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OCR for page 115
IV
CHARACTERIZATION
OF CONTINENTAL CRUST
OCR for page 116
OCR for page 117
Seismic Exploration of the
Continental Basement:
Trends for the 1980's
10
INTRODUCTION
JACK E. OLIVER
Cornell University
In scientific research it is helpful to view one's activities
occasionally from a fresh and different perspective.
Imagine, for example, an astronaut-scientist visiting the
earth from an advanced civilization on another planet and
having the task of reporting the sate of earthly science to
his leaders. I think his report would be mixed in tone. He
would report favorably about some of our efforts to inves-
tigate our surroundings. On one scale, for example, space-
craft leave the earth to explore the solar system and
beyond in an effort that strains our technology. At another
extreme of scale, huge sophisticated devices cause tiny
subatomic particles to collide at high velocity in an effort
to learn more and more about smaller and smaller entities.
Once again our technology is strained. The astronaut
would probably be favorably impressed by progress in
certain methods for exploring the earth sophisticated
laboratory devices, the techniques of the petroleum
industry, perhaps deep-sea drilling vessels. But I think he
would be surprised and dismayed to find that a society of
four billion people confined to the surface ofthe earth has
been content to know so liKle about the rocks a few
~7
hundreds or thousands of meters below it, and from which
it derives much of its livelihood the rocks of the conti-
nental basement. In making this statement, I do not mean
to be critical of those scientists who work very capably
and professionally on this topic but rather of the magni-
tude ofthe total effort directed toward study ofthis part of
the earth, an effort that seems too small in relation to the
need for an inventory of its resources by an expanding and
ravenous society. I hope new advances within the next
decade will ease the mind of the astronaut on this point.
It is not that methods and tools for exploration of the
continental basement are unavailable. We have them;
many of them will be discussed in this paper. The seismo-
logical methods that I shall discuss are but a part of our
overall capability. Others include further mapping, im-
proved and extended field and laboratory studies of sur-
face rocks, drilling for informational purposes, study of
crustal xenoliths, and a variety of geophysical techniques.
The problem is one of focusing scientific interest on the
topic and of devoting an appropriate portion of our efforts
to study of this region, for sound economic as well as
scientific reasons.
OCR for page 118
118
Seismology has been, of course' a major producer of
information on the earth's interior~the major producer of
certain kinds of information including structure and cer-
tain mechanical properties. Therefore, ~ wish to consider
the potential of further seismic exploration of the conti-
nental basement. By continental basement I mean the
entire continental crust below the sediments and Me
uppermost mantle.
From the broadest perspective and for several reasons,
it is clear that the potential of the seismic method for
exploration of the earn, and particularly the continental
basement, is by no means exhausted. First, to do so would
require sources and receivers scattered over and through
the earth at the Nyquist spacing for the shortest waves that
can be detected after propagation through the deep region
of interest. We are certainly far from achieving such a
level of observations at this time and from predicting what
we would observe if we did. Second, we lag in analysis;
we are not able to understand and to make useful conclu-
sions from all the information we obtain now. Third, the
current rate of discovery of new features continues to be
high. From these three points, one can deduce conf~-
dently that a much improved understanding of the earth's
interior remains to be revealed by seismology. The chal-
lenge of seismology is to approach this ideal in an opti-
mum manner given economic and other practical con-
straints. In a sense, the various kinds of seismological
studies represent venous routes toward this goal. Let us
consider some ofthem, more or less in order of decreasing
wavelength.
SPECTRUM OF SEISMIC WAVELENGTHS
Since 1960, seismologists have been observing and stucly-
ing waves of very long periods (up to almost an hour) and
hence very long wavelengths that may be thought of as
corresponding to free oscillations of the earth. Most
studies of free oscillations treat the earth as almost spheri-
cally symmetrical (they include flattening and rotations.
The resolution of differences between continents and
oceans is very limited and the differences are averaged
out in most cases. If some of the very-high-mode, i.e.,
sho~t-waveleng~, Dee oscillations can be observed,
identified, and resolved in sufficient detail, information
on gross structural differences between continents and
oceans including associated mantle structure may be
provided.
Many such higher-mocle oscillations are alcin to their
traveling counterparts, seismic surface waves. Traveling
surface waves offer the opportunity for detennin~ng earn
structure based on one pass of the waves, as opposed to
many passes with consequent averaging with other effects
in the case of free oscillations. Surface waves have been
used regularly in recent years to.detennine cn~staI and
upper-mantle structure, including regional variations of
such structure. Many of the measurements of depth to the
top of the low-velocity zone in the mantle, which is pre-
sumably though not necessarily near or related to the base
JACK E. OLIVER
of the lithosphere. are derived from surface waves. Al-
though the results are sometimes ambiguous and very
dependent on the validity of certain assumptions, there is
probably a good deal more information about the earth
including the continental basement, to be obtained from
surface waves. To utilize surface-wave data for conti-
nental structure will require, as a minimum, more
complete and more closely spaced observations of the
phenomenon, farther attention to focusing and multi-
pathing, integration of surface-wave observations with
those of Other phenomena, and further development of
techniques for using models that depart from flat-lying
layered structures.
In fact, the inadequacy of spherical or flat-layered
models that portray the earth as lacking in lateral hetero-
geneity is growing in importance and has probably
reached the crisis stage in the case of the continental
basement. Our level of understanding has reached the
point where refinement of such models may be more mis-
leading than informative. As a first approximation, geo-
physicists have utilized simplified layered models, not
only in the case of seismic surface waves but also in many
other kinds of seismic studies and in other branches of
geophysics as well. This approach is not without good
reason. Gravity is an important factor in geology, and
hence there is indeed a strong tendency for spherical
layering. Furthermore, layered models or one-dimen-
sional models are relatively easy to handle Mom the theo-
retical point ofview. Such simple models are no longer an
adequate representation ol the continental crust, where a
bewildering variety of structures and rock types is found
in almost any large outcrop (Figure 10.~. Figure 10.1,
taken from a paper by Smithson et al. (1977), but in turn
borrowed from Berthelsen (1960), shows pyroxene granu-
lite layers surrounded by granitic gneiss. The important
point in the present context has to do with the complex
three-dimensional structure of this feature. Resolution of
such complex buried contortions by seismic methods, or
any methods, will be difficult, but on the other hand to
apply flat-lying layered approximations to such structures
is nonsense. We must develop models and observational
and analytical techniques that will provide information an
structures of complexity greater than that of simple layers
and to the level of complexity shown in Figure 10.1, if
possible. Such structures must be probe c] at depth within
the crust, and perhaps the upper mantle.
Seismologists have been moving away gradually from
simple-layered models for some time. Plate tectonics was
a step in this direction. Perhaps more than anything, study
of lateral variations and complex structures will charac-
terize the application of seismology to the study of the
continental basement during the 1980's.
Continuing through the spectrum to shorter wave-
lengths, consider the body waves generated by earth-
quakes, typically in the range of a few seconds per cycle
to a few cycles per second. The body-wave travel-time
method, which has produced so much of our knowledge of
the earth's deep interior, has been applied widely on a
crustal scale for study of the continents. However, the
OCR for page 119
Seismic Exploration of the Continental Basement
FIGURE 10.1 A three-dimensional
structure in the continental basement.
To approximate such structures by [lat-
layered models is of very limited
value. From Smithson et al. ( 1977)
(taken from Berthelsen, 1960).
IBM
1~
traditional, so-called near-earthquake studies have not
produced much new information recently, partly because
superior precision and flexibility may be had by use of
artificial sources and partly because of the limitations of
the flat-layered models usually used in interpretation of
near-earthquake studies.
In recent years there have been some significant depar-
tures from the well-wom path, however. Tightly spaced
detection networks have produced more precise locations
of sources in time and space and more reliable travel-time
data. Models involving lateral variations of structure such
as faults can be considered using powerful computer-
based methods. Techniques involving differential travel
times at networks of stations encompassing a particular
feature have led to investigation of features of unusual
shape' the work, for example, of Aki et al. (1977) and the
group at the U.S. Geological Survey (USGS) flyer, 1973;
Ellsworth and Koyangi, 1977~.
On the basis of the clear signs of life in current study of
body waves, and the renewed emphasis that is likely to
result from new efforts to predict earthquakes and reduce
the earthquake hazard and to detect and identify nuclear
explosions, one can foresee new results, particularly those
emphasizing lateral variations, over the next decade at
least. The keys for application ofthree-dimensional inver-
sion of travel times are tightly spaced, probably movable,
networks and developments in methods for using not only
travel-time differences but also wave character resulting
from attenuation, focusing, conversion, or other effects.
NATURAL SEISMIC SOURCES
In the case of natural seismic sources, perhaps as much
information can be obtained from the sources as from the
wave propagation. Historically, with each increase in pre-
cision of location of hypocenters of earthquakes, our un-
~9
~,~
1.1
. ~
~ it'
_ _
derstanding of tectonics has grown. For example, in the
1950's and earlier, hypocenters of earthquakes in the
southern hemisphere were frequently mislocated by
more than 100 km. At that stage, an earthquake could at
best be associated with a major regional feature such as an
island arc. In the 1960's, with the advent of the World
Wide Standardized Seismic Network (WVItSSN) and other
advances, it became possible to locate hypocenters with a
precision of less than 10 or 20 km. Earthquakes could then
be associated with tectonic features of smaller scale- a rift
valley, a deep earthquake zone, the outer wall of a trench.
Now, with the closely spaced observing networks that are
available in a few areas, earthquake hypocenters can be
located with a precision of less than a kilometer, much
less in some cases. Hence we can now confidently asso-
ciate earthquakes with particular faults, and also associate
properties ofthe quake with properties ofthat fault. Fault-
plane solutions tell us of the orientation of the fault and
the movement; other focal mechanism data tell us of the
change in stress and scale of the movement.
Further improvements in precision of hypocentral loca-
tion may tell us about movements along a particular part
of a fault, progression in fault activity, and complexities
such as asperities and slices. Figure 10.2 shows how an
interpretation of tectonics may be strongly dependent on
the precision of hypocentral data. This figure is taken
from a paper by Barazangi and Isacks (in press). A cross
section of the seismicity through central and northern
Peru is shown twice. In the upper half of the figure only
hypocenters of very high precision are plotted. In the
lower half of the figure other hypocenters located during
the same time period but with lower precision are shown.
There is clearly a great contrast between the structure
defined by the well-located hypocenters and what might
be deduced from the less well-Iocated hypocenters. A
similar effect may be anticipated at smaller scale.
Other modem techniques are telling us of fault move-
OCR for page 120
120
meets that are slower than the abrupt displacements of
typical earthquakes. Such slow movements may fail to
generate short-period seismic waves, may generate only
very long-period seismic waves, or may generate no
detectable seismic wave and may be thought of as epi-
sodes of fault creep. Such studies enhance greatly our
understanding of fault motion. The study of seismicity
and sources is an area of vast potential and one in which
integration of seismology and geology is likely to be
crucial.
One may confidently state that interaction of seismol-
ogy, or geophysics in general, and geology is likely to
increase markedly in the next decade. A recent report by
the NRc Committee on Seismology (1976) notes that much
of the progress in unclerstanding the problem of earth-
quake hazards in recent years has come from geological,
not purely seismological, evidence. Plans to drill deeply
into the San Andreas Fault are afoot. We can anticipate
growing interaction between these disciplines, with
mutual benefit as our understanding of the continental
basement grows.
ARTIFICIAL SEISMIC SOURCES
In general, artificial sources have the a~lvantage of precise
timing, simplicity and control of source Unction, and
flexibility of location and the disadvantage of lower en-
ergy except in the special case of nuclear explosions.
Thus, nonnuclear artificial sources are currently of no
value for study ofthe deep parts ofthe earth's interior, but
for the continental crust ant! the upper mantle they can
indeed provide information, ant! in fact the infonnation
with the best resolution at the level of detail that we now
are seeking.
Seismic studies using artificial sources fall loosely and
somewhat arbitrarily into three classes: (1) refraction, (2)
deep seismic sounding (DSS) or refraction and wide-angle
reflection, and (3) reflection profiling.
The refraction rnetho~ is responsible for much of what
is known about the structure of the deep crust; it provides
the depth to the Mohorovicic discontinuity and typically
a simple model of crustal structure consisting of layers of
different velocities. Seismic refiaction studies have been
carried out in the United States by venous university and
private groups and the USGS. However, almost all of this
work was clone in the 1960's and before. Figure 10.3, from
Prodehl (1977), shows a summary of crustal models
deduced from refraction data for venous parts of the
United States. Nearly flat-lying layered models prevail for
each area, and the differences from one area to another are
illustrated at the bottom of the figure. There is considera-
ble similarity in all the crustal models, and, in fact, the
rather uniform depth of the mantle throughout the conti-
nent is a rather striking feature. There are also substantial
and measurable variations from one province to another
on this gross scale. This figure is a summary of current
knowledge of U.S. crustal structure based on refraction
JACK E. OLIVER
data. Surprisingly, for about the last decade there has
been relatively little activity of this type directed toward
study of the continental basement In the United States.
Abroad, however, the stow Is different. The Soviet
Union has operated a program of crustal exploration at a
very high level of activity since World War II and has
developed the DSS method, in essence the use of re-
fracted and wide-angle reflected waves observed with
closely spaced stations, to determine crystal structure
with resolution better than that obtained by refraction
measurements alone. Furthennore and after a somewhat
slower start, groups of European seismologists have been
using similar techniques and obtaining detailed and
abundant results. Figure 10.4 shows a typical set of seis-
mic refraction data from Europe arid a crustal model
deduced Tom the data. Detailed velocity{lepth function
and a great deal of other seismic information are obtained
that is not explained by the simple model. The work in the
Soviet Union and in Europe is typically characterized by
much more thorough observation through very closely
spaced detecting stations than is the case for the older
studies of this type in the United States. The papers in
Giese et al. (1976) provide a comprehensive survey of
explosion seismology in central Europe. Further seismic
retraction studies in the United States will surely provide
new information on the crust The extent to which the
new information will be coupled with other geological
and geophysical information to provide enhanced under-
standing of the continents will depend on He further
cle~relopment of methods of interpretation to produce
more realistic crustal models than the present simple
layered configurations. The models should present a
more realistic geological picture.
The newcomer on the scene of seismic exploration of
the continental basement is seismic reflection profiling.
In venous countries, within the last decade, this tech-
nique has been applied with some modification to study
of basement to depths as much as 50 or so kilometers,
primarily by the Consortium for Continental Reflection
Profiling (COCORP) project in the United Sates (see Oliver
et al., 1976, for a review). The method uses closely spaced
vibratory or explosive sources, listening arrays of thou-
sands of geophones, and sophisticated computer analysis
of the data. Typically, tens of millions of rays penetrating
the earth are sampled and utilized at each site. The
potential resolution of structural features ofthe deep crust
through reflection profiling is greater than that of any
other method, but it is no small operation. Figure 10 ~
shows a field party in action. Of particular interest is that
the sources of the seismic waves are not explosives but
large truck-mounted vibrators. The V=ROESIS method
(registered trademark of the Continental Oil Company)
has been used in all the cOcORP studies to date. The re-
sults Tom seismic reflection profiling so far clearly indi-
cate vast potential for this method in fixture studies of the
continental basement.
Figure 10.6 shows a seismic reflection profile across the
Rio Grande nR. The abscissa is distance in decameters;
OCR for page 121
Seismic Exploration of the Continental Basement
5
TRENCH COAST ANDES
~ AL 7~ ' 300 ' 'U: su
5 it, I ~ , , ~ IT ~ , ~ - 70C
100:
200L SECTIONS 2,3 AND 4
| CLASS a AND B
I
o o Loo ° deco ° - 7 OC
o ~ o ~ ~ cat
i ~~° °8 ~ 0 ~ ° ~ ~` 'en ~ °°C ~ —
C ~ of oC ~ ~ ~ ' -
-
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BLOC
2, 3 4 200 400 600
~ 00 ~ ~ ~ ~ o _ =~ `~ A i ~ ~ - ~ ~ A ~ ~ ~~ I ~ ~ -— ~ 00
~~ ~ s~ - V amp ~ °-° ^°
tOC
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c
SECTIONS 2 ,3 AND 4 o
| CLASS C AND D
I ~ I !
200
300 ~ -- 00
121
~ of NO 3 ~
o o
o
I o
400
DISTANCE, KM.
1100
[20C
6 1° - - 1 - 1300
FIGURE 10.2 A generalized section through Peru showing by hypocentral locations of high precision in upper view and other
hypocentral locations of lower precision in lower view. Combining these two sets of data can result in an interpretation different from
one based only on the better data. From Barazangi and Isacks (in press).
FIGURE 10.3 A summary by region
of seismic refraction studies of the U.S.
continental crust. Note (1) overall simi-
larity, (2) variations from one province
to another, (3) use of layered models.
From Prodehl (1977, copyrighted '
American Geophysical Union).
- ~ ,/'
T +~ - ~
interior Plains / ~ r
Atlantic Plain
04~' O. ~°~ 0
~ 2 3 ~ 5
_ ~~ _ lo
Velocity tams)
6 ? 8 9 to
OCR for page 122
122
-
-2
FIGURE 10.4 Typical record sec-
tion, travel-time curves, and flat-
layered model based on refraction
study. From Mueller ( 1977, copy-
righted by American Geophysical
Union).
45 ~
~ 5 B 7
the ordinate is two-way travel time in seconds. To convert
time in seconds to depths in kilometers approximately,
multiply by 3. In a gross sense such a section may be
thought of as a section of the eat, but a detailed interpre-
tation of the data requires knowledge of wave propagation
and the data processing involved, so that it is invalid to
assume that the details of a time section such as the one
shown are necessarily or precisely features of the deep
earn. This section, for example, has not yet been sum
jected to migration to position the reflections in their
proper spatial locations. Even so, certain gross features
can be seen in the data that are illustrative of the capabil-
ity of the method. The sedimentary sections and the sedi-
mentary basement boundary are clear near the top of the
section. There is an intragraben horst of substantial size
and other more subtle evidence for faulting of the sedi-
ments. At the time of about 7~ see there is a rather strong
reflector that corresponds to the top of the magma body
first proposed for this area by Sanford et al. (1977) on the
FIGURE 105 Seismic reflection pro
filing park in the field in Wyoming.
JACK E. OLIVER
._ ~
.~ _
' 101) tSO 200 250 ~(km)
STEINBRUNN-SW
8 ~ Vpiltm/ - c)
basis of microearthquake data. Although this particular
section does not show much information at a time corre-
sponding to the crust~nantle boundary, other sections do.
Figure 10.7 shows a line drawing of the data from the
section in Figure 10~6 and another section to the east. The
hvo sections together span the rift valley in the vicinity of
Belen, New Mexico. The top of the basement and various
sedimentary features are clear, as are certain other fea-
tures within the basement itself. The magma body shows
as a strong reflector in parts of both sections. At a time of
about 12 see in the easternmost section there is an arrival
that may be associated with the crust~nantle boundary,
although it is not continuous for a very long distance.
Other profiles in the Rio Grande no area show a stratified
pattern to the reflectors in the vicinity of this boundary.
Much more detail can be found throughout the section in
the original data.
Figure 10.8 shows a line drawing of a short section
talcen as a test of the method in the vicinity of the San
am;
~,= - , ~ . _ ... , .. _, ., — ...
1
.% .-g
-;~
_
-
ILL
~ _
OCR for page 123
Seismic Exploration of the Continental Basement
VP NO.
400 3SO
1 1
FIGURE 10.6 COCORP seismic reflec-
tion profile (unmigrated) across west-
em part of Rio Grande no north of
Socorro. Abscissa Is vibration point
number or distance in decameters; or-
dinate is two-way travel time in sec-
onds. Velocity structure from a nearby
refraction study shown in column on
left. Courtesy Larry Brown, Comell
University.
Andreas Fault. There is great contrast in the data from one
side of the fault to the other. In the fault zone, to a depth
of about 10 or 12 km, diffraction hyperbolas, associated
with discontinuities marking the fault zone, are seen.
Below Mat zone, however, there is a region of little infor-
mation, suggesting that the zone was not penetrated by
the seismic waves, which would be surprising in view of
SOCORRO Ll N E lA
w
WHO
SOCORRO Ll N E I A
123
2s0 200
1 ~
50 100
1 1
so 10
1 1 o
~5
We information from much greater depths on both sides, or
that the zone is so distorted through flow that no coherent
reflected energy was obtained. The latter seems more
probable. At a depth near the crust-mantle boundary,
particularly in the western part of the section, there are
many reflectors, suggesting a complex feature for this
boundary.
ABO PASS Ll NE I
400
5.S
~ - - A,-_
~ ~ Ad. 5 _
6.
7.5
_~
. N~ — ^
200 150 100 so 10 10 5c
'~,=~ '
-` ~
_
. ~ - f
VP ~
00 150 200 250
~-~` Am- - .
,. ~
—10
—IS
FIGURE 10.7 Line drawing based on data of Figure 10.6. See text for discussion. Column on leR shows velocity
structure determined from a nearby seismic refraction study.
OCR for page 124
124
JACK E. OLIvER
Sw
10
0.0-
2 n-
2 - wAr 3 0-
TI~E 4.0-
5.0-
6.+
7.0-
°ARKFIEL3 CAL~F SAF SAF
1 1
HE
~ so ~ ~ 60 ~ 8° ~ `= 110 i~ '= ." i~ '" '70 ' - t~ 2= 2~0 220 2= 24c 2x 2"
I ~ ! ' I I I I ~ I I I ~ ~ I ~ I ~ ~ I ~ 1 1 ~
10.0-
1 1.0-
12.0-
FIGURE 10.8 Line drawing of short test section crossing San Andreas Fault near Parkfield. Distance in decameters.
Tw~way travel time in seconds. S.A.F. indicates boundary of San Andreas Fault zone. See text for discussion.
Figure 10.9 shows a map of the southeasternmost por-
tion of the Wind River range and indicates the line of a
seismic reflection profile made in this area. Line drawings
of parts ofthis section are indicated in Figure 10.10. There
is a great clear of over information of considerable variety.
The most prominent feature is the eastward extension of
We thrust fault on We western boundary of the Wind
Rivers Mountains. This Cult extends to a depth of at least
25 km and more likely to 40 km without much change in
dip. These data, then, seem to resolve the long-standing
geological problem of the origin of the Wind Rivers in We
sense that compressional forces seem to predominate as
opposed to those producing vertical uplift. Substantial
shortening of the crust is clear.
CONCLUS ION
With We possible exception of study of the lower modes
of free oscillations, all the seismic methods described
above have something substantial to contribute to under-
standing of the continental basement. Furthermore, there
are other useful methods and techniques that I will not
discuss here, and in any case the dividing lines between
the various methods are somewhat arbitrary. My purposes
are (l ~ to draw attention to the large unrealized potential
for understanding the continental basement through
application of present-day seismic methods and ¢2J to
point to means for improving that potential through con-
tinuing evolution and improvement of seismological
methods. Suppose, for example, that a method could be
found for artificially generating shear waves of sufficient
amplitude so that the deep crust and upper mantle could
be explored using such waves, or that movable, tightly
spaced seismic arrays could be deployed so as eventually
to cover the continent. Suppose the dee~sea floor, the
continental shelves, and venous remote areas were no
longerareas of essentially no seismic data on earthquakes.
Suppose that seismic profiling were conducted along
long, closely spaced lines spanning the continents. A1-
though this might seem like an ambitious undertaking to
some, one should remember that it is only within about
the last 30 years that there has been significant seismic
exploration of the deposed floor and only about 20 years
since reflection profiling ofthe seafloor began. The ocean
basins, which, of course, occupy a much greater portion of
the earths surface than do Me continents, have in that
short time been crisscrossed innumerable times by seis-
mic reflection profiling. A comparable achievement in the
form of seismic profiling of the continents is within the
grasp of the present generation of geophysicists. Suppose
microprocessors and other electronic developments result
in more sensitive and selective seismographs and new
ways of managing large complex sets of data. Suppose the
seismic reflection method is generalized so that three-
dimensional images rather than tw~dimensional sections
are produced. Suppose comparable advances are made
concurrently in related branches of geophysics and geol-
ogy. Every one of these suppositions is technically
within our reach today.
If even some of these suppositions are fulfilled, I think
we can look forward to a new understanding of Me crust,
one in which deep subsurface structural features become
as familiar to continental scientists as the midocean
edges, the trenches, the seamounts, and the submarine
canyons have become to ocean scientists over the last few
decades. When they are, surely our understanding of the
evolution and the genesis of continents will become more
OCR for page 125
Seismic Exploration of the Continental Basement
30' /5' _ I09°00' 45' W Do' 15' 108°00' 45'
. .~ ~ ~ / ]43°00'
.' .,'.' ~ .. ~ \ . 1
~ ,. .
to,;
got
Jon
Toteraccle Butte
\
f / Aikoli Otter i5
elf P~
p,450
l -~ ~ ~ =~ ~
~ ::: :- :
I; 'T' "I ~ Who r'~
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~~\~'
:/~250
/~00
~50 6'
:~OC~,>~ Oregon Buttes
OContlnenla
Peal
_
,45'
N
30'
_ /5'
42-00'
0 10
FIGURE 10.9 Clap of southeastern Wind River Mountains in Wyoming showing location of COCORP seismic
reflection line.
COCORP wrOM`NG I iNE ~ wtOMiNG L'NE iA
G~_ R`~ ~~.e
S. sr~rro~ ~vu-ffeS
50 `00 Is0 20o 250 100
...a Q,~e, 84,n,
sour~ PaSS C/TY
| sr~rloa au_tes
20 ·m
125
WYOM`NG LINE 2
a.~ ei~—.,. ~ c,'. ~E
s0 It00 150 2nt~ ~—~ ·~ ~ ,~ ~ ~ ,~ 5rA7~— ~RS / 1 \
_' ~ r?
-_~ ~_-- ~ -___ '~ \ _ \ \ \,,,-
1s 0
1 . i 20.0
FIGURE 10.10 Line drawing of section along line of Figure 10.9. Note especially major thrust fault bounding the Wind River uplift
on west. See text for discussion. Courtesy Jonathan Brewer and Scott Smithson, University of Wyoming.
OCR for page 148
148
Imm
llJ
cleft, 100
IOkb Ikb 1 3 lOOb lOb
lOmm ~ , I ~ 1 ' ~
_
101`
l ll
+
!+
m.g.d. "C ~-1.6
-
1 1, 1, 1, 1, 1, 1,
10 9 8 7 6 ~
0 10 10 10 10 10
JO cm~
FIGURE 12.9 Mean grain size versus dislocation density and
differential stress for olivine. Analogous relations can be defined
for plagioclase and clinopyroxene and should be useful in defin-
ing differential stress for defonned granulites. Figure is from
Koldstedt et al. (1976).
An important question that follows from study of the
retrograde hydration in some xenoli~s is the source ofthe
water. Stable-isotope studies (hydrogen and oxygen) may
be able to distinguish alternatives such as hydrothermal
circulation or dehydration of underlying subducted oce-
anic lithosphere. Microcrack studies may also be impor-
tant in defining mechanisms for migration of fluid phases
in the deep crust (Simmons and Richter, 1978~.
Substantial progress on the dynamics of upper-mantle
flow has been made by studies of deformation mech-
anisms of olivine in the laboratory and application to
peridotite xenoli~s (e.g., Kohlstedt et al., 1976~. A paral-
led study using lower crustal granulites deserves atten-
tion. The density of unannealed defects can be examined
using the transmission electron microscope. Experi-
mental work needs to be done to understand the deforrna-
tion mechanisms of feldspar and pyroxene, probably the
most abundant lower crustal minerals. In addition, the
simple relationship between grain size and stress found
in olivine (Figure 12.9) encourages We search for a simi-
lar relationship for feldspars and pyroxenes. One ob-
serves Mat grain size in basic granulites is variable from
locality to locality; could this reflect a variable stress in
ROBERT W. I[AY and SUZANNE MAHLBURG KAY
Me lower crust? If stress and temperature can be deter-
mined for a xenolith, Den strain rate can be calculated,
and questions of lower crustal dynamics can be
addressed.
SUMMARY AND CONCLUSIONS
Xenoliths and magmas can help to answer such questions
as (a) what is the parentage of the lower crust, i.e., per-
centage of original igneous versus sedimentary material,
percentage derived directly from the mantle versus ma-
terial derived from crustal processes; (b) is the lower crust
hydrous or anhydrous; (c) what are the temperature-
pressure regimes in the lower crust; (d) are rocks in the
lower crust at equilibrium with present or past tempera-
ture~ressure conditions; (e) what are the age relations
between various units of the lower crust; and (f) what is
the deformation history and what is the present state of
stress in the lower crust? When knowledge derived from
xenoliths and magmas is combined with drill holes and
geophysical information, a three~imensional picture of
the crust can be constructed. Crustal studies have barely
begun: a great expansion of our ability to answer all these
questions can be expected in the near future.
ACKNOWLE DGM E NTS
We thank D. Smith and E. Padovani for reviews and
members of the Continental Tectonics Panel for discus-
sion and acknowledge Me National Science Foundation
for providing financial support under Grant EAR 77-
13656.
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OCR for page 151
Geochemical Evolution
of the
Continental Crust
l2
INTRODUCTION
GILBERT N. HANSON
State University of New York at Stony Brook
To place limits on possible origins of the earth's conti-
nental crust it is necessary to understand how the earth's
crust has evolved and how the various processes acting
have modified the geochemistry of the pre-existing crust.
Prior to about 3900 million years (m.y.) ago the earth as
well as the moon must have undergone significant infall
of very large extraterrestrial bodies (Smith, 1976~. This
bombardment must have played a significant role in
crustal evolution. However, on the earth the record of this
event has yet to be found. Thus speculations on the geo-
chemical evolution of the continental crust based on the
lithological record must start from 3800 m.y. ago, the age
of the oldest terrestrial rocks found so far.
The main purpose of this chapter is to suggest isotopic
and trace-element approaches useful for studies leading
to a better understanding of the geochemical evolution of
the earth's continental crust. There are a number of recent
papers pertinent to this topic, for example, Lowman
(1976~; Tugarinov and Bilikova (19161; Smithson and
Decker (19741; Smithson and Brown (1977~; Hargraves
151
(1976~; Taylor (in press); Tarney and Windley (1977~;
Armstrong and Hein (19731; Jahn and Nyquist (19761;
Moorbath (1977~; Heier (1973~; Tarney (1976~; Collerson
and Fryer (1978~; Green (1972~; Pankhurst (19771; Brooks
et al. (1976b); O'Nions and Pankhurst (19781; Oversby
(19781; Engel et al. (19741; Shaw (1976), and O'Nions et
al. (1979).
Models for the evolution of the crust can be placed
between two extreme schools ofthought (also see Chapter
15). One is that the continental crust formed early in the
history of the earth (during the Archean) and that only
small fractions of material have been added since then.
The other model is that the continental culst has grown
substantially since the Archean. Both models acknowl-
edge the more or less continuous addition of igneous
rocks into or upon the upper continental crust. There are,
however, two possible sources for this material, the man-
tle or the lower crust. Material added from the mantle
will, of course, increase the mass of the continents,
whereas material derived from the lower cn~st will not
change the mass of the continental crust but only redis-
tribute matter within it.
OCR for page 152
152
The continental crust makes up only 0.3 percent of the
mass of the earth, but it is strongly enriched in elements
such as K, U. Th, Rb, Ba, and Sr (Gast, 19601. Based on
heat-flow data and the abundances of K, U. and Th, Heier
(1973) suggests that the lower crust has lower abundances
of these elements than the upper crust and that granulite-
grade rocks of intermediate composition are reasonable
candidates for the lower-crust composition. A~ more exten-
sive argument for this model is presented by Smithson
and Brown (1977~.
One of the most important factors in any interpretation
of crustal evolution is how mantle convection has changed
with time. In the plate tectonic model, the crust is a pas-
sive feature riding on lithospheric plates, the motions of
which are determined by convection within the astheno-
sphere. The igneous as well as tectonic activity within the
crust is directly or indirectly related to activity in the
asthenosphere. Thus, to understand crustal evolution it is
essential also to understand the present convection re-
gimes ofthe mantle, how these regimes may have evolved
with time, and the possible interactions that various parts
of the mantle may have had with the continental crust.
As a first approximation, the upper mantle may be
divided into two parts: the suboceanic mantle and the
subcontinental mantle. Based on isotope and trace-
element ratios for basalts, there are two principal sources
of magma in the suboceanic mantle: one is the source of
ocean-ridge basalts, the other the source of the ocean-
island basalts. Radiogenic isotope data would suggest that
the sources are separate and have been isolated for some
2000 m.y. (Church and Tatsumoto, 1975; Brooks et al.,
1976a; Sun and Hanson, 1975~. The ocean-ridge types of
basalts appear to be restricted to zones of spreading either
at ocean edges or in marginal basins; basalts ofthe ocean-
island type occur in nearly every tectonic environment in
the oceans and continents (Schwarzer and Rogers, 1974~.
Where the ocean-island-type basalts occur on continents,
there may be little reaction with the continental crust
(e.g., Ross Island, Sun and Hanson, 1976~. The mantle
source for continental basalts (a large and geochemicaIly
variable group of basalts), however, may in some cases
have a history associated with the continents (Peterman et
al., 1970; Leeman, 197~;; Brooks et al., 1976b), and the
source may have interacted or mixed with crustal compo-
nents (Faure et al., 1972; 1974~.
RADIOGENIC ISOTOPES
Some of the key data for understanding the evolutionary
history of sources for igneous rocks are the initial isotope
ratios of Pb, Sr, and Nd. It must be emphasized that the
initial ratios alone cannot be used to tell whether the
immediate source of a rock is the mantle or the crust. The
isotopic ratios only allow an estimation of the U/204Pb,
Rb/Sr, and Sm/Nd ratios of the source and a determination
of the time these ratios existed. If continental evolution
involves input of significant quantities of igneous rocks
| BUSHVELD
In
ID
ran
Or =°~
G PEAT
D I K E ~ =:=—
_
4.6 4.0 3.5 3.0 2.5 2.0 I.S 1.0 0.5 0.0
109 YEAR S
GILBERT N. HANSON
40 707
0.706
0.705
,ISLAND
O704
0.703
BRIDGE
0702
. 0701
. 0.700
0699
FIGURE 13.1 Strontium evolution diagram for mantle with
data for basic and ultrarnaf~c rocks modified from Jahn and Ny-
quist ( 1976), with data for the Great Dyke and Bushveld Complex
from Hamilton (1977). "Island" designates field for ocean-island
basalts. "Ridge" designates field for ocean-ridge basalts.
derived from the mantle, it is important to understand
how the subcontinental and suboceanic mantle regimes
have evolved.
Figure 13.1 shows some schematic mantle evolution
curves for Sr. The large variation in 87Sr/86Sr in modern
oceanic basalts indicates that the suboceanic mantle has
considerable heterogeneity. This heterogeneity may have
also existed in the Precambrian, but the limited number of
basaltic rocks analyzed may not adequately sample the
Precambrian mantle. Hamilton (1977) suggests that the
initial 87Sr/86Sr ratios for the 2100-m.y.-old Bushveld Com-
plex, which vary Tom 0.7056 to 0.7086, may reflect a het-
erogeneous mantle source variably enriched in Rb/Sr and
is not a result of mixing with crustal components. If he is
correct, prior to 2100 m.y. ago the subcontinental mantle
in the vicinity of the Bushveld Complex had been vari-
ably enriched in Rb/Sr for a significant period of time.
Veizer and Compston (1976) have determined initial Sr
isotope ratios on sedimentary carbonates throughout the
geological record. If these values represent carbonates
from oceanic environments, they should indicate the
average Sr isotope ratios of the rocks supplying Sr to the
oceanic environment. It can be seen in Figure 13.2 that
the Sr isotope ratios in the Archean are low, typical of
values assumed for the mantle. This may indicate that if
the continents were extensive in the Archean, either they
had low 87Sr/86Sr ratios and low Rb/Sr ratios or, if the
continents had higher 87Sr/86Sr ratios, the strontium in
the oceanic environment was predominantly derived
from volcanic regimes and thus reflected a mantle source.
After the Archean, the 87Sr/86Sr ratio of the carbonates in-
creases significantly. This would suggest that the conti-
nental source is more exposed and volcanics are less of a
source or that there is significant growth ofthe continental
crust at the end of the Archean. The same evolutionary
OCR for page 153
Geochemical Evolution of the Continental Crust
41~
K2O
N a2o
0.71OI
87s
r
86sr
_ SEDIMENTS
I GNEOU S
ROC K S
SEA WATER
_ .
0.700 _
0 1
109 YEARS
FIGURE 13.2 87Sr/86Sr ratios in sedunenta~y carbonates
(Veizer and Compston, 1976) and K2O/Na2O in sediments and
volcanics (Engel et al., 1974) as a function of We age of the rocks.
relationship can be seen in the K/Na ratio of sedimentary
and volcanic rocks (Engel et al., 1974) and in the rare-
ear~ elements content of sediments (Taylor, in press).
Figure 13.3 is a single-stage mantle growth curve for Pb
on a 207pbl204pb versus 206Pb/204Pb plot. The data from mod-
ern oceanic basalts indicate that there is not a simple
growth curve for the recent mantle. The oceanic basalt
data lie along lines with slopes the ages of which are
approximately 2000 m.y., suggesting that some 2000 m.y.
ago their sources were variably enriched in the 238UI204Pb
ratio relative to the growth curve. Some basalts from pre-
sumed subcontinental mantle show a quite different rela-
tionship. For example, the Pb isotope data for Absaroka
volcanics from Wyoming (Peterman et al., 1970) lie about
a line with a slope of 2800 m.y. These rocks, whether
derived from the mantle or the lower crust, indicate a
source that has had a low 238U/204Pb ratio with respect to
the mantle growth curve since 2800 m.y. ago. This age is
approximately that of the basement rock in this region.
Leeman (1975) found similar results for basalts from the
Snake River Plain as well as from Yellowstone National
Park. He suggests that the trace- and major-element com-
position of the basalts require their derivation from the
mantle. In both studies, the lead and strontium isotopes
are not correlated and cannot be explained by a simple
mixing relation between crust- and mantle-derived end
153
members. These studies suggest that in these regions the
subcontinental mantle has been attached to the conti-
nental crust as a mantle keel since at least 2700 m.y. ago.
The volcanics from these areas have 87Sr/86Sr ratios of
0.704~.709, on the higher end of the oceanic basalts,
suggesting that although their source was depleted in U
relative to Pb it was not depleted in Rb relative to Sr. If
anything, it was enriched.
Based on initial Sr, Pb, and Nd ratios, many granitic and
basaltic rocks would appear to have either a source in the
mantle or a source with only a short history in the crust
(e.g., Moorbath, 1977; McCullogh and Wasserburg, 1978;
DePaolo and Wasserburg, 1976; and O'Nions et al., 19791.
This suggests an episodic and continuous addition of ma-
terial to the crust through time. Further geochemical
study of rocks derived from crustal sources, but with es-
sentially mantle ratios, may make it possible to place
limits on how the crust evolved and the times involved.
Likewise, further geochemical study of rocks derived
from the mantle may allow a characterization of the scale
of heterogeneities in the mantle, show how they are
evolving through time, and help to distinguish parts ofthe
mantle interacting with the continental crust at a given
time. As convection models for the mantle improve, this
inforTnation should allow a direct correlation between an-
cient tectonic regimes and convection in the mantle.
PETROGENESIS OF IGNEOUS ROCKS
Petrogenetic studies emphasizing isotope and trace-
element analyses of a suite of igneous rocks are particu-
larly pertinent for placing limits on the geodynamic fac-
tors in the mantle responsible for tectonic activity in an
area at the time of formation of the suite of rocks. The
purpose of a petrogenetic study of an igneous suite is to
determine the chemical and mineralogical composition of
the source rocks at the time of melting; the history of the
sources prior to melting; the extent of melting; the tem-
perature (T) and pressure (P) or depth conditions during
~6
t5 - 1.0~ U OCEAN R ~ DO E
A, 14
lo
Cal
D 1 3
Cal 1 2
11
ABSAROKA VOLC. _~
_ , , ~ _e~CEAN ISLAND
BASALT S
~ ~ BASALTS
TOO GNEISS. ISUA
/4.o
0/
, ty
4.5,7 xl~y
10
9 10 11 12 13 14 15 t6 17 IS 19 20 2!
206 pb/20 P b
FIGURE 13.3 Mantle growth curve for Pb with selected rock
types plotted (modified from Tatsumato, 1978). Data for Absaroka
volcanics are from Petennan et al. (1970). Data for Amitsoq
gneisses, Isua, W. Greenland, are from Moorbath et al. (1975).
OCR for page 154
154
melting; and modification of the primary melts by dif-
ferentiation, assimilation, metasomatism, or late-stage
fluids. Although a petrogenetic study relies heavily on
major-j minor-, and trace-element analysis and isotopic
ratios, it must be based on rocks for which the field,
geochronological, and petrologic relations are well un-
derstood. The major-element analyses when combined
with modal mineral data allow a comparison with experi-
mental studies for estimating T. P. and volatiles during
melting or differentiation. The isotopic data for Sr, Pb,
and Nd allow an estimate to be made of the history of the
U/Pb, Rb/Sr, and Sm/Nd ratios of the source. Modeling of
Pace elements allows an estimate of the trace-element
composition of the source, the mineral composition of the
residue at the time of removal of the melt, sequences of
fractional crystallization, and the extent of these pro-
cesses. Along with the initial radiogenic isotope ratios, the
trace elements allow an estimate ofthe extent of mixing or
reactions with other melts or rocks (Vollmer, 1976; Lang-
muir et al., 1978~.
To obtain the maximum information, each of the dif-
ferent types of analyses must be made on the same sam-
ples. There are few places where a complete study can be
made in one laboratory, and it may be questioned as to
how many suites of rocks require such detailed analysis.
The extensive data, however, are warranted for selected
suites, because they can lead to a more quantitative in-
sight into crustal evolution. Once the data are available,
the best petrogenetic interpretation may not be immedi-
ately obvious but will probably lead to new approaches
and models.
As an example of tectonic application, the petrogenesis
of granitic rocks in two tectonic settings will be compared.
The two settings are (1) an intrusive granite~reenstone
belt in northeastern Minnesota in which all the rocks
dated give ages of 2700 m.y. (Arth and Hanson, 1975) and
(2) a high-grade Weiss terrane in southwestern Minnesota
with ages as old as 3600 m.y. (G. N. Hanson, State Uni-
versity of New York at Stony Brook, in preparation).
In the northeastern Minnesota greenstone belt the
initial 87Sr/86Sr ratios of basic, as well as granitic, rock are
all between 0.700 and 0.701, suggesting that they were
derived from a mantle source or sources with high Rb/Sr
ratios that existed for only a short period of time prior to
melting. Dacitic and tonalitic rocks have KlRb and Rb/Sr
ratios similar to those of Archean tholeiite and strongly
depleted heavy rare-earth element patterns. The model
Mat best fits the data is that the dacites and tonalites are
derived by partial melting of a tholeiitic parent, probably
derived from an oceanic mantle, leaving a residue of gar-
net and clinopyroxene.
The quark monzonites *om the greenstone belt have
lower K/Rb ratios and higher Rb/Sr ratios than the to-
nalites and dacites, and rare-ear element patterns simi-
lar to that of the tonalites and dacites but with higher
abundances and negative Eu anomalies. The best model
for the origin of the quartz monzonites is partial melting
(upper amphibolite grade) of short-lived (<~;0 m.y.) grey-
wacke. The greywacke consists of dacitic and tholeiitic
GILBERT N. HANSON
detritus derived from within the greenstone belt that has
been enriched in K and Rb by sedimentary processes.
In this greenstone belt all the components are thought
to be derived from either the mantle or from rocks with
short histories outside the mantle. The belt probably
developed on an oceanic crust. If there were a continental
crust underlying the greenstone belt, it was apparently
not a major source for the volcanic or intrusive rocks
analyzed.
In the high-grade gneiss terrane in southwestern Min-
nesota, the 3600-m.y.-old Morton and Montevideo
gneisses were intruded by granitic rocks at 3100, 2600,
and 1800 m.y. (Goldich et al., 1970; S. S. Goldich, North-
ern Illinois University, and J. Wooden, Lockheed E:lec-
tronics Company, in preparation; S. S. Goldich and C. E.
Hedge, U.S. Geological Survey, in preparation). The
gneisses vary from quartz diorite through quartz mon-
zonite, and the intruding granitic rocks are granodiorite to
quartz monzonite. The rare-earth element patterns for the
gneisses and the later intruding granites are all very simi-
lar to one another, suggesting that they have similar
sources. These patterns are quite distinctive from those of
the tonalites but similar to those of the quartz monzonites
from northeastern Minnesota. Based ore the trace-element
abundances and the geological relations, the best model is
that the gneisses and the later granites are derived from
melting of similar sources, presumably the lower conti-
nental crust. This model is supported by Pb isotope data
(Doe and Delevaux, in press), which suggest that the later
granites are derived from related sources with a signifi-
cantly long history in the crust. The K content of the
gneisses, mainly tonalites, is generally lower than that of
the later granites, mainly granodiorites to quartz mon-
zonites. Lower K content for high-grade metamorphic
rocks as compared with those of lower grade is not
unusual (Heier, 1973~. This might imply that the gneisses
originally formed under conditions that led to melts of
lower K content or that the gneisses have lost K since the
time of their origin.
These two examples of petrogenesis would indicate
that although the major-element compositions of quartz
monzonite and quartz diorite are similar in both the intru-
sive granite~reenstone belt and the ancient gneiss ter-
rane, a more careful study oftheir chemistry and relations
shows that the similarity is superficial and that the origins
are probably quite different. The greenstone belt devel-
oped in a short period of time and consists of rocks
derived principally from the mantle or from rocks with a
short history outside the mantle; whereas the gneiss ter-
rane developed over a longer period of time, and the prin-
cipal source for the granite rocks appears to be the melt-
ing of pre-existing crustal sources.
CHEMISTRY OF THE LOWER CRUST
Based on heat-flow data, Heier (1973) suggested that the
lower crust has lower abundances of K, U. and Th than the
upper crust. If the lower crust is made up of granulite-
OCR for page 155
Geochemical Evolution of the Continental Crust
grade rocks of intermediate composition, this could fit a
model of a depleted lower crust and an enriched upper
crust, because most granulite rocks are relatively de-
pleted in K, U. and Th with normal abundances of Sr and
Ba compared with similar rocks of lower grade. This re-
sults in higher K/Rb (commonly 500 or greater) and lower
Sr/Ba (~10), Rb/Sr (~0.02), and U/Pb ratios in granulite
rocks (Tarney and \Vindley, 19771. The depletion in U is
reflected in the low U/Pb ratios found in some granulite-
grade rocks, leading to whole-rock leads that plot along
isochrons below the mantle growth curve and to the left of
the geochron in Figure 13.3.
Is the relative depletion of these elements inherent in
the origin of the types of rocks found in a granulite ter-
rane, or have the rocks in a granulite tenant preferentially
lost some of these elements? If the granulites have lost
these elements, there are two means of transport: as melts
or in aqueous or other solutions. An important difference
between granulite and lower grades of metamorphism is
the lower water content in the granulite facies rocks.
Some of the loss of elements may thus be associated with
the loss of water. One of the more surprising discoveries
was that whereas fluid inclusions in rocks of amphibolite
grade are rich in H2O, fluid inclusion in granulite-grade
rocks have high proportions of CO2 (Touret, 1974), sug-
gesting that the fluids with which they were in contact
during high-grade metamorphism were CO2 rich. Gold-
smith (1976) reminded us that a very important mineral in
the lower crust is scapolite and that scapolite is a mineral
into which substantial fractions of C03, S04, and C1 can be
placed. He suggests that much of the carbonate is em-
placed in the granulite terrane directly from the mantle.
Lloyd and Bailey (1975) in studying peridotite nodules
from the subcontinental mantle have found metasomatic
textures, suggesting that normal lherzolite has been meta-
somatized, resulting in the growth of titaniferous phlogo-
pite, amphibole, diopside-salite, ferroaugite, titanomag-
netite, sphere, perovskite, apatite, and calcite in what was
originally lherzolite. It thus appears that many elements
may be mobile in the mantle and are being added to the
subcontinental mantle in carbonic or aqueous solutions.
Wendlandt and Harrison (1978), for example, found that
under mantle conditions CO2 vapor is 3 orders of magni-
tude more enriched in rare-earth elements than is
aqueous vapor and is also enriched in rare-earth elements
relative to silicate melts.
Shieh and Schwarcz (1974) have shown that the oxygen
isotope ratios in rocks of the amphibolite grade in the
Grenville province are characteristic ofthe~r unmetamor-
phosed equivalents, whereas rocks of the highest meta-
morphic grade have oxygen isotope ratios more indicative
of the mantle. A similar relation has been found for Ar-
chean rocks in the Superior province (Longstaffe and
Schwarcz, 1977~.
Although transporting material in the form of siliceous
melts from the mantle to the crust or from the lower crust
to the upper crust is undoubtedly important in terms of
quantities of material moved, the effects of aqueous or
carbonic vapors or fluids in transporting material within
155
the mantle, from the mantle to the crust, or within the
crust may be significant. Particularly, they may play an
important role in separating elements that behave simi-
larly during magmatic processes. The solubilities of ele-
ments in these fluids and mineral-fluid distribution coef-
ficients must be determined experimentally under a
variety of conditions so that a proper evaluation of these
processes may be made.
MANTLE-CRUST INTERACTION
To understand the evolution of the continental crust it is
necessary to understand how the mantle interacts with the
continental crust. This requires characterizing variations
within the mantle, determining their dimensions, and as-
sessing how the variations are affected by mantle convec-
tion. Sun and FIanson (1975) suggested that Rb~r and
Pb-Pb isochron ages for ocean-island basalts of about
9000 m.y. reflect a real time of separation and isolation of
the mantle sources for ocean-ridge and ocean-island ba-
salts and that they are not the result of simple mixing
between a large-ion-lithophile-element- (~IL) depleted
ocean-ridge source and a -enriched ocean-island
source. This is best shown in a plot of 87Sr/86Sr versus
206Pb/204Pb, in which the ocearl-ridge basalt plots away
from the main trend of the data for the ocean islands and
not at either end of a potential mixing curve. Although
ocean-ridge basalts are only known to occur in spreading
centers, whether at ocean ridges or in marginal basins,
these environments encircle the globe. The ocean-island
basalts are found in continental, island-arc, and oceanic
terranes seemingly unrestricted in their geographic oc-
currence. Thus both sources appear to be ubiquitous but
separated. Until we have better information regarding
convection in the mantle, the simplest model to explain
these observations is a stratified mantle in which the
source for the ocean-ridge basalts is a convecting mantle,
below which is the source for the ocean-island basalts.
This lower source may also be convecting id. Richter,
University of Chicago, personal communication, 1978~.
Applying this model to a continental environment, there
may be a continental mantle keel attached to the conti-
nental crust for hundreds to thousands of millions of years
(Figure 13.41. In this model, starting from the leR side of
Me figure and using the numbers in Figure 13.4: (1) Per-
turbations in the convecting mantle produce upwellir~g,
rifting, and melting of the continental mantle with the
formation of continental basalts. The wide variety ofthese
melts may or may not be a result of reaction with or melt-
ing of the continental crust. (2) Carbonatites or lcimber-
lites may result from melting or instability a: ;r the low-
velocity zone. (3) Ocean-island-type basalts found on the
continents are associated with deep-mantle plumes. (4)
The addition of CON to the lower crust may be a result of
continued production of CO2 over wide areas in the man-
tle that reacts with the granulite-grade rocks in the lower
crust, or it may be episodic, associated with tectonic dis-
turbance.
OCR for page 156
156
FIGURE 13.4 Diagrammatic repre-
sentation of present day mantle.
Figure 13.4 also depicts a subduction zone (far right) on
the continental margin in which there is extensive tec-
tonic activity, (5) the connation of a marginal basin, and (6)
in the arc, volcanism and the intrusion of gabbroic
through granitic plutons. Below the arc there may be
melting of: the subducted plate to produce tonalities; the
subcontinental mantle or the ocean-ridge-type mantle to
produce basalts; magic rocks near the base of the crust to
produce a`nor~ositic or gabbroic plutons and possibly an-
desites; and the intennediate-composit~on continental
crust to produce granitic intrusions. The melting is prob-
ably enhanced by the dehydration of the subducting
plate. In the marginal basin the first volcanics would be
derived by melting of the subcontinental mantle. As rift-
ing proceeds and the marginal basin widens, ocean-ridge-
type mantle becomes the dominant source of basalt.
Detailed petrogenetic studies of suites of modern igne-
ous rocks should allow testing of this and other models.
Similar studies on other geological time spans should al-
low an evaluation of the evolution of mantle regimes,
mantle convection, and the interaction of the mantle with
the continental crust.
ACKNOWLE DGM E NTS
S. R. Hart and I. Wooden reviewed tl~e manuscript. This
report was supported by NSF Grant No. EAR 76-13354
AO1 (Geochemistry).
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GILBERT N. HANSON
1 2 3 4 5 6
~ :— ~ ~
~~ ~3~ ~ ~ = ~~;~
1 {T^HOSPHERE~ .~ .~. . ~~ ~ +~N ~ 0.
. ~ ~~ ~ ~ ~ ~ U ^, ~ ~ ~ U ~~ '4 ~ t~ ~ ~ * ,, ~ ~t- ^ _
4 . - ~ ~ ~ n ~ ,~ .-, i a/ ,,. ~ At; At, ,,-,,,
OCEAN RIDGE SOU RCE
~ ~ ~ ~ .~;~.*f
r~1 B~> ~ /
'1 ~
Iv ~ ~
A
7>1
1 ~1
V ~ ~ ~ L ~ ~ ~ < ~ ~ ~ ~ ~ ~ L
~1 ~ ~ A i, ~ ^~= ~ hi 1~1 ^~^ - ~1
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OCR for page 158
Representative terms from entire chapter:
continental crust
