UNRAVELLING THE NEW ENGLAND OROCLINE AND IMPLICATIONS FOR END PALEOZOIC TO EARLY MESOZOIC OROGENESIS ALONG THE PACIFIC MARGIN OF GONDWANA

Oroclines are map-view bends of originally quasi-linear lithospheric elements. The doubly vergent New England orocline lies within the Eastern Australian segment of the Terra Australis Orogen and developed during late Paleozoic to early Mesozoic Gondwanide orogeny that extended along the Pacific margin of Gondwana. The southern and northern segments of the orocline show counter-clockwise and clockwise rotation respectively of around 180 degrees. The orocline deformed an arc assemblage consisting of a western magmatic arc, an adjoining forearc basin and an eastern subduction complex. Assuming an original linear trend for the system, oroclinal bending has resulted in at least 50% shortening in the New England region from an original length of at least 1200 km. This has in part been accommodated by some 300 km of lateral displacement of the arc onto the Gondwana foreland. The doubly vergent nature of the orocline with the northern and southern segments related to dextral and sinistral convergent regimes respectively, has led to contrasting models of formation. We resolve these conflicting kinematic settings with a model involving buckling about a vertical axis due to northward translation of the New South Wales (New England) segment of the arc system against the Queensland segment that is pinned relative to cratonic Gondwana. Northward motion was driven by coupling between the Gondwana and Pacific plates. A new pulse of magmatic arc activity commenced around 270 Ma and formed a linear belt emplaced mostly within the pre-existing subduction complex in New England, cutting across the orocline and extending into the older forearc and magmatic arc in Queensland. The apparent eastward displacement of the magmatic arc between its pre-310 Ma and its post-270 Ma positions may reflect foreshortening and westward displacement of the upper crust of the arc system rather than a shift in the position of the subduction zone in an asthenospheric reference frame. This requires a detachment, probably thermally induced, above which oroclinal motion and foreland displacement took place. Westward motion of the orocline onto the foreland during buckling and vertical axis rotation may have been buttressed by the rigid oceanic lithosphere of the Pacific plate restricting eastward motion of the thickening arc system.