Rift Valley Wall Research Articles

Near bottom water temperature anomalies: Mid‐Atlantic Ridge Crest at latitude 26°N

Six water temperature anomalies were measured along two profiles each about 10 km in length at the TAG Hydrothermal Field on the east wall of the rift valley of the Mid‐Atlantic Ridge at latitude 26°N. The temperature anomalies were measured using a 3 m‐long vertical array of four thermistors towed within 20 m of the ocean bottom. The anomalies exhibit increases in potential temperature up to 0.127°C above ambient and inverse gradients warming downwards as great as 4.2 × 10−2°C m−1. The temperature anomalies are distributed as point sources in a linear pattern about 7 km in length along known fault zones in the east wall of the rift valley between depths of 2750 and 3200 m below sea level associated with hydrothermal mineral deposits. The characteristics of the temperature anomalies and their geologic setting indicate an origin by discharge of hydrothermal solutions through the faults in the rift valley wall. The magnitudes of the anomalies are consistent with values previously used to estimate a heat output of the order of several megawatts per kilometer of ridge at the TAG Hydrothermal Field.

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

Duration of hydrothermal activity at an oceanic spreading center, Mid-Atlantic Ridge (lat 26°N)

Hydrothermal manganese oxide coats talus on the Mid-Atlantic Ridge at lat 26/sup 0/N until spreading moves the rock past the thermally and structurally active rift-valley wall. Hydrothermal activity is replaced by hydrogenous ferromanganese oxide precipitation on ocean crust older than 0.7 m.y. on the ridge-crest highlands.

Rate of palagonitization and manganese coating on basaltic rocks from the Rift Valley in the Atlantic Ocean near 36°50′N

Various types of rocks consisting of plagioclase basalts, olivine basalts, pyroxene basalts, picritic basalts and volcanoclastics were found in the Rift Valley between 36°52′N and 36°47′N in the Atlantic Ocean. Palagonite and manganese are the main coating materials encountered on the chilled margins of the pillow-lava fragments. Palagonite is often associated with montmorillonite and chlorite. The thickness of palagonite coating the chilled margins increases in the fragments found from the innerfloor axis towards the walls of the Rift Valley. There is a linear correlation between the thickness of palagonite and that of the manganese encrustation. Considering a rate of manganese and palagonite accumulation of about 3μ 10 3 year it is speculated that the age of a hill located near the axis of the inner floor near 36°50′N is between 3,000 and 12,000 years. The relative age of the inner floor near the intersection of the Rift Valley walls is less than 120,000 years. The specimens having a thicker manganese and palagonite crust located at 4–6 km away from the inner floor axis are 130,000–482,000 years old.

Some characteristics of the Rift Valley in the Atlantic Ocean near 36° 48′ north

The Rift Valley between 36° 42′N and 36° 55′N in the Atlantic Ocean is 31 km wide, with half-widths of 12 and 19 km for the western and eastern sides respectively. Both outer edges of the Rift Valley stand about 1500 m above an Inner Floor where very fresh pillow lavas occur. The Inner Floor probably includes the locus of new crust; and its bordering slopes, which are particularly well-defined on the western side, limit to less than about 2.5 km the width of the zone over which new crust may have evolved with little or no vertical displacement. The width of the locus of new crust may be less than 0.5 km between 36° 45′N and 36° 47′N, where the deepest slopes of the Rift Valley walls nearly merge. Near 36° 50′N, the Inner Floor accommodates an approximately 1 km wide, 4 km long Central High, with a height of up to 250 m. In this area, the locus of new crust may also occupy a very narrow zone; it may lie either along the Central High or along a trough flanking the Central High. The magnetic anomaly pattern indicates that, since the beginning of the Brunhes epoch (6.9 × 10 5 yr B.P.), the eastern limb has grown approximately twice as fast as the western limb. Using extrapolated spreading rates, the ages of the outer edges of the Rift Valley are 1.3 and 1.7 m.y. for the eastern and western sides respectively. Comparison with data for the Rift Valley in other parts of the ocean further suggests that the residence time of new crust in the Rift Valley is about 1.5 m.y. Uplift of crust from the Inner Floor, which may be dominated by lithospheric thickening, may thus be primarily a function of age.

Detailed near-bottom geophysical study of the Gorda Rise

A deeply towed instrument package was used to survey the fine details of topography, sediment distribution, and magnetization of the Gorda rise, an active spreading center off northern California. The gross form of the central rift valley is the result of large-scale normal faulting. The surface is broken into long, narrow, tilted steps parallel to the spreading center. Topographic features just outside the rift valley also have a tilted blocky aspect and are lineated and symmetrical with respect to features of the same age on the opposite flank. Flat terrigenous turbidite sediments are found in the valley floor and in deeps on the west flank, as might be expected. They also are found on the tops of the steps high in the rift valley walls, implying that the steps are uplifted to form the walls as they move out from the center. The magnetic field measured near the ocean bottom can be approximately simulated by using straightforward model calculations. The extremely complex short-wavelength anomalies (width less than 1 km) generally are caused by topographic effects, and from them the bulk magnetization of the topographic features was estimated to be about 0.009 emu/cm3 near anomaly 2 and 0.007 emu/cm3 near anomaly 3. Assuming these values of magnetization for the entire magnetic layer, the magnitude of long-wavelength anomalies shows that it must be about 0.5 km thick. In crust formed at a spreading rate of 37 mm/yr, magnetic field polarity reversals are recorded in the magnetic layer as a gradual zone of transition about 2 km wide. Of this width, 0.1 to 0.7 km can be attributed to the time it takes for a reversal to be completed, and the remaining width, about 1.7 km, can be attributed to the process of emplacement of magnetic material. Comparisons with emplacement models show that most magnetic material was emplaced within 2 km of the center (within 0.7 km if a pure dike injection model is assumed). The 2-km width of the emplacement effect acts as a smoothing function for events in the magnetic field, so that the record in the magnetic layer of events of duration less than about 10,000 years should be greatly attenuated. Two medium-wavelength anomalies are interpreted as possible normal polarity events centered at 3.4 and 3.55 m.y.

Block Faulting on the Gorda Rise

A study made of Gorda Rise near 41 degrees 15'N with a novel instrument shows that the rift-valley walls have a tilted steplike profile, often with perched, planar sediments. Topography indicates that the steps were formed by block faulting. Distribution of the steps and the character of their tops suggest that they originated in the central 2 or 3 kilometers of the valley floor and were subsequently moved outward, uplifted, and tilted along with their underlying blocks as the sea floor spread. Gorda Rise is considered a slowly spreading part of the oceanic-rise system. Studies of the Mid-Atlantic Ridge report features that may be similar uplifted blocks.