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. 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. 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 Arc Systems 81 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 82 Tectonic Settings Sea Level (a) (b) (c) 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. 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. Arc Systems 83 Foreland Foreland Basin A ( = mostly 4 ) A ( = mostly 4) Detritus A (= mostly 4 ) C ( = 2 + 3 + minor 1& 4 B ( = 3 + lesser 4 ) B ( = 3 + lesser 4 ) Fold Thrust (a) (b) (c) 4 4 3 2 1 4 3 2 1 3 1 4 3 2 2 1 4 3 2 1 Figure 3.17 Progressive unroofing of an advancing foreland thrust sheet. Modified from Graham et al. (1986). 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. 84 Tectonic Settings High-Stress Type Low-Stress Type (a) (b) Pronounced Bulge Accretionary Prism Large Earthquakes Back-arc Spreading Old Plate Pronounced Graben Structures Trap Sediments Abrasive Subsidence Steep Benioff Zone Collapse & Erosion Shallow Benioff Zon e Erosion Younge Plate Sea Level Uplift Figure 3.18 Idealized cross-sections of high- and low-stress subduction zones. Modified from Uyeda (1983). Arc Processes Seismic reflection profiling and geological studies of uplifted and eroded arc systems have led to a greater understanding of arc evolution and of accretionary prism and fore- arc basin development (Stern, 2002). Widths of modern arc–trench gaps (75–250 km) are proportional to the ages of the oldest igneous rocks exposed in adjacent arcs (Dickinson, 1973). As examples, the arc–trench gap width in the Solomon Islands is about 50 km, with the oldest igneous rocks about 25 Ma, and the arc–trench gap width in northern Japan (Honshu) is about 225 km, with the oldest igneous rocks about 125 Ma. The cor- relation suggests progressive growth in the width of arc–trench gaps with time. Such growth appears to reflect some combination of outward migration of the subduction zone by accretionary processes and inward migration of the zone of maximum magmatic activity. Subduction-zone accretion involves the addition of sediments and volcanics to the margin of an arc in the accretionary prism (von Huene and Scholl, 1991). Seismic profiling suggests that accretionary prisms are composed of sediment wedges separated by high-angle thrust faults produced by the offscraping of oceanic sediment (Fig. 3.19). During accretion, oceanic sediments and fragments of oceanic crust and mantle are scraped off and added to the accretionary prism (Scholl et al., 1980). This offscraping results in outward growth of the prism and controls the location and evolutionary patterns of overlying forearc basins. Approximately half of modern arcs are growing because of offscraping accretion. Reflection profiles suggest deformational patterns are consider- ably more complex than simple thrust wedges. Deformation may include large-scale structural mixing and infolding of forearc basin sediments. Geological evidence for such mixing comes from exposed accretionary prisms, exemplified by parts of the Franciscan Complex in California. Fluids also play an important role in facilitating mixing and meta- somatism in accretionary prisms (Tarney et al., 1991). In addition to accretion to the landward side of the trench, material can be underplated beneath the arc by a process known as duplex accretion. A duplex is an imbricate package of isolated thrust slices bounded on top by a thrust and below by a low-angle detachment fault (Sample and Fisher, 1986). During transfer of displacement from an upper to a lower detachment horizon, slices of the footwall are accreted to the hanging wall (accretionary prism) and rotated by bending the frontal ramp. Observations from seismic reflection pro- files, as well as exposed accretionary prisms, indicate that duplex accretion occurs at greater depths than offscraping accretion. Although some arcs, such as the Middle America and Sunda arcs, appear to have grown by accretionary processes, others, such as the New Hebrides arc, have little if any accretionary prism. In these latter arcs, either little sediment is deposited in the trench or most of the sediment is subducted. One way to subduct sedi- ments is in grabens in the descending slab, a mechanism supported by the distribution of seismic reflectors in descending plates. Interestingly, if sediments are subducted in large amounts beneath arcs, they cannot contribute substantially to arc magma production as con- strained by isotopic and trace element distributions in modern arc volcanics. Subduction erosion is another process proposed for arcs with insignificant accretionary prisms. It involves mechanical plucking and abrasion along the top of a descending slab, Arc Systems 85 which causes a trench’s landward slope to retreat shoreward (Fig. 3.19). Subduction ero- sion may occur either along the top of the descending slab (Fig. 3.19b) or at the leading edge of the overriding plate (Fig. 3.19c). Evidence commonly cited for subduction erosion includes (1) an inland shift of the volcanic front, as occurred in the Andes in the last 100 My; (2) truncated seaward trends and seismic reflectors in accretionary prisms and fore- arc basins; (3) not enough sediment in trenches to account for the amount delivered by rivers; and (4) evidence for crustal thinning such as the tilting of unconformities toward the trench, most easily accounted for by subsidence of the accretionary prism. All of these can be explained by erosion along the top of the descending slab. Subduction erosion rates have been estimated along parts of the Japan and Chile trenches at 25 to 50 km 3 /My for each kilometer of shoreline (Scholl et al., 1980). 86 Tectonic Settings Bedrock Framework: Crystalline Sedimentary Oceanic Crust Small Accretionary Body Sediment Subduction Forearc Basin Subduction Erosion Sediment Subduction Subduction Erosion Cratonic Massif (a) (b) (c) 0 0 0 0 0 0 50 50 50 30 30 0 km km km km 30 km 30 km 2.5:1 2.5:1 2.5:1 Figure 3.19 Sediment subduction and sediment erosion at a convergent plate boundary. Modified from Scholl et al. (1980). Accretion, mixing, subduction erosion, and sediment subduction are all potentially important processes in subduction zones, and any of them may dominate at a given place and evolutionary stage. Studies of modern arcs indicate that about half of the ocean-floor sediment arriving at trenches is subducted and does not contribute to growth of accre- tionary prisms either by offscraping or by duplex accretion (von Huene and Scholl, 1991). At arcs with significant accretionary prisms, 70 to 80% of incoming sediment is subducted, and at arcs without accretionary prisms, all of the sediment is subducted. The combined average rates of subduction erosion (0.9 km 3 /year) and sediment subduc- tion (0.7 km 3 /year) suggest that, on average, 1.6 km 3 of sediment are subducted each year. High-Pressure Metamorphism Blueschist-facies metamorphism is important in subduction zones, where high-pressure, relatively low-temperature mineral assemblages form. Glaucophane and lawsonite, both of which have a bluish color, are common minerals in this setting. In subduction zones, crustal fragments can be carried to great depths (>50 km) yet remain at rather low tem- peratures, usually less than 400° C (Fig. 2.10). A major unanswered question is how these rocks return to the surface. One possibility is by continual underplating of the accre- tionary prism with low-density sediments, resulting in fast, buoyant uplift during which high-density pieces of the slab are dragged to the surface (Cloos, 1993). Another possi- bility is that blueschists are thrust upward during later collisional tectonics. One of the most intriguing fields of research at present examines how far crustal frag- ments are subducted before returning to the surface. Discoveries of coesite (high-pressure silica phase) and diamond inclusions in pyroxenes and garnet from eclogites from high- pressure metamorphic rocks in eastern China record astounding pressures of 4.3 gigapascals (GPa, about 150-km burial depth) at 740° C (Schreyer, 1995). Several other localities have reported coesite-bearing assemblages recording pressures from 2.5 to 3.0 GPa. Also, several new, high-pressure hydrous minerals have been identified in these assemblages, indicating that some water is recycled into the mantle and that not all water is lost by dehydration to the mantle wedge. Perhaps the most exciting aspect of these findings is that for the first time we have direct evidence that crustal rocks (both felsic and mafic) can be recycled into the mantle. Igneous Rocks The close relationship between active volcanism in arcs and descending plates implies a genetic connection between the two. Subduction-zone-related volcanism starts abruptly at the volcanic front, which roughly parallels oceanic trenches and begins 200 to 300 km inland from trench axes adjacent to the arc–trench gap (Fig. 3.20). It occurs where the subduction zone is 125 to 150 km deep, and the volume of magma erupted decreases in the direction of subduction-zone dip. The onset of volcanism at the volcanic front probably reflects the onset of melting above the descending slab, and the decrease in volume of erupted magma behind the volcanic front may be caused by either a longer vertical distance Arc Systems 87 for magmas to travel or a decrease in the amount of water liberated from the slab as a function of depth. The common volcanic rocks in most island arcs are basalts and basaltic andesites; andesites and more felsic volcanics also become important in continental-margin arcs. Whereas basalts and andesites are erupted chiefly as flows, felsic magmas are commonly plinian eruptions in which much of the ejecta are ash and dust. These eruptions produce ash flows and associated pyroclastic (or hyaloclastic) deposits. Arc volcanic rocks are generally porphyritic, containing up to 50% phenocrysts in which plagioclase dominates. Arc volcanoes are typically steep-sided stratovolcanoes composed of varying propor- tions of lavas and fragmental materials (Fig. 3.21). Their eruptions range from mildly explosive to violently explosive and contrast strikingly to the eruptions of oceanic-island and continental-rift volcanoes. Large amounts of water are given off during eruptions. Rapid removal of magmas may result in structural collapse of the walls of stratovolca- noes, producing calderas such as Crater Lake in Oregon. The final stages of eruption in some volcanic centers are characterized by the eruption of felsic ash flows that may travel great distances. Seismic shadow-zone studies indicate that modern magma reservoirs in subduction-zone areas are commonly 50 to 100 km deep. The migration of earthquake hypocenters from depths up to 200 km over periods of a few months before eruption reflects the ascent of magmas at rates of 1 to 2 km/day. The cores of arc systems comprise granitic batholiths as shown in deeply eroded arcs. Such batholiths, composed of numerous plutons, range in composition from diorite to granite with granodiorite often dominating. In contrast to oceanic basalts, arc basalts are commonly quartz normative, with high Al 2 O 3 (16–20%) and low TiO 2 (<1%) contents. Igneous rocks of the tholeiite and calc- alkaline series are typical of both island arcs and continental-margin arcs. 87 Sr/ 86 Sr ratios in 88 Tectonic Settings 0 50 100 150 200 500 400 300 200 100 0 km Depth (km) WEDGEMANTLE ARC Descending Slab Volcanic Front Zone of Partial Melting Metasomatized Mantle Figure 3.20 Cross-section of a subduc- tion zone showing shallow devolatilization of descend- ing slab (short vertical lines) and magma production in the mantle wedge. volcanics from island arcs are low (0.702–0.705), and those from continental-margin arcs are variable, reflecting variable contributions of continental crust to the magmas. Arc basalts also exhibit a subduction-zone component (Hawkesworth et al., 1994; Pearce and Peate, 1995) (depleted Nb and Ta relative to neighboring incompatible elements on a primitive mantle, normalized graph; Fig. 3.8). Arc granitoids are chiefly I-types, typically meta- aluminous, with tonalite or granodiorite dominating. Compositional Variation of Arc Magmas Both experimental and geochemical data show that most arc basalts are produced by partial melting of the mantle wedge in response to the introduction of volatiles (principally water) from the breakdown of hydrous minerals in descending slabs (Pearce and Peate, 1995; Poli and Schmidt, 1995) (Fig. 3.20). Other processes such as fractional crystal- lization, assimilation of crust, and contamination by subducted sediment, however, affect magma composition. Trace element and isotope distributions cannot distinguish between a subducted sediment contribution to arc magmas and a continental assimilation. Arc Systems 89 Figure 3.21 Eruption of Mount St. Helens, southwest Washington, May 1980. [...]... which shear zones are sutures and which are not Foreland and Hinterland Basins Peripheral foreland and hinterland basins are like retroarc foreland basins in terms of sediment provenance and tectonic evolution Major peripheral foreland and hinterland basins developed in the Tertiary in response to the Alpine–Himalayan collisions (Allen and Homewood, 1986) (Fig 3. 23) These basins exhibit similar stages... chert, banded iron formation, carbonate, and— locally—shallow-water evaporites and barite These rocks, which commonly preserve 12 Epiclastic and volcaniclastic sediments with local alkaline volclanics (Pull-apart basin) Epiclastic sediments with abundant granitic provenance 6 Emerging mafic-felsic volcanoes and volcaniclastic sediments (Island arc) Submarine felsic/intermediate volcanic centers with associated... subprovinces of 1.40 to 1 .34 Ga and 1.50 to 1.42 Ga granites occur in the midcontinent region The largest and oldest anorogenic granites in this Proterozoic belt occur in Finland and Russia and date from 1.80 to 1.65 Ga (Haapala and Ramo, 1990) Large anorthosite bodies are associated with some anorogenic granites, and most occur in the Grenville province and adjacent areas in eastern Canada (Fig 3. 27) Although... alluvial fans also may be deposited in foreland basins The Himalayas As an example of a young collisional mountain range, none can surpass the High Himalayas The Himalayan story began some 80 Ma when India fragmented from Gondwana and started on its collision course with eastern Asia Collision began about 55 Ma and is still going on today Prior to collision, Tibet was a continental-margin arc system with... volcanics These changes reflect an evolution from voluminous oceanic eruptions of basalt and komatiite, commonly referred to as a mafic plain (oceanic plateau), to more localized calc-alkaline and tholeiitic stratovolcanoes (volcanic arc) that may emerge with time and to intervening sedimentary basins Archean volcanoes were in some respects similar to modern oceanic volcanoes in arc systems (Ayes and... Figure 3. 29 Generalized geologic map of the Archean Superior province in eastern Canada Modified from Card and Ciesielski (1986) Three general trends observed with increasing stratigraphic height in some late Archean greenstone successions are (1) a decrease in the amount of komatiite, (2) an increase in the ratio of volcaniclastics to flows, and (3) an increase in the relative abundance of andesitic and... rise as salt domes and trap oil and gas Oil and gas may also be Table 3. 2 Oil Reserves in Devonian and Younger Reservoirs Intracratonic Basin (%) Cenozoic Mesozoic Paleozoic Total 5 43 13 61 *14% in the Persian Gulf and 4% elsewhere Passive Margin (%) 8 5 1 14 Foreland Basin (%) 18* 6 1 25 111 112 Tectonic Settings trapped in structural or stratigraphic traps as they move upward in response to increasing... accumulate in peripheral foreland and hinterland basins, which develop in response to uplift and erosion of a collisional zone These basins and the sediments therein evolve in a manner similar to retroarc foreland basins Classic examples of peripheral foreland basins developed adjacent to the Alps and the Himalayas during the Alpine–Himalayan collisions in the Tertiary (Fig 3. 23) During the Alpine collision,... sedimentation and bimodal volcanism, as evident in the Himalayas and Tibet (Dewey, 1988) To some extent, each collisional belt has its own character In some instances, plates lock together with relatively little horizontal transport, such as along the Caledonian suture in Scotland or the Kohistan suture in Pakistan In other orogens, such as the Alps and Himalayas, allochthons are thrust considerable distances and... late Archean Abitibi belt in eastern Canada, contains several greenstone domains and hence can be considered a terrane or, more accurately, a superterrane amalgamated around 2.7 Ga (Fig 3. 28) Subprovinces in the Archean Superior province, such as the WawaAbitibi and Wabigoon subprovinces (Fig 3. 29), can be considered superterranes and represent amalgamations of greenstone terranes of various oceanic settings . zones are sutures and which are not. Foreland and Hinterland Basins Peripheral foreland and hinterland basins are like retroarc foreland basins in terms of sed- iment provenance and tectonic evolution Greenland and into the Baltic shield in Scandinavia and Russia. The granites are massive and relatively undeformed, thus the name anorogenic granite (Anderson and Morrison, 1992; Windley, 19 93) . Many. arcs. Whereas basalts and andesites are erupted chiefly as flows, felsic magmas are commonly plinian eruptions in which much of the ejecta are ash and dust. These eruptions produce ash flows and associated