The Temecula Trough

by Don Layton

Lithosphere (September 1995); Fallbrook Gem and Mineral Society, Inc.; Fallbrook, CA

[Ed. Note: This article won first place in the adult technical/advanced article competition in the California Federation of Mineralogical Societies in 1995.]

An excellent description of the formation of the Salton Trough appeared in the last Lithosphere ["The Salton Sea;" June 1995]. However, if you don't want to travel that far to feast your geological eyes, we have another example of a geological trough much closer to home.

The next time you travel north on Interstate-15 over Rainbow Pass, you can view a smaller example -- the Temecula Trough. Like the Salton Trough, this graben (German for "grave") is a down-dropped block bounded by faults and partially filled with sediments. Actually, it is a continuation of the Elsinore Trough which trends northwest to Corona (and beyond?).

The well-known Elsinore Fault forms the basin margin on the southwest and the steep fault scarp of the Santa Ana Mountains. Two miles southwest of Murrieta, and capping Burro Mesa at an elevation of 1700 feet, are the Santa Rosa Olivine Basalt flows (10-100 feet thick). These same lava flows also cap the Hogback at a similar elevation three miles northeast of Murrieta. This basalt flow has been dated as Late Pliocene-Early Pleistocene -- roughly two million years old.

The reason for mentioning this key marker bed, which has remained essentially horizontal, is that it has also been found in an oil well test hole. The Barnard #2 oil test (Wildomar) encountered 16 feet of Olivine Basalt at a depth of 2450 feet. Thus, we have evidence of at least 3300 feet of subsidence and the accumulation of at least 2400 feet of sediments in the Temecula graben in the last two million years.

You may say, "Big Deal!"; but one of geology's operating principles -- The Present is the Key to the Past -- can be valid in reverse -- The Past is the Key to the Future. The latest tectonic adjustment was the Temecula earthquake of December 4, 1948.

References

Mann, John F., Jr.;

Geology of a Portion of the Elsinore Fault Zone, California (Special Report 43); [California] Division of Mines; October 1955; 22 pages.

Norris, Robert M. and Webb, Robert, W.;

Geology of California; John Wiley & Sons; 1976; pp. 174-180.

Copyright © 1995 by Fallbrook Gem and Mineral Society, Inc.

The preceding article was originally published in the September 1995 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).

Permission to reproduce and distribute this material, in whole or in part, for non-commercial purposes, is hereby granted provided the sense or meaning of the material is not changed and the author's notice of copyright is retained.