No abstract

High-resolution aeromagnetic data acquired over several basins in the central Rio Grande rift, north-central New Mexico, prominently display low-amplitude (5-15 nT) linear anomalies associated with faults that offset basin-fi ll sediments. The linear anomalies give an unparalleled view of concealed faults within the basins that has signifi cant implications for future basin studies. These implications provide the impetus for understanding the aeromagnetic expression of faults in greater detail. Lessons learned from the central Rio Grande rift help to understand the utility of aeromagnetic data for examining concealed faults in sedimentary basins in general. For example, linear anomalies in the rift can be explained entirely by the tectonic juxtaposition of magnetically differing strata rather than the product of chemical processes acting at the fault zone. Differences in layer thickness, depth to the layer(s), and magnetic susceptibility govern the variability of the anomaly shape. Further investigations of these variables using simple models provide graphical, mathematical, and conceptual guides for understanding the aeromagnetic expression of faults, including the criteria for aeromagnetic expression of faults, how to locate fault traces from aeromagnetic anomalies, the effect of fault dip, and how to assess the role of topography. The horizontal gradient method applied to reduced-to-pole aeromagnetic data is particularly effective in mapping fault locations, especially at regional scales. With our new understanding of the aeromagnetic expression of faults, we updated interpretations of faults from the aeromagnetic data for the central Rio Grande rift. These interpretations, along with the guides, should provide direction and fuel for future work in a wide variety of multidisciplinary basin-related topics.

The characteristics and statistical range of fault zone features appear to be predictable and consistent throughout the basin; this predictability can be used in groundwater flow simulations that consider the influence of faults.

The Jurassic Preuss Sandstone, exposed in five thrust plates of the Wyoming‐Idaho‐Utah thrust belt, carries directions of remanent magnetization that group most tightly after only partial unfolding. Field, petrographic, and rock magnetic evidence indicates that the carrier of this magnetization is detrital, low‐Ti titanomagnetite. The detrital titanomagnetite was remagnetized at low temperatures (75°–150°C) probably completely during folding. Anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain. We suggest that low‐temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization, the acquisition of which was enhanced by stress, or a piezoremanent magnetization that involved stress‐induced movement of domain walls during intracrystalline strain, or was a combination of the two mechanisms. Stress may promote remagnetization at temperatures much lower than predicted by current theoretical models. Other mechanisms, such as acquisition of chemical remanent magnetization during folding, deflection of a prefolding magnetization by internal strain, or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the Preuss. The locus of acquisition of synfolding magnetization in the Preuss migrated in conjunction with deformation in the thrust belt. A model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps. A lack of reverse‐polarity directions remains a puzzling feature of the remanence. The remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in Late Cretaceous and early Tertiary (?) periods of predominantly normal polarity.