University of California Press Geology of the Pinnacles National Monument

FAULTS

Three large faults occur within the area mapped, besides a number of smaller faults and numerous fractures. It will be remembered that the San Andreas rift passes approximately 4 miles from the mouth of Bear Valley and trends in a direction of north 40 degrees west. The major faults of the Pinnacles area are roughly parallel to this master fracture.

Miners Gulch fault.—This fault was determined only by physiographic evidence but there seems to be no doubt of its existence. Numerous large pegmatite dikes trending south 30 degrees east with a minor system at right angles occur southwest of the fault but are nearly absent from the other side of the fault. Miners Gulch is very straight, and deeply cut. From the lower borders of Chalone Peaks, the superior elevation of the area southwest of Miners Gulch is very apparent, especially since the higher hills are toward the Salinas Valley and should normally be lower. The rock on both sides is essentially granitic and thus should not permit such a marked difference in erosion. Finally, the change in gradient at the point where this fault intersects Stonewall Canyon is very noticeable. Above the fault, the valley is being sanded up, and below the fault the gradient is very steep. The southwest side of the fault has definitely been elevated by several hundred feet. A sharp bend in Stonewall Canyon at the fault intersection may indicate a horizontal displacement of one-third mile in a direction opposite to that of the San Andreas rift, but evidence for this latter hypothesis is less convincing.

Pinnacles fault.—The important displacement along the west edge of the volcanic ridge has been designated as the Pinnacles fault. It may be easily traced throughout most of its course and trends a few degrees east of due south.

This fault is normal and dips 60 degrees easterly at an excellent exposure 1 mile southwest of Hawkins' Peak. Where exposures are good, it is manifested by the sharp abutment of volcanic breccia against quartz diorite. The vegetation is a little more dense on the side of the granitic rocks and there is generally a steepening of the breccia surface on account of sapping close to the fault. The fault is still active, as is proved by open rock fissures southwest of Chalone Peaks in section 27, as well as by the sharpness of definition along its entire course.

The east side is the downthrow side and sinking may have been, in part, a result of its being loaded with volcanic material. This material was derived, in all probability, from beneath the block which settled. The window of granite south of Chalone Peaks, and the occurrence of rhyolite on the east side of the fault, almost to the bottom of Chalone Creek where the two rock types intersect north of the Pinnacles (one-half mile north of the map boundary) indicates a vertical displacement of from 600 to 800 feet. Movement is considered to be mostly vertical, as no horizontal displacement along the strike is evident.

Chalone Creek fault.—The major fault which follows Chalone Creek for several miles in a southeast-northwest direction has been designated as the Chalone Creek fault. Surface manifestations of this fault are clear and striking. Pink and gray rhyolites are brought into contact with land-laid Temblor deposits to the north of the mouth of Bear Valley, and to the south, granite is brought into contact with arkosic gravels and continental breccias of Temblor age.

The rhyolite along this fault contact is much brecciated but may be partly recemented with silica with or without iron oxide. This fracture accounts for the straightness of Chalone Creek for several miles above the mouth of Bear Valley, and causes water carried in the sands of the combined Bear and Chalone creek channels to appear suddenly in the form of springs, where the fault finally crosses and leaves Chalone Valley. These springs, brought up by the fault below the mouth of Bear Valley, flow throughout the year and are important to ranchers in this vicinity.

Miocene beds northeast of this fault are but slightly tilted and very little folded, suggesting but little lateral compression. It is probable that the fault is normal and the downthrow side is to the northeast. The displacement was not accurately determined, but it must be several hundred feet since the relation between formations was not altered in passing from hills to valleys which show a total relief of this amount. At the extreme east edge of the map, the Chalone Creek fault is intersected at right angles by the Horse Valley fault and becomes less important to the southeast.

At the mouth of Bear Valley, the Chalone Creek fault branches and a minor fracture may be traced due south for a distance of 3-1/2 miles. It brings the granite and rhyolite in contact and has occasioned several large landslides. The west side has been depressed by an amount estimated not to exceed 200 or 300 feet. The fault is lost at the south in a number of cross-faults which extend into the mass of volcanics.

JOINTS

Numerous joints occur within the volcanic rocks and these have had considerable effect upon the weathering. Displacement is often nil, but many of these fractures may be traced continuously for a mile or more. Much of the rhyolite is fractured and slickensided along the east border but these slickensided areas are results of crushing and settling strains rather than of faulting of any considerable displacement.