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When plates collide: convergent-margin geology.

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. The frequent mixing of such "high-grade" metamorphic blocks with materials of lower metamorphic grade (greenschists) presents a perennial problem in accretionary tectonics. Recent ocean drilling penetration of serpentine diapirs and volcanoes in the Mariana forearc (leading edge of the Eurasian plate, in the Philippine Sea) documents intermixed blocks of mid-ocean ridge basalt and blueschist. The metamorphic grade indicates transport of the blueschists from sources 13 to 18 kilometers below the serpentine volcano and suggests accretionary processes are at work in deeper parts of the forearc. These drilling results strongly support field observations in many orogenic belts that accretion and underplating of seamounts and parts of oceanic crust occur over a range of depths.

Ridge Subduction

The effect of the collision or subduction of an active spreading center has been controversial. One can argue that oceanic highs such as spreading ridges provide a principal mechanism of ophiolite emplacement in forearc regions. It can also be inferred that forearcs record unusual thermal events. Ocean drilling in the Chile triple junction penetrated a site previously interpreted as an emplaced ophiolite and discovered, instead, evidence for near-trench in situ volcanism.

Shikoku Basin basalts recovered during Nankai Trough drilling are covered by a thick submarine pyroclastic deposit that dates to about 15 million years ago. This correlates with land geology in southwest Japan, where there is evidence of several contemporary unusual thermal events: near-trench igneous activity including gabbro and granitic rock intrusions, as well as high-temperature metamorphism. The combination of ocean-drilling results and orogenic-belt studies shows the geologic events in the forearc that are associated with the subduction of an active spreading center.

Collision Processes

Collision of major crustal features such as continents and island arcs is considered to be a principal cause of orogenesis that normally results in building mountain chains and thickening the crust. Mountain-building processes, however, are poorly understood. One approach to this problem is to study the eroded sediments that are deposited in the ocean, such as Leg 116 drilling in the Indian Ocean's Bengal fan, which was formed by Himalaya Mountain erosion as perhaps the largest sedimentary deposit in all earth history. Detailed study of heavy mineral assemblages suggests a two-phase uplift of the higher Himalayas, one during the period from 11 to 8 million years ago and the other less than 1 million years ago. Compilation of DSDP and ODP data from various places in the Indian Ocean also reveals a similar two-phase uplift pattern. The general inference of such studies is that mountain-building processes are episodic, and considerably swifter than previously thought.

Ocean Drilling Contributions to Continental Evolution

Accretion of various materials from one plate to the other is a part of the global material cycle. In early earth history, igneous rocks derived from the mantle were progressively assembled and accreted to form continental crusts. Subsequent collision of continental blocks and arcs produced mountains and yielded new sediments. As a result, sedimentary accretionary prisms became a major part of modified continental blocks. Thus ocean drilling should continue to be important not only to marine geoscientists but also to those who study continental geology.

Asahiko Taira went from Japan to Texas where, to his astonishment, everything was flat. After receiving his Ph.D. from the University of Texas at Dallas in sedimentology, he went to Kochi University in Japan, where he encountered the vertically dipping, highly deformed Shimanto accretionary prism. The Shimanto belt research led him further into the study of the deep sea. Since 1985, when he moved to the University of Tokyo, he has been in charge of Japanese ODP operations. He was co-chief scientist for the drilling he describes in Nankai Trough. His current research interest lies in the evolution of arcs and continents.
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Title Annotation:25 Years of Ocean Drilling; tectonic plates
Author:Taira, Asahiko
Date:Dec 22, 1993
Previous Article:Scientific ocean drilling and continental margins: understanding the fundamental transition from continent to ocean.
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