Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

摘要

High-velocity lower crust (HVLC) and seaward-dipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the natureand origin of HVLC is under discussion. Here we provide a comprehensiveanalysis of deep crustal structures in the southern segment of the SouthAtlantic and an assessment of HVLC along the margins. Two new seismicrefraction lines off South America fill a gap in the data coverage andtogether with five existing velocity models allow for a detailed investigationof the lower crustal properties on both margins. An important finding is themajor asymmetry in volumes of HVLC on the conjugate margins. The seismicrefraction lines across the South African margin revealcross-sectional areas of HVLC 4 times larger than at the South American margin, a findingthat is opposite to the asymmetric distribution of the flood basalts in theParaná–Etendeka Large Igneous Province. Also, the position of theHVLC with respect to the SDR sequences variesconsistently along both margins. Close to the Falkland–Agulhas Fracture Zonein the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below theinner SDR wedges while in the northern area, closer tothe Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seawardof the inner SDRs.This challenges the concept of a simple extrusive/intrusive relationshipbetween SDR sequences and HVLC, and it providesevidence for formation of the HVLC at different times during the rifting andbreakup process. We suggest that the drastically different HVLC volumes arecaused by asymmetric rifting in a simple-shear-dominated extension.