Lynn E. Johnson

The first evidence for MORB-like lavas from the outer Mariana forearc: geochemistry, petrography and tectonic implications

Abstract The Mariana forearc, the region between the active Mariana volcanic arc and the trench axis, is considered to be composed of rocks entirely of arc origin. This conclusion was based upon the lack of an accretionary wedge and recovery of arc volcanics from sites throughout the forearc (including the inner trench wall). New chemical data from dredged samples demonstrate that exposures of altered basalts similar to mid-ocean ridge basalt (MORB) and of ocean island basalt (OIB) exist in the Mariana forearc. Several forearc sites including a 2000 m scarp bounding a large graben at 19°30′N, two small scarps in the inner trench wall, and a seamount were dredged. MORB-like and OIB lavas in addition to island arc tholeiite (IAT) and boninite were recovered. All samples exhibit low-temperature seafloor alteration, and some are metamorphosed to lower greenschist facies. IAT samples have low TiO 2 ( 2 of 1–2% and Zr of 60–93 ppm. OIB samples typically have high TiO 2 (> 3%) and Zr (> 180 ppm). Several OIB samples contain modal nepheline and all have high alkali contents. The presence of OIB and MORB-like lavas in the outer forearc may suggest either that entrapment of Philippine Sea or Kula Plate or that obduction of Pacific Plate has occurred in the evolution of the Mariana forearc. However, the intimate association of MORB-like lavas with arc lavas in the fault scarp requires that either IAT magma was injected into entrapped or obducted oceanic crust or that MORB-like magma was injected into an arc crust during forearc rifting or deformation.

Assessment of flash flood warning procedures

Assessment of four alternate flash flood warning procedures was conducted to ascertain their suitability for forecast operations using radar-rainfall imagery. The procedures include (1) areal mean basin effective rainfall, (2) unit hydrograph, (3) time-area, and (4) 2-D numerical modeling. The Buffalo Creek flash flood of July 12, 1996, was used as a case study for application of each of the procedures. A significant feature of the Buffalo Creek event was a forest fire that occurred a few months before the flood and significantly affected watershed runoff characteristics. Objectives were to assess the applicability of the procedures for watersheds having spatial and temporal scale similarities to Buffalo Creek, to compare their technical characteristics, and to consider forecaster usability. Geographic information system techniques for hydrologic database development and flash flood potential computations are illustrated. Generalizations of the case study results are offered relative to their suitability for flash flood forecasting operations. Although all four methods have relative advantages, their application to the Buffalo Creek event resulted in mixed performance. Failure of any method was due primarily to uncertainties of the land surface response (i.e., burn area imperviousness). Results underscore the need for model calibration; a difficult requirement for real-time forecasting.

Upper Cenozoic volcanic rocks in the Mariana Forearc recovered from drilling at Ocean Drilling Program Site 781: Implications for forearc magmatism

A horst block was drilled in the center of the Mariana forearc near 20°N during leg 125 of the Ocean Drilling Program. At this site 781, the drill penetrated a Pleistocene vesicular, porphyritic basalt at 72 m below the seafloor, and the top of the basalt corresponds to a high-amplitude reflection on seismic reflection profiles across the site. The thickness of the basalt unit can only be estimated to be between 13 and 25 m because of poor core recovery (28% to 55%). The presence of an upper glassy chilled zone and a lower, fine-grained margin suggest that the basalt unit is either a single lava flow or a near-surface sill. The basalt is an island-arc tholeiite (IAT) enriched in large-ion-lithophile elements relative to high-field-strength elements, similar to the submarine lavas of the southern Mariana Arc seamounts. The basalt layer, the youngest in situ igneous layer reported from the Izu-Bonin and Mariana forearcs, is enigmatic because of its location more than 100 km from the active volcanic arc. The sediment layers above and below the basalt unit are late Pliocene (about 2.5 Ma) and normally magnetized. The basalt has schlieren - like structures, reverse magnetization, and a K-Ar age of 1.68 ± 0.37 Ma. Thus, the basalt layer is probably a sill fed by magma intruded along a fault zone bounding the horst and graben in the forearc. The geochemistry of the basalt is consistent with a magma source similar to that of rocks from the magmatic axis (or volcanic front) of the island arc, and derived from a mantle source above the subducting Pacific plate.