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.