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Geology

Selected Works

Barbara John

Articles 1 - 6 of 6

Full-Text Articles in Physical Sciences and Mathematics

Cooling Rates And The Depth Of Detachment Faulting At Oceanic Core Complexes: Evidence From Zircon Pb/U And (U-Th)/He Ages, C. B. Grimes, Michael Cheadle, Barbara John, P. W. Reiners, J. L. Wooden Jun 2012

Cooling Rates And The Depth Of Detachment Faulting At Oceanic Core Complexes: Evidence From Zircon Pb/U And (U-Th)/He Ages, C. B. Grimes, Michael Cheadle, Barbara John, P. W. Reiners, J. L. Wooden

Barbara John

Oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid-ocean ridges, but many questions persist about the thermal evolution and depth of faulting. We present new Pb/U and (U-Th)/He zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the Mid-Atlantic Ridge (Ocean Drilling Program (ODP) holes 1270D and 1275D near the 15 degrees 20'N Transform, and Atlantis Massif at 30 degrees N). Closure temperatures for the Pb/U (similar to 800 degrees C-850 degrees C) and (U-Th)/He (similar to 210 …

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Strain Localization Along The Atlantis Bank Oceanic Detachment Fault System, Southwest Indian Ridge, E. A. Miranda, Barbara John Jun 2012

Strain Localization Along The Atlantis Bank Oceanic Detachment Fault System, Southwest Indian Ridge, E. A. Miranda, Barbara John

Barbara John

Microstructural observations and mineral thermometry from in situ samples collected from the Atlantis Bank oceanic core complex (SW Indian Ridge) indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile, semibrittle, and brittle regimes as strain localized along the exposed, now subhorizontal fault surface. Ductile, semibrittle, and brittle fabrics are developed within dominantly gabbroic rocks. Footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor, whereas semibrittle and brittle fabrics are concentrated in the 80 …

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Cooling History Of Atlantis Bank Oceanic Core Complex: Evidence For Hydrothermal Activity 2.6 Ma Off Axis, J. J. Schwartz, Barbara John, Michael Cheadle, P. W. Reiners, A. G. Baines Jun 2012

Cooling History Of Atlantis Bank Oceanic Core Complex: Evidence For Hydrothermal Activity 2.6 Ma Off Axis, J. J. Schwartz, Barbara John, Michael Cheadle, P. W. Reiners, A. G. Baines

Barbara John

We report 26 (U-Th)/He zircon ages from Atlantis Bank, Southwest Indian Ridge, which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting. Samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly (< 1 Ma), yielding cooling rates > 1200 degrees C/Ma, consistent with existing models for the cooling of oceanic crust. (U-Th)/He zircon ages from samples collected along N-S and E-W trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust. These samples have a mean (U-Th)/He zircon age 2.6 Ma younger than their corresponding igneous crystallization ages and …

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Evolution Of The Southwest Indian Ridge From 55 Degrees 45 ' E To 62 Degrees E: Changes In Plate-Boundary Geometry Since 26 Ma, A. G. Baines, Michael Cheadle, H.J.B. Dick, A. H. Scheirer, Barbara John, N. J. Kusznir, T. Matsumoto Jun 2012

Evolution Of The Southwest Indian Ridge From 55 Degrees 45 ' E To 62 Degrees E: Changes In Plate-Boundary Geometry Since 26 Ma, A. G. Baines, Michael Cheadle, H.J.B. Dick, A. H. Scheirer, Barbara John, N. J. Kusznir, T. Matsumoto

Barbara John

[1] From 55 degrees 45'E to 58 degrees 45'E and from 60 degrees 30'E to 62 degrees 00'E, the ultraslow-spreading Southwest Indian Ridge (SWIR) consists of magmatic spreading segments separated by oblique amagmatic spreading segments, transform faults, and nontransform discontinuities. Off-axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the SWIR. Individual magmatic segments show varying magnitudes and directions of asymmetric spreading, which requires that the shape of the plate boundary has changed significantly over time. In particular, since 26 Ma the Atlantis II transform fault grew by 90 km to reach 199 km, …

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Oxygen Isotopic Ratio Of Ocean Zircon, C. Grimes, T. Ushikubo, Barbara John, J. W. Valley Jun 2012

Oxygen Isotopic Ratio Of Ocean Zircon, C. Grimes, T. Ushikubo, Barbara John, J. W. Valley

Barbara John

Meeting abstract for “Oxygen Isotopic Ratio of Ocean Zircon,” June 2009.

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Protracted Construction Of Gabbroic Crust At A Slow Spreading Ridge: Constraints From (206)Pb/(238)U Zircon Ages From Atlantis Massif And Iodp Hole U1309d (30 Degrees N, Mar), C. B. Grimes, Barbara John, Michael Cheadle, J. L. Wooden Jun 2012

Protracted Construction Of Gabbroic Crust At A Slow Spreading Ridge: Constraints From (206)Pb/(238)U Zircon Ages From Atlantis Massif And Iodp Hole U1309d (30 Degrees N, Mar), C. B. Grimes, Barbara John, Michael Cheadle, J. L. Wooden

Barbara John

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30 degrees N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40-1415 m below seafloor in U1309D yield a weighted mean of 1.20 +/- 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 +/- 0.07 Ma and 1.28 +/- 0.05 Ma indicating …

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