International Ocean Discovery Program; Expedition 360 preliminary report; Southwest Indian Ridge lower crust and Moho; the nature of the lower crust and Moho at slower spreading ridges (SloMo Leg 1); 30 November 2015-30 January 2016

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doi: 10.14379/
Author(s): Dick, Henry J. B.; MacLeod, Christopher J.; Blum, Peter; Abe, Natsue; Blackman, Donna K.; Bowles, Julie A.; Cheadle, Michael J.; Cho, Kyungo; Ciazela, Jakub; Deans, Jeremy R.; Edgcomb, Virginia P.; Ferrando, Carlotta; France, Lydéric; Ghosh, Biswajit; Ildefonse, Benoît M.; Kendrick, Mark A.; Koepke, Juergen H.; Leong, James A. M.; Liu Chuanzhou; Ma Qiang; Morishita, Tomoaki; Morris, Antony; Natland, James H.; Nozaka, Toshio; Pluemper, Oliver; Sanfilippo, Alessio; Sylvan, Jason B.; Tivey, Maurice A.; Tribuzio, Riccardo; Viegas, Luis G. F.
International Ocean Discovery Program, Expedition 360 Scientists, College Station, TX
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA, United States
Cardiff University, United Kingdom
Texas A&M University, United States
Japan Agency for Marine-Earth Science and Technology, Japan
Scripps Institution of Oceanography, United States
University of Wisconsin-Milwaukee, United States
University of Wyoming, United States
Pukyong National University, South Korea
Adam Mickiewicz University, Poland
Texas Tech University, United States
Université de Montpellier, France
Université de Lorraine, France
University of Calcutta, India
Australian National University, Australia
University of Hannover, Germany
Arizona State University, United States
Chinese Academy of Sciences, Institute of Geology and Geophysics, China
Tongji University, China
Kanazawa University, Japan
University of Plymouth, United Kingdom
University of Miami, United States
Okayama University, Japan
Utrecht University, Netherlands
Universita degli Studi di Pavia, Italy
Universidade de Sao Paulo, Brazil
Source: Preliminary Report - International Ocean Discovery Program, Vol.360, 50p. Publisher: International Ocean Discovery Program, College Station, TX, United States. ISSN: 2372-9562
Note: In English. 85 refs.
Summary: International Ocean Discovery Program (IODP) Expedition 360 was the first leg of Phase I of the SloMo (shorthand for "The nature of the lower crust and Moho at slower spreading ridges") Project, a multiphase drilling program that proposes to drill through the outermost of the global seismic velocity discontinuities, the Mohorovičić seismic discontinuity (Moho). The Moho corresponds to a compressional wave velocity increase, typically at ∼7 km beneath the oceans, and has generally been regarded as the boundary between crust and mantle. An alternative model, that the Moho is a hydration front in the mantle, has recently gained credence upon the discovery of abundant partially serpentinized peridotite on the seafloor and on the walls of fracture zones, such as at Atlantis Bank, an 11-13 My old elevated oceanic core complex massif adjacent to the Atlantis II Transform on the Southwest Indian Ridge. Hole U1473A was drilled on the summit of Atlantis Bank during IODP Expedition 360, 1-2 km away from two previous Ocean Drilling Program (ODP) holes: Hole 735B (drilled during ODP Leg 118 in 1987 and ODP Leg 176 in 1997) and Hole 1105A (drilled during ODP Leg 179 in 1998). A mantle peridotite/gabbro contact has been traced by dredging and diving along the transform wall for 40 km. The contact is located at ∼4200 m depth at the drill sites but shoals considerably 20 km to the south, where it was observed in outcrop at 2563 m depth. Moho reflections have, however, been found at ∼5-6 km beneath Atlantis Bank and <4 km beneath the transform wall, leading to the suggestion that the seismic discontinuity may not represent the crust/mantle boundary but rather an alteration (serpentinization) front. This then raises the interesting possibility that a whole new planetary biosphere may thrive due to methanogenesis associated with serpentinization. The SloMo Project seeks to test these two hypotheses at Atlantis Bank and evaluate carbon sequestration in the lower crust and uppermost mantle. A primary objective of SloMo Leg 1 was to explore the lateral variability of the stratigraphy established in Hole 735B. Comparison of Hole U1473A with Holes 735B and 1105A allows us to demonstrate a continuity of process and complex interplay of magmatic accretion and steady-state detachment faulting over a time period of ∼128 ky. Preliminary assessment indicates that these sections of lower crust are constructed by repeated cycles of intrusion, represented in Hole U1473A by approximately three upwardly differentiated hundreds of meter-scale bodies of olivine gabbro broadly similar to those encountered in the deeper parts of Hole 735B. Specific aims of Expedition 360 focused on gaining an understanding of how magmatism and tectonism interact in accommodating seafloor spreading, how magnetic reversal boundaries are expressed in the lower crust, assessing the role of the lower crust and shallow mantle in the global carbon cycle, and constraining the extent and nature of life at deep levels within the ocean lithosphere.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; 18 Geophysics, Solid-Earth; Alteration; Atlantis Bank; Atlantis II fracture zone; Boreholes; Cores; Crust; Drilling; Expedition 360; Gabbros; Geochemistry; IODP Site U1473; Igneous rocks; Indian Ocean; International Ocean Discovery Program; Leg 118; Leg 176; Leg 179; Lithosphere; Lower crust; Magmatism; Magnetic properties; Magnetic susceptibility; Mantle; Marine drilling; Metasomatism; Mohorovicic discontinuity; ODP Site 1105; ODP Site 735; Ocean Drilling Program; Olivine gabbro; Paleomagnetism; Peridotites; Petrology; Physical properties; Plutonic rocks; Sea-floor spreading; Serpentinization; Southwest Indian Ridge; Structural analysis; Ultramafics; Upper mantle
Coordinates: S324300 S324300 E0571700 E0571700
Record ID: 2016038197
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