[1] We use data from campaign and continuous GPS sites in southeast Alaska and the neighboring region of Canada to constrain a regional tectonic block model that estimates block angular velocities and derives a self-consistent set of fault slip rates from the block motions. Present-day tectonics in southeast Alaska is strongly influenced by the collision of the Yakutat block. Our model predicts a velocity of 50.3 ± 0.8 mm/a toward N22.9 ± 0.6°W for that block. Our results suggest that the eastern edge of the Yakutat block is deforming. Along this edge, the Fairweather fault accommodates a large portion of the Pacific-North America relative plate motion through 42.9 ± 0.9 mm/a of dextral slip. Further south along the Queen Charlotte fault, our model predicts an average of 43.9 ± 0.6 mm/a of dextral slip and a southward increasing amount of transpression. Strain from the Yakutat collision is transferred far to the east of the strike-slip system. This strain transfer causes the region north of Glacier Bay to undergo a clockwise rotation. South of Glacier Bay and inboard of the Queen Charlotte fault, a smaller but clearly defined clockwise rotation is observed. The heterogeneous block motion north and south of Glacier Bay may indicate the area is undergoing internal deformation and could explain regional patterns of diffuse seismicity. The Northern Cordillera of Canada displays a small northeasterly motion. Our block model suggests that the entire southeastern Alaskanorthwestern Canada margin is mobile.

We present a comprehensive average velocity field for Alaska, based on repeated GPS surveys covering the period 992-2007, and review the major results of previously published papers that used subsets of this data. The spatially and temporally complex pattern of crustal deformation in Alaska results from the superposition of several processes, including postseismic deformation after the 1964 earthquake, spatial variations in plate coupling/slip deficit, translation and rotation of large crustal blocks or plates, and a large slow-slip event in Cook Inlet. Postseismic deformation from the 964 earthquake continues today, mainly caused by viscoelastic relaxation, and causes trenchward motion. The behavior of the shallow seismogenic zone along the Alaska-Aleutian megathrust is characterized by dramatic along-strike variability. The width of the inferred seismogenic zone varies over along-strike distances that are short compared to the width. The alongstrike distribution of locked and creeping regions along the megathrust is consistent with the persistent asperity hypothesis. A large slow-slip event occurred in upper Cook Inlet in 998-200, and a smaller event in the same area in 2005-2006. No sign of slow-slip events has been found in segments that are dominated by creep, which suggests that creep there occurs quasi-statically. The overriding plate in Alaska is subject to considerable internal deformation, and can be described in terms of the independent motions of at least four blocks: the Bering plate, the Southern Alaska block, the Yakutat block, and the Fairweather block.

[1] We use data from campaign and continuous GPS sites in southeast and south central Alaska to constrain a regional tectonic block model for the St. Elias orogen. Active tectonic deformation in the orogen is dominated by the effects of the collision of the Yakutat block with southern Alaska. Our results indicate that~37 mm/yr of convergence is accommodated along a relatively narrow belt of N-NW dipping thrust faults in the eastern half of the orogen, with the present-day deformation front running through Icy Bay and beneath the Malaspina Glacier. Near the Bering Glacier, the collisional thrust fault regime transitions into a broad, northwest dipping décollement as the Yakutat block basement begins to subduct beneath the counterclockwise rotating Elias block. The location of this transition aligns with the Gulf of Alaska shear zone, implying that the Pacific plate is fragmenting in response to the Yakutat collision. Our model indicates that the Bering Glacier region is undergoing internal deformation and could correspond to the final stage of offscraping and accretion of sediments from the Yakutat block prior to subduction. Predicted block motions at the western edge of the orogen suggest that the crust is laterally escaping along the Aleutian fore arc.

We present an updated GPS velocity field for Alaska and western Canada and use it to develop the first regionally comprehensive tectonic block model for the area based on modern geodetic data. The greatest tectonic influences along the southern margin are the translation, collision, and flat slab subduction of the Yakutat block and subduction of the Pacific plate. Northward directed velocities surrounding the Yakutat collisional front are consistent with indenter‐related deformation while southcentral Alaska is undergoing a counterclockwise rotation. Westerly velocities in western Alaska and along the Aleutian forearc suggest that crustal material is escaping into the Bering Sea region. The majority of relative plate motion is taken up along major boundary faults, but right‐lateral strike‐slip faults in interior and western Alaska accommodate part of the motion. Escape tectonics in western Alaska extends as far north as the Kaltag fault. We observe significant motion relative to North America in every part of Alaska, including the North Slope. Evidence of localized right‐lateral shear between the Totschunda and Fairweather faults suggests that strain transfer into interior Alaska has moved away from the eastern Denali‐Chatham Strait system to a more direct corridor. Observed deformation in southcentral Alaska indicates that locked portions of the Yakutat flat slab extend further east and north than previously estimated.

Investigations of tectonic and surface processes have shown a clear relationship between climate‐influenced erosion and long‐term exhumation of rocks. Numerical models suggest that most orogens are in a transient state, but observational evidence of a spatial shift in mountain building processes due to tectonic‐climate interaction is missing. New thermochronology data synthesized with geophysical and surface process data elucidate the evolving interplay of erosion and tectonics of the colliding Yakutat microplate with North America. Focused deformation and rock exhumation occurred in the apex of the colliding plate corner from > 4 to 2 Ma and shifted southward after the 2.6 Ma climate change. The present exhumation maximum coincides with the largest modern shortening rates, highest concentration of seismicity, and the greatest erosive potential. We infer that the high sedimentation caused rheological modification and the emergence of the southern St. Elias, intercepting orographic precipitation and shifting focused erosion and exhumation to the south.

The Yakutat-St. Elias collision in SE Alaska and adjacent Canada represents a prime example of present-day tectonics associated with an indentor corner. Its eastern syntaxis is marked by high exhumation, a sharp structural bend, and strain concentration at the transition from shortening to oblique transpression. Here we present GPS velocity and strain rate fields that cover the syntaxis, including 11 new stations in the core of the St. Elias Mountains. These data are corrected for transient deformation (glacial isostatic adjustment and postseismic and interseismic loading) to produce residual velocities and strain rates representative of long-term tectonics. The main features of these velocity and strain rate fields are a peak in strain rates (strain knot) in the syntaxis at the junction between the main fault systems and a rapid rotation from convergence-parallel to convergence-normal orientations of the velocities and shortening axes around the syntaxis, leading to shortening across the southern Denali Fault. These features are consistent with the strain and tectonic patterns expected near an indentor corner at the transition from shortening to transpression, with a combination of diffuse and localized deformation. The GPS velocities and strain rates show diffuse deformation around the syntaxis, from pure convergence-parallel shortening in the orogenic wedge to oblique extension that accommodates the strain rotation at the front of the syntaxis. This indentor-corner model also results in a near-zero strike-slip rate on the southern Denali Fault and shows no clear evidence for a throughgoing fault hypothesized to link the Fairweather and Totschunda Faults.

No abstract

Biodiversity is believed to be closely related to ecosystem functions. However, the ability of existing biodiversity measures, such as species richness and phylogenetic diversity, to predict ecosystem functions remains elusive. Here, we propose a new vector of diversity metrics, structural diversity, which directly incorporates niche space in measuring ecosystem structure. We hypothesize that structural diversity will provide better predictive ability of key ecosystem functions than traditional biodiversity measures. Using the new lidar-derived canopy structural diversity metrics on 19 National Ecological Observation Network forested sites across the USA, we show that structural diversity is a better predictor of key ecosystem functions, such as productivity, energy, and nutrient dynamics than existing biodiversity measures (i.e. species richness and phylogenetic diversity). Similar to existing biodiversity measures, we found that the relationships between structural diversity and ecosystem functions are sensitive to environmental context. Our study indicates that structural diversity may be as good or a better predictor of ecosystem functions than species richness and phylogenetic diversity.