Transactions of the Royal Society of Edinburgh: Earth Sciences

Research Article

Growth of wedge-shaped plutons at the base of active half-grabens

S. W. Richardsa1 and W. J. Collinsa2

a1 Earth Materials: Structure and Tectonics, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. e-mail: simon.richards@amu.edu.au

a2 Department of Geology, School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia.

ABSTRACT

Combined field and geophysical data show that plutons from the Bega Batholith are elongate, meridional, wedge-shaped bodies which intruded during a period of regional east–west extension in the Palaeozoic eastern Lachlan orogen, eastern Australia. Plutons within the core of the batholith have intruded coeval, syn-rift sediments and co-magmatic volcanics. The batholith is bound by high-temperature, dip-slip faults, and contains several major NE-trending transtensional faults which were active during batholith construction. In the central part of the batholith, the Kameruka pluton is an asymmetric, eastward-thickening, wedge-shaped body with the base exposed as the western contact, which is characterised by abundant, shallow-dipping schlieren migmatites which contain recumbent folds and extensional shear bands. A shallow (<30°), east-dipping, primary magmatic layering in the Kameruka pluton steepens progressively westward, where it becomes conformable to the east-dipping basal migmatites. The systematic steepening of the layering is comparable to sedimentary units formed during floor depression in syn-rift settings. The present authors suggest that the wedge-shaped plutons of the Bega Batholith are the deeper, plutonic expression of a hot, active rift. The batholith was fed and sustained by injection of magma through sub-vertical dykes. Displacement along syn-magmatic, NE-trending faults suggests up to 25 km of arc-perpendicular extension during batholith construction. The inferred tectonic setting for batholith emplacement is a continental back-arc, where modern half-extension rates of 20–40 mm yr−1 are not unusual, and are sufficient to emplace the entire batholith in xs223C1 Ma. This structural model provides a mechanism for the emplacement of some wedge-shaped plutons and is one solution to the ‘room problem’ of batholith emplace

(Received December 16 2003)

(Accepted August 24 2004)

KEY WORDS

  • Bega Batholith;
  • extension;
  • Lachlan Fold Belt;
  • pluton emplacement