Introduction—Lithospheric Discontinuities

Huaiyu Yuan1 and Barbara Romanowicz2

1 ARC Centre of Excellence for Core to Crust Fluid Systems, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia

2 Berkeley Seismological Laboratory, University of California, Berkeley, CA, USA

The origin and evolution of our continents remains one of the present grand challenges in the Earth Science research community. Deciphering the history, internal dynamics, and evolution of the lithosphere, both in continental and oceanic settings, can shed light on how the crust formed, how plate tectonics began, and how our continents became habitable. In the modern Earth, this information can improve our ability to monitor earthquakes, tsunamis and volcanoes, and ultimately alleviate the effects of these natural hazards. In addition, understanding the nature and evolution of the lithosphere has a direct impact on our capability to uncover hidden resources. The internal structure of continents, however, is still poorly understood. The thickness of the continental lithosphere, for example, one of the key parameters in understanding the formation and evolution processes both dynamically and chemically, has been the subject of debate in the community for many years. Identified as one of ten “Grand Challenges” in modern seismology [Lay et al., 2009], understanding the lithosphere–asthenosphere boundary (LAB), the base of the lithosphere or the LAB, is fundamental to determining why Earth has plate tectonics and continents and how Earth processes are controlled by material properties. These are also two of the ten “Grand Research Questions in the Solid‐Earth Sciences” [DePaolo et al., 2008]. The boundary, which must physically exist for plate tectonics to work, has been singularly difficult to pin down [Eaton et al., 2009; Fischer et al., 2010], especially beneath the oldest continental areas (cratons).

Over the past decades progress in detecting the LAB in both continental and oceanic lithosphere has started shedding light on mantle processes that could have formed and subsequently modified the lithosphere, and those that govern its interactions with the asthenosphere. These observations emerged from different fields such as seismology, geochemistry, magnetotellurics, rheology, and geodynamics simulations, which, together, have provided evidence for rapid changes in physical properties at the LAB. A major discovery has been that of the presence of a strong internal discontinuity—stronger than the LAB—in the subcontinental lithospheric upper mantle at a depth of about 100 km on average [e.g., Thybo and Perchuć, 1997], which has rapidly gained global cross‐disciplinary attention and confirmation, especially in the past decade, owing to improved coverage of continent with high quality broadband seismic stations. This feature is observed globally in stable parts of continents [Romanowicz, 2009; Rychert and Shearer, 2009], and is referred to as the midlithospheric discontinuity (MLD) [Fischer et al., 2010]. Little is known regarding the nature of this MLD, and consensus on its origin is the topic of vigorous debates [e.g., Thybo, 2006; Hansen et al., 2015; Karato et al., 2015; Rader et al., 2015; Selway et al., 2015; Chen, 2017; Kennett et al., 2017]. In addition, evidence from multiple seismological techniques has been mounting for the presence of fine‐scale multiple layering within the continental lithosphere [e.g., Bostock, 1997; Hopper and Fischer, 2015; Calò et al., 2016; Kennett et al., 2017]. Interpreting the fine‐scale layering in different geological contexts in terms of continent formation and evolution has now become a multidisciplinary effort that aims to reconcile field observations, laboratory experiments, and geodynamic predictions (Figure 1).

Figure 1 Chemical and seismic layering in the lithosphere. (a) A conceptual model of the subcontinental lithospheric mantle showing potential “markers” (red) of chemical stratification that may originate and feed various mineral systems and provide fluids/melts and pathways.

(Modified from Griffin et al. [2013]. Reproduced with the permission of Springer).

(b) A view of the North American continent from the seismological perspective, based on seismic anisotropy observations. Evolution of the continental lithosphere is envisioned as a multistep process, with older and shallower material having rock fabric/composition distinct from that of the younger and deeper lithosphere.

(Adapted from Gung et al., [2003] and Yuan and Romanowicz [2010].)

The chapters in this volume are intended to present state‐of‐the‐art understanding, challenges, and future research directions concerning the MLD and LAB, in both continental and oceanic settings. In Chapter 1, Karato and Park summarize the current understanding of the MLD and LAB from both the seismic and experimental perspectives. They review possible mechanisms for the observation of sharp seismic discontinuities corresponding to the MLD in cratons and the LAB in active regions, and make a case favoring the EAGBS (elastically activated grain boundary sliding) as a physical process responsible for observed seismic attenuation. Chapters 2–4 focus on the discontinuities in the evolution of the oceanic lithosphere. Evans and coauthors (Chapter 2) provide a review of electromagnetic observations and modeling of the oceanic lithosphere and asthenosphere. They suggest a model of oceanic lithosphere formation and development that reconciles discrepancies between predictions from simple thermal evolution and observations of its electrical structure. Montagner and Burgos (Chapter 3) document the evolution of the oceanic lithosphere by using seismic anisotropy derived from surface‐wave tomography. They suggest that oceanic lithosphere may not be as simple as a single layer, and a stratification between the Moho and the oceanic LAB is necessary. Rychert and coauthors (Chapter 4) summarize commonly used imaging techniques, and discuss recent findings with numerical modeling and experimental constraints in mapping the oceanic lithosphere and the LAB.

Chapters 5–9 target the continent. In Chapter 5, Selway presents a review of the studies of the crust, subcrustal lithospheric mantle, and asthenosphere boundaries from measurements of electrical conductivity. The strong laterally heterogeneous and vertically layered continental lithosphere revealed in resistivity calls for more experimental constraints to better understand these features and discontinuities. In Chapter 6, Priestley and coauthors use empirical velocity to temperature relations to derive a global LAB map from surface‐wave tomography. They further discuss how thick cratonic mantle lithosphere can form by thickening during collisions. Eaton and coauthors (Chapter 7) apply thermal diffusion modeling, combined with mineral physics relations to predict thermochemical erosion processes at the edges of cratonic keels, beneath the Canadian Cordillera and the North China craton. In Chapter 8, Kind and Yuan describe the S‐receiver function technique and its applications to the European and North American continents. They show that rapid signal changes, likely associated with fossil subductions/collisions, may be used to distinguish the Phanerozoic MLD and the cratonic LAB, which occur at similar depths. In Chapter 9, Sun and coauthors summarize an emerging new autocorrelation analysis in imaging fine‐scale lithospheric layering. The small‐scale vertical heterogeneities revealed in mantle lithosphere provide insights on possible roles of MLDs in craton formation and destruction. In Chapter 10, Aulbach summarizes the petrological and tectonic significance of the MLDs in the cratonic lithosphere. This chapter tries to reconcile evidence from seismology and petrology for both ubiquitously present MLDs and rarely detected LABs by considering the role of volatiles in the metasomatized lithospheric mantle.

The scope of work presented in this volume about lithospheric discontinuities is far from complete. For the Moho discontinuity, readers are referred to the recent community effort in, for example, the Tectonophysics Special Volume “Moho: 100 years after Andrija Mohorovicic” [Thybo et al., 2013] and the Tectonics Special Volume “An Appraisal of Global Continental Crust: Structure and Evolution.” For the geochemical perspective and geodynamic implications, and other recent reviews as well as new research that was published during the development of this book see, including but not limited to: the Lithos Special Volumes on the LAB [O'Reilly et al., 2010]; the Geochemistry, Geophysics, Geosystems’ Special Volume “The Lithosphere –Asthenosphere Boundary”; and Liao et al. [2013], Chen et al. [2014], Cooper and Miller [2014], Fischer [2015], Rader et al. [2015], Aulbach et al. [2017], Cooper et al. [2017], Kawakatsu and Utada [2017], Kennett et al. [2017], Mancinelli et al. [2017],...