4.2 Low-temperature thermochronology
We sampled along two structural cross-sections in the central (Guaranda area) and southern parts (Cuenca area) of the Western Cordillera (Fig. 2B, C), spanning an elevation range between 0.9 and 1.4 km. Each transect includes a vertical profile in the hanging wall of the principal reverse fault. We also sampled along a vertical profile in the northern part of the Western Cordillera (Apuela area).
We present 86 AHe single-grain ages for 22 samples, AFT data for 20 samples, and 26 ZHe single-grain ages for 9 samples (Fig. 3; Tables 2, 3, 4; Figs. S2-S7; Tables S3-S24). AHe ages range from 15.4 ± 0.3 to 2.3 ± 0.1 Ma, AFT ages range from 21.7 ± 1.6 to 8.1 ± 1.3 Ma; mean-track lengths are between 13 and 16 µm. For the same samples ZHe ages are younger than AFT ages and range from 23.6 ± 0.3 to 5.9 ± 0.1 Ma (Fig. 3).
Thermochronological ages from the Garanda cross-section are different either side of the Montalvo reverse fault, with mean AHe and ZHe ages being older in the footwall and younger in the hanging wall (Fig. 2B). AFT dates cluster at 20 Ma in the hanging wall and no AFT date is available in the footwall (Fig. 2B). Thermochronological ages from the Cuenca cross-section record a clear pattern with older AHe, AFT and ZHe ages for the samples located in the footwall of the Ponce Enríquez reverse fault and younger ages in the hanging wall (Fig. 2C).
Combined, the samples from the vertical profiles cover an elevation range from 950 to 3100 m (Fig. 3). Early Miocene AFT ages from the Apuela and Garanda areas are similar to the ages of intrusion (Fig. 3A, B). In the Cuenca area AFT ages cluster at ~11 Ma, and are largely invariant with elevation (Fig. 3C). AHe ages from the different sampling sites are consistent along the Western Cordillera and show an age-elevation relationship, with a trend toward younger ages at lower elevations (Fig. 3D, E, F). AHe ages from the Cuenca and Apuela area clearly show a positive age-elevation relationship (Fig. 3D, F), with ages ranging from 8.7 ± 0.1 Ma to 2.3 ± 0.1 Ma. AHe ages from the Garanda area show more intersample grain-age dispersion and do not reveal such a clear trend with elevation (Fig. 3E). Here, most samples are located at higher elevation, and AHe ages are dispersed around 12 Ma. ZHe ages increase with elevation at the scale of the Western Cordillera.