Geologic structure near the Cajon Pass Scientific Drill Hole



The geologic structure near the Cajon Pass drill site is controversial. The Squaw Peak fault was previously interpreted as the edge of a thrust plate of large displacement. This may be incorrect based on the style of deformation and the presence of similar granodiorite on both sides of the fault. Alternatively, mid-Miocene crustal extension on a west-facing listric normal fault may have formed the basin in which the Cajon Formation was deposited. Post Miocene crustal shortening normal to the San Andreas fault may have folded the upper plate of this fault and thrust it northeastward. By this interpretation, steeply inclined beds at the drill site terminate on a fault ramp, and Squaw Peak is the corner of the initially stable block that was broken off and ramped upward.