fault breccia - spectacular display specimen from the Lenwood Fault Zone
A breccia is formed of rock fragments, normally broken by faulting, that have been cemented together. In this case, the rock fragments have been weathered into relief, making these examples fairly spectacular.
Exposed along the Lenwood Fault in San Bernardino County, California, this breccia is composed of angular rock fragments of primarily granitic rock up to one meter in diameter, interpreted as being the product of large rock avalanches that originated on nearby fault escarpments. These breccias are common in Tertiary extensional basins throughout the Basin and Range Province.
The name breccia comes from the Italian for broken. The Lenwood Fault is about 75 km long. It experienced triggered slip due to the Landers earthquake in 1992 and has a slip rate of 0.8 mm/year. An impressive scarp just southwest of Daggett Ridge exposes many large blocks of this breccia.
An excellent display specimen. The pencil is 5 1/2" long, for scale.
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