Publication Date:
2014-11-17
Description:
High-velocity lower crust (HVLC) and seawarddipping
reflector (SDR) sequences are typical features of
volcanic rifted margins. However, the nature and origin of
HVLC is under discussion. Here we provide a comprehensive
analysis of deep crustal structures in the southern segment
of the South Atlantic and an assessment of HVLC
along the margins. Two new seismic refraction lines off
South America fill a gap in the data coverage and together
with five existing velocity models allow for a detailed investigation
of the lower crustal properties on both margins.
An important finding is the major asymmetry in volumes of
HVLC on the conjugate margins. The seismic refraction lines
across the South African margin reveal cross-sectional areas
of HVLC 4 times larger than at the South American margin,
a finding that is opposite to the asymmetric distribution
of the flood basalts in the Paraná–Etendeka Large Igneous
Province. Also, the position of the HVLC with respect to
the SDR sequences varies consistently along both margins.
Close to the Falkland–Agulhas Fracture Zone in the south,
a small body of HVLC is not accompanied by SDRs. In the
central portion of both margins, the HVLC is below the inner
SDR wedges while in the northern area, closer to the Rio
Grande Rise-Walvis Ridge, large volumes of HVLC extend
far seaward of the inner SDRs.
This challenges the concept of a simple extrusive/intrusive
relationship between SDR sequences and HVLC, and it provides
evidence for formation of the HVLC at different times
during the rifting and breakup process. We suggest that the
drastically different HVLC volumes are caused by asymmetric
rifting in a simple-shear-dominated extension.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
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