Caroline Forbes

Synchronous development of Type 2 and Type 3 fold interference patterns: evidence for recumbent sheath folds in the Allendale Area, Broken Hill, NSW, Australia

Abstract Synchronous development of Type 2 and Type 3 fold interference patterns occur in the poly-deformed Broken Hill Inlier. The interference patterns have resulted from the superposition of recumbent F2 folds and ∼N–S-oriented upright F3 folds. The synchronous development of Type 2 and Type 3 fold interference patterns is attributed to variation in the hinge of F2 folds by as much as 90°, suggesting the development of a regional-scale sheath-like fold geometry during D2. Overprinting relationships along the eastern limb of the Pap Synform suggests that it formed part of a recumbent fold hinge that was flattened during horizontal crustal shortening. This has resulted in the development of a modified Type 3 fold interference pattern. The results of this study show that the Type 2 and Type 3 fold interference patterns can develop during the same deformation event in an evolving orogen.

Naturaliste Plateau, offshore Western Australia: A submarine window into Gondwana assembly and breakup

The origin of the submarine Naturaliste Plateau off the southwestern coast of Australia is controversial; previous work supports both oceanic and continental affinities for the basement to volcanic and sedimentary sequences. We report the first evidence of reworked Mesoproterozoic (ca. 1230–1190 Ma) continental crust, based on laser ablation–inductively coupled plasma–mass spectrometry analysis of zircons from granite and orthogneiss samples dredged from the southern margin of the plateau. Thermobarometry of peak metamorphic minerals and electron microprobe chemical dating of monazite reveal that these igneous rocks were metamorphosed to ~700 °C and ~6.5 kbar during the Cambrian Pinjarra Orogeny at ca. 515 Ma. These data confirm a continental origin for a significant swathe of the southern Naturaliste Plateau, and suggest that the protoliths may have affinities to Mesoproterozoic crust within the Albany-Fraser-Wilkes Orogen (Australia-Antarctica). The present Naturaliste Plateau basement beneath its volcanic carapace probably represents a middle-to lower-crustal extensional allochthon exhumed during Cretaceous hyperextensional breakup between Australia and Antarctica.

Dating Prograde Amphibolite and Granulite Facies Metamorphism Using In Situ Monazite U‐Pb SHRIMP Analysis

In situ SHRIMP U‐Pb analysis of monazite grains from pelites within an early‐formed high‐temperature shear zone in the southern Broken Hill Block, Australia, has been used to constrain the timing of prograde amphibolite facies and peak granulite facies metamorphism within the terrain. Geochronological analyses from grains inhabiting texturally distinct localities revealed two distinct age populations, ca. 1619 and ∼1600 Ma. The older age was obtained from grains that occur as inclusions completely enclosed within coarse‐grained K‐feldspar and garnet grains, which evidently armored the monazite inclusions against resetting during younger deformation and metamorphic events. The ca. 1619‐Ma monazite population occurs as part of an amphibolite facies inclusion assemblage hosted within the peak granulite facies mineral assemblage and constrains the timing of prograde amphibolite facies metamorphism within the Broken Hill Block. The younger ∼1600‐Ma monazite age population was from grains within the pervasive shear fabric of the pelites or adjacent to fractures or grain boundaries in the matrix. The ∼1600‐Ma age population represents either a later stage of monazite growth or isotopic resetting during deformation along the high‐temperature shear zone at peak granulite facies conditions.

Development of Type 2 fold interference patterns in the Broken Hill Block: implications for strain partitioning across a detachment during the Olarian Orogeny

A north‐south‐trending relatively high‐strain shear zone active during the earliest stages of the ca 1.60–1.58 Ga Olarian Orogeny bounds domains characterised by different structural geometries in the northern Broken Hill Block. To the west of the shear zone Freyers Metasediments preserve Type 2 ‘mushroom’ fold interference patterns between fold generations F2 and F3. Recumbent F2 folds are highly non‐cylindrical. These were subsequently overprinted by north‐orientated close to tight upright folds (F3). To the east of the shear zone, in the Parnell Formation, F2 folds are not developed and the structural grain reflects the F3 folding event. The shear zone is interpreted to represent a local detachment that accommodated lateral transportation of hangingwall rocks, producing non‐cylindrical recumbent folding during thin‐skinned deformation (D2). F3 folds represent thick‐skinned deformation during the late stages of the Olarian Orogeny, folding rocks above and below the detachment. This resulted in zones of different structural geometry and strain pattern across the detachment.