Subducted oceanic asthenosphere and upper mantle flow beneath the Juan de Fuca slab

Abstract

Many studies have shown that typical oceanic lithosphere is underlain by a well-developed asthenosphere characterized by slow seismic velocities from ~100 to 250 km depth. However, the fate of the oceanic asthenosphere at subduction zones is poorly understood. I show here using shear-wave splitting of S waves emanating from earthquakes in the Juan de Fuca slab that upper mantle asthenospheric anisotropy beneath the slab is consistent with the presence of two distinct subducted asthenospheric layers, one with fast shear trends parallel to the subduction trench, and a second, deeper layer with fast upper mantle fabrics parallel to the motion of the Juan de Fuca plate with respect to the deeper mantle. The consistent orientation of unsubducted Pacifi c asthenospheric anisotropy in the direction of current plate motion implies that the trench-parallel, subslab anisotropy develops when the lithosphere subducts.

Source-side shear-wave splitting and upper-mantle flow beneath the Arakan slab, India-Asia-Sundalind triple junction

“…The Arakan slab event that occurred on Julian day 209, 2008 (Table 1), yielded no suitable waveforms for splitting and will not be discussed further. Splitting delay times are generally high, with a mean of 3.0 s, similar to that found for source-side splitting in the Cascades region (2.9 s; Russo, 2009) and the Carpathian Arc (2.77 s; Russo and Mocanu, 2009). Results in Figure 10 are plotted at surface projections of the point at 200 km depth along their downgoing path from the source event to the receiver station.…”

“…Results in Figure 10 are plotted at surface projections of the point at 200 km depth along their downgoing path from the source event to the receiver station. This procedure allows discernment of variable upper-mantle anisotropy in the source region (Russo, 2009;Russo and Mocanu, 2009), and is predicated on results of numerical studies of S-wave-effective Fresnel zones (Zhao et al, 2000) and the effects of slowly varying anisotropy on observed splitting (Saltzer et al, 2000). As is clear from Figure 10, shear-wave splitting in the vicinity of the Arakan slab is highly variable, and appears to depend strongly on the upper-mantle volume sampled by the downgoing S waves.…”

“…These results were derived from data collected at temporary seismic deployments in China and India, and such deployments are not currently possible in Myanmar. However, the form of the upper-mantle deformation fi eld south of the syntaxis in Myanmar and northeastern India can be determined using shear waves from earthquakes in the Arakan slab itself (e.g., Russo and Silver, 1994;Russo, 2009;Russo and Mocanu, 2009;Russo et al, 2010). In the following, I present results from such a study and show that the deformation of the Arakan slab strongly affects upper-mantle anisotropy locally, and that the larger-scale upper-mantle fl ow in the region is consistent with India's northward motion and fl ow around the northern and southern ends of the Arakan slab, and perhaps also through the southern slab gap.…”

“…Shear-wave splitting due to upper-mantle substation anisotropy can be determined using SK(K)S and PKS phases, and thus the splitting on the receiver side of the S-wave travel path (Fig. 8) can be corrected (Russo and Silver, 1994;Russo, 2009;Russo and Mocanu, 2009). Observed receiver station splitting parameters used to correct for splitting beneath the receiver stations are detailed in Table 2.…”

“…Note, only one receiver station used in this study is considered to be isotropic (KRIS, Schmid et al, 2004), and thus never required correction for receiver-side splitting. Details of the method for correcting known station splitting can be found in Russo and Silver (1994), Russo (2009), and Russo and Mocanu (2009).…”