NEO tectonic models

A variety of tectonic models have sought to explain the variation through time of volcanic and plutonic rock chemistries. In particular, a shift from calc-alkaline chemi stries to tholeiitic and more variable compositions during the latest Carboniferous seems to imply a shift from subduction to back arc extension, with possible cessation of subduction. The observed return to calc-alkaline volcanism during the mid-late Permian suggests renewed subduction. This time period i s also associated with the Hunter Bowen orogeny, which is thought to be the main deformational event responsible for development of the New England fold belt. At least two models seek to explain these magmatic trends in greater detail.

One model deals with the concept of slab rollback, particularly described by Allen (2000). In this model, the relative convergence rate between Australia and the subducting oceanic lithosphere to the east is reduced, allowing the slab dip to increase and inducing extension in the back arc region of the Australian continent (Figure 5.1). An increase in slab dip could feasibly allow some volume of warm, fertile asthenosphere to come into contact with the base of the Australian lithosphere and help to produce volcanic rocks with a more variable geochemical signature than would be observed during normal subduction. Thinning of the crust during this magmatic episode could also help to explain the transition to I-type granite geochemistry by late Permian time (see geochemistry).

Figure 5.1. Schematic diagram showing one interpretation of the evolution of the eastern margin of Australia during the latest Carboniferous and early Permian. In this model, a transition to extension is driven by slab rollback (Allen, 2000).

A second model seeks to explain the transition to back arc extension through a mechanism known as slab break-off (Caprarelli and Leitch, 1998). In this scenario, subduction would have somehow been halted off the eastern coast of Australia during the late Carboniferous, perhaps by attempted subduction of a mid-ocean ridge or seamount, and the remaining portion of subducted oceanic material would break off from the rest of the plate and sink into the underlying asthenosphere (Figure 5.2). This model would involve a similar decrease in convergence between Australia and adjacent oceanic lithosphere that could lead to back arc extension and a large variability in volcanism. One major piece of support for this latter model is a supposed eastward jump in the locus of arc volcanism between late Carboniferous or early Permian, when there may have arguably been no active volcanic arc, and renewal of calc-alkaline volcanism in late Permian time.

Figure 5.2. Time-space diagram of igneous strata in the New England Orogen. Major periods of magmatism, particularly continental margin arc volcanism and plutonism, are represented by the "2" and "14" fields on the diagram. Igneous rocks formed during the intervening time are highly variable in geochemical character and rather sporadically spaced throughout the orogen. However, the most intense arc volcanism appears to have jumped eastward following the variable early Permian magmatism. Caprarelli and Leitch (1998) use this observation to argue for a model of slab breakoff to explain the evolution of the NEO (figure from Caprarelli and Leitch, 1998).

These models can also provide insight into the relationship of observed megafolds, or large-scale, oroclinal folds to the Hunter-Bowen orogeny. In particular, some authors call for strike-slip motion or some component of oblique convergence between Australia and the adjacent oceanic plate, starting in the early Permian, to explain the magmatic pattern. Such plate motions could have been responsible for formation of megafolds or oroclinal folding, as well as displacing parts of tectonostratigraphic terranes northward or southward along roughly N-trending strike-slip faults. In this scenario, the strike-slip faulting and oroclinal folding would at least partially pre-date main thrusting associated with the Hunter Bowen orogeny. This thrusting is fairly well constrained to have occurred during the late Permian through early Triassic. The most often cited alternative to this model is one in which the megafolding and strike-slip faulting occurred coevally with Hunter-Bowen thrusting. This is an unresolved issue in this area (see active research).


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