The Atlantis Bank gabbro massif, SW Indian Ridge; the largest know exposure of the lower crust in the oceans

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Author(s): Dick, Henry J.; Kvassnes, Astri J. S.; Kinoshita, Hajima; MacLeod, Christopher J.; Robinson, Paul T.
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Woods Hole, MA, United States
ReStoneAS, Norway
Japan Agency for Marine-Earth Science and Technology, Japan
Cardiff University, United Kingdom
Chinese Academy of Sciences, Institute of Geology, China
Volume Title: AGU 2017 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2017; American Geophysical Union 2017 fall meeting, New Orleans, LA, Dec. 11-15, 2017. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Until the discovery of oceanic core complexes little was known and much inferred about the lower ocean crust at slow-spreading ridges. Their study shows the ocean crust isn't simply a uniform layer-cake of pillow lavas, sheeted dikes and gabbros, but is highly variable in thickness, composition and architecture, and even absent over large regions. The 660 km2 Atlantis Bank Gabbro Massif in the rift-mountains of the SW Indian Ridge flanking the Atlantis II Transform is the magmatic end member for ocean core complexes, and best approximates 'average' slow-spread crust. Thus it has been a focus for drilling since its discovery in 1986, leading to the current attempt to drill to Moho there (Project SloMo). There are 3 ODP and IODP drill holes on its crest: 1508-m deep Hole 735B, 158-m deep Hole 1105A, and 809.4-m deep Hole U1473. These provide a ∼200 Kyr view of lower crustal accretion at a slow-spread ocean ridge. Here we extend this view to ∼2.7 Myr. Mapping and sampling shows the gabbro massif extends nearly the length of a single 2nd order magmatic ridge segment. With numerous inliers of the dike-gabbro transition at numerous locations, and a crust-mantle boundary, traced for ∼30-km along the transform wall, it would appear to represent a full section of the lower crust. As Moho is at ∼5.5±1 km mbsf near Hole 735B, and ∼4.5 km beneath the transform, it is likely a serpentinization front. The crust-mantle boundary was crossed by dives at 4 locations. In each case gabbros at the base of the crust crystallized from melt that had previously fractionated 50% or more from a likely parent. Thus the gabbro massif must be laterally zoned, and the parental mantle melts had to have been emplaced at the center of the paleo-ridge segment, before intruding laterally to the distal end of the complex. Gabbros on a lithospheric flow line down the center of the massif closely resemble those from the drill holes. This shows that while lateral variations in crustal composition and thickness exist at Atlantis Bank, we can extend the conclusions derived from drilling at Hole U1473 that there is a continuum of accretionary magmatic and tectonic processes for 2.7 Myr, and a centrally located deep hole through the lower crust and mantle there will likely be representative of the 660-km2 Atlantis Bank gabbro massif as a whole.
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Atlantis II fracture zone; Crust; Expedition 360; IODP Site U1473; Indian Ocean; International Ocean Discovery Program; Leg 118; Leg 176; Leg 179; Lower crust; ODP Site 1105; ODP Site 735; Ocean Drilling Program; Oceanic crust; SloMo; Southwest Indian Ridge
Coordinates: S324327 S324318 E0571618 E0571557
S324308 S324308 E0571639 E0571639
S324222 S324222 E0571641 E0571641
Record ID: 2018079984
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