Tectonics, alteration and the fractal distribution of hydrothermal veins in the lower ocean crust

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doi: 10.1016/0012-821X(94)00239-U
Author(s): Magde, Laura S.; Dick, Henry J. B.; Hart, Stan R.
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 129(1-4), p.103-119. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Woods Hole Oceanog. Inst., Contrib. No. 8839. 25 refs.; illus., incl. 3 tables
Summary: Gabbros from Hole 735B preserve the igneous, structural and metamorphic history beneath the Southwest Indian Ridge down to middle amphibolite facies conditions. The gabbro was isolated from the zone of lithospheric necking by detachment faulting, unroofing and block uplift at the inside-corner high of the Atlantis II Transform. Lower crustal accretion, as preserved in the core, is a complex integration of igneous, hydrothermal and tectonic processes. Alteration began at anhydrous granulite conditions as surface faulting penetrated the brittle-ductile transition to form brittle-ductile shear zones. In the amphibolite facies, hydrous alteration around these zones was extensive, while undeformed sections remained virtually unaltered. Amphibole veins formed during the brittle-ductile deformation have a high fractal dimension, reflecting an unclustered distribution, consistent with high strain and cracking rates in the zone of lithospheric necking beneath the ridge. However, the fractal dimensions of the two major gneissic amphibolite zones in Hole 735B are different and suggest that they represent discrete fault zones formed at different times. Below middle amphibolite facies a dramatic drop in the extent of alteration reflects cooling, cracking and alteration under static conditions, similar to layered intrusions. Initially, fracture and vein formation exploited undeformed sections, where elastic strain accumulated during cooling from high temperatures was unrelieved by recrystallization. Plagioclase-diopside veins formed at this time have the same orientation as higher temperature veins, but a mineralogy reflecting much lower rock permeabilities, and a low fractal dimension, reflecting a tight clustering of veins, as would be anticipated for crack formation at low strain rates. Late zeolite and carbonate veins and irregular, smectite-lined, near-vertical cracks also have a very low fractal dimension, reflecting clustering appropriate to low strain, near static conditions. These, however, show no preference for undeformed sections of the core and are oriented with subvertical and subhorizontal maxima. This indicates crack formation under the present day stress field. Thus, elastic strain, due to cooling from high temperatures and extension during lithospheric necking, dissipated after plagioclase-diopside vein formation.
Year of Publication: 1995
Research Program: ODP Ocean Drilling Program
Key Words: 16 Structural Geology; 18 Geophysics, Solid-Earth; Amphibolite facies; Cores; Cracks; Crust; Deformation; Detachment faults; Elastic strain; Facies; Faults; Fractals; Fractures; Gabbros; Granulite facies; Hydrothermal alteration; Igneous rocks; Indian Ocean; Leg 118; Lower crust; Mechanical properties; Metamorphic rocks; Metasomatism; ODP Site 735; Ocean Drilling Program; Oceanic crust; Plutonic rocks; Southwest Indian Ridge; Strike-slip faults; Structural analysis; Tectonics; Transform faults; Veins
Coordinates: S324327 S324318 E0571618 E0571557
Record ID: 1995019345
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands