36. TECTONIC, VOLCANIC, AND PALEOGEOGRAPHIC IMPLICATIONS OF REDEPOSITED REEF FAUNAS OF LATE CRETACEOUS AND TERTIARY AGE FROM THE NAURU BASIN AND THE LINE ISLANDS'

The age of the Pacific Plate at Deep Sea Drilling Project Site 462 is taken to be 148 m.y. , based on the location of the site on Magnetic Anomaly M-26. Drilling at Site 462 however, revealed the presence of a thick volcanic complex made up of a lower sheet-flow unit and an upper sill unit which engulfed and intruded Cretaceous sediments. The igneous complex is entirely normally magnetized, indicating that it probably was emplaced during the Cretaceous long normal magnetic event which spans the period 80 to 112 m.y. ago. A Ar-^Ar date from a sill is 110 ±3 m.y. However, the lower flow unit is probably Barremian. A displaced reef fauna containing a Campanian-Maestrichtian pseudorbitoidid assemblage was found in a volcaniclastic section that represents the final episode of Cretaceous volcanism in the Nauru Basin-Marshall Islands area. The probable sequence of volcanic events at Site 462 appears to be (1) formation of the underlying Pacific Plate 148 m.y. ago, (2) extrusive volcanism in Barremian time, about 115 m.y. ago, (3) sill and flow volcanism accompanied by edifice-building in late Aptian time, about 110 m.y. ago, and (4) a further episode of edifice-building and deep-water volcanism beginning in the Campanian and ceasing by the middle Maestrichtian, about 70 m.y. ago. Application of models relating lithospheric uplift and subsidence to volcanism developed by Crough (1978) and Detrick and Crough (1978), combined with sedimentological and paleontological evidence from Site 462, allows a semi-quantitative reconstruction of the vertical tectonic history of the Nauru Basin. The basin did not subside "normally" since its formation at a ridge crest 148 m.y. ago, but instead was uplifted in Barremian-Aptian time, about 115 to 110 m.y. ago, and it remained relatively shallow until Maestrichtian time, about 70 m.y. ago. Reefs grew in the Marshall Islands area during Campanian-Maestrichtian time; prior to DSDP Leg 61, the oldest reefs in the Marshall Islands were dated as Eocene at Enewetak. Analysis of the stratigraphic distribution of mid-plate volcanism and reef faunas from the Nauru Basin-Marshall Islands area, the mid-Pacific Mountains, the Line Islands, and the central Pacific Basin indicates that this entire area underwent thermally induced uplift between about 115 to 110 and 70 m.y. ago. The region has been subsiding since about 70 m.y. ago. This history of vertical tectonics associated with volcanism is reminiscent of the Darwin Rise, described by Menard (1964). The chronology and areal extent of Cretaceous midplate volcanism in the central Pacific is difficult to reconcile with the Wilson-Morgan hot-spot theory. We suggest that the mechanism proposed by Liu (1980), involving convection-generated stress in the lithosphere, which results in fractures allowing magma access to the surface, is an attractive alternative. Correlation of central Pacific mid-plate volcanism with volcanic events in the Caribbean and South Atlantic indicates that uplift associated with mid-plate volcanism was a major factor in causing the global Cretaceous transgression heretofore ascribed to increases in spreading rates at mid-ocean ridges. The Late Cretaceous reef faunas from the Marshall-Line Islands and the Caribbean have strong affinities, suggesting that these areas were closer together in Late Cretaceous time and that there were shallow-water "stepping stones" between them. This interpretation supports the contention of earlier workers that the present Caribbean Plate was part of the Farallon Plate in Cretaceous time. Redeposited reef faunas found in Oligocene and Miocene turbidite sequences are related to accelerated reef erosion during glacially induced sea-level falls. The Cenozoic reef assemblages, in contrast to the Cretaceous ones, have IndoPacific affinities.