We estimate here a geothermal gradient of only 17¿5 ¿C km-1 for the tilted Grayback fault block in southeastern Arizona when extension began ~25 Ma. This gradient is lower than preextension gradients estimated elsewhere in the Basin and Range, is only about 50% of typical gradients in the Basin and Range today, and needs to be accounted for in models of continental extension. The Grayback block exposes a 12-km-thick crustal section of Proterozoic and Cretaceous granitoids, which was tilted 90¿ during extension between 25 and 15 Ma. Zircon fission-track ages decrease structurally downward (westward) across the block and were all within a zone of partial track annealing prior to tilting and quenching. The zircon age gradient suggests that the 220¿--240 ¿C isotherm migrated downward 5--6 km during Paleogene erosion and regional cooling. Apatite fission-track ages decrease westward from ~83 Ma in the structurally highest crystalline rocks to ~24 Ma at ~6-km paleodepth and then to ~15 Ma another 6 km farther west. Track-length analysis confirms that apatites above the break in slope in age at ~5.7-km paleodepth resided in a zone of partial annealing prior to tilting, and deeper apatites record rapid cooling upon tilting and unroofing beginning ~25 Ma. At that time the 110¿10 ¿C isotherm determined by the depth at which tracks in apatite were fully erased was at a basement paleodepth of ~5.7 km, and the 220¿30 ¿C isotherm as estimated from zircon data resided at a pretilting basement depth of ~12.15 km. From consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17¿5 ¿C km-1. From 25 to 15 Ma the rotating Grayback block cooled rapidly as higher, westward moving blocks unroofed it tectonically at a rate of ~1 km m.y.-1. ¿ American Geophysical Union 1996