Peter R. Vogt

Plumes, subaxial pipe flow, and topography along the Mid-Oceanic Ridge

If plate thickness depends on crustal age, the region of extensive partial melting below the spreading axis will be wider around fast-spreading ridges. The melt region creates a subaxial conduit channeling partial melts away from ridge-centered hot spots. The channel is here modeled by an elliptical pipe of semiminor (vertical) axis 2 × 106 cm (20 km) and semimajor (horizontal) axis KS, where S is spreading half-rate (cgs) and K is a constant of magnitude 1014 to 1015 seconds. This simple analytical model is used to explain the observation that maximum hot spot elevations on the Mid-Oceanic Ridge fall dramatically with increasing spreading rate (there are no Icelands or Afars on the East Pacific Rise!). A hot spot under a fast-spreading ridge has a broad pipe in which to discharge its partial melts; hence, only a slight topographic gradient and a low elevation is needed to discharge the mass flux rising out of the deeper mantle at the hot spot center. A second factor is that partial melts are “used up” faster in the accretion process on fast-spreading ridges. In the simple analytical model, both factors operating together explain the rapid fall of hot spot heights with increasing spreading half-rate. This result indirectly helps confirm the idea of horizontal pipe flow below the Mid-Oceanic Ridge. A theoretical topographic profile through a hot spot on the Mid-Oceanic Ridge is derived from the assumption that the pressure — i.e., topographic — gradient at a distance x from the hot spot is sufficient to supply all the accreting lithosphere downstream of x, out to xn, the limit of topographic hot spot influence. The predicted profile is quadratic in x and concave upward, and resembles several observed profiles where neighboring hot spots are not so close as to confuse the profiles. Some observed profiles are more nearly linear or even convex upward. This could be explained, for example, by downstream increases in viscosity or decreases in pipe dimensions. A hot spot on a ridge spreading at much less than 1 cm/yr half-rate would produce an enormous elevation of the ridge axis, according to our model, because the pipe would be very narrow. Such a large topographic high would create a large gravity potential which would cause the plates to move apart faster, thereby widening the pipe, and reducing the topographic high. The system of ridges and hot spots may thus be self-regulating with respect to plate speeds; this could explain why spreading half-rates on the Mid-Oceanic Ridge are in many areas as low as 1.0 cm/yr but very rarely as low as 0.5 cm/yr.

Discontinuities in sea-floor spreading

Abstract Time variations in the rate and direction of sea-floor spreading, though relatively slight, have occurred simultaneously along much of the Mid Oceanic Ridge at intervals of 10–30 million years. Various criteria by which such discontinuities might be recognized are examined. The recent discovery that transform faults, at least those of small offset, may be created and destroyed repeatedly during the life of an ocean basin suggests several physical models for the creation and destruction of such faults. The shape of the Mid-Oceanic Ridge may represent a minimum-work configuration for a particular spreading rate and direction. When these quantities change the ridge shape adjusts itself to a new configuration if the energy barrier is not too great. The rotation of a ridge segment with respect to the spreading direction may require several million years and depends on spreading rate, ridge orientation, and fracture spacing. Observed rotation rates are about one third or one half the theoretical maximum.

The Terceira Rift as hyper-slow, hotspot-dominated oblique spreading axis: A comparison with other slow-spreading plate boundaries

Abstract We suggest the 550 km long Terceira Rift (TR, Azores Plateau) is the world’s slowest-spreading (hyper-slow, 4 mm/a plate separation; 2.3–3.8 mm/a perpendicular to oblique axial segments) organized accreting plate boundary. In its slightly sinuous (ca. 300 km radius of curvature) axial trace, its oblique spreading angles (ca. 40°–65°), and in frequency and first motions of earthquakes, the TR resembles better-known ‘ultra-’ or ‘super-’ slow spreading ridges (e.g. Gakkel and Southwest Indian ridges). Interpreted simply as volcanically ‘unfilled’ rift valley segments, the inter-island basins (e.g. the 3200 m deep Hirondelle Basin) are slightly wider (30–60 km), but not significantly deeper (1000–2200 m) than the Mid-Atlantic Ridge (MAR) median valley (20–28 mm/a; 10°N–53°N). However, along-axis segmentation wavelengths (ca. 100 km) are double those along the central MAR, but make TR comparable to the ‘ultra-slow’ (15–16 mm/a) Southwest Indian and Gakkel (7–13 mm/a) ridges. If this segmentation wavelength reflects Rayleigh–Taylor instabilities, the viscosity contrast between the overlying axial lithosphere and the partial melt zones is about an order of magnitude greater at ca. 4–16 mm/a than at 20–30 mm/a. The TR differs dramatically from ultra-slow ridges only in the large amplitude of along-strike topography (2000–4000 m; 4200 m total variation) owing perhaps to a copious melt flux from the Azores ‘hotspot’, combined with a spreading-rate-determined greater axial flexural strength and plate thickness, and slower export of volcanics from the rift axis. The probable TR youth (ca. 1 Ma?, requiring less than 4 km new oceanic crust) suggests lack of steady-state spreading conditions, which may explain the published gravity evidence against TR spreading. Absolute plate motions support the creation of the Azores Plateau by successive NE jumps of the rift axis to maintain its position over a fixed ‘hotspot’.

Rhyodacites, andesites, ferro-basalts and ocean tholeiites from the galapagos spreading center

Abstract Volcanic rocks, dredged from depths greater than 1000 m on the Galapagos spreading center, show extreme chemical diversity, including rhyodacites, andesite, ferro-basalts, and low-K oceanic tholeiite. All samples have fresh glassy margins. The ferro-basalts contain up to 18.5% total iron as FeO and up to 3.75% TiO2, while the oceanic tholeiites are as low as 0.02% K2O. The ferro-basalts correlate with the previously proposed zone of high magnetic anomaly amplitudes which flank the Galapagos hot spot, and are consistent with a genesis by shallow fractional crystallization.

Volcano height and plate thickness

Abstract When the height of major young oceanic volcanoes above the basaltic basement are corrected for the buoyancy of their submerged bases, the heights are shown to increase as the square root of basement age. This is compatible with an isostatic model for the height of the magma in the volcanic conduit: As plates thicken with increasing age, the length of magma columns in isostatic equilibrium with the adjacent plate also increases. Volcano height is therefore primarily limited by plate thickness rather than, for example, the speed of a plate over a hot spot or the hot spots productivity.

Hydrothermal manganese crusts from two sites near the Galapagos spreading axis

Abstract Manganese oxide crusts similar to those reported from the Mid-Atlantic Ridge rift valley by Scott et al. (1974) were dredged at two sites near the Galapagos spreading axis on ocean floor estimated from magnetic anomalies to be 2.4 and 0.3 m.y. old. Compared to the typical ocean-floor manganese deposits attributed to precipitation from seawater, the 2–6 cm thick manganese crusts reported here exhibit very low Fe/Mn and low 232 Th/ 238 U ratios, as well as lower transition metal and higher manganese concentrations. The manganese crusts were deposited several orders of magnitude faster than the more common hydrogenous nodules; this fact together with other geochemical characteristics and the geophysical environment suggests the manganese deposits reported here are of hydrothermal origin.

Magnetic and bathymetric evidence for the “Yermak hot spot” northwest of Svalbard in the Arctic Basin

Detailed aeromagnetic data and several new bathymetric profiles collected in the Eurasia Basin off northern Greenland and Svalbard provide evidence for a “Yermak hot spot” which has affected the basement morphology of an ∼200-km-long segment of the Nansen spreading axis since rifting began here about 60 m.y. B.P. Paired aseismic basement ridges (the Morris Jesup Rise and Yermak Plateau) associated with high-amplitude (500 to over 2000 nT, where 1 nT= 1 gamma) complex, sublinear magnetic anomalies were formed at the spreading axis beginning in the lower Tertiary and ending abruptly at anomaly 12–13 time. During maximum hot spot activity (anomaly 13–18 time) the Yermak/Morris Jesup Plateau was apparently a single, Iceland-like volcanic massif emergent above sea level. The flat-topped plateaus subsided to their present ∼1–2 km depth along the standard oceanic crust subsidence curve. From anomaly 12 to 5 time (∼35to10 m.y. B.P.) the Yermak hot spot appears to have been very subdued or dormant in its influence on the ocean crust. Beginning about anomaly 5 time, relatively higher amplitudes began to develop along the same section of spreading axis that had earlier generated the Yermak and Morris Jesup rises. This “Yermak H-zone” has expanded northeastward at about 1 cm/yr, presumably due to the flow of plume-derived mantle material below the spreading axis. The zone is presently over 200 km long (anomaly 1) and the central anomaly locally exceeds 1000 nT in amplitude. At its southwestern end the H-zone appears sharply terminated at the Yermak fracture zone, perhaps because the subaxial flow is dammed there. Although no aseismic ridge is presently being generated by the hot spot, the rift valley floor and rift mountains are relatively shallower (∼4 km and ∼2 km depths) than along the Nansen Ridge to the northeast, where the valley floor locally descends below 5 km depths. Along this “normal” segment of the Nansen Ridge, dramatic along-strike amplitude variations (∼50–500 nT; 50–100 km wavelength) correlate with rift valley depth (3.7–5.3 km), possibly a manifestation of varying basalt productivity associated with the slow spreading (0.5–0.7 cm/yr). The Yermak H-zone has not been sampled, but analogy with similar regions along the Galapagos and Juan de Fuca spreading axes suggest the high amplitudes are caused by exceptionally fractionated, FeTi-enriched basalts of high remanence. Other possible manifestations of the Yermak hot spot include the early Tertiary Kap Washington Group volcanics in northeast Peary Land, Miocene basalts and Paleocene ash horizons on Svalbard, and the generally higher elevation of continental land masses in the area.

Haakon Mosby Mud Volcano provides unusual example of venting

A seafloor mud volcano north of Norway is presenting researchers with an uncommon example of venting and is raising important questions. Seafloor aqueous vents, gas vents, mud volcanoes, and mud diapirs are found in a variety of geological settings. However, scientists did not expect to discover venting at the northern site, now known as the Haakon Mosby Mud Volcano (HMMV). It is considered especially unusual because of its Arctitc Location (72°N), its development largely within glacial marine sediments, and its lack of association either with salt tectonics or with plate subduction. Further, the volcano is posing questions for investigators about the relationship of methane generation and mud volcanism to thick, rapidly deposited sediments; sediment failure; and gas hydrates (GH).

Spreading center jumps and sub-axial asthenosphere flow near the Galapagos hotspot☆

Abstract We have identified a sequence of small rise-axis jumps on the Cocos—Nazca spreading center between 93° and 95.5°W. The locus of jumps has migrated 150 km west along the rise axis, away from the Galapagos Islands, during the last three million years, at an average rate of 50 mm/year. The linear increase in jump distance during this sequence of jumps has resulted in a change in regional azimuth of the rise axis from about 085° to 095°. We visualize this sequence of jumps as a new rift propagating through the Cocos plate, forming a new Cocos—Nazca spreading center. The region affected by these rise jumps appears to correlate with an area of exceptionally high-amplitude magnetic anomalies. The high-amplitude region seems to result from Fe-Ti-rich (FeTi) basalts of high remanent magnetization. We speculate that the development of the new accretion axis and concomitant rise jumps are related to the flow of FeTi basalt-producing asthenosphere away from the Galapagos hotspot. The snout of anomalous asthenosphere has remained nearly stationary, with respect to the Galapagos hotspot, during the last 3 m.y. A northwestward component of flow, reflecting the southward position of the plume center with respect to the spreading axis, might explain why the new spreading center is developing along a more northwesterly azimuth. The rise jumps have resulted in the sort of pattern of asymmetric accretion which is required to substantiate the hotspot hypothesis for the origin of the Cocos and Carnegie ridges. Several other puzzling platetectonic phenomena may be explained by the propagating rift model developed here.

Episodes of sea-floor spreading recorded by the North Atlantic basement

Abstract A fresh compilation of available bathymetric, magnetic and seismic data in the central North Atlantic (west of the Mid-Atlantic Ridge) and north of the Azores (east of the Mid-Atlantic Ridge) reveals a complex episodic history of sea-floor spreading. JOIDES deep drilling results allow approximate dating of these episodes. By the end of the Jurassic, which was characterized by fast but decelerating spreading and few fracture zones, the central North Atlantic had reached about one third of its present width. Even before the end of this episode, close-spaced fracture zones and rough basement began to be formed, and this basement change appeared systematically later from north to south, consistent with the hypothesis that a threshold spreading rate was involved. As the spreading pole lay to the north, a decelerating spreading rate would necessarily bring a northern location through a given threshold rate before a southern one. Near the Jurassic-Cretaceous boundary, the Mid-Atlantic Ridge rapidly grew up to 0.5 km higher in some areas; this is known from basement topography and supported by a band of high-amplitude magnetic anomalies that suggest a thickened pillow lava layer about 50 km wide. This unusual band called the Bermuda Discontinuity, generally separates the rather densely fractured Cretaceous sea floor (about one fracture zone per 50–100 km) on the east flank of the Bermuda Rise from the Jurassic province to the west. Comparison of basement character with crustal structure suggests that, at least at the latitude of Bermuda, the “fast” Jurassic crust is “Pacific” (oceanic layer, normal mantle) whereas the Cretaceous sea floor is “Atlantic” (no oceanic layer, low velocity mantle). Sometime after the start of the Cretaceous, basement south of about 26° N again became “Pacific” (smooth, normal crust) whereas to the north it remained “Atlantic”. Cretaceous fractures yield a good azimuth to the virtual spreading pole, which is considerably southeast of the late Tertiary pole. The Cretaceous ended with the formation of basement elevations such as the Corner Rise. Major changes in spreading pattern probably occurred both at 76 and 60 m.y.B.P., the later date marking the separation of Greenland from Europe. Between about 60 and 40 m.y.B.P. fracture zones trend east-west or even WSW, suggesting the spreading pole moved west at the Cretaceous-Cenozoic boundary. After 40 m.y.B.P. large pole changes are not evident; a slight eastward movement may have occurred about 9 m.y.B.P. concomitant with the disappearance of numerous minor fracture zones. A preliminary comparison between spreading history on Reykjanes Ridge and deep drilling in Rockall-Hatton Basin suggests rapid subsidence following, by a few million years, each of three changes of spreading pattern. It is noteworthy that the three major events in the history of the North Atlantic all fall close to period boundaries, which therefore are not merely stratigraphic boundaries whose significance is restricted to Europe. Noting also the coincidence of the three major events with flood basalt episodes, we propose that discharge from mantle plumes, varying with crude 60 m.y. periodicity, has been the prime cause for the observed North Atlantic evolution.