The second event recorded in the rocks of the Puente Hills began some 23 million years ago. The trench system, which had for so long dominated the formation of Southern California, ended around 28 Ma, when the Pacific Plate transform convergence replaced the Farallon Plate trench system. As the “trailing edge of the Farallon Plate entered the subduction zone,”[ref]Baldridge, Geology of the American Southwest, 215.[/ref] the Pacific Plate was brought into contact with the North American Plate. The Pacific Plate’s collision, in contrast to the Farallon Plate, which had collided with the North American plate almost perpendicularly, was “nearly parallel”[ref]Baldridge, Geology of the American Southwest, 215.[/ref] to the North American Plate. This new transform fault—the “notorious San Andreas Fault of California”[ref]Baldridge, Geology of the American Southwest, 216.[/ref]—would live up to its name; it would transform the greater Los Angeles region and give rise to the greater Puente Hills. This story is recorded in the thick marine rocks that underlie the Hills and in the volcanic rocks that form the boundary between the old plate subduction system and the new plate transform system. When the Pacific Plate slid in behind the remnants of the Farallon Plate, it made contact with the North American Plate, broke off slivers and blocks of continental crust, and began dragging them away toward the northwest. Thus, as the Pacific Plate moved almost parallel to the North American plate boundary, it picked up and began to drag Southern California and the greater Los Angeles region to the northwest.[ref]McPhee, Basin and Range, 180. See Baldridge, Geology of the American Southwest, and T. L. Wright, “Structural Geology and Tectonic Evolution of the Los Angeles Basin,” K.T. Biddle, ed. Active Margin Basins. American Association of Petroleum Geologist Memoir 52. (Tulsa: The American Association of Petroleum Geologists, 1991), 35-134.[/ref]
The first result of this northwesterly transfer and subsequent dragging of chunks of crust caused the Western Transverse Range[ref]“Trending essentially east-west across the regional grain of Southern California is the Transverse Range province, which comprises elongate mountain ranges and valleys, chains of hills, an broad basins that are geologically very complex. The province as a whole resembles the adjoining Coast Range and Peninsular Range regions in several aspects, but is distinguished from them by prevailing east-west structural trends.” Jahns, “Investigations and problems of southern California Geology,” 17). The Transverse Ranges are separated into eastern and western segments. The eastern Transverse Ranges are, from east to west: the Eagle, Pinto, Little San Bernardino, and San Bernardino. The western Transverse Ranges are, from east to west: the San Gabriel, Santa Monica, Santa Susana, Topatopa, Pine, and Santa Ynez Mountains. Only the western Transverse Range blocks were rotated during the switch from a subduction to transform margin. The Peninsular Ranges are, from south to north: Laguna, Vallecito, Santa Rosa, Agua Tibia, San Jacinto, and Santa Ana Mountains. In addition, the Peninsular Range extends, through the Southern California Batholith formation, into the Baja Peninsula of Mexico and is geologically part of the overall Peninsular Range block system. (Richard H. Jahns, “Investigations and problems of southern California Geology,” Section 1 in Chapter I, Geology of Southern California, Richard H. Jahns, ed., California Division of Mines Bulletin 170. (San Francisco: Department of Natural Resources, 1954), 11.[/ref] to break away from the Peninsular Range. At one time the Western Transverse Range had lain parallel to the Peninsular Range with its southern rim, the Channel Islands, lying near San Diego. When the blocks and chunks of crust forming the bedrock of Southern California were transferred from the North American Plate, the Pacific Plate picked up the Transverse Range blocks. In effect, the Pacific Plate, scraping against the North American Plate, violently wrenched the Western Transverse Range from its position next to the Peninsular Range along the continent, rotating it clockwise 110 degrees to its present east to west configuration.[ref]The rotational axis was the San Gabriel to Chino Hills to Cristianitos faults east of the Puente basin. This fault system—though each individual member would remain active between 18 to 0 Ma—was replaced by a series of other faults. Foremost among those beginning 6 Ma was the San Andreas Fault system. When the San Andreas became active, the Pacific Plate continued to drag the Peninsular Range block northward opening the Gulf of California as Baja California was detached from its initial position along mainland Mexico and added to the Pacific Plate. For more information on the rotation of the Transverse Range blocks. See Yeats, “Tectonics of the San Gabriel basin and Surroundings, Southern California,” 1158 – 1182, Baldridge, Geology of the American Southwest, Wright, “Structural Geology and Tectonic Evolution of the Los Angeles Basin,” and Raymond Ingersoll and Peter E. Rumelhart, “Three-stage Evolution of the Los Angeles Basin, Southern California,” Geology. 27(7) July 1999.[/ref] This block rotation stretched, thereby thinning, the remaining crust, drawing lava and the lower basement rock up to fill the gap. This process of rotating is known through the volcanic rocks—the Glendora and El Modeno volcanic rocks—which spewed out of the cracks that had opened in the bedrock of the newly-opened marine basin. These formations were directly laid over the bedrock formations; their story is one of rifting, rafting, and rotation, which would ultimately result in the opening of the Los Angeles Basin and the formation of the thick marine sediments that constitute the roof rock of the Puente Hills.[ref]Peter W. Weigand, et. al., “The Conejo Volcanics and other Miocene Volcanic Suites in Southwestern California,” in Contributions to Crustal Evolution of the Southwestern United States, Andrew Barth ed., Special Paper 365, (Boulder Colorado: The Geological Society of America, 2002), 197. See Ingersoll and Rumelhart, “Three-stage Evolution of the Los Angeles Basin, Southern California,” 595. See also, Baldridge, Geology of the American Southwest, and Andrew J. Meigs and Michael E. Oskin, “Convergence, Block Rotation, and Structural Interference Across the Peninsular-Transverse Range Boundary, Eastern Santa Monica Mountains, California,” in Contributions to Crustal Evolution of the Southwestern United States, Andrew Barth ed., Special Paper 365, (Boulder Colorado: The Geological Society of America, 2002) 279 – 295.[/ref]
Excerpted from Richard H. Ross. “From Rock, Wind, and Water: A Natural History of the Puente Hills.” Claremont Graduate University, 2006.