Evolution of the Western Cordillera in the Andes of Ecuador (Late Cretaceous-Paleogene)

ABSTRACT
The subduction of ocean floor at a convergent plate boundary can be regarded as approximating
a steady-state process. Interruption or cessation of that process commonly follows the arrival
of a buoyant object. Arcs and continents are the familiar kinds of buoyant objects involved
in collisions. I here provide a detailed analysis of a less familiar kind of collision, that of an
oceanic plateau.
The determination of accurate and precise ages for the timing of collision between oceanic
plateaus and continental crust provides an understanding of how the indenting and buttressing
plates respond to the collisional event. The volcanic basement of the Ecuadorian Western
Cordillera (Pallatanga Formation and San Juan Unit) is made up of mafic and ultramafic rocks
with oceanic plateau geochemical affinities. A SHRIMP crystallization age (zircon) of 87.1±1.66
Ma (2s) and 40Ar/39Ar (hornblende) age of 84.69±2.22 Ma (2s) from an accreted fragment of the
plateau, overlap with an 40Ar/39Ar age of 88±1.6 Ma age obtained for oceanic plateau basement
rocks of the Piñon Formation in coastal Ecuador (Luzieux et al., 2006), and a suite of ~92-88
Ma ages reported for oceanic plateau sequences in Colombia and the Caribbean region. These
results are consistent with the idea that the oceanic plateau rocks of the Western Cordillera and
coastal Ecuador are derived from the Late Cretaceous Caribbean Colombia Oceanic Plateau
(CCOP).
Intraoceanic island arc sequences (Pujilí Granite, Rio Cala Group, Naranjal Unit) overlie
the plateau and yield crystallization ages that range between ~85–72 Ma. The geochemistry
and radiometric ages of lavas associated with the Rio Cala Arc, combined with the age range
and geochemistry of their turbiditic, volcanoclastic products indicate that the arc initiated by
westward subduction beneath the Caribbean Plateau, and are coeval with island arc rocks of
coastal Ecuador (Las Orquideas, San Lorenzo and Cayo formations). These island arc units
may be related to the Late Cretaceous Great Arc of the Caribbean.
Paleomagnetic analyses of volcanic rocks, of the Piñon and San Lorenzo formations of the
southern external forearc (Luzieux, 20067), indicate their pre-collisional extrusion at equatorial
or shallow southern latitudes. Furthermore, paleomagnetic declination data from basement and
sedimentary cover rocks in the coastal region (Luzieux, 2007) indicate 20–50o of clockwise
rotation during the Campanian, which was probably synchronous with the collision of the
oceanic plateau and arc sequence with South America.
The initial collision between the South American Plate and the Caribbean Plateau was
synchronous with accelerated surface uplift and exhumation (>1km/my) within the buttressing
continental margin during the Late Cretaceous (c. 75–65 Ma), in an area extending as far inland
as the Eastern Cordillera. The rapid exhumation coincides with the deposition of continental
siliciclastic material in both the fore- and backarc environments (Yunguilla and Tena formations
respectively).
Collectively, this evidence shows that the initial collision between the Caribbean Plateau
and the Ecuadorian margin occurred during the late Campanian–Maastrichtian (73–70 Ma),
and resulted in plugging of the subduction zone, the termination of island arc magmatism, and
deformation of the continental margin.
Magmatism associated with the Campanian–early Maastrichtian Rio Cala Arc, which
erupted through the Pallatanga Formation, ceased during the Maastrichtian and was followed
by the initiation of east-dipping subduction beneath the accreted oceanic plateau. The new
active margin gave rise to the latest Maastrichtian (ca 65 Ma) Silante volcanic arc, which was
deposited in a terrestrial environment.
During the Palaeocene to Eocene, marine conditions were dominant in the area now occupied
by the Western Cordillera, and volcanic rocks of the Macuchi Unit were deposited, possibly as
a temporal continuation of the Silante volcanic arc. This submarine volcanism was coeval with
the deposition of siliciclastic rocks of the Angamarca Group, and the Saguangal Formation,
which were mainly derived from the emerging Eastern Cordillera.
Finally, no evidence exists to support previous hypotheses that the Macuchi Arc accreted in
the Late Eocene, causing structural inversion of the Angamarca Basin. It is geometrically difficult
to suggest that the Macuchi Block accreted in the Late Eocene, and inserted itself between the
Piñon and Pallatanga blocks, which accreted during the Late Cretaceous. Furthermore, volcanic
rocks of the Macuchi Unit are found to be conformably overlain by turbidites of the Angamarca
Group.