The San Gabriel fault zone, which trends northwesterly subparallel with the San Andreas fault for about 90 miles, appears to have a post-late Miocene right-lateral displacement of 15-25 miles. Southwest of this nearly vertical fault, 6 miles northwest of Castaic, Los Angeles County, upper Miocene coarse conglomerates grade southwest into finer sediments from their source, now disappeared, across the fault on the northeast. The conglomerates, which contain boulders of anorthosite and norite, were derived from a near-by basement terrane containing these rock types. A buried source is eliminated because at present, so far as known, all basement rock across the San Gabriel fault from the conglomerates for several miles northeast is covered by sedimentary rocks older than the onglomerates. Basement types exposed still farther northeast do not include anorthosite and norite. In fact, the only in situ occurrence of these rock types known in the entire region is in the San Gabriel Mountains 23 miles southeast of the conglomerates and on the other (northeast) side of the San Gabriel fault zone. It thus seems clear that this was their source area and that the conglomerates have been displaced 15-25 miles relatively northwest.

Large right-lateral displacement is also suggested by the presence of upper Miocene sedimentary breccia now found northeast of the fault about a mile northwest of Castaic. This breccia, containing blocks of gneiss almost exclusively, has been offset from its source now at least 6 miles but probably 15 miles or more northwest. Again, sediments older than the breccia across the fault from its present exposure preclude noteworthy dip-slip displacement.