HR: 1330h
AN: V22C-0598    [PDF]
TI: Structural and palaeomagnetic Analysis of the Koolau Dyke Swarm Exposed in the Kapa'a Quarry, Oahu: Implications for the Structural Evolution of Kilauea - Type Volcanic Rift Zones.
AU: Day, S J
EM: s.day@ucl.ac.uk
AF: Benfield Hazard Research Centre, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST, University of Hawaii, 1680 East-West Road, Honolulu, HI 96822 United States
AU: Knight, M D
EM: mknight@sti-hawaii.com
AF: Science & Technology International, Pacific Guardian Centre 733 Bishop Street, Suite 3100, Honolulu, HI 96813 United States
AB: The Koolau dyke swarms, exposed within the Kailua embayment on the northeastern side of Oahu, have been interpreted as feeders to rift zones in the Koolau volcano, analagous to the rift zones of Kilauea volcano (Walker, 1987), that developed prior to the giant Nuuanu lateral collapse at about 2.0 Ma. Systematic determination of cross-cutting relationships between Koolau dykes has proved difficult because of dense vegetation and rapid weathering in the humid tropical climate of windward Oahu. For this study we have concentrated on exposures of the dykes in the large (450 m wide and 200m deep) Kapa'a quarry, about 3 km NE of the near vertical Nuuanu Pali escarpment. Kapa'a quarry was chosen for this study because it has large, freshly exposed sections nearly perpendicular to the dyke swarm trend in hydrothermally altered but largely unweathered rocks. The hydrothermal alteration indicates that the dykes exposed in the quarry were emplaced well below a high water table resulting from trapping of water between dykes: this high water table may have played a part in the eventual collapse. The excellent exposure at Kapa'a allows systematic, accurate determination of structural data and cross-cutting relationships for every dyke in the measured sections. These data were collected for 171 dykes, of which 10 dykes have so far been sampled for palaeomagnetic analysis (i.e remanence and petrofabrics). Most dykes at Kapa'a dip steeply, from around 70 degrees SW (away from the unstable flank of the volcano) through vertical to 50 degrees NE (towards the unstable flank). NE dipping dykes are more abundant overall. Groups of SW - dipping dykes alternate with groups of subvertical to NE - dipping dykes, suggesting a quasi-periodic alternation of the stress regime within the Koolau volcano. Dyke dilations were typically subhorizontal. A final group of dykes, cross-cutting all the others, dip from 50 degrees NE to as little as 35 degrees NE, and dilated in a steeply SW - plunging orientation implying uplift to the NE side of the dykes and a major change in the stress field within which the dykes were emplaced. All 10 dykes for which palaeomagnetic data are so far available, including 2 of the low-angle group of dykes, show reversed polarity indicating dyke emplacement between 2.6 Ma and (at the latest) 1.77 Ma.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1520 Magnetostratigraphy
DE: 8400 VOLCANOLOGY
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting