Investigators
Lisa C. McNeill, Chris Goldfinger, LaVerne D. Kulm, Kenneth A. Piper, and Robert S. Yeats
Summary
Analysis of multichannel
seismic reflection profiles reveals that listric normal faulting is widespread
on the northern Oregon and Washington continental shelf and upper slope,
suggesting E-W extension in this region. Fault activity began in
the late Miocene and, in some cases, has continued into the Holocene.
Most listric faults sole out into a subhorizontal deecollement coincident
with the upper contact of an Eocene to middle Miocene melange and broken
formation (MBF), known as the Hoh rock assemblage onshore, whereas other
faults penetrate and offset the top of the MBF. The areal distribution
of extensional faulting on the shelf and upper slope is similar to the
subsurface distribution of the MBF. Evidence onshore and on the continental
shelf suggests that the MBF is overpressured and mobile. For listric
faults which become subhorizontal at depth, these elevated pore pressures
may be sufficient to reduce effective stress and to allow downslope movement
of the overlying stratigraphic section along a low-angle (0.1 deg-2.5 deg)
detachment coincident with the upper MBF contact. Mobilization, extension,
and unconstrained westward movement of the MBF may also contribute to brittle
extension of the overlying sediments. No Pliocene or Quaternary extensional
faults have been identified off the central Oregon or northernmost Washington
coast, where the shelf is underlain by the rigid basaltic basement of the
Siletzia terrane. Quaternary extension of the shelf and upper slope
is contemporaneous with active accretion and thrust faulting on the lower
slope, suggesting that the shelf and upper slope are decoupled from subduction-related
compression.
Figure 1. Map showing locations of listric normal faults mapped primarily with multichannel reflection profiles.
Figure 2. Migrated MCS profile WO- 4058 on the Washington upper continental slope. This profile shows detail of the upper portion of one of the Washington normal faults, including offset of the seafloor and of a shallow slide debris package, prominent growth strata, and stratal rolloverinto the fault zone. Click for lager image.
Figure 3) (top) E-W migrated
multichannel seismic reflection profile A-A' on the central Washington
continental shelf and upper slope with (bottom) interpretive line drawing.
See Figure 1 for location. Three major listric faults, A1, A2 and
A3, are crossed by the profile including fault A1 at the head of Grays
Canyon, a target of submersible dives in 1994. Listric faults deform
late Miocene to Quaternary sediments with minor deformation of the uppermost
melange and broken formation. Faults A1 and A2 show evidence of recent
activity including deformed Holocene sediments, seafloor offset, and methane-derived
carbonates resulting from fluid venting. The listric faults sole
out at depth into a decollement close to or at the upper contact of the
melange and broken formation. The faults are characterized by growth
strata and rollover folds. TWTT, two-way travel time. Vertical
exaggeration ~ 2:1 at seafloor.
Figure 4. (left) Dive video frame grab from the DELTA submersible on dive 3417. This image shows the active tip of one of the acrive normal faults on the Washington shelf. Lasers are 10 cm apart for scale. Light toned gray sediment of the scarp face is thought to be Pleistocene in age, with a the in Holocene drape of olive green sediment. Right panel shows dive location (click for larger image).
Cascadia Normal Faulting Mechanisms
The melange and broken formation
appears to control both the normal fault distribution and the timing of
faulting, beginning in the late Miocene, following deposition, uplift,
and erosion of the middle Miocene MBF. The MBF appears to decouple
the overlying continental shelf sediments, characterized by extensional
deformation, from subduction-controlled E-W to NE-SW compressional deformation
evident on the lower continental slope. Two related mechanisms of
decoupling are described below, involving, first, detachment of the basinal
shelf sediments from the MBF, and secondly, mobilization and extension
of the MBF. The upper contact of the MBF, represented by the middle
to late Miocene unconformity and downward transition in acoustic character
from well-stratified to discontinuous reflectors, dips very gently west
throughout much of the continental shelf (Figure 7), with measured slopes
from the midshelf to the shelf break of approximately 0.1 deg-2.5 deg.
These gentle seaward slopes represent the regional dip of this upper contact,
ignoring local vertical variations due to faulting and folding. We
hypothesize that such a shallow surface dip may be sufficient to allow
unstable gravitational sliding on the upper MBF surface due to low basal
friction and consequent detachment of the overlying sediments (Figures
7a and 7b). This mechanism can be used to explain extension along
only the listric normal faults which sole out at depth into the upper MBF
contact. The reduced strength and effective shear stress along a
fault plane or detachment associated with high pore fluid pressures has
been documented by Hubbert and Rubey [1959] in the theory of low-angle
overthrust faulting or gravitational sliding and by Davis et al. [1983]
in the Coulomb theory of the critical tapered wedge. Seaward or downslope
dipping listric normal faults also support gravitational sliding as a mechanism
of extension. The upper MBF contact (middle to late Miocene unconformity)
thus may act as a detachment [Piper, 1994], separating the mobile MBF and
the more rigid post-MBF sediments, along which the late Miocene to Quaternary
section moves downslope (Figure 7b). The listric faults on the Cascadia
margin may therefore be similar to growth faults on the Texas coast, where
faults flatten at depth into low-density, high fluid pressure shale masses
[Bruce, 1973]. Normal faulting at the base of the Guatemalan slope
is also thought to be a result of decoupling through elevated pore fluid
pressures, as encountered during Leg 84 of the Deep Sea Drilling Project
[Aubouin et al., 1982], although this margin is characterized by much steeper
terrain.
Figure
5. Development and mechanisms of listric faulting on the
Cascadia outer shelf. (a) Prior to extension: melange and broken
formation (MBF) deposited at bathyal depths, uplifted, and eroded, and
overlying late Miocene sediments deposited. The upper MBF contact
dips gently seaward. (b) Extensional failure occurring through gravitational
collapse along a detachment separating the MBF and overlying sediments.
Elevated pore pressures within the MBF increase the chance of movement
on the low-dipping failure plane. Dip of the melange surface, a =
0.1deg-2.5deg. (c) Mobilization and extension of the MBF results
in brittle extension of the overlying sediments. East dipping normal
faults also form.
The subhorizontal upper
contact of the MBF on the continental shelf and upper slope suggests mobilization
and redistribution of this unit, aided by gravitationally driven downslope
movement. The MBF may therefore be undergoing mobilization and extension
to the west, where it is apparently unconstrained, with accompanying rigid
or brittle extension of the overlying younger deposits (Figures 7a and
7c). The upper contact may still behave as a detachment, as hypothesized
above, but in this case, both the mobile MBF and the overlying brittle
section undergo extension, with reduced relative displacement between these
two units. There may be an additional detachment at depth within
the MBF, below which no extension occurs, resulting from increases in strength
or decreases in pore fluid pressure. Mobilization and extension of
the MBF comprise a preferred explanation for listric faults which do not
flatten into a sub-horizontal decollement, but penetrate and offset the
MBF unit. Diapiric intrusions throughout the shelf and evidence of
upward movement of the MBF at the shelf edge (Figures 5 and 7c), where
the overlying sedimentary load is reduced, point to significant mobilization.
Extension of both the mobile MBF and overlying brittle sediments explains
the presence of east dipping and apparently upslope dipping normal faults
on the shelf and upper slope (e.g., western end of Figure 5). These
faults are less easily explained by downslope movement on a seaward-dipping
detachment. Extension and thinning of the Hoh beneath the shelf might
be expected to result in net subsidence, which contradicts paleobathymetric
evidence of net uplift during the period of extension [Rau, 1970; Bergen
and Bird, 1972]. This apparent contradiction can, however, be explained
by the counteraction of other factors influencing the uplift history of
the shelf, including sedimentation rates, sediment underplating, and the
variation of subducting slab dip.
Extension Versus Compressional Deformation
Current extension of the
continental shelf and upper slope is contemporaneous with accretion and
thrust faulting on the lower slope of the accretionary wedge. In
addition, extensional faulting appears to be contemporaneous with mapped
fold structures of C. Goldfinger and L.C. McNeill (manuscript in preparation,
1997) and other workers on the continental shelf. In the light of
the evidence for mobile extension, we have reexamined our earlier mapping
and conclude that many of the folds in the vicinity of the normal faults
are rollover folds, drape structures, and folds driven by downslope spreading
of the MBF. These structures could be misinterpreted as purely convergence-related
structures without the high quality data set used for this study.E-W contractile
strain is apparently low on the shelf and upper slope. We hypothesize
that the extensional tectonic regime of this region is isolated by the
mobile MBF from the convergence-related E-W to NE-SW compression on the
lower slope. Extension extends seaward to the upper slope, and the
prominent bulge may mark the seaward edge of the MBF, and therefore extension,
on the central Washington margin. The midslope area, lying between
these two regions of known compression and extension, may act as a transition
zone or, more likely, a distinct change from extension to compression is
located in this area. The seaward extent of the MBF is uncertain,
and the resolution of available data may prevent the identification of
extensional faults on much of the slope. The thickness and strength
of the older MBF are unknown, and therefore the depth to which extension
extends is unclear: a deeper compressional regime may underlie the
extending MBF. The presence of E-W trending folds on the inner continental
shelf suggests that N-S compression and E-W extension are operating simultaneously.
An extreme case of decoupling extending to the plate interface (10-15 km
beneath the shelf) would have significant implications for the extent of
coupling on the subduction zone and hence position and width of the interplate
locked zone. The extent and significance of decoupling induced by
the MBF are the subject of further study and cannot be fully addressed
in this paper.
Conclusions
Listric normal faulting
appears to be the result of (1) downslope movement along a low-angle
deecollement between the uppermost middle Miocene MBF and the overlying
basinal sediments and (2) mobilization and extension of the MBF and consequent
brittle extension of the overlying sediments. Miocene and Pliocene
uplift of the continental shelf may have resulted in oversteepening of
the shelf and further gravitational collapse but was probably not a requirement
for extension. The subsurface distribution of the MBF restricts extension
to the Washington and northern Oregon shelf and upper slope. Contemporaneous
compressional tectonics of the lower slope and extensional tectonics of
the shelf and upper slope are apparently isolated from each other, with
the latter region being decoupled from the E-W compressional forces of
convergence by the underlying mobile material. Such segregation of
extensional and compressional regimes on convergent margins is not unique
to Cascadia, with similar observations on the Peru, Japan, Costa Rica,
and Alaskan margins. Many N-S trending fold structures previously
interpreted as tectonic expressions of convergence-related compression,
including rollover folds, drape folds, and hanging wall synclines, can
be attributed to listric faulting, with E-W extension being the dominant
tectonic style. We conclude that E-W contractile strain is low on
the Washington and northern Oregon shelf and that a transition from extension
to compression occurs in the mid slope region, likely coincident with the
seaward edge of the MBF (Figure 2a). The presence of long-term major
extensional faults, which displace sediments to depths of 2-3 km or greater
throughout much of the northern Cascadia continental shelf and upper slope,
is of importance to the current stability of the margin.
Publications
McNeill, L.C., Piper, K.A., Goldfinger, C., Kulm, L.D., and Yeats, R.S., 1997, Listric normal faulting on the Cascadia continental margin: Journal of Geophysical Research, v. 102, p. 12,123-12,138. 
