Low-temperature thermal evolution of the McArthur Basin and adjacent Proterozoic orogens

Abstract

Palaeozoic intracratonic deformation during the Alice Springs Orogeny presents as a highly cryptic reactivation of central Australia, far inboard from the plate margins with no apparent indications of propagated strain expressed in the intermediate regions. Metamorphism and deformation defining the Alice Springs Orogney has been extensively studied within the Aileron Province, Irindina Province and northern Amadeus Basin, and it has been research in these regions that have dominated the literature on post-Proterozoic reworking of the North Australian Craton following the major tectonothermal events during cratonic amalgamation. This thesis documents the low-temperature thermochronological reactivation history of the North Australian Craton, north of the Aileron Province, aiming to establish a deformational framework for northern Australia away from the traditional zones of tectonism at the plate boundaries. Voluminous extrusion of basaltic lavas during the Kalkarindji Large Igneous Province across the West Australian Craton and North Australian Craton in the Cambrian rapidly heated the uppermost crust. This signal represents the oldest preserved low-temperature heating event within the North Australian Craton, and only appears to be retained in the McArthur Basin. Importantly, this event is recognised exclusively as surficial heating associated with sub-aerial lavas, as the possibility of thermal resetting by comagmatic intrusive suites within the basin has been discounted by undertaking extensive U Pb geochronology of intrusives across the region that are shown to be considerably older and part of the Deirm Derim Galiwinku/Yanliao Large Igneous Province. Following initial rapid heating of the shallow sub- surface, these low thermal conductivity basalts effectively thermally insulated the underlying basin, invoking lower magnitude heating for a longer time period. Widespread structural reactivation and exhumation contemporaneous with the Ordovician Carboniferous Alice Springs Orogeny dominates the Palaeozoic record in the North Australian Craton, with reactivation of similar ages observed more sporadically within the South Australian Craton. Deformation away from the orogenic locus of this event is largely constrained within Palaeoproterozoic crystalline basement that host pre-existing structures which were reactivated in response to changing stress regimes at the distant plate boundaries. The clearest evidence of low-temperature reworking during the Alice Springs Orogeny is observed during the Pertnjara-Brewer Event (ca. 390 360 Ma), with reactivation observed in the Pine Creek Shear-Zone, western McArthur Basin, Tennant region and northern Aileron Province, in addition to the more commonly recognised large-scale deformation and metamorphism observed within the central Aileron Province and northern Amadeus Basin. This broadly coincides with the major Tabberabberan Orogeny of eastern Australia. Considerable cooling is, however, additionally observed away from highly faulted terranes throughout the duration of the Alice Springs Orogeny. In these structurally less favourable regions, cooling is much slower and long-lived in response to longwave exhumation. Such slow cooling has been observed within the Murphy Province, Arnhem Province, western Tennant region and northern Aileron Province during the Ordovician Carboniferous, where all of these localities lack pre-existing or favourably oriented structures for strain accommodation. The youngest reactivation within the North Australian Craton is, somewhat counterintuitively, preserved furthest inboard from the Gondwanan plate boundary, within the central Aileron Province of central Australia. Triassic deformation, corresponding with the timing of the late Hunter-Bowen Orogeny, observed in this area represents the inboard propagation of strain from the eastern margin that was localised in regions of thermally and metasomatically weakened crust, far from the classical accretionary terranes of the Tasmanides. This late reactivation in central Australia is confined within regions of abnormally elevated local heat production by radiogenic elements, which has thermally weakened the lithosphere and made these areas more susceptible to reworking in response to the evolving far-field tectonic events at the plate margin. Hence, a key finding from this thesis is that the thermochronological imprint in high heat producing terranes allows us to expand the timescale of the thermal history of central Australia to incorporate the far-field effects of Mesozoic tectonics at the plate margins. Further to low-temperature thermochronology data collected in this thesis, additional U Pb data has been collected on mafic sills which widely intrude the McArthur Basin, and have implications for tectonic history of northern Australia. Temporally and geochemically similar Mesoproterozoic magmatism in northern Australia and north China at ca. 1330 1295 Ma supports coeval intrusion across both regions, while geochemical evidence indicates a mantle plume derived melt as the probable source. The intrusion of geochemically analogous sills within temporally and stratigraphically comparable sedimentary basins suggests plume activity proximal to a once continuous sedimentary system that covered much of the North Australian Craton and North China Craton, which supports a close connection between the cratons within the supercontinent Nuna in the Mesoproterozoic.