Tectonic implications of post–30 Ma Pacific and North American relative plate motions

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Abstract
The Pacific plate moved northwest rela- tive to North America since 42 Ma. The rapid half rate of Pacific-Farallon spread- ing allowed the ridge to approach the con- tinent at about 29 Ma. Extinct spreading ridges that occur offshore along 65% of the margin (Lonsdale, 1991) document that fragments of the subducted Farallon slab became captured by the Pacific plate and assumed its motion prior to the actual sub- duction of the spreading ridge. This plate- capture process can be used to explain much of the post‐29 Ma Cordilleran North America extension, strike slip, and the in- land jump of oceanic spreading in the Gulf of California. The Pacific and North Amer- icancontactzonelengthenedwitheachsuc- cessive plate capture event, underpinning the parts of western North America directly inland with a strong plate undergoing Pa- cific relative motion. We suggest that much of the post‐29 Ma continental tectonism is the result of the strong traction imposed on thedeeppartofthecontinentalcrustbythe gently inclined slab of subducted oceanic lithosphereasitmovedtothenorthwestrel- ative to the overlying continent. The plate- capture hypothesis is distinctly different from theories involving shallow slab gaps. Kinematic problems associated with shal- low slab-gap models cause us to question

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