Velocity structure of upper ocean crust at Ocean Drilling Program Site 1256

Abstract

[1] We examine shipboard physical property measurements, wireline logs, and vertical seismic profiles (VSP) from Ocean Drilling Program/Integrated Ocean Drilling Program Hole 1256D in 15 Ma ocean crust formed at superfast spreading rates to investigate lateral and vertical variations in compressional velocity. In general, velocities from all methods agree. Porosity is inversely related to velocity in both the logging and laboratory data. We infer that microfracturing during drilling is minor in the upper 1 km of basement, probably due to connected pores and, thus, low effective stress. The closure of porosity to very low values coincides with the depth below which laboratory velocities diverge from logging velocities. We infer that porosity controls velocity in layer 2, lithostatic pressure controls the thickness of seismic layer 2, and the distribution of flow types determines seismic velocity in the upper 200 m of basement. In the sheeted dikes, changes in physical properties, mineralogy, and chemistry define clusters of dikes.Components: 11,976 words, 16 figures.

Expedition 335 summary

“…F9, F12). Shipboard determinations of seismic velocities of discrete samples are in close agreement with in situ measurements by wireline tools to ~1320 mbsf, above the granoblastic dikes interval; below that depth, velocities are significantly lower than the sonic log, and the gabbro velocities range between ~5.3 and 6.4 km/s (Swift et al, 2008). Contrary to expectation, porosity increases and P-wave velocities decrease stepwise downward from the lowermost dikes into the uppermost gabbro in Hole 1256D, as the result of the contact metamorphism of the granoblastic dikes and the strong hydrothermal alteration of the uppermost gabbros (Fig.…”

“…In Hole 1256D, gabbros have been recovered from crust clearly within seismic Layer 2 based on shipboard, wireline, and seismic refraction velocity measurements (Figs. F8, F12) (Swift et al, 2008;Guerin et al, 2008;Gilbert and Salisbury, 2011 …”

The response of two-phase hydrothermal systems to changing magmatic heat input at mid-ocean ridges

“…Data from the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) determined from packer measurements provide permeability estimates to a depth of 1600m. Two sites of particular importance are Site 504B south of the Costa Rica Rift formed about 6 million years ago at a half spreading rate near 36 mm/a, and Site 1256D, which is on 15 million years old crust formed at a rate of 110 mm/a at the East Pacific Rise [Becker, 1989;Fisher, 1998;Swift et al, 2008;Carlson, 2011]. At Site 504B, the total depth is 1562.3m below seafloor, penetrating 275m of sediments, 575m of extrusive basaltic lava, a 200m transition zone, and more than 500m of the underlying layer of intrusive basaltic sheeted dikes [Becker and Sakai, 1989].…”

Lateral continuity of basement seismic reflections in 15 Ma ultrafast-spreading crust at ODP Site 1256

“…The sediment cover of about 275 m has two stratigraphic intervals, an upper silica-rich ~70m thick layer dating to 10 Ma and a lower, older Miocene carbonate layer (Shipboard Scientific Party (2003b) Chapter 2). Refraction velocity increases from 4.8 km/s at the basement surface to ~6.5 km/s to the bottom of the hole (Swift et al, 2008). The vertical velocity gradient decreases to 0.2 km/s per km at about 1500 mbsf, marking the seismic layer 2/3 boundary.…”

“…The crustal thickness is about 5.4km. First arrival travel times from a vertical seismic profile (VSP) in the Hole 1256D indicate (1) 4.6 km/s in a thin, ponded lava flow unit at the basement surface, (2) 4.1 km/s in the underlying sheet flows and (3) an increase in velocity to 6.2 km/s in sheeted dikes near the base of the hole (Swift et al, 2008).…”