The Early Cretaceous Alisitos island arc, located in the western part of the Peninsular Ranges batholith, Baja California, accreted to North America during the mid-Cretaceous. A syn- to post-collisional fold-thrust belt dominated by sinistral transpression and orthogonal convergence developed along the northern and eastern edges of the arc, respectively. Field observations across the fold-thrust belt show a deformation gradient with stronger planar and linear fabrics, fold tightening, and greater finite strain towards the arc–continent suture. Flattening strains dominate and finite strain intensity ranges from 0.08 to 2.71 and generally increases towards the suture. In detail, the fold-thrust belt narrows southward from ∼12 km to ∼3 km. Furthermore, finite strain is heterogeneous reflecting a heterogeneous fold-thrust belt characterized by local high strain zones near faults, folds, and igneous intrusions. Finite strain data and field observations allow several conclusions: (1) the colliding arc deformed significantly as a result of collision; (2) strain contributes to bulk shortening and crustal thickening in the collision zone; (3) geometry, composition, and tectonic setting of the continental margin prior to collision control along-strike variations in the fold-thrust belt; and (4) narrowing of the fold-thrust belt southward is offset by increased deformation in continental margin units.