Abstract Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10–20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c . 1.5 AU and c . 1.0 AU, respectively (1 AU, or Armstrong Unit,=1 km 3 a −1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c . 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c . 60 km 3 Ma −1 km −1 ( c . 60% by subduction erosion). The corresponding global recycling rate is c . 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c . 115 km 3 Ma −1 km −1 . At a global length of 6000 km, recycling at CSSZs is accordingly c . 0.7 AU. The collective loss of CIA crust estimated for modern OMSZs and for older CSSZs is thus estimated at c . 3.2 AU. SZ additions (+2.7 AU) and subtractions (−3.2 AU) are similar. Because many uncertainties and assumptions are involved in assessing and applying them to the deep past, the net growth of CIA crust during at least Phanerozoic time is viewed as effectively nil. With increasing uncertainty, the long-term balance can be applied to the Proterozoic, but not before the initiation of the present style of subduction at c . 3 Ga. Allowing that since this time a rounded-down rate of recycling of 3 AU is applicable, a startlingly high volume of CIA crust equal to that existing now has been recycled to the mantle. Although the recycled volume ( c . 9×10 9 km 3 ) is small ( c . 1%) compared with that of the mantle, it is large enough to impart to the mantle the signature of recycled CIA crust. Because subduction zones are not spatially fixed, and their average global lengths have episodically been less or greater than at present, recycling must have contributed significantly to creating recognized heterogeneities in mantle geochemistry.

Geophysical and geological observations document that beneath the submerged forearc, processes of sediment subduction and subduction erosion move large volumes of material toward the mantle. The conveying system is the subduction channel separating the upper plate from the underthrusting ocean plate. Globally, the zero-porosity or solid-volume rate at which continental debris is shuttled toward the mantle is estimated to be ∼2.5 km 3 /yr. To deliver this volume, the average thickness of the subduction channel is ∼1.0 km. Some deeply subducted material is returned to the surface of Earth as a component of arc magma or as tracks of high- P/T crustal underplates. But over long periods of time (>50 m.y.), most of the removed material is evidently recycled to the mantle. Applying Cenozoic recycling rates to the past astonishingly implies that since 2.5 Ga a volume of continental crust equal to the standing inventory of ∼6 × 10 9 km 3 has been removed from the surface of Earth. This minimum estimate does not include crustal material recycled at continental collision zones nor reliable estimates of recycling where large accretionary bodies form. The volume of demolished crust is so large that recycling must have been a major factor determining the areal pattern and age distribution of continental crust. The small areal exposure of Archean rock is thus probably more a consequence of long-term crustal survival than the volume originally produced. Reconstruction of older supercontinents is made difficult if not unachievable by the progressive truncation of continental edges effected by subduction zone recycling, in particular by subduction erosion.

Abstract Modern convergent and strike-slip margins are crossed by six continent/ocean transects (Fig. 1). Along these margins, continental and oceanic crust are separated by a seismically active tectonic contact, which is the boundary between the North American Plate and the Pacific or Cocos Plates. Four transects cross convergent margins where oceanic crust underthrusts continental crust (A-2, B-2, B-3, and H-2). One transect (B-l) is across the Queen Charlotte Transform Fault, a part of the Pacific-North American plate boundary, which is as long as the better-known San Andreas Fault. Another transect (A-3) crosses a collision zone and the controversial transition fault zone, which is interpreted by some as a highly oblique thrust and by others as inactive during the past 5 m.y. or more, and only recently an active fault. We summarize here the tectonic interpretations of these boundaries made by the transect teams, and results from work completed after the transects were assembled. After transect compilation, the multichannel seismic records across active margins in A-2 and H-2 were reprocessed, and a new record was acquired in the B-2 transect area.

Abstract The Aleutian arc is the arcuate arrangement of mountain ranges and flanking submerged margins that forms the northern rim of the Pacific Basin from the Kamchatka Peninsula (Russia) eastward more than 3,000 km to Cook Inlet (Fig. 1). It consists of two very different segments that meet near Unimak Pass: the Aleutian Ridge segment to the west and the Alaska Peninsula- Kodiak Island segment to the east. The Aleutian Ridge segment is a massive, mostly submerged cordillera that includes both the islands and the submerged pedestal from which they protrude. The Alaska Peninsula-Kodiak Island segment is composed of the Alaska Peninsula, its adjacent islands, and their continental and insular margins. The Bering Sea margin north of the Alaska Peninsula consists mostly of a wide continental shelf, some of which is underlain by rocks correlative with those on the Alaska Peninsula. There is no pre-Eocene record in rocks of the Aleutian Ridge segment, whereas rare fragments of Paleozoic rocks and extensive outcrops of Mesozoic rocks occur on the Alaska Peninsula. Since the late Eocene, and possibly since the early Eocene, the two segments have evolved somewhat similarly. Major plutonic and volcanic episodes, however, are not synchronous. Furthermore, uplift of the Alaska Peninsula-Kodiak Island segment in late Cenozoic time was more extensive than uplift of the Aleutian Ridge segment. It is probable that tectonic regimes along the Aleutian arc varied during the Tertiary in response to such factors as the directions and rates of convergence, to bathymetry and age of the subducting

Abstract The eastern and western continental margins of the Gulf of Alaska are simple plate boundaries separating the Pacific and North American Plates. On the east is the Queen Charlotte–Fairweather transform fault system, along which the Pacific Plate moves northwest with respect to the North American Plate. On the west is the Aleutian Trench, along which the Pacific Plate converges against the North American Plate and produces a subduction zone (Fig. 1). Between the transform and convergent plate boundaries is a tectonically complicated area including the Yakutat Terrane (Jones and others, 1982), which migrated northwest on the Pacific Plate until it collided with the North American Plate. This collision occurred where the transform and thrust fault boundaries meet to form a 90° change in the trend of the plate boundary (Fig. 1). The general transform and convergent plate motion pattern has existed during the Cenozoic history of the Gulf, but variation in rate of plate motion, collision of features on the plate with North America, and subduction of oceanic crust of different age and physical properties have produced variations in the tectonic history. The tectonics produced by the subduction of thousands of kilometers of ocean crust shaped the geologic record of the convergent margin, but only the rudiments of that record are known because of the reconnaissance level of geological knowledge of the submerged margins around the Gulf of Alaska.

Abstract The eastern boundary of the Pacific basin from northern Mexico to Panama is the Middle America Trench, a morphological expression of convergence between the oceanic Cocos Plate and the North American and Caribbean Plates (Fig. 1). A transect of seismic and drill core data across the northern part of the trench off southern Mexico where the Cocos and North American Plates converge is described by Watkins (this volume). In this chapter, I describe a corridor of seismic and drill core data across the Guatemalan convergent margin. The tectonic structure off Guatemala differs from other examples along the Middle America Trench in the lack of accreted rock younger than Eocene. The unique character of the trench between Mexico and Costa Rica is seen in its morphology. North of the Gulf of Tehuantepec (Fig. 1 and Plate 1C), the continental shelf is narrow and much of the continental margin is a steep continental slope. South of the Gulf of Tehuantepec, the continental shelf is underlain by a wide fore-arc basin. At the Santa Elena Peninsula of Costa Rica, the shelf becomes narrow again, and locally the slope of the trench begins at the shore. Thus, the segment of the Middle America Trench between the Gulf of Tehuantepec and the Santa Elena Peninsula is the only one associated with a broad offshore fore-arc basin.

Abstract In 1980, A.W. Bally assembled and edited an innovative three-volume “picture and work” atlas of seismic reflection record sections. This compilation of more than 130 excellent seismic section reproductions provided much new data from the earth's subsurface. For petroleum geologists, academic scientists, and students, it is the field geologist's counterpart of outcrops and type sections. The smaller collection of seismic sections presented in this folio was inspired by Bally's atlas, and we follow his general format and philosophy. Our objective is to provide a reference series of sections for the earth sciences and a vehicle for continuing scientific dialogue relating to modern convergent margins. Our assembled sections represent a single facet of that dialogue by bringing together a series of exceptional seismic sections from the fronts of presently active convergent margins around the Pacific.

Abstract In multichannel seismic reflection record sections, clear imaging is accomplished largely by processing. The data acquired across ocean trenches, where water is from 5 to 8 km deep, the sea floor is irregular, and the geology is complex, must be processed with procedures not commonly used on data from the adjacent continental shelf. We have experimented to develop better processing with data from the eastern Aleutian Trench. One record is used here to illustrate the improvement possible. It contains a complex faulted fold at the base of the slope, and processing is made difficult by the opposing dips of faults and by the changing vergence of folds (Figure 2.1). Careful velocity analysis and a special processing sequence are needed to image the velocity-sensitive areas of these data. In our experience, clear imaging of complex structures is assured only when each step in the processing sequence is accompanied by an appropriate level of velocity analysis. Here we show the difference in imaging a complex structure in the spectrum between first-level or production processing and research-level processing.

Abstract Seismic section L5-7 crosses the outer forearc of the central Aleutian Ridge south of Adak Island, roughly along 175°W longitude. In this region the direction of underthrusting of the Pacific plate beneath that of the ridge is approximately 30° northwest of a direction normal to the regional trend of the Aleutian Island arc and its paralleling trench. Geomorphically, the ridge includes two provinces: the cresting massif of the island arc and the more deeply submerged forearc region about 4 km deep (see idealized structural section, p. 12). The forearc comprises the broad platform of the Aleutian Terrace and the flanking landward slope of the adjacent Aleutian Trench. The Aleutian Ridge is constructed of three principal rock sequences. The ridge's igneous basement—the arc massif—consists mainly of extrusive and intrusive masses and related coarse volcaniclastic rubble of Eocene age (Scholl et al., in press; Vallier et al., in press). These rocks constitute the ridge's lower series. Regional information implies that lower series rocks began to accumulate between 55 and 50 Ma (early to middle Eocene). Rapid magmatic growth of the massif waned between about 45 and 40 Ma, and by at least earliest Oligocene (37 Ma) the submerged flanks of the ridge began to be buried by a thickening blanket of volcaniclastic and pelagic deposits, sandy and silty strata that constitute the ridge's middle series. Middle series sediment continued to accumulate until roughly the end of the Miocene (5 to 6 Ma), when the structural basin of the Aleutian Terrace began to

Abstract The Shumagin segment of the eastern Aleutian convergent margin extends into the Gulf of Alaska from the Alaska Peninsula. Here the shelf is narrower and the Shumagin Islands are much smaller than islands, in the adjacent Kodiak shelf area, but many of the rocks are similar. Insular exposures are comprised of late Cretaceous sedimentary rocks metamorphosed in a low-temperature environment and once buried about 10 km deep. Paleogene rocks that crop out in the Kodiak area may continue into the Shumagin area, but they have not yet been sampled beneath the Shumagin shelf. The Paleogene rocks are cut by a major unconformity that is overlain by upper Miocene rocks in the Kodiak area. A Neogene forearc basin, Shumagin basin (Bruns and von Huene, 1977; Bruns et al., in press), contains about 2.5-km-thick strata of probable late Miocene and younger age. Landward of the basin is the Border Ranges fault (Fisher and von Huene, 1984), which probably isolates the pre-Upper Cretaceous geology of the Alaska Peninsula from that of the Shumagin shelf. The Aleutian volcanic arc forms the backbone of the Alaska Peninsula. Bristol Bay and the Bering Sea shelf constitute the backarc basin. Structurally, the Shumagin margin has all the elements associated with a major subduction zone. However, this segment differs from the central and eastern Aleutian sections (p. 10 and 20, this volume) in having no sediment ponded in the trench axis (von Huene, 1972). Seaward of the trench axis is Zodiak fan, a Paleogene deep-sea fan complex that

Abstract The seismic section across the Eastern Aleutian margin off southern Kodiak Island illustrates structure from the axis of the eastern Aleutian Trench to the edge of the Kodiak shelf. The seafloor morphology includes a flat trench axial area, a lower slope with two main steps, and a sharp topographic break marking the base of the steepened upper slope. The seismic section crosses a deep canyon in the upper slope, connected to one of the relict glacial troughs that cross the Kodiak Shelf (Hampton, 1983). The Kodiak margin is composed of the insular outcrops containing metamorphosed accretion complex of Upper Cretaceous to Eocene age, the Kodiak shelf with the Neogene Albatross basin behind a high at the edge of the shelf named Albatross bank, and the landward slope of the trench. Albatross basin is filled with upper Miocene to Recent sediment 5 km deep (Fisher and von Huene, 1980) and is floored by a subareal erosion surface across landward-tilted Eocene and Oligocene (?) strata. These strata were sampled northeast of the seismic record section at Middleton Island (Rau et al., 1977; Keller et al., 1984), on the seaward flank of Albatross basin (Herrera, 1978), and southwest of it near Sanak Island (Bruns et al., in press). Subsidence of the Miocene regional erosion surface began about 6 Ma and subsequently, about 2 Ma, Albatross bank was uplifted at least 3 km (Fisher and von Huene, 1980; von Huene et al., in press). Thus, the steep upper slope that descends from Albatross bank

Abstract The geologic history of Cenozoic sedimentary and volcanic rocks of the Oregon continental margin is interpreted as having involved episodic periods of underthrusting, transcurrent faulting, and extension between the oceanic and North American plates. The seismic sections across this margin illustrate that both compressional and extensional forces have molded the tectonic framework (Snavely et al., 1980). The Oregon Coast Range and inner shelf is floored by Paleocene to lower Eocene ridge basalt (see diagrammatic cross section), which is interpreted to represent eruptions in an elongate basin formed by rifting of the continental margin (Snavely, 1984). Oceanic islands and sea mounts were constructed on the basaltic ocean floor, and the volcanic flows and breccia that erupted from these centers intertongue with neritic to bathyal sedimentary rocks of early and middle Eocene age. Middle Eocene turbidite sandstone overlaps both the oceanic basalts and the pre-Tertiary rocks of the Klamath Mountains, suturing the Coast Range to North America about 50 Ma. Oblique convergence between the oceanic and the North American plates occurred during most of post-middle Eocene time, but periods of more head-on convergence occurred in the middle late Eocene and late middle Miocene. Sedimentation, punctuated by episodes of volcanism, was virtually continuous in the forearc basin, the axis of which lay along the present inner continental shelf. More than 7000 m of sedimentary and volcanic rocks were deposited in this basin (Snavely et al., 1980), which is located east of seismic lines 4 and 5 (see diagrammatic cross section). The principal structure

Abstract Seismic line MX-16 transects the Middle America Trench on the Pacific coast of Mexico. The bathymetric map reveals a narrow continental shelf, a steep trench slope, and an axial trench about 5 km below sea level. A large submarine canyon cuts the upper slope in the vicinity of line MX-16. This area was extensively drilled during Leg 66 of the Deep Sea Drilling Project (Watkins et al., 1982). Subduction in the area has been active intermittently during the last 100 m.y. (Karig et al., 1978). The convergence rate of the underthrusting Cocos plate relative to the overriding North American plate is inferred to be about 7 cm/yr (Minster and Jordan, 1978). Leg 66 drilling results suggest the continental margin in the area was truncated between 10 and 35 m.y.B.P., perhaps by transcurrent faulting or subduction erosion (Moore et al., 1982). A small accreted wedge subsequently developed by sediment underplating and offscraping (Moore et al., 1982). No forearc basin such as those seen on many trench slopes is evident on seismic line MX-16.

Abstract The seismic section across the Pacific margin of Central America off Costa Rica provides a structural cross section of the southern terminus of the Middle America trench. The section begins less than 10 km from the shoreline of the Nicoya Peninsula and extends for 70 km across the trench slope and trench, and into the ocean basin. Regionally, the trench shallows toward the southeast because of a large sea-floor rise, the Cocos Ridge, at the southern boundary of the Middle America trench. The Cocos and Caribbean plates are converging at about 9 cm/yr nearly normal to the margin (McNally and Minster, 1981), with earthquake activity defining a subduction zone dipping to the northeast (Burbach et al., 1984). Old oceanic crust is exposed on the Nicoya Peninsula in the form of Late Jurassic and Cretaceous volcanic and sedimentary rocks. The structural relationships within these rocks have been reported by a number of different investigators, but their exact origin remains unresolved (Dengo, 1962; Schmidt-Effing, 1979; Lundberg, 1982). The presently subducting ocean floor is of early Miocene age (Hey, 1977).

Abstract

Abstract The New Hebrides island arc strikes northward through the southwestern Pacific Ocean and parallels the trench where the Australia-India plate on the west is subducted beneath the North Fiji basin on the east. Convergence between the oceanic plate and the arc occurs in an easterly direction (N75 − E) as determined from earthquake focal mechanisms (Pascal et al., 1978; Isacks et al., 1981). However, the rate of convergence is poorly constrained because of plate motion beneath the North Fiji basin. Chase (1971) estimates an 8 cm/yr convergence rate at the New Hebrides trench, whereas Taylor et al. (1985) propose a convergence rate between 11 and 20 cm/yr. The d'Entrecasteaux Zone (DEZ) is a large submarine mountain system that extends north and east from New Caledonia to where it collides with the New Hebrides arc. Near the arc the DEZ consists of two subparallel ridges that trend east-west and show considerable topographic relief. Collision of the DEZ with the arc has apparently caused abrupt transitions from a single axial line of volcanic islands south and north of the intersection to triple chains of islands at the intersection. The Aoba basin formed within the arc's summit and lies beneath 2000 to 3000 m of water. Although summit basins are present outside of the collision zone, the Aoba basin has greater bathymetric expressions than any other summit basin. The main objective of this discussion is to describe two seismic lines—-one obtained within and one obtained outside of this collision zone—-to illustrate the contrast in structure

Abstract A 24-fold seismic line is presented that traverses the widest part of the convergent margin off the east coast of North Island, New Zealand. The seismic data were collected in December 1983 as a cooperative project between the New Zealand Department of Scientific and Industrial Research and the United States Geological Survey (USGS). Data were collected aboard the S.P. Lee . The complete line extends approximately 275 km (5100 CDP points), of which a portion across the slope (CDP 2100 1 / N 4500) is shown. A preliminary interpretation of the entire line has been published by Davey et al. (in press). The convergent margin is formed by the westward subduction of the Pacific plate under the Indian plate. Between North Island and Chatham Rise, it corresponds to the western margin of the Hikurangi Trough. Convergence between the two plates in the vicinity of the seismic line is oblique (approximately 30° from normal) and rapid (45 mm/year) and has created an imbricate-thrust accretionary prism that extends for as much as 175 km to the edge of North Island (Lewis, 1980). This accretionary prism contains well-defined trench-slope basins and intervening ridges that lie subparallel to the slope. The seismic source consisted of five Bolt air guns in a tuned array that were shot at 2000 psi manifold pressure. The gun depth was 8.5 m and the shot spacing was 50 m. The receiver system was a Seismic Engineering (SEI) Multidyne 24-fold streamer with a 100-m group interval and 100-m group length towed at an

Abstract The Nankai Trough marks the convergent boundary between the oceanic Philippine plate and the continental Eurasian plate. The trough extends from central Honshu Island, where a triple junction separates it from the Japan Trench, southwest along the platform from which the southwestern Japanese islands emerge. The direction of plate convergence is nearly normal to the regional trend of the continental slope and proceeds at about 2 mm/yr. The northern Philippine plate is covered with a 1.5- to 2-km thick sediment sequence near the area of the trough. The ocean crust flexes downward as it enters the trough, but apparently flexure is not enough to produce the normal faulting commonly found on the seaward slope of deep ocean trenches. The Philippine plate sediment sequence was sampled at DSDP site 297 (Karig et al., 1975). The sediment consists mainly of hemipelagic muds, thin layers of sand and silt, and some local sand turbidites. This sediment sequence begins to fail compressionally in front of the trough, and at the topographic trough it separates into an offscraped sequence above a major decollement and a subducted sequence below it. Approximately 1300 m of soft sediment are thus detached from the subducting plate to form an accretionary complex at the front of the Eurasian plate, whereas the lower 600 m remain attached to the oceanic plate and are subducted below the front of the margin. The structure and tectonic history of the Eurasian plate is poorly known in the area from the front of the margin

Abstract The Japan Trench off northern Honshu Island is associated with a backarc basin, a magmatic arc, a forearc basin, and a continental shelf that is submerged deeper than the usual shelf above 200 m water depth. The latest period of arc magmatism began to build the volcanic backbone of Honshu Island in late Oligocene to early Miocene time, and an intense Miocene period of volcanism produced green tuff, a thick complex of altered felsic to mafic volcanic rock interbedded with marine sediment. Other times of accelerated volcanism occurred in the Pliocene and Pleistocene (Cadet and Fujioka, 1980). The Pacific side of Honshu Island is built on an older continental framework encompassing the Kitakami massif, a large body of Mesozoic and Paleozoic rock overlain by a transgressive Cretaceous sediment sequence. In the adjacent submerged area is a deep basin filled with Cretaceous to Recent sediment (see diagrammatic section). The seaward flank of the basin is truncated by subaerial erosion as is the adjacent 35-km-wide section of Cretaceous rock that comprises the seaward part of the margin. The unconformity marking the top of the Cretaceous complex is prominent in many multichannel records across this margin, and it marks an abrupt change from rocks with a low to rocks with a high acoustic velocity. Paleogene sediment also fills part of the basin, but its seaward extent is limited. Neogene sediment with a basal Oligocene unit underlain in turn by the Cretaceous complex was penetrated at DSDP site 439 (see diagrammatic section), and geophysical