New heat flow data in Arizona suggest that the southern Basin and Range has recently undergone less extension than other area of the Basin and Range province. Heat flow-heat production data imply that magmatism and hydrothermal circulation in the crust and upper mantle beneath sourthern Arizona and northwestern Mexico may be less extensive than along the Rio Grande rift. A heat flow transition between the Basin and the Colorado Plateau occurs within a lateral distance of 60 km in central Arizona. Predicted steady state temperature suggest that the iM discontinuity may be nearly coincident with an isotherm along this transition. The new heat flow measurements from depths greater than 650 m 0.3 to 0.4 HFU difference between the Colorado Plateau and suggest that the mean value for the Arizona Basin and Range is 1.94¿0.12 HFU, indicating as little as 0.3 to 0.4 HFU difference between the Colorado Plateau and the Arizona Basin and Range. This difference could be attributed primarily to widespead magmatic intrusion in the Basin and Range.