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This digital Magnetic Anomaly database and map for the North American continent is the result of a joint effort by the Geological Survey of Canada (GSC), U.S. Geological Survey (USGS), and Consejo de Recursos Minerales of Mexico (CRM). The database and map represent a substantial upgrade from the previous compilation of Magnetic Anomaly data for North America, now over a decade old (Committee for the Magnetic Anomaly Map of North America, 1987). This integrated, readily accessible, modern digital database of magnetic anomaly data will be a powerful tool for further evaluation of the structure, geologic processes, and tectonic evolution of the continent and may also be used to help resolve societal and scientific issues that span national boundaries. The North American magnetic anomaly map derived from the digital database provides a comprehensive magnetic view of continental-scale trends not available in individual data sets, helps link widely separated areas of outcrop, and unifies disparate geologic studies. This booklet outlines the data processing and compilation procedures used to produce the magnetic anomaly database and map that accompany this booklet.

Interpretation of gravity and high-resolution aeromagnetic data reveal the three-dimensional geometry of the Tuscson Basin, Arizona and the lithology of its basement. Limited drill hole and seismic data indicate that the maximum depth to the crystalline basement is approximately 3600 meters and that the sedimentary sequences in the upper ~2000 m of the basin were deposited during the most recent extensional episode that commenced about 13 Ma. The negative density contrasts between these upper Neogene and Quaternary sedimentary sequences and the adjacent country rock produce a Bouguer residual gravity low, whose steep gradients clearly define the lateral extent of the upper ~2000m of the basin. The aeromagnetic maps show large positive anomalies associated with deeply buried, late Cretaceousearly Tertiary and mid-Tertiary igneous rocks at and below the surface of the basin. These magnetic anomalies provide insight into the older (>13 Ma) and deeper structures of the basin. Simultaneous 2.5-dimensional modeling of both gravity and magnetic anomalies constrained by geologic and seismic data delineates the thickness of the basin and the dips of the buried faults that bound the basin. This geologic-based forward modeling approach to using geophysical data is shown to result in more information about the geologic and tectonic history of the basin as well as more accurate depth to basement determinations than using generalized geophysical inversion techniques. CONTENTS

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