HR: 0800h
AN: V21D-0632    [Abstracts]
TI: Constraints on the Timing and Geometry of Kula-Farallon Ridge Subduction and Implications for the Eocene Magmatic and Tectonic History of the Pacific Northwest
AU: * Tepper, J H
EM: jtepper@ups.edu
AF: Geology Dept., University of Puget Sound, 1500 N. Warner, Tacoma, WA 98416-1048 United States
AU: Clark, K P
EM: kclark@ups.edu
AF: Geology Dept., University of Puget Sound, 1500 N. Warner, Tacoma, WA 98416-1048 United States
AU: Wolfe, M R
EM: melissarwolfe@gmail.com
AF: Geology Dept., University of Puget Sound, 1500 N. Warner, Tacoma, WA 98416-1048 United States
AB: Dates on basalts in the WA-OR Coast Ranges and on scattered centers of adakite and near-trench magmatism in WA and Vancouver Island constrain the timing and geometry of subduction of the Kula-Farallon Ridge (KFR) beneath western North America between ~60 and 35 Ma. These data indicate that: (1) Coast Range basalts from widely separated areas show extensive age overlap with no apparent geographic trend over time, (2) 43 Ma adakites in WA are coeval with the Tillamook volcanics in Oregon, and (3) between 48 and 35 Ma the focus of adakite / near trench magmatism migrated northward from the southern Olympic Peninsula to central Vancouver Island. Points (1) and (2) require left-stepping offsets of the KFR, which would have led to multiple ridge-trench intersections and the development of a series of fraternal slab windows (Thorkelson, 1996). The exact geometry of this fraternal window system (FWS) cannot be reconstructed as there are no constraints on the geometry of subducted ridge offsets, but it would have been larger in area than a window resulting from a single ridge intersection (e.g., Breitsbacher et al., 2004). Preliminary calculations of the maximum extent of the KFR FWS indicate it could have extended from SE Oregon to southern BC and as far inland as Montana. Development of this FWS provides an explanation for widespread Eocene volcanism of the Challis event. The arc geochemical signature of many Challis rocks may reflect: (1) the presence of the subducting Farallon plate under some portions of the region, and/or (2) contamination of the lithosphere during earlier subduction episodes (Hooper et al., 1995). Subsequent establishment of the modern Cascade arc at ~39 Ma corresponds in space and time with the reappearance of a subducting slab beneath OR/WA as the KFR migrated northward. This model for the transition from Challis to Cascade magmatism eliminates the need for a steepening or foundering of the subducting Farallon slab, accounts for the width and orientation of the Challis arc, and explains the lack of an east-to-west age progression in Challis age volcanism. Other Eocene phenomena in the Pacific NW that may have resulted from this FWS include bimodal volcanism in WA, high-temperature metamorphism in the North Cascades, S- and A-type plutonism in WA and BC, and development of sedimentary basins.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 8104 Continental margins: convergent
DE: 8157 Plate motions: past (3040)
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005