ACTIVE FATE OF PASSIVE MARGINS: A DEEP SEISMIC PERSPECTIVE

A fundamental tenet of the Wilson cycle is the expectation that passive margins of a nascent ocean basin will eventually encounter a convergent margin during the subsequent closure of that ocean basin. Perhaps the best known active examples of this process are a) the Himalayas, where the Tethyan margin is now intricately bound up in Himalayan megathrusting, b) the Banda Arc, where the northern Australian passive margin is being subducted beneath the Indonesian arc, and c) Taiwan, where the rifted margin of the East China sea is colliding with the Luzon arc. Ancient examples include the Appalachians of the eastern U.S. (Paleozoic) and the Slave Province of the Canadian Shield (Archean). Each of these examples have also been the focus of seismic reflection surveys which have helped define the degree to which passive margin elements have played an active role in the tectonics of collision. For example, COCORP deep reflection surveys in the Appalachians suggest that underthrust rift structures may be reactivated during collision and/or serve to nucleate splay faults in the overlying thrust belt. Collisional reactivation of former extensional and transform structures may explain the distribution of contemporary seismicity in both active (e.g. Taiwan) and supposedly inactive (e.g. eastern U.S., Ghana) regions. Passive margin lithologies carried into the convergence zone must also influence tectonics. For example, water and carbonates in the passive sedimentary accumulations presumably facilitate partial melting. Volcanic supracrustal sequences and/or deep mafic underplates of passive margins should amplify forces driving collision and help stimulate delamination. Attempts to quantify shortening and crustal thickening during collision must factor in the pre-collisional crustal geometries of rifted margins involved. In short, an understanding of the tectonics of active convergence zones requires due recognition of the role of former passive margins in the collision process.