Guides to understanding the aeromagnetic expression of faults in sedimentary basins: Lessons learned from the central Rio Grande rift, New Mexico V.J.S. Grauch U.S. Geological Survey, MS 964, Federal Center, Denver, Colorado 80225-0046, USA Mark R. Hudson U.S. Geological Survey, MS 980, Federal Center, Denver, Colorado 80225-0046, USA ABSTRACT cho, northwest of Albuquerque, New Mexico (Fig. 2). Linear anomalies are interpreted as faults that offset basin-fi ll sediments based on their High-resolution aeromagnetic data acquired over several basins in consistent correspondence to isolated exposures of mapped faults, follow- the central Rio Grande rift, north-central New Mexico, prominently up investigations at individual sites, and geophysical modeling (Grauch, display low-amplitude (5–15 nT) linear anomalies associated with 2001; Grauch et al., 2001, 2006). faults that offset basin-fi ll sediments. The linear anomalies give an As in many sedimentary basins, mapping faults in the central Rio unparalleled view of concealed faults within the basins that has sig- Grande rift is diffi cult because of the extensive alluvial cover. As a conse- nifi cant implications for future basin studies. These implications pro- quence, geologists have used the linear aeromagnetic anomalies to delin- vide the impetus for understanding the aeromagnetic expression of eate partially concealed faults and denote possible locations of totally faults in greater detail. Lessons learned from the central Rio Grande buried faults on geologic maps (e.g., Connell, 2006) and in fault compila- rift help to understand the utility of aeromagnetic data for examin- tions (Machette et al., 1998; Personius et al., 1999). The benefi t of using ing concealed faults in sedimentary basins in general. For example, aeromagnetic data to locate faults in conjunction with geologic mapping linear anomalies in the rift can be explained entirely by the tectonic is exemplifi ed in the Hubbell Spring area, south of Albuquerque (Fig. 3). juxtaposition of magnetically differing strata rather than the prod- The aeromagnetic data reveal a comprehensive view of the fault patterns uct of chemical processes acting at the fault zone. Differences in layer and provide evidence for a signifi cant, previously unknown fault system thickness, depth to the layer(s), and magnetic susceptibility govern located under cover east of the Hubbell Spring fault (“buried faults” on the variability of the anomaly shape. Further investigations of these Fig. 3B). variables using simple models provide graphical, mathematical, and As demonstrated by the Rio Rancho and Hubbell Spring examples, the conceptual guides for understanding the aeromagnetic expression of aeromagnetic data imply that faults are much more numerous than previ- faults, including the criteria for aeromagnetic expression of faults, ously suspected throughout the basins in the central Rio Grande rift. This how to locate fault traces from aeromagnetic anomalies, the effect of inference has signifi cant implications for a variety of structural topics, fault dip, and how to assess the role of topography. The horizontal such as the evolution of structural styles involved in rifting, the amount of gradient method applied to reduced-to-pole aeromagnetic data is par- basin extension, the linkage between intrabasin and basement faults, and ticularly effective in mapping fault locations, especially at regional relations between fault trace length and fault throw. The numerous linear scales. With our new understanding of the aeromagnetic expression anomalies have already led hydrologists to consider the important role of faults, we updated interpretations of faults from the aeromagnetic that faults may play in compartmentalizing basin aquifers and control- data for the central Rio Grande rift. These interpretations, along with ling ground-water fl ow in the central Rio Grande rift (Bartolino and Cole, the guides, should provide direction and fuel for future work in a wide 2002; Heywood et al., 2002). variety of multidisciplinary basin-related topics. Although subtle linear anomalies are associated with faults in other sedimentary basins (Gunn, 1997; papers in Peirce et al., 1998; Smith et Keywords: aeromagnetic surveys, faults, faulting, sedimentary basins, al., 2002), the ones in the central Rio Grande rift are prominent and gen- rifts. erally isolated from interfering anomalies, providing excellent illustrative examples for study. For example, a signifi cant conclusion from earlier INTRODUCTION studies is that the source of the anomalies is the tectonic juxtaposition of magnetically differing strata rather than the product of chemical pro- High-resolution aeromagnetic data acquired over the central Rio Grande cesses acting at the fault zone (Grauch et al., 2001, 2006; Hudson et al., rift (Fig. 1) reveal numerous subtle, northerly trending, linear anomalies 2008). This conclusion suggests that chemical processes are not required that are widespread across the basin areas. The anomalies are best delin- to explain fault-related anomalies. In fact, no defi nitive case study show- eated in shaded-relief images, as demonstrated in an area over Rio Ran- ing that the effects of chemical processes at fault zones are signifi cant Geosphere; December 2007; v. 3; no. 6; p. 596–623; doi: 10.1130/GES00128.1; 17 fi gures; 2 plates; 1 table. 596 For permission to copy, contact [email protected] © 2007 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/6/596/854624/i1553-040X-3-6-596.pdf by guest on 27 September 2021 Aeromagnetic expression of faults in sedimentary basins Figure 1. Regional geology and topography of the central Rio Grande rift, showing the locations of the Albuquerque, Santo Domingo, and southern Española basins. Generalized geology (New Mexico Bureau of Geology and Mineral Resources, 2003; Machette et al., 1998) is overlain on a shaded digital elevation model (U.S. Geological Survey National 30 m digital elevation data). ABQ—Albuquerque. Inset map locates the map area with respect to basins of the Rio Grande rift in Colorado and New Mexico. Geosphere, December 2007 597 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/6/596/854624/i1553-040X-3-6-596.pdf by guest on 27 September 2021 Grauch and Hudson Figure 2. Color shaded-relief image of reduced-to-pole (RTP) aeromagnetic data for the Rio Rancho area (Sweeney et al., 2002). The colors primarily refl ect the broad variations in the data, whereas the illumination (from the west) empha- sizes detailed variations, especially linear features associated with faults. Geologic contacts from New Mexico Bureau of Geology and Mineral Resources (2003). QTs—Quaternary and Tertiary sediments (Santa Fe Group and alluvial cover). QTb—Quaternary and Tertiary basaltic and andesitic rocks, undifferentiated. Mz—Mesozoic sedimentary rocks. Aster- isks indicate volcanic vents. See inset for location. Labeled profi les are shown in Figures 6 and 11. Dashed white boxes show areas of Figures 11 and 12. 598 Geosphere, December 2007 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/6/596/854624/i1553-040X-3-6-596.pdf by guest on 27 September 2021 Aeromagnetic expression of faults in sedimentary basins Figure 3. Geologically versus aeromagnetically mapped faults for an area surrounding the Hubbell Spring fault, south of Albuquerque (see inset for location). Except for isolated exposures of Paleozoic sedimentary rocks in the extreme northeastern corner, the area is mapped as Santa Fe Group and younger surfi cial deposits (Love et al., 1996; Kelley, 1977). (A) Faults mapped before the aeromagnetic data were available are from Machette (1982) and Kelley (1977). (B) Color shaded-relief aeromagnetic image illuminated from the east, with selected features labeled. Note the numer- ous north-trending linear anomalies, showing apparent en echelon and anastomosing patterns. Compare these to the near lack of expression of the 15- to 50-m-high walls of Hells Canyon Wash (light blue outline). (C) Faults inferred from aeromagnetic data indicating numerous concealed faults and revealing the complexity of faulting in the area. Fault interpretation for the apparent dendritic pattern in the northwest corner of the area is supported by geologic mapping (Love et al., 1996). enough to produce anomalies detected by aeromagnetic surveys is known fault geometry, and magnetic properties to the variations in the aeromag- in the literature (Nabighian et al., 2005; Mushayandebvu and Davies, netic signatures. Based on the understanding gained at individual faults, 2006). These considerations suggest that tectonic juxtaposition of faulted we then develop a systematic procedure for mapping faults from aeromag- strata may be a common cause of linear anomalies in sedimentary basins. netic data at scales more regional than 1:24,000. Examples of the utility Given the likelihood that tectonic juxtaposition explains many fault- and limitations of this procedure provide guides to using linear anomalies related aeromagnetic anomalies in sedimentary basins, the next step is to for mapping concealed faults. investigate the relation in greater detail. In particular, we seek to answer The guides in this report can be used in the central Rio Grande rift or in the following questions: other sedimentary basins to examine the concealed portions of individual • What is required for a fault to produce an aeromagnetic anomaly? faults, judge the utility and limitations of aeromagnetic