Late Eocene Extension of the Cascadia Forearc and its Control on Modern Deformation
Abstract
About 50 m.y. ago, a 20-30 km-thick oceanic basalt terrane, now known as Siletzia, accreted to N. America along the Paleogene subduction zone off present day Oregon, Washington and British Columbia. At 42 Ma, 8 m.y. after the collision, the newly accreted crust was rifted during an unusual episode of forearc extension and widespread magmatism. Dominantly oceanic tholeiitic and alkalic basalt magmatism between 42 and 34 Ma produced the Tillamook Volcanics, Cascade Head Volcanics and Yachats Basalt of Oregon, and the Grays River Volcanics of southwest Washington, collectively known as the Tillamook magmatic episode. This burst of magmatism has been interpreted as resulting from the encounter of the leading edge of N. America with a long-lived Yellowstone hotspot, or the product of slab-window magmatism during ridge subduction (or possibly both). The volcanic complexes of the Tillamook episode were fed by a regional dike swarm that crosses the Coast Range at a high angle to the present margin from 43.9° N (south of Eugene, OR) to 46.6° N (south of Centralia, WA), a width of about 300 km. Aeromagnetic anomalies illuminate the broad extent of the dike swarm and their relation to magmatic centers. Scores of linear anomalies reveal a spectacular dike swarm west of Eugene that appears to be associated with the Yachats Basalt volcanic center along the coast. The anomalies follow mapped dikes, but are much more abundant and extend ESE across the entire Coast Range, Willamette Valley, and possibly beneath the Cascade foothills south of Eugene, a distance of 120 km. The dikes and their anomalies are oriented N50-80°W, and are typical of dike trends throughout the province. The forearc has been tectonically rotated about 45° clockwise since the late Eocene, so the original orientation of the dikes was ENE, roughly normal to the original Paleogene convergent margin. The WNW trend of the dikes is parallel to sinistral faults that form clockwise rotating, domino-style fault domains between larger, active NNW dextral faults in the forearc. Aeromagnetic and structural mapping highlight the left-stepping jogs of these sinistral faults where they intersect the master dextral faults. We suggest the fabric of active shear rotation had its origins in the margin parallel extension that produced the regional dike swarms of the Tillamook magmatic episode.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T31C..06W
- Keywords:
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- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8158 Plate motions: present and recent;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY