Seafloor age dependence of Rayleigh wave phase velocities in the Indian Ocean

Abstract

Variations in the phase velocity of fundamental‐mode Rayleigh waves across the Indian Ocean are determined using two inversion approaches. First, variations in phase velocity as a function of seafloor age are estimated using a pure‐path age‐dependent inversion method. Second, a two‐dimensional parameterization is used to solve for phase velocity within 1.25° × 1.25° grid cells. Rayleigh wave travel time delays have been measured between periods of 38 and 200 s. The number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22,272 paths at a period of 40 s. At periods < 100 s, the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half‐space‐cooling model for a mantle potential temperature of 1400°C. The inferred thermal structure beneath the Indian Ocean is most similar to the structure of the Pacific upper mantle, where phase velocities can also be explained by a half‐space‐cooling model. The thermal structure is not consistent with that of the Atlantic upper mantle, which is best fit by a plate‐cooling model and requires a thin plate. Removing age‐dependent phase velocity from the 2‐D maps of the Indian Ocean highlights anomalously high velocities at the Rodriguez Triple Junction and the Australian‐Antarctic Discordance and anomalously low velocities immediately to the west of the Central Indian Ridge.

Hot Upper Mantle Beneath the Tristan da Cunha Hotspot From Probabilistic Rayleigh‐Wave Inversion and Petrological Modeling

“…It is also useful to compare our phase velocities with those for the Indian Ocean from Godfrey et al (2017); although the range of their overlap is narrow, we can see that the Indian Ocean dispersion curve for 20-52 Ma (green line in their Figure 3) and our average for Tristan da Cunha (e.g., Figure 6) are similar. The 20-52 Ma dispersion curve in Godfrey et al (2017) is probably representative mostly of the eastern part of the Indian Ocean, with the spreading along the Southwest Indian Ridge much slower than that along the Southeast Indian Ridge and, thus, with the area from which the curves are computed greater in the eastern part of the ocean. The phase-velocity average for the Indian Ocean thus reflects the rejuvenation of the lithosphere by the Kerguelen hotspot (Godfrey et al, 2017;Schaeffer & Lebedev, 2015).…”

“…The 20-52 Ma dispersion curve in Godfrey et al (2017) is probably representative mostly of the eastern part of the Indian Ocean, with the spreading along the Southwest Indian Ridge much slower than that along the Southeast Indian Ridge and, thus, with the area from which the curves are computed greater in the eastern part of the ocean. The phase-velocity average for the Indian Ocean thus reflects the rejuvenation of the lithosphere by the Kerguelen hotspot (Godfrey et al, 2017;Schaeffer & Lebedev, 2015). Our phase velocities at periods over 40 s, sampling the low velocity zone, are around 4.0 km/s, not as low as the 3.9 km/s reported for the active volcanism part of Hawaii by Laske et al (2011) (their Figure 4).…”

Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone

“…Within oceanic plates the phase velocity is expected to vary with plate age. Previous measurements have typically focused on larger oceanic basins including the Pacific (Nishimura & Forsyth 1988), the Atlantic (James et al 2014), and the Indian (Godfrey et al 2017). Our data set offers the opportunity to compare these measurements with a significantly smaller and younger plate system, but with greater instrument density to resolve finer-scale age dependence.…”

“…Our data set offers the opportunity to compare these measurements with a significantly smaller and younger plate system, but with greater instrument density to resolve finer-scale age dependence. We group the Juan de Fuca and Gorda plates into ages between 0-4 Ma and >4 Ma using the magnetic lineation interpretation of Wilson (1993), comparable to age bins used for the Pacific (Nishimura & Forsyth 1988) and Indian basins (Godfrey et al 2017). Due to the slow spreading rate in the Atlantic, the youngest age bin in the James et al (2014) model is 0-20 Ma, while the maximum age for the Juan de Fuca and Gorda plates is 10 Ma, making comparison between these two data sets difficult.…”

“…Therefore, they can be relatively easily compared to other phase-velocity observations from different regions (e.g. James et al 2014;Godfrey et al 2017) or to alternative models from the same region (e.g. Foster et al 2014;.…”