Faults

greenvilleThere are numerous faults in Coe Park, including a variety of different kinds and ages. Probably the oldest structures are parallel to the rock layers and formed during subduction. The best example of this kind of fault (called a thrust) is seen in the canyon above Paradise Lake (east fork) where chert and basaltic greenstone of the Rooster Comb layer are on top of thin-bedded sandstone of the Burnt Hills terrane. The knife-sharp fault surface is perfectly exposed in the creek bottom. This is almost certainly the single most important geologic feature that is preserved in the park.

In addition to these ancient thrust faults, there are at least two zones of younger, north-trending faults that are steep and probably related to the presently active San Andreas fault system. Usually, these zones do not contain a single fracture but a series of en echelon faults, analogous to a herd of caterpillars all going in the same general direction. Each distinct fault may last for only a mile or two before it fades out and the fault movement is taken up along a new fracture. From near Mt. Diablo south to San Antonio Valley, the Greenville fault is a relatively narrow fault zone, but near the park boundary it splits into two branches. One of these, the Coyote Creek zone, follows that creek as far south as Los Cruzeros and possibly as far as Gilroy Hot Springs. The other segment, the Red Creek zone, follows Red Creek as far south as where it begins to bend to the east, and then continues in a southerly direction down the South Fork of Orestimba Creek at least as far south as the north end of the Kaiser-Aetna Road (near the junction with County Line Road).

Hydrothermal waters (hot springs) along these faults have welled up and leached out some of the minerals of the surrounding sandstones. As a consequence, these younger fault zones are characterized by bleaching and iron-staining.

Thus far, efforts to date these faults have not been very precise. We know that the Red Creek zone offsets some 10-million-year-old dikes (described in the next section), but it is not known if either zone is active today.

 

 

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