Character, extent and significance of broken formation for the Tabberabbera Zone, central Lachlan Orogen

Abstract

Chevron‐folded Ordovician‐Silurian turbidite sequences in the northern Tabberabbera Zone, Lachlan Orogen are cut by steeply, northeast‐ and southwest‐dipping fault zones with black slate (?Warbisco Shale) incorporating tectonic mélange (broken formation) in the hangingwalls. The black slate zones appear laterally discontinuous, have 50–150 m outcrop widths, and are spaced at ∼2–4 km (Rose River section). Dominated by attenuated, homoclinally dipping bedding, local disruption in these zones is shown by block‐in‐matrix tectonic mélange (broken formation). The mélange fabric has anastomosing, stylolitic cleavages typical of pressure solution. Marked flattening and attenuation of sedimentary layering is accompanied by up to 180% subhorizontal elongation within the foliation. Folding of the mélange fabric is shown by local development of small‐scale, steeply plunging isoclinal folds that have reclined geometry, and local development of a crenulation fabric. This reflects shear strain during activation of the broken formation zones as fault zones. Sandstone blocks within the mélange are characterised by subrounded, commonly non‐faceted terminations, and a lack of internal cataclastic deformation. Initial deformation, therefore, involved disruption of unconsolidated sediment, in which deformation was close in time to sedimentation when the sediments were not totally lithified. Associations of broken formation and a disrupted oceanic stratigraphy suggest that Tabberabbera Zone structure developed by thrusting within an oceanic setting.