Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska.

Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼ 2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8-1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼ 100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2-0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50-80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Analyzing the horizontal orientation of the crustal stress adjacent to plate boundaries.

The spatial analysis of horizontal stress orientation is important to study stress sources and understand tectonics and the deformation of the lithosphere. Additional to the stress sources, the geometry of stress fields depends on the underlying coordinate reference system, which causes spatial distortions that bias the analysis and interpretation of stresses. The bias can be avoided when the stress field is decomposed and transformed into the reference frame of its first-order stress source. We present a modified and extended theory based on the empirical link between the orientation of first-order stresses and the trajectories of lateral plate boundary forces. This link is applied to analyze the orientation of horizontal stresses, their patterns, and tectonic structures from the perspective of their first-order source or cause. By using only parameters for the relative motion between two neighboring plates, we model the first-order orientation of the maximum horizontal stress that statistically fits the orientation of [Formula: see text]80% of the global stress data adjacent to plate boundaries. Considerable deviations of the observed stress from the predicted first-order stress direction can reveal the geometry of second-order stresses and confine areas where other stress sources dominate. The model's simple assumptions, independence from the sample size, potential application to regional to global scale analysis, and compatibility with other spatial interpolation algorithms make it a powerful method for analyzing stress fields. For immediate use, the presented method is implemented in the free and open-source software package tectonicr, which is written in the computer language R.