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

Abstract

[1] We present results from a study mapping the seismic layer 2A/2B boundary in young ocean crust located adjacent to the north wall of the Blanco Transform Fault (BTF). We review seafloor features, geochemical and petrologic data, seismic imaging, and seismic structure and conclude that the BTF seismic study region is representative of crust formed at the intermediate spreading Juan de Fuca Ridge. The mean layer 2A two-way travel time beneath the seafloor is 0.37 ± 0.10 s for the 42 × 12 km seismic survey area. Several regions are observed where layer 2A is consistently thin or thick over lateral distances of 5-10 km, both in a ridge-parallel and ridge-perpendicular direction. Layer 2A thicknesses appear more variable in the ridge-parallel (isochron) direction than the ridge-perpendicular (flow line) direction. There is no systematic pattern of layer 2A thickness variability with distance from the BTF, nor is there a correlation between seafloor topography and layer 2A thickness. Calculated mean layer 2A thickness for the seismic study region is 485 ± 135 m assuming an interval velocity of 2.65 km/s. The imaged layer 2A/2B boundary projects on average ∼600-650 m above the top of the sheeted dike complex mapped on the adjacent BTF north wall. We argue that the geologic context of the layer 2A/2B boundary will vary as competing processes of magmatic construction and fracturing (increasing porosity) and crustal thickening, compaction, dike intrusion, and hydrothermal sealing (decreasing porosity) vary as the crust spreads laterally from the ridge axis to ridge flanks.Components: 12,415 words, 15 figures.

Seismic Layer 2A: Evolution and Thickness From 0‐ to 70‐Ma Crust in the Slow‐Intermediate Spreading South Atlantic

“…Spreading rate has been proposed as a predictor of crustal morphology (Morgan & Chen, ), but global studies have not shown a relationship between spreading rate and crustal thickness, except possibly at ultraslow spreading rates (Brown & White, ; Chen, ). We find no correlation when comparing the thickness of layer 2A to spreading rate across the transect (Figure ); this interpretation matches a similar comparison of published layer 2A thickness values (Christeson et al, ).…”

“…Table is a comparison of one standard deviation of layer 2A thicknesses from our overall mean in the South Atlantic with thickness ranges reported by previous studies at slow and intermediate spreading ridge systems (Arnulf et al, ; Arnulf et al, ; Baran et al, ; Blacic et al, ; Christeson et al, ; Hussenoeder et al, ; Nedimović et al, ). While our range is larger than those of other studies, we suggest that this is an expected result given our larger age range.…”

Ocean Crustal Seismic Layer 2C

“…However, as shown in Figure 1 of Christeson et al (2010), the lavas are thicker than layer 2A at all of these sites except Hess Deep and Pito Deep. However, as shown in Figure 1 of Christeson et al (2010), the lavas are thicker than layer 2A at all of these sites except Hess Deep and Pito Deep.…”

“…The layer 2/3 boundary shown in black is taken from the Collins et al (1995) seismic profile. Also shown are profiles from Grevemeyer et al (1998) and Christeson et al (2010), which do not exhibit features that might correspond to layer 2C. Also shown are profiles from Grevemeyer et al (1998) and Christeson et al (2010), which do not exhibit features that might correspond to layer 2C.…”

“…In the first, the transition from layer 2A to 2B is thought to occur within the extrusive pile (Harding et al, 1989;Rohr et al, 1988;Vera et al, 1990); while based on several lines of evidence, it has been proposed that layer 2C is associated with sheeted dike section penetrated by drilling in Hole 504B (Anderson et al, 1982;Becker, 1985;Newmark, 1985;Salisbury et al, 1985), as shown in Figure 1. Christeson et al (2010) among others have noted that this has become the accepted model. Christeson et al (2010) among others have noted that this has become the accepted model.…”