INFERRED THRUST IMBRICATION, DEFORMATION GRADIENTS AND THE LACHLAN TRANSVERSE ZONE IN THE EASTERN BELT OF THE LACHLAN OROGEN, NEW-SOUTH-WALES

Re-examination of the Ordovician geology between Mandurama and Big,aa in the Lachlan Orogen of central western New South Wales has produced new interpretations of the stratigraphy and structural geology. The Ab ercrombie beds have been previously inferred to comprise an Ordovician turbidite package with interbedded black shale bands. Although hamper ed by a paucity of fossil ages, new data suggest that the Ordovician g eology of this region instead represents an imbricate stack of Lower O rdovician turbidites (Adaminaby Group) and Upper Ordovician black shal es (Warbisco Shale). Structural data from the north of this region sug gest that duplication occurred in a D-1 event (with formation of broad ly east-west to west-northwest-trending thrust slices or fold limbs) a nd was accompanied by formation of cleavage and isoclinal folds. Thrus ting of the Adaminaby Group and Warbisco Shale over or under the Lower Ordovician Coombing Formation (southern part of the Molong volcanic b elt) also occurred at this time. East-vergent imbrication and thrustin g and formation of a regional near-meridional steeply west-dipping cle avage occurred in the D-2 event, when D-1 thrusts or folds were folded around overturned (east-vergent) D-2 folds. These new data also sugge st that there is a north-to-south gradient in the intensity of the D-2 deformation, with D-2 effects decreasing from south to north approach ing the Lachlan Transverse Zone. Such a gradient mirrors similar but m ore subtle local changes from the north. Together, they imply that the Lachlan Transverse Zone was a major zone of weakness during north-sou th shortening that resulted in the formation of D-1 structures but was relatively rigid in local areas during the regional D-2 deformation t hat resulted from east-west shortening when it formed a major tear/acc ommodation zone. This D-2 rigidity may be caused by strength imparted by the earlier emplacement of large (variably mineralised) intrusive/v olcanic complexes along the transverse zone.