Abstract : A comparison of detrital zircon ages from
the Cenozoic forearc basins in SW Ecuador–
NW Peru and those derived from the
Amotapes Massif offers a singular opportunity
to shed some light on the evolution of
the NW Andean margin of South America.
An extensive set of pre-Andean U-Pb zircon
ages (older than Late Cretaceous) was
obtained from 18 detrital samples coming
from the Talara, Zorritos, Progreso, and
Gulf of Guayaquil–Tumbes forearc basins,
as well as from two metasedimentary rocks
of the Amotapes Massif. U-Pb age analysis
on detrital zircon grains allows a general
source-to-sink scenario to be determined
for this section of the Andes. These analyses
give consistent insights into the early topographic
growth during the modern Andean
orogeny, the occurrence of proto-Andean
magmatic periods in the western margin of
Gondwana, and the evolution of the Amotapes
Massif within the geodynamical setting
of northwestern South America. Zircon ages
at ca. 320 Ma and Neoproterozoic clusters
for the Amotapes metasediments show significant
similarities along the strike of the
metamorphic belt, thus defining a common
metasedimentary origin, and can be compared
with previously dated rocks from the
Eastern Cordillera of Peru. These strong similarities
demonstrate the presence of a wide,
polyphased metamorphic belt that includes
the western parts of the Eastern Cordillera,
the northern section of the Occidental Cordillera
of Peru, and the Amotapes Massif.
Detrital sediments of the forearc are marked
by a major gap in Early Cretaceous–Jurassic
ages and the minor presence of Ordovician
(Famatinian orogeny) and Carboniferous–
Permian (340–285 Ma) ages. The age spectrum
is not compatible with sediment input
from the eastern Andes and suggests that the
external areas of the Andean chain (e.g., the
Western Cordillera) were already uplifted
prior to or immediately after forearc initiation
during the Paleocene, thus representing
a pervasive drainage divide throughout all of
the Cenozoic. The Amotapes Massif or other
terrains sharing the same age affinities (e.g.,
the Olmos
terrane; Occidental Cordillera of
Peru) are likely to represent the best potential
sources for pre-Andean zircon grains in the
forearc. Age clusters in both source and sink
at ca. 600 Ma, 570–560 Ma, and 520 Ma most
likely define magmatic periods and may represent
first-order constraints for the difficult
correlation between proto-Andean magmatic
and orogenic periods to the north and south
of the Huancabamba deflection. It is unlikely
that the Neoproterozoic zircon grains came
from the Pan African–Braziliano belts, thus
suggesting the onset of significant magmatic
activity in this section of the Andes at around
570–560 Ma and probably ca. 600 Ma. The
520 Ma cluster is compatible with the most
prominent magmatic period in the southern
Andes related to the end of the Pampean
orogeny, thus suggesting the existence of a
regional proto-Andean magmatic arc during
Early Cambrian times.