Abstract

A delamination model applied to the Lachlan fold belt in eastern Australia shows that lithospheric delamination began in the Early Silurian (∼430 Ma), after crustal thickening in the Wagga-Omeo zone. The delamination induced isostatic uplift farther east to produce localized, transient extensional basins and coeval silicic magmas, which formed by partial melting of the lower crust during asthenospheric upwelling. However, the voluminous granitoid intrusions were emplaced during and after compression, reflecting reestablishment of horizontal compression associated with plate convergence. Continued sinking of the slab produced an east. ward-widening orogenic belt where the granite batholiths and associated thermally softened zones of deformation tracked the direction of delamination. Once the slab was completely detached at ∼400 Ma, the compressive stresses were transmitted across the belt, and delamination recommenced about 1000 km westward against the cratonic margin; the process was repeated at 400-370 Ma. Within 60 m.y., the two sinking slabs had converted a 1700-2000-km-wide continental margin into a 700-km-wide orogenic belt. Important effects of delamination on the upper and middle crust are (1) production of regional-scale, low-pressure metamorphic belts of limited crustal thickness; (2) widespread voluminous silicic magmatism; (3) lack of deep-crustal exposures even with high degrees of crustal shortening; and (4) partial replacement of the lower crust by a mafic underplate that cooled completely after deformation ceased.

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