Glacial isostatic adjustment and relative sea level change over the last earthquake cycle in upper Cook Inlet, Alaska, USA

Abstract

Using observations of land surface deformation, sea level change and geophysical modelling, this thesis considers the interactions of ice mass fluctuations and tectonic deformation over the last great earthquake cycle in south central Alaska.

Reconstructions of relative sea level change over the last 900 years, based upon extensive lithological, biostratigraphical and chronological investigations of salt marsh sequences in upper Cook Inlet, Alaska, record changes in marine influence and the direction of sea level that do not fit the expected interseismic model of land level movements. Dating of the sequences suggests the changes in RSL occurred sometime through c. AD 1600 – 1900, during the middle and late phases of Little Ice Age ice mass balance changes. The chronological methods used comprise a multi-method approach: 210Pb, 137Cs, stable lead (206Pb/207Pb) ratios, pollutants associated with the history of regional gold mining and development, tephrochronology and AMS 14C wiggle match dating. The research highlights some of the limitations of applying some of these dating methodologies to recent, high latitude, salt marshes.

GIA modelling identifies part of the GPS measured present day uplift in upper Cook Inlet as attributable to post Little Ice Age (AD 1200 – 1900) glacial isostatic adjustment, with a spatial signal over tens of kilometres. A set of viable Earth models, constrained by GPS data and the pattern of post-seismic displacement quantifies the relative displacement and deformation of the ocean geoid at a series of locations in south central Alaska over the past 1000 years. Modelling results show the asthenosphere viscosity and thickness to be the main rheological controls on relative displacement during the last earthquake deformation cycle.

Integration of the geological data and geophysical model results show RSL in upper Cook Inlet during the last earthquake deformation cycle is a combination of tectonic land-level changes, ‘local’ processes, glacial isostatic adjustment and deformation of the ocean geoid. To fully quantify the relative contribution of each mechanism requires improvements in the methods of RSL reconstruction, dating of recent salt marsh sediments and GIA modelling.