Strain localization in granulite-facies, oxide-bearing mylonitic gabbros from the Atlantis Bank oceanic detachment fault system, Southwest Indian Ridge (IODP Expedition 360)

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Author(s): Viegas, Gustavo; Morales, Luiz; Cassemiro, Paula; Silva, Raylline
Author Affiliation(s): Primary:
University of Brasilia, Institute of Geosciences, Brasilia, Brazil
ETH Zurich, Scientific Center for Optical and Electron Microscopy, Switzerland
Volume Title: European Geosciences Union general assembly 2018
Source: Geophysical Research Abstracts, Vol.20; European Geosciences Union general assembly 2018, Vienna, Austria, April 8-13, 2018. Publisher: Copernicus GmbH on behalf of the European Geosciences Union (EGU), Katlenburg-Lindau, Germany. ISSN: 1029-7006
Note: In English
Summary: The Atlantis Bank oceanic core complex formed during exhumation along an oceanic detachment fault system at the Southwest Indian Ridge (SWIR). The strain regime along the detachment was partitioned between ductile, brittle-ductile and brittle deformation. IODP Expedition 360 cored 789 m of lower crustal rocks consisting of olivine-gabbro, oxide-gabbro and gabbro at Hole U1473A. Crystal-plastic deformation is pervasive throughout the core, with brittle-ductile structures partially overprinting the fabric at shallower levels. Decimeter-wide shear zones observed below 600 mbsf consist mainly of clinopyroxene and olivine porphyroclasts embedded in a fine-grained, recrystallized plagioclase matrix; clinopyroxene clasts are commonly mantled by rims of recrystallized pyroxene and olivine, while the trace of the foliation is defined by trails of a polyphase fine-grained mixture comprising recrystallized pyroxene, olivine, amphibole and Fe-Ti oxides. The microstructure can be divided into three domains based on the degree of recrystallization/segregation of phases during deformation: 1) rigid porphyroclasts, defined by clinopyroxene (±orthopyroxene) and locally olivine which can reach up to 1 mm, 2) fine-grained (∼20 m) recrystallized rims of pyroxene, olivine and minor Fe-Ti oxide, and 3) fine-grained (∼30 m) recrystallized, monomineralic plagioclase bands. Pyroxene porphyroclasts are commonly bent and crosscut by microfractures filled with olivine Fe-Ti oxide; this mixture is also observed as mantling grains along the porphyroclasts' margins. The presence of Fe-Ti oxides results in the development of trails of a recrystallized mixture that progressively grades from the fringes of the clasts to the foliation planes, suggesting a possible rotation/stretching shear component associated with recrystallization. The monomineralic recrystallized plagioclase matrix shows curved straight grain boundaries and homogeneous grain size, with several bulges and pins observed at grain edges. These preliminary results suggest that strain is initially accommodated in the fine-grained plagioclase matrix via a combination of grain boundary migration and subgrain rotation. Fe-Ti oxide-rich, melt/fluid percolation results in the development of interconnected layers in the fine-grained polyphase mixture, which promotes weakening mainly via diffusion and dissolution-precipitation creep during reaction-softening processes. The rheological behavior of the SWIR crust may be drastically affected by melt/fluid percolation throughout shear zones, ultimately leading to strain localization and weakening in initially dry, strong mechanical layers of the lithosphere. [Copyright Author(s) 2018. CC Attribution 4.0 License:]
Year of Publication: 2018
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 05 Petrology, Igneous and Metamorphic; Atlantic Ocean; Atlantis fracture zone; Cores; Expedition 360; Facies; Fracture zones; Granulite facies; IODP Site U1473; Indian Ocean; International Ocean Discovery Program; Marine sediments; Mylonitization; North Atlantic; Sediments; SloMo; Southwest Indian Ridge
Coordinates: S324222 S324222 E0571641 E0571641
Record ID: 2019066725
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from European Geosciences Union, Munich, Germany