Variability in oceanic crustal thickness and structure: Multichannel seismic reflection results from the northern East Pacific Rise

Multichannel seismic reflection data acquired between 8 degrees 50' and 9 degrees 50'N and between 1 degrees 30' and 13 degrees 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 degrees N area is 1.55 to 2.45 s; the total range in the 13 degrees 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 degrees 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 similar 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 degrees N area, and by 1.5 lan in the 13 degrees N area. The majority of this variation is attributed to seismic layer 3 (the lower crust). Segment-scale variation of similar to 1.8 km (similar to 5.5 to 7.3 km thickness) is observed in the 9 degrees N area, with thinnest crust formed between similar to 9 degrees 40' and 9 degrees 50'N and thickest formed between similar to 9 degrees 15 and 9 degrees 20'N. Results imply a three-dimensional pattern of magma supply to the 9 degrees N segment. The OSC at 9 degrees 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 degrees 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 degrees 40' to 9 degrees 50'N.