As featured in the December/January 2020 Pick & Shovel Gazette
By Tracy Repp
California’s complex and storied geologic history just keeps getting more and more interesting as time goes by. This past summer saw a small new chapter open up in that history and it’s a chapter that has only its opening line written.
Before diving into that, let’s take a very brief step back to talk about some of the major events that have shaped California’s complex geologic past.
Probably the first thing to know when trying to understand how the landscape in the Golden State came to be is that California is a place at the mercy of plate tectonic forces. The surface of the Earth is covered in a crust that is broken up into many pieces — some very large like the North American Plate and some very small like the Juan de Fuca Plate (Fig. 1). In very simple terms, these plates “float” around according to flow patterns within the molten and semi-molten rock below.
Fig. 1 - Tectonic plates of the Earth with red arrows indicating direction of plate movement with respect to adjacent plates. (Source: United States Geological Survey)
Interesting, and often violent, events occur when these plates crash into (convergent boundary), pull apart (divergent boundary) or slide past (transform boundary) one another. California is proof of that as the state comprises vastly different pieces of crust stuck together from past plate collisions (Fig. 2).
Fig. 2 - Geomorphic regions of California indicating vastly varied topography and physiography. (Source: Ssalonen, California Geomorphic Provinces)
Some of the bigger collisions led to major episodes of volcanism and folding and faulting as the existing rocks bent and broke under the strain. This activity formed many of the mountainous areas characteristic of California today. But the state also has areas where plates have and are pulling apart. This causes the crust to stretch and thin and sometimes break. When it breaks, pieces or blocks can drop down in elevation with respect to adjacent blocks. These dropped blocks are called grabens and a large one is responsible for the formation of Death Valley.
Because of this violent past, California is riddled throughout with faults. Major fault zones include the San Andreas, which is the largest in California and marks the boundary between the North American and Pacific tectonic plates, and the Garlock, Elsinore and San Jacinto faults, to name a few.
This leads us back to the “new chapter” that I mentioned. It turns out not all fault zones in the state are clearly understood and this was made evident on July 4, 2019, after a 6.4-magnitude earthquake struck near the city of Ridgecrest in the northeast corner of Kern County in Southern California. This was followed some 34 hours later by a 7.1-magnitude earthquake, along with more than 100,000 aftershocks. The earthquakes occurred along a lesser-known fault system previously thought dormant that runs almost perpendicular to the east-west trending Garlock Fault, which is just a few miles to the south (Fig. 3). Most of the earthquakes and aftershocks occurred within the China Lake Naval Air Weapons Base, causing an estimated $2.2 billion in damage.
Fig. 3 - July 4-5, 2019, Ridgecrest earthquake sequence. Red stars represent earthquakes of magnitude M≥6.0, gray dots represent aftershocks, brown lines are the faults and the pink lines are highways. (Modified from: USGS)
Questions emerged following the earthquakes including whether these earthquakes could trigger events on nearby faults such as the Garlock and connecting San Andreas, which are overdue for significant movement as strain in the area continues to build. According to the U.S. Geological Survey, the chances of triggering faults along these larger faults are low, since only a handful of small aftershocks were recorded on the Garlock following the Ridgecrest earthquakes.
The Rand gold mining district is located near where the line of Ridgecrest earthquakes intersects with the Garlock Fault (Fig. 3). According to mindat.org, lode and placer mining has occurred in the Rand district since the 1860s with intermittent development after the 1940s when gold production here peaked. Most of the lode deposits are known to be concentrated along faults in the schist that is widespread in the area. Now the question begs, could the July earthquakes near Ridgecrest have resulted in the formation of new vein deposits of gold in the area?
While there’s no way of knowing for sure whether the Ridgecrest earthquakes formed new deposits of gold, based on historical mine locations and their close relationship to the known faults in this area, there is certainly a possibility. For gold prospectors and miners searching for that ever-elusive dream, that is a very exciting prospect indeed!
Tracy Repp is a professional geologist of 20 years and a part-time writer living from Michigan