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On the continental platform of western Mexico, there is a young island of volcanic origin: Isla Isabel. It is the only volcanic manifestation in the region, contrasting with Islas Tres Marías, which are not volcanic. We aim at characterizing the source of the volcanic activity present in this particular location. Under Isla Isabel there is a bathymetric bulge that rises 60–80 m above the platform, and extends 20 km in the NW–SE direction and 17 km in the perpendicular direction. Isla Isabel is the only emerged portion of the bulge, extending only 1.8 km in the NW–SE direction. The island shows Plio-Pleistocene volcanic activity, including the formation of maars and the presence of mantle xenoliths. Using independent 2D modelling and 3D inversion methods for the gravity and magnetic fields, we analyse the nature of the bulge and its surroundings. A magnetotelluric station yields information about the electrical resistivity under the island, with penetration depths of approximately 20 km. The models are consistent with the presence of dense bodies of varying magnetizations that are interpreted as intrusive bodies. Results support the presence of an intrusion that locally has raised the ocean-floor topography. Volcanic activity projected from the bulge created Isla Isabel; the existence of additional, submerged volcanic centres in the area is most probable. We are inclined to identify the Isabel Bulge as a laccolith.

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