A COMPARISON OF MECHANICAL THICKNESS ESTIMATES FROM TROUGH AND SEAMOUNT LOADING IN THE SOUTHEASTERN GULF OF ALASKA

The southern portion of the Kodiak-Bowie seamount chain in the southeastern Gulf of Alaska presents a unique opportunity to investigate loading on young oceanic lithosphere. Oceanic lithosphere younger than 25 Ma is loaded both by sediments in a deep offshore trough and by seamounts 100-200 km offshore. Free-air gravity anomaly values associated with the trough range from -90 mGal over the trough to +25 mGal over the offshore flexural high, whereas values associated with the volcanic loading range from +80 mGal over the seamounts to -10 mGal over their associated moats. These anomalies are modeled first using elastic beam theory. Gravity anomalies associated with the trough are modeled in terms of elastic bending of a sediment filled trench. Elastic thicknesses associated with the trough range from 12 to 22 km with an estimated uncertainty of +/-5 km. Seamounts are approximated as a series of stacked finite cylinders. Gravity highs over the seamounts are used to model density, while gravity patterns associated with the moat and peripheral bulge are used to model the elastic thickness along the seamount chain. Elastic thicknesses along the seamount chain range from 2 to 5 km (estimated uncertainty range 0 to 7 and 0 to 10 km, respectively). To investigate differences in elastic thickness estimates between the trough and seamount models, we incorporate the effects of finite yield strength. Curvatures implied by the elastic beam models are used to calculate mechanical bending stresses. Bending moments calculated from elastic beam models and yield envelope models agree for flexure models at the Queen Charlotte Trough and most seamounts. The mechanical thickness corresponds to the depth to the 700-degrees-C isotherm assuming a dry olivine rheology and a simple cooling model for oceanic lithosphere. Our results imply that the strength of the lithosphere calculated for both seamount and trench loading corresponds to the same isotherm.