LINKING SUBDUCTION INITIATION, ACCRETIONARY OROGENESIS AND SUPERCONTINENT ASSEMBLY

Age relations for assembly of Gondwana and Pangea indicate that internal collisional orogenesis was synchronous with subduction initiation and orogenesis within accretionary orogens along the margin of these supercontinents.

Final assembly of Gondwana occurred between ca. 570-510 Ma, amalgamating E and W Gondwana. Interior continental collisions were synchronous with a switch from passive margin sedimentation to convergent margin activity along the Pacific margin. Timing of subduction initiation along the Pacific margin ranges from 540-550 Ma evidenced by the oldest supra-subduction zone (SSZ) plutons along the Antarctic segment of the margin. A phase of extension marked by SSZ ophiolite generation at 535-520 Ma is preserved in greenstone successions in E Australia and precedes Ross-Delamerian orogenesis between 520-490 Ma, inboard of the plate margin, and synchronous with final internal-suturing of Gondwana. Emplacement of SSZ igneous bodies continued throughout this period indicating that subduction was on-going.

Pangean assembly occurred between ca. 320-230 Ma, corresponding with the end of convergence within the Altaids accretionary orogen. At the same time major plate boundary reorganization was followed by regional orogenesis along the Pacific margin. In E Gondwana, convergent margin activity terminated at ca. 305 Ma and was replaced by extension and strike-slip activity until ca. 260 Ma. Convergence was then re-established along the plate margin, marked in E Australia by the emplacement of SSZ igneous bodies into the fossil Paleozoic accretionary prism, indicating a stepping out of the plate margin. Synchronous with this phase of plate re-adjustment was the Gondwanide Orogeny (305-230 Ma) affecting the entire Pacific margin of Pangea.

Temporal relations between interior collisional and exterior accretionary orogens during supercontinent assembly suggest a linked history related to global plate kinematic adjustments. Orogenesis in accretionary orogens occurs during ongoing convergence and must involve a transitory coupling across the plate boundary. Correspondence of coupling with, or immediately following, subduction initiation and plate boundary reorganization, suggest it may reflect plate readjustments involving increased relative convergence across the plate boundary.