An anomalous fold belt exists in northern Peru where Tertiary folds trend east rather than northwest. The axis of the Coast batholith continues to trend northwest parallel to the present coast as well as to hypothetical paleotrenches.
East-trending fold axes cross the tectonic grain established by Mesozoic events. K-Ar dates now available infer development of the fold belt within the period 65 to 40 m.y. B.P. North-south compressive forces, effective in producing the Compaccha–Agua Agria shear couple, were responsible for the east-trending folds. Apparent absence of a cratonic basement in this area may have been a factor in localizing the fold belt.
No K-Ar ages for the Coast batholith in this area are younger than 43 m.y. This period of plutonic activity ceased prior to development of the Calipuy volcanic sequence, which is now interpreted to have developed between 30 and 10 m.y. ago.