The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges

Alexander, Robert J.; Harper, Gregory D.

doi:10.1144/gsl.sp.1992.060.01.02

Cited by 26 publications

(46 citation statements)

References 105 publications

(128 reference statements)

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“…15) is another mechanism which can produce jigsaw-puzzle breccias (Sibson, 1986). Jigsaw-puzzle breccias cemented by hydrothermal minerals are commonly associated with oceanic faults in ophiolites (Alexander and Harper, 1992;Alexander et al, 1993;Agar, 1994), but also have been documented in dilational jogs along faults in the Josephine ophiolite (Alexander and Harper, 1992). As noted above, jigsaw-puzzle textures are also common in hydrothermal ore deposits and modern geothermal fields.…”

Section: Discussionmentioning

confidence: 99%

Structural Evolution of Upper Layer 2, Hole 896A

Harper¹,

Tartarotti²

1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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An analysis of the state of stress in the upper ocean crust shows that high hydrostatic pore-fluid pressures will result from the thick overlying water column, providing that the upper crust is sufficiently permeable. High pore-fluid pore pressures will result in very low effective stresses in the upper ocean crust. Such conditions favor deformation by formation of extension fractures and mixed mode (combined extension and shear) fractures rather than by frictional sliding on faults. High pore-fluid pressures are considered very likely to have produced many of the structures in Hole 896A, including jigsaw-puzzle breccias and associated vein networks, cement-supported breccias, and fibrous veins. These structures record an overall volume increase, and their geometry and offset markers indicate formation as extensional fractures or, in some cases, mixed mode fracturing. The geometry and textures of jigsaw-puzzle breccias and associated vein networks suggest formation by longitudinal (axial) splitting, a type of brittle rock failure that occurs in rock deformation experiments done under stress conditions similar to those in the upper ocean crust (σ 2 ' and σ 3 ' < 0). Breccias can form by disaggregation during volume increase associated with extension or mixed mode fractures. The textures of many breccias formed in this way are very similar to those of sedimentary breccias and thus the sedimentary vs. tectonic origin of many breccias in the drill core is uncertain, especially because the margins of the breccias are not usually preserved in core. The fibrous veins generally postdate jigsaw-puzzle breccias and vein networks. They probably formed by repeated cycles of hydrofracturing and sealing of fractures (crack-seal mechanism). The extension directions indicated by both fibrous and nonfibrous veins are highly variable in orientation, even within a single piece, and vein networks suggest simultaneous extension in several directions. In addition, small faults having fibrous slickensides occurring below 334 m below seafloor indicate normal, reverse, and oblique slip. The style of brittle deformation observed in Hole 896A should produce microseismicity characterized by a nonuniform stress field and non-double-couple earthquakes. Such microseismicity characterized geothermal activity in the post-rifting stage at Krafla, Iceland.

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Section: Discussionmentioning

confidence: 99%

Structural Evolution of Upper Layer 2, Hole 896A

Harper¹,

Tartarotti²

1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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“…The Cralice flysch is virtually identical in lithology, petrography, chemistry, and age to the "Catlice" flysch overlying the Josephine ophiolite (Figure 4b) [Harper, 1983[Harper, , 1989 Both the Smith River (Josephine-Galice) and Rogue River subterranes were regionally metamorphosed to low grade and intruded by calc-alkaline magmas during the Nevadan orogeny [Harper and Wright, 1984]. The Catlice Formation in both subterranes has a generally east dipping slaty cleavage, a gently plunging stretching lineation, and variably plunging fold axes [Park, Jones, 1988;Riley, 1987;Jones, 1988;Harper, 1980Harper, , 1989Harper, , 1992.…”

Section: ]mentioning

confidence: 99%

“…The calc-alkaline affinity of the intrusions is apparent from the abundance of hydrous phases, porphyritic textures, and geochemistry [Dick, 1976;Harper, 1980Harper, , 1985Norman, 1984]. The dikes and sills have clearly shared in the Nevadan regional metamorphism, and many are also deformed [Harper, 1984[Harper, , 1992 …”

Section: Amphibolitementioning

confidence: 99%

“…Sample D26 is a sill from the same locality as D24 but is located farther up section within thick bedded graywacke. The sill is largely recrystallized to metamorphic minerals and has fibrous extension veins related to formation of slaty cleavage in the Galice [Harper, 1992]. Relict igneous hornblende gives a well defined isochron age of 146.2+1.0 Ma (Figure 1 lb) which is considered to be an igneous age.…”

Section: Intrusions Cutting the Galice Formationmentioning

confidence: 99%

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Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and ⁴⁰Ar/³⁹Ar geochronology

Harper¹,

Saleeby²,

Heizler³

1994

J. Geophys. Res.

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120

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HAR•E• ET AL.' GE•ONOIX)O• OF THE JOSE•H•E O?H•oLrrE AND ITS EM•LACEMENTThe 4øAr/39Ar ages on igneous minerals will be similar to crystallization ages only if cooling was rapid (e.g., shallow level dikes). Many of the dated plutons are small and were intruded into rocks having ambient temperatures of <350øC. Such small intrusions would be expected to cool quickly below the -500øC closure temperature for hornblende. Many of these plutons, however, consist of two or more phases, so the temperature may have remained elevated by continual addition of heat from new intrusions, especially if substantial volumes of magma were transported through the plutonic complex.Zircon U/Pb age data are presented in Table 2, and the locations of the samples are given in Appendix A (available on microfiche). Notes on analytical procedures and assessment of uncertainties are given in the footnotes of Table 2 Table 2 with Table 1

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“…Thus good constraints on the dip angle of fault scarps cannot be accurately measured from available multibeam bathymetric data. Studies in ophiolites [e.g., Alexander and Harper, 1992;Alexander et al, 1993] suggest that faults can be low-angle and propagate into the mantle and that the OC IC Figure 6. Averaged across-axis profiles in bathymetry and residual Table 1.…”

Section: Kinematic Modelsmentioning

confidence: 99%

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

Escartı́n¹

1996

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Two-thirds of the Earth's surface is oceanic crust formed by magmatic and tectonic processes along mid-ocean ridges. Slow-spreading ridges, such as the Mid-Atlantic Ridge, are discontinuous and composed of ridge segments. Segments are thus fundamental units of magmatic accretion and tectonic deformation that control the evolution of the crust. The objective of this Thesis is to constrain the tectonic processes that occur at the scale of slowspreading segments, to identify the factors controlling segment propagation, and to provide constraints on lithospheric strength with laboratory deformation experiments.In chapter 2, bathymetry and gravity from various areas along the global mid-ocean ridge system are analyzed to quantify systematic variations at the scale of individual segments. There is a marked asymmetry in bathymetry and gravity in the vicinity of segment offsets. We develop a model of faulting to explain these observations. Low-angle faults appear to accommodate tectonic extension at the inside corners of ridge-offset intersections, and result in substantially uplifted terrain with thin crust with respect to that at the outside corners or centers of segments.Results from Chapter 3 indicate that the crust magmatically emplaced on axis is not maintained off-axis. This transition is revealed by both statistical and spectral analyses of bathymetry and gravity. Tectonic extension varies along the length of a segment, resulting in thinning and uplift of the crust at ridge-offset inside corners, and a decorrelation between bathymetry and gravity patterns. Tectonic deformation substantially reshapes the oceanic crust that is magmatically emplaced on-axis, and strongly controls the crustal structure and seafloor morphology off-axis.Satellite gravity data over the Atlantic shown in Chapter 4 reveal a complex history of ridge segmentation, and provides constraints on the processes driving the propagation of segments. The pattern of segmentation is controlled mainly by the geometry of the ridge axis, and secondarily by hot spots. Segments migrate primarily down regional gradients associated with hot spot swells. However, the lack of correlation between gradients and propagation rate, and the propagation up gradient of some offsets, suggest that additional factors control propagation (e.g., variations in lithospheric strength). Most non-transform offsets are short-lived and migrating, while transform offsets are long-lived and stable.Both the propagation of segments (Chapter 4) tectonism along a segment (Chapters 2 and 3) are controlled by the lithospheric rheology. In Chapter 5 I present results from laboratory deformation experiments on serpentinite. These experiments demonstrate that serpentinites are considerably weaker than peridotites or gabbros, display a non-dilatant style of brittle deformation, and strain is accommodated by shear cracking. Serpentinites may weaken the lithosphere, enhance strain localization along faults, and control the style of faulting. Thesis supervisor:Jian Lin Title:Associate...

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The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges

Cited by 26 publications

References 105 publications

Structural Evolution of Upper Layer 2, Hole 896A

Structural Evolution of Upper Layer 2, Hole 896A

Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and ⁴⁰Ar/³⁹Ar geochronology

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

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The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges

Cited by 26 publications

References 105 publications

Structural Evolution of Upper Layer 2, Hole 896A

Structural Evolution of Upper Layer 2, Hole 896A

Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and 40Ar/39Ar geochronology

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

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Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and ⁴⁰Ar/³⁹Ar geochronology