Volcanic breccia and hyaloclastite in blocks from the Nuuanu and Wailau landslides, Hawaii

Steep slopes of giant landslide blocks in the Nuuanu and Wailau landslides expose fragmental volcanic rocks subdivided into monomict and polymic t hyaloclastite and breccia. The various samples form as 1) secondary slopemantling unlithified polymict breccia consisting of clasts set in a mud matrix; 2 ) monomict and polymict hyaloclastite and polymict breccia, with zeolite cement, that form downslope of the shoreline where lava flows enter the sea; and 3 ) monomict breccia formed by tectonic fragmentation of glassy submarine-erupted pillow basalt. Lavas erupted from single volcanoes are highly variable in major-element composition, even during their tholeiitic shield stage, making it difficult to identify which landslide block was derived from which volcano. Low-temperature fluids circulate through the fragmental deposits on the flanks of the volcanoes, partially altering the glass to palagonite and cementing the volcaniclastic rocks with Na- and K-rich zeolites. Volcano spreading along low-angle thrust faults laterally transports deep early submarine pillow lava out to the flank of the volcano where it crops out as tectonic monomict breccia. The faults underlying the landslide blocks are within this tectonized core of the volcano, not simply within the shallow slope deposits of hyaloclastite and breccia. The Nuuanu landslide predates the 1.5 Ma Wailau landslide.