A Seismic Tomography, Gravity, and Flexure Study of the Crust and Upper Mantle Structure of the Hawaiian Ridge: 1

Abstract

The Hawaiian Ridge has long been a focus site for studying lithospheric flexure due to intraplate volcano loading, but crucial load and flexure details remain unclear. We address this problem using wide‐angle seismic refraction and reflection data acquired along a ∼535‐km‐long profile that intersects the ridge between the islands of Maui and Hawai'i and crosses 80–95 Myr‐old lithosphere. A tomographic image constructed using travel time data of several seismic phases reveals broad flexure of Pacific oceanic crust extending up to ∼200–250 km either side of the Hawaiian Ridge, and vertically up to ∼6–7 km. The P‐wave velocity structure, verified by gravity modeling, reveals that the west flank of Hawaii is comprised of extrusive lavas overlain by volcanoclastic sediments and a carbonate platform. In contrast, the Hāna Ridge, southeast of Maui, contains a high‐velocity core consistent with mafic or ultramafic intrusive rocks. Magmatic underplating along the seismic line is not evident. Reflectors at the top and bottom of the pre‐existing oceanic crust suggest a ∼4.5–6 km crustal thickness. Simple three‐dimensional flexure modeling with an elastic plate thickness, Te, of 26.7 km shows that the depths to the reflectors beneath the western flank of Hawai'i can be explained by volcano loading in which Maui and the older islands in the ridge contribute ∼43% to the flexure and the island of Hawai'i ∼51%. Previous studies, however, revealed a higher Te beneath the eastern flank of Hawai'i suggesting that isostatic compensation may not yet be complete at the youngest end of the ridge.

A Seismic Tomography, Gravity, and Flexure Study of the Crust and Upper Mantle Structure Across the Hawaiian Ridge: 2. Ka'ena

“…Together, these studies confirm that volcanic loads to the west of Hawai'i are largely compensated by flexure. However, as discussed in MacGregor et al (2023), previous studies suggest on-going subsidence and a higher T e value beneath of the eastern flank of Hawai'i Island, suggesting that isostatic compensation may not yet be complete at the youngest end of the ridge.…”

“…Presumably then, the high-density and high-velocity core imaged here (Figure 10) is the western edge of the center of volcanism of Ka'ena volcano. Alternatively, the imaged core could be a slice of a more elongate feature that extends considerably along the Ka'ena Ridge, such as is expected for the core-like feature detected beneath the Hāna Ridge (MacGregor et al, 2023). The presence of the core suggests an edifice-level magma chamber or mush zone, which would have evolved considerably in size and elevation during volcano growth.…”

“…The uncertainties in load distribution and corresponding degree and extent of flexure, leave models of mantle properties without substantial constraints. To address this issue, in 2018 we conducted a marine geophysical experiment across the Hawaiian Ridge, as part of a broader study of the seismic structure of the Hawaiian-Emperor Seamount Chain (Boston et al, 2019;Cilli et al, 2023;MacGregor et al, 2023;Watts et al, 2021;Xu et al, 2022). The experiment consisted of two ocean bottom seismograph (OBS) lines for tomographic imaging (Lines 01 and 02 in Figure 1) and seven MCS lines, along with corresponding gravity and multibeam bathymetry data collection (e.g., Watts et al, 2020).…”

“…In this paper, we report results from the seismic and gravity profile located to the western side of the island of O'ahu (Line 02; Figures 1 and 2) across the Ka'ena ridge. This paper is a companion paper to MacGregor et al (2023), which analyzed data collected along Line 01 located across the western flank of the island of Hawaii. Our findings reveal that the Ka'ena Ridge consists of a volcanic edifice of ∼7 km thick, which overlays pre-existing Pacific oceanic crust with a thickness of ∼6 km.…”

“…Early gravity and seismic experiments found evidence for high-density or high-wave-speed bodies within the islands of O'ahu and Hawai'i (e.g., Adams & Furumoto, 1965;Strange, Woollard, & Rose, 1965, Strange, Machesky, & Woollard, 1965Woollard, 1951;Zucca et al, 1982) that were suggested to be each the result of up to a few million years of accumulated mafic and ultramafic intrusive materials. More recently, an updated residual gravity anomaly map of the Hawaiian Ridge (Flinders et al, 2013) and 2-D and 3-D seismic tomographic imaging studies across the island of Hawai'i and its submarine flanks clearly show that the summits and upper rift zones of volcanoes are characterized by high-density and high-velocity materials (Lin et al, 2014;MacGregor et al, 2023;Park et al, 2007Park et al, , 2009. Cumulate core features have been found by previous studies at other seamounts as well, for example, Jimmu guyot on the Emperor Seamounts (Watts et al, 2021;Xu et al, 2022), Jasper seamount (Hammer et al, 1994), Louisville guyot (Contreras-Reyes et al, 2010), Great Meteor seamount (Weigel & Grevemeyer, 1999) and La Reunion Island (Gallart et al, 1999).…”