Geometry, kinematics, and
displacement characteristics of
tear-fault systems: An example
from the deep-water Niger Delta
Nathan P. Benesh, Andreas Plesch, and John H. Shaw
ABSTRACT
We use three-dimensional seismic reflection data and new mapbased structural restoration methods to define the displacement
history and characteristics of a series of tear faults in the deepwater Niger Delta. Deformation in the deep-water Niger Delta
is focused mostly within two fold-and-thrust belts that accommodate downdip shortening produced by updip extension on
the continental shelf. This shortening is accommodated by a
series of thrust sheets that are locally cut by strike-slip faults.
Through seismic mapping and interpretation, we resolve these
strike-slip faults to be tear faults that share a common detachment level with the thrust faults. Acting in conjunction, these
structures have accommodated a north–south gradient in
westward-directed shortening. We apply a map-based restoration technique implemented in Gocad to restore an upper
stratigraphic horizon of the late Oligocene and use this analysis
to calculate slip profiles along the strike-slip faults. The slip
magnitudes and directions change abruptly along the lengths
of the tear faults as they interact with numerous thrust sheets.
The discontinuous nature of these slip profiles reflects the manner in which they have accommodated differential movement
between the footwall and hanging-wall blocks of the thrust
sheets. In cases for which the relationship between a strikeslip fault and multiple thrust faults is unclear, the recognition
of this type of slip profile may distinguish thin-skinned tear
faults from more conventional deep-seated, throughgoing
strike-slip faults.
Copyright ©2014. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received January 25, 2011; provisional acceptance March 30, 2011; revised manuscript
received May 5, 2013; final acceptance June 25, 2013.
DOI:10.1306/06251311013
AAPG Bulletin, v. 98, no. 3 (March 2014), pp. 465–482
465
AUTHORS
Nathan P. Benesh Department of Earth and
Planetary Sciences, Harvard University, 20 Oxford
Street, Cambridge, Massachusetts 02138; present
address: ExxonMobil Upstream Research Company, Houston, Texas; [email protected]
Nathan P. Benesh is a research geologist in the Structure and Geomechanics Group at the ExxonMobil
Upstream Research Company. He previously received his Ph.D. in earth and planetary sciences at
Harvard University. His research interests focus
on geomechanical modeling and the quantitative
evaluation of structures at the trap to regional scale.
Andreas Plesch Department of Earth and
Planetary Sciences, Harvard University, 20 Oxford
Street, Cambridge, Massachusetts;
[email protected]
Andreas Plesch is a senior research associate in the
Structural Geology and Earth Resources Group in
the Earth and Planetary Science Department,
Harvard University. He received his Ph.D. from
Free University, Berlin, Germany. His research interests revolve around three-dimensional modeling and the analysis of structures on the reservoir
to mountain belt scale with a focus on quantitative
aspects.
John H. Shaw Department of Earth and
Planetary Sciences, Harvard University, 20 Oxford
Street, Cambridge, Massachusetts;
[email protected]
John H. Shaw is the Harry C. Dudley professor of
structural and economic geology and chair of the
Department of Earth and Planetary Sciences,
Harvard University. Prior to joining the Harvard
faculty, Shaw worked as an exploration and production geologist. His research interests include
the structural characterization of complex traps
and reservoirs in both conventional and unconventional petroleum systems.
ACKNOWLEDGEMENTS
We thank CGGVeritas for providing the seismic
data used in this study and Landmark Graphics
Corporation for donating the software through its
University Grant Program. We also thank ExxonMobil for its support of this work and Chevron
Corporation for providing additional data sets.
The AAPG Editor thanks the following reviewers for
their work on this paper: Christopher F. Elders,
Stephen J. Naruk, and Sandro Serra.
INTRODUCTION
The Niger Delta, situated in the Gulf of Guinea at
the southern end of the Cretaceous Benue trough
rift basin, represents one of the largest modern
deltas and most productive regions for petroleum
exploration in the world (Doust and Omatsola,
1990; Figure 1). The delta serves as a prime example of gravity-driven tectonic deformation, a
characteristic of many deep-water passive margins
(Evamy et al., 1978; Doust and Omatsola, 1990).
The delta began to form as sediments that shed
from the Niger River gradually filled the Benue
trough during the opening of the equatorial Atlantic, and, by the late Eocene, the delta had begun to prograde on top of the continental margin.
Two distinct fold-and-thrust belts within the delta
accommodate the shortening and downdip deformation produced by gravity-driven extension
on the continental shelf. This gravity-driven extension is caused by rapid sediment deposition
that leads to differential loading and the subsequent formation of large normal faults within the
deltaic deposits on the continental shelf (Wu and
Bally, 2000). This updip extension is fed at depth
onto several detachment surfaces within thick overpressured shales of the early to middle Paleogene
that underlie the postrift deltaic deposits (Bilotti
et al., 2005; Corredor et al., 2005). Shortening in
the deep water began in the late Miocene to the
early Pliocene to accommodate the slip that extends downdip along these detachments. The
thrust faults produced by this shortening occur in
two distinct provinces, termed the “inner fold-andthrust belt” and the “outer fold-and-thrust belt”
(Connors et al., 1998; Corredor et al., 2005).
With the advent of petroleum exploration in the
deep-water Niger Delta in the 1990s, high-quality
two-dimensional (2-D) and three-dimensional (3-D)
seismic reflection data have been acquired, which
have allowed for the investigation of the nature of
these gravity-driven contractional structures. These
studies have mainly focused on the analysis of the
deep-water thrust faults and fault-related folds, including their structural geometries and kinematics
(e.g., Connors et al., 1998; Shaw et al., 2004; Suppe
Figure 1. A generalized map of the main structural provinces of the Niger Delta and a schematic diagram of the stratigraphy in the area
of interest. Also shown are the locations of the three-dimensional (3-D) seismic reflection data volume and two-dimensional (2-D)
seismic survey used in this study. Qua. = Quaternary; Plio. = Pliocene; Oligo. = Oligocene; Paleo. = Paleocene; Cret. = Cretaceous.
466
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
et al., 2004; Corredor et al., 2005), and the effects of
high basal fluid pressures (e.g., Bilotti et al., 2005)
and multiple detachment surfaces (e.g., Corredor
et al., 2005; Briggs et al., 2006) on their structural
styles. It is generally accepted that these thrustrelated structures provide the dominant means by
which shortening is accommodated in the compressional toe of the delta; however, in this study,
we show that transport-parallel strike-slip faults
are also an important factor. We use both highresolution 2-D and 3-D seismic data (Figure 1) to
analyze a system of tear faults in the outer foldand-thrust belt that partition distal contractional
deformation.
The term “tear fault” has long been applied to
strike-slip faults that abruptly terminate thrust
sheets alongstrike (e.g., Twiss and Moores, 1992).
The Jacksboro and Russell Fork faults, which bracket
the Pine Mountain thrust sheet in the southern Appalachian Valley and Ridge Province in the eastern
United States, provide a well-known classic example of a tear-fault system (Mitra, 1988). Beyond
the Appalachians, thin-skinned tear faults that only
involve the shallow sedimentary section have also
been noted in the Canadian Rocky Mountains
(Benvenuto and Price, 1979), the Western Foothills
of Taiwan (Mouthereau et al., 1999), the Santa
Barbara Channel (Shaw and Suppe, 1994), and the
Maracaibo Basin in Venezuela (Escalona and Mann,
2006), among other localities. These tear faults are
commonly recognized based on surface exposure
or interpretations of 2-D seismic lines. The inherently 3-D nature of these faults, however, has made
it difficult to fully constrain their geometries, kinematics, and displacement characteristics. Leduc
et al. (2012) used 3-D seismic data to evaluate a
strike-slip system that accommodates motion on
primarily extensional faults on the northern margin of the Niger Delta, inboard of the outer foldand-thrust belt; however, we know of no other
study in which 3-D seismic data have been applied
to carefully examine the full spatial relationship
and displacement profiles for multiple tear faults
and the thrust faults that they more commonly
terminate. The aim of this article, based on the work
of Benesh (2010), is to more fully characterize the
nature of tear faults and their relationships with
thrust-fault systems using 3-D seismic reflection
data that image such structures in the outer foldand-thrust belt of the Niger Delta. The system we
analyze contains multiple strike-slip faults that
work in tandem with the thrust structures to partition strain and accommodate a gradient in shortening across the fold-and-thrust belt. In addition to
analyzing the geometric and genetic relationships
among the faults, we quantify shortening across the
region, and, by means of a map-view surface restoration, we determine diagnostic slip profiles for the
strike-slip tear faults.
GEOLOGIC SETTING
The focus of this study is a series of tear faults that
occur near the northern termination of the outer
fold-and-thrust belt of the Niger Delta. The outer
fold-and-thrust belt represents the more distal of
the two fold-and-thrust belts that accommodate
outboard shortening driven by gravitational collapse and extension of the deltaic sequence on the
continental shelf. For most of the outer fold-andthrust belt, the primary detachment lies within the
Akata Formation (Corredor et al., 2005), a timetransgressive, thick marine shale that generally sits
atop an Upper Cretaceous sedimentary sequence,
which itself overlies an Early Cretaceous oceanic
crust (Avbovbo, 1978; Knox and Omatsola, 1989;
Wu and Bally, 2000; Figure 1). This shale is believed to be one of the major source rocks for hydrocarbons in the delta and also contains some
locally developed turbidite sands. Additionally,
the formation generally exhibits a low seismic
velocity that reflects fluid overpressures that locally reach 90% of the lithostatic stress (Bilotti
et al., 2005). Above the Akata Formation lies the
Agbada Formation. This formation is Eocene to
Pleistocene in age and is believed to be the dominant petroleum-bearing unit in the delta. In our
study area, the lowest part of the Agbada consists
of channel complexes and basin-floor fans composed of sediments that are believed to be sourced
from the Dahomey trough of the onshore Guinea
Basin. This unit tapers out southward and is
termed the “Dahomey wedge” (Morgan, 2004;
Benesh et al.
467
Figure 2. A representative thrust fault
from the outer fold-and-thrust belt in the
northwestern part of the Niger Delta. This
structure displays the classic characteristics
of a shear fault-bend fold as reflected in
the long and shallowly dipping backlimb.
The interpreted detachment level for this
structure (red dashed line) is above the
Akata Formation, within the Dahomey
wedge. All seismic data presented here are
migrated and depth converted. Data are
owned and provided courtesy of CGGVeritas,
Crawley, UK.
Briggs et al., 2006; Figure 1). The overlying Benin
Formation, found in some coastal regions, is not
encountered in the deep-water region of our study
area.
STRUCTURE
Most of the shortening is accommodated in the
outer fold-and-thrust belt by thrust sheets, commonly referred to as “toe thrusts,” that exhibit a
variety of structural styles including detachment,
fault-propagation, fault-bend, and shear fault-bend
folding (Shaw et al., 2004; Suppe et al., 2004;
Corredor et al., 2005; Higgins et al., 2007; Kostenko
et al., 2008). Many of these folds that develop above
the thrust faults involve a significant component
of pure and/or simple shear, reflected in long backlimbs that typically dip less than the fault ramp
(Serra, 1977; Suppe et al., 2004; Shaw et al., 2005).
468
This type of fold geometry is commonly produced
when a weak hanging-wall stratigraphic interval
undergoes bed-parallel simple shear, or pure shear
localized above the base of the thrust ramp, during
fold growth. Corredor et al. (2005) showed that this
shear was generally localized in the Akata Formation
and the lowermost part of the Agbada Formation for
much of the southern lobe of the outer fold-andthrust belt. We find, however, that, in our northern
study area, shear is limited to the lowermost part of
the Agbada Formation, within the weak Dahomey
wedge, as the basal detachment for the fold-andthrust belt has generally risen to the top of the Akata
Formation (Figure 2). As has been modeled and inferred in the southern Niger Delta (Cobbold et al.,
2001, 2009; Mourgues and Cobbold, 2006), shear
localized within the Dahomey wedge and the location of the basal detachment may arise from local
overpressure near the top and the bottom of the
Akata Formation and the Agbada Formation,
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
Figure 3. A horizontal
slice of seismic data at a
depth of 4461 m (14,640 ft,
left) and an illustrated
structural map (right). The
location of the seismic
data volume is shown in
Figure 1. The dashed lines
indicate the positions of
various figures and regional cross sections discussed in the text. Data
are owned and provided
courtesy of CGGVeritas,
Crawley, UK.
respectively (Bilotti et al., 2005; Briggs et al.,
2006; Guzofski, 2007).
A migrated and depth-converted seismic profile
from a 3-D seismic volume is presented in Figure 2.
These seismic data show a typical toe-thrust structure, or shear fault-bend fold, from the study area.
The interpreted version of the profile also displays
the four primary stratigraphic horizons that we
mapped across the region and used for our structural
characterization. Starting with the deepest level, the
horizons represent the top of the Akata Formation,
the top of the Dahomey wedge, a prominent Agbada
Formation reflection of the late Oligocene, and another prominent Agbada Formation reflection of
the late Miocene. Aided by these mapped horizons,
which we will refer to as the “AF,” “DW,” “LO,”
and “LM” horizons, we can see that the interpreted structure displays the classic characteristics of a shear fault-bend fold; it possesses a
backlimb that dips less than the fault ramp and has
a width much greater than the forelimb of the fold
and greater than the amount of slip on the thrust
ramp (Suppe et al., 2004; Shaw et al., 2005).
A very low bathymetric slope is observed in
the distal part of the outer fold-and-thrust belt; thus,
no significant propensity exists for regional forethrusts (east-dipping faults) to develop in preference
to backthrusts (west-dipping faults; Bilotti et al.,
2005). We find, in fact, a much higher percentage
of backthrusts compared to forethrusts in the study
area. In addition to the thrust faults, we have
mapped a small set of normal faults in the northern
part of our area and three primary strike-slip faults
that crosscut the region striking northeast–southwest (Figure 3). Notably, most of the thrusts in the
area terminate into one or more of these strike-slip
faults. As noted previously, this truncation of
shortening structures is a defining characteristic of
tear faults (Twiss and Moores, 1992).
With the availability of 3-D seismic reflection
data, we were able to directly constrain the 3-D geometry of the tear-fault systems using a combination of vertical sections (inlines and crosslines) and
time and depth slices. As with thrust or normal
faults, we used abrupt fold limb (kink-band) terminations and horizon-fault cutoffs to precisely define the position of the strike-slip fault traces in these
seismic sections (Figures 4, 5).
Vertical seismic sections provide the best means
of constraining the downdip extent of the tear-fault
systems. The section displayed in Figure 6 represents
a typical image for any of the three primary strikeslip tear faults that crosscut the study area. We observe significant vertical offsets across the strike-slip
Benesh et al.
469
Figure 4. Seismic data
illustrating the common
indicators (kink-band and
horizon terminations) that
are commonly used to
identify strike-slip faults in
map view. These data
are from a horizontal slice
at a depth of 4461 m
(14,640 ft). Data are owned
and provided courtesy
of CGGVeritas, Crawley,
UK.
Figure 5. A second example of the interpretation of strike-slip faults in
map view when using
three-dimensional (3-D)
seismic reflection data.
These data are from a
horizontal slice at a depth
of 4461 m (14,640 ft). Data
are owned and provided
courtesy of CGGVeritas,
Crawley, UK.
470
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
Figure 6. A representative cross section
of the strike-slip faults in the study area.
Note the vertical offset apparent for the
LM, LO, and DW horizons that does not
continue past the AF reflection. The red
dashed line represents the likely detachment level. Data are owned and provided
courtesy of CGGVeritas, Crawley, UK.
faults for the Dahomey wedge, late Oligocene, and
late Miocene horizons; however, these offsets do not
extend to the top Akata Formation horizon. In
Figure 6, a vertical offset of approximately 0.25 km
(0.16 mi) observed at the level of the DW horizon decreases to no discernable offset at the AF
horizon, which lies 0.5 km (0.3 mi) deeper. This
abrupt downward decrease in separation suggests
that the tear faults do not extend below the top
Akata Formation. This interpreted fault termination corresponds directly with the basal detachment horizon interpreted from the downward
termination of the thrust sheets (Figures 2, 7). This
observation supports the interpretation of the foldand-thrust belt as a thin-skinned system and suggests that the tear-fault systems serve a function in
accommodating displacement gradients on the basal
detachment that are perpendicular to the general
east–west transport direction.
Perhaps the best constraints on the 3-D geometry of the tear faults are provided by the time and
depth slices, where the strike-slip faults are typically
imaged as discrete linear zones of low coherency.
Structural and stratigraphic features, such as folds,
Benesh et al.
471
Figure 7. Seismic cross
section of a thrust fault
in proximity to the southern strike-slip fault restraining bend. The darkblue dashed line shows
the division between strata
deposited before and
during growth. Data are
owned and provided
courtesy of CGGVeritas,
Crawley, UK.
thrust faults, and channel systems, are abruptly terminated across these strike-slip fault traces. We
developed a 3-D representation of the tear faults
by mapping their traces in a series of closely spaced
time and depth slices, ranging from the sea floor to
the travel times and depths corresponding with
the basal detachment level. The faults have nearly
vertical dips along most of their extents, with significant changes in fault dip occurring in regions of
fault stepovers (Harding, 1985; Sylvester, 1988).
Both extensional releasing and contractional restraining bends are common along these tear-fault
systems. The timing of the deformation of these
strike-slip faults is constrained by syntectonic growth
strata deposited across these restraining and releasing bend systems (Figure 8). In general, an
upper Miocene horizon that sits approximately
0.5 km (0.3 mi) stratigraphically higher than our
mapped late Miocene reflection marks the transition from strata deposited pretectonically to syntectonically. The pretectonically deposited deltaic
section is characterized by laterally continuous
stratigraphic horizons with stratigraphic thicknesses that vary gradually. In contrast, the section
472
deposited syntectonically thins abruptly onto structural highs and includes stratigraphic onlaps and
other features that indicate that the units were
deposited in the presence of bathymetry generated by the growth of the underlying folds and
faults. Notably, the sections deposited during the
growth of the toe thrusts and tear faults are coincident (Figures 7, 8), further supporting our
interpretation that the tear-fault systems are an
important factor in accommodating displacement
gradients in the toe-thrust systems.
REGIONAL SHORTENING
To discern how the patterns of shortening in the
outer fold-and-thrust belt are influenced by the
strike-slip faults, we estimated the total shortening using three cross-sectional transects across the
region (see Figure 3 for map locations). The northern transect (AA′) is located north of the primary
strike-slip faults and encounters only two thrust
faults: one backthrust and one forethrust (Figure 9).
Relative to the Akata Formation reflection, which
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
Figure 8. A seismic
cross section through a
restraining bend of the
southern strike-slip fault
examined in Figure 5. The
division between pretectonically and syntectonically deposited strata is
marked by the dark-blue
dashed line. This horizon
marks the beginning of
the section deposited during the growth of both
the restraining bend and
adjacent thrust faults
(Figure 7). Data are owned
and provided courtesy of
CGGVeritas, Crawley, UK.
lies below the basal detachment, the overlying horizons are all shortened, with a maximum of 0.5 km
(0.3 mi) of shortening reached at the level of the
late Oligocene reflection. Based on the palinspastic
restoration of these sections and shear fault-bend
folding theory, we assess that most of this shortening results from displacement on the detachment and ductile deformation or shear within the
Dahomey wedge, with lesser amounts of shear
between the Dahomey wedge and late Oligocene
horizons.
The central transect (BB′, Figure 10) is located
between the northern and central strike-slip faults
and crosses the greatest number of thrust faults.
Like the AA′ line, most of the faults in this section
sole into the Dahomey wedge; however, several
smaller faults terminate either between the late
Oligocene and Dahomey wedge horizons or at the
Dahomey wedge horizon. This transect has undergone considerably more shortening than the AA′
line. Along the late Oligocene horizon, we measure
3.0 km (1.9 mi) of shortening. As with AA′, most
Figure 9. Uninterpreted
and interpreted regional
seismic profile along transect AA’. The colored
horizons represent the
AF reflection (blue), the
likely detachment level
(red, dashed), the DW
reflection (green), the LO
reflection (orange), and
the LM reflection (dark
red). A maximum of 0.5 km
(1600 ft) of shortening
is measured along the LO
horizon for this transect.
Data are owned and
provided courtesy of
CGGVeritas, Crawley, UK.
Benesh et al.
473
Figure 10. Uninterpreted and interpreted cross section along transect BB′. A maximum of 3.0 km (1.9 mi) of shortening is measured
along the LO reflection (orange) for this transect. Data are owned and provided courtesy of CGGVeritas, Crawley, UK.
of this shortening is produced by shear within the
Dahomey wedge and is reflected in the considerable visible thinning and thickening within that
interval. The shear above the Dahomey wedge
horizon also contributes some amount to the measured maximum shortening.
The southernmost transect (CC′C″) contains
fewer faults than section BB′ but more total shortening (Figure 11). Because the toe-thrust systems
extend westward of the 3-D seismic coverage in this
region, we extended the section using constraints from
2-D seismic reflection data. Measured along the late
Oligocene horizon, CC′C″ exhibits a maximum of
3.8 km (2.4 mi) of shortening that is primarily produced by ductile shear within the Dahomey wedge.
As illustrated by these transects, the contractional structures in this region accommodate a gradient in westward-vergent shortening that increases
from north to south. This gradient is segmented
by the two northern strike-slip faults to produce a
northern low-shortening block, a central transition
block, and a southern high-shortening block. Relative
to the northern low-shortening block, deformation
extends farther to the west in the transition block
Figure 11. Seismic profile for transect CC′C″. The left part of the profile between C′ and C″ represents an extrapolation of surface
geometries from two-dimensional (2-D) seismic lines. For this transect, we measure a maximum of 3.8 km (2.4 mi) of shortening along
the LO horizon (orange). Data are owned and provided courtesy of CGGVeritas, Crawley, UK.
474
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
Figure 12. An offset
channel of the middle to
late Miocene (top), restored (left) and interpreted (right). Restoration
of the offset indicates
that the fault has experienced 752 (±140) m (2470
[±460] ft) of right-lateral
displacement at this location. Data are owned and
provided courtesy of
CGGVeritas, Crawley, UK.
and farther still in the southern high-shortening
block. This westward extension of the deformation
front, coupled with the north–south gradient in
shortening, produces net right-lateral motion along
the strike-slip faults.
RESTORATION OF CHANNEL OFFSETS
In an effort to validate our interpretation of the sense
of slip on the strike-slip faults and to define the faultslip magnitude, we sought to determine piercing
points that directly constrain the accumulated slip at
locations along the strike-slip faults. To this end, we
have identified in the 3-D seismic data two channel systems that formed in the middle Miocene to
late Miocene that cross and are offset by the two
northern primary strike-slip faults. These channels
occur in the oldest part of the syntectonically deposited section. Thus, we recognize that their paths
may have been influenced by the initial formation
of the fault system. As a result, apparent channel
offsets might overestimate true fault slip if the
channels traveled along the traces of the faults. To
avoid this pitfall, we selected sections of channels
with bends and meanders that are not aligned along
Benesh et al.
475
Figure 13. A channel of
the middle to late Miocene that is offset by the
central strike-slip fault
(top), restored to a conformable geometry (left)
and interpreted (right). The
restored offset for this
channel indicates 1710
(+100 or −600) m (5610
[+330 or −1970] ft) of
right-lateral fault slip. Data
are owned and provided
courtesy of CGGVeritas,
Crawley, UK.
the traces of the faults and thus provide more distinct piercing-point offsets. The first of these channels crosses the most northern strike-slip fault and is
composed of two truncated arms of a 180° channel bend (Figure 12). We find that a translation of
752 (±140) m (2470 [±460] ft) along the strikeslip fault trace restores the bend and channel arms
to a conformable position. The sense of translation
agrees with right-lateral fault slip as inferred from
the gradient in shortening across the fold-and-thrust
belt. The estimate of uncertainty that is included in
476
our restoration measurement is derived from two
sources. First, the inherent limitation in the resolution of seismic reflection data at depth (~50 m
[160 ft]) produces some uncertainty in identifying
and tracing the specific amplitude or wavelet that
corresponds to the impedance contrast produced
by the presence of the channel. Second, given the
amount of fault-related deformation that has occurred since the formation of the channel, we recognize that not all parts of the channel trace are
preserved as they cross the fault. Thus, the restoration
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
measurement reflects our best attempt to align
wavelet signatures that we believe were contiguous
or nearly contiguous in a pretectonic setting, within
the limits of the data resolution.
The second channel crosses the central strikeslip fault near the western margin of the 3-D seismic data volume. We find that a translation of
1710 m (5610 ft) restores the offset channel traces
to a conformable position (Figure 13); however,
given the more linear trace and nebulous seismic character of this channel near the fault, we feel
that this restoration measurement is more likely to
represent a near-maximum offset value. Based on
the wavelet and amplitude signatures in the seismic data, we estimate that the restoration slip value
has an uncertainty of +100 or −600 m (+330 or
−1970 ft). Although more uncertainty is observed,
we feel that the primary results of this channel restoration are robust. Like the northern channel system, the sense of offset and translation for this second channel is consistent with right-lateral strike-slip
motion. Moreover, the greater magnitude of slip
along this central strike-slip fault in comparison to
the northernmost fault is in accordance with our
earlier observations of a southward-increasing gradient in westward-vergent shortening across the
fold-and-thrust belt.
restoration the late Oligocene horizon because it
represents a pretectonically deposited unit encompassing the full amount of accumulated shortening and, hence, strike-slip motion, within the
region of interest. After mapping the selected horizon and faults in both the 2-D and 3-D seismic
data sets, we imported the picks into Gocad and
used discrete smooth interpolation to generate
evenly meshed surfaces. We then used a structural
modeling workflow that allowed us to make iterative refinements to the horizon and fault surfaces to
ensure model consistency. Examining the 3-D fault
relationships during this process provided more
support for the interpretation of a shallow common
detachment level for all regional strike-slip and
thrust faults (Figure 14).
The 2-D, or map-view, restoration technique
that we apply is a parametric method that restores
the faulted and folded surface back to a horizontal
datum while minimizing area change and internal
strain (Muron et al., 2005; Plesch et al., 2007). To
calculate the displacements and strains needed to
restore the deformed surface, the method requires
slip directions, but not magnitudes, on at least some
of the faults to be specified. For our restoration, we
elected to maintain the strike-slip faults as free
surfaces and thus specify only that the thrust faults
SURFACE RESTORATION AND SLIP
PROFILE CALCULATION
We seek to make a more comprehensive assessment of slip along the strike-slip faults, although
the recognition of offset channels has provided two
point measurements of slip along the northern tear
faults and supported our conclusion based on the
regional transects that the strike-slip faults segment a north–south gradient in shortening. For this
assessment, we use a 2-D, horizon-based structural restoration technique implemented in Gocad,
a geological computer-aided design–based modeling package (Mallet, 1992; Muron et al., 2005).
This method simultaneously restores folds and
faults across a deformed horizon and therefore calculates a full displacement field for the horizon and
the faults that displace it. We selected for this
Figure 14. Numerically meshed surfaces that represent the
fault systems in the study area (Figure 3). The light green surface
represents the common detachment level shared by both the
thrust and strike-slip faults.
Benesh et al.
477
Figure 15. The deformed (top) and restored
(bottom) surfaces representing our upper horizon of the late Oligocene.
The restoration solution
produces 0.21% contraction when moving from
the undeformed to the
deformed state. Colors
correspond to depth only
in the upper image.
generally slip in a northeast–southwest direction,
parallel with the strike-slip faults and consistent
with piercing-point offsets of channels. The small
normal faults in the northern part of the section
were constrained to slip in a purely dip-slip sense.
With these constraints and the added specification
of a pinpoint, we then used the restoration tool to
calculate the displacement and strain fields required to flatten the horizon and fully recover its
fault offsets. The initial and restored late Oligocene
surfaces are shown in Figure 15.
In general, the restoration yields a smooth displacement field with translation in a direction parallel with the slip on the thrust faults and the traces
of the tear faults. Moreover, the change in area between the deformed and restored state is modest
(0.21%), and the range of local strain magnitudes is
small. In terms of local dilatation, or area change
478
during the restoration, 90% of the surface has
strains of less than ±4%. Higher strains, as much as
±15%, are localized along fault traces. Thus, the
restoration seems to perform well in recovering
both fold and fault offsets while generally maintaining the area of the horizon with geologically
reasonable magnitudes of local strain.
To further constrain the displacement profiles
along the strike-slip faults, we identify pairs of nodes
on the horizon that lie adjacent to each other but
on opposite sides of the strike-slip faults in the
undeformed (restored) state. We then measure
the offset of these adjacent nodes in the deformed
state, thereby specifying the full displacement profiles along the faults. These slip vectors allow us to
define both the strike-slip and dip-slip components
of motion along both the northern (Figure 16) and
central (Figure 17) strike-slip faults. In these figures,
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
Figure 16. Plot of the right-lateral slip (top) and displacement vectors (bottom) for the northern strike-slip fault. The dashed lines
represent the location of intersecting thrust faults: Lines that extend upward from the dark-gray data points represent faults lying to the
north, and lines extending downward correspond to thrust faults to the south. The light-gray area reflects uncertainty; the black diamond
and error bars represent the measured slip value from the channel restoration (Figure 12). The slip vectors correspond to the movement
of the northern block relative to the southern block.
the slip profiles represent right-lateral slip, and each
gray dashed line indicates the location at which a
thrust fault truncates into the strike-slip fault. Lines
that extend downward from the data points represent thrust faults to the south of the strike-slip fault,
whereas lines that extend upward represent thrust
faults on the northern side. The light gray–shaded
region represents the error in these restoration-based
slip measurements. Note that the error does not
constitute a comprehensive evaluation of all possible modeled fault geometries and linkages, but it
reflects the uncertainty inherent in the tracking of a
chosen seismic data horizon and in the generation
of fault and horizon meshes based on those seismic
Figure 17. Plot of the
right-lateral slip (top) and
displacement vectors
(bottom) for the central
strike-slip fault in the
study area. The dashed
lines represent the location of intersecting thrust
faults. The light-gray area
reflects uncertainty; the
black diamond and error
bars represent the measured slip value from
the channel restoration
(Figure 13). The slip vectors correspond to movement of the northern
block relative to the
southern block.
Benesh et al.
479
Figure 18. Block diagrams, slip profiles, and
displacement vectors for a
conventional, throughgoing
strike-slip fault (left) and
thin-skinned tear fault
(right). The illustrated
block diagrams suggest
coeval activity for the strikeslip and thrust faults in
the tear-fault example but
demonstrate the noncontemporaneous nature
of thrust-accommodated
shortening and strike-slip
motion in the conventional
view. The slip profiles and
displacement vectors represent an idealized case for
the traditional strike-slip
fault (left) and reflect our
observations of tear-fault
systems in the Niger Delta
(right).
interpretations. The magnitude of this uncertainty
is approximately 125 m (410 ft).
We observe that the slip profiles for both strikeslip faults display a unique stair-step character. Slip
is not uniform or smoothly varying but instead jumps
abruptly at each location where the strike-slip fault
is met by a thrust fault. These jumps represent the
transition between the footwall and hanging-wall
blocks for a given thrust fault. Because footwall and
hanging-wall blocks have the opposite direction of
motion by nature, moving from one to the other
produces an abrupt change in differential motion
along the strike-slip fault. We also note that thrust
faults extending to the strike-slip fault from the
north and the south tend to have opposite effects.
Naturally, the development of a shortening structure on one side of the fault would increase rightlateral motion, whereas the development of the
same structure on the opposite side would act to
decrease right-lateral slip.
We note that the estimates of slip derived
from channel offsets accord very well with our
restoration-calculated slip profiles (Figures 16, 17).
For the northern strike-slip fault, the channel480
measured slip value of 752 m (2470 ft) is 83% of
the restoration-derived value. For the central strikeslip fault, the channel offset of 1710 m (5610 ft)
represents 103% of the restoration-derived value.
Both channel offset values fall within our estimates of the uncertainties in slip derived from
the restorations.
Finally, we also note that the rake of the slip
vectors and, hence, the proportions of strike-slip
and dip (vertical)-slip displacement, vary abruptly
along the tear faults. Specifically, the rake of the slip
vector is nearly horizontal in regions where thrust
sheets are not present, reflecting that slip is almost
purely strike-slip in these regions. In contrast, the
rake of the slip changes abruptly when a thrust fault
bounds one or both sides of the tear fault because
the component of thrust motion induces a locally
higher component of dip slip on the tear faults. Collectively, these patterns of abruptly changing slip
magnitudes and orientations along the tear faults
clearly distinguish them from more continuous displacement patterns expected on traditional strikeslip faults (Walsh and Watterson, 1988; Kim and
Sanderson, 2005; Figure 18).
Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System
CONCLUSIONS
Although tear faults have long been recognized as
important structural elements in fold-and-thrust
belts, a thorough understanding of their geometric
and kinematic relationship to adjacent thrust faults
and their function in accommodating displacement
gradients has been lacking. In part, this results from
the difficulty in observing or imaging the inherently
3-D nature of the fault interactions. This study has
sought to overcome these limitations by using highquality 3-D seismic reflection data to constrain more
fully the manner in which these tear faults interact
with thrust sheets and to define their displacement
patterns.
We have shown in an example from the northwestern part of the outer fold-and-thrust belt of the
Niger Delta that a north–south increase in westward displacement and shortening is partitioned
by a system of tear faults. The tear faults not only
segment the gradient in shortening across three
large regional blocks, but also accommodate differential motion among the smaller footwall and
hanging-wall blocks associated with individual thrust
sheets. This accommodation produces a unique slip
profile for the tear faults, which exhibit changes
in slip magnitude and orientation occurring at their
intersections with each truncated thrust fault. We
suggest that this type of slip profile is specific to the
development of tear faults and that it may serve as
a useful diagnostic tool that can be used to distinguish tear faults from more traditional strikeslip systems.
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Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System