Crustal structure of the Salton Trough and southern San Andreas Fault based on gravity and magnetic investigations
Abstract
Gravity and magnetic investigations reveal regional and local features of the subsurface structure of the Salton Trough and southern San Andreas Fault. Regional gravity features include a prominent, narrow gravity low in Coachella Valley that decreases in amplitude to the southeast near the north shore of the Salton Sea. At the south end of the Salton Sea, the gravity low has become a large gravity high that has been attributed to crustal thinning and mafic lower crust. The northeast margin of the gravity low is linear and coincides with both the San Andreas Fault and a distinctive magnetic high that is probably related to crystalline basement rocks of the Little San Bernardino Mountains. The southwest margin of the gravity low is also marked by a gravity gradient, but is not linear in map-view and is generally less steep. The gradient mimics the irregular contact between pre-Cenozoic basement rocks and Cenozoic basin fill. Gravity data were collected to augment existing data along transects of the Salton Seismic Imaging Project. Measurement spacing averaged about 800-m, except along the high-resolution seismic line near Salt Creek, where a detailed grid of gravity measurements spaced about 400-m is supplemented by ground magnetic transects. The detailed data reveal a bench in the gravity gradient that is coincident with a narrow (500-m) 100-nT magnetic high that may reflect multiple strands of the San Andreas Fault or interactions with the Hidden Springs fault. These anomalies, however, only extend along strike for about 2-km before diminishing. Along the stretch of the San Andreas Fault between Indio and Salt Creek, the San Andreas Fault is located near the base of the southwest-facing steep gravity gradient, indicating a steep northeast dip of the basement contact, with higher-density rocks northeast of the fault. North of Indio, the gradient becomes more diffuse, suggestive of a gentler northeast dip. About 5-km south of Salt Creek, the fault changes its position relative to the gravity field and is located at the top of a northeast-facing gravity gradient. This is also suggestive of a northeast dip of the basement contact, placing high-density material on the southwest side against low-density rocks on the northeast side of the fault.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP41A0976A
- Keywords:
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- 1219 GEODESY AND GRAVITY / Gravity anomalies and Earth structure;
- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation