A.C.G. Costa

Large-scale active slump of the southeastern flank of Pico Island, Azores

We report evidence for ongoing lateral slump of part of the southeastern flank of the Pico volcanic ridge in the Azores. Data from a high-resolution digital elevation model, field work, GPS, and radar interferometry show that: (1) the slumping sector is several cubic kilometers in size; (2) the structure involves several curved scars with normal fault kinematics; (3) the central part is undergoing little horizontal displacement toward the southeast (1.6 ± 1.3 mm/yr), but significant downward movement (5–12 mm/yr); and (4) the outer part of the southeastern flank of Pico is subsiding faster than the inner parts; this likely reflects recent individualization of a steep seaward-dipping fault in the moving mass. The slump shares similarities with active slumps recognized elsewhere, although the studied area may represent only the proximal part of a much larger complex potentially affecting the deep submarine base of the island. Displacement of the subaerial part of the southeastern flank of Pico seems to be accommodated by the movement and rotation of large blocks along listric normal faults.

Deformation in a hyperslow oceanic rift: Insights from the tectonics of the São Miguel Island (Terceira Rift, Azores)

The evolution of hyperslow oceanic rifts, like the Terceira Rift (TR) in the Azores, is still poorly understood. Here we examine the distribution of strain and magmatism in the portion of the TR making up the Nubia-Eurasia plate boundary. We use Sao Miguel Island because it stretches most of the TR width, which allows to investigate the TRs architecture and shedding light on TRs age and mode of deformation. From topography and structural analysis, and new measurements of 380 faults and dikes, we show that (1) Sao Miguel has two main structural directions, N150 and N110, mostly concentrated in the eastern part of the island as an onshore continuation of the faults observed offshore in the NE (N110 faults) and SW (N140) TR walls; (2) a new N50-N80 fault system is identified in Sao Miguel; (3) fault and dike geometries indicate that eastern Sao Miguel comprises the TRs northern boundary, and the lack of major faults in central and western Sao Miguel indicates that rifting is mostly concentrated at master faults bounding the TR. Based on TRs geometry, structural observations and plate kinematics, we estimate that the TR initiated between 1.4 and 2.7u2009Ma ago and that there is no appreciable seafloor spreading associated with rifting. Based on plate kinematics, on the new structural data, and on Sao Miguels structural and volcanic trends, we propose that the eastern two thirds of Sao Miguel lie along a main TR-related transform fault striking N70-N80, which connects two widely separated N130-N150 TR-trending segments.