On spreading rates and crustal structure at mid-ocean ridges

Understanding the velocity structure of mid-ocean spreading ridge is a key factor to investigate the evolution of oceanic lithosphere. In order to explore the characteristics of velocity structure of crust with different spreading rates, we select global 152 1-D seismic wave velocity structure profiles of both on and off-axis oceanic crust with spreading rates at fast (full spreading rate >90 mm . a(-1)), slow (full spreading rate 20 similar to 50 mm . a(-1)) and ultra-slow (full spreading rate <20 mm . a(-1)). The study is based on statistical analysis of average values of global on and off-axis oceanic crust with different spreading rates. It shows that: (1)The average oceanic crust thickness of fast, slow and ultra-slow spreading ridges is 6. 4 km,7. 2 km and 5. 3 km, respectively, where the oceanic layer 2 shows similar thickness value, while the layer 3 has large variations. In addition, the oceanic crust thickness variation ranges are 4. 9 similar to 8. 1 km,4. 6 similar to 8. 7 km and 4. 2 similar to 10. 2 km from fast to ultra-slow ridges. The slower the spreading rates, the larger variations of the oceanic crust thickness are observed. (2) At a given depth, oceanic crust with fast spreading has faster velocity compared to the middle and slow one, this is probably related to the formation mechanism of oceanic crust at different spreading rates. (3)The thickness of oceanic crust at age older than 10 Ma is 0. 3 km thicker compared to the one younger than 10 Ma. This is smaller but comparable to previous studies, which indicates that there is probably correlation between the seafloor age and oceanic crust thickness.