An Extensionally Fractured Upper Lithosphere on Io

The Jovian moon Io features hundreds of paterae, broad depressions often filled with lava. The largest such example, Loki Patera, features a large, polygonal, and fractured island in its center. This island may reflect the fragmentation of the upper Ionian lithosphere by a shallow magma body, facilitated by a great number of shallow extensional structures. We propose a model for the Ionian lithosphere in which the upper portion is heavily deformed by joints, normal faults, and graben resulting from combined tidal, subsidence, and thermal stresses, and a lower portion rife with major thrust faults formed by horizontal compression of deeper crustal levels from the continued burial of surface units. Some of those thrusts may be reactivated normal faults. An extensionally fractured upper lithosphere accounts for the Loki Patera island, for observations of more than 175 paterae with straight edges, for the fact that almost 95% of Io’s surface is covered with volcanic flow and plains units, and for widespread instances of mass wasting within Io’s gigantic mountain blocks. Additional, high-resolution image and topographic data are required to test this hypothesized model for the moon’s lithosphere.