Barry Drummond

New evidence of Tasmania's tectonic history from a novel seismic experiment

In March 1995, 44 land-based recorders were deployed throughout Tasmania, SE Australia, to record seismic energy from an encircling array of marine normal-incidence reflection shot lines. We invert refraction and wide-angle reflection traveltimes for crustal structure, with the principal outcome being a map of the Tasmanian Moho. Key tectonic inferences from this map include: (1) the Arthur Lineament metamorphic belt in NW Tasmania overlies a major change in crustal thickness (over 5 km) and probably represents the NW limit of deformation in Tasmania during the Mid-Late Cambrian Tyennan Orogeny, (2) thickening of the crust beneath central northern Tasmania may be associated with the juxtaposition of the Eastern and Western Tasmania Terranes during the Mid-Devonian Tabberabberan Orogeny, and (3) the difference in crustal thickness between the east and west coasts reflects the presence of differing strain regimes during the Cretaceous break-up of Gondwana.

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 ...

Fluid reservoirs in the crust and mechanical coupling between the upper and lower crust

An important observation associated with seismic activity on the Nagamachi-Rifu Fault is the existence of tabular, fluid rich zones at mid-crustal levels. These zones resemble the “bright spots” seen in many seismic images of the crust worldwide. The aim of this paper is to develop the mechanical foundations for the formation of such zones. To do so requires an understanding of the distribution of pore fluid pressure in a deforming crust. In a hydrostatically stressed porous material, the pore fluid pressure should equal the mean stress in order to keep the pores from collapsing. Past discussions of this subject imply very high pore fluid pressures, two to three times lithostatic. Considerations of plastic yielding together with continuity arguments, particularly at the plastic/viscous transition, suggest that pressures closer to lithostatic are more the norm. Particularly just below the plastic/viscous transition in compressive regimes, this leads to collapse of porosity with an associated collapse in permeability resulting in an over-pressured region comprising that part of the lower crust that is characterised by high mean stress. The base of the plastic region is at a strong discontinuity in stress difference where localised deformation occurs. Tabular, dilatant fluid filled regions develop at and above this zone in close association with dilatant tensional zones in the hanging-walls of faults and diffuse shear zone development in the upper to mid crust. Some of these dilatant zones ultimately develop into listric transitions between steeply dipping, upper crustal faults and shear zones associated with the plastic/viscous transition. These zones are also the sites of strong mineral alteration that may, particularly in ancient examples, also contribute to the delineation of “bright spots” in seismic images. For high geothermal gradients another class of fluid filled layers, in the form of “stagnant fluid zones”, develops below the region of high mean stress in the viscous lower crust. Mineral alteration associated with this second class of fluid rich layers is predicted to be asymmetric in distribution as opposed to the first class that would be homogeneous in the mode of alteration.

Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Palaeozoic

Abstract The island of Tasmania in southeast Australia consists of a number of stratotectonic elements. The relationships between these elements are largely obscured by younger cover of the Tasmania Basin, which contains extensive dolerite sills that limit the ability of potential field techniques to map basement. Therefore the development of a robust tectonic model for Tasmania has been inhibited. To assist in the development of a tectonic model, a deep seismic reflection program undertaken offshore around the entire island was designed to map the large-scale structures of Tasmania at depth. The airgun seismic energy was also recorded at a number of seismographs deployed across the island, allowing low resolution 3D tomographic imaging. Short reflection profiles were recorded onshore across structures which could not be imaged by the offshore profiling. This paper focuses on eastern Tasmania. In the seismic sections, the Proterozoic basement in the southeast is mostly featureless, except for large rotated blocks with weakly reflective boundary faults, indicating extension of the Tyennan Element by block faulting. The deposition of the sedimentary succession of the Adamsfield–Jubilee Element was related to this extensional event. In the northeast, a reflective lower crust is interpreted to represent thrust slices of previously highly extended continental crust and possibly fragments of oceanic crust. The Early Palaeozoic sedimentary succession of the Northeast Tasmania Element formed across the inverted margin. The apparently complex geology of eastern Tasmania therefore fits into an extensional model where continental extension eventually led to the formation of very thin continental crust and possibly oceanic crust to the east. The extension was probably related to Late Neoproterozoic extension recorded elsewhere in Australia. The region was subsequently shortened, probably in a northeast–southwest direction, with most shortening accommodated in the seismically reflective, probably oceanic part of the crust, and little or no shortening in the block-faulted, and extended continental crust.

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.

The role of crustal fluids in the tectonic evolution of the Eastern Goldfields Province of the Archaean Yilgarn Craton,Western Australia

Gold deposits in the Archaean Eastern Goldfields Province in Western Australia were deposited in greenstone supracrustal rocks by fluids migrating up crustal scale fault zones. Regional ENE-WSW D2 shortening of the supracrustal rocks was detached from lower crustal shortening at a regional sub-horizontal detachment surface which transects stratigraphy below the base of the greenstones. Major gold deposits lie close to D3 strike slip faults that extend through the detachment surface and into the middle to lower crust. The detachment originally formed at a depth near the plastic-viscous transition. In orogenic systems the plastic-viscous transition correlates with a low permeability pressure seal separating essentially lithostatic fluid pressures in the upper crust from supralithostatic fluid pressures in the lower crust. This situation arises from collapse in permeability below the plastic-viscous transition because fluid pressures cannot match the mean stress in the rock. If the low permeability pressure seal is subsequently broken by a through-going fault, fluids below the seal would flow into the upper crust. Large, deeply penetrating faults are therefore ideal for focussing fluid flow into the upper crust. Dilatant deformation associated with sliding on faults or the development of shear zones above the seal will lead to tensile failure and fluid-filled extension fractures. In compressional orogens, the extensional fractures would be subhorizontal, have poor vertical connectivity for fluid movement and could behave as fluids reservoirs. Seismic bright spots at 15–25 km depth in Tibet, Japan and the western United States have been described as examples of present day water or magma concentrations within orogens. The likely drop in rock strength associated with overpressured fluid-rich zones would make this region just above the plastic-viscous transition an ideal depth range to nucleate a regional detachment surface in a deforming crust.