Mauna Loa's submarine western flank: Landsliding, deep volcanic spreading, and hydrothermal alteration

Abstract

Four new remotely operated vehicle dives carried out by Monterey Bay Aquarium Research Institute (MBARI) reveal a heterogeneous distribution of lithologies and compositions along a transect across the submarine west flank of Mauna Loa, from the outer scarp of the frontal bench to the upper flank. The frontal bench is composed predominantly of volcaniclastic sediments, ranging from very fine‐grained monomictic hyaloclastites to coarse‐grained, compositionally mixed volcaniclastic breccias. The predominance of subaerially derived clasts suggests accumulations of landslide deposits, probably emplaced along a regional shear plane preserved in cataclastic breccias with local foliations and grain trails. Repeated packages of inversely graded strata are interpreted to reflect thrust imbrication of the resulting volcaniclastic apron during volcanic spreading of Mauna Loa's western flank, similar to that now documented along Kīlauea's south flank. Many of the rocks from the bench show evidence for alteration, ranging from low‐grade burial diagenesis to higher‐grade hydrothermal alteration, including phases never before observed in submarine Hawaiian rocks, including epidote, talc, sphene, and corrensite. Alteration is concentrated in deformed zones, denoting pathways for fluid flow into or out of the volcanic edifice. Formed at depth, the altered rocks were subsequently transported along low‐angle thrust faults into the bench and exposed along high‐angle fractures and faults. The upper submarine flanks are draped by subaerially erupted, submarine emplaced pillow lavas and interbedded hyaloclastites, generated by shoreline‐crossing lava flows. Basalt glasses indicate Mauna Loa origin but imply earlier compositions than present‐day lavas, consistent with Ar‐Ar ages suggesting eruption 0.28 ± 0.10 Ma. Late stage detachment of a nearshore slump produced the 'Ālika 2 debris avalanche that broke through the frontal bench, perhaps portending the evolution of the active Hilina slump now present on Kīlauea volcano's south flank.