Multichannel seismic reflection data acquired between 8°50′ and 9°50′N and between 12°30′ and 13°30′N along the East Pacific Rise provide a three‐dimensional view of the young oceanic crust. Seafloor‐to‐Moho reflection travel times vary by up to 0.9 s within our study areas; the total range of crustal travel times in the 9°N area is 1.55 to 2.45 s; the total range in the 13°N area is 1.60 to 2.05 s. The variation is systematic, indicating thinner crust locally associated with overlapping spreading centers (OSCs) and, in the 9°N area, segment‐scale variation along crustal isochrons. Crustal travel time is found to be a valid proxy for oceanic crustal thickness. Outside of the axial low‐velocity volume, thickness can be calculated from time to ∼500 m. Even in the axial region thickness can be calculated to <1 km, if low‐velocity zone position is known. Crustal thicknesses calculated from travel times vary by 2.6 km in the 9°N area, and by 1.5 km in the 13°N area. The majority of this variation is attributed to seismic layer 3 (the lower crust). Segment‐scale variation of ∼1.8 km (∼5.5 to 7.3 km thickness) is observed in the 9°N area, with thinnest crust formed between ∼9°40′ and 9°50′N and thickest formed between ∼9°15 and 9°20′N. Results imply a three‐dimensional pattern of magma supply to the 9°N segment. The OSC at 9°03′N is associated with major disruptions of the segment‐scale pattern, in the form of local thin areas within the discordant zone; the smaller OSC at 12°54′N is not associated with dramatic changes in thickness of the surrounding crust. In the absence of OSCs, the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment. Thicker crust does not always correlate with shallower ridge bathymetry, broader axial cross section, or more negative mantle Bouguer or subcrustal gravity anomaly. Variable thickness of the crust‐mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result. We hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick Moho transition zone and a subcrustal gravity low. Such a diapiric upwelling center appears to be now located beneath the East Pacific Rise near 9°40′ to 9°50′N.

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An integration of geophysical data from the Pacific Plate reveals plate bending anomalies, massive intraplate shearing and deformation, and a lack of oceanic crust magnetic lineaments in different regions across the Gulf of Alaska. We argue that farfield stress from the Yakutat Terrane collision with North America is the major driver for these unusual features. Similar plate motion vectors indicate that the Pacific plate and Yakutat Terrane are largely coupled along their boundary, the Transition Fault, with minimal translation. Our study shows that the Pacific Plate subduction angle shallows toward the Yakutat Terrane and supports the theory that the Pacific Plate and Yakutat Terrane maintain coupling along the subducted region of the Transition Fault. We argue that the outboard transfer of collisional stress to the Pacific Plate could have resulted in significant strain in the NE corner of the Pacific Plate, which created pathways for igneous sill formation just above the Pacific Plate crust in the Surveyor Fan. A shift in Pacific Plate motion during the late Miocene altered the Yakutat collision with North America, changing the stress transfer regime and potentially terminating associated strain in the NE corner of the Pacific Plate. The collision further intensified as the thickest portion of the Yakutat Terrane began to subduct during the Pleistocene, possibly providing the impetus for the creation of the Gulf of Alaska Shear Zone, a > 200 km zone of intraplate strike‐slip faults that extend from the Transition Fault out into the Pacific Plate. This study highlights the importance of farfield stress from complex tectonic regimes in consideration of large‐scale oceanic intraplate deformation.