43713-G8
The Effect of Late Cenozoic Glaciation on the Evolution of the Olympic Mountains
This project seeks to determine if the topographic evolution of the Olympic Mountains is controlled by patterns of surface erosion and thus climate. The alternative hypothesis is that tectonic forces determine the pattern of rock uplift. As these processes are inherently linked with feedback in both directions causality can be difficult to establish. There are therefore two end-member explanations for non-uniform rock uplift patterns: (1) uplift reflects tectonic processes independent of erosion rates such that high erosion rates simply respond to high tectonic uplift rates (passive erosion); and (2) erosion rates vary spatially as a function of climate and geomorphic process, and tectonic processes respond with a rock uplift rate that reflects the surface erosion rate (erosional forcing).
The Olympic Mountains of NW Washington State is an example of an accretionary wedge that does not exhibit spatially-uniform rock uplift rates. Zircon fission track dating (Brandon and Vance, 1992), apatite fission track dating (Brandon et al, 1998), and U-TH/He dating of apatite (Batt and Brandon, 2001) indicate that the Olympic Mountains are exhuming at near steady, but spatially variable, rates. In this project, I have attempted to address whether or not hypothesis two can explain the pattern of rock uplift observed via analytical and numerical modeling of surface processes in the range.
Part 1: Numerically coupled glacial and tectonic uplift model.
This project has sponsored the first coupled model (ICE couple) that links a dynamic model of crustal deformation with a fluvial/glacial erosion model. The deformation model is a two dimensional Finite Element model (based on PLASTI; see Willett, 1999) that treats the convergent wedge as having a plastic rheology. The surface model uniquely contains both fluvial and glacial processes, and uses a finite element scheme to solve for both erosion and ice flow over a two dimensional landscape (based on ICE Cascade; see Tomkin and Braun, 2002).
The initial results of this coupled model have been published in JGR- Earth Surface Processes (“Coupling glacial erosion and tectonics at active orogens:
A numerical modeling study” Tomkin, 2007). The study concluded that precipitation in fluvially dominated systems controls rock uplift as predicted by analytical models, that the amount of glacial erosion in linearly related to the area of glacial coverage, that glaciated orogens are more sensitive to the precipitation rate, that the so-called “glacial buzzsaw” may arise out of glacial sliding dominated erosion processes, spatially non-uniform rock uplift rates are predicted by glacial erosion models, but that glacial erosion does not necessarily concentrate erosion at the center of the range (as observed in the Olympics) – the spatial pattern of erosion in the Olympics depends on the temporal pattern of glacial coverage.
Part 2: Analytically coupled glacial and tectonic uplift model
In addition to producing the above numerical model, the project produced the first analytical model that couples glacial and critical wedge tectonics. This model was published in Earth and Planetary Science Letters (“Climate and tectonic controls on glaciated critical-taper orogens”, Tomkin and Roe, 2007). It was found that glacially dominated systems (such as the Olympics) are much more sensitive to climate change – both in terms of temperature and in the rate of precipitation – than equivalent fluvial orogens. This has significant consequences for orogen development: in most cases, orogen-width is predicted to be super-linearly related to the precipitation rate. Erosion patterns are predicted to be highest at, and below, the ELA, reinforcing the need for temporal as well as spatial ice reconstructions. We also made the first response time prediction for the Olympic Mountains: the range should respond to changes in climate with an e-folding time of approximately 1.5Myrs.
Part 3: Olympic Mountains Ice Reconstruction
Given the importance of ice coverage over testing the hypothesis, investigations were made in constraining the climate and glacial history of the range. LGM ice extent is not diagnostic on the eastern side of the range, as cordilleran ice occupied Puget Sound and pushed up-valley, but western LGM glacial extent can be used to test climate models. In conjunction with a linear precipitation model (Anders et al, 2005) that determines precipitation patterns, a climate sensitivity test was performed and tested against the observed moraine patterns. This has restricted LGM mean summertime sea-level temperatures to a narrow range (7-8 degrees C) but leaves some latitude for LGM precipitation (~25-75% of the modern). In this way we have produced the first dynamic reconstruction of glacial ice extent for the Olympic mountains (Tomkin, Hellwig and Anders, in prep.). Field work has also commenced, focusing on the weak constraints on intermediate glacial extents (such as during the Younger Dryas and Little Ice Age) and eastern moraines, with initial ground-truthing of moraine reconstructions and dating (including lichenometry).
