--> ABSTRACT: Hybrid Modeling of Gravity and Geomagnetic Anomalies: A Tool for Improved Paleothermal Reconstruction in Areas Adjacent to Igneous Activity, by Yasuto Itoh, Yasuhiko Nagasaki, Ned R. Timbel, Bruce T. May, Peter J. J. Kamp, Thomas L. Thompson; #91020 (1995).

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Hybrid Modeling of Gravity and Geomagnetic Anomalies: A Tool for Improved Paleothermal Reconstruction in Areas Adjacent to Igneous Activity

Yasuto Itoh, Yasuhiko Nagasaki, Ned R. Timbel, Bruce T. May, Peter J. J. Kamp, Thomas L. Thompson

Igneous activity has a substantial impact on hydrocarbon maturation in active margin settings and rifted continental margins, where intrusions are commonly associated with continental breakup. Apatite fission track analysis has shown that doleritic intrusions cause widespread thermal effects resulting from high initial temperatures. To assess the extent of thermal effect, the nature of igneous rocks in and under sedimentary basins must be known. Because of strong magnetization and wide range of Curie temperature as a function of composition, geomagnetic anomaly patterns are a key to the volume and type of deeply seated igneous rocks. Considering the effect of time decay on remanent magnetization under elevated temperatures, some rock types can produce a magnetic anomaly i sedimentary basins. Gravity measurement should then be combined with magnetics to verify the assumed lithology and emplacement form.

Pattern analyses around Japan are presented as example interpretations for an active margin setting. A granitic belt on the fore-arc slope of Northeast Japan, for instance, holds deflected remanent magnetization before the Miocene fan-shaped opening of the Japan Sea, without analogous gravity response. Thus, nature of igneous rocks may be determined by examining gravity and magnetic data within a geological framework. The modeling of hybrid anomalies is also applied in the oil-producing offshore basins of New Zealand, on the basis of GEOSAT gravity mapping, air-borne magnetic anomaly, and regional seismic data. Igneous activity is an overlooked source of paleothermal history data that, combined with maturity modeling and fission track analysis, may provide important clues about hydroc rbon potential.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995