Magazine article Oceanus

When Plates Collide: Convergent-Margin Geology

Magazine article Oceanus

When Plates Collide: Convergent-Margin Geology

Article excerpt

On the modern globe, Earth's tectonic plates mostly converge in deep sea trenches or collisional troughs. (See Oceanus Winter 1992/93 for a discussion of "Island Arcs, Deep-Sea Trenches, and Back-Arc Basins.) Ocean drilling has provided fundamental information about colliding-plate processes, including accretion of sediments and volcanic edifices from underthrusting to overriding plates, emplacement of rocks that have been altered by the forces at work in colliding-plate zones, and the nature of continental collisions. It has opened new avenues for comparative studies of modern and ancient earth processes. Recent plate-tectonic models indicate that many areas known as "orogenic belts," where Earth's crust has been deformed by such mountain-building phenomena as thrusting, folding, and faulting, have evolved through convergent-plate-margin processes such as formation of accretionary prisms, accretion of various exotic terranes, and the collision of arcs and continents.

Accretionary Prisms

The seafloor-spreading concept posed the question of the fate of sediments on descending oceanic plates, and the ocean drilling program offered an opportunity to study the nature of sediment deformation in the deep trenches. DSDP investigations demonstrated that oceanic plate sediments progressively adhere to the leading edge of the overriding continental plate, forming an "accretionary prism." Drilling results also show that sediments from the descending plate are underplated onto the overriding plate, apparently thickening and lifting the prism. The figure on page 96 shows seismic reflection and drilling data for the Nankai accretionary prism, where coring penetrated the incoming sedimentary sequence completely, transecting the frontal thrust, the decollement zone (zone of detachment that separates accreted and underthrust sediments), and underthrust deposits to the ocean basement. The Nankai drilling provided basic trench stratigraphy, including small-scale structural features that develop during initial deformation, and it allowed measurement of frontal thrust displacement and decollement zone thickness. In addition to clarifying the geology of initial deformation, the deep coring shows a sharp increase in porosity of mudstone across the decollement, indicating that the decollement is a zone of overpressured pore fluid.

We know from studies of orogenic belts on land that they contain large volumes of highly disrupted and deformed clastic sediments (mostly turbidites) with minor amounts of apparently interlayered basalts, cherts, and tuffs. Detailed stratigraphic work in the Shimanto belt of Japan, for example, showed an orderly sequence before disruption: oceanic basement (basalts), pelagic sediments, hemipelagic sediments with silicic tephras and muddy turbidites, and coarser grained turbidites, basically similar to that found in the Nankai Trough. Identification of such stratigraphy in the orogenic belts is a key to the recognition of ancient accretionary prisms.

Analysis of small-scale structures in the Nankai cores showed that they faithfully recorded the geophysically determined direction of plate convergence. This verification of the connection between small-scale structural development and plate motions lends a whole new level of credibility to studies that claim this correlation in ancient rocks.

Exotic Terrane Accretion and Blueschist Emplacement

Recent advances in the study of orogenic belts include discovery of many exotic geologic bodies such as fragments of oceanic plateaus or island arcs that have traveled great distances to their present position. Recent drilling in the Vanuatu forearc of the southwest Pacific (the leading edge of Fiji microplate) unequivocally demonstrates the accretion of sediments and mid-ocean ridge volcanic rocks as discrete thrust sheets that form a frontal accretionary prism.

Many orogenic belts are characterized by metamorphic rocks called blueschists that have been formed under high-pressure and low-temperature conditions. …

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