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Slab Pull and the Seismotectonics of Subducting Lithosphere

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Slab Pull and the Seismotectonics of Subducting Lithosphere REVIEWS OF GEOPHYSICS, VOL. 25, NO. 1, PAGES 55-69, FEBRUARY 1987 Slab Pull and the Seismotectonicsof Subducting Lithosphere WILLIAM SPENCE National Earthquake Information Center, U.S. Geolo•ticalSurvey, Denver, Colorado This synthesislinks many seismic and tectonic processesat subduction zones, including great subduc- tion earthquakes, to the sinking of subducted plate. Earthquake data and tectonic modeling for subduc- tion zones indicate that the slab pull force is much larger than the ridge pusJaforce. Interactions between the forces that drive and resist plate motions cause spatially and temporally localized stressesthat lead to characteristic earthquake activity, providing details on how subduction occurs. Compression is localized acrossa lockedinterface thrust zone, because both the ridgepush and the slabpull forcesare resisted there. The slab pull force increaseswith increasing plate age; thus becausethe slab pull force tends to bend subducted plate downward and decreasethe force acting normal to the interface thrust zone, the characteristicmaximum earthquake at a given interface thrust zone is inverselyrelated to the age of the subductedplate. The 1960 Chile earthquake (M w 9.5), the largestearthquake to occur in historic times, began its rupture at an interface bounding oceanic plate < 30 m.y. old. However, this rupture initiation was associatedwith the locally oldest subductinglithosphere (weakest coupling); the rupture propagated southward along an interface bounding progressivelyyounger oceanic lithosphere,terminating near the subductingChile Rise. Prior to a great subduction earthquake, the sinking subductedslab will cause increasedtension at depths of 50-200 km, with greatest tension near the shallow zone resisting plate subduction.Plate sinking not only leads to compressionalstresses at a locked interface thrust zone but may load compressionalstresses at plate depths of 260-350 km, provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle. This explains K. Mogi's observations of M •_ 7 thrustearthquakes at depthsof 260-350km, immediatelydowndip and within 3 yearsprior to five great, shallow earthquakes of northern Japan. The slab pull model explains the lower layer of double seismic zones as due to tension from the deeper, sinking plate and the upper layer as due to localized in-plate compression, as plate motion is resisted by the bounding mantle. Just downdip of the interface thrust zone,there occurs an aseismic20ø-50 ø dip increaseof subductedplate. This slabbend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change. Resistanceto subductionprovided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone. INTRODUCTION Ridge Push and Slab Pull Although the slab pull force is recognizedas dominant in Horizontal density contrasts, resulting from cooling and moving tectonic plates, mechanisms by which this force is transmitted through locked subduction zones have received thickening oceanic lithosphere, produce the ridge push (sliding little attention. This is becausemany investigators believe that plate) force which contributes significantly to plate motions most of the slab pull force is balanced by friction between the [Hales, 1969; Andersonet al., 1976; Lister, 1975; Hager, 1978; plate and the surroundingmantle. Reasonsfor the persistence Solomon et al., 1980; Hager and O'Connell, 1981]. The nega- of this view include (1) the perception that great, interface tive buoyancy of subducted oceanic lithosphere (the slab pull thrust earthquakesat subductionzones primarily are caused force) is established as the major driving force for plate mo- by compression from the oceanic side, (2) results from glacial tions [Elsasset, 1969; McKenzie, 1969; Isacks and Molnar, rebound studies indicating that mantle viscosity is high 1971' Smith and Toks6z, 1972; Forsyth and Uyeda, 1975; Schu- enough to inhibit deep plate motions from influencing shallow bert et al., 1975; Solomon et al., 1975; Vlaar and Wortel, 1976; tectonics, and (3) calculations of vertical forces acting at Richter, 1977; Chapple and Tullis, 1977' Carlson, 1981, 1983]. trenches, based on observed trench topographies, indicating Both a trench suction force and back arc spreading have much smaller forcesthan are predicted from thermal modeling been suggestedas mechanismsfor the trenchward movement of subducted plate. In this paper a synthesisof earthquake and of the leading edges of overriding plates [Forsyth and Uyeda, 1975' Carlson, 1983' Chase, 1978' Karig, 1974], but these tectonic data weakens the main objections to accepting the mechanisms are shown to be related to the sinking and sea- dominance of the slab pull force in driving the earthquake cycle. Also, this synthesis provides the conceptual framework ward propagation of subducted plate [Garfunkel et al., 1986]. for reinterpretation of several important dynamic properties of Hot spot push also may contribute to driving plate motions subduction zones. [Morgan, 1972; Chase, 1978]. Forsyth and Uyeda [1975] determined that the slab pull FORCES THAT MOVE LITHOSPHERIC PLATES force, Fse, is 10 times more important than the ridge push Plate-driving forces largely are due to within-plate density force, F ae, in moving oceanic lithospherebut also suggested that most of the slab pull force is balanced by viscous dissi- contrasts. These forces largely determine how plates move at pation in the mantle rather than a large component being subduction zones and consequently determine how stresses balanced near shallow subduction zones. A linear inversion of accumulate there. plate velocities relative to specificsubduction zone geometries by Carlson [1983] indicatesthat the slab pull force is about 3 This paper is not subjectto U.S. copyright.Published in 1987 by times more significant than the ridge push force in determin- the American Geophysical Union. ing plate velocities. Paper number 6R0645. The slab pull force cannot initiate subduction. However, as 55 56 SPENCE: SLAB PULL TABLE 1. Great Normal-Faulting Earthquakes, 1930 to the Present Date Location M o, dyn cm Depth Reference Jan. 15, 1931 Oaxaca 3.5 X 1027 shallow Singh et al. 1985] March 2, 1933 Sanriku 4.3 X 1028 shallow Kanamori [1971] Nov. 4, 1963 Banda Sea 3.1 X 1028 100 km Osada and Abe [1981] May 26, 1964 SouthSandwich Islands 6.2 X 1027 120 km Abe [1972a] May 31, 1970 CentralPeru 1.0 X 1028 60 km Abe [1972b] June22, 1977 Tonga 2.3 x 1028 65 km Silver and Jordan [1983] Aug. 19, 1977 East Sundaarc 2.4-4.0 x 1028 shallow Spence[1986] initially subducted lithosphere enters the upper mantle, it is tion indicate that at shallow subduction zones the maximum apparent that slab sinking, due to the slab pull force, becomes tensional stressesof slab pull origin are significantly greater increasingly dominant in subduction processes.The motions than the maximum compressional stresses of ridge push of plates and subducted lithospheres are important compo- origin. nents of global patterns of mantle flow [Hager et al., 1983; SLAB PULL AS A CAUSE OF SUBDUCTION Garfunkel et al., 1986]. gONE EARTHQUAKES Earthquake Evidencefor Relative Size of In order to providea frameworkfor the developmentof this Slab Pull and Ridge Push Forces paper, the tectonicmodel advocatedhere is summarized.Sub- Although modeling of plate motions implies that the slab sequentsections provide observationalsupport for the model pull force is the dominant force at subduction zones, earth- or deal with model refinements. quake data have not been directly used to confirm this con- How can tensional stressesarising from slab pull forceslead clusion. The episodic nature of subduction is implicit in the to thrust-faultingearthquakes at an interfacethrust zone when widely accepted seismicgap hypothesis [Fedotov, 1965; Mogi, such earthquakes imply that compression acts across this 1968; Kelleher et al., 1973; McCann et al., 1979; Sykes and interface? Consider the analogy of pulling a massive,rigid box Quirtmeyer, 1981]. In the time interval between repeating across a rough floor. While the body forces within the box great earthquakes at a seismicgap, plate motion there is large- would be tensional, a strong box would not deform signifi- ly blocked, and stressesaccumulate. An examination of the cantly but would tend to translate,with compressionalstresses sizes of earthquakes downdip and updip of interface thrust localized at the interface between the box and floor. In the zones, outside of the times of great interface thrust earth- caseof a subductingoceanic lithosphere, the slab pull force is quakes, should indicate the plate strains there and the relative resisted by the overriding plate, causing compressionat the size of the slab pull and ridge push forces acting at shallow interface thrust zone. subduction zones. Prior to a great subductionearthquake, the subductedplate The four great normal-faulting earthquakes at depths great- slowly sinks,increasing extensional stresses at depthsof 50 to er than 50 km in Table 1 occurred downdip of the correspond- 200 kin, with the greateststresses at shallowdepths. Although ing interface thrust zones. Also, the great Oaxaca earthquake slab pull stressesare guided updip, these stressesare dimin- probably is below the primary interface thrust zone. Although ished by forces resisting subduction, leaving 5-10% of the shallow-dipping lithosphereexists downdip of the central Peru grossslab
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