Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise

Abstract

The determination of melt distribution in the crust and the nature of the crust-mantle boundary (the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.

Sill to surface: Linking young off-axis volcanism with subsurface melt at the overlapping spreading center at 9°03′N East Pacific Rise

“…In addition, though the illustration suggests the presence of a dacitic magma body overlying the sub-axial magma sill, the location of dacitic magma is not truly known. Detailed images and information on the seismically determined distribution and geometry of subsurface melt can be found in Kent et al (2000) and Singh et al (2006). model U-Th differentiation ages.…”

“…However, this melt "sill" is not clearly a continuous melt-rich region, but it is instead complex in 3-D geometry and distribution and may include a network of isolated melt-rich regions away from the melt accumulation underlying the axial graben (cf., Fig. 3d of Singh et al, 2006).…”

“…The overlapping spreading center (OSC) at 9103′N EPR presents an ideal opportunity for studying the relationships between volcanism and magmatism because the subsurface melt distribution and seafloor morphology have recently been well-characterized in a number of geophysical and geological studies (e.g., Bazin et al,a low velocity anomaly interpreted as melt in the uppermost mantle extending across the OSC, with the lowest velocities centered beneath the eastern limb (Dunn et al, 2000;Toomey et al, 2007), melt accumulation in the overlying crust (Singh et al, 2006), and a mid-crustal mush zone at the northern end of the overlap basin at~9108′N (Bazin et al, 2003;Crawford and Webb, 2002). In addition, a 3-D seismic reflection study reveals a~4 km wide, upper-crustal melt sill beneath the eastern limb that extends westward into the northern portion of the overlap basin (Kent et al, 2000).…”

Root zone of the sheeted dike complex in the Oman ophiolite

“…[2] A small melt lens has been imaged by multichannel seismic profiles in the late 1980s below fast spreading centers, extending <1 to 4 km on each side of the ridge axis and locally as thin as 30 m [e.g., Detrick et al, 1987;Harding et al, 1989;Collier and Singh, 1997;Singh et al, 1998Singh et al, , 2006Kent et al, 2000]. This discovery has raised the question of how this lens relates to the overlying sheeted dike complex, as previously described from oceanic drilling [e.g., Alt et al, 1993Alt et al, , 1996Bach et al, 2003] and in tectonic windows such as Hess Deep [Karson et al, 1992[Karson et al, , 2002b and fracture zones [e.g., Juteau et al, 1995;Karson et al, 2002a].…”

Temporal and petrogenetic constraints on volcanic accretionary processes at 9-10 degrees north East Pacific Rise

“…In contrast to many large MOR discontinuities, the large overlapping spreading center (OSC) at 9º03'N East Pacific Rise (EPR), a 2 nd order discontinuity with an ~8 km offset and ~27 km of overlap, has been the subject of extensive tectonic and geophysical study Combier et al, 2008;Dunn et al, 2001;Sempere and Macdonald, 1984;1986a,b;Toomey et al, 2007;Tong et al, 2002;Singh et al, 2006;White et al, 2009), including the first detailed 3-D multi-channel seismic reflection survey of a MOR (Kent et al, 2000). However, only limited geochemical and petrologic work has been done to date and these studies (Langmuir et al, 1986;Natland and Melson, 1980;Natland et al, 1986a, b) indicate that this tectonic discontinuity delimits a significant magmatic discontinuity.…”