Rapid Uplift Of Southern Alaska Caused By Recent Ice Loss

Abstract

Changing surface loads, such as melting glaciers, can induce deformation of the Earth's crust. The speed of the Earth's response to load changes and the pattern of deformation they cause can be used to infer material properties of the lithosphere and mantle. Rapid uplift of southern Alaska has been measured with tide gauges, Global Positioning System (GPS) measurements and studies of raised shorelines. With multiple sites uplifting at rates in excess of 25 mm/yr, these measurements reveal the world's fastest regional uplift. Southern Alaska has over 75000 km2 of glaciers, the rapid melting of which is contributing more to global sea level rise than Greenland. Southern Alaska also has intense tectonic activity, and uplift driven by tectonics has been suggested to be comparable with that driven by glacial unloading. The majority of the uplift measurements examined here are located along the strike-slip portion of the Pacific - North America plate boundary. GPS measurements show little compressional strain associated with tectonic forcing. Tide gauges indicate long term linear uplift rates within the strike-slip regime, contrasting with tectonically influenced non-linear uplift to the northwest, where the Pacific Plate subducts beneath North America. Dating of raised shorelines within southeast Alaska show that the rapid uplift there began simultaneously with glacial unloading ~1790 AD. These observations indicate that the tectonic contribution to the uplift in southeast Alaska is small. Multiple independent studies are used here to constrain the load changes in southern Alaska over the past ~1000--2000 yrs. A detailed model of the advance, standstill and retreat phases of the Little Ice Age glaciation is used as input to a simple viscoelastic Earth model. This model can match the pattern and magnitude of the region's uplift observations with a low degree of misfit, verifying that the region's uplift can be entirely attributed to glacial isostatic rebound. Furthermore, the uplift observations require at the 95% confidence level a three-layer Earth model consisting of a <math> <f> 50⁺³⁰<inf>-25</inf></f> </math> km thick elastic lithosphere, an asthenosphere with viscosity eta A = (1.4 +/- 0.3) x 1019 Pa s and thickness <math> <f> 110⁺²⁰<inf>-15</inf></f> </math> km, overlaying a viscous upper mantle half-space (etaum = 4 x 1020 Pa s).