Advances in Geophysics (eBook)

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2015 | 1. Auflage
326 Seiten
Elsevier Science (Verlag)
978-0-12-802436-2 (ISBN)

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The critically acclaimed serialized review journal for over 50 years, Advances in Geophysics is a highly respected publication in the field of geophysics. Since 1952, each volume has been eagerly awaited, frequently consulted, and praised by researchers and reviewers alike. Now in its 56th volume, it contains much material still relevant today--truly an essential publication for researchers in all fields of geophysics. - Contributions from leading authorities - Informs and updates on all the latest developments in the field

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Chapter One

Polarized Plate Tectonics

Carlo Doglioni∗,1 and Giuliano Panza§,¶     ∗Dipartimento di Scienze della Terra, Università Sapienza, Roma, Italy     §Dipartimento di Matematica e Geoscienze, Università di Trieste, ICTP-SAND group, Trieste, Italy and Institute of Geophysics, China Earthquake Administration, Beijing, China     ¶International Seismic Safety Organization (ISSO)
1 Corresponding author: E-mail: [e-mail] 

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 supporting generalized asymmetries along the plate boundaries that point to a polarization 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 compression. 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 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 polarized. All this supports a general tuning of the Earth's geodynamics and mantle convection by astronomical forces.

keywords

Asymmetric plate boundaries; Geodynamic mechanisms; Plate kinematics; Tectonic equator; Westward drift of the lithosphere

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 mechanism and on the evidence that light continental lithosphere is subducted in continent–continent collision areas (Anderson, 2007a; Mueller & Panza, 1986; Panza & Mueller, 1978; Panza, Calcagnile, Scandone, & Mueller, 1982; Panza & Suhadolc, 1990; Pfiffner, Lehner, Heitzmann, Müller, & 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 (5500–6000 °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 kinematics (Anderson, 2007);

2. plates do not move randomly as required by a simple Rayleigh–Bénard 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 sufficient 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 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 homogeneous mantle, tomographic images are not indication of cold and hot volumes 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 underlying mantle, being this rotation decoupled in the low-velocity asthenospheric 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 superadiabatic condition of the LVZ (low-velocity zone) (Anderson, 2013) due to the thermal buffer of the overlying LID (lithospheric mantle) and shear heating. 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 internal 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 pressure >3 GPa, >100 km (Green, Hibberson, Kovacs, & Rosenthal, 2010) as in 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, 2011; Scoppola, Boccaletti, Bevis, Carminati, & Doglioni, 2006. The LVZ is a layer in which the viscosity can be extremely lowered (Dingwell et al., 2004; Hirth & Kohlstedt, 1995; 2003; Mei, Bai, Hiraga, & Kohlstedt, 2002) and far lower than GIA estimates, when computed under horizontal shear (Hansen et al., 2012; Riguzzi et al., 2010; Scoppola et al., 2006). It could reach values as low as 1012 Pa s (Jin, Green, & Zhou, 1994). Therefore, the LVZ represents the basal fundamental decollement of plate tectonics and it is well expressed worldwide (Rychert & Shearer, 2009) by the slowdown of seismic waves (Figure 4). The westerly polarization of the lithosphere motion relative to the underlying mantle controls a diffuse asymmetry along plate boundaries, which are shaped by the “eastward” relative mantle flow with respect to the overlying lithosphere. Velocity gradients are the by-product of the lateral viscosity variations in the low-velocity layer, i.e., the decollement plane. The lower the viscosity, the faster westward motion of the overlying lithosphere (e.g., the Pacific plate). Low viscosity in the LVZ can be generated both by higher than standard ambient temperature (Figure 5) and the presence of fluids (Figure 3). Velocity gradients among plates determine tectonics at plate margins and related seismicity (Doglioni, 1993a). The horizontal component of the solid Earth's tide pushes plates to the “west.” When faults reach the critical state, the vertical component of tides may trigger the earthquake due to variations of g. The same variation acts in opposite directions as a function of the fault type (Riguzzi, Panza, Varga, & Doglioni, 2010): it can determine the increase or decrease of σ1 (maximum principal compressive stress) in extensional environments, or of σ3 (minimum principal compressive stress) in compressional tectonic settings.

Figure 1 Main...

Produkt-Details

• Erscheint lt. Verlag: 7.4.2015
• Mitarbeit: Herausgeber (Serie): Renata Dmowska
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geologie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geophysik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik
• ISBN-10: 0-12-802436-4 / 0128024364
• ISBN-13: 978-0-12-802436-2 / 9780128024362