Lithospheric Discontinuities

Huaiyu Yuan, Barbara Romanowicz, Huaiyu Yuan, Barbara Romanowicz

• Publisher: American Geophysical Union
• Year: 2018
• Print ISBN: 9781119249719
• eBook ISBN: 9781119249733
• Edition: 1
• Topic: Ciencias biológicas
• Subtopic: Ecología

A multidisciplinary update on continental plate tectonics and plate boundary discontinuities

Understanding the origin and evolution of the continental crust continues to challenge Earth scientists. Lithospheric Discontinuities offers a multidisciplinary review of fine scale layering within the continental lithosphere to aid the interpretation of geologic layers. Once Earth scientists can accurately decipher the history, internal dynamics, and evolution of the continental lithosphere, we will have a clearer understanding of how the crust formed, how plate tectonics began, and how our continents became habitable.

Volume highlights:

• Theories and observations of the current state of tectonic boundaries anddiscontinuities
• Contributions on field observations, laboratory experiments, and geodynamic predictions from leading experts in the field
• Mantle fabrics in response to various mantle deformation processes
• Insights on fluid distribution using geophysical observations, and thermal and viscosity constraints from dynamic modeling
• Discontinuities associated with lithosphere and lithosphere-asthenosphere boundary
• An integrated study of the evolving physical and chemical processes associated with lithosphere asthenosphere interaction

Written for academic and researchgeoscientists, particularly in the field of tectonophysics, geophysicists, geodynamics, seismology, structural geology, environmental geology, and geoengineering, Lithospheric Discontinuities is a valuable resource that sheds light on the origin and evolution of plate interaction processes.

1
On the Origin of the Upper Mantle Seismic Discontinuities

Shun‐ichiro Karato and Jeffrey Park

Department of Geology and Geophysics, Yale University, New Haven, CT, USA

ABSTRACT

Recent high‐frequency seismological studies revealed enigmatic structures in the upper mantle: the mid‐lithosphere discontinuity (MLD) in the old continents and the oceanic lithosphere–asthenosphere boundary (LAB) at relatively shallow depth. Both discontinuities occur at relatively low temperatures (~1000 °C) making it difficult to attribute them to partial melting. Also the inferred relatively sharp and large velocity drop at these boundaries is difficult to explain with conventional smooth temperature‐dependent elastic and anelastic properties from an absorption‐band Q model. Models that go beyond these two mechanisms are needed to explain these characteristics of the MLD and the LAB. Three models are reviewed: (a) layered anisotropy, (b) layered composition, and (c) elastically accommodated grain‐boundary sliding (EAGBS). A layered anisotropy model is difficult to reconcile with the presence of these discontinuities for diverse ray geometries, and a layered composition model is not supported by the petrological/geochemical observations. EAGBS can explain globally observed substantial velocity drop at a modest temperature. This model implies that the link between seismic‐wave velocity and the long‐term creep strength is indirect, and predicts a peak in attenuation near these boundaries. However, experimental studies on EAGBS and geophysical tests are incomplete. Directions of future studies to test these models are suggested.

1.1. INTRODUCTION

Major seismic discontinuities both bound the transition zone (~410 to ~660 km) and occur within it. In this depth range, the seismic discontinuities are attributed mostly to phase transformations in the major mantle minerals [e.g., Ringwood, 1991]. In contrast, the volumetrically dominant upper‐mantle mineral olivine suffers no phase transformations shallower than ~410 km. (There is a phase transformation in pyroxene ((Mg,Fe)SiO₃), the second most abundant mineral in the upper mantle, but the influence of this phase transformation on seismic‐wave velocity is relatively minor [e.g., Matsukage et al., 2005].) Several seismic features interpreted as discontinuities, however, have been reported in the upper mantle [e.g., Thybo and Perchuć, 1997; Kawakatsu et al., 2009; Rychert and Shearer, 2009; Tauzin et al., 2010; Beghein et al., 2014; Hopper and Fischer, 2015; Wei and Shearer, 2017], among which we focus on two well‐documented discontinuities in the upper mantle: the lithosphere–asthenosphere boundary (LAB) in both oceanic and continental upper mantle and the mid‐lithosphere discontinuity (MLD) in the continental upper mantle.

At both the LAB and the MLD seismic velocities decrease with depth. Both the LAB and the MLD have been attributed to the onset of partial melting [e.g., Anderson and Spetzler, 1970; Thybo, 2006], layering in anisotropy [e.g., Yuan and Romanowicz, 2010b], layering in chemical/mineralogical composition [e.g., Selway et al., 2015], the smooth temperature dependence of seismic velocities including the influence of anelastic relaxation [e.g., Gueguen and Mercier, 1973; Faul and Jackson, 2005], and physical dispersion associated with temperature and water‐dependent anelasticity facilitated by elastically accommodated grain‐boundary sliding [Karato, 2012; Karato et al., 2015]. These models differ in their implications for the geological significance of the LAB and the MLD. Therefore it is important to understand which mechanisms may cause these discontinuities.

In this chapter, we review geophysical observations on upper mantle discontinuities and their interpretation in terms of elastic and nonelastic properties of rocks. In the next section (section 1.2), we review seismological observations on these discontinuities and emphasize the importance of using a broad range of mutually complementary seismological approaches (e.g., high‐frequency body waves, low‐frequency surface waves). Also, we will briefly summarize magnetotelluric (MT) observations on electrical conductivity as far as it is related to the MLD and the LAB (see also Chapters 2 and 5). In section 1.3 we review geological (and petrological) observations related t...

Table of contents

• Cover
• Table of Contents
• PREFACE
• Introduction—Lithospheric Discontinuities
• 1 On the Origin of the Upper Mantle Seismic Discontinuities
• 2 The Evolution of the Oceanic Lithosphere: An Electromagnetic Perspective
• 3 Lithospheric and Asthenospheric Structure Below Oceans from Anisotropic Tomography
• 4 Seismic Imaging of the Base of the Ocean Plates
• 5 Electrical Discontinuities in the Continental Lithosphere Imaged with Magnetotellurics
• 6 A Lithosphere–Asthenosphere Boundary—a Global Model Derived from Multimode Surface‐Wave Tomography and Petrology
• 7 Frayed Edges of Cratonic Mantle Keels: Thermal Diffusion Timescales and Their Predicted Imprint on Mantle‐Velocity Structure
• 8 Perspectives of the S‐Receiver‐Function Method to Image Upper Mantle Discontinuities
• 9 Continental Lithospheric Layering Beneath Stable, Modified, and Destroyed Cratons from Seismic Daylight Imaging
• 10 Cratonic Lithosphere Discontinuities: Dynamics of Small‐Volume Melting, Metacratonization, and a Possible Role for Brines
• INDEX
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