Along-axis variation in crustal thickness at the ultraslow spreading Southwest Indian Ridge (50 degrees E) from a wide-angle seismic experiment

作者:Niu, Xiongwei; Ruan, Aiguo*; Li, Jiabiao; Minshull, T. A.; Sauter, Daniel; Wu, Zhenli; Qiu, Xuelin; Zhao, Minghui; Chen, Y. John; Singh, Satish
来源:Geochemistry Geophysics Geosystems, 2015, 16(2): 468-485.
DOI:10.1002/2014GC005645

摘要

The Southwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thin crust, and extensive outcrops of serpentinized peridotite. Previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the SWIR, from the Prince Edward (35 degrees 30E) to the Gallieni (52 degrees 20E) fracture zones. Here we present a new analysis of wide-angle seismic data along the ridge 49 degrees 17E-50 degrees 49E. Our main conclusions are as follows: (1) we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient, underlain by a layer 3 with variable thickness and low velocity gradient; (2) the crustal thickness varies from approximate to 5 km beneath nontransform discontinuities (NTDs) up to approximate to 10 km beneath a segment center; (3) the melt supply is focused in segment centers despite a small NTD between adjacent segments; (4) the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust. Our observation of thick crust at an ultraslow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate, and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate, and that mantle temperature or chemistry (or both) must vary significantly along axis.