Low-temperature (<110°C) thermal history of the Mt Isa and Murphy Inliers, northeast Australia: evidence from apatite fission track thermochronology *

Abstract

The paucity of Phanerozoic rock sequences in the Proterozoic Mt Isa and Murphy Inliers in northern Australia renders it difficult to determine their Phanerozoic tectonic histories. However, thermochronological methods provide a means to assess this problem. Apatite fission track ages vary from 390 to 218 Ma, and mean track lengths range between 11.1 and 13.6 mm. These results record a non-linear cooling history below about 110 + 108C. Forward modelling of the fission track data suggests that the first episode of relatively rapid cooling since the Early Carboniferous occurred between ca 350 and 250 Ma at rates of *0.98C/million years and was synchronous with intracontinental deformation associated with the Alice Springs Orogeny and tectonics at the contemporaneous eastern margin of Australia. This event resulted in 2 km of exhumation across both inliers. Cooling rates also increased after 100 Ma, although estimates of the amount of cooling and exhumation across trhe inliers obtained by forward modelling of the fission track data exceed those determined by constraints provided by the 40 Ar/ 39 Ar ages of weathering profiles. This observation adds to the growing amount of evidence that the fission track annealing model employed overestimates cooling rates at low temperatures (560 -708C). The spatial variation of apatite fission track data within the Mt Isa Inlier indicates the three major structural belts, the Western Fold Belt, Kalkadoon -Leichhardt Belt and the Eastern Fold Belt, which are separated by major north -south faults, experienced similar thermal histories on a regional scale. This suggests that the main faults bounding the belts have not experienced major reactivation in a vertical sense, as a single large-scale fault plane, since ca 350 Ma. However, adjacent smaller scale, faultbounded blocks within the structural belts demonstrate variable cooling histories, suggesting that reactivation of favourably oriented minor faults in the inlier, including segments of the major faults, has occurred over this time interval. Variations in apatite fission track data show that up to 1.5 km of vertical displacement occurred along parts of the Mt Isa Fault Zone and the Pilgrim Fault between 350 and 250 Ma.

Low‐temperature thermal history of the McArthur Basin: Influence of the Cambrian Kalkarindji Large Igneous Province on hydrocarbon maturation

“…390–300 Ma which are much slower than those typically observed in uplifted or incised terrains, but higher than for quiescent cratonic regimes (e.g., Fairbridge & Finkl, 1980; Summerfield & Hulton, 1994). Given these slow cooling rates, the samples from the central McArthur Basin appear to have not been significantly influenced by the Alice Springs Orogeny, and contrast with many thermal history profiles documented across northern Australia (e.g., Glorie, Agostino, et al, 2017; Nixon, Glorie, Collins, Whelan, et al, 2021; Spikings et al, 2006). Instead, we attribute Devonian–Carboniferous cooling in the central McArthur Basin to gradual erosion of overlying sediments and volcanics of the Georgina Basin and Kalkarindji LIP, which are now only sparsely preserved within the McArthur Basin region (Abbott et al, 2001; Ahmad & Munson, 2013; Dunn, 1963; Glass & Phillips, 2006).…”

“…Instead, we attribute Devonian–Carboniferous cooling in the central McArthur Basin to gradual erosion of overlying sediments and volcanics of the Georgina Basin and Kalkarindji LIP, which are now only sparsely preserved within the McArthur Basin region (Abbott et al, 2001; Ahmad & Munson, 2013; Dunn, 1963; Glass & Phillips, 2006). Unlike major structural reactivation observed in the adjacent Pine Creek Orogen and Murphy Province (Nixon, Glorie, Collins, Whelan, et al, 2021; Spikings et al, 2006), any Devonian–Carboniferous deformation in the McArthur Basin appears to have been minor, at most serving to elevate the region above the peritidal to shallow marine conditions experienced during Georgina Basin deposition (Kruse & Radke, 2008) and into subaerial exposure, without the creation of meaningful topographic relief.…”

“…Intracontinental orogenesis associated with north‐south shortening and metamorphism expressed in the Amadeus Basin and Aileron and Irindina provinces (Haines et al, 2001; Shaw et al, 1992; Shaw & Black, 1991) is contemporaneous with large‐scale structural disturbances and erosional events which extend far beyond the orogenic centre. Low‐temperature cooling during the Alice Springs Orogeny is frequently observed in both basement (Boone et al, 2016; Glorie, Agostino, et al, 2017; Glorie et al, 2019; Hall et al, 2016, 2018; Nixon, Glorie, Collins, Whelan, et al, 2021; Quentin de Gromard et al, 2019; Reddy et al, 2015; Spikings et al, 2006) and basin (Tingate & Duddy, 2002; Wells, 1970) regions within both the North and South Australian cratons, implying the existence of a continental‐scale thermal perturbation coincident with this event. The presence of widespread syn‐deformational sedimentation has been observed across numerous central Australian basins (Haines et al, 2001), which demonstrates a complex deformational regime in which regions of uplift and erosion were spatially separated from contemporaneous siliciclastic deposition.…”

Intracontinental Fault Reactivation in High Heat Production Areas of Central Australia: Insights From Apatite Fission Track Thermochronology

“…Low-temperature data from the eastern Murphy Province presented by Spikings et al (2006) suggests cooling in this region persisted for notably longer than that observed in the western Murphy Province. Cooling in the eastern Murphy Province continued through ca.…”

“…Permian cooling is additionally preserved further east in the Palaeoproterozoic Mount Isa Inlier, at the eastern edge of the NAC (Spikings et al., 1997). This younger cooling progression is most likely linked to the inboard propagation of strain during the later stages of the Tasmanide accretion at the east Australian margin (Spikings et al., 1997, 2006), most likely during the Lachlan Orogeny and Hunter‐Bowen Orogeny (e.g., Glen et al., 2009; Jessop et al., 2019; Rosenbaum, 2018). It is, however, notable that the exposed eastern Murphy Province appears to display elevated heat production (Figures 6a and 6c) corresponding to an elevated geothermal gradient estimated at ∼33°C/km (Supporting Information ), causing the lithosphere to become thermally weakened.…”