Anomalously thin crust in oceanic fracture zones: New seismic constraints from the Kane Fracture Zone

Abstract

Three detailed and carefully positioned seismic refraction experiments have recently been carried out along the Kane fracture zone near 24°N, 44°W in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large Atlantic fracture zones. These three experiments, when combined with earlier studies, result in reversed structural control along about a 300‐km‐long segment of the Kane fracture zone extending from the ridge‐transform intersection out onto crust about 25 m.y. old. An analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and/or velocity. However, along most of the eastern fracture zone trough, upper mantle‐type velocities are found at depths of only 2‐3 km below the seafloor, less than half the typical depth to Moho in the ocean basins. This anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities, relatively high‐velocity gradients throughout most of the thinned crustal section (about 1–2 s−1) and a distinct crust‐mantle boundary. The very thin crust appears to be confined to the deepest parts of the Kane fracture zone trough (<10 km wide), although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone. A specially designed delay time experiment at the intersection of the Kane fracture zone and the Mid‐Atlantic Ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness (about 3 km) except beneath the nodal basin where the crust may be less than 1 km thick. The anomalous crust in the intersection area appears to extend at least part way down the median valley. We interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge‐transform intersections, although tectonic processes may also play a part, at least locally, in thinning the crust. Our results support a “ribbon” model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous, so‐called normal crust 50–100 km wide extend away from spreading centers, each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough.

Seismic Stratigraphy in a Transverse Ridge, Atlantis II Fracture Zone

“…Based on explosive refraction data, Detrick and Purdy (1980) and Cormier et al (1984) suggested that high-velocity material occurred within the transverse ridge north of the Kane Fracture Zone (offset of 150 km), but they could not resolve the thickness of overlying crust. Using gravity and bathymetry data, Louden and Forsyth (1982) determined that the crust beneath the ridge was about one-half normal crustal thickness (~3 km).…”

“…Away from fracture zones, laminated zones and dipping reflectors spaced 5-to 10-km apart appear in lower crustal portions of multichannel profiles (NAT Group, 1985;McCarthy et al, 1988;White et al, 1988). On slowspreading ridges, refraction data indicate that the variability of crustal and Moho seismic structure increases near fracture zones (Detrick and Purdy, 1980;Cormier et al, 1984;Mutter et al, 1984;White et al, 1984;Minshull et al, 1988). These data suggest that beneath fracture zone troughs the crust is often thinner, velocities are lower, and ocean layer 3 velocities (6.8 to 7.1 km/s) are uncommon.…”

Fracture Zone Drilling in the Southwest Indian Ridge

“…In this model, less magma will reach the edges of the spreading cells (i.e., fracture zones) leading to thinner crustal sections. Such thinning of oceanic crust in the vicinity of fracture zones has been demonstrated by seismic studies along the ridge axes (e.g., Detrick and Purdy, 1980;Fox et al, 1980;Cormier et al, 1984). In some cases, the crustal thickness, particularly beneath nodal basins, may be less than 5% to 10% of normal sections.…”

Introduction

“…These and subsequent geophysical studies (Purdy et al, 1979;Schoutenetal., 1985) demonstrated the general character of the ridge in this area and the diverse rock types that are exposed in the median valley and transform fault walls. Geophysical studies in the 1980's documented large variations in crustal structure in the Kane Transform (Detrick and Purdy, 1980;Louden and Forsyth, 1982;Cormier et al, 1984) and along the axis of the median valley south of the ridge-transform intersection (Purdy and Detrick, 1986). Additional dredging , deep-towed ANGUS camera surveys and Alvin submersible investigations (Karson and Dick, 1983) provided constraints on the distribution of major rock units in the median valley near the ridge-transform intersection.…”