Evolution of the boundary between the western and central Lachlan Orogen: Implications for Tasmanide tectonics

Abstract

Differences in oblique overprinting, along‐strike complexity as well as structural, metamorphic and timing constraints suggest that the boundary between the western and central subprovinces of the Lachlan Orogen, currently designated by the Governor Fault, cannot be a single structure. Previously limited data on the nature and kinematics of the fault/shear systems defining the boundary have led to varying scenarios for the tectonic evolution of the Lachlan Orogen. These scenarios either involve large‐scale strike‐slip displacement along the boundary with subsequent overthrusting or convergence of oppositely vergent thrust‐systems with limited strike‐slip translation. Geometrical constraints, fabric chronology and kinematic indicators in both the Mt Wellington (Melbourne Zone) and Governor (Tabberabbera Zone) Fault Zones indicate that maximum displacements relate to thrusting and duplex formation, followed by minor strike‐slip faulting perhaps in response to slightly oblique collision of the Melbourne and Tabberabbera structural zones. Collision of these zones took place between ca 400 and 390 Ma. At Howqua, structural relationships indicate that collision involved northeast‐directed thrusting of the Melbourne Zone (Mt Wellington Fault Zone) over the Tabberabbera Zone (Governor Fault Zone), and was followed by regional, northwest‐trending, open folding. These structures overprint the dominant fabrics and metamorphic assemblages that are interpreted to relate to disruption and underthrusting of Cambrian oceanic/arc crust during closure of a marginal basin. Major deformation in the Tabberabbera Zone took place from ca 445 Ma and was associated with mélange formation, underplating and imbrication or duplexing (Governor Fault Zone, East Howqua segment). At slightly higher crustal levels, and following deposition of Upper Ordovician black shale and chert sequences ( ca 440 Ma), Tabberabbera Zone evolution included offscraping of a serpentinite body (Dolodrook segment) that may have been either a Marianas‐style seamount or transform fault zone within the Cambrian oceanic/arc crust. Major thrusting in the Mt Wellington Fault Zone was underway sometime after ca 420 Ma, and in contrast to the Governor Fault Zone, no mélange or broken formation was produced, metamorphism was at slightly higher temperatures and deformation probably occurred under higher strain states.