Advancements in understanding the aeromagnetic expressions of basin-margin faults—An example from San Luis Basin, Colorado

Abstract

Advancements in aeromagnetic acquisition technology over the past few decades have led to greater resolution of shallow geologic sources with low magnetization, such as intrasedimentary faults and paleochannels. Detection and mapping of intrasedimentary faults in particular can be important for understanding the overall structural setting of an area, even if exploration targets are much deeper. Aeromagnetic methods are especially useful for mapping structures in mountain-piedmont areas at the margins of structural basins, where mineral exploration and seismic-hazard studies may be focused, and where logistical or data-quality issues encumber seismic methods. Understanding if the sources of aeromagnetic anomalies in this context originate from sedimentary units or bedrock is important for evaluating basin structure and/or depth to shallow exploration targets. Advancements in aeromagnetic acquisition technology over the past few decades have led to greater resolution of shallow geologic sources with low magnetization, such as intrasedimentary faults and paleochannels. Detection and mapping of intrasedimentary faults in particular can be important for understanding the overall structural setting of an area, even if exploration targets are much deeper. Aeromagnetic methods are especially useful for mapping structures in mountain-piedmont areas at the margins of structural basins, where mineral exploration and seismic-hazard studies may be focused, and where logistical or data-quality issues encumber seismic methods. Understanding if the sources of aeromagnetic anomalies in this context originate from sedimentary units or bedrock is important for evaluating basin structure and/or depth to shallow exploration targets. Although explorationists have surmised that subtle, narrow, linear aeromagnetic anomalies or gradients are caused by intrasedimentary faults, the nature of the magnetic sources has been debated. A common and intuitive explanation for the linear anomalies considers that the magnetic properties of the fault zone have been altered by secondary chemical processes, either through the growth or destruction of magnetic minerals. However, comprehensive, multidisciplinary studies of partially exposed intrasedimentary faults in basins within the central Rio Grande Rift, New Mexico, have shown that the anomalies can be completely explained by the tectonic juxtaposition of strata of differing magnetic properties at the fault (summarized in Grauch and Hudson, 2007 , 2011 ). Whereas a reduction in magnetic susceptibility was detected at some fault zones in the laboratory, the slight reduction and small volume of material affected were insufficient to produce aeromagnetic anomalies ( Hudson et al., 2008 ). A key finding of the studies summarized by Grauch and Hudson (2007 , 2011 ) is that multiple magnetic contrasts (sources) can be vertically stacked at one fault. This situation requires rethinking of common assumptions when modeling faults as simple steps or when interpreting depth-estimation results. The multiple contrasts in these case studies arise from the tectonic juxtaposition of stratified sediments with differing magnetic properties. Multiple, vertically stacked magnetic sources also can occur where volcanic layers are interbedded with the sedimentary section or where faults offset both shallow bedrock and its overlying sedimentary cover. The latter situation is common at basin margins. Herein, we summarize and expand on an investigation of the sources of aeromagnetic anomalies related to faults along the eastern margin of the San Luis Basin, northern Rio Grande Rift, Colorado ( Grauch et al., 2010 ). Similar to the faults examined in the central Rio Grande Rift, magnetic sources can be completely explained by tectonic juxtaposition and produce multiple, vertically stacked magnetic contrasts at individual faults. However, the geologic sources are different. They arise from both the sedimentary cover and the underlying bedrock rather than from stratified sediments. In addition, geologic evidence for secondary growth or destruction of magnetic minerals at the fault zone is lacking.