The magmatic rifting of Santos Basin: aeromagnetic mapping of dykes,
terranes and marginal structures and the interplay between tectonism and
volcanism
Abstract
We present a new magnetic map integrating continental and oceanic
features at Santos Basin, in order to investigate the connection between
onshore and offshore tectonics and magmatism at the early stages of
rifting. The magnetic and seismic data evidence the offshore continuity
of the Florianópolis and Serra do Mar dyke swarms while the Ponta Grossa
Dyke Swarm was discontinued by the marginal structures. This brings to
evidence a different timing of formation of the latter relative to the
early opening of the margin. A very high amplitude magnetic lineament -
the Santos Marginal Anomaly (SAMA) reaches up to 1500 nT and trend
NE-SW, with local E-W inflections forming the Cananéia and Guaratiba
Arcuate Anomalies. Our modeling suggests that SAMA can be associated
with dykes and syn-rift extrusives linked at depth to larger magmatic
bodies at the proximal and necking domains. Based on the increasing
width and amplitude of the magnetic anomalies oceanwards, we propose a
mechanism where the igneous accretion augmented as rifting evolved from
continent to ocean, creating a volcanic rift axis. This rift was
segmented by crustal discontinuities that probably nucleated on
preexisting rheological contrasts, forming tectono-magmatic segments
during the early extensional stages. The onshore-offshore relationships
highlight that where there was an agreement between basement fabric and
dyke orientation, inheritance may have influenced their emplacement,
although it was not a limiting factor for their formation. The regional
geodynamics of the Gondwana opening was the first order control, with
deviations of the magmatically influenced rift by inheritance.