Slow Long‐Term Exhumation of the West Central Andean Plate Boundary, Chile

Abstract

We present a regional analysis of new low‐temperature thermochronometer ages from the Central Andean fore arc to provide insights into the exhumation history of the western Andean margin. To derive exhumation rates over 10 million‐year timescales, 38 new apatite and zircon (U‐Th)/He ages were analyzed along six ~500‐km long near‐equal‐elevation, coast parallel, transects in the Coastal Cordillera (CC) and higher‐elevation Precordillera (PC) of the northern Chilean Andes between latitudes 18.5°S and 22.5°S. These transects were augmented with age‐elevation profiles where possible. Results are synthesized with previously published thermochronometric data, corroborating a previously observed trenchward increase in cooling ages in Peru and northern Chile. One‐dimensional thermal‐kinematic modeling of all available multichronometer equal‐elevation samples reveals mean exhumation rates of <0.2 km/Myr since ~50 Ma in the PC and ~100 Ma in the CC. Regression of pseudovertical age‐elevation transects in the CC yields comparable rates of ~0.05 to ~0.12 km/Myr between ~40 and 80 Ma. Differences between the long‐term mean 1‐D rates and shorter‐term age‐elevation‐derived rates indicate low variability in the exhumation history. Modeling results suggest similar background exhumation rates in the CC and PC; younger ages in the PC are largely a function of increased heat flow and consequently an elevated geothermal gradient near the arc. Slow exhumation rates are suggestive of semiarid conditions across the region since at least the Eocene and deformation and development of the Andean fore arc around this time.

Effect of changing vegetation and precipitation on denudation – Part 2: Predicted landscape response to transient climate and vegetation cover over millennial to million-year timescales

“…We kept the rock uplift rate temporally and spatially uniform across the domain at 0.2 mm yr −1 (Table 1). Studies of the exhumation and rock uplift history of the Chilean Coastal Cordillera are sparse at the latitudes investigated here; however, existing and in progress studies further to the north are broadly consistent with the rock uplift rate used here (Juez-Larré et al, 2010;Avdievitch et al, 2018). The EarthShape focus sites are situated in similar granitic lithologies (Oeser et al, 2018), which allows for the assump- tion that the same critical shear stress, baseline diffusivity, and fluvial erodibility can be used.…”

“…The focus areas shown in Fig. 1a were chosen due to their similar granitic lithology and geologic and tectonic history (Andriessen and Reutter, 1994;McInnes et al, 1999;Juez-Larré et al, 2010;Maksaev and Zentilli, 1999;Avdievitch et al, 2018), in addition to the large gradient in climate and vegetation cover over the region (Fig. 1b, c).…”

“…In contrast, a positive step change in mean annual precipitation leads to an initial increase in fluvial shear stress, which initially causes headward incision of river channels and leads to a wave of erosion that propagates upstream and increases channel slope values (Fig. 8b, see also e.g., Bonnet and Crave, 2003). The increase in channel slopes leads to an increase in the hillslope diffusive flux adjacent to the channels that then propagates upslope.…”

“…The exceptions to this are Istanbulluoglu and Bras (2005) who also considered the lag time for vegetation regrowth on hillslopes after a mass wasting event and Yetemen et al (2015) who considered more complex hydrology in their models, but on a smaller spatial scale. However, numerous studies (Ledru et al, 1997;Allen and Breshears, 1998;Bachelet et al, 2003) document that vegetation cover changes in tandem with climate change over a range of timescales (decadal to million year). Missing from previous landscape evolution studies, is consideration of not only how transient vegetation cover influences catchment denudation, but also how coeval changes in precipitation influence catchment-wide mean denudation.…”

Response of a Rheologically Stratified Lithosphere to Subduction of an Indenter‐Shaped Plate: Insights Into Localized Exhumation at Orogen Syntaxes

“…(a) Predicted apatite fission track (AFT) ages for model 11 characterized by weakly coupled rheological structure and no upper plate advance contribution ( V Adv = 0%) in the total shortening and relatively narrow indenter width (150 km). (b–d) measured thermochronological cooling ages: (b) the Southeast Alaska (Arkle et al, ; Benowitz et al, , ; Ferguson et al, ; Haeussler et al, ; O'Sullivan et al, ; Plafker et al, ; Spotila et al, ); (c) the Olympic Mountains of the Cascadia subduction zone (Brandon et al, ; Currie & Grist, ; England et al, ; Johnson et al, ; Reiners et al, ); and (d) the South American subduction zone (Andriessen & Reutter, ; Avdievitch et al, ; Barnes et al, ; Gunnell et al, ; Juez‐Larré et al, ; Maksaev & Zentilli, ; McInnes et al, ; Noury et al, ; Schildgen et al, , ; Wipf, ; Wipf et al, ). White circles indicate the concentric zones of vertical advection (bull's‐eye pattern) in the modeled (a) and observed (b, c) AFT ages fields.…”