James H. Natland

Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A

On Leg 78A we drilled Sites 541 and 542 into the seaward edge of the Barbados Ridge complex, and Site 543 into the adjacent oceanic crust. The calcareous ooze, marls, and muds at Sites 541 and 542 are lithologically and paleontologically similar to the upper strata at Site 543 and are apparently offscraped from the down-going plate. A repetition of Miocene over Pliocene sediments at Site 541 documents major thrust or reverse faulting during offscraping. The hemipelagic to pelagic deposits offscraped in the Leg 78A area include no terrigenous sand beds, but they contain numerous Neogene ash layers derived from the Lesser Antilles Arc. Hence, this sequence is quite unlike the siliciclastic-dominated terranes on land that are inferred to be accretionary complexes. The structural fabric of the offscraped deposits at Sites 541 and 542 is disharmonic, probably along a decollement, with an underlying acoustically layered sequence, suggesting selective underthrusting of the latter. The acoustically layered sequence correlates seismically with pelagic strata cored at Site 543 on the incoming oceanic plate. Cores recovered from the possible decollement surface at both Sites 541 and 542 show scaly foliation and stratal disruption. Approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits. In the incoming section, a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata. In the Leg 78A area, the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the Barbados Ridge complex.

The origin of the Marquesas fracture zone ridge and its implications for the nature of hot spots

Abstract The Marquesas Island chain departs by 20–30° from the trends of other late Tertiary hot-spot traces and appears more intermittent in activity than most others in having produced volcanos only in the period 1.4–6.4 Ma. In order to explain the Marquesas data, it has been proposed that hot spots can move relatively rapidly with respect to one another and that most plumes are episodic phenomena. We reevaluate these hypotheses based on geophysical data from the Marquesas fracture zone ridge collected during a recent oceanographic expedition to the Marquesas Islands. Our preferred explanation based on analysis of Seabeam bathymetric, seismic, gravity, and magnetic data across the ridge is that it is a young volcanic construct, 20 km wide and almost 2 km high, formed recently as the Marquesas hot spot encountered the Marquesas fracture zone. Examination of regional bathymetric maps reveals that topographically identical ridges also appear along the Galapagos fracture zone and the Nova-Canton Trough at positions corresponding to the predicted trace of the Marquesas plume based on the trends of the Hawaiian and Society island chains. Using these observations, we conclude that the Marquesas hot spot is not moving rapidly with respect to the other Pacific hot spots, nor is it intermittent in nature. We prefer a model in which the Marquesas plume is simply too weak to penetrate normal oceanic lithosphere unless given an easy conduit to the surface, such as a fracture zone. The surface expression of weak plumes such as the Marquesas is therefore strongly controlled by the thermal and mechanical state of the overlying oceanic lithosphere, an effect which must be taken into account before interpreting hot-spot traces in terms of intrinsic plume properties.

Temporal and Geochemical Variability of Volcanic Products of the Marquesas Hotspot

The Marquesas archipelago is a short, NW-SE trending cluster of islands and seamounts that formed as a result of volcanic activity over a weak hotspot. This volcanic chain lies at the northern margin of a broad region of warm and compositionally diverse mantle that melts to build several other subparallel volcanic lineaments. Basalts dredged from submerged portions of volcanoes along the Marquesas lineament decrease in age from northwest to southeast. The new sample age distribution yields a volcanic migration rate significantly slower than that expected for Pacific plate motion over a stationary Marquesas hotspot. This and the aberrant orientation of the chain indicate deflection of the plume by westward upper mantle flow. The interaction of this weak plume with upper mantle flow accounts for the temporal and spatial patterns in Marquesan volcanism. The compositions of subaerial and submarine basalts reflect the mixing of at least two mantle sources, distinguished by Sr, Nd, and Pb isotope and trace element compositions. There is a consistent evolutionary pattern at each volcano, from early tholeiitic to later alkalic basalt eruptions. Tholeiitic and transitional lava compositions can be derived by variable degrees of partial melting of a source composed of depleted mid-ocean ridge basalt mantle (DMM) and mantle characterized by radiogenic Pb (HIMU). Alkalic lava compositions appear to be dominantly the result of smaller degrees of melting of a more radiogenic mantle source (EMII). Large-scale melting of the lower lithosphere or upper mantle (DMM+HIMU) entrained within a sheared, thermally buoyant plume (EMII) could produce the tholeiitic and transitional basalts found in the main shields of the volcanoes, while alkalic basalts could result from melting of mantle of EMII composition at the edges of the hotspot.

Fissure control on volcanic action in the Pacific

Many modern linear volcanic chains on the Pacific plate can be backtracked to the general region of large Mesozoic seamount provinces in the far western Pacific, but those provinces differ in many ways from their younger counterparts in volcano arrangement, spacing, linearity, and age progression. Over the past 165 m.y., regions of volcanism have become narrower, more linear, and more consistently age-progressive. Rather than viewing this trend as a consequence of changes in the number, location, duration, and intensity of mantle plumes, we propose instead that five sets of stresses in the upper mantle combine to produce the patterns of great fissures and systems of fissures through the lithosphere along which seamount provinces and linear chains form. These are (1) lateral lithospheric contraction; (2) subduction-driven flow of the asthenosphere, which presently is antiparallel to Pacific plate motion, but which can be uneven in velocity and somewhat divergent in direction; (3) stresses in the plate produced by changes in patterns, relative orientations, and rates of subduction; (4) accumulation of low-density material and melt at places of lateral heterogeneity of density, viscosity, and composition in the asthenosphere; and (5) volcanic loading and redistribution of mass during differentiation at volcanic centers. Of these, the present direction of propagation of linear volcanic chains is most strongly controlled by contractive stresses and the pattern of subduction-driven asthenospheric counterflow, with volcanic spacing and volume arising from different combinations of the other stresses. Eruption of midplate seamounts results from buoyancy contrasts in the uppermost asthenosphere stemming from mantle heterogeneity, slightly variable temperature gradients, and the ponding of melt. Melt generally present in the low-velocity zone flows into shear gashes and then pools in traps beneath permeability barriers at the base of the lithosphere, with higher melt concentrations occurring above regions of greater mantle fertility and/or higher temperature. The lithospheric stress regime coupled with magma overpressure then induces lithospheric fracturing and eruption. Eruption, even above fertile mantle, may be stifled when the combination of lithospheric stresses is compressive, and fairly voluminous, even above refractory asthenosphere, when lithospheric stresses are extensional. Extremely voluminous volcanism occurs when a strongly extensional regime and/or a lithospheric thin spot coincides with asthenosphere having both high mantle fertility and higher temperature (e.g., oceanic plateaus at triple junctions). The relative importance of the five sets of stresses has varied in the past. Assuming that linear island chains propagate mainly in the local direction of least principal stress in the underlying lithosphere, the changes in fissure patterns through time are probably related to the increasing proportion and distribution of subduction boundaries on the northern and western margins of the plate as it grew in size to become the largest on Earth. This initiated counterflow in the asthenosphere and caused changes in the orientations of spreading ridges, the number and pattern of transform faults, the existence and distribution of microplates, and the magnitudes and orientations of the stress fields. Stress concentrations in the plate were far from uniform. Stress differences led to varieties of fracture patterns, which prompted the release of magma along volcanic ridges of varied morphology, size, and distribution of volcanic centers. The orientation and magnitude of counterflow in the asthenosphere and stress contrasts across the plate increased with the growth of subduction boundaries. Fissure patterns accordingly became more consistently oriented. Important events at the boundaries of the plate, such as the Australian-Asian collision and the demise of the Kula ridge in the northern Pacific changed the direction of asthenospheric counterflow, midplate stress orientations, and the patterns of stress concentration, and these conditions—rather than shifts in plate motions over fixed hotspots—resulted in changes in directions of volcanic chains.

Cretaceous Drowning of Reefs on Mid-Pacific and Japanese Guyots

Reefs dredged on guyots of the Mid-Pacific Mountains and the Japanese Seamounts yield middle Cretaceous fossils, indicating that submergence killed off the fauna of the reefs sometime during the Albian-Cenomanian. Eustatic rise of sea level is probably responsible.

Galapagos hydrothermal mounds: stratigraphy and chemistry revealed by deep-sea drilling

The Gal�pagos mounds sea-floor hydrothermal system is at least 300,000 years old and once produced manganese-poor sediments, which nearly blanketed the area of the present mounds field. Present-day mound deposits are limited manganese-rich exposures, suggesting that the system has changed from rock-to water-dominated and has diminished in intensity with time.