HR: 1330h
AN: S42A-0146    [PDF]
TI: Tectonic Surfing: Evidence of Rapid Landward Transport in the Cascadia Subduction Wedge, NW Washington State
AU: Stewart, R J
EM: rjstew@u.washington.edu
AF: University of Washington, Earth and Space Sciences, Seattle, WA 98195 United States
AU: * Brandon, M T
EM: mark.brandon@yale.edu
AF: Yale University, Kline Geology Laboratory, New Haven, CT 06520-8109 United States
AU: Campbell, K A
EM: ka.campbell@auckland.ac.nz
AF: University of Auckland, Department of Geology, Private Bag 92019, Auckland, 00000 New Zealand
AU: Nesbitt, E A
EM: lnesbitt@u.washington.edu
AF: University of Washington, Burke Museum of Natural History and Culture, Seattle, WA 98195 United States
AU: Pazzaglia, F J
EM: fjp3@lehigh.edu
AF: Lehigh University, Earth and Environmental Sciences, Bethlehem, PA 18105 United States
AB: There is considerable evidence now that over the last 15 m.y., the Cascadia subduction wedge along the Olympic Peninsula has been in a flux steady state, where the accretionary influx is balanced with the erosional outflux from the forearc high. This steady state condition implies that the size and shape of the wedge have also been in steady state as well. If correct, we can use the steady-state flux F$_{a}$$\sim$45 km$^{2}$/m.y. and the present shape of the wedge to predict the average transit time $\tau$ through the wedge, given by $\tau(x)$ = A(x) /F$_{a}$, where x is the landward distance from the front of the wedge and A(x) is the area of the wedge seaward of x. Given the present bathymetry of the wedge, z(x), we can relate age to depth ($\tau$ to z) for accreted material moving through the wedge. These relationships predict an age-depth history that is remarkably similar to that determined for the Coastal unit of the Olympic Structural Complex (OSC) (Hoh assemblage), which consists of accreted sediments presently exposed along the west coast of the Olympic Peninsula. For this comparison, we have determined new estimates of paleo-water depth using paleobathymetric interprations for benthic forams by Ingle (1980), together with published benthic foram reports by W. Rau. The Coastal OSC was deposited at 16.5 Ma at water depths $>$2000 m, which is similar to the 2500 m depth of the modern trench. The Coastal OSC is highly deformed and imbricated, consistent with its origin by subduction accretion. Thermal indicators show that the unit never experienced temperatures $>\sim$100 C, so this structural unit must have been accreted near the front of the wedge and transported landward just beneath the surface of the wedge. Exploration wells on the shelf west of the Olympic Peninsula drilled through a coherently bedded sequence and into the underlying Coastal OSC. The base of the coherent sequence is middle and late Miocene (15 to 10 Ma) and was deposited at $>$2000 m water depth, which indicates deposition in a post-accretionary lower slope basin. The Quillayute Formation, which overlies the Coastal OSC on land, is late Miocene ($\sim$10 Ma) age, and was deposited at $\sim$1000 m depth. The upper part of the Quinault Formation, which also overlies the Coastal OSC onland, was deposited at 150 m at $\sim$4.5 Ma. The Coastal OSC is currently exposed near sea level along the west coast of the Olympic Peninsula. Geomorphic evidence in Pazzaglia and Brandon (2001) indicates that these rocks are currently moving landward at a rate of $\sim$3 km/m.y. All of these estimates compare well with our simple quantitative model, which assumes only frontal accretion in a steady state wedge. If correct, the Coastal OSC has moved landward in the direction of convergence by 135 km since its initial accretion at 16.5 Ma. Rapid landward transport may be a common feature at other accreting subduction wedges.
DE: 8102 Continental contractional orogenic belts
DE: 8150 Plate boundary--general (3040)
SC: Seismology [S]
MN: 2003 Fall Meeting