Geology Reference
In-Depth Information
5
Marine Magnetic Anomalies
Abstract
This chapter is devoted to the techniques of collecting and analysing
magnetic data on oceanic crust. These data represent the main source for
the determination of global plate motions during the Cenozoic and part of
the Mesozoic. I describe the methods to design magnetic surveys, process
the raw data, and build isochron maps of oceanic regions.
magnetic isochrons. These lines are used in turn
in the determination of the finite reconstruction
(Euler) poles that describe the relative position of
two plates in the geologic past (e.g., Klitgord and
Schouten 1986 ; Matias et al. 2005 ).
In Chap. 4 , we have seen that the observed
magnetic field at a location r near the Earth's
surface at time t , T D T ( r , t ), results from the
superposition of three basic component fields.
The main contribution comes from the core field,
F D F ( r , t ), whose model is represented by an
IGRF for the corresponding epoch. This field
varies from being horizontal with magnitude
30,000 nT near the Equator to vertical and with
magnitude 60,000 nT near the poles; the root
mean square (rms) magnitude of the field vector
over the Earth's surface is 45,000 nT. This field
also varies in time, on a timescale of months and
longer, as a consequence of the secular variation,
which is on average 80 nT/year. The second
contribution is the generally small external field
S D S ( r , t ), associated with solar and ionosphere
activity and with currents induced in the crust,
which has variable magnitude. In favourable
conditions, the S q variations determine changes
5.1
Magnetic Anomalies
Marine magnetic anomalies represent a
fundamental data component in plate kinematics
studies, as they furnish most of the basic
information that is necessary to unravel the
evolution of the world's oceanic basins through
the geological time. Vine and Matthews ( 1963 )
were the first to discover that the magnetic
signature of sea-floor spreading is represented by
long stripes having alternate polarity of crustal
magnetization. Starting from this pioneer work,
the analysis of the magnetic anomaly field across
the world's oceans has been a formidable tool
for the study of the spreading history along the
oceanic ridges and the determination of global
plate motions. In particular, if we assume that
the highly magnetic 2A layer of the oceanic crust
can be represented by an assembly of long prisms
having uniform remnant magnetization, then
the boundaries between adjacent blocks having
opposite directions of magnetization (normal
or reversed) are magnetic lineations that can
be combined with fracture zone trends to form
Search WWH ::




Custom Search