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How Mantle Slabs Drive Plate Tectonics

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How Mantle Slabs Drive Plate Tectonics See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/11093558 How Mantle Slabs Drive Plate Tectonics Article in Science · November 2002 DOI: 10.1126/science.1074161 · Source: PubMed CITATIONS READS 280 350 2 authors: Clinton P. Conrad Carolina Lithgow-Bertelloni University of Oslo University of California, Los Angeles 119 PUBLICATIONS 3,502 CITATIONS 126 PUBLICATIONS 4,840 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Shear-driven upwelling (SDU) and Intraplate Volcanic Fields View project The link between Subduction Parameters and Surface Topography View project All content following this page was uploaded by Carolina Lithgow-Bertelloni on 31 May 2014. The user has requested enhancement of the downloaded file. R EPORTS criterion that corresponds to a maximum allowable rate of trench migration at the How Mantle Slabs Drive Plate surface (17). We defined three models for the slab pull force that used rollback rate Tectonics cutoffs of 10% or 25% (20) and included or excluded material below 660 km (Fig. 1). Clinton P. Conrad* and Carolina Lithgow-Bertelloni Slab pull is applied normal to subducting plate boundaries and totals 1.9 ϫ 1021 N The gravitational pull of subducted slabs is thought to drive the motions of for all upper-mantle slabs. By comparison, Earth’s tectonic plates, but the coupling between slabs and plates is not well slab suction shear tractions from both up- established. If a slab is mechanically attached to a subducting plate, it can exert per- and lower-mantle slabs total 1.6 ϫ a direct pull on the plate. Alternatively, a detached slab may drive a plate by 1021 N. Thus, the slab pull force can be exciting flow in the mantle that exerts a shear traction on the base of the plate. comparable to, or even more important From the geologic history of subduction, we estimated the relative importance than, the slab suction force as a plate- of “pull” versus “suction” for the present-day plates. Observed plate motions driving mechanism. are best predicted if slabs in the upper mantle are attached to plates and Because the slab pull and slab suction generate slab pull forces that account for about half of the total driving force plate driving mechanisms act differently on on plates. Slabs in the lower mantle are supported by viscous mantle forces and subducting and overriding plates, they drive plates through slab suction. should generate different patterns of sur- face plate motion. To constrain the relative Although the motions of Earth’s tectonic subduction for each of the nine most tec- importance of these mechanisms, we com- plates are generally accepted to be the sur- tonically active present-day subduction pared observed plate motions (17) with pre- face expression of convection in Earth’s zones (Fig. 1). The excess weight of this dicted plate motions that we computed mantle (1), the mechanism by which mantle material should generate the slab pull force, from the slab pull and slab suction driving convection drives plate motions has been but only if the slab remains mechanically mechanisms (Fig. 2). We predicted plate the subject of debate for some time (2–7). coherent within the mantle. Rapid trench velocities by first computing resisting shear Mantle convection may be driven primarily migration and the phase transition at 660 tractions that are induced by viscous flow by the descent of dense slabs of subducted km may cause slab coherency to deteriorate in the mantle (6, 15) and then enforcing the oceanic lithosphere (8–10), which are the (19). Thus, we estimated slab pull forces on no-net-torque approximation for each plate most prominent density heterogeneities in the basis of the excess weight of all con- (2). The predicted plate velocity field de- the mantle (11, 12). The motions of the nected slab material, subject to an inclusion pends on the mantle viscosity structure, surface plates have also been attributed to the pull from descending slabs (2–7, 13), but it is not clear whether the two are directly coupled, or whether induced man- tle flows transmit stresses from slabs to plates. If slabs remain mechanically at- tached to the surface plates as they subduct, Central(Cocos) AmericaSouth(Nazca) AmericaJava(Indian-Aus.) - BengalNew(Indian-Aus.) HebridesTonga,(Pacific) KermadecMarianas,(Pacific) Izu-BoninAleutian(Pacific) NE Japan,Kamchatka Kurile,Philippine, (Pacific)(Philippine) SE Japan then slabs can act as stress guides that 0 transmit the downward pull of dense mantle slabs directly to plate boundaries (14). 500 These “slab pull” forces drive subducting plates toward subduction zones. Alterna- tively, if slabs in the mantle are not well 1000 attached to the surface plates, then de- scending slabs may induce mantle circula- tion patterns that exert shear tractions at the 1500 base of nearby plates. These “slab suction” Depth (km) forces have been shown to cause both sub- 2000 Slab Model 1: ducting and overriding plates to move to- Upper Mantle, 10% Rollback ward subduction zones (1, 5–8, 15). Slab Model 2: To determine the relative importance of 2500 Whole Mantle, 10% Rollback the slab pull and slab suction forces for the Slab Model 3: Whole Mantle, 25% Rollback mantle, we examined a model of present- 3000 day slab locations (6, 16, 17) that was 0 50 100 0 50 100 0 50 100 0 50 100 0 50 100 0 50 100 0 50 100 0 50 100 0 50 100 computed from Cenozoic (18) and Mesozo- Average Slab Thicknesses (km) ic (6) plate reconstructions. Using the his- Fig. 1. Profiles of the slab material attached to the nine subduction zones used in this study.Slab tory of subduction since the Mesozoic, we ␬ 1/2 thickness is expressed as 2( tc) , where tc is the average age of the slab at the time of subduction defined the material in the slab location and ␬ϭ10Ϫ6 m2 sϪ1 is the thermal diffusivity.Although average slab thicknesses are shown here, model that has been part of continuous variations in thickness along the length of each slab are important and are included in the determination of the pull forces on plates.In slab model 1 (red), slab material is restricted to the upper mantle.In slab models 2 and 3, slab material is permitted in the lower mantle but restricted Department of Geological Sciences, University of to a departure from verticality represented by 10% trench rollback (model 2, green) or 25% trench Michigan, Ann Arbor, MI 48109, USA. rollback (model 3, blue).The plate to which each slab is attached is shown in parentheses.The total *To whom correspondence should be addressed.E- excess mass of attached slabs is 1.3 ϫ 1020,2.9ϫ 1020, and 7.0 ϫ 1020 kg for models 1, 2, and mail: [email protected] 3, respectively. www.sciencemag.org SCIENCE VOL 298 4 OCTOBER 2002 207 R EPORTS which we take to be the one that gives the tion that plates with subduction zones move at 3 moving overriding plates that surround the best fit to the geoid for the slab heteroge- to 4 times the rate of plates without subduction faster subducting plates in the Pacific basin. neity model (6). Our calculations did not zones (18, 22). This difference in plate speeds, The most important discrepancy between the include any possible lateral variations in which has been the subject of both controversy predicted and observed fields is that the slab viscosity, which can affect the relative cou- and study (18, 21), cannot be produced by the pull mechanism causes overriding plates to pling of continental and oceanic plates to slab suction mechanism acting alone, because move slowly away from subduction zones mantle flow (21). This simplification the pattern of mantle flow excited by downgo- (Eurasia, North America, and South America should have a greater effect on patterns of ing slabs exerts shear tractions equally (sym- in Fig. 2C), whereas these plates move slowly plate motions driven by slab suction than metrically) on both subducting and overriding toward subduction zones in the observed field on those of plate motions driven by slab plates. In addition, the slab suction mechanism (Fig. 2B). This motion in the wrong direction pull, because the former plate-driving causes subducting plates such as the Pacific, occurs because the mantle flow pattern gen- mechanism depends directly on plate-man- and the smaller Cocos and Nazca plates in erated by the motion of subducting plates tle coupling while the latter does not. particular, to move too slowly relative to the toward subduction zones typically causes If the driving torques on plates are generated others—and, in the case of the Nazca plate, in overriding plates to move in the same direc- by the slab suction mechanism acting alone, the the wrong direction. tion as subducting plates. This counterintui- predicted plate velocity field (Fig. 2A) differs Slab pull forces acting by themselves also tive asymmetrical effect of the slab pull from the observed plate velocity field (17) (Fig. cause subducting plates to move toward sub- mechanism is the key to understanding the 2B) in several ways. Most important, the slab duction zones (Fig. 2C), but in this case they difference between the observed speeds of suction model causes overriding plates to move move more quickly than the overriding subducting and overriding plates. toward neighboring subduction zones at speeds plates. The pattern of plate speeds (Fig. 2C) is Compared to observed plate motions, over- comparable to those of the subducting plates similar to that of the observed plate motions riding plates in the slab suction model move too (Fig.
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