Elaboración de fichas de recolección de datos

Values obtained for the depth of the analytic shelf edge (ASE) off most sectors of the islands differed considerably from the global sea level lowstands in recent Glacial Maxima, suggesting thick on-shelf sediment wedges, different emplacement times and vertical crustal movements. An examination of shore-normal sub-bottom profiles off the SW corner of the Almoxarife Formation (the only area where a few sub-bottom profiles effectively extended down to the beginning of the island slope) reveals a thick sediment wedge present over a sub-bottom reflector that intersected the seafloor surface at 114 m depth and constitutes a palaeo-shoreline (Figure 34). This reflector is interpreted as the palaeo-horizon planed by surf erosion on the island flank during past sea level fluctuations which has been buried by a sediment wedge with a maximum thickness of 35 m. The location of its distal palaeo-shoreline at 114 m depth further suggests that (if any) the area may have only suffered a minor uplift of ~16 m, rather than the massive ~75 m uplift suggested by the ASE location.

Figure 34: Sand wedge on southern Faial shelf. A. Seimic profile with delineated bedrock horizon (a-b). B: Location of the profile and local bathymetry. Note the location of the analytic shelf edge (55 m depth on A and red line on B) and the surfacing of the eroded bedrock horizon only at 114 m depth. Outcrops of this bedrock horizon are visible in the area along the 110-130 m depth stratum (white arrows).

The lack of outer shelf seismic data around the rest of the island precludes locating the edge of the planed bedrock and obtaining a more robust indicator of the isostatic behaviour.

Despite this data shortage, the presence of a thick sediment wedge likely accounts for the shallow location the ASE off the S and SW margins of the Cedros Complex, as suggested by the identification of mid-shelf sediment thicknesses of 40 m in Varadouro Bay (Quartau, 2002). Contrastingly, the Ribeirinha complex and the northern margin of the Faial-Pico passage displayed ASEs (-170 m and -183 m, respectively) significantly deeper than the glacial maxima lowstands that occurred since the emergence of this edifice (800 kyr BP). The apparent continuity between these two sectors of the shelf edge suggests that they may represent the margin of an early shallow-water edifice straddling from north-eastern Faial (the oldest part of the island) to the northern half of the present Faial-Pico passage. In these circumstances, the seemingly planed structure on the northern margin of the Faial-Pico passage is interpreted as a subsided early shelf of this edifice (Figure 5).

Disregarding the sediment probably accreted on the outer shelf of an edifice of this age, a depth at edge of approximately 180 m implies that the edifice has subsided at least ~50 m since the Pleistocene lowstand where surf action planed it.

It is hypothesized that the sinking of this early edifice may be associated with crustal warping generated by sequential loading resulting from the formation of neighbouring islands (McNutt & Menard, 1978; Lambeck, 1981). In this particular case, subsidence could be a response to the emplacement of São Jorge island (~30 km to the NE), which emerged 200 kyr later than the Ribeirinha complex. If one assumes the loading has generated not only subsidence but also tilting of Faial towards the NE, the same process could also account for the excessive dip (12º) of the subaerial slopes of the Ribeirinha Complex pointed out by Pacheco (2001), which exceeds the typical sloping angle (<10º) of shield volcanoes (Cas & Wright, 1987; Francis, 1993).

A concurrent uplift of the opposite island margin is not absolutely necessary in the present case, as at least part of vertical movement may have been accommodated within the Pedro Miguel graben. However, the tilting process could help explain the minor uplift suggested by the location of the planed bedrock margin at 114 m.

The occurrence of the shelf edge at a mere 21-25 m depth off the Capelo peninsula is a consequence of the young age of this edifice rather than of tectonic or major sedimentation processes and will be discussed in 5.3.1.

The ASE of 134 m off the western sector of the Cedros Complex roughly matches the lowstand at the Last Glacial Maximum but the sample is too small to derive any sound conclusions and no seismics data exist to assess the thickness of the sediments accumulated on the outer shelf.

5.1.2. Faults

Whilst active faults are generally well known over the islands of Faial and Pico, the same is not true for the surrounding seafloor. The high resolution of swath data contributed to a more precise mapping of some of these structures, which were either expressed in displacements of the seafloor surface or in volcanic lineaments.

The faults with the best on-shelf expression were those projecting seaward from the Pedro Miguel graben on northeastern Faial, which confirms previous suggestions by Berthois (1953) and Nunes (1999). However, not all of the Pedro Miguel graben faults were topographically expressed on the seafloor surface along their seaward projections. Only those presenting headwalls that were not buried too deep by the coating of frequently reworked sediments could be pinpointed.

Often, these faults were clearer in the centre of the passage, where stronger currents appear to prevent thick sedimentation. Many of the fault extensions towards eastern Faial were capped by sediments, whilst towards Pico they were mostly covered by the lava flows.

A previously unreported dextral strike-slip fault straddles the Espalamaca-Madalena ridge and trends N55W. It does not strictly belong to any of Faial’s dominant fault families (which strike N60-80W and N10-30W according to Madeira & Brum da Silveira (2003) and its connection to onshore faults could not be established from seafloor topography.

Volcanic activity is commonly associated with the faults and is manifest as small cones and possible fissural extrusions. Some older extrusions (e.g., VC5 and VC6 on the Espalamaca Fault) appear to have been heavily eroded by waves. The largest volcanic activity was associated with the faults underlying the Baixa do Sul ridge where a large edifice encloses a basin on the southern half of the passage (Figure 23).

With the exception of the NNW-SSE trending faults, the surficially-expressed faults identified in the Faial-Pico passage did not confirm the interpretation of the local submerged tectonics deduced by Dias & Matias (2006) from Faial’s 1998 aftershock epicentre sequence.