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On the Role of Subducting Oceanic Plateaus in the Development of Shallow flat Subduction

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On the Role of Subducting Oceanic Plateaus in the Development of Shallow flat Subduction Chapter 6 On the role of subducting oceanic plateaus in the development of shallow flat subduction Abstract Oceanic plateaus, aseismic ridges or seamount chains all have a thickened crust and their subduction has been proposed as a possible mechanism to explain the occurrence of flat subduction and related absence of arc magmatism below Peru, Central Chile and at the Nankai Trough (Japan). Its extra compositional buoyancy could prohibit the slab from sinking into the mantle. We investigated this effect using a numerical thermo-chemical convection model, simulating the subduction of an oceanic crust that contains an oceanic crustal plateau of 18 km thickness. With a systematic variation of the important physical parameters, we examined the physical circumstances that are required to obtain shallow flat subduction. Metastability of the basaltic crust in the eclogite stability field is of crucial importance for the slab to remain buoyant throughout the subduction process. The mod- eling results show that in a 44-Ma old subducting plate, basalt must be able to survive a Ó 7¼¼ temperature of 6¼¼ C to keep the plate buoyant sufficiently long to cause a flat slab segment. In addition, we found that the maximum yield stress in the slab must be limited to about 6¼¼ MPa to allow for the necessary bending to the horizontal. Young slabs show flat subduction for larger parameter ranges than old slabs, since they are less gravitation- ally unstable and show less resistance against bending. Furthermore, hydrous weakening of the mantle wedge area and lowermost continent are required to allow for the necessary deformation of a change in subduction style from steep to flat. The maximum flat slab extent, found in the models, is about 300 km. This is sufficient to explain the observed shallow flat subduction near the Nankai Trough (Japan), but additional mechanisms, such This chapter has been submitted for publication to Tectonophysics(2001). 67 68 Chapter6 as lithospheric doubling, need to be invoked to explain the flat slab segments up to 5¼¼ km long below Peru and Central Chile. 6.1 Introduction The subduction of regions of oceanic lithosphere with overthickened crust (aseismic ridges, oceanic plateaus or seamount chains) seems to coincide spatially and temporally with the absence of arc volcanism (McGeary et al., 1985). In some of these cases, the slab dips more shallow or even horizontal at a depth of approximately 100 km. For example, gaps in recent active volcanism at low-angle subduction below Peru, Central Chile and at the Nankai trough near Japan seem to be related to the subduction of the Nazca Ridge, Juan Fernandez seamount chain and Palau-Kyushu Ridge, respectively (Cross and Pilger, 1982; 7¼¼ McGeary et al., 1985). For these areas, a flat slab is observed at ¾¼¼ km (Nankai) and km (Peru and Chile) from the trench (Sacks, 1983; McGeary et al., 1985). In fact, flat subduction occurs along about ½¼± of the modern convergent margins, and most of these regions correlate with a subducting plateau or aseismic ridge (Gutscher et al., 2000b). Under such circumstances, absence of arc volcanism is explained by the disappearance of the mantle wedge with decreasing dip angle: contact of the hydrated subducted crust with the hot asthenosphere is absent, which stops partial melting and associated volcan- ism. Flat subduction of an even larger scale has been proposed to explain the formation of the Rocky Mountains during the Laramide orogeny (Dickinson and Snyder, 1978; Bird, 1988), although for this event no evidence of subducting oceanic plateaus is available. The correlation between the subduction of oceanic plateaus and the occurrence of flat subduction suggests these plateaus to be anomalously buoyant. The overthickened crust is underlain by a proportionally overthickened harzburgite layer. During subduction, the gabbroic or basaltic crust transforms into eclogite. Eclogite and harzburgite have com- positional buoyancies of opposite sign relative to undepleted mantle material (Irifune and Ringwood, 1993), that approximately cancel each other within an oceanic lithosphere. Only before eclogitisation, the subducting slab has a compositional buoyancy, which is dependent on the crustal thickness. If the eclogite formation would occur under near- equilibrium conditions, shallower than 70-80 km depth (Kirby et al., 1996a), slabs would lose their compositional buoyancy at this shallow depth, regardless of their crustal thick- ness. Only a kinetically hindered reaction from basalt to eclogite would cause the slab to preserve (some of) its compositional buoyancy to greater depth. Therefore, the hypothesis that buoyant plateaus cause flat subduction implicitly assumes a significant kinetic delay of eclogitisation of the crust. The kinetics of the basalt-to-eclogite transition has been subject to research for several decades. Ahrens and Schubert (1975) report no significant reaction rates under completely dry conditions, but geologically fast reaction rates with the addition of water. More re- cent experiments (e.g. (Holland, 1980) vs. (Hacker et al., 1993)) do not reach consensus about the metamorphic reaction rates in the oceanic crust under subduction conditions (Hacker, 1996). Geological observations (pT-paths) suggest a wide range of possible re- Ó action rates (Hacker, 1996; Austrheim, 1998): complete transformation at 45¼ Candin- 6.2 Model description 69 Ó complete transformation at 8¼¼ C are both recorded. The degree of hydration is probably at least as significant as the ambient temperature and pressure and dry rocks can remain metastable for a long time (Rubie, 1990; Austrheim, 1998). Furthermore, deformation both enhances the reaction rates and facilitates the access of fluids (Saunders et al., 1996). Incomplete reaction to eclogite has also been suggested on the basis of seismological studies (Abers, 2000; Kirby et al., 1996a). Increased buoyancy of the overthickened crust is not the only proposed mechanism to explain shallow flat subduction of the plateaus. Non-hydrostatic pressure forces (Jischke, 1975) or a slab suction force (Stevenson and Turner, 1977; Tovish et al., 1978) and active overriding of the overlying continent (lithospheric doubling) (Vlaar, 1983; van Hunen et al., 2000) are proposed as alternative mechanisms. In fact, the largest flat slab seg- ments (Central Chile and Peru) occur below the fastest overriding plate motion (Jarrard, 1986). This suggests that lithospheric doubling plays a significant role in the creation of a horizontally subducting slab. However, the striking correlation between the subducting plateau locations and the sites of shallow flat subduction suggests an important influence of these plateaus on the subduction process. Although several mechanisms have been proposed for the absence of arc volcanism above subducting plateaus, for the preservations of the compositional buoyancy of the plateaus and for the dynamics of the slab flattening, quantitative models are rare or absent. In this paper, we determine the viability and importance of shallow flat slab development as a consequence of a subducting ‘buoyant plateau’, using numerical modeling. 6.2 Model description 6.2.1 Governing equations and rheology description To study the dynamics of a thickened crust within a subducting oceanic lithosphere, we have developed a two-dimensional finite element numerical model. We adopt the ex- tended Boussinesq approximation (EBA) (Ita and King, 1994) and use a non-dimensiona- lization scheme as in (van den Berg et al., 1993). In non-dimensional form, the governing equations are: @ Ù =¼ j j (6.1) X @ ´ ¯_ µ @ ¡È =´ÊaÌ Êb · ÊcC µÆ ij i k k iÞ j (6.2) k X d @Ì Êb k k · Ù @ Ì Di´Ì · Ì µÛ Di´Ì · Ì µ @ @ Ì ­ j j ¼ ¼ j j k @Ø Êa dØ k Di = À ¨· (6.3) Êa @C · Ù @ C =¼ j j (6.4) @Ø Equations 6.1 to 6.4 describe conservation of mass, momentum, energy and composition, respectively. Symbols are explained in Table 6.1. 70 Chapter6 Symbol Meaning Value used Dimension Ò ½ A Èa × k;Ñ Pre-exponential flow law parameter 6 ½ ½ A ½¼ Ã × kiÒ Kinetic pre-exponent C Composition parameter ½ ½ c ½¾5¼ Âkg Ã Ô specific heat Di «g h=c ¼:47 Dissipation number = Ô £ ½ E  ÑÓÐ k;Ñ activation energy £ ½ E  ÑÓÐ kiÒ kinetic activation energy À non-dimensional radiogenic heat production k material index Ñ deformation mechanism index Ò k;Ñ viscosity stress exponent Ò 5 Ý yield stress exponent ½ É Âkg Ä Latent heat release across a phase transition ½ ¿ 8:¿½4¿ ÂÃ Ñ Ê gas constant ¿ 7 Êa «¡Ìh ½:8 ¢ ½¼ Thermal Rayleigh number = ¿ 7 Ægh = ¾:4 ¢ ½¼ Êb Phase Rayleigh number ¿ 7 Êc ¡ gh = ¾:4 ¢ ½¼ Compositional Rayleigh number c Ì non-dimensional temperature Ì ¾7¿=¡Ì ¼ non-dimensional surface temperature ½ G  ÑÓÐ ¡ Gibbs free energy between basalt and eclogite È ¡ non-dimensional hydrodynamic pressure Ì ¾¿¼¼ à ¡ temperature contrast across model domain Ø non-dimensional time Ø ½¼ Åa ØÖ transition time for the eclogitisation reaction Ì Ù =´Ú; Ûµ Ù non-dimensional velocity ½ £ ¿ Ñ ÑÓÐ Î activation volume k;Ñ Y growth function of the eclogitisation reaction 5 ½ ¿ ¢ ½¼ à « thermal expansion coefficient k k phase functions for all mantle phase transitions ½ ­ ¿ ÅÈaà 4¼¼ Clapeyron slope 400 km phase transition ½ ­ ¾:5 ÅÈaà 67¼ Clapeyron slope 670 km phase transition ½ G  ÑÓÐ ¡ Gibbs free energy difference between reaction products ¡ c compositional density relative to mantle material ¿ 4¼¼ kg Ñ basalt ¿ 77 kg Ñ harzburgite à ÆÌ temperature increase due to latent heat release Ñ ÆÞ phase transition deflection ¿ Æ ¾7¿ kg Ñ 4¼¼ density difference across the 400 km phase transi- tion ¿ Æ ¿4¾
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