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Controls of geometry, location of magmatic arcs, and of arc and back-arc regions

TIMOTHY A. CROSS Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77001 REX H. PILGER, JR. Department of , Louisiana State University, Baton Rouge, Louisiana 70803

ABSTRACT States), intra-arc extension (for example, convergence rate, direction and rate of the ), foreland absolute upper-plate motion, age of the de- Most variation in geometry and angle of and thrust belts, and Laramide-style scending plate, and subduction of aseismic inclination of subducted oceanic tectonics. ridges, oceanic plateaus, or intraplate is caused by four interdependent factors. - chains. It is crucial to Combinations of (1) rapid absolute upper- INTRODUCTION recognize that, in the natural system of the plate motion toward the and active , the major factors may interact and, overriding of the subducted plate, (2) rapid Since Luyendyk's (1970) pioneer attempt therefore, are interdependent variables. De- relative plate convergence, and (3) subduc- to relate subduction-zone geometry to some pending on the associations among them, in tion of intraplate island-seamount chains, fundamental aspect(s) of plate kinematics a historical and spatial context, their aseismic ridges, and oceanic plateaus and dynamics, subsequent investigations effects on the geometry of subducted litho- (anomalously low-density oceanic litho- have suggested an increasing variety and sphere can be additive, or by contrast, one sphere) cause low-angle subduction. Under complexity among possible controls and variable can act in opposition to another conditions of low-angle subduction, the resultant configurations of subduction variable and result in total or partial cancel- upper surface of the subducted plate is in zones. Most of these investigations have lation of the normal effects of each variable contact with the base of the overlying plate, examined cause and effect relations of bi- acting independently. the wedge of low-density is variate systems. For example, attractive but The interaction of these variables produc- replaced by subducted lithosphere, and the imperfect correlations have been reported es observed variations in geometry of sub- width of the arc-trench gap either is signifi- between convergence rates and dip of the duction zones, principally with respect to cantly increased or a magmatic arc is not inclined seismic zone (Luyendyk, 1970; Tov- angle of subduction and the depth to which developed within the overlying plate. The ish and Schubert, 1978), between the vol- oceanic lithosphere has been subducted. In fourth factor is age of the subducting ume of accreted along turn, subduction-zone geometry and its evo- lithosphere. Subduction of young litho- and the width of the arc-trench gap (Dickin- lution through time is a principal control on sphere produces two opposing tendencies: son, 1973; Karig and Sharman, 1975; Karig the space-time distribution of magmatic (1) low-angle subduction and increased arc- and others, 1976; Jacob and others, 1977), arcs, as well as other major tectonic fea- trench distance, owing to its low density; between the direction and rate of absolute tures, such as the elevation of the upper and (2) decreased arc-trench distance, owing upper-plate motion and presence or absence plate above subducted lithosphere, regional to its higher temperature. of back-arc spreading (Morgan, 1972; subsidence and consequent accommodation Two factors of secondary importance Chase, 1978a; Uyeda and Kanamori, 1979), of sediment within continental interiors, the contribute to variation in subduction-zone and between dip of the inclined seismic zone occurrence of Laramide-style tectonics and geometry and arc-trench distance. Accre- and curvature of island-arc and trench sys- foreland fold-and-thrust-belt deformation, tion of sediment in trenches depresses the tems (Frank, 1968; Tovish and Schubert, variations in the petrochemistry of sub- upper portion of the subducting oceanic 1978). duction-related , and extension in plate and causes the trench axis to migrate This report describes the major factors back-arc and intra-arc regions. seaward. Prolonged subduction thickens which control the geometry of subducted the upper plate, depresses the isotherms in oceanic lithosphere and analyzes the conse- CONTROLS OF SUBDUCTION-ZONE the subducted plate, and may create a quent variations in space-time distribution GEOMETRY broader arc. Both factors increase the arc- of magmatic arcs. We recognize four prin- trench gap. cipal factors or variables which control the To isolate the principal factors control- The four primary factors also control geometry of subduction zones, and we ling subduction-zone geometry and to de- development of other tectonic elements, believe that most of the observed variations termine their interactive effects, we have such as regional subsidence (for example, in subduction-zone geometry can be ex- examined the characteristics of contempor- the Amazon basin and a portion of the Cre- plained by the interaction among these var- ary subduction systems in which two or taceous Interior Seaway of western United iables. The major variables are relative more of the possible variables are constant

This article is included in a set of papers presented at a symposium on "Subduction of oceanic plates," held in November 1979.

Geological Society of America Bulletin, v. 93, p. 545-562, 9 figs., 1 table, June 1982.

545

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and only one or two are changing. We pre- ticular subduction systems from otherwise zone geometry and the principal effects of sent selected examples of subduction sys- systematic reations observed in bivariate each, acting independently, are summarized tems which empirically demonstrate the plots. For example, bivariate plots of con- in Table 1 and shown schematically in Fig- major controls of subduction geometry, vergence rates against inclination of the ure 1. The first four factors are regarded as how those controls interact, and some of Benioff zone show a systematic trend for primary controls, whereas the last two are the consequences of those interactions. most subduction systems, but others plot subordinant in importance with respect to Although this approach has not yielded well off the trend (for example, Tovish and their influence on the geometry of the entire quantitative measures of the various effects Schubert, 1978). Often, the source of these subduction system and on other geologic related to isolated or combined controlling discrepant values is related to other major and tectonic responses to subduction pro- factors, it does provide estimates of the rela- factor(s), such as subduction of an aseismic cesses. We propose that the four major con- tive importance among the factors of each ridge, modifying the effect of relative con- trols and their effects constitute a general subduction system. Further, it provides a vergence rates. empirical model for understanding and means of understanding departure of par- The factors which control subduction- interpreting the geometries, kinematics, and

TABLE 1. FACTORS AFFECTING THE GEOMETRY OF SUBDUCTION ZONES

Factor Possible effects Contemporary examples Associated phenomena

A. Convergence rate Increased rate decreases angle of Trans-Mexican Increased rate increases down-dip length of subduction, depresses isotherms, and in- (COCO-NOAM). (3-5) clined seismic zone. (6, 7) increases width of arc-trench gap (1-4) B. Absolute motion of Increased motion toward the trench de- Trans-Mexican volcanic belt Rapid overriding and low-angle subduction upper plate creases angle of subduction. Arc-trench (COCO-NOAM) versus Cen- creates compressional stress regime in upper separation either increases or the arc is ex- tral American arc (COCO- plate; crustal shortening (Cordilleran or Lara- tinguished and a new arc develops 600 to C'ARB). (3, 4, 5, 10) mide style) results. Retrograde motion creates 1,000 km inland from the trench. Slow or extensional stress regime in upper plate; retrograde motion permits steeper subduc- back-arc and/or intra-arc extension results. tion and seaward migration of the trench. (3, 4, 11-15) (3, 4, 8, 9) C. Subduction of aseis- Reduced average density and consequent Aseismic ridges: Nazca and Buoyant lithosphere resists subduction. In- mic ridges, intraplate relative buoyancy of lithosphere reduces Cocos Ridges. (4) creased area of interface increases coupling island-seamount subduction angle. Very low-angle subduc- between upper and lower plates. Compres- chains, or oceanic tion is common. is extin- I ntraplate seamount chains: sional stress regime usually is produced in plateaus guished, but a new one may form 600 to Juan Fernandez Ridge, Louis- upper plate, and -rooted thrusting 1,000 km inland from the trench. ville Ridge, and Kodiak-Bowie (Laramide style) may result. Isostatic subsi- (4, 16-18) seamount chain. (4) dence above subducted ridge creates peri- cratonal basins. (19) If absolute upper-plate motion is retrograde, back-arc spreading rate is retarded. D. Age of descending Young lithosphere is relatively buoyant Trans-Mexican volcanic belt Subduction of young lithosphere generally plate and subducts at reduced angle. In various (COCO-NOAM). (3-5, 24) results in back arc and intra-arc compression. combinations with other factors, subduc- Subduction of old lithosphere generally re- tion of young lithosphere will cause Andean arc (NAZC-SOAM sults in back-arc and intra-arc extension. to migrate trenchward, land- and ANTA-SOAM). (4) Down-dip length of inclined seismic zone ward, or cease entirely. (4, 20-24) decreases with decreasing age. (4, 7, 20, 21) Sandwich arc (SOAM-SCOT). E. of sediment Flattens the inclined seismic zone at Circum-Pacific and northern Weight of accretionary depresses in trenches shallow levels only. Arc-trench separation Indian arcs. (25, 26, 28) oceanic plate prior to subduction. (27) is increased by seaward migration of the trench (25-28) F. Duration of subduc- Additive effects of accretion (E.) and Circum-Pacific arcs. (3) Thickens upper plate? (29) tion and age of arc(?) of isotherms due to prolonged subduction of old lithosphere increases the arc-trench separation. (29, 30)

References: (1) Luyendyk, 1970; (2) Tovish and Schubert, 1978; (3) Cross and Pilger, 1978a ; (4) this report; (5) Molnar and Sykes, 1969; (6) Isacks and others, 1968; (7) W. J. Morgan, cited in Deffeyes, 1972; (8) Hyndman, 1972; (9) Moberly, 1972; (10) Jordan, 1975; (11) Burchfiel and Davis, 1975; (12) Brewer and others, 1980; (13) Morgan, 1972; (14) Chase, 1978a; (15) Uyeda and Kanamori, 1979; (16) Kelleher and McCann, 1976, 1977; (17) Pilger, 1977, 1981;(18) Isacks and Barazangi, 1977; (19) Cross and Pilger, 1978b; (20) Molnar and Atwater, 1978; (21) England and Wortel, 1980; (22) DeLong and Fox, 1977; (23) Pilger and Henyey, 1979; (24) Truchan and Larson, 1973; (25) Karig and others, 1976; (26) Karig and Sharman, 1975; (27) Worzel, 1976; (28) Jacob and others, 1977; (29) James, 1972; (30) Dickinson, 1973.

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dynamics of modern and ancient subduc- modeling of the load applied by accreted prisms are formed only when voluminous tion systems and their associated magmatic material, Karig and others (1976) concluded are carried to or are otherwise arcs. Thus, low-angle subduction results that the geometry of the upper, shallow available on the subducting ocean floor. from combinations of: (1) rapid absolute segment of the inclined seismic zone is con- Inasmuch as accretionary loading affects upper-plate motion toward the trench, (2) trolled by the load of the accretionary the subducting plate only in the region rapid relative plate convergence, (3) subduc- prism on the subducting plate. Accretion- between the inner slope of the trench and tion of anomalously low-density oceanic ary processes are dependent on the relative the upper-slope discontinuity (Karig and lithosphere, and (4) subduction of young balance between rates of convergence and others, 1976), the geometry and dip of the oceanic . Ancillary effects of low-angle sediment supply. Under rapid-convergence remainder of the subducting plate are inde- subduction include landward displacement situations, which independently induce flat- pendent of the affects of loading. Through of the magmatic arc or cessation of sub- tening of the Benioff zone, accretion is rapid this reasoning, we regard accretionary load- duction-related magmatism, compressional and is principally tectonic in origin. Under ing as subordinate to the other four factors tectonics within and behind the arc, and slow-convergence situations, tectonic accre- in controlling the geometry of the entire widespread subsidence in pericratonal re- tion is slow, but sedimentary filling or subduction zone. gions. Normal or steeper subduction results accretion is variable and depends on the The second factor which may act as a from combinations of: (1) slow or retro- rate of sediment supply. Hamilton (1978, subordinate control of subduction-zone grade absolute upper-plate motion, (2) slow 1979) observed that large accretionary geometry is that of prolonged subduction of relative plate convergence, and (3) subduc- tion of normal-density and old oceanic PRINCIPAL CONTROLS ON GEOMETRY OF SUBDUCTED LITHOSPHERE lithosphere. Ancillary effects of steeper subduction include development of a mag- CONVERGENCE RATE ABSOLUTE MOTION OF UPPER PLATE matic arc close to the trench and exten- sional tectonics within and behind the arc. |-f-150 - 600-H FAST I KM The two factors, load of an accretionary FAST prism and prolonged subduction, that we regard as subordinant to the other four have been suggested by others and are reviewed briefly here. In a review of arc-trench systems, Karig and Sharman (1975) noted an empirical correlation between the width of the arc- trench gap and the volume (width) of accreted sediment and slices of along the inner margin of the trench. This correlation corresponds to a flattening of the inclined seismic zone at shallow depths between the trench axis and the front of the volcanic arc. Karig and others (1976) demonstrated that the width of this flat- SUBDUCTION OF ASEISMIC RIDGES AGE OF OCEANIC LITHOSPHERE tened portion of the inclined seismic zone is proportional to the arc-trench separation OLD MOO . 150 KM-^j for most subduction systems. Several in- vestigators have suggested or inferred that accretion of sediment and slices of oceanic crust along the inner trench slope loads and depresses the subducting plate, reduces the angle of subduction in the shallow section of the Benioff zone, and causes the trench axis to migrate seaward; the direct result is an increase in the arc-trench separation (for example, Karig and Mammerickx, 1972; Burk, 1972; Dickinson, 1973; Hamilton, 1973; Seely and others, 1974; Karig and Sharman, 1975; Jacob and others, 1977; Hamilton, 1978). Expanding upon these suggestions, Worzel (1976) showed that the Figure 1. Schematic illustration of the subduction model, showing the effects of each weight of the accretionary prism was suffi- major control acting independently. Drawn approximately to scale. Only subduction of cient to depress the subjacent lithosphere oceanic lithosphere beneath continental lithosphere is considered. Solid triangles indicate isostatically. From observed geological and positions of volcanic arcs. "T" indicates position of trench axis. Lengths of arrows are geophysical data and from mathematical proportional to velocities of plates in a direction normal to strikes of trenches.

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oceanic lithosphere. James (1971) suggested ing controls on subc uction-zone geometry. lute and relative components, the two con- that prolonged subduction of cold oceanic Acting independently, convergence-rate trols are always interdependent and only lithosphere depresses isotherms within the variation is presumed to control the dip of rarely can their independent effects on subducting plate and the overlying . the inclined seismic zone and, consequently, subduction-zone geometry be ascertained. Consequently, the zone of magma genera- the arc-trench separation in the following In contrast to relative plate motions, tion is displaced downward and laterally manner. Oceanic lithosphere is metastable which are described by angular rotations away from the trench, such that it underlies because it is denser than the underlying about geometrically constructed poles of portions of the overlying plate at progres- asthenosphere. Under constant conditions relative motion, absolute plate motions are sively greater distances from the trench and of slow relative convergence, gravitational described relative to some external (inde- a broader arc is created. James also inferred, force predominates, the plate descends at a pendent) reference, such as the Earth's cen- and Dickinson (1973) suggested, that this steep angle, and the separation of the vol- ter or axis of rotation, which is presumed effect may be augmented by gradual thick- canic arc and trench is small. By contrast, fixed or which is stationary compared to ening of the overlying plate through accre- fast relative convergence results in a lesser relatively rapid motions of lithospheric tion of subduction-related volcanic and component of gravitational sinking, smaller plates. A common way of determining plutonic rocks. A crude, empirical correla- angle of subduction, wider spacing and vectors (or angular rotations) of absolute tion between crustal thickness and separa- depression of isotherms, and wider arc- plate motion is with respect to the hot-spot tion of the arc and trench, as well as trench separation. reference frame, assuming it is stationary duration of subduction and arc-trench sepa- One region that exhibits the effects of over tens of millions of years, relative to ration, was shown by Dickinson (1973). We variation in relative convergence rates is rapid motions of lithospheric plates (for have not been able to extend these observa- along the Middle American trench where example, Minster and others, 1974; Molnar tions beyond an empirical level nor to the is siubducting beneath the and Francheteau, 1975). One can also incorporate the possible effects of pro- (Fig. 2). Relative regard material in the asthenosphere as longed subduction into the subduction convergence rates increase from ~ 5.5 cm/yr moving diffusely and more slowly than the model for two principal reasons. First, as in the north to ~7.0 cm/yr in the south overlying lithosphere (Jacoby, 1970; Elsas- Dickinson (1973) noted, it is usually diffi- along the trench, a. consequence of close ser, 1971). In this case, the deep mantle is cult to determine whether increased arc- proximity to the relative-motion pole (Min- nearly stationary, and motion of lithosphere trench separation is the result of accretion ster and others, 1974). Absolute motions of relative to the deep mantle may be defined (seaward trench motion), prolonged sub- the North American and Cocos plates are as absolute plate motion. duction (continentward arc motion), or of a almost invariant along the subduction Along a convergence boundary, absolute combination of the two. Second, and more boundary. The age of the Cocos plate motions of the adjacent plates may aug- importantly, these empirical correlations increases from north to south along the ment, leave unchanged, or oppose the phys- are necessarily derived from study of older trench. As predicted by the subduction ical effects on subduction-zone geometry and often inactive subduction systems. In model, the arc-trench distance increases induced by relative convergence rates. In these instances, it is not possible to un- progressively from north to south, corres- order that the absolute motion of a plate equivocally demonstrate that the arc-trench ponding to increasing relative convergence may influence or control subduction-zone separation is the product of prolonged sub- rates. The influence of age variation of the geometry independently of relative plate duction plus accretion, or of another cause, Cocos plate on subduction-zone geometry motion, the lithosphere must not be coupled such as angle of subduction, which is con- and arc-trench distance is discussed subse- directly to asthenosphere flow, at least trolled by the other four principal factors. quently. At this stage, it is sufficient to note along a wide margin parallel to the conver- that increased arc-trench distance associated gence boundary. This assumption is implicit with subduction of progressively younger Relative Convergence Rates in our analysis. The relative influence or and relatively more buoyant oceanic lithos- dominance of the two controls is most phere is not observed along the Middle Luyendyk (1970) first proposed an inverse directly related to the absolute motion of American trench. correlation between convergence rate and the upper plate. As noted by Solomon and angle of subduction. This relation was others (1975), all subducting plates advance based on examination of four arc-trench Absolute Plate Motion toward the trench in an absolute-motion systems in the western Pacific. More re- frame, whereas upper plates move at vary- cently, Tovish and Schubert (1978) demon- Absolute plate motion, particularly that ing rates and directions toward and away strated that, when more arc-trench systems of the upper plate, is a second major control from convergence boundaries in the ab- were included in such an analysis, the inverse of subduction-zone geometry. It is not read- solute-motion frame. On the basis of this relation between convergence rate and in- ily apparent that absolute motion of one difference, we argue that the absolute clination of the subducting plate was exhi- or more plates can be treated or expressed motion of the upper plate dominates over bited by some arc-trench systems, but other differently than relative motions of the same relative convergence rates in controlling the systems did not conform to the anticipated plates. Nor is it readily apparent that abso- geometry of subduction zones. relation. In addition, they observed that lute motions of two plates along a conver- Absolute upper-plate motion controls the dips were constant throughout the length of gence boundary produce physical changes geometry of subduction zones dynamically each of five subduction zones, although the in the configuration of the subduction zone by modifying the effects induced by gravita- convergence rates varied by as much as 2.7 that are conceptually independent of the tional sinking of subducting lithosphere. cm/yr along a single system. These excep- configuration induced by relative motions, Rapid absolute upper-plate motion toward tions to the general relation observed by that is, by relative convergence rates. Be- the trench causes active overriding of the Luyendyk (1970) are important in that they cause motions of lithospheric plates along trench and the subduction zone, reduced reflect the interactions of the other remain- any plate boundary always have both abso- influence of gravitational sinking of the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/6/545/3434514/i0016-7606-93-6-545.pdf by guest on 02 October 2021 Figure 2. Plate-tectonic setting of Middle America, showing absolute motion of the faulting, positions of active volcanoes (A), and Quaternary volcanic rocks (stippled North American and Caribbean plates and the relative motion of the North American, pattern) are indicated. Age of Cocos plate in millions of years, as interpreted from Caribbean, and Cocos plates, as determined by Minster and others (1974) and Jordan magnetic anomalies, also shown. (1975). Plate boundaries modified from Molnar and Sykes (1969). Area of normal

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subducting lithosphere, and consequent Chase, 1978b). As discussed previously, the nic crust beneath aseismic ridges reduces formation of a shallowly inclined subduc- arc-trench distance increases progressively the average density of the lithosphere tion zone (Hyndman, 1972; Moberly, 1972). from north to south in Mexico. A minor such that lithosphere bearing an aseismic The major consequences are landward dis- gap in active arc volcanism and a substan- ridge is no longer gravitationally unstable. placement of the volcanic arc or, in some tial offset of the volcanic arc toward the Consequently, as oceanic lithosphere bear- cases, cessation of magmatism along the trench occur at the boundary between the ing an aseismic ridge or former arc and development of a new arc North American and Caribbean plates (Fig. encounters a convergence boundary, it is several hundred kilometres away from the 2). Southward from that boundary, the vol- buoyed up in the subduction zone, and trench (Cross and Pilger, 1978a). With slow canic arc in is parallel to low-angle subduction ensues. Alternatively, absolute upper-plate motion toward the the trench, and the arc-trench separation is the ridge may accrete to the upper plate and trench, gravitational sinking of the subduct- narrow and constant. The sudden narrow- a new subduction zone may develop out- ing lithosphere predominates and results in ing of the arc-trench gap reflects contrasting board of the ridge. Because aseismic ridges a more steeply inclined subduction zone, absolute motions of the North American are the same age as surrounding oceanic oceanward migration of the trench relative and Caribbean plates. The subduction crust, little or no thermal contrast exists to the upper plate, and a narrow arc-trench model predicts shallow subduction of the between the ridges and adjacent oceanic separation (Moberly, 1972). Retrograde Cocos plate beneath North America where crust, and the buoyancy of ridges relative (away from the trench) absolute motion of the North American plate is overriding the to adjacent crust is unlikely to be age- the upper plate exerts little or no control on trench. Conversely, the Cocos plate should dependent. It is the difference in average the geometry of the subduction zone, and descend at a steeper angle beneath the density of lithosphere bearing aseismic the angle of descent is determined by the where the upper plate is ridges versus that of normal lithosphere that interaction of the other major controls and nearly stationary in the absolute-motion is critical in assessing the relative buoyancy gravitational sinking of the subducted litho- frame. Observations of foci dis- of the ridge. sphere. The contrast between generally tributions in this region support these pre- It is difficult to predict how deeply the steeply dipping, westward-verging subduc- dictions; the inclination of the seismic zone buoyancy effect should persist. Experimen- tion zones of the western Pacific and mod- is steeper beneath the Caribbean plate than tal evidence, summarized by Ahrens and erately dipping, eastward-verging subduc- beneath the North American plate (Molnar Schubert (1975), suggests that oceanic crust tion zones of the eastern Pacific (for and Sykes, 1969, Figs. 10 and 11). should transform to much denser in example, Isacks and Barazangi, 1977) may the depth interval of 40 to 80 km. Such a reflect control by differences in absolute Subduction of Aseisimic Ridges, Oceanic transformation of anomalously thick crust upper-plate motions. In the western Pacific, Plateaus, and Island-Seamount Chains should increase the density contrast between upper-plate absolute motion is retrograde oceanic lithosphere and asthenosphere and or highly oblique with respect to strikes of Aseismic ridges, oceanic plateaus, and should reinforce the gravitational instability trenches. By contrast, in the eastern Pacific, intraplate island-seamount chains of var- of the descending plate. Assuming that this the North and South American plates are ious origins stand as topographic promi- transformation occurs, the experimental overriding the trenches in the absolute- nences or as broad, topographically positive work predicts that aseismic ridges and motion frame (Morgan, 1972; Minster and areas relative to surrounding oceanic crust. oceanic plateaus should subduct at steeper others, 1974; Chase, 1978b). By virtue of the anomalous crust and angles than normal oceanic plates, owing to The convergence boundary along Mexico mantle which underlie them, they are likely their greater average density. This predic- and Central America provides a rare exam- to produce anomalous effects on the con- tion differs from observed relations; we ple where the effects of upper-plate absolute figuration of subduction zones when sub- consider possible resolutions of this discre- motion are distinguishable from the other ducted. pancy subsequently. major controls on the geometry of subduc- Aseismic ridges apparently form syn- In contrast with aseismic ridges, island- tion zones. Along the Middle American chronously with adjacent oceanic crust, as seamount chains form in an intraplate trench, the Cocos plate subducts beneath indicated by DSDP data (Sclater and position subsequent to formation of sur- two plates, the North American and the Fisher, 1974), subsidence curves (Detrick rounding lithosphere and in association Caribbean (Fig. 2). In the vicinity of the and others, 1977), isostatic gravity anomo- with a thermal anomaly of uncertain origin three-plate boundary, the age of the Cocos lies (Kogan, 1979; Detrick and Watts, 1979; (for example, Morgan, 1972; Jarrard and plate is essentially constant (Herron, 1972) Watts and others, 1980), and plate recon- Clague, 1977; Watts and others, 1980). As and the rates of relative convergence be- structions (Pilger and Handschumacher, a consequence of their intraplate origin, tween the Cocos-North American and 1981). Kelleher and McCann (1976, 1977) island-seamount chains lack a substantially Cocos-Caribbean plate pairs are compara- reviewed evidence supporting the presence thickened crustal root; any increase in crus- ble. Of the four major controls on subduc- of anomalously thick crust or "structural tal thickness of such chains is confined to tion-zone geometry, only the absolute roots" beneath aseismic ridges. Subsequent the volcanic edifice built on the floor. motions cf the upper plates are radically dif- studies, based on gravity, seismic refraction, The thermal anomaly responsible for the ferent. Whereas, in an absolute-motion and isostatic modeling, have recognized chains also produces a decrease in density, frame, North America is overriding the that anomalously thick crust is characteris- thickness, or both, of the lithosphere be- Middle American trench (for example, tic of aseismic ridges and oceanic plateaus neath young chains. This response is mani- Minster and others, 1974), the Caribbean (Kogan, 1979; Detrick and Watts, 1979). fested in a distinctive thermal of plate is moving slowly subparallel to and Kelleher and McCann (1976, 1977) sug- lithosphere adjacent to individual edifices slightly away from the trench (Jordan, 1975; gested that the increased thickness of ocea- (Crough, 1978; Detrick and Crough, 1978).

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Consequently, lithosphere containing an more buoyant than normal lithosphere. Subduction of aseismic ridges produces the island-seamount chain has an average den- However, all aseismic ridges and intraplate most dramatic effects on the configuration sity lower than that of normal oceanic island-seamount chains are not equally of subduction zones in the eastern Pacific, lithosphere and is relatively more buoyant. buoyant. Differences in relative buoyancy where lithosphere generally is young, where The thermal anomaly associated with the arise from differences in absolute age, con- aseismic ridges are continuous and well formation of an island-seamount chain trast in age with respect to adjacent lithos- developed, and where upper plates are decays with age of formation of the chain phere, size, and crustal thickness. Upon overriding the subduction zone in the (Detrick and Crough, 1978). Thus, litho- subduction, the buoyancy effect for both subduction zone in the absolute-motion sphere bearing island-seamount chains in- normal oceanic lithosphere and lithosphere frame. By contrast, in the western Pacific, creases in thickness, average density, and which includes an island-seamount chain is where lithosphere generally is old, where gravitational instability as a function of age likely to be age dependent. That is, old aseismic ridges are thinner and discontinu- from the time of chain formation in a lithosphere and old island-seamount chains ous, and where upper plates are retreating manner similar to that observed for litho- have a less pronounced buoyancy effect from trenches in the absolute-motion frame, sphere generated at spreading ridges (Parker than young lithosphere and young island- the effects of aseismic-ridge subduction are and Oldenburg, 1973). seamount chains. This relation conforms limited to retarded back-arc spreading and Despite their different origins, both with tentative conclusions that young to slight decrease in the inclination of the aseismic ridges and intraplate island-sea- oceanic lithosphere subducts at lower subducting plate (Vogt and others, 1976; mount chains represent density anomalies angles than old lithosphere (DeLong and Isacks and Barazangi, 1977). These rela- in oceanic plates, and both are relatively Fox, 1977; Molnar and Atwater, 1978). tionships are considered in the following analyses of four subduction systems. 80° 700 W Nazca and Juan Fernandez Ridges. Stu- dies of earthquake-hypocenter distribution have demonstrated that the is subducting beneath South America at angles of 25° to 30° in the intermediate to deep zone along most of the convergence boundary (Isacks and Molnar, 1971; Stau- der, 1973, 1975; Barazangi and Isacks, 1976, 1979; Isacks and Barazangi, 1977). Except for two conspicuous breaks, Quaternary stratovolcanoes form a continuous volcanic chain along the western margin of the above what Barazangi and Isacks (1976) interpreted as the line of contact between the upper surface of the descending Nazca plate and the overlying wedge of par- tially molten asthenosphere. The gaps in recently active volcanism occur in northern and central Peru and central Chile and coincide with positions of very low-angle (< 10°) subduction of the Nazca plate in the intermediate depth interval (Fig. 3; Bara- zangi and Isacks, 1976). Barazangi and Isacks (1976) attributed the absence of vol-

Figure 3. Peru-Chile subduction zone. Contours (in kilome- tres) on top of inclined seismic zone and location of active volcanoes from Barazangi and Isacks (1976) and Isacks and Barazangi (1977). Focal-mechanism solutions of shallow within the from Stauder (1973, 1975). All indicate horizontal, east-west compression. Centers of shaded quadrants are the P axes; centers of unshaded quadrants are T axes. Thin solid line is 3-km topo- graphic contour; note inverse correlation between width of Andes and dip of the inclined seismic zone.

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canism in these two regions to displacement The major factors responsible for the two tions of the beneath South of the wedge of partially melted astheno- very low-angle subduction segments along America and demonstrated that the very sphere by the very low-angle subducting the Peru-Chile trench, as well as the asso- low-angle subduction segment and other lithosphère and consequent direct superpo- ciated gaps in Quaternary volcanism along geologic features in Peru correspond in time sition of two lithospheric plates. At the sites the Andean chain, are subduction of the and space to progressive subduction of the of very low-angle subduction, the Andes Nazca Ridge opposite southern Peru and Nazca Ridge (Fig. 4). Thus, we conclude form a narrow, near-coastal range the Juan Fernandez seamount chain oppo- that thickened oceanic crust beneath the instead of a broad, high plateau (for exam- site central Chile. Barazangi and Isacks Nazca Ridge and consequent reduction in ple, the Altiplano of the central Andes). (1976) and Isacks and Barazangi (1977) average lithospheric density has caused low- Earthquakes with compressional focal noted the correspondence between subduc- angle subduction beneath Peru. mechanisms within the western South tion of the Nazca ridge and the anomalous In addition, Pilger (1981) showed that if American plate are restricted to the sites of low-angle subduction segment beneath an off-ridge "hot-spot" origin for the Juan very low-angle subduction and are absent or Peru. Pilger (1977, 1981) expanded upon Fernandez seamount chain is assumed, the undetected in segments of steeper (-30°) and documented this observation and sug- plate reconstructions place the subducted subduction (Fig. 3; Stauder, 1973, 1975). gested that this correspondence reflected portion of that chain in the zone of very Additional evidence for compressional a fundamental geneti.c relationship. He sug- low-angle subduction beneath central Chile. stress in overlying a low- gested that the Nazca and Tuamotu Ridges Etuoyancy of the lithosphere beneath the angle subduction segment occurs 500 km were generated contemporaneously by a Juan Fernandez Ridge, owing to its relative east of the Peru-Chile trench where the single source, such ELS a "hot spot" (Easter youth, is proposed as the mechanism pro- Argentine Pampean ranges comprise a Island of Morgan, 1972) or ducing the central Chilean low-angle sub- reverse-block-faulted of late Ceno- other fundamental process, centered on or duction segment. zoic age (Fig. 3; Stoll, 1964). near the symmetrically spreading Nazca- There is an apparent difficulty with the Pacific Ridge. Pilger concluded that the It is possible to qualitatively resolve the buoyancy hypothesis for aseismic ridge geometry of the subducted portion of the relative contributions of those factors which subduction that requires resolution. In the Nazca Ridge could be reconstructed by control the geometry of the subducted case of the Nazca Ridge, we observe that assuming it was initially a mirror image of lithosphere beneath western South Amer- low-angle subduction extends to a depth of the Tuamotu Ridge. Using plate reconstruc- ica, particularly the configuration of the 150 km, well in excess of the 40- to 80-km tions and the assumption of initial mirror two anomalous, very low-angle subduction depth at which the transformation of crust imagery, Pilger mapped the successive posi- segments. Relative convergence rate is es- to eclogite is expected from experimental sentially constant at ~11 cm/yr along the South A'merican-Nazca plate boundary (Minster and others, 1974), as is the age of the Nazca plate (about 41 to 46 m.y. B.P. at the Peru-Chile trench between 35° S and 15° S latitudes; Pitman and others, 1974). Absolute motion of each plate is constant, with South America moving toward the trench at about 2.2 cm/yr and the Nazca plate advancing toward the trench at about 10° s 9.3 cm/yr (Minster and others, 1974; Chase, 1978a, 1978b). Moderate-angle (-30°) sub- duction characteristic of most of the con- vergence boundary, in all probability, is - 20° attributable to the effects of rapid conver- gence, overriding of the trench by the South American plate, and the relative youth of the subducting Nazca plate. Other factors which may affect subduction-zone geometry on a global scale also may contribute to moderate-angle subduction along South America in comparison to generally steeper inclinations of subduction zones having westward vergence. These include hydrody-

namic forces (Jischke, 1975), and tidal drag 1 30° 60° W (Bostrom, 1971; Nelson and Temple, 1972; Figure 4. Reconstruction of the relative positions of the Nazca and South American Bostrom and others, 1974), although Jor- plates at 0,4.2,9,19.5, 25.5, and 35.6 m.y. B.P. (corresponding magnetic-anomaly numbers dan (1974) has argued that tidal forces are shown by numerals iin parentheses). Outline of subducted portion of Nazca Ridge is shown inadequate to significantly influence plate by stippled pattern, assuming mirror-image symmetry with the Tuamoto Ridge. Modified motions. from Pilger (1981).

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data (Ahrens and Schubert, 1975). The tively, the absence of subducted oceanic approximately constant from about 15 m.y. absence of steeper subduction in the vicinity crust would preclude of B.P. in the northwest to about 20 m.y. B.P. of the Nazca Ridge projection indicates, by amphibolitic or eclogitic material, a mecha- in the southeast, the region of principal contrast, that either the transformation to nism that Green and Ringwood (1968) and concern (Fig. 5; Hey and others, 1977). Hey eclogite has not occurred or that the basaltic Marsh (1976) have proposed for generation (1977) and Hey and others (1977) described component, which presumably would trans- of andesitic magma along "normal" subduc- the evolution of the Cocos-Nazca spreading form to eclogite, is absent from the sub- tion zones. ridge and of the Cocos plate. They con- ducted segment. The first postulation might Cocos Ridge. The subduction boundary cluded that the Cocos Ridge was generated be explained by low initial abundance of separating the Caribbean and Cocos plates by a "hot-spot" or other fundamental pro- volatiles within the oceanic crust which are provides another example of the influence cess centered on the Cocos-Nazca spreading necessary to catalyze the transformation. of a young aseismic ridge on the configura- center. Thus, the Cocos Ridge is the same Alternatively, the crust of the aseismic ridge tion of the subducting plate. Absolute age as the relatively young Cocos plate. The may be sheared away from the mantle por- motions of the two plates were discussed subduction model predicts that the Cocos tion of the descending plate at shallow previously; relative convergence rate along plate should be relatively buoyant where the depths. Such an event was suggested by Nur the boundary is approximately constant at Cocos Ridge intersects the Central Ameri- and Ben-Avraham (1981) in another con- about 7.5 cm/yr (Jordan, 1975). The age of can trench and that it should subduct at a text. If phase transformation of basaltic the Cocos plate as it enters the trench is much reduced angle; magmatism should crust to eclogite is the norm and is responsi- ble for the increase in angle of dip typically 90° 85° 80° observed in subduction zones involving "normal" oceanic lithosphere, then absence of transformed oceanic crust (that is, sub- ducted lithosphere consisting only of peri- dotitic mantle) could result in reduced average density and low-angle subduction. Both of these alternative hypotheses are 15° speculative. Direct evidence is lacking for the possible existence of sheared-off aseis- mic-ridge crust along the Peruvian segment of low-angle subduction. However, there is indirect evidence that such events may occur (see also Nur and Ben-Avraham, 1981). In the Oregon Coast Range, an apparent aseismic-ridge terrane was identi- fied by Simpson and Cox (1977). They cited the evidence indicating that this tectonically anomalous element formed as oceanic crust 10° and and postulated that an aseismic ridge or island-seamount chain was sheared off the subducting and was accreted to North America. Physical separation, by shearing or other mechanism, of subducted aseismic-ridge crust from the subjacent mantle provides an explanation, alternative to that of Bara- zangi and Isacks (1976), for the correspond- ing absence of arc volcanism. Anderson and 5° others (1976) suggested that dehydration of subducted oceanic crust may provide the necessary to partially melt astheno- sphere or the peridotitic mantle at the base of the upper plate. However, subducted litho- sphere consisting only of mantle would be depleted in free water relative to normal oceanic lithosphere and would be Figure 5. Plate-tectonic setting of the Central American volcanic arc. Active and Qua- relatively stable at shallow depths («5 150 ternary volcanoes (stippled pattern) from Macdonald (1971). Age of Cocos plate (in m.y. km). Consequently, dehydration of sub- B.P.) from Hey and others (1977). 150-km (solid line) and 100-km (dotted line) contours on ducted mantle lithosphere would not occur top of inclined seismic zone and segments of subducted lithosphere (dashed lines) from Carr and water would not be released. Alterna- and others (1979).

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either cease or be displaced significantly NW KM KM from the trench axis. Carr and others (1979) 100 200 100 200 100 KM 200 , à , aa — summarized the observed relations between f- Quaternary volcanism and dip of the in- clined seismic zone in Central America and documented the segmented of the 100 subducting plate. From the triple-plate CENTRAL WESTERN EL SALVADOR boundary at the northwest to the point of 8 intersection of the Cocos Ridge and the 200 trench, the subducted plate dips between 45° and 60° within the intermediate depth interval (Figs. 5, 6). Within the same region, SE the axis of the continuous Quaternary vol- TOP OF canic chain is parallel to and situated 150 to BENIOFF 200 km away from the trench. In central ZONE Costa Rica, immediately north of the inter- 100 \ section of the Cocos Ridge and Central s EASTERN WESTERN \ CENTRAL American trench, the dip of the Benioff NICARAGUA COSTA RICA \ COSTA RICA SÉ zone in the intermediate depth interval is

abruptly and substantially reduced and the 200 volcanic axis is displaced landward (Figs. 5, Figure 6. Cross sections normal to the trench axis of segments along the Central Ameri- 6). Farther to the southeast, opposite the can subduction zone. Compiled from Carr (1976), Carr and others (1979), Stoiber and Carr Cocos Ridge, historically active volcanoes (1973), and Molnar and Sykes (1969). are absent. In summary, moderate-angle subduction in the intermediate depth inter- 1974); Chase, 1978a, 1978b Uyeda and enters the trench also is remarkably uniform val from Guatemala to Costa Rica is a pro- Kanamori, 1979), thus contributing a slow, at about 45 m.y. B.P. (Pitman and others, duct of relatively low convergence rate, strongly oblique (with respect to the strike 1974). absence of an actively overriding upper of the trench) component to the relative The Aleutian and the Alaskan plate, and subduction of young lithosphere. motion. The age of the as it Peninsula comprise a continuous volcanic The notable decrease in dip of the Benioff zone and the abrupt cessation of volcanism 170 E 180" 170 W 160 W 150°W 140°W 130°W occur in the region where the aseismic Cocos Ridge intersects the trench, while all other factors presumed to control the 60 N geometry of the subducted lithosphere 50 N remain constant. A complicating factor in the analysis of the Cocos Ridge requires mention. Lonsdale and Klitgord (1978) sug- gested that the ridge has only recently (< 1 m.y. B.P.) encountered the Middle America trench, so that the observed seismic effects 180 W 170 W 50 N lecfw 150 W 140 W may not be representative of prolonged subduction of an aseismic ridge. Longitude of 180° W 170°W 160°W 150 W Kodiak-Bowie Seamount Chain. Subduc- -i r • Actual —Trench Distance tion of intraplate island-seamount chains O03 affects the geometry of the subducted plate (Q a> 0 Volcano—Trench Distance Corrected Ora < 200 observing the effects of subduction of sea- £ mount chains is the Pacific-North Amer- ican plate boundary along the Aleutian B trench. Convergence rates are nearly uni- 7000 6000 5000 form over the length of the , Distance from Volcano to Pole of Rotation (Km) from about 7 cm/yr on the west to about 6 Figure 7. Plate-tectonic setting of Aleutian and Alaskan volcanic arc. 150-km contour on cm/yr on the east (Fig. 7; Minster and oth- top of inclined seismic zone, locations of volcanoes (solid circles), and maximum width of ers, 1974). In the absolute-motion frame, accreted sedimentary prism since the (diagonal-lined pattern), from Jacob, North America is moving to the southwest Nakamura, and Daivies (1977). Positions of the Kodiak seamount and Mount McKinley at about 2.3 cm/yr (Minster and others, are indicated by "K" and "X," respectively.

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chain north of the Aleutian trench. The arc- 175 E 180° 175° Isacks (1976) for the Peru-Chile trench trench separation increases progressively l r region. 10 • V0LCAN0S - from west to east, from a minimum of about 10°S Louisville Ridge. As age of an intraplate INOI- 170 km to a maximum of about 570 km in cm/yr seamount chain increases, the buoyancy •)— INDI-PCFC the Gulf of region (Fig. 7). The effect of the chain as it enters a subduc- horizontal separation between the trench / 200/ tion zone should be reduced. The inter- and the 150-km contour of the top of the /m 3oo ym^ section of the Louisville Ridge with the 15°S - inclined seismic zone (as projected to the sur- Tonga-Kermadec trench in the western face) also increases from west to east, Pacific illustrates this prediction. In con- reflecting the gradual decrease in dip of the trast to the other areas cited in this review, subducting plate. Jacob and others (1977) the lithosphere in this part of the Pacific is showed a correlation between the increase old, although poorly determined at >80 20°S - of the arc-trench distance and the width of m.y. B.P., and its gravitational instability, the accreted sedimentary prism on the lead- owing to its greater average density, ap- ing margin of the North American plate. proaches the maximum for oceanic lithos- They attributed much of the increase in arc- phere. The Louisville Ridge is probably no trench distance and the gradual decrease in younger than 36 m.y. B.P., on the basis of 25°S - dip of the subducting plate at shallow levels isotopic ages of samples collected from the (< 100 km) to accretion of the sedimentary ridge, and it may be older (Jarrard and prism since the mid-Miocene. The max- Clague, 1977). Watts and others (1980) imum estimated widths of the accretion- determined that the Louisville Ridge was ary prism vary from 100 to 200 km in generated on old (>35 m.y. B.P.) litho- the Gulf of Alaska region. Removal of 30°S - sphere in an off-spreading-ridge position. these values, equivalent to correcting for Covergence rates between the Pacific and the maximum amount of increase in arc- Indian plates along the Tonga-Kermadec trench distance attributable to post-mid- trench change uniformly from ~ 7.5 cm/yr in Miocene accretion, makes the arc-trench the south to ~9.9 cm/yr in the north (Fig. 8; distance uniform along most of the subduc- Minster and others, 1974; Chase, 1978a). tion boundary (Fig. 7; Jacob and others, 35°S - With such a configuration, but in the 1977). absence of an intraplate seamount chain, Even with this correction, a major anom- Figure 8. Plate-tectonic setting of Tonga- the subduction model predicts a fairly sim- aly, in the form of an abrupt increase in the Kermadec subduction zone. Contours ple, uniform subduction zone with a steep arc-trench separation and a decrease in the on top of inclined seismic zone (in kilo- inclination (possibly progressively shallower dip of the subducting plate, occurs in the metres), from Isacks and Barazangi (1977). to the north) and a well-developed continu- region of intersection of the Kodiak-Bowie Absolute-motion (INDI-hotspot) and ous volcanic arc on the upper plate. Figure seamount chain and the trench. This anom- relative-convergence (INDI-PCFC) vectors 8 illustrates the generalized of aly also corresponds with a conspicuous gap calculated from Minster and others' (1974) the seafloor, the positions of Quaternary in the volcanic chain centered about the Mt. model; lengths of arrows are scaled to volcanoes, and contours on the top of the McKinley region. Despite the absence of calculated rates. Thin contours show posi- inclined seismic zone. It is clear that the Quaternary volcanism, a well-developed tions of Coalville, Lau, Tonga, and Kerma- geometry of the subduction zone is not sim- inclined seismic zone lies beneath this dec subsea ridges. ple, as predicted, and therefore that sub- region to depths greater than 150 km. The ducted portions of the Louisville Ridge strike of the Kodiak-Bowie seamount chain seamount chain, the northern extension of have modified its character. is oblique to the magnetic-anomaly patterns the Kodiak-Bowie seamount chain. As the The subduction zone to the south of the of the northeastern Pacific. Isotopic ages of Pacific plate passed over the Bowie "hot Louisville Ridge, opposite the Kermadec samples collected from several seamounts spot," the lithosphère thinned by melting of trench, dips about 12° in the intermediate show an age progression from ~ 23 m.y. the base of the lithosphere. Replacement of and deep levels (Isacks and Barazangi, B.P. at Kokiak seamount to Recent at the base of the lithopshere by lower-density 1977), as indicated by the spacing of hypo- Bowie seamount in the southeast (Turner asthenosphere and concomitant thinning of center depth contours (Fig. 8). The dip pro- and others, 1973; Jarrard and Clague, mantle caused that portion of the plate to be gressively shallows to about 58° in the 1977). These ages are considerably younger more buoyant than surrounding "normal" intermediate level to the north of the Louis- than the ages of the surrounding sea floor lithosphere (Detrick and Crough, 1978). ville Ridge-trench intersection (Isacks and (Pitman and others, 1974) and indicate that Low-angle subduction in the Gulf of Alaska Barazangi, 1977). Moreover, the subducted the seamount chain was generated by an region was a result of subduction of this lithosphere progressively flattens at deeper off-ridge "hot spot" or other process unre- relatively buoyant lithosphere. Absence of levels opposite the . Kelleher lated to crustal formation along spreading volcanism in the Mt. McKinley region may and McCann (1976) previously noted that ridges. reflect the direct contact of the subducted Quaternary and active volcanoes and major We attribute this anomalous region of plate with the base of the North American earthquakes in the subducted lithosphere shallow subduction and absence of volcan- plate and consequent displacement of as- are clustered between 18°S and 23° S lati- ism to subduction of an intraplate island- thenosphere, as suggested by Barazangi and tudes and between 28° S and 32° S latitudes.

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In the intervening gap, approximately op- mally induced, and the sea floor is in gence rates and absolute upper-plate mo- posite the Louisville Ridge-trench intersec- isostatic equilibrium (Sclater and others, tion. As a spreading ridge approaches a tion, Quaternary volcanoes are absent and 1971). Initially, the thin lithosphere consists trench and progressively younger litho- major earthquakes are rare. The breadth of predominantly of light oceanic crust which sphere is subducted, we anticipate gradual the volcanic and seismic gap and the modi- forms near the ridge crest. With time, the decrease in dip of the Benioff zone and fied geometry of the subduction zone to the lithosphere thickens, cools, and becomes migration of the arc away from the trench. north of the Louisville Ridge is explained denser, and the sea-floor elevation is gradu- As the thermal effect becomes dominant by oblique subduction of the Louisville ally reduced (Parker and Oldenburg, 1973). over the buoyancy effect, partial melting Ridge and its presumed continuation to the Most of this thickening occurs within the occurs progressively closer to the trench and northwest beneath the . Isacks mantle, whereas the crustal thickness is the arc migrates back toward the trench, and Barazangi (1977) summarized the re- produced early and increases only slightly even though low-angle subduction con- sults of plate reconstructions which place through time. As lithosphere ages, it con- tinues. Finally, at some critical point, arc the initial intersection of the Louisville sists of progressively lower ratios of crust to magmatism ceases as warm, young litho- Ridge extension and the Tonga trench to mantle. Consequently, the average litho- sphere is absorbed prior to significant the north of its present position. Since the sphere density is a function of age, and magma generation. This critical point is Pliocene, the point of intersection has young lithosphere is relatively more buoy- dependent not only on lithosphere age, but migrated southward as successive sea- ant than older, denser lithosphere. co-varies with convergence rate and abso- mounts comprising the Louisville Ridge The anticipated effect of lithospheric age lute upper-plate motion. In cases where rela- were subducted obliquely. Subduction of on subduction zone: geometry is derived tive plate motions require subduction of a this anomalous oceanic lithosphere was from these generalizations. As old, dense spreading ridge and recently formed litho- concomitant with the opening of the Lau lithosphere enters a subduction zone, it sphere on its oceanward flank, there will be basin. sinks more rapidly than young lithosphere, a delay before arc magmatism is renewed. Resurgent volcanism will occur near the In summary, local modification of because it is gravitati onally unstable. Other trench, and as progressively older litho- subduction-zone geometry and other ancil- factors being equal, old lithosphere tends to sphere is subducted, the arc will migrate lary effects occur as aseismic ridges, oceanic subduct at steeper angles than young litho- away from the trench. plateaus, and intraplate island-seamount sphere. Consequently , a magmatic arc with a chains are subducted. Despite the different narrow arc-trench separation develops ap- In the previous discussion of the Cocos- origins of these ridges, the lithosphere proximately parallel to the trench. Under North American plate boundary (Fig. 2), we which contains them has a reduced average otherwise similar conditions, young litho- noted the correlation between increasing density relative to surrounding lithosphere sphere will subduct at shallower angles convergence rates and greater arc-trench devoid of such anomalous elements. The because it is thinner, hotter, and more buoy- separation from north to south along the modifications are a consequence of the ant. In contrast to subduction of old litho- Middle American trench. The age of the reduced average density and consequent sphere, the relation between arc magmatism Cocos plate also increases from north to buoyancy relative to surrounding litho- and subduction is not as simple when young south. With respect to arc magmatism, it is sphere. In contrast to lithosphere bearing lithosphere is subducted. Low-angle sub- clear that the age-related buoyancy effect is aseismic ridges and oceanic plateaus, the duction caused by relative buoyancy of subordinate to the opposing effect of age- average density of lithosphere carrying young lithosphere produces effects similar related temperature. Although recently island-seamount chains is strongly depend- to those induced by rapid convergence and formed, relatively buoyant lithosphere is ent on ages of the crust and the island- rapid upper-plate motion toward the trench. subducting beneath South America in the seamount chains. As shown in the cited These effects are either increased arc-trench north, magma generation occurs early, close examples, with progressive increase in age, separation or cessation of volcanism by dis- to the trench. This reflects the dominance of the buoyancy effect is progressively reduced. placement of subcontinental asthenosphere the age-related temperature effect, slow Nonetheless, even old chains on old litho- (Barazangi and Isacks, 1976). However, convergence, and moderate-angle subduc- sphere cause the Benioff zone to dip at shal- because young lithosphere also is hotter tion. To the south, as progressively older lower angles and may inhibit generation of than old lithosphere, partial melting of the lithosphere is subducted, the age-related arc magma. The relative buoyancy of oceanic crust and generation of arc magma tendency toward steeper subduction is offset aseismic ridges and some oceanic plateaus is may occur more quickly and closer to the by increasing convergence rates and de- related to anomalously thick crust created trench. Moreover, young lithosphere may creasing temperatures of the oceanic plate. during the formation of the ridges. This be absorbed into the asthenosphere before Observed relations of subduction-zone thickness and, therefore, the buoyancy appreciable low-density magma is produced. geometry and arc magmatism along the effect are not age dependent. Greater crustal In this case, arc magmatism ceases. Sub- Peru-Chile trench north of the thicknesses of aseismic ridges may be related duction of young lithosphere thus produces (Fig. 9) are compatible with our predictions, to formation on slow spreading centers or two opposing tendencies with respect to arc but they do not provide an unambiguous to mantle sources contributing anomalous magmatism: (1) low-angle subduction and test of them. Absolute motions of the Nazca volumes of magma at isolated positions increased arc-trench distance, owing to its and South American plates along the sub- along spreading centers. low density; and (2) decreased arc-trench duction boundary are essentially constant. distance, owing to its higher temperature. Convergence rates also are essentially con- Age of Lithosphere This rationale prompts the following pre- stant (<2% deviation from maximum value dictions regarding relations of arc magma- of ~ 11 cm/yr at 39° S latitude), and they are Elevation of the sea floor along and mar- tism to subduction kinematics through time notably higher than along the Middle ginal to oceanic spreading ridges is ther- and under conditions of constant conver- American trench. The age of the Nazca

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plate increases uniformly northward from neath South America. Thus, South America and combined infracrustal compression the Chile Rise to about 30° S latitude. is overriding the western flank of the former with supracrustal extension. These tectonic Excluding that portion of the subduction Chile Rise, and progressively older litho- styles result from different stress regimes in zone influenced by the Juan Fernandez sphere is being subducted. The lithosphere the crust of the upper plate imposed upon Ridge, from 20° S to 40° S latitudes the adjacent to the trench is younger than 19 pre-existing thermomechanical properties Nazca plate subducts at moderate (-30°) m.y. B.P. (anomaly 6; Weissel and others, of the crust. The stress regime is related to angles beneath South America (Stauder, 1977); therefore, the subducted portion of the geometry of subducting lithosphere. The 1973; Barazangi and Isacks, 1976). From the is young, warm, and four factors inferred as the principal con- about 40° S to the intersection of the Chile thin. Convergence rate is slow, about 2 trols on the geometry of subducting litho- Rise with the Peru-Chile trench at 46° S lati- cm/yr, and most of the convergence is sphere, therefore, also are inferred to tude, seismicity is less intense and generally attributable to westward motion of South control the stress regime along the conver- is restricted to shallow levels and the geome- America. No magmatic arc is present east of gent margin of the upper plate and the try of the subducted plate is less well the trench. Under these conditions of slow deformational style within and landward of defined. Corresponding to the southward convergence and subduction of young litho- the volcanic arc. decrease in age of the Nazca plate, the arc- sphere, the warm oceanic crust is being The degree of compressional stress trans- trench separation gradually decreases slight- assimilated into the asthenosphere without mission through the crust of the upper plate ly from 30° S to 43° S latitudes. To the attendant magma generation. The subduc- is related to the area of interface and degree south, active volcanism terminates opposite tion model predicts that, as progressively of coupling between the upper and lower the position where young lithosphere ( ~ 14 older lithosphere is subducted, diffuse vol- plates and, thus, to the subduction angle. m.y. B.P.; anomaly 5b) is subducted (Weis- canism will occur within a broad region of Low-angle subduction and efficient trans- sel and others, 1977; LaBrecque and others, South America, and gradually a volcanic mission of compressional stress are induced 1977). Cessation of arc volcanism in this arc will develop close to the trench. by combinations of rapid absolute upper- region is probably a result of high conver- plate motion toward the trench, rapid gence rates, shallow subduction of the CONTROLS ON THE TECTONIC relative convergence, subduction of young young, relatively buoyant Nazca plate, and ENVIRONMENT OF lithosphere, and subduction of anomalous- vertical juxtaposition of continental and CONVERGENT-PLATE MARGINS ly low-density lithosphere. Regional exten- oceanic lithosphere. However, available sional stress regimes within the crust are seismic data are insufficient to resolve this Along contemporary convergent-plate induced by steeper subduction (reduced suggestion. margins, deformation within upper plates at coupling between the upper and lower South of the Chile Rise, the Antarctic- varying distances from trenches may be plates) and retrograde upper-plate absolute Nazca spreading ridge has been subducted assigned to three principal styles of defor- motion. Extensional stress regimes at and the Antarctic plate is subducting be- mation. These are compression, extension, supracrustal levels may occur above a region of compressional stress where iso- statically uplifted crust has greater gravi- tational potential to expand in the absence of lateral confining stress. Within these regional stress regimes, deformational behavior is also partially controlled by thermomechanical properties of the crust, which are a product of its geo- logic history. Crustal shortening at depth in relatively cold, elastic regions at great dis- tances from the trench (Laramide style) is a response to very low-angle subduction, rapid trenchward absolute motion of the upper plate, and subduction of aseismic ridges. Cordilleran style crustal shortening by fold and thrust deformation is a response to moderate or low-angle subduction and

Figure 9. Plate-tectonic setting of southern Andean subduc- tion zone. Age of the Nazca plate (shown by patterns) based on magnetic anomalies identified by Herron and Tucholke (1976), Weissel and others (1977), and Pitman and others (1974) and presented according to the time scale of LaBrecque and others (1977). Positions of identified magnetic anomalies shown by numerals.

1050

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underthrusting of the foreland beneath the are absent in the two segments, in contrast and stress transmission between the sub- ductile, isostatically uplifted back-arc. Usu- to the continuous chain of volcanoes of the ducting and overriding plates. ally, this compressional deformation is central Andes. In addition, the Andes are Determination of crustal stress regime accompanied by extensional deformation at markedly narrower along the two segments, and deformational style in the Cascades high crustal levels in the arc and back-arc by comparison with the Altiplano of the volcanic arc is complicated by its proximity regions, where the isostatically uplifted central Andes. to two triple-plate junctions, a consequence crustal mass expands in the absence of lat- Regional intra-arc compression along the of the small size of the subducting Juan de eral confining stress. Extensional deforma- Aleutian and Alaskan volcanic chain is Fuca plate. However, east-west extension tion within the crust of back-arc and indicated by preferred orientations of dike within the arc apparently is the principal intra-arc regions is related to steep subduc- swarms and distribution of volcanic cones deformational style, as indicated by numer- tion, but it requires retrograde absolute and craters (Nakamura and others, 1977). ous north-south trending normal faults and motion of the upper plate. Using this method, Nakamura and others fissure zones that cut Pleistocene volcanics Contemporary back-arc extension is well (1977) determined that the maximum hori- of the High Cascades Group (Hammond, documented behind the Izu-Bonin, Ryukyu, zontal compressive stress is parallel with the 1979). Mariana, and Tonga arcs in the western direction of North America-Pacific conver- Volcanic centers of the trans-Mexican Pacific (for example, Karig, 1974; Herman gence. By contrast, in the back-arc region of volcanic belt show a subtle northeast- and others, 1979), within the the Aleutian volcanic chain, maximum southwest alignment on a regional scale in the Indian Ocean (Curray and others, horizontal compressive-stress indicators are (King, 1969). Through application of the 1979), and behind the in the oriented approximatdy east-west, or paral- techniques of Nakamura and others (1977), South Atlantic (Barker, 1970; Barker and lel to the strike of the arc. Nakamura and we infer a parallel maximum horizontal- Griffiths, 1972). Intra-arc extension is pres- others (1977) inferred a vertical maximum, stress orientation and note that this orienta- ent in the central part of the Middle Ameri- an east-west intermediate, and a north- tion is approximately parallel to the can arc (Dewey and Algermissen, 1974) and south minimum principal stress, thus plac- direction of North America-Cocos conver- occurred in the Basin-Range (Scholz and ing the back-arc region in a dominantly gence (for example, Chase, 1978a). We sug- others, 1971; Cross and Pilger, 1979) and extensional stress regime. In the back-arc of gest that the crust of the back-arc and proto-Gulf of California (Karig and Jensky, Alaska, coincident with the region of low- intra-arc regions of Mexico is dominated 1972) regions during the late Tertiary. angle subduction, maximum horizontal by a regional compressive stress field, compressive-stress indicators maintain an Megard and Philip (1976) concluded that with the principal stress oriented northeast- approximate parallelism with the direction the central Andes is a region of intra-arc southwest. Possible foreland fold and thrust of North America-Pacific convergence. extension. Other data suggest a coeval belt development in the transitional crust Contemporary stress fields in back-arc and compressional stress regime within the same beneath the Gulf of Mexico is marked by intra-arc regions of the Aleutian-Alaska segment, but displaced to the east. Pub- the northwest-southeast-trending Mexican and the Andean subduction systems are lished cross-sections by Barth (in Zeil, 1979, Ridges fold belt (for example, Buffler and remarkably similar with respect to geometry p. 141-142) show thrust faults of late others, 1979). of subducting lithosphere. Cenozoic age along the eastern flank of the This review of selected subduction sys- central Andes Altiplano. Shallow-focus The state of stress and deformational tems shows clear correlations between the (<70 km) earthquakes along the east flank style within and behind other volcanic arcs geometries of subduction zones and ob- of the Altiplano (for example, Barazangi are less certain than the examples discussed served stress regimes and deformational and Isacks, 1976) may reflect contemporary above. The tectonic character of some of styles within the leading margin of the movement along these faults. However, these was termed "simple" by Molnar and upper plate. We infer that the four major focal-mechanism solutions for these earth- Atwater (1978), in the sense that neither factors that control the geometry of subduc- quakes have not been determined, and the extension nor compression ("Cordilleran" tion zones also control the stress regime state of stress and sense of faulting are not style) is dominant in back-arc or intra-arc within the upper plate and influence the known. Few, if any, shallow-focus earth- regions. As they noted, the information deformational style within and landward of quakes have been detected within the Alti- base is not sufficient to definitively charac- the arc. plano (Barazangi and Isacks, 1976). terize the tectonic style of intra-arc and There is a direct correspondence between Contemporary regional compression in back-arc regions of Sumatra and Java, the low-angle subduction and evidence of com- back-arc and intra-arc regions, with the Cascades, and the trans-Mexican volcanic pressive stress and compressional tectonics. principal stress component oriented per- arc. Barazangi and Isacks (1976) and Megard pendicular to strikes of trenches, is doc- Wilcox and others (1973) ascribed late and Philip (1976) suggested that low-angle umented along fewer subduction systems. Cenozoic compressional deformation in the subduction results in mechanical or dynam- The two segments of very low-angle back-arc region (foreland basin of Hamil- ic coupling and more efficient transmis- subduction along the Peru-Chile trench, ton, 1979) of Sumatra to wrench- tec- sion of compressive stress between the in northern Peru and central Chile, are tonics. Alternatively, we tentatively regard descending and overriding plates than does characterized by east-west horizontal com- the region as a foreland fold and thrust belt steeper subduction. This general relation pressive stress within the shallow (<90 km) analogous to the Canadian Rockies, on the was applied successfully to explain several part of the upper plate (Stauder, 1973, basis of the dimension of the deformational major Laramide events in western North 1975). These zones of compressive stress zone, the anomalous moderate-angle dip America. These include basement-rooted extend up to 700 km from the trench. As (30°; Fitch, 1970) of the subducting plate, thrusting in the central Rocky previously discussed, Quaternary volcanoes and the probability of moderate coupling (Burchfiel and Davis, 1975; Brewer and

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others, 1980) and a shift in locus and reduc- erate angles (~30°) beneath the central of Oregon and Washington would be pre- tion in volume of magmatism from the Andes, and absolute-motion models indi- dicted because of the slow subduction of the to the cate active overriding of the trench by young , but existing evi- (Cross and Pilger, 1978a). An ancillary South America (Minster and others, 1974; dence indicates regional extension. These effect of very low-angle subduction is Chase, 1978a). This subduction configura- discrepancies arise because the analysis regional isostatic subsidence of the upper tion generates compressional stress across neither includes an evaluation of the other plate due to subcrustal cooling and re- the base of the crust beneath the Altiplano. controls on the dip of the subducting plate placement of low-density asthenosphere by Compressional deformation occurs along nor considers the effects of upper-plate higher density oceanic lithosphere (Cross the eastern margin of the Altiplano, where absolute motion on the style of deformation and Pilger, 1978b). This effect is reflected by the South American foreland is under- within the upper plate. the contrast in elevations between the broad thrusting to the west beneath the Altiplano. Observed contrasts in tectonic style along Altiplano of the central Andes, which over- Uplift of the Altiplano is the product of the three segments of the Middle American lies a segment of moderate-angle subduc- westward underthrusting of the foreland, arc are correctable with combined varia- tion, and the lower elevations of the elevated temperature and isostatic compen- tions in the four major controls of sub- Amazon basin and by the narrow width of sation above the wedge of partially molten, duction-zone geometry. The subduction the Andes in central Peru. Cross and Pilger low-density asthenosphere, and addition of zone beneath the trans-Mexican volcanic (1978b) also explained regional subsidence magma from depth. Uplift and addition of belt is inclined at a moderate angle 20°), a of the central Rocky Mountains area during magma require expansion of the upper crust consequence of relative youth of the Cocos the Late Cretaceous by this mechanism. of the Altiplano. Thus, both crustal shorten- plate, relatively rapid convergence, and Another evident relation between the ing and extension may occur concurrently southwestward absolute motion of the major controls on the geometry of subduc- along the same subduction segment, but at North American plate. On the basis of tion zones and tectonic style was observed different structural levels or at different dis- moderate-angle and shallow subduction by Morgan (1972) and discussed by Chase tances from the trench. (earthquake foci have maximum depths of (1978a). All areas of back-arc extension Molnar and Atwater (1978) argued that about 150 km; Molnar and Sykes, 1969), we overlie segments of steep subduction, and the major control on the tectonic environ- infer that the two plates are moderately the absolute motion of the upper plate ment of back-arc regions, whether exten- coupled and that compressive stress is either is parallel to the trench or is retro- sional or compressional, is the age of the transmitted into the crust of Mexico in the grade with respect to the trench. In the Sco- subducting lithosphere. They noted that back-arc and intra-arc regions. Intra-arc tia Sea, back-arc spreading is observed subduction zones in the western Pacific are extension in Central America is, in all prob- (Barker, 1970; Barker and Griffiths, 1972), characterized by subduction of old, dense ability, primarily a response to slow, slightly but the absolute motion of the upper plate is lithosphere and back-arc extension, whereas retrograde absolute motion of the Carib- uncertain. However, the analysis by Forsyth subduction zones in the eastern Pacific are bean plate. In the absence of upper-plate (1975) requires very slow upper-plate mo- characterized by subduction of young litho- motion toward the trench, the older Cocos tion if the plate beneath the Scotia Sea is sphere and back-arc compressional tec- plate is gravitationally unstable, descends at coupled to the Antarctic plate. To the tonics (either Cordilleran style or "simple"). steep angles, the trench migrates seaward, extent that the descending plate tends to England and Wortel (1980) expanded and and the minimum principal stress within the anchor the subduction zone, retrograde quantified the approach of Molnar and Central American crust is oriented normal upper-plate motion requires crustal exten- Atwater (1978) by combining convergence to the strike of the trench. Uplift of the arc sion within the upper plate. The tendency rate and dip of the inclined seismic zone to above the wedge of partially molten asthen- for back-arc or intra-arc extension is in- estimate sinking velocity. England and osphere provides additional gravitational creased by (1) the gravitational instability of Wortel (1980) concluded that a combination potential for extension. This combination the descending lithosphere and consequent of increasing lithospheric age and in- results in extension within the hot, ductile seaward migration of the trench, and (2) the creasing descent velocity favors back-arc or volcanic-arc region. The zone of extension increased potential of the elevated crust in intra-arc extension, while subduction of within the central segment of the Middle the arc region to expand in the absence of younger lithosphere at low rates of descent American arc apparently ends in southern lateral confining stress. favors compressional (Cordilleran style) Costa Rica. In this area, back-arc thrusts are developed which continue offshore to Other subduction systems exhibit neither tectonics. the southeast along the base of the Carib- dominantly compressional nor dominantly The analysis of England and Wortel bean of Panama (J. extensional deformation. These are asso- (1980) shows attractive correlations between Case, 1980; oral commun.). The transition ciated with moderate to steeply dipping age of subducting lithosphere, descent velo- from intra-arc extension to back-arc shor- subduction zones and absolute-upper plate city, and tectonic style within the upper tening in Costa Rica corresponds with the motion toward the trench. Evidence for plate. However, there are several areas intersection of the aseismic Cocos Ridge contemporaneous compressional and ex- which do not conform with these cor- and the Middle American trench. We inter- tensional deformation often is found at dif- relations. Major discrepancies include pret back-arc thrusting as a direct conse- fering structural levels or at varying dis- the Sandwich (Scotia Sea) and Central quence of enhanced coupling between the tances from the trench along the same American arcs, in which relatively young Cocos and Caribbean plates and transmis- subduction segment. The central Andes lithosphere is subducting, but in which sion of compressive stress through the crust provide an example of possible coexistence appear dominant in of the upper plate. This is a result of of extensional and compressional deforma- back-arc and intra-arc regions. Similarly, increased resistance to subduction of the tion. The Nazca plate is subducting at mod- compressional deformation in the Cascades

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thicker crust and more-buoyant lithosphere potential for extensional deformation. Geophysical Journal, v. 57, p. 537-555. beneath the Cocos Ridge, as suggested by Rapid upper-plate motion toward the Barker, P. F., 1970, of the Scotia Sea region: Nature, v. 228, p. 1293-1296. Silver (1979), and in partial analogy with trench tends to flatten the subducting plate, Barker, P. F„ and Griffiths, D. H„ 1972, The subduction of the Nazca Ridge beneath increases the area of interface and degree of evolution of the Scotia Ridge and Scotia Sea: Peru. coupling between the plates, and creates a Royal Society of London Philosophical In summary, the four major factors which compressional regime (or reduces the exten- Transactions, v. 271, p. 151-183. control the geometry of subducting litho- sional stress) within the upper plate. Crustal Bostrom, R. C., 1971, Westward displacement of the lithosphere: Nature, v. 234, p. 536-538. sphere also control or affect the state of shortening and extension may result at dif- Bostrom, R. C., Sherif, M. A., and Stockman, R. stress and tectonic style in the upper plate. ferent structural levels within the same seg- H., 1974, Deformation of earth's lithosphere In general, the state of stress is most closely ment, as in the central Andes. Subduction with reference to tidal couples: American related to the inclination of the subducting of anomalously low-density lithosphere (for Association of Petroleum Geologists Me- plate and reflects the degree of coupling example, aseismic ridges, intraplate island- moir 26, p. 463-485. Brewer, J. A., Smithson, S. B., Oliver, J. E., and across the subduction interface and the seamount chains, or very young lithosphere) Brown, L. D., 1980, The Laramide : effectiveness of stress transmission through augments the tendency toward low-angle Evidence from COCORP deep crustal seis- the upper plate. Stress may be transmitted subduction and causes compressive stress mic profiles in the Wind River Moun- to the surface directly, by mechanical coup- and crustal shortening to dominate in the tains, Wyoming: , v. 62, ling over an interface of stress, or upper plate. Displacement of the low- p. 165-189. Buffler, R. R„ Shaub, R. J., Watkins, J. S., and indirectly, by hydrodynamical coupling via density asthenospheric wedge by anomal- Worzel, J. L., 1979, Anatomy of the Mexi- the fluid asthenospheric wedge between the ously low-density lithosphere, particularly can ridges, southwestern Gulf of Mexico, in upper and lower plates. aseismic ridges, may cause surficial subsi- Watkins, J. S., Montadert, L., and Dicker- dence and create a pericratonic sedimentary son, P. W., eds., Geological and geophysical Under moderate- or steep-angle subduc- basin (for example, a portion of the West- investigations of continental margins: Amer- tion conditions, a regional extensional stress ican Association of Petroleum Geologists ern Interior Seaway and the Amazon basin; regime within the crust arises from isostatic Memoir 29, p. 319-328. Cross and Pilger, 1978b). uplift, consequent increased gravitational Burchfiel, B. C., and Davis, G. A., 1975, Nature potential for extensional deformation, and We regard both the extensional and com- and controls of Cordilleran orogenesis, western : Extensions of an thermal expansion (increased volume/mass pressional stresses as being broadly distrib- earlier synthesis: American Journal of Sci- ratio) of the crust. These are induced in two uted within the crust. The actual location of ence, v. 275-A, p. 363-396. ways: (1) isostatic compensation and ther- deformation, whether intra-arc, back-arc, Burk, C. A., 1972, Uplifted eugeosynclines and mal expansion above the wedge of partially or both, is controlled by pre-existing ther- continental margins: Geological Society of molten, low-density asthenosphere; and (2) momechanical properties of the crust. America Memoir 132, p. 75-85. uplift, thermal expansion, and increased Carr, J. J., 1976, Underthrusting and active faulting in northern Central America: Geo- volume by injection of magma from depth. ACKNOWLEDGMENTS logical Society of America Bulletin, v. 87, Surficial uplift and near-surface extensional p. 825-829. deformation within a dominantly compres- We are particularly indebted to Gordon Carr, J. J., Rose, W. I., and Mayfield, D. G., 1979, Potassium content of and depth sional regime also may be induced by Gastil for initially suggesting and encourag- to the seismic zone in Central America: underthrusting of the foreland beneath the ing preparation of this report. Helpful Journal of Volcanology and Geothermal volcanic arc and immediate back-arc region comments and criticisms were offered by Research, v. 5, p. 387-401. (for example, the Altiplano). Extensional Gordon Gastil, Dick Phillips, Kevin Biddle, Chase, C. G., 1978a, Extension behind island arcs and motions relative to hot spots: Journal of stresses are reinforced by seaward migration and two anonymous reviewers. We thank Geophysical Reserch, v. 83, p. 5385-5387. of the trench, a function of lithosphere den- them for their time and courtesy. sity (age and presence or absence of anom- 1978b, Plate kinematics: The Americas, east Africa, and the rest of the world: Earth and alous crust), and absolute upper-plate Letters, v. 37, p. 355-368. motion. Slow or retrograde motion of the REFERENCES CITED Cross, T. A., and Pilger, R. H„ Jr., 1978a, upper plate enhances the tensional stress Constraints on absolute motion and plate regime and permits more-pronounced intra- Ahrens, T. 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