Continental Growth in Accretionary Orogens

Paleozoic crustal growth, structure, and metallogeny of the Lachlan Orogen, Eastern Australia

Assembly of the Tasmanides of Eastern Australia involved accretion of the Delamerian, Lachlan and New England orogens between ca. 520 and 260 Ma. The Lachlan Orogen was constructed in an oceanic setting when subduction of paleo-Pacific plates alternated from slab rollback and backarc basin formation to intense shortening and basin closure. Lachlan accretion involved deformation of three thrust-systems that formed during shortening of a massive turbidite fan and island arc complex within a >1500 km wide marginal ocean basin. An east–verging fold-thrust wedge in the western Lachlan formed in two stages between 455-439 Ma and 410-395 Ma. In the central Lachlan a SW-propagating thrust wedge was active between 440-395 Ma in front of a 440-410 Ma magmatic arc. Final deformation in the Ordovician Macquarie volcanic arc of the eastern Lachlan is younger (400-340 Ma), although subduction had started in the east before ca. 445 Ma. Extensional basins and inverted rifts are prominent in the central and eastern Lachlan Orogen.Syntectonic and post-tectonic felsic and mafic magmatism throughout the Lachlan reflects subduction geometries, areas of crustal thickening, basaltic underplating, periods of extension, and stabilization of the crust.Strain rates associated with tectonic shorting in the western Lachlan, defined by integrating 40Ar/39Ar and strain measurements, were about 10-14 s-1. Metallogenic associations across the Lachlan Orogen are correlated with tectonic setting and the geodynamics of evolving basins, magmatic arcs, forearcs, and accretionary prisms.

Map of Gondwana at about 530 Ma showing the location of the Tasmanides. SF, Sao Francisco craton, RDP, Rio de le Plata craton

Map of eastern Australia showing the composite Tasmanides including the Paleozoic Lachlan Orogen.

(a) Map showing the major tectonic elements important for understanding the tectonic setting of the Lachlan Orogen (after Gray and Foster, 2004). The bold lines show locations of major faults and the finer gray lines show the orientation of the major structural grain. (b) Map showing the average grade of regional metamorphism across the Lachlan (after Gray and Foster, 2004). Eastern metamorphic complex abbreviations: CMC-Cooma, CaMc-Cambalong, JMV-Jerangle and KMC-Kuark (c) Map showing the age and location of major Paleozoic granitic plutons in the Lachlan (after Foster and Gray, 2000). (d) Map showing the age of major deformation and metamorphism for different locations across the Lachlan (after Foster and others, 1999).

Reconstructions of the tectonic evolution of the Lachlan Orogen (after Foster and Gray, 2000, 2008; Gray and Foster, 2004)

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Cartoon showing the location and timing of mineral deposits in the Lachlan with respect to the evolving tectonic settings (after Bierlein and others, 2002).

 

Strain rate in Paleozoic thrust sheets, the western Lachlan Orogen, Australia : strain analysis and fabric geochronology

David Foster and David Gray

It is possible to calculate average orogenic strain rates when mica cleavage or syndeformational veins can be dated, and finite strain can be estimated. Deformation of accretionary-style thrust sheets in the western Lachlan Orogen occurred by chevron folding and faulting over an eastward propagating decollement. Based on 40Ar/39Ar dates of white micas, that grew below the closure temperature, this deformation started at ~457 Ma in the west and ended at ~378 Ma in the east, with apparent “pulses” of deformation at about 440, 420 and 388 Ma.

The 40Ar/39Ar data from thrust sheets in the Bendigo structural zone show that deformation progressed from early buckle folding, which started at 457-455 Ma through to chevron fold lock-up and thrusting at 441-439 Ma. Based on retrodeformation, the total average strain for this thrust sheet is -0.67, such that the bulk shortening across the thrust sheet is 67%. This amount of strain accumulated over a duration of ~16 Ma gives a minimum strain rate of 1.3 x 10-15 s-1 and a maximum strain rate of 5.0 x10-15 s-1, based on fan thickness considerations. The total shortening on this thrust sheets is between ~310 km and ~800 km, which gives a decollement displacement rate between ~19 mm a-1 (minimum) and ~50 mm a-1 (maximum). If deformation occurred in pulses ~457-455 and ~441-439 Ma then the calculated strain rate would be on the order of 1 x 10-14 s-1. These strain rates are similar to convergence rates in western Pacific back arc basins, and shortening rates in accretionary prisms, and turbidite-dominated thrust systems like Taiwan.