A detailed synthesis of the chronology and spatial distribution of magmatic activity along a transect of the Andes of central Peru (around 11°S latitude) and some new K-Ar radiochronological data are presented. This huge data set is compared with independent geophysical data and reconstructions dealing with the interaction dynamics between the oceanic Nazca plate (previously Farallon) and the South American continent.
The data strongly suggest that magmatic activity has been discontinuous. All periods of high convergence rate (> 10 cm/yr) between Nazca (Farallon) and South American plates are characterized by important magmatic activity. In contrast, the intervals of magmatic quiescence or low magmatic activity are systematically associated with low convergence rate. Two main exceptions to these rules are: (1) The last period of Coastal Batholith emplacement, between 75 and 59 Ma, seems to be associated with a lower convergence rate (5 to 7 cm/yr) and a very weak compressive deformation. (2) Magmatic activity is absent in central Peru during the last 3 m.y. in a context of high convergence rate. This has been interpreted as a consequence of the lack of an asthenospheric wedge above an abnormally flat slab. The change from a “normal” slab dip (≈30°) to the “flat” slab is associated with the subduction of the Nazca ridge and not with a rejuvenation of the slab.
The formation of the Copara-Casma (Aptian-Albian) aborted marginal volcanic basin appears to be contemporaneous with the rifting of the South Atlantic Ocean, an extensional tectonic regime and a steeply dipping slab. The beginning of emplacement of the Coastal Batholith (Albian to Paleocene) is synchronous with the onset of active spreading in the Atlantic Ocean, the change to a compressive regime along the Andean margin, and a decrease of slab dip. Channeling of magmas through deep-seated lithospheric structures along the axis of the Casma basin may have played a role during the emplacement of the Coastal Batholith. From Albian to Mio-Pliocene times, the eastward migration of the trenchward magmatic front did not exceed 50 km. The main change in the geometry of the magmatic belt is its rapid broadening, from a narrow (≈40 km) to a wide (>150 km) belt in the early and mid-Eocene. This change is contemporaneous with the main period of compressive deformation and corresponds to a decrease in the dip of the slab. It is not associated with a rejuvenation of the slab nor with a higher trenchward motion of the overriding plate. Rather, it appears to be related to an increase in the rate of convergence and to a change from a very oblique to a nearly normal convergence with respect to the continental margin.
Tectonic erosion at the trench cannot be invoked at least for the last 40 m.y. but probably occurred during the first episodes of emplacement of the Coastal Batholith. No conclusion may be drawn for the period between ca. 75 to ca. 45 Ma.
