Heterogeneous Neogene cooling and exhumation of the Los Cabos block, southern Baja California: Evidence from fission-track thermochronology

Abstract

The Los Cabos block is a massif of Mesozoic crystalline basement at the southern tip of the Baja California Peninsula and has long been considered a distinct tectonostratigraphic terrane separated from the rest of the peninsula by the La Paz fault along its western margin. Although the region is cut by an extensive array of active north-northwest–striking normal faults, none of them correspond to the proposed La Paz fault. The largest fault in the array and arguably one of the dominant Neogene structures in the Gulf extensional province is the San Jose del Cabo fault, which is an east-dipping normal fault that has a strike length of ∼150 km, a topographic escarpment in excess of 1000 m, and forms the eastern limit of the Los Cabos block. Apatite and zircon fission-track data demonstrate a marked difference in Neogene tectonism between the two margins of the Los Cabos block. Samples from the western margin, in the footwall of the proposed La Paz fault, indicate rapid Paleocene cooling (∼20 °C/m.y.), followed by essentially slow monotonic cooling (∼2–3 °C/m.y.) through the Tertiary. In contrast, samples from the eastern margin, in the footwall of the San Jose del Cabo fault, record rapid cooling (to ∼45 °C/m.y.) related to tectonic exhumation across the fault commencing in the mid-Miocene (∼10–12 Ma), when the sampled rocks were at ∼150 °C. The San Jose del Cabo fault accommodated ∼5.2–6.5 km of exhumation at rates as high as ∼1.5–2 mm/yr, but averaged 0.4–0.7 mm/yr. Continental rifting in the southern Gulf extensional province postdated the southward passage of the Rivera triple junction (ca. 12 Ma), which implies that driving forces were likely dominated by far-field plate kinematics and coupling between the Pacific, Farallon, and North America plates. This continental rifting has persisted through many inferred reconfigurations of plate motion: northward rotation of relative-motion vector (8 Ma), eastward migration of wrenching from the Pacific to the Gulf extensional province (6 Ma), and onset of sea-floor spreading (3.6 Ma).