Abstract:
The southern Central American volcanic front lies on the SW edge of the Caribbean Plate,
inboard of the subducting Cocos and Nazca Plates. It is one of the most studied intra-oceanic convergent
margins around the world, which is generally interpreted to have developed in the late Cretaceous along
an oceanic plateau (the Caribbean Large Igneous Province or CLIP) and to be currently undergoing a
regime of subduction erosion. In the last decades a particular effort has been made to understand
subduction-related processes on the basis of geophysical and geochemical studies.
In southern Costa Rica and western Panama accretionary complexes and structures at the base of
the volcanic front have been exposed in response to subduction of aseismic ridges and transforms. Onland
exposures are located as close as to 15 km from the trench and provide a unique opportunity to better
understand some of the processes occurring along the subduction zone. We provide new constraints on
the origins of these exposures by integrating a comprehensive field work, new geochemical, sedimentary
and paleontological data, as well as structural observations based on remote imaging. A new Campanian
to Eocene tectonostratigraphy is defined for the forearc area located between the Osa Peninsula (Costa
Rica) and the Azuero Peninsula (Panama). Our results show that the outer margin is composed of a
complicated arrangement of igneous complexes and overlapping sedimentary sequences that essentially
comprise an arc basement, primitive island-arc rocks, accreted seamount fragments and accretionary
mélanges.
Evidences are provided for the development of the southern Central American arc on the top an
oceanic plateau. The subduction initiation along the SW edge of the Caribbean Plate occurred in the
Campanian and led to formation of primitive island-arc rocks characterized by unusual geochemical
affinities broadly intermediate between plateau and arc affinities. The arc was mature in the Maastrichtian
and was forming a predominantly continuous landbridge between the North and South Americas. This
allowed migration of terrestrial fauna between the Americas and may have contributed to the Cretaceous-
Tertiary crisis by limiting trans-equatorial oceanic currents between the Pacific and the Atlantic.
Several units composed of accreted seamount fragments are defined. The nature of the units and
their structural arrangement provide new constraints on the modes of accretion of seamounts/oceanic
islands and on the evolution of the margin since subduction initiation. Between the late Cretaceous and
the middle Eocene, the margin recorded several local episodes of seamount accretion alternating with
tectonic erosion. In the middle Eocene a regional tectonic event may have triggered strong coupling
between the overriding and subducting plates, leading to higher rates of seamount accretion. During this
period the situation along the margin was very similar to the present and characterized by subducting
seamounts and absence of sediment accretion. The geological record shows that it is not possible to
ascribe an overall erosive or accretionary nature to the margin in the past and, by analogy, today, because
(1) accretionary and erosive processes exhibit significant lateral and temporal variations and (2) it is
impossible to estimate the exact amount of material tectonically eroded from the margin since subduction
initiation.
In southern Costa Rica, accreted seamount fragments point toward a plume-ridge interaction in
the Pacific in the late Cretaceous/Paleocene. This occurrence of accreted seamount fragments and
morphology of the Pacific Ocean floor is indicative of the formation of the Cocos-Nazca spreading
system at least ~40 Ma prior to the age proposed in current tectonic models.
In Panama, we identified a remarkably-well preserved early Eocene oceanic island that accreted
in the middle Eocene. The accretion probably occurred at very shallow depth by detachment of the island
in the trench and led to an exceptional preservation of the volcanic structures. Exposures of both deep and
superficial parts of the volcanic edifice have been studied, from the submarine-shield to subaerialpostshield
stages. The stratigraphy allowed us to distinguish lavas produced during the submarine and
subaerial stages. The lava compositions likely define a progressive diminution of source melting and a
decrease in the temperature of erupted melts in the latest stages of volcanic activity. We interpret these
changes to primarily reflect the progressive migration of the oceanic island out of the melting region or
hotspot.