Relationship Between the High-Amplitude Magnetic Anomalies and Serpentinized Fore-Arc Mantle in the Cascadia Subduction Zone

Abstract

A zone of significant high-amplitude magnetic anomalies is observed without a comparable gravity high along the Cascadia margin and is spatially correlated with the low-velocity fore-arc mantle wedge. This wedge is interpreted to be serpentinized fore-arc mantle and is further considered to be the main source of the high-amplitude magnetic anomalies. To test this hypothesis, the magnetization-density ratio (MDR) is estimated along the Cascadia margin to highlight the physical characteristics of serpentinization (reduced density and increased magnetization). Interestingly, high MDR values are found only in central Oregon, where slab dehydration and fore-arc mantle serpentinization (50–60% serpentinization) are inferred in conjunction with sparse seismicity. This result may indicate either poorly serpentinized fore-arc mantle (low degree of serpentinization) or that the fore-arc mantle is deeper than the Curie temperature isotherm for magnetite in northern and southern Cascadia. This finding means that magnetic anomaly highs and serpentinized fore-arc mantle may not always be correlated in subduction zones. On the other hand, the MDR pattern suggests segmentation of the Cascadia subduction zone, which is consistent with several previous geological and geophysical observations.