PHANEROZOIC CRUSTAL GROWTH IN OCEANIC TURBIDITE-DOMINATED OROGENS OF GONDWANA: INSIGHTS INTO PRECAMBRIAN CRUSTAL GROWTH PROCESSES

Convergent margins involving accretion of large turbidite fans on oceanic crust represent important sites of continental growth and recycling. Neoproterozoic-Phanerozoic orogens of Gondwana containing voluminous packages of deformed, mildly metamorphosed (sub-greenschist to lower amphibolite facies) continental or arc-derived turbidites, and belts of meta-basalt, provide insight into crustal growth and recycling in Archean greenstone belts and Proterozoic accretionary orogens. Accreted crust in turbidite-dominated orogens consists of an upper layer of recycled continental and juvenile detritus underlain by a lower layer of tectonically imbricated and underplated basaltic rocks. The Tasman Orogen, Australia, is the archetypal example of this style of orogeny in which rapid continental growth from juvenile additions took place episodically in Paleozoic time. In the Lachlan belt of the Tasmanides monotonous assemblages of turbidite and belts of greenstone, with similar depths of exposure (10-15 km), crop out over the entire width of southeastern Australia. Moderate to high P/T ratios indicate metamorphism under relatively low geothermal gradients with the exception of areas dominated by syn- to post orogenic plutons. Tasminide felsic igneous rocks are dominated by juvenile sources with variable contamination by the sediment. The Neoproterozoic-Cambrian Damara Orogen, Namibia, is similar to the Lachlan except that is contains a greater volume of post orogenic plutons, whereas the Mesozoic Rangitatan Orogen, New Zealand, is transitional to classic accretionary wedges. Structural style is dominated by steep to vertical fabrics within thick thrust sheets. Classic ophiolites are absent, but oceanic lithosphere is represented by thrust-bound belts of metamorphosed mafic igneous rock. These orogens share important elements with Archean turbidite-greenstone belts and Proterozoic accretionary orogens, including Proterozoic greenstone belts. Important differences remain including the occurrence of komatiite in the Archean belts, but these younger systems offer insight into the preservation of tectonic assemblages including ophiolites, structural style, low metamorphic grade, and neutral buoyancy for hundreds of millions of years after stabilization.