Although subduction of the Nazca plate beneath the Peruvian continental margin has been in progress for nearly 200 Ma, the Andean cycle in western Peru is dominated by periods of contemporaneous rifting and large scale crustal extension. It began with the formation of the west Peruvian trough (WPT), a major depositional structure and one of a series of 'marginal' basins that developed along the entire Pacific coast of South America during the Cretaceous. The western element of the WPT north of Lima is the Albian Casma basin, composed of nearly 9000 m of mostly basaltic to intermediate volcanic rocks and minor intrusives. Facies analysis of the basinal fill is consistent with a spreading system in a relatively isolated, deep-sea environment. The Tertiary is marked by two major episodes of extensional, subaerial volcanism: the c. 53-15 Ma Calipuy Group, followed in the Late Miocene-Pliocene (c. 7.6-4.65 Ma) by the more easterly Yungay volcanic rocks. Although both were extruded during periods of crustal extension, the prevailing tectonic regimes differ in detail. Thus the acid-intermediate Calipuy Group was extruded from fissure-type volcanoes over a long period with only minor plutonic activity, whereas the more easterly Yungay volcanic rocks, located furthest from the present trench, are closely associated with major batholith intrusion within a strike-slip transtensional system. Although the volcanic rocks of the Casma-Calipuy-Yungay groups are dominantly 'calc-alkaline' in the classic sense, the role of extension has clearly been fundamental in their genesis. In contrast, their relation to contemporaneous subduction is less clear.