Late Miocene rejuvenation of central Idaho landscape evolution: A case for surface processes driven by plume-lithosphere interaction

Abstract

We utilized field measurements of erosion rates and topographic analyses to constrain the timing and magnitude of landscape rejuvenation on the western flank of the Rocky Mountains in central Idaho, United States. Deeply incised canyons of the Clearwater, Salmon, and Snake Rivers dissect a broad region of roughly 8 × 10 4 km 2 . Along the Salmon River, an observable break in slope separates relict landscapes of low relief (<400 m valley depth) from high-relief landscapes (1200-1600 m valley depth) adjusting to base-level forcing. The 10 Be cosmogenic radionuclide concentrations in river sediment record basinwide erosion rates that increase from 0.05 mm/yr ± 0.008 in the low-relief topography to 0.12 mm/yr ± 0.016 in the adjusting, high-relief landscapes over the last 10 3 -10 4 yr and are consistent with longer-term estimates of erosion. Using the covariance of erosion rates and channel morphology, we calibrated a 1-dimensional river incision model to constrain the dynamics of incision along the Salmon River. More than 10 5 model runs explored uncertainty and assumptions and found that increased incision initiated roughly 9.5 ± 2 Ma and persists to the present. New constraints on the distribution of erosion processes at locations within a 400 km transect across central Idaho suggest northward surface tilting. In light of these data, we offer a new hypothesis that attributes late Miocene landscape rejuvenation of central Idaho to surface uplift driven by density changes in the mantle-lithosphere precipitated by the Yellowstone plume. We demonstrated the hypothesis through a simple model of flexure of an elastic plate subject to a buried buoyant load, and we found that density changes extending 200 km north of the Snake River Plain can reproduce the south-north distribution of uplift with reasonable values for elastic thickness and anomalous density.

Spatially Variable Increase in Rock Uplift in the Northern U.S. Cordillera Recorded in the Distribution of River Knickpoints and Incision Depths

“…Beranek et al () show this capture occurring near the confluence between the Salmon and North Fork Salmon Rivers sometime between 4 and 2 Ma (Figures and in Beranek et al, ). Based on the sharp westward turn in the Salmon River near this confluence (NF Salmon in Figure a), Anderson () similarly suggested that this sharp turn was caused by drainage capture “by headward erosion of a vigorous stream from the west.” Although Anderson's hypothesis is qualitative, it is possible this western stream's vigor was caused by the rock‐uplift variations proposed by Larimer et al (). Much like the drainage capture scenarios we consider (Figure ), drainage capture along the upper Salmon River would cause incision that decreases downstream.…”

“…An increase in rock uplift could be achieved through a range of alternative drivers, however. One possibility is that flexural isostasy is ongoing due to the region's suspected lithospheric loss (Camp & Hanan, ; Darold & Humphreys, ; Hales et al, ), as proposed by Larimer et al (). These authors also modeled the flexure of an elastic plate subjected to a buoyant load to demonstrate that density changes extending 200 km north of the Snake River Plain could cause the proposed northward surface tilting.…”

“…This drainage capture increased the discharges of the lower Snake River (Link et al, 2014), and the Snake River's drainage area continued to grow as the North American plate moved W-SW over the Yellowstone plume (Pierce & Morgan, 1992). A recent study by Larimer et al (2018), however, found the erosion rates and morphologies of certain Salmon River tributaries to be inconsistent with this driver. These authors used in situ 10 Be concentrations of fluvial sediment to show that the erosion rates of adjusted and relict tributaries of the Salmon River near Burgdorf, ID (Figure 1a) differ by a factor of about 2.4.…”

“…Is incision caused by far-field base level fall (e.g., gradual decay of long-lived high topography), drainage reorganization, or recent changes in rock uplift? Based on previous work (Larimer et al, 2018;Vogl et al, 2014), we anticipate that both spatial patterns in rock uplift and variations in rock properties have the potential to influence knickpoint migration in this region. To help distinguish between these differing effects, we (1) present example numerical models demonstrating how knickpoint elevations depend on both rock properties influencing stream power model parameters (Whipple & Tucker, 1999) and spatiotemporal patterns in either rock uplift or base level fall and (2) highlight previous analytical work by Royden and Perron (2013) as a framework for understanding transient bedrock river behavior.…”

“…These findings also coincide with the threefold increase in exhumation rates around 11–8 Ma found through fission‐track data in the Boise Mountains (BM in Figure a; Sweetkind & Blackwell, ) and (U‐Th)/He dating in Pioneer‐Boulder Mountains (PBM in Figure a; Vogl et al, ). Larimer et al () further argued that this sustained increase in erosion may be driven by density changes in the mantle‐lithosphere facilitated by the Yellowstone plume. Indeed, Zhou and Liu () also argued for a geodynamic source of uplift in central Idaho, with dynamic uplift around 14–11 Ma causing an ~800‐m increase in elevation by the present.…”