Geophysical Unit of Menlo Park, Calif. (GUMP)
This report presents an updated isostatic gravity map, with an accompanying discussion of the geologic significance of gravity anomalies in the Santa Ana 30 by 60 minute quadrangle, southern California. Comparison and analysis of the gravity field with mapped geology indicates the configuration of structures bounding the Los Angeles Basin, geometry of basins developed within the Elsinore and San Jacinto Fault zones, and a probable Pliocene drainage network carved into the bedrock of the Perris block. Total cumulative horizontal displacement on the Elsinore Fault derived from analysis of the length of strike-slip basins within the fault zone is about 5-12 km and is consistent with previously published estimates derived from other sources of information. This report also presents a map of density variations within pre-Cenozoic metamorphic and igneous basement rocks. Analysis of basement gravity patterns across the Elsinore Fault zone suggests 6-10 km of right-lateral displacement. A high-amplitude basement gravity high is present over the San Joaquin Hills and is most likely caused by Peninsular Ranges gabbro and/or Tertiary mafic intrusion. A major basement gravity gradient coincides with the San Jacinto Fault zone and marked magnetic, seismic-velocity, and isotopic gradients that reflect a discontinuity within the Peninsular Ranges batholith in the northeast corner of the quadrangle.
Download the pamphlet as a 25-page PDF file (36.6 MB).
Download the map as a ~50" x 36" PDF file (31.5 MB).
Download the data as an Excel woorkbook with all of the principal facts of the data used to generate the contours on the map (1.4 MB).
For more about gravity and magnetic data and methods, visit the Geophysical Unit Menlo Park website
For more about geologic mapping in Southern California, visit the SCAMP website
For more about earthquake studies in Southern California, visit the USGS Pasadena Field Office
For questions about the content of this report, contact Vicki Langenheim
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