Sources of along-strike variation in magnetic anomalies related to intrasedimentary faults: A case study from the Rio Grande Rift, USA

Abstract

Normal faults within sedimentary basins are commonly associated with subtle linear features in high-resolution aeromagnetic data. Many of these anomalies arise from the tectonic juxtaposition of sedimentary units of differing magnetic properties. In detail, the anomalies are quite variable in character, even along the strike of individual faults. To understand this variability, we examine the well-exposed San Ysidro fault in the central Rio Grande rift, USA, using magnetic-property measurements and geophysical models. We find that the sources of along-strike anomaly variability arise mainly from (1) differences in levels of erosion that bring different juxtaposed units to the surface and (2) magnetic properties that vary along strike within individual units. Uneven levels of erosion have the most significant effect, owing to the presence of several discrete magnetic sources distributed at different levels of the fault. Although variable fault throw affects the extent to which contrasting units are in contact across the fault, the effect appears to be overwhelmed by the other factors. The results at the San Ysidro fault, especially the possibility of multiple magnetic sources at a single fault, should aid in the geologic interpretation of linear magnetic anomalies related to intrasedimentary faults in general.