Stijn Glorie1, Kate Agostino1 and Mark Pawley2
1 Department of Earth Sciences, University of Adelaide
2 Geological Survey of South Australia, Department of the Premier and Cabinet
Download this article as a PDF (1.7 MB); cite as MESA Journal 84, pages 19–22
Introduction
Low-temperature thermochronology comprises a series of dating techniques to examine the low-temperature thermal evolution and shallow crustal exhumation history of an area. The various techniques are effective for different temperature ranges dependent upon the closure temperatures and or closure temperature windows for the different thermal properties of a mineral that is being measured. This article summarises the results of our recently published study into the low-temperature thermal history of the Musgrave Province, South Australia (Glorie et al. 2017). We utilised apatite fission track thermochronology and apatite and zircon (U–Th–Sm)/He thermochronology to constrain the low-temperature thermal evolution of a portion of the Musgrave Province. This article also aims to illustrate the use of low-temperature thermochronology to understand the exhumation history of a geological terrane.
Low-temperature thermochronology methods
Apatite fission track thermochronology is based on the measurement of nuclear damage trails, or fission tracks, which are caused by the spontaneous fission decay of 238Uranium (238U) within the crystal lattice (Wagner and Van den haute 1992). The tracks are only completely retained at temperatures ≤60 °C. Between ~60 °C and ~120 °C, the fission tracks shorten or anneal. Above ~120 °C the crystal lattice is able to completely repair itself from radiation damage, removing the tracks (Green et al. 1985). In 2015 the Department of Earth Sciences at the University of Adelaide successfully obtained funding under the Australian Research Council Linkage Infrastructure Equipment and Facilities scheme to install an Autoscan System for semi-automatic fission track analysis. This has enabled the collection of a large database of thermochronological measurements in a number of studies across South Australia and internationally (e.g. Hall et al. 2016a, 2016b).
The apatite fission track method is complemented by analysis of apatite and, if available, zircon, via the (U–Th–Sm)/He method. Apatite and zircon (U–Th–Sm)/He low-temperature thermochronology is based on the diffusivity of radiogenic 4He through a mineral crystal lattice during certain temperature ranges. For apatite, this method records the timing of thermal events between ~45 °C and ~75 °C (Ehlers and Farley 2003), whereas zircon grains record cooling ages of between ~130 °C and ~200 °C (Wolfe and Stockli 2010).
These techniques can be complemented by other thermochronological methods such as 40Ar/39Ar to understand the thermal history of a region. The choice of methods depends upon the mineralogical assemblages available within the rock types being studied. These techniques are particularly powerful when a suite of samples are collected along and across structures as this allows the timing of fault-induced exhumation to be estimated.
Geological background
The Musgrave Province is an east–west-trending basement inlier located in central Australia, which is bounded by Neoproterozoic to Mesozoic sedimentary rocks of the Amadeus, Officer, and Eromanga basins (Wade et al. 2008; Raimondo et al. 2010; Howard et al. 2015; Smithies et al. 2015). Previous studies have focused on the magmatic and high-grade metamorphic histories of the Musgrave Province, highlighting the c. 1200–1120 Ma Musgravian Orogeny that affected the entire province (e.g. Camacho and Fanning 1995; Aitken and Betts 2009; Smithies et al. 2011; Tucker et al. 2015; Wong et al. 2015), the c. 1090–1040 Ma extensional Giles Event, and the c. 600–540 Ma transpressional Petermann Orogeny that resulted in the current east–west-trending grain of the province (e.g. Raimondo et al. 2010; Walsh et al. 2013; Maier et al. 2015). In addition, younger reworking is also documented in the form of shear zone reactivation and localised deformation during the long-lived but apparently episodic c. 450–300 Ma Alice Springs Orogeny (Haines, Hand and Sandiford 2001; Buick, Storkey and Williams 2008).
However, the thermochronological record of the younger events is not well studied or constrained. In particular, no low-temperature thermochronological studies have been conducted to investigate the timing of exhumation to shallow crustal levels.
Sample rationale and methods
The purpose of the study was to examine the exhumation history across the east-trending structural grain of the eastern Musgrave Province. Specifically, the study aimed to test the hypothesis that the Petermann and Alice Springs orogenies were the main causes for Phanerozoic exhumation in the region. In addition, the research aimed to test if any thermal events occurred subsequent to the Alice Springs Orogeny. To achieve this, a suite of samples was collected from Paleo- and Mesoproterozoic granitoids in a northeast-trending traverse across the Marryat and Coglin faults (Fig. 1). This aimed to evaluate potential exhumation across these structures. Methods used were apatite fission track, and apatite and zircon (U–Th–Sm)/He, along with U–Pb dating of apatite, a technique with closure temperatures of between ~450 °C and ~550 °C.
Results
Apatite U–Pb dating on six samples yielded consistent results of c. 1075–1025 Ma. Apatite fission track analysis indicates that several discrete thermal events affected the study area, inducing cooling through apatite fission track closure temperatures (~60–120 °C), and is supported by additional apatite and zircon (U–Th–Sm)/He data. These events occurred during the: Late Neoproterozoic (c. 550 Ma), when cooling from deep crustal levels to temperatures <200 °C occurred; Silurian–Devonian (c. 450–400 Ma); Late Carboniferous (c. 310–290 Ma); and Triassic – Early Jurassic (localised).
Discussion and conclusion
The data reveals four thermal events, described below, that affected the eastern Musgrave Province.
- Thermal resetting of granites to temperatures up to ~500 °C occurred during mantle-derived magmatism of the Giles Event (c. 1090–1040 Ma).
- Neoproterozoic cooling is likely to be related with exhumation and denudation during the Petermann Orogeny.
- The Silurian–Devonian and Late Carboniferous cooling phases suggest there were several phases of cooling during the Alice Springs Orogeny. These are likely associated with the exhumation of granitoids of the eastern Musgrave Province to shallow crustal depths.
- A Triassic – Early Jurassic thermal event observed throughout the study area is thought to be related to elevated geothermal gradients at that time. However, more data is needed to further constrain this potential Mesozoic thermal event.
The high sample density across the structural architecture of the study area also reveals patterns of fault reactivation and resulting differential exhumation.
The results indicate shallower exhumation levels in the centre that are represented by younger apatite fission track ages (c. 250 Ma), and deeper exhumation towards the margins of the transect where older apatite fission track ages (c. >300 Ma) are recorded (Figs 1, 2). The observed differential exhumation patterns match with existing seismic data and fit a model of an inverted graben system between the Coglin and Marryat faults. Furthermore, the data suggests that the reactivation of this graben system occurred relatively late in the Alice Springs Orogeny.
The results highlight that the eastern Musgrave Province records a complex Phanerozoic low-temperature thermal history, revealing the poorly appreciated tectonic evolution of inland Australia. The study also shows the value of low-temperature thermochronology as a tool for understanding the fault and exhumation history of an area.
Acknowledgements
Rian Dutch, James Hall, Martin Danišík, Noreen Evans and Alan Collins were part of the team for the original study and are acknowledged for their input. Anthony Reid provided a review for this article.
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