Geological Mapping in the San Gabriel Mountains

Funding from the USGS and California Geological Survey continues for geological mapping in the crystalline basement of the San Gabriel Mountains.  Numerous Cal Poly Pomona undergraduate students  have contributed to the mapping efforts since 1990.  The objectives are to use multiple piercing lines in the basement to reconstruct the movement histories of several major late Cenozoic faults within the range (for example, check out the ambitious senior thesis of Brent Norum, B.Sc., 1997).  In general, Late Miocene structures such as the San Gabriel Fault, Punchbowl Fault, and the Sawpit-Clamshell Fault are displaced by left-lateral faults (Stoddard Canyon, San Antonio Canyon, and San Dimas Canyon faults), which in turn are overprinted by dextral faults of the modern  San  Andreas and San Jacinto systems.  Kinematic relationships between these faults and major structures in the Los Angeles basin are proving to be intriguing (please see PDF of my recent GSA Special Paper; also peruse the maps below).  Analyses of deformed alluvial terraces and landslides provide additional constraints on possible Holocene fault activity. 

**View a Photo Gallery of San Gabriel Mountains projects showing outcrops and student assistants in action

**View senior theses completed on geology and hydrology of the San Gabriel Mountains and vicinity

Geologic Mapping in Progress, Showing Bedrock Geology and Major Faults

in the San Gabriel Mountains, and Hypothetical Palinspastic Reconstructions: 

 

Bodies of Pelona-Orocopia-Chocolate Mountains Schist  (shown above in gray) are

displaced on strands of the San Andreas-San Gabriel Fault system

 

 

View the senior thesis of Shaun Wilkins (2004) to learn more about geology of Middle Fork

and North Fork Lytle Creek canyons, in the eastern part of the above map

 

View the senior thesis of Jeff DeLand (2002) to learn more about geology of the Mendenhall Gneiss

north of the San Gabriel fault, in the west-central part of the above map

 

The maps below show a proposed sequential palinspastic reconstruction of the

eastern San Gabriel Mountains block.  See Nourse (2002) for more details and sharper graphics:

 

 

 

 

 

The map above shows  a proposed Middle Miocene configuration of the San Gabriel Mountains, Los Angeles basin, and Ventura basin upon restoration of strike- slip displacements and block rotations. See Nourse (2002) for details.