Ophiolite accretion in the Lachlan Orogen, Southeastern Australia

Slivers of dismembered Cambrian ophiolites are preserved in major fault zones of the turbidite-dominated, western and central Lachlan Orogen of southeastern Australia. Geometrical, chronological, and metamorphic constraints indicate that these slivers did not undergo classic Tethyan-style obduction but were incorporated into evolving turbidite wedges as offscraped slices, imbricated fault slivers or duplexes, and as blocks in mélange. The processes involved include decoupling of the oceanic crustal stratigraphy predominantly above the moho, formation and metamorphism of serpentinite/talc matrix mélange up to blueschist conditions, and accretion via underplating or offscraping during underthrusting. These processes have produced major, large-scale fault zones within the chevron folded turbidites that show marked along-strike variation in structural complexity that relate to magnitude of displacement. The fault zones may have formed preferentially where topographic highs occurred in the oceanic crust, promoting fold and fault nucleation within irregularities in the layering. These fault zones have predominantly brittle character due to their formation under a low geothermal gradient. Duplex-like structures in the fault zones and the inferred development through buckling of, then faulting of, the oceanic lithosphere, have analogues in modern oceanic environments (e.g. Indian Ocean) and young subduction systems (e.g. Sulu Sea, southeast Asia). They are interpreted to have formed due to convergence in a non-collisional, intra-oceanic setting during closure of a Cambrian backarc basin.