Nicholas G. Direen

Volcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ∼70 000 km² Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ∼90 000 km² Naturaliste Plateau, located ∼170 to ∼500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. ⁴⁰Ar/³⁹Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high ⁸⁷Sr/⁸⁶Sr (up to 0.78), high ²⁰⁷Pb/²⁰⁴ Pb ratios (15.5–15.6), low ¹⁴³Nd/¹⁴⁴Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km².

Vintage 2-D (two-dimensional) seismic reflection surveys from the sparsely explored Mentelle Basin (western Australian margin) have been reprocessed and integrated with a recent high-quality 2-D seismic survey and stratigraphic borehole data. Interpretation of these data sets allows the internal geometry of the Mentelle Basin fill and depositional history to be reanalysed and new insights into its formation revealed. Basin stratigraphy can be subdivided into several seismically defined megasequences separated by major unconformities related to both breakup between India-Madagascar and Australia-Antarctica in the Valanginian-Late Hauterivian and tectonically-driven switches in deposition through the Albian. Resting on the Valanginian-Late Hauterivian breakup unconformity are several kilometre-scale mounded structures that formed during Late Jurassic to Early Cretaceous extension. These have previously been interpreted as volcanic edifices although direct evidence of volcanic feeder systems is lacking. An alternative interpretation is that these features may be carbonate build-ups. The latter interpretation carries significant climatic ramifications since carbonate build-ups would have formed at high palaeolatitude, ~60° S. Soon after breakup, initial subsidence resulted in a shallow marine environment and deposition of Barremian-Aptian silty-sandy mudstones. As subsidence continued, thick successions of Albian ferruginous black clays were deposited. Internally, seismic megasequences composed of successions of black clays show previously unresolved unconformities, onlapping and downlapping packages, which reflect a complex depositional, rifting and subsidence history at odds with their previous interpretation as open marine sediments. Southwestwards migration of the Kerguelen hotspot led to thermal contraction and subsidence to the present day water depth (~3000 m). This was accompanied by Turonian-Santonian deposition of massive chalk beds, which are unconformably overlain by pelagic Palaeocene-Holocene sediments. This substantial unconformity is related to the diachronous breakup and onset of slow spreading between Australia and Antarctica, which may have led to the reactivation and inversion of basement faults and was followed by rapid seafloor spreading from the Middle Eocene to the present.

Australia's western continental margin formed during Gondwana breakup between Australia – Antarctica and India, leaving behind an evolving plate boundary with significant new magmatic crust accreted to the continental fragments. In this context, the margin segment from the outer Exmouth Plateau to the Gascoyne Abyssal Plain has been repeatedly studied as an example of volcanic margin formation. However, many recent analyses based on selective interpretation of geophysical datasets have come to conflicting conclusions regarding both the tectonic architecture and the age of breakup on this margin. These differences profoundly impact on models for both the global plate circuit and for the timing and extent of magmatism implicated in volcanic breakup processes. We present new interpretations of the distribution of magmatic and pre-rift rock packages in this margin, based on the integrated interpretation of two deep crustal transects with existing seismic-reflection, refraction, gravity, magnetic and geochemical data. Interpretations are constrained by data from sparse Ocean Drilling Program and petroleum-exploration drilling, and dredging. We find evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling extensional fault-controlled basins developed in a broad volcanic-margin transition zone between the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the form of seaward-dipping reflector sequences, and are dense and magnetised. Most significantly, these packages give rise to potential-field anomalies that have previously been interpreted as due to seafloor spreading. Recognition of these packages in a volcanic-margin transition zone has implications for the recognition of the inboard edge of unequivocal oceanic crust, the oceanic – volcanic-margin boundary. The main locus of igneous activity in the volcanic-margin transition zone off the Exmouth Plateau is spatially offset from a previously recognised high-velocity zone, suggesting that these two phenomena may not be temporally related. Seismically imaged differences in total thinning and partitioning of thinning, between upper and lower crust, provide support for models of depth-dependent thinning previously proposed for this margin.

Evidence of remanent magnetism was observed in aeromagnetic images fromThe Granites/Tanami Inlier in the Northern Territory. An investigation was undertaken to verify whether remanent magnetism existed, and if so, what mineral and/or system caused it. Alternating field demagnetisation and Curie Temperature analyses was employed to calculate the magnetic properties of whole rocks. Optical petrology and electron microscopy were then used to determine the minerals present and their chemical compositions. The two dominant magnetic minerals in this region are multidomain magnetite and monoclinic pyrrhotite, confirmed by petrological analysis. Chemical analysis determined the magnetite and pyrrhotite were pure with no substitution of elements. Alternating field demagnetisation and Curie Temperature analyses show monoclinic pyrrhotite holds the remanent magnetism, but has a low magnetic susceptibility. The study indicates there is remanent magnetism in the Tanami region. If there is inadequate understanding of magnetic data, misinterpretation of magnetic anomalies could occur.