Thermochronometric constraints on the timing and rates of late Miocene-Pliocene exhumation in flat-slab subduction zone of north-central Chile

Abstract:

The timing and rate of uplift and topographic evolution in many parts of the Andes is still under debate. One of these areas is the segment of flat-slab subduction of north-central Chile (27-32°S). Geomorphological studies along this segment define two levels of Oligo-Miocene low-relief surfaces that are incised and partially covered by gravel sequences. These surfaces are interpreted as pediments and suggest that two pulses of uplift and incision occurred during the Oligo-Miocene time; one during the Oligocene-early Miocene and another during the late Miocene-Pliocene. Existing data from apatite fission-track show exhumation during the Oligocene-early Miocene in the entire segment, however, only south of 29.5°S does a second pulse of exhumation occur in the late Miocene-Pliocene. Most low-temperature thermochronometers may be insensitive to low rates of exhumation in the more arid north, however the increased sensitivity of ⁴He/³He thermochronometry (70-40°C) has successfully been applied to unravel the denudation history and landscape evolution in arid mountain areas. This work provides low-temperature thermochronometry data (including ⁴He/³He thermochronometry) indicating a late Miocene-Pliocene pulse of exhumation at ~29°S. Calculated exhumation rates are very slow prior to 15-10 My, ~ 20 m/My, then increase to ~ 50 m/My from ~10 My to present. These denudation rates are in agreement with those calculated from cut and fill volumes between pediments that were sealed by ash layers of known ages. These new thermochronometric data suggest that the topographic evolution of the northern segment of the studied region was not only controlled by the Oligocene – early Miocene deformation, but also by a late Miocene period of uplift and valley incision. This late Miocene deformation/exhumation may be a consequence of the initiation of flat-slab subduction, leading to a decrease of convergence rates and underthrusting of the mountain's core.