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

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 seafloor 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.     

Rifting of oceanic crust at Endeavor Deep on the Juan Fernandez microplate

The development of an anomalously deep rift appears to be a common characteristic of the evolution of microplates along the East Pacific Rise, including the Galapagos, Easter, and Juan Fernandez microplates. We investigate crustal rifting at Endeavor Deep on the Juan Fernandez microplate using bathymetry, gravity and side scan sonar data. An initial phase of lithospheric extension accompanied by extensive subsidence results in the formation of a very deep rift valley (up to 4 km of relief, 70 km long and 20 km wide). Morphological observations and gravity data derived from GEOSAT satellite altimetry show the subsequent initiation of crustal accretion and development of a mature spreading center. Recent models of the kinematics of microplate rotation allow the amount of opening across Endeavor Deep over the past 1 m.y. to be quantified. We develop a simple mechanical model of rifting involving block faulting and flexural response to explain the gravity signature over the rift valley. The Bouguer gravity anomaly is asymmetric with respect to the surface topography and requires that a shallow-dipping fault on the western wall of the valley dominate the extension at Endeavor Deep. Consideration of three similar microplate rift valleys leads us to suggest that asymmetric rifting is the characteristic process forming microplate deeps.     

New Observations on the Distribution of Past and Present Hydrothermal Activity in the TAG Area of the Mid-Atlantic Ridge (26°08′ N)

Seafloor acoustic and photographic imagery combined with high- resolution bathymetry are used to investigate the geologic and tectonic relations between active and relict zones of hydrothermal venting in the TAG (Trans-Atlantic Geotraverse) hydrothermal field at 26°08N on the Mid-Atlantic Ridge (MAR). The TAG field consists of a large, currently active, high-temperature mound, two relict zones (the Alvin and Mir zones), and an active low-temperature zone. The active mound and the Alvin relict zone lie along a series of closely-spaced, axis-parallel (NNE-trending) faults in an area of active extension east of the neovolcanic zone. The Alvin zone extends for 2.5 km along these faults from the valley floor onto the eastern wall, and consists of at least five mounds identified using DSL-120 sidescan sonar and bathymetric data. The existence of sulfide structures on most of these mounds is verified with near-bottom electronic still camera (ESC) images from the Argo-II deep-towed vehicle, and is confirmed in at least one case with collected samples. Two of these mounds were previously unidentified. The existence of these mounds extends the length of the Alvin zone by ~0.5 km to the south. Much of the Alvin relict zone appears to be buried by debris from a large mass wasting event on the eastern wall of the median valley. The Mir zone, located on normal fault blocks of the eastern valley wall, cannot be clearly identified in the sidescan data and no structural connections from it to the active mound or Alvin zone can be discerned. The active mound is located at the intersection of an older oblique fault set with the younger axis- parallel faults which extend into the Alvin relict zone, and no fresh volcanics are observed in the vicinity of the mound. The fact that both the active mound and the Alvin relict zone lie along the same set of active, axis-parallel faults suggests that the faults may be a major control on the location of hydrothermal activity by providing pathways for fluid flow from a heat source at the ridge axis.