(U-Th)/He - derived thermochronological constraints on the post - middle Miocene tectonic history of the Ecuadorian Andes

2005 
Introduction Numerous apatite fission -track (APT) monocompositional thermal history (t-T) reconstructions from the Andean cordilleras have identified a single period of continuai, linear cooling during 9± 1 0 Ma along the entire orogen. However, the lack of sensitivity of the AFT method at temperatures <,....(jO°C (Laslett et al., 1987), coupled with arnbiguity regarding the initial conditions of track annealing (e.g. the Jength of an unannealed fission track; Jonckheere, 1996) suggests the AFT method may have frequently failed to resolve post9 Ma cooling histories . Consequently, the post 9 Ma cooling and exhumation rates reported in many cases are underestimates as they simply average rates from more than one event over a time which is longer than the cumulative time of the individual events . The aim of this contribution is to try to improve the resolution of the thermal histories during the Miocene and post-Miocene period from the central and northern Eastern Cordillera of Ecuador using the (UTh) /He thermochronological rnethod, which can provide precise quantitative information about the thermal histories of rocks at temperatures lower than ~ 90°C. An enhanced spatial knowledge of the timing and rates of cooling during the Miocene and later in the Ecuadorian Andes is useful for identifying and quantifying the driving forces responsible for the development of the contemporaneous Andean Orogen , including the prominent Interandean Depression in the Northern Andes.
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