5.2 Implications for the timing of Carnegie Ridge subduction
The subduction of bathymetric highs has been suggested to trigger
widespread deformation and uplift in non-collisional orogens (e.g.,
Lallemand et al., 1992; von Huene et al., 1997; Dominguez et al., 1998;
Pubellier et al., 1999; Ramos, 2005; Martinod et al., 2013; Spikings and
Simpson, 2014; Georgieva et al., 2016) and island arcs (Taylor et al.,
1987; Mann et al., 1998; Rosenbaum and Mo, 2011; Freisleben et al.,
2021). In addition, the subduction of oceanic aseismic ridges also
appears to cause changes in the geochemical composition of magmatism
with a shift to a more juvenile isotope signal (Chiaradia et al., 2020).
Our thermochronological data from the Western Cordillera in Ecuador
clearly document the onset of a regional uplift and exhumation phase
that started at ~6-5 Ma (Fig. 4). Our data suggest that
the Ponce Enríquez and Montalvo reverse faults are major structures
controlling rock uplift and exhumation in the Western Cordillera.
Importantly, these faults appear to be associated with transpression and
the extrusion of the North Andean Sliver along the dextral
Puna-Pallatanga-Cosanga strike-slip fault. Although the North Andean
Sliver may have initiated as early as 15 Ma due to oblique subduction
(e.g., Alvarado et al., 2016), the acceleration of northward motion of
this block suggests increased coupling at the plate interface after the
interception of the Carnegie Ridge (Witt et al., 2006). Importantly,
earlier cooling and exhumation phases identified in the Eastern
Cordillera and the Coastal Cordillera between 15 and 7 Ma (Spikings et
al., 2005, 2010; Brichau et al., 2021) that have been associated with
subduction of the Carnegie Ridge (i.e., Spikings et al., 2010), were
synchronous with deformation and uplift recorded at 17-15 Ma, 13 Ma, and
9-8 Ma in the sub-Andes of northern Peru (e.g., Mégard, 1984; Sébrier et
al., 1988; Moreno et al., 2020). These earlier cooling phases, however,
are not recorded in the Western Cordillera, suggesting that earlier
uplift and exhumation in other parts of Ecuador were independent from
aseismic ridge subduction processes.
Initial Carnegie Ridge subduction at ~6-5 Ma is
consistent with geodynamic reconstructions and with the results of
studies of the offshore morphology and subsidence history of the Gulf of
Guayaquil that propose an onset of ridge subduction at 5 Ma and document
ongoing ridge subduction by ~1.8 Ma (e.g., Witt et al.,
2006; Collot et al., 2019). Onset of ridge subduction at
~6-5 Ma is also compatible with the timing of regional
compressional tectonic inversion of the forearc (Daly, 1989). Moreover,
an eastward shift of volcanism at 6-5 Ma in northern Ecuador (Barberi et
al., 1988) and a subsequent change in magma chemistry also support the
notion of onset of Carnegie Ridge subduction at this time (e.g., Bourdon
et al., 2003; Samadiego et al., 2010).
Taken together, our thermochronological and geochronological data
suggest that the subduction of the Carnegie Ridge triggered shortening
and exhumation in the Ecuadorian Andes starting at ~6-5
Ma (Fig. 5). However, a delay between the onset of ridge subduction and
exhumation processes in the high Andes is possible. Thus, we cannot
exclude an onset of Carnegie Ridge subduction at 9-8 Ma as previously
suggested by Daly (1989) and Gutscher et al. (1999) based on plate
kinematic reconstruction. Finally, Hafnium isotopic compositions of
zircons in Ecuador become more juvenile at 8-6 Ma (George et al., 2020)
and could also reflect the arrival of the Carnegie Ridge at the trench
and ensuing changes in volcanic arc geochemistry at that time.