Tesis Doctorales con investigaciones realizadas en ACG http://hdl.handle.net/11606/55 2026-04-12T16:09:40Z 2026-04-12T16:09:40Z Buchs, David M. http://hdl.handle.net/11606/366 2017-02-13T20:14:07Z 2008-01-01T00:00:00Z

Late Cretaceous to Eocene geology of the South Central American forearc area (southern Costa Rica and western Panama): Initiation and evolution of an intra-oceanic convergent margin Buchs, David M. 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.

2008-01-01T00:00:00Z Gillies, Cameron Scott http://hdl.handle.net/11606/365 2017-02-13T20:06:52Z 2008-01-01T00:00:00Z

Corridor use, habitat selection, and route choice by forest birds in the fragmented tropical dry forests of Costa Rica Gillies, Cameron Scott Tropical forests hold the majority of the world’s biodiversity, but face tremendous threats from agricultural expansion. Among these forests, dry forests have already undergone extensive clearing and only 2% of the original tropical dry forest remains in Mesoamerica. In such fragmented landscapes, the movement of individuals among subpopulations is fundamental to long term population persistence. Despite the importance of movement, little is known about how forest-dependent birds move through fragmented areas and use connecting habitat elements, like corridors. To address this deficiency, I translocated individuals of two species of forest birds with differing forest dependence in three treatments in the fragmented tropical dry forests of northwestern Costa Rica: along a riparian corridor, along a fencerow, or across pasture. I then followed their return trajectories with unprecedented resolution, recording positions approximately every 15 min for up to four days. Detailed route information yielded four main conclusions. First, riparian corridors facilitated the movement of the forest specialist barred antshrike (Thamnophilus doliatus). In riparian corridor treatments, returns to their original territories were faster and more likely, they selected forest habitat more strongly, and they traveled further from the forest edge. Second, fencerows were not sufficient corridors for the specialist, which generally chose longer routes in forest rather than more direct routes via fencerows. Third, individuals adjusted their behaviours based on habitat context. In addition to changes by the specialist in riparian corridor treatments (above), the generalist rufous-naped wren (Campylorhynchus rufinucha) selected forest more strongly in riparian corridor treatments and selected edge habitat more strongly with decreases in forest cover. Finally, the specialist chose more forested steps when they were far from their territories and when in forest habitat. They preferred steps ending in stepping stones (isolated trees) when available routes had low forest cover, but avoided them when forest cover was higher. I conclude that forested habitat and corridors benefit the movement of forest specialist birds and the conservation of these habitats will be important in this landscape and likely others. Furthermore, stepping stones may be an important element for the movement of birds through the most inhospitable matrix where forested alternatives do not exist.

2008-01-01T00:00:00Z