Mega-Regional Mapping of Seaward-Dipping Reflectors Along Cretaceous, Conjugate Volcanic Margins of South America and West Africa Using Satellite Gravity Data

Abstract:

Seaward dipping reflectors, or “SDRs”, are seaward-inclined, packages of igneous crust that underlie the transition from continental to oceanic crust and form during the rifting phase of volcanic passive margins. On the conjugate margins of South America and west Africa, previous studies using seismic reflection have shown that SDR belts of Barremian-Aptian age range in width from 91 to 117 km and in thickness from 10 to 20 km. The composition of SDRs where known from deep drilling on analogous volcanic margins can vary with mixtures of stratiform, subaerial volcanic flows separated by volcaniclastic units and non-volcanic marine and non-marine sedimentary units. Due to heterogeneity of SDR compositions, the Bouguer anomaly alone is not revealing for their mapping, In this presentation, we show how SDRs can be mapped in map view at mega-regional scales using the total horizontal gradient of the Bouguer anomaly which effectively detects the sloping or faulted edges of the SDRs relative to the adjacent areas of normal oceanic crust and thinned continental crust. In order to validate our proposed SDR maps we have georeferenced six, 200-400 km-long, deeply-penetrating, seismic reflection lines spaced at distances of 500-600 km in the offshore areas of the Pelotas and Punta del Este basins in southern Brazil and Uruguay and the Walvis, Luderitz, and Orange basins of Namibia in South America. As an additional constraint on our interpretation of the SDRs we have created 2D gravity models along each of these six lines. Our predicted SDRs using total horizontal gradient is either exact or within 10 kms of the seismically-imaged boundaries - especially on the less structurally complex, seaward margins. We also compare the gravity results to magnetic mapping which on its own is not diagnostic - but offers some support of the gravity results.