Structure and Geology of the Lachlan Fold Belt
The Lachlan terrane consists of three lithotectonic assemblages each with differences in rock-type, metamorphic grade and structural history and geological evolution (Gray 1997). The western and central subprovinces are dominated mainly by metamorphosed quartz and shale turbidite sequences, while the eastern subprovince consists of quartz and shale turbite sequences as well as mafic volcanics and limestones.
Geology
The western subprovince is made up predominantly of "a thick sequence of unfossiliferous sediments, including slate, rare carbonaceous slate, siltstone and greywacke showing effects of low-grade regional metamorphism and localized higher grade metamorphism associated with ... granite intrusions" (Brown et al 1982). Grading, cyclicity and fine-scale cross-bedding all suggest in these rocks a turbidite origin. Turbidite ages range from Ordovician in the west to Early Carboniferous in the east. These turbidites were most likely sourced from the Delamerian mountain chain to the west on the margin of eastern Gondwanaland. These turbidites flowed out over a vast area (2000 km across and 2-5 km thick) underlain by an oceanic crust basement (Fergusson and Coney 1992).
Two characters of granites occur in this subprovince. Early Devonian granites are small and relatively equideminsional shaped bodies. Late Devonian granites, on the other hand, are larger and have elongated east-west shapes. These Late Devonian granites are considered post tectonic and are associated with volcanic cauldron complexes and ignimbrite flows and crustal extension and delamination (Gray 1997).
Structural style
The
western subprovince is characterized by upright chevron folds with a west-dipping
axial-plane cleavage. Steep west dipping thrust faults are
also common, thus forming an east verging fold thrust system.
Thrust faults all sole into a regional detachment at a depth of 4-6
km, and have been confirmed by deep seismic imaging (Glen 1992).
Structural analysis of this area suggest a minimum of 50% total
shortening (Fergusson and Coney 1992).
Geology
The central subprovince is also dominated by quartz-rich turbidite sequences ranging in age from Ordovician in the west younging to Late Silurian in the east. One of the most striking features, however, of this region is the fault-bounded NNW-trending Wagga/Omeo Metamorphic Belt (Gray 1997). This metamorphic zone is "characterized by amphibolite and greenschist-grade, steeply dipping, foliated pelitic schists with local development of gneiss (Coney 1991)" and extends almost 500 km with a width of 150 km. This metamorphism represents high temperature-low pressure and seems to be more of a regional contact aureole, as is supported by a extensive granite intrusions in this area. Peak metamorphic conditions reached in this area were T ~ 700o C and P ~ 3.5kbar (Morand 1990). These turbidite sequences were also most likely sourced from the Delamerian mountain chain on the eastern margin of Gondwanaland.
Granite intrusions, as stated above, are extensive in this region, leading to a regional contact metamorphism of the metasedimentary rocks. Granite ages are mostly Silurian in the western part of the subprovince to Late Silurian in the northwestern portion. Granite bodies are generally elongate NNW-trending.
Structural Style
The
central subprovince lies between two major fault systems, the
Gilmore "Suture" on the east and the Jackermaroo fault
system in the west. "Both fault systems show evidence of
considerable strike-slip movement" and structures within the
belt "suggests that the whole belt has undergone southerly
translation both in the Early Silurian and in the Middle Devonian
(Glen 1992)." Strike-slip movement of a vast terrane of
quartz-rich turbidite sequences from the north also explains the
repetition of the sediment ages and the "duplication of the
strike-width of the fold belt" resulting in the extreme width
for the palinspastically restored Lachlan turbidite blanket (Gray 1997)."
Geology
The eastern subprovince marks an end to the vast deep water turbidite sequences. This area is characterized by turbidite sequences to the far west but grades into platform carbonates and clastics intermixed with felsic volcanics and extensive granitization of mainly I-type granites. Rock ages range from Silurian to Lower-Carboniferous with the same younging towards the east trend seen in all parts of the Lachlan Fold Belt. This grading into shallow water facies and volcanics marks an important transition to a shallow island arc complex accreting onto the Gondwanan margin. Shallow basins caught between the accreting complex and the continent show multiple sedimentation cycles, each cycle grading upwards from clastic and volcanics into limestones or limey mudstones, with the discordances evident at the bases of sequences indicative of basin subsidence due possibly to extensional episodes within the overall convergence (Glen 1992).
Structural Style
This
area, as are all in the Lachlan Fold Belt, is characterized by a
roughly north-south trending structural grain. Mostly west
dipping thrust faults intermixed with tightly folded bedding, forming
an east vergent system (Gray 1997). As discussed above,
inversion of shallow marine basins is common, and possibly attests to
multiple phases of compression and extension.
Questions? Comments? Please contact fwagner@geo.arizona.edu