Volcanic and tectonic evolution of the Cascade Volcanic Arc, central Oregon

From 22 to 0 Ma, σ3 in the Cascade arc rotated clockwise from approximately N–S to E–W. σ1 rotated from subhorizontal to vertical at about 7 Ma, producing an extensional stress regime at 7–0 Ma. Rotation of σ1 and σ3 was likely a response to decreasing influence of ENE to NE compression at the Juan de Fuca plate-North American plate (JDFP-NAP) boundary relative to N-S compression and attendant continental extension produced at the Pacific plate (PP)-North American plate boundary. Decreases in orthogonal convergence rate, convergence angle, and length of the convergent margin relative to the NAP-PP transform boundary caused the stress rotations. Volcanic production decreased by a factor of 3 from the interval 35–17 Ma to 16.9–7.4 Ma, probably reflecting a decrease in convergence rate. Volcanic production increased at 7.4–0 Ma even though convergence rate continued to decrease. The extensional stress regime at 7–0 Ma promoted mafic volcanism that caused the increased volcanic production. Volcanic production is therefore a function of convergence rate and upper plate stress regime. The volcanic front migrated progressively eastward from 35 to 0 Ma as the volcanic belt narrowed. The narrowing was caused primarily by steepening slab dip at depths greater than 100 km. Eastward migration was likely caused by decreasing shallow (0–100 km) slab dip resulting from thinning of the NAP. Uplift of the Western Cascades province in the early Pliocene may have been caused by vigorous flow into the mantle wedge accommodating an increase of free rollback rate of the subducted plate at 4 Ma.

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