High-Resolution Marine Magnetic Mapping of the Portuguese Nearshore:
Unraveling Geological Domains, Faults and Magmatic Structures
Abstract
The SW Portuguese margin has been intensively studied, particularly for
rifting, tectonic inversion and tectonic reactivation of the Atlantic
passive margin. In this work we bring new data on the continental shelf,
usually not acquired by the heavier geophysical methods (e.g. airborne
or low resolution deep seismic-magnetic surveys). These new data allow
casting a clear light bridging between the geological structures onshore
and offshore. The geology of this margin went through the Variscan
orogeny of Paleozoic age, the North Atlantic rifting, the Late
Cretaceous alkaline magmatism (intrusive and extrusive), the Alpine
tectonic inversion and the Quaternary reactivation of the passive
margin. We present results from the compilation of a series of marine
magnetic surveys conducted along the Portuguese nearshore from 2014 to
2019. Magnetic data were acquired with 1 nautic mile line separation,
resulting in near full coverage of the nearshore along a 120 km long
margin segment, from Sintra to Odeceixe. For a large part of the
surveyed area, ultra-high resolution seismics and multibeam bathymetry
were simultaneously acquired. Magnetic data were processed to produce
high resolution mapping of magnetic anomalies, and also to enhance both
shallow and deep structures, using several derivative and filtering
techniques. We combine the interpretation of high-resolution magnetic
mapping with the interpretation of ultra-high resolution and vintage
deep penetration seismic data to infer the local and regional expression
of tectonic structures and magmatic bodies. Our results allow:
identifying the offshore extension of important faults, e.g. the
Grândola, Pinhal Novo and Messejana faults; resolving previously
blurry-imaged magmatic structures, e.g. Sines and Cabo Raso anomalies;
identifying faults recycled from the Paleozoic through Present;
constraining the relation between magmatic intrusions and faults; and
bringing constraints to the discussion of magmatic emplacement.