A. Hildenbrand

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.

Palaeomagnetic study of a subaerial volcanic ridge (Sao Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

Magma genesis controlled by tectonic styles in the northern part of the Arabia plate during Cenozoic time

Abstract Widespread lava fields in the northern part of the Arabian platform are the subject of an open geodynamic debate on the origin of the intraplate volcanism. We present new K–Ar ages and whole-rock geochemical data for lava flows from Syria, which allow us to propose a new model of volcano-tectonic evolution highlighting how tectonics have controlled magma genesis in the region during the last 18 Ma. The Cenozoic Syrian lavas are alkaline and subalkaline rocks, typical of magma erupted in such a continental intraplate context. Basaltic samples from different Syrian volcanic provinces show significant variations in terms of incompatible trace element signatures. Crustal contamination plays a negligible role during magma migration and differentiation, and crystal fractionation is essentially restricted to olivine and clinopyroxene. Our results suggest that the Syrian lavas have been generated by variable degrees of partial melting (c. 1–10%) from different levels of a locally heterogeneous lithospheric mantle. The light/medium rare Earth element (LREE/MREE) ratios not only illustrate how the degree of partial melting has changed spatially and temporally during the last c. 18 Ma, but also indicate that the degree and the style of extensional tectonics has changed through time. We conclude that the Cenozoic Syrian volcanism is a consequence of extensional tectonics, under periodical influence of the north- and eastwards convergence at the Arabia–Eurasia margin, which induces rotational tectonic styles. This controls the partial melting at various depths in the mantle. The volcanism of northern Arabia developed in the framework of the Red Sea rifting and initiated at the same time as the southern Red Sea volcanism. It extends up to historical time, progressively smoothed to the north in a contradictory relation with the compressional/extensional setting of the Arabia–Eurasia margin. Supplementary material: Further information on the analytical results is available at http://www.geolsoc.org.uk/SUP18681.

Morpho-structural analysis of Harrat Al Sham volcanic field Arabian plate (Syria, Jordan, and Saudi Arabia): methodology and application

We present here a digital methodology aimed at identifying the morphometry of monogenic volcanic cones including edifice height, slope angle, and volume. We recognize more than 800 monogenic volcanic cones from the Harrat Ash Shaam volcanic province (HASV, northern Arabian Plate) to the north of the Arabian plate by using Landsat7 Enhanced Thematic Mapper plus and digital elevation models (Shuttle Radar Topography Mission, SRTM data). Our results show that the volcanic cones fall into six clusters, each one having a NW-SE orientation parallel to the Red Sea rifting. The density of the cones is most likely controlled by fissure distribution and a possible rotation in the HASV lithospheric block, rather than by the age of the volcanic activity. Interaction with crustal lithologies also plays a negligible role during magma migration. However, the morphology of the monogenic volcanic cones appears to be significantly modified by erosion processes, and both the height and slope of the various cones thus can be used to estimate their relative ages. From such morphological criteria, HASV monogenetic volcanoes in Jordan and Saudi Arabia are older than those in the Golan Heights and the Jabal Al Arab Mountain in Syria, thus, supporting the hypothesis of a northward migration of the recent explosive activity. Our new data, together with the available sediment thickness data, give rise to a new volcano-tectonic interpretation. We show that variations in the cumulative volume of monogenic volcanic cones in several zones of the study area can be related with the sediment thickness (basement depth), and therefore, we propose that the sediment thickness most likely affects the properties of magma before eruption. The reactivation of a deep previous fault perhaps also explains the higher volume of monogenic volcanic cones, focused along a well-defined alignment. This study, thus, gives insight into the relations between sediment thickness, the volcanism, and tectonics. Our results based on new digital treatment of remote sensing data are in overall good agreement with the volcano-tectonic evolution of HASV. This allows us to recommend this methodology as a potential tool to study volcanic cones of other inaccessible regions, either on Earth or other terrestrial planets.

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications: Palaeomagnetic study of S. Jorge Island

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.