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J. L. Morton

Publications and source records attributed to J. L. Morton.

12 recordsLinked to original sources
USGS2012

Volcanism and massive sulfide formation at a sedimented spreading center, Escanaba Trough, Gorda Ridge, northeast Pacific Ocean

Seismic‐reflection profiles over the sediment‐filled Escanaba Trough at the southern Gorda Ridge reveal a series of volcanic centers that pierce the sediment. The volcanic edifices are 3 to 6 km in diameter and are spaced at 15 to 20 km intervals along the axis of the trough. The volcanic intrusions have uplifted the sediment as much as 100 m, and in places fresh basaltic flows are exposed at the sea floor. Unoxidized pyrrhotite‐rich massive‐sulfide samples were obtained from the flanks of one of these volcanic centers. The composition and form of the sulfide samples suggest significant interaction between hydrothermal fluids and sediment at depth, and deposition of sulfide within the sediment pile.

Geophysical Research Letters↗
USGS1999

No spreading across the southern Juan de Fuca ridge axial cleft during 1994-1996

Direct-path acoustic measurements between seafloor transponders observed no significant extension (−10±14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44°40′ N and 130°20′ W). The acoustic path for the measurement is a 691-m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full-spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (∼ 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft-parallel 714-m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5±7 mm/yr) between the 1994 and 1996 surveys.

Geophysical Research Letters↗
USGS1991

Subalkaline andesite from Valu Fa Ridge, a back-arc spreading center in southern Lau Basin: petrogenesis, comparative chemistry, and tectonic implications

Tholeiitic andesite was dredged from two sites on Valu Fa Ridge (VFR), a back-arc spreading center in Lau Basin. Valu Fa Ridge, at least 200 km long, is located 40-50 km west of the active Tofua Volcanic Arc (TVA) axis and lies about 150 km above the subducted oceanic plate. One or more magma chambers, traced discontinuously for about 100 km along the ridge axis, lie 3-4 km beneath the ridge. The mostly aphyric and glassy lavas had high volatile contents, as shown by the abundance and large sizes of vesicles. An extensive fractionation history is inferred from the high SiO2 contents and FeO* MgO ratios. Chemical data show that the VFR lavas have both volcanic arc and back-arc basin affinities. The volcanic arc characteristics are: (1) relatively high abundances of most alkali and alkaline earth elements; (2) low abundances of high field strength elements Nb and Ta; (3) high U/Th ratios; (4) similar radiogenic isotope ratios in VFR and TVA lavas, in particular the enrichment of 87Sr 86Sr relative to 206Pb 204Pb; (5) high 238U 230Th, 230Th 232Th, and 226Ra 230Th activity ratios; and (6) high ratios of Rb/Cs, Ba/Nb, and Ba/La. Other chemical characteristics suggest that the VFR lavas are related to MORB-type back-arc basin lavas. For example, VFR lavas have (1) lower 87Sr 86Sr ratios and higher 143Nd 144Nd ratios than most lavas from the TVA, except samples from Ata Island, and are similar to many Lau Basin lavas; (2) lower Sr/REE, Rb/Zr, and Ba/Zr ratios than in arc lavas; and (3) higher Ti, Fe, and V, and higher Ti/V ratios than arc lavas generally and TVA lavas specifically. Most characteristics of VFR lavas can be explained by mixing depleted mantle with either small amounts of sediment and fluids from the subducting slab and/or an older fragment of volcanic arc lithosphere. The eruption of subalkaline andesite with some arc affinities along a back-arc spreading ridge is not unique. Collision of the Louisville and Tonga ridges probably activated back-arc extension that ultimately led to the creation and growth of Valu Fa Ridge. Some ophiolitic fragments in circum-Pacific and circum-Tethyan allochthonous terranes, presently interpreted to have originated in volcanic arcs, may instead be fragments of lithosphere that formed during early stages of seafloor spreading in a back-arc basin. ?? 1991.

Chemical Geology↗
USGS1988

Relic magma chamber structures preserved within the Mesozoic North Atlantic crust?

The North Atlantic Transect seismic reflection data, collected southwest of Bermuda, have been reinterpreted following post-stack migration and reveal two major intracrustal reflections. The shallower of these two events, located ∼1 s below the igneous basement, is a subhorizontal, undulating surface that in some places is continuous for as much as 10 km. On the basis of its position within the section and its laterally discontinuous nature, we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro. A second set of lower crustal reflections, dipping ∼20°-40° eastward, is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection Moho. Several lines of evidence argue against these features being either artifacts or out-of-the-plane events. Instead, their presence may be ascribed either to crustal-penetrating fault zones or to mafic-ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo-spreading center. Because of the laminated character of these events and their typical occurrence within 1.0 to 1.5 s of the reflection Moho, we prefer a compositional versus a structural interpretation for their origin. The gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred; although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities, significant departures are recognized elsewhere. Similarly, the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the Blake Spur fracture zone. An interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal-thinning model of Mutter and others.

Geological Society of America Bulletin↗
USGS1986

Petroleum associated with polymetallic sulfide in sediment from Gorda Ridge

A sediment sample, impregnated with asphaltic petroleum and polymetallic sulfide, was dredged from the southern end of Gorda Ridge (the Escanaba Trough) off northern California, within the offshore Exclusive Economic Zone of the United States. The molecular distributions of hydrocarbons in this petroleum show that it was probably derived from terrestrial organic matter in turbidite sediment filling the Escanaba Trough. Hydrothermal activity at the Gorda Ridge spreading center provided the heat for petroleum formation and was the source of fluids for sulfide mineralization.

California↗