Nature of the seismic lithosphere‐asthenosphere boundary within normal oceanic mantle from high‐resolution receiver functions

Abstract

Receiver function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness, and age dependence of the seismic wave speed decrease thought to mark the lithosphere‐asthenosphere boundary (LAB). We use a combination of frequency‐dependent harmonic decomposition of receiver functions and synthetic forward modeling to provide new seismological constraints on this “seismic LAB” from 17 ocean‐bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ∼1300 K isotherm but a lower isotherm (∼1000 K) is suggested in the Daito ridge, the Izu‐Bonin‐Mariana trench, and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ∼70 km. The age dependence of the seismic LAB depth is consistent with either a transition to partial‐melt conditions or a subsolidus rheological change as the causative factor. The age dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust, a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the subsolidus model invoking anelastic relaxation mediated by temperature and water content, but is not readily explained by a partial‐melt model. The Ps conversions display negligible two‐lobed or four‐lobed back azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean‐bottom observations indicate a subsolidus elastically accommodated grain‐boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long‐term ductile deformation, the seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere corresponding to a change in the long‐term rheological properties.

Structure of the oceanic lithosphere and upper mantle north of the Gloria Fault in the eastern mid‐Atlantic by receiver function analysis

“…The lithosphere‐asthenosphere boundary (LAB) beneath the oceans is often imaged using surface waves [e.g., Romanowicz , ; Takeo et al , , ; Lin et al , ], SS waveforms [e.g., Rychert et al , ], RFs employing land stations [e.g., Li et al , ; Kumar and Kawakatsu , ], or OBSs [e.g., Kawakatsu et al , ; Olugboji et al , ]. Most of the discussed models of the LAB [ Kawakatsu et al , ; Fischer et al , ; Olugboji et al , ] include thermal control, changes in rheology, dehydration, anisotropy, or partial melt.…”

Effects of crystal preferred orientation on upper-mantle flow near plate boundaries: rheologic feedbacks and seismic anisotropy

“…Interpretations of observed seismic anisotropy provide one means to infer mantle flow patterns (McKenzie 1979;Ribe 1989;Long & Becker 2010) and what they might imply about, for example, the nature and extent of coupling across the lithosphere-asthenosphere boundary (Nicolas & Violette 1982;Tommasi et al 2000;Olugboji et al 2016), or the interactions between partial melting and flow in the vicinity of plumes or plate boundaries (Holtzmann & Kendall 2010). Experimental results (Karato 2008;Boneh et al 2015;Hansen et al 2016) and natural sample measurements (Ben Ismail & Mainprice 1998) of olivine deformation provide constraints on numerical simulations of mantle polycrystal behaviour under various applied stresses (Wenk et al 1991;Tommasi 1998;Dawson & Wenk 2000;Kaminski & Ribe 2002;Castelnau et al 2008;Li et al 2014;Goulding et al 2015).…”

Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

“…Around this critical frequency, the value of which depends on temperature, pressure, and grain size, elastically accommodated grain-boundary sliding (EAGBS) causes a peak in attenuation and a major reduction in shear modulus, which consequently decreases both Vp and Vs. (For example, at 1000°C and 4 GPa, for a grain size of 5 mm, the critical frequency is~5 Hz, or~0.2 s.) At lower frequencies (longer periods, seconds to hundreds of seconds for the specified conditions), diffusionally accommodated grain-boundary sliding (also termed "absorption band behavior") leads to a slow but continual increase in attenuation and decrease in shear modulus. Though much of the current research on anelasticity has been specifically targeted at understanding the cause of midlithospheric discontinuities (MLDs; e.g., Karato et al, 2015;Selway et al, 2015) and at characterizing the nature of the LAB (e.g., Olugboji et al, 2013;Olugboji et al, 2016), the proposed models of anelastic controls on seismic observables can, and indeed should, be applied more broadly when formulating geodynamic interpretations of observed seismic anomalies. The solid line is the median value of all profiles, and the shaded regions indicate the range of extracted profiles (darker region is the 25th-75th percentile; lighter region is the full range).…”