Structural, geophysical, and thermochronologicaldata from the transition zone between the Sierra Nevada and the Basin andRange province at latitude ~39¿N suggest ~100 km westward encroachmentof Basin and Range extensional deformation since the middle Miocene. Extension,accommodated primarily by east dipping normal faults that bound west tilted,range-forming fault blocks, varies in magnitude from 150% in the Wassuk and Singatse Rangesto the east. Geological and apatite fission track data from exhumed uppercrustal sections in the Wassuk and Singatse Ranges point to rapid footwallcooling related to large magnitude extension starting at ~14--15Ma. Farther to the west, geological and thermochronological data indicatea younger period of extension in the previously unextended Pine Nut Mountains,the Carson Range, and the Tahoe-Truckee depression initiated between 10 Maand 3 Ma, and incipient post-0.5 Ma faulting to the west of the Tahoe-Truckeearea. These data imply the presence of an extensional breakaway zone betweenthe Singatse Range and the Pine Nut Mountains at ~14--15 Ma, formingthe boundary between the Sierra Nevada and Basin and Range at that time. Inaddition, fission track data imply a Miocene preextensional geothermal gradientof 27 ¿ 5¿C km-1 in the central Wassuk Rangeand 20 ¿ 5¿C km-1 in the Singatse Range,much higher than the estimated early Tertiary gradient of 10 ¿ 5¿Ckm-1 for the Sierra Nevada batholith. This might pointto a significant increase in geothermal gradients coupled with a likely decreasein crustal strength enabling the initiation of extensional faulting. Apatitefission track, geophysical, and geological constraints across the Sierra Nevada-Basinand Range transition zone indicate a two-stage, coupled structural and thermalwestward encroachment of the Basin and Range province into the Sierra Nevadasince the middle Miocene.