Three-dimensional Structure of the Crust and Mantle beneath the Island of Hawaii

The Island of Hawaii is the youngest and southeasternmost member of a quasi-linear, age-ordered, chain of volcanic islands and seamounts which extends for 3500 km across the northwestern Pacific sea floor. Similarities between the Hawaiian chain and at least six other linear island chains of the Pacific basin, including their nearly unidirectional and uniform rate of extension, WNW orientation, and formation atop comparatively ancient Pacific lithosphere far from plate boundaries, strongly imply a common mode of formation for these mid-plate island chains. This study addresses the question of the origin of Hawaiian-type shield volcanoes through analysis of the deep internal velocity structure of the Island of Hawaii. The velocity structure of the crust and mantle underlying the Island of Hawaii is investigated using the arrival times of P-waves recorded by a seismograph network located on that island. Relative travel times of these waves contain abundant evidence for the existence of lateral heterogeneities within both crust and mantle beneath the island. Although lower mantle structural variations can explain limited aspects of these data, requisite structures are found to be improbable. Absolute travel time residuals for both P and PKP average 0.4 sec late, which indicates that the cumulative effect on all departures in velocity from the mean earth encountered by these teleseismic rays is small. Inconclusive evidence does suggest that the steepest rays are delayed by up to 1.5 sec. A flexible modeling technqiue for these data is developed to give a quantitative image of laterally heterogeneous structure beneath Hawaii. This method extends the three-dimensional inversion modeling of Aki et al. (1977) by incorporation of ray tracing in three-dimensionally heterogeneous media, and through allowance for a wide variety .f medium characterizations. Iterative convergence of the solution is found to be rapid when using either initially homogeneous or heterogeneous models. Solutions generated using this technique also agree well with single-step solutions. Application of this method to P-wave travel time data collected from seismographs on the Island of Hawaii determines a high resolution, three-dimensional image of crust and mantle structure to depths in excess of 160 km. Crustal structure of this young volcanic island is dominated by the presence of high velocity instrusive dikes and sills in the summit complexes and radial rift zones of the five shield volcanoes. Mantle structure within the underlying lithosphere, or to about 75 km depth, contains acentralized low velocity zone with typical horizontal dimensions of about 50 km which is flanked by higher velocities in the surrounding offshore zone. The velocity contrast between the low velocity zone and the encircling highs averages 3 to 4%. This contrast increases markedly in the asthenosphere to upwards of 10% AV/V. However, the most intense low velocity region lies to the east of the island, and coincides with the axis of the Hawaiian Island chain as extrapolated from the older islands. The validity of the three-dimensional structure determined by teleseismic data is tested through modeling of-crust and uppermost mantle structure using travel times of local earthquakes. A broad and statistically significant correlation exists between these independently determined velocity models, although some significant discrepancies also exist. The structural relationships indicated by the lateral velocity heterogeneities supports other geophysical and geochemical evidence that the tholeiitic basalts erupted on Hawaii are derived from a source region below the lithosphere. The vertical continuity of the centralized low velocity zone in the lithosphere, which underlies the principal volcanic summits, with more intense low velocity zones in the asthenosphere indicates that the basalts originate no shallower than about 100 km and may be in transit through both asthenosphere and lithosphere from a deepermantle source. The deepest-seated lateral heterogeneities resolvable by thedata align in a broad low velocity zone which is elongated parallel to the Hawaiian chain axis. Because the most intense regions of this low velocity feature are not directly associated with any overlying volcanism, lie ahead of the island chain along the direction of chain growth, and extend well over 80 km into the asthenosphere, _I___I1ILMIYPIIYI_1_1~1__~____---~-~li -i-LILILl~rX