ALVIN investigation of an active propagating rift system, Galapagos 95.5� W

Abstract

Abstract. ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5°W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.-1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seaf.oor spreading center, an extensively faulted and fissured volcanic ridge, develops ~10 km (~200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.

Propagating rift model for the V‐shaped ridges south of Iceland

“…If the inward facing scarps are the pseudofaults, they would in some ways be similar to the structures at the Galapagos 95.5°W propagator, where there is a topographically low area at the propagating rift tip bounded by inward facing pseudofault scarps. This tip depression eventually evolves to a V‐shaped trough, bounded on the other side by the gradually developing constructional axial ridge [ Hey et al , 1986, 1992; Kleinrock and Hey , 1989]. In this case the troughs bounding the A′ propagator could be tip depression wakes.…”

“…If the outer boundary of this E scarp trough is the pseudofault, this would require an ∼2–3 Ma time lag between initial rifting and the formation of the E scarp at the constructional axis. In the Galapagos area, the time lag is ∼200,000 years, and the resulting constructional ridge forms much more gradually than the abrupt A and E scarps [ Hey et al , 1986, 1989, 1992; Kleinrock and Hey , 1989].…”

“…The troughs could perhaps instead be failed rift depressions, with thinner crust than normal for this area. In the Galapagos area there are clearly defined failed rift grabens thought to result from the transitional spreading period during the required deceleration from the full rate to zero on the failing rift [ Hey et al , 1986, 1989, 1992; Kleinrock et al , 1989; Wilson and Hey , 1995]. Deep failed rift grabens are also formed in the superfast spreading 28°–29°S dueling propagator area between the Easter and Juan Fernandez microplates [ Hey et al , 1995; Korenaga and Hey , 1996; Martinez et al , 1997].…”

“…Both the intermediate spreading Galapagos failed rift grabens and the superfast dueling propagator failed rift grabens are typically ∼10 km wide, wider than the Reykjanes Ridge jumps we model. The Galapagos 95.5°W failing rift lavas are predominantly highly magnesian [ Christie and Sinton , 1981; Hey et al , 1989, 1992], as are the Reykjanes V‐shaped trough samples [ Taylor et al , 1995].…”

Geochemical evidence for low magma supply and inactive propagation at the Galápagos 93.25°W overlapping spreading center

“… Christie and Sinton [1981] and Sinton et al [1983, 1991] showed that most propagating rifts are associated with along‐axis gradients to deeper bathymetry and increased extent of shallow level fractionation at the propagating tip; these gradients can extend for a few tens to a few hundreds of kilometers behind the rift tip. In contrast, along failing rifts the extent of shallow level fractionation typically decreases toward the rift tip, while the rift axis also increases in depth [ Sinton et al , 1983, 1991; Yonover et al , 1989; Hey et al , 1992]. However, near 93.25°W, MgO increases toward the tip of each limb and within the overlap zone, and the overlapping ridge axes become shallower than those outside the overlap zone on either side.…”