Oceanic layer

Layer 2 is variable in its thickness, in the range 1.02.5 km. Its seismic velocity is similarly variable in the range 3.4-6.2 km s-1. This range is attributable to either consolidated sediments or extrusive igneous material. Direct sampling and dredging of the sediment-free crests of ocean ridges, and the necessity of a highly magnetic lithology at this level (Section 4.2), overwhelmingly prove an igneous origin. The basalts recovered are olivine tholeiites containing calcic plagioclase, and are poor in potassium, sodium, and the incompatible elements (Sun et al., 1979). They exhibit very little areal variation in major element composition, with the exception of locations close to oceanic islands (Section 5.4).

North America

Atlantic Ocean North American basin

Mid - Atlantic ridge

Atlantic Ocean North American basin

Mid - Atlantic ridge

Acoustic basement

Pleistocene sand and clay

Acoustic basement

Pleistocene sand and clay

Site 105

Horizon A Horizon A* Horizon 3" Basement

Cenozoic hemipelagic mud -s Upper Cretaceous

Lower Cenozoic multicoloured clay

•. Cretaceous black clay

Late Jurassic and Neocomian limestone Basalt

Figure 2.18 (a) Major seismic reflectors in the western Atlantic Ocean. (b) Corresponding lithologies determined by deep sea drilling (after Edgar, 1974, Fig. I. Copyright © 1974, with kind permission of Springer Science and Business Media).

Three subdivisions of layer 2 have been recognized. Sublayer 2A is only present on ocean ridges near eruptive centers in areas affected by hydrothermal circulation of sea water, and ranges in thickness from zero to 1 km. Its porous, rubbly nature, as indicated by a P wave velocity of 3.6 km s-1, permits such circulation. The very low velocities (2.1 km s-1) of the top of very young layer 2 located on the Mid-Atlantic Ridge (Purdy, 1987) probably indicate a porosity of 30-50%, and the much higher velocities of older layer 2 imply that the porosity must be reduced quite rapidly after its formation. Sublayer 2B forms the normal acoustic basement of layer 1 when sublayer 2A is not developed. Its higher velocity of 4.8-5.5 km s-1 suggests a lower porosity. With time layer 2A may be converted to layer 2B by the infilling of pores by secondary minerals such as calcite, quartz, and zeolites. Sublayer 2C is about 1 km thick, where detected, and its velocity range of 5.8-6.2 km s-1 may indicate a high proportion of intrusive, mafic rocks. This layer grades downwards into layer 3.

The DSDP/ODP drill hole 504B, that drilled through the top 1800 m of igneous basement in 6 Ma old crust on the Costa Rica Rift, in the eastern central Pacific, encountered pillow lavas and dikes throughout. It revealed that, at least for this location, the layer 2/3 seismic boundary lies within a dike complex and is associated with gradual changes in porosity and alteration (Detrick et al., 1994).

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