New perspectives on subduction processing east of, and correlations across the Straight Creek-Fraser River fault, Central Cascades of Washington State
Peter B. Davis, Pacific Lutheran University
James H. MacDonald, Jr, Florida Gulf Coast University
Iris Hernandez, Pacific Lutheran University
Abstract
The dextral strike-slip Straight Creek-Fraser River fault system (SCF), a prominent feature of Pacific Northwest Tectonics, offsets several accretion and subduction related units at least 90 km. Two exposures immediately east and west of the SCF have been interested to have been continuous, in particular the Easton Metamorphic Suite that contains what appears to be the Darrington phyllite and overlying Shuksan blueschist-greenschist. These rocks are in contact with Hicks Butte intermediate igneous complex that includes calc-alkaline hornblende quartz diorite with an age of 150.0 ± 6 Ma. on zircon is mantled by a symplectitic gabbro that yields a zircon age of 153.6 ± 2.0 Ma. The Pl+Ol+Mt->Hbl+Sp+Opx assemblage suggests rapid cooling from 1100˚C to a ~700˚C at pressures less than 5.5 kbars. These are crosscut by a high Sr/Y ratio dacite with zircon age of 144.0 ± 2.4 Ma. that is incorporated into regional strain fabrics are in turn cross cut by a post-tectonic dacite ~ 137.3 ± 2.4 Ma. on zircon.
Proximal Easton Gln+Ep+Chl+Ab+Q blueschists contain omphacite inclusions in albite suggest peak pressure conditions within a subduction channel. A subsequent thermal peak in a Zo+Chl+Mt+Q greenschist assemblage both crosscuts and is incorporated into the regional deformational fabric near the pluton. Regional strain analysis of EBSD on plagioclase in symplectite lacks a CPO, suggesting sub-magmatic diffusion creep, while Prism-a slip in quartz in the gabbro and intrusive 144 Ma. dacite shows 550°–650°C. This mechanism mingling suggests high mean ductility with low contrast passive folding, suggesting the 144 Ma. dacite dates age of regional deformation. The Easton displays a range from basal-a to rhomb-a to prism-a slip (~400-650˚C) that shows a thermal gradient away from the pluton that mirrors P/T cooling path gabbroic symplectite through the greenschist contact areole on blueschist that all show simple shear verging to the north.
Detrital zircon U/Pb analysis of the underlying Darrington phyllite yield peaks from 133-145 Ma with a youngest age of 125 Ma. signifying basic closure. Ar/Ar on muscovite from near the Hicks Butte complex and several kilometers north yield a ~125-127 Ma. age suggesting that the Darrington was thrust under the Shuksan ~10 Ma. after the last subduction processing. The 142-148 Ma. The Darrington Ar/Ar muscovite age west of the SCF is 20 Ma. older than the youngest zircons to the east, and an opposite subduction vergence suggest that the correlation of the Easton units across the SCF has become problematic.
Session
Subduction zone geology