The Nuuanu and Wailau giant landslides: insights from paleomagnetic and anisotropy of magnetic susceptibility (AMS) studies

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, 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 subaerial lavas of the Koolau volcano as well as the deep-sea sediments on top of the Nuuanu and Wailau landslides by means of magnetostratigraphy and magnetic anisotropy studies to investigate the volcanic evolution, the plumbing of the Koolau volcano and the timing of the Nuuanu detachment. In addition, we have conducted deep-sea magnetostratigraphic studies of a 20 and 7 m piston core recovered by the RV Kana Keoki and the RV Bertha Ann, as well as three 7 m piston cores recovered by the RV Kairei to understand the origins of the giant landslides. 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. Parts of the Koolau complex are a sheeted dike swarm as in ophiolites as demonstrated by geologic and anisotropy of magnetic susceptibility (AMS) studies. On the other hand, the magnetostratigraphic results of the subaerial part of the Koolau complex indicate that at least one of the normal Reunion Subchrons (ca. Reunion II 2.15±0.04 to 2.11±0.04 Ma) had been registered at two different locations. Deep-sea cores have recorded several reversals. The oldest one is the top of the Olduvai Subchron (ca. 1.78 Ma). Land and deep-sea paleomagnetic and plumbing investigations indicate that the collapse of the Koolau volcano had to be relatively rapid in a period of less than 0.5 Ma due to forceful injection of dikes associated with extension and gravitational effects. Therefore, the timing of the main collapse of the Koolau volcano that originated the Nuuanu landslide had to occur between 2.1 and 1.78 million years ago based on the magnetostratigraphic evidence.