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10.1146/annurev.earth.30.091201.141132
Annu. Rev. Earth Planet. Sci. 2002. 30:347–84
DOI: 10.1146/annurev.earth.30.091201.141132
c 2002 by Annual Reviews. All rights reserved
Copyright °
GEOLOGIC STRUCTURE OF THE UPPERMOST
OCEANIC CRUST CREATED AT FAST- TO
INTERMEDIATE-RATE SPREADING CENTERS
Jeffrey A. Karson
Division of Earth and Ocean Sciences, Nicholas School of the Environment and
Earth Sciences, Duke University, Durham, North Carolina 27708-0230;
e-mail: jkarson@duke.edu
Key Words seafloor spreading, lavas, dikes, faulting, accretion
■ Abstract Geological investigations of major fault scarps (“tectonic windows”)
and DSDP/ODP Drill Holes provide direct views of the uppermost oceanic crust generated at fast- to intermediate-rate spreading centers. These areas reveal a consistent
upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing (“inward”) dip toward the spreading center at which they formed and
dikes in the lavas and underlying sheeted dike complex showing a similar degree of
“outward” dip. Widespread fracturing, faulting, and hydrothermal metamorphism accompanied magmatic construction. These geological relationships can be interpreted
in terms of dramatic, asymmetrical, subaxial subsidence of upper crustal rock units that
diminishes across the very narrow (few kilometers wide) zone of lava accumulation
and dike intrusion at the ridge axis. This type of crustal structure is in accord with some
existing models of spreading but augments these idealized views with more realistic
geological complexity.
INTRODUCTION
The oceanic crust is generally considered to consist of a relatively simple layered sequence of rock units that reflect accretionary processes at different structural levels of mid-ocean ridge spreading centers (Figure 1). From the top down,
these include the following sequence: basaltic pillow lavas, sheeted dike complex,
gabbroic to ultramafic plutonic complex; together defining a magmatic crustal
assemblage typically 5–7 km thick. This assemblage overlies residual upper mantle peridotites from which melts required to build the crustal units were derived.
Collectively, these rock units are incrementally accreted to the diverging edges of
lithospheric plates at mid-ocean ridge spreading centers. This layered sequence
and the internal structures of its major rock units provide important constraints on
the mode of crustal construction beneath spreading centers.
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Figure 1 (a) Generalized internal structure and (b) interpretation of oceanic crust derived
from studies of ophiolite complexes and interpretations of marine seismic data. Note that
seismic data can resolve features only tens of meters thick and hundreds of meters long.
(c) Outcrop photos of upper crustal rocks from ophiolites: top, pillow lavas, Macquarie
Island; middle, sheeted dike complex, Oman; bottom, gabbroic rocks, Bay of Islands.
Most of the data supporting this view have come from dredged rock samples,
shallow crustal drilling, and widely scattered submersible dives. These data have
provided a basis for the interpretation of the marine seismic studies of oceanic
crust and upper mantle that have delineated a layered velocity structure that is
laterally continuous on the scale of marine seismic experiments (a few kilometers)
(Bratt & Purdy 1984, Christeson et al. 1994, Harding et al. 1989, Vera & Diebold
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1994). Detailed information on the composition and internal structure of oceanic
crust has mainly come from ophiolite complexes—fault-bounded masses of mafic
and ultramafic rocks interpreted as remnants of oceanic crust and upper mantle
found in orogenic belts (e.g., Nicolas 1989). Ophiolite complexes are commonly
highly dismembered, but many of them can be reconstructed as the type of layered
structure expected in oceanic lithosphere (Casey & Karson 1981, Casey et al.
1983, Harper 1984, Moores & Vine 1971, Pallister & Hopson 1981). Although the
specific tectonic provenance of these assemblages is a continuing topic of debate
(Karson 1998, Moores 1982, Moores et al. 2000, Shervais 2001), they clearly have
formed in environments of tectonic extension and magmatic construction similar
to that of spreading centers and thus probably provide suitable analogs for many
aspects of the geology of the oceanic crust and upper mantle.
Recently, various marine research tools, including high-resolution side-scan
sonar, remotely operated vehicles, and submersibles, have been used to examine
extensive exposures of the oceanic crust along major fault scarps. The view through
these “tectonic windows” has permitted direct investigations of the geology of the
oceanic crust at a scale and resolution approaching that possible in outcrop studies
of ophiolite complexes. For the first time, it is now possible to go beyond the
ambiguities of the provenance and reconstruction of ophiolites to study the oceanic
crust directly. This approach eliminates many of the uncertainties surrounding
ophiolites but is hampered by its own family of limitations related to studies of the
deep seafloor. These seafloor studies have documented tremendous diversity in the
geologic structure of oceanic crust. These are most apparent in tectonic windows
in crust formed at slow spreading rates of <30 mm/year (Auzende et al. 1989,
Karson 1998, Karson & Dick 1983, Lagabrielle et al. 1998), but recent studies of
tectonic windows into crust formed at faster rates have provided new details on
the geology of the uppermost crust formed in these environments.
I review the geology of the upper 2000 m of the oceanic crust formed at
intermediate- to fast-spreading ridges, which are commonly perceived as the simplest types of spreading centers—ones that are likely to conform to traditional
models and to generate a geologically simple layered crust. The details of the
geology of a few key areas have been accumulating over the past two decades,
and this is an attempt to synthesize these results. From this perspective, I hope
that readers will be reminded of how basic geological observations can inform our
view of how mid-ocean ridge spreading centers work. I also hope to bring to light
some of the problems inherent to traditional representations and models of the
oceanic crust and how an appreciation for the complexities of geologic structures
can help constrain processes operating beneath the seafloor at spreading centers.
An important message in this regard is that despite the rather simple geology of
lava flows and minor faults seen at the surface of fast- to intermediate-spreading
ridges (Fornari et al. 1998, Macdonald 1998, Perfit & Chadwick 1998), there are
processes at work in the subsurface that create geological complexity and chaos.
These processes have important implications for many aspects of oceanic crust
and spreading centers that are current areas of aggressive research.
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INTERMEDIATE- TO FAST-SPREADING
CRUST—A GLOBAL CONTEXT
Mid-ocean ridges are the sites of the most vigorous fluxes of matter and energy from
the interior to the surface of the Earth. They are the sites of the most voluminous
volcanism on our planet as well as its most dramatic manifestation of extensional
tectonics. Hydrothermal fluxes driven by magmatic heat through fractures in the
crust change both the compositions of the rocks as well as the chemistry of the
oceans. Seafloor spreading, probably initially at high rates, has been operating
since Archean time, continuously “repaving” two thirds of our planet with new
seafloor as older lithosphere is recycled at subduction zones (Burke et al. 1976,
Bickle 1978). Although most of the length of the mid-ocean ridge system on Earth
today is spreading slowly (<30 mm/year, full-rate), most of the oceanic crust of
our planet was formed at intermediate- to fast-spreading rates (50–200 mm/year).
Thus, to understand the geology of the Earth’s crust beneath the sea, it is essential
to know about this faster-spread type of crust.
Although there have been many geological studies of fast- to intermediatespreading mid-ocean ridges (e.g., the East Pacific Rise—EPR), comparatively
few have examined the internal structure and composition of the crust at depth.
Because of the relatively high magma budget (volume of magma relative to plate
separation rate) of these spreading centers, fault scarps with only relatively small
relief occur. These typically expose only the upper 100–200 m thickness of the lavas
of the uppermost crust. Closely spaced normal faults and intervening talus ramps
commonly obscure much of this material. Major tectonic windows in intermediateto fast-spread crust are rare compared with those of slow-spread crust. However,
there are a few large tectonic windows into intermediate- to fast-spread crust at
places such as microplate boundaries at the Hess Deep Rift (Lonsdale 1988) and the
Blanco Transform Fault (Embley & Wilson 1992). Additional exposures occur at
Microplate boundaries at Pito Deep (Francheteau et al. 1994) and Endeavor Deep,
and an abandoned spreading center at Mathematicians Ridge (Batiza & Vanko
1986) (Figure 2). At present, only reconnaissance investigations have been done
in these last three areas. In addition, several holes drilled by the Deep Sea Drilling
Project (DSDP) and the Ocean Drilling Program (ODP) have penetrated substantial
thicknesses of upper crustal lavas. Only a single drill hole at DSDP/ODP site 504B
has penetrated to more than a couple hundred meters through the lavas and into
underlying sheeted dikes. This review is based on the few intensively studied
areas that provide major constraints on the internal structure of the upper oceanic
crust.
THREE KEY AREAS
Three areas stand out in terms of having extensive geological and geophysical
data sets that describe vertical upper crustal sections of the upper 2000 m of the
oceanic crust generated at intermediate- to fast-spreading ridges (Table 1). These
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Figure 2 Location map for areas referred to in this review. Note especially locations
of DSDP/ODP Hole 504B, the Hess Deep Rift, and the Blanco Transform Fault where
the most extensive studies of uppermost oceanic crust formed at intermediate- to fastspreading rates have been made.
include deep drilling and borehole studies at DSDP/ODP Hole 504B south of
the Costa Rica Rift (Alt et al. 1993, Anderson et al. 1985, Becker et al. 1988,
Cann et al. 1983, Dick et al. 1992), the north wall of the Blanco Transform Fault
(BTF) at the southern end of the Juan de Fuca Ridge (Juteau et al. 1995, Karson
et al. 2002b, Naidoo 1998), and the north wall of the Hess Deep Rift (HDR) in
the equatorial Pacific (Francheteau et al. 1992, 1990; Karson et al. 1992, 2002a)
(Figure 2).
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TABLE 1 Summary of key parameters for three key study areas of uppermost oceanic crust
Blanco Transform
Hess Deep Rift
Hole 504B
Crustal age
0.6–1.6 Ma
1–1.5 Ma
5.9 Ma
Spreading half-rate
30 mm/year
66 mm/year
36 mm/year
Spreading center
provenance
Juan de Fuca Ridge
(S. cleft segment)
East Pacific Rise
(Pac/Nazca Ridge)
Costa Rica Rift
(segment center)
Exposure of upper
crustal units
Faulted north wall
Blanco Transform
Fault
Faulted north wall
Hess Deep Rift
Drill hole in valley
between abyssal hills
Sedimentary units
Few m of pelagic
ooze, chalks,
and turbidites
∼2 m pelagic ooze
and chalks
274.5 m of pelagic
sedimentary rock
Upper lavas
200–500 m
∼100–300 m
∼200 m
Lower lavas
300–900 m
500–600 m
∼400 m
Lava/dike
transition zone
100–200 m
20–200 m
∼200 m
Deepest lavas
∼1500 m
∼800 m
780 m
Dip of flows in
upper/lower
lavas
Inward: mostly
30◦ –50◦ (outcrops)
Inward: mostly
20◦ –40◦ (outcrops)
Not evident in core
or borehole records
Magnetic and
paleomagnetic
constraints on
tilting
Inward dipping
reversal boundaries
(magnetometer)
Dikes rotated to
outward dip
(oriented samples)
Lavas tilted to
dip inward: 20◦ –30◦
(magnetic
inclination)
Sheeted dike
complex
∼500 m
300–>1000 m
>1056 m (base
not penetrated)
Dip of dikes in
sheeted dike
complex
Mean 40◦ outward;
Later steeper dikes
cut earlier lower
angle dikes
Mean 70◦ outward;
later steeper dikes
cut earlier lower
angle dikes
2 groups: earlier tilted
dikes, mean 68◦ ;
later steeper dikes,
mean 82◦
Gabbroic rocks
>200 m
>1000 m
Not penetrated
References
(Juteau et al. 1995,
Karson et al.
2002b, Naidoo
1998)
(Francheteau et al.
1990, 1992; Karson
et al. 1992, 2002a)
(Alt et al. 1993,
Anderson et al. 1985,
Becker et al. 1988,
Cann et al. 1983,
Dick et al. 1992)
Hole 504B (Figure 2) is essentially a one-dimensional sample of the upper
oceanic crust, and it is difficult to judge how detailed geological features of the
core and walls of the borehole might be related to the larger-scale structures of the
surrounding crust. Core recovery of only approximately 25% in the lava units and
approximately 10% in the sheeted dikes and the small size of many core pieces
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hamper detailed reconstructions. There is a huge leap of scale between the core
(a few cm across) and those of geological structures of spreading centers or geophysically defined crustal features. Comparison with nearby Hole 896A (Figure 3)
demonstrates significant crustal variations over approximately 1-km distances (Alt
et al. 1993, Dilek 1998). Despite these limitations, Hole 504B provides an important glimpse of the internal structure of upper oceanic crust without possible
complications arising from exposure along a tectonic window.
The BTF and HDR scarps (Figure 2) have very extensive exposures of the
upper oceanic crust that represent flow-line-parallel natural cross sections of the
upper 2 km of the oceanic crust oriented normal to isochrones defined by magnetic
anomalies and seafloor lineaments. In both areas, extensive high-resolution sidescan sonar surveys provide a broad-scale (kilometers) view of the seafloor
Figure 3 Summary of possible relationships at DSDP/ODP Hole 504B and ODP
Hole 896A located approximately 1 km to the south on a basement high (Alt et al.
1993). White and black bars on Hole 504B show averaged magnetic inclinations and
inferred dips of lavas (Schouten & Denham 2000).
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morphology along these scarps. These data allow more detailed observations to
be placed in the context of major fault scarps, seafloor lineaments, talus accumulations, and other large-scale features. Submersible observations and sampling in
upslope transects, spaced at approximately 1 km along the scarps, permit major
rock units to be mapped for tens of kilometers (Figure 4). Along-strike variations
in crustal structure occur in both areas, and these may reflect temporal variations
in spreading processes at their respective spreading centers. In addition to direct submersible observations, typically on the scale of a few meters, electronic
mosaics of video and digital still-camera images provide an intermediate scale
of observation with fields of view from several meters to several tens of meters across (Figure 5). Integrated studies along the BTF and HDR scarps on the
scale of meters to tens of kilometers help to put the much more detailed studies
of the vertical sample at Hole 504B and other shallow drill sites into a broader
framework. Indeed, many enigmatic aspects of the geology at Hole 504B are understood as part of a range of spreading processes when viewed in this broader
context.
In this review, I focus on these three areas because of the wealth of information
available there. In broad terms, the geology of the upper crustal rocks in all three
areas is very similar. Although there are some significant differences, they can be
viewed as variations on a theme of seafloor spreading that is significantly different
from the prevailing views currently popularized in textbooks and many current
research papers. It is very significant that the complex structures found in each of
these areas and for tens of kilometers along major scarps are so similar despite
the very different tectonic settings in which they are exposed. The broadly similar
deformation structures found in these three areas (Figure 6) argues persuasively
that these structural assemblages are the products of seafloor spreading and that
they have not been strongly modified by later tectonics.
The geological relationships found in these areas can be interpreted in terms of
the processes of magmatic construction, faulting, and hydrothermal alteration that
create and modify the oceanic crust at a limited range of spreading rates. The more
complicated geology revealed in these tectonic windows compared with traditional
views of oceanic crust (Figure 1) implies correspondingly more complicated and
interesting processes attending seafloor spreading. Major features and key references for each of these areas are outlined in Table 1. Details of the geology in
each of them are discussed in the context of major rock units in the following
section.
GEOLOGY OF UPPER CRUSTAL ROCK UNITS
Although workers have defined somewhat different rock units in the various study
areas, I attempt to group the various rock associations in a way that makes sense in
terms of their distinctive modes of construction and subsequent evolution. These
groupings are also appropriate for comparison with geophysical expressions of the
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Figure 4 Highly generalized cross sections of the Hess Deep (Karson et al. 2002a) and Blanco Transform (Karson et al. 2002b) scarps
showing significant along-strike variations in the thickness of major rock units that may correlate with variations in spreading processes.
Spreading centers are to the left for both panels. Bold lines and dark gray boxes show relative locations of submersible dives and ROV
surveys. The light gray bar shows the extent of high-resolution side-scan sonar data in each area. Note inward-dipping magnetic polarity
reversal boundaries for B, J, and 2 for the Blanco Transform Fault (Tivey 1996, Tivey et al. 1998).
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upper crust and studies of many ophiolites. For each unit, I discuss the features
common to all of the study areas, followed by some comments regarding local
variations. The rock units are upper basaltic lavas, lower basaltic lavas, lava/dike
transition zone, and the sheeted dike complex. Here, I ignore associated sedimentary rocks that have accumulated on top of the lava and that locally drape the
faulted crustal sections, as these are not part of the initial spreading history of the
crust. In many places, only part of the complete vertical section is preserved. In
some scarps, the uppermost part of the basaltic lava unit has been degraded by
slope failure, leaving a ragged edge modified by recent mass wasting. Transects
that upslope away from scarp edges commonly encounter constructional volcanic
surfaces created at the spreading center, with variable thicknesses of overlying
sedimentary material making it unlikely that important details of this unit would
be missed. The lower units are commonly partially buried by talus derived from
the overlying units. Exposures of massive, jointed gabbroic rocks occur in limited
areas beneath the sheeted dike complex at the BTF and HDR scarps, and they are
not described in detail here.
In many places, this vertical sequence of rock units can be observed just as
in the upper crustal portions of many ophiolites (Casey et al. 1981, Harper 1984,
Moores & Vine 1971, Pallister 1981). Faulting associated with the exposure of
this sequence may have modified the thickness of units in some places. At present,
it is not possible to quantify this effect. However, cross-cutting dikes and mineralized fault zones provide strong evidence that most of the tectonic deformation
evident in these units took place at or very near to the spreading center and hence,
represents an integral part of the seafloor spreading process. This subaxial deformation includes tectonic rotation of lava flows and dikes on various scales as well
as ubiquitous and locally intense brittle deformation. The tilting of rock units in
the three areas considered is systematically related to their respective spreading
centers. To facilitate comparisons, the conventional terminology of inward- and
outward-dipping orientations is used to indicate planar structures that dip toward
or away from the local spreading center, respectively.
The details of the igneous petrology and overprinting hydrothermal metamorphism are generally not distinguishable in seafloor outcrops; however, investigations of samples collected from well-constrained geologic settings have
revealed correlations with large-scale crustal structures that may be attributed to
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
−
Figure 5 Typical outcrop images of upper crustal rock units. (a) Upper basaltic lava
unit with pillow, lobate, sheet flow morphologies dipping inward; Blanco Transform
Fault. (b) Lower basaltic lava unit with inward-dipping, fractured lavas; Hess Deep
Rift. (c) Lower basaltic lavas (or transition zone?) with fractured lavas and dikes cut by
massive, jointed, outward-dipping dikes; Hess Deep Rift. (d ) Sheeted dike unit with a
panel of parallel, outward-dipping dikes; Hess Deep Rift. Spreading center is to left in
all images.
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Figure 6 Generalized columnar sections and key geological features of uppermost oceanic crust at the Blanco Transform Fault, Hess
Deep Rift, and DSDP/ODP Hole 504B. See Table 1 and text for additional details and references. Despite very different spreading rates and
post-spreading histories, a similar structural geometry is present in all of these. Note especially downward-increasing inward dip of lavas,
outward-dipping dikes, and intense fracturing throughout the lower lavas and sheeted dikes. Hess Deep Rift and Blanco Transform Fault
columns represent summaries of features found in each study area assuming lateral continuity of rock units on the scale of individual dive
transects.
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temporal variations in spreading processes. These relationships are discussed in a
later section.
Upper Basaltic Lavas
This unit is defined as the uppermost part of the basaltic lava pile where constructional volcanic features are only slightly modified by burial and the collapse of
pore space generated during eruptive processes. The top of the unit is the lava
surface created at the spreading center but may include some minor off-axis (more
than several kilometers away) lavas as well. Flat-lying sedimentary material blankets this surface. The lower contact is gradational, and I define it at a point where
tilting, fracturing, and dike intrusion have caused substantial modification of the
volcanic architecture. The total thickness and character of this unit varies between
the three sites discussed here and on a more local scale across the BTF and HDR
scarps. Basaltic lavas of this upper unit show the morphologies documented by
many geological investigations of surface features along active spreading centers
(e.g., Kurras et al. 2000, Perfit & Chadwick 1998). The cross-sectional perspective
from drill holes and fault scarps leads to somewhat different conclusions regarding
the eruptive style and hence eruptive processes.
The lavas consist of dominantly pillow lavas (Figure 5a) in all three areas
as well as in several other drill sites and minor escarpments. The pillows are
typically 0.5–2 m across and vary from solid, tightly packed assemblages to hollow,
partially filled, or collapsed forms. Radial contraction joints are present almost
everywhere. Hyaloclastites formed by spalling of glassy rinds during expansion
and contraction of the pillows are a common constituent. Lobate lava flows are
difficult to distinguish from elongate pillows and lobes of sheet flows but appear to
be very common. Tabular sheet flows range from a few tens of centimeters to a few
meters in thickness. They have nearly flat tops and undulating bases that are molded
over underlying lava surfaces. Breccias have diverse textures and origins including
(a) sedimentary breccias from indurated mass wasting deposits, (b) hyaloclastite
and related breccias generated during eruptions, and (c) “jigsaw-puzzle” breccias
and net veins produced by hydraulic fracturing (Alt et al. 1993).
From cross-sectional views of submarine escarpments and drill holes it is difficult to unambiguously determine the thickness of individual lava flows or eruptive
units because of the lack of interstratified sedimentary material, poorly exposed
flow contacts, and the impracticality of fine-scale geochemical mapping. Examination of drill core and downhole geophysical records at Hole 504B as well as
digital mosaics from the BTF and HDR scarps suggests that individual sheet flows
are 0.5–4 m thick. Variably fractured pillow lava sequences are commonly as much
as 10–20 m thick (Adamson 1985).
Lava flow surfaces defined by flattened pillows and lobate flows and sheet flow
surfaces are typically near horizontal ( ±<20◦ ), and no consistent sense of tilting
is present in the top of the unit. The overall impression is one of a chaotic pile of
basaltic lava lobes and minor sheet flows with substantial porosity derived from
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eruptive processes. Large-scale, cavernous porosity seen in surface exposures with
lava lakes, collapse pits, and lava tunnels has not been observed. This probably
reflects the rapid and efficient collapse of pore space with burial or infilling by
later lava flows (Fornari et al. 1998).
Deeper in the unit, the lavas appear to be densely compacted and flow surfaces
dip consistently toward the ridge axis. Dips vary substantially but generally lie in
the range of 20◦ –40◦ . Sheet flows are excellent paleo-horizontal reference markers
and in many places are subparallel to surfaces defined by flattened pillows and
lobate flows. The dipping sheet flows in particular demonstrate that significant
tectonic rotations have occurred (Karson et al. 2002a).
Intrusive features are relatively rare. Steeply dipping dikes at this crustal level
have sinuous traces and lobate or budded edges in some places. High-level sheeted
dike swarms locally reach within 200 m of the top of the section. The abundance
of sills is uncertain because they are difficult to separate from sheet flows where
contacts are poorly exposed. Most careful examinations have concluded that tabular
units at this crustal level are flows rather than sills.
Fault zones in this unit appear to be widely spaced (hundreds of meters) and
steeply dipping. They cut the lavas and are marked by distinctive walls of fault
breccias in outcrops and intensely fractured and veined intervals of high porosity
in boreholes. Although numerous small faults and shear fractures are present in
this material, major faults such as might be expected as the subsurface expression
of abyssal hills have not been identified. This may be a result of syn-tectonic
volcanism or growth faulting.
At Hole 504B, this unit appears to consist of 70% pillow lavas, 13% each thin
flows and massive units, and 4% dikes. If breccias are systematically undersampled
by drilling, they could make up as much as 17% of the unit (Dilek 1998). These materials have been subjected to low-temperature (<100◦ C) interaction with seawater
and are only slightly altered (5%–15%). The upper 100–200 m of the hole has very
low compressional wave velocities corresponding to seismic layer 2A and high
porosities, 10%–12%. Nearby Hole 896A is similar but has a lower proportion of
pillow lavas relative to flows (about equal amounts) and significantly more breccia
(∼50%). Paleomagnetic studies show that stable inclinations are scattered but do
not deviate greatly from predicted values, indicating that little if any tectonic tilting
has affected the basaltic material at this level (Pariso & Johnson 1989, Schouten
& Denham 2000). A similar relationship is inferred from borehole studies of ODP
Hole 801 in the Western Pacific (Pockalny & Larson 2002).
Upper lavas at the BTF range from <100- to 700-m thick. Typical thickness in
dive transects that reach the top of the scarp are 200–500 m. Rapid lateral variations
in thickness are suggested by closely spaced (∼1 km) dives. Pillow lavas dominate
the unit. Just as seen in the rift valley of the Juan de Fuca Ridge (Chadwick &
Embley 1994), sheet flows up to 2–3 m thick are also common but probably <15%.
Breccias appear to occur only locally at this crustal level. The lava flows become
more consistently inward-dipping and the dip increases downward (Karson et al.
1999).
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The least deformed lavas at the HDR are only <100- to 200-m thick. They
are mostly pillow lavas and lesser lobate flows. Sheet flows up to approximately
3-m thick occur near the top of the section in a few places. Thin sheet flows
<0.5-m thick are widely dispersed deeper in the section. Overall sheet flows are
estimated to constitute less than 5% of the total volume of lavas in this unit. Sheet
flows, flattened pillows, and lobate flows dip persistently inward 20◦ –30◦ . Steeply
dipping dikes cut the lavas in many places. Good exposures of this unit also occur
on the south wall of the rift and include inward-dipping lava flows and sparse
outward-dipping dikes.
Lower Basaltic Lavas
The lower part of the lava unit is similar to the overlying lavas, but differs in that it is
much more densely compacted and more intensely deformed and metamorphosed.
The upper boundary of this unit is gradational and difficult to map in a systematic
way. The lower contact with the underlying transition zone is also gradational over
approximately 100 m and is probably best defined by an increase in dikes. This
unit is approximately 500 m thick in the three study areas but reaches >900 m in
some places on the BTF scarp. Across most of the HDR scarp most of the lavas
are of this type.
The proportions of various lava flow morphologies appear to be the same as in
the upper lavas, but fracturing obscures these features in most outcrops. Locally
intense fracturing is developed over many tens of square meters and discrete faults
cut the sequence. Widespread breccias are the result of in situ fracturing with
relatively small displacements. Locally well-preserved lavas grade laterally or
vertically into much more damaged areas (Figure 5b). In many places, fracturing
is so intense that flow morphologies are no longer recognizable. Even with largescale digital mosaics it is difficult to trace individual lava flows for more than a few
meters across scarp faces. Where intact flow morphologies can still be discerned,
pillow and lobate flows clearly dominate. Basaltic material of this unit has also
been affected by heterogeneous, greenschist facies hydrothermal metamorphism.
In general, metamorphic effects are more widespread and reach higher temperature
assemblages (typically greenschist facies) than in the overlying lavas. Compaction,
metamorphism, and vein filling increase the density of the material.
Preserved lava flows dip persistently inward and in many places steeply (>50◦ ).
Typically, the intensity of fracturing correlates with increasing dip. Despite the
tilting, relatively steeply outward-dipping dikes cut this assemblage (Figure 5c).
In some places, tabular lava flows dip so steeply that, apart from their distinctive
flow bottoms, they resemble inward-dipping dikes. Fracturing and the proportion
of dikes both increase downward.
Discrete faults are more obvious at this level than in the upper lavas. They cut
sharply truncated sections through the lavas and appear to be plastered with a layer
of fault breccia to cataclasite, some of which is very intensely altered, suggesting
that these faults focused hydrothermal outflow (Gillis et al. 2001, Saccocia & Gillis
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1995). Resistant, hydrothermally cemented fault breccias protrude from slopes as
jagged, spiny layers of material.
At Hole 504B the porosity decreases to <10% and the seismic velocity increases
in kind downward through this unit. Although lava flow contacts are difficult to
document from this perspective, paleomagnetic studies show that stable inclinations are scattered and are overall skewed toward values consistent with 20◦ to
30◦ tectonic (inward-dipping) rotation in the lower 400 m of the lava section, with
dips locally greater than 50◦ near the base of this unit (Pariso & Johnson 1989,
Schouten & Denham 2000).
The lava section at the BTF is remarkably well exposed and continuous. It
reaches a total thickness approaching 1500 m in some places. Lava flows in the
lower part of the BTF section include flattened pillow and lobate flows as well
as numerous originally horizontal sheet flows. The flows dip persistently inward
and define a downward-steepening pattern increasing from 20◦ –30◦ in the top of
the unit to 40◦ –60◦ (and locally steeper) toward the base of the unit (Karson et al.
2002b). Cross-cutting dikes are relatively rare.
The lower lavas at the HDR are intensely shattered across most of the scarp.
Inward dips increase downward and show an increasingly scattered pattern with
most lavas dipping between 20◦ and 40◦ . Cross-cutting dikes are mostly steeply
outward dipping or near vertical. Paleomagnetic investigations on oriented samples document that like lavas, the dikes have been tectonically rotated rather than
intruded in their current outward dipping attitudes (Hurst et al. 1994). Outcrops on
the south wall of the HDR have very similar structures, including variably tilted,
outward-dipping dikes.
Lava/Dike Transition Zone
This unit of mixed lavas and dikes is described from many studies of ophiolites and
seafloor escarpments. Although it is well developed in some places, continuous
exposures in major escarpments show that it is not a laterally continuous, mappable rock unit. Where the lava/dike contact is gradational, it appears to be highly
variable in its internal structure, proportion of dikes and lavas, metamorphism,
thickness, and relation to bounding rock units. In many places, this transition zone
is on the order of 100- to 200-m thick. Both the lower and upper contacts tend to
be gradational. The upper contact is defined in terms of a rapid increase in the proportion of dikes relative to lavas. The density of fractures also increases sharply.
A transition zone is identified at Hole 504B, where it coincides with downhole
changes in porosity, mineralogy, and seismic velocity across a fault zone. A similar interval occurs in some places at both the BTF and HDR scarps, but it essentially
pinches out along strike into areas where sheeted dikes crop out within a few tens
of meters of lower lava units with a very low proportion of dikes. Elsewhere at
both of these scarps, high-level sheeted (vertical) dike swarms (at least tens of
meters across) protrude upward hundreds of meters into the lower and even the
upper lavas.
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Where visible, lava flows dip moderately to steeply inward, and most dikes
dip steeply to moderately outward. Some workers identify gently inclined massive
tabular basaltic units as sills (Francheteau et al. 1992, Juteau et al. 1995, Naidoo
1998). Given the locally extreme tilting of dikes, variable tilting of lava flows, and
poorly preserved and commonly fractured contacts, the proportion of possible sills
is uncertain.
Fracturing is variable but locally very intense in this unit. Areas of pervasive,
close-spaced (few centimeters) fracturing are common. The pervasiveness and
temperature of hydrothermal metamorphism is also highly variable here. In general, metamorphic recrystallization and veining combine to decrease the porosity
and increase the seismic velocity of this rock unit (Alt et al. 1996).
At Hole 504B, shipboard scientific parties have identified a discrete transition
zone 209-m thick (Alt et al. 1993, Becker et al. 1988). A fault zone occurs at the
top of this interval. A rapid increase in actinolite and hornblende signal higher
metamorphic temperatures, at least in this vertical section. Paleomagnetic studies
show that some of the largest values of stable inclinations occur in this unit and
correlate with very intense fracturing (Schouten & Denham 2000). They suggest
inward tilting of lavas of 40◦ to 50◦ .
At the BTF scarp, the transition zone is poorly exposed and not clearly evident
in many places. Where units below the lower lavas are exposed, the sheeted dike
complex passes rapidly upward (over a few tens of meters) into lower lavas, as
defined above.
The transition zone is highly variable along the HDR scarp. In some places it
appears to be absent, whereas in other areas it is at least 200-m thick. The relief
and variable character of its upper and lower contacts and the intense fracturing at
this crustal level make this unit very difficult to map across the scarp.
Sheeted Dike Complex
Sheeted dike complexes have been studied extensively in ophiolite complexes
(Kidd 1977, Nicolas & Boudier 1992, Pallister 1981, Varga 1991) and have been
observed in some oceanic escarpments (Auzende et al. 1989, Francheteau et al.
1992, Karson 1998). In most cases, sheeted dike complexes are defined as assemblages of subparallel basaltic dikes (Figure 5d ) that have less than 10% intervening
screens of basaltic or gabbroic material. These screens are potentially important
as indicators of how the sheeted dike complex has grown vertically during its
construction (see below).
The width of individual dikes in ophiolite complexes is typically 0.5 to 2 m
(Kidd 1977). In oceanic escarpments, dike margins are very difficult to discern.
Fracturing is commonly concentrated along the margins, and ferro-manganese hydroxide coatings obscure textural details such as chilled margins. In sheeted dike
complexes where dikes are commonly split by later dikes, thicknesses are likely
underestimated in visual estimates. Along the BTF and HDR scarps, direct measurements and visual estimates show that individual dikes are typically ∼1-m wide
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in both the sheeted dike complex and in the lava units. Significantly wider dikes
(a few meters wide) are very rare. At Hole 504B, dike widths are calculated based
on assumed dips constrained by contacts in cores, borehole records, and paleomagnetic data. Two populations occur: About half of the dikes are 1–2 m wide
and half are 2.5–7 m wide (Adamson 1985, Becker et al. 1988). Given the very
low recovery rate in the sheeted dike complex, it is likely that some dike margins
were missed and that some units recorded are multiple dikes.
Traditionally, individual dikes in sheeted dike complexes are considered to have
been intruded vertically beneath spreading centers and to remain vertical as they
move off axis. Most current models of intermediate to fast seafloor spreading
assume vertical dikes. Reconstructions of ophiolite complexes commonly rely
on the assumption that dikes provide a vertical reference plane. Some ophiolites
document tectonic rotations of blocks of sheeted dike complexes between normal
faults (Harper 1982, Varga 1991). Although there are good mechanical reasons why
dikes are very likely to be originally intruded in a vertical orientation, postintrusion
rotations may tilt the dikes in ways that provide important information on subaxial
tectonic processes.
Observations at the three study areas examined here all show that dikes in
sheeted dike complexes, at least in these areas, are not vertical. Instead, they dip
outward, that is, away from the spreading centers where they formed. Dikes occur
in discrete packages or “panels” of subparallel dikes a few meters to perhaps
as much as 100 meters across that are bounded by intensely fractured, diffuse
cataclastic deformation zones or discrete faults, many of which are subparallel to
local dike margins. Within the panels, slip on dike margins is common. Fracturing
is pervasive and locally so intense that individual dikes and their margins can no
longer be discerned in outcrops. Less fractured, later dikes intrude this assemblage.
Some of these are steeply dipping, but many tend to follow older tilted dikes or fault
zones, making cross-cutting age relations difficult to detect in individual outcrops
or submersible views.
Drilling at Hole 504B penetrated >1000 m of a sheeted dike complex. A vertical
drill hole and core are clearly not the ideal perspectives for sampling the steeply
dipping dike margins expected in oceanic crust, yet very significant results were
obtained. Some of these results were difficult to reconcile with simple models
of seafloor spreading, but these may now be viewed in the context of broader
exposures in tectonic windows. For example, very little has been made of the fact
that approximately 200 dike margins were intersected by the vertical drill hole,
demonstrating that the dikes are not vertical. Observations at the BTF and HDR
show that this is normal, at least in these areas.
At Hole 504B, dike margins are not well represented. Drilling reports show that
only a very small number of dike margins were recovered, and borehole imaging
data do not show many convincing steeply dipping features that could be dike
margins (Anderson et al. 1985). Careful geochemical and magnetic logging of
these records, however, has identified ∼200 basaltic dikes (Becker et al. 1988,
Dilek et al. 1996). Assuming a typical dike width of approximately 1 m and a
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drill hole is vertical, the average dip of the dike margins is calculated as ∼70◦ ,
in good agreement with the magnetically corrected dips of cored dike margins.
Stable magnetic inclinations of core samples are scattered over 30◦ –40◦ but tend
to be skewed toward dips away from the Costa Rica Rift; that is, they dip outward.
This is consistent with the sense of tectonic rotation inferred from the inclinations
in the overlying lavas. Two discrete dike populations appear to be present: a more
gently dipping (average 68◦ outward dip) and a significantly steeper population
(average 82◦ outward dip). The more gently dipping dikes are inferred to have been
tilted prior to the intrusion of the later steeper dikes (Becker et al. 1988). Other
interesting observations include intense and pervasive fracturing, the occurrences
of dikes in subparallel clusters, and sharply downward increasing metamorphic
grade and pervasiveness.
Porosity slowly decreases downward in the sheeted dike unit and has values below approximately 3%. Seismic velocities increase downward and reach velocities
and gradients typical of seismic layer 3 at a depth of 1500 m into the basement,
still in the sheeted dike complex (Detrick et al. 1994). The velocity increase at
this depth is correlated with a change in metamorphic mineral assemblage with
increasing hornblende and decreasing chlorite (Alt et al. 1996).
This important one-dimensional sample can now be viewed in the broader
context of variations that occur along tectonic windows. In retrospect, it now
seems very likely that Hole 504B sampled the same type of intensely fractured
and variably rotated dike complex that is exposed at HDR and less clearly at the
BTF. The intense fracturing present at this crustal depth may account for the very
low recovery rates in this interval. Although the type of crustal structure drilled at
Hole 504B is well within the range of geologic variations seen in these tectonic
windows, significant variations on this theme also exist.
At the BTF, the sheeted dike complex is poorly exposed. Talus and debris slide
breccias cover much of it. Initial investigations found few individual dikes and
interpreted much of the material below the lava units as massive diabase (Juteau
et al. 1995, Naidoo 1998). Reexamination of outcrop data shows that, in fact,
highly fractured sheeted dikes are present in subparallel swarms that dip moderately outward and in some places dip as gently as 20◦ . These grade into pervasively
shattered materials both vertically and laterally (Karson et al. 2002b).
Outcrops of the sheeted dike complex along the north wall of the HDR are the
most extensive and best exposed yet found on the seafloor (Francheteau et al. 1992,
Karson et al. 2002a). The sheeted dike complex appears to be more or less continuously exposed for at least 30 km along the scarp corresponding to about 0.5 Ma
of EPR spreading history. Widely spaced submersible dives in the area indicate that
these exposures may extend many kilometers farther along the scarp. The upper
contact with lower lavas or transition zone rocks is extensively exposed, but talus
covers the lower contact over many areas. The combination of large-scale outcrop
views afforded by mosaics constructed from digital still-camera surveys with the
Argo II ROV and detailed observations and sampling with the submersible Alvin
have provided important new information on the nature of sheeted dike complexes
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and provided a context within which to consider more limited exposures such as
at the BTF or Hole 504B.
The upper contact of the sheeted dike complex is highly variable and has hundreds of meters of relief over lateral distances as small as 1 km. In contrast, the
lower contact varies by only approximately 100 m vertically over the 4 km of the
scarp where it is exposed (Figure 4). Sheeted dikes grade upward into overlying
units by a decrease in the proportion of dikes. In some places, this occurs over
only a few tens of meters, but elsewhere may be spread over as much as 200 m. In
some transects, sheeted dikes occur at very high levels of the crust, locally reaching within 200 m of the top of the crustal section. In these high-level swarms, the
dikes are very steeply outward dipping to vertical and have steep lateral contacts
with surrounding lava units. In some places, downward dike terminations result
in vertically alternating lava and sheeted dike units. Dike morphologies (steps,
bridges, and horns) and geochemistry support the interpretation that these are areas of lateral dike injection rather than upward extensions of the underlying dike
complex or repetitions created by scarp-parallel normal faulting.
The main part of the sheeted dike complex has a very complicated but broadly
consistent internal structure. It consists of discrete packages or “panels” of subparallel dikes, typically a few meters to tens of meters across, separated by intensely
shattered dike rock or screens of other material. The panels are tilted so that dike
margins within them dip outward, that is, away from the spreading center where
they formed. Dips are highly variable, ranging from nearly vertical to as low as 20◦ ,
with an average dip of about 60◦ . Panels with steeply inward-dipping dikes occur
in a few places but are rare. Paleomagnetic studies of oriented samples show that
these dikes have been variably rotated from the orientations in which they were
intruded (Hurst et al. 1994). Rotations and small displacements have been accommodated by slip across the bounding intensely fractured areas. Detailed sampling
shows that nearly all dike margins and fracture-bounded pieces within the panels have sheared, polished, and slickensided surfaces, indicating some amount of
internal deformation.
Less fractured, massive dikes cut across the fractured material and across the
panels of tilted dikes. Most of these are moderately to steeply dipping, though
low-angle sheets and dikes injected parallel to tilted dikes within the panels have
been observed. Variable tilting and cross-cutting relations suggest that crustal
construction involved a more or less continuous interval of spatially overlapping
intrusion, fracturing, and rotation rather than two discrete episodes of dike intrusion
before and after deformation. Detailed investigation of the geochemistry of dikes
in the sheeted dike complex indicates that dikes tapping different magma sources
have intruded the complex, probably along strike (Stewart et al. 2002). This type
of complex internal structure in the sheeted dike unit is persistent over the entire
30-km breadth of the study area, indicating that this type of structure was generated
at the EPR for at least 0.5 Ma.
Metamorphic effects in the sheeted dike complex along the HDR scarp are not
as systematic as those documented at Hole 504B (Gillis et al. 2001). Hydrothermal
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metamorphism is highly variable laterally and vertically on a broad scale because
it is strongly influenced by fracture porosity, faulting, and dike intrusion. However,
it provides an important reminder that the well-documented vertical metamorphic
zonation at Hole 504B (Alt et al. 1996) is likely to be a single one-dimensional
sample through a much more complex three-dimensional system. Spatial patterns
of metamorphism and crustal structure hint at possible systematic variations, discussed below.
BUILDING THE UPPERMOST OCEANIC CRUST
It has long been recognized that in order to construct a substantial thickness (hundreds of meters) of volcanic material in a region of relatively low relief (tens
of meters) like the East Pacific Rise, substantial subaxial subsidence must occur
(Cann 1974, Dewey & Kidd 1977). Several existing models account for this and
generally predict bending of upper crustal units by isostatic compensation permitted by slip on vertical fault surfaces with displacements decreasing laterally
away from the axial region (Cann 1974, Hooft et al. 1996, Kidd 1977, Pálmason
1973, Rosencrantz 1982, Schouten & Denham 2000, Schouten et al. 1999). Axial
“loading” of lavas is generally considered to be the driving mechanism for this
deformation. However, the thickening of basaltic lavas at spreading centers also
requires the creation of “accommodation space” by the rearrangement of subaxial
material. Geological observations from the three study areas examined here support
models in which the lava pile thickens rapidly at ridge axes by bending of upper
crustal rock units to create inward-dipping patterns of lava flows. However, they
also show that similar rotations accommodated by intense fracturing and faulting
affect the underlying sheeted dikes and possibly deeper gabbroic units as well. The
observed geometries are even more complicated than those shown in most current
models. The more extensive crustal deformation associated with these areas may
have important implications for the porosity structure of the upper oceanic crust
and related phenomena.
Creating Accommodation Space
The growth of lava accumulations at spreading centers can be viewed as analogous to the creation of “accommodation space” in the formation of sedimentary
basins (Gibbs 1983). The three study areas considered above show varying total
thicknesses of lava, typically approximately 800 m, but locally as much as 1500 m.
Screens of basaltic material in underlying dike complexes indicate even somewhat
greater original lava thickness. Because even the lowest part of the volcanic unit
must have been erupted on the seafloor, the total thickness of the lava unit is a measure of net vertical subsidence (relative to the seafloor) that occurred as overlying
basaltic material accumulated (Figure 7). Where an axial high exists, additional
subsidence equal to this relief must also occur.
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Subsidence accommodating the thickening of the basaltic lava unit demands
substantial vertical mass transport (equal to the thickness of lavas) of material beneath the lavas (Figure 7). The subsidence must be most rapid beneath the spreading
axis and diminish over the width of the area of active volcanic construction, a few
kilometers for fast- to intermediate-rate ridges (Perfit & Chadwick 1998, Perfit et al.
1994). In the uppermost axial crust deformation occurs in the brittle regime. At
deeper levels, vertical mass transport could possibly be accommodated by ductile
flow in hotter, weaker material or even by hypersolidus viscous flow in a subaxial
melt lens or crystal mush. Detailed examination of the gabbroic rocks immediately
beneath sheeted dike complexes could provide constraints on these processes.
Various modes of deformation can accommodate subsidence in the uppermost
crust (Figure 8). Some models suggest bending of lavas and vertical slip on deeper,
dike-parallel faults (Cann 1974, Hooft et al. 1996, Kidd 1977, Schouten & Denham
2000). A kinematic equivalent is slip on conjugate inward- and outward-dipping
normal faults (Cann 1974, Dewey & Kidd 1977). Bending of upper crustal units
(resulting in inward-dipping lavas) in the brittle regime creates accommodation
space and could be accomplished by a number of different fracture kinematics.
Geological observations at the three study areas examined here provide direct
support for these general types of models. However, they also demonstrate that
fracture-accommodated rotation of upper crustal units affects the sheeted dike
complex and possibly part of the underlying gabbroic rocks.
A systematic pattern of progressive asymmetric subsidence and accretion would
produce the types of patterns of inward-dipping lava flows and outward-dipping
dikes found in the study areas (Figure 7). In detail, these patterns are not as regular
and systematic as suggested by the models, but this is to be expected given possible
fluctuations in magmatic and tectonic processes and the variability of depositional
slopes in pillow lava–dominated volcanic assemblages. Although the three study
areas show similar upper crustal structures (Figure 6), there is no way to tell
at present if these are representative of the vast areas of crust generated at fast- to
intermediate-spreading rates.
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
−
Figure 7 Subsidence model for intermediate- to fast-spreading ridges. (a) Inferred
lateral variations in lava accumulation and dike intrusion at intermediate- to fastspreading ridges. Width of accretion zone probably roughly correlates with spreading
rate, but also probably fluctuates with magma budget and pressure. Dike intrusion is
likely to be sharply focused beneath the axis but to extend off axis for at least 1 km
based on the gradational nature of the lava/dike contact (Kidd 1977). (b) Highly generalized patterns of lava flow dips and dike dips develop from progressive subaxial
subsidence during accretion (Dewey & Kidd 1977). Note the trajectory of a point at
the base of the lava unit (bold black dots) illustrating subsidence during lateral spreading near the axis. (c) Geological representation of above model with structures found
in upper crustal sections of the oceanic crust. Bending to accommodate the subsidence
probably causes extensive fracturing of both lava and dike units.
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Figure 8 Mechanisms that can accommodate subsidence beneath spreading centers. Vertical mass transfer under the influence of gravity probably occurs as a result of the following:
(a) plate separation causing normal faulting and/or more distributed failure, (b) thermal
contractional cooling of hot axial crust, (c) transfer of magma from deflating axial magma
chamber and surface loading of lavas (note that lateral transport in dikes may separate these
effects along axis), and (d ) tectonic extension above subsurface dikes. All of these mechanisms cause axial subsidence that can permit lavas to thicken rapidly beneath the neovolcanic
zone. Mechanisms that result in laterally diminishing subsidence can produce inward-dipping
lavas and outward-dipping dikes. Outcrop-scale rotations require slip on listric faults (or kinematically equivalent cataclastic zones) or on planar faults that also rotate (i.e., domino-style
block rotation).
370
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Geological observations provide the following details on the mode of deformation accommodating subaxial subsidence. Although these are most spectacularly
displayed at the HDR, similar structures occur at the BTF and can also be inferred
from Hole 504B. Throughout the upper crustal units, deformation is manifest in a
number of related deformation styles. Crustal accretion and subaxial deformation
integrate to produce a complex upper crustal structure (Figure 9).
Rotations
The most obvious manifestation of crustal deformation is the tectonic rotation
of originally horizontal lava flows and originally steeply dipping dikes to inwardand outward-dipping orientations, respectively. These rotations are clearly
evident in outcrops at both the HDR and BTF. Although pillow lavas and
lobate flows can accumulate on surfaces with substantial slopes (Ballard & Moore
1977), flat-topped sheet flows provide reliable paleo-horizontal markers. Although
sheet flows represent only a small proportion of the total volume of lavas at the
HDR and BTF scarps, they are widely interspersed in the lava piles. The sheet
flows are commonly subparallel to lobate and pillow flow surfaces defined by
hummocky flow surfaces and flattened lava lobes. These provide important reference markers for tectonic rotations. In the top of the upper pillow lavas, flow
surfaces do not have a consistent dip direction; however, downward persistent inward dips through this unit demonstrate increasing tectonic rotation. Lava flow
surfaces dipping inward as much as 60◦ –80◦ in the lower lavas are evidence of
very large rotations. It should be noted that in detail the dip patterns are highly
variable between the study areas and even across the HDR and BTF scarps. In
some places, very steep dips are present at shallow crustal levels, for example, in
the upper lavas, whereas elsewhere, nearly flat-lying lavas occur even in the lower
lavas or transition zone. In the best-exposed vertical transects, dips of lava flows
are commonly variable and do not necessarily increase monotonically downward.
This may be attributed to the variable rotation of many relatively small blocks
(tens to hundreds of meters across) bounded by fault zones (see below). Similar
rotations have been inferred from paleomagnetic data on cores from 504B (Pariso
& Johnson 1989, Schouten & Denham 2000). There is a very large scatter in these
inclination data (tens of degrees) over short vertical intervals (meters to tens of
meters). This could be due to variable block rotations as seen in the HDR and BTF
outcrops.
In order to thicken the basaltic volcanic unit to a few hundred meters within
the region of active volcanism (a few kilometers wide), the rate of subsidence
would have to be about one half of the lateral spreading rate (tens of millimeters
per year). Subsidence as well as the rate of accumulation of lavas would diminish rapidly away from the axis as the crust accelerates to the half-spreading rate
(Figure 7).
Individual dikes in the lava units and in the sheeted dike unit generally dip
outward. Many dikes are oriented at a high angle to nearby lava flows, but later,
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steeper dikes cutting previously rotated dikes and lava flows show that rotation and
dike intrusion occurred close to the spreading axis. These types of cross-cutting
relationships are widely developed at the HDR (Karson et al. 1992, 2002a) but less
obvious at the BTF (Karson et al. 2002b). A similar geometry can be inferred for
dikes at Hole 504B (Becker et al. 1988).
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Distributed Fracturing
The most pervasive mode of deformation is distributed fracturing. Rather than discrete faults, distributed fracturing occurs over broad areas of many tens of square
meters. The boundaries of these intensely fractured intervals grade into less fractured lavas and dikes. The geometry of these volumes of fractured material is not
yet well known, but they do not appear to have a preferred size, shape, or orientation. They appear to engulf large volumes of less damaged upper crustal material.
Displacements across these areas have not been measured because of a lack of
suitable markers; however, they clearly accommodate relative block rotations of
10◦ –30◦ . Blocks of less damaged material in these areas include variably fractured
lavas ± dikes in the lower lava unit and panels of variably tilted and fractured
sheeted dikes at deeper levels.
Areas of intense cataclasis or “crush zones” have been described in dike complexes of ophiolites, oceanic drill holes, and other environments with dense dike
intrusion (Agar & Klitgord 1995, Dilek 1998, Karson & Brooks 1999, Williams
& Malpas 1972). Exposures at the HDR and BTF show that these areas are not
related to a decoupling horizon, as suggested for the Troodos Ophiolite (Agar &
Klitgord 1995), but rather that most of the lower lavas, transition zones, and sheeted
dike complexes are laced with a complex network of these “fuzzy” fault zones
(Figure 9).
Faulting
A third important group of deformation structures are fault zones parallel or at very
low angles to dike margins. These suggest that domino-style rotations of dikes or
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
−
Figure 9 Schematic diagrams of spreading geometry at intermediate- to fastspreading ridges (Karson et al. 2002a,b). Despite differences in spreading rate, crust
generated at these types of spreading centers have similar upper crustal structures.
Lavas are thicker in crust formed at slower rates. Note that most of the volcanic succession is erupted very close to the ridge axis and subsides rapidly as it is buried
beneath a veneer of relatively undeformed lavas. The upper lavas are erupted on the
ridge flanks or flow down the flanks from the axis. Below the weakly faulted uppermost
lavas, lower lavas with inward-dipping flows and the sheeted dike complex with dominantly outward-dipping panels of dikes, are intensely fractured. Subaxial subsidence
and vertical mass transfer create “accommodation space” for the growth of the lava
unit, which is typically 500–100 m thick. Dike intrusion, block rotation, and fracturing are the dominant deformation mechanisms. Local high-level, vertical dike swarms
mark intervals of intense magmatic construction with both vertical and lateral injection
of magma. Underlying gabbroic rocks are weakly deformed and probably crystallized
during or after the brittle deformation of the overlying units but still beneath the axial
summit region. Normal faults bounding abyssal hills may cut the entire upper crustal
assemblage and superimpose outward tilts on these structures. Ornament as in previous
figures. No vertical exaggeration.
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panels of sheeted dikes is another common structural element. Dike orientations are
highly variable between panels of subparallel, outward-dipping dikes, compatible
with rotations that would accommodate subaxial subsidence as the extrusive layer
spread laterally. Dike-parallel slip is the main mechanism by which panels of
sheeted dikes are internally deformed. Block rotation by slip on dike margins is
well known in extensional regimes including ophiolites (Varga 1991), where dikes
impart a strong mechanical anisotropy to the crust.
Given the relief and lateral continuity of rift valley bounding faults, abyssal hill
lineaments, and inward- and outward-facing fault scarps mapped on the seafloor
(Carbotte & Macdonald 1994), it would be reasonable to expect to find major faults
cutting the upper oceanic crust. Large continuous faults have not been mapped in
either the HDR or BTF scarps, even where they are intersected by abyssal hill
lineaments. This may be a function of the spacing of dives and other detailed
surveys along the scarps. High-resolution side-scan sonar surveys do not resolve
these types of features either. At the HDR, a single large, outward-dipping, dikeparallel fault zone was found. This fault zone is marked by hydrothermally altered
fault breccias a few meters thick within a wider interval of faulted and slickensided
dike rock. It is not clear how this fault zone is related to the structural evolution of
this crustal section.
Development of the Sheeted Dike Complex
The sheeted dike complex is a composite assemblage of incremental intrusions that
probably formed before, during, or possibly after most of the brittle deformation
of the upper crustal units. Dikes were probably initially intruded in a very narrow
region beneath the spreading axis, a region substantially narrower than the width
of the neovolcanic zone (e.g., Kidd 1977). Initially, vertical swarms of dikes were
fragmented and rotated to create the array of tilted panels of dikes seen in the HDR
and BTF outcrops and probably drilled at Hole 504B.
Massive, weakly jointed dikes cut across the shattered and rotated dikes in the
sheeted dike complex as well as intensely fractured basaltic material in the lower
lavas and transition zone in all three areas. The geochemistry of even relatively
late, cross-cutting dikes correlates strongly with that of the rest of the lavas and
dikes in all three areas (Naidoo 1998, Sparks 1995, Stewart et al. 2002), suggesting
that the entire assemblage was generated at the nearby spreading center. Off-axis
volcanism does not appear to have been significant in any of these areas. Thus, the
rotation and intense fracturing of upper crustal material is inferred to have occurred
beneath the spreading axis, or at least within the limits of axial dike intrusion.
As noted above, the vertical separation between the deepest lava screens and
shallowest gabbro screens in the sheeted dike complex at the HDR is only 200–
300 m. This relationship might be related to a number of different possible scenarios: Observations of dramatic upward protruding, relatively late dike swarms and
sparse dikes in deeper gabbros suggest that the vertical dimensions of the sheeted
dike complex grew substantially during accretion of the overlying lavas. Dikes
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may have been added by vertical or lateral injection or both, and increased the
initial vertical extent of the dike complex. It is also possible that the lava and gabbro screens may have been brought closer together by substantial crustal thinning
in this interval that is now masked by later dikes. Yet another possibility is that a
high-level gabbro sill was intruded in the dike complex prior to the intrusion of
the latest dikes. Geological details required to explore these possibilities are not
yet available.
Very little in the surface geology or geophysical expression of fast- to
intermediate-spreading ridge axes suggests that the type of complex structures
described above are developing beneath their rather subdued axial surfaces. The
upper crustal geology at the three sites discussed here provides evidence of a dynamic tectonic environment in which brittle deformation and vertical mass transfer
akin to caldera collapse are major processes. Syntectonic intrusion of dikes and
extrusion of lavas have resulted in an exceedingly complex geologic structure that
has been anticipated by some models but is not apparent in most ophiolites.
LATERAL (TEMPORAL) VARIATIONS AND LITHOLOGIC
ASSOCIATIONS
Variations in the geologic structure of the uppermost oceanic crust occur along
the length of both the BTF and HDR scarps (Figure 4). Because these scarps
are oriented parallel to seafloor spreading flow lines, these along-strike variations
may be interpreted in terms of temporal variations in spreading processes. This
of course assumes that these cross sections are not intersected by propagating
rifts, overlapping spreading centers, or off-axis eruptions that might result in age
reversals or other structural complications in these sections.
In both of these areas there appear to be tendencies for specific types of geological features to be grouped together. Although sampling and mapping are incomplete, there appears to be a more or less continuous spectrum of these variations
between two general end-members. At one extreme, structures suggest tectonic
deformation and subsidence/thickening of the lava unit, whereas at the other, robust magmatic construction, dike intrusion, and lateral spreading dominate. These
different types of spreading are probably a function of both magma budget and
magma pressure. The basic characteristics of these end-members are outlined below (Figure 10).
Low Magma Pressure
Most of the crustal sections at both the HDR and BTF appear to have been constructed under conditions of relatively low magma pressure. This may be related
to their specific tectonic settings wherein decreased magmatism is anticipated. In
these areas, thick lava piles are intensely fractured and have moderately to steeply
inward-dipping lava flows. Dikes are sparse in the lavas and the transition zone
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Figure 10 Schematic (two-dimensional) model for end-member modes of spreading suggested by major variations in upper crustal structure seen in tectonic windows. (a) Low magma
budget (low magma pressure) with limited, relatively deep-level dike intrusion (vertical or
lateral) resulting in subsidence of overlying units accommodated by fracturing and block
rotation. Subsidence permits the volcanic layer to thicken relative to the sheeted dike unit.
(b) High magma budget (high magma pressure) results in high-level dike swarms (fed vertically and/or laterally), mainly lateral spreading, and a relatively thin volcanic layer. Undeformed high-level dikes cut highly fractured overlying lavas and dikes. (a) and (b) may also
correspond to structures at segment ends and centers, respectively. Symbols as in previous
figures. Note that the axial topography is greatly exaggerated (after Rivizzigno et al. 1999).
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is relatively sharp. Dikes in the lavas and the sheeted dike complex are fractured
and dip outward (Figure 10). Although sampling is limited, these types of areas at
the HDR appear to have rather diffuse, relatively low-temperature hydrothermal
metamorphism (Gillis et al. 2001). These characteristics suggest subsidence and
thickening of the lava units. Although several different mechanisms could influence this subsidence (Figure 8), the dike distribution suggests that deep-level dike
intrusion (limited by magma pressure?) may have been the dominant mechanism.
This type of structure might be expected to form most commonly near segment
ends or during the waning stages of magmatic events. The section drilled at Hole
504B appears to have this type of structure.
High Magma Pressure
A few areas, especially well defined at the HDR scarp, have a somewhat different
crustal structure. In these places, the lavas are thinner (<500 m) and not so highly
fractured. There are abundant dikes in the lavas including high-level swarms of
subvertical dikes. The highest temperature metamorphic assemblages have been
found in these areas (Gillis et al. 2001). This type of structure could be interpreted
in terms of dominantly shallow dike intrusion driven by elevated magma pressure. High-level dike intrusion would result primarily in lateral spreading of the
uppermost lavas, with only limited subaxial extension and subsidence, perhaps as
surficial fissure swarms (Figure 10). The delivery of magma via closely spaced
intrusions would also cause local heating of the crust and strongly focused hydrothermal activity in these areas. This type of structure might be expected to form
most commonly near segment centers or during the waxing stages of magmatic
events.
Correlation with Axial Features
Intermediate- to fast-spreading ridges vary significantly in their along-axis morphologies from broad axial highs to narrower or rifted examples. These forms
vary systematically with respect to along-axis discontinuities, but it is not yet clear
if or how individual segments also vary with time. Several studies have sought
to link axial morphology with other parameters such as axial magma chamber
dimensions, hydrothermal activity, etc. (Carbotte et al. 2000, Macdonald 1998).
Some authors conclude that the key factor is magma supply, whereas others emphasize magma pressure. The variations in the internal structure of the uppermost
crust noted above suggest that both of these are important considerations. Magma
pressure can control the height of dike intrusion, and hence, whether an individual
dike intrusion event will result mainly in lateral spreading (high-level intrusion)
or subsidence (deep-level intrusion) of the upper crustal units. However, for multiple dikes to be intruded in high-level swarms, the volume of magma available
must also be relatively large over a short time interval. Areas with thinner lavas
and high-level swarms likely correspond to broader, inflated ridge morphologies
with relatively shallow, robust magma chambers, and areas with thicker, subsided
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lava piles would be generated beneath rifted or deflated morphologies with deeper,
smaller magma chambers.
SUMMARY AND IMPLICATIONS
Geological observations provide direct evidence of the internal structure of the
uppermost oceanic crust formed at fast- to intermediate-spreading rates and, by
inference, processes that created it at corresponding oceanic spreading centers.
Key features include (a) inward-dipping lava flows varying from near horizontal near the seafloor to steeply dipping at depth, (b) outward-dipping dikes in
variably rotated and deformed panels of subparallel dikes, (c) variable and locally very intense fracturing and faulting through most of both the lava and
sheeted dike units, and (d ) syn- to postdeformational cross-cutting dikes. This
structural assemblage differs from traditional reconstructions of the internal structure of ophiolite complexes, raising some difficult questions: How accurate are
the reconstructions of ophiolite complexes? Are reference frames, such as assumed vertical dikes, appropriate in all cases? Are ophiolites generated in accretionary environments that are somehow different from relatively fast-spreading
ridges? Are the available areas known from tectonic windows and Hole 504B
representative of crust produced at these rates? The answers to these questions
must await the next generation of investigations in both oceanic crust and
ophiolites.
The structural details described above are mostly well below the resolution of
most marine seismic investigations, but the intense fracturing found at all three
sites would probably strongly influence the seismic structure of the uppermost
crust. For example, many investigations interpret seismic layer 2A, at the top of
the crust, as a relatively low velocity layer corresponding to basaltic lavas with
a high porosity inherent to their eruptive style (Carbotte et al. 1997; Christeson
et al. 1996, 1994; Hooft et al. 1997). Underlying higher velocity is thought to
have lower porosity resulting from hydrothermal sealing and dike intrusion. Intense fracturing related to the bending and subsidence of the upper crustal rock
units would create high fracture porosity and hence lower velocities in all upper
crustal rock units regardless of their mode of emplacement. The base of seismic layer 2A might be defined by the depth of unhealed fractures that cut across
both lavas and dikes. The thickness of lavas in all three areas does not compare
well with typical thickness of layer 2A in mature crust (Christeson et al. 1994).
Typical seismic layer 2A thickness of <600 m would include both upper and
lower lavas as well as dike swarms and even possibly sheeted dikes at the HDR
and BTF. Thus, using seismic layer 2A as a proxy for the thickness of extrusive
basaltic units is likely to be inaccurate. Furthermore, the significant variations in
the thickness of lava units along the HDR and BTF scarps occur over <1-km
distances (Figure 4) and are unlikely to be resolved by marine seismic studies.
The seismic structure of the upper crust is likely to be defined by a combination
of factors including constructional and fracture porosity that lower velocities and
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dike intrusion and hydrothermal mineralization that lower porosity and increase
velocities. The spatial patterns of these features in the upper crust appear to vary
in a complex way in depth and along flow lines. Although their integrated effects
may result in a broad-scale (hundreds of meters) horizontally layered structure,
this generalized view probably masks significant geological variations that reflect
important variations in spreading processes. For example, 1 km along a flow line
corresponds to ∼15,000 years for the HDR (and ∼30,000 years at the BTF or crust
near Hole 504B), and significant spreading events may occur within these time
intervals.
Increased fracture porosity in the upper oceanic crust is also of interest from
a biological perspective. Recent investigations suggest that microbial life inhabits
the pore space in the upper oceanic crust (Kelley 2002). Extensive fracture porosity
in the upper oceanic crust greatly expands the possible range and pervasiveness of
space that might be occupied by this deep biosphere.
In retrospect, the contributions of marine seismology and ophiolite studies have
been extremely valuable in shaping the understanding of the oceanic crust. These
ideas have in turn contributed to the view of processes occurring at mid-ocean
ridge spreading centers. Recent direct observations of the oceanic crust bring a new
perspective and new details to light, but with possible pitfalls. Tectonic windows
may not be exposing representative crustal geology and even deep drilling that
may sidestep this concern may miss important features because of low recovery
rates and the very limited area sampled. In any case, more data from an increasing
number of sites will help determine what is normal and what is an anomaly.
Although increasingly detailed investigations are possible in seafloor outcrops,
limitations on sampling and observations persist. The continuing cross-pollination
of ideas and data between seafloor observations and ophiolite studies has been
extremely productive and will be required to make the next generation of advances
in understanding the details of seafloor spreading.
ACKNOWLEDGMENTS
The data and interpretations presented in this paper stem from so many studies and
collaborations that it would be impossible to adequately acknowledge them all.
John Delaney, Debbie Kelley, Maurice Tivey, Thierry Juteau, and Monie Naidoo
generously provided input on the Blanco Transform area. Steve Hurst, Emily Klein,
Bob Varga, and the Hess Deep ’99 Scientific Party all contributed to the collection
of data and development of ideas presented here. Peter Lonsdale raised many
difficult questions for me concerning Hess Deep and thereby improved the end
result. Without the sustained and meticulous collection, analysis, and reporting
of drilling results by DSDP and ODP scientists for Hole 504B, there is no way
those results could be viewed in the context of more recent studies. Thanks to
Kevin Burke for his very helpful review of this paper. This particular synthesis
was made possible with support from National Science Foundation grant OCE
907308.
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Visit the Annual Reviews home page at www.annualreviews.org
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