Upper crustal seismic structure of the slow spreading Mid‐Atlantic Ridge, 35°N: Constraints on volcanic emplacement processes

Abstract

[1] The upper crustal seismic structure of the slow spreading Mid-Atlantic Ridge is studied using a genetic algorithm-based waveform inversion of multichannel streamer data. Four single-ship multichannel profiles from 35°N are analyzed: one in the rift valley and three in the rift mountains along 0.7, 1.6, and 1.9 Ma crust. A layer 2A horizon is continuously imaged along three profiles and is associated with a sharp velocity increase from extrusives to dikes. Its depth and regularity in the rift valley indicate that most of the extrusive section is built on the inner valley floor through a pattern of deposition and fault-bounded uplift into the rift mountains. Its variability along one line, however, shows that this process is disrupted during tectonically dominated periods. A thickening of layer 2A toward the Oceanographer fracture zone may be the result of along-axis magma transport. The interval of rapid velocity increase at the base of layer 2A thins with age, a possible response to enhanced hydrothermal mineralization within the zone of mixed dikes and extrusives. Transition zone ($200 m) and off-axis layer 2A thicknesses (350-600 m) are similar to those at other spreading centers. This indicates that equivalent extrusive volumes are produced at all spreading rates along a relatively narrow zone of dike emplacement. However, differences in on-axis layer 2A thickness between this area and fast spreading ridges suggest that the exact pattern of thickening varies between spreading regimes. Relative to fast spreading ridges, the moderate velocity increase with age recorded in the upper crust (from 2.3 to >2.7 km s À1 within $2 Myr) may be due to a less active hydrothermal system and hence slower porosity reduction. INDEX TERMS: 3035 Marine

Crust and Lithospheric Structure - Seismic Structure of Mid-Ocean Ridges

“…The low-velocity layer, seismic layer 2, is usually observed throughout the top 1-2 km of the crust (Barclay et al, 1998;Canales et al, 2000a;Dunn et al, 2005;Hooft et al, 2000;Hosford et al, 2001;Hussenoeder et al, 2002) and is generally accepted to be the result of a high proportion of pores and cracks at these depths. The low-velocity layer, seismic layer 2, is usually observed throughout the top 1-2 km of the crust (Barclay et al, 1998;Canales et al, 2000a;Dunn et al, 2005;Hooft et al, 2000;Hosford et al, 2001;Hussenoeder et al, 2002) and is generally accepted to be the result of a high proportion of pores and cracks at these depths.…”

“…The low-velocity layer, seismic layer 2, is usually observed throughout the top 1-2 km of the crust (Barclay et al, 1998;Canales et al, 2000a;Dunn et al, 2005;Hooft et al, 2000;Hosford et al, 2001;Hussenoeder et al, 2002) and is generally accepted to be the result of a high proportion of pores and cracks at these depths. The seismic structure of the upper crust is laterally variable and appears to reflect a variety of processes, such as tectonic modification, volcanic flux variations, and crustal aging (Arnulf et al, 2012;Barclay et al, 1998;Dunn et al, 2005;Hosford et al, 2001;Hussenoeder et al, 2002;Magde et al, 2000). The seismic structure of the upper crust is laterally variable and appears to reflect a variety of processes, such as tectonic modification, volcanic flux variations, and crustal aging (Arnulf et al, 2012;Barclay et al, 1998;Dunn et al, 2005;Hosford et al, 2001;Hussenoeder et al, 2002;Magde et al, 2000).…”

Three‐dimensional seismic structure of the Mid‐Atlantic Ridge (35°N): Evidence for focused melt supply and lower crustal dike injection

“…Several observations indicate that the upper 3–4 km of crust is cool and brittle. A seismic low‐velocity layer observed throughout the top 1–2 km of the crust [ Barclay et al , 1998; Hooft et al , 2000; Canales et al , 2000a; Hosford et al , 2001; Hussenoeder et al , 2002], known as seismic layer 2, is observed here as well as worldwide in the oceans and is generally accepted to be the result of a high proportion of pores and cracks at these depths [e.g., Spudich and Orcutt , 1980; Detrick et al , 1994; Swift et al , 1998]. An analysis of the ratio of P to S wave velocity in the shallow crust and the detection of seismic anisotropy, an indicator of widespread ridge‐parallel extension cracks, reveals that the upper 2 km of crust is pervaded with cracks and is thus relatively cool and brittle [ Barclay et al , 2001; Barclay and Toomey , 2003].…”

“…The seismic structure of the upper crust is laterally variable and appears to reflect a variety of processes such as tectonic modification, crustal accretion variations, and crustal aging [ Barclay et al , 1998; Magde et al , 2000; Hosford et al , 2001; Hussenoeder et al , 2002]. In contrast to fast spreading ridges where the axis of eruption is fairly narrow (∼1–2 km), formation of new crust appears to occur over the entire width of the axial valley [ Barclay et al , 1998].…”

Crustal magma plumbing within a segment of the Mid-Atlantic Ridge, 35°N

“…tectable given our ray set. Seismic re£ection pro¢les in our study area have determined one-dimensional shallow (to depths of 800 m) velocity structures for 11 locations, ranging in age from 0 to 1.9 Ma [23]. Ray tracing through these structures we ¢nd a maximum 1-way variation of 20 ms for vertically propagating P-waves.…”