Abstract
Abstract High-resolution aeromagnetic surveys over forearc basins can detect faults and folds in weakly magnetized sediments, thus providing geologic constraints on tectonic evolution and improved understanding of seismic hazards in convergent-margin settings. Puget Sound, Washington, and Cook Inlet, Alaska, provide two case histories. In each lowland region, shallow-source magnetic anomalies are related to active folds and/or faults. Mapping these structures is critical for understanding seismic hazards that face the urban regions of Seattle, Washington, and Anchorage, Alaska. Similarities in aeromagnetic anomaly patterns and magnetic stratigraphy between the two regions suggest that we can expect the aeromagnetic method to yield useful structural information that may contribute to earth-hazard and energy resource investigations in other forearc basins.
Highlights
The populated lowlands of Puget Sound, Washington, and Cook Inlet, Alaska, lie within seismically active forearc basins in a dynamic convergent margin (Fig. 1)
Similarities in aeromagnetic anomaly patterns and magnetic stratigraphy between the two regions suggest that we can expect the aeromagnetic method to yield useful structural information that may contribute to earth-hazard and energy resource investigations in other forearc basins
While it is reasonable to postulate the contribution of deep serpentinized mantle to the total aeromagnetic signature in both Puget Sound and Cook Inlet, it is evident that long-wavelength anomalies arise mainly from mid- to upper-crustal sources, namely magnetic basement and intrusive rocks related to the arc settings discussed above
Summary
The populated lowlands of Puget Sound, Washington, and Cook Inlet, Alaska, lie within seismically active forearc basins in a dynamic convergent margin (Fig. 1). Magnetic susceptibility measurements at five locations between Seattle and Everett indicate susceptibilities ranging from 1 to 2.5 mSI (induced magnetizations of 0.05 to 0.12 A/m), with an overall average value of 2 mSI (0.09 A/m; Blakely et al, 2004) In both the Cook Inlet and Puget Lowland, shallowsource aeromagnetic anomalies are predicted to arise from structural or topographic features involving surficial or near-surface geologic units. Portions of the fluvial Tertiary section (distinct units within the Blakely Harbor Formation in Puget Sound, as described above, and parts of the Sterling Formation in Cook Inlet) are sufficiently magnetic to produce moderate to large amplitude magnetic anomalies with short to moderate wavelengths. This steeply dipping conglomerate produces a dike-like magnetic anomaly (the northern high in Fig. 11), shorter
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