Velocity structure from forward modeling of the eastern ridge‐transform intersection area of the Clipperton Fracture Zone, East Pacific Rise

Abstract

Abstract. In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge-transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two-dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low-velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to "fill in" the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma-starved, displays the crustal thickness and apparent signal attenuation characteristic of a plentiful, but perhaps episodic, magma supply.

Variations in the geochemistry of magmatism on the East Pacific Rise at 10°30′N since 800 ka

“…In contrast, an axial reflector has not been detected beneath the ridge axis at 10º30 0 N, although the presence of a crustal low-velocity zone suggests that melt is present, possibly in a mush zone [24]. At 10º30 0 N, the crustal thickness is essentially normal (¾5.5 km [24]), despite the inferred low magma supply. On the other hand, recent studies on the EPR have shown that crustal thickness does not vary in any simple way with magma budget as inferred from ridge axis depth or cross-sectional area [29,30].…”

“…Much of the EPR between 9º and 14ºN is underlain by a seismic reflecting horizon which has been interpreted as evidence for the presence of magma chambers at a depth of between 1 and 2.5 km beneath the seafloor [1]. In contrast, an axial reflector has not been detected beneath the ridge axis at 10º30 0 N, although the presence of a crustal low-velocity zone suggests that melt is present, possibly in a mush zone [24]. At 10º30 0 N, the crustal thickness is essentially normal (¾5.5 km [24]), despite the inferred low magma supply.…”

“…[4]). Early seismic studies did not detect a magma reflector beneath the EPR at 10º30 0 N [1], although recent seismic profiling of the eastern ridge-transform intersection has revealed the presence of a low-velocity zone consistent with the presence of partial melt, possibly as a magma lens that is too small to be resolved seismically [24]. Thompson et al [5] argue that focussed melt supply, 0 N (squares).…”

“…The 10º30 0 N region of the EPR is an ideal place to study temporal changes in the chemistry of MORB erupted on a single ridge segment, because the structure of this ridge segment is relatively simple, and the along-axis geochemical variation is well known. In addition, this region of the EPR has been the focus of several detailed geological and geophysical studies, which have provided insights into the structure of the crust and the dimensions of magma bodies beneath this ridge segment [24][25][26]. The samples we have analyzed were studied previously by Batiza et al [21], who showed that the variation in major element chemistry of lavas dredged from the flanks of the EPR at 10º30 0 N is much greater than in lavas from the seafloor up to 50 km from the ridge axis at 9º30 0 N and 11º20 0 N. Batiza et al [21] argued that at 9º30 0 N and 11º20 0 N, steady-state magma chambers beneath the ridge axis have buffered the compositions of the lavas erupted over the past ¾800 ka, whereas at 10º30 0 N the rate of melt supply has been relatively low over the same period, leading to larger variations in the average temperature and composition of the magma chamber beneath the ridge.…”

Crustal Velocity Structures Imaged from Four-Component OBS Data Across the Southern Gagua Ridge in the Western Philippine Sea

“…This linear oceanic ridge separates the Huatung Basin off SE Taiwan from the oceanic crust of the West Philippine Basin (Murauchi et al 1968). Fracture zones near the Gagua Ridge, northwestward extending to the Taitung Canyon , are similar to those ridges with fracture zones in the North Atlantic (Detrick et al 1993), the South Ocean (Collot et al 1995), the Central Pacific and the East Pacific Rise (Begnaud et al 1997). The Gagua Ridge formation in the western Philippine Sea and how the Gagua Ridge relates to the origins of Huatung Basin and West Philippine Basin have been the focal points of several investigations.…”