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Open-File Report 94–16

Geologic Map of the Julian 7.5' Quadrangle, San Diego County, California

By Victoria R. Todd

Thumbnail of mapAbstract

The Julian 7.5' quadrangle lies within the Jurassic-Cretaceous Peninsular Ranges batholith of southern California and Baja California. Four granitic plutonic units and one gabbroic unit, most comprising a number of individual plutons, have been mapped in the Julian quadrangle and informal names have been assigned. The formal name Cuyamaca Gabbro has been retained. In addition to these plutonic units, metasedimentary and minor metavolcanic rocks occur in steeply dipping tabular bodies, or screens, within and between plutons.

Jurassic plutons in the Julian quadrangle underwent synkinematic metamorphism with the result that plutonic contacts and foliation are concordant with those in the surrounding metamorphosed country rocks. Foliation in Jurassic plutons consists of the planar orientation of recrystallized mineral grains and aggregates; deformation textures include augen gneiss, mylonitic gneiss, and mylonite. Structural studies indicate that a significant part of this deformation took place in the Cretaceous and, therefore, the regional foliation in this part of the batholith clearly postdates intrusion of many Cretaceous plutons.

First posted June 2, 2015

For additional information, contact:
GMEG staff, Geology, Minerals, Energy, & Geophysics Science Center—Tucson, Arizona
U.S. Geological Survey., c/o University of Arizona
ENRB Bldg, 520 N. Park Ave, Rm 355
Tucson, AZ 85719-5035
http://geomaps.wr.usgs.gov/gmeg/

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Suggested citation:

Todd, V.R., 2015, Geologic map of the Julian 7.5' quadrangle, San Diego County, California: U.S. Geological Survey Open-File Report 94–16, 18 p., 1 sheet, scale 1:24,000, https://dx.doi.org/10.3133/ofr9416

ISSN 0196–1497


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