Shoreline Slope Breaks Revise Understanding of Hawaiian Shield Volcanoes Evolution

Abstract Slope gradient maps of swath bathymetry around the Hawaiian islands locate 12 shield slope breaks associated with former shorelines that are now submerged, ranging in age back to 5 Ma. The age of their drowning correlates with the waning of tholeiitic shield volcanism that ceases to repave the shoreline sometime between the beginning and the end of the late‐shield stage. Slope breaks on Mauna Loa's northeast and southwest rifts are consistent with it being in the waning stages of shield building. Superposition of shorebreaks shows that the Hualālai shield is older than Mauna Kea's. We find evidence for three volcanic shields forming Ka'ena Ridge, for a simultaneous waning of Maui‐Lāna'i‐Kaho'olawe shield building and the initiation of Kohala shield building by 1.3 Ma, and for Mahukona growing to just above sea level about 0.6 Ma. A contiguous shoreline slope break formed ~1.8 Ma at the end of the West Moloka'i late‐shield stage and the beginning of the East Moloka'i late‐shield stage. At its western end the shield dips obliquely away from, rather than toward, this slope break. Similarly, the short Hualālai slope break plunges north, which is opposite to that expected for volcanic loading to the south. Shields locally sloping away from paleoshorelines may be related to landslides causing flexural back tilting away from their unloaded footwall. The Nu'uanu and Wailau slides and Pololū slump were mostly shield flank events that cut marginally into their former shorelines, enough to preferentially nucleate headward erosion by streams.