Hi All. Petra doesn’t know it yet, but she is going to be taking another field trip. She only has herself to blame. She keeps me busy by having me search for material for current and future use. Occasionally I find something not related to what she has me searching for that I feel might be useful to someone else.

It appears that the San Andreas may be something, or may have been something other then a right lateral strike slip fault in the area of Point Arena. This comes from the finding of what appears to be rocks of the Franciscan Complex west of the fault in the Gualala Block.

If they are indeed Franciscan rocks, then their incorporation into material west of the SAF requires a more complex model of fault evolution than that of a simple right-lateral slip.

The nature of deformation along the SAF is the existence of compressional structures and uplifted Quaternary marine terraces at several locations along the northern segment of the fault. One of these locations is the Gualala block, which is flanked along its western perimeter by a flight of wave-cut marine platforms that vary in altitude, indicating both uplift and differential tilting since the time of formation.

At Point Arena, extensive outcrops of thrust faults indicate that compression is occurring, even though the orientation of the SAF at this location, which makes a slight clockwise bend, would suggest that extension might occur, not compression. This same kind of turn can be seen at Black Mountain just south of San Francisco.

Another significant problem is related to the timing and rate of slip along the right-lateral fault. Only one historic event has occurred on the northern segment of the SAF. The rupture trace of this event, the 1906 San Francisco earthquake, was about 435 km (271.875 miles), from San Juan Bautista to Point Delgada (Prentice et al., 1999). (Recent determination that rupture did indeed extend all the way to Point Delgada was the result of a 1995 Keck project.

If the fault in this area is indeed compressional then there is something much deeper that has not been seen. Take Care…Don in creepy town.

• Crustal Deformation