CHAPMAN CONFERENCE

Detachments in Oceanic Lithosphere: Deformation, Magmatism, Fluid Flow and Ecoysystems

Agros, Cyprus 8-16 May 2010

Conveners Juan Pablo Canales, Woods Hole Oceanographic Institution, Massachusetts (USA) Javier Escartín, Centre National de la Recherche Scientifique (CNRS), IPGP, Paris (France)

Program Committee Donna Blackman, Scripps Institution of Oceanography, La Jolla, California (USA) Joe Cann, University of Leeds (UK) Mathilde Cannat, Centre National de la Recherche Scientifique (CNRS), IPGP, Paris (France) Gretchen Früh-Green, ETH, Zurich (Switzerland) Barbara John, University of Wyoming, Laramie (USA) Gianreto Manatschal, Ecole et Observatoire des Sciences de la Terre-Université Louis Pasteur, Strasbourg (France) Andrew McCaig, University of Leeds (UK) Eleni Morisseau, Cyprus Geological Survey Department, Lefkosia (Cyprus) Stelios Nicolaides, Cyprus Geological Survey Department, Lefkosia (Cyprus) Robert Reves-Sohn, Woods Hole Oceanographic Institute, Massachusetts (USA) Anna-Louise Reysenbach, Portland State University, Oregon (USA) Cindy Van Dover, Duke University, Beaufort, North Carolina (USA)

Financial Sponsors

The conference organizers acknowledge the generous support of the following sponsors:

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Chapman Conference on Detachments in Oceanic Lithosphere: Deformation, Magmatism, Fluid Flow, and Ecosystems

Agros, Cyprus  8–16 May 2010

Meeting At A Glance

Saturday, 8 May 17:00-19:30h Registration 19:30-21:30h Dinner

Sunday, 9 May 07:00-08:30h Breakfast 08:30-09:00h Introduction to Conference 09:00-10:30h General Session 10:30-11:00h Coffee Break & Registration 11:30-12:30h General Session 12:30-14:00h Lunch 14:00-15:30h Introduction of Field Trip 1 15:30-16:00h Coffee Break 16:00-19:30h Poster Session I (put up posters) 18:00-19:30h Poster Reception 19:30-21:30h Dinner

Monday, 10 May 07:00-08:30h Breakfast 08:30-18:00h Field Trip 1 – Introduction to the Troodos Ophiolite 18:00-19:30h Poster Session I 19:30-21:30h Dinner

Tuesday, 11 May 07:00-08:30h Breakfast 09:00-10:30h General Session 10:30-11:00h Coffee Break 11:30-12:30h General Session 12:30-14:00h Lunch 14:00-15:30h General Session 15:30-16:30h Introduction of Field Trips 2 and 3 16:30-17:00h Coffee Break 17:00-19:30h Poster Session I (put down posters) 19:30-21:30h Dinner

Wednesday, 12 May 07:00-08:30h Breakfast 08:30-18:00h Field Trip 2 – Group A: Western Limassol Forest Field Trip 2 – Group B: Eastern Limassol Forest 18:00-19:30h Poster Session II (put up posters) 19:30-21:30h Dinner

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Thursday, 13 May 07:00-08:30h Breakfast 08:30-18:00h Field Trip 3 – Group A: Eastern Limassol Forest Field Trip 3 – Group B: Western Limassol Forest 18:00-19:30h Poster Session II 19:30-21:30h Dinner

Friday, 14 May 07:00-08:45h Breakfast 08:45-09:00h Introduction to Breakout Groups 09:00-10:00h Breakout Groups 10:00-10:30h Panel Discussion 10:30-11:00h Coffee Break 11:00-12:00h Breakout Groups 12:00-12:30h Panel Discussion 12:30-14:00h Lunch 14:00-16:30h Keynote Talks 16:30-17:00h Coffee Break 17:00-19:30h Poster Session II (put down posters) 18:00-19:30h Poster Reception 19:30-21:30h Dinner

Saturday, 15 May 07:00-08:30h Breakfast 08:45-09:00h Introduction to Breakout Groups 09:00-10:00h Breakout Groups 10:00-10:30h Panel Discussion 10:30-11:00h Coffee Break 11:00-12:30h Open Discussion, Summary, and Perspectives 12:30-14:00h Lunch 19:30-21:30h Dinner

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SCIENTIFIC PROGRAM

Saturday, 8 May

17:00-19:30h Registration Apollon Room

19:30-21:30h Dinner Dionysos Restaurant

Sunday, 9 May

07:00-08:30h Breakfast Dionysos Restaurant

08:30-09:00h Introduction to Conference – J. P. Canales & Apollon Room J. Escartín

09:00-10:30h The Detachment Fault Zone and the Core Complex Apollon Room Beneath It

09:00-09:45h Cannat, Mathilde “Oceanic Detachment Faults and the Exhumation of Gabbros and Mantle-derived Peridotites at Mid-ocean Ridge”

09:45-10:30h Cheadle, Michael “Fault Rheology, Footwall Deformation andGeochronologic Constraints on the Process of Detachment Faulting at Mid-Ocean Ridges”

10:30-11:00h Coffee Break & Registration Rodon Cafateria

11:00-12:30h Geophysical Framework Apollon Room

11:00-11:45h Morris, Anthony “Palaeomagnetic Insights Into Oceanic Detachment Fault Processes at Mid-ocean Ridges”

11:45-12:30h Sohn, Robert “Geophysical Constraints on the Structure of Detachment Faults: Stone Cold Facts, Red Hot Topics, and Everything In-between”

12:30-14:00h Lunch Break Rodon Restaurant

14:00-15:30h Introduction to Field Trip 1 Apollon Room

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15:30-16:00h Coffee Break Rodon Cafateria

16:00-19:30h Poster Session I Apollon & Athina Rooms

18:00-19:30h Poster Reception Apollon & Athina Rooms

19:30-21:30h Dinner Rodon Restaurant

Monday, 10 May

07:00-08:30h Breakfast Dionysos Restaurant

08:30-18:00h Field Trip 1: Introduction to the Troodos Ophiolite The first field trip offers an overview of the Ophiolite, visiting classic ophiolite sections including lavas, sulphide deposits, dykes, gabbros, and the Lemithou Detachment.

18:00-19:30h Poster Session I Apollon & Athina Rooms

19:30-21:30h Dinner Rodon Restaurant

Tuesday, 11 May

07:00-08:30h Breakfast Dionysos Restaurant

09:00-10:30h Implications for Continental (Metamorphic) Core Apollon Room Complexes and Detachment Faulting During Continental Breakup

09:00-09:45h John, Bobbie “Oceanic and Continental Detachment Fault Systems: How Similar Are They?”

09:45-10:30h Manatschal, Gianreto “The Role of Detachment Faults During Crustal Thinning, Mantle Exhumation and Continental Break-up at Magma-poor Rifted Margins?”

10:30-11:00h Coffee Break Rodon Cafateria

11:00-12:30h Fluid Flow in Detachment Faults and Core Complexes Apollon Room

11:00-11:45h McCaig, Andrew “Hydrothermal Systems and Detachment Faulting”

11:45-12:30h Früh-Green, Gretchen “Serpentinization of the Oceanic Lithosphere:Consequences for Hydrothermal Activity and Biogeochemical Cycles”

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12:30-14:00h Lunch Break Rodon Restaurant

14:00-15:30h Ecology and Microbiology of Hydrothermal Systems Apollon Room Associate with Detachment Faulting

14:00-14:45h Kelley, Debbie “Linkages Among Serpentinization and Life in Ultramafic-hosted Hydrothermal Systems: The Lost City Hydrothermal Field”

14:45-15:30h Perner, Mirjam “Microbial communities and metabolisms from basalt- and ultramafic-hosted vents”

15:30-16:30h Introduction to Field Trips 2 and 3 Apollon Room

16:30-17:00h Coffee Break Rodon Cafateria

17:00-19:30h Poster Session I Apollon & Athina Rooms

19:30-21:30h Dinner Rodon Restaurant

Wednesday, 12 May

07:00-08:30h Breakfast Dionysos Restaurant

8:30-18:00h Field Trip 2 Group A: Western Limassol Forest Group B: Eastern Limassol Forest The participants will be separated into two groups to visit the Limassol forest to examine two major detachments, the tectonic structures associated to them, and their mode of formation and emplacement.

18:00-19:30h Poster Session II Apollon & Athina Rooms

19:30h Dinner Rodon Restaurant

Thursday, 13 May

07:00-08:30h Breakfast Dionysos Restaurant

6 08:30-18:00h Field Trip 3 Group A: Eastern Limassol Forest Group B: Western Limassol Forest The participants will be separated into two groups to visit the Limassol forest to examine two major detachments, the tectonic structures associated to them, and their mode of formation and emplacement.

18:00-19:30h Poster Session II Apollon & Athina Rooms

19:30h Dinner Rodon Restaurant

Friday, 14 May

07:00-08:45h Breakfast Dionysos Restaurant

08:45-09:00h Introduction to Breakout Groups Apollon Room

Apollon, Demetra Breakout Groups. We will divide in 3 groups. All & Artemis Room groups will discuss the following:

How and Under Which Conditions Does Detachment-faulting Lead to the Formation and Development of OCCs?

What are the Mechanisms and Conditions that Promote and Sustain Strain Localization Over Long Periods of Time, and the Development of an OCC?

What are the Similarities and Differences Between Oceanic and Continental Core Complexes?

What Can We Learn About Continental Breakup and Initiation of Seafloor Spreading From OCCs?

10:00-10:30h Panel Discussion Apollon Room

10:30-11:00h Coffee Break Rodon Cafateria

11:00-12:00h Breakout Groups. We will divide in 3 groups. All groups will discuss the following:

Apollon, Demetra What is the Lithological Structure of OCCs and its Variability & Artemis Room at Scales of Meters to Tens of Kilometers?

What is the Link Between Deformation and Magmatic Emplacement?

12:00-12:30h Panel Discussion Apollon Room

7 12:30-14:00h Lunch Break Rodon Restaurant

14:00 – 16:30h New Frontiers in OCC Research Apollon Room

14:00-15:15h Kelemen, Peter “Evaluating In Situ Mineral Carbonation in Near-Seafloor Peridotite for CO2 Capture and Storage

15:15-16:30h Reysenbach, Anna-Louise “The microbiology of OCC’s: Implications for life in the subsurface and elsewhere in the solar system”

16:30-17:00h Coffee Break Rodon Cafateria

17:00-19:30h Poster Session II Apollon & Athina Rooms

18:00-19:30h Poster Reception Apollon & Athina Rooms

19:30h Dinner Rodon Restaurant

Saturday, 15 May

07:00-08:45h Breakfast Dionysos Restaurant

08:45-09:00h Introduction to Breakout Groups Apollon Room

Apollon, Demetra Breakout Groups. We will divide in 3 groups. All & Artemis Rooms groups will discuss the following:

What Drives Hydrothermal Circulation in OCCs and Detachment Faults? What are the Feedbacks Between Fluid Circulation, Deformation, and Magmatic Processes? What is the Biodiversity and Characteristics of Ecosystems Associated with OCCs, and How are They Related to the Style of Hydrothermal Circulation within OCCs and Along Detachment Faults?

10:00-10:30h Coffee Break Rodon Cafateria

11:00-12:00h Open Discussion, Summary, and Perspectives Apollon Room

12:30-14:00h Lunch Rodon Restaurant

19:30h Dinner Rodon Restaurant

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POSTER SESSION SCHEDULE

Session I: 16:30-19:30h, Sunday, 9 May; 18:00-19:00h, Monday, 10 May; 16:30-10:30h, Tuesday, 11 May  Session II: 18:00-19:30h, Wedenesday, 12, and Thursday, 13, May; 17:00-19:30h, Friday, 14 May

APOLLON AND ATHINA ROOMS

POSTER SESSION I

1. Abe, N. High Temperature Metamorphism and Petrological Insights of An Abyssal Chromitites From Site 1271, ODP Leg 209, MAR 15 20 N 2. Joens, N. Melt-rock Interaction and its Impact on Serpentinization of Abyssal Peridotites

3. Christofferson, The Igneous Architecture of IODP Hole C. U1309D: ~10’s Meter Scale Units Suggest Accretion by Small Volume Intrusive Events 4. Efimov, A. Strontium in Primitive Gabbros of Deep Holes 735B and U1309D: Incompatibility With Magmatic Models of Formation of the Third Seismic Layer of Ocean 5. Godard, M. Geochemistry of IODP Site U1309 Gabbroic Series (Atlantis Massif): Evidence for High and Cyclic Magmatic Activity 6. Grimes, C. Isotopic Insights Into Length-scales, Initiation Depths and Extent of High- temperature Hydrothermal Circulation Associated With Oceanic Detachment Fault Systems 7. Kurz, M. Noble Gases as Tracers of Mantle Deformation

8. Andreani, M. Tectonic Structure and Internal composition of the Rainbow Massif, Mid-Atlantic Ridge 36°14'N 9. Baines, G. The Tectonic Setting and Evolution of the Atlantis Bank Oceanic Core-complex

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10. Schroeder, T. Footwall Lithology Controls on Strain Localization Mechanisms and the Geometry of Oceanic Detachment Faults 11. Escartin, J. Strain Localization Along Detachment Faults: Mechanical Role of Serpentinite, Talc, Tremolite and Chlorite 12. Ildefonse, B. The Atlantis Massif : An Illustration of the Interplay Between Core Complex Development and Magmatic Activity 13. Karson, J. Detachment Shear Zone Exposed on the S. Wall of the Atlantis Massif Core Complex, Mid-Atlantic Ridge 30°N 14. Pressling, N. A Continuous Structural Characterisation of Atlantis Massif Using an Integrated Analysis of Oriented Downhole Imagery and Logging Data 15. Morishita, T. Direct Observations of the 25°S Oceanic Core Complex Along the Central Indian Ridge 16. Ohara, Y. Godzilla Mullion Developed in a Slow- spreading Environment: New Age and Petrologic Data Constraints 17. Dannowski, A. Two End-members of Oceanic Core Complex formation 18. Xu, M. Heterogeneous Seismic Velocity Structure of the Upper Lithosphere at Kane Oceanic Core Complex, Mid-Atlantic Ridge 23°17’N- 23°37’N 19. Canales, J. Advanced Seismic Imaging of Oceanic Core Complexes 20. Henig, A. Comprehensive Shallow Coverage of Seismic Velocity Structure at Atlantis Massif Oceanic Core Complex 21. Blackman, D. Refraction Constraints on 2~5 km Subseafloor Depths, Atlantis Massif MAR 30°N 22. Tivey, M. The Magnetism of Oceanic Core Complexes – the Kane OCC 23. Sauter, D. The Magnetic Signature of Non-volcanic Domains on the Southwest Indian Ridge Between 61E and 67E 24. Tominaga, M. The Origin of the Smooth Zone in Early Cretaceous North Atlantic Magnetic Anomalies 25. Denny, A. Geologic Evolution and Structural Control of the Lost City Hydrothermal Field 26. Castelain, T. Hydrothermal Fluid Flow in Oceanic Gabbros, IODP Site 1309, Mid-Atlantic Ridge: Strontium and Oxygen Isotopic Data.

10 27. Frueh-Green, G. Faults, Fluids and Alteration During the Evolution of an Oceanic Core Complex: Constraints From the Atlantic Massif 28. Delacour, A. Serpentinization Along the Mid-Atlantic Ridge (13°-36°N) 29. Lilley, M. From Mantle to Microbes: The Cycling of Volatiles in Ridge Environments 30. German, C. Hydrothermal Activity Along the Ultra-Slow Spreading Mid-Cayman Rise

31. Garzetti, F. Timing of Exhumation of a MOR-type Gabbroic Intrusion (Internal Ligurian ophiolites, Italy) 32. Sanfilippo, A. Interplay Between Tectonic and Magmatic Events During the Exhumation of a Gabbro- peridotite section to the Sea-floor (Northern Apennine ophiolites, Italy) 33. Ma, C. Petrogenesis of Piemont-Ligurian Oceanic Plagiogranites (albitites), Western Alps

34. Sawyer, D. Aspects of Mantle Exhumation During Magma Starved Rifting at the Galicia Margin

35. Canovas, P. Thermodynamic Constraints on Biomolecule Synthesis and Metabolism Due to Alteration of Ultramafic Host Rock 36. Reysenbach, A. Host-rock Composition Shapes the Microbial Community Structure Associated With Hydrothermal Vent Deposits 37. Hurst, S. Similarities in Structure of Uppermost Oceanic Crust of the Hess Deep Rift and Pito Deep with the Sheeted Dike Complex of the Troodos Ophiolite 38. Katzir, Y. Fault-related Oceanic Serpentinization in the Troodos Ophiolite, Cyprus: Implications for a fossil Oceanic Core Complex 39. Ebert, Y. Ridge-transform Intersection: Inferences From Paleomagnetism and Structure of the Troodos Ophiolite, Cyprus 40. Granot, R. Evidence From the Troodos Ophiolite for Partitioning of Vertical-axis Rotations ('torsional detachment') Across the Dike- gabbro Boundary

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POSTER SESSION II

41. Achenbach, K. Lithosphere Accretion and the Deepest Roots of Detachment Faults: Evidence from Abyssal Peridotites Beneath Atlantis Bank, SWIR 42. Hansen, L. Mylonitic Detachment Faulting Along the Mid-Atlantic Ridge at the Kane Fracture Zone Oceanic Core Complex 43. Hellebrand, E. Pervasive Reactive Melt Migration Before Core Complex Formation: Evidence From IODP Hole U1309D, Atlantis Massif 44. Casey, J. Evidence For High Degrees of Mantle Melting, Yet Extensive Development of Oceanic Core Complexes and Significant Evidence of Low Magmatic Production on the Mid-Atlantic Ridge between 12°-16°N 45. Klein, F. CO2-metasomatism and the Formation of Talc in Oceanic Detachment Faults

46. Harigane, Y. Microstructural Development of the Ultramafic Rocks from Godzilla Mullion in the Parece Vela Basin 47. Loocke, M. Godzilla Mullion: Plagioclase Systematics of a Back-arc Core Complex

48. Dick, H. Tale of Two Core Complexes: Contrasting Crustal Architecture and Fault Geometries

49. Schoolmeesters, Cooling History of the Atlantis Massif and N. Kane Oceanic Core Complexes at the Slow-spreading Mid-Atlantic Ridge 50. Reston, T. Detachment Faulting and the Formation of Oceanic Core Complexes 51. Cann, J. Oceanic Core Complexes in the Mid- Atlantic Ridge for 200 km South of the Kane Fracture Zone 52. Cannat, M. Lateral and Temporal Variations in the Degree of Mechanical Weakening in the Footwall of Oceanic Detachment Faults 53. Miranda, E. The Role of Strain Partitioning in the Development of an Oceanic Detachment Fault System: Insights From Relict Oceanic Core Complexes in the Western Mirdita Ophiolite, Albania

12 54. Gaëlle, P. New Variscan 40Ar-39Ar Ages for the Danubian Ophiolite 55. MacLeod, C. Detachment Faulting and Melt Transport Through the Mantle Lithosphere: Evidence from the Lizard ophiolite, SW England 56. MacLeod, C. Quantitative Constraint on Tectonic Rotations at Oceanic Core Complexes From a Structural and Palaeomagnetic Study of Oriented Rock Cores from the Mid-Atlantic Ridge at 15°45’N 57. Searle, R. Life Cycle of Oceanic Core Complexes, 1: Overview 58. MacLeod, C. Life Cycle of Oceanic Core Complexes, 2: Melt Emplacement and Detachment Fault Termination Mechanisms 59. Mallows, C. Life Cycle of Oceanic Core Complexes, 3: Crustal Structure and Melt Supply Variability on the Mid-Atlantic Ridge at 13N 60. Unsworth, S. Life Cycle of Oceanic Core Complexes, 4: Mantle Melting Controls on Amagmatic Crustal Accretion 61. Searle, R. Life Cycle of Oceanic Core Complexes, 5: Internal Structures and Comparisons 62. Buck, W.R. Magmatic Influences on Axial Relief and Oceanic Core Complex Formation 63. Choi, E. 3D Numerical Models for the Formation of Oceanic and Continental Core Complexes 64. Olive, J. Emplacement of Plutonic Bodies in Oceanic Core Complexes 65. Schouten, H. Tectonic vs Magmatic Extension in the Presence of Core Complexes at Slow- spreading Ridges From a Visualization of Faulted Seafloor Topography 66. Morris, A. Palaeomagnetic Constraints on the Evolution of the Atlantis Massif Oceanic Core Complex (Mid-Atlantic Ridge, 30°N) 67. Rough, M. H2 Production From Ultramafic-Bearing Rock Assemblages: An Experimental Study at 420°C, 500 Bars 68. Fontaine, F. Hydrothermal Circulation in and Around Detachment Faults: Inferences From Three-dimensional Numerical Modeling. 69. Boulart, C. Emerging Technologies for in Situ Dissolved Methane Measurements in Hydrothermal Vents 70. Rona, P. Detachment Faulting Chronology in Oceanic Crust Using Radiometric Dating of Associated Hydrothermal Deposits: TAG Hydrothermal Field, Mid-Atlantic Ridge 26○N

13 71. Cole, E. Understanding Hydrothermal Fluid Flow Along Oceanic Detachment Faults - Kane Oceanic Core Complex, 23°N Mid Atlantic Ridge 72. Tucholke, B. Manifestations of Hydrothermal Venting at Kane Megamullion 73. Picazo, S. Modes of Deformation in Ultramafic Rocks Exhumed in the Footwall of Detachment Faults at Slow-spreading Ridges and Ocean Continent Transitions 74. Konstantinou, A. Evolution of Magmatism During the Development of a Continental Metamorphic Core Complex 75. Bronner, A. Magnetic Anomalies In The Ocean- continent Transition Zone From The Iberian-Newfoundland Margins : An Alternative View 76. Lasseigne, A. Rifting Mechanisms, Mantle Exhumation and the Initiation of Seafloor Spreading: Evidence from the Deep Galicia Basin 77. Lartaud, F. Fossil Bivalves in the Rainbow Area: New Insight Into the Diversity and Evolution of Chemosynthetic Communities. 78. Shaar, R. An Antithetic Tilt of the Plutonic and the Sheeted Dikes Sections Evidenced by a Paleomagnetic Study Near the Village of Agros, Cyprus

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ABSTRACTS

Poster I-1 Achenbach, Kay L.; Cheadle, Michael High Temperature Metamorphism J.; Dick, Henry J.; Swapp, Susan M. and Petrological Insights of an Abyssal Chromitites From Site 1271, K.L. Achenbach, Department of Earth ODP Leg 209, MAR 15 20 N Sciences, Durham University, Durham, UNITED KINGDOM; M.J. Cheadle, S.M. Abe, Natsue Swapp, Department of Geology & Geophysics, University of Wyoming, IFREE, JAMSTEC, Yokosuka, JAPAN Laramie, WY; H.J. Dick, Geology & Geophysics Department, Woods Hole Dunite and gabbroic materials, the rocks Oceanographic Institution, Woods Hole, compose the moho transition zone, are MA recovered from 15 20 N FZ, the Mid- Atlantic Ridge (MAR) during (ODP) Leg The deformation conditions in the 209. Several podiform chromitites were uppermost mantle beneath the roots of recovered at Site 1271, both Hole 1271A a mid-ocean ridge detachment fault are and B, south of 15 20 N FZ during the not well understood. Here we use cruise. The primary chromites in the abyssal peridotites from the footwall of a chromitites have moderately high-Cr# detachment fault to investigate the (0.52 and 0.48 for Hole 1271A and latest stages of the lithospheric accretion 1272B, respectively) and chromian at an oceanic core complex. Twelve spinels in the surrounding chromitites spinel lherzolites were collected using have similar composition. It is the Kaiko unmanned submersible along considered that podiform chromitite is a single 1.5-km dive track on the formed by two kind of melt (melt-mantle western flank of the Atlantis Bank interaction, therefore there should be oceanic core complex adjacent to the plenty volume of melt in the mantle Atlantis II Transform, south of the beneath this area. Abundant of gabbro Southwest Indian Ridge (SWIR). All and dunite were recovered from this samples were examined area suggest that main volume of melt petrographically; nine were analyzed for body were consumed in the upper crystal lattice-preferred orientation mantle by melt-wallrock interaction. On (LPO) using electron backscatter the other hand, chromite grains have diffraction (EBSD), and two were thick rims of Cr-magnetite or completely analyzed for clinopyroxene trace replaced by magnetite without chromite element compositions. All the samples core. Cr content elevation in the Cr- are protogranular to coarse magnetite rim occurred with Fe- porphyroclastic in texture, and are thus enrichment. These chemical typical of mantle rocks deformed in the modifications suggest that the chromitite asthenosphere. LPO data support this from Site 1271 were metamorphosed at interpretation, as the LPOs of all amphibolite facies because significant Al- constituent minerals are fairly weak missing from chromite cores is taken (olivine M-index = 0.05–0.14; place above 550 degree Celsius. orthopyroxene M-index = 0.05–0.14; clinopyroxene M-index = 0.04–0.07). Poster II-41 Furthermore, olivine LPOs preserve slip Lithosphere Accretion and the in the (010)[100] system, which is Deepest Roots of Detachment characteristic of the high-temperature, Faults: Evidence from Abyssal low-stress, low-water-content Peridotites Beneath Atlantis Bank, deformation expected beneath mid- SWIR ocean ridges. Statistically significant differences in LPO strength imply variations in degree of mantle

15 deformation. The rocks preserve melt- The ultramafic-hosted Rainbow rock interaction textures that are hydrothermal field is located on a massif essentially undeformed, and the at a non-transform offset between the prevalence of these melt-related AMAR and South AMAR ridge segments, features varies from rock to rock and is at 36°14'N along the Mid-Atlantic Ridge not related to variations in LPO strength. (MAR). This massif shares some Plagioclase is absent in the peridotites, characteristics with other oceanic core and spinel is present in some melt- complexes (OCCs), like the common related features. Textures preserved in dome morphology, although no the peridotites show varying degrees of corrugated surface has been observed post-deformation melt-rock interaction. so far. During the two MoMARDREAM The available trace element data also cruises (in July 2007 on R/V Pourquoi suggest variable amounts of melt-rock Pas ? and in Aug-Sept 2008 on R/V interaction; the slightly more enriched Atalante), we studied the structural clinopyroxene grains are found in the context of the hydrothermal system and sample with the greatest textural the lithology of the massif. Hydrothermal evidence for melt-rock interaction, and and basement rock samples were the more depleted grains are found in recovered by dredging and diving (with the sample with the least textural the manned submersible Nautile and the evidence for melt-rock interaction. ROV Victor), complementing prior Thus, the Atlantis Bank sampling realized during the FLORES peridotites reveal a history of (1997) and IRIS (2001) cruises. The heterogeneous deformation during the tectonic of the Rainbow Massif is latest stages of asthenospheric dominated by a N-S trending fault upwelling. The last stages of pattern on its western flank and a series deformation in these rocks occurred at of SW-NE fault-related ridges temperatures of 1100oC or more, at the crosscutting the massif. Available focal lithosphere-asthenosphere boundary. mechanisms indicate that tectonic This deformation was subsequently extension is perpendicular to N30-50° overprinted by melt-rock interaction at trending-direction. The active high- depths of ~25km or greater (based on temperature hydrothermal system is published thermal models). Thus, at the located in the area were these two time that deformation localized into systems crosscut. Mantle peridotites discrete shear zones of the detachment (spinel harzburgites and dunites) were fault, mantle deformation was already the predominant rock type recovered at somewhat heterogeneous on the scale of the footwall and show petrographic hundreds of meters and melt was still characteristics of both melt-rock and actively percolating through the mantle. fluid-rock interaction. Melt-rock interaction is evidenced by typical interstitial texture and high REE Poster I-8 concentrations (1 x Chondrite). Variable Tectonic Structure and Internal degrees and styles of serpentinization Composition of the Rainbow Massif, and deformation (commonly undeformed Mid-Atlantic Ridge 36°14'N or displaying talc-serpentine foliated schist into faults) were identified in the Andreani, Muriel; Ildefonse, Benoit; hydrated peridotites. Serpentinites are Delacour, Adelie; Escartin, Javier; frequently oxidized and can be highly Godard, Marguerite; Dyment, Jerome enriched in iron oxide veinlets, also indicated by the high FeO contents of M. Andreani, LST, ENS - Université Lyon some samples. In addition to ultramafic 1, Lyon, FRANCE; B. Ildefonse, M. rocks, gabbros, cm- to dm-thick Godard, GM, CNRS- Université gabbroic veins, and basalts (including Montpellier 2, Montpellier, FRANCE; A. fresh glass) were recovered from talus Delacour, DTP, OMP-Université Paul and sediments on the SW and NE massif Sabatier, Toulouse, FRANCE; J. Escartin, flanks. Massive chromitite was also J. Dyment, IPGP, CNRS, Paris, FRANCE recovered in one dredge haul. The inferred lithological variability of the

16 Rainbow massif footwall is thus growth since at least 30 Ma was consistent with that of OCCs studied accommodated by long-lived asymmetric along the MAR and Southwest Indian spreading and was under transtension Ridge. The stockwork of the ~8-20 Ma. Once detachment faulting hydrothermal system has been sampled commenced, spreading was highly on two locations on the western side of asymmetric such that the detachment the present-day hydrothermal field, fault formed the principal plate along N-S trending normal fault scarps boundary. U-Pb zircon dating suggests and within the talus underneath. It is that the lithosphere was up to 18-km- mainly made of massive sulfides, thick and that magmatism continued strongly altered serpentinites, and relatively unabated. However, crustal breccias containing clasts of iron sulfide accretion was focused at depth in the and/or iron oxide impregnated footwall of the fault leading to relatively serpentinites. Our preliminary enhanced accretion of the lower crust interpretation indicates that the Rainbow and reduced volcanism in the axial massif has been exhumed by tectonic valley. The 1.5 km lower crustal section extension, but this tectonic context is in ODP Hole 735B accreted rapidly, and much more complex than that of other cooled to below 365°C in <0.5 Ma, but well-studied OCCs. The massif is located anomalously young thermochronologic within a non-transform offset and does ages suggest later heating by not develop in continuity with the crust hydrothermal circulation along fault accreted along one flank of the ridge zones at ~8-9 Ma. These results require axis. This context would explain the a tectonothermal event that significantly obliquity of both the cross-cutting faults post-dates detachment faulting. This and the focal mechanisms and the lack event and the faulting which uplifted of evidence of single detachment fault Atlantis Bank to sea-level were likely responsible for the exhumation of the related to tectonics along the Atlantis II massif. Transform, and specifically to the final stages of long-lived transtension caused Poster I-9 by a rotation in the direction of plate- The Tectonic Setting and Evolution spreading 19.6 Ma. of the Atlantis Bank Oceanic Core- complex Poster I-21 Refraction Constraints on 2~5 km Baines, Graham; Schwartz, Joshua J.; Subseafloor Depths, Atlantis Massif Cheadle, Michael J.; John, Barbara MAR 30°N

E.G. Baines, School of Earth & Blackman, Donna K.; Collins, John A. Environmental Sciences, University of Adelaide, Adelaide, South Australia, D.K. Blackman, IGPP, Scripps Institution AUSTRALIA; J.J. Schwartz, Department Oceanography, La Jolla, CA; J.A. Collins, of Geological Sciences, Alabama, MGG, Woods Hole Oceanographic Tuscaloosa, AL; M.J. Cheadle, B.E. John, Institution, Woods Hole, MA Department of Geology and Geophysics, University of Wyoming, Laramie, WY The seismic structure of the oceanic core complex comprising Atlantis Massif, Atlantis Bank on the southwest Indian 30°N on the Mid-Altantic Ridge, is Ridge (SWIR) formed 10-13 Ma by assessed through analysis of OBS oceanic detachment faulting. Oceanic refraction data. The source-receiver detachment faults are scarce in the geometry for a 1997 experiment immediate vicinity, leading us to suggest provides coverage that extends to 4-8 that detachment faulting at Atlantis km subseafloor depths. For assessing Bank was triggered by a short-lived the depth to mantle velocities, a 40-km regional increase in the full-spreading cross-axis line is most relevant; we have rate along the SWIR and was also considered 2 shorter along-strike preferentially localized adjacent to the lines over the central and southern parts Atlantis II Transform Fault, whose of the domal core. Velocity structure

17 results are based on inversion of first Toulouse, FRANCE; H. C. Seat, M. arrival travel-times and we have tested Cattoen, Th. Bosch, LOSE-ENSEEIHT, robustness via a suite of contrasting Toulouse, FRANCE; Ph. Arguel, LAAS- starting models where initial Moho depth CNRS, Toulouse, FRANCE; B. Dubreuil, is varied (3-6 km subseafloor). The OBS Ph. Behra, LCA-INPT-ENSIACET, data alone require near-seafloor (< 0.5- Toulouse, FRANCE; M. Aufray, CIRIMAT 1 km) high velocity gradients within the -INPT-ENSIACET, Toulouse, FRANCE Central Dome and upper km structure that is similar to that determined from Hydrothermal systems along mid-ocean much denser multi-channel seismic ridges are a significant source of refraction modeling. No clear PmP or Pn methane for the Deep Ocean, which can arrivals that would mark a strong be related to different geochemical contrast across a sharp Moho have been processes (e.g. Lein et al. 2000; Charlou identified in the data. Modeling all et al. 2002). Among them, arrivals as Pg, there is no requirement serpentinization can produce high that mantle velocities be very shallow (< methane concentrations in hydrothermal 3 km) anywhere along the lines. fluids and strongly impact the gas flux to However, velocities > 7.5 km/s are the water column. However dissolved determined to occur at depths of 4-5 km gas and fluid fluxes are notoriously on the inside corner lithosphere. This difficult to measure, leading to indicates that the crust mantle interface inaccuracies in the estimate of the is shallower than normal beneath the amount of methane produced by OCC and this finding is consistent with hydrothermal systems. Moreover, inferences based on prior 3-D gravity significant uncertainties exist on the modeling. In contrast, the depths distribution and the patchiness of the determined for material > 7.5 km/s marine sources. These issues can be (essentially fresh, olivine dominated rock addressed by in situ chemical sensing to which would likely be of mantle origin) in gain new insights on the geographical both the axial zone and the outside distribution of the vents and on the time corner appear to be deeper, and likely variation of geochemical processes at typical for young Atlantic crust. One of hydrothermal sources. Different sensing the along-strike lines provides the first technologies and sensors currently exist velocity information on the western but suffer from limitations (time portion of the Southern Ridge. Initial response, hysteresis, (Boulart et al., In inversions suggest that velocities and Press), which can be bypassed by optical gradients there are somewhat lower methods associated to “intelligent” than those determined previously for polymers. It is shown that dissolved the central portion of the Southern Ridge methane can be detected and accurately (Canales et al., 2008; also corroborated measured by a surface plasmon in our coarser refraction coverage) and resonance sensor associated with a signficantly lower than recently methane specific binding polymer determined (Henig et al., 2009) for the (Boulart et al. 2008). We describe a new eastern portion of this part of the domal detector based on this property, using core of Atlantis Massif. optical fibres and infrared detection. Tests of detection and calibrations are Poster II-69 performed in the laboratory under Emerging Technologies for in Situ different environmental conditions Dissolved Methane Measurements in (temperature, pressure, pH). Responses Hydrothermal Vents of the methane specific polymer are also studied with respect of the same Boulart, Cedric C.; Chavagnac, V.; conditions. We present here the first Seat, H. C.; Cattoen. M.; Bosch, Th.; results from the laboratory-based Arguel, Ph.; Dubreuil, B.; Aufray, M.; experiments. Behra, Ph.; Castillo, A.; Gisquet, P.

C. Boulart, V. Chavagnac, A. Castillo, P. Gisquet, LMTG-CNRS-UPS-IRD,

18 Buck, W. Roger; Choi, Eunseo Poster II-75 Magnetic Anomalies in the Ocean- Lamont-Doherty Earth Obs., Columbia continent Transition Zone from the Univ, Palisades, NY Iberian-Newfoundland Margins : An Alternative View Observations indicate that slow spreading ridges operate in several Bronner, Adrien; Sauter, Daniel; modes including the formation of core Manatschal, Gianreto; Munschy, Marc complexes and simple models reproduce these modes. Mid-ocean ridges also Institut de Physique du Globe, show a continuous variation in across- Strasbourg, FRANCE axis bathymetric relief from axial highs to deep axial valleys. However, models Some of the magnetic anomalies from that fix the fraction of plate separation the deepest part of the Iberian- accommodated by magma (M) cannot Newfoundland margins have been explain the observed range of axial interpreted as seafloor spreading depths. For M=1 the axial depth is anomalies. M0 up to M22 magnetic controlled by the density structure (or anomalies have been identified in the buoyancy) of the axis and so we term zone of exhumed mantle within the this depth, Dbuoy. If M < 1 the axial ocean-continent transition. This valley depth is equivalent to the depth interpretation is strongly debated. The produced by tectonic stretching alone magnetic anomalies are often weak, and (here termed Dstretch). A resolution to there is no agreement about the source. this problem may come from Either gabbro bodies or serpentinized consideration of magma input on a peridotites have been proposed to be segment scale. Assuming that most responsible for these magnetic dikes are fed from a magma chamber at anomalies. Moreover, kinematic a segment center allows simulation of reconstructions using these isochrons how magma might be distributed with have shown to be inconsistent with distance from the center. At a given some geological constraints. Here we distance from a segment center we then propose an alternative model based on fix calculate the magma input into 2D forward modeling of magnetic anomaly cross-sectional numerical models of profiles using seismic refraction and drill spreading that treat faulting and dike hole data. We show that these magnetic intrusions in a mechanically consistent anomaly profiles can be explain without way. In these models dikes open in inversion of the Earth magnetic field. We response to the lithospheric stress field use only positive magnetizations after a set time of spreading with no assuming that magmatic and tectonic dikes. The amount of magma delivered processes occur during the quiet into the a given cross section depends Cretaceous magnetic period. We propose on the extensional stresses there and that the main magnetic anomaly (J this in turn depends partly on the depth anomaly) in the ocean continent of the spreading axis. The deeper the transition area of the Iberian- axis the larger the extensional stresses Newfoundland margins might be so that more magma is pulled into the explained by underplated magmatic that area. Though the model results bodies and surface lava flows. These depend on several parameters, including magmatic bodies and lavas may result the time interval between dike events, from the magmatic and tectonic event the axial lithospheric thickness and the which marks the transition to seafloor magma source pressure-volume spreading. relation, the main result is simple. Since the magma input depends on the axial Poster II-62 depth a complete range of axial depths Magmatic Influences on Axial Relief is possible. The deeper the axis the and Oceanic Core Complex more magma is supplied to dikes in an Formation event. As long as M<1 the axis will deepen, but M should increase with

19 increasing axial depth. If M=1 the axis Poster II-51 will cease deepening. If the is no depth Oceanic Core Complexes in the Mid- where magma volumes are sufficient to Atlantic Ridge for 200 km South of accommodate all spreading then the the Kane Fracture Zone axial depth equals Dstretch. Large offset faults only form for M<1 cases when the Cann, Johnson R.; Smith, Deborah; axis is at its maximum depth. Escartin, Javier; Schouten, Hans

Poster I-19 J.R. Cann, School of Earth and Advanced Seismic Imaging of Environment, University of Leeds, Leeds, Oceanic Core Complexes UNITED KINGDOM; D. Smith, H. Schouten, Geology and Geophysics, Canales, Juan Pablo Woods Hole Oceanographic Institution, Woods Hole, MA; J. Escartin, Groupe de Woods Hole Oceanographic Institution, Geosciences Marines, CNRS-IPGP, Paris, Woods Hole, MA FRANCE

Oceanic lithosphere formed along slow- Recent surveys of the Mid-Atlantic Ridge and ultra-slow spreading centers is have identified bathymetric features that highly heterogeneous at many different distinguish the different tectonic scales. This compositional and elements that make up the oceanic crust structural heterogeneity arises from (Smith et al., 2008). Fault-dominated temporal and spatial variations in mantle regions show a blocky topography composition and thermal structure, consisting of domal core complexes that magma supply, efficiency of melt are often corrugated, symmetrical linear extraction, tectonic extension, or a ridges with steep outward- and inward- combination of any of these factors. facing slopes and oval basins on the This heterogeneity is well exposed at outward side of the ridges. We interpret oceanic core complexes (OCCs), which the domal core complexes as exposures are deep sections of the oceanic of long-lived detachment faults that lithosphere exhumed to the seafloor by have been rotated to low angles as they long-lived detachment faults. Deep emerge, and the linear ridges as highly drilling at OCCs thus offers a unique rotated fault blocks with the steep opportunity to sample the lower crust outward facing slopes representing and upper mantle of slow-spreading volcanic seafloor that was erupted lithosphere. However, the importance of originally close to horizontal in the the geological processes inferred from median valley floor. Sampling of the one-dimensional view that deep serpentinite, gabbro and metabasalt is drilling provides can only be assessed by common in fault-dominated regions. understanding the complex, three- Magmatic-dominated segments show dimensional structure of OCCs. Seismic linear ridges with steep inward-facing reflection and refraction are powerful scarps and well-preserved volcanic methods to image the deep structure of features such as cones and terraces. We OCCs at several scales and depths. interpret these as slices of the median Here we present advances made during valley floor uplifted by faulting, but the last two years on seismic imaging of scarcely rotated. The axes of the OCCs along the flanks of the Mid-Atlantic volcanic ridges are commonly curved Ridge. Travel-time tomography of long- towards segment ends. Plutonic rocks offset (6 km) multichannel seismic data are not recovered from magmatic reveals the heterogeneity of the shallow segments. The axis of the median valley (<1.5 km) structure of OCCs at scales of floor always shows a volcanic a few to tens of kilometers, while morphology; the differences develop at waveform tomography has the potential the edges of the median valley and on to image the internal structure of OCCs its walls. We apply these new criteria to at scales comparable to those of seafloor a reanalysis of a well-surveyed area of observations. the Mid-Atlantic Ridge running south for over 200 km from the Kane Fracture

20 Zone and reaching out more than 100 50% of the seafloor formed in magma- km (about10 Ma) on each side. The area starved regions of ultraslow ridges. covers about 50,000 km2 of Atlantic Large vertical displacements along axial Ocean floor. The segmentation of this normal faults are required to exhume area was first studied by Gente et al. these deeply-derived rocks. In this (1995), who showed that the pattern of presentation, I will give an overview of segmentation is complex, with spreading the geological-geophysical observations segments apparently growing and made in ultramafic seafloor areas of the shrinking on times scales of around 1 MAR and Southwest Indian Ridge Ma. Our analysis shows that within this (SWIR). These observations indicate that area about half of the ocean floor carries corrugation-bearing Oceanic Core the fault-dominated signal. Along the Complexes are but a particular case of 205 km of active spreading axis, five axial exhumation structure, probably spreading segments can be identified. forming in conditions of intermediate Of these one (35 km long) is spreading magmatic activity. This raises the issue magmatically on both flanks, while the of the extent to which observations other four are spreading by faulting on made on corrugated, or un-corrugated, one flank and magmatically on the exhumation faults can be used to other. This indicates that at present constrain generalized models of oceanic about 40% of the new ocean lithosphere detachments and their evolution. generated in this area is fault- Another issue that is often left aside in dominated. conceptual models of oceanic detachment faults, is that of the References: concomittant deformation in the lower, ductile part of the axial lithosphere. I will Gente et al.,1995, Characteristics and discuss these two issues, trying to evolution of the segmentation of the Mid outline end-member hypotheses, and Atlantic Ridge between 20° and 24°N their possible consequences in terms of during the last 10 million years. Earth lithospheric architecture. and Planetary Science Letters, 129, 55- 71. Poster II-52 Lateral and Temporal Variations in Smith, et al., 2008, Fault rotation and the Degree of Mechanical core complex formation: Significant Weakening in the Footwall of processes in seafloor formation at slow- Oceanic Detachment Faults spreading mid-ocean ridges (Mid-Atlantic Ridge, 13_–15°_N): Geochemistry, Cannat, Mathilde; Escartin, Javier; Geophysics, Geosystems, v. 9, p. Lavier, Luc; Picazo, Suzanne Marie; Q03003, doi:10.1029/2007GC001699. Sauter, Daniel

Oral – Saturday 8th, 09:00 M. Cannat, J. Escartin, S. M. Picazo, Oceanic Detachment Faults and the Géosciences Marines, CNRS-IPGP, Paris Exhumation of Gabbros and Mantle- Cedex 05, FRANCE; L. Lavier, Jackson derived Peridotites at Mid-ocean School of Geosciences, University of Ridges Texas, Austin, TX, United States; D. Sauter, EOST, CNRS-Université de Cannat, Mathilde Strasbourg, Strasbourg, FRANCE

Equipe de Géosciences Marines, CNRS- The motivation for the work presented in UMR7154, Institut de Physique du this poster (partially published in Cannat Globe, 4 place Jussieu 75252 Paris cedex et al., EPSL 2009) is that, although a lot 05 France of attention has been rightly given in recent years to mid-ocean ridge Serpentinized peridotites and gabbros detachments that produce corrugations are exposed over roughly 25% of the in their footwall, these represent only a seafloor formed at the slow-spreading few percent of the seafloor formed at Mid-Atlantic Ridge (MAR), and up to slow and ultraslow ridges. These

21 corrugated detachments therefore Poster I-35 accommodate only a fraction of the large Thermodynamic Constraints on vertical movements needed to expose Biomolecule Synthesis and serpentinized peridotites and gabbros Metabolism Due to Alteration of over roughly 25% of the seafloor formed Ultramafic Host Rock at the slow-spreading Mid-Atlantic Ridge (MAR), and up to 50% of the seafloor Canovas, Peter A.; Shock, Everett L. formed in magma-starved regions of ultraslow ridges. In the poster, we show P.A. Canovas, E.L. Shock, School of examples of the variability of the relief Earth and Space Exploration, Arizona that is associated WITH exposures of State University, Tempe, AZ; E.L. Shock, serpentinized peridotites and gabbros Department of Chemistry & formed at both the MAR and the Biochemistry, Arizona State University, ultraslow Southwest Indian Ridge Tempe, AZ (SWIR). These examples illustrate two characteristics of corrugated The alteration of igneous basement detachments relative to other large rocks produces disequilibria that lead to offset axial normal faults : their exposed microbial habitat generation in seafloor footwall has a very subdued relief when hydrothermal systems. The evolution of connected to a detachment that is still fluid compositions and mineral alteration active on-axis (Smith et al., Nature assemblages constrains the availability 2006), but commonly has a prominent of energy for microbes and influences relief when found off-axis. The subdued the viability of various metabolic on-axis relief indicates very important pathways. By combining theoretical flexural rotation over short distances, models of rock alteration and fluid consistent with extremely low mixing with biomolecule synthesis and mechanical strength of the fault’s metabolism, a quantitative approach can footwall (Smith et al., Gcubed 2008), be taken toward predicting microbial which may also account for the metabolisms as functions of formation of corrugations (Spencer, temperature, pressure, fluid composition Geology 1999). The pronounced off-axis and the extent of rock alteration. These relief is taken to indicate that the results can be presented in the form of strength of the fault’s footwall increased affinity diagrams where conditions of significantly at the termination stage. real systems can be plotted to link the nonequilibrium thermodynamics of Following Buck (Tectonics 1988), we water-rock alteration with the hypothesize that stresses related to nonequilibrium processes of metabolism bending of the plate cause internal and biosynthesis. To this end, deformation and damage in the footwall reaction path modeling was performed of the fault, which is associated with on host rock assemblages with weakening. As a possible mechanism for representative solid solution enhanced footwall weakening while compositions of olivine, orthopyroxene corrugated surfaces form, we propose and clinopyroxene. Comparisons the formation of weak shear zones between assemblages reveal large coated with hydrous minerals such as contrasts in dissolved hydrogen, silica, talc, amphibole, chlorite and serpentine, and pH. Dissolved hydrogen in mantle-derived ultramafics next to concentrations in particular change gabbro intrusions. If this hypothesis is dramatically in response to small correct, the amount of footwall changes in the initial mineral weakening and roll-over along axial assemblages, with large variations detachment faults at slow spreading centered around host rocks dominated ridges may be controlled both by the by olivine. In a biochemical/synthesis distribution of gabbros intrusions in context, the results suggest that energy exhumed ultramafics, and by access to supply and demand will be modulated by hydrothermal fluids in the footwall of the a variety of mineral sources of ferrous detachment. iron and the diversity of biochemical reactions microorganisms couple to

22 mineral alteration. For instance, in some Houston, TX; R. Searle, Geology systems the synthesis of alkanes, Department, Durham University, Cardiff, alkenes and some carboxylic and amino UNITED KINGDOM; C.J. MacLeod, acids is thermodynamically favored Geology Department, Cardiff University, without the input of energy from Cardiff, UNITED KINGDOM; B. Murton, microorganisms or their need to couple National Oceanography Center, synthesis to other reactions. However, University of Southampton, this is almost never the case for energy- Southampton, UNITED KINGDOM intensive biomolecules such as carbohydrates, purines, and pyrimidines he region between 12°N and 16°N on (Shock and Canovas, 2010). In order to the Mid-Atlantic is characterized by at make these compounds, microorganisms least 10 near-axis core complexes need to couple their syntheses to other exposing ultramafic and gabbroic rocks. reactions. In many hydrothermal We have extended sampling programs systems the energy released by from previously surveyed northern methanogenesis is more than sufficient regions from 14° to 16°N to four newly for the biosynthesis of nicotinamide discovered core complexes (Smith et. dinucleotide (NAD), adenosine al., 2006) and adjacent ridge segments triphosphate (ATP), and other between 14 and 12°30'N on a cruise of biomolecules central to metabolism. the research vessel James Cook in Other energetic couplings with respect to March-April 2007. We examine the biomolecules may be explored as well. distribution of lithologies sampled in the For example, under conditions entire region, the newly discovered core associated with low temperature complexes, the ridge and core complex alteration environments, thermodynamic morphotectonics along the segments, calculations indicate there is sufficient and the results of geochemical analysis drive to support the reduction of NAD via of basalts, gabbroic rocks and mantle hydrolytic oxidation of ferrous minerals. assemblages. Geochemical results from Reactions using ferrosilite to meet these assemblages may help to explain energetic demands are more or less why the basalt characteristics can show favorable than those using fayalite variations symptomatic of high degrees depending on the conditions where they of melting (e.g., low Na 8.0) and why occur. Overall, microbes will meet their the mantle compositions are among the energetic demands through various most strongly depleted along the MOR coupled biogeochemical pathways, (also indicating high degrees of melting), depending on the details of the system yet the evidence of mantle unroofing they inhabit. As a consequence, coupled and a thin magmatic crust persist metabolic processes will shape the throughout such a broad area. reaction paths, rates and extents of Integrated studies of major element, microbial alteration that various systems trace element, and isotopic variations undergo, as well as their overall among basalts, gabbroic rocks and biological productivity and diversity. igneous and residual ultramafic rocks in the region indicate that 1) the enriched Poster II-44 basalts have positive Ta-Nb anomalies, Evidence For High Degrees of Mantle enriched relative to La and Th, 2) Melting, Yet Extensive Development basalts have relatively high SiO2 of Oceanic Core Complexes and abundances compared to the global Significant Evidence of Low average, 3) basalts show a HIMU Magmatic Production on the Mid- isotopic influence, and 4) bulk major Atlantic Ridge between 12°-16°N element abundances and mineral chemistry in mantle rocks indicate that Casey, John F.; Searle, Roger; they are among the most depleted, MacLeod, Christopher J; Murton, although variably refertilized, residual Bramley mantle assemblages sampled to date along MORs. We suggest that much of J.F. Casey, Earth and Atmospheric the regional variation in major and trace Sciences, University of Houston, element data, as well as isotopic data

23 and the unusual regional geology compositions (0.70687 to 0.70904), (multiple core complexes and ridge compared with MORB compositions morphotectonics) reflecting magma deeper in the hole (>1 km). A key supply can be explained by melting of a question is how fluid with a seawater sub-axial mantle that contains two end isotopic signature passed through members, one highly depleted and the partially altered gabbros to reach thin other enriched. These subaxial mantle ultramafic horizons. Samples of components appear to involve ancient individual minerals and alteration zones recycled ocean crust and lithospheric have been extracted from thin sections mantle. We also examine melt using a microscope-mounted drill. entrapment within the mantle Seawater signature is observed in lithosphere in the subaxial region and serpentines and prehnites, as well as in local to regional basalt geochemical amphibole filling vugs, and corona variations associated with the core textures. Intermediate values are seen complexes. in amphiboles replacing clinopyroxene and chlorite. Plagioclase usually shows a Poster I-26 MORB-like value except when altered to Hydrothermal Fluid Flow in Oceanic albite at vein contacts. Alteration Gabbros, IODP Site 1309, Mid- occurred at temperatures ranging from Atlantic Ridge: Strontium and 250 to 350°C (from calibration curves of Oxygen Isotopic Data Zheng, 1993) except for corona textures for which temperatures are >= 500°C Castelain, Teddy; McCaig, Andrew; (estimated from Cole and Ripley, 1999). Cliff, Bob; Delacour, Adelie; Fallick, These data suggest that fluid percolating Anthony through the gabbros was significantly more radiogenic in strontium than the T. Castelain, A. McCaig, B. Cliff, School whole rock analyses might suggest. of Earth & Environment, Institute of Where fluid flowed through high flux Geophysics & Tectonics, Leeds, UNITED pathways with little exchange with wall KINGDOM; A. Delacour, Lab. de rocks, minerals isotopically similar to Dynamique Terrestre et Planétaire, seawater were precipitated. Serpentinite Toulouse, FRANCE; A. Fallick, SUERC, layers were altered either by fluid Glasgow, UNITED KINGDOM passing along such high flux pathways, or at significantly lower temperatures IODP Hole U1309B and D were drilled in (200-250°C) than the gabbros, as the the footwall of the Atlantis Massif estimates from the oxygen isotope ratios detachment fault at the Mid-Atlantic suggest. Ridge 30°N. The cores are composed of gabbroic rocks interlayered with Oral – Saturday 8th, 09:45 ultramafic rocks, with basalt/diabase Fault Rheology, Footwall sills in the upper 130 m. The dominant Deformation and Geochronologic alteration occurred in the greenschist Constraints on the Process of facies and is characterised by Detachment Faulting at Mid-Ocean amphiboles replacing pyroxenes and Ridges formation of corona textures between olivine and plagioclase. At depths > 750 Cheadle, Michael; Barbara E John mbsf, alteration is restricted to veins, fractures and lithological contacts. Whole Dept of Geology and Geophysics, University rock oxygen isotope values range from of Wyoming, Laramie, WY, United States 5.6 ‰ to 1.5 ‰, indicating variable degrees of interaction with seawater at The nature of oceanic detachment faults temperatures generally > 250°C. is just beginning to be understood. Gabbroic rocks and diabases exhibit a These faults may localize in both range of Sr isotope ratios from MORB peridotite and gabbro dominated values (0.70261) to intermediate ratios footwalls, and typically reveal a down- (0.70429), while six serpentinites from temperature deformation history, < 350 mbsf show seawater-like culminating in a ubiquitous low-

24 temperature talc-chlorite-tremolite fault the hanging wall of the fault. Field schist +/-cataclasite assemblage. observations and lab experiments However, the high-temperature suggests that fault weakening due to deformation history seems to vary. In water ingress and the formation of peridotite-dominated footwalls such as “weak” secondary hydrous minerals at Kane (mid-Atlantic Ridge (MAR)), including serpentine, chlorite, tremolite peridotite mylonites commonly form an and talc may lead to continued slip on anastomosing network of thin (1-2m?) existing faults rather than the formation shear zones that extend up to 400m of new faults. Hydration and hence fault below the principal slip surface exposed weakening likely occurs deeper, at at the seafloor. These presumably higher temperatures, where evolved accommodate the transition from gabbros are in contact with peridotite. distributed deformation in the upwelling However, the depth of penetration of the asthenosphere to the brittle deformation hydrothermal circulation remains a associated with detachment fault slip. At major question. Geochronolologic data the gabbro dominated Atlantis Bank confirm the results of geomagnetic (Southwest Indian Ridge), the studies that show that many oceanic detachment fault comprises a 100m core complexes are formed during thick network of anastomosing shear periods of significant asymmetric zones consisting of granulite- to spreading and consequent plate amphibolite-grade gabbro mylonites, boundary migration, raising the question overprinted by a narrow zone of brittle of whether detachment faults form as a deformation with the typical fault rock consequence of plate boundary assemblage of gouge, cataclasite and migration or vice-versa. Further, the schist. The concentration of high- progressive decrease in age of gabbro temperature strain in Fe-Ti oxide intrusions toward the detachment fault gabbros suggests that the shear zones termination supports the concept of root into hyper-solidus melt lenses. continuous footwall casting, with Additional shear zones occur between gabbros continuously crystallizing in the 470 and 950m below the fault surface footwall as the fault slips. exposed at the seafloor, highlighting that high-temperature deformation can Poster II-63 occur throughout the footwall. Some of 3D Numerical Models for the this deformation shows a reverse sense Formation of Oceanic and of shear and is associated with the ~60o Continental Core Complexes of bending associated with unroofing the footwall. Elsewhere, for example at the Choi, Eunseo; Buck, W. Roger center of Atlantis Massif (MAR), gabbro- hosted detachment fault zones are much Division of Marine Geology and thinner (<10-20m?), and high Geophysics, Lamont-Doherty Earth temperature mylonites are rare. This Observatory, Palisades, NY rarity of gabbro mylonites is expected to be the norm, because of the narrow Oceanic core complexes (OCCs) appear temperature window for plastic to be a product of a unique mode of deformation. Typically this window is lithospheric extension but also stand on defined by the solidus of evolved melts a common mechanical ground with the (~850°C for oxide bearing gabbros), and continental counterparts. Our on-going the brittle plastic transition of 'dry' project is to study the extension of plagioclase (~750°C). continental lithosphere using a combination of geological observations There remain fundamental questions as and modeling. Until recently it has been to why large slip detachment faults impractical to consider three- form. Compelling evidence suggests that dimensional numerical models of the detachment faults originate as normal, long-term development of extensional high-angle axial valley faults. Modeling faults. We plan to use a newly suggests sustained slip may be partially developed 3D numerical modeling code facilitated by limited magma addition to to study controls on faulting during

25 oblique continental extension including macro units, compiled from observations the role of pluton emplacement in the of the archive halves of the entire core, formation of domal cores of continental and refined using other available data core complexes (CCCs). Our efforts will including magnetic susceptibility. The be beneficial to the OCC studies in two core contains mostly gabbro, olivine ways. The methodology can be applied gabbro and trocolitic gabbro (83.2% to OCCs in a straightforward fashion. In total). Oxide gabbro comprises 5.4% of particular, the 3D code can be used to the core and appears intermittently explore the thermo-mechanical within the larger gabbro bodies. These conditions for the formation of OCCs and diagrams show that the thickness of the to simulate their evolution in time and individual magmatic units is on the order space. In addition, if new results are of 1 to a few 10’s of meters, significantly successfully acquired from our project smaller than implied by previous on continental lithosphere, contributions lithology diagrams (Blackman et al, will be made for the comparison 2006). Contacts between many of these between OCCs and CCCs in the light of units are sharp and thus many of the these new findings. In this conference, units are interpreted to result from small we will present preliminary models for injections of melt as opposed to in-situ OCC and CCC formation and attempt to fractionation. However these units are compare them. often cut and disaggregated by later intrusive bodies, and hence it is likely Poster I-3 that individual gabbro units were initially The Igneous Architecture of IODP somewhat larger. Downhole plots of Hole U1309D: ~10’s Meter Scale magmatic fabric dip are consistent with Units Suggest Accretion by Small the units having initially been intruded Volume Intrusive Events as sills, and subsequently rotated at least ~40-50°, as constrained by Christofferson, Christian A.; Cheadle, paleomagnetic data (Morris et al., Michael J.; John, Barbara E.; Grimes, 2009). We note that the thickest unit in Craig B. the core (in macro unit II) is 27 m thick (~20m after correcting for dip) placing C.A. Christofferson, M.J. Cheadle, B.E. an upper limit on the thickness of John, Department of Geology and individual injections. U-Pb zircon dating Geophysics, University of Wyoming, has shown that macro unit II is older Laramie, WY; C.B. Grimes, Department than the overlying macro unit I. Macro of Geosciences, Mississippi State unit II was formed during a period of University, Mississippi State, MS magmatic activity at ~1.24 Ma, whereas macro unit I was intruded around 1.17 Gabbroic rocks form a significant part of Ma with some components intruding the footwall of many oceanic core macro unit II. This relationship requires complexes. The decreasing age of these that the depth of magma intrusion was gabbros down dip from the breakaway not constant but shallowed with time, to the termination of the detachment with macro unit I intruded ~1-1.5 km fault suggests that they are continuously shallower than macro unit II. Using all accreted as the detachment fault that available data, we propose the gabbros bounds the core complex slips (Baines et of Atlantis Massif are composite bodies, al., 2008). But how are these large (>1 formed of ~500 m thick gabbro macro km diameter) gabbro bodies units which grew episodically (~ every constructed, and at what scale and 70,000 yrs) by relatively small (1 to a frequency is melt added to the magma few 10’s meters) repeated sill-like system? Here we report a detailed injections of melt. We conclude that the lithologic analysis of IODP Hole U1309D magma chamber that formed these drilled into the Atlantis Massif core gabbros was relatively small at any one complex on the Mid-Atlantic Ridge in time. We argue that these bodies were order to address these questions. We emplaced at varying depths in the present new, high resolution (1 m scale) footwall and consequently, if the downhole diagrams for the three igneous detachment fault is rooted in the magma

26 chamber, that the depth of detachment Hansen (2007) showed that brittle faulting may have varied with time. deformation is localized in the upper reaches of the section (< 200 mbsf), Poster II-71 close to the principal slip surface, Understanding Hydrothermal Fluid whereas crystal plastic deformation is Flow Along Oceanic Detachment distributed as localized shear zones Faults - Kane Oceanic Core Complex, throughout the upper 500m of the 23°N Mid Atlantic Ridge footwall. More detailed petrographic examination of the samples will help Cole, Elizabeth; John, Barbara E.; constrain the down-temperature Cheadle, Michael J.; Teagle, Damon; deformation and metamorphic history of Banerjee, Neil the fault system. It is highly likely that fluid flow was pervasive through zones E. Cole, B.E. John, M.J. Cheadle, that experienced the greatest brittle Geology and Geophysics, University of deformation, leading to the highest Wyoming, Laramie, WY; D. Teagle, degrees of mineral alteration and National Oceanography Centre, serpentinization. Gabbro and peridotite University of Southampton, that host high temperature mylonitic Southampton, UNITED KINGDOM; N. fabrics may have acted as seals to focus Banerjee, Earth Sciences, University of this fluid flow. The resulting reaction Western Ontario, London, Ontario, weakening and generation of minerals CANADA including talc and serpentine likely facilitated the protracted history of slip Hydrothermal fluid flow is implicated in along the detachment fault. Together, large slip oceanic detachment faults; the microstructural analysis (including however, the depth and mechanisms of mineral deformation mechanisms and fluid circulation both along the fault zone estimated temperatures), and the and into the footwall remain poorly isotopic data, will quantify fluid-rock understood. The Kane Ocean Core ratios and will help constrain the thermal Complex (OCC) on the Mid-Atlantic history and depth of fluid flow along and Ridge is cut by two moderate-dip faults, through both peridotite and gabbro- which penetrate the detachment fault hosted detachment faults. surface as exposed at the seafloor. Samples collected during R/V Knorr Poster I-17 Cruise 180-2 (Tivey et al., 2004) from Two End-members of Oceanic Core these fault scarps provide 600-1200m Complex Formation deep cross-sections through the detachment fault zone and into the Dannowski, Anke; Grevemeyer, Ingo; footwall, affording a unique opportunity Planert, Lars A. to conduct a detailed study of hydrothermal fluid flow associated with FD 4 - Geodynamics, Leibniz Institute of the detachment system. Previous Marine Sciences (IFM-GEOMAR), Kiel, isotopic studies (Sr, Nd, and O) have GERMANY been used to trace fluid flow through oceanic detachment fault zones, Oceanic core complexes (OCC) are however existing work has focused on predominantly generated at slow- or whole-rock analyses. In contrast, we are ultra-slow spreading ridge flanks. conducting Sr- and O- isotopic studies of Basically they are formed during mineral separates from samples episodes of limited magma supply and collected from both peridotite- and hence rifting dominated spreading, gabbro-dominated fault scarps that cut exposing deeper crustal rocks. While the Kane detachment system, to geophysical studies show that the constrain the highest temperatures of general lithology is similar at many hydrothermal circulation. This study, OCCs the shallower structures seem to together with detailed microstructural differ strongly. We compare seismic analysis, will help elucidate the full modelling results of two oceanic core history of fluid flow in the core complex. complexes along the Mid-Atlantic Ridge

27 (MAR), formed near 5° S and at 22.19° Catherine; Ildefonse, Benoit; Dyment, N. They show significant differences Jerome; Fouquet, Yves within the shallow structures while the deeper structures show similar A. Delacour, M. Cannat, S.M. Picazo, C. characteristics. The OCC near 5° S is Mevel, J. Dyment, Geosciences marines, capped by a smooth, corrugated surface, IPGP, Paris, FRANCE; A. Delacour, DTP, which is covered with serpentinite OMP-UNIV. PAUL SABATIER, Toulouse, (found in a dredge), interpreted as a FRANCE; M. Andreani, LST, ENS-Univ. gouge smeared along the corrugated slip Lyon, Villeurbanne, FRANCE; M. Godard, surface, facilitating sustained slip on this B. Ildefonse, Geosciences Montpellier, fault. Below, both seismic data and Univ. Montpellier, Montpellier, FRANCE; dredged rocks indicate gabbroic crust. In Y. Fouquet, Ifremer, DRO-GM, Plouzane, contrast, the OCC at 22.19° N carries FRANCE rafted basaltic blocks on top of the detachment fault. Serpentinites have Serpentinization consists in the been found on the southern steep flank hydration of mantle peridotites, mainly of the OCC facing to the segment end. exposed in magma-starved areas along The available data suggest that the bulk slow-spreading centers. Serpentinites is made of gabbroic rocks at both OCCs outcropping along the Mid-Atlantic Ridge and the crust-mantle boundary is (MAR) are distinguished, from one site constrained by seismic wide-angle to another, by strong variations in reflections, indicating a profound change mineral assemblages, chemical and of lithology about 4 km below seafloor. isotopic compositions. In order to We seem to have two styles of OCC characterize the origin of these formation: In the first case, upper crust variations, we conducted a detailed (layer 2) is completely transferred to the petrologic and geochemical study of conjugate ridge flank, while, in the serpentinites from four sites located second case basaltic crust remains as between 13° and 36°N along the MAR rider blocks on the inactive part of the (Ashadze, Logatchev, MARK and detachment fault, at the footwall. The Rainbow). These four sites were selected plate boundary in the shallower part is because: 1) they are made of variably following the active fault until it is altered and deformed serpentinites and rotated away from its optimum angle gabbroic rocks, exposed on the seafloor and a new fault is created (Mohr- through a detachment fault, 2) their Coulomb criterion). These observations dome-like morphology has been may suggest differences in the thermal interpreted as an oceanic core complex, regime of the ridge axis. In a cooler and 3) they host active high- setting, the brittle regime may reach temperature hydrothermal system. down to the mantle, favouring injection Harzburgites and dunites were of serpentinites into the fault zone and predominantly recovered at these sites hence weakening the fault. In and primary minerals include olivine, consequence, a single fault remains orthopyroxene, Cr-spinel and, locally, active during core complex formation. If minor clinopyroxene. Olivine is replaced heat flow is higher the fault will not predominantly by serpentine and reach into the mantle but stay within the magnetite with various textures (e.g., lower crust, which hinders deep mesh, ribbon). Orthopyroxene is serpentinisation. A stronger fault may commonly altered to bastite, while support creation of new faults and rider relatively unaltered core may be blocks. preserved. Serpentine compositions vary between sites (i.e., Fe, Mg, Al, Cl, S) Poster I-28 and depend on the primary phases (e.g., Serpentinization Along the Mid- Fe enrichment in bastite). Aluminous Atlantic Ridge (13°-36°N) phases, such as tremolite and/or chlorite, are scarce or absent. In Delacour, Adelie; Andreani, Muriel; contrast, carbonate veins are abundant Godard, Marguerite; Cannat, Mathilde; and ubiquitous and form, together with Picazo, Suzanne Marie; Mevel, serpentine veins, the two predominant

28 vein types. Logatchev and Ashadze The southern wall of the Atlantis Massif serpentinites show four generations of Oceanic Core Complex is host to the serpentine veins, identical to the V1-V4 novel serpentinite-hosted Lost City vein series identified at MARK (Andreani Hydrothermal Field (LCHF). The LCHF is et al., 2007). Sulfide mineral situated near the summit of the Atlantis assemblages are specific to each site Massif, ~ 20 km west of the Mid-Atlantic suggesting variations in redox Ridge and ~ 10 km north of the floor of conditions, fluid fluxes and fluid the Atlantis Transform Fault. The core of chemistry during serpentinization. In the field is located on a down- dropped spite of their high degrees of bench, ~70 m below the summit of the serpentinization, most studied samples massif and is cut by a young, high- have compositions close to the mantle angle, north-south trending normal fractionation array, Rainbow and fault. The summit of the massif is Logatchev having the most refractory marked by a ~ 2 m thick, well-lithified protolith (avg. Al2O3/SiO2 < 0.02). All pelagic cap rock that lies unconformably serpentinites are characterized by on top of the detachment fault. Direct enrichment in Pb, Sr and U due to observations from the human occupied interaction with seawater-derived fluids. submersible Alvin (2000 and 2003), high Rare Earth Element (REE) patterns show definition imagery from the ROV variable enrichments in Light REE (LREE; Hercules (2005), and high-resolution (5 0.01 to 0.1 x chondrite) and highly m) bathymetry from the AUV ABE variable Heavy REE contents between (2003) have been compiled into a high- sites: Rainbow serpentinites show high resolution geologic map of the LCHF and LREE concentrations (1 x chondrite) adjacent Atlantis Massif. Hydrothermal while MARK serpentinites have depleted activity is dominantly focused along the compositions (0.01 x chondrite), similar ~60 m-tall, 100 m-long carbonate to that of the less altered mantle edifice called Poseidon, which is actively peridotites. Variability in mineralogical venting 90°C fluid. To the south and and chemical compositions of west hydrothermal activity rapidly serpentinites is discussed on the basis of declines, and within 200 m dies off primary compositions of peridotites and completely. North and east of Poseidon, vicinity of basalts and/or gabbros, extent hydrothermal flow is characterized by of fluid-rock interaction (fluid fluxes and egress of diffuse fluids from small, chemistry) and serpentinization abundant fissures that are conditions (temperature and redox predominantly constrained to a 100 m conditions). Andreani et al. (2007) G3, thick, highly deformed mylonite zone. doi:10.1029/2006GC001373. Here, gently dipping, anastomosing networks of fractures channel Poster I-25 hydrothermal fluids that issue from near Geologic Evolution and Structural vertical serpentinite walls. Beyond the Control of the Lost City extent of current hydrothermal activity Hydrothermal Field there is evidence of at least two relict fields, to the south and west Denny, Alden; Kelley, Deborah; Fruh- respectively, that are not contiguous Green, Gretchen; Ludwig, Kristin; with the current active field. This Karson, Jeffrey Alan strongly suggests that the LCHF has undergone multiple stages of field A. Denny, D. Kelley, School of development progressing to the Oceanography, University of northeast during its >100,000 years of Washington, Seattle, WA; G. Fruh- venting activity. Concurrent uplift, Green, Department of Earth Sciences, faulting, and intense mass wasting along ETH-Zurich, Zurich, SWITZERLAND; K. the southern wall of the massif, along Ludwig, Consortium for Ocean with seismic activity along the Atlantis Leadership, Washington, DC; J.A. Transform Fault facilitates migration of Karson, Department of Earth Sciences, venting to the northeast. Carbonate- Syracuse University, Syracuse, NY; filled fissures, which cut the cap rock, mark these nascent venting sites.

29 Poster II-48 continuously rafted upwards into the Tale of Two Core Complexes: zone of active diking and then emplaced Contrasting Crustal Architecture and to the seafloor from beneath a volcanic Fault Geometries carapace. These geologic relationships require then, that the detachment fault Dick, Henry continuously rooted at the dike-gabbro transition. Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, By contrast, mapping and seismic MA refraction at the Kane Megamullion indicate that focused melt flow from the ABSTRACT BODY: The Kane mantle to the crust occurred at different Megamullion on the MAR, and the points in space and time beneath the Atlantis Bank Core Complex on the SWIR ridge segment giving rise to ephemeral are two end-members for oceanic core magmatic centers, from which melts complex formation and crustal accretion. were transported along the rift axis to Both expose plutonic basement form the volcanic carapace extending underlying nearly the length of a over the rift segment. The gabbro bodies magmatic accretionary ridge segment. appear to represent much smaller The discontinuous thin lower crust intrusions than the Atlantis Bank massif, exposed at Kane contrasts to the >700- likely little more than 100 to 200 km2. km2 1.5-km+ thick Atlantis Bank gabbro While intercalated dikes and gabbros are massif, clearly showing more robust also exposed, so are outcrops of the magmatism at the latter. However, the crust-mantle transition zone. This SWIR spreading rate is 54% of the MAR suggests that the detachment geometry rate, the offset of the Atlantis II FZ is evolved, rooting at the dike-gabbro 46% greater and Na8 of the spatially transition for a time, and then as associated basalts 16% greater – all of magmatism waned, plunging downwards which predict thinner crust at Atlantis to exhume the base of the crust and the Bank, and thicker crust at the Kane shallow mantle. Laterally, along strike, Megamullion. At the same time, the however, even as the detachment average composition of Kane and rooted at the dike-gabbro transition, Atlantis II transform peridotites are massive mantle peridotite was exposed nearly identical showing that the degree for extended periods of time where the of depletion of the mantle from which crust appears to have consisted of pillow the crust was extracted is similar. This is lavas and sheeted dikes directly best explained by a more fertile parent overlying mantle peridotite. This raises mantle beneath the SWIR, and the possibility that the detachment fault demonstrates that crustal thickness geometry varied along strike, and may predicted by inverting MORB account for why the core complex compositions can be significantly in consists of a series of apparently error. independent domes rather than a single large turtleback reflecting the changing The Atlantis Bank core complex extends architecture of the crust through time. some 37.5 kilometers in the spreading direction, forming a single large dome of Poster I-39 turtleback that was later modified by Ridge-transform intersection: transform parallel faulting. It represents inferences from paleomagnetism continuous emplacement of gabbroic and structure of the Troodos ocean crust for 3.8 Ma: for at least 2.5 ophiolite, Cyprus Ma this occurred on a single fault without significant tectonic disruption or Ebert, Yael; Agnon, Amotz; Abelson, cross faulting. Numerous outcrops of Meir; Ron, Hagai intercalated sheeted dikes with gabbro screens are exposed along the length of Yael Ebert, Amotz Agnon, Hagai Ron, the complex, demonstrating that The Hebrew University Of Jerusalem, gabbros crystallized at depth were Jerusalem, Israel; Meir Abelson,

30 Geological Survey of Israel, Jerusalem, Poster I-4 Israel Strontium in Primitive Gabbros of Deep Holes 735B and U1309D: Taking advantage of the deeply incised Incompatibility with Magmatic Troodos ophiolite exposes all levels of Models of Formation of the Third the oceanic crust (pillow lavas, dikes, Seismic Layer of Ocean gabbro), we reconstruct 3D deformation of the fossil oceanic ridge- transform Efimov, Alexander A.; Puchkov, Victor intersection (RTI),. In order to N. reconstruct the crustal deformation we have collected paleomagnetic directional A.A. Efimov, Institute of Geology an data. Twenty-one sites were sampled in Geochemistry, Ekaterinburg, RUSSIAN both the upper and the lower crust. The FEDERATION; V.N. Puchkov, Institute of data indicate contrasting patterns of Geology, Ufa, RUSSIAN FEDERATION deformation between the upper and the lower crust. The sheeted dike complex is Primitive olivine gabbros and troctolites, twisted about a vertical axis, as which have no chemical analogues expected for an inside corner. Twist among ocean basalts, constitute the saturates at 90°,at a location major part of sections of IODP holes indistinguishable from the previously 735B and U1309D, drilled on uplifts of inferred RTI corner (MacLeod et al., the 3rd layer in the Indian and Atlantic 1990). The inside corner in the gabbro oceans [1, 2, 5]. The whole rock discerned by paleomagnetic analysis abundances of Sr are not discussed by (Granot et al., 2006), is found further off authors of publications [2, 5]. Gabbro RTI. Between these two domains in the represents a kind of a mechanical gabbro, the outside-corner tilt and the mixture of femic minerals which inside-corner twist, a domain 7 km wide practically do not contain Sr, and shows the original Troodos mean vector plagioclase with Sr concentration which (TMV) (indicating negligible rotation). is constant for each hole (nearby 300 This domain was interpreted as the fossil and 150 ppm respectively). The partition neo-volcanic zone (NVZ) in the gabbro coefficient of Sr between plagioclase layer (Granot et al., 2006). Our crystal and magmatic liquid always paleomagnetic results from the dikes exceeds 1. Hence, Sr concentration in and the gabbro can be reconciled by a late plagioclase and in late liquids should 'torsional detachment' as suggested by irreversibly decrease upsection. Strict Granot et al. (2006), where twisted linear dependence between plagioclase dikes overly the non-rotated gabbro. In amount and whole rock Sr concentration addition, we identify a contrasting allows to calculate SrPL for any sample behavior between the dikes and the in a series. Calculated SrPL in both gabbro at the very close vicinity of the series is practically constant and does RTI: where the sheeted dikes have not depend on depth. It is possible to nearly reached their final inside-corner make two important conclusions. The twist, the underlying gabbro shows first: there are no signs of decreasing Sr features of outside corner deformation, upsection in the studied sections of the i.e. tilting about a horizontal axis. This 3rd layer predicted by magmatic model observation can be explained by an of crystallisation of a magma basin. The inclined boundary between the second: whole rock abundances of Sr in transform-controlled inside corner and the sections are linear function of the divergence-controlled outside- amount of plagioclase with constant Sr corner. Such an inclined “torsional concentration. This fact is inexplicable in detachment” can also explain the case of the assumption of crystallisation location offset between the RTI in the of magmatic liquid in one or several dikes level (MacLeod et al., 1990) and chambers. Each section of the 3rd layer the more easterly RTI in the gabbro represents a single whole, a product of (Granot et al., 2006). an only 'creation event'. But there are no reasons to consider it as a product of solidification of a magma chamber (or

31 chambers). Data of marine geophysics Oceanic detachments requires efficient speak against a presence of any strain localization along a fault zone over significant magma chambers in mid- periods of up to a few Myrs. Geological ocean ridges. Incompatibility of these sampling of the fault zone yields highly facts with predictions of magmatic deformed fault rocks with a complex models is obvious, and it cannot be interaction with coeval fluid circulation. ignored. The facts mean equality Lithologies are very variable, with fault (uniformity) of chemical potential Sr at schists composed of talc, serpentinite, the moment of formation of sections. It tremolite and chlorite in different allows to doubt that sections were proportions and amounts. Experimental formed by crystallisation of a magmatic data and characterization of deformation liquid. Formation of the 3rd layer can be mechanisms exists for serpentinite and connected with the process of recycling talc. Both rocks display low friction of ancient oceanic lithosphere [4, 6]. coefficients that decrease siginificantly Mantle eclogite as a residual product of with increasing T (from ~0.2 to <0.1 for partial melting of ancient oceanic crust talc), and exhibit very efficient strain in subduction zones and plumes can be localization at high T in the brittle or protolith of primitive gabbro [3]. semibrittle deformation regimes. Full Decompression and sharp inversion of crystal plasticity was not achieved under density during an eclogite-to-gabbro the conditions tested, i.e., the von Mises retrograde transformation leads to an criterion was never satisfied. The ascent and merging of gabbro bodies rheology of other phases (chlorite, within more dense residual peridotite. tremolite), however, is unconstrained Finally the spreading process can lead to experimentally, but their weak nature is formation of the global 3rd seismic layer supported by the microstructures of of ocean. References 1. Blackman D.K. natural fault rocks. They often show a et al.//Proc. IODP. Vol. 304/305. 2006. close association of chlorite and 2. Dick H.D.B., Natland J.H., Miller D.J. tremolite with talc and serpentinite, each et al.//Proc. ODP, Init. Reports. 1999. phase often displaying different modes Vol. 176./ 3. Efimov A.A., Puchkov V.N. of deformation (brittle, plastic) operating About origin of the ophiolite association. at the same conditions. Tremolite Sverdlovsk, 1980. 63 pp. (In Russian) 4. displays either intragranular fracturing, Hofmann A.W.//Nature. 1997. Vol. 385. or ductile deformation by syntectonic P. 219-229. 5. Natland J.H., Dick growth and later kinking. Talc is either H.J.B.//J. Volcanol. Geotherm. Res. static or syntectonic, with semi-brittle 2001. Vol. 110. P. 191-233. 6. Sobolev deformation through intragranular A.V., Hofmann A.W., Nikogosian sliding and kinking. Chlorite probably I.K.//Nature. 2005. Vol. 434. P. 590- reduces fault strength when found with 597. syntectonic tremolite. Serpentine schists are rare, suggesting that deformation is Poster I-11 systematically associated with fluid flow Strain Localization Along and metasomatization along the fault. Detachment Faults: Mechanical Role of Serpentinite, Talc, Tremolite and Owing both to their weakness and to Chlorite their unusual mechanical behavior, these fault rocks can control strain localization Escartin, Javier; Andreani, Muriel; along detachments, which may Hirth, Greg accommodate up to 50% of the total plate separation when they operate J. Escartin, Geosciences Marines (IPGP), near-axis. Their weakness can be further Centre National de la Recherche enhanced by elevated pore fluid Scientifique (CNRS), Paris, FRANCE; M. pressure during deformation, as Andreani, Lab. Sciences de la Terre, suggested by the geochemistry, Univ. Lyon, Lyon, FRANCE; G. Hirth, deformation microstructures, and the Dept. Geological Sciences, Brown paragenesis of these rocks. The elevated University, Providence, RI and sustained rates of seismicity associated with detachments, and its

32 extent from 2 to 7 km b.s.f. (TAG the seafloor and (iii) the extent of the detachment), indicates that these hydration processes in the lithosphere materials are not ubiquous or are not well constrained. In this work we homogeneously distributed, and/or that investigate these issues by means of some of them display velocity numerical models of hydrothermal weakening; tremolite’s characterisitc circulation. The models consider cataclastic microstrutures may reflect convective heat and mass transfers in this mode of seismogenic deformation. and around moderate-angle (50°) Talc, which is stable at T<750°C and detachment faults in a three-dimensional potentially present to the base of the geometry. The effect of the permeability lithosphere, and serpentinites, stable at structure is specifically and T<600°C, have a velocity-strengthening systematically investigated by (i) behavior that would promote aseismic varying the permeability ratios between slip instead. Small amounts of these the fault zone itself, and the foot- and alteration products along the fault zone hanging-walls, (ii) considering the can significantly weaken the presence or absence of a high- detachment, and an heterogeneous and permeability top layer, and (iii) enabling discontinuous distribution may reduce for a more or less sophisticated fault the rupture size of earthquakes, plane geometry as suggested by promoting the sustained seismicity rates seismic studies (planar versus convex- observed at detachments. upward, 'spoon-like' fault plane, e.g., deMartin et al., 2007). The various Poster II-68 simulated three-dimensional flow Hydrothermal Circulation in and geometries and thermal regimes are Around Detachment Faults: then discussed in the light of the Inferences From Three-dimensional characteristics of the venting styles Numerical Modeling (e.g., high- versus low-temperature vent fields, distance from the point of Fontaine, Fabrice J.; Cannat, Mathilde; emergence of the fault at the seafloor) Escartin, Javier observed at known peridotites-hosted sites. IPGP-CNRS, Paris, FRANCE Poster I-27 Oceanic low-angle detachment faults Faults, Fluids and Alteration During expose mantle rocks that are often the Evolution of an Oceanic Core hydrated by serpentinisation processes, Complex: Constraints from the which may play an important role on Atlantic Massif global element cycles (mass exchanges with the ocean, recycling of water and Frueh-Green. Gretchen L.; Boschi, other components in the mantle and Chiara; Delacour, Adélie; Dini, Andrea; subduction zones). Robust, long-lived Kelley, Deborah high- and low-temperature hydrothermal vent fields have been G.L. Frueh-Green, C. Boschi, A. discovered along segments of slow- Delacour, ETH Zurich, Zurich, spreading ridges (e.g. MAR) in SWITZERLAND; C. Boschi, A. Dini, CNR, association with detachment faults. To Pisa, ITALY; A. Delacour, DTP, OMP- explain vent longevity and the striking Univ. Paul Sabatier, Toulouse, FRANCE; correlation with detachment faults, D. Kelley, School of Oceanography, ascending hydrothermal fluids may University of Washington, Seattle, WA exploit seismically active detachment faults as flow conduits (McCaig et al., We present a petrologic and geochemical 2007). However, the thermal regime in overview that highlights the interplay and around detachment faults is still between deformation, fluid flow and poorly known and parameters controlling mass transfer during emplacement and (i) the generation of high- and low- alteration of the Atlantis Massif (AM) and temperature fields, (ii) their distance to the ultimate formation of the Lost City the point of emergence of the fault at Hydrothermal Field (LCHF). The fact that

33 IODP unexpectedly recovered 1400m of products of long-lived tectonic and gabbro 5 km north of the peridotite- hydrothermal activity during formation hosted LCHF is testament to how of the massif as an OCC. Thus, the spatially variable magmatic accretion chemical signatures used to infer processes and fluid-rock interaction sources and fluid fluxes may not be fully must be in OCCs. Within the southern representative of the present-day wall, gabbroic rocks (~30%) occur as conditions in the basement peridotites lenses interspersed with pervasively that directly feed the LCHF. However, serpentinized spinel harzburgites. The the comprehensive picture given by the harzburgites have depleted geochemical data of the fluids, compositions, and most samples show hydrothermal deposits, and basement slightly enriched REE contents, rocks at Lost City is that interaction of interpreted as the result of melt seawater with both variably fresh entrapment prior serpentinization. peridotites and serpentinized peridotites Detachment faulting is marked by talc- is crucial to the formation of volatile- amphibole-chlorite metasomatism and rich, Lost City-type systems, and that heterogeneous distribution of high strain transform-related normal faulting and deformation. The metasomatic zones mass wasting facilitate seawater reflect interaction between gabbroic and penetration necessary to sustain ultramafic rocks at T <400°C, with hydrothermal activity over tens of localized, channeled circulation of thousands of years. oxidizing, Si-Al-Ca-rich fluids and mass transfer during high strain deformation. Oral – Tuesday 11th, 11:45 Serpentinization in the peridotite-rich Serpentinization of the Oceanic domains continued during uplift and was Lithosphere: Consequences for influenced by transform-related normal Hydrothermal Activity and faulting, volume expansion and mass Biogeochemical Cycles wasting along the southern wall. High integrated fluid-rock ratios at 150-250°C Frueh-Green. Gretchen L.; Boschi, produced enrichments in B and U, Chiara; Delacour, Adélie; Kelley, changes in Sr- and Nd-isotope ratios Deborah towards seawater values, and highly depleted bulk rock O-, H-, and B-isotope G.L. Frueh-Green, C. Boschi, A. compositions in the basement. In Delacour, ETH Zurich, Zurich, addition, B-isotope analyses of the SWITZERLAND; C. Boschi, CNR, Pisa, hydrothermal fluids and precipitates at ITALY; A. Delacour, DTP, OMP-Univ. Paul Lost City indicate that brucite is a Sabatier, Toulouse, FRANCE; D. Kelley, significant, temporally variable, reservoir School of Oceanography, University of for Mg and B in these systems. Washington, Seattle, WA

Bulk rock O-, Sr- and Nd-isotope ratios Exposure of mantle rocks and of the gabbroic-dominated central dome serpentinization of peridotite are integral indicate variable degrees of seawater components of slow- and ultra-slow interaction at temperatures spreading ridges. Mantle-dominated predominantly under greenschist facies. lithosphere is a highly reactive chemical In contrast to the southern wall, the and thermal system, in which interaction degree of fluid flow, alteration and mass with seawater during serpentinization transfer in the gabbroic domain of the has significant geophysical, geochemical central dome is limited and strongly and biological consequences for the controlled by veining, igneous contacts, global marine system. At slow spreading and zones of brittle faulting. Despite ridges, seawater reacts with peridotite differences in sampling approaches, the as detachment faults unroof mantle two areas are distinctly different on a material, and variably serpentinized scale of a few kilometers from N to S. peridotite hosts black smoker type and Our samples obtained by submersible cooler off-axis hydrothermal systems. from the southern wall likely represent Here we review serpentinization the “outer skin” of the system and end- processes as fundamental to

34 understanding the exposure of mantle high pH and reducing conditions of such material and the evolution of oceanic systems dictate that any carbonate lithosphere formed at slow spreading species in the fluids are either reduced ridges, and discuss open questions or precipitated as carbonate before related to heat and fluid flow and the venting on the seafloor, and thus consequences of serpentinization for represents an important sink of biogeochemical cycles. dissolved (inorganic and organic) carbon from seawater. The mineral assemblages and textures In contrast to basalt-hosted of oceanic serpentinites typically record hydrothermal systems, where progressive, static hydration reactions conceptual models of the fluid pathways that take place under a wide range of and subsequent reactions and element temperatures, lithospheric depths, fluid uptake are relatively well constrained, compositions and redox conditions. The less is known of the fluid flow and products and sequence of reaction paths or the source and flux of serpentinization reactions will be heat in off-axis, serpentinite-hosted influenced by the flux of seawater hydrothermal systems. The importance penetration into the lithosphere, the bulk of high temperature, peridotite-hosted protolith compositions, the presence or systems (such as Rainbow) in terms of absence of magmatic intrusions and/or heat, fluid and chemical fluxes to the trapped gabbroic melts, and structure oceans at slow and ultraslow spreading (e.g., detachment faults, cataclastic fault ridges has been relatively well studied. zones). In turn, these factors influence However, more detailed studies are vent fluid chemistry, volatile contents, necessary to better constrain fluid and type of hydrothermal deposits, and pathways and the importance of lower distinguish the Lost City hydrothermal temperature diffuse flow and off-axis field from other known peridotite-hosted hydrothermal systems for chemical and hydrothermal systems, such as the thermal exchange between the Rainbow and Logatchev systems, on the lithosphere, hydrosphere and biosphere. Mid-Atlantic Ridge. Poster I-31 Serpentinization processes have major Timing of Exhumation of a MOR-type consequences for long-term, global Gabbroic Intrusion (Internal geochemical fluxes by acting as a sink Ligurian ophiolites, Italy) for H2O, Cl, B, U, S, and C from seawater and a source of Ca, Ni and Garzetti. Fabio; Tribuzio, Riccardo; possibly Cr to hydrothermal fluids, and Tiepolo, Massimo by producing extremely reduced fluids. In addition, seafloor weathering of F. Garzetti, R. Tribuzio, Dipartimento di serpentinized abyssal peridotites may Scienze della Terra, Università di Pavia, result in Mg loss. The formation of H2 Pavia, ITALY; R. Tribuzio, M. Tiepolo, during serpentinization is generally Istituto di Geoscienze e Georisorse, CNR, attributed to the production of magnetite U.O. Pavia, Pavia, ITALY during olivine hydration and is described by simplified model reactions with end- The ophiolites from Internal Ligurian member phases. In reality, units of Northern Appennine are serpentinization involves solid solutions lithosphere remnants of a Middle-Upper and metastable reactions governed by Jurassic embryonic ocean. These local variations in the activities of ophiolites are characterised by gabbroic elements such as Si, Mg, Fe, Ca, and C. intrusions in mantle peridotites and bear Serpentinization of harzburgitic structural and compositional similarities peridotites at temperatures below to modern (ultra-)slow spreading ridges. ~200°C and low fluid-rock ratios The gabbro-peridotite association shows produces high alkaline, Ca-rich fluids evidence for exhumation at the seafloor with elevated concentrations of through a polyphase tectono- abiogenic CH4 and H2, as exemplified by metamorphic evolution, starting from the Lost City hydrothermal system. The high temperature ductile conditions. In

35 this work, we wish to measure the McDermott, Jill; Nakamura, Ko-ichi; exhumation rate of the gabbros, from Seewald, Jeffrey; Smith, Julie; Van their intrusion into the mantle to their Dover, Cindy; Whitcomb, Louis; Yoerger, exposure at the seafloor. This may Dana provide new information on the tectonic processes associated with opening of C.R. German, A. Bowen, J. Kinsey, J. (ultra-)slow spreading ridges. McDermott, J. Seewald, D. Yoerger, Geochronological determinations have WHOI, Woods Hole, MA; M. Coleman, Jet been thus carried out through single- Propulsion Laboratory, CalTech, grain U-Pb zircon datings by laser Pasadena, CA; D. Connelly, NOCS, ablation ICP-MS. On the basis of field Southampton, UNITED KINGDOM; D. relationships relative chronology, we Honig, C. Van Dover, DUML, Beaufort, distinguished four main events for the NC; J. Huber, J. Smith, MBL, Woods formation and exhumation of the Hole, MA; M. Jakuba, University of gabbros: (1) formation of gabbroic Sydney, Sydney, New South Wales, plutons. The age of gabbro intrusion was AUSTRALIA; K. Nakamura, AIST, determined through zircons separated Tsukuba, JAPAN; L. Whitcomb, Johns from plagioclase-rich pegmatoid lenses Hopkins University, Baltimore, MD and from albitite dykelets displaying irregular hornblende-rich contacts Thirty years after the first discovery of against the host gabbro. (2) high submarine venting the vast majority of temperature deformation in ductile shear the global Mid Ocean Ridge remains zones, which form a low angle with unexplored for hydrothermal activity. Of respect to the igneous layering. (3) high particular interest are the world’s temperature brittle deformation slowest-spreading ridges which were the associated with melt injections. The last to be demonstrated to host high- ductile shear zones are crosscut at high temperature hydrothermal activity and angles by thin hornblende veins and which may host systems rooted in igneous dykes. The geochemical ultramafic lithologies that are signature of vein hornblende indicates particularly relevant to pre-biotic that the gabbros interacted with chemistry and the origins of life. Here, seawater-derived fluids. Dykes are we report the first systematic basalts with chilled margins and albitites exploration for and characterization of showing sharp planar boundaries to the hydrothermal venting along the short host gabbros. The age of this event was (~110 km), deep (> 5000 m), ultra- determined through zircons collected slow-spreading (<20 mm yr-1) and from the second albitite pulse. (4) low geographically isolated Mid-Cayman Rise temperature brittle deformation leading (MCR). From a combination of CTD to the exposure of the gabbros at the casts and tow-yos together with in situ seafloor. The U-Pb zircon datings observations from the Nereus hybrid document a long time interval (Ma- AUV/ROV, we show that the MCR, one of scale) between solidification of the Earth’s deepest and slowest-spreading gabbros and its uplift to sub-seafloor ridges, appears to play host to all three conditions. This implies a very slow distinct styles of hydrothermal venting exhumation rate for the gabbro known anywhere else in the deep intrusion, in agreement with a “passive” ocean:– (a) neovolcanically-hosted high model of mantle uplift and melting. temperature venting on the rift-valley floor in the northern segment at 4950m Poster I-30 - the deepest site yet known in the Hydrothermal Activity Along the oceans; (b) tectonically (± Ultra-Slow Spreading Mid-Cayman ultramafically)-hosted high-temperature Rise venting at the southern end of the MCR and (c) toward the center of the MCR, a German, Christopher R.; Bowen, low-temperature hydrothermal site Andrew; Coleman, Max; Connelly, located at the summit of a long-lived Douglas; Honig, David; Huber, Julie; detachment fault. The latter may Jakuba, Michael; Kinsey, James; represent only the second ever 'Lost

36 City' style vent-site to be found even after a common parental MORB melt. though it is predicted that they could be (Lack of systematic) downhole variations widespread. Our work, funded through indicate that the gabbroic series were NASA’s ASTEP program, was carried out built by multiple magma injections. In aboard the RV Cape Hatteras in October- detail, textural and geochemical November 2009. As well as being of variations in olivine-rich troctolites and interest to InterRidge and ChEss gabbronorites suggest chemical (Census of Marine Life) this cruise also interaction (assimilation?) between the represented the first scientific field parental melt and the intruded program funded to use WHOI’s new lithosphere. hybrid deep submergence vehicle, Nereus, in both AUV and ROV mode. We carried out mass balance calculations The discoveries we have already made, to determine the bulk hole geochemical together with follow-on investigations composition of Site U1309 gabbroic planned by the UK in 2010, are poised to rocks. Bulk hole major element render the MCR an ideal natural composition is characterized by high laboratory for continuing studies of Mg# (76) compared to estimates of ultra-slow spreading ridges and the wide primitive MORB melt composition (~73). variety of hydrothermal systems that This may reflect under-sampling of the they host. most evolved gabbroic components and/or assimilation of (reaction with) Poster I-5 olivine-rich rocks. Calculations carried Geochemistry of IODP Site U1309 out to test the effects of sampling biases Gabbroic Series (Atlantis Massif): and of extended assimilation processes Evidence for High and Cyclic result in bulk hole compositions close to Magmatic Activity that of primary MORB magma for both major and trace elements. This suggests Godard, Marguerite; Ildefonse, Benoit that although troctolites and gabbros form a cumulate series indicating there CNRS - Universite Montpellier, has been local separation of melt and Montpellier, FRANCE solids, the most evolved end-members of the Site U1309 gabbroic series are IODP Site U1309 was drilled at Atlantis complementary to the cumulate series. Massif, an oceanic core complex, at These results suggest that (i) there was 30°N on the Mid-Atlantic Ridge (MAR). no large scale removal of melts from this We present the results of a bulk rock gabbro section and (ii) Site U1309 geochemical study (major and trace gabbroic series do not represent the elements) carried out on 228 samples complementary magmatic product of representative of the different lithologies 30°N volcanics. sampled at this location. The occurrence of such a large Over 96% of Hole U1309D is made up of magmatic sequence implies that high gabbroic rocks: olivine-rich troctolites and lasting magmatic activity is (>70% modal olivine) and troctolites associated with the formation of Atlantis having high Mg# (82-89), high Ni (up to Massif. Our results suggest that almost 2300 ppm) and depleted trace element all melts feeding this magmatic system compositions (Yb 0.06-0.8 ppm); olivine stay trapped into the intruded gabbros and gabbros (including lithosphere during the OCC formation, gabbronorites) with Mg# of 60-86 and but can occasionally reach the surface. low trace element contents (Yb 0.125- Hence, trace element compositions of 2.5 ppm); and oxide gabbros and clinopyroxene and plagioclase leucocratic dykes with low Mg# (<50), poikiloblasts in olivine-rich troctolites low Ni (~65 ppm) and high trace indicate that they crystallized from the element contents (Yb up to 26 ppm). same depleted MORB melt, which has The main geochemical characteristics of compositions similar to that of diabases Site U1309 gabbroic rocks are consistent crosscutting the upper part of the with a formation as a cumulate sequence borehole and to basalts in the MAR 30 °

37 N axial region (Drouin et al, 2009). This the RTI. Although the nature of the is consistent with an episodic upper detachment is unclear, our results imply crustal feeding system in the Atlantis that this motion was accommodated Massif area. across a narrow zone located at or close to the dike-gabbro transition zone. East Drouin et al. 2009, Chem. Geol., 264: of the intact zone, a clockwise rotation in 71-88, the gabbro increases gradually away doi:10.1016/j.chemgeo.2009.02.013. from the axial zone, similar to that of the overlying dikes, indicating coupling Poster I-40 of the lower crust with the brittle upper Evidence from the Troodos Ophiolite oceanic crust at the inside corner. for Partitioning of Vertical-axis Similarly, coupling is also inferred for Rotations ('Torsional Detachment') the outside corner. The transition from Across the Dike-gabbro Boundary the decoupled layers of sheeted dikes and gabbros in the axial-zone to the Granot, Roi; Abelson, Meir; Ron, Hagai; dike-gabbro coupling in the Agnon, Amotz inside/outside corners is in keeping with deepening of the brittle-ductile transition R. Granot, Institut de Physique du Globe away from the hot axial zone. de Paris, Paris, FRANCE; R. Granot, H. Ron, A. Agnon, Institute of Earth Poster I-6 Sciences, Hebrew University, Jerusalem, Isotopic Insights into Length-scales, ISRAEL; M. Abelson, Geological Survey Initiation Depths and Extent of of Israel, Jerusalem, ISRAEL High-temperature Hydrothermal Circulation Associated with Oceanic The gabbro suite of the Troodos ophiolite Detachment Fault Systems is exposed where an extinct spreading axis (Solea graben) intersects a fossil Grimes, Craig B.; John, Barbara E.; oceanic transform (Arakapas transform). Cheadle, Michael J.; Valley, John W.; This is a unique exposure of deep crustal Wooden, Joseph L. rocks formed at both inside- and outside-corners of a ridge-transform C.B. Grimes, Dept. of Geosciences, intersection (RTI). Remanence directions Mississippi State University, Mississippi from 24 gabbroic sites, positioned along State, MS; B.E. John, M.J. Cheadle, flowline direction, were used as Dept. of Geology and Geophysics, indicators for rigid body rotation of the University of Wyoming, Laramie, WY; lower crust. These data complement C.B. Grimes, J.W. Valley, WiscSIMS, studies that have reconstructed the Dept. of Geoscience, University of brittle deformation of the upper crust Wisconsin, Madison, WI; J.L. Wooden, and together they facilitate a USGS-Stanford Ion Microprobe Facility, surprisingly coherent structural model Stanford, CA (please see Granot et al., EPSL, 2006, for a complete description). The spatial Within the last decade, the application of distribution of lower crustal rotations isotopic dating using zircon recovered allow recognition of three regions to from in situ ocean crust has begun which deformation is partitioned: 1) a providing exciting insights into the western region (outside corner) that complex accretion processes occurring at experienced primarily tilt about mid-ocean ridges. Here, Pb/U and (U- horizontal axis 2) a central region with Th)/He zircon ages of gabbroic crust negligible rotation (interpreted as the exposed in the footwalls of several axial zone) and, 3) an eastern area large-offset, low-angle normal fault (inside corner) where vertical axis systems along the Mid-Atlantic Ridge are rotations are dominant. Clockwise- combined to investigate footwall cooling rotated dikes are found at the very close histories and apparent length-scales of vicinity of the intact central domain, detachment fault systems. Ti-in-zircon suggesting a 'torsional detachment' crystallization temperatures, taken with between the upper and lower crust at the closure temperature of the (U-

38 Th)/He system in zircon bracket the interaction between seawater and rocks acquisition temperature of magnetic along these faults must have post-dated remanence, defining cooling rates of the formation and crystallization of the 1000-2000°C/m.y over the temperature latest stage magmas. range of ~800-220°C. As these large- offset faults serve as the plate boundary Poster II-42 while active, the fault slip rate is taken Mylonitic Detachment Faulting Along to be equivalent to the plate-spreading the Mid-Atlantic Ridge at the Kane rate at the time of faulting. Assuming Fracture Zone Oceanic Core Complex the footwall was denuded along a single continuous fault system, and using these Hansen, Lars N.; Cheadle, Michael J.; slip-rates, the time interval defined by John, Barbara E.; Dick, Henry J.; the difference in Pb/U and (U-Th)/He Tucholke, Brian E.; Tivey, Maurice A. zircon ages can be used to estimate the distance between the ~800 and 220°C L.N. Hansen, University of Minnesota, isotherms, and therefore the length- Minneapolis, MN; M.J. Cheadle, B.E. scale of the active fault system. At the John, University of Wyoming, Laramie, Atlantis Massif core complex (30° N, WY; H.J. Dick, B.E. Tucholke, M.A. MAR) these data suggest a detachment Tivey, Woods Hole Oceanographic fault length of 10±2 km between the Institution, Woods Hole, MA 800 and 200°C isotherms. Lengths of two other fault systems at 15°N, MAR, In contrast to oceanic core complexes are calculated to be 9.5±3 km (Hole (OCCs) on the Southwest Indian Ridge 1275D) and 5±3 km (Hole 1270D) km. and in the Parece Vela basin, some OCCs Published Pb/U and (U-Th)/He zircon on the Mid-Atlantic Ridge (e.g., Atlantis ages from Atlantis Bank (SWIR) allow Massif) show restricted high- calculation of a similar fault length of temperature deformation. Thus, low- 7.7±4 km. Using the calculated fault temperature deformation processes have lengths and a fault initiation dip of been considered the controlling 50°(consistent with paleomagnetic and processes for Atlantic detachment faults. bathymetric constraints) we estimate We challenge this apparent dichotomy the fault systems along the MAR were between Atlantic OCCs and other OCCs active to depths of up to 8 km (~8 km by demonstrating the existence of for Atlantis Massif, 8 and 5.5 km at mylonitic shear zones several hundreds 15°N). A lower initiation dip of 35° of meters thick at the Kane OCC. (based on magnetic constraints) at Atlantis Bank gives a fault depth of ~6.3 In 2004, the RV Knorr collected samples, km. These depth estimates for active bathymetric data, and geophysical data faulting in slow spreading environments from the Kane Fracture Zone at 23°N on are consistent with microseismicity the Mid-Atlantic Ridge. Samples were studies, and the existence of high collected by submersible and by temperature fault rocks associated with dredging from the complex's surface and many oceanic detachment faults. The from high-angle fault scarps that cut the apparent differences in calculated fault complex. Examination of the deformed initiation depths may reflect variations in samples by hand sample analysis, the thermal structure beneath the ridge petrography, electron backscatter systems, with deeper faults indicating a diffraction, and geothermometry deeper ~800°C isotherm. Such deep confirms that the Kane OCC is bounded rooting fault systems potentially serve by a detachment fault system that as conduits for high temperature initiated at high temperatures (>700°C) hydrothermal fluid circulation into the and rooted below the brittle-plastic deep crust. However, igneous zircon transition. Fault rocks reveal a history of record nearly constant, mantle-like deformation from granulite and oxygen isotope ratios, and provide no amphibolite through subgreenschist indication of interaction between altered facies, including brittle cataclasis. Slopes rocks or seawater and the zircon- of the breakaway ridge (>23° to the forming melts. Thus, any significant west and >22° to the east) give

39 constraints on the initial dip of the Poster II-46 detachment fault. Assuming this ridge Microstructural Development of the formed by flexural uplift, these slopes Ultramafic Rocks from Godzilla suggest the detachment fault formed Mullion in the Parece Vela Basin with a dip >45°. Harigane, Yumiko; Michibayashi, We present two cross sections through Katsuyoshi; Ohara, Yasuhiko Y. the detachment fault based on samples Harigane, Kanazawa Univ., Kanazawa, collected from secondary high-angle JAPAN; K. Michibayashi, Shizuoka Univ., normal fault scarps. One section, Shizuoka, JAPAN; Y. Ohara, through Cain Dome, is dominated by Hydrographic and Oceanographic peridotite and shows a 450-m thick zone Depart., Tokyo, JAPAN of discrete ductile shear zones overprinted by a 200-m thick zone of Godzilla Mullion, the world’s largest semi-brittle and brittle deformation. This oceanic core complex, occurs in the mylonitic envelope is on a similar scale Parece Vela Basin spreading ridge to those observed in Hole 735B at (Parece Vela Rift), the Philippine Sea. Atlantis Bank on the Southwest Indian Large amount of lithospheric mantle and Ridge [e.g., Dick et al., 2000, EPSL] and lower oceanic crustal rocks, including is consistent with the mylonitic sequence fault rocks from mylonite to cataclasite, reported by Karson [1999, Trans. Roy. has been recovered from the surface of Soc. Lond.] at the northern termination Godzilla Mullion (e.g., Ohara et al., of the Kane OCC. An analysis using 2009). Harigane et al. (2008) recently grain-size piezometry and analyzed the plastically deformed geothermometry to construct gabbroic rocks from the breakaway area, deformation mechanism maps suggests and revealed the development of a strain rates for the gabbronorites and ductile shear zone in the lower oceanic the peridotites of 10−12 s−1, and 10−13 crust. They argued that the s−1 respectively. The other section, microstructural development of the through Adam Dome, is dominated by gabbroic rocks occurred during uplift- gabbroic rocks and exhibits little high- related cooling of the gabbro body and temperature deformation. Although that a primary shear zone developed several discrete, high-temperature shear near the breakaway area at depth under zones are present, this section lies <4 anhydrous conditions at high km from the breakaway, and is therefore temperatures above 850 degree. In this inferred to have undergone primarily contribution, we present the brittle deformation in the shallow crust. microstructural development of the Finally, the presence of deformed and ultramafic rocks from the expanded undeformed diabase dikes, peridotite region of Godzilla Mullion. The ultramafic mylonites that have been intruded by a rocks consist of serpentinized peridotite, gabbroic melt, and Fe-Ti oxide serpentinite, serpentinite schist and talc microstructures indicative of deformation schist. These ultramafic rocks were with a melt present suggest that recovered from nine dredge sites during detachment faulting was coeval with cruises KR03-1 and KH07-2, and one magmatism. Thus, the Kane OCC dive site by the submersible Shinkai provides evidence that detachment 6500 during YK09-05 along a flow line of faults are consistent with a rolling-hinge Godzilla Mullion. In serpentinized detachment fault model and can be peridotites, the serpentine is typically influenced by high-temperature lizardite with minor chrysotile, consisting processes including mylonitization and of mesh texture, bastite and cross- interaction with partially molten rocks cutted vein in microstructure. The regardless of location/ridge system. original microstructures of peridotites are preserved as pseudomorph, enabling us to classify them into four types: weakly sheared peridotite, moderately sheared peridotite, mylonite, and ultramylonite. The presence of mylonite

40 and ultramylonite along the surface of Modena, ITALY; A. von der Handt, Godzilla Mullion suggests an occurrence Geoscience Institute, University of of shear zone(s) within the deeper part Freiburg, Freiburg, GERMANY of the oceanic lithosphere. Furthermore, crystallographic-preferred orientation Drilling at Hole U1309D (IODP Legs (CPO) data for olivine in mylonite were 304/305) penetrated 1415 m into the measured using a SEM-EBSD system at footwall of the Atlantis Core Complex. Shizuoka University. Olivine CPO is More than 96% of all recovered rocks characterized by a strong alignment of are gabbroic and only minor peridotite [100] oblique 45 degree to the lineation slivers were found. The complexly and [001] in a girdle normal to the stacked gabbroic sequence ranges from foliation, indicating [100](001) pattern. primitive olivine-rich troctolites to Serpentinite schists and talc schists (olivine) gabbros and gabbronorites to contain a well-developed schistosity that highly evolved oxide gabbros and is overprinted by kinking, whereas one leucocratic dikes. of the talc schists shows mylonitic texture including asymmetric texture Most lithologies preserve evidence for and shear band. These serpentinite pervasive, and partly multistage, schists and talc schists are similar to reactive melt transport. The most clearly those of the detachment fault-related visible overprint occurred after formation schists described from OCCs in the Mid- of cumulus grains and is manifested by a Atlantic and Southwest Indian Ridges Ti enrichment at cpx grain boundaries. (e.g., MacLeod et al., 2002; Escartín et Prior to that, a more focused reactive al., 2003). The presence of such schists melt transport occurred, leading to cpx along the surface of Godzilla Mullion formation at the expense of a indicates that they were derived from compacted olivine-plagioclase shear zone(s) in the cold (i.e., shallow) framework. These reactions occur oceanic lithosphere at greenschist facies throughout the entire core. condition. Such developed shear zone(s) in both deeper and shallower oceanic Since their recovery, the origin of the lithosphere could play an important role olivine-rich troctolites (ORT) has been on the structural evolution of the intensely debated, and a possible mantle Godzilla Mullion in the Parece Vela Rift. origin was discussed on board We interpret that these ultramafic rocks (Blackman et al 2006). According to were formed in the shear zone(s) in both textural criteria, the ORT resemble deeper and shallower oceanic primitive cumulates, albeit fine grained. lithosphere, presumably associated with In contrast to these textural constraints, detachment faulting throughout the geochemical and microstructural criteria Godzilla Mullion during tectonic suggest that may represent reacted and extension within the Parece Vela Rift. impregnated mantle peridotites (Blackman et al 2006; Suhr et al 2008; Poster II-43 Drouin et al 2009; Drouin et al, in Pervasive Reactive Melt Migration press). Suhr et al (2008) argue that the Before Core Complex Formation: ORT are former mantle rocks which were Evidence From IODP Hole U1309D, converted to dunite at the base of the Atlantis Massif upper magmatic unit. Later, as melts derived from the lower magmatic unit Hellebrand, Eric; Suhr, Guenter; percolated, they were gradually Johnson, Kevin; Brunelli, Daniele; von equilibrated to a more evolved chemistry der Handt, Anette and transformed to a fine grained ORT. The main arguments against a cumulate E. Hellebrand, K. Johnson, A. von der nature of the ORT are the lack of any Handt, Geology & Geophysics, University systematic down-hole trend in of Hawaii, Honolulu, HI; G. Suhr, compatible elements within the ORT, its Geology & Mineralogy Inst., University of distinctly fine-grained microstructure, Cologne, Cologne, GERMANY; D. the high Cr-content of cpx, and its Ni- Brunelli, DST, University of Modena, rich olivine composition. The high NiO

41 for a given Mg/(Mg+Fe) in the ORT can uppermost ~km of Atlantis Massif, an be modeled by simple equilibration of oceanic core complex (OCC) at 30°N on relict mantle olivine with a somewhat the Mid Atlantic Ridge. Our analysis evolved melt. Independent from the includes all MCS lines sampling the compatible element behavior, strongly massif. We employ the Synthetic On- depleted trace element abundances in Bottom Experiment (SOBE– Harding et olivine have also been proposed to al., 2007; Arnulf et al., 2009) processing support a mantle origin of these rocks method to downward continue all lines (Drouin et al., 2009). to the seafloor, producing models with unprecedented structural detail in the However, these criteria are not upper few hundred meters subsurface. sufficiently conclusive and appear to be Our velocity inversions for axis-parallel inconsistent with the extreme lines covering the domal core show heterogeneity observed within the ORT- velocities within a few hundred meters hosted Cr-spinels. Not only do the of the seafloor ranging 5.5-6.5 km/s. spinels resemble compositions observed Based on the continuity of this structure in primitive MORB, they are entirely we interpret the signature to indicate a unlike any peridotitic spinel found on the large gabbroic body. This petrologic ocean floor or in the mantle section of inference is consistent with deep drilling ophiolites. The exact origin will continue that retrieved a 1.4 km nearly- to be subject of further study. continuous gabbroic section from the central dome of Atlantis Massif However, independent of whether the (Blackman et al., 2006). The continuity ORT directly crystallized from a melt or of this velocity feature does not support whether they formed from by reaction the idea of a discreet fault or lithological with a mantle protolith, proportionally contact between the two main structural large amounts of melt are required to components of the massif- the central form these rocks. Considering that the dome and the southern ridge. Also occurrence of sparse truly mantle apparent from our velocity models is a harzburgites at this location requires an structural unit of few hundred meter earlier period of low magma supply and thickness with comparatively lower presumably thin magmatic crust, this seismic velocities, more akin to normal pulse of high melt supply must predate young Atlantic shallow crustal structure, or at best coincide with the onset of core capping the western, northern, and complex formation. southernmost portions of the OCC. The spreading-parallel MCS lines show Poster I-20 significant lateral variation in velocity Comprehensive Shallow Coverage of structure between the earliest-exhumed Seismic Velocity Structure at side of the massif, where velocities are Atlantis Massif Oceanic Core lower (2.5-3.5 km/s in the upper few Complex hundred meters), and the most recently exhumed side, where velocities are Henig, Ashlee S.; Blackman, Donna K; comparatively higher in the few hundred Harding, Alistair J; Kent, Graham M; meters immediately subseafloor (4.5-6 Canales, Juan Pablo km/s). This lateral variation can be attributed to an increase in permeability A.S. Henig, D.K. Blackman, A.J. Harding, and alteration of the subsurface with an Scripps Institution of Oceanography, La increase in age of exposure, or could be Jolla, CA; G.M. Kent, Nevada a result of different lithologies within the Seismological Lab, University of Nevada, massif. Recently completed analysis of Reno, Reno, NV; J. Canales, Woods Hole the MCS line traversing the hanging wall Oceanographic Institution, Woods Hole, to the detachment reinforces prior MA interpretations of a volcanic composition for the overlying block on the eastern Using multichannel seismic (MCS) data, edge of the massif. Thus, our velocity we finalize our investigation of the models suggest significant lateral detailed velocity structure of the heterogeneity within the upper (~1 km)

42 structure of the Atlantis Massif. of Troodos, including north of Spilia and Likewise, we observe large vertical south of Lemithou. Overall, these velocity gradients >4 s^-1, structural patterns at Hess Deep and in the upper few hundred meters of Pito Deep have been interpreted in structure, but never observe fresh terms of fluctuations in the relative mantle velocities >7.5 km/s within the contributions of dike intrusion and coverage area of the MCS refraction, subsidence/volcanic construction during down to ~1 km depth. crustal accretion. Large parts of the Troodos Ophiolite must have accreted in Poster I-37 a manner and environment similar to the Similarities in Structure of fast-spread crust at Hess Deep and Pito Uppermost Oceanic Crust of the Deep and thus may have been created Hess Deep Rift and Pito Deep with in a fast spreading environment. the Sheeted Dike Complex of the Troodos Ophiolite Poster I-12 The Atlantis Massif : An Illustration Hurst, Stephen D. of the Interplay Between Core Complex Development and Geology, Univ. of Illinois, Urbana, IL Magmatic Activity

Direct observations of the uppermost 2 Ildefonse, Benoit; Godard, Marguerite; km of the oceanic crust exposed at the Drouin, Marion Hess Deep Rift and Pito Deep reveal substantial similarities with structure of B. Ildefonse, M. Godard, Géosciences the sheeted dike, extrusive section and Montpellier, CNRS - Université uppermost gabbros of the Troodos Montpellier 2, Montpellier, FRANCE; M. Ophiolite. At the largest scale, all Drouin, Laboratoire Géosciences locations show dramatic variations in the Réunion- IPGP, Université de la Réunion, thickness of both basaltic volcanic and Saint-Denis, REUNION sheeted dike rock units. In addition, all have extensive (>= 10km width) Oceanic Core complexes expose exposure of moderately dipping ( 40- intrusive crustal rocks via detachment 70°) sheeted dike units. In Troodos, the faulting, and are generally associated west side of the Solea Graben is the with serpentinized mantle peridotite on most extensive and best exposure of the the seafloor. All of the ODP/IODP drilling tilted dikes and is underlain by a at 4 different OCC, including Hole horizontal detachment fault separating U1309D, have recovered only gabbroic the gabbro from the dikes. sections, with almost no serpentinized Paleomagnetic evidence and slickenside peridotite. Ildefonse et al. (2007, analysis of small faults in the uppermost doi:10.1130/G23531A.1) have proposed gabbro and lowermost sheeted dikes a possible revised model for OCC indicate that the dikes rotated 40-90° development in which the "core" of the consistent with extension on this and OCC domes represents a period of parallel faults, accounting for up to 40 greater than typical mafic intrusion in xtension. No such detachment has yet overall magma-poor regions of been definitely observed at either Hess slow/ultra-slow spreading ridges. Deep or Pito Deep. At Hess Deep in Exposure of the gabbroic intrusion(s) is particular, lowermost dikes and enabled by deformation that localizes uppermost gabbros appear as a predominantly within the serpentinized gradational contact with interfingering of peridotite that surrounds them. The diabase and gabbro. However, similar proposed model was different from gradations and interfingering is exposed previous published models in that OCC in Troodos just to the east of the Solea represent the tectonic and morphologic Graben, northwest of Spilia. Other expression of the magma-rich end- interesting similarities include the member of a fundamental mode of several km wide area of massive diabase crustal accretion- the intrusion of gabbro observed at Pito Deep and several areas plutons at depth, in an heterogeneous

43 "plum pudding" type of crust; it is Poster I-2 consistent with recent seafloor Melt-rock Interaction and Its Impact observations and numerical modeling on Serpentinization of Abyssal suggesting that OCC do not represent Peridotites the magma starved end-member of mid- ocean ridges. Joens, Niels; Bach, Wolfgang; Klein, Frieder; Schroeder, Timothy An illustration of the vigorous magmatic activity associated with OCC is the N. Joens, W. Bach, F. Klein, University of occurrence of olivine-rich troctolites in Bremen, Bremen, GERMANY; F. Klein, Hole U1309D. We present a Woods Hole Oceanographic Institution, petrostructural (EBSD) and in-situ Woods Hole, MA; T. Schroeder, geochemical (EPMA, LA-ICPMS and LA- Bennington College, Bennington, VT HR-ICPMS) study of olivine-rich troctolites (ol > 70%; 5.5 % of Interaction of peridotites and seawater recovered section) and associated leads to serpentinization, which affects gabbros (Drouin et al., 2009, the physical and chemical characteristics doi:10.1016/j.chemgeo.2009.02.013; of oceanic lithosphere. We examined submitted). Trace element compositions abyssal serpentinites exposed in the of clinopyroxene and plagioclase footwalls of major detachment faults on poikiloblasts in olivine-rich troctolites the flanks of the slow-spreading Mid- indicate that they crystallized from the Atlantic Ridge near the 15°20′N fracture same depleted MORB melt. Olivine trace zone (Ocean Drilling Program Leg 209, element compositions appear too Site 1270). The study is aimed at depleted in light REE to be in equilibrium understanding the relations between with plagioclase and clinopyroxene. magmatism, hydration reactions, shear Olivine crystallographic preferred zone formation and fluid flow. orientations are weak, and misorientations are consistent with The examined serpentinites are hydrated deformation by dislocation creep with harzburgites with an alteration degree of activation of the high-temperature (010) >90%. Olivine is replaced by serpentine [100] slip system, commonly described minerals and magnetite in a typical in asthenospheric mantle. The fabrics mesh texture, whereas bastite after also display a relatively strong orthopyroxene is almost magnetite free. uncommon [001] concentration that we Brucite has not been identified. We have interpret as resulting from abundant specifically picked out samples that are melt impregnation. The joint study of crosscut by narrow (<15mm) shear geochemical processes and zones. Within these shear zones rocks microstructures in olivine-rich troctolites are of strongly foliated appearance and suggest a complex crystallization history consist of magnesium-rich chlorite and in an open system with percolation of tremolitic/actinolitic amphibole. In large volume of MORB-type melt that addition, these "fault schists" contain postdate olivine crystal-plastic porphyroclasts of brownish deformation. This is consistent with the magnesiohornblende as well as formation of OCC associated with accessory apatite and zircon. Whole-rock relatively strong magmatic activity, and geochemical and oxygen isotope with the crystallization of most of melt in analyses have been perfomed on the Atlantis Massif lithosphere without separated fault schists and associated basaltic counterpart erupted on the serpentinite host rocks. In accordance seafloor. Although the mantle origin of with results from mineral chemistry, Ti- the olivine is difficult to demonstrate in-zircon thermometry and reaction path unequivocally, we propose that olivine- modeling, fault schists are interpreted to rich troctolites represent the ultimate represent hydrated former plagiogranitic residue of melt-mantle reaction melt veins. processes. The presence of evolved melts within abyssal peridotites has important consequences on both the chemical and

44 the structural evolution of detachment Detachment faults and their associated fault systems. The fundamentally core complexes occur both in continental different hydration behaviour of rifts and at mid-oceanic ridges, and plagiogranites and peridotites leads to represent a fundamental mode of crustal an early high-T (≈500°C) hydration of extension. Both continental and oceanic plagiogranitic melt veins and subsequent detachment faults are characterized by focussing of shear zones on the hydrated corrugated, domal topography; rock portions. At these temperatures the exposures of the fault surface extend surrounding peridotite is still stable tens of kilometers in the slip direction, under hydrous conditions. Whole-rock with dips ≤20°. In each system, the oxygen isotope analyses of fault schists faults comprise a network of indicate that the high temperature shear anastomosing fault zones, locally zones are zones of increased fluid flow comprising mylonite, overprinted by a and thus represent the starting point for damage zone of cataclasite, breccia and effective serpentinization upon further gouge one – to a few hundred meters cooling to temperatures below ca. thick, exhibiting a progressive down- 350°C. temperature continuum in deformation. Both form at strain rates ~10−12– Increased fluid flow along melt- 10−14/sec and accommodate asymmetric impregnated shear zones might suggest extension that the composition of nearby issuing hydrothermal vent fluids is strongly Despite these similarities, the two influenced by phase relations in systems exhibit significant differences, hydrated melt veins. However, due to controlled by their environments of their overall small volume and high fluid- formation. Oceanic core complexes form rock mass ratios, their influence on vent in thinner lithosphere, with higher fluid compositions of hydrothermal vent geothermal gradients, dominated by systems is supposedly insignificant. In olivine and feldspar rheology. In contrast, they have a comparatively contrast, deformation associated with large impact on chemical composition continental detachment faults is and phase relations of adjacent controlled by quartz and feldspar ultramafic host: the high chemical rheology. Oceanic detachment faults potential gradient between shear zone- also have a more intimate association hosted fluids and serpentinite host rocks with magmatic accretion. Hydrothermal impresses a metasomatic zoning. An circulation and consequent alteration are important effect is prevention of brucite more pronounced in oceanic detachment formation due to increased silica activity. systems, dominating the low- Apart from that, rare earth elements are temperature fault evolution, and likely strongly enriched at the vein interface. seismicity. Oceanic detachment faults In combination, our multidisciplinary are rolling-hinge type normal faults. In study suggests that evolved melts contrast, some continental detachment intruding into mantle rocks exert a systems were clearly initiated at low hitherto underestimated influence on angle (dips <20°). Detachment faults fluid-rock interactions within detachment cutting oceanic lithosphere are non- faults. conservative; footwalls are much more extensive than the hanging wall of the Oral – Tuesday 11th, 09:00 fault. Beneath continental core Oceanic and Continental complexes lower crustal flow maintains Detachment Fault Systems: How crustal thickness despite significant Similar Are They? extension. Lower crustal flow beneath oceanic core complexes is limited and John, Barbara E.; Cheadle, Michael J. restricted to the environs of the magma chamber. Dept of Geology and Geophysics, Laramie, WY One final contrast between the detachment systems is that mylonitic rocks, and hence crystal-plastic shear

45 zones, are relatively common in peridotites by extreme, localized tectonic continental and rare in oceanic core extension during seafloor spreading over complexes. Following Hirth et al (1998), the past 1.5-2 Ma. The shear zone is we argue that this is a simple about 100 m thick and can be traced for consequence of crustal rheology. In at least 3 km in the tectonic transport continental core complexes the onset of direction. Within this zone, cm-dm thick plasticity of quartz is ~300°C. The domains of mylonitic serpentinized temperature window for mylonite peridotites and metasomatic talc- and/or formation therefore extends many amphibole-chlorite-rich rocks occur hundreds of degrees, and several million together with less deformed peridotites years at cooling rates of ~100°C/m.y. In and metagabbros and reflect strain contrast, oceanic core complexes, which localization and focused fluid flow. Fresh host gabbroic footwalls, have a narrow high-T peridotite mylonites locally temperature window for plastic preserve unaltered olivine relicts, deformation. Typically this window is suggesting an early, high-T history of defined by the solidus of evolved melts the fault zone. The detachment shear (850°C for oxide bearing gabbros), and zone is cut by numerous steeply dipping the brittle plastic transition of 'dry' normal faults and shear zones that plagioclase (750°C). Consequently the predate the development of an aerially timescales for formation of mylonitic extensive sheet of sedimentary breccias shear zone formation are short, due to and carbonates that cap the massif. The the high cooling rates (2000°C/m.y.) late faults localize serpentinization- associated with core complex driven hydrothermal outflow at the Lost denudation. City Vent Field located at the crest of the massif. Structural features reveal a Poster I-13 history of intense noncoaxial shear with Detachment Shear Zone Exposed on strain localization and focused fluid flow the S. Wall of the Atlantis Massif followed by coaxial flattening of the Core Complex, Mid-Atlantic Ridge massif with serpentinization and 30°N volumetric expansion. Debris slide breccias and pelagic limestones are Karson, Jeffrey A.; Kelley, Deborah; interpreted in terms of the progressive Frueh-Green, Gretchen L. rotation of the shear zone from moderately inclined toward the median J.A. Karson, Department of Earth valley axis to its present, broadly Sciences, Syracuse University, Syracuse, arched, subhorizontal form. Active NY; D. Kelley, School of Oceanography, faulting and mass wasting help maintain University of Washington, Seattle, WA; ongoing serpentinization and resulting G.L. Frueh-Green, Department of Earth hydrothermal venting. Sciences, ETH-Zürich, Seattle, SWITZERLAND Poster I-38 Fault-related Oceanic High-resolution bathymetry and near- Serpentinization in the Troodos bottom observations document the Ophiolite, Cyprus: Implications for a subhorizontal detachment shear zone at Fossil Oceanic Core Complex the top of the Atlantis Massif Oceanic Core Complex near the intersection of Katzir, Yaron; Nuriel, Perach; Abelson, the Mid-Atlantic Ridge and the Atlantis Meir; Valley, John W; Matthews, Alan; Transform Fault. Faulting and mass Spicuzza, Michael J; Ayalon, Avner wasting along the transform valley wall expose a cross section of the lower plate Y. Katzir, P. Nuriel, Geological and of the core complex that contrasts with Environmental Sciences, Ben Gurion results from IODP deep crustal drilling University of the Negev, Be'er Sheva, on the crest of the massif about 5 km to ISRAEL; M. Abelson, A. Ayalon, the North. The shear zone is inferred to Geological Survey of Israel, Jerusalem, be responsible for the unroofing of lower ISRAEL; J.W. Valley, M.J. Spicuzza, crustal gabbros and upper mantle Geoscience, University of Wisconsin,

46 Madison, WI; A. Matthews, Earh Kelemen, Peter; Matter, Jürg; Streit, Sciences, The Hebrew University of Lisa; Paukert, Amelia Jerusalem, Jerusalem, ISRAEL Lamont Doherty Earth Observatory, Heavily serpentinized ultramafic rocks Columbia University, Palisades NY 10964 occur adjacent to a major axis-parallel fault, the Amiandos Fault (AF), at a Recent studies have shown that reaction fossil ridge-transform intersection (RTI) of tectonically exposed peridotite with in the Troodos ophiolite (Fig. 1). So far, the atmosphere, ground water, and serpentinization and faulting were not seawater, is actively forming solid considered to be related to the carbonates, both in ophiolites and on the Cretaceous ocean spreading history of seafloor. Despite the fact that this the Troodos ophiolite, but instead were process increases the solid volume, interpreted as associated with late, geochronological data show that emplacement-related tectonics and porosity, permeability and reactive diapirism. Here, petrographic and stable surface area are maintained, or even isotope tracers (δD, δ18O) of water-rock increased, during reaction over tens to interaction are examined in three hundreds of thousands of years. We've profiles across the AF to determine the proposed that this occurs because the spatial distribution, temperature, and volume increase due to olivine the type of water involved in carbonation is large enough to create serpentinization in the Troodos RTI. Two large stresses that, in turn, fracture the distinct serpentinization events were rock. identified: 1) Low-δ18O serpentine (4.6 to 6.6‰) that occurs close to the AF Kinetic studies of olivine carbonation probably formed by fault-localized show that the rate is maximized near hydrothermal (100-200°C) alteration 185°C over a range of CO2 partial initiated by deep infiltration of seawater pressures from surface conditions to during seafloor spreading. 2) This was hundreds of bars. When the reaction is 18 overprinted by pervasive high-δ O catalyzed by fluids with high NaHCO3 (10.6-12.6‰), low-δD (–70 to –86‰) concentration at 185°C and > 70 bars low-temperature hydration and PCO2, the rate is millions of times faster chrysotile veining that may have than the natural process at surface occurred during ophiolite emplacement. conditions. Post-magmatic decrease of δ18O(plagioclase) in non-amphibolitized The combination of natural rates and gabbros in the footwall of the AF kinetic data suggests the possibility of suggests high-temperature, off-axis enhancing natural peridotite carbonation gabbro-water interaction and focused reactions in order to create a significant fluid flow extending to the lower crust sink for anthropogenic CO2 emissions to through the AF zone. The Amiandos the atmosphere. An engineered Fault was thus active during seafloor technique would be to create access for spreading, operating as a detachment CO2-rich fluids, or seawater, to circulate fault in a core complex structure, and through peridotite at about 185°C. We progressively exhuming deeper levels of have considered two methods: (1) use the oceanic lithosphere. This scenario is of nearly-pure CO2 captured from fossil supported by additional observations fuel extraction and from power plants, or such as proximity to RTI and spatial (2) use of surface seawater, with return association with highly-rotated blocks in of CO2 depleted water to the surface. the sheeted dikes. The first is hundreds or thousands of times more efficient, in terms of kg CO2 Oral – Friday 14th, 14:00 converted to solid carbonate per kg Evaluating In Situ Mineral peridotite per second, and provides Carbonation in Near-Seafloor substantial exothermic heat production Peridotite for CO2 Capture and that can maintain a rock volume at the Storage optimal temperature for reaction. However, the second could be much less

47 expensive, because it avoids the costs of this and other planets and in the industrial CO2 capture and transport, present-day biosphere, the tectonic and provides a method for geologic CO2 processes that form oceanic core capture as well as storage. Both complexes, the nature of oceanic crust methods would benefit from, and the the at slow spreading ridges, and the origin second method requires, access to a and fate of gabbroic intrusions emplaced large volume of peridotite that is already into peridotite. close to the optimal temperature for olivine carbonation. Oral – Tuesday 11th, 14:00 Linkages Among Serpentinization Tens of percent of the seafloor at slow and Life in Ultramafic-hosted spreading ridges is thought to be Hydrothermal Systems: The Lost underlain by tectonically exposed City Hydrothermal Field peridotite in oceanic core complexes. Many near-ridge, near-seafloor Kelley, Deborah S.; Brazeton, William peridotites remain at high temperature J.; Proskurowski, Giora; Früh-Green, due to the presence of gabbroic Gretchen L.; Baross, John A.; Lilley, intrusions. Together with many other Marvin D.; Ludwig, Kristin A.; Lang, workers, we propose accelerated study Susan Q. of near seafloor peridotites in this context. What are natural rates of peridotite carbonation on and near the D. S. Kelley, W. J. Brazeton, J. A. seafloor, beneath established Baross, M. D. Lilley, School of hydrothermal vents and in lower Oceanography, Univ. of Washington, temperature settings? What is the Seattle, USA; G. Proskurowski, Woods subsurface evolution of carbonate veins, Hole Oceanographic Institution, Woods fractures, permeability, reactive surface Hole MA, USA; G. L. Früh-Green, Susan area, and fluid flow in peridotites? How Q. Lang, ETH Dept. Earth Sciences, might peridotite carbonation be Zurich, SWITZERLAND; Kristin A. Ludwig enhanced? What would be the least ,Department of Public Affairs, expensive and most effective way to Consortium for Ocean Leadership, transport surface waters to seafloor Washington D.C. peridotites? What can be learned from For over two decades, high-temperature, natural hydrothermal convection, about acidic black smoker systems were inducing and maintaining fluid flow believed to be the norm along the Mid- without substantial pumping? Atlantic Ridge (MAR). However in 2000, While the proposed research outlined the discovery of the Lost City here is catalyzed by the potential for CO2 Hydrothermal Field hosted not in the capture and storage, it is also inspired axial valley, but instead ~15 km west of by fundamental science questions of the MAR, fundamentally changed our great importance, ranging from the views about submarine hydrothermal physical processes of fluid transport to systems and where and how life can global geochemical cycles. For example, exist in the oceans. Extreme attenuation what are the regimes in which increasing of the crust and formation of long-lived solid volume causes reaction-driven detachment faults may be key to cracking, versus the regimes in which formation of present-day Lost City-type porosity, permeability and reactive systems. surface area decrease, greatly limiting reaction progress? What is the global Lost City is characterized by extreme flux of subducting carbon in near- conditions never before seen in the seafloor peridotites, and what is its fate? marine environment: venting of pH 9- Also, there are many synergies between 11, >90°C, metal-poor fluids from research on peridotite carbonation and carbonate edifices that tower 60-m related fields of current interest, above the seafloor. The young, including the processes of aragonite- and brucite-dominated serpentinization, the potential role for chimneys are devoid of metals and are serpentinization in the origin of life on characterized by highly sinuous flow

48 channels with porosity >50%. The high- were recovered from Lost City vent pH fluids are dominated by bicarbonate fluids, perhaps reflecting the presence of and carbonate species. When these a hot, subseafloor biosphere beneath the fluids mix with seawater, carbonate ions field. Bacteria, phylogenetically related 2+ combine with Ca from the vent fluid to to CH4- and S-oxidizers, are mostly form CaCO3. In contrast to nascent found in the oxygenated, outer walls of chimneys, old deposits are dominated by chimneys where fluid chemistry is calcite and have porosity values < 5%. substantially different than the chimney Progressive bathing of the inactive interiors. Results from pyrosequencing structures in seawater results in show that the archaeal and bacterial incorporation of Co, Cr, V, Ni, Sr and U biofilm communities underwent dramatic into the structures. U/Th dating of active changes as environmental conditions in and extinct carbonate structures shows the chimneys changed over a 1,200-year that hydrothermal activity has operated period. for >120,000 years. Long-lived flow is likely facilitated by seismic activity Considering the global distribution of associated with the Atlantis Transform ultramafic environments and the Fault, volume expansion and fluid over- potential importance of these systems to pressuring within the serpentinite the origin of life and to models of Earths’ basement rocks, and an abundant earliest microbial ecosystems, study of supply of fresh mantle material within Lost City has the potential to yield new the south face of the Atlantis Massif. discoveries with implications for understanding the linkages between Moderate to low-temperature (<150- abiotic water-rock reactions and 200°C) fluid-rock reactions in the microbial evolution. underlying ultramafic rocks result in alkaline fluids with high concentrations Poster II-45 of abiogenically produced H2 (up to 15 CO2-metasomatism and the mmol/kg), CH4 (1-2 mmol/kg), and Formation of Talc in Oceanic other low molecular weight Detachment Faults hydrocarbons that support novel microbial communities. The Lost City Klein, Frieder fluids contain higher concentrations of formate and acetate than any other non- Woods Hole Oceanographic Inst., Woods sedimented hydrothermal system Hole, MA studied to date. While formate is likely abiogenic, acetate concentrations Dynamic recrystallization of serpentinite strongly correlate with H2 across the to talc schist is a prominent feature of field, indicating that it likely results from oceanic detachment faults and crucial for microbial activity. The anoxic, warm localizing shear stress; however, (>80°C) interior zones of the chimneys conditions and processes during talc harbor biofilms containing 106-109 cells formation remain poorly constrained. To g/CaCO3: >80% of these cells belong to transform magnesian serpentine a single phylotype of archaea called Lost (Mg/Si=1.5) to talc (Mg/Si=0.75) City Methanosarcinales (LCMS). massive removal of Mg or addition of Si Surprisingly, simultaneous is required. As the solubility of Mg is methanogenesis and anaerobic CH4- very low in fluids with pH>3, oxidation may be occurring in the LCMS metasomatic enrichment of silica biofilms. Inactive carbonate deposits, or transported away from gabbroic chimneys that are only weekly venting, intrusions appears to be a plausible contain anaerobic methanotrophic explanation. Based on petrographic organisms (ANME-1), providing strong observations and phase petrological support that ecological succession has examinations an additional, equally occurred within the chimneys. In plausible process to form talc in oceanic contrast to the carbonate-hosted detachment faults is explored. Talc- communities, Thermococcus species magnesite veins and schists crosscutting affiliated with black smoker systems serpentinite on land are commonly

49 attributed to the ingress of CO2-rich Hansen, L.D., et al., 2005. Canadian fluids, either related to magmatic Mineralogist, 43: 225-239. degassing or decarbonation of Proskurowski, G. et al., 2008. Science, sediments. The simplified succession of 319: 604-607. Seewald, J.S., et al., P.J., mineral assemblages during progressive 2003. Earth Planet. Sci. Lett., 216: 575- carbonation of serpentinite is as follows: 590. Shipboard Scientific Party, 2006. (1) serpentine brucite, (2) serpentine Proc. IODP, magnesite, (3) magnesite talc, and 10.2204/iodp.proc.304305.103.2006. finally (4) magnesite quartz (e.g., Hansen et al., 2005). Talc-magnesite Poster II-74 altered serpentinite, recently described Evolution of Magmatism During the in rocks from the Atlantis Massif (IODP Development of a Continental Leg 304 Hole 1309B, Shipboard Metamorphic Core Complex Scientific Party, 2006), suggests that CO2-metasomatism is taking place in Konstantinou. Alexandros; Strickland, oceanic serpentinites as well. Here, Ariel A. thermodynamic calculations are used to explore carbonation of oceanic Geological and Environmental Sciences, serpentinite in relation to fluid Stanford University, Stanford, CA composition and temperature. Vent fluids emanating from high-T hybrid The Basin and Range Province (BRP) of ultramafic/mafic-hosted hydrothermal the Western U.S. is a classic example of systems (e.g., Logatchev, Rainbow) are a broad, active, continental rift where strongly enriched in CO2 (10 – 16 structures related to differing amounts mmol/kg) compared with seawater of extension can easily be studied for (Charlou et al., 2002), though at T ≥ holistic models of the progressive 350°C phase relations indicate that CO2 extension of the brittle crust. concentrations will not suffice to drive Metamorphic core complexes represent carbonation. Magmatic degassing during windows into the processes that occur crystallization of a gabbroic intrusion near or beneath the DBTZ, and are may foster carbonation even at very exhumed to the surface by the process high temperatures, as CO2 of extension. Plutonic and volcanic rocks concentrations are expected to increase are often exposed within and around dramatically (cf. Seewald et al., 2003). these complexes and are intimately In vicinity of Hole 1309B, the Lost City linked to the extensional process, but hydrothermal field emanates low-T, the exact relationship is highly debated. CO2-poor (< 26 µmol/kg) fluids This study focuses on the Cenozoic (Proskurowski et al., 2008). As magmatic history of the Albion - Raft minimum CO2 concentrations necessary River - Grouse Creek Metamorphic Core for mineral carbonation decrease Complex (ARG) and its relation to markedly with decreasing T, at T≤100°C extensional faulting. The Raft River CO2 concentrations in a µmolal range detachment fault is an impressive would be sufficient to destabilize brucite feature, with exposures of rocks which in favor of magnesite. However, low underwent ductile deformation CO2/3He ratios at Lost City indicate that extending for ~35 km in the transport mantle CO2 has been removed before direction of the detachment. Upper venting; assuming the predicted, much plate faults cut a thick higher CO2 concentrations of up to 4.1 stratigraphic/structural section and mmol/kg (Proskurowski et al., 2008), merge with this fault at depth. The map phase relations indicate a relations are such that together with thermodynamic drive for talc-magnesite geophysical data, the exposures of formation at T≤ 175°C. In combination, ductile crust can be projected beneath there may be a synergistic effect of Si- supracrustal sequences for a more and CO2-metasomatism on weakening accurate understanding of the large- oceanic detachment faults from high to scale geometry of the detachment low T. Charlou, J.-L., et al., 2002. system. Early magmatism in the Chemical Geology, 191: 345-359. Grouse Creek Mountains is represented

50 by plutons and possibly volcanic rocks Indian Ridge and St. Paul’s Rocks that range in age from 41 to 35 Ma (equatorial Mid Atlantic Ridge) suggests (Egger et al., 2003; Brooks et al., that noble gas abundances are strongly 1995). This was followed by slightly influenced by mantle deformation (Kurz, younger magmatism, composed of Warren, and Curtice, Chem. Geol. granitic plutons that range in age 266(2009)10-18). In both sample between 30.4 and 25.3 Ma (Egger et al., suites, the most deformed rocks 2003; Strickland et al., in press). These (mylonites and ultramylonites) have the plutons represent large degrees of highest helium abundances when crustal melting and increasing T’s of the compared with related undeformed crust through time, but mostly stalled at samples. The relationship between mid-crustal depths without erupting and texture and helium abundance in predated the onset of actual Basin and peridotites suggests that metamorphism Range extension. The latest magmatic is an unrecognized but important control event is composed of bimodal lavas - on noble gas distribution in the mantle high temperature rhyolitic vitropheres and crust. The helium and neon isotopic and basalts- that are late Miocene in age compositions of the peridotites are (9.1 to 7.7 Ma), and up to 800 m thick, primarily mantle-derived, which shows the Jim Sage volcanic suite. These lavas that the gases were incorporated at were erupted broadly during slip on the depth in the mantle, and are not Raft River detachment which, based on strongly influenced by atmospheric thermochronology, was active during the gases. The St. Paul’s Rocks mylonites Middle to Late Miocene (14 – 8.5 Ma) have helium contents equivalent to gas- (Egger et al., 2003; Wells et al., 2000). rich MORB glasses (6 x 10-6 to 3.8 x 10- The basin that originally formed by this 5 cc STP He/gram), placing them among fault system filled rapidly(?) with ~2000 the most gas-rich mantle-derived rocks. m of Cenozoic sediments that are cut He/Ne ratios in St. Paul’s Rocks also and displaced by younger, rotational vary widely (~ 20x) with deformation normal faults, and strata now dip about and mineralogy, with the highest He/Ne 40 degrees to the west. The sedimentary ratios (and helium concentration) found section fines upward from a basal in the finest grained ultramylonite conglomerate to lacustrine deposits that peridotite. Based on the He/Ne ratios, record unroofing of footwall rocks and we infer that the most likely gas near its top is interleaved with the late enrichment mechanism is diffusive Miocene Jim Sage volcanic suite that trapping within defects at pressure in dips more gently (10-15 degrees) to the the mantle. This requires that west and is cut only by small deformation takes place under gas-rich displacement normal faults. conditions, and would have important implications for gas distribution in the Poster I-7 mantle and crust, gas permeability of Noble Gases as Tracers of Mantle fault zones, and the preservation of Deformation ambient gas characteristics by deformation. Additional studies in Kurz, Mark D.; Warren, Jessica M. different tectonic settings are presently under way to test the relationship M.D. Kurz, Marine Chemistry and between noble gases and deformation, Geochemistry, WHOI, Woods Hole, MA; and expore the use of noble gases as J.M. Warren, Department of Terrestrial tracers of deformation. Magnetism, Carnegie Institution, Washington, DC Poster II-77 Fossil Bivalves in the Rainbow Area: Due to their inertness and diverse New Insight into the Diversity and isotopic compositions, the noble gases Evolution of Chemosynthetic are extraordinary tracers of mantle Communities sources, as well as diffusion and degassing processes. A recent study of Lartaud, Franck; Little, Crispin T.S.; de ultramafic rocks from the Southwest Rafelis, Marc; Bayon, Germain;

51 Ildefonse, Benoit; Dyment, Jerôme; Le Bathymodiolus azoricus are abundant Bris, Nadine and co-occur with a few specimens of Phreagena and Thyasira. This diversity L., Franck, Le Bris, N., Lab. of bivalves is not seen in the living vent Ecogéochimie des Environnements community at Rainbow. Benthiques (LECOB), FRE 3350, Observatoire Océanologique Banyuls sur The isotopic signatures of bivalve shells Mer, UPMC Paris 06, Banyuls sur Mer, from both DR 9 and DR 11 suggest FRANCE; Little, C.T.S., School of Earth influence of oxidized methane on the and Environment, University of Leeds, sediment pore water DIC, which is Leeds, UNITED KINGDOM; de Rafelis, consistent on Oceanic Core Complexes M., Lab. Biominéralisations et environments. There is a contrast Environnements sédimentaires, ISTeP, between Phreagena shells, which are UMR 7193, UPMC Paris 06, Paris, enriched in 13C, and Thyasira shells, FRANCE; Bayon, G., Départemet de which are highly depleted in 13C Géosciences Marines, IFREMER, suggesting different mechanisms of Plouzané, FRANCE; Ildefonse, B., carbon fixation for these bivalves. At off- Géosciences Montpellier, UMR 5243, axis sites like DR 9 and DR 11, both Université Montpellier 2, Montpellier, sediment cover and the availability of FRANCE; Dyment, J., Lab. Géosciences methane and sulphide, and possibly Marines, UMR 7154, Institut de Physique even hydrogen, as electron donors for du Globe de Paris, Paris, FRANCE autotrophic symbionts offer a wide variety of suitable habitats for Hydrothermal circulation at ultramafic- chemosynthetic species. The spatial hosted sites supports a large variety of distribution and geochemical diversity of high- and low-temperature hydrothermal serpentine-hosted habitats might have vents and associated ecosystems. Along favoured a more diverse fauna to the Mid-Atlantic Ridge (MAR), different colonize these habitats, and could have types of habitats for chemosynthetic- played a major role in the ability of based organisms have been identified in chemosynthetic vent and seeps species a serpentinization context, e.g. the high to disperse over ocean basin scales. temperature vents at Rainbow and Logatchev, and the low temperature, Poster II-76 off-axis Lost City vents. Each displays a Rifting Mechanisms, Mantle certain degree of isolation and endemic Exhumation and the Initiation of taxa. Much remains to be understood Seafloor Spreading: Evidence from about the temporal dynamics and the Deep Galicia Basin biogeography of these communities over geological time scales. During the Lasseigne, Amy; Sawyer, Dale MOMARDREAM_08 cruise (August 2008), numerous dead bivalve shells and Earth Science, Rice University, Houston, associated carbonates were dredged TX from close to the active Rainbow vent field (36°N). These fossils point to past Our study focuses on Iberia’s Deep hydrothermal activity on top of a heavily Galicia Basin (DGB) which formed by sedimented ultramafic structure, 2.6 km East-West rifting of Iberia from east of the Rainbow field (site DR 9) at Newfoundland. The DGB is a magma- 24 ± 0.2 kyr, and on the slope of the starved, fault-bounded block rifted same structure, 1.2 km north-east to margin basin. Because it is free from Rainbow field (site DR 11) at 192 ± 12 significant volcanic and salt deposits and kyr. At the younger site abundant shells has been relatively sediment starved of the vesicomyid bivalve genus over its history, the DGB is an ideal Phreagena, previously unknown from the place to study rifting processes with MAR, are distributed over a large area both seismic and drilling technologies. and associated with rarer specimens of the thyasirid bivalve Thyasira. At the Several MCS seismic cruises, ODP and older site, specimens of the mussel IODP drilling legs, and geophysical and

52 submersible studies have developed a beneath S. These reflections below S rich database along Iberia and its could be interpreted as seaward dipping conjugate margin. Within the DGB, the reflectors, but there are two problems crustal fault blocks and sediment are with that interpretation: 1) their widely interpreted to overlie a prominent apparent northward dip is not in seismic reflection called S. The basin’s agreement with the expected east-west continental crust here is unusually thin, extension, and 2) these reflections lie in often less than 2 km thick over a wide material that is most commonly area. Most interpretations make S a interpreted as serpentinized upper boundary separating the faulted and continental mantle. With further rotated continental crust from examination we hope to describe these serpentinized upper mantle; and often, sub-S reflections with length, true dip, S is interpreted as the seismic and S-offset; map out S’s geometry in expression of a major detachment fault. space; quantify extension in both Given that, the extent and role of dimensions; and compare findings to apparent low-angle faulting on this existing extension models for the Deep margin are still not fully understood. Galicia Basin. Proposed models for extension in the DGB vary from true low-angle Poster I-29 detachment faulting, to rolling-hinge From Mantle to Microbes: The models, to polyphase faulting, or large- Cycling of Volatiles in Ridge scale mass wasting. However, the Environments current literature is based largely on the interpretation of east-west, dip oriented Lilley, Marvin D.; Frueh-Green, seismic profiles, and the complexity of Gretchen L. this problem argues for a more 3D approach. M.D. Lilley, University of Washington, Seattle, WA; G.L. Frueh-Green, ETH- In this study, we utilize a grid of 2D Zurich, Zurich, SWITZERLAND seismic strike and dip lines over the well-developed S reflector area in order There are fundamental differences in the to examine the along-strike variation of nature of the volatiles produced in the margin. The profiles are processed basaltic and ultramafic-hosted using Prestack Kirchhoff Depth Migration hydrothermal systems. Olivine-rich and a layer stripping approach with the gabbros and mantle peridotites are hope of extracting true geometries and significant components of the seafloor in dips. Focusing on two North-South slow- and ultraslow-spreading ridge profiles overlying S, we have already environments. The alteration of these made a few surprising observations. For rocks has fundamental geophysical and one, the geometries of some crustal biogeochemical consequences for the fault blocks seem strangely “pod- global marine system and is critical in shaped” when looking at individual strike cycling reduced volatiles from the lines. The fault blocks therefore do not mantle to the biosphere. Thus, it is seem elongated parallel to the margin important to understand the extent and but highly broken up in both dimensions. conditions of alteration (e.g., Also, the amount of pre-rift sediment temperature, fluid flux, oxidation states) making up the stretched continental and the impact of these processes for crust is greater than expected, often the formation and speciation of C-O-H-S making up the entire thickness of crust fluids as nutrients for biological activity. and lying in direct contact with S. S itself Emphasis will be placed on how fluid has a more irregular geometry than chemistries in peridotite-hosted previously realized, having large steps hydrothermal systems (Rainbow, which often correlate to the edges of the Logatchev, Lost City and Saldanha along crustal rotated fault blocks. Other the MAR) differ from those in black smaller steps in S appear unrelated to smoker systems, and on the role of the crust above, but rather associate serpentinization in generating highly with a series of north-dipping reflections reduced, volatile-rich hydrothermal

53 fluids and in the biological communities Poster II-47 they may support. Serpentinization Godzilla Mullion: Plagioclase produces elevated concentrations of Systematics of a Back-arc Core primarily abiogenic methane (up to 2.5 Complex mmol/kg) and hydrogen (up to 16 mmol/kg) that have important Loocke, Matthew P.; Snow, Jonathan consequences for vent microbial E.; Ohara, Yasuhiko communities and for the existence of a deep H2-based biosphere. Varying fluid M.P. Loocke, J.E. Snow, Earth and fluxes influence redox conditions and the Atmospheric Sciences, University of stabilities of oxides, sulfides, and FeNi Houston, Katy, TX; Y. Ohara, alloys, which together with microbial Hydrographic and Oceanographic activity, can significantly affect C-H-S Department of Japan, Tokyo, JAPAN cycles in these systems. In addition, recent experimental studies and vent Godzilla Mullion is the largest known fluid data provide evidence for abiotic example of a large low-angle oceanic synthesis of organic acids and other detachment fault, or Oceanic Core organic compounds through Fischer- Complex (OCC). It is located in the back Tropsch type reactions during arc of the Izu-Bonin-Mariana (IBM) serpentinization and provide constraints system, at the southern tip of the Parece on alternative pathways for the Vela Rift in the Philippine Sea and covers formation of early membranes and the an area measuring roughly 125km by origin of life. At moderate temperatures, 55km. A total of 11 academic such as at Lost City, serpentinization expeditions have been conducted in the produces high pH fluids that promote area, including two recent dredging carbonate precipitation and produce cruises and a submersible dive cruise on large, porous hydrothermal structures. the megamullion itself. Peridotites The fluids in this system contain high recovered from the mullion are divided concentrations of H2 and CH4 but very petrographically into Fertile, Depleted, low concentrations of CO2.This and Plagioclase-bearing groups (1). production of reduced volatiles and Plagioclase rimming spinel (or rather its variable mixing with ambient seawater low temperature pseudomorphs) is a in the subsurface and in near-vent strong indicator of melt stagnation in the environments is an important process in lithosphere (2). For this reason, the creating strong chemical gradients that extant thin section collection was provide micro-niches for distinct surveyed to determine the prevalence communities of H-, S- and CH4-utilizing and geographic distribution of melt archaea and bacteria which also results impregnation on Godzilla Mullion. A total in high organic carbon contents (up to 1 of 178 thin sections were studied from wt%) in the hydrothermal structures. the Kairei KR 03-01, Hakuho Maru KH 07-09 and Yokosuka YK 09-05 cruises. In contrast, basalt-hosted hydrothermal In these, 45.56% of all peridotites were fluids typically contain much higher found to be plagioclase bearing. This concentrations of CO2 and relatively compares with the worldwide abyssal lower concentrations of H2 and CH4. The peridotite average of ~20%(2). The exception to this generalization is that mullion is divided up into three regions, during (and for a short period after) the proximal region ( closest to eruptions, basaltic-hosted fluids can termination), the medial region, and the exhibit very high H2 (up to 40 mmol/kg) distal region ( furthest from the concentrations, higher than any yet seen termination)(3).Observations by region in ultra-mafic systems. Volatile provide that 53.45% ( 62 out of 116 concentrations and isotopic signatures samples) in the proximal region (15 from several basalt hosted mid-ocean dredges), 11.76% ( 2 out of 17 ridge environments will be compared samples) in the medial mullion( 3 and contrasted with those from the dredges), and 25% (7 out of 28) in the ultra-mafic hosted systems mentioned distal mullion ( 5 dredges) show of above. evidence of plagioclase impregnation.

54 Unpublished major element Europe (Schmid et al, 2004). Here we compositions of spinels from the report recent field research on the samples studied support these plag- ophiolite suite located between systematics. The distal, depleted portion Montgenèvre (SE France) and Serra del of the mullion represents a robust Parco (NW Italy). mantle section that was still producing abundant melt and can be compared to The Montgenèvre-Serra del Parco typical oceanic spreading with its ophiolite originated as an oceanic crust relatively “normal” percentage of plag fragment of Piemont-Ligurian ocean (a impregnation. The medial, fertile portion segment of the Mesozoic Tethys ocean). of the mullion represents a steep falloff This ophiolite suite was exposed to in melt productivity, to a minimum of surface before the Alpine orogeny or about 5%(4). The proximal, heavily during its earliest stage, as testified by plag-impregnated portion of the mullion fission-track ages (Schwartz et al, 2007) represents an increasing stagnation of on zircon (118±1 Ma) and on apatite melt into a lithosphere that was (67±9 Ma). Unlike most other ophiolites progressively thickening. This coincides in the Alps, Montgenèvre-Serra del Parco with a change in character of the massif rocks didn't undergo strong ductile surface in the proximal region from a deformation and metamorphism during domed striated mullion surface to a the Tertiary Alpine orogeny. This massive, slab-type exhumation more ophiolite comprises serpentinized typical of ultraslow spreading ridges. peridotite, gabbro and pillow lavas, and (1) Ohara, et al., (2003) G3. 4 (7), was intruded by diorite and albitite 8611, 10.1029/2002GC000469. veins/dikes. This rock assemblage is not (2) Dick (1989) Geol Soc. Lond. Spec. consistent with the typical mid-ocean Pub. 42:71-105. ridge extension model, since neither a (3) Ohara, et al., (2009), Eos Trans. continuous and thick basaltic layer nor a AGU, 90(52), Fall Meet. Suppl. true sheeted-dike complex ever existed Abst.Num. T33D-06 (Lemoine et al, 1987). This phenomenon (4) Hellebrand, et al., 2001 Nature has been attributed to the asymmetric development of a dipping detachment Poster I-33 fault which resulted in mantle Petrogenesis of Piemont-Ligurian denudation. Oceanic Plagiogranites (Albitites), Western Alps The albitite is composed of albite (90%), hornblende (tremolite - actinolite), Ma, Changqian; Guillot, François; minor quartz, and accessory minerals Chen, Ling including allanite, apatite and zircon. Petrogenesis of albitites is highly C. Ma, L. Chen, State Key Laboratory of controversial. It is suggested that these GPMR, China University of Geosciences, albitites represent partial melts of Wuhan, Hubei, CHINA; C. Ma, F. Guillot, gabbros and diorites during oceanic Géosystèmes FRE 3298, Université Lille crust detachment based on the following 1, CNRS-Lille 1, SN5, F-59655 facts: (1) compared with the extensive Villeneuve d'Ascq Cedex, Lille, FRANCE ductile deformation of serpentinized peridotite, gabbro and diorite, the The European Alps recorded the closure albitite has typical fine-grained history of several ocean basins during magmatic texture preserved from ductile the convergence between African Plate deformation; (2) the albitite veins and European Plate and it is classically always cut through mylonitized gabbros; proposed that the Alps are the products (3) the zircon U-Pb age (148±2Ma) of of two orogenies, a Cretaceous one the albitites is younger than the followed by a Tertiary one. The former is crystallization ages of diorites (zircon U- related to the closure of an embayment Pb age, 156±3Ma) and gabbros intruded of the Meliata ocean into Apulia, while into the mantle peridotites (whole rock the latter is due to the closure of the Sm-Nd isochron age, 198±22Ma)( Costa Alpine Tethys between Apulia and and Caby, 2001); (4) According to the

55 Al/(Mg+Fe) vs. Ca/(Mg+Fe) diagram (Roberts et al., 1993, EPSL 116, 101- (Altherr et al, 2000), the albitite has 112). Within the mantle section similar chemical features with products stratigraphically beneath the mantle- of partial melting of mafic rocks. These crust transition is a major mylonite data indicate that the albititic melt is shear zone that displays the same formed after the emplacement of the orientation and kinematics as the ophiolite suite. It is suggested that we smaller-scale structures above, and is should be prudent when employing the hence (accounting for the tilting of the age of oceanic plagiogranite or albitite nappe) interpreted as a ridge-parallel as ophiolite emplacement age or oceanic structure with normal displacement. crust age. This, the Carrick Luz shear zone, is a 200m-wide zone of intensively and This study is financially supported by the pervasively deformed gabbro mylonite Key International S &T Cooperation that forms a dyke-like body within project of China (2007DFA21230), China undeformed mantle peridotite. Xenoliths Geological Survey(Grant of peridotite within the gabbro mylonite 1212010918002) and the Nature demonstrate that the gabbro is a mafic Science Foundation of Hubei Province intrusion into the peridotite. Within the (2009CDA004). Nicolas Tribovillard, shear zone a continuum of fault rocks while he was leader of the Géosystèmes from discrete ultramylonite shear planes Research team, has funded our field through to cataclasites and fault gouges, travels. all with the same kinematics, attests to progressive exhumation on the Poster II-55 structure. Chilled MORB dykes cross-cut Detachment Faulting and Melt the shear zone and demonstrate that the Transport through the Mantle deformation occurred on the ocean floor. Lithosphere: Evidence from the Following Allerton & MacLeod (1998, Lizard ophiolite, SW England Geol. Soc. London Spec. Publ. 148, 29- 42), I interpret the Carrick Luz shear MacLeod, Christopher C. J. zone as a plate boundary-scale, ridge- parallel detachment fault within the School Earth & Ocean Sciences, Cardiff mantle lithosphere beneath the Lizard University, Cardiff, UNITED KINGDOM spreading axis, into which ascending mafic melt was intruded and channelled, The Lizard Complex is a dismembered and onto which deformation was thereby Devonian ophiolite that forms one of a efficiently localised. Capture and series of thrust nappes within the channelling of melt by detachment Variscan belt of SW England. Within the faults, as inferred here, provides a largest contiguous thrust sheet it mechanism for efficiently weakening the preserves mantle peridotites, a ‘Moho’ lithosphere beneath the ridge axis, and transition into a gabbroic sequence and demonstrates how detachment faults thence the base of a sheeted dyke may control melt transport through the swarm with N-MORB composition. The mantle lithosphere. Lizard Complex is thought to have formed in a small incipient ocean basin, probably close to a magma-poor ocean- Poster II-58 continent transition, broadly analogous Life Cycle of Oceanic Core to the tectonic setting inferred for the Complexes, 2: Melt Emplacement Mesozoic Alpine Tethyan ophiolites. and Detachment Fault Termination Metre-scale mylonitic shear zones within Mechanisms gabbros in the mantle-crust transition zone are cut by chilled MORB dykes, MacLeod, Christopher C. J.; Searle, documenting exhumation and uplift on Roger; Murton, Bramley the seafloor. Cross-cutting relations between dyke sets are consistent with C.J. MacLeod, School Earth & Ocean rotation on ridge-parallel structures with Sciences, Main Building, Cardiff the same kinematics as the mylonites University, Cardiff, UNITED KINGDOM;

56 B. Murton, National Oceanography the footwall of the OCC detachment. We Centre, Southampton, UNITED propose that, at this stage in the KINGDOM; R. Searle, Earth Sciences, evolution of an OCC, the mechanism of Durham University, Durham, UNITED magma emplacement into the KINGDOM lithosphere must change fundamentally. From this time onwards variations in the By means of a near-bottom sidescan flux of melt delivered from the mantle sonar, bathymetry and sampling study become decoupled from the magmatic of active oceanic core complexes (OCCs) component of seafloor spreading, and on the Mid-Atlantic Ridge in the 13°N melt addition instead contributes to the region we examine the controls on velocity of the plate on the OCC flank. formation, evolution and subsequent This ‘footwall capture’ mechanism of termination of large-offset detachment melt emplacement can explain the faults at slow-spreading mid-ocean common observation of gabbro plutons ridges. In this contribution we focus on in exposed OCC detachment footwalls. the role of melt supply and mechanisms An OCC detachment is ultimately of melt emplacement in controlling how terminated when the flux of melt OCCs are terminated. We have delivered to the footwall is sufficient to quantified the relative contribution of overwhelm it. This may not necessarily tectonic and magmatic spreading by be related directly to a temporal systematically measuring heaves on increase in mantle melt supply, but to faults as far as magnetic anomaly C2n, the spatial and temporal focusing of and show that OCC formation is magma within the footwall. OCCs in the triggered by a reduction in magma 13°N region are in the process of being supply relative to the symmetrically- terminated today by the along-strike spreading inter-OCC regions. Whereas propagation of the neovolcanic zones the inter-OCC regions have robust from the inter-OCC regions, neovolcanic zones, volcanism is demonstrating that lateral redistribution suppressed or absent on the hanging of melt along axis, presumably by walls of the active OCCs. This suggests dyking, is an efficient mechanism for that plate separation becomes focused terminating detachments. onto the active detachment faults, an assertion supported by dating and/or Poster II-56 magnetic studies of OCCs elsewhere, Quantitative Constraint on Tectonic which show that the extension Rotations at Oceanic Core accommodated by the OCC detachments Complexes From a Structural and is consistently in excess of half the total Palaeomagnetic Study of Oriented plate separation. Under such Rock Cores From the Mid-Atlantic circumstances the detachment must Ridge at 15°45’N lengthen with time and the active (sub- surface) portion of the fault must MacLeod, Christopher C. J.; Carlut, therefore migrate progressively towards Julie; Escartin, Javier; Horen, Hélène; the axial valley and, ultimately (at least Morris, Antony at depth) across it, and past the adjacent neovolcanic zones. This C.J. MacLeod, School Earth & Ocean necessary geometrical evolution has Sciences, Cardiff University, Cardiff, significant implications for the nature of UNITED KINGDOM; J. Carlut, Laboratoire melt emplacement into the lithosphere. de Géologie, Ecole Normale Supérieure In the near-symmetrically spreading de Paris, Paris, FRANCE; J. Escartin, regions new melt is emplaced into the Laboratoire de Géosciences Marines, hanging walls of the valley-wall faults, in Institute de Physique du Globe de Paris, the centre of the axial valley. In Paris, FRANCE; H. Horen, Département contrast, once an active detachment de Géologie, Université de Picardie Jules migrates across the axial valley at Verne, Amiens, FRANCE; A. Morris, depth, melt delivered to the axis here School of Geography, Earth and (either laterally or from below) will Environmental Sciences, University of instead be preferentially intruded into Plymouth, Plymouth, UNITED KINGDOM

57

The sub-surface geometry of Mallows, Christopher; Searle, Roger detachment faults at slow-spreading mid-ocean ridges is debated: are they Earth Sciences, Durham University, planar features that form and slip at low Durham, UNITED KINGDOM angles, as often inferred for their continental equivalents, or do they We present the results of a combined initiate at steep angles and then flatten deep-towed sidescan sonar and in response to flexural unloading as magnetic survey with shipboard gravity displacement proceeds, as predicted in and extensive seafloor sampling across ‘rolling hinge’ conceptual models? An active Oceanic Core Complexes on the essential difference is that significant Mid-Atlantic Ridge between 13N and rotation of the footwall should occur in 14N. Sidescan sonar data have been the rolling hinge but not the planar fault mapped to reveal variability in the width model. This can be tested using of the neo-volcanic zone (NVZ), which is palaeomagnetism. Previous attempts to entirely absent opposite active OCC address this question have relied upon formation. Furthermore, the youngest interpretation of azimuthally unoriented volcanic material (identified from drillcores, from which only the sidescan and magnetic data) exists in inclinations of remanences can be close proximity to the axial valley-wall considered. Anomalous inclinations have faults, suggesting these faults may been put forward as evidence for capture upwelling melt at depth and substantial rotations about horizontal transport it away from the ridge axis. In ridge-parallel axes. Although plausible, some instances there is a mismatch these interpretations are very non- between the location of the NVZ and unique: a rotation about an axis of any central magnetic anomaly high, which orientation can generate the inclination can be explained by temporal variations changes observed, and most of these in the locus of melt emplacement. solutions require only small rotation Geomagnetic polarity transition widths magnitudes. We here present show this zone of emplacement to vary palaeomagnetic and structural data from along-axis between 2 and 5 km wide. 3D a unique set of azimuthally oriented modelling of the gravitational residual cores, collected using the 'BRIDGE' mantle Bouguer anomaly (RMBA) seabed rock drill, from the 15°45’N indicates that OCCs are associated with oceanic core complex on the Mid-Atlantic high density (~2900 kg.m3) material at Ridge. From these we quantitatively shallow depth. The overall RMBA constrain the permissible axes and structure is remarkably similar to that magnitudes of rotation of the which is observed at Kane Massif and detachment fault footwall without a Fuji Dome. In addition, circular lows are priori assumption about the orientation observed within the RMBA along the of the axis. We show that significant ridge axis, suggesting discrete centres of rotations (64° ±16°) have indeed melting. On the flanks conjugate to occurred, about a sub-horizontal, near active OCCs, RMBA highs are observed ridge-parallel axis, consistent with the which show that significant reductions in previous inferences, and robustly melt emplacement are recorded on both supporting the rolling hinge models. We plates. further discuss the geological implications of this result for the Oral – Tuesday 11th, 09:45 structural, magmatic and thermal history The Role of Detachment Faults of the 15°45'N massif and inferences for During Crustal Thinning, Mantle oceanic core complexes in general. Exhumation and Continental Break- up at Magma-poor Rifted Margins? Poster II-59 Life Cycle of Oceanic Core Manatschal, Gianreto; Lavier, Luc Complexes, 3: Crustal Structure and Melt Supply Variability on the Mid- G. Manatschal, UdS-CNRS, IPGS-EOST, Atlantic Ridge at 13N Strasbourg, FRANCE; L. Lavier, UTIG,

58 Jackson School of Geosciences, Austin, presence of fluids and magma and other TX factors. How these factors control the strain distribution and how they evolve Extensional detachment faults are widely in time and space during rifting is, regarded as playing an important role in however, not yet understood and explaining extreme crustal thinning, remains the subject of ongoing research. mantle exhumation and continental break-up at magma-poor rifted margins. Oral – Tuesday 11th, 11:00 However, how important are these Hydrothermal Systems and structures, when and how do they form, Detachment Faulting how well can they account for geological and geophysical observations made McCaig, Andrew along deep-water rifted margins, and how can they be compared with Institute of Geophysics and Tectonics, continental and oceanic core complexes? School of Earth and Environment, Mapping of rift-related detachment faults University of Leeds, LS2 9JT, UNITED at the present-day Iberia-Newfoundland KINGDOM and the ancient Western Pyrenees-Bay of Biscay and Alpine Tethys rifted It has long been recognised that black margins shows that these structures are smoker hydrothermal systems on slow complex and can not be described by spreading ridges are larger, longer lived, one single low-angle detachment fault as and have higher heat outputs than those commonly shown in the classical on fast spreading ridges. A number of Wernicke type simple shear models. active hydrothermal systems in the Detachment faults form in brittle layers Atlantic occur several km away from the and are interacting with decollements in neovolcanic zone (NVZ) and have been ductile layers in the crust. Only when the linked to detachment faulting. These crust is thinned to less than 10 km and hydrothermal systems challenge the whole crust is brittle, detachment classical assumptions about the depth of faults can cut from the surface into hydrothermal circulation, the mantle. Fluids are intimately linked with permeability structure of the crust and detachment faulting and are responsible upper mantle, and the causal for the breakdown and hydration of relationships between magmatism, feldspar and olivine resulting in weak tectonics and hydrothermal circulation. fault zones that can localize deformation Detachment-related hydrothermal and modify the bulk rheology of the systems can be divided into three types: extending brittle layers. Although (1) Black smoker systems located on the detachment faults occur in the crust, hangingwall of detachments (eg. TAG); they seem to be more prominent in the (2) Black smoker systems located on ocean continent transition of magma- exposed detachment fault surfaces poor rifted margins, where they can (Logatchev, Ashadze, Rainbow); (3) Low explain the occurrence of subcontinental temperature systems located on mantle over tens to hundreds of exposed detachment fault surfaces (Lost kilometres separating continental crust City). from oceanic crust. In conclusion, TAG is located in basaltic rocks about extensional detachment faults observed 4.5 km east of the neovolcanic zone. On at rifted margins are by far more the basis of microseismicity, the vent complex as previously proposed in the field is inferred to be underlain by a existing models. Detachment faults are west-dipping convex-upward polyphase structures that occur in the detachment fault which steepens to 70° highly extended and hydrated parts of beneath the NVZ. Fluid is inferred to be the brittle crust and upper mantle. The channelled up the fault from gabbroic formation of detachment faults seems to intrusions at 7 kmbsf. TAG has an be strongly linked with the bulk estimated heat output of 1000 MW, rheological evolution of the extending which is too large to be easily lithosphere that depends on the initial maintained in steady state, and there is composition, thermal structure, geochronological evidence for episodic

59 behaviour over more than 100kyrs. Fluid ductile transition (700-750 °C) will occur flow up detachments is supported by in the upper 1-2 km of the footwall, studies of exposed (inactive) suppressing mylonite development. Thus detachment faults, which commonly hydrothermal circulation could have a show talc-tremolite-chlorite assemblages profound influence on ridge crest and strong isotopic alteration consistent rheology and tectonic style. A model is with high fluxes of seawater-derived presented in which hydrothermal fluid at black smoker temperatures. In circulation drives the depth of magma contrast, there is little evidence for high emplacement downward into the fluid fluxes in detachment fault footwall of a steep low-displacement footwalls, although hydration over a fault, triggering a change to (cold) range of temperatures is observed. detachment faulting as magma is Type 2 systems are more difficult to trapped beneath the fault. explain, particularly Rainbow, which has a very high heat output and is thought Poster II-53 to have been active for the last 8-12,000 The Role of Strain Partitioning in the years on the basis of sediment profiles. Development of an Oceanic Rainbow is situated on a probable Detachment Fault System: Insights detachment fault at a non-transform From Relict Oceanic Core Complexes offset, and appears to be localised by in the Western Mirdita Ophiolite, steep faults cutting the detachment. Albania Clearly, fluid circulation occurs in the ultramafic footwall, and not in the Miranda, Elena; Johnson, John; Dilek, detachment fault itself. Unfortunately, Yildirim neither passive nor active seismic experiments have been conducted on E. Miranda, J. Johnson, Geological type 2 systems, and the internal Sciences, California State University structure of the detachment footwalls, Northridge, Northridge, CA; Y. Dilek, location of active deformation, and the Geology, Miami University, Oxford, OH location of any magma chambers are all conjectural. We use microstructural and EBSD The Lost City Field (Type 3) is analyses to evaluate the strain superimposed on the deformation and localization processes that accommodate metamorphism characteristic of oceanic detachment faulting in the detachment faulting, and vents at much Krabbi and Puka relict oceanic core lower temperatures. The main role of complexes of the middle Jurassic detachment faulting appears to be in Western Mirdita Ophiolite, Albania. bringing serpentinising mantle rocks Mantle peridotites, lower crustal containing 9and possibly releasing) gabbros, and amphibole schists are significant heat close to the sea floor, exposed along the surfaces of the dome- although topographic effects related to shaped Krabbi and Puka massifs and core complex formation may also be comprise the structurally shallow important. portions of the footwall of an oceanic Detachment faults can be divided into detachment fault system. These footwall two endmembers, a high temperature rocks are separated from the upper crust type characterised by extensive volcanic rocks in the hanging wall by an mylonites formed at 800-950 °C (e.g. oceanic detachment fault that crops out Atlantis Bank, SWIR), and a low along the topographic base of the dome- temperature type characterised by talc- shaped massifs. We document fabric tremolite-chlorite-serpentine schists and development in the footwall rocks an absence of mylonitic deformation in exposed along the massif surfaces to the footwall (30° N and 15°-45’ N, evaluate the role of strain partitioning in Atlantic). A simple 1-D thermal model the development of a localized oceanic shows that if a detachment is the locus detachment fault. Abundant melt- for discharge of fluids at black smoker impregnated lherzolitic peridotites are temperatures, as in the TAG model overprinted by laterally extensive above, rapid cooling below the brittle- granulite-grade mylonitic fabric

60 development, and less abundant Yokosuka, JAPAN; K. Okino, Ocean Res. gabbroic rocks with rare magmatic and Inst., Univ. Tokyo, Tokyo, JAPAN; K. granulite- and amphibolite-grade Takai, SUGAR, JAMSTEC, Yokosuka, mylonitic fabrics occur close to an JAPAN; H. Kumagai, IFREE, JAMSTEC, intrusive contact with the peridotites. Yokosuka, JAPAN Tremolite-chlorite schists with relict pyroxene are interlayered with spinel- We directly observed and recovered bearing peridotite mylonites, and are samples along a transect from near the found in a 100 m-thick zone of termination extending towards the semibrittle deformation in the uppermost breakaway of an oceanic core complex, part of the footwall beneath the 23 km x 13 km in size, along the Central detachment fault surface. The peridotite Indian Ridge at 25°S (25OCC), where fabrics in the footwall suggest that melt the spreading rate is about 50 mm/year, intrusion (impregnation) was closely using SHINKAI 6500 of the Japan followed by extensive high-temperature Agency for Marine-Earth Science and ductile deformation, and the rare Technology (JAMSTEC). The bathymetric magmatic and mylonitic fabrics in the corrugations with wavelengths of ~1-1.5 gabbros indicate that this high- km and striations at centimeter-scale temperature deformation may have parallel to the spreading direction are been preferentially accommodated well developed in the 25OCC. Samples within the peridotites during the earliest recovered from the 25OCC were basalts stage of detachment faulting. The ( doleritic rocks), gabbros, granites, interlayered schists and peridotite serpentinized peridotites and their mylonites are indicative of semibrittle deformed equivalents. Highly deformed deformation, which was preferentially rocks, cataclastic breccias to mylonitic accommodated within peridotites during schists were restrictedly recovered from this later stage of detachment faulting, the sheet-like structures on the top and was promoted by fluid flow along surface of the 25OCC whereas less the shear zones associated with deformed rocks were recovered from the detachment faulting. Our results immediately beneath the top surface. suggest that strain localization Basaltic samples beneath the top surface associated with detachment faulting may have undergone variable degrees of be partitioned into the lithological units alteration and weathering. The gabbros that constitute a greater proportion of (olivine gabbro, gabbro, gabbro norite, the footwall, and that fabric and oxide-rich gabbro) have magmatic development in the less abundant textures, usually no or very minor plastic footwall units may be restricted to those deformation except for very localized rocks near unit contacts owing to millimeter-scale altered zones of rheologic contrasts between different tremolite/actinolite and/or chlorite. Less rock types. deformed, granular serpentinites were recovered along the ridge-facing slope. Poster I-15 Samples recovered from the top surface Direct Observations of the 25°S of the 25OCC show that the striated Oceanic Core Complex Along the features are associated with high-strain Central Indian Ridge cataclastic breccias to mylonitic schists of peridotites, gabbros, basalts and their Morishita, Tomoaki; Soda, Yusuke; combination origin. Talc occurs as two Sawaguchi, Takashi; Nakamura, different textures: static replacing Kentaro; Okino, Kyoko; Takai, Ken; primary minerals, and syn-deforming Kumagai, Hidenori textures showing shape-preferred orientations, i.e., talc schists. It should T. Morishita, FSO, Kanazawa Univ., be emphasized that peridotitic clasts in Kanazawa, JAPAN; Y. Soda, Dep. Earth serpentine/talc schists show mylonite Sci., Waseda Univ., Tokyo, JAPAN; T. texture characterized by reduction of Sawaguchi, Natural Sci. Lab., Toyo olivine grain size and elongated Univ., Tokyo, JAPAN; K. Nakamura, orthopyroxene grains. Gabbroic clasts Precambrian Eco. Lab., Jamstec, also display the mylonite textures

61 characterized by grain size reduction of IODP Expedition 304/305 sampled a 1.4 plagioclase. Tremolites/actinolites are km faulted and complexly layered abundant in well-foliated, altered footwall section on the central dome of gabbros. Basalts inhomogeneously the Atlantis Massif oceanic core complex. suffered from chloritization. Breccias, The core (Hole U1309D) is dominated by consisting of many kinds of clasts and gabbroic lithologies with minor their-derived minerals in the matrix of ultramafic rocks and reflects the tremolite/actionlite, chlorite or talc, interplay between magmatism and develop micro-faults with preferred deformation prior to, during, and orientations. Our study revealed that subsequent to a period of footwall gabbros and peridotites were tectonically displacement and denudation associated exposed as oceanic core complexes in with slip on an oceanic detachment fault. the intermediate-spreading ocean Palaeomagnetic analyses of the gabbroic ridges. Detachment might initiate in sequences at Atlantis Massif provide ductile regime of peridotites and valuable information on the evolution of gabbros, probably around the Moho. the section. High unblocking Hydrothermal fluids were locally temperature components of reversed infiltrated along the fault plane during polarity are seen throughout the the deformation, resulting in the gabbroic sequences. In a number of formation of secondary hydrous intervals, however, the gabbros exhibit a minerals. Strain localization along the complex remanence structure with the fault plane was enhanced by the presence of lower temperature normal presence of the secondary hydrous and reversed polarity components, minerals. The talc-rich deformed rocks suggesting an extended period of were probably formed by reactions remanence acquisition during different between serpentinites and mafic rock- polarity intervals. Remanence derived, SiO2-rich hydrothermal fluids. characteristics suggest that these Deformation continued to occur in the multicomponents were acquired by a brittle regime at low temperatures thermal mechanism. However, there during the exhumation, resulting in platy appears to be no correlation between landform with lineations. remanence structure and either the igneous stratigraphy or the distribution Poster II-66 of alteration in the core. Instead, the Palaeomagnetic Constraints on the remanence data are consistent with a Evolution of the Atlantis Massif model in which the footwall section Oceanic Core Complex (Mid-Atlantic acquired magnetizations of different Ridge, 30°N) polarity during a protracted cooling history spanning two geomagnetic Morris, Antony; Pressling, Nicola; Gee, reversals. Subtle differences in Jeffrey; John, Barbara E.; MacLeod, magnetite grain sizes appear to have Christopher J. controlled the temperature range and number of components recorded by A. Morris, N. Pressling, School of different samples. These data are used Geography, Earth & Environmental as a marker for tectonic rotation of the Sciences, University of Plymouth, footwall section in order to address the Plymouth, UNITED KINGDOM; J. Gee, debate over the sub-surface geometry of Scripps Institution of Oceanography, oceanic detachment faults. Competing University of California (San Diego), La models involve either: (a) displacement Jolla, CA; B.E. John, Department of on planar, low-angle faults with little Geology & Geophysics, University of tectonic rotation; or (b) progressive Wyoming, Laramie, WY; C.J. MacLeod, shallowing by rotation of initially steeply School of Earth & Ocean Sciences, dipping faults as a result of flexural Cardiff University, Cardiff, UNITED unloading (the “rolling-hinge” model). KINGDOM; N. Pressling, School of Ocean This simple test is limited, however, by & Earth Science, University of the lack of full palaeomagnetic vectors, Southampton, Southampton, UNITED since only magnetic inclination data are KINGDOM normally available from unoriented

62 ODP/IODP drill core samples. For the including a priori selection of an axis of first time we have overcome this rotation. limitation by independently reorienting Nevertheless, remanences have been core pieces to a true geographic used to infer whether detachment fault reference frame by correlating structures footwall sections have experienced in individual pieces with those identified rotation during displacement in several from oriented imagery of the borehole oceanic core complexes (OCCs), wall. This allows reorientation of including: Atlantis Bank (Southwest paleomagnetic data and subsequent Indian Ridge; ODP Legs 118 and 176), tectonic interpretation without the need Fifteen-Twenty Fracture Zone (Mid for a priori assumptions on the azimuth Atlantic Ridge (MAR); ODP Leg 209 & of the rotation axis. Results indicate a JR63 cruise), the MARK area (MAR; ODP 46°±6° counterclockwise rotation of the Leg 153) and Atlantis Massif (MAR; IODP footwall around a MAR-parallel Expedition 304/305). Rotations are horizontal axis trending 011°±6°. This invoked to explain differences between provides unequivocal confirmation of the the mean palaeomagnetic inclination and key prediction of flexural, rolling-hinge the expected axial geocentric dipole models for oceanic core complexes, value at the sampling site, but the whereby faults initiate at higher dips and amount of rotation depends critically on rotate to their present day low angle the choice of rotation axis. These geometries. uncertainties even extend to cases where there is no significant difference Oral – Saturday 8th, 11:00 between observed and expected Palaeomagnetic Insights into inclinations, as substantial rotation may Oceanic Detachment Fault Processes still have occurred around a rotation axis at Mid-ocean Ridges oblique to the expected direction (resulting in initial steepening and Morris, Antony subsequent shallowing of the magnetic vector back to its unrotated inclination). School of Geography, Earth & Robust quantification of detachment Environmental Sciences, University of footwall rotation therefore requires Plymouth, UNITED KINGDOM knowledge of both palaeomagnetic inclinations and declinations. An Palaeomagnetic remanence directions approach adopted recently at Atlantis have been used as markers for tectonic Massif OCC is to independently restore rotation in numerous studies of oceanic IODP core pieces to their original crustal geodynamic processes. In azimuths by correlating core structures ophiolite terranes, a combination of with features seen on oriented three-dimensional exposures, fully geophysical imagery of the borehole oriented palaeomagnetic samples, and wall. This provided the first fully oriented supporting field structural data allows palaeomagnetic dataset from a section rotations to be quantified even in areas of lower oceanic crust, and that have experienced complex tectonic demonstrated that the Atlantis Massif histories (e.g. sequential seafloor detachment fault initiated at a steep dip spreading-related and emplacement- of c. >50° and progressively rotated to related deformation events). By lower angles during displacement. Such contrast, palaeomagnetic analysis of footwall rotation has also been oceanic rocks collected by ODP/IODP demonstrated definitively at an OCC drilling is hampered by azimuthally associated with the Fifteen-Twenty unoriented core samples and a reduced Fracture Zone, where fully oriented 1 m scope for structural characterization. cores have been obtained using the Under these circumstances, unique BRIDGE sea-bed rock drill system tectonic interpretations are impossible (confirming an earlier, inclination-only- and a number of assumptions must be based interpretation of data from ODP made in order to analyse Leg 209). In this case, integration of palaeomagnetic inclination data, palaeomagnetic data with structural kinematic data obtained from the same

63 oriented cores provides a powerful consists of a huge corrugated method for simultaneously quantifying detachment fault surface and what was both the amount and axis of rotation called the “hanging wall massif”. Next to and their associated uncertainties. this “massif”, is a large alkaline axial volcano that erupted at ~4.6 Ma Finally, palaeomagnetic analyses of (Ishizuka et al., 2004). Ishizuka et al. ODP/IODP samples from the MARK area interpreted this volcano as “post- and Atlantis Massif have identified spreading”: erupting several million unique examples of young, lower crustal years after cessation of spreading at gabbros that record complex, ~12 Ma. 2. New observations 2-1. multicomponent remanences acquired in New age dating We have new different geomagnetic polarity chrons. radiometric ages by Ar-Ar on basalts and Thermal demagnetization of these U-Pb on gabbroic rocks from Godzilla remanences yields important information Mullion that indicate a significant decline on permissible OCC footwall and asymmetry in spreading rate for the temperatures at chron boundaries in terminal phase of PVB spreading. The these sections. In conjunction with new basalt ages indicate ~1.9 cm/y for constraints from radiometric dating and unroofing of the core complex, and ~1.2 closure temperatures, these data can cm/y for spreading of the conjugate provide valuable constraints on footwall crust. 2-2. Secular variation in cooling histories. peridotite petrology The peridotite petrology indicates a secular variation at Poster I-16 Godzilla Mullion. Those from its distal Godzilla Mullion Developed in a end (i.e., near the breakaway zone) are Slow-spreading Environment: New moderately depleted (spinel Cr# = 0.3- Age and Petrologic Data Constraints 0.4), while those from the medial part are more fertile (spinel Cr# =0.14- Ohara, Yasuhiko; Snow, Jonathan E.; 0.22), and those from the proximal part Ishizuka, Osamu; Tani, Kenichiro (i.e., near the termination) show abundant evidence for melt Y. Ohara, Hydrographic and impregnation, with plagioclase Oceanograph Department of Japan, pseudomorphs and elevated TiO2 and Tokyo, JAPAN; Y. Ohara, O. Ishizuka, K. spinel Cr#. The high abundance of Tani, Institute For Research on Earth plagioclase peridotites at the proximal Evolution, Japan Agency for Marine- part could be related to stagnation of Earth Science and Technology, melts in a thickened lithospheric upper Yokosuka, JAPAN; J.E. Snow, mantle leading up to cessation of Department of Geosciences, University spreading. 2-3. Important new of Houston, Houston, TX; O. Ishizuka, sampling result In 2009 we recovered Geological Survery of Japan, AIST, peridotites as well as basalts from what Tsukuba, JAPAN was called the “hanging wall massif”. We interpret this to indicate that 1. Introduction The world’s largest magmatism declined to the extent that oceanic core complex, Godzilla Mullion, Tucholke et al.’s (2008)’s lower limit for occurs along the extinct spreading detachment formation was crossed, and center of the Parece Vela Basin (PVB) in a different rifting mechanism took over. the Philippine Sea. Based on poorly 3. New scenario for Godzilla Mullion constrained magnetic data, spreading evolution The new age data indicate that was thought to be intermediate (8.8–7.0 the terminal phase of PVB spreading was cm/y full-rate); ceasing at ~12 Ma. not intermediate, with a significant Although a higher magmatic budget is decline and asymmetry accompanying expected for a fast- to intermediate- formation of Godzilla Mullion. This spreading ridge, the PVB shows features should have had profound effects on the indicating a smaller magmatic budget thermal structure of the lithosphere in such as oceanic core complexes and the rift, including progressive abundant abyssal peridotites (Ohara et lithospheric thickening, decreasing al., 2001; 2003). Godzilla Mullion degree of partial melting, and increased

64 melt stagnation. This is consistent with conjugate side of the ridge axis. We the peridotite recovery from what was derive scaling relations to estimate the called the “hanging wall massif”, and expected crustal thickness on both sides may explain morphotectonics similar to of the axis, as a function of that at ultraslow-spreading ridges. M_B, M_D and Based on this scenario, the 4.6 Ma axial total height of the magma injection alkaline volcano is not “post-spreading”, zone. Based on these relations, we can but the final product of PVB spreading expect gabbro bodies up to 3-4 km thick and magmatism at a terminal ultraslow in OCC footwalls, consistent with recent phase. seismic observations. Finally, we suggest that the decoupling of magma Poster II-64 emplacement via along-axis propagation Emplacement of Plutonic Bodies in of dikes and the emplacement of Oceanic Core Complexes gabbroic bodies at depth can explain the large discrepancies in size and continuity Olive, Jean-Arthur L.; Behn, Mark; of plutonic bodies observed in OCCs, and Tucholke, Brian account for OCC formation at elevated magma supply. E J.L. Olive, MIT/WHOI Joint Program, Cambridge, MA; M. Behn, B.E. Tucholke, Oral – Tuesday 11th, 14:45 Geology and Geophysics, Woods Hole Microbial Communities and Oceanographic Institution, Woods Hole, Metabolisms From Basalt- and MA Ultramafic-hosted Vents

We present new thermo-mechanical Perner, Mirjam; Seifert, Richard; models for the development of oceanic Strauss, Harald; Petersen, Sven core-complexes (OCCs) in which magma is emplaced in both the brittle Perner, M, Microbiology, Univ. of lithosphere and ductile asthenosphere. Hamburg, Hamburg, GERMANY; Seifert, The magmatic material is tracked R., Biogeochemistry and Marine throughout the formation and evolution Chemistry, Univ. of Hamburg, Hamburg, of the OCC to quantify the partitioning of GERMANY; Strauss, H., Geology, Univ. crustal emplacement in the footwall of Münster, Münster, GERMANY; Petersen, the detachment, as well as on the S., IFM-GEOMAR, Kiel, GERMANY conjugate side of the ridge axis. The rate of magma injection is defined by High-temperature vent fields along the the ratio of the magmatic to the total Mid-Atlantic Ridge (MAR) are extension, M, and set to different values characterized by two different modes of above and below the brittle-ductile hydrothermal circulation. In basalt- transition, M_B and hosted systems fluids rise through M_D, respectively. This gabbros or basaltic lavas, often over parameterization allows for the long distances along deep-reaching possibility that the intrusion and along- detachment structures. Similar axis propagation of dikes may be detachments are likely responsible for decoupled from the emplacement of the formation of hydrothermal fields gabbroic bodies at depth. Similar to several kilometres away from the previous studies, we find that OCC neovolcanic zone in areas of mantle- formation requires M_B ~ derived rocks, where the fluids pass 0.5, but is not sensitive to through the cooling ultramafic footwall. M_D, or to variations in Consequently, fluid chemical signatures M_D through time. differ considerably according to the host Further, we show that all magma rocks in the subseafloor. The effluents injected in the brittle layer is accreted on from basalt-hosted settings are the side of the ridge axis opposite the generally enriched in sulfide, whereas OCC, while magma injected in the ultramafic regimes discharge H2 and CH4 ductile layer is emplaced symmetrically enriched fluids. The dilution of the high- in the OCC footwall and on the temperature endmember solutions with

65 oxygenated ambient seawater also environment (<5 nM) and a very low contributes to vent fluid chemistry by hydrogenase diversity. In most of the admixing oxygen, nitrate and sulfate to ten fluid incubations H2 could stimulate the reduced emissions. The fluid-rock microbial CO2 fixation, while only in one interactions and the mixing processes incubation CO2 fixation could be likely denote geochemical constraints on significantly enhanced by spiking the the diversity and activity of local fluids with sulfide. This indicates the microorganisms. importance of H2 for stimulating The hydrothermal fluids of the basalt- microbial CO2 fixation in H2-poor as well hosted vent fields at 5°S and 9°S as H2-rich vent environments at (Lilliput hydrothermal field) along the moderate temperatures. The results also MAR are hallmarked by comparatively suggest that the availability of H2 in the low H2 concentrations. In contrast, those fluids is not the sole determinant for the of the ultramafic-hosted vent fields at diversity and activity of H2-oxidizing 15°N (Logatchev hydrothermal field) and microorganisms. Other parameters such at 8°S (Nibelungen field) along the MAR as mixing processes influencing the are considerably enriched in H2. Diffuse oxygen availability and temperatures of and hot hydrothermal effluents from two the effluents likely contribute to the H2-poor sites in the basalt-hosted metabolic diversity and activity of H2- southern MAR fields and from four H2- oxidizing microorganisms in these vent rich localities within the ultramafic- regimes. hosted Logatchev field were used for

comparing the diversity of H2-oxidizing Poster II-73 microbes. To asses the microbial Modes of Deformation in Ultramafic metabolic activities, hydrothermal fluids Rocks Exhumed in the Footwall of from seven basalt-hosted vents at 5°S Detachment Faults at Slow- and 9°S and from three ultramafic- spreading Ridges and Ocean hosted sites in the Logatchev and the Continent Transitions Nibelungen fields were spiked with H2 or sulfide and supplemented with Picazo, Suzanne M.; Cannat, Mathilde; radioactively labeled inorganic carbon. Manatschal, Gianreto; Delacour, Adélie; The fluids were incubated at moderate Escartin, Javier; Silantiev, Sergei; temperatures and microbial hydrogen or Fouquet, Yves

sulfide consumption as well as CO2 fixation rates determined. S.M. Picazo, M. Cannat, J. Escartin, The enzyme catalyzing the microbial CNRS-IPGP, Paris, FRANCE; G. consumption of H2 is the hydrogenase Manatschal, CNRS-IPGS, Strasbourg, and its encoding gene is found in FRANCE; A. Delacour, LDTPOMP, phylogenetically diverse microbial Université Paul Sabatier, Toulouse, lineages. Only few phylogenetically FRANCE; S. Silantiev, Laboratory of diverging hydrogenase genes were Geochemistry of Igneous and identified in the two H2-poor effluents Metamorphic Rocks, Institut of Russian from the basalt-hosted sites and were Academy of Sciences, Moscow, RUSSIAN associated with those of mesophiles FEDERATION; Y. Fouquet, Géosciences only. On the contrary, a high Marines-Géochimie Métallogénie, hydrogenase diversity is illustrated in Ifremer, Brest, FRANCE the four investigated H2-rich Logatchev Mantle exhumation by detachment habitats. Here the hydrogenase genes faulting occurs both at mid-ocean ridges are affiliated with those of distinct and in the distal parts of rifted mesophilic and (hyper)thermophilic continental margins in Ocean Continent microbes, reflecting the broad thermal Transition (OCT). In this poster, we range at which microbial H2-oxidation is present deformation structures in putatively performed. The highest ultramafic rocks sampled in the footwall microbial H2-consumption rates were of detachments, within 100-200m of the exhibited from a basalt-hosted venting main fault, in the 13°-15°N region of the sample characterized by extremely low Mid-Atlantic Ridge (MAR-13N), and in H2 concentrations in the natural the fossil remnants of a Jurassic OCT in

66 the Swiss Alps (Totalp and Tasna). Our form thin shear zones next to the MAR work is carried out on dredged detachment in the Alpine outcrops. samples and on samples recovered during ROV dives (Serpentine cruise; These results suggest that syntectonic 2007). For each type of deformation, we magmatic injections and subsequent use the number of affected samples brittle-ductile shearing of their hydrous relative to the total number of ultramafic alteration products are a characteristic samples in our collection (247 samples) of detachment faulting leading to mantle as an indicator of the distributed exhumation in the MAR-13N region. By character of this deformation. contrast, deformation observed near detachments in the Alpine fossil OCTs is Ductile shear zones with recrystallization primarily brittle and amagmatic. of primary minerals (olivine, pyroxenes) are rare both in the Alps and in the Poster II-54 MAR-13N collection (<2% of the New Variscan 40Ar-39Ar Ages for samples). In both settings, these ductile the Danubian Ophiolite shear zones are not clearly linked with the detachment faults. Plissart, Gaëlle; Monnier, Christophe; Brittle deformation with serpentine and Maruntiu, Marcel; Neubauer, Franz; carbonate-filled veins and occasional Diot, Hervé clast-supported cataclasites is widespread in the Alpine outcrops and Plissart, G., Monnier, C., Diot, H., its intensity increases toward the University of Nantes, Nantes, FRANCE; detachment. In the MAR-13N ultramafic Plissart, G., University of Brussels, samples, this brittle imprint is also Brussels, BELGIUM; Maruntiu, M., IGR, present (50 samples or 20%). In Bucharest, ROMANIA; Neubauer, F., addition, the MAR samples commonly Dep. of Geog and Geol, Salzburg, (115 samples or 46,5%) display variably AUSTRIA sheared amphibole and chlorite veins, with occasional apatite and zircon, which we interpret as hydrated magmatic The Danubian ophiolite consists of four veins. Chlorite, tremolite after primary dismembered massifs cropping out in hornblende, and less common talc grow the Southern Carpathians and the during this deformation but are also Balkans mountains and belonging to the locally kinked and fractured. The veins Upper Danubian Alpin unit: Tisovita Iuti appear to form in brittle cracks in in Romania, Deli Jovan and Zaglavak in variably serpentinized peridotite host Serbia and Tcherni Vrah in Bulgaria. rock. Next to the veins, pre-existing These four massifs would represent the serpentine is replaced by talc. relics of a unique oceanic domain delineating a 160 km-long tectonic In a few MAR-13N samples (45 samples suture dismembered by Oligocene Alpine or 18%), we observe intense brittle- tectonics. Although the age of this ductile deformation in tremolite-chlorite- ophiolite has been considered as Late serpentine schists, with occasional talc. Proterozoic in age (Haydoutov 1989), These samples have seen large strains the Deli Jovan massif has recently been with coeval fluid circulation, and are dated to the Lower Devonian (U-Pb similar to those described in the main zircon age of 405 ± 2.6 Ma; Zakariadze detachment fault at other MAR locations et al., 2006). In this case, the Danubian (D’Orazio et al., EJM 2004, Boschi et al., ophiolite could be considered as a Ofioliti 2006, Escartin et al., Gcubed remarkable marker of a Variscan paleo- 2008). ocean according to its size (~ 500 km2) and its lithological diversity. Finally matrix supported cataclasites are observed in rare (only 4) samples from This study is focused on the the MAR-13N collection, but we have not transformation of gabbroic rocks, observed gouges similar to those that evidenced by a continuous petrographic sequence from the amphibolitized to

67 listvenitized gabbros (zoisite + calcite + of Deli Jovan Massif (NE Serbia). – (Cr-) chlorite + (Cr-) muscovite Ofioliti, 31 (2): 223-224. assemblage). The geothermometry on chlorite from these metagabbros Poster I-14 (Plissart et al., 2009) suggests a two- A Continuous Structural stage transformation: a common and Characterisation of Atlantis Massif large scale retrograde ocean-floor Using an Integrated Analysis of metamorphism in the transitional Oriented Downhole Imagery and amphibolite/greenschist facies occurring Logging Data at ~ 450°C, locally superimposed by a listvenitization process at ~ 280°C. This Pressling, Nicola; Morris, Antony; latter event suggests important John, Barbara E; MacLeod, Christopher geochemical remobilizations linked to J. seawaterderived fluid/rock interactions. N. Pressling, School of Ocean and Earth New 40Ar-39Ar ages have been Science, University of Southampton, performed on one hornblende from Southampton, UNITED KINGDOM; N. amphibolitized gabbros and two Crmicas Pressling, A. Morris, School of from listvenites. The amphibole analyse Geography, Earth and Environmental gives a plateau age of 472.2 ± 3.2 Ma, Sciences, University of Plymouth, whereas the Crmicas ones give plateau Plymouth, UNITED KINGDOM; B.E. John, ages of 372.6 ± 1.3 Ma and 360.6 ± 1.2 Department of Geology and Geophysics, Ma. These results are consistent with a University of Wyoming, Laramie, WY; Variscan age for the Danubian ophiolite C.J. MacLeod, School of Earth and Ocean and confirm the two-stage Sciences, Cardiff University, Cardiff, transformation of gabbros. UNITED KINGDOM Amphibolitization processes could be considered as a very early stage Continuous wireline logging data are probably linked to normal faulting at the invaluable when less than 100% of ridge, whereas listvenitization ones drilled core material is recovered. The might be viewed as a late event. In this data provide information on missing case, listvenitization processes could units, record the true depth of features either be related to deformation and and uniquely constrain spatial transformation occurring (1) within the orientation. Only by fully integrating accretionary wedge, (2) during an intra- continuous, oriented logging data and oceanic obduction or (3) during a discrete, finer-scale core data can we continental obduction. The first develop a complete structural hypothesis seems more likely interpretation for drill holes that is not considering the large amount of water limited by sampling bias. necessary to generate the listvenites and the widespread presence in the area Integrated Ocean Drilling Program of sedimentary thrust slices. (IODP) Expedition 304/305 sampled the Atlantis Massif oceanic core complex at Haydoutov, I. (1989): Precambrian the intersection between the Mid-Atlantic ophiolites, Cambrian island arc, and Ridge and the Atlantis Transform fault at Variscan suture in the South Carpathian- 30° N. Hole U1309D penetrated Balkan region. – Geology, 17(10):905- 1415.5m into the central dome of the 908. massif, which exposes the corrugated Plissart, G., Féménias, O., Maruntiu, M. detachment fault surface denuding the Diot, H., Demaiffe,D. (2009): Mineralogy lower crust and upper mantle. The and geothermometry of gabbro-derived recovered section is dominated by listvenites in the Tisovita Iuti ophiolite, gabbro compositions that are complexly Southwestern Romania. – The Canadian faulted and layered on a variety of Mineralogist, 47:81-105. scales, reflecting the complicated Zakariadze, G.S., Bayanova, T.B. & interplay between magmatic and Karpenko, S.F. (2006): Age, composition tectonic processes controlling the and origin of the paleooceanic gabbros formation, evolution and deformation of

68 oceanic crust at slow-spreading ridges. UNITED KINGDOM; C. Ranero, Barcelona Center for Subsurface Imaging, Instituto The average core recovery at Atlantis de Ciencias del Mar, CSIC, Barcelona, Massif was 74%. Therefore, to augment SPAIN and constrain structural interpretations based on limited core material, we used The corrugated surface of oceanic core the Formation MicroScanner (FMS) complexes (OCCs) are though to wireline logging tool that measures represent the slip surface of large-offset microresistivity contrasts in the normal faults, commonly called immediate vicinity of the borehole wall detachment faults, that cut to depth formation. The data are presented as an beneath the median valley of the unwrapped image of the borehole spreading centre. A detachment fault cylinder, and inclined planar structural represents a normal fault that has features that intersect the borehole, remained active even as large offsets such as faults or veins, are shown as develop, resulting in the rotation of the darker (more conductive) sinusoidal exhuming footwall of the fault to traces. The true dip and azimuth of relatively low-angles: the rolling hinge these features can be calculated directly model. However there are two different due to the inclusion of an accelerometer types of rolling hinge model. The key and magnetometer on the toolstring, difference is whether the angle at which which record the position and spatial slip along the fault is no longer possible orientation (with respect to magnetic occurs beneath or above top basement. north) of the tool within the borehole, If beneath top basement, the upper respectively. portion of the fault locks up, while the deeper root zone remains active, 4324 distinct structural features have requiring that a new fault splays off the been identified in the FMS images root zone, cutting at a steep angle between 97 and 1415mbsf (metres through the hangingwall to the surface. below sea floor). Distinctly different In so doing, it transfers a slice of the structural trends are seen across the five hangingwall to the footwall; the slice is sub-units that are based on petrological then rafted to the surface. Repetition of and geochemical observations of the the process results in a series of stacked recovered core. In addition, variations in rafted blocks on a rotated, inactive the borehole dimensions are used to series of fault segments, resembling a define 115 zones of borehole breakout, single detachment fault (we term a with a cumulative extent of 434.76m Rolling Hinge Type 1 or RH1 (31% of the total drilled). Such regions detachment) on which several fault often correspond to areas of poor blocks have moved. However none of recovery and are consequently poorly these blocks will have moved at low characterised using core samples. The angle, but only been passively rotated extensive FMS-based structural database during the rafting process to low-angle. allows the variation in fracture networks In the alternative rolling hinge model and areas of weakness to be quantified (RH2), the angle at which the fault locks at a high-resolution, leading to improved up is not reached until the slip surface understanding of the hydrothermal fluid has been exhumed above top basement. flow and melt pathways in the footwall This allows movement on the entire fault section. zone to continue, resulting ins the progressive unroofing of deep crustal Poster II-50 and mantle rocks until such time as the Detachment Faulting and the fault is abandoned for other reasons. Formation of Oceanic Core Such a RH2 system explains the Complexes formation of many oceanic core complexes that are observed. We see Reston, Timothy J.; Ranero, Cesar evidence for both types of rolling hinge detachment along the Mid-Atlantic T.J. Reston, Geography Earth and Ridge. In some cases, such as the North Environmental Sciences, Birmingham, Atlantis Inside Corner, it can be shown

69 that a detachment fault that forms an that using these different approaches, exhumed oceanic core complex near the thermophilic methanogens are segment end continues towards the essentially absent at Lucky Strike. It is segment middle beneath an increasing likely that the high concentrations of number and thickness of small fault hydrogen at Rainbow can support a suite blocks. We suggest that this represent of hydrogen oxidizers including an along strike transition from a RH2 methanogens, whereas, because of the detachment at the segment end to a unusual hydrothermal vent chemistry, RH1 detachment further toward the and in particular the very low hydrogen segment middle. The transition between concentrations at Lucky Strike, the the two being related to the depth at hydrogen is below the threshold which the fault rotates to its lock-up concentration that would support angle. The identification of an RH1 methanogen growth. These results detachment here was however only continue to expand our understanding possible because of its along strike on the tight coupling between transition to a RH2 detachment; geochemistry and biology in the deep- elsewhere RH1 detachments may exist sea. beneath a continuous layer of small rafted blocks and not be recognized from Oral – Friday 14th, 15:15 bathymetric data. The Microbiology of OCC’s: Implications for Life in the Poster I-36 Subsurface and Elsewhere in the Host-rock Composition Shapes the Solar System Microbial Community Structure Associated With Hydrothermal Vent Reysenbach, Anna-Louise Deposits Portland State University, Biology Reysenbach, Anna-Louise; Flores, Department, Portland, OR 97201, USA Gilberto; Campbell, James; Kirshstein, Julie; Podar, Mircea; Steinberg, Josh; Oceanic core complexes provide long- Voytek, Mary lived, high and low temperature, hydrothermal circulation, rich in A.L. Reysenbach, G. Flores, J. Steinberg, hydrogen. These systems support a Biology, Portland State Univ, Portland, unique microflora whose diversity is OR; J. Kirshstein, M. Voytek, USGS, tightly coupled to the geochemistry of Reston, VA; J. Campbell, M. Podar, Oak the hydrothermal fluids. Comparisons Ridge National Labs, Oak Ridge, TN with mafic- and ultramafic- hosted systems have revealed the prevalence of Newly formed deep-sea hydrothermal methanogens and other hydrogen- vent deposits are rapidly colonized by a oxidizing chemolithotrophs at ultramafic range of autotrophic and heterotrophic sites like Rainbow and Logachev. High thermophiles. The assembly and throughput DNA sequencing approaches structure of these microbial communities have provided clues into the patterns of are likely influenced by the different diversity associated with these systems, hydrothermal fluid chemistries and point to a much greater diversity of generated at periodotite (ultramafic) and novel microorganisms from deep-sea basalt (mafic) hosted vent fields. We vents. In addition, although some of the explored the microbial phylogenetic metabolisms may be similar, each vent diversity associated with deposits from field has its very unique microbial Lucky Strike (mafic) and Rainbow signature. Interestingly, the microbial (ultramafic) vent fields using high diversity detected at Lucky Strike points throughput pyrosequencing of 16S rRNA to the hypothesized subsurface gene amplicons. Additionally we biosphere below the lava lake, as no compared the distribution of sulfate methanogens were detected, yet reducers (using the functional gene dsr) elevated methane is measure in these and methanogens (using mcr as the vent fluids. Studies of hydrothermal diagnostic gene for this group) and show systems have implications for our

70 understanding of what fuels the precipitates and the development of microbial subsurface biosphere and the faulting in space and time. At least four extent of the microbial world on Earth large (diameter hundreds of m; height and elsewhere in the solar system. tens of m) massive sulfide mounds ranging in state from young-hot to old Poster II-70 cold and in age up to over 100,00 years, Detachment Faulting Chronology in as well as manganese crusts, are Oceanic Crust Using Radiometric distributes in a 5 by 5 km area of the Dating of Associated Hydrothermal hanging wall of an inferred detachment Deposits: TAG Hydrothermal Field, fault zone involving the east wall of the Mid-Atlantic Ridge 26○N axial valley. Radiometric dating of seafloor deposits from the mounds and Rona, Peter sub-seafloor deposits recovered from the young-hot sulfide mound by ODP Leg IMCS, Rutgers University, New 158 drilling indicate that the mound has Brunswick, NJ undergone relatively short intervals of high-temperature activity (hundreds of Hydrothermal systems at ocean ridges years) and longer intervening inactive involve seawater (ubiquitous), heat intervals (thousands of years) for the sources (magma, mantle upwelling, past 50,000 years. A field-wide high- mining heat from crust. exothermic temperature hydrothermal event mineral reactions), rocks of ocean crust affected all four sulfide mounds about and upper mantle, and permeable 50,000 years ago. This chronology and pathways through these rocks for distribution of hydrothermal precipitates convective circulation of the heated may record the development and state seawater. Detachment faulting can of the detachment fault zone which provide pathways for the hydrothermal provided the pathways for the circulation. Limited evidence including hydrothermal circulation that produced heat budgets and dating of hydrothermal the mineralization . Our TAG team has precipitates indicate that hydrothermal proposed to the IODP a second leg to systems at ocean ridges are episodic extend the drilling/logging to sample the with relatively short on and long off whole sequence of mounds in order to intervals. The inferred episodicity is address gaps in the chronologic record influenced by the rate of extraction of hydrothermal activity and to use this versus replenishment of heat and information to elucidate the sealing of circulation pathways by development of the detachment fault hydrothermal precipitation. The zone. observation that ancient and modern volcanogenic massive sulfide (VMS) Poster II-67 deposits tend to occur at the intersection H2 Production From Ultramafic- of fault systems and the rapidity of self- Bearing Rock Assemblages: An sealing by hydrothermal precipitation Experimental Study at 420°C, 500 suggest that faulting is a primary control bars of episodicity where adequate heat sources exist. Manganese crusts and Rough, Mikaella E.; Seyfried, William massive sulfides are precipitated from E. low- and high-temperature solutions, respectively, in seafloor hydrothermal Geology and Geophysics, University of fields. The distribution and chronology of Minnesota, Minneapolis, MN these precipitates in a hydrothermal field on an ocean ridge record of the Black smoker fluid issuing from Pacific distribution and state of the faulting that ocean vent sites illustrate the provided the pathways for the predictably close linkage between the associated hydrothermal circulation. chemistry and temperature of vent fluids The TAG hydrothermal field near 26oN and the sub-seafloor magma chambers on Mid-Atlantic Ridge provides a setting that underlie these systems at shallow to test relations between hydrothermal depths. Yet hydrothermal venting at

71 slow-spreading systems in the Atlantic predicted; instead it limits the reactivity periodically reveal an association with of the minerals. Therefore, the large ultramafic-bearing lithologies and quantities of H2 produced in the coexisting detachment faulting that troctolite experiment indicate that large contains hard to constrain chemistry and H2 concentrations result from the temperature conditions. Systems like reaction of olivine in an Al-bearing Rainbow reveal low pH fluids with system at all temperatures. This temperatures measured at the seafloor concludes that the fluid production in >360°C, high integrated rates of flow, ultramafic-bearing environments high concentrations of methane and necessitates a more complex rock other dissolved organics, dissolved H2 matrix, and can result from the concentrations 1-2 orders of magnitude hydration of fayalitic-olivine at higher than typical of basaltic temperatures above 400°C. hydrothermal systems, and high dissolved transition metal Poster I-31 concentrations. Interplay Between Tectonic and Magmatic Events During the The complex nature of the vent fluids Exhumation of a Gabbro-peridotite cannot be explained by reacting Section to the Sea-floor (Northern seawater with pure basalt or pure Apennine ophiolites, Italy) ultramafic rock at vent exit temperatures, indicating that a more Sanfilippo, Alessio; Tribuzio, Riccardo complex rock-matrix and reaction model must be used to explain slow-spreading A. Sanfilippo, R. Tribuzio, Dipartimento hydrothermal vent systems. Drill core di Scienze della Terra, Università di samples obtained from uplifted lower Pavia, Pavia, Pavia, ITALY; R. Tribuzio, crust-upper mantle fault zones indicate C.N.R. - Istituto di Geoscienze e that gabbros frequently intrude into Georisorse, Unità di Pavia, Pavia, Pavia, more olivine rich rocks producing rock ITALY compositions like troctolite and olivine- rich (70% olivine) troctolite. The Scogna-Rocchetta Vara ophiolite from Northern Apennine is a Middle- We conducted an experiment on Upper-Jurassic sequence made up of plagioclase-rich troctolite rock powder mantle peridotites intruded by different (80-150µm) at 420°C, 500 bars with a gabbroic bodies and topped by a 0.8 mol/kg NaCl fluid to observe the sedimentary cover mostly made of evolution of saline solutions in contact gabbroic breccias and radiolarian cherts. with troctolitic rocks in the sub-surface This ophiolite thus lacks the basalt layer, reaction zone. The experiment yielded Al similar to the “magma-starved sections” and H2 levels higher than Rainbow, from the Mid-Atlantic Ridge (e.g. at 14°- similar Si concentrations, and Ca, Fe, K, 16° N). The mantle peridotites are and Li levels that were much lower than clinopyroxene-poor depleted lherzolites Rainbow vent fluid concentrations. affected by a spinel facies deformation that produced a foliation sub- The quantity of H2 produced was perpendicular to the stratification of previously attributed to the dissolution sedimentary cover. The lherzolites of fayalitic olivine and resulting locally contain thin pyroxenite levels, precipitation of magnetite at attributed to infiltration of MORB-type temperatures below 400°C, yet the rock melts under spinel facies conditions. powder used in this experiment was Plagioclase-orthopyroxene veinlets, dominated by plagioclase and contained associated with minor clinopyroxene, are less olivine than previously conducted frequent in the lherzolites and parallel to experiments using powdered-olivine-rich the spinel facies foliation. These veinlets rock assemblages (Allen, 2003). document that decompression of the Seyfried et al. (2008) illustrated that an mantle section to plagioclase facies experiment with only silica and olivine conditions was associated with melt does not react to completion as impregnation by silica-oversatured

72 melts. The lherzolite stuctures are Poster I-23 replaced by dunite bodies containing The Magnetic Signature of Non- spinel trails that are nearly parallel to volcanic Domains on the Southwest the plagioclase-orthopyroxene veinlets. Indian Ridge Between 61E and 67E Spinel from the dunites have low Cr#, similar to spinel from replacive dunites Sauter, Daniel; Cannat, Mathilde; of (ultra-)slow spreading ridges. The Mendel, Véronique peridotites are crosscut by two generations of gabbroic bodies, which D. Sauter, V. Mendel, Institut de both retain a MORB-type geochemical Physique du Globe de Strasbourg, CNRS- signature. The first generation is UMR 7516, University of Strasbourg, represented by gabbroic dykes Strasbourg, FRANCE; M. Cannat, Equipe (troctolites to clinopyroxene-rich de Géosciences Marines, CNRS- gabbros) that are concordant with the UMR7154, Institut de Physique du mantle structures and display diffuse Globe, Paris, FRANCE contacts to the host rock. The dykes are post-dated by clinopyroxene-rich gabbro We investigate the magnetic signature of sills showing sharp contacts with the non-volcanic seafloor areas along the host lherzolites, which are elongated Southwest Indian Ridge between 61E nearly parallel to the stratification of and 67E and analyze their relationship sedimentary cover. The gabbroic sills are with crustal thickness variations and most likely associated with the past to present ridge segmentation. This development of a large gabbroic part of the Southwest Indian Ridge is an intrusion that mostly consist of coarse end-member for the global ridge system grained olivine-bearing gabbros and in terms of low melt supply, thin crust, minor foliated troctolites. The gabbroic and ultraslow-spreading rates. It is intrusion contains up to 50 m thick characterized by large expanses of lenses of mantle lherzolite remnants and smooth seafloor that show no evidence up to 80 m thick bodies made up of for a volcanic upper crustal layer. Off- olivine-rich troctolites locally intruded by axis, the amplitudes of magnetic gabbroic sills. The olivine-rich troctolites anomalies C5n and C6n are on average are texturally and compositionally higher over volcanic seafloor areas similar to those from oceanic core where thicker crust is inferred (low complexes from Mid Atlantic Ridge and residual Mantle Bouguer Anomalies document a process of infiltration of RMBA values) and lower over smooth MORB-type melts into an olivine-rich non-volcanic seafloor with inferred matrix. The gabbroic intrusion shows thinner crust (high RMBA values). Local sparse ductile shear zones that are standard deviation of the magnetization, nearby parallel to the basement- a proxy for magnetization contrast, is on sediment contact. The ductile evolution average higher for volcanic seafloor than comprises early re-crystallisation of for smooth non-volcanic topography clinopyroxene and plagioclase (± Ti- suggesting that the contribution of the pargasite) at T ~850 °C, followed by an basaltic upper crustal layer to the amphibolite facies event characterised production of magnetic anomalies by hornblende and plagioclase at T ~700 remains important in off-axis regions. °C. The subsequent brittle evolution was However, the distribution of crustal initially associated with injection of magnetization in volcanic seafloor and MORB-type basalt dykes. The following smooth seafloor domains overlap exhumation of the gabbro-peridotite broadly, and smooth seafloor or sequence to the sea-floor is correlated corrugated domains generally show with calcite- and hematite-bearing reasonably good records of the structures in both gabbros and mantle- geomagnetic polarity history. derived serpentinites. Furthermore, along isochron variations of crustal magnetization show that smooth seafloor domains locally have higher crustal magnetization values over thick crust areas than volcanic seafloor

73 domains. This indicates that basalts are have been driven by large topographic not the only source of the magnetic relief created at the boundary between anomaly record in our study area. The the half-thickness, extended, continental lithologies in smooth seafloor area are crust of the Galicia Bank and crust inferred from the small number of composed of mantle rocks exhumed to available dredges to be mostly the seafloor (Zone of exhumed serpentinized peridotites. Gabbros and continental mantle; ZECM) after the diabase samples are very scarce in these continental crust has separated and dredges. Gabbros may, however, be before normal seafloor spreading is present in smooth seafloor areas that established. The slide material is have not been sampled so far. Although interpreted to consist of blocks of Galicia the variations of the thickness and the Bank extended continental crust and intrinsic magnetization of gabbros are pre-sliding sediment. A portion of this not well constrained, we infer from slide material may have been emplaced tentative two-dimensional forward over the ZECM. We examine possible models that an alternative source, consequences of this controversial besides gabbros, is needed to explain hypothesis for the well-known and the magnetic signal of smooth non- widely interpreted drilling and seismic volcanic seafloor areas. An induced data from the Galicia Bank Basin and component of magnetization is present Iberia Abyssal Plain. locally in some high RMBA smooth seafloor areas, but it is not systematic The widely accepted model for magma- and it is weak over most parts of the starved rifting and the initiation of survey area. Serpentinized peridotites seafloor spreading in this region posits are likely carriers of this induced that the prominent S reflector magnetization component. We propose corresponds to a low angle detachment that both gabbros and serpentinized fault forming the base of the extended peridotites may contribute to the continental crust, overlies serpentinized variable amplitude of magnetic upper mantle rocks and is the fault anomalies measured over smooth non- which exhumes continental mantle at volcanic seafloor. This work has been the final stage of breakup. In the large partially published in Sauter D., Cannat mass-wasting model, the S reflector M. and Mendel V., 2008, Magnetization corresponds to the surface of separation of 0-26.5 Ma seafloor at the ultraslow of a slump, deposited upon already spreading Southwest Indian Ridge 61- exhumed continental mantle. In either 67°E. Geochem. Geophys. Geosyst., 9, model, mantle exhumation associated Q04023, doi:10.1029/2007GC001764. with the separation of continental crust is an essential feature. In either case, Poster I-34 the rifting processes at this margin show Aspects of Mantle Exhumation similarities to the processes of slow During Magma Starved Rifting at the spreading rate ocean crust formation. Galicia Margin We have applied pre-stack depth Sawyer, Dale; Lasseigne, Amy migration to a number of additional 2D MCS profiles in this area. They include Earth Science, Rice University, Houston, both dip and strike profiles, offering TX more three-dimensional insight into the rifting and exhumation process. The new Recently, we have interpreted seismic images suggest more lateral complexity profiles crossing the west and south in the S-reflector than has previously sides of the Galicia Bank, west of Spain been noted. Abrupt lateral changes in in the Atlantic Ocean, to suggest a reflectivity in S seem to correspond to heretofore unrecognized role of very places where dipping reflectors in the large mass wasting events presumed to mantle intersect S. We interpret these to have occurred late in the process of be mantle faults offsetting S that formed continental breakup and the initiation of before or during the final stage of seafloor spreading. These events could continental separation. The places where

74 mantle faults intersect S also seem to 1309D, which together with U-Pb zircon correspond to the along strike ages constrain the cooling rate of boundaries of the pod shaped fault gabbros emplaced into the center of bounded blocks in the overlying crust. It Atlantis Massif. These cooling rates will is not yet clear to us whether this is be compared to those determined by because they formed simultaneously or Grimes et al. (2008) for samples from the presence of one controlled the the south wall of Atlantis Massif, formation of the other. adjacent to the Atlantis transform. Downhole variation in (U-Th)/He age, Poster II-49 taken with the present day geothermal Cooling History of the Atlantis gradient constrained by the bottom hole Massif and Kane Oceanic Core temperature of 120-140°C, also limits Complexes at the Slow-spreading the orientation of the ~200oC isotherm Mid-Atlantic Ridge as the core complex was denuded. These data may also place constraints Schoolmeesters, Nicole; Cheadle, on the depth of hydrothermal circulation Michael J.; John, Barbara E.; Grimes, beneath the detachment fault. At the Craig B. Kane oceanic core complex, we will examine the lateral, along strike, N. Schoolmeesters, M.J. Cheadle, B.E. variation in cooling rate and cooling John, Geology and Geophysics, history from the center to the edge of University of Wyoming, Laramie, WY; the core complex near the transform C.B. Grimes, Geosciences, Mississippi valley wall. These cooling rates must State University, Mississippi City, MS reflect the unroofing history of the core complex. Assuming the detachment fault Understanding the cooling history of the that bounds the core complex slipped at lower crust exposed in oceanic core a similar rate along strike of the ridge complexes helps establish denudation segment, any lateral variation in cooling rates, the depth of gabbro emplacement, rates will likely reflect variation in depth and subsequent hydrothermal of the 700-900°C isotherms and hence circulation. Here we use newly acquired constrain along strike variation in the thermochronometic data to constrain the depth of faulting and/or depth of gabbro crystallization history of gabbros hosted emplacement. Gabbros near the in the footwalls of the Atlantis Masssif transform fault likely crystallized at a core complex at 30°N (IODP Hole deeper depth due to being near that 1309D), and the Kane oceanic core cooler transform boundary, whereas complex at 23°N, on the Mid-Atlantic gabbros at the ridge segment center Ridge. Combined U-Pb zircon likely crystallized at a shallower depth, crystallization ages taken with (U-Th)/He consistent with the 600-700 kyr older U- zircon ages allow determination of the Pb zircon ages from samples near to the cooling history of rocks sampled from transform fault. these core complexes. The closure temperature for U-Pb in zircon from Poster II-65 oceanic gabbros ranges between ~900- Tectonic vs Magmatic Extension in 700°C, and the closure of the (U-Th)/He the Presence of Core Complexes at system for zircon is ~200°C in these Slow-spreading Ridges From a rapidly cooled rocks. Intermediate Visualization of Faulted Seafloor temperatures can be potentially Topography constrained by either multi-component remanent magnetization (300-550°C) Schouten, Hans; Smith, Deborah; and/or U-Pb ages in sphene (700- Cann, Johnson R; Escartin, Javier 550°C). Thus thermochronometry can constrain the detailed cooling history H. Schouten, D. Smith, Department of from ~900°C to 200°C (John et al., Geology and Geophysics, Woods Hole 2004). We have determined (U-Th)/He Oceanographic Institution, Woods Hole, ages for 6 samples from depths ranging MA; J.R. Cann, School of Earth and between 40 and 1040 mbsf in IODP Hole Environment, University of Leeds, Leeds,

75 UNITED KINGDOM; J. Escartin, Groupe Oceanic detachment faults serve locally de Geosciences Marines, CNRS - IPGP, as plate boundaries to accommodate Paris, FRANCE plate separation and tectonically denude gabbro and mantle peridotite to the We develop a forward model of the seafloor during magma-reduced generation of faulted seafloor spreading. Drilling and submersible topography (visualization) to estimate sampling suggest that oceanic the relative roles of tectonic and detachment faults form different magmatic extension in the presence of patterns of seafloor morphology and core complexes at slow-spreading employ different strain localization ridges. The visualization assumes mechanisms depending on whether their flexural rotation of 60° normal faults, a footwalls are dominated by gabbro or constant effective elastic thickness of mantle periodotite. Three ODP/IODP drill young lithosphere, Te, and a continuous sites at detachment faults displaying infill of the depressed hanging wall by classic corrugated domal “core complex” lava flowing from the spreading axis. We morphology, 735 (Atlantis Bank), 1275 obtain a new estimate of Te = 0.5 - 1 (15°20’ Fracture Zone), and U1309 km from the shapes of the toes of 6 (Atlantis Massif), contain dominantly well-documented oceanic core gabbro in their footwalls. Five ODP drill complexes. We model an 80-km-long sites (1268, 1270, 1271, 1272 & 1274) bathymetric profile in the Equatorial north and south of the 15°20’ Fracture Atlantic across a core complex and the Zone Mid-Atlantic Ridge offset have ridge axis at 13°20'N and estimate the footwalls comprising dominantly mantle variation in tectonic extension, which peridotite with small (<50 m wide) yields the variation in the fraction of gabbro intrusions. These faults form upper crust extension by magmatic bathymetric ridges ~ 15-40 km long diking at the ridge axis, M. Core complex (parallel to the spreading axis) with formation appears to be stable for all ~2000 m of vertical relief and 10-15 values of M < 0.5. The visualization km-wide flanks instead of the classic shows how gabbro emplaced at the base core complex morphology. The fact that of the lithosphere during extension by these ridges have narrower flanks than magmatic diking is partitioned to each core complexes suggests that these side of the spreading axis, and predicts a faults are shorter-lived and undergo less high probability of finding gabbros in the displacement than core complex domes of core complexes that correlates detachment faults. Both types of with M. From the bathymetry of the detachment faults are present in the Kane Megamullion and of the conjugate large area of magma-reduced spreading sea floor east of the axis we estimate M to the north and south of the 15°20’ in a similar manner and compare its Fracture Zone, but only 3% of the variation with the lithology and seafloor has the core-complex geophysics of this oceanic core complex. morphology. This suggests that core- complex style detachment faults Poster I-10 represent a special case of oceanic Footwall Lithology Controls on detachment faulting, in which a greater Strain Localization Mechanisms and proportion of plate separation is the Geometry of Oceanic accommodated by magma intrusion than Detachment Faults is the case for the ridge-like detachment faults. Schroeder, Timothy J.; Cheadle, Michael J.; John, Barbara E. High-temperature strain localization at both types of detachment faults is T.J. Schroeder, Division of Natural correlated with intrusions of melt or Science and Mathematics, Bennington magma-derived fluids. At the Atlantis College, Bennington, VT; M.J. Cheadle, Bank and the Atlantis Massif core B.E. John, Geology and Geophysics complexes, plastic strain is at least Dept., University of Wyoming, Laramie, locally concentrated into horizons of Fe- WY Ti oxide gabbro that may have formed

76 by infiltration of highly-evolved extent and duration of “tectonic magmatic segregations. Strain spreading” at the Mid-Atlantic Ridge. localization may be the result of the OCC detachment faults initiate as high- rheologic contrast between low-ductile- angle (~65°) normal faults no different strength Fe-Ti oxides and comparatively from surrounding valley-wall faults and, stronger silicates. The minimum like them, rapidly flatten to dips of ~30° temperature of penetrative deformation in response to flexural unloading. in these mylonites is estimated to be However, on some faults displacement between 350°C and 700°C by two-oxide persists for 1-2 Ma rather than being geothermometry of undeformed transferred inward to a new fault. This ilmenite-magnetite exsolution lamellae; leads to enhanced footwall uplift, indicating that brittle strain during final flattening of the fault to horizontal or denudation was partitioned away from beyond, and thus creation of a the oxide-gabbros. Strain localization at detachment. This long-term persistent several peridotite-footwall, non-core- slip is associated with local waning of complex detachment faults ridge the ridge-axis magma supply. It is appears to have occurred within small aided by strain localisation and fault intrusions of either oxide gabbro or weakening resulting from seawater plagiogranite. Tectonic mixing and penetration and talc formation along the chemical reaction between the magmas fault zones. We propose this is easier and host peridotite formed amphibole- where crust is thin or heterogeneous chlorite schists and cataclasites. Strain with peridotite occurring at shallow appears to have remained localized into levels. Our data show that ridge-axis these schists until final footwall volcanism is suppressed and locally denudation based on the presence of absent when core complexes are active. overprinting low-temperature Thus the majority of plate separation is metamorphic assemblages and accommodated by tectonic slip on a discontinuous deformation textures. single detachment, rather than magmatic accretion. This results in Poster II-57 highly asymmetric seafloor spreading. Life Cycle of Oceanic Core Effectively, at initiation of the Complexes, 1: Overview detachment the plate boundary jumps from the ridge axis to the detachment Searle, Roger; MacLeod, Christopher fault then, as a consequence of J.; Murton, Bramley; Casey, John F.; asymmetric spreading, gradually Mallows, Chris; Harris, Michelle migrates back toward the ridge axis. Detachment faults are terminated by the R. Searle, C. Mallows, Earth Sciences, emplacement of renewed magma into Durham University, Durham, UNITED their footwalls, either from below or, as KINGDOM; C.J. MacLeod, School of we demonstrate at MAR 13°N, by Earth & Ocean Sciences, Cardiff propagation of neovolcanic ridges University, Cardiff, UNITED KINGDOM; laterally across them from adjacent B. Murton, M. Harris, National magmatically robust segments. We Oceanography Centre, Southampton, demonstrate clear differences in internal UNITED KINGDOM; J.F. Casey, structure of OCC footwalls, including a University of Houston, Houston, TX smooth dome that we interpret as extruded mantle and a rough, strongly Results from a near-bottom geophysical faulted massif that we interpret as mafic and sampling study of the Mid-Atlantic crust. Ridge at 13°N – 14°N identify the critical controls on oceanic core complex Poster II-61 development and evolution, and Life Cycle of Oceanic Core increase our knowledge of their internal Complexes, 5: Internal Structures structure. We demonstrate how spatial and Comparisons (~1–10km) and temporal (~100ka– 1Ma) variations in magma flux to the Searle, Roger ridge axis directly control the formation,

77 Durham University, Durham, UNITED structural reconstruction of the plutonic - KINGDOM sheeted dike interface. Here we report a paleomagnetic study aiming at I have completed geophysical studies, quantitative reconstruction of decoupled including high-resolution sonar imaging, rotation of the two sections. Following a of oceanic core complexes in two areas: structural mapping of Agar and Klitgord the Mid-Atlantic Ridge at 13°-14°N, and (1995), we sampled 15 paleomagnetic the South West Indian Ridge at 64°E. sites in the plutonic and the dikes The sonar images provide sections along the two sides of a unprecedentedly detailed views of the detaching “crush-zone”. Six to eight 2Ma-old FUJI Dome OCC (SWIR), and field-drilled oriented cores were collected two active OCCs (MAR). The surface at each site and structural attitude were morphology is particularly clear in the carefully determined. Alternating field young, actively slipping OCC at MAR (AF) and thermal demagnetization were 13°18’N. The part proximal to the used to isolate the characteristic spreading axis displays a smooth, remanent paleomagnetic vectors. The striated dome that we interpret as behavior in the demagnetization extruded mantle, while the more distal experiments suggests that characteristic part, which we interpret as the mafic magnetization in both sections is carried crust is rougher, strongly deformed and by fine-grained magnetite, whereas faulted. If this interpretation is correct, sometimes, a secondary component the 13°18’N OCC thus presents an carried by iron-sulfides overprints the outstanding exposure of a complete signal. The mean paleomagnetic vector crustal and upper mantle section. of the sheeted-dikes shows a declination Having identified these features on the of 321°, an inclination of 64°, and α95 active OCC, I can recognise them, of 9° for 5 sites. The mean though not necessarily all together, in paleomagnetic vector of the plutonic other OCCs, both in sidescan and in section shows a declination of 289°, an bathymetric images. Bathymetry from inclination of 6°, and α95 of 6° for 9 the SWIR also shows previously sites. The paleomagnetic vector analysis unreported topographic features similar clearly demonstrates a rotational to those reported by Smith & Cann partitioning of two independent domains (2006) at MAR 13°-14°N, including large separated by the “crushed zone”. In complex massifs that may be linked order to quantify the amount of rotation OCCs, and a comparison of these will be for each domain with respect to the given. initial state in the axial zone, we test the hypothesis that the dikes were initially Poster II-78 vertical at formation within the An Antithetic Tilt of the Plutonic and spreading axis, where they had acquired the Sheeted Dikes Sections magnetic remanence. Since dikes appear Evidenced by a Paleomagnetic Study in both of the domains, we rotated the Near the Village of Agros, Cyprus paleomagetic vector of each dike according to the present structural Shaar, Ron; Ron, Hagai; Agnon, attitude. A tilt corrected average vector Amotz; Abelson, Meir; Ebert, Yael yields declination of 308°, inclination of 30°, and α95 of 12°. Thus, the tilt R. Shaar, H. Ron, A. Agnon, Y. Ebert, correction may provide the initial state The Institute of Earth Sciences, The before the antithetic tilt initiated. Hebrew University, Jerusalem, ISRAEL; Furthermore, the tilt corrected M. Abelson, Geological Survey of Israel, inclination is very close to the mean Jerusalem, ISRAEL paleomagnetic vector of Troodos (TMV, 274/36° α95 =12.3°), supporting this The exposure of the plutonic and the approach. All together, the results sheeted dikes sections in the area of the suggest a composite rotation that can be village of Agros allows a unique described synthetically as two events: opportunity for a three-dimensional an antithetic tilt of ~60° about a independent paleomagnetic versus horizontal axis, and a clockwise rotation

78 about a vertical axis of ~35°. patient may present conflicting symptoms, which can preclude an Oral – Saturday 8th, 11:45 unambiguous diagnosis. Moreover, Geophysical Constraints on the despite the obvious similarities, it is Structure of Detachment Faults: nevertheless true that every detachment Stone Cole Facts, Red Hot Topics, system has its own unique quirks that and Everything In-between confound the urge to generalize. Finally, the number of active detachment faults Sohn, Robert that have been rigorously studied remains unacceptably low, particularly in Woods Hole Oceanographic Institution, the ocean, where the simplest and Woods Hole, MA 02543 easiest to understand systems are to be found. For all these reasons there Geophysical measurements provide remain significant gaps in our unique constraints on the sub-surface understanding, and at the most structure of detachment faults, with the fundamental level we do not understand primary advantage being the ability to the mechanics of crustal accretion and image active systems in situ. Whereas extension on dome-shaped detachment geological and geochemical studies of faults. The dome shape presents a space exhumed samples constitute a kind of accommodation issue, and the nature of post-mortem on ‘dead’ core complexes, crustal deformation at the intersection of geophysical techniques are capable of a detachment fault with normative imaging and observing ‘living’ ridge-parallel fault systems is anybody’s detachment faults. The combination of guess. Although low-angle detachment passive microearthquake data and faults are ubiquitous in the geologic active-source seismic record there are but a few observations refraction/reflection data, along with of earthquakes on shallow dipping potential field gravity and magnetics planes. Seismic images of core data, have demonstrated that complexes typically reveal large domains detachment faults, both oceanic and of mantle lithologies, but efforts to drill continental, are fundamentally different these features have invariably result in geological structures compared to the recovery of gabbro. Are these ‘conventional’ extensional systems. In apparent discrepancies simply endemic particular, detachment faults roll over features of low-angle faults and core from steep (70-80°) dips at depth in the complexes, or are they threads of lithosphere to shallow (20-30°) dips in evidence that threaten to unravel the the shallow crust, and they have arcuate whole paradigm? epicenter patterns indicative of dome- shaped fault surfaces. In addition, the Poster I-22 detachment footwall is composed of The Magnetism of Oceanic Core lower-crustal and upper mantle rocks Complexes – the Kane OCC that slip against an essentially stationary hanging wall, which is typically Tivey, Maurice; Williams, Clare M. composed of highly-tectonized volcanics – at least in oceanic systems where Tivey, Woods Hole Oceanographic primary lithologies are preserved. These Institution, Woods Hole, MA; C. basic characteristics are supported by Williams, BP Exploration, Sunbury-on- numerical models, and as more data Thames, UNITED KINGDOM become available, it is striking how similar this basic structure is for both Marine magnetic anomalies can provide continental and oceanic systems, despite important insight into the structure, significant variations in geologic setting. history, and lithologic composition of oceanic core complexes (OCCs) while If geophysical methods enjoy the paleomagnetic measurements can advantage of working on living patients, constrain crustal rotation and the timing then so to do they suffer the drawbacks. of fault motion as well as the source and The images may be equivocal, and the provenance of crustal magnetic

79 properties. Detailed magnetic central dome. The polarity boundary in measurements of OCCs both in terms of the northern region is interpreted to be anomaly measurements and rock a rotated cooling isotherm in a magnetic results from surface sampling predominantly gabbroic layer. Two- and drilling show a wide range of dimensional modeling of core complex behavior leading to a somewhat complex thermal structure indicates that a 580C picture of OCC crustal magnetization. Curie cooling isotherm with an initial dip For example, magnetic anomaly polarity of ~165 deg. at the ridge axis can be reversals can either continue rotated by up to 45 degrees, to a uninterrupted across some OCCs (e.g. maximum dip of ~120 deg. after 3 Ma. Kane or Atlantis Bank 735B) or are In contrast, the polarity boundary in the severely disrupted in terms of coherency serpentinized peridotites along the and anomaly amplitude (e.g. Mid central region likely reflects an alteration Atlantic Ridge 12-14N or TAG). front. Assuming that the serpentinized Similarly, paleomagnetic results reveal peridotites experienced the same univectoral distribution in some drilled tectonic rotation as the northern sections (Atlantis Bank, 735B) or show a gabbros, the initial dip of the polarity complex directional history (e.g. MARK boundary near the ridge axis would be area). The Kane OCC on the southern close to vertical (~76 deg.). This side of Kane Transform Fault exposes boundary geometry may result from both gabbro and serpentinized perturbed isotherms that dip steeply peridotites and also records an towards the ridge axis in the footwall of uninterrupted magnetic polarity the detachment fault due to seawater boundary (C2r.2r/ C2An.1n, ~2.581 penetration. Ma). The Kane OCC thus provides a superb opportunity to investigate the relationship between crustal Poster I-24 magnetization, its initial formation and The Origin of the Smooth Zone in subsequent evolution as part of a Early Cretaceous North Atlantic detachment fault system. Rock Magnetic Anomalies magnetic measurements from Kane OCC samples collected by ROV show that Tominaga, Masako; Sager, William both the outcropping gabbro (NRM 1.5 A/m) and serpentinized peridotite (NRM W M. Tominaga, Geology and 4.7 A/m) have sufficient magnetization Geophysics, Woods Hole Oceanographic intensity and stability to act as the Institution, Woods Hole, MA; M. source of the magnetization as well as Tominaga, W.W. Sager, Oceanography, an ability to record a coherent magnetic Texas A&M University, College Station, signal. Similarly, two transform parallel, TX near-bottom magnetic profiles acquired across the northern and central domes Late Jurassic-Early Cretaceous marine of Kane OCC using the autonomous magnetic anomalies observed in the vehicle ABE document the seafloor North Atlantic exhibit an abrupt change location of the reversal boundary. We in character in M5-M15 (130-140 Ma) used an Analytic Signal approach to crust. The anomalies are smoother with extract information from these magnetic low amplitudes, and are difficult to anomalies to determine the range of dip correlate among nearby profiles. angle solutions for the reversal Previous studies suggested that the boundary, assuming an initial explanation for the origin of this smooth remanence direction parallel to the zone is diminished resolution and geocentric axial dipole. Curiously, the anomaly interference due to slow polarity boundary is determined to dip spreading rates. Our new magnetic away from the spreading axis (134 deg. modeling of these anomalies indicates ±14 deg.) in the northern gabbro- that neither slow spreading rates alone dominated dome but towards the nor slow spreading rates in combination spreading axis (41 deg. ±17 deg.) in the with a decrease in geomagnetic field serpentinized peridotite-dominated intensity can explain the basic character

80 of the smooth zone. Combined with structures that mimic the form of basalt other geophysical evidence, we suggest pillows. The slabby marlstones contain that this smooth zone is explained by a Fe-Mn-rich interbeds that, like the less coherent magnetic source produced mound cement, are of hydrothermal by a thinned crustal basalt layer or a origin. The slabs in some places are on non-basaltic magnetic source layer the flanks or bases of mounds and in resulting from low melt supply during a other places are on relatively smooth period of ultra-slow spreading. seafloor. The latter form is often cracked in linear to polygonal patterns, Poster II-72 with upturned ridges produced at the Manifestations of Hydrothermal cracks. Geochemical data indicate that Venting at Kane Megamullion some of the upper Fe-Mn layers in the slabs, as well as outer coatings on rock Tucholke, Brian E.; Humphris, Susan; and breccia samples, were precipitated Dick, Henry J. directly from seawater. Thus, at least in these samples, there is no evidence for Geology & Geophysics, Woods Hole recent hydrothermal venting. We Oceanographic Inst., Woods Hole, MA propose that the debris mounds were formed where point-source fluid venting Kane megamullion was formed between occurred through the feather edge of the about 3.3 Ma and 2.1 Ma by a detachment hanging wall near the active detachment fault that extended up to 35 fault trace. The fluids cemented the km south from the eastern inside corner hanging-wall debris and bonded it to the of Kane FZ. The northern margin of the emerging footwall, while surrounding, megamullion is coincident with the south unconsolidated debris wasted away wall of the Kane transform. The down the sloping (30-40 deg.) fault detachment exhumed sheeted dikes, surface. The mounds that consist diabase sills, gabbros, and mantle predominantly of Fe-Mn-rich sediments peridotites, as documented from probably formed updip from the fault imaging and sampling by ROV Jason, the trace. The cemented slabby marlstones autonomous vehicle ABE, and dredging. may have formed in association with Manifestations of hydrothermal more diffuse venting of hydrothermal cementation and sedimentation occur on fluids through fractures in the footwall the detachment surface as: 1) cemented as it was exhumed and subjected to mounds of rock debris and sediment extensional bending stress in its upper enriched in Mn-Fe-rich hydrothermal part. Precipitation of hydrothermal Fe- precipitates, and 2) cemented, slabby Mn oxyhydroxides within mounds and marlstones with Mn-Fe-rich layers. The within accumulating pelagic carbonates mounds range from conical to elongate may have caused internal expansion or ridge-like, with heights of 1 to 8 that led to observed pillow-like flows and meters and flank slopes of 30 deg. to polygonal crack-and-ridge deformation, near-vertical. Mound compositions respectively. Our observations indicate range from nearly pure rock debris that there are pathways of focused (mostly basalt, but also gabbro and hydrothermal fluid flow that are serpentinite), to polymict breccia, to intimately associated with major normal nearly pure sediment. Thin-section and faults. In addition, widespread diffuse XRD analyses show two kinds of (and locally focused) fluid flow may be breccias: 1) serpentinite clasts cemented commonly associated with fractures with carbonates in which microfossils are formed in the emerging, bending replaced by Fe-Mn oxyhydroxides, and footwalls of the faults. 2) basaltic clasts cemented with quartz. Fe/Mn ratios, and trace- and rare-earth- element concentrations indicate that the Fe-Mn oxyhydroxides in serpentine breccia cement are of hydrothermal origin. The Fe-Mn oxyhydroxides in some sedimentary mounds show flow

81 Poster II-60 material melts very rapidly at depth, Life Cycle of Oceanic Core producing the group 1 basalts, leaving Complexes, 4: Mantle Melting the mantle column depleted and Controls on a Magmatic Crustal dehydrated. This promotes a brief hiatus Accretion in melting, allowing spreading to be taken up by detachment initiation. When Unsworth, Samantha; Murton, melting resumes, Group 2 basalts are Bramley; Taylor, Rex generated from the dry, depleted source. Thus, periods of low melt S. Unsworth, B. Murton, R. Taylor, production at amagmatic regions, National Oceanograpy Centre, University resulting directly from mantle of Southampton, Southampton, UNITED compositional characteristics, drive the KINGDOM transition from magmatic to tectonic spreading. Additionally, we show that Abstract Slow-spreading ridges are often melt produced at the axis today is characterised by variable volcanic cover, partially intruded along the detachment with some magmatically robust areas, fault as dykes, reducing the amount of and other areas which experience little volcanism expressed at the axis, and volcanism. Here, spreading is giving the impression of magma accommodated via extension along low- starvation during the active lifetime of angle detachment faults which expose the core complex. mantle peridotite to form structures known as oceanic core complexes Poster I-18 (OCC). Little is known about the Heterogeneous Seismic Velocity mechanisms which control the transition Structure of the Upper Lithosphere between these two spreading states. We at Kane Oceanic Core Complex, Mid- investigate these mechanisms in detail Atlantic Ridge 23°17’N-23°37’N at the Mid-Atlantic Ridge (MAR,12°60’N- 15°20’N). The geochemistry of the Xu, Min; Canales, Juan Pablo; Tucholke, extrusives along the neovolcanic axis Brian E; DuBois, David yields no evidence to suggest that melt production is reduced at OCCs, despite M. Xu, MIT-WHOI Joint Program, Woods the lack of volcanism at these locations. Hole, MA; J.P. Canales, B.E. Tucholke, However, the geochemistry of off-axis D. DuBois, Woods Hole Oceanographic basalts, erupted at the time of OCC Institution, Woods Hole, MA initiation (13°30’N, -44°95W), is substantially different. Two groups are We use multichannel seismic (MCS) data identified here, with bimodal to constrain the shallow seismic velocity compositions bracketing those of the structure of the Kane Oceanic Core neovolcanic axis. The first consists of Complex (OCC). The high-resolution volatile-rich, CPX-bearing E-MORB, and traveltime seismic tomographic models the second of more N-MORB like show large lateral variability in P-wave compositions with flat REE profiles. High velocity within the upper ~0.5-2.0 km Yb/Lu ratios, taken as a proxy for the lithosphere, and these variations residual garnet in the melting zone, correlate to first order with observed suggest that group 1 are high-pressure, variations in lithology, documented by in low melt fraction products. Group 2 have situ basement samples and seafloor lower Yb/Lu ratios consistent with spinel morphology. Lithological interpretation phase melting, and compositions of the velocity models indicates that reflecting higher degrees of mantle there is marked lateral variability in melting. Group 1 have Pb, Nd and Sr distribution of gabbroic intrusions, isotope ratios with a HIMU flavour. serpentinized peridotites, and basalts at These lie off the local neovolcanic axis scales of a few km to ~10 km. trend, indicating that during OCC Serpentinized peridotites appear to initiation; mantle sources were dominate the central and older parts of geochemically distinct. We conclude the OCC. High-velocity gabbros are that this enriched and HIMU source consistently (but not exclusively)

82 present closer to the termination of the Kane detachment fault and toward the ends of the OCC. The structure of the lithosphere exhumed by the Kane detachment fault is far from the standard ophiolite-based Penrose model, and it does not show segment-centered magmatism that is commonly interpreted at slow-spreading ridges. If the gabbros exhumed toward the termination of the OCC were emplaced deep (ca. 10 km) beneath the spreading axis, they may have constituted a weak zone that focused initiation of the Kane detachment fault. Alternately, as the OCC footwall was being exhumed the gabbros may have been emplaced due to dynamic changes in melt supply, changes in mantle fertility, or decompression melting. Late-stage volcanism is clearly associated with a major high-angle normal fault that cuts the detachment surface; this volcanism may have been stimulated or enhanced by bending stresses in the bending footwall. The shape of the large-scale corrugated morphology of the OCC is nearly invariant in the dip direction across major changes in basement lithology, indicating that once established, the form of the Kane detachment fault was highly resistant to modification. http://www.agu.org/pubs/crossref/2009 /2009GC002586.shtml

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