The Gulf of California provides an active rift example to test the role of rift obliquity in continental rupture. Continental rifts require focused strain to rupture and form an ocean basin. Strike-slip faults are ubiquitous in oblique rifts and focused transtensional strain adjacent to these faults may be a catalyst for rupture. To test this hypothesis, I completed structural mapping, fault-kinematic analysis, geochronology, basin analysis, and paleomagnetism of pre- and syn-rift rocks exposed in coastal Sonora, México. Sedimentary basins record ∼16 km of west-northwest-directed transtension across the Kino-Chueca Shear Zone. Onset of transtension in the study area is estimated to be ca. 7 Ma and lasted for approximately 1 million years. This represents a significant portion (∼28%) of plate boundary deformation over this time interval. Dextral shear was progressively localized within this zone of extensional deformation, and together shear and extension acted to focus lithospheric-scale strain into a narrower zone.