2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

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Symposium 1: Structural Geology, Tectonics and Geodynamics
by thermally activated diffusion). The grain size aliquots yield very precise concordant 238 U/ 206 Pb ages that range between
137.87 ± 0.34 and 94.53 ± 0.47 Ma. Furthermore, a positive age-to-grain size correlation implies that volume diffusion is
the dominant process controlling Pb loss. We have obtained a set of time (t)-temperature (T) solutions (Figure 1) for the
preliminary data using a controlled random search procedure provided by the HeFTy software (version 1.6.7; Ketcham
2009), using the diffusion parameters of Cherniak et al. (1991; Activation Energy of 54.6 Kcal/mol; Absolute Diffusivity of
1.27 X 10 -4 cm 2 s -1 ). Geological constraints for the t-T solutions provided considerable freedom, and are i) zircon crystallization
at ~247 Ma during a thermal spike that caused widespread anatexis (Figure 1, A), and ii) cooling during 75-65 Ma as a
consequence of the collision of the CLIP. The thermal history solutions satisfy the U-Pb ages obtained from four size aliquots.
The best fit t-T solutions (Kolmogorov-Smirnoff goodness of fit > 0.4) reveal a period of Early Cretaceous heating
from ~200°C to ~510°C during 140-100 Ma (Figure 1, B), at rates of ~10°C/my, followed by a period of rapid cooling from
~510°C to ~300°C that started at 100-95 Ma (Figure 1, C).
We propose that the Jurassic trench migrated westward, accompanied by slab rollback that forced the arc axis to migrate
oceanwards, and drove rapid extension in the upper plate forming the Alao/Salado Basin. We attribute heating of the
Triassic basement rocks to i) asthenospheric upwelling that accompanied extension and the formation of transitional
crust, and ii) basin sedimentation burial. Rapid cooling at ~100Ma is attributed to exhumation that accompanied rock
uplift, which was driven by closure of the marginal Alao Basin during the rapid westward migration of the South American
Plate during the Early Cretaceous.
The thermal history solutions obtained from apatite U-Pb data acquired from a single rock corroborate independent geological
observations. Additional U-Pb thermochronology is scheduled to further assist in constraining the tectonic evolution
of the Ecuadorian and Colombian margin prior to 75 Ma.
Figure 1: Time-temperature (t-T) history produced for 4 apatite 238U/206Pb ages.
REFERENCES:
Cherniak, D.J., Lanford, W.A., Ryerson, F.J. (1991). Lead diffusion in apatite and zircon using ion implantation and
Rutherford Backscattering techniques. Geochim. Cosmochim. Acta 55, 1663–1673.
Schoene, B., & Bowring, S.A. (2007). Determining accurate temperature-time paths from U-Pb thermochronology: an
example from the SE Kaapvaal Craton, Southern Africa. Geochim. Cosmochim. Acta 71, 165-185.
Spikings, R.A., Seward, D., Winkler, W., Ruiz, G.M. (2001). Low-temperature thermochronology of the northern Cordillera
Real, Ecuador: Tectonic insights from zircon and apatite fission track analysis. Tectonics 19, 649–668.
Spikings, R.A., Crowhurst, P.V.,Winkler, W., Villagomez, D. (2010). Syn- and post-accretionary cooling history of the
Ecuadorian Andes constrained by their in-situ and detrital thermochronometric record. Journal of South American
Earth Sciences 30 (2010) 121e133
Swiss Geoscience Meeting 2011
Platform Geosciences, Swiss Academy of Science, SCNAT