Anisotropy of Magnetic Susceptibility (AMS) and Remanent Magnetization Directions (NRM) determined from Intrusive Magma Flow: Volcanic Evolution of the Koolau Dike Complex. <p> HR: <span class="hr"> 13:55h<br></span> AN: <span class="an"> GP12A-02 <a href="../absbook/fm03-GP12A.pdf">[PDF]</a><br></span> TI: <span class="ti"> Anisotropy of Magnetic Susceptibility (AMS) and Remanent Magnetization Directions (NRM) determined from Intrusive Magma Flow: Volcanic Evolution of the Koolau Dike Complex. <br></span> AU: <span class="au"> Knight, M <br></span> EM: <span class="em"> mknight@sti.com<br></span> AF: <span class="af"> Science and Technology International, 733 Bishop St, Suite 300, Honolulu, HI 96813 United States<br></span> AU: <span class="au"> * Herrero-Bervera, E <br></span> EM: <span class="em"> herrero@soest.hawaii.edu<br></span> AF: <span class="af"> SOEST-HIGP Petrofabrics and Paleomagnetics Laboratory, University of Hawaii, 1680 East West Rd., Honolulu, HI 96822 United States<br></span> AB: <span class="ab"> The Koolau volcano on the northeastern flank of the island of Oahu in Hawaii is the source area for one of the largest landslides on Earth (3,000 to 4,000 km3), the Nuuanu debris avalanche. The offshore expression of this slide is an extensive rubbly field of debris extending approximately 230 km from the island across the Hawaiian Deep and onto the Hawaiian Arch. We have studied the intrusives of the Koolau volcano by means of Anisotropy of Magnetic Susceptibility (AMS) and paleomagnetism (NRM) studies to investigate the plumbing of the Koolau volcano, the volcanic evolution and to constrain further the timing of the Nuuanu detachment (Herrero-Bervera et al., 2002). We have re-examined 71 dikes from the Knight and Walker (1988) study and 10 additional dikes sampled in the Kapaa quarry located within the deeply and perhaps the youngest collapsed part of the Koolau Volcano. The results derived from these investigations indicate that the plumbing of the Koolau volcano is characterized by a very coherent dike complex and by a high dike injection nature. The petrofabrics yielded coherent flow azimuths regardless of their time of emplacement. Nearly 50 percent of the dikes studied (30) gave paired AMS plots and were fed by lateral flow (near horizontal magma flow, Kmax between 5 to 30$^{o}$ of inclination). In very few cases the magma flow was near vertical. We were able to measure the NRM of 79 dikes of which 63 dikes are characterized by a high degree of magnetic stability. At least 20 dikes were stepwise demagnetized by alternating fields from 5 to 100 mT. The demagnetization diagrams obtained showed a stable direction of remanence for most samples. In all cases, the characteristic component (ChRM) was clearly defined from at least seven successive directions isolated during stepwise demagnetization. The mean direction calculated for each intrusive revealed the existence of reversed polaritites except for two dikes that are characterized by normal polarities. The overall results (AMS and NRM) indicate that the dike injection within the Koolau Dike Complex (KDC) was the main cause of the triggering of the giant Nuuanu landslide (synchronous denudation by seaward creep on normal to listric faults). <br></span> DE: <span class="de"> 1518 Magnetic fabrics and anisotropy<br></span> DE: <span class="de"> 1519 Magnetic mineralogy and petrology<br></span> DE: <span class="de"> 1520 Magnetostratigraphy<br></span> DE: <span class="de"> 1535 Reversals (process, timescale, magnetostratigraphy)<br></span> DE: <span class="de"> 1599 General or miscellaneous<br></span> SC: <span class="sc"> Geomagnetism and Paleomagnetism [GP]<br></span> MN: <span class="mn"> 2003 Fall Meeting<br></span></p></body></html>