Serving professionals in engineering, environmental,
and groundwater geology since 1957

MONTHLY DINNER MEETING
(joint meeting with SCGS)

DATE: Tuesday, July 15, 2003
LOCATION: Wyndham Garden Hotel, 3350 Ave. of the Arts, Costa Mesa
TIME: 6:00 pm--Social Hour, 7:00 pm—Dinner, 8:00 pm--Talk
RESERVATIONS: Call Leighton & Associates (949) 250-1421 ex. 570 by noon, Friday,
July 11. PLEASE INDICATE YOUR CHOICE OF SALMON or
LONDON BROIL when you call in.
COST: $22 per person with reservations, $27 at the door, $12 for students.

SPEAKER: Eldon Gath
TITLE: Uplift of the Puente Hills and Santa Monica Mountains

ABSTRACT

The Santa Ana Mountains (SAM) are a 1.7 km high mountain range that have not yet been provided with an uplift mechanism. The uplift issue is not just geological curiosity, but it could be of extreme importance to the safety of the citizens of Orange and Riverside counties. Recent geomorphic mapping and analysis are providing some constraints on patterns and rates of uplift, and attempting to shed light on the uplift mechanism and seismic hazard implications. This work is using a new method of quantitative analysis from the Puente Hills based on relationships between drainage basin area and age, then applied to the Santiago Creek drainage basin. In the Puente Hills, the basin age was calculated by measuring the right-lateral strike-slip displacement of each basin’s primary stream, and regressing it against the basin’s area.! Because the Whittier fault’s slip rate is known (2.5 mm/yr), the age of the channel can be calculated by retro-deforming it.! So, if you calculate the basin area, you can calculate the age of the initiation of the stream forming the drainage basin. The PH also have three fill terraces and three higher erosional surfaces. From this analysis, the Puente Hills have been rising 0.4 mm/yr since their emergence ~1 Ma.! The SAM have three well developed erosional surfaces preserved on them, as well as a suite of four fluvial fill terraces preserved in Santiago Creek, a drainage trapped between the uplifting SAM and a parallel Loma Ridge. By correlation of the terraces with the marine eustatic sea level curve, (similar to the San Joaquin Hills) we were able to estimate a 0.3 mm/yr uplift rate for the SAM and an emergence age of ~3.6 Ma.! Santiago Creek formed ~2.4 Ma in conjunction with the initiation of the Loma Ridge structure, a hanging wall structure that formed in response to compressional buckling of sedimentary strata on the flanks of the uplifting Santa Ana block. Hanging wall block faulting appears to have deflected Santiago Creek northerly ~1,200 m along five discrete block margin faults. The termination of the Elsinore fault into the Chino and Whittier faults leaves at least 1-2 mm/yr of north-vergent strain unaccounted for. We speculate that this missing strain is being transferred into uplift of the SAM, with complex interaction among other northvergent structures in the southern California area. Come, hear, argue.
Biography

Eldon Gath is the President of Earth Consultants International (ECI), a geological consulting firm formedin 1997. Eldon has nearly 25 years of professional consulting experience with southern California firms Pacific Soils Engineering and Leighton and Associates before forming ECI.! He received his BS in Geology from the University of Minnesota in 1978, and has been in and out of all southern California graduate schools ever since.! Eldon is a past president of the South Coast Geological Society, a past national president of theAssociation of Engineering Geologists, and is currently serving on the National Research Council committee to develop a research agenda for the National Earthquake Engineering Simulation program.! This talk is based on research for his PhD at UCI, led by Dr. Lisa Grant of the Dept. of Environmental Analysis and Design, and admirably assisted by Eric Runnerstrom.