Publications

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2009
Fisher, RL.  2009.  Meanwhile, Back on the Surface: Further Notes on the Sounding of Trenches. Marine Technology Society Journal. 43:16-19.   10.4031/MTSJ.43.5.7   AbstractWebsite

In the early 1950s there remained a number of still-young scientifically trained civilians, recently discharged from the U.S. Navy, who found the small but established (or gestating) U.S. oceanographic institutions to be exciting, fecund, and rewarding intellectually and career-wise. Broad sponsorship from the enlightened Office of Naval Research (ONR) provided use of converted small but fully seagoing vessels. Ad hoc teams applied or invented techniques with deck hardware, surplus World War II explosives, and laboratory electronic rigs to investigate the oceans in regions and environments key to the Navy’s potential missions but also to examine several crustal structures and tectonic processes fascinating to human curiosity, sometimes deemed “basic research.” One war-born example known to all is the observational explosion made possible by SCUBA; many of us did become saltwater dermatologists in those icy, pre-wetsuit days. For some, it was not enough.

1997
Fisher, RL, Goodwillie AM.  1997.  The physiography of the Southwest Indian Ridge. Marine Geophysical Researches. 19:451-455.   10.1023/a:1004365019534   AbstractWebsite

Seafloor morphology of the southwestern Indian Ocean is dominated by the very slow-spreading Southwest Indian Ridge. Traversing diagonally the whole area, the ridge extends 7700 km from the Bouvet triple junction to the Rodrigues triple junction. Along the ridge the topographic expression varies in a series of seven or more 'provinces.'

1992
Mahoney, J, Leroex AP, Peng Z, Fisher RL, Natland JH.  1992.  Southwestern Limits of Indian-Ocean Ridge Mantle and the Origin of Low Pb-206 Pb-204 Midocean Ridge Basalt - Isotope Systematics of the Central Southwest Indian Ridge (17-Degrees-E-50-Degrees-E). Journal of Geophysical Research-Solid Earth. 97:19771-19790.   10.1029/92jb01424   AbstractWebsite

Basalts from the Southwest Indian Ridge reflect a gradual, irregular isotopic transition in the MORB (mid-ocean ridge basalt) source mantle between typical Indian Ocean-type compositions on the east and Atlantic-like ones on the west. A probable southwestern limit to the huge Indian Ocean isotopic domain is indicated by incompatible-element-depleted MORBs from 17-degrees to 26-degrees-E, which possess essentially North Atlantic- or Pacific-type signatures. Superimposed on the regional along-axis gradient are at least three localized types of isotopically distinct, incompatible-element-enriched basalts. One characterizes the ridge between 36-degrees and 39-degrees-E, directly north of the proposed Marion hotspot, and appears to be caused by mixing between hotspot and high epsilon(Nd), normal MORB mantle; oceanic island products of the hotspot itself exhibit a very restricted range of isotopic values (e.g., Pb-206/Pb-204 = 18.5-18.6) which are more MORB-like than those of other Indian Ocean islands. Between 39-degrees and 41-degrees-E, high Ba/Nb lavas with unusually low Pb-206/Pb-204 (16.87-17.44) and epsilon(Nd) (-4 to +3) are dominant; these compositions are not only unlike those of the Marion (or any other) hotspot but also are unique among MORBs globally. Incompatible-element-enriched lavas in the vicinity of the Indomed Fracture Zone (approximately 46-degrees-E) differ isotopically from those at 39-degrees-41-degrees-E, 36-degrees-39-degrees-E, and both the Marion and Crozet hotspots. Thus, no simple model of ridgeward flow of plume mantle can explain the presence or distribution of all the incompatible-element-enriched MORBs on the central Southwest Indian Ridge. The upper mantle at 39-degrees-41-degrees-E, in particular, may contain stranded continental lithosphere, thermally eroded from Indo-Madagascar in the middle Cretaceous. Alternatively, the composition of the Marion hotspot must be grossly heterogeneous in space and/or time, and one of its intrinsic components must have substantially lower Pb-206/Pb-204 than yet measured for any hotspot. The origin of the broadly similar but much less extreme isotopic signatures of MORBs throughout most of the Indian Ocean could be related to the initiation of the Marion, Kerguelen, and Crozet hotspots, which together may have formed a more than 4400-km-long band of juxtaposed plume heads beneath the nearly stationary lithosphere of prebreakup Gondwana.

Royer, JY, Sclater JG, Sandwell DT, Cande SC, Schlich R, Munschy M, Dyment J, Fisher RL, Mueller RD, Coffin MF, Patriat P, Bergh HW.  1992.  Indian Ocean plate reconstructions since the Late Jurassic. Geophysical Monograph. 70( Duncan RA, Rea DK, Kidd RB, von Rad U, Weissel JK, Eds.).:471-475., Washington, DC, United States (USA): American Geophysical Union, Washington, DC
1989
Leroex, AP, Dick HJB, Fisher RL.  1989.  Petrology and geochemistry of MORB from 25-degrees-E to 46-degrees-E along the Southwest Indian Ridge: Evidence for contrasting styles of mantle enrichment. Journal of Petrology. 30:947-986.   10.1093/petrology/30.4.947   AbstractWebsite

The 1984 PROTEA expedition, leg 5, to the central Southwest Indian Ridge recovered basaltic lavas from fracture zones and ridge segments between 25°E and 48°E. In terms of petrography and major element variations the samples are unremarkable for ocean ridge basalts and range from aphyric to highly plagioclase phyric and from primitive (mg-number = 70) to moderately evolved (mg-number = 40) in composition. Multiply saturated (i.e., olivine, plagioclase, and clinopyroxene) basalts are common within this region. There is no systematic difference in compositional characteristics between basalts dredged from fracture zone walls and those dredged from ridge segments, and fractional crystallization has played an important role in controlling the overall range in lava composition in both tectonic environments.

Mahoney, JJ, Natland JH, White WM, Poreda R, Bloomer SH, Fisher RL, Baxter AN.  1989.  Isotopic and geochemical provinces of the Western Indian-Ocean spreading centers. Journal of Geophysical Research-Solid Earth and Planets. 94:4033-4052.   10.1029/JB094iB04p04033   AbstractWebsite

Basalt glasses from the Central Indian Ridge are distinct isotopically from mid-ocean ridge basalts (MORB) of the Indian Ocean triple junction and western few hundred kilometers of the Southeast Indian Ridge. In particular, very low 206Pb/204Pb and high 87Sr/86Sr signatures, which characterize the latter region, are absent over most of the Central Indian Ridge. In turn, lavas from the unusually deep eastern 1100–1500 km of the Southwest Indian Ridge are different chemically and isotopically from those of the above areas. A rather abrupt eastern boundary to Southwest Indian Ridge-type compositions occurs at or very near the geographic triple junction. This provinciality in western Indian Ocean ridge basalts partly mirrors fundamental regional differences in the underlying mantle but, at least between the eastern Southwest Indian Ridge and the western Southeast Indian Ridge and triple junction, also may reflect variations in extent and depth of melting in a vertically zoned upper mantle. A pronounced low εNd, high 206Pb/204Pb, high 87Sr/86Sr anomaly exists on the Central Indian Ridge at the Marie Celeste Fracture Zone and on the adjacent ridge segment to the south. Despite the great distance (>1100 km) of Réunion Island from the ridge, this zone appears to demark a region of mantle containing substantial Réunion hotspotlike material. Several old (35–60 m.y.) Deep Sea Drilling Project basalts which erupted on the ancestral Central Indian Ridge also record a significant Réunion hotspotlike influence, whereas a 46-m.y.-old sample that formed farther from the presumed locus of the hotspot possesses isotopic values identical to many present (non-Marie Celeste area) Central Indian Ridge MORB. The variably expressed and/or heterogeneous low 206Pb/204Pb material partly responsible for the isotopic distinctiveness of Indian Ocean ridge basalts may have entered into the Indian MORB mantle as a result of continental lithospheric remobilization preceding the breakup of Gondwana, particularly from the portion that would eventually become Greater India.

Bloomer, SH, Natland JH, Fisher RL.  1989.  Mineral relationships in gabbroic rocks from fracture zones of Indian Ocean ridges; evidence for extensive fractionation, parental diversity, and boundary-layer recrystallization. Magmatism in the ocean basins. 42( Saunders AD, Norry MJ, Eds.).:107-124., London, United Kingdom (GBR): Geological Society of London, London Abstract

Evolved two-pyroxene gabbros and ferrogabbros predominate in gabbroic suites dredged from five fracture zones on the SW and Central Indian Ridges. Compositions of olivines (Fo56-Fo85) and plagioclases (An10-An80) and the generally low magnesium numbers of orthopyroxene (0.58–0.84) and clinopyroxene (0.50–0.89) indicate that most gabbros crystallized from liquids more fractionated than those represented by basalts from adjacent ridge segments and the fracture zones themselves. This disparity, the paucity of diabases and gabbros, the absence of more magnesian gabbros and olivine-spinel cumulates, and the abundance of serpentinite in the dredge collections suggest that the fracture-zone gabbros crystallized in small magma bodies such as dykes or sills. These were emplaced laterally from large central magma chambers along ridge axes towards the fracture zones beneath a carapace of less fractionated basalt.Rare olivine gabbros, however, contain magnesian clinopyroxene (magnesium number >0.84) and orthopyroxene (magnesium number >0.70), compositions which are more magnesian than those of pyroxenes that have been crystallized experimentally from certain ocean-ridge basalts at 1 atm. It has been suggested that such high-magnesium pyroxenes result from moderate- to high-pressure fractionation. However, the magnesian pyroxenes in the Indian Ocean samples follow olivine and plagioclase in the crystallization sequence and have compositions appropriate for crystallization at low pressure. For example, they match compositions of phenocrysts in refractory (low-Na2O, low-TiO2) siliceous (51–53% SiO2) basalts from Deep Sea Drilling Project sites in Eocene-Cretaceous portions of the Indian Ocean. Such refractory basalts have not been studied experimentally, but the early crystallization of bronzite at low pressure in one of them is consistent with the siliceous and magnesian composition of the host glass.Alternatively, the more magnesian pyroxenes in the gabbros may have resulted from a process of in situ boundary-layer recrystallization (resorption and recrystallization) of minerals on the walls of small magma bodies as high-temperature magmas were repeatedly injected along them. This mechanism is suggested by phase relationships in simplified basaltic systems at low pressure and by the compositions of rounded (resorbed) mega-crystal minerals which occur in many tholeiites from the Indian Ocean and elsewhere.

1988
Bloomer, SH, Fisher RL.  1988.  Arc Volcanic-Rocks Characterize the Landward Slope of the Philippine Trench Off Northeastern Mindanao. Journal of Geophysical Research-Solid Earth and Planets. 93:11961-11973.   10.1029/JB093iB10p11961   AbstractWebsite

Five dredge hauls from the landward flanks (9300–3600 m) of the Philippine Trench at 10°30′N, northeast of Mindanao, recovered densely plagioclase-clinopyroxene phyric basalt, basaltic andesite, and andesite, with small amounts of diabase. These rocks are all transitional from an arc-tholeiitic to arc-calcalkaline volcanic series and have low Ni and Cr (10–30 and 30–70 ppm, respectively), low TiO2 (0.7–1.2%), and high incompatible element concentrations (Sr, 260–530 ppm; Ba, 170–410 ppm). A single dredge on the offshore slope of the trench included aphyric tholeiitic basalt similar to N-type ocean ridge basalts sampled from other locations in the West Philippine Basin (TiO2, 1.0–1.2%; Ni, 70–80 ppm; Cr, 130 ppm; Ba, 2–10 ppm; Sr, 70–85 ppm). The landward samples are distinct from the offshore samples and are identical in texture and composition to arc volcanic rocks from southeast and northeast Mindanao. The Mindanao rocks are interpreted to be part of the Samar-East Mindanao arc, which collided with the Central Mindanao-Sangihe island arc in the late Oligocene/early Miocene. The volcanic rocks exposed on the landward slope of the Philippine Trench are, apparently, part of the eastern margin of the Samar-East Mindanao arc. After the collision the Philippine Trench was initiated immediately adjacent to the arc massif; there is no evidence of any accretion of material from the subducted Philippine Sea crust. The polarity of the Samar-East Mindanao arc is unknown, but there is no evidence of either a deformed volcanoclastic apron or an older forearc terrane along the trench. Such materials must have been limited in extent prior to collision, have foundered and been subducted as the trench was initiated, or have been faulted or rotated so that they are not locally exposed.

Fisher, RL.  1988.  A Proposal for Modesty. International Hydrographic Review. 65:163-165.   10.1130/0091-7613(1987)​15<583:APFM>​2.0.CO;2   AbstractWebsite

In preparing a review of a manuscript for Geology I was struck and a bit put off by a situation increasingly displayed in professional journals that publish the results of marine geological-geophysical field research. The authors "named" two undersea features, both of which had been explored and shown in illustrations by earlier investigators; neither name seemed particularly apt or obviously required. Unwittingly, they join too many arguably parvenu scientists who offhandedly baptize a deep-sea topo- graphic feature (or even a plate-tectonics process model such as "triple junction" or "transform" or "subduction zone") that may have been known and well-explored—even if possibly unnamed—earlier, or even one bearing a long-established name in another language. They gin up a name, place it on an illustration, perhaps mention it in the text, get it by a harassed editor and into the technical literature, and consider that feature "named" for posterity. On the other tack, some editors, editorial boards, or technically specialized reviewers apparently know so little about historical courtesy, significant commemoration, or even good taste that the seafloor is becoming littered, and the literature of marine geology-geophysics clut- tered, with personal, in-group, self-aggrandizing, back-scratching, or trite unimaginative ("14°N Fracture Zone") names or ugly acronyms ("GOFAR Fracture Zone").

1987
Bloomer, SH, Fisher RL.  1987.  Petrology and Geochemistry of Igneous Rocks from the Tonga Trench: A Non-Accreting Plate Boundary. Journal of Geology. 95:469-495. AbstractWebsite

Petrologic and geochemical examination of a varied suite of intermediate, mafic, and ultramafic rocks dredged from the deep flanks of the Tonga Trench between 200S and 210S show that the landward slope has not developed by accretion of material from the subducted Pacific plate.

1986
1984
Dick, HJB, Fisher RL.  1984.  Mineralogic studies of the residues of mantle melting; abyssal and alpine-type peridotites. Kimberlites I: The mantle and crust-mantle relationships. ( Kornprobst J, Ed.)., Amsterdam, Netherlands (NLD): Elsevier Sci. Publ., Amsterdam Abstract

Abyssal peridotites dredged from the ocean ridges range from diopside-poor harzburgite to lherzolite but all contain diopside-saturated enstatite, indicating that melting of the abyssal mantle was constrained by the pseudo-invariant point olivine + enstatite + diopside + spinel + melt. Alpine-type peridotites overlap the range for abyssal peridotites, but extend to far more depleted and enriched compositions. Many contain enstatite undersaturated with respect to diopside. Frequently alpine-type peridotites contain highly magnesian Al-poor and Cr-rich minerals lying outside the abyssal range. Melting of many alpine peridotites, therefore, has occurred well into the three phase field olivine + enstatite + spinel + melt under different conditions to those of abyssal peridotites.

Dick, HJB, Fisher RL, Bryan WB.  1984.  Mineralogic Variability of the Uppermost Mantle Along Mid-Ocean Ridges. Earth and Planetary Science Letters. 69:88-106.   10.1016/0012-821x(84)90076-1   AbstractWebsite

Modal analyses of 273 different peridotites representing 43 dredge stations in the Atlantic, Caribbean, and Indian Oceans define three separate melting trends. Peridotites dredged in the vicinity of “mantle plumes” or hot spots have the most depleted compositions in terms of basaltic components, while peridotites dredged at locations removed from such regions are systematically less depleted. The modal data correlate well with mineral compositions, with the peridotites most depleted in pyroxene also having the most refractory mineral compositions. This demonstrates that they are the probable residues of variable degrees of mantle melting. Further, there is a good correlation between the modal compositions of the peridotites and the major element composition of spatially associated dredged basalts. This demonstrates for the first time that the two must be directly related, as is frequently postulated. The high degree of depletion of the peridotites in basaltic major element components in the vicinity of some documented mantle plumes provides direct evidence for a thermal anomaly in such regions—justifying their frequent designation as “hot spots”. The high incompatible element concentrations in these “plume” basalts, however, are contrary to what is expected for such high degrees of melting, and thus require either selective contributions from locally more abundant enriched veins and/or contamination by a volatile-rich metasomatic front from depth.

Stakes, DS, Taylor, H. P. J, Fisher RL.  1984.  Oxygen-isotope and geochemical characterization of hydrothermal alteration in ophiolite complexes and modern oceanic crust. Geological Society Special Publications. 13( Gass IG, Lippard SJ, Shelton AW, Eds.).:199-214., London, United Kingdom (GBR): Geological Society of London, London   10.1144/GSL.SP.1984.013.01.17   AbstractWebsite

Stable isotopic, geochemical, and mineralogic variations in plutonic and hypabyssal rocks from oceanic crust (mainly from the Indian Ocean) and ophiolitic terranes (principally the Semail complex, Oman) are very similar. Several stages of seawater-oceanic crust interaction are recognized in these gabbros and diabases. Isotopic and chemical compositions of secondary mineral assemblages reflect changing temperatures and water-rock ratios, and record the effects of (i) pervasive seawater-hydrothermal circulation associated with the main stage of crustal formation at an oceanic spreading centre, (ii) subsequent alteration associated with off-axis volcanism (upper pillow-lava sequences), and (iii) progressively lower-temperature alteration associated with hydrothermal ‘ageing’ of the oceanic crust. Along the contact between the high-level, isotropic gabbro of the ophiolite, and the overlying sheeted dyke complex, repeated stoping of hydrothermally altered roof rocks into the magma chamber appears to be a ubiquitous process. Stoping is a maximum where the roof is intruded by large (off-axis) gabbro-diorite-plagiogranite bodies which may be 60% xenoliths. Abundant quartz-epidote-sulphide veins originate near these silicic intrusions and alter the overlying sheeted dyke complex. Diabase from the sheeted dyke complex in Oman, typically much more altered than dredged oceanic rocks of similar texture, exhibits the integrated effects of both the axis and off-axis hydrothermal systems.

1983
Fisher, RL, Sclater JG.  1983.  Tectonic evolution of the Southwest Indian Ocean since the Mid-Cretaceous: plate motions and stability of the pole of Antarctica/Africa for at least 80 Myr. Geophysical Journal International. 73:553-576.   10.1111/j.1365-246X.1983.tb03330.x   AbstractWebsite

The Southwest Indian Ridge is the boundary where seafloor is being created between the African and Antarctic plates. East of the newly-recognized Du Toit Fracture Zone at 27°E it trends north-easterly and is cut by a series of six deep near north-south clefts marking transform faults. This series starts with the dual Prince Edward Fracture Zone near 35°E and includes Discovery II (also dual) at 42°E, Indomed at 46°E, and Gallieni at 52°E, terminating with the spectacular Atlantis II and Melville Fracture Zones at 57°30′E and 60°0′E respectively. The trend of these fracture zones is compatible with an instantaneous pole of relative motion for Antarctica/Africa at 8.4°N, 42.4°W. Between 35°E and 55°E the development of the Southwest Indian Ridge has separated the elevated Madagascar Ridge from the discontinuous Crozet Plateau that was contiguous to it prior to Eocene time. In this entire sector the ocean floor overall is elevated with respect to ocean floor of equivalent age elsewhere. Between these features anomalies 0–22 (0–53 Ma) have been identified north and 13–26 (35–60 Ma) south of the ridge axis. These anomalies, and 0–34 (0–80 Ma) located just to the west in the Mozambique Basin, lie at the same distance from the ridge axis as a similar sequence east of the Bouvet triple junction. The similarity in distance from the ridge axis of these anomaly sequences is evidence that Africa and East Antarctica have been separated by a single plate boundary from 80 Ma to the present. The pole representing the motion of these two plates at the present has not moved in totality far from the position it occupied in the Late Cretaceous, 80 Ma. Furthermore, calculation of intermediate finite rotation poles for four intervals within this span reveals only minor (< 15°) excursions from the present pole. The spatial reconstruction of the Southwest Indian Ridge at 80 Ma (anomaly 34) places the ridge midway between Africa and Eastern Antarctica and lying at right angles to the north-trending Mozambique Ridge. The separation of Antarctica from Africa following initial opening in the Jurassic may have been as straightforward as it appears to have been from 80 Ma to the present.

1982
Fisher, RL.  1982.  The Indian Ocean south of the Equator. General Bathymetric Chart of the Oceans (GEBCO). , Ottowa: Canadian Hydrographic Service Abstract
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1981
Sclater, JG, Fisher RL, Patriat P, Tapscott C, Parsons B.  1981.  Eocene to recent development of the South-west Indian Ridge, a consequence of the evolution of the Indian Ocean Triple Junction. Geophysical Journal International. 64:587-604.   10.1111/j.1365-246X.1981.tb02686.x   AbstractWebsite

The South-west Indian Ridge, the contact between the African and Antarctic plates, lies between the Bouvet Triple Junction in the South Atlantic and the Indian Ocean Triple Junction about 2100 km east of Madagascar. From the vicinity of Prince Edward Island at 40° E it trends north-easterly and it is segmented by a suite of deep north-south gashes terminating on the north-east with two spectacular meridional fracture zones, the ‘Atlantis II’ and the ‘Melville’, at 57° 30′E and 60° 30′E respectively. From there north-east to the Indian Ocean Triple Junction at 25° 30′S, 70° 00′E the ridge trends N75° E; it is characterized by a triangle of rough topography with the triple junction at the eastern apex. From all available data an instantaneous pole of relative motion for Africa/Antarctica was computed; it lies at 8.4° N, 42.4° W, with a rate of 0.15° Myr−1.Since the marked change in the direction and rate of spreading in the Madagascar, Crozet and Central Indian Basins that occurred in the Eocene (44 Ma, Anomaly 19), the poles of relative motion for the African, Indian and Antarctic plates have changed very little. We fixed the Africa/Antarctica and Africa/India poles and computed that for India/Antarctica. We justified this pole by comparisons of predicted isochrons with observed magnetic lineations and determined the tectonic history of the triple junction. Since the Eocene (44 Ma, Anomaly 19), this junction has moved as rapidly east-wards with respect to Africa as Antarctica has moved south. The resultant geometry and slow spreading account for the triangle of rough topography produced by the South-west Indian Ridge east of the Melville Fracture Zone. The triple junction evolved as a stable ridge—ridge—ridge type with the South-east Indian Ridge remaining approximately constant in length. It was not resolved whether this constancy in length is maintained by frequent ridge jumps or by oblique spreading on the South-west and Central Indian Ridges near the triple junction.

1980
Tapscott, CR, Patriat P, Fisher RL, Sclater JG, Hoskins H, Parsons B.  1980.  The Indian Ocean Triple Junction. J. Geophys. Res.. 85:4723-4739.: AGU   10.1029/JB085iB09p04723   AbstractWebsite

The boundaries of three major plates (Africa, India, and Antarctica) meet in a triple junction in the Indian Ocean near 25∞S, 70∞E. Using observed bathymetry and magnetic anomalies, we locate the junction to within 5 km and show that it is a ridge-ridge-ridge type. Relative plate motion is N60∞E at 50 mm/yr (full rate) across the Central Indian Ridge, N47∞E at 60 mm/yr across the Southeast Indian Ridge, and N3∞W at 15 mm/yr across the Southwest Indian Ridge; the observed velocity triangle is closed. Poles of instantaneous relative plate motion are determined for all plate pairs. The data in the South Atlantic and Indian oceans are consistent with a rigid African plate without significant internal deformation. Two of the ridges at the triple junction are normal midocean spreading centers with well-defined median valleys. The Southwest Indian Ridge, however, has a peculiar morphology near the triple junction, that of an elongate triangular deep, with the triple junction at its apex. The floor of the deep represents crust formed at the Southwest Indian Ridge, and the morphology is a consequence of the evolution of the triple junction and is similar to that at the Galapagos Triple Junction. Though one cannot determine with precision the stability conditions at the triple junction, the development of the junction over the last 10 m.y. can be mapped, and the topographic expressions of the triple junction traces may be detected on the three plates.

Goslin, J, Segoufin J, Schlich R, Fisher RL.  1980.  Submarine topography and shallow structure of the Madagascar Ridge, Western Indian Ocean. Geological Society of America Bulletin. 91:741-753.   10.1130/0016-7606(1980)91<741:stasso>2.0.co;2   AbstractWebsite

Bathymetric, seismic-reflection, seismic-refraction, and magnetic-anomaly data suggest that the Madagascar Ridge consists of two different domains. North of 31°S, both the sea floor and the basement topography appear to be locally and regionally complex. Small sediment-filled pockets are present between numerous basaltic basement highs. Large-scale normal faulting shapes the western flank; on the east, the Late Cretaceous fracture zones of the Madagascar Basin penetrate deeply into the ridge. South of 32°S, an extensive area of thick undeformed sediments is found over the central part of the ridge. Two prominent seismic horizons — the upper one being attributed to an unconformity between lower Eocene–upper Oligocene and lower Miocene sediments — can be mapped throughout this area. The steep western slope reflects the Mozambique Basin fracture-zone trends. Both the topography and sediment cover at the southeastern limit are closely related to the north-south–trending fracture zones of the Southwest Indian Ridge. The character of magnetic anomalies is in accord with this regional subdivision. An east-west–trending rise situated near 32°S and marked by the 4,000-m isobath marks the former boundary of two spreading systems that are related to the Central Indian Ridge and the Southwest Indian Ridge. The shallow structural data and our interpretation of those data favor an oceanic nature for the entire Madagascar Ridge.

1979
Hedge, CE, Futa K, Engel CG, Fisher RL.  1979.  Rare-Earth Abundances and Rb-Sr Systematics of Basalts, Gabbro, Anorthosite and Minor Granitic-Rocks from the Indian-Ocean Ridge System, Western Indian-Ocean. Contributions to Mineralogy and Petrology. 68:373-376.   10.1007/bf01164522   AbstractWebsite

Basalts dredged from the Mid-Indian Ocean Ridge System have rare earth, Rb, and Sr concentrations like those from other mid-ocean ridges, but have slightly higher Sr87/Sr86 ratios. Underlying gabbroic complexes are similar to the basalts in Sr87/Sr86, but are poorer K, Rb, and in rare earths. The chemical and isotopic data, as well as the geologic relations suggest a cumulate origin for the bulk of the gabbroic complexes.

1977
Mammerickx, J, Fisher RL, Emmel FJ, Smith SM.  1977.  Bathymetry of the east and southeast Asian seas. ( Geological Society of A, Ed.)., Boulder, Colo: Geological Society of America Abstract
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Batiza, R, Rosendahl BR, Fisher RL.  1977.  Evolution of Oceanic-Crust: 3. Petrology and chemistry of basalts from East Pacific Rise and Siqueiros Transform Fault. Journal of Geophysical Research. 82:265-276.   10.1029/JB082i002p00265   AbstractWebsite

Basalt samples obtained from the Siqueiros transform fault/fracture zone and the adjacent East Pacific Rise are mostly very fresh oceanic tholeiite and fractionated oceanic tholeiite with Fe+3/ Fe+2 ∼ 0.25; however, alkali basalts occur in the area as well. The rocks of the tholeiitic suite are ol + pl phyric and ol + pl + cpx phyric basalts, while the alkali basalts are ol and ol + pl phyric. Microprobe analyses of the tholeiitic suite phenocrysts indicate that they are Fo68–Fo86, An58–An75, and augite (Ca34Mg50Fe16). The range of olivine and plagioclase compositions represents the chemical variation of the phenocryst compositions with fractionation. The phenocyrsts in the alkali basalts are Fo81 and An69. The suite of tholeiites comprises a fractionation series characterized by relative enrichment of Fe, Ti, Mn, V, Na, K, and P and depletion of Ca, Al, Mg, Ni, and Cr. The fractionated tholeiites occur on the median ridge (which is a sliver of normal oceanic crust) of the double Siqueiros transform fault, on the western Siqueiros fracture zone, and on the adjoining East Pacific Rise, while the two transform fault troughs contain mostly unfractionated or only slightly fractionated tholeiite. We suggest that the fractionated tholeiites are produced by fractional crystallization of more ‘primitive’ tholeiitic liquid in a crustal magma chamber below the crest of the East Pacific Rise. This magma chamber may be disrupted by the transform fault troughs, thus explaining the paucity of fractionated tholeiites in the troughs. The alkali basalts are found only on the flanks of a topographic high near the intersection of the northern transform trough with the East Pacific Rise.

1976
Shepard, FP, Marshall NF, McLoughlin PA, Fisher RL.  1976.  Sediment Waves (Giant Ripples) Transverse to West Coast of Mexico. Marine Geology. 20:1-6.   10.1016/0025-3227(76)90071-2   AbstractWebsite

A 45-km belt of large symmetrical sediment waves extends along the west coast of Mexico west of Manzanillo between depths of 320 and 770 m. They are thought to be the result of a strong subsurface current that changes seasonally from southeast to northwest.

1975
Engel, CG, Fisher RL.  1975.  Granitic to Ultramafic Rock Complexes of Indian-Ocean Ridge System, Western Indian-Ocean. Geological Society of America Bulletin. 86:1553-1578.   10.1130/0016-7606(1975)86<1553:gturco>2.0.co;2   AbstractWebsite

During five expeditions of the Scripps Institution of Oceanography to the western Indian Ocean, more than 4,500,000 sq km of the Central Indian Ridge and its branching Southeast Indian Ridge and Southwest Indian Ridge were explored by bathymetric, magnetic, and seismic-reflection profiling. In some 2,800,000 sq km of this region, igneous rocks of the crust, lower crust, and possible upper mantle are exposed by faulting or volcanism. Fifty-six dredge hauls of these igneous rocks were obtained, largely from the major cross-fractures (transform faults) or clefts trending athwart the volcanically active ridges. From north to south, the cross-fractures most intensively sampled were the Vema Fracture Zone, which crosses the crestal area near 9°S, Argo Fracture Zone near 13°30′S, Marie Celeste Fracture Zone near 17°30′S, and the newly delineated “Melville Fracture Zone” trending north-south for more than 600 km near 60°30′E on the Southwest Indian Ridge.

1974
Sclater, JG, Fisher RL.  1974.  Evolution of East Central Indian-Ocean, with Emphasis on Tectonic Setting of NinetyEast Ridge. Geological Society of America Bulletin. 85:683-702.   10.1130/0016-7606(1974)85<683:eoteci>2.0.co;2   AbstractWebsite

The meridional Ninetyeast Ridge in the eastern Indian Ocean separates the deep Central Indian Basin from the deeper Wharton Basin (or Cocos Basin–West Australian Basin) to the east. The flattish-summited ridge extends slightly east of north from near 32° S. directly to 7° S. where it appears segmented as a series of en echelon northeast-southwest–trending highs, then in a northerly direction disappears beneath the sediments of the Bengal Fan system near 9° N. Linear parallel to subparallel troughs border this linear ridge on the east side; on the west, from results of magnetic observations and preliminary deep drilling information, Ninetyeast Ridge apparently is bonded to the Indian plate. A second extensive north-south topographic rise and magnetic boundary zone, herein named the Investigator Fracture Zone, lies near 98° E. in the Wharton Basin. Easterly trending magnetic-anomaly lineations identified as numbers 5 through 16 and numbers 21 through 33b, increasing in age northward and with spreading rates variable through time, have been recognized in the Central Indian Basin. East of Ninetyeast Ridge in the Wharton Basin, anomalies 19 through 27, with spreading rates varying in concert with those of comparable age west of the ridge, have been found to increase in age toward the south. Older anomalies 28 through 33 have been identified in both basins; their divergent trends provide evidence that spreading rates decrease markedly westward during the time span they cover in the Late Cretaceous.From deep-sea drilling information supplementing and supporting magnetic, topographic, and gravity data obtained principally by research ships and PROJECT MAGNET since 1962, we interpret Ninetyeast Ridge to be an extrusive pile with a low-density shallow root, rather than a horst or an uplift resulting from the convergence of plates. The trough system that is partially buried with sediment east of the ridge and the north-south Investigator Fracture Zone several hundred kilometers farther to the east are remnants of formerly active transform faults that marked huge relative offsets between the spreading centers separating the Indian and Antarctic-Australian plates from anomaly 33b (Late Cretaceous) to anomaly 19 (Eocene) time. During the Late Cretaceous, Ninetyeast Ridge and Chagos-Laccadive Ridge had similar settings, marking paired offsets of an active spreading center around the southern tip of India. Both features terminated as active transform faults with the cessation of north-south spreading and the commencement of northeast-southwest spreading close to the time of anomaly 11 (Oligocene). The here-interpreted oceanic data is strong but not conclusive support for fitting India to Enderby Land in Antarctica in the Early Cretaceous. With presently available information, we have been unable to establish the precise time at which the spreading center in the Wharton Basin ceased to function.