Listric Normal Faulting on the Cascadia Continental Margin
 
Lisa C. McNeill1, Kenneth A. Piper2, Chris Goldfinger3, LaVerne D. Kulm3, and Robert S. Yeats1
 
1Department of Geosciences, Oregon State University, Corvallis, OR 97331
2Minerals Management Service, Camarillo, CA 93010
3College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 
*This paper is published in full in the Journal of Geophysical Research, v. 102, p. 12,123-12,138 PDF
 
 

Abstract
        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 of the Cascadia subduction zone showing the study area (boxed) and underlying lithologies of the Oregon and Washington continental shelves (modified from Snavely [1987] and Palmer and Lingley [1989]).  The northern Oregon and Washington shelf are underlain by melange and broken formation (MBF) of the accretionary complex (Hoh rock assemblage of Rau [1973]) which extends landward onto the Olympic Peninsula, whereas the central Oregon shelf is underlain by oceanic basalt of the Siletz River Volcanics. Industry boreholes penetrating the MBF are also shown.  PA, Point of the Arches, BCT, Big Creek Thrust.  Submarine fans:  NF, Nitinat Fan; AF, Astoria Fan.  Position of Ocean Drilling Program Site 888 is shown.  Plate convergence vector from De Mets et al. [1990].

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Figure 2.  Map of the study area locating major normal faults mapped in this study.  Listric normal faults indicated by "L".  The 200 m (approximating the shelf edge) and 1000 m bathymetric contours represented as dashed lines.  Normal faults active in the latest Pleistocene and Holocene (deforming youngest sediments and/or seafloor) shown as solid lines; normal faults active in the Pliocene and early Pleistocene shown as dashed lines.  Bar and ball indicate downthrown side of fault.  Submarine canyons are labeled (Juan de Fuca to Astoria) and A-A', B-B', and C-C' represent seismic reflection profiles shown in Figures 3, 4, and 5, respectively.  Industry boreholes on the continental shelf and in Grays Harbor area (open circles):  1, Pan American P-0141; 2, Sunshine Medina; 3, Union Tidelands; 4, Shell P-0155; 5, Shell and Pan American P-0150; 6, Shell P-075; 7, Shell P-072.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 


 

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 2 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.  (Top) E-W migrated multichannel seismic reflection profile B-B' on the central Washington upper slope and shelf edge with (bottom) interpretive line drawing.  See Figure 2 for location.  Listric faults B1 and B2 on the upper slope deform late Miocene to Holocene sediments and offset the seafloor, with seaward facing scarps of ~25 m height.  Fault B2 is characterized by growth strata.  TWTT, two-way travel time.  Vertical exaggeration ~ 2:1 at seafloor.
 

Figure 5.  (Top) E-W migrated multichannel seismic reflection profile C-C' on the southern Washington midshelf off Willapa Bay with (bottom) interpretive line drawing.  See Figure 2 for location.  Listric faults C1 and C2 deform mid-Miocene (melange and broken formation) to Pliocene sediments and are overlain by the undeformed Plio-Pleistocene unconformity.  The growth strata are uplifted by a mud diapir rooted in the melange to the west.  Note also an east dipping normal fault to the west of the diapir.  TWTT, two-way travel time.  Vertical exaggeration ~ 2:1 at seafloor.
 

Figure 6.  Video frame from the Delta submersible of fault scarp A1 (see Figure 3) at the head of Grays Canyon.  Video camera points NE and two dots represent parallel laser beams placed 20 cm apart.  The scarp is ~ 0.5-1 m in height.  Scarp is relatively free from bioturbation and hemipelagic sediment, and angular boulders are observed at the base of the scarp, both suggesting recent activity.
 
 

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 7.  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.

References

Aubouin, J., et al., Leg 84 of the Deep Sea Drilling Project: Subduction without accretion: Middle America Trench off Guatemala, Nature, 297, 458-460, 1982.

Bergen, F.W., and K.J. Bird, The biostratigraphy of the Ocean City area, Washington, in The Pacific Coast Miocene Biostratigraphic Symposium, Proceedings, pp. 173-191, Econ. Paleontol. and Mineral. Pac. Sect., 1972.

Bruce, C.H., Pressured shale and related sediment deformation:  Mechanism for development of regional contemporaneous faults, AAPG Bull., 57, 878-886, 1973.

Davis, D., J. Suppe, and F.A. Dahlen, Mechanics of fold-and-thrust belts and accretionary wedges, J. Geophys. Res., 88, 1153-1172, 1983.

De Mets, C., Gordon, R.G., Argus, D.F., and Stein, S., Current plate motions, Geophys. J. Int., 101, 425-478, 1990.

Hubbert, M.K., and W.W. Rubey, Role of fluid pressure in mechanics of overthrust faulting, 1, Mechanics of fluid-filled porous solids and its application to overthrust faulting, Geol. Soc. Am. Bull., 70, 115-166, 1959.

Palmer, S.P., and Lingley, W.S., An assessment of the oil and gas potential of the Washington outer continental shelf, Washington State Division of Geology and Earth Resources, Report WSG 89-2, 88p, 1989.

Piper, K.A., Extensional tectonics in a convergent margin - Pacific Northwest offshore, Washington and Oregon, AAPG Bull., 78, 673, 1994.

Rau, W.W., Foraminifera, stratigraphy, and paleoecology of the Quinault Formation, Point Grenville-Raft River coastal area, Washington, Wash. Div. Mines Geol. Bull., 62, 41 pp., 1970.

Rau, W.W., Geology of the Washington coast between Point Grenville and the Hoh River, Washington Division of Geology and Earth Resources Bulletin, 66, 58 p., 1973.

Snavely, P.D., Jr., Tertiary geologic framework, neotectonics, and petroleum potential of the Oregon-Washington continental margin, in Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins-Beaufort Sea to Baja California, edited by D. W. Scholl, A. Grantz, and J.G. Vedder, pp. 305-335, Circum-Pac. Counc. for Energy and Miner. Resour., Houston, Tex., 1987.


 
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