Everett D. Jackson

Hawaiian-Emperor Chain and Its Relation to Cenozoic Circumpacific Tectonics

The Hawaiian Ridge and Emperor Seamounts appear to form a single chain of tholeiitic shield volcanoes that erupted sequentially on the sea floor of the central Pacific Ocean during Tertiary and Quaternary time. The chain cuts obliquely across the older Cretaceous structural patterns of that sea floor. While the pattern of the chain as a whole is linear, the individual volcanoes lie on short, sigmoidal, en echelon loci that are subparallel with respect to each other and that may represent extensional features in the crust and upper mantle. In general, the order of eurption progressed from northwest to southeaśt along the chain, but the rate of progression of volcanism along individual loci is nonlinear where best studied in the southeastern part of the chain. Furthermore, simultaneous eruptions appear to have occurred within a distance along the chain of about 200 to 400 km. The available data are consistent with a genesis related to the motion of the Pacific crust over a melting spot in the mantle. This melting spot, which may be due to either excess heat or pressure release, appears to have a diameter of about 300 km and is presently centered slightly north of the island of Hawaii. We concur with the idea that the bend in the Hawaiian-Emperor chain probably reflects a significant change in the motion of the Pacific plate. Our best estimate of the age of the Hawaiian-Emperor bend, based on the existing radiometric data, is 24.6 ± 2.5 m.y., which correlates with a time of increased tectonic activity in the western Pacific island arcs and along the northern and eastern boundaries of the Pacific plate. The vector change in the motion of the Pacific plate (with respect to the melting spot) that is required to produce the bend is about 12 cm/yr in a west-southwest-ward direction.

The Vourinos Ophiolite, Greece: Cyclic Units of Lineated Cumulates Overlying Harzburgite Tectonite

Re-examination of the Vourinos ophiolite shows it to be composed of metamorphic tectonites, cumulates, plagiogranites, dikes, and lava. The contact between the tectonites and the cumulates is exposed and sharp. Beneath the cumulate contact, the rocks have been highly deformed and complexly folded; above that contact, they simply have been tilted vertically to expose a stratiform complex 1,500 m (4,800 ft) thick. The stratiform intrusion is characterized by cyclic units, rich in olivine at the base, and rich in feldspar at the top. Some cumulus diorites are present at the top of the section which grade into quartz diorites (plagiogranites) with hypautomorphic textures. Lineate lamination characterizes the cumulates and may indicate the direction or orientation of the Mesozoic mid-oceanic ridge crest with respect to the present position of the complex. A siliceous dike swarm cuts the upper part of the stratiform complex. The section suggests that in the case of Vourinos, a large magmatic chamber formed at a mid-oceanic ridge crest and that intrusion was a much more important process than extrusion in the formation of oceanic crust in that area. The reported presence of cumulates in many other ophiolite complexes suggests that these relations may obtain generally at most or all spreading ridges. The contact between the tectonites and the cumulates of the complex would not have corresponded with seismic M.

Contributions to the Petrography and Geochronology of Volcanic Rocks from the Leeward Hawaiian Islands

Petrographic and chemical analyses of basalt from Nihoa Island, Necker Island, French Frigate Shoals, and Midway Atoll, all in the leeward part of the Hawaiian chain, confirm that these islands are subaerial remnants of tholeiitic shield volcanoes similar to those that form the principal Hawaiian Islands. Chemistry suggests that Gardner Pinnacles may be part of the alkalic cap on a tholeiitic shield. Weighted mean potassium-argon ages of 7.0 ± 0.3 m.y. for Nihoa, 10.0 ± 0.4 m.y. for Necker, 11.7 ± 0.4 m.y. for French Frigate, and 17.9 ± 0.6 m.y. for Midway demonstrate that the ages of these volcanoes increase northwestward, continuing the trend of increasing age away from the active volcano of Kilauea shown by the main islands. The increase in age with distance along the chain, however, appears to be nonlinear. The results support the general hypothesis that the volcanoes of the Hawaiian chain have a common origin and were formed as the Pacific plate moved northwestward over a melting spot in the mantle.

Calculated geochronology and stress field orientations along the Hawaiian chain

Abstract A new method has been discovered for calculating ages of the main shield building stages of volcanoes along the Hawaiian chain from Kilauea to the Hawaiian-Emperor bend. The method is based on a graphical technique for hypothetical subtraction of distance intervals that theoretically represent regions of simultaneous volcanism along adjacent or nearly en-echelon loci of volcanism. Distances along the chain, measured from Kilauea, when progressively foreshortened by the distances of hypothetical “collapse” and plotted versus existing age data are found to give linear age-distance relationships. A calibration graph is presented that agrees closely with the measured ages in 17 of the 20 existing dated volcanoes. The criterion for simultaneous activity on different loci is based on the concept of equal azimuths of synchronous volcanic propagation within coeval segments of the chain. This is the predicted relationship when magmatic fluids inject the lithosphere along directions normal to a nearly horizontal least principal stress. It appears that the Pacific plate has been subjected to oscillatory, but principally clockwise, rotations of horizontal stress components during the last 40 m.y.

Isotopic investigations of xenoliths and host basalts from the Honolulu volcanic series

Abstract Oxygen, carbon and strontium isotope ratios point to a ground-water origin for the carbonates found in Hawaiian Volcanic Series rocks. Strontium isotope ratios in the xenoliths and host basalts indicate a genetic relationship between them.

Structure and Petrology of a Cumulus Norite Boulder Sampled by Apollo 17 in Taurus-Littrow Valley, the Moon

A glass-coated half-meter-size boulder was sampled by the Apollo 17 crew at station 8 near the foot of the Sculptured Hills. The rock proved to be a coarse-grained (0.5-cm) plagioclase-orthopyroxene cumulate, and the samples are the only true norites returned from the lunar surface. Photographs of the boulder showed it to contain at least nine structural surfaces and four glass veins. Orientation and inspection of three of the returned samples resulted in the identification of six surfaces and one vein. One of the structural surfaces visible in the boulder was identified as primary cumulus planar lamination, which was folded through an angle of at least 35° between two oriented samples, whereas fracture sets representing the other surfaces were coincident. The boulder is believed to be a sample of the deeper highlands or submare lunar crust, derived from a depth of 8 to 30 km and somewhat shock-metamorphosed during at least two excavation events. The chemical composition of the norites, when determined, should be of special interest in view of the large amount of literature concerning glass, cataclasite, hornfels, and “basalt” of noritic composition returned by other Apollo missions. However, the cumulus texture of the boulder precludes its being representative of any magmatic liquid composition, suggests that the lunar crust is heterogeneously layered, and that plagioclase sank, not floated, in magmatic liquids that formed the lunar crust.

Petrology of Unshocked Crystalline Rocks and Shock Effects in Lunar Rocks and Minerals

On the basis of rock modes, textures, and mineralogy, unshocked crystalline rocks are classified into a dominant ilmenite-rich suite (subdivided into intersertal, ophitic, and hornfels types) and a subordinate feldspar-rich suite (subdivided into poikilitic and granular types). Weakly to moderately shocked rocks show high strain-rate deformation and solid-state transformation of minerals to glasses; intensely shocked rocks are converted to rock glasses. Data on an unknown calcium-bearing iron metasilicate are presented.

Geologic Setting and Petrology of Apollo 15 Anorthosite (15415)

Apollo 15 sample number 15415, popularly called the “Genesis Rock,” is coarse-grained anorthosite composed largely of calcic plagioclase with small amounts of three pyroxene phases. The rock was found as a clast in a piece of friable soil breccia on the lip of Spur crater, a small young crater on the lower slopes of the Apennine Mountains. The mode of occurrence of sample 15415 indicates that it has undergone at least two, and possibly three or more, fragmentation events. These events are reflected in the texture of the rock by shattered and granulated minerals. An earlier thermal metamorphic event is represented by irregular bands of coarsely recrystallized plagioclase and minor pyroxene that cross larger plagioclase grains. Preliminary observations of textural relations of the large plagioclase grains are consistent either with accumulation of plagioclase followed by overgrowth of cumulus grains and post-cumulus crystallization of minor interstitial pyroxene, or with metamorphic recrystallization that eradicated original textures. Any of the events in the complex history of this rock may have affected apparent radiometric ages. Comparative abundance of similar, though smaller, pieces of anorthositic rock in the area and dominance of originally coarse-grained gabbroic-anorthositic clasts in breccia at Spur crater suggest that sample 15415 is the least-deformed member of a suite of similar rocks that were ejected from beneath the regolith at Spur crater.

Lunar 'dunite', 'pyroxenite' and 'anorthosite.'

Abstract Monomineralic aggregates of olivine, clinopyroxene, orthopyroxene and plagioclase with granoblastic textures are widespread minor constituents of Apollo 14 breccias. Recrystallization is commonly incomplete within these aggregates, leaving relict material that clearly indicates single-mineral-grain sources for the aggregates. The aggregates are not, therefore, properly characterized by igneous rock names, nor can any conclusions regarding differentiation be drawn from them. Average sizes of the aggregates indicate source rocks with grain sizes mostly larger than 1 to 5 mm, a few clasts of which occur in the breccias; the proportions of the different types of aggregates suggest dominantly feldspathic source rocks.