MAGMA FLOW AND INFLATION WITHIN THE WAI’ANAE VOLCANO (OAHU, HAWAII, USA): IMPLICATIONS FROM MAGNETIC FABRIC DATA OF DIKES

Volcanic structures deform due to the transient variations in internal magma chamber overpressure, dike intrusion and induced rifting. Furthermore, the accumulation of magma in shallow levels and repeated dike intrusions over long periods (e.g. Ma) typically results in the development of loading and stress effects in the structure and the progressive inflation and change in the topography. These accumulated effects tend to cause an imbalance of the equilibrium of the volcanic edifice and may promote a subsequent destabilization and failure of the whole or part of the structure. This work focus on the characterization of the magma flow directions by means of the anisotropy of magnetic susceptibility (AMS) in dikes of the central part of the Wai’anae volcano on Oahu, that have experienced destabilization and flank collapse.

Associated rock-magnetism data evidence composite magnetic mineralogy with dominant Single Domain (SD) and Pseudo-Single Domain (PSD) particles of titanomagnetite. Magnetic fabric is clearly related to the superimposition of normal and inverse fabrics in part of the data giving “abnormal” composite fabrics. After rejection of these “abnormal” data, the determination of the magnetic zone axis, which underlines the effect of imbrication in dike margins, yielded flow direction in most studied dikes with a predominance of the vertical to sub-vertical directions.

The inferred dominantly vertical to sub-vertical magma flow of dikes within the most internal parts of the volcano suggests a process of magma accumulation in different levels beneath the central part of the edifice. That is confirmed by subhorizontal 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.