Page 39 - Global Tectonics

26 CHAPTER 2

North America
Atlantic Ocean
Continental margin Mid - Atlantic ridge
North American basin

Acoustic basement
Pleistocene sand and clay
Site 105
Horizon A Cenozoic hemipelagic mud
Upper Cretaceous
Horizon A* Lower Cenozoic multicoloured clay
Horizon ß Cretaceous black clay
Basement 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. 1. Copyright © 1974, with kind permission of Springer Science and Business
Media).

Three subdivisions of layer 2 have been recog- associated with gradual changes in porosity and altera-
nized. Sublayer 2A is only present on ocean ridges tion (Detrick et al., 1994).
near eruptive centers in areas affected by hydrother-
mal circulation of sea water, and ranges in thickness
from zero to 1 km. Its porous, rubbly nature, as
−1
indicated by a P wave velocity of 3.6 km s , permits 2.4.7 Oceanic layer 3
−1
such circulation. The very low velocities (2.1 km s )
of the top of very young layer 2 located on the Mid- Layer 3 is the main component of the oceanic crust and
Atlantic Ridge (Purdy, 1987) probably indicate a represents its plutonic foundation (Fox & Stroup, 1981).
porosity of 30–50%, and the much higher velocities Some workers have subdivided it into sublayer 3A, with
−1
of older layer 2 imply that the porosity must be a velocity range of 6.5–6.8 km s , and a higher velocity
−1
reduced quite rapidly after its formation. Sublayer 2B lower sublayer 3B (7.0–7.7 km s ) (Christensen & Salis-
forms the normal acoustic basement of layer 1 when bury, 1972), although the majority of seismic data can
sublayer 2A is not developed. Its higher velocity of be explained in terms of a layer with a slight positive
−1
4.8–5.5 km s suggests a lower porosity. With time velocity gradient (Spudich & Orcutt, 1980).
layer 2A may be converted to layer 2B by the infi lling Hess (1962) suggested that layer 3 was formed from
of pores by secondary minerals such as calcite, quartz, upper mantle material whose olivine had reacted with
and zeolites. Sublayer 2C is about 1 km thick, where water to varying degrees to produce serpentinized peri-
−1
detected, and its velocity range of 5.8–6.2 km s dotite, and, indeed, 20–60% serpentinization can explain
may indicate a high proportion of intrusive, the observed range of P wave velocities. However for
mafic rocks. This layer grades downwards into oceanic crust of normal thickness (6–7 km) this notion

layer 3. can now be discounted, as the value of Poisson’s ratio
The DSDP/ODP drill hole 504B, that drilled through for layer 3A, which can be estimated directly from a
the top 1800 m of igneous basement in 6 Ma old crust knowledge of both P and S wave velocities, is much
on the Costa Rica Rift, in the eastern central Pacifi c, lower than would be expected for serpentinized perido-
encountered pillow lavas and dikes throughout. It tite. In fact, Poisson’s ratio for layer 3A is more in accord
revealed that, at least for this location, the layer 2/3 with a gabbroic composition, which also provides
seismic boundary lies within a dike complex and is seismic velocities in the observed range. It is possible,