Orogenic concepts - Application and definition: Lachlan Fold Belt, Eastern Australia

The orogenic framework for the Paleozoic Lachlan Fold Belt of southeastern Australia was based originally on local and/or regional unconformities in the rock record with the assumption that these reflected orogenic belt-wide tectonic events. Tectonic evolution was related to four main orogenic 'pulses' starting at approx 450 Ma and ending with stabilization by 340 Ma. Maximum thermal activity reflected by crystallization and cooling ages from granitic plutons was between 420 and 340 Ma with a peak at 400 to 410 Ma. (40)kr/Ar-39 geochronologic data from cleaved slates and mica-bearing quartz veins in the chevron-folded turbidites of the western subprovince show that deformation was diachronous and progressed from the west at 440 Ma to the east at 410 to 390 Ma. The end Ordovician-Silurian event is now recognized as being the most significant and most widespread orogenic event to affect the Lachlan Fold Belt, It occurred as two migrating waves of subduction-related west-to-east deformation within the western and eastern/central subprovinces respectively. The Middle Devonian event (approx 380-370 Ma), formerly considered to be the paroyxsmal cataclysmic orogenic event for the Lachlan Fold Belt, has now markedly reduced significance and only represents limited deformation due to amalgamation of the western and central/eastern subprovinces of the ford-belt and attainment of freeboard at this time. Dating of mica growth during penetrative deformation, at low to medium metamorphic grades, can be used to constrain the timing and patterns of deformation during tectonic evolution of orogenic belts. The orogenic framework of the Lachlan Fold Belt must be redefined as continuous over the extent of the belt from Late Ordovician through to Late Devonian times. Apparent episodic events, which dominate the rock record as unconformities and facies changes, only reflect ''local'' instability due to changing accommodation mechanisms at the local to regional scale, and changing plate position with time.