Seismicity re-evaluation of the northern Red Sea

The Aqaba subnetwork of five vertical short-period stations of the seismological observatory of King Saud University was installed in late 1986 along the eastern side of the Gulf of Aqaba, northern Red Sea. During the first six years (1986 August to 1992 July) of the subnetwork operation, 400 microearthquakes were detected. Of these, 93 events were recorded by most of the subnet stations and were located. Their epicentres lie in the northern part of the Red Sea between latitudes 25.5 N and 27.5 N and longitudes 33.5 E and 36 E along the axial depression of the Red Sea where the large intrusions (deeps) are located. Magnitudes of the locatable events ranged from 2.1 to 4.8. Two intensive swarms of about 200 microearthquakes occurred in February and June of 1992. The February swarm is the first intensive sequence observed in the surveying area since the establishment of the KSU network. Frequency-magnitude analysis of the recorded events for the period 1986-1992 yielded 3.543 for a and 0.658 for b . These relatively higher b values (0.658) are a good indication of the crustal heterogeneity under the spreading zone of the northern Red Sea. USGS and KSU data together show 3.41 for a and 0.49 for b . This study, together with historical data, confirms that the area is very seismically active and that the activity is mainly of swarm type, and may be attributed to the subsurface magmatic activity and spreading centres that are usually associated with strike-slip and normal faulting, respectively.     

Extension across the Indian–Arabian plate boundary:the Murray Ridge

Seismic reflection profiles from the Murray Ridge in the Gulf of Oman, northwest Indian Ocean, show a significant component of extension across the predominantly strike-slip Indian–Arabian plate boundary. The Murray Ridge lies along the northern section of the plate boundary, where its trend becomes more easterly and thus allows a component of extension. The Dalrymple Trough is a 25 km wide, steep-sided half-graben, bounded by large faults with components of both strike-slip and normal motion. The throw at the seabed of the main fault on the southeastern side of the half-graben reaches 1800 m. The northwest side of the trough is delineated by a series of smaller antithetic normal faults. Wide-angle seismic, gravity and magnetic models show that the Murray Ridge and Dalrymple Trough are underlain by a crystalline crust up to 17 km thick, which may be continental in origin. Any crustal thinning due to extension is limited, and no new crust has been formed.
We favour a plate model in which the Indian–Arabian plate boundary was initially located further west than the Owen Fracture Zone, possibly along the Oman continental margin, and suggest that during the Oligocene–Early Miocene Indian Ocean plate reorganization, the plate boundary moved to the site of the present Owen Fracture Zone and that motion further west ceased. At this time, deformation began along the Murray Ridge, with both the uplift of basement highs, and subsidence in the troughs tilting the lowest sedimentary unit. Qalhat Seamount was formed at this time. Subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produce the present basement topography. The normal faulting was accompanied by hanging-wall subsidence, footwall uplift, and erosion. Flat-lying recent sediments show that the major vertical movements have ceased, although continuing earthquakes show that some faulting is still active along the plate boundary.     

Post‐seafloor spreading magmatism and associated magmatic hydrothermal systems in the Xisha uplift region,northwestern South China Sea

Submarine magmatism and associated hydrothermal fluid flows has significant feedback influence on the petroleum geology of sedimentary basins. This study uses new seismic profiles and multibeam bathymetric data to examine the morphology and internal architecture of post‐seafloor spreading magmatic structures, especially volcanoes of the Xisha uplift, in extensive detail. We discover for the first time hydrothermal systems derived from magmatism in the northwestern South China Sea. Numerous solitary volcanoes and volcanic groups occur in the Xisha uplift and produce distinct seismic reflections together with plutons. Sills and other localized amplitude anomalies were fed by extrusions/intrusions and associated fluid flow through fractures and sedimentary layers that may act as conduits for magma and fluid flows transport. Hydrothermal structures such as pipes and pockmarks mainly occur in the proximity of volcanoes or accompany volcanic groups. Pipes, pockmarks and localized amplitude anomalies mainly constitute the magmatic hydrothermal systems, which are probably driven by post‐seafloor spreading volcanoes/plutons. The hydrothermal fluid flows released by magma degassing or/and related boiling of pore fluids/metamorphic dehydration reactions in sediments produced local overpressures, which drove upward flow of fluid or horizontal flow into the sediments or even seafloor. Results show that post‐seafloor spreading magmatic activity is more intense during a 5.5 Ma event than one in 2.6 Ma, whereas the hydrothermal activities are more active during 2.6 Ma than in 5.5 Ma. Our analysis indicates that post‐seafloor spreading magmatism may have a significant effect on hydrocarbon maturation and gas hydrate formation in the Xisha uplift and adjacent petroliferous basins. Consequently the study presented here improves our understanding of hydrocarbon exploration in the northwestern South China Sea.