C. Spaggiari

Chapter 3.6 The Narryer Terrane, Western Australia: A Review

Publisher Summary This chapter presents a review of the Narryer terrane, Western Australia. The Narryer Terrane occupies an area of ∼30,000 km2 in the northwestern corner of the Archean Yilgarn Craton of Western Australia. It is one of the earliest crustal terranes on the Earth, containing rocks with U-Pb zircon ages ranging up to 3730 Ma, the oldest known rocks in Australia, and detrital zircons up to 4404 Ma, the oldest terrestrial material on the Earth. Initial geochronological investigations on granitic gneisses in the vicinity of Mt. Narryer, using Rb-Sr whole-rock techniques, established an age of 3348 ± 43 Ma using multicomponent samples. An initial investigation of the Mt. Narryer area commenced using the SHRIMP ion microprobe, including not only a study of the ancient gneisses, but also of the detrital zircon population in the granulite facies quartzites and conglomerates. This led to the exciting discovery of four detrital zircon cores with ages ranging from 4110 to 4190 Ma, the oldest crustal remnants identified on the Earth at that time. The Jack Hills metasedimentary belt lies ∼60 km NE of Mt. Narryer and is approximately 90 km long with a pronounced sigmoidal curvature, typical of a dextral shear zone. It is found that the margins of the belt are sheared and have steep dips, indicating that the belt itself is also steeply dipping.

Cooling and exhumation along the curved Albany-Fraser orogen, Western Australia

The Albany-Fraser orogen of Western Australia exhibits a distinct 45° primary (preorogenic) curvature. Consequently, northwest-southeast compression during Mesoproterozoic orogeny was orthogonal to orogenic strike in the east of the orogen, but was oblique in the west. This produced different structural settings in the east and west of the orogen, with a greater component of dextral transpression in the west. We report new 40 Ar/ 39 Ar thermochronology from the east Albany-Fraser orogen, and compare these results to the cooling history of the west to examine how cooling varies between the differently striking domains of a curved orogen. The 40 Ar/ 39 Ar analyses of hornblende, muscovite, and biotite grains encompass a range of metaigneous and metasedimentary lithologies from two lithotectonic domains. The eastern Biranup Zone yields five hornblende cooling ages at ca. 1190 Ma and seven muscovite and biotite cooling ages between ca. 1171 and 1158 Ma. Hornblende and biotite in the southwestern Fraser Zone record cooling between ca. 1217 and 1205 Ma, and the central Fraser Zone reached 40 Ar/ 39 Ar biotite closure temperature at ca. 1157 Ma. Slow 8.2–9.5 °C/m.y. cooling in the eastern Biranup Zone commenced 20 m.y. earlier than 22–33 °C/m.y. cooling in the west Albany-Fraser orogen. The differences in cooling rate are a result of the different structural settings in the east and west. However, similar mica 40 Ar/ 39 Ar cooling ages in the east and west record a convergence in cooling history. This suggests that exhumation had become increasingly decoupled from compressional tectonics, instead driven by more passive processes related to isostatic rebound and erosion.

Crustal surface wave velocity structure of the east Albany-Fraser Orogen, Western Australia, from ambient noise recordings

S U M M A R Y Group and phase velocity maps in the period range 2–20 s for the Proterozoic east AlbanyFraser Orogen, Western Australia, are extracted from ambient seismic noise recorded with the 70-station ALFREX array. This 2 yr temporary installation provided detailed coverage across the orogen and the edge of the Neoarchean Yilgarn Craton, a region where no passive seismic studies of this scale have occurred to date. The surface wave velocities are rather high overall (>3 km s−1 nearly everywhere), as expected for exposed Proterozoic basement rocks. No clear signature of the transition between Yilgarn Craton and Albany-Fraser Orogen is observed, but several strong anomalies corresponding to more local geological features were obtained. A prominent, NE-elongated high-velocity anomaly in the northern part of the array is coincident with a Bouguer gravity high caused by the upper crustal metamorphic rocks of the Fraser Zone. This feature disappears towards longer periods, which hints at an exclusively upper crustal origin for this anomaly. Further east, the limestones of the Cenozoic Eucla Basin are clearly imaged as a pronounced low-velocity zone at short periods, but the prevalence of low velocities to periods of ≥5 s implies that the uppermost basement in this area is likewise slow. At longer periods, slightly above-average surface wave velocities are imaged below the Eucla Basin.