Geoscience Reference
In-Depth Information
Figure 9.5.
(a) Velocity-
depth models from the
North Atlantic, grouped
according to the age of
the lithosphere. All these
models were determined
from seismic-refraction
data by calculating
synthetic seismograms
and adjusting the model
until the travel times and
the amplitudes agreed
well with the data. (From
White (1984).) (b) A
velocity-depth model
obtained from the East
Pacific Rise correlated
with the DSDP hole 504B
drilled into 6.6-Ma oceanic
crust formed at the Costa
Rica Rift with a
half-spreading rate of
3.0-3.4 cm/yr
−1
. Hole
504B at 2200 m is the
deepest hole drilled into
oceanic crust. (From Bratt
and Purdy (1984).)
Layer 1
, the sedimentary layer, thickens with distance from the ridge axis.
The P-wave velocity at the top of the sediments is generally close to 1.5 km s
−
1
(the velocity of sound in sea water) but increases with depth as the sediments
consolidate. (This layer is not shown in Fig. 9.5.)
At the top of
layer 2
, the
volcanic layer
, there is a sudden increase in P-wave
velocity to approximately 4 km s
−
1
. Reflection profiling has shown that the top of
layer 2, the
oceanic basement
,isvery rough for crust that was formed at a slow-
spreading ridge. It was proposed in the 1970s that layer 2 should be subdivided
into three layers, 2A, 2B and 2C, with P-wave velocities of 3.5, 4.8-5.5 and
5.8-6.2 km s
−
1
, respectively, and this terminology is often used in the literature.
However, layer 2 is probably best described as a region of the oceanic crust in
