Typical Mid-ocean Ridge Research Articles

Ultramafic and mafic dredge samples from the equatorial Mid-Atlantic ridge and fracture zones

Dredge samples selected for study from the Vema Romanche, and Chain fracture zones include serpentinized spinel lherzolites, ilmenite norites, oceanic tholeiites, and a teschenite. The serpentinized ultramafic rocks from the fracture zones are chemically and mineralogically similar to spinel lherzolites (or harzburgites) found as ultramafic tectonites or cumulates. The primary mineral assemblage is olivine (Fo89.3–91.6) orthopyroxene (En87.0–88.8) clinopyroxene (Wo38.4–49.0 En46.4–55.5 Fs4.0–6.1), and a spinel rich in Cr and Al. These rocks apparently represent depleted upper mantle exposed at the base of the fracture zones or diapirically intruded along the fracture zones, or, alternatively, ultramafic cumulates such as in ophiolite complexes. The TiO2-rich norites (TiO2 4.00–5.88 wt %) are iron-rich (FeO + 0.9 Fe2O3 17.3–18.2 wt %) and compositionally resemble the late-stage Skaergaard rocks. Major minerals are plagioclase (An37–68), orthopyroxene (En58–70), clinopyroxene (Wo40–44 En15–17Fs41–43), low-Mg ilmenite, and low-Ti magnetite. Low K2O oceanic tholeiites from the central rift valley on the ridge are typical midocean ridge basalts. Megacrysts of plagioclase that include olivine grains are believed to be of cognate origin. The teschenite, which closely resembles the Black Jack sill teschenite, contains high-Ca titaniferous augite (Wo46–48En23–31Fs15–31), plagioclase (An10–75), ilmenite, titaniferous magnetite, analcite, and zeolites. The diversity of rock types and the occurrence of alkalic basalt imply complex plutonic processes occurring within the fracture zones.

On the Driving Forces of Plate Tectonics

Sumnznry We suppose that the plates are pulled along on top of an effectively viscous asthenosphere by their cold dense sinking leading edges, and that they also tend to slide down the flanks of ocean ridge systems. Using reasonable literature values of density, viscosity and thickness, we find that a typical strong subduction zone pulls about seven times as hard as a typical mid-ocean ridge pushes. With the simplifying assumptions that other driving forces are much smaller, and the return current in the asthenosphere is everywhere (anti)parallel to the plate velocity, we perform a torque balance for each major plate in order to find its angular velocity, thus finding a set of relative angular velocities to compare with observations. The directions fit quite well but not the magnitudes. On the additional hypothesis that oceanic lithosphere thickens with age, owing to heat diffusion, the driving forces will be greater for large old plates. The fit is thereby greatly improved: predicted speeds are from 0.81 to 1.38 times the observed and the COCOS plate is not anomalous. The model predicts tension in plates near their subduction zones. The Red Sea and African rift appear to have begun spreading as a crack moved into the former African +Arabian plate from a re-entrant corner off the Gulf of Aden which would have concentrated the stress. Gondwanaland may possibly have been split in a similar way.

Seismicity and tectonics of the east Pacific Ocean

The epicenters of earthquakes in the east Pacific Ocean for the period 1961–1972 were evaluated and in some instances recomputed to create a file of accurate epicenters. These epicenters were used to delineate many fracture zones, two triple junctions, and the axial trend of the mid-oceanic ridges more accurately than has been done previously. The Pacific-Antarctic ridge is cut by transverse fracture zones with associated earthquake activity. Three active en echelon fractures are delineated where the Eltanin fracture zone bisects the ridge; but north of the fracture at 50°S, between 37° and 49°S, earthquakes are located only on possible extension of the fractures. The Chile ridge is interpreted as a typical mid-oceanic ridge structure of rifted ridge sections offset by fractures east of 94°W and large active transverse fractures to the west. Two unusual seismicity patterns were found near 32°S, 111°W and 24°S, and 112°W, where the largest earthquakes were not located on the ridge but to the east of it, and the main seismic pattern. North of 20°S almost all the located earthquakes were associated with fracture zones except in the area of the triple junction at 2.2°N, 102.2°W. The Galapagos rift zone is interpreted as a typical oceanic ridge with fracture zones and offset ridge segments except for an apparent alignment of the volcanic related earthquake in the Galapagos Islands with those located along the fracture zone at 91°W.