Thick crust, thin Moho transition zone and negative velocity gradient in the mantle along a 30.6 Ma segment in the South Atlantic Ocean at 31oS

Oceanic crust is formed by basaltic melt produced through decompression melting of ascending mantle at mid–ocean ridges. This oceanic crust is separated from the residual mantle by the Mohorovičić (Moho) discontinuity. Determining the crustal and mantle velocities and the structure of Moho transition zone is critical for understanding the mantle melting, melt extraction and migration and crustal accretion along mid–ocean ridges.

 

We used seismic full waveform inversion (FWI) to analyse the ocean bottom seismometer (OBS) data from the 2016 CREST experiment from the South Atlantic Ocean at 31^oS that samples the 30.6 Ma crust formed along the Mid–Atlantic Ridge at a slow–spreading rate (half–spreading rate of 24 mm/year). Seven four–component OBSs were deployed at ~10 km interval along the seismic profile, and the airgun array source was shot at 150 m interval. The high–quality OBS data show clear crustal refraction arrivals (Pg) up to ~35 km offsets, strong Moho reflection arrivals (PmP) at ~20–65 km offsets but absence of mantle refraction arrivals (Pn), indicating the presence of a relatively thin Moho transition zone (MTZ) and a negative velocity gradient in the mantle.

 

We performed two-dimensional elastic FWI of the pressure data recorded by hydrophone to constrain fine–scale crustal and mantle velocity. The starting model for FWI was obtained from a previous study of joint tomography of manually picked travel times of Pg and PmP arrivals. We progressively inverted the OBS seismic data in FWI from 3.0–4.5 Hz data to 3.0–6.5 Hz data to gradually update the subsurface velocity. The preliminary FWI result shows a uniformly thick crust of 7.1 km along the profile, ~1 km thicker than the global mean of oceanic crust. This observation indicates a relatively uniform mantle upwelling along the ridge and ~20^oC higher mantle temperature at the time of crustal formation. The lower–upper crustal ratio is ~2.5, suggesting the upper crust was formed by a magma reservoir in the mid–crust. The lower crust is heterogeneous where high and low velocity layering is observed, indicating lower crustal accretion by the in–situ crystallisation of melt sills. Assuming the depths of 7.2 and 8.0 km/s velocity contours as the top and bottom of the MTZ, respectively, the thickness of the MTZ varies between 0.6 and 1.2 km with an average of ~0.9 km. A ~1 km–thick layer with velocity up to 8.2 km/s lies beneath the MTZ, possibly due to the presence of a thin dunite–rich layer. Further below, the upper mantle velocity gradually decreases with depth, which could be due to the mantle anisotropy and/or the presence of frozen gabbroic sills in the mantle.