HIGH-TEMPERATURE, LOW-PRESSURE (HTLP) METAMORPHISM AND EXTENSIONAL OROGENESIS IN THE EASTERN LACHLAN FOLD BELT (LFB), SE AUSTRALIA

Metamorphic and magmatic longevity in the eastern LFB was initiated and sustained by episodes of prolonged extension, intermittently interrupted by transient periods of compressive deformation.

Upright fabrics, folds and extensive thrusting are renowned as evidence for compression-dominated orogenesis and have long been applied to the LFB. However, these structures are evidence of short lived events, and are not representative of the principal orogenic processes operating during the 450-390 Ma mid-Palaeozoic history of the LFB.

The eastern LFB is an elongate (>800 km x 300 km), N-S trending orogen, characterised by extensive tracts of multiply deformed, greenschist facies metasediments and younger, intervening, orogen-parallel basins. These orogen-parallel basins dominate the northern LFB, but translate southwards into contemporaneous, sub-basinal batholiths and HTLP metamorphic complexes. Furthermore, the main basin margin faults continue southwards where they also form the bounding structure to the batholiths and HTLP metamorphic complexes. Incremental pluton construction and HTLP metamorphism occurred at the base of these extensional basins during repeated, ~8-10 Ma long periods of extension, driven by east-directed retreat of a west-dipping subducting slab. Lithospheric thinning, asthenospheric decompression melting and underplating by mafic magmas associated with this extension generated a perturbed crustal geotherm, HTLP metamorphism and granite magmatism.

The arrival of buoyant oceanic crust, and a switch from steep- to shallow-slab subduction caused compressive deformation, lithospheric thickening and removal of the primary heat source for metamorphism and magmatism. A compilation of isotopic ages from pre-, syn- and post-tectonic plutons across the eastern LFB, together with detailed structural magmatic and metamorphic information, suggest that compressive deformation is short lived (~2 Ma) relative to extension, but upright fabrics, folding and faulting are preserved and ‘appear’ as the prominent structures of the LFB.

Following subduction of the buoyant crust, the orogenic system reverted to extension, resulting in the formation of a new, but adjacent belt of HTLP metamorphism, batholith construction and basin formation in the upper crust.