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, total magnetic intensity (TMI) data. Many of these anomalies arise from the tectonic juxtaposition of sedimentary units of differing magnetic properties. In detail, the anomalies can be 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 detailed magnetic-property measurements, geophysical models based on geology, and Euler analysis. We find that along-strike anomaly variability arises mainly from (1) multi-levelled magnetic contrasts at the fault that are variably sampled by uneven levels of erosion, and to a lesser extent from (2) magnetic susceptibilities that vary along strike within individual units, and (3) variable throw and dip of the fault that produces differences in the extents to which contrasting units are in contact. The multi-levelled magnetic contrasts arise from the juxtaposition of different strata across the fault at discrete depths. Locations of magnetic sources along the fault estimated from Euler analysis of the TMI data reflect the variations in depths to the shallowest sources along strike. Variations in clustering of the Euler solutions suggest that the sources have variable geometry (structural index). The results at the San Ysidro Fault demonstrate the important and complex role of multi-levelled magnetic sources in understanding anomalies associated with faulted geologic layers in general. The potential for multiple sources suggests that the use of simple model geometries to represent faults may not always be appropriate.