Seaward-dipping reflectors and the continent-to-ocean transition along the South American volcanic passive margin

Abstract

Thick packages of lavas forming seaward-dipping reflectors (SDRs) are diagnostic features of volcanic passive margins. SDR sequences form during the final stages of continental breakup, amid the transition from continental to oceanic crust (i.e. the COTZ). Despite their significance to continental breakup studies, the formation mechanism of SDRs is still debated. The ambiguity surrounding their formation is likely due to the paucity of high-quality depth-migrated seismic imagery along volcanic margins, however. Here, ~ 22,000 km of high-quality 2-D pre-stack depth-migrated seismic data, recorded by ION, are used to document the SDR geometry offshore Argentina, Uruguay and Brazil. The geological interpretation of these data is constrained using detailed reflection-based velocity analysis of long-offset (10.2 km) seismic gathers, conducted along three seismic profiles. Together, the measured reflection geometry and velocity structure provide novel insights into the formation of three new SDR facies (Type I, IIa & IIb SDRs). Type I SDRs are fault-bounded and associated with anomalously high-velocity bodies (6.5 - 7 km/s) both beneath and at the down-dip end of their diverging wedges. This facies is identified along the entire margin. Another set of (Type II) SDRs are observed outboard of, but onlapping, the Type I facies. Type II SDRs are not typically fault-bound and are not associated with high-velocity bodies. In the south, the Type II SDRs are short (IIa) with average lengths of 2 ± 2 km. Conversely, closer to the Paraná flood basalt province, Type II SDRs are longer (IIb) with average lengths of 27 ± 23 km. The Type I SDRs are interpreted as lava flows confined to continental half-grabens and sourced from point-source intrusive magmatic bodies, while Type II SDRs are erupted from linear sub-aerial spreading centres. The difference in the average length of the Type II SDRs is likely due to variations in the elevation of the source along the margin. Where the southern Type IIa lavas flow into standing bodies of water and the northern Type IIb typically do not. Interpreting volcanism using the facies presented in this thesis provides novel insights into; (1) the structure and formation of the COTZ, (2) the formation of early post-rift basins and (3) provides possible constraints on relative sea-level during magma-influenced continental breakup.