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9
INTRODUCTION
Smoluchowski's Dilemma Revisited.
A Note on the
Fluid Pressure History of the
Central Appalachian Fold-Th~ust Belt
.
TERRY ENGELDER
The Pennsylvania State University
ABSTRACT
Cross-fold joints in the Central Appalachian fold-thrust belt propagated during
periods of abnormally high fluid pressure prior to tectonic compaction and the
development of first-order Alleghanian structures in the valley and ridge. These
early joints, found in both the valley and ridge province and the plateau province,
are organized in sets forming patterns that correlate across the Allegheny Front.
Examples of early joints are found in the Devonian Brallier and Trimmers Rock
Formations of the Pennsylvania Valley and Ridge and in the Genesee Group of
the Appalachian Plateau. One interpretation is that high fluid pressures were
generated by topographically driven flow across the Appalachian Basin as a con-
sequence of uplift of the core of the Appalachians early in the Alleghanian
Orogeny. The high fluid pressures accompanying this topographically-dnven
flow system later facilitated the development of first-order structures in the valley
and edge. Later joint sets that do not correlate across the Allegheny Front are
more likely to be a consequence of fluid pressure pulses developed during local
tectonic compaction and the development of first-order Alleghanian structures.
These later joint sets vary in number and orientation from location to location.
Smoluchowski's (1909) famous dilemma is that thrust
sheets are too wide for emplacement by "dry" frictional
sliding. Theoretically, the back end of wide thrust sheets
should collapse under the large tectonic stress necessary to
push the entire thrust sheet against frictional resistance.
140
The most popular solution to Smoluchowski's dilemma
was presented by Hubbert and Rubey (1959) and Rubey
and Hubbert (1959), who pointed out that the theoretical
width of thrust sheets is greatly increased by an increase in
fluid pressure and concomitant reduction in effective nor-
mal stress across the basal decollement. Because fric-
tional resistance is directly proportional to effective nor
OCR for page 141
FLUID PRESSURE HISTORY OF THE CENTRAL APPALACHIAN FOLD-THRUST BELT
mat stress, a reduction in effective normal stress has the
net effect of reducing the push (i.e., tectonic stress) neces-
sary for emplacement of wide thrust sheets. Lower tec-
tonic stress reduces the tendency for fracture and thicken-
ing at the back end of the thrust sheet (Davis et al., 19831.
A reduction in effective normal stress occurs if the base
of the thrust sheet cuts through a stratigraphic section
containing abnormally high fluid pressures. How this high
fluid pressure evolves is still subject to debate. Two
mechanisms for generating high fluid pressures as men-
tioned by Hubbert and Rubey (1959) are artesian flow and
mechanical compaction of water-filled pores. In their
companion paper Rubey and Hubbert (1959) focus on the
generation of abnormally high fluid pressures by three
mechanisms: (1) the uplift of sealed sand lenses, (2) tec-
tonic compaction, and (3) compaction by overburden
weight. They gave no further consideration to artesian
flow. There are, of course, other mechanisms such as
aquathermal pressuring (Barker, 1972), diagenetic dewa-
tering of clays (Schmidt, 1973), and generation of CO2 and
CH4 during the breakdown of hydrocarbons (Spencer,
1987~.
Rubey and Hubbert (1959) do not examine artesian
flow as a mechanism for generation of abnormal fluid
pressures in foreland fold-thrust belts. By implication
they consider it less important than tectonic compaction as
a source for abnormal fluid pressures in overthrust terrain.
The problem is that tectonic compaction occurs well after
the initiation of thrusting. The development of overthrust
terrain would be greatly facilitated if high fluid pressures
developed before the onset of thrusting. Artesian flow
may permit such a buildup in fluid pressure. Furthermore,
experience in the Alberta Basin suggests that it should be
taken seriously as a model for generating high fluid pres-
sures in foreland basins (Toth, 1980~.
Artesian flow is commonly understood to be groundwa-
ter flow from a topographically high recharge area to a
topographically low discharge area. This type of flow,
also called topographically driven flow, is modeled by
Toth (1962, 1980) using a flow net first illustrated by
Hubbert (1940~. One consequence of topographically
driven flow is that the discharge area is subject to pore
water pressures in excess of hydrostatic developed be-
cause the mechanical energy per unit volume of pore fluid
is highest in the recharge area and lowest in the discharge
area. A fortuitous combination of aquitards and topogra-
phy can lead to near-lithostatic fluid pressures in the dis-
charge area (Engelder and Bethke, 1985~. A topographi-
cally driven flow system is steady state; leakage is bal-
anced by recharge. This is in direct contrast to compac-
tion-driven flow, where fluid pressure gradually returns to
hydrostatic once compaction stops.
Are there geological structures that enable the geologist
141
to distinguish topographically driven flow from other
mechanisms including tectonic compaction that might have
been the source of high fluid pressures in a foreland fold-
thrust belt? In principle, regional joint sets could serve as
such structures. This paper presents further evidence
supporting the regional correlation of cross-fold joint sets
(Joints with normals subparallel to regional fold axes) in
the Appalachian foreland fold-thrust belt and then deals
with the geological consequences of regionally developed
cross-fold joint sets in terms of a mechanism for generat-
ing the necessary pore-fluid pressure.
CORRELATION OF CROSS-FOLD JOINTING
ACROSS THE CENTRAL APPALACHIAN
FORELAND FOLD-THRUST BELT
Cross-fold joints are very prominent in Upper Devonian
outcrops along the edges of the Finger Lakes of New York
State (Figure 9.1~. By the first decade of the twentieth
century geologists recognized that these cross-fold joints
were organized into more than one set (Sheldon, 1912~.
While tracing these cross-fold joints along strike of the
New York Plateau for more than 200 km, Parker (1942)
recognized that they maintained an orientation normal to
fold axes despite a 30° change in strike of the fold axes.
Parker made no judgment about whether cross-fold joints
on either end of the map area are part of the same joint set.
Nickelsen and Hough (1967) were the first to map joints as
systematic sets in the Central Appalachians. They identi-
fied five cross-fold joint sets in sandstones of the Appala-
chian Plateau in Pennsylvania. By extrapolating to New
York State, they identified three joint sets in Parker's map
area. On mapping in the Appalachian Valley and Ridge,
Nickelsen (1979) and Orkan and Voight (1985) attempted
to correlate joint sets between the valley and ridge and
plateau of Pennsylvania. Orkan and Voight (1985) identi-
fied six cross-fold joint sets in the valley and ridge (Figure
9.2~.
Nickelsen and Hough's (1967) and Orkan and Voight's
(1985) technique for correlation of joints along strike
depends largely on the orientation of joints. Their as-
sumption is that joints of one set have similar orientations
over large regions. If a suite of joints at an outcrop is
misoriented by, say, 15° from an established joint set, this
suite belongs to another joint set regardless of its orienta-
tion with respect to local structures. As is illustrated in
Figure 9.2, the consequence of this assumption is that
members of a joint set do not change orientation even as
fold axes swing through the Central Appalachians. On a
regional basis the change in strike of fold axes is accom-
modated by the overlap of joint sets of different orienta-
tions. The notion for overlapping joint sets is supported
by outcrops containing more than one joint set. Nickelsen
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Representative terms from entire chapter:
fluid pressures
142
\
\
\ ~
\
\
NEW YORK ;
PENNSYLVAN IA , '
,'2 ..,...r', ROCHESTER
.
.. ..
.....
~ BUFFALO ~7~
TERRY ENGELDER
.,
.. . SYRACUSE
. .
~'-~
:' - LEROY~=~3~ ~
t\~-~1,:
, .. it\ \, W~
\-~-__ ~
~'~
FLUID PRESSURE HISTORY OF THE CENTRAL APPALACHIAN FOLD-THRUST BELT
has many of the same difficulties as correlation along
strike of the Appalachians, both Nickelsen and Hough
(1967) and Orkan and Voight (1985) feel that joints corre-
late across the Allegheny Front. [The Allegheny Front,
which is the boundary between the Appalachian Valley
and Ridge and Plateau, is largely controlled by the south-
eastern edge of the Silurian salt basin where decollement
faulting climbed up from the Cambrian shales into the salt
beds. Low strength of the salt changed the character of the
Appalachian foreland tectonics from duplex structures of
the valley and ridge to layer parallel shortening of the
Appalachian Plateau (Davis and Engelder, 1987~.] A
correlation may be based on the common occurrence of a
clockwise rotation of joint propagation in both the valley
and ridge and the Appalachian Plateau. [Although the
clockwise rotation of joint propagation is common through-
out the region, Helgeson and Aydin (1989) report that a
counter-clockwise rotation is well developed in some
outcrops.] At Bear Valley Strip Mine Nickelsen (1979)
identified eight stages of deformation with the first three
being two phases of jointing followed by layer parallel
shortening. These prefolding events witness a prefolding
compression that rotates clockwise (Geiser and Engelder,
1983; Engelder, 1985~. All along the Allegheny Front
from Williamsport to State College, Pennsylvania, early
I,, A A A ~ D
~ N e w ... . .. .. .. '; ... . .'. . ..
143
cross-fold jointing shows a sequence indicating a clock-
wise rotation of compression (Lacazette, The Pennsylva-
nia State University, personal communication). This same
sequence is well displayed in the Devonian Brallier For-
mation at Huntingdon, Pennsylvania.
The Devonian Brallier Formation of the central Appa-
lachian Valley and Ridge is equivalent in age and litho-
logic composition to the Genesee Group of the Appala-
chian Plateau. Of all the lithologies in the valley and ridge
from Cambrian carbonates up through Carboniferous flu-
vial deposits, none carry joints that more closely resemble
those seen on the Devonian section of the Appalachian
Plateau. At an outcrop just south of Huntingdon, Pennsyl-
vania, the Brallier dips to the southeast at about 15°. Like
joints in the sandstone-shale beds of the Genesee Group
on the Appalachian Plateau, two sets of cross-fold joints
cut the Brallier, with the finer-grained beds carrying joints
striking about 140° and the coarser beds carrying joints
striking about 158°. The relative time of propagation of
the joint sets may be determined using a joint spacing
criterion developed by DeGraff et al. (1987) in the Genesee
Group at Taughannock Falls, New York. Toward the
north end of the Brallier outcrop joints in siltstone beds
can be seen propagating upward from joints in shale beds.
Based on the spacing criterion, joints in the silty shale
FIGURE 9.2 Orkan and Voight's (1985) map
of regional joint sets within the Central Appa-
lachian fold-thrust belt. Sets A through E are
those of Nickelsen and Hough (1967~. Set F
was identified by Orkan and Voight (1985~.
Regional joint sets are based on the data of
Nickelsen and Hough (1967) and Engelder and
Geiser (1980~.
144
(140°) propagated prior to those in the sandstone (158°)
and, hence, show the same clockwise rotation as seen
throughout the Appalachian foreland.
Aside from the fact that these joints look like those
found in the flat-lying Devonian rocks of the Appalachian
Plateau, two pieces of evidence suggest that the cross-fold
joints in the Brallier preceded folding. First, the joints
have been rotated to dip between 84° and 87° to the south-
west. If the present dip of the Brallier is removed, these
joints are vertical, presumably the orientation at which
they propagated. Second, some joints in the silty shale
beds are decorated with slickensides and fibrous calcite,
indicating a left-lateral shear. This is the type of slip
expected for the compression responsible for later folding.
Furthermore, the orientation of the calcite fibers indicates
that slip direction has a shallower plunge than bedding
dip. These are some of the same arguments used by
Nickelsen (1979) to demonstrate early jointing at Bear
Valley.
It is likely that early joint sets propagated prior to the
formation of the Allegheny Front. The Allegheny Front
became significant only with the development of first-
order structures of the valley and ridge, an event that took
place long after early joint propagation, as shown by Nick-
elsen (1979) at Bear Valley. Joints correlate across the
Allegheny Front largely because that structural front did
not exist at the time the joints formed. The mechanism for
early joint propagation must precede and be independent
of the development of first-order structures of the Appala-
chian Valley and Ridge. In summary, the Upper Devonian
shales and siltstones of the entire Central Appalachian
foreland contain Alleghanian cross-fold joints that predate
both major tectonic compaction and the development of
first-order folds.
FLUID PRESSURE AND JOINTING
The regionally developed cross-fold joints of the Appa-
lachian foreland formed at depth in the crust of the Earth,
where the propagation of such joints requires the develop-
ment of effective tensile stresses within the rock (e.g.,
Nickelsen, 1979; Narr and Currie, 1982~. Effective tensile
stresses are possible under conditions of cooling (Voight
and St. Pierre, 1974), curvature above the neutral fiber of
a fold (Price, 1974), or significantly high fluid pressures
(Secor, 1965~. In the foreland portion of mountain belts
where folding would favor the propagation of strike joints
(joints striking parallel to fold axes), curvature can be
ruled out as a likely driving mechanism for cross-fold
joints. At full depth of burial rocks have not cooled
appreciably, so thermal cracking can also be ruled out as a
driving mechanism. In contrast, a growing body of evi-
dence suggests that high fluid pressure serves as the driv
TERRY ENGELDER
ing mechanism for joints at depth. Joints filled with such
minerals as quartz, calcite, and chlorite are often cited as a
manifestation of fluid pressure-driven joint propagation
(i.e., hydraulic fracturing; e.g., Beach, 1977~. The mul-
tiple fracture of crack-seal veins (e.g., Ramsay, 1980) and
the repeated arrest of joints during propagation (e.g.,
Engelder, 1985) are fracture-related structures associated
with the cracking of rock under the influence of high fluid
pressure.
An understanding of the extent to which joints correlate
in both time and space is critical to identifying the mecha-
nisms for generation of high fluid pressures in a mountain
belt. Although rapid joint propagation occurs on the scale
of outcrops, the timing of joint propagation at different
locations across a foreland is less certain. Presently it is
not clear whether fluid pressure increases simultaneously
everywhere across a foreland or whether high fluid pres-
sure occurs as local pulses affecting only small parts of the
mountain range at one time. This, of course, leads to
uncertainty about whether joint sets of the same orienta-
tion should correlate across distances of tens to hundreds
of kilometers. Certainly, based on data discussed above,
current dogma for the Appalachians is that early joint sets
do correlate over large distances (e.g., Nickelsen and Hough,
1967; Engelder and Geiser, 1980; and Orkan and Voight,
1985).
Figure 9.2 suggests that joint sets, such as "set A,"
extend from the Great Valley of Pennsylvania to the far
reaches of the Appalachian Plateau near Buffalo, New
York. This is the Orkan and Voight (1985) interpretation
of regional joint sets where joint development cuts across
tectonic boundaries such as the Allegheny Front. If a
correlation across the Allegheny Front is valid, the evolu-
tion of a high pore pressure must have been a foreland-
wide event. Not all mechanisms for generation of high
pore pressure are regional in extent. Although deposi-
tional and diagenetic mechanisms for the generation of
high fluid pressure may be regional, they are considered
unlikely mechanisms for a regional pore pressure event in
the Appalachian fold-thrust belt because the earliest cross-
fold joint sets are Alleghanian and, hence, developed long
after deposition and diagenesis of the foreland sediments
containing the joints. Although tectonic compaction af-
fects an entire foreland, it was not uniform, as indicated by
a variation is strain. The upper crust does not have the
strength to simultaneously compact across the foreland
until the core of the mountain belt has built into a sizable
wedge (Davis et al., 1983~. Furthermore, strain in fore-
lands is much too low to have been continuously active at
plate tectonic rates for the duration of an erogenic event
such as the Alleghanian orogeny in the Appalachians. For
these reasons tectonic compaction seems unlikely to have
contributed to a foreland-wide pore pressure event of the
FLUID PRESSURE HISTORY OF THE CENTRAL APPALACHIAN FOLD-THRUST BELT
type required for early jointing. The only mechanism for
generating a regional pore pressure event that cannot be
rejected out of hand is the topographically driven flow
system. Therefore, it is assumed to be the most likely
source for high fluid pressures causing the simultaneous
development of a joint set across the foreland, particularly
during early stages of foreland development.
DISCUSSION: OROGENIC PULSES
AND THE GENERATION OF ABNORMALLY
HIGH FLUID PRESSURE
Early foreland-wide joint sets may be reconciled with
topographically driven flow systems. In this case a re-
gional flow system may have formed in response to uplift
of mountains to the southeast of the Great Valley. To
generate the high pressures for joint propagation, such a
topographically driven flow system is, of course, going to
require regional aquitards and a significant topographic
gradient across the foreland. Because the upper Paleozoic
section of the Central Appalachians developed very few
through-going thrust faults, it may have served as a re-
gional aquitard. Furthermore, evidence is accumulating
that suggests that during the Alleghanian orogeny the
Central Appalachians southeast of the Allegheny Front
was quite thick (Levine, 1983; Paxton, 1983; Orkan and
Voight, 1985~. Vitrinite reflectance and fission track data
suggest that the Devonian and Carboniferous of New York
and Pennsylvania may have been buried to a depth of 6 km
(Friedman and Sanders, 1982~. Current studies of crustal
flexure suggest that external forces were necessary for the
magnitude of crustal depression necessary for the depth of
burial found in Pennsylvania (Beaumont, 1981~. Such
crustal loading can be accomplished during continent-
continent collisions. The Alleghanian Orogeny was a period
during which the continent of Africa collided with North
America, producing continental edges having a topogra-
phy similar to the India-Asia collision. This interpretation
of regional joint sets requires that uplift at the core of the
mountain belt preceded the development of first-order
structures in the foreland. The early development of a
regional flow system with elevated pore pressures facili-
tates later thrusting and the development of first-order
structures, particularly in the discharge area of the fore-
land.
Regardless of their correlation, everyone agrees that
some cross-fold joints in the central Appalachian foreland
fold-thrust belt propagated early and are organized into
discrete sets rather than being distributed randomly or
uniformly. The existence of multiple joint sets indicates
that the syntectonic stress field changed in orientation
during the evolution of the foreland fold-thrust belt. This
regional organization of joints leads to the inference that
145
joint propagation took place during punctuated events.
Not only did the orientation of the stress field change with
time but the magnitude of the effective stress varied. Fluid
pressures were not continuously at a level necessary for
joint propagation, but rather some poorly understood events
caused fluid pressure to fluctuate up and down throughout
a region. Such events took place a finite number of times
during the development of the foreland portion of the
Central Appalachians.
Mountain belts include a complex combination of dia-
chronous structures superimposed over periods as long as
1 billion years ago. During the evolution of foreland fold-
thrust belts, deformation is punctuated rather than continu-
ous. Punctuated events called orogenic pulses are identi-
fied on the basis of the appearance of an arbitrarily chosen
set of structures within the mountain belt. For example, a
regionally developed disjunctive cleavage may be attrib-
uted to one orogenic pulse, whereas a second cross-cutting
cleavage may be attributed to a later orogenic pulse. With
few exceptions the duration of an orogenic pulse is ex-
tremely difficult to measure.
The intensity of an orogenic pulse is often correlated
with the finite strain within rocks or the regional shorten-
ing associated with folding and faulting. Orogenic pulses
become increasingly hard to discriminate as the finite strain
or regional shortening decreases. Although the case may
be argued that major structures such as folds are the signa-
ture of a single orogenic pulse, multiple joint sets within
folds are themselves witness for multiple orogenic pulses
prior to the folding event. Regional joints, particularly
sensitive indicators of individual orogenic pulses, are
commonly found in the unmetamorphosed foreland where
more than one set may cross-cut. Cross-fold joints may
propagate even during very mild orogenic pulses and in
many instances before significant bed rotation. These
mild orogenic pulses in the foreland may reflect uplift
events in the core of the mountain belt or periods of rapid
tectonic compaction. Unlike faults, folds, or finite strain
markers of any sort, the propagation of joints is so close to
instantaneous that one moment in the history of mountain
building is recorded. The convenience of joint sets is that
stress trajectories associated with an orogenic pulse can be
mapped with reasonable confidence (Ode, 1957~.
The development of several joint sets suggests that
fluid pressures were not continuously lithostatic through-
out the Alleghanian orogeny. If fluid pressures were
continuously at lithostatic during realignment of the stress
field, then joints should have a uniform distribution of
orientations rather than appear as isolated joint sets.
Multiple joint sets suggest that fluid pressures rise to
lithostatic levels during short-lived events before pore fluids
leak off to drop the pressure well below that needed for
joint propagation. Fluid pressures rise again once the
10
Fluid Pressure History in
Subduction Zones: Evidence from Fluid
Inclusions in the
Kodiak Accretionary Complex, Alaska
PETER VROLIJK~
GEORGIANNA MYERS2
University of California, Santa Cruz
ABSTRACT
Fluid inclusions offer a simple and elegant means to examine the history of fluid
pressure, temperature, and composition at intermediate levels in subduction zones.
At deeper levels ductile deformation mechanisms tend to disrupt fluid inclusion
relationships in veins. Fluid inclusions provide a direct record of the fluid phase
and apparently preserve relatively short-term changes in fluid pressure. More-
over, fluid inclusions are found in veins that can be directly tied to the develop-
ment of structural fabrics in the rock. In syntectonic veins of the Kodiak accre-
tionary complex, Alaska, fluid inclusions record fluctuating fluid pressures. These
fluctuations reflect a history of fracture opening, continued fracture growth, and
creation of an interconnected fracture network, with inferred fluid flow toward
shallower levels within the subduction zone.
Fluid inclusions also record fluid temperature at the time of inclusion entrap-
ment and thereby indicate the temperature of fluids involved in the deformation of
rocks in the decollement zone during active subduction. Data from the Kodiak
accretionary complex indicate that fluid temperatures at 10 to 15 km in the ancient
Kodiak subduction zone were two to three times higher than predicted by conduc-
tive heat flow models. The cause of this temperature anomaly appears to be the
migration of warm fluids along the decollement zone from deeper structural
levels.
These data, in conjunction with emerging data from both modern and ancient
subduction complexes, suggest that high fluid pressures are common in subduc
iCurrent Address: Exxon Production Research Company, Houston, Texas
2Current Address: Minnesota Pollution Control Agency
148
FLUID PRESSURE HISTORY IN SUBDUCTION ZONES
149
lion zones and that the presence of high fluid pressures has a profound influence
on the geology at convergent margins. The manifestations of high fluid pressures
include the style of structural deformation, heat flow, modifications of diagenesis
and metamorphism by tectonically driven fluid flow, and effects on the global
water and geochemical budgets.
INTRODUCTION
High fluid pressures are probably more common in
subduction zones than in any other tectonic environment.
In subduction zones water-rich sediments are rapidly
dragged with the subducting oceanic crust to great depth.
The sediments directly overlying the oceanic crust, which
have the greatest potential for being subducted, are typi-
cally fine "rained and pelagic with low intergranular per-
meability (Moore, 1975; Shepherd and Bryant, 1983~. The
rapid sediment burial afforded by plate convergence and
subduction, in combination with the common occurrence
of fine-grained sediments, should theoretically lead to the
pervasive development of high fluid pressures (Walder
and Nur,1984~.
Understanding the fluid pressure history in subduction
zones has regional and global significance. Many aspects
of the geology, including the shape of accretionary wedges
(Davis et al., 1983), the development of thrust faulting
(Hubbert and Rubey, 1959), fold vergence (Seely, 1977),
heat transport (Reck, 1987; Vrolijk et al., 1988), the di-
agenetic and metamorphic history (Etheridge et al., 1983;
Ritger et al., 1987), and the support of benthic organisms
(Suess et al., 1985), are affected by high fluid pressures
and resulting fluid flow. The global water and geochemi-
cal budgets also depend on the fluid pressure distribution
in subduction zones because mass balance calculations
suggest that water must return to the ocean through sub-
duction complexes to prevent the world's oceans from
being subducted into the mantle (e.g., Fyfe et al., 1978; Ito
et al., 1983~.
Fluid pressures appear to play a direct role in mineral
diagenesis (e.g., Bird, 1984; Koster van Groos and Gug-
genheim, 1984, 1986, 1987) and therefore may in part
determine the depth at which water is expelled from hy-
drous minerals. In the presence of high fluid pressure,
montmorillonite retains interlayer water to much higher
confining pressures than under hydrostatic conditions (Hall
et al., 1986; Colten-Bradley, 1987; Koster van Groos and
Guggenheim, 1987~. Smectites in ocean crust and sedi-
ments subducted beneath accretionary prisms may lose
water according to the fluid pressure distribution. Deter-
mining where water-rich minerals dehydrate in a subduc-
tion zone also has important implications for the genesis
of arc magmatism (e.g., Gill, 1981~.
EVIDENCE FOR HIGH FLUID PRESSURES
Investigations in modern subduction zone environments
have led to the discovery of several tantalizing results. On
DSDP Leg 78A in the Barbados Ridge Complex, Moore
and Biju-Duval (1984) inferred high, near-lithostatic fluid
pressures along the decollement zone, or detachment hori-
zon, between the subducting Atlantic and overnding Car-
ibbean plates. In the Japan trench, von Huene et al. (1980)
attributed lower sediment density in highly fractured cores
to fracture porosity and overpressuring. Similarly, wells
in subduction zone forearc settings record fluid pressures
far above hydrostatic (Shouldice, 1971; Hottman et al.,
1979~. The common occurrence of mud volcanoes and
mud diapirs in accretionary complexes also suggests the
presence of high fluid pressures (Shouldice, 1971; von
Huene, 1972; Brown and Westbrook, 1987; Langseth et
al., 1988~. Whereas none of these studies have determined
the distribution of fluid pressures in subduction zones,
they strongly suggest that high fluid pressures may be
common.
Indirect evidence for high fluid pressures in modern
environments comes from pore water chemical and ther-
mal data (e.g., Yamano et al., 1982; Davis and Hussong,
1984; Kulm et al., 1986; Moore et al., 1987; Ritger et al.,
1987; Gieskes et al., 1989~. Both types of data indicate
anomalies along subduction zones, and further considera-
tion of these data suggest that anomalies can be sustained
only by relatively rapid fluid flow; which is likely aided
by high fluid pressures.
Mechanical models of subduction zones also suggest
that high fluid pressures are present (e.g., Davis et al.,
1983; Shi and Wang, 1988~. In addition, a conceptual
model of subducting sediments suggests that fluids cannot
easily escape along sedimentary layers toward the seafloor
(Figure 10.1~. In the model presented in Figure 10.1 a
tectonically undisturbed sedimentary layer extends from
in front of the deformation front far beneath the accretion-
ary prism; this bed remains within the subducting plate
throughout the model. The geometry of the subduction
zone is modeled after the Eastern Aleutian trench (von
Huene et al., 1985~. During subduction a fluid pressure
gradient will develop along the sedimentary bed because
of the increased load of the accretionary prism, which will
encourage fluid flow from beneath it. However, in a fixed
