Oceanic Moho Research Articles

Evidence for the subducting lithosphere under southern Vancouver Island and western Oregon from teleseismic P wave conversions

Long‐period teleseismic P waves recorded at VIC (Victoria, British Columbia) and COR (Corvallis, Oregon) show anomalously large Ps conversions and later arriving P‐to‐S reverberations not observed from typical continental crustal sections or from previously proposed structures for these stations determined from refraction surveys. The timing and large amplitude of the Ps phase, relative to direct P, suggests a high velocity‐contrast interface at 45 to 50 km depth under VIC and COR forming the base of a distinct low velocity zone. This interface is proposed to be the oceanic Moho which is being subducted under North America. Off azimuth Ps recorded at COR is consistent with a 20° eastward dip for the interface. Horizontal particle motion at both sites show evidence for lateral heterogeneity in local crustal structure. The distinct low velocity zone and its negative velocity gradient with depth has important consequences for refraction interpretation in the region since the usual assumption of increasing velocity with depth is violated. Crustal thicknesses derived from such misinterpretation may be overestimated. In principle, this type of structure suggests a solution for the Vancouver Island crustal thickness problem in which the observed positive Bouguer gravity anomaly is inconsistent with the 50 km thick crustal thickness derived from previous refraction work.

Petrologic nature of the oceanic Moho

The oceanic Mohorovicic discontinuity may be well within the tectonized harzburgite unit found at the base of most ophiolites. The model developed indicates that the Moho is a boundary separating partially (35%) serpentinized ultramafic rocks above from fresh tectonized ultramafic rocks below.

Petrologic nature of the oceanic Moho: Comment and reply

Petrologic nature of the oceanic Moho: Comment and reply

Petrologic nature of the oceanic Moho: Comment and reply

Petrologic nature of the oceanic Moho: Comment and reply

Dynamics of the active plate boundary in southwest colombia according to recent geophysical measurements

Deep seismic sounding refraction studies made during the Nariño Project, combined with other recent geophysical and geological data, show many features of the active boundary between the Nazca Plate and the South American Plate in Colombia. Two sea-land profiles, 600 km in length, reveal a shallow anticline about 200 km offshore, topped by a pronounced graben. In this area of lithospheric bending we have observed the beginning of a small dip in the Moho and some shallow seismicity within the uppermost mantle. The oceanic Moho can be followed by clear P n-arrivals down to depths of 50 km below the coastal sediments and further down to about 200 km by seismicity studies. The coastal plains are interpreted as overlaying a hidden trench. The West Andes, being oceanic according to geological investigations, are also found to be oceanic by their high velocity—high density crustal composition. They seem to be carried to their present position by a process of obduction and introduction, which always seems to accompany a general subductive process. The chain of andesitic volcanoes in the Central Andes in situated about 150 km off a hidden trench and 150 km above the subducted oceanic crust. The Central Andes show a continental structure with rather low crustal velocities indicating high temperatures and strong inhomogeneities, possibly caused by frictional heating and magmatic activity. In addition to subduction, lateral strike-slip displacements seem to have taken place during the last sixty million years.

The Eastern End of the Azores-Gibraltar Plate Boundary

Summary The known relative motion of the African and Eurasian plates at the eastern end of the Azores-Gibraltar plate boundary implies consumption of oceanic lithosphere at the low rate of 1–1.5 cm/yr. This section of the plate boundary, which crosses the area 34–38deg;N, 6–14° W, has none of the characteristic features of zones of oceanic lithosphere consumption (e. g. arcuate trench, island are volcanics, deep seismicity). A block of oceanic crustal and upper mantle material has been upthrust to form a 120 km long, 5 km high ridge, called Gorringe Ridge. Basalts, gabbros and ultramafics have been dredged from the northern face of this ridge. A mechanism for slow consumption is proposed which accounts for the anomalous features of this part of the plate boundary, and for the formation of Gorringe Ridge. Four deep seismic refraction lines show the major characteristics of the crustal structure to be 3–4 km thick sediment sequences, low upper mantle velocities of 7.3-7.6 km s−1 and typical oceanic moho depths of about 11 km below sea level. Continuous seismic reflection profiles show 1–2 km of basement relief in the Western Horseshoe and Tagus Abyssal Plains and define a zone of acoustically opaque sediment in the Eastern Horseshoe Abyssal Plain. The free-air gravity anomaly contour map shows 480 mgal. of relief. Closely-controlled models for the structure of Gorringe Ridge devised to fit north-south gravity profiles across the plate boundary require a large body with a density of at least 3.0 g cm−3 beneath the ridge to explain its associated 390 mgal. free-air anomaly. The plate boundary is not well defined by the relocated epicentres but crustal structures suggest a 100 km southward offset immediately west of Gorringe Ridge. The calculation of finite difference poles of rotation for the African plate relative to Eurasia shows there have been two major periods of relative motion across the plate boundary in this region during the past 72 My. The slow compressive phase of the last 10 My was preceded 60–72 My before present by right lateral strike-slip motion at a rate of 5.5 cm/yr. The direction of these relative motions was respectively perpendicular and parallel to 060°, a trend which is predominant in the bathymetry of this area and in the geology of southern Spain. This suggests that for the past 72 My a single line of weakness has existed in the lithosphere along which all motion between the African and Eurasian plates has been accommodated.

High-Pressure Assemblage Changes Near Moho Depths: ABSTRACT

Laboratory studies have been carried out on synthetic mineral systems representative of basalt, on natural basalts and eclogites, and on peridotites reconstructed from natural minerals. The transformation from basalt through pyroxenite to eclogite was found to take place over a broad pressure range, approximately 4 to 8 kb, depending on bulk composition and temperature. The mineralogical changes involve complex solid solutions of phases varying in velocity of transmission of compressional waves from about 5.5 to 8.5 km./sec. The series of assemblage changes do not appear to be accompanied by marked changes in velocity of compressional waves. High-pressure changes in hydrous sediments which produce assemblages of uniquely dense hydrous minerals or metamorphic assemblages w ll probably have End_Page 364------------------------------ similar characteristics. Therefore such assemblage changes are probably not responsible for a sharp continental Moho discontinuity but may contribute to a deeper than average, gradual, continental Moho. Assemblage changes are known for certain bulk compositions in the region of pressure and temperature believed to exist near the oceanic Moho; however, they do not pertain to the rocks believed to exist there. Although the Moho discontinuity is not sufficiently well resolved, the subtle high-pressure assemblage changes in a varying heat flow and geothermal gradient imposed on a complex structural configuration do not appear to be as critical as significant bulk compositional changes in determining the character of a discontinuity. End_of_Article - Last_Page 365------------