The study area is along the continental divide, within the Mesozoic fold-and-fault belt of western Peru. The rocks exposed are Early Jurassic to early Tertiary sedimentary strata; neither pre-Early Jurassic nor Tertiary volcanic rocks are exposed.

Structurally the area can be divided into two zones separated by the Tranca reverse-fault zone. The eastern zone has northwest-trending folds and reverse faults; the overturn of the fold axial planes and the direction of overthrusting are toward the northeast. The faults are in dextral en echelon array. The western zone shows tight folding, and imbricate reverse faulting. In both zones decollement at depth may control the structural pattern. The age of the tectonism is early to middle Tertiary. Comparisons are shown between the types of deformation in the study area and those elsewhere in the same fold-and-fault belt. A series of post-tectonic stocks of granodiorite-diorite composition of middle Tertiary age or younger is present in the area.