Controls of subduction geometry, location of magmatic arcs, and tectonics 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 Geology, Louisiana State University, Baton Rouge, Louisiana 70803 ABSTRACT States), intra-arc extension (for example, convergence rate, direction and rate of the Basin and Range province), foreland absolute upper-plate motion, age of the de- Most variation in geometry and angle of fold and thrust belts, and Laramide-style scending plate, and subduction of aseismic inclination of subducted oceanic lithosphere tectonics. ridges, oceanic plateaus, or intraplate is caused by four interdependent factors. island-seamount chains. It is crucial to Combinations of (1) rapid absolute upper- INTRODUCTION recognize that, in the natural system of the plate motion toward the trench and active Earth, 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 asthenosphere 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 sediment accreted along trenches 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 magma, 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 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 546 CROSS AND PILGER 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 volcanic belt 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. Volcanic arc 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 basement-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).