Sierra de Hualfn is a 22-km (14-mi)-long basement-cored uplift with an approximately 66-km² (26-mi²) exposure of the basement-cover interface. The uplift is composed of Ordovician granite, overlain by and locally thrust over Tertiary sedimentary rocks. The geometry of the uplift is that of a faulted anticlinorium in granite basement that is delineated by the basement unconformity. Detailed mapping of the basement unconformity documents large-scale folding of granite basement, the result of fault-propagation folding associated with a thrust-fault tip initially located deep in the basement.

Our findings at Sierra de Hualfn indicate that homogeneous granite can fold as a deformable body by stress-induced development and/or exploitation of joints, faults and fractures, and an unconformity-parallel fracture foliation in the uppermost basement. Specifically, folding is achieved through systematic coordinated movements involving (1) reactivation of joints as faults and mode I fissures, (2) fracturing and faulting near large displacement faults, and (3) flexural shear of the uppermost basement. The extent to which joints, microfractures, and fracture foliation are reactivated determines the deformability of granite. At Sierra de Hualfn, the deformability of granite is such that the folding of the basement is consistent with trishear kinematics.

Our observations contradict standard models of basement-cored uplifts that assume that the fault tip is located at the basement-cover interface. We postulate that the folded shape of Sierra de Hualfn and of some uplifts in the Rocky Mountain foreland can be attributed to basement distortion taking place in advance of a propagating fault tip below the basement-cover unconformity.