Joint inversion of gravity and magnetic anomalies of eastern Canada

The power spectra and degree correlation of the surface topography and free -air gravity anomalies of eastern Canada show that the gravity anomalies ar e subdivided into three parts. The short-wavelength components (30-170 km, shorter than 30 km are not well resolved) largely arise from density pertur bations in the crust and to a lesser extent from the surface topography and Moho undulation, whereas the contribution of intracrustal sources to the i ntermediate-wavelength components (170-385 km) is comparable with that of t he topography. The long-wavelength components (385-1536 km) are overcompens ated at the Moho. We present a crustal model for the intermediate- and long wavelength components which takes into account the surface topography, den sity perturbations in the crust, and Moho undulation with a certain degree of isostatic compensation. The general characteristics of this model resemb le the crustal structure revealed from seismic measurements. The reduced-to -pole magnetic anomalies of eastern Canada show no pronounced correlation w ith the topography and with the vertical gradient of the gravity anomalies, suggesting that the source bodies are within the crust and Poisson's relat ionship does not hold over the entire area. Assuming that the magnetic anom alies arise from induced magnetization, lateral variations of magnetic susc eptibility of the crust are determined while taking into account the effect s of the surface topography and the Moho undulation of our crustal model. T he intermediate- and long-wavelength components of the susceptibility contr asts delineate major collision zones as low-susceptibility regions. We inte rpret this in terms of thermal demagnetization of the high-magnetic crustal roots beneath the collision zones.