Paul Lennox

Palaeomagnetism and tectonic rotation of the Hastings Terrane, eastern Australia

The Hastings Terrane comprises two or three major fragments of the arc‐related Tamworth Belt of the southern New England Orogen, eastern Australia, and is now located in an apparently allochthonous position outboard of the subduction complex. A palaeomagnetic investigation of many rock units has been undertaken to shed light on this anomalous location and orientation of this terrane. Although many of the units have been overprinted, pre‐deformational magnetizations have been isolated in red beds of the Late Carboniferous Kullatine Formation from the northern part of the terrane. After restoring these directions to their palaeohorizontal (pre‐plunging and pre‐folding) orientations they appear to have been rotated 130° clockwise (or 230° anti‐clockwise) when compared with coeval magnetizations from regions to the west of the Hastings Terrane. Although these data are insensitive to translational displacements, a clockwise rotation is incompatible with models previously proposed on geological grounds. While a...

Devonian‐Carboniferous stratigraphy of the southern Hastings Block, New England Orogen, eastern Australia

Devonian and Carboniferous rocks within the Hastings Block represent an arc‐related basin fill located, out of place, on the outboard margin of the subduction complex and juxtaposed along strike from the fore‐arc basin of the southern New England Orogen. The stratigraphic sequence, structural style and history of rocks in the southern part of the Hastings Block differs from that in the northern part of the block. The stratigraphy of the southern part of the Hastings Block consists of a succession of nine formations (eight new) ranging in age from Late Devonian (Frasnian) to Late Carboniferous (Namurian). A further five formations (three new) ranging in age from Siluro‐Devonian to Late Devonian (Famennian) are confined to fault‐bounded blocks. Three of the latter are probably correlative with parts of the continuous Frasnian‐Namurian succession. The Port Macquarie Block is considered part of the southern Hastings Block because it contains two closely comparable formations. Late Devonian formations include ...

Barry Granodiorite and Sunset Hills Granite: Wyangala‐style intrusion at the margin of a regional ductile shear zone, northern Lachlan Fold Belt, New South Wales

The Barry Granodiorite is a weakly deformed I‐type, and the Sunset Hills Granite is a moderately deformed S‐type, granite. Both granites were passively intruded into an already foliated greywacke and volcanic sequence. Emplacement may have been facilitated by faults related to the oblique opening of the late Early Silurian Hill End Trough. The granites display a dominant foliation which formed during the late Middle Devonian and subsequently was reoriented during the Early Carboniferous. The Barry Granodiorite and Sunset Hills Granite are on the margin of north‐south ductile shear zones related to the Wyangala Batholith. These granites and the adjacent Carcoar Granodiorite have undergone reorientation during movement on ductile shear zones either due to megakinking during late‐stage north‐south shortening, or southeastward movement of the southern margin of the west‐northwest‐trending Lachlan Transverse Zone.

Structural evolution and granite chronology of the central Molong Zone, Eastern Lachlan Fold Belt, Australia

The details of the deformation of the Molong Zone after its development are poorly constrained because of the paucity of dates on significant structural features, the difficulty of mapping faults that are poorly exposed, and the limited understanding of the timing and kinematics on key faults. Bedding and foliation in the Ordovician metavolcanics and greywackes in the northern Molong Zone record pre-granite Benambran deformation and up to three post-granite foliation-forming events (Bowning – Bindian, Tabberabberan and Kanimblan). There appears to have been multiple cycles of foliation development along meridional trends. The granites were intruded along pre-existing faults and overall this area shows post-granite southward and anticlockwise movement during dominant east – west shortening. Fault movement occurred during sedimentation, prior to, during and after granite intrusion. Many major faults have a complex history involving a significant component of dextral strike-slip movement sometimes after normal, dip-slip histories. The granodiorites and granite intruded at different structural levels and were progressively uplifted during generally east-directed thrusting between the bounding Wongalong – Werribee and Copperhannia – Godolphin Fault Systems. Zircons from the Carcoar Granodiorite were dated using single crystal and SHRIMP dating methods and recorded a crystallisation age of 425.6 ± 2.3 Ma (mean) and 434.4 ± 5.5 Ma, respectively. Conventional U – Pb on three zircon grains from the Barry Granodiorite gave a crystallisation age of 425.8 ± 1.9 Ma. U – Pb in zircon SHRIMP age for the Sunset Hills Granite yielded a crystallisation age of 430.1 ± 3.7 Ma. The Ar – Ar hornblende radiometric ages for the granodiorites suggest that there is a 2 – 28 million years interval between granite crystallisation and hornblende closure. This is unlikely given these are high-level granites and suggests that intercalibration problems between the U – Pb and Ar – Ar radiometric methods may in part explain the apparent extended period between crystallisation and closure of hornblende.

Kinematic history of serpentinites in the faulted margins of the Hastings Block, New England Orogen, eastern Australia

Serpentinites along the boundaries of the Hastings Block, southern New England Orogen, eastern Australia, contain S, C and C′ structures that provide information on the kinematic history of faults bounding the block. The serpentinite bodies at Yarras on the western boundary of the Hastings Block within the Ralfes Creek, Taylors Creek and Old Highway-Yarras Mountain Trail Faults display dominantly sinistral strike-slip or oblique-slip movement. Serpentinite bodies between these faults record dextral strike-slip and oblique-slip movement histories as well as sinistral strike-slip or oblique-slip and rare normal, dip-slip movement. The Mt George Serpentinite consists of a northeast-oriented section folded and displaced by movement on a west-northwest section of the Peel–Manning Fault System. Serpentinite structures suggest the west-northwest section initially underwent dextral strike-slip, which reoriented the northeast-oriented section, and subsequently sinistral strike-slip movement. Preliminary studies of serpentinites elsewhere around the periphery of the Hastings Block indicate localised effects due to readjustment of the block during its interaction with the Nambucca Block on its northern margin. The structures observed in the serpentinites are thought to have occurred during and after the Hunter–Bowen Orogeny and prior to stitching of the accretionary–subduction and forearc by the 228 Ma Werrikimbe Caldera.

Tectonic implications of two Rb/Sr biotite dates for the Tia Granodiorite, southern New England Fold Belt, NSW, Australia

New Rb/Sr biotite dates for the Tia Granodiorite of 264 ± 2.6 Ma and 262 ± 2.6 Ma are considerably younger than the 290 ± 4 Ma Rb/Sr whole rock dates from the Hillgrove Plutonic Suite (including the Tia Granodiorite). The whole rock dates mark the approximate emplacement age of the plutons. The Tia Granodiorite exhibits two distinct biotite fabrics both of which formed during west over east movements on a ductile shear zone directly east of the pluton. Structural analysis suggests that the earliest fabric, formed synchronous with emplacement of the granodiorite in the hanging wall of the ductile shear zone. The later fabric probably formed during a major faulting event that juxtaposed this area against unmetamorphosed Permian strata of the Manning Basin to the west and the Yarrowitch Block to the east.

Contrasts in morphogenesis and tectonic setting during contemporaneous emplacement of S- and I-type granitoids in the Eastern Lachlan Fold Belt, southeastern Australia

Abstract Integrative gravity and structural modelling of Ordovician-Silurian granitoids in the Eastern Lachlan Fold Belt (southeastern Australia) revealed contrasts in emplacement mode and deformation style between coeval S- and I-type granites. The NNE-SSW directed contraction during the Benambran event of the Lachlan Orogen caused dextral movement along two major strike-slip faults (Carcoar Fault/Copperhannia Thrust) and simultaneous formation of both transtensional pull-apart and transpressional shear zones. The geometry and deformation style of the plutons and country rock, their spatial relationship at depth to adjacent faults and the structural history of both the granites and country rocks suggest a genetic linkage between magma emplacement and synmagmatic deformation. Synchronously, the Carcoar Granodiorite was emplaced into a transtensional pull-apart structure and the Barry Granodiorite and Sunset Hills Granite intruded transpressional shear zones. The I-type Carcoar and Barry granites are square to tabular, wedge-shaped bodies exhibiting a weak deformation; whereas the S-type Sunset Hills Granite is an elongated, tabular to sheet-like pluton showing a moderate deformation degree. The contrasts in 3D shape, emplacement mode and deformation style between the I- and S-type granites are due to differences in nearfield stress regime, geometry of the emplacement sites, intrusion level with respect to thermal and rheological conditions, and in their response to deformation. This response is in part controlled by the proportion of resistant/non-resistant minerals in the granite and host rock. This study demonstrates that distinctive emplacement modes can operate simultaneously in different parts of a fault system under contrasting deformation conditions.

Dating deformation using crushed alkali feldspar: 40 Ar/ 39 Ar geochronology of shear zones in the Wyangala Batholith, NSW, Australia

Diffusion parameters have been estimated for K-feldspar in and adjacent to mylonite shear zones in the Wyangala Batholith. The parameters obtained suggest that deformation during mylonitisation would have caused argon systematics to reset because diffusion distances were reduced by cataclasis, deformation and/or recrystallisation. However, the mineral lattice remained sufficiently retentive to allow subsequently produced radiogenic argon to be retained. 40Ar/39Ar geochronology is thus able to constrain operation of these biotite-grade ductile shear zones to the period from ca 380 Ma to ca 360 Ma, at the end of the Tabberabberan Orogeny.

Quartz veining in simply folded arenites, Cape Liptrap, southeast Victoria, Australia

Quartz veins in a simply folded sequence of interbedded arenite and mudstone at Cape Liptrap, Victoria, indicate the importance of the local stress fields in controlling the orientation of quartz veins in the folding of a multilayered sequence. Because most of the quartz veins formed by hydraulic extension, the effective stress field at any point closely constrained the orientation of the veins. The veins formed in sets consisting of many subparallel, dimensionally comparable, almost synchronously formed quartz veins. Planar sets aligned either (sub) parallel or (sub) normal to the fold axis, formed preferentially in the hinge zone, whereas single, en echelon sets oriented (sub) normal to the fold axis formed on the limbs. Most sets oblique to the fold axis are present on the limbs of folds and, from cross‐cutting relationships, appear to have formed prior to fold formation. Silica depletion in the quartz arenites and high fluid to rock ratios, together with high uniform oxygen isotope results for the qua...

History of faulting in the Northern Hastings Block, southern New England Orogen

ABSTRACT Brittle failure is common in the Devonian to Permian rocks in the Northern Hastings Block (NHB) and is manifested by faults of different orientation and kinematic histories, but the timing of fault movement is not well defined. In this study, faults in the NHB were analysed with the map pattern of cross-cutting faults used to estimate the relative time of movement and relationship to other faults. We defined five episodes of faulting or fault reactivation that affected the NHB. The Yarras Fault System on the southwestern side of the NHB and the Parrabel Fault and related faults on the eastern side of the NHB are the two major fault systems responsible for transporting and rotating the NHB in the late Carboniferous. Faults on the eastern, northeastern and northern part of Parrabel Dome started and stopped moving after emplacement of the Hastings Block and before the intrusion of the Werrikimbe Triassic granitoids. We suggested that the movement on the major bounding faults is related to the accommodation of the NHB to the folding and cleavage development in the adjoining Nambucca Block, and is associated with the earliest part of the Hunter–Bowen Orogeny. Limited dextral movement on the extensions of the Taylors Arm Fault System caused minor displacements in the northeastern part of the NHB during the Late Triassic. Some small faults cut the Triassic granitoids or Triassic Lorne Basin sediments indicating tectonic activity continued post-Triassic.