The lachlan belt of eastern Australia and Circum-Pacific tectonic evolution

There is considerable evidence that the Pacific Ocean basin has had a remarkable permanency at least throughout the Phanerozoic. The orogenic systems that have evolved around its margins are accretionary continental margin orogens and show little or no evidence of continental collisions in their evolutionary history. This is in dramatic contrast to the Circum-Atlantic and Tethyan realms which have experienced repeated openings and closures of, or successive transfer of continental fragments across, ocean areas that were relatively never large. In other words, the Wilson Cycle has dominated tectonic evolution of Atlantic and Tethyan realms, but has not been important in the Circum-Pacific. The northeastern margin of the Pacific Ocean is the North American Cordillera which is a “classic” continental margin-accretionary system dominated by a well-developed complex miogeoclinal terrace, significant fringing or “exotic” arc-trench systems and other “oceanic” accretions progressively consolidated into North America from mid-Paleozoic times, but mainly from mid-Meso-zoic times to the present. The northwestern margin of the Pacific Ocean is the collage of Asia which was produced by Tethyan tectonics, not Pacific tectonics — i.e., the progressive transfer of Gondwanaland fragments across Tethys to Baltica-Siberia. Only since the early Mesozoic have minor Pacific accretions, such as Japan, produced the present margin. The southeastern, southern, and southwestern margins of the Pacific Ocean are South America, Antarctica, and Australia, respectively. Through Paleozoic-early Mesozoic times they were joined and a very enigmatic Pacific margin orogenic system extended for 20,000 km from northwestern South America to northeastern Australia. The Lachlan Fold Belt in particular, and the Tasman belt in general, are important windows into that enigma. Lack of a well-developed through-going miogeocline is notable, and late Precambrian but mostly extensive lower Paleozoic, fairly deep-marine turbiditic and occasionally submarine volcanic facies are common along the margin, often directly juxtaposed against the cratonic interior. The tectonic evolution is dominated by prolonged histories of first late Precambrian to Late Cambrian then Early Silurian-Early Mesozoic convergent to transpressive and accretionary tectonics, often accompanied by extraordinary magmatism, which progressively consolidated a considerable “oceanic” to “quasi-continental” real estate into the Gondwanaland