Abbas Babaahmadi

Kinematics of the demon fault: implications for Mesozoic strike-slip faulting in eastern Australia

The Demon Fault is a north-striking, dextral strike-slip fault that displaces Carboniferous to Middle Triassic rock units in eastern Australia. We present results from aeromagnetic gridded data, satellite images, digital elevation models, and field observations, which provide constraints on the geometry and kinematics of the Demon Fault. The fault can be subdivided into four steeply dipping segments (DF1 to DF4) with oblique reverse-dextral kinematics. Earlier NW-striking faults, which affected Early and Middle Triassic magmatic rocks, are displaced by the Demon Fault, implying that activity along the Demon Fault has occurred during the latest Middle Triassic or later. The amount of dextral offset along the different parts of the Demon Fault is decreased towards the south. This suggests that either different segments have experienced different histories of reactivation, or that deformation toward the south has partitioned into minor splays, which operated as reverse faults and resulted in a lesser dextral displacement. We propose that the Demon Fault and other sub-parallel faults operated in conjunction with the development of basins in eastern Australia from Late Triassic to Early Cretaceous. The pattern of minor faults and their kinematics indicate that recent activity along the Demon Fault has occurred during dextral transpression, which we attribute to mid-Cretaceous contractional deformation, or possibly even Cenozoic deformation associated with collisional processes at the northern boundary of the Australian plate.

Alternating episodes of extension and contraction during the Triassic: evidence from Mesozoic sedimentary basins in eastern Australia

Triassic to Lower Cretaceous continental sedimentary basins occur in eastern Australia, but the tectonic and structural evolution of these basins is not fully understood. Using gridded aeromagnetic data, seismic reflection data and field observations, we conducted a structural analysis aimed at characterising major faults and deformation style in these sedimentary basins. Our results show evidence for two alternating episodes of rifting during the Triassic. An earlier episode of rifting, which took place in the Early Triassic to early Late Triassic, is inferred based on synsedimentary normal faults in the Nymboida Coal Measures and the boundary West Ipswich Fault System in the Esk Trough. This phase of rifting was followed by a contractional event that resulted in tilting, folding, and thrust faulting. Evidence of synsedimentary normal faults and bimodal volcanism indicates that another rifting phase occurred during the Late Triassic and resulted in the development of the Ipswich Basin. From the latest Late Triassic to the Early Cretaceous, the accumulation of continental sediments in the Clarence-Moreton Basin was accompanied by subsidence. We suggest that the alternating rifting episodes and contraction were ultimately controlled by plate boundary migration and switches between trench retreat and advance during the Triassic.

The Red Rock Fault zone (northeast New South Wales): kinematics, timing of deformation and relationships to the New England oroclines

The formation of the northern part of the New England oroclines has previously been linked to dextral strike-slip faulting, but hitherto no concrete evidence has been shown to support this suggestion. We studied an exposure of a fault zone in the Red Rock headland, northeastern New South Wales, and we present here structural observations from the fault zone complemented by geochronological constraints on the timing of faulting and geochemical data that inform us on the nature of the co-seismic fluids. Our observations show evidence for dextral strike-slip faulting, with a reverse kinematic component, along a subvertical fault plane oriented NNE–SSW. Rb–Sr and 40Ar/39Ar ages from fault gouge samples indicate that brittle faulting occurred at 286.5 ± 1.5 Ma with possible reactivations at 267.0 ± 9.6 Ma and 264 ± 11 Ma. Oxygen and hydrogen stable isotope geochemistry indicates that the fluids that circulated in the fault zone were associated with a deep crustal source. Based on these results, we conclude that the Red Rock Fault zone is likely an exposed segment of a larger fault system, that was active at 288–285 Ma (Early Permian). The timing of faulting was contemporaneous with the development of the New England oroclines, raising the possibility that oroclinal bending was accompanied by strike-slip faulting. Whether faulting was associated with local deformation at the limb of the Coffs Harbour Orocline, or with a larger-scale wrench tectonic zone remains unresolved.

Structure and kinematics of the Louth-Eumarra Shear Zone (north-central New South Wales, Australia) and implications for the Paleozoic plate tectonic evolution of eastern Australia

ABSTRACT The ∼E–W-trending Olepoloko Fault and ∼ENE-trending Louth-Eumarra Shear Zone in north-central New South Wales are approximately orthogonal to the dominant ∼N–S-trending structural grain of Paleozoic eastern Australia. These structures have been interpreted to represent the boundary between the Thomson and Lachlan orogens, but their exact geometry and kinematics remain unclear owing to the scarcity of surface exposure. Using gridded aeromagnetic data and limited field mapping, we obtained new data on the tectonic history of the Louth-Eumarra Shear Zone, which seems to represent a broad zone of dextral shearing with a component of crustal thickening indicated by the recognition of kyanite growth in a mica-schist. The timing of deformation is relatively poorly constrained, but at least a component of the dextral shearing appears to be coeval or younger than the age of displaced late Silurian and Early Devonian granitoids. Additional indicators for dextral kinematics farther north, along the ∼ENE-trending Culgoa Fault, suggest that the width of the zone that was subjected to dextral deformation is possibly >100 km. This raises the possibility that a large component of dextral displacement was accommodated in this region. In a broader geodynamic context, we discuss the possibility that the precursor of the Louth-Eumarra Shear Zone and Olepoloko Fault originated from segmentation between the northern and southern Tasmanides, perhaps during the Cambrian. The existence of such a discontinuity may have buttressed the process of oroclinal bending in the Silurian. The observed dextral kinematics has possibly resulted from reactivated deformation during the Tabberabberan and Alice Springs orogenies.

The development of a Triassic fold-thrust belt in a synclinal depositional system, Bowen Basin (eastern Australia)

A synclinal depositional system in eastern Australia (Taroom Trough, Bowen Basin) was affected by folds and thrusts, but the structural style associated with this deformation is not fully understood. Using gridded aeromagnetic data and 2D seismic reflection data, we conducted a structural analysis that unravels the geometry and kinematics of major thrust faults in the eastern-central part of the Taroom Trough. Major structures are the east-dipping Cockatoo, Miles and Taroom faults, and west-dipping Burunga and Glebe faults. Our results show that west-dipping thrusts have a listric geometry that produced gentle hanging-wall anticlines. A north-striking gentle symmetric syncline and anticline pair is also observed to the west of the Burunga Fault. These observations indicate that the deformation in the central Taroom Trough was controlled by decollements in the basement rocks. The decollements and resultant structures were likely developed in response to mild contraction of the synclinal depositional system during the last phase of the Permian-Triassic Hunter-Bowen Orogeny (HBO). The last phase of the HBO also resulted in the reactivation of pre-existing east-dipping Cockatoo and Miles faults. The bulk longitudinal strain, however, in the eastern-central Bowen Basin was low (~2.8% shortening), with the deformation restricted to a relatively narrow zone. In contrast, deformation in the northern Bowen Basin was distributed in a wider fold-thrust belt that accommodated a higher amount of strain. This change in the pattern of deformation along the eastern part of the Bowen Basin could possibly be explained by along-strike variations in the rates of trench advance.

Kinematics of orocline-parallel faults in the Texas and Coffs Harbour oroclines (eastern Australia) and the role of flexural slip during oroclinal bending

The Texas and Coffs Harbour oroclines are defined by a Z-shaped curvature in the southern New England Orogen (eastern Australia), but the geometry and kinematics of faults around these oroclines, as well as their possible role during oroclinal bending, have hitherto not been understood. Using aeromagnetic and open file seismic data, as well as field observations, the pattern, geometry and kinematics of fault systems, have been investigated. Fault traces with a strike-slip component are oriented parallel to the curved magnetic and structural fabrics of the Texas and Coffs Harbour oroclines. Our observations show evidence for sinistral or sinistral-reverse, dextral (or dextral-reverse) and normal kinematics along NW-striking faults. The dominant kinematics along NNE- and NE-striking faults is dextral or dextral-reverse. The timing of faulting is not well constrained, but the ubiquitous recognition of orocline-parallel faults may suggest that a flexural slip mechanism operated during oroclinal bending in the early–middle Permian (ca 299–265 Ma). Our observations indicate that many of the orocline-parallel faults, with strike-slip separation, were reactivated during the Mesozoic and Cenozoic, as indicated by the recognition of displaced Triassic granitoids, Mesozoic sedimentary rocks and Cenozoic basalts.

The evolution of a Late Cretaceous???Cenozoic intraplate basin (Duaringa Basin), eastern Australia: evidence for the negative inversion of a pre-existing fold???thrust belt

The Duaringa Basin in eastern Australia is a Late Cretaceous?–early Cenozoic sedimentary basin that developed simultaneously with the opening of the Tasman and Coral Seas. The basin occurs on the top of an earlier (Permian–Triassic) fold–thrust belt, but the negative inversion of this fold–thrust belt, and its contribution to the development of the Duaringa Basin, are not well understood. Here, we present geophysical datasets, including recently surveyed 2D seismic reflection lines, aeromagnetic and Bouguer gravity data. These data provide new insights into the structural style in the Duaringa Basin, showing that the NNW-striking, NE-dipping, deep-seated Duaringa Fault is the main boundary fault that controlled sedimentation in the Duaringa Basin. The major activity of the Duaringa Fault is observed in the southern part of the basin, where it has undergone the highest amount of displacement, resulting in the deepest and oldest depocentre. The results reveal that the Duaringa Basin developed in response to the partial negative inversion of the pre-existing Permian–Triassic fold–thrust belt, which has similar orientation to the extensional faults. The Duaringa Fault is the negative inverted part of a single Triassic thrust, known as the Banana Thrust. Furthermore, small syn-depositional normal faults at the base of the basin likely developed due to the reactivation of pre-existing foliations, accommodation faults, and joints associated with Permian–Triassic folds. In contrast to equivalent offshore basins, the Duaringa Basin lacks a complex structural style and thick syn-rift sediments, possibly because of the weakening of extensional stresses away from the developing Tasman Sea.

Style and intensity of Late Cenozoic deformation in the Nagoorin Basin (eastern Queensland, Australia) and implications for the pattern of strain in an intraplate setting

Eastern Australia was affected by late Cenozoic intraplate deformation in response to far-field stress transmitted from the plate boundaries, but little is known about the intensity and pattern of this deformation. We used recently surveyed two-dimensional seismic reflection lines and aeromagnetic data, and data from the recently released Australian Stress Map, to investigate the structure of the Nagoorin Basin in eastern Queensland. The western margin of the Nagoorin beds was displaced by the Boynedale Fault, which is a NNW-striking SW-dipping oblique strike-slip reverse fault with a vertical throw of c. 900 m and c. 16 km sinistral displacement. A significant part of this large sinistral displacement is interpreted to have occurred prior to late Cenozoic time. Several low-angle (<30°) thin-skinned thrusts with a flat-ramp geometry also displaced the Nagoorin beds, which are interpreted to have developed along detachment surfaces in oil shales and claystone. The Boynedale Fault is a segment within longer NNW-striking faults that include the North Pine and West Ipswich fault systems in eastern Queensland. These NNW-striking faults are potentially active, and may accommodate neotectonic thrust movement in response to the present-day NE–SW orientation of S. Results of this study, in conjunction with previous information on sedimentary basins in eastern Australia, indicate that Cenozoic contractional deformation is stronger at the continental margins, possibly due to the presence of pre-existing rift-related structures.