Structure of the Tucson Basin, Arizona from gravity and aeromagnetic data

Abstract

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

Guides to understanding the aeromagnetic expression of faults in sedimentary basins: Lessons learned from the central Rio Grande rift, New Mexico

“…The linear anomalies coincide with shallow faults that offset basin sediments and are superposed on large-magnitude anomalies related to volcanic and igneous rocks that comprise the basin fl oor (Smith et al, 2002). Lack of similar linear anomalies in high-resolution aeromagnetic data over the Tucson basin, Arizona, is explained by a thick cover of young sediments that were not cut by earlier extensional faulting (Rystrom, 2003).…”

Reconstruction of Mid‐Cenozoic Extension in the Rincon Mountains Area, Southeastern Arizona, USA, and Geodynamic Implications

“…A 29‐km‐long, east‐west seismic reflection profile that passes just north of the Exxon well (Figure ) reveals some of the shallow structure associated with the central Tucson basin and the area penetrated by the well (Wagner & Johnson, , ). Reflections reveal a bedrock shoulder ~2–3 km east of the Exxon drill hole that coincides with a steep lateral Bouguer gravity gradient (Rystrom, ). This shoulder was interpreted as the footwall block of a normal fault that dips westward under the deeper units penetrated by the Exxon drill hole (Figure ; Wagner & Johnson, ).…”