Ground motion and tectonics in the Terceira Island: Tectonomagmatic interactions in an oceanic rift (Terceira Rift, Azores Triple Junction)

Abstract

The interpretation of high-resolution topography/bathymetry, GPS and InSAR data, and detailed structural geology indicate that: (1) Terceira developed at the intersection of two major volcano-tectonic lineaments: WNW-ESE (local TR's direction) and NNW-SSE (submarine chain of volcanoes, here firstly recognised and coined Terceira Seamount Chain). (2) Terceira is affected by four main fault systems: the ca. N165° (normal faults dipping to east and west, mostly across the middle of the island), the N140° (normal faults mostly making up the Lajes Graben), the N110° (faults with oblique striations – normal dextral, making up the main volcanic lineament), and the more subtle N70° (the transform direction related to the Nubia/Eurasia plate boundary). Seismicity, GPS data and faults displacing the topography indicate that all systems are active. (3) The whole island is subsiding at a rate of ca. 5mm/yr, as attested by both GPS and InSAR data, which is exceptionally high for the Azores islands. Common explanations like thermal contraction, or bending of the lithosphere, or magmatic processes, or collapse of the island under its own weight likely cannot justify the observed subsidence rate. The estimated average of TR's subsidence rate is also not enough, therefore we conclude that the measured 5mm/yr can be a peak. (4) The fault geometry and kinematics are consistent with the current direction of maximum extension in the Azores (ca. N65°), and the rotation of Nubia relative to Eurasia. (5) Given that the NE shoulder of the Lajes Graben is moving upwards at 5mm/yr and sits directly on the TR's NE shoulder, we conclude that the TR's shoulder is moving up, most likely as a result of the elastic rebound associated with rifting. The elastic rebound in both NE and SW TR's shoulders is most likely responsible for the observed ridge morphology all along the TR.