Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

Abstract

SUMMARY A grid of 32 across‐axis and five axis‐parallel multichannel seismic (MCS) reflection profiles were acquired at an axial volcanic ridge (AVR) segment at 57° 45′N, 32° 35′W on the slow‐spreading Reykjanes Ridge, Mid‐Atlantic Ridge, to determine the along‐axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure, the along‐axis continuity and segmentation of melt supply to the crust, the development of faulting and the thickness of oceanic layer 2A. Seismic reflection profiles acquired at mid‐ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events. In this paper, we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed MCS profiles. The 37 MCS profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2, to the axial magmatic system and to the faults which dismember each AVR as it ages through its tectono‐magmatic life cycle and which form the median valley walls. The layer 2A event can be mapped around the entirety of the survey area between 0.1 and 0.5 s two‐way traveltime below the seabed, being thickest at AVR centres, and thinning both off‐axis and along‐axis towards AVR tips. Both AVR‐parallel and ridge‐parallel trends are observed, with the pattern of on‐axis layer 2A thickness variation preserved beneath relict AVRs which are rafted off‐axis largely intact. Each active AVR is underlain by a mid‐crustal melt lens reflection extending almost along its entire length. Similar reflection events are observed beneath the offset basins between adjacent AVRs. These are interpreted as new AVRs at the start of their life cycle, developing centrally within the median valley. The east–west spacings of relict AVRs and offset basins is ∼5–7 km, corresponding to a life span of the order of 0.5–0.7 Myr, during which AVRs appear to undergo multiple 20–60 Kyr tectono‐magmatic cycles.

Temporal and spatial cyclicity of accretion at slow-spreading ridges-evidence from the Reykjanes Ridge

“…It also shows that the main fault terraces are separated by relatively unfaulted regions and that the seabed topography is asymmetric about the ridge axis. In addition, Topping (2002) and Peirce et al (2005), interpreting a grid of multichannel seismic reflection profiles acquired across the RAMESSES AVR, show that there is a distinct asymmetry in the faulting pattern with terraces separated at 2–5 km (200 000–500 000 yr) intervals which also correspond to periods of enhanced magmatism as evidenced by a correspondingly thicker extrusive layer within the upper crust. These observations, in the context of Thatcher & Hill's (1995) study, lend further support to cyclic rather continuous accretion and tectonism.…”

Mapping of seismic layer 2A/2B boundary above the sheeted dike unit at intermediate spreading crust exposed near the Blanco Transform

“…[8] Tectonic lineations, mapped from GLORIA sidescan data [EEZ-Scan 84 Scientific Staff, 1986], are primarily oriented parallel or orthogonal to the Juan de Fuca Ridge axis (Figure 3b). Oblique lineations east of the survey area are interpreted as the trace of an inner pseudofault produced by ridge propagation that intersects the BTF at the saddle between the West and East Blanco Depressions [Peirce et al, 2007], Galapagos Ridge [Blacic et al, 2004], Juan de Fuca Ridge [Canales et al, 2005;Van Ark et al, 2007], Southeast Indian Ridge [Baran et al, 2005], and East Pacific Rise [Carbotte et al, 1997[Carbotte et al, , 2000Harding et al, 1993;Kent et al, 1994]. Lava thicknesses at tectonic windows are from studies at the Blanco Transform [Juteau et al, 1995;Karson et al, 2002b;Tivey et al, 1998], Hess Deep [Francheteau et al, 1990[Francheteau et al, , 1992Karson et al, 1992Karson et al, , 2002a, Pito Deep [Francheteau et al, 1994;Karson, 2005;Morgan et al, 2005], and Endeavor Deep (minimum thickness as sheeted dike complex may not be exposed) .…”

Three‐dimensional geometry of axial magma chamber roof and faults at Lucky Strike volcano on the Mid‐Atlantic Ridge

“…More recent seismic reflection studies have revealed evidence for crustal melt lenses or bodies beneath slow spreading centers. The Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics at 57°45′N [e.g., Sinha et al , ; Peirce et al , ] revealed a seismic reflector 2.5–3 km beneath the seafloor on the Reykjanes Ridge underlain by a larger low‐velocity zone interpreted as a magma chamber that could contain up to 20% melt. Subsequently, the Seismic Study for Monitoring of the Mid‐Atlantic Ridge (SISMOMAR) seismic reflection experiment imaged a bright crustal reflector beneath the central volcano of the Lucky Strike segment of the MAR, interpreted as the roof a magma chamber [ Singh et al , ].…”