Age of tilted reefs, Hawaii

Abstract

Submerged carbonate reefs are preserved as a series of submarine terraces between Molokai and Hawaii along a 200‐km span of the southeastern Hawaiian Ridge. Limestones from two of the terraces have been sampled from submersibles and dated radiometrically at 13 and 120 ka. Recognition that the terraces are tilted permits assignment of about a dozen terraces from 150 to 1300 m depth to eight general reef platforms. These reefs were drowned by the combined effects of island subsidence and sea level rise at the end of successive glacial stages from 13 to 647 ka. The platforms are tilted 5 m/km southeast toward the locus of volcanic loading centered on the island of Hawaii. The end of the active period of volcanic shield building is represented on each nonactive volcano by a submerged change in slope. Dating of this slope change by reference to the dated reefs provides independent evidence that shield building ended at about the same time that the volcano changed from eruption of tholeiitic to alkalic basalt. The variation in depth of the dated tilted reefs along the ridge indicates that the end of subsidence follows the end of shield building by about one half million years. The present northwest margin of major subsidence probably crosses southeast Maui. During the last half million years the rate of southeast migration of the beginning of both subsidence and of tholeiitic volcanism does not appear to have increased. However, the rate of southeast migration of the end of both subsidence and of tholeiitic volcanism has apparently increased. This shortening of the life history of the younger volcanoes implies an increase in eruption rates.

Summary of the Oahu, Hawaii, Regional Aquifer-System Analysis

“…Ground-water use averaged about 394 Mgal/d from all sources in 1980 (Nakahara, 1980). Of this, about 81 percent, or 318 Mgal/d, was pumped from the volcanicrock aquifers of the island, with 238 Mgal/d being pumped from the southern Oahu area alone.…”

“…Moore and Campbell (1987), in a study of tilted, deeply submerged reefs in the Hawaiian islands, concluded that subsidence ended about 0.5 Ma after the end of shield-building volcanism. Moore (1987) provides a comprehensive summary of this evidence for the Hawaiian islands, together with new evidence and analysis, and concluded that most of the volcanoes have subsided 6,500 to 13,000 ft. Lavas that originally were erupted subaerially and have textural characteristics of subaerial lavas have since been carried to great depths by this subsidence.…”

Subsidence of Puna, Hawaii inferred from sulfur content of drilled lava flows

“…The Hilo tide gage, located 30 km north-northwest of the drill hole area, has recorded an average absolute subsidence rate of 2.4 mm/yr for the past 38 years (Moore, 1987). A similar rate (2.5 mm/yr) was determined for the past half million years on the basis of the age of drowned reefs off Kohala volcano on the northern part of the island (Moore and Campbell, 1987). We assume that the subsidence rate at the drill hole area is similar to that at Hilo because a compilation of seismic evidence indicates that depression of the base of the crust is similar at Hilo and the drill hole area (Moore, 1987, his fig.…”

Submarine landslides: Selected studies in the U.S. Exclusive Economic Zone

“…The 1 ,300-m-deep terrace is about 800 m deeper than a 500-m-deep terrace that formed about 850,000 years ago (Moore and Campbell, 1987). If the Molokai subsidence rate was comparable to that of northern Hawaii (2.5 millimeters per year (mm/yr)) during the period between formation of the 1 ,300-m-deep terrace and the 500-m-deep terrace, then the estimated age of the 1 ,300-m-deep terrace is about 1.17 million years.…”