Baja California, Domain 1

Abstract

Ophiolites occur in Baja California along the outer coast from San Benito and Cedros Islands through the Vizcaíno Peninsula to Magdalena and Santa Margarita Islands. This is a mountainous region with altitudes up to 920 m (3018 ft) on the Vizcaíno Peninsula, >300 m (∼1000 ft.) on Magdalena Island, and about 550 m (∼1800 ft) on Santa Margarita Island. The ophiolite of Calmalli, which is geologically distinct from ophiolites on the outer coast, is in low hills (mostly <500 m, or 1640 ft) near El Arco, about midway from Guerrero Negro to the Gulf of California. Ophiolites of the outer coast are in the Cochimí terrane, whereas the ophiolite of Calmalli is in the Alisitos terrane (Sedlock et al. 1993, Sedlock 2003). Mafic rocks of the Peninsular Ranges batholith that extends from California into Baja California are included in this domain. A major feature of the Peninsular Ranges is this batholith with plutons that range in composition from granite to gabbro, with tonalite the most common composition. Also, gabbro is common in the “western zone” of the batholith (Sedlock 2003). This zone is mostly southwest of the Elsinore fault zone in the California and north of the Agua Blanca fault in Baja California. All the ophiolites are in desert areas. Mean annual temperatures are about 20°C, and mean annual precipitation is about 10 cm on Cedros Island and along the outer coast of Baja California Sur and about 15 cm in the ophiolite of Calmalli locality (Hastings and Turner 1965). The precipitation falls mostly in winter in the Cedros Island and Puerto Nuevo localities, in September in the Magdalena–Margarita locality, and in both September and in winter in the Calmalli locality. Fog and dew are common along the outer coast around Santa Margarita and Magdalena Islands. Drought persists for most of each year at all the localities (Hastings and Humphrey 1969; fig 13-3). The gabbro belt in the northern part of the Peninsular Ranges has been added to this domain. Descriptions of the geology, climate, soils, and vegetation of the gabbroic plutons are given in section 13.8, describing the Los Pinos locality.