higher values (χ ≥ 1 SI), the approximation errors for nonellipsoidal bodies become excessive. This paper reports a new method that allows for calculation of the magnetic field from arbitrary finite bodies with high magnetic susceptibility while minimizing approximation errors caused by the use of self-demagnetization corrections for nonellipsoidal bodies. This technique uses a segmented model defined by spherical elements (or voxels) of arbitrary diameter and an iterative computation of the magnetic field at the center of each voxel in free space and then with respect to the surrounding voxels.

Simultaneous modelling of the phase and amplitude components of downhole magnetometric resistivity data

Abstract The downhole magnetometric resistivity (DHMMR) technique is an effective method for follow-up exploration of massive and disseminated sulphide deposits. The method comprises the “in-hole” measurement of low-amplitude, low-frequency magnetic fields associated with galvanic current flow between two current electrodes. This paper presents methods for the simultaneous modelling of both the magnetometric resistivity (MMR) amplitude and phase (or magnetic-induced polarisation [MIP]) response for DHMMR data. Analyses of the MIP response and the interactions between conductive and polarisable bodies and their host are calculated using variations of the Cole–Cole model for complex impedance. The inphase and quadrature components provide symmetric signatures at the target depth, but some results are counter-intuitive when expressed as the MIP (phase) response. These methods are used to provide an interpretation of DHMMR data obtained from the Flying Doctor Prospect near Broken Hill, New South Wales.

Cavity Detection Using Single-fold Frequency Analysis Editorial

A 4-channel, single-fold seismic acquisition system can be used to measure frequency domain response along paved roads. Shallow membrane delaminations and voiding defects are identified by a dominant mode consistent with membrane vibration and high-frequency attenuation above 100Hz. The results correlate well with GPR profiles, and are especially useful for near-surface imaging where metallic clutter noise corrupts EM measurements.