Abstract
In order to investigate the role of dikes in the volcanic evolution and the triggering mechanisms of catastrophic mass wasting volcanoes, we have sampled for a pilot study, seven dikes within the Wai’anae volcano, Oahu, Hawaii. The width of the dikes ranged between 0.4 and 2.5 m. This work focuses on the characterization of the magma flow directions using anisotropy of magnetic susceptibility (AMS) data in dikes of the inner part of the Wai’anae volcano. This part is now exposed, because this volcano experienced destabilization and flank collapse. Rock magnetism data show composite magnetic mineralogy, corresponding when plotted on the Day diagram to be dominated by single domain (SD) and pseudo-single domain particles of pure titanomagnetite, suggesting possible inverse magnetic fabric associated with the SD grains. The obtained magnetic fabric does not reflect such grain sizes and is probably partly related to the presence of different magnetic phases, resulting in part of our samples as having “abnormal” fabrics. We therefore used a simple criterion to eliminate most of the abnormal fabrics in order to analyze the magnetic fabric data in a clearer way. After rejection of most of the abnormal data, the determination of the magnetic zone axis, which underlines the effect of imbrication in dike margins, yielded reliable magma flow directions in most of the studied dikes, with a predominance of vertical to subvertical AMS directions. The inferred dominantly vertical to subvertical magma flow of dikes (feeding from below) within the most internal parts of the volcano, suggests a process of accumulation of new magma at different levels within the inner part of the edifice. This process was enhanced by subhorizontal magma flow toward the volcano center in two other dikes. Such accumulation helps to explain the inflation, subsequent destabilization, and flank collapse of the Wai’anae volcano.
Highlights
Dike swarms are present in all basaltic volcanic systems, but in only a few cases erosion allows us to view them directly
To take into account the probable superposition of normal and inverse fabrics, a criterion was first applied to reject data with a clearly abnormal fabric: Sample data corresponding to angular differences between the minimum axis and the dike plane lower than 45° were not retained for determination of the zone axis
Alteration may deeply modify the magnetic fabric of dikes
Summary
Dike swarms are present in all basaltic volcanic systems, but in only a few cases erosion allows us to view them directly. MZA magnetic zone axis, Km mean susceptibility, Kmax maximum susceptibility, Kmin minimum susceptibility, D and I declination and inclination of the axes, respectively, P′ and T corrected anisotropy degree and shape parameter of Jelinek (1981), Int, N and Inv intermediate, normal and inverse magnetic fabric, respectively susceptibility for all dikes is 15.5 ± 13.8 10−3 SI units, corresponding to a large scatter in susceptibility values for the sample scale with a bimodal distribution (Fig. 2d), indicating possibly heterogenous magma sources in two populations of samples. Knight and Walker (1988), who studied mafic dikes from the collapsed Koolau volcano (Oahu, Hawaii), confirmed the agreement between the orientation of the principal axes of AMS and macroscopic flow related structures (such as surface lineations consisting of parallel, millimeter-high wrinkles projecting out from the weathered chilled margins and cross sections that show imbricated sheet joints along dike margins (see for example Knight and Walker 1988, figure 3). Kint axes are too scattered to be used instead of Kmin to highlight an imbrication
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