作者: James S. McClain , John A. Orcutt , Mark Burnett
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摘要: In 1982 we undertook a seismic refraction experiment, known as the MAGMA expedition, to examine detailed structure of East Pacific Rise near 12°50′N. This segment rise, where full spreading rate is about 110 mm/yr, projected trace O'Gorman fracture zone and site an overlapping center well “black smoker” hydrothermal activity. this paper describe analysis subset travel time data collected during namely from profiles which were oriented normal rise axis. These provide set roughly equivalent those on other experiments sample cross-sectional rise. We have modeled these using two-dimensional ray-tracing program. found that velocities in young oceanic crust are rather high, with velocity gradients 4.0–5.5 s−1 uppermost crust. The highest at seafloor occur beneath axis itself seem decrease ages 0.1 Ma. must result increase porosity upper may coincide development abundant surface fissures spreads. does not appear penetrate deeper than 0.5 km reverse alteration fills pores cracks. fact under suggests circulation responsible for black smokers confined seismically unresolved conduits, consistent high temperatures discrete nature vents. Our best model includes magma chamber some 4 wide extending Moho within 1.1 seafloor. far smaller many models predicted but larger inferred sites Rise. discrepancies probably arise because steady state feature changes cooling perhaps episodic supply mantle or along