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[1] The timing of collision of the Cocos Ridge at the Middle America Trench remains one of the outstanding questions in the tectonic evolution of the Central American convergent margin. New analyses of the tectonic geomorphology of the Cordillera de Talamanca, the extinct volcanic arc inboard of the Cocos Ridge, coupled with low temperature thermochronometry data, provide insight into the cooling and erosional history of the arc from late Miocene to present. We identify a low-relief surface at high elevation along the northeastern flanks of the range, which represents a relict erosional landscape cut across shallow plutonic rocks of the arc edifice. Longitudinal profiles of rivers on this surface are isolated from steep downstream sections by prominent knickzones that are interpreted to reflect a migrating wave of transient incision generated during differential rock uplift of the range. Reconstruction of pre-incision profiles suggests that rock uplift during the growth of the Cordillera de Talamanca is no greater than $2 km. This inference is corroborated by results from our apatite (U-Th)/He and apatite fission track analyses along an elevation transect on Mt. Chirripó, the highest mountain in the Cordillera de Talamanca. Low-temperature cooling ages overlap significantly with published high-temperature 40 Ar/ 39 Ar ages; the combined results imply that rapid cooling in the late Miocene was related to secular cooling of a shallow pluton, rather than exhumation. Our results imply that rapid incision along downstream channel segments, differential rock uplift, and growth of the Talamanca as a bivergent orogenic wedge associated with the onset of Cocos Ridge subduction are relatively young characteristics of the range. A review of previously published radiometric ages and revised plate reconstructions for the late Miocene further suggest that the cessation of arc volcanism in both the Cordillera de Talamanca and the Cordillera Central of western Panama was coeval with the initiation of oblique subduction of the Nazca plate during the late Miocene. Thus, we conclude that the cessation of arc volcanism in the late Miocene (>5-8 Ma) and the onset of Cocos Ridge collision (<3 Ma) are separate events that reflect recent changes in the configuration of the plate boundary system.

Structural and thermochronologic studies of the western margin of the central Andean Plateau show changing styles of deformation through time that give insights into tectonic evolution. In southwest Peru, uplift of the plateau proceeded in several distinct phases. First, NW striking, NE dipping reverse faults accommodated uplift prior to ∼14–16 Ma. Subsequent uplift of the plateau relative to the piedmont (between the plateau and the Pacific Ocean) occurred between ∼14 and 2.2 Ma and was accommodated by NW striking, SW dipping normal faults and subparallel monoclinal folds. The youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block. Although the uplift magnitude associated with phase 1 is unknown, phases 2 and 3 resulted in at least 2.4–3.0 km of uplift. Up to 1 km of this may have occurred during phase 3. Geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift.

Lesser Antilles arc lavas have trace element and radiogenic isotope characteristics indicative of a continent-derived contribution. It is hotly debated whether this continental signature represents terrigenous sediment that has been subducted with the Atlantic plate and added to the magma sources in the mantle wedge or portions of the sub-arc crust that are assimilated during magma ascent. Here we present Mo isotope data for Lesser Antilles arc lavas and sediments off-board the Lesser Antilles trench. Sequences of black shales, present in the subducting sediment piles, are highly enriched in Mo and have unusually high 98 Mo/ 95 Mo. Despite their low mass fraction in the sediment package (<10 % in DSDP Site 144), they dominate the Mo content and isotopic composition of the bulk sediment subducting at the Lesser Antilles trench. We show that lavas from the southern part of the Lesser Antilles arc also have high 98 Mo/ 95 Mo ratios, implicating the addition Manuscript Click here to download Manuscript G38344-Freymuth-eXtyled_revised.doc

The Taiwan orogenic belt is often treated as a steady, southward-propagating orogenic system with an essentially constant erosion rate of 4–6 mm/yr over the past 5 m.y. We present 4 new age-elevation transects from the Central Range based on 19 new and 86 previously published fission track and (U-Th)/He dates of completely reset detrital zircon and apatite grains. The age-elevation curves and thermal models imply slow cooling prior to ca. 2–1.5 Ma (at exhumation rates of ∼0.1 mm/yr), an increase in exhumation rates from ca. 2–1.5 Ma to ca. 0.5 Ma (2–4 mm/yr), and possibly a further acceleration in exhumation from ca. 0.5 Ma to present (4–8 mm/yr). Three transects from three different latitudes in the eastern Central Range yield similar results, each showing punctuated exhumation with progressively faster rates.