Seismic structure of an oceanic core complex at the Mid‐Atlantic Ridge, 22°19′N

Abstract

[1] We present results from a seismic refraction and wide-angle experiment surveying an oceanic core complex on the Mid-Atlantic Ridge at 22°19′N. Oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks, exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections. Tomographic inversion of our seismic data yielded lateral variations of P wave velocity within the upper 3 to 4 km of the lithosphere across the median valley. A joint modeling procedure of seismic P wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle. A gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust, while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks, caused by un-roofing lower crust rocks. Under the core complex crust is approximately 40% thinner than in the median valley and under the conjugated eastern flank. Clear PmP reflections turning under the western ridge flank suggest the creation of a Moho boundary and hence continuous magmatic accretion during core complex formation.

The velocity structure of a fossil spreading centre in the Southwest Sub‐basin, South China Sea

“…A similar feature was observed in the Southwest Indian Ridge at 50°E (Zhao et al, 2013), the Mid-Atlantic Ridge at 22°19′N (Dannowski et al, 2010) and the Parece Vela back-arc basin , and oceanic detachment faulting was proposed to explain this feature Dannowski et al, 2010;Zhao et al, 2013). In most cases, a high-velocity anomaly, strong crustal thinning and Moho uplifting are observed beneath an oceanic detachment fault, and the crustal thickness ranges from 2.5 to 5.5 km Dannowski et al, 2010;Zhao et al, 2013). The lithospheric structure resulting from an oceanic detachment fault shows lateral variations in velocity, crustal thickness and Moho depth across the spreading centre Escartin et al, 2008;Dannowski et al, 2010;Zhao et al, 2013).…”

“…The oceanic detachment faults are well established along magma-starved ridges with half-spreading rate of less than 40 mm/year (Canales et al, 2000;Ildefonse et al, 2007;Escartin et al, 2008;Dannowski et al, 2010;Zhao et al, 2013). The oceanic detachment faults are well established along magma-starved ridges with half-spreading rate of less than 40 mm/year (Canales et al, 2000;Ildefonse et al, 2007;Escartin et al, 2008;Dannowski et al, 2010;Zhao et al, 2013).…”

“…7c) shows an asymmetric lithospheric structure across the fossil spreading centre in the SWSB. The lithospheric structure resulting from an oceanic detachment fault shows lateral variations in velocity, crustal thickness and Moho depth across the spreading centre Escartin et al, 2008;Dannowski et al, 2010;Zhao et al, 2013). Therefore, a possible explanation of this asymmetric lithospheric structure along the profile T1 is oceanic detachment faulting.…”

“…However, contradictions remain. Additionally, partial melting is usually used to explain the low mantle velocities at the active spreading environment ridge (Grevemeyer et al, 1998;Dannowski et al, 2010). Additionally, partial melting is usually used to explain the low mantle velocities at the active spreading environment ridge (Grevemeyer et al, 1998;Dannowski et al, 2010).…”

The Dynamics of Continental Breakup and Extension

“…These rocks are generally assumed to be pieces of the hangingwall carried on top of the moving footwall (e.g., Rehrig and Reynolds, 1980). Similar 'rider' blocks have been recognized in oceanic core complexes by bathymetric and seismic studies (Dannowski et al, 2010;Reston and Ranero, 2011).…”