Deep-sea hydrothermal vents possess complex ecosystems and abundant metallic mineral deposits valuable to human being. On-axial vents along tectonic plate boundaries have achieved prominent results and obtained huge resources, while nearly 90% of the global mid-ocean ridge and the majority of the off-axial vents buried by thick oceanic sediments within plates remain as relatively undiscovered domains. Based on previous detailed investigations, hydrothermal vents have been mapped along five sections along the Southwest Indian Ridge (SWIR) with different bathymetry, spreading rates, and gravity features, two at the western end (10°–16°E Section B and 16°–25°E Section C) and three at the eastern end (49°–52°E Section D, 52°–61°E Section E and 61°–70°E Section F). Hydrothermal vents along the Sections B, C, E and F with thin oceanic crust are hosted by ultramafic rocks under tectonic-controlled magmatic-starved settings, and hydrothermal vents along the Section D are associated with exceed magmatism. Limited coverage of investigations is provided along the 35°–47°E SWIR (between Marion and Indomed fracture zones) and a lot of research has been done around the Bouvet Island, while no hydrothermal vents has been reported. Analyzing bathymetry, gravity and geochemical data, magmatism settings are favourable for the occurrence of hydrothermal systems along these two sections. An off-axial hydrothermal system in the southern flank of the SWIR that exhibits ultra-thin oceanic crust associated with an oceanic continental transition is postulated to exist along the 100-Ma slow-spreading isochron in the Enderby Basin. A discrete, denser enriched or less depleted mantle beneath the Antarctic Plate is an alternative explanation for the large scale thin oceanic crust concentrated on the southern flank of the SWIR. 相似文献
For small earthquakes, focal depths can be estimated jointly when epicenters are located using the arrival times of Pg and Sg waves recorded at seismic stations close to the event. However, if regional network coverage is sparse, this approach does
not give accurate results. An alternative solution is the use of the regional depth-phase modeling (RDPM) method when such
depth phases are available. Small, shallow earthquakes can generate Rg waves, the amplitudes of which approximately attenuate exponentially with focal depth; whereas, the amplitudes of Sg waves are, on average, less dependent on focal depth. Based on these features, a method using the maximum power spectral
ratio (MPSR) between the Rg and Sg segments was developed to determine focal depth. Tests show the focal depth solutions obtained by the MPSR and RDPM methods
for five events in an earthquake swarm and one event acquired by inspection are in good agreement. The error in the MPSR-determined
focal depth caused by the error in the epicentral distance is in the order of 0.1 km. The error in the focal depth when using
a default focal mechanism is in the order of 0.5 km. The quality factor, Q does not generate a significant error. Using the average of focal depths can provide a more reliable solution. Using an azimuth
of approximately 45° from the strike direction to generate the synthetic ratio curve can reduce the error. As with any other
earthquake locating technique, a reasonable regional crustal model is required when the MPSR method is used. Case studies
show that the MPSR method can be used to successfully determine focal depths for events as small as mN 1.6. 相似文献
The basic parameters for the earthquake with a moment magnitude (MW) of 5.2 on the 23rd of June 2010 have been investigated. The earthquake occurred on a hidden fault in the northwest direction about 60?km north-northeast of Ottawa in the Western Quebec Seismic Zone (WQSZ) and had a focal depth of about 21?km. The focal mechanism was a thrust type with strike in the northwest direction and dipping in the northeast direction. The relative relocations of seven larger aftershocks show that the source rupture area was about 6?km2. The b value of the aftershock sequence was 0.8?C1.0, and the decay rate of the aftershocks was faster than normal cases. The dominant seismogenic depths are about 12 to 22?km in most parts of the WQSZ, while the seismogenic depth along the Ottawa?CBonnechere Graben can be as deep as 28?km. Based on the seismic activity in the WQSZ and vicinity since 1961, it seems that the periods of moderate earthquakes are about 6?C10?years. 相似文献
<正>Objective The Shanwang Basin is a small Cenozoic sedimentary basin located in Linqu county,Shandong province.The Shanwang Formation,especially the diatomaceous shale member,contains diverse and finely preserved flora and fauna fossils(Fig.1).Previous paleontological study and radiometric dating show that it was formed in the Miocene.However,on the precise age of the formation,there are such different opinions as Late Miocene,Middle 相似文献
Recently, garnet pyroxenite enclaves within peridotites occurring near Raobazhai, Huoshan County, have been discovered. The garnet pyroxenite is small pods, decimeters in size, enclosed within intensively serpentinized peridotites. Major mineral components comprise: garnet (Prp25–35), sodium augite (Jd10–25) with a small amount of ilmenite. There are two stages of retrometamorphism: the retrogressive granulite facies mineral assemblage is superimposed by that of amphibolite facies. The host rocks of the garnet pyroxenite are spinel peridotites, including spinel harzburgite and lherzolite. Due to intensive serpentinitization, only 5%–40% of the relic olivine (Fo92–93) are preserved. The orthopyroxenes are Mg-rich (En87–93) with bending of cleavages and granulation at their margins showing intracrystalline plasticity. On the basis of garnet-clinopyroxene Fe−Mg exchange equilibrium geothermometry proposed by Ellis & Green (1979) and Krogh (1988)KD=4.06–5.28;T=793–919°C,P=1.5 GPa are estimated for the garnet pyroxenite. It is inferred that the peridotites are mantle rocks about 60 km in depth. During the exhumation of the orogenic belt, it was tectonically emplaced into the lower crust in the solid state and then uplifted to the shallow depth. Obviously, this kind of garnet pyroxenite must be petrogenetically related to its host rock. The REE distribution pattern and the Ni−Co−Sc diagram reveal that they are chemically equivalent to the basaltic melt and ultramafic residua respectively derived from partial melting of mantle rocks.