Weakly magnetic crust in the Canadian Cordillera

Current models of continental crust favor an increase in magnetization with depth. Here we report a counter example from the Canadian Cordillera where almost a full thickness of non-magnetic continental crust is suggested by joint interpretation of magnetic and seismic data. The magnetic field over the Cordillera is characterized by complex, short-wavelength (< 100 km) anomalies associated with intrusive, metamorphic and volcanic rocks that occur at shallow depths (< 5 km) within accreted terranes. The long-wavelength (> 100 km) portion of the Cordilleran field is subdued and mainly featureless, and suggests a lack of magnetic sources at greater depths. Seismic reflection and refraction data from three major transects in the Yukon and British Columbia, Canada support this interpretation and indicate that sedimentary-like formations make up the majority of the crust. The dominance of shallow, upper crustal magnetization in the Canadian Cordillera contrasts with the generally-held view that the lower continental crust is the primary source for long-wavelength magnetic anomalies. Sources for these anomalies are often assumed to be located in the lower crust when surface magnetizations are insufficient to produce such anomalies or no correlation exists between the magnetic field and the mapped surface geology. The Canadian Cordillera appears to be an example of a non-magnetic lower crust overlain by a more magnetic upper crust that is, however, not magnetized strongly enough to produce significant long-wavelength magnetic anomalies.