The unique low-temperature sensitivity of the apatite (U-Th)/He (AHe) technique provides valuable new information relating to the late-stage thermal evolution of metamorphic core complexes in the southern Basin and Range Province. In all the ranges studied, the data show greater complexity than previously seen from other thermochronometers. In the Chemehuevi and northern Sacramento Mountains, a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall. This is followed by essentially age-invariant segments of 15 ± 1 Ma, reflecting an increase in the rate of detachment fault movement. The timing of the change suggested by the data agrees with recently published AHe results from the nearby Harcuvar Mountains, located ∼125 km to the southeast, suggesting that an underlying regional mechanism triggered this change. Plate reconstructions indicate that the Pioneer-Mendocino fracture zone migrated northward through the area at this time, leaving a slab window in its wake. We suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this, combined with the extensional strain regime present, resulted in an increased rate of slip within the actively extending metamorphic core complexes. This agrees well with timing constraints for extension onset in the central Basin and Range Province of southern Nevada, and suggests that weakening of the southern Basin and Range Province at 15 ± 1 Ma was partially responsible for the onset of extension in the central Basin and Range. These findings demonstrate that improved constraints on changes in extension rate within the Basin and Range Province should be possible through the application of the AHe technique to other metamorphic core complex footwalls.