TubB Canyon Desert Conservancy
Geology of the Tubb Canyon 7.5' Quadrangle, Anza-Borrego Desert, Eastern San Diego County, California
Excerpt below published in January 2009, based on a report by David L. Wagner from data compiled 1996-1997.
The Tubb Canyon Quadrangle lies (mostly) within the Anza Borrego Desert State Park. The geologic mapping of Tubb Canyon was part of the Department of Parks and Recreation General Plan Inventory of Natural Features of the Anza Borrego Desert. Cretaceous granitic rocks of the Peninsular Ranges Batholith and prebatholithic metamorphic rocks of uncertain age underlie most of the quadrangle. These rocks are exposed primarily in the upland areas.
The lowlands, Borrego Valley, Yaqui Flat, Yaqui Meadows, and Grapevine Canyon, are underlain by mostly granitic detritus including silt, sand, and gravel eroded from the surrounding uplands. Major structural features of the quadrangle are the Borrego Shear Zone and the San Felipe Fault Zone. The Borrego Shear Zone is believed to represent deformation that occurred during emplacement of granitic rocks of the Peninsular Ranges Batholith in Cretaceous time some 90 to 100 million years ago. Some deformation may have also occurred during the opening of the Salton Trough in Miocene time, 10 to 20 million years ago.
The San Felipe Fault Zone traverses the southern part of the quadrangle in Yaqui Flat. It is a complex zone of many faults and appears to have had both right lateral and vertical displacement. Although the San Felipe Fault Zone may have been active as recently as 500,000 years ago, it does not appear to have a significant potential for generating an earthquake.
Landslides are conspicuous geologic features in the area covered by the Tubb Canyon Quadrangle. Several different types of landslides have been occurring over the past several million years. There are huge ancient landslides mapped on the steep slopes of Pinyon Ridge. Planar zones of weakness, formed during the deformation that formed the Borrego Shear Zone, are parallel to slopes above Borrego Valley.