Low‐temperature thermochronology of northern Baja California, Mexico: Decoupled slip‐exhumation gradients and delayed onset of oblique rifting across the Gulf of California

Abstract

The northern Gulf Extensional Province displays key structural relationships that characterize the magnitude, direction, and timing of Neogene rift‐related transtension during the opening of the Gulf of California. Apatite fission track and (U‐Th)/He thermochronology from the Sierra San Felipe document moderate cooling (4°C/Myr–7°C/Myr) during the early Paleogene associated with progressive unroofing caused by erosional downwearing of the ancestral Peninsular Ranges. Beginning at ∼45–35 Ma, a period of tectonic quiescence with low cooling rates (≤1°C/Myr) marks the development of a regional Oligocene‐Miocene peneplain. Rift‐related exhumation began at ∼9–7 Ma and attains ∼2.5 km in the hinges of two antiformal megamullions. Decoupling between exhumation and finite displacement in certain fault segments is explained by vertical deflections associated with extension‐perpendicular folding of the fault surfaces. The main faults of the detachment system were active contemporaneously, thus forming a mechanically linked array of large‐displacement normal faults in the hanging wall of the Main Gulf Escarpment. The Late Miocene onset of transtension in the Sierra San Felipe suggests that widespread deformation may only have localized in the Gulf Extensional Province between ∼9 and 7 Ma, some ∼3–5 Ma after a major plate reorganization associated with cessation of subduction in the trench to the west. Between ∼12 Ma and ∼9–7 Ma, plate boundary shearing was likely distributed between the continental borderland west of Baja California and the southern Basin and Range province in Mexico.