Abstract The Albany–Fraser Orogen in southwestern Australia preserves an important thermo‐tectonic record of Australo‐Antarctic cratonic assembly during the Mesoproterozoic. New petrologic and thermobarometric data from the Coramup Gneiss (a 10 km wide zone of high strain rocks within the NE‐trending eastern Albany–Fraser Orogen) indicate at least two high‐grade metamorphic events during 1345–1140 Ma convergence and amalgamation of the West Australian and Mawson cratons. The first event (M1) involved c . 1300 Ma granulite facies metamorphism of the Coramup Gneiss (M1a: 800–850 °C, 5–7 kbar), followed by burial and recrystallization under high‐ P conditions (M1b: 800–850 °C, c . 10 kbar) prior to high‐ T decompression (M1c: 700–800 °C, 7–8 kbar) and the 1290–1280 Ma emplacement of Recherche Granite sills. The second event (M2) entailed high‐ T , low‐ P metamorphism within dextral D2 shear zones (M2a: 750–800 °C, 5–6 kbar), followed by fluid‐present amphibolite facies M2b retrogression. Subsequent sinistral D3 mylonites and pseudotachylites are considered contemporaneous with similar structures in the adjacent Nornalup Complex that postdate the c . 1140 Ma Esperance Granite. Our petrological and thermobarometric data permit two end‐member P – T ‐time relationships between M1 and M2: (1) a single post‐M1b event involving continuous M1b–M1c–M2a–M2b cooling and decompression, and (2) a two‐stage post‐M1b evolution involving M1c metamorphism during the waning stages of an event unrelated causally or temporally to subsequent M2a metamorphism and D2 deformation. In a companion paper, new structural and U–Pb SHRIMP zircon data are presented to support a two‐stage P – T evolution for the Coramup Gneiss, with M1 and M2, respectively, reflecting thermo‐tectonic activity during Stage I (1345–1260 Ma) and Stage II (1215–1140 Ma) of the Albany–Fraser Orogeny.
In stable continental regions (SCRs), the process of probabilistic seismic‐hazard assessment (PSHA) remains a scientific and technical challenge. In producing a new national hazard model for Australia, we developed several innovative techniques to address these challenges.
The Australian seismic catalog is heterogeneous due to the variability between magnitude types and the sparse networks. To reduce the resulting high epistemic uncertainty in the recurrence parameters, a and b , the magnitudes of pre‐1990 earthquakes have been empirically corrected to account for changes in magnitude formulas around 1990. In addition, existing methods for estimating recurrence parameters (e.g., maximum likelihood estimation) were found to be unstable. To overcome this problem, a new method was developed that removes outlier earthquakes before applying a regression.
The incorporation of a model of episodic seismicity into the new hazard model required deviation from the more conventional method of PSHA. The selection of the maximum earthquake magnitude M max is based on the analysis of surface ruptures from paleoearthquakes, with M max thought to vary between geological domains (e.g., 7.2–7.6 in nonextended SCR and 7.4–7.8 in extended SCR). The sensitivity of PSHA to M max, source zone boundary location, recurrence parameters, and ground‐motion prediction equations (GMPEs) was examined in this study. The hazard was found to be generally insensitive to M max in the estimated preferred magnitude range. The uncertainty in recurrence parameters was found to contribute a variation in hazard comparable to the epistemic uncertainty associated with the different GMPEs used in this study. For sites near source zone boundaries, a similar variation in hazard was observed by reasonable changes in the position of the boundaries. Aleatory variability and epistemic uncertainty in GMPEs are routinely incorporated in PSHAs, as is variation in M max. However, the uncertainties in recurrence parameters and source zone boundaries are generally given less attention.
Introduction Australia is classified as a Stable Continental Region (SCR) in terms of its plate tectonic setting and seismicity (Johnston et al. 1994). While such settings produce only approximately 0.2% of the world’s seismic moment release, large and potentially damaging earthquakes are not uncommon (e.g. Crone et al. 1997). In the last four decades five locations in Australia are documented as having experienced surface rupturing earthquakes (Figure 1).
<p>The mechanisms that lead to the localisation of stable continental region (SCR) seismicity, and strain more generally, remain poorly understood. Recent work has emphasised correlations between the historical record of earthquake epicentres and lateral changes in the thickness, composition and/or viscosity (thermal state) of the lithospheric mantle, as inferred from seismic velocity/attenuation constraints. Fluid flow and the distribution of heat production within the crust have also been cited as controls on the location of contemporary seismicity. The plate margin-centric hypothesis that the loading rate of crustal faults can been understood in terms of the strain rate of the underlying lithospheric mantle has been challenged in that a space-geodetic strain signal is yet to be measured in many SCRs. Alternatives involving the release of elastic energy from a pre-stressed lithosphere have been proposed.</p><p>The Australian SCR crust preserves a rich but largely unexplored record of seismogenic crustal deformation spanning a time period much greater than that provided by the historical record of seismicity. Variations in the distribution, cumulative displacement, and recurrence characteristics of neotectonic faults provide important constraint for models of strain localisation mechanisms within SCR crust, with global application. This paper presents two endmember case studies that illustrate the variation in deformation characteristics encountered within Australian SCR crust, and which demonstrate the range and nature of the constraint that might be imposed on models describing crustal deformation and seismic hazard.</p><p>The ~0.5 m high 2018 M<sub>W</sub> 5.3 Lake Muir earthquake scarp in southwest Western Australia is representative of a class of ruptures in the Precambrian SCR of Australia where the scarps are isolated from neighbouring scarps and there is little or no landscape evidence for recurrence of morphotectonic earthquakes, or of the construction of regional tectonic relief. In contrast, scarps in the Phanerozoic SCR of eastern Australia typically occur within a scarp-length of neighbouring scarps, and demonstrate extended histories of recurrence of morphotectonic events. For example, the ~75 km-long Lake George fault scarp is associated with a vertical displacement of ~250 m which accrued as the result of many morphotectonic earthquakes over the last ca. 4 Myr. The scarp links into neighbouring scarps, forming a belt-like arrangement that defines the topographic crest of the southeast Australian highlands. The limited data available indicates that recurrence is highly episodic, with periods of fault activity potentially coinciding with changes at the plate boundaries.</p>
SHRIMP U–Pb zircon isotopic data have been obtained for four samples collected from granitoids and paragneisses in the Fraser Complex, a large composite metagabbroic body cropping out in the Mesoproterozoic Albany‐Fraser Orogen of Western Australia. The data are combined with the results of field mapping and petrographic analysis to revise a model for the geological evolution of the Fraser Complex. Three main phases of deformation are recognised in the Fraser Complex (D1–3) associated with two metamorphic events (M1–2), which involve four distinguishable episodes of recrystallisation. The first metamorphic event recognised (M1a/D1) reached granulite facies and is characterised by peak T ≥800°C and P = 600–700 MPa. A syn‐M1a/D1 charnockite has a U–Pb SHRIMP zircon age of 1301 ± 6 Ma, which also provides an estimate for the age of intrusion of Fraser Complex gabbroic rocks. Disequilibrium textures comprising randomly oriented minerals (M1b), consistent with approximately isobaric cooling, formed in various lithologies in the interval between D1 and D2. Post‐D1, pre‐D2 granites intruded at 1293 ± 8 Ma and were foliated during the D2 event, which culminated in the burial of the Fraser Complex to depths equivalent to 800–1000 MPa. Following burial, pyroxene granulites on the western boundary of the complex were pervasively retrogressed to garnet amphibolite (M2a). An igneous crystallisation age of 1288 ± 12 Ma from a syn‐M2a aplite dyke suggests that retrogression may have occurred only a few millions of years after the peak of granulite facies metamorphism. Exhumation to depths of less than ∼400 MPa occurred within ∼20–30 million years of the M2a pressure peak. Associated deformation (D3) is characterised by the development of mylonite and transitional greenschist/amphibolite facies disequilibrium textures (M2b). Keywords: Albany‐Fraser OrogenFraser ComplexgeochronologyMesoproterozoic
Probabilistic seismic hazard analyses in Australia rely fundamentally on the assumption that earthquakes recorded in the past are indicative of where earthquakes will occur in the future. No attempt has yet been made to assess the potential contribution that data from active fault sources might make to the modelling process, despite successful incorporation of such data into United States and New Zealand hazard maps in recent years. In this paper we review the limited history of paleoseismological investigation in Australia and discuss the potential contribution of active fault source data towards improving our understanding of intraplate seismicity. The availability and suitability of Australian active fault source data for incorporation into future probabilistic hazard models is assessed, and appropriate methodologies for achieving this proposed.
Abstract Analysis of TanDEM‐X and Shuttle Radar Topography Mission (SRTM) data reveals geomorphic evidence for 292 fault‐propagation fold scarps across the Miocene Nullarbor and Pliocene Roe Plains in south‐central Australia. Vertical displacements (VD) are determined using topographic profiling of a subset ( n = 48) of the fold traces. Fault dips (mean = 44 +16/−14° at 1σ) are estimated from seismic reflection data; the mean dip is assigned to faults with unknown dip and combined with VD to estimate net displacements (ND) and average net displacements (AD) for each fault. AD exceeds single‐event displacements estimated from fault‐length scaling regressions, indicating the identified faults have hosted multiple earthquakes. Combining AD with (i) faulted surface ages (Nullarbor ~10–5 Ma, Roe ~2.5 Ma), (ii) ages of faulted erosional–depositional features (e.g. relic Late Miocene dune fields and Pliocene paleochannels), and (iii) onset of the neotectonic regime in Australia at ~10 Ma yields average slip rates from <0.1 m Myr −1 to >17 m Myr −1 (mean = 1.1 m Myr −1 ). Summation of displacements across faults yields crustal horizontal shortening rates lower than geodetically detectable resolution (≤0.01 mm yr −1 ) since the Late Miocene. The ca. 10 Myr‐long record of neotectonic faulting on the Nullarbor Plain provides important insights into earthquake spatial–temporal behaviours in a slowly deforming intraplate continental region.
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