A Low-Relief Shield Volcano Origin for the South Kauaʻi Swell

Garrett Ito^1*, Michael O. Garcia¹, John R. Smith², Brian Taylor¹, Ashton Flinders³, Brian Jicha⁴, Seiko Yamasaki⁵, Dominique Weis⁶, Lisa Swinnard¹, Chuck Blay⁷

^*Corresponding Author: gito@hawaii.edu, 808-956-9717

¹Department of Geology and Geophysics, SOEST, University of Hawaiʻi, Honolulu, Hawaiʻi 96822, USA

²Hawaii Undersea Research Laboratory, SOEST, University of Hawaiʻi, Honolulu, Hawaiʻi 96822, USA

³Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, 02882, USA

⁴Dept. of Geoscience, University of Wisconsin, Madison WI 53706,USA

⁵Tono Geoscience Center, Japan Atomic Energy Agency, Gifu, 509-5102, Japan

⁶Pacific Centre for Isotopic and Geochemical Research, Department of Earth Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4

⁷TEOK, Kauaʻi, Hawaiʻi, USA

Abstract

The South Kauaʻi Swell (SKS) is a 110 km x 80 km ovoid bathymetric feature that stands >2 km high and abuts the southern flank of the island of Kauaʻi. The origin of the SKS was investigated using multibeam bathymetry and acoustic backscatter, gravity data, radiometric ages, and geochemistry of rock samples. Most of the SKS rock samples are tholeiitic in composition with ages of 3.9-5.4 Ma indicating they were derived from shield volcanism. The ages and compositions of the SKS rocks partially overlap with those of the nearby Niʻihau, Kauaʻi and West Kaʻena volcano complexes. The SKS was originally described as a landslide; however, this interpretation is problematic given the ovoid shape of SKS, its relatively smooth, flat-to-convex surface, and the lack of an obvious source region that could accommodate what would be one of Earth’s most voluminous (6 x 10³ km³) landslides. The morphology, size, and the surrounding gravity anomaly are more consistent with the SKS being a low-relief shield volcano, which was partially covered with a small volume of landside debris from south Kauaʻi and later with some secondary volcanic seamounts. A shield origin would imply that Hawaiian and possibly other hotspot shield volcanoes can take on a wider variety of forms than is commonly thought, ranging from tall island-building shields, to smaller edifices such as Kaʻena Ridge and Mahukona, to even lower-relief volcanoes represented by the SKS and possibly the South West Oʻahu Volcanic Field.

Multibeam bathymetry data around Kauaʻi and Niʻihau from our survey (KM0718) and from the Hawaiʻi Multibeam Synthesis (http://www.soest.Hawaiʻi.edu/HMRG/ Multibeam/index.php), with 111 m grid spacing, colored and illuminated from the NW. Contours are every 0.5 km from depths of 4.5 km to 2 km. JASON dive numbers are labeled; sample locations are marked with red circles; dredge numbers (KS1-3) are labeled with locations marked by triangles. SKS is outlined with dashed line and its central axis is marked by large arrows. Gravity anomaly highs (≥ +80 mGal) overly the inferred magmatic centers of the two shield volcanoes are marked by translucent yellow patches [Flinders et al., 2010]. White symbols mark locations of identified slope breaks, interpreted as the paleo-shorelines of Kauaʻi (triangles) and Niʻihau (circles) [Flinders et al., 2010]. Dotted lines indicate the sections where the slope break continues between the white symbols. Small black arrow marks where a slope-break of Kauaʻi’s terrace lies above a slope-break of Niʻihau’s terrace.

Perspective views of (a) existing bathymetry (b) reconstructed bathymetry in the absence of SKS, with Kauaʻi’s paleo-shoreline interpolated between the identified paleo-shorelines (Figure 2), and (c) reconstructed bathymetry with Kauaʻi’s paleo-shoreline protruding southward so that it would account for the full volume of the SKS above the shown abyssal seafloor. Contour interval is 0.5 km and vertical exaggeration is 3:1 (a1) Close-up and (a2) profile (location marked by A-A’ of inset map) of Kauaʻi’s existing southern flank. (b1) Close-up and (b2) profile of Kauaʻi’s southern flank reconstructed in (b). No vertical exaggeration in (a2) and (b2).