Syn- and post-accretionary cooling history of the Ecuadorian Andes constrained by their in-situ and detrital thermochronometric record

Abstract

New 40Ar/39Ar, apatite fission track and (U–Th)/He data from the late Cretaceous indenting and buttressing margins of Ecuador have been combined with previous thermochronological studies to constrain the timing of syn- and post-accretionary tectonic events in the Ecuadorian Andes to within ±1 Ma. Our interpretations are more accurate than previous hypotheses because i) they are more sensitive to lower temperatures (<60 °C), ii) we directly compare data obtained from in-situ and detrital rocks, and iii) they are constrained by recently published palaeomagnetic, stratigraphic and geochronological data. The response of the buttressing Ecuadorian margin to the collision of the Caribbean Plateau and its overlying arc was diachronous. Exhumation occurred as an immediate response to collision at ∼75 Ma, south of S1°30′, whereas the northern region started to exhume at ∼65 Ma, suggesting that accretion may have been oblique. Elevated cooling and exhumation rates within specific massifs dispersed along the entire length of the Ecuadorian cordilleras, during 43–30 and 25–18 Ma, are attributed to i) an increase in convergence rates between the Farallon and South American plates during 42–37 Ma, and an increase in spreading rates in the southern Atlantic ocean, and ii) a change in the vector of the subducting plate, which changed from ESE to E at 25 Ma in response to fragmentation of the Farallon Plate. Previous suggestions that Eocene reactivation of the buttressing margin were driven by collision of the Macuchi Arc are shown to be incorrect. 40Ar/39Ar, zircon fission track, and apatite fission track and (U–Th)/He analyses from the Eastern Cordillera north of S1°30′ reveal well defined periods of rapid cooling and exhumation at 15 Ma, 9–7 Ma and 5.5–0 Ma. Apatite (U–Th)/He data reveals late Miocene-Recent cooling and exhumation (≤1.3 km) of the southern Eastern Cordillera by a lower quantity than that experienced to the north (≥3.5 km). These distinguishable differences in cooling and exhumation are attributed to the collision of the Carnegie Ridge with the northern SOAM Plate at 15 Ma, and the subsequent subduction of high topography along the ridge at ∼5 Ma, which reactivated the serpentinised Campanian suture via dextral transcurrent displacement.