EARTH AS AN EVOLVING PLANETARY SYSTEM Part 3

80 Tectonic Settings Figure 3.15 Franciscan melange near San Simeon, California. Large fragments of greenstone (metabasalt) (left) and graywacke (center) are enclosed in a sheared matrix of serpen-tine and chlorite. Courtesy of Darrel Cowan. shear zone subparallel to a subducting slab. Fragments of oceanic crust and trench sedi-ment are scraped off the descending plate and accreted to the overriding plate. Gravitational slumping may produce olistostromes or debris flows on oversteepened trench walls or along the margins of a forearc basin. Debris flows, in which clay minerals and water form a single fluid possessing cohesion, are probably the most important transport mechanism of olistostromes. Arc Systems 81 Forearc Basins Forearc basins are marine depositional basins on the trench side of arcs (Fig. 3.14), and they vary in size and abundance with the evolutionary stage of an arc. In continental-margin arcs, such as the Sunda arc in Indonesia, forearc basins can be up to 700 km in strike length. They overlie the accretionary prism, which may be exposed as oceanic hills within and between forearc basins. Sediments in forearc basins, which are chiefly tur-bidites with sources in the adjacent arc system, can be many kilometers in thickness. Hemipelagic sediments are also of importance in some basins, such as in the Mariana arc. Olistostromes can form in forearc basins by sliding and slumping from locally steepened slopes. Forearc-basin clastic sediments may record progressive unroofing of adjoining arcs, as shown by the Great Valley Sequence (Jurassic–Cretaceous) in California (Dickinson and Seely, 1986). Early sediments in this sequence are chiefly volcanic detri-tus from active volcanics, and later sediments reflect progressive unroofing of the Sierra Nevada batholith. Volcanism is rare in modern forearc regions, and neither volcanic nor intrusive rocks are common in older forearc successions. Arcs Volcanic arcs range from entirely subaerial, such as the Andean and Middle America arcs, to mostly or completely oceanic, such as many of the immature oceanic arcs in the southwest Pacific. Other arcs, such as the Aleutians, change from subaerial to partly oceanic along the strike. Subaerial arcs include flows and associated pyroclastic rocks, which often occur in large stratovolcanoes. Oceanic arcs are built of pillowed basalt flows and large volumes of hyaloclastic tuff and breccia. Volcanism begins rather abruptly in arc systems at a volcanic front. Both tholeiitic and calc-alkaline magmas characterize arcs, with basalts and basaltic andesites dominating in oceanic arcs and andesites and dacites often dominating in continental margin arcs. Felsic magmas are generally emplaced as batholiths, although felsic volcanics are common in most continental-margin arcs. Back-Arc Basins Active back-arc basins occur over descending slabs behind arc systems (Fig. 3.14) and commonly have high heat flow, relatively thin lithosphere, and in many instances, an active ocean ridge enlarging the size of the basin (Jolivet et al., 1989; Fryer, 1996). Sediments are varied depending on basin size and nearness to an arc. Near arcs and rem-nant arcs, volcaniclastic sediments generally dominate, whereas in more distal regions, pelagic, hemipelagic, and biogenic sediments are widespread (Klein, 1986). During the early stages of basin opening, thick epiclastic deposits largely representing gravity flows are important. With continued opening of a back-arc basin, these deposits pass laterally into turbidites, which are succeeded distally by pelagic and biogenic sediments (Leitch, 1984). Discrete layers of air-fall tuff may be widely distributed in back-arc basins. Early stages of basin opening are accompanied by diverse magmatic activity, including 82 Tectonic Settings felsic volcanism, whereas later evolutionary stages are characterized by an active ocean ridge. As previously mentioned, many ophiolites carry a subduction-zone geochemical signature and thus appear to have formed in back-arc basins. Subaqueous ash flows may erupt or flow into back-arc basins and form in three prin-cipal ways (Fisher, 1984). The occurrence of felsic, welded ash-flow tuffs in some ancient back-arc successions suggests that hot ash flows enter water without mixing and retain enough heat to weld (Fig. 3.16a). Alternatively, oceanic eruptions may eject large amounts of ash into the sea, which falls onto the seafloor and forms a dense, water-rich debris flow (Fig. 3.16b). In addition to direct eruption, slumping of unstable slopes com-posed of pyroclastic debris can produce ash turbidites (Fig. 3.16c). Because of the highly varied nature of modern back-arc sediments and the lack of a direct link between sediment type and tectonic setting, scientists cannot assign a distinct sediment assemblage to these basins. It is only when a relatively complete stratigraphic succession is preserved and detailed sedimentologic and geochemical data are available that ancient back-arc successions can be identified. Inactive back-arc basins, such as the western part of the Philippine plate, have a thick pelagic sediment blanket and lack evi-dence for recent seafloor spreading. Remnant Arcs Remnant arcs are oceanic aseismic ridges that are extinct portions of arcs rifted away by the opening of a back-arc basin (Fig. 3.14b) (Fryer, 1996). They are composed chiefly of subaqueous mafic volcanic rocks similar to those formed in oceanic arcs. Once isolated Figure 3.16 Mechanisms for the origin of subaque-ous ash flows (from Fisher, 1984). (a) Hot ash flow erupted on land flowing into water. (b) Ash flow forms from column col-lapse. (c) Ash turbidites develop from slumping of hyaloclastic debris. Sea Level (a) (b) (c) Arc Systems 83 by rifting, remnant arcs subside and are blanketed by progressive deepwater pelagic and biogenic deposits and distal ash showers. Retroarc Foreland Basins Retroarc foreland basins form behind continental-margin arc systems (Fig. 3.14a), and they are filled largely with clastic terrigenous sediments derived from a fold–thrust belt behind the arc. A key element in foreland basin development is the syntectonic character of the sediments (Graham et al., 1986). The greatest thickness of foreland basin sedi-ments borders the fold–thrust belt, reflecting enhanced subsidence caused by thrust-sheet loading and deposition of sediments. Another characteristic of retroarc foreland basins is that the proximal basin margin progressively becomes involved with the propagating fold–thrust belt (Fig. 3.17). Sediments shed from the rising fold–thrust belt are eroded and redeposited in the foreland basin only to be recycled again with basinward propaga-tion of this belt. Coarse, arkosic alluvial-fan sediments characterize proximal regions of foreland basins and distal facies by fine-grained sediments and variable amounts of marine carbonates. Progressive unroofing in the fold–thrust belt should lead to an “inverse” strati-graphic sampling of the source in foreland basin sediments (Fig. 3.17). Such a pattern is well developed in the Cretaceous foreland basin deposits in eastern Utah (Lawton, 1986). In this basin, early stages of uplift and erosion deposited Paleozoic carbonate-rich, clastic sediments followed by quartz- and feldspar-rich detritus from the elevated Precambrian basement. Foreland basin successions also typically show upward coarsening and thickening terrigenous sediments, a feature that reflects progressive propagation of the fold–thrust belt into the basin. Foreland Fold Thrust (a) 4 3 2 1 (b) 4 3 2 1 (c) 4 3 1 Foreland Basin A ( = mostly 4 ) B ( = 3 + lesser 4 ) A ( = mostly 4) 4 3 2 1 C ( = 2 + 3 + minor 1& 4 B ( = 3 + lesser 4 ) A (= mostly 4 ) 4 3 2 1 Figure 3.17 Progressive unroofing of an advancing foreland thrust sheet. Modified from Graham et al. (1986). 84 Tectonic Settings High- and Low-Stress Subduction Zones Uyeda (1983) suggested that subduction zones are of two major types, each representing an end member in a continuum of types (Fig. 3.18). The relatively high-stress type, exem-plified by the Peru–Chile arc, is characterized by a pronounced bulge in the descending slab, a large accretionary prism, relatively large shallow earthquakes, buoyant subduction (producing a shallow dipping slab), a relatively young descending slab, and a wide range in composition of calc-alkaline and tholeiitic igneous rocks (Fig. 3.18a). The low-stress type, of which the Mariana arc is an example, has little or no accretionary prism, few large earthquakes, a steep dip of the descending plate that is relatively old, a dominance of basaltic igneous rocks, and a back-arc basin (Fig. 3.18b). In the high-stress type, the descending and overriding plates are more strongly coupled than in the low-stress type, explaining the importance of large earthquakes and the growth of the accretionary prism. This stronger coupling, in turn, appears to result from buoyant subduction. In the low-stress type, the overriding plate is retreating from the descending plate, opening a back-arc basin (Scholz and Campos, 1995). In the high-stress type, however, the overriding plate is either retreating slowly compared with the descending plate or perhaps converging against the descending plate. Thus, the two major factors contributing to differences in subduction zones appear to be (1) relative motions of descending and overriding plates and (2) the age and temperature of the descending plate. Figure 3.18 Idealized (a) cross-sections of high- and High-Stress Type low-stress subduction zones. Modified from Uyeda (1983). Sea Level Pronounced Bulge Uplift Accretionary Prism Large Earthquakes (b) Low-Stress Type Back-arc Spreading Pronounced Graben Structures Trap Sediments Old Plate ... - tailieumienphi.vn