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The Macquarie Arc of the eastern Lachlan Orogen, SE Australia, is a good example of a mafic-intermediate intraoceanic arc that, after accretion, became sandwiched between coeval Ordovician craton-derived, turbidite and black shale terranes. Until very recently, it was assumed that the arc and the turbidite terranes formed adjacent to each other. However, recognition of major faults between the packages, the lack of provenance mixing, the different zircon patterns and the local presence between them of ?suture zone' MORB-like basalts suggests that they formed at distances greater than several hundreds of kilometres.
The Macquarie Arc began about 490 Ma and evolved in four magmatic phases separated by hiatuses. Early-Middle Ordovician arc development occurred in response to west-dipping subduction, so that the arc lay on the Gondwana plate A short interval of east-dipping subduction may have occurred in the early Late Ordovician. In the middle-late Late Ordovician, subduction could have been to the east or west, or the arc may have been under transtension.
Clues to the processes involved in accretion come from studies of the flanking craton-derived turbidite and shale terranes. These show such similar stratigraphies that they must have been adjacent (end on or side on) in the Ordovician before deformation in the Benambran Orogeny and before they became separated by the arc. The simplest model is one in which the turbidite and shale terrane east of the arc is allochthonous and migrated northwards in the Late Ordovician, along the eastern margin of Gondwana, into a position outboard of the Macquarie Arc, thereby blocking subduction. Ensuing oblique compression drove the eastern turbidite and black shale package into the arc and drove the arc into its backarc basin and led to a combination of thrusting and major strike-slip faulting within, inboard and outboard of the arc at 443-439 Ma and again at 435-430 Ma in the multiphase Benambran Orogeny. Orogen- parallel strike?slip deformation is thus an important component in the formation of this accretionary orogen.
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