Rn. Harris et Ds. Chapman, A COMPARISON OF MECHANICAL THICKNESS ESTIMATES FROM TROUGH AND SEAMOUNT LOADING IN THE SOUTHEASTERN GULF OF ALASKA, J GEO R-SOL, 99(B5), 1994, pp. 9297-9317
The southern portion of the Kodiak-Bowie seamount chain in the southea
stern Gulf of Alaska presents a unique opportunity to investigate load
ing on young oceanic lithosphere. Oceanic lithosphere younger than 25
Ma is loaded both by sediments in a deep offshore trough and by seamou
nts 100-200 km offshore. Free-air gravity anomaly values associated wi
th the trough range from -90 mGal over the trough to +25 mGal over the
offshore flexural high, whereas values associated with the volcanic l
oading range from +80 mGal over the seamounts to -10 mGal over their a
ssociated moats. These anomalies are modeled first using elastic beam
theory. Gravity anomalies associated with the trough are modeled in te
rms of elastic bending of a sediment filled trench. Elastic thicknesse
s associated with the trough range from 12 to 22 km with an estimated
uncertainty of +/-5 km. Seamounts are approximated as a series of stac
ked finite cylinders. Gravity highs over the seamounts are used to mod
el density, while gravity patterns associated with the moat and periph
eral 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 c
alculate 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 thic
kness corresponds to the depth to the 700-degrees-C isotherm assuming
a dry olivine rheology and a simple cooling model for oceanic lithosph
ere. Our results imply that the strength of the lithosphere calculated
for both seamount and trench loading corresponds to the same isotherm
.