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Crystallization depth beneath an oceanic detachment fault (ODP Hole 923A, Mid-Atlantic Ridge)

Lissenberg, Cornelis Johan, Rioux, Matthew, MacLeod, Christopher John, Bowring, Samuel A. and Shimizu, Nobumichi 2016. Crystallization depth beneath an oceanic detachment fault (ODP Hole 923A, Mid-Atlantic Ridge). Geochemistry Geophysics Geosystems 17 (1) , pp. 162-180. 10.1002/2015GC006027

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Abstract

Oceanic detachment faults are increasingly recognised as playing an integral role in the seafloor spreading process at slow and ultraslow spreading mid-ocean ridges, with significant consequences for the architecture of the oceanic lithosphere. Although melt supply is considered to play a critical control on the formation and evolution of oceanic detachments, much less well understood is how melts and faults interact and influence each other. Few direct constraints on the locus and depth of melt emplacement in the vicinity of detachments are available. Gabbros drilled in ODP Hole 923A near the intersection of the Mid-Atlantic Ridge and the Kane transform fault (23°N; the MARK area) represent magmas emplaced into the footwall of such a detachment fault and unroofed by it. We here present U-Pb zircon dates for these gabbros and associated diorite veins which, when combined with a tectonic reconstruction of the area, allow us to calculate the depths at which the melts crystallized. Th-corrected single zircon U-Pb dates from three samples range from 1.138 ± 0.062 Ma to 1.213 ± 0.021 Ma. We find a crystallization depth of 6.4 +1.7/-1.3 km, and estimate that the melts parental to the gabbros were initially emplaced up to 1.5 km deeper, at <8 km below the seafloor. The tectonic reconstruction implies that the detachment fault responsible for the exposure of the sampled sequence likely crossed the ridge axis at depth, suggesting that melt emplacement into the footwall of oceanic detachment faults is an important process. The deep emplacement depth we find associated with ‘detachment mode' spreading at ∼1.2 Ma appears to be significantly greater than the depth of magma reservoirs during the current ‘magmatic mode' of spreading in the area, suggesting that the northern MARK segment preserves a recent switch between two temporally distinct modes of spreading with fundamentally different lithospheric architecture. This article is protected by copyright. All rights reserved.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Publisher: American Geophysical Union.
ISSN: 1525-2027
Funders: NERC
Date of First Compliant Deposit: 30 March 2016
Date of Acceptance: 11 December 2015
Last Modified: 04 Jun 2017 08:46
URI: http://orca-mwe.cf.ac.uk/id/eprint/84467

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