Oceanic core complexes and crustal accretion at slow-spreading ridges

Abstract

Oceanic core complexes expose gabbroic rocks on the seafl oor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown. Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of the section to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. We propose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridotite: emplacement of a large intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic core surrounded by deformed serpentinite. Oceanic core complexes may therefore refl ect processes associated with relatively enhanced periods of mafi c intrusion within overall magma-poor regions of slow-and ultra-slow-spreading ridges.

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“…An estimated 40% or more of the Atlantic Ocean floor was formed by interlinked processes of asymmetric extension, magmatism, and detachment faulting resulting in exposure of mantle and lower crustal rocks in OCCs (Dick et al, 2008;Cannat et al, 2006;Smith et al, 2006Smith et al, , 2008Ildefonse et al, 2007;Escartín et al, 2003;Tucholke et al, 2008). OCCs incorporate highly reactive olivine-rich rocks that interact with seawater over a range of temperatures to produce both high-and low-temperature hydrothermal systems (Douville et al, 1997(Douville et al, , 2002Charlou et al, 1998Charlou et al, , 2002Sagalevich et al, 2000;Kelley et al, 2005;McCaig et al, 2007).…”

Oceanic Core Complex Development in Modern and Ancient Oceanic Lithosphere: Gabbro‐Localized versus Peridotite‐Localized Detachment Models

“…The ultramafic rocks recovered from these fault surfaces have been used to invoke a limited role of magmatism during core complex formation, leading to the development of many "amagmatic" models of oceanic detachment faulting (Tucholke and Lin 1994;Tucholke et al 1998;Escartín et al 2003). In contrast, the detachment-fault surfaces of the Atlantis Bank (SWIR) and Kane (MAR) core complexes are dominated by gabbroic and troctolitic rocks and fault schists (Karson and Lawrence 1997;MacLeod et al 1998;Arai et al 2000;Kinoshita et al 2001;Matsumoto et al 2002), leading to models of core complex development that emphasize moderate levels of magmatism during detachment faulting (Dick et al 1992Cannat et al 1997;Karson 1999;MacLeod et al 2003;Ildefonse et al 2007). However, recent deep drilling of two core complexes thought to have developed during reduced magma supply, the Atlantis Massif and the 15Њ45ЈN, has revealed long sections (∼1400 and ∼200 m, respectively) of gabbroic rocks in the footwalls of both massifs Kelemen et al 2007).…”

“…However, recent deep drilling of two core complexes thought to have developed during reduced magma supply, the Atlantis Massif and the 15Њ45ЈN, has revealed long sections (∼1400 and ∼200 m, respectively) of gabbroic rocks in the footwalls of both massifs Kelemen et al 2007). The recovery of gabbroic rocks in these drill cores has prompted revision of the amagmatic models of OCC development (Ildefonse et al 2007), leading to new models that suggest that core complexes form when 30%-50% of the total extension is accommodated by magmatic accretion, requiring significant magma intrusion into the footwalls of OCCs during detachment faulting (Tucholke et al 2008). In light of these new models, we present two end-member models of core complex development, neither of which require amagmatic extension; instead, they differ in (1) the relative proportions of gabbro and peridotite rocks in the footwall and (2) the nature of fabric development within their respective detachment-fault shear zones.…”

A short electromagnetic profile across the Kane Oceanic Core Complex

“…The fault zone appears to be thin, possibly on the order of tens of meters or less [Escartín et al, 2003;Schroeder and John, 2004;Karson et al, 2006], and shows variable composition and degree of alteration (from talc schists to deformed mafic and ultramafic rocks). Some OCC models predict extensive serpentinite bodies while others suggest that serpentinites are essentially restricted to the fault that uplifts the gabbro footwall [Ildefonse et al, 2007]. Thus, the extent of serpentinization, the distribution of lithologies (gabbro, peridotite) and composition of the fault zone and footwall should provide important clues as to the mechanisms of OCC formation.…”