Bruce Goleby

Crustal architecture of central Australia based on deep seismic reflection profiling

Abstract The crustal architecture of central Australia is interpreted from deep seismic reflection profiling conducted by the Australian Geological Survey Organisation in two surveys in 1985 and 1993. The seismic traverses, oriented normal to the main surface structures, ran north-south in central Australia, and crossed parts of the Arunta Block, Amadeus Basin, Musgrave Block and Officer Basin. The present crustal fabric was set in place by the end of the Mesoproterozoic (by about 1100 Ma). Reactivation of the structures took place in a continental intraplate setting mainly during the Middle-Late Palaeozoic Alice Springs Orogeny, but also during other orogenic events. In the Arunta Block. the crust is dominated by major north-dipping planar structures, interpreted as thick-skinned thrusts. Many of these thrusts cut deep into the crust, and at least one, the Redbank Thrust Zone, appears to cut and offset the Moho. In contrast, in the northern Musgrave Block, limited field mapping and teleseismic data suggest that the major crustal-scale planar structures are south-dipping. In the central to southern Amadeus Basin, deformation is essentially thin-skinned, with north-directed thrusting confined to the sedimentary succession. Thus, the deep seismic profiles in central Australia show a present day crustal architecture that is the response of the crust to Mesoproterozoic terrane amalgamation and to later reactivation by intraplate deformational events. Therefore. central Australia is a model for intraplate cratonic deformation that occurs in continental crust that is cold. thick and strong.

Seismic reflection profiling in the Proterozoic Arunta Block, central Australia: processing for testing models of tectonic evolution

Abstract The region extending from the northern Amadeus Basin to the Northern Arunta Province is characterised by a large change in Bouguer gravity (1400 μ s −2 ), abrupt changes in teleSeismic travel-time residuals and large thrust structures in which rocks from the lower crust are now exposed at the surface. Deep Seismic reflection data, other geophysical data and geological mapping within this region have contributed to the development of a crustal model that would result from “thick-skinned” tectonic processes, in which deformation occurred along moderately dipping thrusts or shear zones that extended from the surface down to at least the crust-mantle boundary. Two important features of this model have emerged with the help of unconventional stacking techniques. The first is a series of reflections from the Redbank Deformed Zone that can be traced from surface outcrop to depths of more than 30 km. The second is evidence of a fault extending through the crust and displacing the Moho.

New constraints on the seismic structure of West Australia: Evidence for terrane stabilization prior to the assembly of an ancient continent?

We present a new, near-comprehensive survey of the variations in seismic structure across the West Australian craton at the scale of the main terrane groups. Analyzing data from distant earthquakes recorded at temporary and permanent stations located across the region, we found the best-fi tting structure by modeling the conversions from P- to S-wave motion (the receiver function) that take place as the seismic energy travels upward through the lithosphere. Such methods can be used to delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited, and they provide an effective alternative to active-source seismic techniques for deep crustal targets. The seismic structure is consistent within several of the individual Archean terranes, most notably the Pilbara, Murchison, and Southern Cross. These terranes are underlain by lower crust of low seismic velocity and show a sharp seismic Moho. The structure shows signifi cant contrasts between neighboring terranes; thus, major tectonic units have a velocity profi le that is a signature of that terrane or terrane group. We infer that the seismic structure of the Archean crust and upper mantle was fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units that formed the West Australian craton.

Seismic reflection and refraction profiling across the Arunta Block and the Ngalia and Amadeus Basins

In order to investigate the tectonic evolution of the Arunta Block and the Ngalia and Amadeus Basins, a regional north‐south seismic reflection line 420 km long from the Northern Arunta Province to the southern part of the Amadeus Basin, and an east‐west refraction profile over 400 km within the Arunta Block, were recorded by the Bureau of Mineral Resources in 1985. The most significant basement features observed in the reflection data are prominent bands of northerly dipping reflections originating from beneath the Northern Arunta Province and the Ngalia Basin at times of between 4 and 10 s. In this region, reflected energy with frequencies as high as 100 Hz is present at two‐way times of 5–6 s, implying that the rocks have high Q to depths of at least 18 km. The character of the reflections changes markedly with varying frequency, which suggests that they arise by interference phenomena, probably associated with laterally varying lamellar structures. Deep crustal features on the reflection profiles from...

Seismic reflection imaging of mineral systems: Three case histories

Mineral deposits can be described in terms of their mineral systems, i.e., fluid source, migration pathway, and trap. Source regions are difficult to recognize in seismic images. Many orebodies lie on or adjacent to major fault systems, suggesting that the faults acted as fluid migration pathways through the crust. Large faults often have broad internal zones of deformation fabric, which is anisotropic. This, coupled with the metasomatic effects of fluids moving along faults while they are active, can make the faults seismically reflective. For example, major gold deposits in the Archaean Eastern Goldfields province of Western Australia lie in the hanging‐wall block of regional‐scale faults that differ from other nearby faults by being highly reflective and penetrating to greater depths in the lower crust. Coupled thermal, mechanical, and fluid‐flow modeling supports the theory that these faults were fluid migration pathways from the lower to the upper crust. Strong reflections are also recorded from two ...

Deep Seismic profiling in central Australia

Abstract Deep Seismic profiling undertaken in central Australia comprised expanding spread reflection profiling, long-range refraction work and a small-scale three-dimensional refraction survey, as well as the more usual near-vertical incidence reflection profiling. Scismic reflection sections within the Arunta Block in central Australia show abundant northerly dipping events that are interpreted as reflections from dipping faults, many of which are evident in surface geological mapping. The Redbank Deformed Zone, a major thrust feature, has been imaged to depths of at least 30 km and defines a marked change in reflection character. South of the Redbank Zone, steep, northerly dipping reflections are absent; sub-horizontal basement reflections are prevalent below the Southern Arunta Province and Amadeus Basin. Seismic refraction profiles indicate that the crust is more than 50 km thick below both the boundary between the Southern and Central Arunta Provinces and the northern part of the Amadeus Basin. Expanding reflection spreads and a three-dimensional refraction survey have added fine details to the results in regions of special geological interest and include the resolution of complicated velocity variations in the sedimentary rocks of the northern part of the Amadeus Basin and the measurement of anisotropy in the granulites of the Central Arunta Province adjacent to the Redbank Zone.

Review of recent results from continental deep seismic profiling in Australia

Abstract The Australian Geological Survey Organisation regularly collects 450–500 km of onshore deep seismic reflection data and up to 4500 km offshore each year in Australia. These recordings are made in a wide range of tectonic provinces, including, in the last few years, late Palaeozoic-Mesozoic intracontinental and Palaeozoic-Mesozoic-Cenozoic continental margin extensional basins, moderately deformed Palaeozoic transtensional basins and compressional fold belts, and Archaean greenstone terranes. Several of these provinces are major petroleum exploration provinces, whereas others contain significant mineral deposits. The primary purpose of the deep seismic profiling program is to resolve the tectonic history of the Australian continent, and thereby to encourage exploration for hydrocarbons and mineral resources in Australia. On the northwest Australian continental margin, major basin systems including the Bonaparte Basin, formed as a result of complex interactions since the Carboniferous, involving episodes of extension followed by strike-slip movements and inversion, which reactivated both the primary extensional and ancient basement structures. Off southeastern Australia, basins such as the Gippsland Basin formed as part of a linked transtensional system related to movement on a common mid-crustal detachment complex. On continental Australia, the Bowen Basin, in the northeast, was deformed by thrust faults that root in a major E-dipping detachment that flattens in the middle crust. The Cobar Basin, in the southeast, is a case where the seismic data support a detachment model in which the upper plate displacement vector can be calculated by plate reconstructions linking the geometry of the detachment surface with that of the basin. The greenstone terranes within the Eastern Goldfields region of Western Australia show crustal-scale fault systems that are planar and steep dipping, more in keeping with those interpreted in data from other Precambrian provinces rather than those of the Palaeozoic provinces.

3D isochronal modelling of reflections from the deep crust: application to reflection profiling in central Australia

Abstract Reflection profiling of the whole crust across ancient terranes encounters a range of interpretational problems and ambiguities, especially with regard to the geometry of faults. Such problems can be addressed by Seismic modelling with full inclusion of diffractions and a suitable modelling procedure for such three-dimensional problems is the isochronal technique described by Cao and Kennett (1989). Such modelling has been used to investigate the validity of some of the interpretations of a deep crustal refection profile in central Australia involving planar faults of moderate dip penetrating most of the crust and considerable Moho topography. Even with a relatively short (4 km) field spread it would be possible to detect the reflected energy from faults with dips of about 40°. The character of the reflections also suggests considerable variability in piysical properties within the fault zones. The largest fault, the Redbank Zone, has significant displacement of the Moho. But most of crustal faults appear to sole out into the crust-mantle interface. This gives rise to an undulating Mcho for which Seismic modelling corresponds closely to the observed data.

Tomographic reconstruction of upper crustal velocity variations in the Arunta Block, central Australia

Abstract A Seismic refraction/reflection programme of crustal/upper-mantle investigations was recently undertaken by the Australian Bureau of Mineral Resources within the Proterozoic Arunta Block, central Australia. One experiment involved the successive deployment of sixteen portable Seismic recorders on two approximately orthogonal, intersecting traverses. Shots detonated on one traverse were recorded on the other traverse to give three-dimensional coverage. Shot-receiver distances varied from 1 to 39 km. P-wave first-arrival times were processed using an OCSIRT (overlapping-cell simultaneous iterative reconstruction tomography) technique to produce a map of upper crustal velocity inhomogeneity. Synthetic data, using the same shooting geometry, were also processed as an aid to interpretation. The final tomograms exhibit a velocity variation from 5.5 to 6.4 km s −1 . The higher velocities (> 5.9 km s −1 ) are associated with mafic and felsic granulites of the Central Arunta Province that have been thrust over amphibolite facies gneisses and migmatites of the Southern Arunta Province. The lower velocities ( −1 ) are generally associated with wave paths through the mylonites of the Redbank Thrust Zone and migmatitic gneisses of the Southern Arunta Province.

The interpretation of expanding spread reflection profiles: examples from central and eastern Australia

Abstract The Australian Bureau of Mineral Resources has recorded expanding reflection spreads coincident with regional deep Seismic reflection profiles on land, both in sedimentary basins and in areas of exposed basement rocks. Arrival times of refracted, reflected and diffracted signals have been matched with synthetic times computed by ray tracing to give one- or two-dimensional velocity models for the regions immediately below the expanding spreads. Significant lateral variations in Seismic velocity in both sedimentary and basement rocks over distances of about 20 km are widespread, making it difficult to fit refraction, near-vertical incidence and moderate- to wide-angle reflection data with reasonably simple velocity models. One expanding spread recorded across the Nebine Ridge in southern Queensland shows P-to-SV and SV-to-P converted reflections from a bright spot in the coincident near-vertical incidence reflection data at a depth of about 17 km. Another expanding spread recorded in the Amadeus Basin has provided a detailed velocity model for the 9 km thick sedimentary section. The resolution of deep basement velocities is best below the Surat Basin, although the absence of persistent, continuous reflections and the presence of reflections from outside the vertical plane through sources and receivers limits the reliability of estimates of basement velocities.