Integrated Ocean Drilling Program Expedition 305 preliminary report; oceanic core complex formation, Atlantic Massif 2; oceanic core complex formation, Atlantis Massif, Mid-Atlantic Ridge; drilling into the footwall and hanging wall of a tectonic exposure of deep, young oceanic lithosphere to study deformation, alteration, and melt generation; 8 January-2 March 2005

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doi: 10.2204/
Author(s): Blackman, Donna; Ildefonse, Benoit; John, Barbara E.; Ohara, Yasuhiko; Miller, D. Jay; MacLeod, Christopher J.; Delius, Heike; Abe, Natsue; Beard, James S.; Brunelli, Daniele; Delacour, Adélie G.; Escartin, Javier; Fryer, Patricia B.; Halfpenny, Angela; Hansen, Heidi-Elisabeth; Harris, Amber C.; Hasebe, Akihiro Tamura; Hellebrand, Eric; Ishimaru, Satoko; Johnson, Kevin T. M.; Karner, Garry D.; Linek, Margaret; Mason, Olivia U.; Michibayashi, Katsuyoshi; Nozaka, Toshio; Rosner, Martin; Suhr, Guenter; Tominaga, Masako; Yamasaki, Toru; Zhao, Xixi; Einaudi, Florence; Abratis, Michael W.; Andal, Eric S.; Andreani, Muriel; Awaji, Shunsaku; Charney, Allison; Christie, David; Drouin, Marion; Frost, Bryce R.; Gee, Jeffrey S.; Godard, Marguerite; Grimes, Craig B.; Hayman, Nicholas W.; Hirose, Takehiro; Hirth, James Gregory; Maeda, Jinichiro; McCraig, Andrew M.; Morris, Antony; Nakagawa, Tatsunori; Searle, Roger C.; von der Handt, Anette
Integrated Ocean Drilling Program, Expedition 304 Scientists, College Station, TX
Author Affiliation(s): Primary:
Scripps Institution of Oceanography, La Jolla, CA, United States
Université de Montpellier II, France
University of Wyoming, United States
Hydrographic and Oceanographic Department of Japan, Japan
Integrated Ocean Drilling Program, United States
Cardiff University, United Kingdom
University of Leicester, United Kingdom
Japan Marine Science and Technology Center, Japan
Virginia Museum of Natural History, United States
Laboratoire Pierre Sue-DRECAM, France
Eidgenössische Technische Hochschule Zürich, Switzerland
Université Pierre et Marie Curie, France
University of Hawaii at Manoa, United States
Source: Preliminary Report (Integrated Ocean Drilling Program), Vol.305, 78p. Publisher: IODP Management International, College Station, TX, United States. ISSN: 1932-9423
Note: In English. 40 refs.
Summary: Integrated Ocean Drilling Program Expedition 305, a joint science program with Expedition 304, was designed to investigate the processes that control formation of oceanic core complexes, as well as the exposure of ultramafic rocks in very young oceanic lithosphere. Prior studies indicated that two main drill sites on Atlantis Massif, on the western rift flank of the Mid-Atlantic Ridge (MAR) at 30°N, could provide key constraints on the structure of the detachment fault zone, rock types exposed at shallow structural levels in the footwall, and their alteration history, as well as that of the volcanic succession in the hanging wall. Expedition 305 deepened Hole U1309D in the footwall of Atlantis Massif to 1415.5 meters below seafloor, with high recovery (average = 74.8%) of dominantly gabbroic rocks. Hole U1309D was logged twice, providing the opportunity for unprecedented core-logging integration for a deep borehole in the oceanic lithosphere. The recovered rocks range from dunitic troctolite, troctolite, (olivine) gabbro, and gabbronorite to evolved oxide gabbro that locally contains abundant zircon and apatite, and diabase. The texture of the dunitic troctolite suggests a cumulate origin. The gabbroic suite from Hole U1309D is among the most primitive recovered from the MAR, with Mg# ranging from 67 to 87. Although alteration mineral assemblages record cooling of gabbroic rocks from magmatic conditions to zeolite facies, a low-temperature phase that reflects alteration at temperatures <500°C is most significant. The overall trends in alteration and the changes in secondary mineralogy downhole suggest that there may be two separate secondary processes that have affected the footwall in the vicinity of Hole U1309D. In the upper ∼840 m, seawater-rock interactions may pervade the gabbroic sequence. Below that depth, the nature of and the fluctuations in degree and style of metamorphism are related to fluids of a different composition percolating along fault/ductile deformation zones. Hence, the core records an extensive history of gabbroic rock-fluid interaction, possibly including magmatic fluids. One of the prominent features of the rocks from Hole U1309D is the lack of extensive amphibolite facies alteration and deformation. This contrasts strongly with the gabbroic suite recovered from Ocean Drilling Program Hole 735B, at the Southwest Indian Ridge. The rocks recovered in Hole U1309D show very little deformation, and any deformation related to a major detachment fault system must have occurred at low temperature and must be strongly localized in the very upper part of the hole. This, together with very minor deformation in the amphibolite facies, is not consistent with the classical "core complex" interpretation of the corrugated, domal massifs on the seafloor resulting from surface exposure of a detachment fault that roots deeply at the base of the lithosphere. In addition, shipboard paleomagnetic measurements indicate there has been no significant net tectonic rotation (≤15°) of the footwall. This seems to preclude a rolling hinge model for the uplift of the core of Atlantis Massif along a single concave, normal fault. The ∼1.4 km sequence of dominantly gabbroic rocks is inconsistent with the initial prediction that the footwall was composed of an uplifted mantle section where serpentinization was responsible for lower densities/seismic velocities in the upper few hundred meters. A more complex model than that put forward before Expeditions 304 and 305 will be required. The fact that we did not reach fresh mantle peridotite, together with the known exposures of serpentinized mantle along the southern ridge of the massif, supports models of complicated lateral heterogeneity in slow-spreading oceanic crust. We have, however, placed a constraint on the magnitude of this heterogeneity-gabbro bodies in this setting can exceed 1.5 km in thickness.
Year of Publication: 2005
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; 18 Geophysics, Solid-Earth; Atlantic Ocean; Atlantis Massif; Basalts; Boreholes; Bouguer anomalies; Cataclasites; Cores; Crust; Deformation; Detachment faults; Diabase; Downhole methods; Drilling; Expedition 305; Expeditions 304/305; Fault zones; Faults; Foot wall; Gabbros; Geochemistry; Gravity anomalies; Hanging wall; Hydrothermal alteration; IODP Site U1309; Igneous rocks; Integrated Ocean Drilling Program; Lithosphere; Lithostratigraphy; Magnetic properties; Magnetic susceptibility; Major elements; Marine drilling; Metamorphic rocks; Metamorphism; Metasomatism; Microorganisms; Mid-Atlantic Ridge; North Atlantic; Oceanic crust; Paleomagnetism; Peridotites; Petrofabrics; Physical properties; Plutonic rocks; Serpentinization; Temperature; Trace elements; Troctolite; Ultramafics; Volcanic rocks; Well logs
Coordinates: N301000 N301100 W0420600 W0420700
Record ID: 2008016933
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