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Polarized Plate Tectonics ARTICLE IN PRESS Polarized Plate Tectonics Carlo Doglioni*,1, Giuliano Panzax,{ *Dipartimento di Scienze della Terra, Universita Sapienza, Roma, Italy x Dipartimento di Matematica e Geoscienze, Universita di Trieste, ICTP-SAND group, Trieste, Italy and Institute of Geophysics, China Earthquake Administration, Beijing, China { International Seismic Safety Organization (ISSO) 1Corresponding author: E-mail: [email protected] Contents 1. Introduction 2 1.1 Mantle Stratigraphy 8 2. Tectonic Equator and Westward Drift of the Lithosphere 10 3. Asymmetries along Opposite Subduction Zones 25 4. Asymmetries along Rift Zones 52 5. Mediterranean Geodynamics 62 6. Mechanisms of Plate Tectonics 95 6.1 Mantle Convection 98 6.2 Slab Pull 102 6.3 Astronomical Tuning 122 7. Discussion 132 8. Conclusions 144 Acknowledgments 147 References 148 Abstract The mechanisms driving plate motion and the Earth’s geodynamics are still not entirely clarified. Lithospheric volumes recycled at subduction zones or emerging at rift zones testify mantle convection. The cooling of the planet and the related density gradients are invoked to explain mantle convection either driven from the hot interior or from the cooler outer boundary layer. In this paper we summarize a number of evidence sup- porting generalized asymmetries along the plate boundaries that point to a polariza- tion of plate tectonics. W-directed slabs provide two to three times larger volumes to the mantle with respect to the opposite E- or NE-directed subduction zones. W-directed slabs are deeper and steeper, usually characterized by down-dip compres- sion. Moreover, they show a shallow decollement and low elevated accretionary prism, a steep regional monocline with a deep trench or foredeep, a backarc basin with high heat flow and positive gravity anomaly. Conversely directed subduction zones show antithetic signatures and no similar backarc basin. Rift zones also show an asymmetry, e.g., faster Vs in the western lithosphere and a slightly deeper bathymetry with respect to the eastern flank. These evidences can be linked to the westward drift of the Advances in Geophysics, Volume 56 ISSN 0065-2687 © 2015 Elsevier Inc. http://dx.doi.org/10.1016/bs.agph.2014.12.001 All rights reserved. 1 j ARTICLE IN PRESS 2 Carlo Doglioni and Giuliano Panza lithosphere relative to the underlying mantle and may explain the differences among subduction and rift zones as a function of their geographic polarity with respect to the “tectonic equator.” Therefore also mantle convection and plate motion should be polar- ized. All this supports a general tuning of the Earth’s geodynamics and mantle convec- tion by astronomical forces. 1. INTRODUCTION Plate tectonics provides the tectonic framework supporting Wegener’s hypothesis of continental drift. Objections to the original formulation of continental drift and plate tectonics have been focused on the driving mech- anism and on the evidence that light continental lithosphere is subducted in continentecontinent collision areas (Anderson, 2007a; Mueller & Panza, 1986; Panza & Mueller, 1978; Panza, Calcagnile, Scandone, & Mueller, 1982; Panza & Suhadolc, 1990; Pfiffner, Lehner, Heitzmann, Muller,€ & Steck, 1997; Schubert et al., 2001; Suhadolc, Panza, & Mueller, 1988). Therefore, the origin of plate tectonics and the mechanisms governing the Earth’s geodynamics are still under debate. The most accepted model for the dynamics of the planet is that tectonic plates are the surface expression of a convection system driven by the thermal gradient from the hot inner core (5500e6000 C) and the surface of the Earth, being the shallowest about 100 km the upper thermal boundary layer, a <1300 C internally not convecting layer called lithosphere. However, during the last decades it has been shown that: 1. mantle convection driven from below cannot explain the surface kine- matics (Anderson, 2007); 2. plates do not move randomly as required by a simple RayleigheBénard convective system, but rather follow a mainstream of motion (Crespi, Cuffaro, Doglioni, Giannone, & Riguzzi, 2007; Doglioni, 1990); 3. plate boundaries rather show asymmetric characters (Doglioni, Carminati, Cuffaro, & Scrocca, 2007; Panza, Doglioni, & Levshin, 2010). Currently accepted engines for plate tectonics do not seem to supply suf- ficient energy for plate’s motion and do not explain the globally observed asymmetries that from the Earth surface reach mantle depths. Moreover, convection models are generally computed as deforming a compositionally homogeneous mantle, whereas the Earth is chemically stratified and laterally highly heterogeneous (e.g., Anderson, 2006). Based on these assumptions, it is here reviewed an alternative model of plate tectonics, in which the internal ARTICLE IN PRESS Polarized Plate Tectonics 3 heat still maintains possible mantle convection, but mostly dominated from above (Anderson, 2001). The lithosphere is active in the process, likely sheared by astronomically forces. Moreover, in a not chemically homoge- neous mantle, tomographic images are not indication of cold and hot vol- umes but also of chemical heterogeneity (Anderson, 2006; Foulger et al., 2013; Tackley, 2000; Thybo, 2006; Trampert, Deschamps, Resovsky, & Yuen, 2004). The Earth is subject to the secular cooling of a heterogeneous and stratified mantle, and to astronomical tuning. The combination of these two parameters is here inferred as the main controlling factor of plate tectonics. The planet is still hot enough to maintain at about 100 km depth the top of a layer where partial melting determines a low-velocity and low-viscosity layer at the top of the asthenosphere, allowing partial decoupling of the lithosphere with respect to the underlying mantle (Figure 1). The Earth’s rotation and the tidal despinning generate a torque acting on the lithosphere, and producing a net westerly directed rotation of the lithosphere with respect to the under- lying mantle, being this rotation decoupled in the low-velocity astheno- spheric layer (LVZ) where some melting occurs (Figure 2). Melting is testified by petrological models, slowing down seismic waves and magnetotelluric data both beneath oceans and continents (e.g., Crépisson et al., 2014; Naif, Key, Constable, & Evans, 2013; Panza, 1980 and references therein) favored by the presence of water (Figure 3), the super- adiabatic condition of the LVZ (low-velocity zone) (Anderson, 2013)dueto the thermal buffer of the overlying LID (lithospheric mantle) and shear heat- ing. The overlying lithosphere acts as an insulator for the heat dissipated by the underlying mantle due to the pristine heat from the Earth’s early stage of magma ocean, plus the heat delivered by radiogenic decay from the inter- nal mantle itself (Hofmeister & Criss, 2005; Lay, Hernlund, & Buffet, 2008; Rybach, 1976). Moreover, in the LVZ has been inferred the presence of a large amount of H2O delivered by the pargasite mineral when located at pres- sure >3GPa,>100 km (Green, Hibberson, Kovacs, & Rosenthal, 2010)asin Figure 3. The asthenosphere is in superadiabatic condition, having a potential temperature (Tp) possibly higher than that of the underlying mantle, which has been inferred to be subadiabatic and less convecting (Anderson, 2013). The viscosity of the asthenosphere is generally inferred by inverting uplift rates of postglacial rebound or glacial isostatic adjustment (GIA) e.g., Kornig & Muller (1989). However, the viscosity is the resistance to flow under a shear and postglacial rebound has two limitations: (1) it computes the vertical movements in the mantle and (2) thin layers such as the LVZ may be invisible by the GIA (for a discussion, see Doglioni, Ismail-Zadeh, Panza, & Riguzzi, 4 ARTICLE IN PRESS Figure 1 Main nomenclature and parameters used to describe the uppermost 450 km of the Earth, after Doglioni and Anderson (2015). Data after Jin et al. (1994), Pollitz et al. (1998), Anderson (1989, 2007a, 2013), Doglioni et al. (2011), Tumanian et al. (2012) and references therein. The gray bands are a gross representation of the uncertainties involved. M, Moho. The lithosphere and the seismic lid (LL) are underlain by Carlo Doglioni and Giuliano Panza aligned melt accumulations (AMA), a low-velocity anisotropic layer (LVZ) that extends from the Gutenberg (G) to the Lehmann (L) discon- tinuity. Their combination forms LLAMA. The amount of melt in the global LVZ is too small to explain seismic waves speeds and anisotropy unless the temperature is w200 C in excess of mid-ocean ridge basalt (MORB) temperatures, about the same excess required to explain Hawaiian tholeiites (Hirschmann, 2010) and oceanic heat flow. This suggests that within-plate volcanoes sample ambient (local) boundary layer mantle. The lowest wave speeds and the highest temperature magmas are associated with the well-known thermal overshoot. The most likely place to find magmas hotter than MORB is at this depth under mature plates, rather than in the subadiabatic interior. Most of the delay and lateral variability of teleseismic travel times occur in the upper 220 km. ARTICLE IN PRESS Polarized Plate Tectonics 5 Figure 2 In the upper asthenosphere or lower lithosphere and the seismic lid with aligned melt accumulations, the geotherm is above the temperature of mantle solidus, and small pockets of melt can cause a strong decrease of the viscosity. In the low- velocity zone (LVZ) of the asthenosphere the viscosity can be much lower than the present-day estimates of the asthenosphere viscosity based on the postglacial rebound; in fact the
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