Mesozoic-Pleistocene fission-track ages on rocks of the Venezuelan Andes and their tectonic implications
Published:January 01, 1984
B. P. Kohn, R. Shagam, P. O. Banks, L. A. Burkley, 1984. "Mesozoic-Pleistocene fission-track ages on rocks of the Venezuelan Andes and their tectonic implications", The Caribbean-South American Plate Boundary and Regional Tectonics, William E. Bonini, Robert B. Hargraves, Reginald Shagam
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A total of 45 fission-track ages are reported on 22 apatite, 21 zircon, and 2 sphene concentrates from 23 rock samples from the Venezuelan Andes. Apatite ages range from about 1.4 to 24.0 Ma and are all interpreted as cooling ages related to uplift and erosion of cover rocks. Nine of the apatite ages are on one granitic pluton and adjacent rocks which occur through relief of 2,360 m, and the plot of age versus elevation approximates a straight-line slope of 0.8 km/Ma uplift. Because that rate far exceeds accepted rates of thermal diffusion in rocks, updoming of isotherms is inferred. In that case, elevation-age curves may not give accurate rates of uplift. Moreover, the time span of uplift is likely to be greater than the range of ages actually measured. It is suggested that the problem of updoming of isotherms in interpreting apatite ages may best be resolved by multiple sampling as near as possible to the vertical at two or more sites located at different ranges of elevation.
On the basis of the apatite ages, it is suggested that uplift of the Andes involved first uplift of the leading (northwestern) margin in the Oligocene to Miocene, followed by uplift of the trailing (southeastern) margin in the late Miocene, and in turn by rapid uplift of the central Andes in Pliocene-Pleistocene time. This view differs significantly from previous models of Tertiary Andean uplift.
Zircon ages range from about 60 to 172 Ma with a strong grouping in the range of about 81 to 113 Ma. Ages of 60 and 61 Ma (early Paleocene) are interpreted as uplift ages (and possibly a third age at 72 Ma) and are in conformity with regional stratigraphic evidence. The remaining ages are viewed as mixed ages, that is, ages modified by lengthy residence in the depth zone of partial track annealing. As such they do not have specific geologic significance. Two sphene ages (about 139 and 159 Ma) are also interpreted as mixed ages.