PLATE MOTION REORGANIZATION, MANTLE WEDGE DYNAMICS, GRAVITATIONAL COLLAPSE OF CORDILLERA OROGENS AND THE FORMATION OF CONTINENTAL RIBBONS AND INTERVENING OCEANIC BASINS

100 Myr ago, along the Pacific margin of East Gondwana, the coincidence between change in plate motion and the switch from contractional to extensional tectonics suggests that a fundamental link exists between tectonic regimes at the Earth’s surface and stresses imposed by buoyant mantle wedges on Cordilleran orogens.

Plate reconstruction in a mantle reference frame shows that from 135 to 105 Ma East Gondwana moved eastward toward the Pacific-Phoenix plate. This convergence was largely accommodated by the subduction under East Gondwana, and the formation of the Zealandia Cordillera along the edge of the Australia/East Antarctic craton. From 105 to 95 Ma, as East Gondwana remained stationary, the basal traction that produced and sustained Zealandia decreased significantly allowing for oceanward gravitational collapse and the formation of metamorphic core complexes (MCC) in New Zealand and further south in Marie Byrd Land where MCC flank the West Antarctic rift system. Interestingly, from 95 to 65 Ma, extension continued with the separation of New Zealand and Campbell plateau from Antarctica and the initiation of the opening of the Tasman Sea and Ross Sea, whereas further north the Fairbank and New Caledonia basins formed leading to the formation of continental ribbons. The temporal relationship between the gravitational collapse and opening of the Tasman Sea, Ross Sea and extension in the West Antarctic rift system suggests that they are dynamically linked.

A series of 2D thermo-mechanical numerical experiments was designed to assess the effects of the buoyancy of the mantle wedge overlying a Cordillera Orogen, in a context of decrease of absolute plate velocity. Results show that stresses from the destabilization and the buoyant rise of the mantle wedge can drive trench retreat providing space for gravitational collapse of the Cordillera and the fragmentation of the active margin via the formation of continental ribbons and intervening oceanic basins.