Resumen: The recovery of calcareous breccia and other carbonate-cemented rocks from Mont dels Oliva in the Balearic Promontory indicates that both this seamount and the adjacent Mont Ausias Marc, east of Ibiza and Formentera islands, are of continental origin. Recent multibeam echosounder mapping and high-resolution seismic reflection profiles indicate
that the third high, Emile Baudot Seamount, is located within a field of more than 118 pinnacles. Based on their magnetic signature and the recovery of basalt in the vicinity of the pinnacles suggest that the features are of volcanic origin. A basalt sample dredged from Emile Baudot Seamount indicates an igneous origin for this seamount. The limited sampling, the chemistry of the rock recovered and the size of the seamount itself (18 by 3 km), however, does not preclude the possibility that Emile Baudot may be a fractured uplifted continental block intruded by the basalt. The K/Ar age of 1.46ñ0.18 my yielded by the basalt sample verifies the earlier proposal by Acosta et al. (Mar. Pet. Geol. 128 (2001a) 349) that the volcanism that created part or all of Emile Baudot Seamount and the Southwest Majorca Volcanic Field associated with the seamount took place in the Pleistocene. This volcanism is probably related to decompression resulting from the normal faulting that led to the subsidence of the low (Central Depression) between Ibiza and Majorca (Balearic Islands).
Palabras clave: Mineralogía, Arcillas Cerámicas, Santa Cruz de Mudela, Microscopía Electrónica, Plasticidad, Mineralogy, Building Clays, Santa Cruz de Mudela, Electron Microscopy, Plasticity.
Resumen: A series of clastic dikes and tubular vents were identified in southern Tenerife (Canary Islands). These features
are the result of seismic liquefaction of a Holocene sand deposit, as the consequence of a high intensity
paleoearthquake. The peak ground acceleration (pga) and magnitude of the paleoearthquake
generating these liquefaction
features were estimated by back calculation analysis. A representative value of 0.30 ñ 0.05 g was obtained
for the pga. From this, an earthquake intensity of IX was estimated for the liquefaction site. Magnitude bound
methods and energy based approaches were used to determine the magnitude of the paleoearthquake, providing a
moment magnitudeM = 6.8. The zone in which the liquefaction structures are found has undergone tectonic uplift
and is affected by two faults. One of these faults was responsible for displacing Holocene materials. Dating of the
uplifted sand formation indicates an age of 10,081 ñ 933 years, the liquefaction features ranging from this age to
3490 ñ 473 years BP. This paleoearthquake was of much greater magnitude than those known historically. Faults
with neotectonic activity are significant features that should be borne in mind when assessing the seismic hazards
of the Canary Islands, presently considered as low and mainly of volcanic origin
Resumen: A complete multibeam coverage of the sea floor of Mallorca Channel, in the western Mediterranean, was recorded during
the Spanish Exclusive Economic Zone surveys in 1995, 1996 and 1997. These data, combined with previous high-resolution
seismic reflection profiles, allow an assessment of the geomorphology of the area. The
channel seafloor is disrupted by a fault
complex and pockmarks. Motion along the faults split the sea floor into a series of undulations separated by narrow V-shaped
notches. Faulting may be a consequence of recent seaward gravitational sliding that occurred along a soft surface at the top of a
late Messinian–early Pliocene unit and a late Messinian evaporite. These units have been tilted during recent subsidence of the
Mallorca Channel at the same time that the insular shelf was uplifted along a fault at the shelf’s edge. The set of pockmarks in
the channel sequence were probably formed by the expulsion of gas of hydrothermal origin, and expulsion may have been
enhanced by the faulting. This gas seepage could be an additional factor leading to sediment instability.
Resumen: We infer that the Alboran Basin, the first western Mediterranean Basin found after crossing Gibraltar, is an orogenic float underlained by a de´ collement system, a multi-layered ductile shear extending from 10km to between 30 and 40km below sea level. This float was formed as consequence of the
collision of the African–Eurasian plates in the Oligocene–late Miocene. Synchronous with this compression the float experienced basin wide crustal thinning and subsidence about 25 m/year ago by subcrustal processes. Since latest Miocene the float has undergone compression due to the continuous convergence of Eurasia and Africa. The faults created as a result of this compression are dominated by a conjugate system of northeast trending left-lateral and northwest right-lateral strike–slip faults. This deformation is taking place under a simple shear mechanism. Associated with the northwest and northeast lateral faults are zones of compression trending west and east of north extending from the base of the basin’s north upper slope to the Alboran Ridge. The initial morphology of the Alboran Ridge on the southern side of the Alboran Basin was due to the construction of a volcanic edifice at the northeast end of the ridge and igneous activity along northeast trending fractures southwest of the edifice. At the northeast end of the Alboran Ridge motion along a right-lateral fault cutting across the ridge led to sediment collapse and the creation of a prominent embayment on the ridge’s northwest flank. Deformation is more subdued in the western than in the eastern part of the Alboran Basin, a tectonic style due either to differences in sediment rheology or that the accommodation of the convergence of Africa and Iberia is more diffused and attenuated in the west than in the east.
Resumen: As part of the ‘National Hydrographic and Oceanographic Research Plan for the Spanish Exclusive Economic
Zone’, multibeam bathymetry and seismic reflection profiles were obtained in the Canary Islands aboard the R/V
Hespérides. The submarine flanks of the Anaga offshore extension of Tenerife
Island are here studied to analyze its
geomorphology. In the north sector of the Anaga submarine massif, the extension of the Anaga Debris Avalanche
has been mapped for the first time, and a volume of 36 km3 was calculated. The relationship between the Anaga
and Orotava Debris Avalanches is also described. Faulting has been recognized as a key process for the occurrence
of debris avalanches and the growth of volcanic lineaments. Moreover, faulting affects previous structures and the
channelling of debris flows. Structural analysis shows the typical radial pattern of an oceanic island. In addition, a
NE-SW dominant direction of faulting was obtained, consistent with the Tenerife Island structural trend seen in the
Anaga Massif and Cordillera Dorsal. NW-SE and E-W are two other main trends seen in the area. Special interest
is manifest in two long faults: ‘Santa Cruz Fault’ bounds the southern edge of Anaga offshore Massif with a length
of 50 km and a direction that changes from NE-SW to almost E-W. The Güimar Debris Avalanche was probably
channeled by this fault. The ‘Guayotá Fault’ was recognized in several seismic profiles with a N-S direction that
changes towards NW-SE at its southern end. This fault affects the more recent sediments with a vertical offset of
25–30 m, along 60 km. It has been interpreted as a transpressive strike-slip fault.
Palabras clave: Anaga massif; Bathymetry; Debris avalanche; Fractures; Tenerife island
