Fractionation and mantle heterogeneity in basalts from the Peru-Chile Trench

Scheidegger, K. F.; Kulm, L. D.; Corliss, John B.; Schweller, W. J.; Prince, Roger A.

doi:10.1016/0012-821x(78)90056-0

Cited by 14 publications

(20 citation statements)

References 27 publications

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“…old; and (3) fault blocks of oceanic crust in the Peru-Chile Trench (dredges 7, 10, 11, 18) believed to be about 50 m.y. old (Scheidegger and Stakes, 1977;Scheidegger and others, 1978). Dredge haul 52 from the EPR contains over 300 samples spanning almost the entire range of textures and alteration effects; it thus provides an opportunity to evaluate how these factors vary in a single section.…”

Section: Sample Selection and Analytical Methodsmentioning

confidence: 99%

“…as oceanic crust about 50 m.y. old entered the trench (Scheidegger and others, 1978). Basalts exposed along the individual scarps have experienced highly oxidative conditions since the faulting occurred.…”

Section: Oxidative Alteration At Low Temperaturementioning

confidence: 96%

“…If primary plagioclase feldspar is involved in alteration, Ca-and Al-rich secondary minerals (phillipsite, analcite, potassium feldspar, and calcite) would also be expected (Scheidegger and Stakes, 1980). Examples of such compositional and mineral changes accompanying oxidation of old hydrothermally altered basalts can be best documented for pillow basalts dredged from fault scarps within the Peru-Chile Trench (Scheidegger and Stakes, 1977;Scheidegger and others, 1978). The prominent fault scarps (~1 km vertical offsets) are believed to have developed during the past 0.5 rri.y.…”

Section: Oxidative Alteration At Low Temperaturementioning

confidence: 97%

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Temporal variations in secondary minerals from Nazca plate basalts, diabases, and microgabbros

Stakes

Scheidegger

1981

Geological Society of America Memoirs

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The mineralogy and chemistry of secondary phases observed in altered basalts from the Nazca plate provide a record of temporal variations in the processes that accompany the aging of oceanic crust. The earliest formed phase in vein and vesicle fillings is Fe-and Mg-rich, Al-poor saponite; it is the most abundant alteration mineral in younger rocks dredged from near the East Pacific Rise, and it lines many fractures and vesicles in much older pillow basalts recovered from the plate. Saponite formation is enhanced by low pH values, high water-rock ratios, and Mg-and Fe-rich solutions, conditions that persist until fractures and other pore spaces become closed and the hydrothermally induced circulation becomes negligible. During this phase of alteration, constituents derived from hightemperature leaching of underlying holocrystalline diabasic and gabbroic rocks are redistributed, both into secondary minerals found in overlying pillow basalts and into sea water. Holocrystalline, upper layer 2 basalts containing abundant smectite are richer in Mg, Na, K, and Ti and poorer in Ca relative to unaltered fresh glasses. During the waning of the hydrothermal system, lower temperatures and slightly alkaline and oxidizing solutions are indicated by the formation of calcite, ferric oxides and hydroxides, and celadonite. Calcite commonly fills all remaining void spaces in hydrothermally altered rocks. If such hydrothermally altered crust is re-exposed to oxygenated sea water by tectonic uplift or by other means, intense low-temperature oxidation can dramatically affect its bulk chemical composition and secondary mineralogy. Earlier formed saponites and ferrosaponites are destroyed and replaced by celadonite and ferric oxides, and bulk-rock compositions show pronounced losses in Mg and enrichments in Fe and K relative to fresh glasses. It is argued that secondary mineral assemblages in altered layer 2 crustal rocks are very sensitive indicators of conditions of alteration. With few exceptions, evidence of nonoxidative, hydrothermal alteration is ubiquitous in upper layer 2 crustal rocks recovered from many different locations on the Nazca plate.

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Section: Sample Selection and Analytical Methodsmentioning

confidence: 99%

Section: Oxidative Alteration At Low Temperaturementioning

confidence: 96%

Section: Oxidative Alteration At Low Temperaturementioning

confidence: 97%

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Temporal variations in secondary minerals from Nazca plate basalts, diabases, and microgabbros

Stakes

Scheidegger

1981

Geological Society of America Memoirs

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“…However, basalt pillow lavas were commonly dredged from the axial ridges in the trench (Scheidegger and others, 1978), which suggests a relatively shallow depth of faulting and consequently the occurrence of relatively thin thrust sheets. The model proposed here (Fig.…”

Section: Thickness Of Thrust Sheetsmentioning

confidence: 99%

“…Rather, this segment of oceanic crust has numerous structurally weak layers of basalt breccia and/or sediment interlayered between basalt flows. Dredging of several fault scarps 500 m to 1,100 m high within the Peru-Chile Trench yielded only pillow basalts and volcanic breccia with no coarse-grained rocks (Scheidegger and others, 1978), indicating a relatively weak crust composed of many separate flow units. Such lithologies are characteristic of the upper few hundred metres of crustal layer 2.…”

Section: Thrust Faulting and Uplift Modelmentioning

confidence: 99%

Tectonics, structure, and sedimentary framework of the Peru-Chile Trench

Schweller¹,

Kulm²,

Prince³

1981

Geological Society of America Memoirs

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A comprehensive data set of more than 200 profiles across the Peru-Chile Trench between 4° and 45° S is used to describe the morphology and shallow structure of the trench axis and the downbending oceanic plate just prior to subduction. Five morphotectonic provinces (4°-I2°, 12°-17°, 17°-28°, 28°-45°S) show distinct changes in trench depth, axial sediment thickness, oceanic plate fault structures, and dip of the seaward trench slope. In general, the northern and southern regions are characterized by relatively shallow axial depths, moderate to thick trench axis turbidites, and a gently dipping seaward trench slope that exhibits minor normal faults. The deeper central area is almost barren of axial sediments and bends downward more steeply prior to subduction; bending has developed an extensive network of major faults with up to 1,000 m vertical offset on the seaward slope.Two systems of faulting occur in conjunction with subduction. Bending of the oceanic plate causes extensional stress and brittle failure of the upper oceanic crust, resulting in step faults, grabens, and tilted fault blocks on the seaward trench slope. Extensional faulting begins near the outer edge of the trench and develops progressively toward the trench axis. Basaltic ridges and tilted, uplifted trench fill at several locales along the trench can both be explained by thrust faulting. Compressional stress due to plate convergence occasionally can be transmitted seaward from beneath the continental margin through the oceanic plate, emerging as thrust faults within the oceanic crust near the trench axis. Axial turbidites are commonly tilted landward as they are uplifted, probably as a result of downward curving of the underlying thrust fault. Faulting of the oceanic crust prior to and during subduction may have important implications for evolution of convergent continental margins.

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The genesis of mid-ocean ridge basalt

Wilkinson¹

1982

Earth-Science Reviews

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Fractionation and mantle heterogeneity in basalts from the Peru-Chile Trench

Cited by 14 publications

References 27 publications

Temporal variations in secondary minerals from Nazca plate basalts, diabases, and microgabbros

Temporal variations in secondary minerals from Nazca plate basalts, diabases, and microgabbros

Tectonics, structure, and sedimentary framework of the Peru-Chile Trench

The genesis of mid-ocean ridge basalt

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