40Ar/39Ar and K–Ar age constraints on the timing of regional deformation, south coast of New South Wales, Lachlan Fold Belt: problems and implications*

The Lachlan Fold Belt is well known for its complex deformational history, with a number of orogenic events and associated structural styles recognised. Recent syntheses of this orogenic evolution have emphasised the difficulty of identifying fold-belt-wide deformational episodes and have inferred the existence of diachronous deformation zones with an overall merging of episodic orogenic events (Gray & Foster 1997; Gray et al. 1997). 40Ar/39Ar ages have been utilised in an attempt to clarify the timing of deformation in the Lachlan Fold Belt because, in many areas, stratigraphic constraints are either absent or too broad to permit precise timing of individual deformation events (Foster et al. 1998, 1999; Offler et al. 1998; Bierlein et al. 1999). Debate over the implications of the 40Ar/39Ar age data has focused on the interpretation of deformation models that are at least partly constrained by the radiometric ages. Dispute has been mainly over diachronous deformation related to subduction complex settings versus episodic deformation in a convergent margin setting (VandenBerg 1999). A significant proportion of the 40Ar/39Ar data derives from step-heating analyses of low-grade, whole-rock slates, mostly from the western Lachlan Fold Belt (Foster et al. 1998, 1999). Studies of similar samples from the eastern extremity of the Lachlan Fold Belt are limited to the study by Offler et al. (1998), who reported 40Ar/39Ar ages of 450 3 Ma and 445 2 Ma from samples in the Ordovician turbidite succession near Narooma and Bermagui, respectively. These ages were regarded as a precise constraint on the timing of intense underplating-related deformation associated with development of a subduction complex in the Late Ordovician at the eastern extremity of the Lachlan Fold Belt. In this account we report the results of a 40Ar/39Ar and K–Ar study of samples from the south coast of New South Wales (Figure 1). We have found that the 40Ar/39Ar data are influenced by inherited mica that was not recrystallised during metamorphism associated with deformation, as well 40Ar/39Ar and K–Ar age constraints on the timing of regional deformation, south coast of New South Wales, Lachlan Fold Belt: problems and implications*