The Maria fold-and-thrust belt (MFTB) is a narrow belt of Mesozoic crystal shortening that trends east-west across west central Arizona and adjacent southeastern California. It is characterized by generally south vergent folds and thrusts that commonly displace Proterozoic crystalline rocks over deformed and metamorphosed Paleozoic and Mesozoic strata. The MFTB is cut by a south to southeast trending belt of mid-Tertiary extensional deformation. Extension was characterized by large displacements on low-angle normal faults, known as detachment faults, and by isostatic uplift of mylonitic midcrustal rocks now exposed in metamorphic core complexes. The geometry and style of extensional deformation change along the extensional belt and reveal the influence of the MFTB. Several extensional features are spatially coincident with the root zone of MFTB thrusts: (1) an areally extensive west-northwest trending belt of denuded Tertiary mylonitic fabrics in the Whipple and Harcuvar metamorphic core complexes, (2) a style of extension characterized by minimum extensional dismemberment of the upper plate and maximum denudation and uplift of deep-seated lower plate rocks, (3) an abrupt bend in the belt of arched, uplifted rocks below detachment faults, and (4) an abrupt bend in the trend of the breakaway zone of the detachment faults and an associated lateral ramp in the detachment fault system. The first two features may be the result of isostatic uplift of a crustal root (downward protruding Moho bulge) inherited from crustal thickening within the MFTB, and the other two features may reflect, at the time of detachment fault initiation, the influence of stresses in the crust associated with flexural support of the buoyant Mesozoic crustal root. ¿ American Geophysical Union 1990