Abstract
AbstractOceanic lithosphere provides an ideal location to decipher the nature of the lithosphere‐asthenosphere system which is vital to our understanding of plate tectonics. It is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to the conductive cooling models. Yet this simple realization fails to explain various observations. For example, old oceanic lithosphere does not subside as predicted. Further, precise imaging of the lower boundary of the oceanic lithosphere has proven challenging. Here we use SS precursors to image the discontinuity structure across the Pacific Ocean using 24 years of teleseismic data. We image a sharp pervasive velocity discontinuity (3–15% drop over <21 km) at 30–80 km depth that increases in depth with age from the ridge to at least 36 ± 9 Myr along the 1100°C conductive cooling isotherm. Beneath seafloor >36 Myr, there is no age‐depth dependence, and we image the discontinuity at an average depth of 60 ± 1.5 km. The amplitude and sharpness of the boundary suggests that a compositional variation and/or layered carbonatitic melt may be required to explain our observations rather than temperature alone. The strength and pervasiveness of the boundary suggest that it is likely related to the lithosphere‐asthenosphere boundary. An additional deeper discontinuity at 80–120 km depth is imaged intermittently that in most cases likely represents a continuing negative velocity gradient in depth.
Highlights
We focus on seismically imaging the Pacific lithosphere using SS precursors, which are sensitive to the structure near the bounce points (Figure 1)
We image some complexities: locations with a shallow positive discontinuity (20–36 km), locations with no discontinuities, and locations where the negative gradient continues to greater depth
Positive discontinuity, we took out the Moho at 7 km depth that was assumed for the rest of the bins, since these shallow positive discontinuities may represent thickened crust
Summary
The concept of the lithosphere-asthenosphere system is well defined as the rheological boundary between the rigid lithosphere that transfers coherently and the weaker asthenosphere [Barrell, 1914; Daly, 1940], but its nature still remains enigmatic since various studies using a variety of geochemical and geophysical techniques have proposed different mechanisms to define the boundary [Artemieva, 2006; Jones et al, 2001; Karato, 2012; Kawakatsu et al, 2009; Moorkamp et al, 2010; Regan and Anderson, 1984; Rychert and Shearer, 2011]. Heat flow, and gravity studies have shown that seafloor subsides according to half-space cooling (HSC) for oceanic lithosphere 70 Myr). This apparent deviation from the half-space cooling model has been attributed to additional heat source [Parsons and Sclater, 1977; Smith and Sandwell, 1997] possibly caused by small-scale convection [Dumoulin et al, 2001; Huang and Zhong, 2005; Parsons and Mckenzie, 1978] and/or hot spot alteration [Korenaga and Korenaga, 2008]
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