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OCR for page 128
COCORP and Fluids in the Crust
8
INTRODUCTION
During a little more than a decade, COCORP (Consor-
tium for Continental Reflection Profiling) has collected
deep seismic reflection data in the United States for pro-
files totaling over 8000 km in length (Figure 8.1~. These
data, though trivial in quantity compared to the huge vol-
ume of industrial seismic data for the sedimentary basins,
128
JACK E. OLIVER
Cornell University
ABSTRACT
Using the seismic reflection profiling technique, COCORP has probed the conti-
nental basement along lines traversing a wide variety of major geological fea-
tures. The results are correspondingly diverse, and they are informative on
various aspects of crustal geology, including the geology of fluids, the particular
aspect emphasized in this chapter. There are at least three ways in which
COCORP data may relate to the understanding of fluids in the basement. First,
some anomalously strong reflectors, typically near-horizontal and at depths of
about 20 km, may correspond to pockets of magma or other fluids trapped within
the basement. Second, as suggested by others, some or even all basement
reflectors may correspond to zones of fluid-fi~led porous rocks. Third, new
insights into structures suggest that tectonic models include inferences about the
role of fluids in the crust. For example, fluids expelled tectonically from sedi-
ments buried in erogenic belts may play a role in migration of hydrocarbons,
transport of minerals, diagenesis, crustal rheology, chemical Demagnetization, and
a variety of other phenomena.
constitute the largest and most comprehensive set of land-
based seismic reflection data on the deep crust of the
continents anywhere. BIRPS (British Institutions Reflec-
tion Profiling Syndicate) has collected a comparable quan-
tity of marine data on the deep crust beneath the continen-
tal shelf surrounding Great Britain, and at the time of this
writing some 20 countries, including the United States and
the United Kingdom, have surveyed a total of 20,000 to
OCR for page 129
COCORP AND FLUIDS IN THE CRUST
I NW _
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25,000 km of seismic reflection profiles of the deep conti-
nental crust throughout the world. This total is exclusive
of any proprietary industrial data on the deep crust.
This quantity of data corresponds to exploration of only
a small fraction of the total volume of the continents.
Nevertheless, these data represent early exploration of a
new frontier, and as such they have already provided sur-
prising and important information on many aspects of
continental crustal geology. This chapter, however, passes
over most of these aspects and, because of the nature of
this volume, focuses solely on just one aspect of crustal
geology fluids in the crust. Furthermore, the chapter is
based almost exclusively on COCORP data, all taken in
the 48 contiguous states. For a review of COCORP stud-
ies in general, see Brown et al. (1986~.
COCORP observations relate to the study of fluids in
the crust in at least three different ways and even more if
various indirect lines of reasoning not emphasized here are
pursued. First, the observations occasionally reveal, at mid-
crustal depths, exceptionally strong, near-horizontal re-
flectors that are probably associated with magmas and/or
other fluids. This information is some of the very best
evidence in existence on spatial locations of fluids in deep
basement rocks at the present time. Second, the COCORP
observations reveal a variety of weaker reflectors through-
out the continental basement, some, or even all according
to one hypothesis, of which may be related to fluids in
some manner. Third, COCORP data provide novel infor-
mation on the structure and tectonics of the crust and
thereby stimulate thinking about crustal tectonic models
that have implications about fluid behavior in the crust.
This chapter discusses these topics in order and with some
129
FIGURE 8.1 Map showing deep seismic
profiles surveyed by COCORP from the
inception of the project through December
1986.
emphasis on the third point because of the nature of this
volume.
STRONG REFLECTORS IN THE MID-CRUST
In 1975 an early COCORP survey sensed a strong re-
flector at a depth of about 20 km in the crust beneath the
Rio Grande Rift north of Socorro, New Mexico. This
location is just where Allan Sanford (Sanford et al., 1973,
1977), basing his ideas on earthquake and other geophysi-
cal and geological data, had earlier postulated that a magma
body exists. The COCORP reflection survey supported
Sanford's hypothesis, delineated certain features of the
body, and, if Sanford's well-supported hypothesis is in-
deed correct, provided a type example of reflection data
for a magma body in the mid-crust.
Later, while profiling in Death Valley, COCORP de-
tected a strong reflection that in many ways resembles that
type example from beneath the Rio Grande Rift. The
Death Valley reflector is at about the same depth, is nearly
horizontal, and is in a similar extensional tectonic environ-
ment. An obvious possible, though not unrefutable, inter-
pretation is that another magma body exists beneath that
part of Death Valley (de Voogd et al., 1986~. In fact, the
data also reveal a dipping reflector that rises from the
vicinity of the proposed magma body and extends to near
the surface at a place where recent volcanic activity has
occurred. It has thus been proposed (de Voogd et al.,
1986; Serpa et al., 1987) that this dipping reflector corre-
sponds to a fault zone that acted as a conduit for rising
magma. Whether the fault zone continues to hold magma,
in its lower if not upper reaches, is not resolved.
OCR for page 130
130
Since the Death Valley survey, COCORP has found at
least two other comparable reflectors, one in Nevada and
one in Arizona. Both locations are in the Basin and Range
Province. These two reflectors are somewhat less promi-
nent features, but they are of the same general character
and at about the same depth as the Rio Grande Rift and
Death Valley reflectors. They may also correspond to
magma at some stage of cooling.
These strong mid-crustal reflectors, coupled with addi-
tional observations by COCORP and by others on various
aspects of the buried continental crust, are prompting new
speculation about the formation and evolution of the deep
crust and its role in the evolution of modern surface geol-
ogy. The subject of the structure, composition, evolution,
and tectonics of the deep continental crust and uppermost
mantle is a particularly important and exciting one at pres-
ent. The speculation takes many forms and produces diverse
hypothetical models. Figure 8.2 shows one example that
corresponds to an old idea that magmas generated in the
mantle intrude the mid-crust near the top of a zone of
earlier intrusions. Subsidence and phase change continu-
ally convert the rocks at the base of the crust to denser and
higher-velocity phases that become uppermost mantle or
"anomalous" mantle. In a similar model (not shown)
intrusion occurs at the base of the crust and successive
intrusions build up from below, adding to anomalous
mantle. Other ideas to explain the nature of the deep crust
without major intrusion are also under consideration; the
two above are mentioned here because of their bearing on
fluid penetration of the crust and their partial base in
COCORP data.
The examples cited above of strong mid-crustal reflec-
tors all occur in the Basin and Range Province, a region
characterized by extension in the most recent tectonic
~ , ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ ' · ' · ' · ' · ' · ' · ' · ' ~
/\'l\~/\~/\~/~/~/~/~'l\'l\'l\~/\'l\~/\~/\~/\~/\~/\~/\~/~/\~/
,', i',, A,, ;',', <', ,U\P ,P\E OR\, CRUST,',-`',', `',',n',',-\'
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_ \ _ ~ _ \ _ \ _ \ _ \ _ \ _ \ _ \ _ \ \ _ \ _ \ \ _ \ _ \ _ \ _ \ \ _ \ _ \ ' \
~ _ _~
FIGI~E 8.2 An example of one hypothesis proposed to explain
the structure of the lower crust in some areas. Magma from
mantle intrudes crust near top of zone of previous intrusion.
Deeper portion of zone of intrusions is converted to mantle-like
material (anomalous mantle) at base of crust. This hypothesis is
rid --a designed to explain structure and
dynamics of lower crust and uppermost mantle.
but one example of many
JACK E. OLIVER
episode. COCORP has acquired data elsewhere on still
another strong mid-crustal reflector, this one of somewhat
different seismic character and somewhat different mod-
ern tectonic character (Brown, 1987~. The reflector lies
beneath the coastal plain of Georgia (i.e., a province dis-
tinctly different tectonically from the Basin and Range
Province). This reflector is called the Surrency bright
spot "Surrency" because that is the name of a small town
nearby and "bright spot" because the reflection resembles
in many ways a bright spot of the type familiar to the
petroleum industry. Oil industry bright spots typically
occur in shallow sedimentary rocks. They are commonly
drilled and commonly produce hydrocarbons. The typical
bright spot is a consequence of occurrence of fluids that
affect the acoustic impedance of porous rocks and of grav-
ity which tends to make boundaries between fluids flat and
horizontal. In spite of the striking similarity, the Surrency
bright spot differs dramatically from a typical oil industry
bright spot in that it occurs at a reflection time of about
6 s (i.e., a depth of about 15 to 18 km) and hence must be
well within the basement that lies beneath the sediments of
the coastal plain of Georgia.
The Surrency bright spot, which is described in more
detail elsewhere (Brown, 1987), is located in the Paleozoic
suture zone between former African and North American
crust. How to interpret this unusual and very robust obser-
vation is not completely clear. Because of lack of any
evidence for young volcanism or tectonic movements within
the coastal plain, it seems unlikely that there is magma in
the crust at this location. A lithologic boundary may be
considered as an explanation, but a strikingly flat and
horizontal boundary between two lithologies of such great
impedance contrast as would be necessary to produce the
observed reflection also seems unlikely in this highly
deformed zone. Perhaps the most reasonable suggestion is
that fluids caught up and trapped in the collision process,
or trapped later in the deformed rocks of the collision
zone, account for the observation. The fluids might be
water, He, CO2, hydrocarbons, or something else-just
what is uncertain at present. But it seems obvious that
information on such prominent, almost certainly fluid-
related, features of the crust must be important to anyone
attempting to understand the geology of fluids in the base-
ment. Unfortunately, with only a small fraction of the
total crustal volume so far explored, it is not possible to
say whether this occurrence is unique or one of tens or
hundreds or thousands. Nor is it known whether, if there
are many such features, all are in suture zones or some are
distributed through other parts of the crust as well. Detec-
tion of such features is so easy and straightforward techni-
cally, however, that a great opportunity for further study is
readily apparent from this first striking observation.
OCR for page 131
COCORP AND FLUIDS IN THE CRUST
THE REFLECTIVE CRUST
In addition to the rare, near-horizontal, strong reflectors
of the mid-crust just discussed, in most areas the entire
continental crust is generally reflective to some degree.
Flat and curved reflectors, dipping reflectors, laminated
reflectors and diffractors are common in the basement,
although generally they are weaker than those in sedimen-
tary rocks. Reflections within Phanerozoic, and in some
cases Proterozoic, sedimentary basins are studied inten-
sively by the petroleum industry. Such reflectors may
correspond to lithologic changes, time-stratigraphic bounda-
ries, faults, fluids, etc., and, because of the great industrial
effort, understanding of them is relatively advanced. For
reflectors within the crystalline basement, most not drilled
and most so deep as to be undrillable, identification and
interpretation are normally less certain. Sometimes a
particular reflector can be identified by tracing it to the
surface. Sometimes the spatial configuration of a reflector
gives it a place in an interpretation of geologic structure
that in turn gives confidence to speculation on the nature
of the reflector. Commonly, however, it is not known for
certain just what physical properties produce a particular
reflector at a particular place. Furthermore, it is likely that
a combination of factors is involved.
One possible explanation is that some, or in the extreme
even all, such basement reflectors correspond to fluid-
filled fractures in the crust. From drilling (Koslovski,
1984; Clarke et al., 1986) it is known that, at least for one
location, fluid-filled fractures prevail to depths of at least
11 km. That such fractures are reflective was demon-
strated by Mair and Green (1981) and Green and Mair
(1983) for a case of very shallow reflectors in a granitic
intrusion. Matthews and Cheadle (1986) suggested that all
reflectors throughout the basement are associated with
fluids, an extreme but nevertheless provocative sugges-
tion. Among other things, this suggestion implies that the
reflection Moho, which is commonly taken as the base of
the reflective zone, marks the base of fluid-filled fractures,
and hence fluids, in the crust. An explanation in terms of
a mobile deep fluid boundary is thereby provided for the
vertical mobility of the Moho that is now well demon-
strated by seismic reflection studies. This hypothesis is
not without its difficulties. For example, mere draining of
fluids from a part of the lower crust would likely not
elevate velocities to sub-Moho levels, and hence the hy-
pothesis would not account for the observed coincidence
in reflection and refraction Mohos at most places.
Clearly the reflective crust is not yet fully understood
and remains in a speculative stage. It would be risky, for
example, to base hydrologic or geochemical studies of the
crust on the concept that all basement reflectors necessar
131
fly indicate fluid-filled fractures of the crust, but it would
also be risky to ignore the possibility that many of them
do.
A SYNTHESIS OF INFORMATION ON
CRUSTAL STRUCTURE, TECTONICS, AND
FLUIDS IN THE CRUST
The COCORP data provide information on the struc-
ture and evolution of the continental crust. Many other
sources contribute important, different, and highly diverse
information on this subject. All such information must
ultimately be fit into a common story. What follows is an
attempt to synthesize observations from a wide variety of
disciplines in earth science, all related in some way to the
story of fluids in the crust. The role of COCORP data in
this synthesis arises primarily from its contribution to the
tectonic model and not so much from direct detection of
fluids in the crust as described earlier.
The reader should be aware that (a) much of what
follows is speculative. The hypothesis to be presented
may be incorrect; some parts are surely controversial at
present. Although a substantial amount of evidence in
support of the hypothesis is presented, there remains much
opportunity for further testing. Should the hypothesis be
more or less correct, however, it will be of considerable
importance because it links many geological observations
of the continents to the great global geodynamical pro-
cesses through the mechanism of plate tectonics. (b) Parts
of what follow will be familiar to some, for the hypothesis
and synthesis incorporate some concepts that have been
proposed and discussed, though not necessarily agreed
upon, previously. This chapter should not be mistaken as
a claim for originality with regard to those concepts. Instead
the chapter is an attempt at synthesis on an unusually
broad scale. Appropriate references to concepts proposed
earlier are provided insofar as possible. (c) The author is
not a specialist in most of the wide variety of disciplines
from which observations are drawn and would appreciate
notification from specialists of any case in which data
from a specialty have been misinterpreted, particularly if
that misinterpretation affects the testing of the hypothesis.
The synthesis of this chapter uses as a framework a hy-
pothesis that may be described in general terms as follows
(Oliver, 1986~.
At the surface of the Earth, continents (and smaller land
masses) drift about within the large sea that occupies most
of the surface. The continents have limited freeboard, and
their margins are generally awash in the sea. The conti-
nents are porous and permeable, at least in their upper
parts, and are for the most part saturated with fluids. When
two landmasses converge and collide, the process is an
OCR for page 132
132
asymmetric one. One landmass is partially subducted
beneath the other. Put another way, the margin of one
landmass is commonly buried beneath the accretionary
wedge on the leading edge of the other. According to the
hypothesis, fluids from the sediments of the buried conti-
nental margin are expelled and travel toward the interior
of the partially subducted continent carrying heat, miner-
als, and organic compounds (Figure 8.3~. These fluids
may leach or deposit materials as they travel; hence, they
redistribute materials and leave a lasting record of their
passage. As an illustration of the hypothesis, the Appala-
chian orogen can serve as the type example.
The Appalachian orogen is a consequence of conver-
gence and closing of the proto-Atlantic Ocean during
Paleozoic time. The process included several progenies
and culminated in the collision of the North American and
African continents. Subduction during this closing was
generally to the east, so a large accretionary wedge or
thrust sheet was forced onto and over the sediments of the
North American margin. Some sediments were buried,
some were carried along with the thrust sheet, and some
were reworked to become younger strata. The load of the
thrust sheet caused a large depression near the sheet and a
more distant forebulge, both changing dynamically as the
process evolved. The depression, or foreland basin, filled
with sediments from the thrust sheet. Some of these sedi-
ments were also buried by the advancing sheet. The sedi-
ments involved were generally porous and full of pore
fluids, and they included hydrous minerals. Perhaps one-
third to one-half of the total volume was made up of water
in the pores and in the minerals. As the sediments were
buried, water was expelled, some from shallow depths at
modest temperatures and some from greater depths at higher
temperatures. According to this hypothesis, some of these
fluids, or brines, were expelled into rocks of adjacent parts
of the continent.
Some of the fluids may be an important part of phe-
nomena of the metamorphic core of the orogen, perhaps
producing veins, dikes, and intrusions and facilitating thrust-
ing, but they are not the phenomena of interest here. This
chapter is about fluids that are expelled into sediments of
FIGURE 8.3 Block diagram of orogen as
accretionary wedge or thrust sheet over-
ndes preexisting continental margin. Sub-
duction is to the east. Heavy arrows sche-
matically illustrate flow of tectonic brines
expelled from buried sediments. Gas and
anthracite deposits are closer to orogen
than oil and bituminous coal, respectively.
Continental crust is -35 km thick; hori-
zontal dimension of diagram is ~500 km
(from Oliver, 1986~.
JACK E. OLIVER
the platform and foreland basin, perhaps eventually reach-
ing the more distant continental interior. These fluids
carry heat, minerals, and organic compounds, some from
the marginal sediments and perhaps some leached from
sediments along the way. Expelled by the tectonic load,
and perhaps by heating that produces pressure of volatiles,
perhaps by other volume changes, they leave the orogen
and travel long distances in the style of long-distance
hydrologic flow (Figure 8.3~. Hydrologic flow for which
meteoric waters are the source is thought to carry fluids
for many hundreds of kilometers through parts of the
continent, although the details of that flow are not yet well
known (Sun, 1986~. Fluids ejected tectonically into the
continent may propagate regionally in similar manner.
How much mixing occurs between water from meteoric
sources and tectonic fluids is not specified here, but it
seems that such mixing will be dependent on various local
parameters and so may vary greatly from one region to
another. Nor does this hypothesis specify such matters as
the chemistry of the migrating brines or the particular
place and time when organics mature into hydrocarbons.
Such maturation presumably may occur at many places-
in the sedimentary trough of the margin prior to subduc-
tion, during the subduction process, or in the foreland
basin sediments almost independently of the tectonics. It
remains to be seen whether data on the details of migration
for a particular hydrocarbon province can be fit into this
hypothesis, but, as shown later, more general information
on hydrocarbon migration seems compatible with it.
Fluids injected into the hydrologic system must perturb
the thermal regime. Precisely what form such perturba-
lions take cannot easily be specified in the absence of
information on parameters such as channeling, depths,
rates of subduction, etc. Nevertheless, as the temperature
of tectonic fluids from depth is likely to be high, the
vertical temperature gradient is likely to be increased,
thereby enhancing certain kinds of metamorphism.
Not all features or processes that might be associated
with this model are closely specified here. At this stage of
development it seems prudent to maintain flexibility in the
hypothesis, or model, so as to preserve the opportunity to
/ 91TUMiN~S ANTHRACITE '`-;
\ THRUST \~ SHEET
'' .',,.: ·:--'', v ': it_
CONT I N ENTAL ~RIJ9T
~ /'
Forebulge
Subsidence due to load
| ~ TECTONIC BRINES
SCHEMATIC:
NOT
TO
SCALE
OCR for page 133
COCORP AND FLUIDS IN THE CRUST
refine the model as observations dictate. Let us now
consider some of the data that provide support for the
model. All observations reported here are taken from the
literature. The data are many; only a summary is pre-
sented here. For a more complete discussion of the obser-
vations, see Oliver ( 1990~.
Coal
It is consistently observed that in foreland basins coal is
generally metamorphosed to higher grade near the oro-
genic belt; the grade decreases more or less gradually
toward the continent. Thus, in Pennsylvania anthracite is
found to the east and successively lower grades to the
west. This point has been demonstrated not only for the
Appalachian orogen but also for the Ouachita and Cordil-
leran orogens by Thom (1934~. This pattern is conven-
tionally explained by deeper burial, and hence the higher
temperatures needed to metamorphose the coal, near the
orogen. In such explanations the deep burial is followed
by sometimes rapid uncovering to bring the coal near the
surface where it is mined (Levine, 1986~. Some coal
geologists are not in accord with the burial hypothesis,
however, and have suggested lesser depths of burial and
anomalous sources of heat such as intrusions or hydrother-
mal fluids (Damberger, 1974; Haquebard and Donaldson,
1974~. An alternative explanation is that a pulse of heat
carried by tectonic fluids caused the coal to be metamor-
phosed at depths shallower than those usually called upon
under the burial depth hypothesis.
Brines
There is a great deal of evidence that suggests or is
compatible with the idea that a pulse of hot mineral-laden
brines propagated through permeable sediments of the
continent at the time of orogeny in a nearby erogenic belt.
The evidence comes from study of Mississippi Valley-
type (MVT) lead-zinc ores, fluid inclusions, certain occur-
rences of dolomite, paleoremagnetization, and some other
sources.
After some years of controversy and discussion, eco-
nomic geologists seem to be settling on the origin of the
strata-bound sphalerite and galena deposits commonly
called MVT Pb-Zn ores as, quoting Guilbert and Park
( 1986), "deposition from connate basinal water that moved
updip in response to compaction or other loading pres-
sure." Some geologists call upon expulsion of fluids from
compacting basins to provide such brines. The view of
this chapter is that basins subjected to tectonic deforma-
tion and burial are more likely to expel fluids in greater
volumes than are simple compacting ones, although there
is no reason to conclude that compaction might not be a
]33
source in some cases, and of course some basins may be
affected by both self-compaction and tectonic activity.
The map pattern of MVT lead-zinc occurrence, shown
in Figure 8.4, illustrates that such occurrences bear a spa-
tial relation to erogenic belts similar to that for hydrocar-
bons (see later section) except that, for reasons unknown,
Pb-Zn deposits tend to occur between basins rather than
within them. MVT deposits tend to occur in carbonates
with a strong bias toward dolomites (Anderson and Mac-
Queen, 19824. This observation suggests a common ori-
gin for MVT Pb-Zn and some dolomites.
The subject is not free of controversy, but in recent
years at least sedimentologists have come to believe that
some dolomites are formed at depth as a consequence of
passage of basinal brines through limestones (see Zenger
and Dunham, 1980, for a review). This view is in contrast
to the hypothesis that all dolomites are formed at or near
the surface. Gregg (1985) proposed that circulation of
basinal brines through an underlying sandstone caused a
6-m layer of dolomite at the base of a limestone and shale
horizon in southeastern Missouri. He associated this dolo-
mitization with the emplacement of nearby MVT Pb-Zn
ores and also with the mineralizing waters that Leach et al.
(1984), on the basis of fluid inclusion data, had postulated
as flow from the Arkoma Basin. In other words, dolomi-
tization is apparently related to MVT ores in some cases
because of related origin of both as a consequence of
mineralizing migrating brines that may have originated in
erogenic belts. Thus, brines producing the MVT deposits
of Missouri likely originated during the Ouachita orogeny
to the south, although the possibility of some effects from
the Appalachian orogen to the east must also be kept in
mind.
The dating of emplacement of MVT Pb-Zn deposits is
not easy, but some information is available. Sphalerite
(ZnS) twins in deformation. Taylor et al. (1983) inter-
preted sphalerite twinning in intermediate layers of multi-
layered sphalerite in eastern Tennessee to indicate deposi-
tion during the Alleghenian orogeny and attributed the
mineralization to brines expelled from the shale basin to
the east. Beales et al. (1980) used paleomagnetic data to
determine an Upper Pennsylvanian age for MVT ores and
speculated on the role of tectonic processes in ore forma-
tion and hydrocarbon migration. Such dates, as available,
for MVT ore deposition support the tectonic fluids hy-
pothesis.
Fluid inclusions are an important source of information
on the history of fluids in the crust. Roedder (1984)
prepared an excellent summary of this subject, and his
book includes many points relevant to the subject matter
of this chapter. For example, Roedder noted that evidence
from fluid inclusions in Ohio, New York, Iowa, Missouri,
and Tennessee are similar and hence suggest that scattered
OCR for page 134
134
FIGURE &.4 Map, modified from Cathles
and Smith (1983), showing Mississippi
Valley-type lead-zinc occurrences. This
map is an approximation. Occurrences in
small quantities may be more widespread.
occurrences of sphalerite, etc., formed from the same kinds
of fluids as MVT ores. This point implies a possible pulse,
or pulses, of mineralizing brines that covered large re-
gions. Roedder also noted that organic material is com-
mon in the inclusions of both ores and scattered minerals.
Leach (1973) attributed fluids from sphalerite in a coal
mine and MVT PB-Zn to a single episode of fluid flow.
Leach et al. (1984), Rowan et al. (1984), and Leach and
Rowan (1986) studied fluid inclusions from Missouri,
Kansas, Arkansas, and Oklahoma and found lateral gradi-
ents in temperature increasing to the south. They suggest
that fluids expelled tectonically from the Ouachita Arkoma
basin at 200° to 300°C migrated northerly to deposit MVT
Pb-Zn. Much evidence from fluid inclusion studies sup-
ports the concept of tectonic expulsion of fluids from
erogenic belts.
Certain aspects of diagenesis, a term used here in its
general sense to include epigenesis, of sediments may be a
consequence of migrating tectonic fluids. Morton (1985)
dated, by the Rb-Sr method, diagenetic illite from the
Upper Devonian black shales of Texas and found a date
equivalent to the time of the Ouachita orogeny. He sug-
gested that brines from the Ouachita tectonic zone were
JACK E. OLIVER
. , ,
1 1
1
1
_. ~ ma_ . _ _
~J-i
`__} J.~\ ,- -\
~"`';'`~
\ / ~ LEAD-ZINC OCCURRENCES ~ ~
c, x~ All rot ~\J
\\ ~~'~ _~
J ~ ~
, .
~` ~ ~!'
. . . .
~ - ~ ' ~,; ::.: TECTONIC 8ELtS
0 200 ItlLo - 7EnSk
~ ~ ~ BAS I NS
the cause and, following Dickinson (1974), that hydrocar-
bons may also have migrated to West Texas at that time.
Authigenesis, the enlargement by overgrowth, of potas-
sium feldspars in Cambro-Ordovician rocks of western
Maryland was dated by Hearn and Sutter (1985) using the
40Ar/39Ar method. They found ages corresponding to the
Alleghenian orogeny and suggested that brines from the
erogenic zone produced the authigenesis and affected
hydrocarbon migration and ore deposition. Hearn et al.
(1985) obtained results that were consistent with and that
extended these conclusions for feldspars in Pennsylvania,
Virginia, and Tennessee.
Authigenesis by migrating brines is playing a role of
growing importance in the subject of paleoremagnetiza-
tion, a major new dimension in the field of paleomagnet-
ism. In recent years paleomagneticists have demonstrated
widespread demagnetization of sedimentary rocks for large
parts of North America (Van der Voo, 1986~. One pos-
sible explanation is based on thermally activated viscous
magnetization. The other, and apparently the preferred
one, is chemical demagnetization through authigenesis. The
demagnetization is dated as the time of the Alleghenian
orogeny in some areas. Van der Voo and French (1977)
OCR for page 135
COCORP AND FLUIDS IN THE CRUST
found Alleghenian dates in Virginia, West Virginia, and
Pennsylvania. Scotese et al. (1982) found similar dates
from a study of Silurian and Devonian rocks in New York.
McCabe et al. (1983) studied demagnetization of folded
Silurian-Devonian rocks in New York using the fold test
and found that demagnetization occurred during folding
(i.e., during the Alleghenian orogeny). McCabe et al.
(1984) found similar dates in Quebec, Ontario, and New
York.
Kent and Opdyke (1985) revised an earlier study con-
trasting paleolatitudes of the New England-Canadian
Maritime region and the North American craton and showed
an error as a result of paleoremagnetization that negated
earlier and enigmatic conclusions about major relative
tectonic movement between these two provinces. To
summarize, there is widespread and growing evidence from
studies of paleomagnetism that supports the concept of
authigenesis of magnetic minerals in sediments of the
continental interior as a consequence of brines migrating
at the time of orogeny.
Hydrocarbons
In the preceding portion of this chapter evidence is
presented in support of the concept of long-distance
migration of brines expelled from erogenic belts. This
section considers the possible effects on hydrocarbons,
particularly the spatial distribution of hydrocarbons as a
consequence of that brine migration.
Petroleum geologists are divided over the question of
long-distance migration of hydrocarbons. Some believe
little or no such migration occurs, others call for substan-
tial migration, and still others take intermediate positions.
This chapter takes the position that for some hydrocar-
bon provinces those unaffected by tectonic fluid flow-
lateral migration is typically modest at most. For those
provinces affected by tectonic fluids, however, lateral
migration up to distances of many hundreds of kilometers
may occur. The spatial pattern of hydrocarbons through-
out the world may be examined in this framework.
A second factor influencing the occurrence and abun-
dance of hydrocarbons in the case of tectonically influ-
enced deposits is the nature of material on the down-going
slab of the subduction zone. If the sediments are volumi-
nous and organic-rich, as in the case of a large delta, the
development of hydrocarbons is enhanced. Of course,
many other factors that are part of the rich literature of
petroleum geology but not discussed here, such as seals,
traps, and the nature of organic materials in source rocks,
influence the spatial pattern of hydrocarbon deposits;
however, the emphasis here is on active tectonism and the
factors discussed above.
As an example consider the Gulf Coast province, which
135
holds one of the Earth's largest hydrocarbon deposits.
Drainage from a large part of the North American conti-
nent has resulted in a large and complex delta with organ-
ics of terrestrial and marine origin. Organics are buried to
maturation conditions and converted to hydrocarbons. The
hydrocarbons migrate vertically and, at most, to modest
distances (perhaps tens of kilometers) horizontally and are
found in traps relatively near to where the hydrocarbons
were formed.
Now imagine what would happen if the Gulf of Mexico
were to close, so that the North American continent was
partially subducted to the south beneath an accretionary
wedge on the leading edge of a landmass impinging from
the south. Fluids from the Gulf Coast sediments would be
expelled and, according to the hypothesis, travel toward
the interior of the continent (i.e., to the north). The fluids
would be brines carrying minerals and both organics and
hydrocarbons. Some organics may have already matured;
some might encounter conditions for maturation along the
way. The foreland basin resulting from the tectonic load-
ing would fill with sediments. These sediments might
produce some of their own hydrocarbons that might or
might not be affected by the flow of tectonically generated
fluids. Nevertheless, the general pattern of the hydrocar-
bon occurrences that resulted would reflect the effect of
the migrating fluids.
This experiment need not be done solely in the mind. It
may represent about what happened at the time of the
Ouachita orogeny. Thus, the spatial pattern of the hydro-
carbon province extending from West Texas through
Oklahoma and Kansas (Figure 8.5) may be at least partly
a consequence of the Ouachita orogeny and the expulsion
of fluids from the Ouachita erogenic belt. The pattern is
sharply defined by a smooth boundary along the south and
east sides. Along the north and west it has the feather edge
that might be expected of the squeegeeing effect of Ou-
achita overthrusting. This view resembles that of Salis-
bury (1968), who suggested years ago that such a phe-
nomenon occurred in the case of the Marathon erogenic
belt and associated hydrocarbons to the north of that belt.
Gas in the Ouachita hydrocarbon province tends to occur
near the orogen, oil farther away. The hypothesis suggests
that some hydrocarbons of this province have not migrated
much, but others have traveled as organics or hydrocar-
bons long distances from where the former continental
margin lies buried beneath the Ouachita orogen.
To the east and north the similarity of the pattern of the
boundary of hydrocarbon occurrences in Kentucky and
Illinois with that of the Ouachita orogen raises the specu-
lation that some flushing of these hydrocarbons to the
north may have occurred.
In like manner one can interpret other hydrocarbon
provinces of the map of Figure 8.5. Williston Basin, Michi
OCR for page 136
~-
~ art
1
`,~ Oil fields or scattered
occurrences
`~ Gas fields or scattered
occurrences
Tectonic belts
JACK E. OLIVER
N_._
__ ~
V~ ~ By, - ~.~.~. .~ By." ' . ' =\
I _ ,, ~ .,, . , . . . . . . ..... ~ . ,
FIGURE 8.5 Map, generalized from PennWell map (Wilkerson,
1982), showing regions of oil and gas fields in the United States
and adjoining parts of Canada and Mexico. Also shown are
erogenic belts. The spatial distribution of hydrocarbons in the
gan Basin, and California hydrocarbons are found essen-
tially where they were formed, with only modest migra-
tion within the particular basin. Appalachian oil has
migrated to the west as described by Woodward (1958) as
a consequence of burial of an organic-rich continental
margin by convergence and collision during Paleozoic
time. The oil of the Findlay arch in Ohio and Indiana was
part of the westward migration in the early Paleozoic and
concentrated in the arch when it was formed as a forebulge
resulting from the loading of the crust by later Appala-
chian tectonics.
Albertan hydrocarbons migrated to the east, the conse-
quence of burial of an organic-rich margin beneath over-
thrust slices from the west. Demaison (1977) proposed
such an explanation for the Alberta tar sands (and also for
large tar sands elsewhere). He called upon burial of a
foreland basin delta, with source to the east, beneath thrust
Appalachians, Alberta, the Cordilleran province, the West Texas-
Oklahoma-Kansas province, and possibly the Illinois basin are in
part a consequence of fluid migration from nearby erogenic belts
according to the hypothesis discussed here.
ing from the west that drove the hydrocarbons updip through
the permeable beds of the delta. The pattern of Cordillera
hydrocarbons, though grossly similar to that of Alberta, is
not so simple. This region has been disrupted by more
recent tectonics in the foreland basin that must have af-
fected the spatial distribution of hydrocarbons. Further-
more, the Cordillera hydrocarbons may not be associated
with a subducted continental margin as rich in organics as
that of Alberta. Nevertheless, the pattern is not in dis-
agreement with the hypothesis.
Information on directions of migration of hydrocarbons
in the 48 states is summarized in a map by Momper (1978)
reproduced in Figure 8.6. Note that all of the directions on
Momper's map are compatible with those predicted by the
tectonics-based hypothesis discussed here, even to the extent
of asymmetrical migration from the Illinois basin as op-
posed to symmetrical migration from other interior basins
OCR for page 137
COCORP AND FLUIDS IN THE CRUST
(i.e., Williston and Michigan). The surprisingly good fit
of an independently collected set of data with a hypothesis
based initially on entirely different considerations must be
considered strong support for the hypothesis.
Similar reasoning about the spatial distribution of hydro-
carbons may be applied to other parts of the world. For
example, the petroliferous deltas of Africa and the rifts of
China are places unaffected by major compressional orog-
eny and also where hydrocarbons have not migrated far
from the place of maturation.
The hydrocarbons of the Middle East, as was pointed
out by Dickinson (1974) in one of the first and most
important early attempts to relate hydrocarbon accumula-
tions to plate tectonics, likely migrated updip to the west
from the Zagros erogenic belt as a consequence of subduc-
tion of the leading edge of the African continent, which
then included the Arabian peninsula, to the east beneath
the overriding Asian plate. Dickinson focused on hydro-
carbon migration and not the associated brines. However,
in the context of this chapter, one might speculate that, as
a consequence of the paleodrainage of the African conti-
nent, a large organic-r~ch delta existed on the leading edge
of the African plate that was subducted in the Zagros, with
the consequence that fluids from the delta were expelled
into parts of Iran and the Arabian peninsula. The Red Sea,
a later feature, was not part of the proposed paleodrainage
pattern. An interesting point here concerns the Persian
Gulf, which has been in existence through much of the
period of subduction and hence through the period of
proposed hydrocarbon migration. Consequently, in this
area there was not topographic relief with elevations de
LONG - DISTANCE Old MIGRATION
NINE GENERAll2ED EXAMPlES IN U.S.A.
:~ ~:
~.:
SOURCE PROVINCE '-_ L ~its ~
PH0ttA.
AlSEtrA. loll mopl
llllSTON. O \
WlillSION, M ~lillNoISe M
ANADAlKO, ~ MICHIGAN. I`l ~ i ~
(it) ~41~AND, ~APPALACHIA {Mull;)
"C\
137
creasing from the erogenic belt into the foreland basin
(i.e., towards Arabia). Thus, conditions were not favorable
for hydrologic flow of meteoric waters from the erogenic
belt toward the foreland basin in a manner comparable to
that described by Garven and Freeze (1984) for Alberta.
Consequently, such hydrologic flow is not likely the cause
of hydrocarbon migration in the Middle East, and the
migration is more likely a consequence of tectonic expul-
sion and updip transport of already formed hydrocarbon
fluids and perhaps immature organic materials.
The locations of the hydrocarbons of the North Slope of
Alaska may be at least partly a consequence of expulsion
of fluids from the overthrust zone or subduction zone of
the Brooks Range to the south. Difficulties of associating
source rock and oil (Magoon and Claypool, 1985) in this
region may be a consequence of failure to make full use of
the tectonic framework as a basis for sample selection and
interpretation of data.
Within the context of this hypothesis one might ask
why greater accumulations of hydrocarbons are not found
in India in the foredeep of the Himalayas, for example. A
possible reason is that a large delta never existed on the
leading edge of India, the edge that was subducted beneath
the Himalayas. As India drifted across the Tethys, the
paleodrainage was limited to the Indian subcontinent. The
area of this subcontinent is not very large, and the major
drainage may have terminated at other parts of the margin,
a hypothesis that could be tested by offshore seismic stud-
ies. Prior to the time of drift the portion of India that made
up the margin of Gondwanaland may not have been the
site of a major delta.
FIGURE 8.6 Map of Momper (1978)
showing summary of information on mi-
gration direction of hydrocarbons in the
United States. Note general agreement
between the migration directions of this
map and those suggested by the hypothe-
sis on fluid expulsion from erogenic belts
as described in the text.
S - SOURC E AREA
- MIGRATION pArMs
OCR for page 138
138
The eastern margin of the United States seaward of the
Appalachian hydrocarbon province is not very productive
of hydrocarbons, whereas the Gulf Coast that occupies a
similar position relative to the Ouachita hydrocarbon prov-
ince is. According to the hypothesis, this situation occurs
because the Gulf Coast is and has been the deposition area
for drainage of a large part of the North American conti-
nent and the East Coast has not. Of course, other factors
such as climate, conditions of deposition, and marine
sources must be involved.
To summarize this section, it appears that a large amount
of evidence from a wide diversity of specialties favors the
hypothesis that fluids expelled from erogenic belts play an
important role in many geological phenomena such as the
spatial distribution of certain hydrocarbon and mineral
deposits, authigenesis, diagenesis, coal metamorphism,
dolomitization, and paleoremagnetism. The case is not
closed, however, for there remains opportunity for further
testing, and the attempt at such broad-based synthesis is so
new that many aspects require further attention. Should
the hypothesis be more or less correct, however, a means
for relating many types of geological observations of the
continent to plate tectonics, and hence the great geody-
namical processes of the Earth, may be in sight.
ACKNOWLEDGMENTS
This chapter, as are all papers based on COCORP data,
is highly dependent upon the efforts of many scientists and
others who are part of the COCORP project. The CO-
CORP project is funded by National Science Foundation
grant EAR-8418157 and by the Cornell Program for the
Study of the Continents. The story of fluids expelled from
erogenic belts is not strictly a part of the COCORP project
but is a speculative synthesis stimulated by COCORP and
other data. This is contribution no. 79, INSTOC, Cornell
University.
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Representative terms from entire chapter:
foreland basin
