Geology of the pre-Cretaceous rocks in a portion of the Santa Ana Mountains

The Santa Ana Mountains, a northern extension of the Penninsular Range, lie between the Elsinore Trough on the East and the coastal plain on the West. The area concerned in this investigation is in a northern part of the range in Orange County, California. Both igneous and metasedimentary rocks are present. Metasediments include the Santa Ana formation, which has been subdivided into two members, and the Hough formation. The oldest rocks have been dated as Triassic. Unconformably overlying the Santa Ana is the Hough formation which is believed to be Jurassic in age. Igneous rocks of pre-Upper Cretaceous age are intruded into the meta-sediments and are largely responsible for their metamorphism. Overlapping the metasediments from the west are Cretaceous rocks, the basal unit of which is the Trabuco formation. Structurally the Santa Ana Mountains resemble the Sierra Nevada, but on a smaller scale. They are essentially a tilted fault block. The older sediments were contorted into broad open folds late in Triassic time, but subsequent to this there has been little folding of consequence. Uplift of the east face of the mountains along the Elsinore fault occurred early in the Tertiary and has continued spasmodically up to the present. Later in the Tertiary there was normal faulting within the area which displaced Cretaceous and Triassic rocks