Quantifying canyon incision and Andean Plateau surface uplift, southwest Peru: A thermochronometer and numerical modeling approach

Abstract

[1] Apatite and zircon (U-Th)/He ages from Ocoña canyon at the western margin of the Central Andean plateau record rock cooling histories induced by a major phase of canyon incision. We quantify the timing and magnitude of incision by integrating previously published ages from the valley bottom with 19 new sample ages from four valley wall transects. Interpretation of the incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to subduction and volcanism. Furthermore, the large magnitude of incision leads to additional threedimensional variations in the thermal field. We address these complications with finite element thermal and thermochronometer age prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages. Comparison of 275 model simulations to observed cooling ages and regional heat flow determinations identify a best fit history with 0.2 km of incision in the forearc region prior to $14 Ma and up to 3.0 km of incision starting between 7 and 11 Ma. Incision starting at 7 Ma requires incision to end by $5.5 to 6 Ma. However, a 2.2 Ma age on a volcanic flow on the current valley floor and 5 Ma gravels on the uplifted piedmont surface together suggest that incision ended during the time span between 2.2 and 5 Ma. These additional constraints for incision end time lead to a range of best fit incision onset times between 8 and 11 Ma, which must coincide with or postdate surface uplift.

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

“…In part, due to lower local relief, sampling at very close horizontal distances while maintaining meaningful elevation differences is not possible in the Calama area where the horizontal VT extent is ~23 km, compared to ~5 km in Putre. Schildgen et al (2007, 2009) accounted for long‐wavelength lateral changes in the closure temperature isotherms in southern Peru by using depth below a paleosurface instead of elevation, but estimating an accurate paleosurface for samples outside of a clearly incised canyon is unrealistic in our case. With the available thermochronometer data, and the lack of a paleosurface, we are unable to conduct a meaningful 2‐D or 3‐D thermal model of the region to correct for lateral variations in the closure isotherm depth and therefore resort to calculating exhumation rates based on a 1‐D modeling approach (see below).…”

“…Present‐day precipitation rates are recorded as low as <1 mm/yr in the coastal cities of northern Chile (e.g., Schulz et al, 2012). The implication of arid climate in Andean mountain building mechanisms is critical as it is associated with the absence of significant and widespread erosion, and removal of surface material can compensate for tectonically induced rock uplift, weaken the thermal integrity of the crust, or initiate an isostatic response (Gregory‐Wodzicki, 2000; Jordan et al, 2010; Schildgen et al, 2009; Zeitler et al, 2001). Additionally, the lack of sediment supply to lubricate subduction in the trench has been suggested to act as a roughening mechanism, enhancing the transfer of compressional forces from the subducting plate to the overlying crust (Lamb & Davis, 2003).…”

“…Though as of late, 2‐D and 3‐D models have become fairly commonplace in evaluation of such data sets (e.g., Schildgen et al, 2009), our 1‐D approach is useful for isolating our parameter of interest, exhumation, without the need for simulating individual faults or assuming steady state topography. Our justification for using a 1‐D modeling approach comes from two factors.…”

“…In the fore arc of the Central Andes, ~15°–25°S, the significance of thermochronometric data has been subject to various interpretations; mineral cooling ages have been used to track the timing, depth, and spatial shifts of pluton emplacement (Andriessen & Reutter, 1994; McInnes et al, 1999), rock uplift induced by subduction of oceanic ridges (Juez‐Larré et al, 2010; Wipf et al, 2008), regional erosion and exhumation (Maksaev & Zentilli, 1999; Schildgen et al, 2009), and changes in the thermal structure (Juez‐Larré et al, 2010). While rock cooling rates are intrinsically tied to the rate of erosion, as well as rock and surface uplift, the comparison of these values is highly sensitive to changes in geothermal parameters and whether an erosion‐uplift system is in an exhumational equilibrium state (England & Molnar, 1990; Ring et al, 1999; Willett & Brandon, 2002).…”

“…While directly equating cooling rates with erosion and uplift may be specious, with thoughtful sampling and consideration of thermal properties in a kinematic model, mineral cooling ages can track, to a first order, the rates of rock exhumation and thus the erosion history of a region. Previous thermochronometric studies quantifying exhumation in the Central Andean fore arc have often focused on canyon incision rates as proxies for timing of surface uplift (Cooper et al, 2016; Hoke et al, 2007; Schildgen et al, 2007; Schildgen et al, 2009). Rapid and sustained river incisions have been suggested as mechanisms for inciting tectonic deformation.…”