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ABSTRACT: Magnetic Susceptibility Record of Volcanogenic Input to the Izu-Bonin Arc

T. L. Janecek

Detailed magnetic susceptibility measurements were obtained on whole-core sections (at 5-10 cm intervals) from cores drilled during Leg 126 of the Ocean Drilling Program along the Izu-Bonin Arc. These rapid and nondestructive log-type analyses are used (1) to provide a temporal record of the style of volcanic input into the forearc and backarc regions; (2) to delineate individual gravity flow units; (3) as an excellent tool for hole to hole and site to site correlations, and, (4) to identify sediments disturbed by coring processes. Peaks in magnetic susceptibility correlate with the relative concentrations of ferromagnetic or paramagnetic minerals, either as solitary phases or as minute inclusions in basaltic glass.

Magnetic susceptibility data, combined with mass accumulation rate (MAR) data, provide a record of the types of volcanogenic input into the region. For example, the volcanogenic sediments in the backarc regions are bimodal in nature, consisting of thick units of felsic vitric silt and darker grains of mafic glass, ferromagnesian minerals, and lithic fragments, and thus have distinctly different magnetic characteristics. The magnetic susceptibility and MAR data show that the flux of felsic material increased dramatically during the last 250,000 years and document the increase in explosive arc volcanic activity during this interval.

The magnetic susceptibility data not only delineate types of volcanic input but, based upon variations in composition and grain size, also differentiate individual gravity flow units as well as divisions within Bouma sequences. The use of magnetic susceptibility data as a tool for site to site correlations and for identifying sediments disturbed by coring processes will also be presented.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990