Tectonic evolution of the Lachlan Orogen, southeast Australia: historical review, data synthesis and modern perspectives

Abstract

The Lachlan Orogen, like many other orogenic belts, has undergone paradigm shifts from geosynclinal to plate-tectonic theory of evolution over the past 40 years. Initial plate-tectonic interpretations were based on lithologic associations and recognition of key plate-tectonic elements such as andesites and palaeo-subduction complexes. Understanding and knowledge of modern plate settings led to the application of actualistic models and the development of palaeogeographical reconstructions, commonly using a non-palinspastic base. Igneous petrology and geochemistry led to characterisation of granite types into 'I' and 'S', the delineation of granite basement terranes, and to nonmobilistic tectonic scenarios involving plumes as a heat source to drive crustal melting and lithospheric deformation. More recently, measurements of isotopic tracers (Nd, Sr, Pb) and U-Pb SHRIMP age determinations on inherited zircons from granitoids and detrital zircons from sedimentary successions led to the development of multiple component mixing models to explain granite geochemistry. These have focused tectonic arguments for magma genesis again more on plate interactions. The recognition of fault zones in the turbidites, their polydeformed character and their thin-skinned nature, as well as belts of distinct tectonic vergence has led to a major reassessment of tectonic development. Other geochemical studies on Cambrian metavolcanic belts showed that the basement was partly backarc basin-and forearc basin-type oceanic crust. The application of 40 Ar-39 Ar geochronology and thermochronology on slates, schist and granitoids has better constrained the timing of deformation and plutonism, and illite crystallinity and b o mica spacing studies on slates have better defined the background metamorphic conditions in the low-grade parts. The Lachlan deformation pattern involves three thrust systems that constitute the western Lachlan Orogen, central Lachlan Orogen and eastern Lachlan Orogen. The faults in the western Lachlan Orogen show a generalised east-younging (450-395 Ma), which probably relates to imbrication and rock uplift of the sediment wedge, because detailed analyses show that the décollement system is as old in the east as it is in the west. Overall, deformation in the eastern Lachlan Orogen is younger (400-380 Ma), apart from the Narooma Accretionary Complex ( ca 445 Ma). Preservation of extensional basins and evidence for basin inversion are largely restricted to the central and eastern parts of the Lachlan Orogen. The presence of dismembered ophiolite slivers along some major fault zones, as well as the recognition of relict blueschist metamorphism and serpentinite-matrix mélanges requires an oceanic setting involving oceanic underthrusting (subduction) for the western Lachlan Orogen and central Lachlan Orogen for parts of their history. Inhibited by deep weathering and a general lack of exposure, the recent application of geophysical techniques including gravity, aeromagnetic imaging and deep crustal seismic reflection profiling ...

Crustal complexity in the Lachlan Orogen revealed from teleseismic receiver functions

“…North-south shortening (generally < 5%) was interactive with east-west shortening during the Lachlan Orogen evolution (e.g. Gray & Foster 2004). However, regional structural relationships between northsouth and east-west shortening suggest that the major north-south structural grain of the Lachlan Orogen results from overall east-west shortening (e.g.…”

“…However, regional structural relationships between northsouth and east-west shortening suggest that the major north-south structural grain of the Lachlan Orogen results from overall east-west shortening (e.g. Gray & Foster 2004). The anisotropy could be due to a strong N-S structural trend in the eastern Lachlan Orogen, which extends from surface to Moho with a variable dip (e.g.…”

“…The variations in the crustal thickness and the intermediate and broad transition between crust and mantle beneath the Lachlan Orogen may be related to the presence of underplating at the base of the crust (e.g. Drummond & Collins 1986;Shibutani et al 1996;Gray & Foster 2004;Fontaine et al 2013). They may also result from existing thick piles of Ordovician mafic rocks present in the mid and lower crust .…”

“…Collins 2002a;Direen & Crawford 2003;Gray & Foster 2004). There is ongoing debate as to the exact causes and the nature of accretion.…”

“…We show evidence for both crustal anisotropy and a dipping Moho in an area south of the earlier seismic reflection studies. *Corresponding author: fabrice.fontaine@univ-reunion.fr Gray & Foster (2004) with the location of the SoCP (Southern Cratons to Paleozoic) seismic network and the permanent seismic stations from Geoscience Australia (GA) and GEOSCOPE networks. Key to marked feature: NVP, Newer Volcanic Province.…”