Contrasts in morphogenesis and tectonic setting during contemporaneous emplacement of S- and I-type granitoids in the Eastern Lachlan Fold Belt, southeastern Australia

Abstract

Abstract Integrative gravity and structural modelling of Ordovician-Silurian granitoids in the Eastern Lachlan Fold Belt (southeastern Australia) revealed contrasts in emplacement mode and deformation style between coeval S- and I-type granites. The NNE-SSW directed contraction during the Benambran event of the Lachlan Orogen caused dextral movement along two major strike-slip faults (Carcoar Fault/Copperhannia Thrust) and simultaneous formation of both transtensional pull-apart and transpressional shear zones. The geometry and deformation style of the plutons and country rock, their spatial relationship at depth to adjacent faults and the structural history of both the granites and country rocks suggest a genetic linkage between magma emplacement and synmagmatic deformation. Synchronously, the Carcoar Granodiorite was emplaced into a transtensional pull-apart structure and the Barry Granodiorite and Sunset Hills Granite intruded transpressional shear zones. The I-type Carcoar and Barry granites are square to tabular, wedge-shaped bodies exhibiting a weak deformation; whereas the S-type Sunset Hills Granite is an elongated, tabular to sheet-like pluton showing a moderate deformation degree. The contrasts in 3D shape, emplacement mode and deformation style between the I- and S-type granites are due to differences in nearfield stress regime, geometry of the emplacement sites, intrusion level with respect to thermal and rheological conditions, and in their response to deformation. This response is in part controlled by the proportion of resistant/non-resistant minerals in the granite and host rock. This study demonstrates that distinctive emplacement modes can operate simultaneously in different parts of a fault system under contrasting deformation conditions.