Folding and Faulting in the Calico Mountains
June 11, 2013
Photographer: Dave Lynch
Summary Author: Dave Lynch
Rocks are hard and brittle, right? No necessarily. With enough time and pressure, most rocks behave like taffy, twisting and bending in graceful arcs. Such deformations are most evident in sedimentary rocks, where the once-horizontal layers deposited in lakes are subsequently lithified and then distorted. Upward bending produces anticlines, while downward bending makes synclines. If the rocks are stressed to the breaking point they fracture and shift creating faults.
The Eastern California Shear Zone (ECSZ) is one of the most fascinating geological areas in the world. This area has experienced folding, faulting, igneous intrusions, metamorphism, lake deposits, limestone deposition, earthquakes and landslides. The region’s tortuous history and current tectonics may help indicate the future: the boundary between the North American and Pacific Plates – currently the San Andreas Fault – may shift north in a few million years and run from the Salton Trough through Death Valley up to Lassen Peak along a path known as Walker Lane.
The Calico Mountains in the ECSZ have some terrific geological exposures. Shown here are two anticlines in sandstone, siltstone and limestone. The first shows a sharp bend by over 90 degrees, and the second shows a different part of the same fold containing a small fault that occurred after the anticline formed. The offset in the colorful bedding is easily seen where the layers are displaced along the fault. Photos taken April 21, 2013, during a field trip as part of the Desert Symposium.