Seismic velocities in the uppermost oceanic crust: Age dependence and the fate of layer 2A

被引:85
作者
Carlson, RL [1 ]
机构
[1] Texas A&M Univ, Dept Geol & Geophys, College Stn, TX 77843 USA
关键词
D O I
10.1029/97JB03577
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
It has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to 'typical' crustal velocities (similar to 5.2 km s(-1)) over a period of 30-40 million years, with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation. Some recent seismic studies, however, suggest that the seismic velocities actually rise rapidly, nearly doubling in <10 million years and remain essentially unchanged thereafter. On the basis of a new compilation of published high-quality seismic survey data, this study finds the following: In crust <1 million years old, seismic velocities are commonly <3 km s(-1), while in crust between 1 and 5 million years old, velocities range from 2 to >5 km s(-1), and velocities <3 km s(-1) are not observed in crust older than 5 Ma. There is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years. Median and average velocities increase from 2.3 to 4.4 km s(-1) by similar to 7.5 Ma and remain unchanged thereafter. If layer 2A is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks (layer 2B, 5.2 km s(-1)), it follows that layer 2A does not disappear but is a persistent feature of the oceanic crust. These results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation.
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收藏
页码:7069 / 7077
页数:9
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