This study investigates possible mechanisms that can account for the intraplate deformation in central Australia and the Canning Basin during the Devo-Carboniferous Alice Springs Orogeny. The intraplate orogeny in central Australia seems to have occurred without the association of a significant collisional orogenic event at the plate boundary. In contrast, the present-day Tian Shan may be viewed as a consequence of the plate boundary collision which has produced the Himalayas and the Tibetan Plateau. The experiments presented in this paper examine a mechanism that produces intraplate thickening and thinning of the crust but leaves the boundary relatively undeformed. A thin viscous sheet approximation of continental lithosphere is used to demonstrate that a clockwise rotational northern boundary acting on a lithospheric sheet with an internal weak zone may produce crustal thickening in the region representing central Australia, and thinning in the region representing the Canning Basin. In this model a clockwise rotation of the northern boundary may be induced either by an eastward shear traction or by a clockwise bending moment. The relation between rotation of the boundary and maximum crustal thickening factor is, to first order, independent of the way in which deformation is driven. It depends primarily on the relative dimensions of the intracratonic weak zone. For plausible estimates of thin viscous sheet geometry and variation of lithospheric strength within the sheet, it is inferred that clockwise rotation of the northern Australian block of order 20–25° is required to produce a maximum crustal thickening factor in central Australia of order 1.67. These calculations indicate that the depth-averaged strength of the lithosphere in central Australia prior to the Alice Springs Orogeny was of order B0 = 0.8 × 1013 Pa s1/3, assuming plausible estimates of plate boundary force of 5 × 1012 Nm−1 and orogenic time span of 100 Ma. Based on a simplified approximate model for lithospheric strength this strength coefficient corresponds to a Moho temperature in central Australia of order 520°C. The concentration of deformation in this relatively narrow zone that stretches E-W across the continent, implies that the blocks to the N and S are much stronger, a difference which might be explained by a decrease in Moho temperature of order 60°C. If Moho temperatures prior to the Alice Springs orogeny were higher than those estimated above, the required deformation may have been compressed into a shorter period than 100 Ma, or may have been episodic rather than continuous.
Abstract. Amber is chiefly known as a preservational medium of biological inclusions, but it is itself a chemofossil, comprised of fossilised plant resin. The chemistry of today's resins has been long investigated as a means of understanding the botanical sources of ambers. However, little is known about the chemical variability of resins and consequently about that of the ambers that are derived from particular resins. We undertook experimental resin production in Araucariacean plants to clarify how much natural resin variability is present in two species, Agathis australis and Wollemia nobilis, and whether different resin exudation stimuli types can be chemically identified and differentiated. The latter were tested on the plants, and the resin exudates were collected and investigated with Fourier-transform infrared attenuated total reflection (FTIR-ATR) spectroscopy to give an overview of their chemistry for comparisons, including multivariate analyses. The Araucariacean resins tested did not show distinct chemical signatures linked to a particular resin-inducing treatment. Nonetheless, we did detect two separate groupings of the treatments for Agathis, in which the branch removal treatment and mimicked insect-boring treatment-derived resin spectra were more different from the resin spectra derived from other treatments. This appears linked to the lower resin viscosities observed in the branch- and insect-treatment-derived resins. However the resins, no matter the treatment, could be distinguished from both species. The effect of genetic variation was also considered using the same stimuli on both the seed-grown A. australis derived from wild-collected populations and on clonally derived W. nobilis plants with natural minimal genetic diversity. The variability in the resin chemistries collected did reflect the genetic variability of the source plant. We suggest that this natural variability needs to be taken into account when testing resin and amber chemistries in the future.
Some liquid plant exudates (e.g. resin) can be found preserved in the fossil record. However, due to their high solubility, gums have been assumed to dissolve before fossilisation. The visual appearance of gums (water-soluble polysaccharides) is so similar to other plant exudates, particularly resin, that chemical testing is essential to differentiate them. Remarkably, Welwitschiophyllum leaves from early Cretaceous, Brazil provide the first chemical confirmation of a preserved gum. This is despite the leaves being exposed to water twice during formation and subsequent weathering of the crato formation. the Welwitschiophyllum plant shares the presence of gum ducts inside leaves with its presumed extant relative the gnetalean Welwitschia. This fossil gum presents a chemical signature remarkably similar to the gum in extant Welwitschia and is distinct from those of fossil resins. We show for the first time that a water-soluble plant exudate has been preserved in the fossil record, potentially allowing us to recognise further biomolecules thought to be lost during the fossilisation process. A wide variety of vascular plants produce fluid exudates 1 e.g. resins and gums, with each group differing in chemical definitions (Table 1). Due to similarity in physical appearance distinguishing exudates based on chemistry is vital, for example gums and resins are visually similar resulting in these terms being used interchangeably 1. However, their chemical definitions are very different (Table 1); resins are composed of lipid-soluble terpenoids 1,2 , while gums are complex, highly branched (non-starch) water-soluble polysaccharides 3. A common example of this misunderstanding is the Eucalyptus tree, which is known as a gum tree, but nuclear magnetic resonance analysis of the Eucalyptus exudate shows its composition to be polyphenolic and is therefore actually a kino 4 (Table 1). Differences between gum and resin can also be seen in the functional roles within the plant. The main roles of resins are to respond to wounding, as a defence against pathogens and to dissuade herbivory by insects and other organisms 2,5,6. Gum is involved in food storage, structural support, and also for wound sealing, but there is no common role across taxa 3. Further confusion arises as some plants, e.g. Boswellia and Commiphora species, even produce exudates with a mixture of polysaccharide and resin components (the gum resins myrrh and frankincense respectively) 1. Until now only fossilised plant resin (ambers) 7 and latex filaments have been reported preserved in the fossil record 8,9. While the fossilisation of fluid exudates might seem unlikely, the fossilisation of resin is relatively common, and extends back some 320 million years to the Carboniferous 10 , but chemically confirmed gums have never been reported. The Early Cretaceous (~120 million year old) Crato Formation 11 of northeast Brazil (Supplementary Fig. S1) is a well-known laminated limestone deposit that yields exceptionally preserved vertebrates, arthropods, and plants from the ...
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