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Structural and Petrologic Evolution of the Lihue Basin and Eastern Kauai, Hawaii

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Structural and Petrologic Evolution of the Lihue Basin and Eastern Kauai, Hawaii Structural and petrologic evolution of the Lihue basin and eastern Kauai, Hawaii Peter W. Reiners* Department of Geological Sciences, Box 351310, University of Washington, Bruce K. Nelson } Seattle, Washington 98195 Scot K. Izuka U.S. Geological Survey, Water Resources Division, 677 Ala Moana Boulevard, Suite 415, Honolulu, Hawaii 96813 ABSTRACT The topography of the eastern part of the Makaleha Olokele Mtns. Hawaiian island of Kauai is dominated by the Plateau Lihue basin, a large (~110 km2) semicircular depression bounded by steep cliffs and partly Nonou filled by late rejuvenated-stage (or post- Ridge erosional stage) volcanic material. As with other large, semicircular basins on ocean- island volcanoes, the subsurface geology and origin (e.g., structural collapse vs. fluvial ero- sion) of the Lihue basin are poorly understood. New analyses of samples collected from east- Lihue ern Kauai and drill holes within the basin doc- ument several important features of the late- Basin stage geologic evolution of Kauai. First, thick (>300 m) sequences of rejuvenated-stage Kalepa Koloa Volcanics in the Lihue basin show sys- Ridge tematic, basin-wide geochemical trends of in- creasingly incompatible elements with time, indicating a gradual decrease in the extent of Palikea Haupu partial melting of mantle sources with time. Ridge Ridge Second, beneath the rejuvenated-stage vol- canics in the basin, a thin layer of postshield al- kalic stage lavas (e.g., hawaiites and mug- Figure 1. Landsat multispectral scanner image of Kauai taken in 1978, provided courtesy of earites) overlies older shield-stage tholeiitic NASA’s Virtually Hawaii project, sponsored by the Hawaii Space Grant Consortium. The flat, lavas of the Napali Member, indicating that the low-lying, semicircular area on the eastern side of the island is the Lihue basin. The high cliffs on Lihue basin formed by structural collapse, not the western margin of the basin are the eastern side of the central massif, including the peaks of fluvial erosion. Third, a large (~2–5 km3) ma- Kawaikini and Waialeale. The large rejuvenated-stage volcanic vent, Kilohana, can be seen in trix-supported breccia, interpreted as deposits the middle of the Lihue basin. Also note the two small ridges on the eastern margin of the basin of one or more debris flows, is within the reju- (Kalepa and Nonou Ridges), the Haupu Ridge to the south, and the Makaleha Mountains, with venated-stage volcanics throughout the basin, the summit plateau, to the north of the basin. For latitude and longitude, see Figure 2. and correlates with surficial exposures of the Palikea Breccia west of the basin. Isotopic compositions of the bulk breccia are similar to ites. Clasts within the breccia are dominantly INTRODUCTION those of tholeiites from the east side of Kauai, hawaiite and alkali gabbro. The source region and distinct from those of west Kauai tholei- of the breccia in the steep cliffs and highlands The Lihue basin of eastern Kauai is a large of the central massif to the west of the basin (~110 km2) topographic depression enclosed must contain magmatic products of an exten- by steep-walled cliffs to the west and open to *Present address: Division of Geological and sive postshield alkalic stage, including hawaiite the east (Figs. 1 and 2). Such U-shaped basins Planetary Sciences, M.S. 100-23, California Institute of Technology, Pasadena, California 91125; e-mail: flows and one or more large intrusive bodies or are common features of ocean-island volca- [email protected]. ponded sequences of alkali gabbro. noes, including those in Hawaii, Réunion, and GSA Bulletin; May 1998; v. 111; no. 5; p. 674–685; 8 figures; 2 tables. 674 LIHUE BASIN AND EASTERN KAUAI, HAWAII Hanalei lying tholeiitic lava flows of the Haupu Member Haena Dry Cave 94PRNP16 2 94PRNP17 are restricted to a small (~1.6 km ), ovoid, fault- bounded structure in the southeast corner of the island that has generally been interpreted as a flank caldera or pit crater (Macdonald et al., Lihue 1960). The Makaweli Member is predominantly Olokele Basin tholeiitic lavas, with two capping flows of hawai- Member ite and mugearite (Clague and Dalrymple, 1988), all of which apparently ponded in a graben-like Puu depression in the southern part of the island. The Lua NEK Nonou ridge Olokele Member is also composed of dominantly SW 94PRNP33,34 PKR tholeiitic lavas with one known hawaiite flow, NWK and includes a thick section of flat-lying, appar- HTZ ently ponded lavas in the central part of the is- land. The inferred structural feature that bounds the Olokele Member is interpreted as a lava-filled Wiliwili Camp summit caldera (Macdonald et al., 1960). The 94PRMK56 Palikea Puu Kee three hawaiite and mugearite lavas in the Olokele Ridge 94PRNP24 and Makaweli Members represent the only previ- ously known lavas of the postshield alkalic stage on Kauai, and have ages of about 3.9 Ma (Clague Figure 2. Generalized geologic map of Kauai (after Macdonald et al., 1960; Clague and Dal- and Dalrymple, 1988). The rejuvenated-stage rymple, 1988) showing the primary lithologies and locations of the drill holes and other sample Koloa Volcanics represent the latest volcanism on locations in and around the Lihue basin. A generalized cross section through the trace of A–A′ Kauai, and consist of alkalic basalt, basanite, is shown in Figure 5. nephelinite, and melilitite lavas and tuffs erupted primarily in the central and eastern parts of the is- land (Macdonald et al., 1960; Clague and the Canary Islands, and have been interpreted trends in the rejuvenated-stage lavas in the Dalrymple, 1988; Maaløe et al., 1992). Radio- both as remnant calderas (Stearns and Vaksvik, basin (not previously observed in rejuvenated- metric ages of lavas of the Koloa Volcanics range 1935; Marti et al., 1996) and as downdropped stage volcanic rocks from any Hawaii volcano) from about 0.5 to 3.6 Ma; lavas from the eastern blocks resulting from giant landslides or suggest a long-term, systematic decrease in ex- part of the island are younger than 1.5 Ma slumps (Presley et al., 1997; Gillot et al., 1994; tent of partial melting through the rejuvenated (Clague and Dalrymple, 1988). Carracedo, 1994, 1996; Gillot and Nativel, volcanic stage. Kauai is distinct from other Hawaiian shield 1989; Watts and Masson, 1995; Holcomb and volcanoes in the abundance and scale of observed Searle, 1991; Lénat et al., 1989; Duffield et al., GEOLOGY OF KAUAI AND and inferred large faults and structural-topographic 1982). The origin of the Lihue basin is particu- THE LIHUE BASIN features. The caldera inferred from the distribution larly enigmatic because of the enormous topo- of thick, flat-lying flows of the Olokele Member is graphic relief (as much as 1 km) on its western Kauai is the second oldest and northernmost of more than 21 km in diameter, which would make it margin and the presence of a positive Bouguer the main Hawaiian islands, and has generally by far the largest in the Hawaiian islands. North- gravity anomaly (generally observed at shield been interpreted as the eroded remnants of a sin- south–trending normal faults bisect the western volcano summit calderas) in the northwest part gle large shield volcano (Dana, 1849; Hinds, part of the island, delineating both the Makaweli of the basin (Krivoy et al., 1965), which is off- 1930; Macdonald et al., 1960), the main shield- graben in the south, and a scarp, at least 800 m set from the inferred central summit caldera building stage of which lasted from before about high, against which younger flows ponded in the about 20 km northwest of the middle of the 5.1 Ma to about 4.0 Ma (McDougall, 1979; northwestern part of the island (the Kalalau fault, basin (Macdonald et al., 1960). We present Clague and Dalrymple, 1988). The lavas that Macdonald et al., 1960). GLORIA side-scan sonar stratigraphic and geochemical results from sec- compose the island have been divided into two imagery indicates the presence of at least two large tions and samples from the margins of the basin main stratigraphic groups: the older Waimea debris fields on the north, south, and possibly east and from deep (>300 m) drill holes in the cen- Canyon Basalt, comprising tholeiitic and a few sides of the island, resulting from giant landslides tral and southern part of the basin that reveal rare hawaiitic and mugearitic lava flows, and the (Moore et al., 1994). All of these structural- several important features of the geologic and rejuvenated-, or posterosional-, stage Koloa Vol- topographic (and bathymetric) features indicate a petrologic evolution of eastern Kauai. In partic- canics, comprising highly alkalic, primitive lavas geologic history of the island that may be more ular this work shows that (1) lithostratigraphic and related volcanic rocks (Fig. 2) (Macdonald complicated than previously recognized. Recent evidence supports an origin by large-scale et al., 1960; Clague and Dalrymple, 1988). The work has established that Sr, Nd, and Pb isotope structural collapse (not fluvial erosion) for the older Waimea Canyon Basalt is further subdi- compositions of tholeiites on the east and west Lihue basin; (2) clasts from a thick debris-flow vided into four members, the oldest and most sides of Kauai are distinct, which has been used in layer in the basin suggest that the center of the widespread of which is the tholeiitic lavas of the conjunction with structural and paleomagnetic ar- island contains a large quantity of intrusive or Napali Member. The Haupu, Makaweli, and guments to suggest the presence of two distinct slowly cooled rocks of a postshield alkalic vol- Olokele Members are primarily younger lavas shield volcanoes on the island (Holcomb et al., canic stage more voluminous than previously that apparently ponded within enclosed or par- 1995, 1997).
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