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June 2017 Meeting of the Chapter

***Wednesday, June 21, 2017 ***

Download the Chapter's June Newsletter


Speaker: Jonathan C. Matti
Geologist, U.S. Geological Survey

Date: Wednesday, June 21, 2017
Social hour: 5:30 - 6:30
Dinner: 6:30 - 7:15
Announcements: 7:15 - 7:30
Talk and Q&A: 7:30 - 8:30

Location: Pinnacle Peak Steakhouse
2533 La Cadena Drive South
Colton, CA

Meal: You have a choice of a steak, salmon, ribs, chicken, or vegetarian meal.

Cost $25 with RSVP by 5pm Tuesday, June 20, 2017,
$10 per student with RSVP and proof of valid student ID,
$30 for walk-ins without RSVP


The San Gorgonio Pass region is well known as a structural knot in the San Andreas Fault zone that evolved in Quaternary time. Accordingly, the 'knot' has been widely investigated for its role in influencing regional fault patterns and regional strain budgets.

However, San Gorgonio Pass is not so well known as a region where several key elements of the San Andreas Fault system potentially come together in a way that informs the late Cenozoic structural and landscape evolution of southern California. Important elements include: (1) the SAF zone itself, a continental transform that evolved at ~6-5 Ma; (2) the coeval West Salton Detachment and its supradetachment sedimentary fill; (3) the Salton Trough that developed as the Gulf of California Transtensional Province propagated northwestward (landward) beginning at ~6.3 Ma; (4) the poorly understood escarpment of the Little San Bernardino Mountains - the NE margin of the evolving Salton Trough; (5) the enigmatic Banning Fault, an older element of the San Andreas Fault system (sensu lato) that preceded the advent of the San Andreas Fault (sensu strictu); (6) the left lateral Pinto Mountain Fault zone; (7) clockwise-rotating panels of the Eastern Transverse Ranges. Tectonic events within these structural systems yielded distinctive landscapes, all of which need to be integrated in order to understand the late Cenozoic geologic history of southern California over the last 10 million years. San Gorgonio Pass is a region where all these structural systems and their landscape elements seem to converge.

Questions explored in this talk include: (1) Were Pliocene and Miocene sedimentary rocks of the San Gorgonio Pass region deposited on the hangingwall of the West Salton Detachment? (2) How did the WSD interact geometrically and kinematically with the San Andreas Fault? (3) Did this interaction guide the structural evolution of the northwest head of the Salton Trough through time and space? (4) Has San Gorgonio Pass always marked the northwest head of the Salton Trough? (5) How did the coextensive and coeval evolution of the San Andreas Fault and the West Salton Detachment lead to the Little San Bernardino Mountains escarpment? (6) Can we reconstruct a late Cenozoic paleogeographic setting that accommodates the San Jacinto Mountains block, the Santa Rosa Mountains block, the Salton Trough, and the Little San Bernardino Mountains?

San Gorgonio Pass - where it all comes together- may provide answers to these questions and many others.
Speaker Biography:
A Geologist with the U.S. Geological Survey since 1975, Jonathan C. Matti received his B.S. and M.S. degrees in Geology from the University of California, Riverside in 1969 and 1971, and received his Ph.D. in Geology from Stanford University in 1978.

A general geologist, emphasizing regional geologic framework relating to geologic hazards and earth resources. His early research interest was Middle Paleozoic marine carbonate rocks in the Great Basin. Which was short lived, as his early USGS work took him to southern California. There he cut his teeth on plutonic and metamorphic rocks, and was introduced to faults of the San Andreas system in the San Bernardino and San Jacinto Mountains. He also was introduced to surficial earth materials ('soils' and landslides), which play a critical role in deciphering active and recently active tectonic systems.

In his career he has participated in a variety of applied- and basic-research investigations, including quadrangle-mapping in Las Vegas Valley (beneath all the casinos and golf courses), mineral-resource assessments of public lands in southern California, a regional evaluation of seismically induced liquefaction in the San Bernardino Basin, slip-rate analyses of strike-slip and thrust-fault zones, geologic studies in support of ground-water investigations in fault-bound basins, geologic mapping on the Twentynine Palms Marine Corps base, a modern geologic map of Joshua Tree National Park (coauthor), and the geologic framework and history of the San Andreas Fault system - both long term and short term (neotectonic framework).