Structure and Composition of the South Kona Landslide, Mauna Loa Volcano, HI
Juli Morgan (Rice University)
Dave Clague (MBARI)
Grant Garven (Johns Hopkins U.)
Four 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 (Figure 1, blue lines).
The frontal bench is composed of volcaniclastic sediments (Figure 2), 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 (Figure 3).
Many of the rocks from the bench show evidence for alteration, ranging from low-grade burial diagenesis to higher grade hydrothermal alteration (Figure 4), including phases never before observed in submarine Hawaiian rocks, e.g., epidote, quartz, and actinolite. Alteration is concentrated in coarse-breccia deposits and 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 (Figure 5). 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 (Figure 5), similar to that now documented along Kilauea's south flank. 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 Alika 2 debris avalanche that broke through the frontal bench, perhaps portending the evolution of the active Hilina slump on Kilauea's south flank.
Morgan, J.K., D.A. Clague, D. Borchers, A. Davis, and K.L. Milliken, in press, Mauna Loa's submarine western flank: Evidence for deep volcanic spreading and hydrothermal alteration, Geochem. Geophys. Geosyst.
Morgan, J.K., and D.A. Clague, 2003, Volcanic spreading on Mauna Loa volcano, HI: Evidence from accretion, alteration, and exhumation of volcaniclastic sediments, Geology, 30, 411-414.
Borchers, D., Morgan, J.K., Clague, D.A., Davis, A., and Moore, G.F., 2003, Composition and structural origin of the midslope bench along Mauna Loa's submarine western flank, Hawaii, EOS Trans. AGU, 84, Fall Meet. Suppl., Abstract V22C-0600.
Morgan, J.K., Clague, D.A., and Davis, A., 2001, Submarine structure and stratigraphy of the South Kona slump, Hawaii: Results from the MBARI 2001 Hawaii expedition, EOS Trans. AGU, 82, Fall Meet. Suppl., 1283.
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