Tréhu, A.M., Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.)
Proceedings of the Ocean Drilling Program, Scientific Results Volume 204
2. SEISMIC SEQUENCE STRATIGRAPHY AND
TECTONIC EVOLUTION OF SOUTHERN
HYDRATE RIDGE1
Johanna Chevallier,2, 3 Anne M. Tréhu,2 Nathan L. Bangs,4
Joel E. Johnson,2, 5 and H. Jack Meyer6
ABSTRACT
This paper presents a seismic sequence and structural analysis of a
high-resolution three-dimensional seismic reflection survey that was
acquired in June 2000 in preparation for Ocean Drilling Program (ODP)
Leg 204. The seismic data were correlated with coring and logging results from nine sites drilled in 2002 during Leg 204. The stratigraphic
and structural evolution of this complex accretionary ridge through
time, as inferred from seismic-stratigraphic units and depositional sequences imaged by the seismic data, is presented as a series of interpreted seismic cross sections and horizon time or isopach maps across
southern Hydrate Ridge. Our reconstruction starts at ~1.2 Ma with a
shift of the frontal thrust from seaward to landward vergent and thrusting of abyssal plain sediments over the older deformed and accreted
units that form the core of Hydrate Ridge. From ~1.0 to 0.3 Ma, a series
of overlapping slope basins with shifting depocenters was deposited as
the main locus of uplift shifted northeastward. This enigmatic landward migration of uplift may be related to topography on the subducted plate, which is now deeply buried beneath the upper slope and
shelf. The main locus of uplift shifted west to its present position at
~0.3 Ma, probably in response to a change to a seaward-vergent frontal
thrust and related sediment underplating and duplexing. This structural and stratigraphic history has influenced the distribution of gas hydrate and free gas by causing variable age and permeability of sediments
beneath and within the gas hydrate stability zone, preferential path-
1 Chevallier, J., Tréhu, A.M., Bangs,
N.L., Johnson, J.E., and Meyer, H.J.,
2006. Seismic sequence stratigraphy
and tectonic evolution of southern
Hydrate Ridge. In Tréhu, A.M.,
Bohrmann, G., Torres, M.E., and
Colwell, F.S. (Eds.), Proc. ODP, Sci.
Results, 204: College Station, TX
(Ocean Drilling Program), 1–29.
doi:10.2973/odp.proc.sr.204.121.2006
2 COAS, Oregon State University,
Ocean Administration Building 104,
Corvallis OR 97331, USA.
Correspondence author:
trehu@coas.oregonstate.edu
3 Present address: Wintershall AG,
Friedrich-Ebert-Strasse 160, D-34119
Kassel, Germany.
4 Institute for Geophysics, University
of Texas at Austin, Austin TX, USA.
5 Present address: Department of Earth
Sciences, University of New
Hampshire, 56 College Road, James
Hall 121, Durham NH 03824, USA.
6 Northwest Natural Gas, 220
Northwest Second Avenue, Portland
OR 97209, USA.
Initial receipt: 26 April 2005
Acceptance: 25 May 2006
Web publication: 2 November 2006
Ms 204SR-121
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
2
ways for fluid migration, and varying amounts of decompression and
gas dissolution.
INTRODUCTION
Washington
44°48'
N
River
Site 175
Oregon
2000
250
0
44°
Columbia
44°36'
Pacific
plate
Site 892
NHR
500
Astoria
Fan
1000
Juan de Fuca
plate
Site 174
46°
1500
Multichannel 3-D seismic data were collected during summer 2000
on the Thomas Thompson (Tréhu and Bangs, 2001). The survey covers a
4 km × 9 km region that includes the southern summit of Hydrate
Ridge and an adjacent slope basin to its east (Fig. F1C). The ship was
navigated by Global Positioning System (GPS) provided by Racal (in-
B
48°
N
1000
3-D Seismic Data
A
0
50
DATA
F1. Location of Hydrate Ridge,
p. 17.
1500
SHR
42°
California
200 km
44°24'
125°24'W
Line
NS
44.60°
N
EW 230
EW 250
125°12'
124°48'
90
1200
Site 1246 Site 1252
Site 1244
Site 1245
1240
Site 1247
Site 1248
EW 300
125°00'
0
Western boundary of
MBARI EM300 survey
C
2
124°
1
128°
NS3
100
Line
0
132°W
300
0
Line
NS
40°
Site 1251
Site 1249
Site 1250
EW 343
800
Seismic volume
1 km
44.55°
125.20°W
125.15°
125.10°
125.05°
125.00°
D
Fig. F4A
1
3
Depth (km)
Accretion of sediments to the North American plate in response to
oblique subduction of the Juan de Fuca plate beneath North America at
a rate of ~4 cm/yr has formed a large accretionary prism composed of
Pliocene and Pleistocene strata deformed into north–south-striking
folded thrust slices (e.g., Seely et al., 1974; Snavely, 1987; MacKay et al.,
1992; Goldfinger et al., 1996; McNeill et al., 2000). Hydrate Ridge, a 10km-wide, 15-km-long ridge located 17.5 km east of the Cascadia deformation front ~100 km offshore Newport, Oregon, is one of these thrust
slices (Fig. F1). Deep-penetration seismic-reflection data indicate that it
is composed of several juxtaposed thrust folds and faults and has a
steep western flank that has repeatedly been subjected to slope instability (Tréhu et al., 1999, Tréhu et al., this volume; Johnson et al., this
volume).
Gas hydrates are present in the shallow sediments beneath Hydrate
Ridge, as indicated by a widespread bottom-simulating reflection (BSR)
(e.g., Tréhu et al., 1999) and by observations of gas hydrates, methane
bubbles, and hydrate-related authigenic carbonates near the northern
and southern summits (e.g., Bohrmann et al., 1998; Tréhu et al., 1999;
Suess et al., 2001; Torres et al., 2002; Tryon et al., 2002; Johnson et al.,
2003). Recent studies have focused on determining the amount of gas
hydrate present (Milkov et al., 2003; Tréhu et al. 2004a), the dynamics
of gas hydrate formation near the summit (Milkov et al., 2004; Torres et
al., 2004a), the plumbing system feeding the summit deposits (Tréhu et
al., 2004b), and the response of the gas hydrates to changes in sea level
and water temperature (Bangs et al., 2005). Logging data and core recovery from Ocean Drilling Program (ODP) Leg 204 show that gas hydrate deposits within the gas hydrate stability zone (GHSZ) range from
massive to patchy to absent, with hydrate generally filling, on average,
<5% of the pore space but comprising as much as 25% of the sediment
volume in a focused deposit at the southern summit (Tréhu et al.,
2004a).
The primary objective of this paper is to reconstruct the stratigraphic
and structural development of southern Hydrate Ridge (SHR) through a
detailed analysis of three-dimensional (3-D) seismic data that were collected as a site survey for Leg 204. The lithologic characteristics of seismic facies and the timing of events are constrained using
sedimentological and biostratigraphic data from ODP Leg 204 (Tréhu,
Bohrmann, Rack, Torres, et al., 2003). The influence of structure and lithology on the free gas and gas hydrate distribution is also discussed.
Seaw
ard
5
Sedi
men
t su
bduc
ted
7
Landward verging
verg
Accreted, underthrust,
duplexed sediment
ing
in th
e pa
st ~0
.4 m
Active duplexing?
.y.
9
6.25
0
6.25
12.50
Distance from deformation front (km)
18.75
25.00
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
cluding differential GPS [dGPS] corrections from a weighted average of
four Racal base stations). These raw fixes were recorded at 1 Hz. They
were smoothed to remove the rolling and pitching of the ship for derivation of the location and the velocity of the ship at each update and
were corrected to compensate for the offset of the source from the GPS
antenna. The streamer’s geometry was determined by four bird compasses positioned at 150-m intervals. These records were used to reposition each single receiver channel. There was no tail buoy for
independent determination of the location of the end of the streamer.
The depths and the wing angles of the birds were monitored by the
watch-stander using a graphical display. Shots were fired at 15-m intervals based on the smoothed velocity estimate from the dGPS fixes. The
source was two generator-injector (GI) guns, each set at 45/45 in3,
towed at a depth of 2.5 m, and fired simultaneously. They proved to be
a good broadband source, producing usable energy from 10 to 240 Hz.
The signal was recorded on a portable, 600-m-long, 48-channel
streamer, which was towed at a depth of 2.5 m.
The bridge used the Nav99 program written by Mark Weidenspahn
(University of Texas Institute for Geophysics) to steer the source location down the selected line. Nav99 also provided a minimum radius
turn path between lines, which were always shot heading east in the
northern half of the grid and heading west in the southern half. Recorded trace length was 6 s. The 3-D survey grid consisted of 81, 11-kmlong east-west sections that were shot 50 m apart in a racetrack pattern.
During the cruise, 3-D fold was monitored based on streamer reconstruction to identify locations where additional data were needed in order to obtain adequate fold in each bin. This resulted in reshooting 18
lines because of variable degrees of streamer feathering or navigational
problems resulting from strong currents. Onboard the Thomas Thompson, the raw SEGD field data were copied from 3480 tapes to SEGY disk
files using Sioseis. The SEGY disk files were then converted to Paradigm
Geophysical’s FOCUS internal format. Seven bad channels were deleted
at this stage. Data were processed at sea as two-dimensional (2-D) lines
for quality control and a first look at the data. Source navigation and
streamer location data for the 3-D grid were converted to universal
transverse Mercator (UTM) coordinates and written in UKOOA90 format. These navigation data were converted into FOCUS format 3-D
navigation traces and binned using the FOCUS 3-D QC/binning package. Fold maps were generated and optimized for 50 m × 25 m and 25 m
× 12.5 m bin sizes. In both cases, the optimum grid azimuth was 348.5°.
On land, data were sorted into 12.5 m × 25 m bins for the 3-D processing and renumbered from 200 to 360 to accommodate interpolated
lines. A velocity analysis was done for every 10th line and every 100th
common midpoint (CMP). A high-pass filter with a ramp from 15 to 25
Hz was applied to the traces to remove noise resulting from choppy seas
and a large swell. The data were then corrected for normal moveout
(NMO), the inner trace was muted (to attenuate the seafloor multiple),
and the data were stacked. A 3-D poststack Kirchhoff migration using
stacking velocities was applied. This yields the 3-D volume, which is 4
km wide × 9 km long and contains a frequency range of 20 to 180 Hz
with a dominant frequency of 125 Hz, resulting in a nominal resolution
of 3 m (1/4 wavelength). The continuity of north-south and time slices
through the data volume, however, is degraded by static effects due to
tidal changes in water depth, swell, and changes in the towing depth of
the streamer and/or GI guns.
3
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
4
2-D Seismic Profiles
Six regional 2-D lines were recorded on the same multichannel
streamer (Tréhu and Bangs, 2001). The tracks of the three north-south
lines used during this study are shown on Figure F1C. The shot interval
for these lines was 37.5 m, and data were sorted into 12.5-m bins, resulting in 16- to 24-fold data. A high-pass filter was applied with a ramp
from 25 to 35 Hz. The data were corrected for NMO, stretch mute was
done with a 35% maximum stretch, and the data were stacked assuming a constant velocity of 1500 m/s. A velocity of 1500 m/s was also
used for frequency-wave-number migration of the data.
ODP Leg 204
F2. Lithologic units, p. 18.
E-W transect
N-S transect
Unit S.V
300
Unit S.VII
early Pleistocene
1.0
1.2
1.6
IV
A
A
2.4
IV
B
400
>1.6
1251
IA
0.1
IB
ID
0.3
II
Unit S.II
1.0
II
VI
t S.
Uni
K
1.6
Unit S.IV
III
1.7
2.0
III
Unit S.VII
middle Pleistocene-Holocene
B'
V
Sequence stratigraphic analysis is based on the identification of depositional sequences, which are stratigraphic units composed of a relatively conformable succession of genetically related strata bounded by
two major unconformities (Mitchum, 1977). This method is usually
used to reconstruct the effects of sea level rise and fall on the history of
deposition on continental margins. Seismic stratigraphic analysis of
SHR strata differs from traditional sequence stratigraphy in that the
stratal succession was not significantly influenced by sea level changes.
SHR is located on the continental slope at 800 m depth (Fig. F1), and
the sediments composing the core of the ridge originated from accretion of abyssal plain sediment originally deposited at ~2900 m depth.
Deposition of the overlying slope basin sediments was controlled by
the formation and evolution of the accretionary wedge fold-thrust belt
B
1.0
III
B
500
USE OF SEQUENCE STRATIGRAPHY
IN A TECTONICALLY ACTIVE ENVIRONMENT
0.3
0.5
1252
D
Unit S.IB
m. Pleist.-Hol.
IB-C
(0.5)
III
A
E
C
II
0.1
IC
U
0.3
0.5
IIA
BSR
K
IIB
late Plio. early Pleistocene
IIIB
IIIB
200
III
A
II 0.3
Unit S.IA
I
early Pleist.
III
A (1.6)
Y
1244
I
l. Plio..
Unit S.III
(1.6)
1246
m. Pleist.-Holocene
III
W
middle Pleist.-Hol.
I
early Pleist.
1245
I
II
late Plio.
Pleist.-Hol.
Pleist.-Hol.
1247
I-II 0.3
0.5
middle Pleist.-Holocene
A
III
A (1.6)
1248
I-II 0.3
early Pleistocene
BSR
early Pleistocene
100
II 0.5
m. Pleist.-Hol.
N
1249
1250
I
Y
early Pleist. Pleist.-Hol.
S
0
Depth (mbsf)
Nine sites were drilled within the boundaries of the 3-D survey during ODP Leg 204 (Fig. F1C). The maximum coring depth ranged from
90 to 540 meters below seafloor (mbsf). Sediment classification was recorded based on visual observation and core description, smear slide
analysis, and correlation with physical property data (Shipboard Scientific Party, 2003a). Mudstones and siltstones of turbidite origin, debrisflow deposits, and intervening hemipelagic clays dominate the lithologies observed in the Leg 204 cores. Classification of sediments into
lithologic units was based on sometimes subtle changes in major and
minor lithologies, changes in biogenic content, changes in magnetic
susceptibility, and changes in the frequency and thickness of turbidite
layers. Absolute ages of strata were determined based on the bioevents
distinguished in the cores and presented in the Leg 204 Initial Reports
volume (Tréhu, Bohrmann, Rack, Torres, et al., 2003). The biostratigraphic zones were inferred from the first and last occurrences of diatoms and calcareous nannofossils. Biostratigraphic ages and lithologic
units determined by the Leg 204 shipboard sedimentologists are summarized in Figure F2. Although this lithologic and biostratigraphic information was used to aid in the interpretation of the seismic data,
seismic units (also shown in Fig. F2) were determined independently
from the lithologic units based on seismic stratigraphic packages, as
outlined in “Use of Sequence Stratigraphy in a Tectonically Active
Environment.” The boundaries between lithologic and seismic units
generally coincide.
1.0
1.6
III
2.0
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
F3. Nomenclature for growth
strata, p. 19.
A
Fold crest
Growth strata
limb
Fore
system. In spite of the difference between the processes controlling
changes in sediment deposition patterns, similar types of angular unconformities can be distinguished based on the geometry of the strata
and their termination (Fig. F3). The unconformities bound sets of relatively concordant strata. Each of these sets is interpreted to represent
one depositional sequence. Each seismic unit is subdivided into a few
depositional sequences. Characteristics of the sequences are controlled
by syn- and postdepositional tectonic activity.
Seismic sequence analysis of the depositional sequences mapped at
SHR was used to reconstruct the sedimentary and tectonic events that
deformed the sediments in the SHR fold and thrust system and the concomitant active basins and to derive a relative time-correlation of these
events. The method included evaluating the relationship between the
synsedimentary thrust and fold systems responsible for the formation
of the ridge and the migration of the intervening slope basins. The primary tectonic processes observed on SHR are thrust-related folding of
the strata and migration of the thrust system. These processes control
the location and growth of sediment depocenters and result in tilting
of the strata on the limbs of folds and/or along the flanks of subsiding
depocenters.
5
b
klim
Bac
Pregrowth strata
Thrust fold
B
C
Overlap
3
Offlap
3
2
D
1
1
Uplift < Accumulation
Uplift > Accumulation
E
Overlap, then offlap
2
Offlap, onlap, then overlap
3
3
2
2
1
1
Uplift < Accumulation
then
Uplift > Accumulation
Uplift > Accumulation
then
Uplift < Accumulation
INTERPRETATION OF SEISMIC UNITS
AND STRUCTURES
Seismic Units
The seismic units fall into two different groups: sediments deposited
on the abyssal plain and transferred to the accretionary complex and
sediment deposited in slope basins overlying the accreted sediments.
F4. East-west slices through the
3-D seismic volume, p. 21.
A
Site
1245
W
2,000
1.1
3,000
Site
1246
4,000
Site
1244
5,000
Site
1252
Distance (m)
6,000
EW Line 230
7,000
E
8,000
9,000
10,000
11,000
1.100
1.200
B'
B
A
1.5
C1.400
D
1.500
K
1.600
A'
Dome
A''
1.7
1.300
1.700
Fold F
S.VII
U
DBF1
0
S.VI
DBF2
Anticline B
200
1.900
1.9
F2
2.1
S.VII
400
2.100
F1
2.200
2.300
2.3
600
2.400
W
1.1
3,000
4,000
5,000
EW Line 250
1.3
1.5
Dome
F
1.7
1.9
2.1
2.3
2.5
E
Distance (m)
2,000
F2
6,000
1.100
1.200
1.300
1.400
1.500
Anticline
1.600
A
1.700
1.800
1.900
2.,000
2.100
2.200
2.300
2.400
2.500
7,000
K
8,000
K
9,000
B
S.VI
S.VII
10,000
U
0
200
400
600
Depth (m)
B
Depth (m)
Two-way traveltime (s)
C
Y
1.3
Two-way traveltime (s)
In this section we describe the characteristics of the seismic units and
relate them to the lithologic units described by Leg 204 shipboard sedimentologists (Shipboard Scientific Party, 2003a). We illustrate our conclusions by showing several interpreted transects through the seismic
volume (Fig. F4A–F4B, F4C–F4D), selected details of the seismic data,
and maps of key stratigraphic horizons or fault surfaces. Further details
are discussed in Chevallier (2004).
Although velocity constraints are available from ocean-bottom seismometers deployed during the 3-D seismic survey (Arsenault et al.,
2001), from sonic logs during Leg 204 (Lee and Collett, this volume;
Guerin et al., this volume), and from vertical seismic profiles (VSPs)
during Leg 204 (Tréhu et al., this volume), velocities within the strata
imaged by the 3-D survey and discussed in this paper are generally
<1800 m/s and the interval velocity above the BSR is <1600 m/s. Because of the difficulty of interpolating the velocity field to define velocity at all data points in the 3-D data set and degradation of data quality
because of variable stretch due to local velocity variations (some of
which are real and some of which may be artifacts), data are presented
here with a vertical axis of two-way traveltime. Because the average velocities are uniformly low, the effect on stratigraphic and structural patterns is small. A constant velocity of 1600 m/s in the subsurface is
adequate to estimate horizon depths and isopach thicknesses throughout the 3-D data volume.
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
6
Abyssal Plain Sediments of the Astoria Fan
Unit S.VII represents deep-sea fan sediments older than 1.6 Ma (late
Pliocene) and Unit S.V represents younger deep-sea fan sediments of
age 1.6–1.1 Ma (Fig. F2). Unit S.VII is characterized by a high degree of
lithification and microfracturing (Shipboard Scientific Party, 2003b,
2003d, 2003e), consistent with its long history of burial and deformation. Internal structures in these units are poorly preserved. Glauconiterich layers near the top of this sequence indicate extended periods of
exposure on the seafloor (Shipboard Scientific Party, 2003b, 2003c,
2003d, 2003e).
Unit S.V is composed of nannofossil-rich silty claystone interspersed
with thick turbidites containing wood fragments that was deposited
rapidly (~70 cm/k.y.) (Shipboard Scientific Party, 2003c) in the trench as
part of the Astoria Fan. The top of this seismic unit is defined by Horizon A, a 2- to 4-m-thick, coarse-grained, volcanic-ash-rich turbidite
layer that appears as an bright reflection in the 3-D data. Drilling revealed that Horizon A contains abundant free gas and plays a critical
role in transporting methane to feed methane vents and formation of
massive gas hydrate at the summit (Tréhu et al., 2004b).
Slope Basin Sediments
F5. Seismic Line NS3, p. 23.
EW Line 230
EW Line 250 EW Line 300
1.0
Two-way traveltime (s)
At Sites 1244 and 1252, Unit S.VII is overlain by sediments with an
age that overlaps Unit S.V but have characteristics that are similar to
Unit S.VII. We call this Unit S.VI. Seismically, it is difficult to distinguish Unit S.VI from Unit S.VII. Sediments recovered from this zone are
relatively lithified and fractured and contain abundant glauconite similar to Unit S.VII, reflecting exposure on the seafloor and a very low sedimentation rate. Combining biostratigraphic data with the structural
reconstruction presented in “Tectonic History of Southern Hydrate
Ridge,” p. 9, we conclude that these sediments were deposited on the
lower slope of the overriding plate near a seaward-vergent frontal thrust
fault.
Slope-basin sediments younger than ~1.0 Ma overlie Units S.VII–S.V
(Figs. F2, F4). These sediments (Units S.IV to S.IA) filled a migrating series of depocenters created by temporal and spatial variations in the locus of uplift. They are composed mostly of turbidites interlayered with
hemipelagic silty clays. The main distinction between the lithologic
units is related to the frequency and thickness of turbidites. Units S.III
and S.II in the western part of the survey were deposited in the lower
slope basin formed by Fold F (Fig. F4). Distinct reflections B and B′ (Fig.
F4A), found at the base of and within Unit S.II, result from coarsegrained gas-rich horizons similar to Horizon A. Horizons B and B′ host
gas hydrate or free gas, depending on whether they were sampled
within or beneath the GHSZ (Tréhu et al., 2004a; Guerin et al., this volume). Units S.III and S.II are thickest in the saddle between the northern and southern summits of Hydrate Ridge (Fig. F5).
In the eastern part of the survey region, Unit S.IV is a locally thick
deposit that developed contemporaneous with Units S.III and S.II in association with Anticline B (see “The Dome and Anticlines A and B,”
p. 7). It is separated by an unconformity (U in Figs. F2, F4) from Units
S.IB and S.IA, which fill a regional basin that has been deepening to the
south with time as SHR has been uplifted (Fig. F4). The lithologic unit
corresponding to Unit S.IV is distinguished from Units S.IA and S.IB by
a decrease in biogenic elements and an increase in the terrigeneous ma-
Site
1244
EW Line 343
SHR
1.2
C
S.IB
1.4
1.6
S.II
NHR
B
K
S.VI
B'
Dome
S.VII
1.8
2.0
6,000
7,000
8,000
9,000
10,000
Distance (m)
11,000
12,000
13,000
14,000
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
7
terial. It is also characterized by rapid deposition (~160 cm/k.y.) and a
high frequency of locally derived turbidites generated by rapid changes
in seafloor topography, which resulted in slope instability.
Structure
F6. Map of Horizons A′ and A,
p. 24.
1.250
400
1.302
1.354
1.406
1.458
250
1. 4
0
300
1.510
300
1.562
350
Two-way traveltime (s)
1.3
400
500
600
100
E
3,000
Distance (m)
W
2,000
Y
Time 1: flattening on Horizon A''
(between Time 1 and Time 2,
sediments thin to the west against
a local bulge)
1.2
1.700
Horizon A time map; contour interval = 0.025 s
Distance (m)
W
1.665
1 km
Horizon A' time map; contour interval = 0.025 s
2,000
E
3,000
Time 2: flattening on Horizon A'
(end of the activity on the
local bulge)
1.3
1.4
A
1.5
1.4
1.5
1.6
A'
1.7
1.6
1.8
A''
1.7
1.9
1.8
2.0
Distance (m)
W
2,000
1.5
Distance (m)
W
3,000
2,000
Time 3: flattening on Horizon A
(Fold F begins to grow and deform
older strata)
1.4
Two-way traveltime (s)
E
E
3,000
Present time 3:
(no flattening)
1.2
1.3
1.6
1.4
1.7
1.5
1.8
1.6
1.9
1.7
2.0
1.8
<-1.2 m.y.
2.1
F7. Time map to Unconformity K,
p. 25.
A
NHR
1.1 s
1.1
900
1.3
1.4
200
B
1.5
1. 6
250
250
00
300
Dome
900
700
800
500
400
300
600
100
1.7
1.8
350
200
1.6
300
A
Two-way traveltime (s)
700
800
400
500
600
300
200
100
1.2
200
350
1.9
2.0
1
.6
00
1 km
Unconformity K time map; contour interval = 0.1 s
B
20
0
0
10
Line
35
0
N
Trac
e
Fold F
0
90
1.0
Dome
Two-way traveltime (s)
1.0
Anticline A
Anticline B
1.2
1.4
The Dome and Anticlines A and B
1.6
3-D view of the top of the
accretionary complex, which includes
Unconformity K to the east and Fault F2
to the west.
1.8
2.0
2.0
F8. Isopach maps of Units S.III,
S.II, S.IB, and S.IA, p. 26.
900
800
500
700
600
300
400
350
Unit S.III time isopach map; contour interval = 0.05 s
0.05
350
0
W on seismic stratigraphic relationships illustrated
in Figure F3F on Line 230.
Two-way traveltime (s)
800
900
00
0.0
0.050
0.15
300
0.10
Two-way traveltime (s)
500
700
400
600
200
300
0.20
250
300
200
0.30
200
0.25
Schematic evolution of Units II and IB based
E
0.35
0.100
100
200
250
100
Two-way traveltime (s)
A
1.2
1.4
Unit S.II
1.6
S.VII
1.8
1 km
2,500
3,500
4,000
4,500
900
800
500
700
400
600
300
200
100
250
0.050
0.0
00
250
300
0.050
0.0
00
300
350
Two-way traveltime (s)
0.100
200
800
900
500
600
700
400
300
200
350
100
0.150
00
0.1
0.050
250
0.0
00
900
800
600
500
700
200
300
350
400
S.II
S.VII
Unit S.IB time isopach map; contour interval = 0.05 s
3,000
3,500
4,000
4,500
Distance (m)
300
350
100
1.6
2,500
200
300
1.4
900
800
200
250
1.2
1.8
1 km
700
500
600
400
200
100
300
Unit S.II time isopach map; contour interval = 0.05 s
Two-way traveltime (s)
Two-way traveltime (s)
200
C
Two-way traveltime (s)
3,000
Distance (m)
B
1.2
1.4
S.II
1.6
S.VII
1.8
2,500
1 km
3,000
3,500
4,000
4,500
0
05
0
250
0
0.20
0
0.
250
0.250
0.050
0.10
0
300
350
300
900
800
700
500
600
200
300
400
350
Unit S.IA time isopach map; contour interval = 0.05 s
Two-way traveltime (s)
800
900
200
15
25
0.
0.000
0.
400
600
500
700
200
100
Two-way traveltime (s)
200
300
Distance (m)
D
100
Middle to late Pleistocene slope-basin strata lap onto a large domeshaped feature (which we call the Dome) composed of Units S.VII and
S.VI. The top of this feature was mapped and is referred to as Unconformity K (Figs. F2, F4, F7). The relief on Unconformity K is much greater
than the present-day seafloor relief, and the shape of the Dome is
nearly circular, centered on the present-day southern summit (Fig. F7).
The 2-D lines suggest an even more pronounced bulge beneath northern Hydrate Ridge. A secondary elongated bulge on the eastern flank of
the Dome is referred to as Anticline A. Anticline A has a distinct topographic signature at present. Both the Dome and Anticline A are characterized by onlapping of the overlying strata against the paleorelief (Fig.
F8E). Relatively rapid growth of the Dome at ~1.0 Ma is inferred from
the abrupt pinching-out of Unit S.III to the east (Fig. F4, F8A). Units
S.VI and S.VII are too deformed to reconstruct the early evolution of the
Dome.
200
1 km
B
The most prominent fault-related fold (Fold F) deforms Unit S.IV and
controls the geometry of Unit S.IV and S.III strata in the western part of
the survey region (Fig. F4). The fold is a north-northeast-striking, doubly plunging fold (Fig. F6A), which may be cored by Fault F1 and probably developed because of drag along Fault F2 (Fig. F4A). The seismic
survey encompasses only its northern half, which is plunging by ~3°.
The interpretation of Fold F as a thrust-cored, landward-vergent fold
formed at the deformation front is based on (1) reconstruction of the
fold’s evolution inferred from the geometry of the strata (Fig. F6B),
which suggests a thrust fold in the hanging wall and formation of a basin overlying the footwall of Fault F2, and (2) similarity to the imbricated, landward-vergent thrust faults studied by Flueh et al. (1998) on
the accretionary prism offshore Washington, which have similar amplitude and wavelength to Fold F. Two splays (F1 and F2) of the deeperrooted thrust fault system could be mapped. Splay F1 was inferred from
the small interlimb angle of Fold F (Fig. F4A). Splay F2 is a conspicuous
termination surface in the deeper stratigraphy (Fig. F4). It disrupts
younger strata than does Splay F1, suggesting more recent activity than
Splay F1.
1.613
300
500
400
300
Fold F and Faults F1 and F2
200
100
350
Two-way traveltime (s)
250
Strike line
of F2
500
100
200
400
500
200
600
Hinge line
of the fold
300
200
100
Hinge line
of the fold
200
300
A
0
The primary structural features in the study area (shown on Fig. F4)
are (1) Fault/Fold F, which represents landward-vergent deformation
front during an earlier stage of development of Hydrate Ridge; (2) the
“Dome,” a broad uplift of Units S.VI and S.VII that has been intermittently active and now forms the core of Hydrate Ridge; (3) Anticlines A
and B, which are localized uplifts of Units S.VI and S.VII; and (4) Fault
System E, which forms the eastern boundary of the “Dome.” In this section, we reconstruct the history of activity of these features through detailed examination of the topography of key unconformities and
seismic stratigraphic patterns such as those shown in Figure F3.
1 km
1.2
C
1.4
1.6
1.8
Monocline
Offlapping strata
S.II
S.VII
2,500
3,000
3,500
4,000
Distance (m) Conformable strata
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
8
Anticline B (Fig. F4A–F4B), in the northeast part of the survey region,
has only a slight signature in the topography but represents dramatic
topography on Unconformity K (Fig. F7). Progressive fanning of the
strata of Unit S.IV on the eastern flank of Anticline B suggests that the
major growth period of the feature occurred at 1.0 to 0.3 Ma (Fig. F2). A
sharp unconformity at the top of Unit S.IV indicates that Anticline B
represented a bathymetric high ~0.2 m.y. ago, which is the minimum
age of Unit S.IB. The progressive tilting and bending of Units S.II and
S.IB and onlap of these onto the flanks of Anticline B suggest progressive subsidence of the feature relative to the Dome and eventual burial
by Unit S.IA since ~0.2 Ma. This is probably a result of reinitiation of
uplift of the Dome at ~0.3 Ma. This stage of uplift probably continues at
present.
Syn- and Posttectonic Deformation Recorded within Unit
S.II Strata
F9. Time map to Horizon B and
seismic data from Line 230, p. 27.
A
900
1.10
1.15
200
1.20
B
250
250
300
1.25
1.30
300
1.35
1.40
900
700
800
500
350
600
400
300
100
200
350
Two-way traveltime (s)
600
700
800
500
400
200
300
100
Hinge line of the fold
200
1.45
Horizon B time map; contour interval = 0.05 s
B
W
4,000
1 km
Distance (m)
Site
1246
1.50
E
Site
1244
5,000
6,000
1.2
Two-way traveltime (s)
Contours on the time map of Horizon B, located within Unit S.II,
show an anticline plunging to the north-northeast; the fold is slightly
asymmetrical, with the northeast limb dipping more steeply than the
western limb (Fig. F9A). The hinge line of the fold corresponds to the
crest of SHR and lies slightly to the east of and approximately parallel to
Fault F2 where defined by Horizon A, suggesting continuing thrusting
on Fault F2 through at least the time of deposition of Unit S.II. The eastern limb of the fold is more prominent and is disrupted by clear northsouth-striking normal faults that extend to the seafloor (Fig. F10B).
Thickening of the interval between the stratigraphic Horizons B and B′
on the downdropped side of the faults (Fig. F9B) suggests synsedimentary faulting.
Fault System E
1.3
1.4
C
1.5
B'
B
F10. Sequence of reconstructed
cross sections, p. 28.
Slice 230
W
E
Slice 300
W
E
1.2 Ma (deposition of Horizon A; transition from seaward to landward vergence and initiation of Fault F2)
1000 m
250 m
A deeply buried normal fault system accommodates the subsidence
of the eastern basin relative to SHR. Two north-south-striking faultsplays are shown on Figure F4C and F4D. The offsets of the faults average ~0.2 s and sum to a total offset of 0.5 s (~387 m) in Unit S.VI in the
southern part of the survey. Offset decreases to zero to the north, where
these faults are no longer imaged (Fig. F4A). It is possible, however, that
the vertical offsets observed in Figure F4C and F4D are the result of
wrench faulting along a strike-slip fault system. The maximum offset of
600 m recorded during the 3-D survey precludes imaging of a steeplydipping strike-slip fault where there is no significant component of dipslip displacement.
The geometry of Units S.IA and S.IB, which are draped over Fault System E, suggests syndeformation sedimentation accompanying uplift of
the Dome. This can be interpreted to result from either rapid uplift of
the Dome since 0.3 Ma in response to underplating and duplexing of
oceanic crust and sediments at depth (Tréhu et al., this volume), from
clockwise block rotation of Hydrate Ridge, which would result in downdrop of the basin relative to the Dome (Johnson et al., this volume), or
from a a combination of these mechanisms.
Dome
Dome
F2
F2
1.0 Ma (deposition of Horizon B'; deformation front at F2; development of Fold F and overlying lower slope basins;
second depocenter east of Anticline A; initiation of Anticline B)
Dome
A
B
B
Dome
Fold F
Fold F
0.3 Ma (deposition of Horizon C; reactivation of Dome uplift along a north-south axis )
B
A
Dome
B
Dome
Fold F
0.2 Ma (deposition of Horizon D; rapid uplift of the Dome and relative subsidence of Anticline B; massive slope instabiity filling eastern basin)
B
Dome
Fold F
Dome
F2
B
E
Present (continuing uplift of Dome, relative subsidence of B along Fault System E, rapid deposition in eastern basin)
Main depocenter
Dome
Fold F
Dome
B
F2
E
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
TECTONIC HISTORY OF SOUTHERN
HYDRATE RIDGE
Figure F10 summarizes the tectonic history of SHR as inferred from
the stratigraphic and structural interpretations presented in “Interpretation of Seismic Units and Structures,” p. 5. This reconstruction was
achieved by sequential removal of the effects of sedimentation and deformation. Structures of interest were unfolded by sequential flattening
of key horizons, resulting in the geometric adjustment of overlying layers. The changes in geometry of the overlying layers through time were
then used to indicate the timing of various folds throughout the region.
Plane strain was assumed, allowing application of the constant bed
length and the constant area rules. Some unit boundaries had to be approximated, for they are poorly constrained by the data. Horizontal
original bedding was assumed except in cases of clear buttress unconformities associated with Anticline B and Fault System E. We also interpreted the absence of depositional sequences to be evidence for
paleobathymetric highs that either impeded the deposition or caused
slope failure events. The cross sections correspond to east-west Lines
230 (Fig. F4A) and 300 (Fig. F4C) from the 3-D survey. These two lines
cut across most of the ODP sites, maximizing temporal constraints.
Line 230 represents the northern part of the survey area where the slope
basins formed by Units S.III, S.II, and S.IB are prominent and provides
detailed information on the late Pleistocene geology. Line 300 crosses
the shallowest part of Fold F and the Dome and provides better resolution for the tectonic reconstruction of early Pleistocene events. Comparison of the sections illustrates the important north-south variations
within the survey area.
Late Pliocene sediments originally deposited as part of the deep-sea
Astoria abyssal fan occur in the core of the proto-Dome (Unit S.VII),
which was probably formed at the toe of the accretionary complex
overlying a seaward-verging frontal thrust (Johnson et al., this volume). Rapid deposition of Astoria Fan sediments at the base of the slope
continued in the early Pleistocene with deposition of Unit S.V. Simultaneously, sediments of Unit S.VI were deposited on the lower slope, albeit at a slow rate. Glauconite-rich layers recovered at Sites 1251 and
1252 near the top of Unit S.VII and within Unit S.VI support the inference of very slow sedimentation in the eastern part of the study region
during this time period (Shipboard Scientific Party, 2003e). The wavy
and chaotic seismic character of Unit S.VI suggests pervasive compressional deformation throughout the proto-Dome during this time period, but the details are not well determined.
Sometime between the deposition of Horizon A′ and Horizon A,
(~1.2 m.y., based on biostratigraphy), the vergence of thrusting at the
deformation front changed to landward, which thrust sediments of
Unit S.V over the older sediments of Unit S.VII along Fault F2. The upper panels of Figure F10 depict this transition. Development of Fault F2
was accompanied by development of a drag fold, Fold F, creating a basin that was filled by sediments of Unit S.III. This model is supported by
the linear character and north-south strike of the hinge line of Fold F,
indicating east-west strain.
Fold F was active throughout deposition of Unit S.III, as indicated by
the divergence of the strata on the flanks of the fold (Figs. F4, F6). Analysis of the internal geometry of onlap and truncation of strata within
Unit S.III indicates that three paleobathymetric highs controlled the
9
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
distribution of Unit S.III: Fold F to the west, the Dome to the south, and
Anticline A to the east. This episode of sedimentation extended from
~1.0 to ~0.5 Ma, although timing of the transition from Unit S.III to S.II
is poorly constrained. Inconsistencies between mapping of coherent
seismic sequences and biostratigraphic constraints during this time period may be due to redeposition of sediments shed from the topographic highs because of slope instability. This pattern of sedimentation
continued from ~0.5 to ~0.3 Ma with deposition of Unit S.II. As Anticline B grew relative to Anticline A, it became the eastern boundary
structure of the basin in which Unit S.II was deposited.
Biostratigraphic data indicate that sediments of Unit S.IV, which are
found on the eastern and southern flanks of Anticline B, are coeval
with Units S.III and S.II. The strata of Unit S.IV are divergent as the result of the synsedimentary activity of Anticline B and unconformably
overlie S.VII strata at a very small angle (Fig. F4), indicating low relief of
Anticline B at 1.0 Ma and major uplift soon after 1.0 Ma, during the
deposition of Unit S.IV. Uplift of Anticline B appears to have stopped
relative to the Dome since 0.3 Ma, as the eastern basin was filled with
Units S.IA and S.IB. The lapping onto and thinning of Unit S.II on the
western flank of Anticline B and the rapid deposition of coeval Unit
S.IV on the eastern flank suggest that either there was a continuous
high between Anticline A and Anticline B that did not provide any accommodation space for sediment deposition in this region or that one
or more major slope collapses occurred between 1.0 and 0.3 Ma along
Horizon K on the northern flank of Anticline A. The latter would also
explain the sudden truncation of Unit S.VI strata to the north as observed on 2-D Line NS3 (Fig. F5).
Unit S.IV strata are truncated at the erosion Surface U (Fig. F4), indicating another major erosion event at ~0.3 Ma. Because the erosion surface is currently dipping to the south and there is no indication for a
tilt of the strata to the south, we suggest that the Event U corresponds
to a slope failure along the southern flank of Anticline B that truncated
Unit S.IV. This event possibly occurred during the final stage of uplift of
Anticline B. That Anticline B has been inactive since ~0.2 Ma is indicated by onlap of Unit S.IA.
The northeastern migration of uplift from Anticline A to Anticline B
is enigmatic, since the deformation front was probably migrating west
during this time as sediment was accreted. The abrupt southern termination of the Anticline B uplift is also difficult to explain in the context
of a north-south-trending subduction zone. We speculate that uplift followed by subsidence may have been due to subduction of a topographic
feature riding on the Juan de Fuca plate. Flemings and Tréhu (1999) inferred the presence of a deeply subducted ridge or chain of seamounts
beneath the upper slope of the Cascadia subduction zone from 42° to
44°N based on modeling of gravity and magnetic data. If this buried
structure is attached to the subducting plate, it would have been beneath the crest of Hydrate Ridge at ~1 Ma and would have moved
northeast relative to the upper plate. Subduction of seamounts is
known to have a impact on the structure of the continental margin offshore Central America (e.g., Ranero and von Huene, 2000; von Huene
et al., 2000) and Nankai (e.g., Park et al., 2004). In this model, the first
episode of uplift of the Dome (~1 Ma) and the uplift of Anticline B are
attributed to passage of the subducted ridge. Both of these episodes of
uplift were followed by sedimentation over the crest of the former topographic high, consistent with collapse of the margin in the wake of the
subducted ridge.
10
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
11
Reactivation of uplift of Unit S.VII along the present-day axis of Hydrate Ridge since ~0.3 Ma is inferred based on eastward migration of the
depocenter for Units S.II and S.IB (Fig. F8). This uplift along a northsouth axis was accompanied by normal faulting in Unit S.II. We interpret this to be due to a reinitiation of seaward-vergent thrusting farther
to the west accompanied by underthrusting of sediment and formation
of deeply buried sediment duplexes at depth beneath Hydrate Ridge
(Tréhu et al., this volume).
The development of Fault System E, which disrupts Unit S.VI beneath the southeastern flank of SHR, is the most recent structural feature we discuss. Offlapping of the Unit S.IB strata shows an increase in
the uplift rate relative to the sedimentation rate since ~0.2 Ma. Within
Unit S.IB, the increased abundance of the pelagic sediments with time
is observed (Shipboard Scientific Party, 2003b). This is interpreted to result from the isolation of the top of the ridge as it is uplifted out of the
region of turbidity current deposition and into a regime of pelagic
drape sedimentation. An increase in dip of the strata within Unit S.IA
with time and the two debris flow deposits (DBF1 and DBF2 on Fig. F4),
as well as the thinning of the beds against the flanks of the Dome provide evidence for the ongoing relative subsidence of the eastern basin
during the deposition of Unit S.IA. This may be due to rotation of Hydrate Ridge in a clockwise direction, with extension in the basin adjacent to southern end of the ridge, as discussed by Johnson et al. (this
volume).
IMPACT OF STRATIGRAPHIC AND STRUCTURAL
HISTORY ON GAS HYDRATE DISTRIBUTION
F11. Map of BSR amplitude, p. 29.
N
200
S.III
0.40
0.35
S.VII
S.IB
S.V
S.II
250
0.30
250
S.V
0.25
0.20
300
300
S.VI
no
BSR
350
900
800
700
500
600
300
200
400
Intersection of the BSR
with Unconformity K
0.15
0.10
350
1 km
0.05
0
Relative amplitude
800
0.45
900
600
Horizon B
Horizon B'
700
300
200
100
Intersection of the BSR
with Horizon A
200
100
Gas hydrates form when methane-rich fluids enter the GHSZ. At
SHR, the distribution of gas hydrates is highly heterogeneous (Tréhu et
al., 2004a). Generally, gas hydrate comprises <5% of the pore space, averaged over the GHSZ. However, massive gas hydrate deposits, estimated to comprise ~25% of the total volume, are present in the upper
20–30 m at the summit of the ridge. Very little hydrate (<2% of pore
space) was found within the sediments of the eastern basin, except for
immediately above the base of the GHSZ. We note that no drilling was
permitted at the crest of Anticline B. In this section, we discuss how the
geologic history outline in the previous sections has influenced largescale patterns in gas hydrate distribution.
The BSR is a conspicuous reflection with negative amplitude through
most of the 3-D survey. Its amplitude, however, varies greatly. Figure
F11 shows the variation in relative amplitude of the BSR overlain by the
boundaries between the stratigraphic units on the BSR surface. It shows
several distinct zones of high amplitude.
The most prominent high-amplitude anomaly is associated with the
buried Anticline B and lies within Unit S.VII. We note that a drill site
that was originally located on this anomaly was moved to Site 1252 by
the ODP Pollution Prevention and Safety Panel. We interpret the relatively uniform character of this amplitude anomaly to indicate the
widespread presence of free gas throughout these older, seismically incoherent, and presumably pervasively fractured sediments and relatively high and heterogeneous gas hydrate content within the GHSZ.
Another pronounced bright spot is located in sediments of Unit S.VI
on the western flank of SHR. This bright spot is associated with the lo-
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
cal circular high that was active prior to deposition of Horizon A. This
relict structure, which may have been a mud volcano in the trench, as
is observed in subduction zones elsewhere, appears to remain as a preferential methane migration path ~1 Ma since it was active. No drilling
data are available to test this hypothesis.
The summit region is characterized by a relatively large zone containing numerous linear and patchy bright spots in the tightly folded and
deformed sediments of Unit S.VI. We interpret these patterns to reflect
variable permeability controlled by discrete stratigraphic horizons and
faults.
Where the base of the GHSZ falls in the younger sediments of Units
S.III, S.II, and S.I, the BSR is generally faint or absent except where
coarse-grained horizons like A, B, and B′ exist to provide paths for gas
migration. Where these horizons intersect the BSR, strong anomalies are
observed. These may, at least in part, indicate constructive interference
between two negative polarity reflections. The numerous small normal
faults that offset strata in Unit S.II may also contribute to the bright BSR
and to relatively high and heterogeneous (1%–8%) gas hydrate content
within the GHSZ (Tréhu et al., 2004a; Weinberger et al., 2005).
In summary, the strongest BSR anomalies are associated with the oldest sediments. We infer that these have the potential to deliver free gas
to the GHSZ more readily than the younger strata for at least two reasons. First, these older sediments have generally undergone greater uplift, leading to a large decrease in the solubility of methane in the pore
fluids because of depressurization. Based on the methane content of
abyssal plain sediments drilled at Deep Sea Drilling Project (DSDP) Site
174, these sediments should be supersaturated in methane when uplifted to the depth at which they are now found (Claypool and Kaplan,
1974; Claypool et al., this volume; Tréhu et al., this volume). Second,
pervasive fracturing of recovered samples, the variable character of geophysical logs from these zones, and very low gradients in chloride concentration (Shipboard Scientific Party, 2003b, 2003d, 2003e; Torres et
al., 2004b) suggest that the accretionary complex sediments are permeable and that the pore fluid within them is well mixed, allowing efficient transport of gas into the GHSZ. We note that at northern Hydrate
Ridge, where venting and authigenic carbonate development is widespread over the summit of the structure, sediments of Units S.VII and
S.VI are present very near the surface, consistent with the association
we document here of older sediments with venting. Our results also
demonstrate the value of high-resolution seismic data in identifying
relict buried structures within the older sediments that focus fluid flow.
CONCLUSIONS
We have reconstructed the geologic history of SHR using techniques
adapted from seismic sequence stratigraphy and ages determined from
biostratigraphic analysis of Leg 204 samples. Between 1.6 and 1.2 Ma,
sediments from the abyssal plain were accreted to the margin along a
seaward-vergent thrust fault. At ~1.2 Ma, vergence direction changed
and Fault F2, a landward-vergent fault overlain by a drag fold (Fold F)
thrust deep-sea fan sediments over the recently accreted sediments.
This episode of landward vergence lasted to ~0.3 Ma. During this time
period, uplift in the accretionary complex shifted toward the northeast,
from Anticline A on the eastern flank of the present ridge to Anticline
B, which is now buried beneath the sediments of the slope basin east of
12
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
Hydrate Ridge. This migrating uplift resulted in shifting depocenters
and overlapping slope basins. Early to middle Pleistocene strata (Units
S.IV, S.III, and S.II) of these basins consist of recycled abyssal plain sediments shed from the bathymetric highs and ponded into the depocenters. The cause of this northeastward shift in uplift while the
deformation front migrated west remains enigmatic but may be related
to passage of a ridge or seamount on the subducting plate. A second reorganization of the deformation front, to seaward thrusting farther to
the west, began sometime prior to 0.3 Ma, probably resulting in reactivation of uplift of the modern Hydrate Ridge in response to sediment
underplating and duplexing. Normal faulting in the slope basin sediments at the crest accompanied this phase, which likely continues today.
Because of this history, a wide range of lithologies currently falls
within the gas hydrate stability zone at SHR. We note a correlation between BSR amplitude and sediment age, with high-amplitude BSR
anomalies correlated with uplifts of the older, lithified, and fractured
accreted sediments that were originally deposited on the abyssal plain.
Gas to feed gas hydrate formation in these sediments is probably exsolved from pore water as a result of decreased gas solubility due to tectonic uplift. We infer that the gas migrates into the gas hydrate stability
zone through pervasive fracture permeability in the accretionary complex and that gas migration and gas hydrate formation in the overlying
slope basin sediments is limited to a few discrete permeable horizons or
faults.
ACKNOWLEDGMENTS
We thank the crew of the Thomas Thompson for their efforts to maintain what must have seemed to them to be a strange track line while we
were acquiring the 3-D seismic survey. We also thank John Diebold,
Scholtz, Mark Weiderspahn, and Steffan Saustrup for keeping the GI
guns, acquisition system, and data quality control hardware and software going. Finally, we thank the ODP staff and crew of the JOIDES Resolution for a job well done. This research used samples and/or data
provided by the Ocean Drilling Program (ODP). ODP is funded by the
U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Acquisition and analysis of the seismic data were funded by grants to OSU
(JOI-U.S. Science Support Program [USSSP] F01558, NSF OCE-9907205,
and NSF OCE-0002410) and to UTIG (NSF OCE-9907205). Data processing was performed using Kingdom Suite software donated to OSU by
Seismic Micro-Technology, Inc.
13
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
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2004GL021437
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SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
17
Figure F1. A. Location of Hydrate Ridge in the Cascadia accretionary complex. The path of sediments from
the Columbia River, which form the Astoria Fan and fill the subduction zone trench, is also shown (blue
arrow). Deep Sea Drilling Project (DSDP) Sites 174 and 175 originally documented the influence of the Astoria Fan on development of the accretionary complex. B. Detailed bathymetric map of the accretionary
complex in the vicinity of Hydrate Ridge. This part of the accretionary complex is characterized by strong
along-strike variability in topography, in contrast to the region to the north, which is characterized by
long, linear ridges. The location of ODP Site 892, drilling on northern Hydrate Ridge in 1992, and the limits
of the 3-D seismic survey discussed in this paper are shown. SHR = southern Hydrate Ridge. C. Detailed
topographic map in the vicinity of the 3-D seismic survey. Contour interval is 20 m. Locations of drill sites
for Leg 204 and the position of seismic lines shown in this paper are shown. Line numbers within the 3-D
volume extend from 200 at the northern edge of the survey to 350 at the southern edge and from 100 at
the western edge to 833 at the eastern edge. MBARI - Monterey Bay Aquarium Research Institute. D. Depthconverted seismic section from the site survey for Leg 146 (Tréhu et al., this volume). Box shows area detailed in Figure F4A, p. 21.
A
B
2000
Site 892
NHR
1000
250
0
44°
Site 175
Oregon
44°36'
Pacific
plate
500
Astoria
Fan
0
50
plate
Site 174
44°48'
N
1000
46°
Co
Juan de Fuca lumbia Rive
r
1500
Washington
1500
48°
N
SHR
42°
California
200 km
44°24'
124°
125°24'W
Line
Line
NS1
128°
44.60°
N
124°48'
90
1200
Site 1246 Site 1252
Site 1244
Site 1247
Site 1251
Site 1248 Site 1249
EW 230
EW 250
125°00'
0
Western boundary of
MBARI EM300 survey
C
125°12'
NS3
100
Line
0
132°W
300
0
NS2
40°
Site 1245
EW 300
1240
Site 1250
EW 343
800
Seismic volume
1 km
44.55°
125.20°W
125.15°
125.10°
125.05°
125.00°
D
Fig. F4A
1
Landward verging
Depth (km)
3
Sea
war
d
5
ime
7
nt s
ubd
ucte
d in
9
6.25
0
ver
g
Accreted, underthrust,
duplexed sediment
ing
Sed
6.25
the
pas
t ~0
Active duplexing?
.4 m
.y.
12.50
Distance from deformation front (km)
18.75
25.00
E-W transect
N-S transect
N
0.3
0.5
B
1.0
B'
III
B
Unit S.V
300
Unit S.VII
early Pleistocene
Depth (mbsf)
1.0
1.2
1.6
IV
A
A
2.4
IV
B
400
Unit S.IB
D
IA
0.1
IB
ID
0.3
II
Unit S.II
II
.VI
it S
Un
K
1.0
1.6
Unit S.IV
III
1.7
2.0
III
Unit S.VII
middle Pleistocene-Holocene
III
A
II
m. Pleist.-Hol.
IB-C
(0.5)
middle Pleist.-Hol.
II 0.3
C
Unit S.IA
I
1251
0.1
IC
U
0.3
0.5
IIA
BSR
K
IIB
late Plio. early Pleistocene
200
III
A
Y
I
1252
early Pleist.
IIIB
II
1244
l. Plio..
IIIB
I
1246
early Pleist.
III
A (1.6)
I
E
late Plio.
Unit S.III
(1.6)
1245
middle Pleist.-Holocene
III
III
A (1.6)
1247
m. Pleist.-Hol.
Pleist.-Hol.
Pleist.-Hol.
I-II 0.3
I-II 0.3
0.5
early Pleistocene
A
II 0.5
1248
early Pleist. Pleist.-Hol.
BSR
early Pleistocene
Y
100
1249
1250
I
0
W
m. Pleist.-Holocene
S
1.0
1.6
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
Figure F2. Comparison of lithologic units defined by the shipboard sedimentologists during Leg 204 (Shipboard Science Party, 2003a) with the
seismic units defined in this paper. Lithologic units are shown as columns at each site. Ages determined by shipboard paleontologists, in millions
of years, are shown as red numbers along the sides of the lithologic columns. Ages in parentheses are not compatible with the seismic data and
are attributed to reworking of older sediments. Seismic stratigraphic units represent coherent sediment packages that can be mapped between sites
and are labeled as Unit S.VII to S.IA. These are separated by seismic stratigraphic Horizons A, B, B′, C, D, and Y and major angular unconformities
K and U. The position of significant structures (Anticlines A and B; Fold F; Faults F1, F2, and E; and the “Dome”) and of the bottom-simulating
reflection (BSR) are also shown.
III
2.0
V
500
>1.6
18
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
19
Figure F3. A. Nomenclature for growth strata deposited during thrust folding. B–E. Model of overlap,
offlap, and onlap. Predictable geometries of syntectonic strata result from different ratio between the relative rates of crestal uplift and coeval sedimentation rates. Crestal uplift is measured with respect to either
the base of the syntectonic strata adjacent to the fold or the position of the correlative marker beds found
in both the anticline and the adjacent syncline. (B) When rates of accumulation are consistently greater
than the rate of crestal uplift, overlap will occur. (C) Lower rates of accumulation versus uplift lead to offlap.
(D) Reversals in the relative magnitude of these rates cause a switch in the bedding geometry. (E) Onlap
occurs following offlap and a change to more rapid accumulation rates (A–E modified from Burbank and
Verges, 1994). (Continued on next page.)
A
Fold crest
l
Fore
Growth strata
b
Pregrowth strata
i mb
klim
Bac
Thrust fold
B
Overlap
3
C
Offlap
3
2
D
1
1
Uplift < Accumulation
Uplift > Accumulation
Overlap, then offlap
E
2
Offlap, onlap, then overlap
3
3
2
2
1
Uplift < Accumulation
then
Uplift > Accumulation
1
Uplift > Accumulation
then
Uplift < Accumulation
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
20
Figure F3 (continued). F. Examples of these characteristic seismic stratigraphic patterns in the 3-D data
from southern Hydrate Ridge. BSR = bottom-simulating reflection.
F
W
Syntectonic basin fill
ns3,
E
Site 1244 '
Distance (m):
Site 1252
2500
3000
3500
4000
4500
1.100
Two-way traveltime (s)
1.200
C
1.300
B
1.400
Seafloor
A
S.II
1.500
1.600
1.700
S.VII
1.800
Line:
Trace:
222
614
222
624
222
634
222
644
Line:
Trace:
226
519
226
529
226
539
226
549
226
559
226
569
1.3
1.4
Seafloor
S.IA
1.5
S.IB
Seafloor
S.IA
1.4
S.IB
BSR
1.5
Anticline B
Onlap and overlap
Divergence
C
Line: 230
Trace: 434
230
444
230
454
230
464
230
474
230
484
1.3
Parallel bedding
overlain by
onlap
S.IB
BSR
1.4
S.II
Two-way traveltime (s)
Two-way traveltime (s)
B
222
654
Two-way traveltime (s)
A
1.5
200 m
A
Site
1245
W
2,000
1.1
3,000
Site
1246
4,000
Site
1244
5,000
6,000
EW Line 230
Site
1252
Distance (m)
7,000
E
8,000
9,000
10,000
11,000
1.100
1.200
B'
B
A
1.5
C1.400
Dome
A''
D
1.500
K
1.600
A'
1.7
1.300
1.700
Fold F
S.VII
U
DBF1
0
S.VI
DBF2
Anticline B
200
1.900
1.9
F2
2.1
S.VII
400
2.100
F1
2.200
2.300
2.3
600
2.400
W
2,000
Two-way traveltime (s)
1.1
3,000
4,000
5,000
EW Line 250
1.3
1.5
Dome
F
1.7
1.9
2.1
2.3
F2
6,000
1.100
1.200
1.300
1.400
1.500
Anticline
1.600
A
1.700
1.800
1.900
2.,000
2.100
2.200
2.300
2.400
2.500
7,000
K
8,000
K
9,000
B
S.VI
S.VII
10,000
U
0
200
400
600
21
2.5
E
Distance (m)
Depth (m)
B
Depth (m)
Two-way traveltime (s)
C
Y
1.3
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
Figure F4. A, B. East-west slices through the 3-D seismic volume. Overlays show seismic stratigraphic units. Color code and letters to identify
seismic units are the same as in Figure F2, p. 18, which shows correlation of seismic units with lithologic units and biostratigraphic ages. Important
structural features are also labeled and identified as in Figure F2, p. 18. The depth axis for A and B was derived from correlation with Site 1245.
DBF = debris flow deposit. (Continued on next page.)
C
Site
1249
W
3,000
1.1
Site
1251
Distance (m)
4,000
5,000
6,000
7,000
EW Line 300
8,000
9,000
E
10,000
11,000
1.100
1.200
1.300
S.VI
1.400
Anticline A
1.5
1.500
D
Dome
Fold F
1.600
S.VII
1.7
1.800
2.,000
W
2.200
400
2.300 Fault
System
2.400
E
600
E
Distance (m)
2,000
1.1
200
Anticline B
2.100
2.3
D
U
1.900
1.9
2.1
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
EW Line 343
1.3
Fold F
1.5
Dome
S.VI
Anticline A
0
1.7
1.9
2.1
2.3
200
F2
Fault
System
E
400
Depth (m)
Two-way traveltime (s)
0
1.700
Depth (m)
1.3
Two-way traveltime (s)
C
A
A'
A''
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
Figure F4 (continued). C, D. The depth axis on C and D was derived from Site 1251, Lines 300 and 343.
22
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
23
Figure F5. Seismic Line NS3 showing thinning of Units S.I and S.II toward the northern and southern summits of Hydrate Ridge. The location of this regional 2-D seismic profile is shown in Figure F1C, p. 17. NHR
= northern Hydrate Ridge. SHR = southern Hydrate Ridge.
EW Line 230
EW Line 250 EW Line 300
Two-way traveltime (s)
1.0
Site
1244
EW Line 343
SHR
1.2
C
S.IB
1.4
1.6
S.II
NHR
B
K
S.VI
B'
Dome
S.VII
1.8
2.0
6,000
7,000
8,000
9,000
10,000
Distance (m)
11,000
12,000
13,000
14,000
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
24
Figure F6. A. Map of Horizons A′ and A showing the strike of Fold F and Fault F2. B. Temporal evolution
of Fold F shown by a series of seismic sections which have been progressively flattened on Horizons A″, A′,
and A. That the presence of a local bulge active prior to ~1 Ma influences fluid flux at present is suggested
by the large amplitudes beneath this structure and the large amplitude and slight relative uplift of the bottom-simulating reflection (BSR) here.
1.250
1.354
600
500
1.302
1.406
1.458
1. 4
0
0
250
300
1.510
300
1.562
350
W
Distance (m)
2,000
Two-way traveltime (s)
1.2
1.3
1.665
1 km
Horizon A' time map; contour interval = 0.025 s
B
600
400
1 km
500
1.613
300
200
100
500
400
300
200
100
350
Horizon A time map; contour interval = 0.025 s
E
3,000
2,000
Y
Time 1: flattening on Horizon A''
(between Time 1 and Time 2,
sediments thin to the west against
a local bulge)
Distance (m)
W
1.3
1.4
1.700
E
3,000
Time 2: flattening on Horizon A'
(end of the activity on the
local bulge)
A
1.5
1.4
1.5
A'
1.6
1.7
1.6
A''
1.7
1.8
1.9
1.8
2.0
W
Distance (m)
2,000
Two-way traveltime (s)
1.4
1.5
E
Time 3: flattening on Horizon A
(Fold F begins to grow and deform
older strata)
2,000
1.2
E
3,000
Present time 3:
(no flattening)
1.3
1.6
1.4
1.7
1.5
1.8
1.6
1.9
1.7
2.0
1.8
2.1
Distance (m)
W
3,000
<-1.2 m.y.
Two-way traveltime (s)
250
400
200
Strike line
of F2
300
100
400
500
Hinge line
of the fold
300
200
200
100
Hinge line
of the fold
200
A
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
25
Figure F7. A. Time map to Unconformity K, which marks the boundary between seismic Units S.VI and
S.VII and younger overlying strata. This map has been extended north of the 3-D survey using the 2-D profiles. It shows the circular nature of the Dome uplift and Anticlines A and B. NHR = northern Hydrate Ridge.
B. 3-D perspective of the top of the accretionary complex. East of southern Hydrate Ridge, this surface is
defined by Unconformity K; to the west, it is defined by Fold F and Fault F2 (Fig. F6A, p. 24). The northward
plunge of Fold F and the southward plunge of Anticline B are apparent.
A
NHR
1.1 s
1.1
200
B
1.5
1. 6
250
250
00
300
Dome
300
A
900
800
700
600
500
400
300
200
1.6
1.7
1.8
350
100
1.4
350
1.9
2.0
1
.6
00
1 km
Unconformity K time map; contour interval = 0.1 s
B
20
0
Line
35
0
0
10
N
Trac
e
Fold F
0
90
1.0
Dome
Anticline A
1.2
1.4
1.6
3-D view of the top of the
accretionary complex, which includes
Unconformity K to the east and Fault F2
to the west.
Anticline B
Two-way traveltime (s)
1.0
1.8
2.0
2.0
Two-way traveltime (s)
1.3
900
800
700
600
500
400
300
200
200
100
1.2
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
26
Figure F8. Isopach maps of Units (A) S.III, (B) S.II, (C) S.IB, and (D) S.IA showing the eastward migration
of depocenters as the Dome underwent continuing uplift. E. Schematic evolution of Units S.II and S.IB
based on seismic stratigraphic relationships. Evidence of continuing uplift is also seen in the stratigraphic
patterns within these units, which show conformable strata downdip of offlapping strata which overlie a
monocline. This reconstruction is based on the portion of 3-D Line 230 shown in Figure F3F, p. 20.
00
0.15
300
0.10
0.0
0.050
300
900
800
700
600
500
400
300
200
350
Unit S.III time isopach map; contour interval = 0.05 s
0.05
350
0
W on seismic stratigraphic relationships illustrated
in Figure F3F on Line 230.
Two-way traveltime (s)
0.20
250
Two-way traveltime (s)
900
800
700
600
500
400
300
200
250
0.30
200
0.25
Schematic evolution of Units II and IB based
E
0.35
0.100
100
200
100
Two-way traveltime (s)
A
1.2
1.4
Unit S.II
1.6
S.VII
1.8
1 km
2,500
4,000
4,500
900
800
700
600
500
400
300
200
100
250
0.050
0.0
00
250
300
350
0.0
0.050
00
300
900
800
700
600
500
400
300
200
350
Two-way traveltime (s)
0.150
00
0.1
0.050
0
0.0
0
300
350
900
800
700
600
500
400
300
350
200
S.II
S.VII
Unit S.IB time isopach map; contour interval = 0.05 s
3,000
3,500
4,000
4,500
Distance (m)
250
100
1.6
2,500
200
300
1.4
900
800
200
250
1.2
1.8
1 km
700
600
500
400
300
100
200
Unit S.II time isopach map; contour interval = 0.05 s
C
Two-way traveltime (s)
200
0.100
200
100
Two-way traveltime (s)
3,500
Distance (m)
B
Two-way traveltime (s)
3,000
1.2
1.4
S.II
1.6
S.VII
1.8
2,500
1 km
3,000
3,500
4,000
4,500
200
0
25
0.
0.20
05
0
250
0.
250
0.250
0.050
0.10
0
300
350
300
900
800
700
600
500
400
300
200
350
Unit S.IA time isopach map; contour interval = 0.05 s
Two-way traveltime (s)
900
800
0.
15
0
0.000
0
700
600
500
400
300
100
200
100
Two-way traveltime (s)
D
200
Distance (m)
1 km
1.2
C
1.6
1.8
Monocline
Offlapping strata
1.4
S.II
S.VII
2,500
3,000
3,500
4,000
Distance (m) Conformable strata
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
27
Figure F9. A. Time map to Horizon B showing a northeast-plunging anticline. This is interpreted to result
from reactivated uplift of southern Hydrate Ridge (SHR). The location of the seismic section shown in part
B is indicated by a thick black line. B. Detail of seismic data from 3-D Line 230 near Sites 1244 and 1246
showing normal faulting in Units S.II and S.IB. This faulting appears to be syndepositional and to continue
to the seafloor, which shows a pattern of small northeast-trending ridges in this region (Johnson et al.,
2003). These faults appear to provide pathways for migration of gas hydrate into the GHSZ, as indicated by
relatively large but laterally variable gas hydrate content (Tréhu et al., 2004a) and the absence of a double
BSR (Bangs et al., 2005) in this region.
A
900
1.10
1.15
200
1.20
B
250
250
300
1.25
1.30
300
1.35
350
1.40
900
800
700
600
500
400
300
200
100
350
1.45
Horizon B time map; contour interval = 0.05 s
B
W
4,000
Site
1246
Distance (m)
5,000
1 km
1.50
E
Site
1244
6,000
Two-way traveltime (s)
1.2
1.3
1.4
C
1.5
B'
B
Two-way traveltime (s)
800
700
600
500
400
300
200
200
100
Hinge line of the fold
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
28
Figure F10. Sequence of reconstructed cross sections along east-west seismic Lines 230 and 300. Retrodeformation of the strata to their original geometry was achieved using rules of constant length and surface
and vertical and horizontal reference pin lines (marked by a black lines and red nail). Color code and labels
for structures are the same as in Figure F2, p. 18.
Slice 230
W
E
Slice 300
W
E
1.2 Ma (deposition of Horizon A; transition from seaward to landward vergence and initiation of Fault F2)
250 m
1000 m
Dome
Dome
F2
F2
1.0 Ma (deposition of Horizon B'; deformation front at F2; development of Fold F and overlying lower slope basins;
second depocenter east of Anticline A; initiation of Anticline B)
Dome
A
B
B
Dome
Fold F
Fold F
0.3 Ma (deposition of Horizon C; reactivation of Dome uplift along a north-south axis )
B
A
Dome
B
Dome
Fold F
0.2 Ma (deposition of Horizon D; rapid uplift of the Dome and relative subsidence of Anticline B; massive slope instabiity filling eastern basin)
B
Dome
Fold F
Dome
F2
B
E
Present (continuing uplift of Dome, relative subsidence of B along Fault System E, rapid deposition in eastern basin)
Main depocenter
Dome
Fold F
Dome
B
F2
E
J. CHEVALLIER ET AL.
SEISMIC SEQUENCE STRATIGRAPHY AND TECTONIC EVOLUTION
29
Figure F11. Map of bottom-simulating reflection (BSR) amplitude with contours showing boundaries between seismic stratigraphic units along this surface. Polarity of this reflection is everywhere negative. Orange lines indicate the intersection of seismic unit boundaries with the BSR. Labels for unit boundaries are
the same as in Figure F2, p. 18.
N
0.45
200
S.III
S.V
S.II
250
0.35
S.VII
S.IB
0.40
0.30
250
S.V
0.25
0.20
300
300
S.VI
no
BSR
350
0.10
900
800
700
600
500
400
300
200
100
350
Intersection of the BSR
with Unconformity K
0.15
1 km
0.05
0
Relative amplitude
900
800
700
Horizon B
Horizon B'
600
300
200
200
100
Intersection of the BSR
with Horizon A