Geology Reference
In-Depth Information
(a)
(b)
Layer 1
2.0
(L 1)
sediment
fractured basalt
massive basalt with dykes
2.0
0
3.5
5.2
(L 2A)
Layer 2
5.0
(L 2B)
dykes with massive basalt
2
(L 2C)
6.1
metagabbros and gabbros
with pockets of plagiogranite and
protrusions of serpentinite
(L 3A)
6.8
4
Layer 3
6.7
gabbros and metagabbros with
serpentinite protrusions and
pockets of cumulate ultramafics
6
(L 3B)
7.3
Fig. 5.28
Velocity (km s
-1
) structure of
typical oceanic lithosphere in terms of
layered structures proposed in 1965 (a) and
1978 ( b), and its geological interpretation.
(From Kearey &Vine 1990.)
anomalous mantle (close to ridge axis)
serpentinized ultramafics
harzburgite and lherzolite
8.1
<6.9
7.8
Mantle 8.1
8
Mantle
granitic or granodioritic material. Lower crustal veloc-
ities are normally in the range 6.5-7.0 km s
-1
and may
represent any of a variety of igneous and metamorphic
rock types, including gabbro, gabbroic anorthosite and
basic granulite. The latter rock type is regarded as the
most probable major constituent of the lower crust on
the basis of experimental studies of seismic velocities
(Christensen & Fountain 1975).
Expanding spread profiling (ESP) is designed to ob-
tain detailed information relating to a localized region
of the crust. The shot-firing vessel and recording vessel
travel outwards at the same speed from a central position,
obtaining reflected and refracted arrivals from subsurface
interfaces out to large offsets. Thus, in addition to near-
normal incidence reflections such as would be recorded
in a conventional common mid-point (CMP) reflection
survey, wide-angle reflections and refracted arrivals are
also recorded from the same section of crust. The com-
bined reflection/refraction data allow derivation of a
highly-detailed velocity-depth structure for the local-
ized region.
Expanding spread profiles have also been carried out
on land to investigate the crustal structure of continental
areas (see e.g.Wright
et al
. 1990).
In constant offset profiling (COP), the shot-firing and
recording vessels travel along a profile line at a fixed,
wide separation. Thus, wide-angle reflections and re-
fractions are continuously recorded along the line. This
survey technique facilitates the mapping of lateral
changes in crustal structure over wide areas and allows
continuous mapping of the types of refracting interface
that do not give rise to good near-normal incidence
reflections and which therefore cannot be mapped
adequately using conventional reflection profiling. Such
interfaces include zones of steep velocity gradient, in
contrast to the first-order velocity discontinuities that
constitute the best reflectors.
5.11.4 Two-ship seismic surveying: combined
refraction and reflection surveying
Marine surveys, usually single-ship experiments, have
shown the ocean basins to have a crust only 6-8 km
thick, composed of three main layers with differing
seismic velocities. This thickness and layering is main-
tained over vast areas beneath all the major oceans.
The results of deep-sea drilling, together with the re-
cognition of ophiolite complexes exposed on land
as analogues of oceanic lithosphere, have enabled the
nature of the individual seismic layers to be identified
(Fig. 5.28).
Specialized methods of marine surveying involving
the use of two survey vessels and multichannel recording
include
expanding spread profiles
and
constant offset profiles
(Stoffa & Buhl 1979). These methods have been devel-
oped for the detailed study of the deep structure of the
crust and upper mantle under continental margins and
oceanic areas.
