Page 40 - Global Tectonics

THE INTERIOR OF THE EARTH 27

however, that all or at least part of layer 3B, where rec- Table 2.3 Correlation of ophiolite stratigraphy with the
ognized, consists of serpentinized ultramafi c material. oceanic lithosphere (after Gass, 1980 with permission
The concept of a predominantly gabbroic layer 3 is from the Ministry of Agriculture and Natural Resources,
in accord with models suggested for the origin of Cyprus).
oceanic lithosphere (Section 6.10). These propose that
layer 3 forms by the crystallization of a magma chamber Complete ophiolite Oceanic
or magma chambers, with an upper layer, possibly cor- sequence correlation
responding to sublayer 3A, of isotropic gabbro and a
lower layer, possibly corresponding to 3B, consisting of Sediments Layer 1

cumulate gabbro and ultramafic rocks formed by crystal Mafi c volcanics, commonly

settling. This layering has been confirmed by direct pillowed, merging into } Layer 2
observation and sampling by submersible on the Vema Mafic sheeted dike complex

Fracture Zone in the North Atlantic (Auzende et al.,
High level intrusives
1989). }
Trondhjemites Layer 3
Gabbros
Layered cumulates
2.5 OPHIOLITES Olivine gabbros } — Moho —
Pyroxenites
Peridotites
The study of oceanic lithosphere has been aided by Harzburgite, commonly Upper mantle
investigations of characteristic rock sequences on land serpentinized ± lherzolite,
known as ophiolites (literally “snake rock”, referring to dunite, chromitite
the similarity of the color and texture to snakeskin; see
Nicolas, 1989, for a full treatment of this topic). Ophio-
lites usually occur in collisional orogens (Section 10.4),
and their association of deep-sea sediments, basalts,
gabbros, and ultramafic rocks suggests that they origi-

nated as oceanic lithosphere and were subsequently
thrust up into their continental setting by a process
known as obduction (Dewey, 1976; Ben-Avraham et al.,
1982; Section 10.6.3). The complete ophiolite sequence
(Gass, 1980) is shown in Table 2.3. The analogy of ophi-
olites with oceanic lithosphere is supported by the gross
similarity in chemistry (although there is considerable
difference in detail), metamorphic grades correspond-
ing to temperature gradients existing under spreading
centers, the presence of similar ore minerals, and the
observation that the sediments were formed in deep
water (Moores, 1982). Salisbury & Christensen (1978)
have compared the velocity structure of the oceanic
lithosphere with seismic velocities measured in samples
from the Bay of Islands ophiolite complex in New-
foundland, and concluded that the determined velocity
stratigraphies are identical. Figure 2.19 shows the cor-
relation between the oceanic lithosphere and three
well-studied ophiolite bodies.
At one time it seemed that investigations of the Figure 2.19 Comparison of oceanic crustal structure
petrology and structure of the oceanic lithosphere with ophiolite complexes (after Mason, 1985, with
could conveniently be accomplished by the study of permission from Blackwell Publishing).