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

摘要

High-velocity lower crust (HVLC) and seawarddipping\udreflector (SDR) sequences are typical features of\udvolcanic rifted margins. However, the nature and origin of\udHVLC is under discussion. Here we provide a comprehensive\udanalysis of deep crustal structures in the southern segment\udof the South Atlantic and an assessment of HVLC\udalong the margins. Two new seismic refraction lines off\udSouth America fill a gap in the data coverage and together\udwith five existing velocity models allow for a detailed investigation\udof the lower crustal properties on both margins.\udAn important finding is the major asymmetry in volumes of\udHVLC on the conjugate margins. The seismic refraction lines\udacross the South African margin reveal cross-sectional areas\udof HVLC 4 times larger than at the South American margin,\uda finding that is opposite to the asymmetric distribution\udof the flood basalts in the Paraná–Etendeka Large Igneous\udProvince. Also, the position of the HVLC with respect to\udthe SDR sequences varies consistently along both margins.\udClose to the Falkland–Agulhas Fracture Zone in the south,\uda small body of HVLC is not accompanied by SDRs. In the\udcentral portion of both margins, the HVLC is below the inner\udSDR wedges while in the northern area, closer to the Rio\udGrande Rise-Walvis Ridge, large volumes of HVLC extend\udfar seaward of the inner SDRs.\udThis challenges the concept of a simple extrusive/intrusive\udrelationship between SDR sequences and HVLC, and it provides\udevidence for formation of the HVLC at different times\udduring the rifting and breakup process. We suggest that the\uddrastically different HVLC volumes are caused by asymmetric\udrifting in a simple-shear-dominated extension.\ud