Gravity and magneitc anomalies in the San Juan Basin area

Abstract

Regional gravity and magnetic maps that include the San Juan Basin area (e .g., Cordell, 1984: Keller and Cordell , 1984) demonstrate that there arc many significant anomalies present. Here , we present a series of re latively detailed maps that enhance these anomalies and briefly comment on the ir geological significance . Our primary goal is to present these maps for the use of those interested in this region and to illustrate their util ity. GRAVITY AND AEROMAGNETIC ANOMALY MAPS The geophysics group in the Department of Geological Sciences at The University of Texas at El Paso maintains an extensive database of gravity and magnetic measurements . These data were used to construct Bouguer gravity and total magnetic intensity anomaly maps of the San Juan Basin area. The San Juan Basin area contains more than 21,000 gravity and 17,000 magnetic data points. The data sets were processed to form a 2-km grid using a minimum curvature algorithm. A fi rst order polynomial surface was removed from both graips before they were filtered. The filtering algorithm employed was a general ized bandpass/strikepass fi lter based on the Hartley transform (Adams et al. , 1991 ). The maps were filtered to pass wavelengths from 20 km to 800 km . In addition, a gravity map was fi ltered to reject strikes from 85° to 225° east of north . These strikes are perpendicular to the prominent cast-northeast-trending gravity low that crosses the basin. The filtered gravity maps (Figs. I and 2) are dominated by maxima associated with basement uplifts. The Nacimiento and Zuni uplifts are of primary interest here. The northwest-trending gravity high associated with the Zuni uplift indicates that this feature is much larger than the actual Precambrian outcrop area . As di scussed by Woodward et al. (this volume), the Nac imiento upli ft and its bounding fau lts have been of interest for many years. The gravity maxima associated with this uplift correlate very well with the outcrop area. The steep gravity gradient FIGURE I . Residual gravity anomaly map, filtered to pass wavelengths from 20 to 800 km. on the west side of the uplift attests to the steep dip and large throw on th is fault. There is a distinct regional east-northeast grain in the gravity map shown in Fig . I. This trend correlates with the dominant anomaly in the aeromagnetic map discussed below. and gravity anomalies with this trend were enhanced to produce the map shown in Fig. 2. This map demonstrates the importance of Precambrian features, which reflect the evol ution of the crust in this region. The dominant feature on the aeromagnctic anomaly map (Fig . 3) is the northeast-trend ing low that extends across the San Juan Basin. passing just north of the Nacimiento uplift. Cordell and Keller ( 1984) showed that th is anomaly passes through the Rio Grande rift onto the Great Plai ns. This anomaly is offset as it crosses the rift and the Sangre de Cristo Mountai ns, indicating the presence of a series of right lateral strike-s lip faults (Cordell and Keller, 1984). It also follows the general structural grain of the Precambrian basement in the region (Grambling et al., 1988) and generally correlates with outcrops of the Ortega Group in the Sangre de Cristo Mountains. Such metasedimentary rocks would be expected to have a relatively low magnetic susceptibility, and thus this negative anomaly probably correlates with a thick mctasedimentary sequence . Cordell and Grauch ( 1985) undertook a sophisticated analysis of this anomaly in the San Juan Basin area and mapped a northeasttrending zone of supracrustal rocks, which probably corresponds to FIGURE 2. Residual gravity anomaly map filtered to pass wavelengths from 20 to 800 km, and strikepass filtered to pass strikes of 85°-225°E. 134 KELLER and ADAMS