Crustal velocity structure north of the Mendocino Triple Junction

Abstract

A 140‐km‐long refraction/wide‐angle reflection profile recently recorded by Stanford University and the U. S. Geological Survey imaged the subducting Gorda slab beneath northern California. The profile, which is subparallel to the coast from Cape Mendocino northward, indicates that the North American plate is 13‐ to 14‐km‐thick along the coast north of Cape Mendocino. The crust is characterized by relatively uniform, low velocities of ≤6 km/s interpreted as Franciscan rocks. Two strong reflections define the upper and lower boundaries of the subducting Gorda crust. Our data indicate that the subducting Gorda crust thickens northward from Cape Mendocino from 7‐km‐thick just north of Cape Mendocino to 10‐km‐thick 120 km to the north. This change in thickness is coincident with a change in velocity from 6.7 km/s south to 6.2 km/s north. Mantle velocities of 7.7 km/s are observed for offsets greater than c. 80 km. We interpret our model to indicate that the Gorda slab is not imbricated on a crustal scale beneath our profile, that sediments and/or a tectonically thickened oceanic layer 2 are present to the north but not in the vicinity of Cape Mendocino, and that proximity to the Mendocino triple junction affects the way sediments are subducted.

Genetic algorithms and their use in Geophysical Problems

“…The subducting Gorda Plate is believed to vary in thickness from 7 km at the southern end of the MTJ to 10 km in the North, decreasing in velocity from 6.7 km/s to 6.2 km/s at the same time (Beaudoin and Magee, 1994). are not as reliable as the estimates of Benioff zone dip.…”

“…The North American Plate in the region of the MTJ is an accretionary complex of Mesozoic to Cenozoic origin (the Franciscan, manifesting itself in the Coast Ranges and Klamath Mountains) which is overlain by the Eel River Basin, a sedimentary forearc basin of Cenozoic origin (Beaudoin and Magee, 1994).…”

“…Velocities in these crustal units are believed to be relatively uniform, in the range of 5.5-5.8 km/s in the West and 6.0-6.5 km/s in the East (Beaudoin and Magee, 1994;Benz et al, 1992). The subducting Gorda Plate is believed to vary in thickness from 7 km at the southern end of the MTJ to 10 km in the North, decreasing in velocity from 6.7 km/s to 6.2 km/s at the same time (Beaudoin and Magee, 1994).…”

“…km/s (Beaudoin and Magee, 1994) and as high as 8.0 km/s (Benz et al, 1992), and the underlying mantle velocity V3 is usually estimated to be from 8.0 to 8.2 kmls.…”

Migration of the Mendocino triple junction and ephemeral crustal deformation: Implications for California Coast range heat flow

“…Over a region extending 200 km north of the triple junction the crust thickens from ∼15 km thick in the north to its maximum thickness of ∼40 km at the MTJ, adjacent to the southern edge of the Gorda slab (Figure 2). The crustal thickness then decreases to ∼20 km over a distance of 150 km south of the triple junction [ Beaudoin et al , 1994; Verdonck and Zandt , 1994; Beaudoin et al , 1998; Villasenor et al , 1998].…”