Relations Among Subduction Parameters
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REVIEWS OF GEOPHYSICS, VOL. 24, NO. 2, PAGES 217-284, MAY 1986 Relations Among Subduction Parameters RICHARD D. JARRARD Lamont-DohertyGeological Observatory, Palisades, New York Clues to the dynamicsof the subductionprocess are found in the many measurableparameters of modern subductionzones. Based on a critical appraisal of the geophysicaland geologicalliterature, 26 parametersare estimatedfor each of 39 modern subductionzones. To isolate causalrelationships among theseparameters, multivariate analysis is appliedto this data set.This analysisyields empirical quantita- tive relationsthat predict strain regime and strike-slipfaulting in the overridingplate, maximum earth- quake magnitude,Benioff zone length, slab dip, arc-trenchgap, and maximum trench depth. Excellent correlation is found betweenlength of the Benioff zone and the product of convergencerate and age of the downgoingslab. This relationshipis consistentwith the conductiveheating model of Molnar et al. (1979), if the model is modified in one respect.The rate of heating of the slab is not constant; it is substantiallyslower during passageof the slab beneath the accretionaryprism and overriding plate. The structural style in the overriding plate is determined by its stressstate. Though the stressstate of overridingplates cannot be quantified, one can classifyeach individual subductionzone into one of seven semiquantitativestrain classesthat form a continuumfrom stronglyextensional (class 1, back-arc spread- ing) to strongly compressional(class 7, active folding and thrusting). This analysis indicates that strain classis probably determinedby a linear combinationof convergencerate, slab age, and shallow slab dip. Interplate coupling, controlled by convergencerate and slab age, is an important control on strain regime and the primary control on earthquake magnitude.Arc-parallel strike-slipfaulting is a common feature of convergentmargins, forming a forearc sliver betweenthe strike-slipfault and trench. Optimum conditionsfor the developmentof forearc sliversare oblique convergence,a compressionalenvironment, and a continental overriding plate. The primary factor controlling presenceof strike-slip faulting is coupling;strongly oblique convergenceis not required.The rate of strike-slipfaulting is affectedby both convergenceobliquity and convergencerate. Maximum trench depth is a responseto flexure of the underthrustingplate. The amount of flexural deflectionat the trench dependson the vertical component of slab pull force, which is very sensitiveto slab age and shallow slab dip. Shallow slab dip conformsto the cross-sectionalshape of the overriding plate, which is controlled by width of the accretionaryprism and duration of subduction.Deep slab dip is affected by the mantle trajectory establishedat shallow depth but may be modified by mantle flow. Much of the structuralcomplexity of convergentmargins is probably attributable to terrane juxtaposition associatedwith temporal changesin both forearc strike- slip faulting and strain regime. Empirical equationsrelating subductionparameters can provide both a focusfor future theoretical studiesand a conceptualand kinematic link betweenplate tectonicsand the geologyof subductionzones. CONTENTS Product of convergencerate and slab age ................ 249 Intermediate dip and the product of slab age and convergence Introduction ............................................ 217 rate ................................................ 252 The data ................................................ 218 Earthquake magnitudemodel ............................. 253 Choice of subduction zones ............................. 218 Strain regime models..................................... 254 Arc curvature ......................................... 221 Convergence rate ...................................... 254 Benioff zone geometry ................................. 222 Slab age .............................................. 255 Compression/extension................................. 224 Absolute motion ....................................... 256 Convergencerates ..................................... 230 Absolute motion and slab age ........................... 258 Absolute motions ...................................... 231 Convergencerate and slab age .......................... 259 Slab age .............................................. 232 Slab dip .............................................. 262 Arc age............................................... 232 Convergence rate, intermediate dip, and either slab age or Trench depth .......................................... 235 absolute motion ..................................... 263 Strike-slip faulting ....................................... 235 Slab dip models ......................................... 266 Characteristics ........................................ 235 Radius of curvature .................................... 267 Leading and trailing edges of forearc slivers .............. 238 Convergencerate ...................................... 267 Continental versus oceanic strike-slip faulting............. 238 Slab age .............................................. 267 Mechanism ........................................... 240 Convergencerate and slab age .......................... 268 Effect of strike-slip faulting on slip vectors ............... 241 Convergencerate, slab age, and absolute motion .......... 268 Implications for Southeast Asian plate motions ........... 243 Mantle flow ........................................... 269 Forearc slivers and terrane motions ..................... 245 Aseismic ridges ....................................... 270 Statisticalanalysis ....................................... 245 Accretion, arc age, and mantle flow ..................... 270 Methods and limitations ................................ 245 Crust of overriding plate ............................... 273 Correlation between independentvariables ............... 248 Trench depth models..................................... 273 Models of slab length .................................... 249 Conclusions ............................................. 276 Convergence rate ...................................... 249 Slab age .............................................. 249 INTRODUCTION The subductionprocess involves a complex interplay of Copyright1986 by the AmericanGeophysical Union. forcesthat affect plate motions and determinethe structural Paper number 5R0903. style behindtrenches. Clues to the dynamicsof the subduction 8755-1209/86/005R-0903515.00 processare found in the many measurableparameters that 217 218 JARRARD: RELATIONS AMONG $UBDUCTION PARAMETERS describe modern subduction zones. Traditionally, the major important step precedingapplication of the relationshipsto causalrelationships within subductionzones have beensought the prediction of past unknown parametersis the testing of through either theoreticalor qualitative analyses.Quantitative the models on the historiesof relatively well known regions physicalmodels have been unable to establishthese relation- (e.g.,western North America). shipswith confidence,due to uncertaintiesin essentialphysical The Pacific margins are now known to be largely a mosaic quantities(e.g., mantle viscosityand shearstress) and physical of displacedterranes, with complex geologichistories some- complexity of the subduction process.Many qualitative re- how related to changingrates and directionsof convergence. lationshipshave been proposed,usually based on a plausible Progressin describingthe kinematics of plate convergence, causal relationship and an apparent correlation between two both Pliocene-Pleistocene[Le Pichon, 1968; Minster et al., subduction parameters for modern subduction zones. Often 1974; Chase, 1978a; Minster and Jordan, 1978] and Tertiary theseproposed relationships are mutually exclusive. [Atwater and Molnar, 1973; Engebretson,1982], has not been The variety of structural environments and plate interac- accompanied by comparable progress in predicting terrane tions at convergentmargins is well illustrated by a brief com- motions [e.g., Stone et al., 1982; Alvarez et al., 1980]. In this parison of the Marianas and Peru-Chile plate boundaries.The paper I consider only one group of terrane motions: those Marianas is characterized by slowly converging,old oceanic occurringwithin the overriding plate. Two other types of ter- crust that is underthrustinga tensional overriding plate at an rane motion are beyond the scope of this paper: accretion almost vertical dip, accompaniedby relatively modest maxi- resultingfrom buoyant portions of the underridingplate col- mum earthquakes (M,• = 7.2). In contrast, the Peru-Chile liding with the trench [e.g., Nur, 1983] and the diffuse defor- plate boundary is characterized by rapidly converging, mation behind trenches that results from continent-continent younger oceaniccrust that is underthrustinga compressional collision [Dewey and Bird, 1970]. Instead, the emphasisis on overriding plate at a nearly horizontal dip, accompaniedby relationshipsduring "normal" subduction. very large (M,• = 8.6-9.5)earthquakes. Most other current The scope of this project is necessarilymore limited than convergentmargins can be describedas lying somewhereon a the entire range of relations among subduction parameters. continuum between the Marianas and Peru-Chile extremes Discussionof the tectonicsof the accretionaryprism is beyond [Uyeda and Kanamori, 1979]. the scope of this paper; some of the recent advancesin this This apparent continuum