Samuel G. Roy

The influence of crustal strength fields on the patterns and rates of fluvial incision

Gradients in the bedrock strength field are increasingly recognized as integral to the rates and patterns of landscape evolution. To explore this influence, we incorporate data from fault strength profiles into a landscape evolution model, under the assumption that erodibility of rock is proportional to the inverse square root of cohesion for bedrock rivers incised by bedload abrasion. Our model calculations illustrate how patterns in the crustal strength field can play a dominant role in local fluvial erosion rates and consequently the development of fluvial network patterns. Fluvial incision within weak zones can be orders of magnitude faster than that for resistant bedrock. The large difference in erosion rate leads to the formation of a straight, high-order channel with short, orthogonal tributaries of low order. In comparison, channels incising into homogeneous strength fields produce dendritic drainage patterns with no directional dependence associated with erodibility gradients. Channels that cross the strength gradient experience local variations in knickpoint migration rate and the development of stationary knickpoints. Structurally confined channels can shift laterally if they incise into weak zones with a shallow dip angle, and this effect is strongly dependent on the magnitude of the strength difference, the dip angle, and the symmetry and thickness of the weak zone. The influence of the strength field on drainage network patterns becomes less apparent for erodibility gradients that approach homogeneity. There are multiple natural examples with drainage network patterns similar to those seen in our numerical experiments.

Dynamic links among rock damage, erosion, and strain during orogenesis

We provide model evidence for a previously unexplored positive feedback between tectonic strain and fluvial erosion by considering rock erodibility as a function of shear damage. Plastic shear strain permanently damages the upper crust within planar shear zones, providing a greater ease for detachment and transport by fluvial processes. The subsequent rapid erosion of exposed shear zones reforms the topographic stress field in a way that encourages continued accommodation of strain, a positive feedback response that becomes more prominent with greater shear damage. Based on model results and natural examples, rock strength heterogeneity plays a major role in the evolution of drainage network patterns and the capability for valley-scale strain localization in active orogens.

Multi-scale characterization of topographic anisotropy

We present the every-direction variogram analysis (EVA) method for quantifying orientation and scale dependence of topographic anisotropy to aid in differentiation of the fluvial and tectonic contributions to surface evolution. Using multi-directional variogram statistics to track the spatial persistence of elevation values across a landscape, we calculate anisotropy as a multiscale, direction-sensitive variance in elevation between two points on a surface. Tectonically derived topographic anisotropy is associated with the three-dimensional kinematic field, which contributes (1) differential surface displacement and (2) crustal weakening along fault structures, both of which amplify processes of surface erosion. Based on our analysis, tectonic displacements dominate the topographic field at the orogenic scale, while a combination of the local displacement and strength fields are well represented at the ridge and valley scale. Drainage network patterns tend to reflect the geometry of underlying active or inactive tectonic structures due to the rapid erosion of faults and differential uplift associated with fault motion. Regions that have uniform environmental conditions and have been largely devoid of tectonic strain, such as passive coastal margins, have predominantly isotropic topography with typically dendritic drainage network patterns. Isolated features, such as stratovolcanoes, are nearly isotropic at their peaks but exhibit a concentric pattern of anisotropy along their flanks. The methods we provide can be used to successfully infer the settings of past or present tectonic regimes, and can be particularly useful in predicting the location and orientation of structural features that would otherwise be impossible to elude interpretation in the field. Though we limit the scope of this paper to elevation, EVA can be used to quantify the anisotropy of any spatially variable property. We quantify topographic anisotropy using multidirectional multiscale variograms maps.Our method takes advantage of GPU acceleration through parallel CUDA code.Spatial anisotropy signals reflect distinct tectonic, climatic, erosional conditions.

Strain-rate estimates for crevasse formation at an alpine ice divide: Mount Hunter, Alaska

Abstract Crevasse initiation is linked to strain rates that range over three orders of magnitude (0.001 and 0.163 a-1) as a result of the temperature-dependent nonlinear rheological properties of ice and from water and debris inclusions. Here we discuss a small cold glacier that contains buried crevasses at and near an ice divide. Surface-conformable stratigraphy, the glacier’s small size, and cold temperatures argue for limited rheological variability at this site. Surface ice-flow velocities of (1.2-15.5) ± 0.472 m a- 1 imply classic saddle flow surrounding the ice divide. Numerical models that incorporate field-observed boundary conditions suggest extensional strain rates of 0.003-0.015 a- 1 , which fall within the published estimates required for crevasse initiation. The occurrence of one crevasse beginning at 50 m depth that appears to penetrate close to the bed suggests that it formed at depth. Field data and numerical models indicate that a higher interior stress at this crevasse location may be associated with steep convex bed topography; however, the dynamics that caused its formation are not entirely clear.

Industrial-age doubling of snow accumulation in the Alaska Range linked to tropical ocean warming

Future precipitation changes in a warming climate depend regionally upon the response of natural climate modes to anthropogenic forcing. North Pacific hydroclimate is dominated by the Aleutian Low, a semi-permanent wintertime feature characterized by frequent low-pressure conditions that is influenced by tropical Pacific Ocean temperatures through the Pacific-North American (PNA) teleconnection pattern. Instrumental records show a recent increase in coastal Alaskan precipitation and Aleutian Low intensification, but are of insufficient length to accurately assess low frequency trends and forcing mechanisms. Here we present a 1200-year seasonally- to annually-resolved ice core record of snow accumulation from Mt. Hunter in the Alaska Range developed using annual layer counting and four ice-flow thinning models. Under a wide range of glacier flow conditions and layer counting uncertainty, our record shows a doubling of precipitation since ~1840 CE, with recent values exceeding the variability observed over the past millennium. The precipitation increase is nearly synchronous with the warming of western tropical Pacific and Indian Ocean sea surface temperatures. While regional 20th Century warming may account for a portion of the observed precipitation increase on Mt. Hunter, the magnitude and seasonality of the precipitation change indicate a long-term strengthening of the Aleutian Low.

Rock failure and erosion of a fault damage zone as a function of rock properties: Alpine Fault at Waikukupa River

ABSTRACT Erosion rates in the hanging wall of the Alpine Fault are high, keeping pace with rock uplift over time frames of 104–106 years. On shorter time frames, prediction of temporal and spatial distribution of erosion is challenging and must account for local conditions and parameters including rock strength, topographic stresses and failure conditions. Constrained by field observations of rock strength, we use 3D mechanical models to predict where and by what mechanism slope failure and erosion are likely to take place along the Waikukupa section of the Alpine Fault. Shear failure is favoured along the base of slopes and where pore pressure is high. Tensile failure is favoured along ridges, higher on slopes and when pore pressure is moderate. A dry material with a high degree of rock strength heterogeneity promotes bedrock gully development, whereas distributed failure is more likely to occur when the material is saturated.

Topographic controlled forcing of salt flow: Three‐dimensional models of an active salt system, Canyonlands, Utah

The Grabens within Canyonlands, Utah is an active salt system primarily driven by differential unloading due to incision of the Colorado River. However, many other conditions exist in the region that potentially influence regional deformation, including the gentle dip of the evaporite deposits, unconfined salt within the river canyon, weaknesses in the overburden, and topographic gradients on various scales. Three-dimensional numerical models were built to test the scale at which salt responds to these parameters individually and as a whole. Topography has a large influence on salt flow on both a regional and local scales and predicts the formation of existing structures in the region on consistent spatial scales without the influence of overburden weakening or salt geometry. Topography also has a large influence on the direction of salt flow, which acts to divert salt away from the canyon at the edge of the Grabens and enhance salt flow within the Grabens. Unconfined salt within the canyon regionally alters displacement rates and patterns, indicating a clear shift in strain before and after incision of the river into the Paradox formation. On a local scale, there is a strong coupling between overburden weakening and salt flow patterns, where salt responds to individual structures and less to regional drivers. All these driving forces create an ensemble of feedbacks that alters the strain field and structural development through time.