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

By
S. W. Richards
S. W. Richards
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W. J. Collins
W. J. Collins
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Published:
January 01, 2004

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 ∼ 1Ma. This structural model provides a mechanism for the emplacement of some wedge-shaped plutons and is one solution to the ‘room problem’ of batholith emplacement.

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GSA Special Papers

The Fifth Hutton Symposium on the Origin of Granites and Related Rocks

S. Ishihara
S. Ishihara
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W.E. Stephens
W.E. Stephens
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S.L. Harley
S.L. Harley
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M. Arima
M. Arima
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T. Nakajima
T. Nakajima
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Geological Society of America
Volume
389
ISBN print:
9780813723891
Publication date:
January 01, 2004

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