San Andreas Fault Displacement of a Distinctive Granite

The San Andreas Fault is one of the most famous geologic features of the world. It is common knowledge that the sides of the fault, relative to each other, are moving. This post concerns a distinctive granite that has been displaced approximately 160 km (100 mi.) by this fault.

Map showing locations (in red) of the distinctive granite that has been offset by the San Andreas Fault in Southern California. The arrows show the relative sense of offset along the sides of the fault. Artwork is by R. Squires.

The rock that has been displaced is, indeed, a type of the igneous rock granite. To be more precise, however, it is a megaporphyritic monzogranite. "Megaporphyritic" means that the crystals are of vastly different sizes. A monzogranite is the commonest type of granite with roughly equivalent amounts of orthoclase and plagioclase, along with abundant biotite.

Megaporphyritic monzogranite hand specimen (20 cm length) from Mill Creek Canyon. The large pink crystals are orthoclase, the white ones are plagioclase, and the black ones are biotite. 

The orthoclase crystal just right of the center of the above picture show twinning. Crystal twinning occurs when two separate crystals of the same substance share some of the same crystal lattice. Instead of a normal single crystal, the crystalline structure appears doubled. There are several minerals that commonly have twinned crystals, and orthoclase is one of them.

An exceptionally large crystal (4.5 cm length) of orthoclase from the Mill Creek Canyon locale.

The rocks, which are found in the Liebre Mountain area in northern Los Angeles County and also in the Mill Creek Canyon area of San Bernardino County, are believed to have been part of the same intrusive magma body that crystallized during the Triassic, about 215 million years ago. The two rock masses, which were both derived from melted continental crust of Precambrian age, share identical chemical compositions and geologic age. They both also suffered a Late Cretaceous thermal event about 70–75 million years ago.

Geologists refer to localities that show precise offsets along faults as "piercing points." Liebre Mountain is 100 km northwest of the Mill Creek area. This distance of offset is less than the generally accepted distance of about 240 km of offset along the San Andreas Fault.

Several years ago, I noticed a boulder-size piece of granite in the office of one of my colleagues, Dave Liggett. Over the years, I kept admiring the rock, and, one day, he kindly gave me some nice representative specimens. These are the same specimens illustrated here, and they are from the Mill Creek Canyon locale. He also provided the background story of this rock. 

Epidote (the coating form)

Ever since I started collecting minerals, I noticed green splashes of a material coating various pieces of granite and other various rocks. I learned later that the coating consists of the common mineral epidote (pronounced “ep-i-dote”).

Epidote is a calcium, aluminum, iron, hydroxyl-silicate mineral typically found in metamorphic-rock areas where alteration or replacement took place in association with hydrothermal fluids. Epidote is especially common in fractures or joints. Fibrous crystals of epidote can be dark-green, black, or even yellow.

The epidote I find, however, has a very distinctive pistachio or pea-green color. It occurs primarily as surface coatings on cobbles and boulders of biotite-rich granite, which weather out from sedimentary rock conglomerates, as shown below. The name “epidote” is derived from a Greek word meaning “increase,” in reference to its crystalline shape.

Epidote coating a clast (maximum dimension 5 cm) of granodiorite
found on a hiking trail in the Santa Clarita area, Los Angeles
County, Southern California.

Preferred orientation of Turritella

Fossil shells belonging to the shallow-marine gastropod Turritella are prone to have been preferentially aligned by waves and currents because their shells are long an straight. I used the first slide below in one of my earlier posts (July 24, 2014) on the subject of "Taphonomy of Mollusks Shells." Taphonomy is the study of post-mortem processes (waves, currents, bored by other organisms, etc.) that affect shells.

Eocene Turritella andersoni lawsoni shells in the Llajas Formation
of Simi Valley, Ventura County, Southern California. The
longest shell is 6 cm long. These shells occur in situ, in
a bed of silty fine-grained sandstone.

More Eocene T. andersoni lawsoni shells from the Llajas Formation
of Simi Valley. The longest shell is 6 cm long. This slab is a piece of
loose rock ("float") from the formation.