5.1 Late Miocene rock uplift and exhumation in the western Ecuadorian Andes
Thermochronological and geochronological data from the Western Cordillera indicate rapid cooling after the early Miocene crystallization of the intrusions (Fig. 4). Initial early Miocene rapid cooling likely corresponds to post-magmatic cooling via thermal relaxation of the intrusions (e.g., Murray et al., 2019), although coeval exhumation cannot be excluded. The most recent cooling histories documented by the Cuenca and the Apuela profiles are consistent and record an isothermal phase followed by a second cooling phase starting at ~6-5 Ma. The isothermal phase suggests that little exhumation occurred in the Western Cordillera between 15 and 6-5 Ma. The second cooling phase in the Western Cordillera is synchronous with the onset of the last cooling phase recorded in the Eastern Cordillera (starting at 5.5 Ma; Spikings and Crowhurst, 2004; Spikings et al., 2010), and with rapid cooling in the Coastal Cordillera between 6 and 5 Ma (Brichau et al., 2021). This late Miocene cooling in both cordilleras is also contemporaneous with the formation of alluvial-fan deposition in the basins to the west and east of the Andes (e.g., Alvarado et al., 2016), supporting the idea that this cooling phase was associated with erosional exhumation. The cooling rate and the geothermal gradient of 30°C/km derived from the modeling of our thermochronological data suggest exhumation rates of ~0.5 km/Myr for the last 6 Myr, with total exhumation of ~3 km achieved since 6 Ma.
In contrast to the simple two-stage cooling history of the Western Cordillera, previous thermochronological studies from the Eastern Cordillera and the Coastal Cordillera have suggested multiple phases of exhumation starting at 15, 9, and 6 Ma, and starting at 6 and 2 Ma, respectively (Spikings et al. 2010; Brichau et al., 2021). AFT data from the Interandean Cuenca Basin suggest that it experienced a cooling history similar to the Eastern Cordillera, including a major cooling event at 9 Ma accompanied by shortening (Steinmann et al., 1999). The AHe ages along the Garanda and Cuenca cross-sections are younger in the hanging wall of the reverse faults, whereas they are older in both the footwall and at high elevations in the hanging wall (Fig. 2B, C). This AHe age pattern suggests uplift of the hanging walls of the Montalvo and Ponce Enríquez reverse faults and internal deformation of the North Andean Sliver. Together with thermal histories obtained for vertical profiles (Fig. 4), the spatial age patterns suggest that shortening along these faults controlled uplift and exhumation in the Western Cordillera during the late Miocene synchronous with shear-zone reactivation in the Western Cordillera (Spikings et al., 2005), and deformation and exhumation along the Eastern Cordillera and the Subandes (Spikings and Crowhurst, 2004; Spikings et al., 2010).
Coltorti et al. (2000) suggested that an extensive low-relief surface in the Western Cordillera at 3500 m was at sea level during the early Pliocene (~5.3 Ma) but became dissected because of uplift in the middle and late Pliocene. In light of our new thermochronological data these observations suggest that cooling in the Western Cordillera involving a total of 3 km of exhumation must have been associated with rock and surface uplift starting at ~6 Ma. In southern Ecuador a low surface elevation in the Western Cordillera prior to 6 Ma is compatible with marine sediments in the inter-Andean basins deposited between 15 and 9 Ma (Steinmann et al., 1999; Hungerbühler et al., 2002). Finally, the combination of sedimentological and thermochronological data from the Western Cordillera and the Eastern Cordillera (i.e., Winkler et al., 2005) suggests limited structural uplift in the Western Cordillera prior to 6 Ma. Thus, the present-day topography of the Ecuadorian Andes, including two parallel, high-elevation mountain ranges separated by an intermontane depression, must have initiated at ~6 Ma.