Mechanical decoupling and thermal structure at the East Pacific Rise axis 9°N: Constraints from axial magma chamber geometry and seafloor structures Violaine Combier ⁎, Satish C. Singh, Mathilde Cannat, Javier Escartin Equipe de Géosciences Marines, Institut de Physique du Globe de Paris, CNRS-UMR 7154, France ABSTRACT ARTICLE INFO Article history: Received 17 March 2008 Accepted 22 March 2008 Available online 9 April 2008 Editor: G.D. Price Keywords: magma chamber melt sill seismic derived bathymetry EPR 9 N OSC We study the relationships between the seafloor structures and the axial magma chamber geometry in the 9°N overlapping spreading center (OSC) area on the fast spreading East Pacific Rise (EPR). Our observations are based on a new high resolution bathymetric map of the 9°N OSC area derived from picks of the seafloor arrival on 3D seismic data, and on previously published data that constrain the presence and distribution of melt below the 9°N OSC. Differences in the orientation of structures between the seafloor and the magma chamber indicate a sharp change in principal stress directions with depth, suggesting that the brittle crust above the melt sill is decoupled from the melt sill itself and the ductile crust underlying it. The stress-field within the brittle upper crust results from a local interaction of the two overlapping spreading centers, whereas the stress-field in the crust below the melt sill corresponds to the regional stress-field imposed by plate separation. Given this mechanical structure of the crust, the melt sill shape and location appear to be controlled by the following factors: the location of the deep melt source below the melt sill, the ambient stress-field at the depth of the melt sill, and the stress-field in the brittle upper crust above the melt sill, which thermally shapes the roof of the melt sill through repeated eruptions. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The nature and geometry of crustal magma chambers at fast spreading ridges have been revealed by numerous experiments for the past 30 years. However, the coupling between the magma chamber at depth and the plate boundary on the seafloor has yet to be understood, particularly in the region of the 9°N overlapping spreading center (OSC) of the East Pacific Rise (EPR) where the geometries of seafloor and magma chamber structures are complex. Indeed, seismic studies have revealed the existence of a quasi- ubiquitous seismic reflector beneath the EPR axis (e.g. Detrick et al., 1987; Kent et al., 1993a,b; TERA Group et al., 1997), which is underlain by a low velocity zone (Harding et al., 1989; Toomey et al., 1990; Vera et al., 1990; Dunn et al., 2000). This reflector is interpreted as the roof of a magma sill containing a high melt fraction (Collier and Singh, 1997; Singh et al., 1998), overlying a crystal mush in the mid- and lower- crustal sections (Harding et al., 1989; Toomey et al., 1990; Vera et al., 1990; Dunn et al., 2000). The melt sill is believed to be the source of dykes feeding eruptions at the axis (e.g. Tolstoy et al., 2006). It is 800 to 2300 m deep below the seafloor, 300 to 4100 m wide (data compiled by Hooft et al., 1997) and 50 to 300 m thick (Kent et al., 1993a; Collier and Singh, 1997; Singh et al., 1998). In parts of the ridge far from discontinuities, seafloor structures and melt sill geometry show little variation in the along-axis direction, and the melt sill is usually centered beneath the ridge axis that marks the location of the plate boundary (Kent et al., 1993a). By contrast, at the 9°N OSC, which is a discontinuity of the EPR axis, the geometries of seafloor structures and melt concentrations in the crust are three- dimensional (3D). Crustal melt concentrations, including the melt sill, are not necessarily centered beneath the ridge axis but are offset up to 2 km away from the axis (Kent et al., 2000; Crawford and Webb, 2002; Tong et al., 2002; Bazin et al., 2003; Singh et al., 2006); the melt sill width varies in the along-axis direction, reaching an anomalously large width of 4 km (Kent et al., 1993b, 2000). In the present paper, we study the geometrical relationships between seafloor and magma chamber structures at the 9°N OSC. Our observations are based on a new high-resolution bathymetric map of the 9°N OSC area that is derived from picks of the seafloor arrival on 3D seismic data, and on previously published seismic data concerning crustal melt concentrations at the 9°N OSC. We interpret the data in terms of mechanical and thermal structure of the ridge axis, and investigate the factors controlling the shape and location of the melt sill. 2. 9°N overlapping spreading center At fast to intermediate spreading rates, lateral offsets of 1 to 30 km in the ridge axes are accommodated by OSCs rather than by transform Earth and Planetary Science Letters 272 (2008) 19–28 ⁎ Corresponding author. Full postal address: Equipe de Géosciences Marines, Institut de Physique du Globe de Paris, cc 89, 4 place Jussieu, 75252 Paris cedex 05, France. Tel.: +33 1 44 27 99 71; fax: +33 1 44 27 99 69. E-mail addresses: combier@ipgp.jussieu.fr (V. Combier), singh@ipgp.jussieu.fr (S.C. Singh), cannat@ipgp.jussieu.fr (M. Cannat), escartin@ipgp.jussieu.fr (J. Escartin). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.03.046 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl