Page 86 - Global Tectonics

SEA FLOOR SPREADING AND TRANSFORM FAULTS 73

4.1 SEA FLOOR magnetic and acid igneous and sedimentary rocks are
usually only weakly magnetic. A full account of the
SPREADING magnetic surveying method is given in Kearey et al.
(2002).
On land, magnetic anomalies reflect the variable

geology of the upper continental crust. The oceanic
crust, however, is known to be laterally uniform (Section
4.1.1 Introduction 2.4.4) and so unless the magnetic properties are hetero-
geneous it would be expected that marine magnetic

By the late 1950s much evidence for continental drift anomalies would reflect this compositional uniformity.
had been assembled, but the theory was not generally
accepted. Up to this time, work had concentrated upon

determining the pre-drift configurations of the conti- 4.1.2 Marine magnetic
nents and assessing their geologic consequences. The
paths by which the continents had attained their present anomalies
positions had not been determined. In order to study
the kinematics of continental drift it was necessary to Magnetic surveying is easily accomplished, and mea-
study the regions now separating once juxtaposed con- surements have been carried out from survey vessels
tinents. Consequently, at this time interest moved from since the mid 1950s both on specific surveys and

the continents to the intervening ocean basins. routinely on passage to the locations of other oceano-
Any kind of direct observation of the sea fl oor, such graphic investigations.
as drilling, dredging, or submersible operations, is time A most signifi cant magnetic anomaly map (Fig. 4.1)
consuming, expensive, and provides only a low density was constructed after detailed surveys off the western
of data. Much of the information available over oceanic seaboard of North America (Mason & Raff, 1961; Raff &
areas has therefore been provided by geophysical Mason, 1961). The magnetic field was shown to be any-

surveys undertaken from ships or aircraft. One such thing but uniform, and revealed an unexpected pattern

method involves measuring variations in the strength of stripes defined by steep gradients separating linear

of the Earth’s magnetic field. This is accomplished using regions of high amplitude positive and negative anoma-
either fluxgate, proton precession, or optical absorption lies. These magnetic lineations are remarkably persis-

magnetometers, which require little in the way of ori- tent, and can be traced for many hundreds of kilometers.
entation so that the sensing element can be towed Their continuity, however, is interrupted at major oceanic

behind the ship or aircraft at a sufficient distance to fracture zones, where the individual anomalies are offset
minimize their magnetic effects. In this way total fi eld laterally by distances of up to 1100 km.
values are obtained which are accurate to ±1 nanotesla Subsequent surveys have shown that magnetic linea-
(nT) or about 1 part in 50,000. Magnetometers provide tions are present in virtually all oceanic areas. They are
a virtually continuous record of the strength of the geo- generally 10–20 km wide and characterized by a peak-
magnetic field along their travel paths. These absolute to-peak amplitude of 500–1000 nT. They run parallel to

values are subsequently corrected for the externally the crests of the mid-ocean ridge system (Chapter 6),
induced magnetic field variations which give rise to a and are symmetrical about the ridge axes (Fig. 4.2).

diurnal effect, and the regional magnetic fi eld arising The source of these linear magnetic anomalies
from that part of the magnetic field generated in the cannot be oceanic layer 1, which is made up of non-

Earth’s core. In theory the resulting magnetic anoma- magnetic sediments. They cannot originate at a depth
lies should then be due solely to contrasts in the mag- corresponding to layer 3 as sources solely within this
netic properties of the underlying rocks. The anomalies layer would be too deep to generate the steep anomaly
originate from the generally small proportion of ferro- gradients. The source of the anomalies must there-
magnetic minerals (Section 3.6.2) contained within the fore be, at least in part, in oceanic layer 2. This con-
rocks, of which the most common is magnetite. In clusion is consistent with the basaltic composition of
general, ultramafi c and mafi c rocks contain a high pro- layer 2 determined by dredging and drilling (Section
portion of magnetite and thus give rise to large mag- 2.4.6), since basalt is known to contain a relatively
netic anomalies. Metamorphic rocks are moderately high proportion of magnetic minerals. The magnetic