High temperature structural relationships in re-oriented cores from the Atlantis Bank, Southwest Indian Ridge (SWIR) and Atlantis massif, Mid-Atlantic Ridge (MAR)

Author(s): Deans, J. R.; Yoshinobu, A. S.; Miranda, E.
Author Affiliation(s): Primary:
Texas Tech University, Lubbock, TX, United States
Other:
California State University Northridge, Northridge, CA, United States
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: We present new microstructural data and the most extensive geographically-reoriented fabric results from two slow-spreading oceanic core complexes to a) further constrain the onset of detachment faulting and magma emplacement, and b) constrain the kinematics of high- to moderate-temperature deformation at slow-spreading mid-ocean ridges. High temperature fabrics (including supersolidus and subsolidus foliations/lineations; amphibolite-facies veins) were re-oriented to the ridge reference frame by correlating structural measurements of fractures in the core (and, therefore, fabrics in the core) and fractures in the core wall identified using the Formation MicroScanner (FMS) logging tool. Four ODP/IODP Holes at core complexes meet the necessary requirements (i.e., structural measurements and FMS logs) to reorient high temperature fabrics: Holes 735B and 1105A from Atlantis Bank, SWIR and 1309B and 1309D from Atlantis Massif, MAR. The Atlantis Bank and Atlantis Massif are bathymetric highs with lower crustal and upper mantle rocks, exposed by a currently low angle detachment fault, providing a cross section through the oceanic crust. Both complexes have been sampled by ODP/IODP drilling, submersible dives, and dredging. The Atlantis Bank was drilled by three ODP Legs over two holes, Hole 735B is 1.5 km deep and 1105A is 160 m deep. Both Holes recovered ≈90% with rocks ranging from olivine gabbro to oxide gabbros. The Atlantis Massif was drilled by two IODP Legs with two main holes, Hole 1309B is 102 deep and 1309D is 1.4 km deep with recoveries of ≈50% and ≈75% respectively. The rocks recovered range from troctolite to oxide gabbro. All rock types recovered at both complexes have magmatic and crystal plastic fabrics. Fabrics were reoriented in Hole 735B over the interval of 90-600 mbsf and in Hole 1309D over the interval 98-380 mbsf. Core pieces from Hole 1309D were reoriented by Morris et al. (2009), who did not reorient the fabrics. The reorientation results suggest that at Atlantis Bank and Atlantis Massif for magmatic and crystal plastic fabrics there is no systematic orientation at any depth. This is in contrast to the commonly perceived notion of all deformation being caused by detachment fault. This result must be reconciled with the presence of ultramylonites at depths greater than a km below the detachment surface; the shear zones at depth can have either a normal or reverse sense of shear; and the observed highly deformed greenschist grade detachment shear zone. If the detachment fault was active during emplacement of basaltic magma then the fabrics would most likely be adjacent to the fault, the majority of fabrics would have a normal sense of shear, they would increase in abundance and intensity towards the fault, and their orientation would be parallel to the fault. This suggests the strain field that caused the magmatic fabrics through ultramylonites in the Atlantis Bank and Atlantis Massif was not the detachment fault. This is in contrast to the greenschist grade deformation, which is almost always localized and defines the detachment surface. These results suggest that both core complexes formed in similar ways at high temperature where extension was dominated by intrusive processes, then by lower temperature slip along a detachment surface.
Year of Publication: 2013
Research Program: ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Atlantis II fracture zone; Indian Ocean; Leg 179; Marine sediments; ODP Site 1105; ODP Site 735; Ocean Drilling Program; Sediments; Southwest Indian Ridge
Coordinates: S324308 S324308 E0571639 E0571639
S324327 S324318 E0571618 E0571557
Record ID: 2015050967
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