Jul 10, 1984 - and Lundquist, 1982; Nicolas and Violette,. 1982]. Olivine and ..... Anderson, R. N., J. Honnorez, K. Becker, A. C.. Adamson, J.C. Alt, R.
JOURNALOF GEOPHYSICALRESEARCH,VOL. 89, NO. B7, PAGES 6126-6138,
JULY 10,
1984
GEOLOGIC AND SEISMIC VELOCITY STRUCTURE OF THE CRUST/MANTLE TRANSITION IN
THE BAY OF ISLANDS J.
A.
Karson
and
OPHIOLITE J.
A.
COMPLEX
Collins
Department of Geology and Geophysics, WoodsHole OceanographicInstitution J.
F.
Casey
Department of Geosciences, University corrections
Abstract. Geological investigations of the Bay of Islands Ophiolite Complex show that while a typical layered ophiolite suite is present, the thickness of major lithologic units is extremely variable from place to place. The composition
and internal
structure
tions
("Moho")
transition
reconstructed
plex internal transition
for
seismic structure.
from mafic
meters)
terrane
com-
Recent
results
over
lateral
from well
distances
exposures of ancient
oceanic
that conside.rable variations
lithologies
thicknesses
from a sharp geologic
discontinuity
structural
varia-
controlled
of
tens
of kilo-
meters regardless of spreading rate or age [Purdy, 1982, 1983; Bratt and Purdy, in press, !984]. Alternatively, studies of ophiolites, subaerial
The crust/mantle
to ultramafic
varies across the ophiolite and seismic velocity
this
suggest an extremely
two-dimensional
seismic refraction experiments suggest that the seismic structure of oceanic crust is laterally homogeneous on the seismic scale (a few kilo-
of map-scale (kilometers
lithosphere
for
in sediment thickness and basement topo-
graphy.
across) lithologic units as well as the contacts that bound them are laterally variable. Inferred velocity-depth functions of the crust/mantle as oceanic
of Houston
on scales
lithosphere,
in lithologic
of centimeters
suggest
unit
to kilometers
may b• present in contemporary oceanic crust.
to a complexly
Reconstructions
of the velocity
structure
of
interlayered transition zone as muchas 3 km thick. The mafic/ultramafic transition is charac-
ophiolites have been used to help interpret the seismic structure of oceanic regions [Peterson et
terized
al., 1974; Salisbury and Christensen, 1978; Spudich and Orcutt, 1980; Nichols et al., 1980; Hale et al., 1982; Kempher and Gettrust, 1982a,
units
by laterally on
the
kilometers
long.
order
thick
discontinuous of
hundreds
of
lithologic meters
and up to several
to
a few
kilometers
These units (megalenses) may be composed
b]. The ophiolite analogy must be applied cautiously, however, because there is a disparity in
of lithologies with higher or lower seismic velocities than those of the surrounding units. Solid-state deformation has produced seismically anisotropic materials in the upper mantle and lower
crustal
units.
Substantial
slopes of up to at least
relief
12ø over lateral
the
and
to the understanding
and are signifi-
of crustal
observation
used
in
seismic
studies
averages
over
lateral
distances
on the scale of the seismic experiment, typically tens of kilometers, whereas laboratory measurements of rock velocities are usually carried out on cylindrical samples only several centimeters long and 1-2 cm in diameter. Cracking of the upper oceanic crust on scales larger than the
geologic unit contacts. Although these types of geological features are striking to the field cant
of
ments represent
dis-
tances of about 10 km occur on the top of the crust/mantle transition zone as well as other
geologist working in ophiolites
scales
and those used in field and laboratory studies of ophiolites. Seismic refraction velocity measure-
accretion
scale
processes, many of them occur on such a small
of laboratory
samples results
in seismic
velocities of the shallow crust being significantly lower than laboratory values measured at the appropriate confining pressure [Schreiber and Fox, 1977; Kirkpatrick, 1979; Salisbury et al.,
scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere.
•979]. Introduction
However, at crustal depths greater than a few kilometers, where increased lithostatic pressure results in crack closure, the mineralogy and texture of the various rock units present are considered to be the dominant factors controlling the velocity structure. Laboratory measured velocities of lithologies expected at deep crustal and upper mantle levels are similar to the seismic velocities reported for the deep crust and upper
Our understanding of the seismic structure of the oceanic crust has evolved from constant veloc-
ity,
"layer
cake" models to more complex models
involving velocity and lateral-velocity
Spudich and Orcutt,
gradients, low-velocity zones, heterogeneities [e.g.,
1980].
Purdy [1982, 1983],
however, has pointed out that our knowledge of the lateral variability of seismic structure on scales of tens of kilometers has not kept pace with
the
above
advances.
He has
cautioned
mantle [e.g., Spudich and Orcutt, 1980; Fox and Stroup, 1981]. Thus the geological and velocity structure of the middle crustal to upper mantle portions of ophiolites may serve as a useful guide in revealing the type of seismic structure that should be expected in marine seismic experiments. In a general sense, the crust/mantle transition or Moho must correspond to the depth at which mafic lithologies with crustal velocities (less than 48.0 km/s) give way to ultramafic litho-
that
apparent lateral variability might well be an artifact of poor experiment location in relation to fracture zones and incomplete knowledge of
Copyright
1984 by the American Geophysical Union.
Paper number 4B0522. 0148-0227 / 84 / 004B-0522 •05.00
logies with upper mantle velocities 6126
(generally
Karson
et al.:
Structure
of the
>8.0 kin/s). The geological expression of the Moho as seen in ophiolites indicates that the crust/mantle transition is far from a simple lithologic contact. In this paper, we present
evidence for complex lateral and vertical variability in the geologic and seismic velocity structure
at the fossil
crust/mantle
transition
(Moho) preserved in the Bay of Islands Ophiolite. We first present a description of the Bay of Islands Ophiolite with emphasis on geologic features which may influence seismic velocity structure
in the lower crust
and upper mantle.
Rock
velocity data are then used to define the seismic
Crust/Mantle
initially
detached from a ridge-transform
farther Massif
the Coastal Complex. Structural continuously and systematically nants of the ophiolite slab and the context of the preobduction temporal and spatial variations
logy and seismic velocities at the crust/mantle
Vertical
transition
the Ophiolite
structure
of oceanic
inter-
from the fracture zone. The Lewis Hills contains the contact between the BOIC and
of this
of the seismic
6127
section along the inactive portion of a fracture zone. The lithosphere within the slab becomes progressively older from SW to NE. The geology of the individual massifs and the apparent displacements across major tear faults suggest that from SW to NE the massifs come from progressively
velocity structure at different points in the ophiolite. Finally, we discuss the possible implications of the observed variability im geofor the interpretation
Transition
slab
of oceanic
Section
variations occur across the remwhen viewed in geometry, reflect in the evolution
lithosphere.
Through
Complex
lithosphere.
In general, the BOIC has a relatively Geology of the Bay of Islands
Complex
stratiform
internal
structure
well-developed ophiolite
simple
that is typical
of
complexes. From base to
top, the layered sequemceconsists of layered
Geologic Settimg
ultramafic
TheBayof IslandsOphioliteComplex (BOIC)is
tectonites
(mostly harzburgite),
layeredultramaficcumulates (dunites,wehrlites,
oneof the world'sbest exposed,complete sections clinopyroxemites, etc.), interlayeredmaficand of ancientoceaniclithosphere[Church and ultramaficcumulates (locally referred to as the
Stevens,1971;Dewey andBird, 1971;Williams, "mafic/ultramafic transitionzone"whereit 1973;Casey et al., 1981;Casey andKarson,1981]. represents an intervalgrading downward from100%
The BOIChas excellent outcrops of the entire
to 100%ultramafic rocks), layered mafic cumulates
dikes, isotropic and layered cumulategabbroic rocks, layered cumulateultramafic rocks, and
metagabbro,etc.); variably metamorphosed sheeted diabasedikes; andvariably altered basaltic
in a continuous
lower parts of the cumulate units are variably
sequence of lithologic units that constitutethe ophiolite suite' pillow lavas, sheeteddiabase residual ultramafic tectonites. stratiform
recognized in ophiolites Conferemce Participants,
These units occur
succession
that
is
worldwide [Penrose 1972]. The BOIC crops
(gabbro,olivine-gabbro,troctolite, etc.); nonlayeredgabbroicrocks(gabbro,hornblende-gabbro,
pillow lavas and minor sediments (Figure 3). tectonized conditions
out in a series of allochthonous klippen which represent the dissected remnants of a once continuous slab of oceanic lithosphere at least 100 km long and 20 km wide (Figure 1), and is a highlevel nappe in the Taconic Humber Arm Allochthon
al.,
of the western Newfoundland Appalachians. It lies structurally above a compressionally telescoped
parallel contacts
Cambro-Ordovicianrifted
(Atlantic
type) conti-
The
under anhydrous, high-temperature [Karson, 1977; Casey, 1980; Casey et
1981].
Im the reconstructions
presented
assumptions have been made. First,
here,
two
undeformed
diabase dikes in the complex are assumed to have formed in a vertical attitude approximately
to a spreadimg ridge axis. Second, between major rock umits in the BOIC are
taken as approximately horizontal over short
nental margin [Williams, 1973, 1975]. The age of the ophiolite is determined by isotopic dates on zircons from the gabbroic units at 485 + 2 Ma
distances (about 100-300 m) in order to construct columnar sections. The fracture zone assemblage formed in an approximately vertical, dextral
[DunningandKrogh, 1983] althoughsomewhat older ages (4504 Ma) have also been reported [Mattinson,
shear-zoneprior to the formation of adjacent parts of the BOICalong its eastern edge (Figure
1976; Jacobsenand Wasserburg,1979].
A basal
2; Karson [1977], Karson and Dewey[1978], and
metamorphic aureoledatedat 470+ 5 Maby 40Ar/
Karson[1984]).
marks the approximate age of obduction of the complex. Within the ophiolite slab a steeply dipping
ed through recent 1:15,000 scale mapping of the ophiolite allochthon and related structural studies [Karson, 1977, 1982; Casey, 1980; Casey
39Ar techniques [Dallmeyer and Williams, 1975]
These geometrical relations have been establish-
linear belt of variably deformedand metamorphosed and Karson, 1981; Casey et al., 1981, 1983]. They ophiolitic lithologies, called the Coastal Comdiffer from someassumptionsused by previous plex, is interpreted as a fossil oceanic fracture zone that transects the ophiolite allochthon [Karson and Dewey, 1978; Karson, 1982, 1984;
workers [Salisbury and Christensen, 1978; Christensen and Salisbury, 1982] that have been shown to be in error due to lack of geological
Karson et al.,
1983]. The geometric relationships
control [Casey et al.,
of structures
in the Coastal Complex and BOIC help
1979; Caseyand Karson,
1981; J. F. Casey et al.,
unpublished manuscript,
provide someimportant constraints on the evolution of this terrane as well as the time/space
1984]. The present reconstruction of the crust/ mantle transition (shown in preliminary form in
relations amongthe separate outcrop areas during formation (Figure 2; Karson [1977,1984], Casey and
the work of Casey et al. [1981]) reveals a much more complex structure than that indicated by earlier studies.
Karson [1981], amdCasey et al. [1983]). From the internal geometric relations of the
ophiolite it appearsthat the ophiolite slab was
The middle crustal to upper mantle sections of the oceanic lithosphere are preserved across the
6128
Karson et al.:
Structure of the Crust/Mantle Transition
EXPLANATION
PARA-ALLOCHTHONOUS SEDIMENTARYROCKS
)'.•
Shale, Sandstone, M•lange
BAY OF ISLANDS OPHIOLITE COMPLEX
•
BasalhcPillowLavas
•
Sheeted Dlabase Dike Complex (hnes show mean dike trends)
•--_• Non-layered Gabbrolc Rocks --E• LayeredGabbrmc Rocks
•
Interlayered Maflc/Ultramafic Cumulates ('Transition Zone')
• Massive Dumte Cumulates :.[•'•-] ß Layered UltramaficCumulates qL--•']Harzburglte Tectomtes COASTAL
•
MOUNTAIN
COMPLEX
Amph•bolHe, Metagabbro, Quartz-D•or•te, Serpentlnlte, Dlabase, Basalt ALLOCHTHONOUSAND PARA-AUTOCHTHONOUS
SEDIMENTARY
[•
ROCKS
ARM
Sandstone, Shale,Limestone
• •
H•gh-Angle Fault Thrust Fault
½
BasalMetamorphic Aureole
•
UnconformHy
.
o ---_.J
5
BAY
OF
I SL
A AID $
io ß
KM
BLOW ME DOWN MOUNTAIN
LEWIS HILLS
Fig. 1. entire
Generalized geological mapof the Bay of Islands Ophiolite Complex.
length of the BOIC. The upper parts of
the section have been removed by erosion in some places. In order to reconstruct the geologic
structure in various places it is necessary to assume some average
upper lithologic restored
parts
crustal
units of
the
structure
where
have been lost. sections
the
The
determine
the
crustal depth assigned throughout each vertical section and therefore the lithostatic pressure at any given point. In order to do this we have attempted to estimate the missing portions of the
sections by assuminga similar geologic structure to nearby exposures.
It
is possible
that
changes in lithostatic pressure do not cause significant variations in seismic velocity for rocks at pressures corresponding to middle to deep crustal levels (i.e., >100 MPa).
the
contacts between the missing upper crustal units had relief that has not been recognized, resulting
in errors in estimates of crustal thickness in the incompletesections. Small errors in these
reconstructions makelittle difference in confin-
ing pressure(43 MPaper 100m) andsuchminor
Lateral
Variation
in
the
Internal
Structure of the 0phiolite
The geologic contacts shownin Figures 1 and 3 are not as simple as they appear.
Some contacts
tend to be sharp (occuring over less than a meter) whereas others are gradational (ocurring over tens to hundreds of meters). In most cases, individual contacts vary in style and may change character over distances as small as a few tens of meters along strike. Discontinuousminor units, not
shownhere becauseof their small scale (less than
a few tens of metersthick), pinchout andswell
alongsome contacts. Complex interlayeringof
Karson et al.:
Structure
of the Crust/Mantle
/
6129
result of unevenly distributed layering, minor intrusive bodies, ductile shear zones, and mineralogic variations. Thus even the mappable
lithologic their
0
Transition
50km ,/•/
units referred to here are variable in
internal
structure
and composition.
Even the most laterally continuous map units tend to pinch and swell across the ophiolite allochthon. +1.0
Thickness
km and
more
variations
occur
both
on the
across
order
massifs and on a broader scale with proximity the Coastal Complex fracture zone assemblage. Thus thickness
/
It
/ ,/ / 2.
tectonic
both
along
and
1981].
is not clear
which,
if
any,
of the contacts
mappedthrough the BOIC can be used as a horizontal
datum.
Here
we have
chosen
to use the
of the depleted upper mantle (harzburgite)
•
Schematic map representation
obduction
occur
to
across the spreading direction. This implies that the individual geological units, although usually laterally continuous, have complex, laterally variable, undulating upper and lower surfaces
[Casey et al.,
Fig.
variations
of
individual
setting
top
sec-
of the pre-
of the Bay of Islands
SPFe. E'ADING DIF•E'CT/ON
Ophiolite Complex [after Karson and Dewey, 1978; Karson, 1983; Casey et al., 1983]. The BOIC is a comparatively simple stratifo• ophiolite sequence.
The adjacent
CC is a variably
deformed
INTRUSIVE/EXTRUSIVE CARAPACE
and metamorphosedophiolite suite that is interpreted to be derived from an oceanic fracture
(MogmoChomberRoof)
zone,
lithologies
(Figure 4) occurs at the contacts
between the lower crustal mantle
(ultramafic)
units,
between major lithologic
(gabbroic)
- MAGMATIC
and upper
and the contacts
PLUTONIC
units are defined by the
Wells ond Floor)
points at which greater than 90%of a single lithology is present. In the case of the mafic/ ultramafic transition contacts are defined
COMPLEX
(MegmeChomberCeding,
zone, the upper and lower by the points at which the
proportionof maficto ultramaficrocksis 90% and 10% respectively. From the map (Figure 1) and sections it is
obvious that while a particular lithologic unit
RESIDUAL UPPER MANTLE
may represent a largepartof thesection in some
places, itmay be entirely absent inothers. discontinuous units pinch out gradually along
Typically, at deep crustal levels these laterally
BASAL
- METAMORPHIC
strike (both parallel and perpendicular to the inferred spreading direction) and tend to pinch
out near their in
upper or lower contacts
the middle
of another
rather
than
unit.
Within someof the major units,
AUREOLE
large-scale
Fig. 3. Generalized columnar section of the BOIC showingthe usual succession of lithologic units.
laterally discontinuousbodies ("megalenses") of (Ornament as in Figure 1, except t, trondhjemite higher or lowerdensity lithologies occur(Figure5). bodies; squigglesamphibolites.) Note that at The largest of these megalensesare a few hundred meters
thick
and a few kilometers
long.
They are
different locations, these units have variable thicknesses,
may be absent,
above and below.
as 1.0 •n/s
was formed
with
of interlayered
the enveloping
mafic and ultramafic
within potentially Lewis Hills rocks
are
units.
Megalenses
rocks occur
higher velocity dunites in the
Massif and megalenses of ultramafic found on the North
Arm Mountain
and
Table Mountain Massifs (Figure 1). (For detailed descriptions of these megalenses see Karson [1977], Casey and Karson [1981], and Elthon et al.
[1982]).
Within the map units we define here (Figure 3), considerable heterogeneity occurs. This is the
or may have different
types of geological contacts with other units
usually composedof interlayered lithologies that collectively have velocity contrasts of as much
from
The basal crustal
metamorphic aureole
rocks
welded
to
the
ophiolite during obduction. The "residual upper
mantle" harzburgites represent oceanic upper mantle depleted by partial melting and extraction of basaltic liquids. The "magmatic" units represent these basaltic
liquids
that
crystallized
as
parts of either a rapidly chilled intrusive/extrusive carapace that formed the roof of a magma chamber or the sequence of plutonic rocks that formed along the ceiling, walls, and floor of the magma chamber.
6130
Karson
et
al.:
Structure
of the
Crust/Mantle
Transition
Fig. 4. Fine-scale lithologic layering in the mafi½/ultramafi½ transition zone of the Bay of Islands Complex. Lighter layers are gabbroi½ and darker layers are •ltramafi½ in composition. Note the fine lamination on the scale of approximately 1 ½m or less and also thicker layers up to perhaps a few tens of centimeters thick. Layering on all scales tends to be laterally discontinous with an overall lensoid form and length
to thickness
ratios
of 10:1-100:1.
tion as a horizontal reference line (Figure 6). This results in significant relief on various unit contacts and locally steep slopes; however, major unit contacts line up well across the entire complex. Assumingapproximately a 1.0-1.5 km thickness of volcanics plus dikes for all sections from the
3-7),
Blow me Down and North
Arm Massifs
no unreasonable seafloor
in those
areas.
(sections
relief
Reconstructions
is implied
of the upper
portions of sections from the Lewis Hills
and
Table
there-
Mountain
Massifs
cannot
be
fore do not help to constrain
made
and
the relative
verti-
cal alignment of those sections. The contact along the top of continuous ultramafic sections, corresponding to the top of the
upper mantle, has relief km within
individual
distances
of
less
of as much as nearly 3.0
massifs; than
about
that 15-20
is,
over
km.
This
produces slopes of as much as 15 • between the columnar
sections
shown.
On a scale
more compar-
able with seismic refraction experiments (i.e., a few tens of kilometers), slopes of 5•-10 • are common. Locally, however, steep slopes exceeding 30 • occur. We wish to emphasize that these relationships are not the result of faulting or folding related to obduction of the complex. These are
structures
that
once
existed
in
the
oceanic
crust. The slopes and topographic features can be smoothed out to some extent by elevating or depressing the groups of columnar sections from the individual massifs relative to one another; however, this only imposes relief on other contacts or on the paleo-seafloor. Adjacent sections
tive
within
the
massifs
to one another
cannot
be shifted
rela-
because they are not separated
See hammer for
scale.
by faults with large displacements across unit contacts. Only a minor amount of tilting might be tolerated. The columnar sections presented in Figure 6 represent the vertical lithologic successions at different positions in the ophiolite allochthon. They are arranged from SE to NW (left to right) from youngest to oldest. In a general way, the distance from the fracture zone (Coastal Complex) increases in the same direction from massif to massif. Thus collectively the sections describe the crustal structure along a transect that extends from near a ridge-transform intersection across isochrons into older crust at a low angle to the inactive portion of the fracture zone. The angle of the transect relative to the fracture zone is not well constrained but appears to be greater than about 20 •. Therefore, when viewed on a broad scale, the lateral variations in structure probably reflect a component of variability related to seafloor spreading at different distances
from
the
fracture
zone.
Sections
from
individual massifs are constructed from positions along seafloor spreading flow lines; that is, normal
to
the
mean
trend
of
dikes
in
that
area.
Variations within these groups of sections reflect temporal variations in seafloor spreading at a single position on a ridge axis. Mafic/Ultramafi½
The contact
Transition
between mafic
Zone
and ultramafi½
lith-
ologies in the BOIC, which should correspond to the Moho, is not a sharp contact between two distinct
rock units.
Instead,
it
is an interval
Karson
et al.:
Structure
of the
Crust/Mantle
Transition
6131
in the BOIC. Instead of a single,
smooth,
increase in ultramafics downward, there are several megalenses of interlayered mafic and ultramafic
rocks
enclosed
section of dunite.
within
a thick
few hundred meters
thick
and are separated
comparable thicknesses of dunite. layered
on a much coarser
ultramafic
(•3
km)
The megalenses are up to a
transition
scale
zones
the BOIC, we regard this
by
<hough interthan the mafic/
observed
Lewis Hills
elsewhere
in
assemblage
as an anomalously thick transition zone. This unusual structure may be a result of processes occurring durimg crustal formation near the
adjacent fracture
zone [Karson, 1977, 1982; Casey
and Karson, 1981]. Whether such a structure in the oceanic crust would appear to be a velocity
gradient rather than a zone of discrete
velocity
reversals would be determined by the nature the seismic experiment (see below). Scale
and
of
Distribution
of Lith01ogic Layering In nearly all
locations outcrop-scale layering
(a few centimeters
to a few meters
thick)
occurs
in the lower crustal and upper mantle lithologies. In the lower part of the gabbroic unit, the mafic/ultramafic transition zone, and the ultramafic cumulates, igneous cumulate layering
is ubiquitous although in someplaces these lithologies
have been affected
solid state deformation.
by high-temperature
Layer thicknesses range
from less than 1.0 cm (Figure
4) to several
tens
Fig. 5. Oblique air photograph looking due north across the Lewis Hills Massif. Scale is approxi-
of meters (Figure 5), and layering at different scales may dominate in different places. This layering is conspicuously discontinuous, and
mately four kilometers from south to north.
individual layers cannot be traced for more than
Steeply dipping, interlayered mafic and ultramafic
50-100 m along strike.
enclosed within cumulate dunite.
ness of ratios of 10'1 to 100'1.
Individual layers have an
lithologies define a north-south trending megalens overall lensoid form with typical length to thickMegalens is com-
posed of interlayered gabbroic (white) and wehrlite (dark) lithologies and has been offset by several east-west trending strike-slip faults. Dunite crops out both east and west of the megalens; that is, both structurally below and above it.
of finite
width
across which there
is a gradual
changedownwardfrom 100%mafic to 100%ultramafic rocks.
The overall
thickness
of this
interval
varies from less than 50 m (e.g., Figure 6, sections 3-5) to more than 1.0 km (e.g., Figure 6, section 6). Across this mafic/ultramafic transition zone there is a gradual increase in the
proportion and thickness of ultramafic relative
to mafic
layers
with
depth.
layers The most
con,non lithologies in this unit include dunite, wehrlite, clinopyroxenite, troctolite, anorthosite, olivine-gabbro, and gabbro. On an outcrop scale these rock types are intimately and complex-
ly interlayered.
On a broader scale (up to a few
hundred meters), megalenses of dominantly mafic or ultramafic lithologies occur. In some places, these wedge out abruptly along strike, whereas elsewhere they grade laterally into interlayered assemblages with different proportions of mafic to
ultramafic
between layers
may be gradational
or
(ratio layering), changes in the specific mineral phases present (phase layering) or a combination of any of these. The most commontype of layering is a type of phase layering with sharp contacts [Casey and Karson, 1981]. In some places, megalenses of interlayered rocks with a limited compositional range occur within map-scale layered assemblages. Where megalenses of interlayered mafic and ultramafic rocks occur
within ultramafic
cumulate units,
for example in
the Lewis Hills Massif, a very thick (•3 km) transition zone with first-order layering defined by the megalenses
Beneath this
occurs.
sequence of variably
deformed igne-
ous rocks the depleted upper mantle unit is also strongly layered. This unit consists mainly of harzburgite metamorphic tectonites with layering defined by layers a few centimeters to tens of meters thick with varying proportions of ortho-
pyroxene relative have gradational
orthopyroxenite
to olivine. contacts.
These generally Thinner
dunite
layers and crosscutting
and
veins up
a transition
to a few tens of centimeters are also present. These layers and veins tend to have sharp contacts with one another and the harzburgite country
seen
rocks.
materials.
The Lewis Hills Massif contains zone that is different from those
Contacts
sharp. Gradational contacts are defined by gradual changes in grain size, changes in the proportion of various mineral phases present
elsewhere
6132
Karson et al.:
Structure
of the
Crust/Mantle
Transition
A km
NORTH ARMMTN,
Oi 2 J- LEWIS HILLS
BLOW ME DOWN MTN.
3
-------
4
6
EXPLANATION
4
• '--'--,-------
PARAALLOCHTHONOUS SEDIMENTARY ROCKS MTN, BAY OF ISLANDS OPHIOLITE COMPLEX •i• •,•TABLE '.'.i-'•-] Shale, Sandstone, M•lange -• Non-layered Gabbrmc Rocks
5
::;)
"'-
MT Z
6
9
•
8
•
•
•
I• '•1 I_•.1
Sheeted Oiabase O•ke Complex (hnes show mean d•ke trends)
I '-[ 'J I•.•1
I'. '1
I0
Bosolhc PillowLavas
•
JO ••'F-•-'• Layered Gabbrmc Rocks M...... Oun,te Cumulates
•
'•
•
•
Interlayered Mafic/Ultromafic Cumulates ('Translhan Zone')
'•
Layered Ult.... fic Cumulates
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l Fig. 6. (a) Geological sections through various parts of the Bay of Islands Complex showing some of the variations that occur along the crust/ mantle (Moho) transition zone (MTZ) from SW to NE (left to right) across about 90 km of this slab of oceanic lithosphere. Geologic control for the sections comes from Karson [1977], O'Connell [1979], Casey [1980], Rosencrantz [1980], and J. Karson and J. Casey, (unpublished data , 1983). See Figure 1 for locations of sections. (b) Velocity-depth functions for
each of the columnar sections
shown in Figure
Serpentinization
6a.
See text
the gabbroic section is fresh and unaltered except along a few widely
At different
for explanation.
structural
levels of contemporary
oceanic lithosphere, different factors in addition to primary mineralogy may influence seismic velocity. At shallow crustal levels, cracks and fractures clearly play an important role. At deep
zones.
spaced fault
zones and shear
The ultramafic rocks in the ophiolite are
variably serpentinized, but the most intense alteration occurs along the basal thrust fault and other relatively late fault zones. In general the freshest ultramafics occur at the top of the
crustal and upper mantle levels, serpentinization
section.
of ultramafic material may lower seismic velocities and control the position of the Moho [Hess, 1962; Christensen, 1966; Clague and Straley, 1977; Nichols et al., 1980]. In the Bay of Islands
discontinuous with depth the serpentinization of the basal ultramafic section cannot be related to the same event(s) responsible for the alteration of the upper units. We consider it unlikely that hydration from lateral migration of fluids played
Complex, serpentinization,
although
now wide-
spread, appears to have been a very late (syn- or post-abduction) event, and therefore we have used the anhydrous velocities of the various rock units
in
our
reconstruction.
hydrated lithologies in the complex. The upper units (pillow lavas, sheeted dikes, and uppermost gabbroic rocks) are variably altered by hydrothermal
metamorphism similar
to that
described
in
[glthon and Stern, 1978] and in
DSDP cores [Anderson et al.,
an important role in alteration of the ultramafic rocks without affecting the immediately overlying mafic
rocks.
Although serpentinization
The evidence for the lack of significant preabduction serpentinization comes from several separate observations. Perhaps the most obvious indication is the regional distribution of
other ophiolites
Becausethe hydration of the complexis
1982].
The bulk of
late
fault
zones,
the earliest
is concentrated along movement on many
of these faults appears to have occurred prior to any pervasive serpentinization. Textures of rocks along these faults show fine-grained anhydrous ultramafic mylonites overgrown by static and/or deformed serpentine pseudomorphs and veins. Apparently, little if any serpentinite occurred in these areas during the initial stages of abduction. Furthermore, the basal metamorphic aureole welded to the sole of the ophiolite includes
Karson
et
al.:
Structure
of
the
Crust/Mantle
Transition
6133
high-grade amphibolites and pyroxene-hornfelses
[e.g.,
indicating temperatures in the adjacent peridotite in excess of the upper limits for serpentinite during that metamorphism. No dehydration (deserpentinization) textures have been found in any of the ultramafic lithologies examined, regardless of proximity to fault zones. Finally, preliminary results of a paleomagnetic
section of ultramafic and mafic cumulates are deformed and recrystallized with a similar structural style and metamorphic fabric orientation to that of the immediately underlying harzburgites [Karson, 1977, 1982; Casey, 1980; Casey et al., 1981, 1983; glthon et al., 1982]. In these areas oliv.ine-rich
study of chemical remanent magnetization of
highly
serpentinizedperidotites (J. CaseyandM. Titus, unpublisheddata, 1982) indicate that samplesfrom
rich or plagioclase-rich lithologies. Thus althoughmost of the ultramafic cumulates, transi-
various
probably not significant serpentinization of the ultramafic parts of the Bay of Islands Ophiolite
tion zone lithologies, and even parts of the layered gabbroic units are pervasively deformed the distribution of strain through these units is much more inhomogeneous than in the harzburgites. Thus we expect the same orientation of the anisotropy of the upper mantle rocks to occur in the lower crustal lithologies but to become progressively less important upward through the mafic/ ultramafic transition zone units. Garmany [1981] cites evidence for increased anisotropy with depth
prior to obduction.
in the upper mantle of the Pacific.
parts of the folded
and faulted
slab have a commonpaleopole.
ophiolite
This requires
that
serpentinization occurred after the obductionrelated folding of the complex. The paleopole determined is consistent with an Upper Ordovician
pole position for North America corresponding to the time interval These
during or just
observations
indicate
after
that
obduction.
there
was
A possible exception may have
occurred in the vicinity of the Coastal Complex fracture zone where serpentinites occur at all crustal levels and are cut by dikes that predate
obduction [Karson, 1984].
We wish to emphasize
that we do not interpret the metamorphic relations in the Bay of Islands Complex to be typical of all contemporary oceanic lithosphere, but rather as one possibility that is likely to exist. This particular structure is probably more typical of fast-spreading ridges. Crust formed at slowspreading ridges appears to be much more faulted and heterogeneous with even shallow-level
exposures of serpentinite
1971; Melson et al.,
[Aumento and Loubat,
1978; OceanographerTransform
that
George, 1978], parts of the lower crustal
ultramafic
rocks
tend
deformed than interlayered
such anisotropy
extends
to be much more
clinopyroxene-
at least
We suggest locally
up-
ward into lower crustal levels. This anisotropy has been considered in assigning the velocity bounds for the affected units in Figure 6. Velocity of
the
Structure
Crust-Mantle
Transition
The lateral variability in lithology within the Bay of Islands Complex (BOIC) results in a similar variability in rock velocity. Figure 6 shows the ten lithological sections which we consider to be representative of lateral variations that occur
across the ophiolite.
Compressional wave velocity
Tectonic Research Team, 1984].
Vp andshear wave velocity Vs profilesfor the crust-mantle transition in each of these sections
Anisotropy
are plotted as a function of depth immediately beneath the corresponding section. Reference to Figure 6 shows that we have not attempted to pre-
Laboratory measurements of mafic and ultramafic rocks with pervasive deformation fabrics have demonstrated their seismic anisotropy and the relationships between velocity and mineral pre-
ferred orientations
[Birch,
1960, 1961;
Christensen, 1966, 1971; Christensen and Ramananantoandro, 1971; Baker and Carter, 1972: Peselnick et al., 1974]. In ophiolite complexes the pervasively deformed and recrystallized upper mantle harzburgites have a fairly consistently oriented fabric that would produce a systematic-
ally oriented velocity anisotropy in the oceanic lithosphere [Nicolas et al., 1973; Christensen and Salisbury, and Lundquist,
Olivine
1979; Karson, 1982; Christensen 1982; Nicolas and Violette, 1982].
and pyroxene preferred orientations
produce maximumvelocities
the spreading direction dikes
in the ophiolite),
parallel tions
would
parallel
to
(normal to the trend of intermediate
velocities
to spreading centers or magnetic linea-
(parallel
velocities
generally
to ophiolite
in a vertical
dikes),
orientation
sent a single velocity-depth profile for each of the sections; instead we have presented an envelope of possible values. We justify this approach on the grounds that (1) most of the rock types considered are anisotropic, (2) Our depth scale could be in error locally by as much as 2.0 km
where sections are incomplete (Figure 6), and hence our choices
also be in error
of confining
locally,
pressures
(3) lateral
would
changes in
rock type such as varying olivine content in the gabbros occur rapidly and randomly • and are otherwise impossible to account for, and (4) the velocity envelopes in Figure 6 are narrow enough to reflect the major vertical lithological changes which are likely to have important implicatiu•L• for
the
seismic
structure
of
the
crust/mantle
transition, namely, the variable thickness of the transition zone between the layered gabbros and the underlying ultramafic components and changing
and slowest
crustal
(but
The V. and Vo profiles for the layered gabbro unit were constructed using the results of velocity measurements on samples of gabbro, olivine gabbro, troctolite and anorthosite from
see
thickness.
Elthon et al. [1982] and Casey et al. [1983]). These relationships are in agreement with the locally observed seismic anisotropy of the Pacific oceanic upper mantle [Morris et al., 1969; Raitt et al., 1969; Keen and Barrett, 1971; Shor et al., 1973; Bibee and Shor, 1976]. Obviously, some complications are expected near oceanic fracture
various parts of the BOIC shown in Table I [Salisbury and Christensen, 1978; Christensen and Salisbury, 1982; Karson, 1982; J. Karson and J. Casey, unpublished data, 1979]. Measurements were
zones [Karson, 1982].
carried out on both dry and water-saturated
In the BOIC, as well as many other ophiolites
samples of cylindrical
shape (several
centimeters
6134
Karson et al.:
Structure
of the Crust/Mantle
Transition
TABLE 1. Minimum and Maximum Velocity Bounds for BOIC Lower Crust and Upper Mantle Lithologies
Lithology
Minimum Vp
Layered Gabbro Wehrlite Dunite Harzburgite
In kilometers
Maximum Vp
6.7 7.9 8.2 8.1
Minimum Vs
7.3 8.4 8.7 8.5
per second.
See text
Maximum Vs
3.5 4.5 4.7 4.7
4.0 4.8 5.0 5.0
for explanation.
Fifty-five compressional wavevelocity Vp and26 shearwavevelocity V$
measurements.
Velocities
from Salisbury
and Christensen
[1978],
Christensen
and Salisbury [1979, 1982], Christensen and Lundquist [1982], Karson [1982], J. Karson and J. Casey (unpublished data,
long and about 1-3 cm in diameter) at room temperature and at confining pressures ranging from 1.5 to 2.5 kbars. The velocity difference between water saturated and dry samples is negligible at these confining pressures [e.g., Schreiber and Fox, 1977]. To allow for the effects of anisotropy, compressional and shear wave velocities were measured in three mutually perpendicular directions for 35 and 16 of the samples, respectively. The minimum compressional and shear wave
velocities
(6.7 and 3.5 km/s, respectively)
were
calculated by taking the mean of the velocity minima reported for each of these three-valued measurements and subtracting the standard devia-
tion.
Similarly, the maximum boundson Vp and
Vs (7.3 and 4.0 km/s, respectively) mined by taking
were deter-
the mean of the velocity
maxima
1979),
and Nichols
[1978].
Discussion
In the "layer cake" picture of the seismic structure of the oceanic crust, the crust/mantle boundary was defined by the abrupt vertical transition between compressional velocities in the range 6.9-7.3 km/s and compressional velocities greater than 8.0 km/s. This abrupt boundary has been referred to as the "Moho discontinuity" and has usually been interpreted as a rapid downward change in density and chemical composition from silicic and basic crustal lithologies to ultrabasic lithologies characteristic of the upper mantle.
It
may lie
has also
and correspond
tinized
been suggested
that
beneath the basic/ultrabasic to the boundary
the Moho
transition
between serpen-
and anhydrous ultrabasic
material
[Hess,
and adding the standard deviation. Wherever possible, measurementsmadeat confining pressure of
1962; Claque and Straley, 1977; Nichols et al., 1980]. As seismic aquisition and interpretation
1.5 and 2.5 kbars were used to construct
methods have improved,
the lower
refraction
studies
have
andupperbounds onVpandVs, respectively. shown that theoceanic crust/mantle boundary is, Twelveof the 18 single-valued Vpmeasurements in manyareas,bestdescribed as a smooth gradiand 18 of the 20 single-valued Vs measurements
ent between the above velocity ranges, suggest-
fall within the given bounds. For simplicity we have modeled the velocity strucure of the transition zone as a linear gradi-
ing a gradual transition between basic and ultrabasic lithologies. This Moho transition zone can vary from 0 km to 2 km in thickness and is typi-
ent between 100% layered
cally
gabbro, and 100% dunite
or harzburgite. The velocities for the dunites and harzburgites (confining pressure = 2.0 kbars)
modelled as a linear
velocity
gradient.
On
the basis of seismic refraction studies, marine seismologists have also identified both low- and
are reconstructed (anhydrous) rather than measured high-velocity zonesat the base of the crust (serpentinized) values and are based on the work [Sutton et al., 1971; Lewis and Snydsman,1977; of Christensen and Lundquist [1982]. These reconstructed velocities are generated by using the known modal composition of these rocks and the known velocities of their constituent minerals [e.g., Christensen and Salisbury, 1975]. The V_
andVs envelopes for the dunitelayer are 8.2-• 8.7 km/s and 4.7-5.0
km/s, respectively
and for
the harzburgite are 8.1-8.5 km/s and 4.7-5.0 km/s, respectively. The V_ and V• bounds for the wehrlite unit are 7.9-8.4 km/s and 4.5-4.8 km/s, respectively at 2.0 kbars. These figures are based in part on measurements reported by Nichols [1978] for clinopyroxenite. The lower bound is
for a wehrlite
of composition 50% olivine
(Vo
8.2, Vs = 4.7 km/s) and 50%clinopyroxene (V%
7.7, Vs = 4.3 km/sec.),and
the upper bound
for a composition 70%olivine (V = 8.7, Vs =
5.0 km/s)and30% clinopyroxene •velocityas above). rocks BOIC.
These modal compositions are typical
found in the deep crustal
sections
Spudich and Orcutt, 1980]. Normal-incidence seismic reflection profiles collected on oceanic crust with multichannel hydrophone arrays have further defined the nature of the crust/mantle transition. On some of these
profiles high-amplitude eventscanbe recognized at depths of 2-3 s two-way travel as reflections
of the
from the
crust/mantle
transition
[Grow and Markl, 1977; Herron et al., 1978; Stoffa et al., 1980; Diebold et al., 1981], suggesting an abrupt change in velocity and density. An
alternative
explanation has been proposed by
MacKenzie and Orcutt [1982]. In their model, the crust/mantle transition consists of a 1- to 2-kmthick stack of interbedded, thin (4100 m), highand low-velocity layers. Synthetic modelling has shown
of
time beneath
oceanic basement and these have been interpreted
that
seismic
refraction
studies
cannot
distinguish between such a model and a model consisting of a smooth velocity transition zone of the same thickness [Spudich and Orcutt, 1980].
Karson et al.:
Structure of the Crust/Mantle Transition
6135
kilometers
Fig. 7. Schematic blockdiagramillustrating the internal structure of the oceanic lithosphererepresented in the Bayof IslandsComplex [after Caseyet al., 1981]. (Ornamentas in Fig. 6.) Note variable nature of geologiccontactsbetweenlithologic units, lateral discontinuity of someunits, laterally varying thickness of units,
high-andlow-densitymegalenses within majorunits, andtopography on nearly all major geologiccontacts. Bold lines showthe top andbottomof the Mohotransition zone. This interval varies in thickness and in the geologic units it contains.
At normal incidence, however, constructive interference
of the reflections
from the thin
layers
can result in high-amplitude returns. Fuchs [1969] has proposed a similar model for the continental Moho transition zone and has shown that high-amplitude normal-incidence reflections can
be generated from high- and low-velocity layers with thicknesses corresponding to one quarter the seismic
It
of
wavelength.
can be seen that
the seismic picture
of the
Moho transition zone in the oceans is a complex one; but how is the velocity structure of the Moho transition zone in the BOIC related to the observations noted above? The geologic sections
and velocity-depth profiles constructedfor the
BOIC(Figure 6) showthe following features: (1)
near the fracture
zone marked by the Coastal
Complex. The timing of serpentinization in this region relative to faulting appears to indicate local deep hydration while the complexwas still in an ocean basin [Karson, 1982, 1984].
Second,
the regional variations in lithologic unit thicknesses demonstrate
that
thinning
of mafic crustal
layers and thickening of ultramafic cumulateunits occurs
for
near
the
fracture
the observed
zone.
thinning
This
could
of the crust
account
near
oceanic fracture zones [e.g., Fox et al., 1980]. Finally, the degree of velocity and fabric anisotropy varies from layer to layer and across horizontal lithologic unit contacts but has a fairly consistent
orientation
with
respect
to the paleo-
spreadingdirection (but see Caseyet al. [1983]).
a variable thickness (0-1 km) of the velocity and geologic (mafic/ultramafic) transition zone at
Therefore
crustal
propagation direction. For example, refraction experiments directed parallel to the spreading
the crust/mantle boundary; (2) a reconstructed thickness which varies
from about 3-4 km
(sections 3, 4, 5) to about 6-7 km(sections 6, 7); (3) thin (
