Regional Magnetic Anomalies, Crustal Strength, and World Thrust Belt Geometries

Regional geometries of continental thrust belts reflect the interaction of compressional tectonic forces and the strength of the upper lithosphere. Regional magnetic anomalies can be used to identify deep thermal and compositional characteristics that influence crustal and upper mantle strength. Detailed geophysical interpretation in the northern Cordillera of North America (Alaska and Canada) reveals a spatial association of distinct magnetic highs with inflections in the thrust belt front. Regional heat flow maps for the northern Cordillera also show that the deep-source magnetic highs coincide with regions of lower heat flow and therefore a deeper Curie temperature isotherm. A deeper Curie isotherm means that a thicker portion of the crust and upper mantle can contribute to long-wavelength magnetic anomalies. Association of these characteristic magnetic highs of the northern Cordillera with positive gravity anomalies suggests a mafic composition to the deep crust and may also indicate a relatively depleted, and therefore refractory, underlying mantle. A mafic lower crust and refractory mantle may combine to create a “buttress” to tectonic forces and limit the overall width of the regional thrust belt. A world data viewer, based on NASA World Wind, allows for the correlation and examination of regional magnetic features of the World Magnetic Anomaly Map and global topography and bathymetry to evaluate the correlation of deep-source magnetic highs with thrust belt geometries. Regional, deep-source, positive magnetic anomalies are found to correlate with inflections in regional thrust belt geometries in the major active thrust belts of the world (North America, South America, Eurasia, India subcontinent). Based on the North American Cordilleran examples, these positive magnetic anomalies probably identify buttresses of stronger lithosphere.