BULLETIN OF CANADIAN PETROLEUM GEOLOGY
VOL. 47, NO. 4 (DECEMBER, 1999), P. 391-411
Basement reactivation in the Alberta Basin: Observational constraints and mechanical rationale
GERALD M. ROSS
DAVID W. EATON
Geological Survey of Canada
3303 33rd Street NW
Calgary Alberta T2L 2A7
Department of Earth Sciences
University of Western Ontario
London, Ontario N6A 5B7
ABSTRACT
The Precambrian crystalline basement of western Canada commonly is hypothesized to have been reactivated during the Phanerozoic and “basement control” is used as an explanation for a variety of anomalous features of the sedimentary section including abrupt facies changes, the orientation of reef trends and clastic strandlines, development of
fracture porosity, the localization of hydrothermal fluids and the accumulation of hydrocarbons. The inference of basement control is based almost universally on spatial coincidence of patterns observed on potential field maps and satellite images with those seen on maps of the sedimentary section. Lithoprobe seismic reflection data provide a framework
for testing these concepts by first unravelling the nature and geometry of Precambrian basement structures and then
examining their relationship to Phanerozoic tectonostratigraphic elements.
Based on interpretations of Lithoprobe crustal seismic profiles, aeromagnetic anomaly data and comparisons with
Phanerozoic structure in the Alberta Basin, we can place an upper limit on the degree of direct control (in terms of seismically resolvable displacements) of Phanerozoic patterns by reactivation of Precambrian structures. Faults and shear
zones that formed during the collisional assembly of the basement in the Precambrian can be mapped with both regional aeromagnetic anomaly data and crustal reflection profiles. These structures rarely are reactivated in the Phanerozoic
Alberta Basin except in close proximity to the present Cordillera (e.g. Vulcan structure) and locally on the crest of the
Peace River Arch area of northwest Alberta. With the exception of a region on the southeast flank of the Peace River
Arch, the orientations of faults in the sedimentary section do not appear to coincide with underlying basement structures. These Phanerozoic faults exhibit little spatial collocation with antecedent fabrics within the basement and are
associated only locally with an offset of the basement-cover contact. Thus, direct basement control of the presence and
orientation of faults in the sedimentary section seems unlikely.
We attribute the general paucity of basement reactivation in the Alberta Basin to the thermal history and consequent
strength of the region’s lithospheric mantle, which has acted as the main, load-bearing layer in the lithosphere and has
limited stress transmission into the crust. This contrasts with areas of western Canada where the thermal structure of the
mantle has been perturbed by Phanerozoic tectonic events, such as initiation of the Williston Basin ca. 500 Ma and the
break-up of western Canada ca. 700 Ma, where basement structures clearly are reactivated.
RÉSUMÉ
L’hypothèse commune selon laquelle le socle cristallin du Précambrien de l’ouest du Canada a été réactivé pendant
le Phanérozoïque et le “contrôle du socle” est utilisé pour expliquer une variété de caractères anomaliques de la coupe
sédimentaire, incluant des changements abrupts de faciès, l’orientation d’alignements récifaux et des lignes de rivage
clastiques, le développement de porosité de fracture , la localisation des fluides hydrothermaux et l’accumulation d’hydrocarbures. La déduction d’un contrôle par le socle est presqu’universellement basée sur la coïncidence spatiale entre
les patrons observés sur des cartes de champs de potentiel et des images satellititaires comparativement aux cartes de
la colonne sédimentaire. Les données de réflexion sismique de Lithoprobe fournissent un cadre d’ensemble pour évaluer ces concepts en éclaircissant d’abord la nature et la géométrie des structures du socle du Précambrien pour ensuite
examiner leur relation avec les éléments tectonostratigraphiques du Phanérozoïque.
En se basant sur les interprétations des profils sismiques de la croûte de Lithoprobe, les données d’anomalies aéromagnétiques et les comparaisons avec la structure du Phanérozoïque dans le bassin de l’Alberta, nous pouvons placer
une limite supérieure quant au degré de contrôle direct (en terme de déplacements sismiquement résolubles) des patrons
du Phanérozoïque par réactivation des structures du Précambrien. Les failles et zones de cisaillement qui se sont formées durant l’assemblage collisionnel du socle au Précambrien peuvent être cartographié à la fois par les données
d’anomalies aéromagnétiques régionales et les profils de réflexion crustale. Ces structures sont rarement réactivées dans
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
391
G.M. ROSS and D.W. EATON
392
le bassin du Phanérozoïque de l’Alberta, à l’exception de celles situées à proximité de la Cordillère actuelle (ex. la structure Vulcan) et localement sur la crête de l’arche de Peace Riverau nord-ouest de l’Alberta. A l’exception d’une région
sur le flanc sud-est de l’arche de Peace River, l’orientation des failles dans la coupe sédimentaire ne semble pas coïncider avec les structures sous-jacentes du socle. Ces failles du Phanérozoïque montrent peu de corrélation avec les fabriques antérieures à l’intérieur du socle et ne sont associées que localement avec un rejet du contact socle-couverture.
Donc, un contrôle de la présence et de l’orientation des failles dans la coupe sédimentaire semble peu probable.
Nous attribuons la rareté générale de réactivation du socle dans le bassin de l’Alberta à l’histoire thermique et à la
force conséquente du manteau lithosphérique régional, qui a agit comme la principale couche mise en charge dans la
lithosphère et a limité la transmission de contrainte dans la croûte. Ceci contraste avec les régions de l’ouest du Canada
où la structure thermale du manteau a été perturbée par les événements tectoniques du Phanérozoïque comme l’initiation du bassin de Williston approximativement à 500 Ma et le démembrement de l’ouest du Canada à approx. 700 Ma.
où les structures de socle sont clairement réactivées.
Traduit par Lynn Gagnon.
INTRODUCTION
Reactivation of pre-existing structures plays a fundamental
role in continental deformation (Holdsworth et al., 1997). Recent
reviews have examined the role of reactivation in the context of
relatively dramatic tectonic processes such as orogenic evolution
and localization of rifts and their inversion (Reading et al., 1986;
Cooper and Williams, 1989; Buchanan and Buchanan, 1995).
Reactivation of structures within cratonic basins can be viewed
as an end-member component within the spectrum of reactivation and continental deformation because the products of such
deformation commonly are subtle. Nonetheless, certain lessons
and principles learned from well-understood examples of structural reactivation in more tectonically active settings can be
applied to cratonic regions. Cratonic areas with sedimentary
cover actually offer a unique record of cratonic deformation.
Although a general lack of basement outcrop beneath sedimentary basins precludes physical examination of the mechanical
properties of a basement fault or shear zone, the sedimentary
cover can provide a relatively precise history of motion and constrains the timing and magnitude of displacements, a constraint
not generally available from exposed basement regions.
The concepts of “basement reactivation” and “basement
control” on sedimentary patterns in sedimentary basins, and the
Western Canada Sedimentary Basin (WCSB) in particular,
have been entrenched in the literature for more than 40 years
(Sikabonyi and Rodgers, 1959). In western Canada, the term
“basement control” commonly is used as an explanation for
long, linear or comparably anomalous trends within the sedimentary section and as a cornerstone of certain play concepts
in the petroleum industry (e.g. Greggs and Greggs, 1989;
Andrichuk, 1961; Churcher and Majid, 1989; Mountjoy, 1980;
Keith, 1970; Paukert, 1982; Stoakes, 1987; Hart and Plint,
1993; Donaldson et al., 1998). The practical appeal of this
explanation is clear; if it could be demonstrated conclusively
that the structural grain of the basement controlled overlying
sedimentation patterns, then a very cost-effective exploration
concept would be in place since basement fabric can be
mapped accurately and inexpensively with magnetic anomaly
surveys. Ironically, until recently, very little was known of the
structure of basement from a seismic perspective, as most
industry seismic records end in the shallow basement (2–4 s)
where the effects of multiple reverberations from the sedimentary section all but obscure details of basement structure (Eaton
et al., 1995; Eaton et al, this volume, Edwards and Brown, this
volume; Lemieux, this volume; Dietrich, this volume).
With the completion of public domain aeromagnetic coverage in the Alberta Basin, the acquisition of Lithoprobe crustal
seismic sections and publication of the Western Canada Basin
Atlas (Mossop and Shetsen, 1994), the geoscience community
is now in a position to address crucial questions of if, and how,
basement structures have influenced the sedimentary section.
Much of the evidence of reactivation in sedimentary basins
comes from establishing or inferring a geometric connection
between old and new structures. Such approaches involve seismic and potential field data sets but rarely examine the underlying thermo-mechanical aspects that could link basement and
sedimentary cover in a unified response to stress.
The purpose of this paper is two-fold. The first objective is
to link and integrate cross-sectional (seismic reflection) and 2D
map (aeromagnetic) data in order to compare deformation fabrics of basement and overlying sedimentary rocks and assess
whether the fabric of the basement has influenced the sedimentary section. Establishing “geometric similarity” between successive deformation events is the first step in documenting and
understanding reactivation in sedimentary basins. The second
objective is to examine these results in the context of mechanical properties of basement structures and the lithosphere, and
their response to stress. Reference to recent numerical and laboratory studies of deformation provide a potential framework
for interpreting basement reactivation. These approaches are
used to develop a rationale and general explanation for
observed basement reactivation in a cratonic sedimentary
basin, with the Alberta Basin as an example.
CONCEPTUAL APPROACHES
A critical element that has been absent in many previous
studies of basement control and reactivation in the WCSB has
been a consideration of the mechanical properties of basement
rocks and fault zones and their interaction with stress fields.
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
393
Fig. 1. Isopach map of the Western Canada Sedimentary Basin showing the location of the Alberta Basin and Williston Basin. The column on
the right gives a general scheme for the evolution of Phanerozoic tectonic events recorded in the Cordillera that influenced sedimentation in the
Alberta Basin.
The role of rock strength and the evolution of the stress field
through time are essential to evaluate the mechanics of reactivation of older structures. As pointed out by Pruscha (1988)
and Holdsworth et al. (1997) the parallelism of structures within basement and sedimentary cover, commonly touted as geometric evidence for cause and effect relationship, offers no
compelling evidence of reactivation.
Tectonic Evolution of WCSB and Sources of Stress
The sources of stress in an intracratonic basin setting likely
are the result of loading associated with the basin fill and/or far
field stresses related to tectonic processes at the plate margin
(Zoback et al., 1989). For much of the Phanerozoic in the
Alberta Basin we can make only broad inferences about the
nature of elastic strain and paleostress, based in part on the
knowledge of the tectonic setting at the time of sedimentation
and the relative contribution of local and plate driving forces to
the ambient stress field. It is not always straightforward to
relate plate margin tectonic styles to a particular far-field stress
because of local changes in stress. For example foreland basins
form in convergent margins but are characterized locally by
extension (Bradley and Kidd, 1991). Similar difficulties of a
more intractable nature apply to understanding paleostress in a
basin with over 500 m.y. depositional record and, bordered by
a complex plate margin such as the Alberta Basin. Nonetheless,
an appreciation for the types of possible stresses, their evolution in orientation, their sense and magnitude in time, is important to bear in mind when marshalling evidence for, or against,
basement reactivation.
The Alberta Basin was deposited during three main tectonic
phases, as recorded in the adjacent Cordilleran orogenic belt
(Fig. 1). During the first phase the Alberta Basin was a cratonic ramp to a passive margin in the Cordillera that formed as a
result of late Precambrian-early Paleozoic extension and continental break-up. Subsequent thermal contraction of the rifted
lithospheric mantle drove subsidence and sediment accumulation on the passive margin into the Ordovician (Bond and
Kominz, 1984). The second phase of evolution is recorded in
the Devonian through early Jurassic strata that may represent a
number of loosely constrained tectonic environments but
includes the Devonian-Mississippian Antler event (an oblique
slip orogen and arc?) and subsidence during development of
outboard arcs and marginal basins. Dynamic topography
(buoyancy contrasts in the mantle) may have been an important
contributing factor to subsidence at this time (Pysklywec and
Mitrovica, 1997). The third phase records foreland basin development during contraction and formation of the Cordilleran
fold and thrust belt. Regional extension and plate reorganization in the Eocene terminated growth of the Rocky Mountain
fold-thrust belt and adjacent foreland which was dominated by
isostatic recovery and uplift throughout the Tertiary (Price,
1994).
Basement Structure
In this paper, we will use the term “basement structure” to
describe Precambrian structural fabrics within the crystalline
basement. Some of these are shear zones that have maintained
their coherence during ductile deformation and some are faults
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
G.M. ROSS and D.W. EATON
394
or fault zones where coherence has been lost and slip in the
brittle regime has occurred. For the basement, such structures
are mapped most accurately using aeromagnetic anomaly data
and moderately to shallowly dipping structures can be imaged
on seismic reflection profiles. Much of the reflection fabric
seen on Lithoprobe seismic lines is the result of Precambrian
orogenic activity that left a structural imprint during the assembly of the basement more than 1.8 Ga (e.g. Ross et al., 1995).
These collisional fabrics, which consist of crustal scale shear
zones, nappes, etc., are the fabrics and boundaries that are
imaged by potential field surveys which, for the most part,
dominate the magnetic signal from the basement (Fig. 2; Ross
et al., 1994; Pilkington et al., 2000). These Precambrian structures contrast with “basement faults” that are observed on shallow seismic profiles in the basin that offset the Phanerozoic
section, including the basement-cover contact. We use the term
“intrabasement” fault to refer to Precambrian structures within
the basement and to discriminate them from “suprabasement”
faults that affect only the Phanerozoic cover and locally the
basement-cover contact (Fig. 3). Although subtle, the semantic
distinction between these classes of basement structures is critical because it influences how potential field data are used for
exploration, for example, or how to evaluate structural controls
within the sedimentary section (Fig. 3).
POTENTIAL FIELD EXPRESSION
Potential field techniques and especially aeromagnetic surveys are used commonly to map basement structure beneath
sedimentary basins. The Alberta Basin represents an excellent
laboratory to assess the potential role of intrabasement structures because the magnetic anomaly signatures of the exposed
basement (Canadian Shield) can be traced unambiguously into
the subsurface (Fig. 2). As outlined by Sprenke et al. (1986)
and Ross et al.(1993), among many others, magnetic anomaly
studies from the shield faithfully reflect the geologic and structural fabric of the uppermost crust, outlining variations in magnetic susceptibility. This offers the potential for structural calibration of magnetic anomaly surveys not generally available in
sedimentary basins floored by less magnetic basement. With
the exception of local intrasedimentary anomalies (e.g. Ross et
al., 1997) the dominant contribution to the magnetic anomaly
field of western Canada is from the basement (Ross et al.1993;
Pilkington et al.2000). Thus magnetic anomaly maps can provide the template of deformation patterns and structural architecture in the basement that can be used to examine basement
influence on the sedimentary basin.
Total field magnetic anomaly data can be filtered to accentuate the internal structure and edges of magnetic sources.
Depending on the wavelength of the filter used, some of the
distracting effects of depth and extent of a magnetic source can
be removed and only the shallow contributions isolated. These
types of approaches, when coupled with the modern acquisition
technologies, result in high resolution aeromagnetic surveys
(HRAM; Peirce et al., 1998a; Leblanc and Morris, this volume). While these surveys may permit recognition of intrased-
imentary anomalies associated with hydrothermal precipitation
(Peirce et al. 1998b), on the basis of their relatively shorter
wavelength, they are incapable of detecting basement faults on
the basis of displacement at the basement-sediment interface
(Dobrin and Savit, 1988).
Deformation Maps
One approach to assessing whether Precambrian structural
grain has influenced Phanerozoic deformation is to examine the
degree of spatial correspondence between the deformation patterns. This can be illustrated for select areas and intervals of the
Phanerozoic of the Alberta Basin where public domain maps of
faults and fault fabric in the sedimentary section are well
known. The first example is from the well-known Peace River
Arch region of northern Alberta, a long wavelength basement
topographic anomaly that is cored by crystalline basement (Fig.
2). Based on regional stratigraphic patterns, the Arch is known
to have been a positive topographic feature prior to the
Devonian, during which time it was progressively onlapped and
then evolved into a depositional embayment (O’Connell et al.,
1990; Eaton et al., this volume). The central and western part of
the Arch is broken by a series of normal faults that define the
Dawson Creek Graben Complex (DCGC), a series of extensional structures that formed in the Carboniferous (Barclay,
1990; Richards et al., 1994). A comparison of the mapped
Carboniferous faults with the magnetic anomaly data (total field
and vertical derivative; Fig. 4A, B) shows that for the most part
the major Carboniferous faults that define the Fort St. John
graben, a part of the DCGC, are oriented at a high angle to the
basement fabric. However several faults in the southeastern part
of the DCGC are collinear with high magnetic gradient anomalies, including the Dunvegan, Tangent, Rycroft and Teepee
faults. Although important, these faults comprise a small subset
of the DCGC. Furthermore, many other linear magnetic anomalies are not associated with Carboniferous normal faults and,
indeed, the strike of the magnetic anomalies is at a high angle to
the strike of the faults. These data do not provide a basis to
determine if a given magnetic gradient anomaly is collinear with
a Carboniferous normal fault, but it is clear that most normal
faults do not coincide with a magnetic anomaly.
A second example is from southern Alberta, where the
Geological Survey of Canada acquired high resolution aeromagnetic data under the auspices of the Federal-Provincial Mineral
Development Agreement. The data was acquired over a region of
Archean crust characterized by northwest-trending potential field
fabrics (Fig. 5). Filtering of the total field data using vertical
derivative calculations resulted in recognition of a series of linear,
short wavelength positive anomalies that have been interpreted as
part of an Eocene dyke swarm that was intruded into southern
Alberta (Ross et al., 1997). The emplacement of the dyke swarm
must have occurred during a period of crustal extension and therefore the dyke swarm can be interpreted as recording a pattern of
strain associated with extension. Importantly, there is a consistent
angular discordance between the strike of the dykes and the strike
of the underlying basement fabric suggesting that the stress field
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
395
Exposed
Canadian
Shield
Fig. 2. Compilation of aeromagnetic anomaly data for the Western Canada Sedimentary Basin with the isopachs from Fig. 1 plotted. The
Lithoprobe seismic lines are labelled and color coded. P: PRAISE; C: CAT; S: SALT; PRA: Peace River Arch; SGA: Sweet Grass Arch; WB: Williston
Basin. The reflection in Fig. 7 is from the east end of the CAT line. Magnetic anomaly maps in Figs. 4 and 5 come from the Peace River and southern Alberta areas, respectively.
that produced the extension during dyke emplacement was not
influenced by pre-existing structures in the basement.
The results of these examples are interpreted to demonstrate
clearly that there commonly is a low degree of spatial coincidence between Precambrian basement structure and younger
Phanerozoic deformation patterns. Thus, if there is a causeand-effect relationship between these deformation patterns,
then it must be both indirect and spatially variable.
SEISMIC REFLECTION CONSTRAINTS
Seismic reflection data provide one of the few lines of evidence that can be used to establish a direct link between
Precambrian basement structure and Phanerozoic faults (Fig. 3).
Use of seismic reflection profiles for this purpose requires that
recording times are sufficiently long to recognize and map basement structures (rather than merely fragments of basement
structures), and that the profile length exceeds the target depth,
in order to ensure reasonable aperture for migration. Because at
least some basement shear zones in Alberta appear to root in the
lower crust or Moho (Ross et al., 1995), these considerations
imply minimum recording times (~16 s) and profile lengths
(~40 km) that rarely are met for non-Lithoprobe seismic data.
Two major impediments to interpretations of basement reactivation using the Lithoprobe seismic data are the ubiquitous
presence of multiple reverberations in the shallow basement
and the limited resolving power of the data in the shallow part
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
G.M. ROSS and D.W. EATON
396
Fig. 3. Basement reactivation concept model. (A) Precambrian tectonic processes generate the structure and fabric of the crystalline basement
which, for example, consists of structures formed during plate collision such as thrust faults, folds and strike-slip faults. Stippled region represents
a plutonic body. Arrow shows plate subducting to the left with an orogen in hanging wall. (B) Post-orogenic erosion removes part of the basement.
The boxed area is shown in C. (C) Detail of area in B showing the contrast between intrabasement and suprabasement faults in the context of reactivation. Intrabasement faults are those faults that clearly are reactivated Precambrian structures that controlled sedimentation, whereas suprabasement faults are faults that offset the basement-cover contact but do follow pre-existing Precambrian structures.
of the crust. Multiple reverberations represent wave energy that
remains trapped in the sedimentary section, locally masking
primary reflections for 1–2 s below the near-basement reflection. Lateral variations in the degree of multiple contamination
are related to the presence or absence of strong intrasedimentary reflectors and slight differences in data processing parameters between individual seismic lines. Fig. 6 shows a typical
velocity analysis plot, illustrating the nature of multiple contamination of the shallow basement. The left panel shows a common midpoint gather (CMP 529 from SALT-25, located about
7 km from the north end of the profile) after application of the
first-break mute and 0.2 s AGC operator. Beside it, the right
panel shows a velocity spectrum computed by semblance
velocity analysis. Dark areas in the velocity spectrum indicate
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
coherent events, each of which corresponds to a distinct hyperbolic event in the CMP gather. By identifying primary reflections in the velocity spectrum, a stacking velocity curve is
defined (dashed line), which almost invariably increases
monotonically with time.
Two bands of coherent events in the velocity spectrum do
not fall along the inferred stacking velocity trend. The first
extends from ~1.2 to 3.5 s, with velocities between 3.0 and 3.3
km/s. This group of events corresponds to intrabed multiple
reflections that originate in the Lower Cretaceous, probably
from coal beds. The stacking velocity for these multiple reflections increases slowly with time, but generally remains close to
the stacking velocity of the strong primary reflection at 1.2 s.
In the CMP gather, these multiples have hyperbolic moveout
like the primary events, but with greater curvature due to their
lower stacking velocity. The multiples contaminate both deeper reflections from Paleozoic and the shallow basement to a
time of 3.5 s. Similar trends of intrabed multiples are evident in
Lithoprobe velocity spectra throughout Alberta, although the
stratigraphic level at which the multiples originate varies with
location. Even with a judicious choice of stacking velocity
function, the multiple reflections dominate the stacked section
between 2 and 4 s in areas where primary reflections in the
shallow basement are weak, or are superimposed on basement
reflections where the basement reflectivity is greater. New
techniques for multiple attenuation (Weglein, 1999) offer
promise that future reprocessing may be successful in attenuating multiple contamination in these data, to facilitate more
detailed interpretation of the shallow basement.
Dipping reflectors in the basement also produce coherent
events that deviate from normal stacking velocities. Unlike
multiple reflections, however, the stacking velocity for dipping
reflections is greater than expected for the observed arrival
time (Fig. 6b). Dip-moveout processing (Hale, 1984) provides
a way to preserve dipping reflections without using artificially
high stacking velocities.
In general, the lateral resolution of migrated seismic data is
several times larger than the spatial sampling interval, which for
the Lithoprobe data varies between 12.5 and 25.0 m. However,
vertical resolution is perhaps the most critical limitation of the
Lithoprobe seismic reflection data which is affected by rock
velocity as well as the bandwidth and dominant frequency of the
data. For the Lithoprobe transect, the top of basement reflection
has a dominant frequency of ~30 Hz, implying vertical resolution
of about 25 m for both thin beds and fault displacement assuming
velocities of ~5000 m/s (Hope et al., this volume). Faults in the
sedimentary section with displacements smaller than this might
be difficult to recognize seismically, although such small displacements could produce sedimentologically important relief in
a shallow water epicratonic basin such as the Alberta Basin. An
additional bias of the seismic reflection method is its limited resolution of structures with a steep dip (>70°). Such structures can
be recognized either by offset of reflections on either side of the
fault plane or by a loss of coherent reflections due to wave scattering along the fault plane (e.g. Hajnal et al., 1997).
397
Recognition of Basement Faults
A fundamental challenge for the present study is the development of robust criteria for recognizing basement faults and
shear zones. In the case of seismic profiles in sedimentary
basins, primary evidence for faulting comes from offset stratigraphic reflections. Secondary seismic evidence for faulting is
provided by diffractions (in unmigrated data) caused by bed
terminations, fault shadows, fault-related folding, abrupt
changes in bed thickness and (occasionally) fault plane reflections (Sheriff and Geldart, 1995). For the most part, basement
rocks lack suitable stratigraphic marker reflections but do generate reflections caused by lithologic layering (e.g. gneissic
fabrics, intrusions, transposition) and seismic interpretations in
crystalline rocks rely on geometric interpretation of these fabrics. There is a tendency in some interpretations, either implicit or explicit, to regard any dipping reflection in the basement
as a fault zone and thus a potential zone of weakness. This
axiomatic tendency is not supported entirely by comparisons
with seismic data from the exposed crystalline terranes.
Like many crustal seismic reflection profiles from shield
regions and orogenic belts, the Alberta Basement Transect seismic reflection data reveal abundant reflectivity from the crystalline crust, from a number of possible sources. There is strong
evidence that distinct, laterally continuous reflections can be
produced by sills and other sheet-like intrusions (Mandler and
Clowes, 1998). Reflection fabrics also may be produced by
compositional layering within highly strained crystalline rocks,
such as layered gneisses that are typical of crystalline terranes
(e.g. Hynes and Eaton, 1999). Such layering may be the result
of transposition of original lithologic layering, or it may be produced during intense deformation such that relatively strongly
sheared rocks are separated by zones of less intense shearing.
Shear and fault zones may also be the source of some of this
reflected energy, either by virtue of juxtaposition of rock types
with contrasting seismic velocities or by deformation-induced
changes in seismic velocity within the shear zone itself (Hurich
and Smithson, 1988).
Alberta Basement Transects seismic reflection data have
yielded a number of spectacular examples of major
Paleoproterozoic basement structures that are recognizable on
the basis of offset reflection markers, analogous to sedimentary
markers, or by reflection geometries that are characteristic of
displacement processes. Discrete offsets of the Winagami
Reflection Sequence, a regionally extensive sequence of subhorizontal reflections observed along the PRAISE transect and
interpreted as tabular intrusions in the middle crust (Ross and
Eaton, 1997), provide a rare example of direct evidence for a
basement fault (Fig. 7a). An example of secondary evidence for
a basement fault is characteristic fold-and-thrust type geometries and juxtaposition of reflection panels with different dips
observed in reworked Archean crust of the Hearne Province on
the Central Alberta Transect (CAT) survey (Fig. 7b; Ross et al.,
1995). These structures are interpreted on the basis of truncations of reflections that define hanging wall-footwall cutoffs. In
the case illustrated in Fig. 7b, it is unknown if the structure
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
G.M. ROSS and D.W. EATON
398
Fig. 4. (A) Total field aeromagnetic data for the Peace River Arch area.
represents the sheared lower limb of a nappe or an actual fault;
the latter implies slip in the brittle regime and has implications
for potential reactivation. High angle fault zones, recognized on
the basis of lack of reflectivity (due to wave scattering; Hajnal
et al., 1997) that separate crust with different reflection characteristics, has been observed along part of the Vulcan structure.
These types of structural features are common within the crust
of the Alberta Basin and thus represent basement discontinuities
that could conceivably be reactivated during the Phanerozoic.
Seismic Reflection Recognition of Basement Reactivation
In order to test, within the limits of seismic resolution,
whether pre-existing basement structures have been reactivated
during the Phanerozoic, we have searched through the 2000 km
of Lithoprobe seismic reflection data looking for two types of
evidence:
(1) reflection zones in the basement that subcrop along the
Precambrian unconformity and project into known (or inferred)
Phanerozoic faults; and
(2) secondary evidence for basement faulting that implies
the presence of a nonreflective fault zone that is aligned with a
known (or inferred) Phanerozoic fault.
Implicit in the first class of evidence is the assumption that
a given subcropping basement reflection represents a fault zone
and not another type of planar reflective element such as lithologic layering or a dyke, for example. Given this assumption, it
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
399
Fig. 4. (B) First vertical derivative of the same, emphasizing the boundaries of magnetic domains. The magnetic fabrics in both maps correspond to Precambian structure in the basement. The overlay shows the map pattern of Carboniferous faults mapped from seismic grids
(Halbertsma, 1994). Most faults show a poor spatial correlation with the Precambian patterns except for the Dunvegan, Rycroft, Tangent and Teepee
faults. Magnetic data are public domain data held by the National Geophysical Data Centre and are plotted from a grid of 400 m. PRAISE lines 11
and 12 are shown in yellow and blue in (A) and (B), respectively.
is also necessary to show that the inferred fault zone is conformable with reflection fabrics that formed during earlier
deformation. In the absence of such evidence, it is impossible
to distinguish between a reactivated basement fault and a basement fault that formed during the Phanerozoic as a new tectonic feature. In the case of the latter type of seismic evidence, the
presence of seismic markers in the crystalline crust (e.g. the
Winagami Reflection Sequence (WRS)) provides a potential
calibration test for reactivation. For example, any observed displacements of deep markers that exceed the displacement of
Phanerozoic fault displacement, or observed differences in the
polarity of faulting along a common surface, would provide
conclusive evidence for geometric reactivation (Holdsworth et
al., 1997).
Fig. 8 is an example of where dipping reflections in the
basement intersect the base of the sedimentary section and
illustrates some of the interpretive ambiguities that confront the
geologist. Two discrete topographic steps occur along the basement surface where the dipping reflections intersect the sedimentary section. Both of the steps have controlled onlap of the
sedimentary section, which in this case is Cambrian age strata.
These steps could have been produced by two contrasting
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
G.M. ROSS and D.W. EATON
400
Fig. 5. Location and regional aeromagnetic data for southern Alberta. The map of the vertical derivative of aeromagnetic anomaly data for the
Cypress Hills survey of southern Alberta is shown as the grey scale plot. The broad anomalies in the vertical derivative map represent basement
sources whereas the narrow north- and northwest-trending linear anomalies represent shallow dykes in the sedimentary section. See text for discussion (from Ross et al. 1997).
mechanisms, reactivation or erosion. Reactivation requires that
the dipping reflections in the basement correspond to discrete
faults that were reactivated to produce the steps. Alternatively,
the steps could have been produced by differential erosion of a
layered basement which could imply that the dipping reflections represent simply lithologic layering with no structural significance. The point is that even when dipping reflections in the
basement can be traced into the sedimentary section their
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
influence on the sediments from a mechanical perspective is
not always clear-cut.
Examples. In the following examples segments of seismic
reflection data are used to compare and contrast the expression
of basement structures and their possible relationships to stratal
patterns in the sedimentary section.
Along the CAT line in east-central Alberta Eaton et al.
(1995; Fig. 9) showed an example of seismic reflection data
which contained diffractions from reflective basement interpreted as evidence for a fault or truncation of layering. The
steep curvature of the diffractions and their persistence after
migration suggest that the diffractions arise from structure that
is out of the plane of the seismic line. Nonetheless, the diffractions in the shallow crust overlie crust with clear structural cutoffs, although these cannot be traced to the base of the sedimentary section. The zone of diffractions coincide with deflections of the top-of-basement reflection and disruption of
Cambrian reflections. There is no resolvable disturbance of
reflections younger than Middle Cambrian. This is an example
of where anomalous basement reflections coincide with anomalous features in the sedimentary section, although it is uncertain if the disruption of the sedimentary reflections is the result
of a fault-controlled or purely erosional topography.
In the Peace River Arch area of northwest Alberta several
faults of Carboniferous age are collinear with magnetic fabric,
especially when examined in relation to the first vertical derivative of the magnetic data (Fig. 4). The Dunvegan Fault is the
most prominent of these and it appears to correspond in part
with the boundary between the Chinchaga and Ksituan
domains. On a seismic reflection profile from the sedimentary
part of the crust (Fig. 10), the Dunvegan Fault is an east-dipping normal fault with ca. 50 ms of displacement (about 112 m,
based on 4500 m/s velocity in nearby log data) that occurred
predominantly in the Carboniferous with lesser movement in
the Permian and Triassic (Eaton et al., this volume). The
Dunvegan Fault also coincides with the linear trend of diagenetic dolomites in the Wabamun Formation (Halbertsma, 1994)
and the Cretaceous Fox Creek Escarpment (O’Connell, 1988)
although neither of these features are resolved on the reflection
data. There is good reflectivity in the underlying basement that
shows an antiformal cutoff marking the shear zone boundary
between the Ksituan and the Chinchaga domains. The projection of the Dunvegan Fault into the basement shows that there
is no seismically resolvable displacement in the basement that
could be considered an antecedent structure to the Dunvegan.
Thus, although there is strong spatial coincidence on the map
view, there is no resolvable (i.e. greater than 25 m displacement) direct relation between basement structures defined by
reflection fabrics and the location of the Phanerozoic
Dunvegan Fault. One possible solution to this conundrum is to
argue that the Ksituan-Chinchaga domain boundary may have
focused anomalous levels of elastic strain in the sedimentary
section and thereby controlled the location of the
Carboniferous Dunvegan Fault (e.g. Heller et al., 1993).
Additionally, there may be steep (and therefore seismically
401
Fig. 6: (a) CMP gather 529 from SALT line 25, after application of
first-break mute and 0.2 s AGC operator, and its associated velocity
spectrum (b) 1 = primary reflections, 2 = intrabed multiples from the
Lower Cretaceous, 3 = dipping basement reflection, 4 = basement
reflections that help to define the stacking velocity curve (dashed line).
indistinguishable) faults that originate from the KsituanChinchaga domain boundary and these have led to the observed
Phanerozoic reactivation. An apparent lack of displacement in
the basement could either be the result of multiples that mask
the basement structure, or the fault displacement in the
Phanerozoic could be partitioned into smaller, seismically
unresolvable “horsetail” splays in the basement.
The Vulcan structure of southern Alberta originally was
proposed as evidence of basement reactivation because of the
interpretation of a rift or aulacogen of Precambrian age on the
basis of seismic reflection and refraction data (Kanasewich et
al., 1969; Clowes and Kanasewich, 1972). This structure was
crossed twice (line 25 in the east and line 32 in the west) during the recent acquisition of seismic reflection data in southern
Alberta (Figs. 5 and 11). Interpretation of the crustal data suggests that there is little, if any, evidence for a pre-Phanerozoic
rift and that the likely origin of the Vulcan structure is that it
marks the suture and site of crustal indentation between the
Medicine Hat Block to the south and the Loverna Domain to
the north (Eaton et al., 1999). Where the Vulcan structure projects into the base of the sedimentary section on line 25, it is
marked by inferred thrusts that bound north-dipping and southdipping reflection domains. There are no public domain fault
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
402
G.M. ROSS and D.W. EATON
Fig. 7: Segments of seismic reflection data that show examples of displacements within the crystalline basement. (A) Fault offset of subhorizontal reflections of the Winagami reflection sequence (northern Alberta, line 13 of PRAISE survey). (B) Northwest verging thrust nappes in eastcentral Alberta (CAT line 10) showing dramatic structural discontinuity. B: top of basement; D: core of antiformal structure; M: Moho. Both sections
are migrated data plotted with no vertical exaggeration.
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
403
Fig. 8. (A) An example of dipping reflectivity in the basement (arrows; fabric dips left) that intersects the base of the sedimentary section and
the possible interpretations of these relationships (Line 14B; PRAISE survey). The points of intersection are associated with steps along the basement surface that have influenced onlap of Cambrian strata (long arrow). Dashed line is top of basement. Note the strong multiples (subhorizontal
“reflections” in the shallow basement to depths of about 3 s. The data are migrated, which accounts for the steep apparent dip of
the basement
reflections. (B) Basement reflectivity is produced by contrasts in seismic velocity between granitic gneiss and schistose rocks. Topography along
the basement surface controls the geometry of overlying sedimentary strata. (C) Model of passive erosional origin for basement topography produced by preferential erosion of schistose rocks that could represent metasedimentary layers in the basement or sheared equivalents of the granitic
gneiss. Onlap of sedimentary strata represents progressive burial of basement topography. (D) Model for active origin of basement topography produced by fault reactivation of basement shear zones during and following deposition of sediments. The inflection of the layers in the sedimentary
strata could be interpreted either as drape or as structural displacement below the limit of resolution of seismic data.
maps for this region but the reflection data from the sedimentary section (line 25) show no evidence for faults within the
sedimentary section that could root in the Vulcan structure. In
contrast, the Vulcan structure on line 32 corresponds to a steep
structural zone that appears to separate different reflection
domains on either side of the Vulcan structure. Line 32, which
is close to the Cordilleran thrust front, shows evidence for a late
contractional fault that is spatially coincident with the Vulcan
structure. Unfortunately, data quality within the basement part
of the profile is poor and precludes matching the Phanerozoic
structure with the Precambrian structure although there are dipping reflections in the basement that are coincident with the
Phanerozoic faults. Nonetheless, it appears as though deformation was collocated within the Vulcan structure on line 32 in
the late Cretaceous-Eocene (see also Lemieux (this volume) for
a regional perspective). Further west into the Cordillera the
Vulcan structure is interpreted to coincide with a prominent
north-facing basement ramp that has a long-lived and pronounced influence on basin stratigraphy (Fig. 11c; Norris and
Price, 1966). Thus there appears to be a evidence of increasing
degree of reactivation of basement structure towards Cordillera
along the strike of the Vulcan structure. A similar conclusion
can be drawn from the work of Brandley et al. (1996) who, on
the basis of isopach maps constructed from field studies, found
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
404
G.M. ROSS and D.W. EATON
Fig. 9. Example of reflection data from central Alberta showing correlation of diffractions with surface roughness along basement unconformity. (a) migrated crustal section from line 10 plotted at true scale. FC: footwall cut-off; R: ramp of dipping reflections; DC: diffractions in upper crust.
(b) Enlargement of region in box in (a). Offset of Cambrian reflections at station 3200 at 1.35 s illustrates possible faulting of sedimentary layers.
PDU: pre-Devonian unconformity; D: Deadwood Formation (Upper Cambrian); E: Earlie Formation (Middle Cambrian); TBR: top-of-basement reflection (from Eaton et al. 1995).
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
405
Fig. 10. Crustal (upper panel) and shallow (lower panel) profiles across the Phanerozoic Dunvegan Fault and the underlying Ksituan-Chinchaga
domain boundary. The crustal profile shows the major basement domain boundary as a cutoff of a hanging wall antiform in the Chinchaga Domain
(east) against a footwall of sheared Ksituan granitoid rocks (west). The structural boundary projects to the base of the sedimentary section ca. 5 km
west of the Phanerozoic Dunvegan Fault. The Dunvegan Fault is an east-dipping normal fault with shows most of its displacement occurred during
Carboniferous time (Debolt Formation, for example). Extension of the normal fault into basement shows that it intersects, without apparent displacement, dipping reflections in the Chinchaga Domain. Both seismic reflection sections are migrated data plotted with no vertical exaggeration.
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
406
G.M. ROSS and D.W. EATON
Fig. 11. Changes in Phanerozoic stratigraphy along strike of the Vulcan structure. See Fig. 5 for line and cross-section locations. (A) Seismic
profile from line 25 showing virtually no structure in the Phanerozoic that coincides with the Vulcan structure. The magnetic axis of the structure is
shown as “VS” but the structure itself is defined seismically by changes in the dip of crustal reflectivity; these boundaries are
shown as dashed
lines (from Eaton et al., 1999). (B) Line 32 from just east of the thrust front shows deformation of at least Cretaceous age that affects virtually the
entire seismic section. The structure overlies the northern part of the Vulcan magnetic low but may be part of a family of inverted extensional faults
in the foreland basin (see Lemieux, this volume). The origin of steeply dipping reflectivity in the basement (arrows) is uncertain but is spatially coincident with overlying stratigraphic anomalies. (C) Schematic stratigraphic sections along profile C-C' (Fig. 5) showing the persistent down-to-thenorth displacement throughout late Precambrian and lower Paleozoic time across the Cordilleran extension of the Vulcan structure, inferred to be
the Moyie and Dibble Creek faults. (from Price et al., 1972).
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
407
DISCUSSION
Fig. 12. (A) Plot of geotherms as a function of time based on relaxation time following a thermal perturbation such as rifting (from
Beekman et al., 1997). Note that little change in the geotherm occurs
after 300 Ma.
evidence for basement control of Mississippian stratal patterns
in the Cordilleran fold-and-thrust belt but no evidence further
east in the Alberta Basin.
Several other recent studies have addressed the issue of basement reactivation in Alberta through the combined use of seismic
reflection data, potential field anomaly maps and isopach/facies
maps. Edwards and Brown (this volume), in a study of the distribution of reef tracts in central Alberta, including the RimbeyMeadowbrook trend, found no evidence to suggest a direct basement control on the reef tract. A similar conclusion was reached
by Dietrich (this volume) in his study of Lower Paleozoic stratigraphy imaged on the CAT 1992 profile through central Alberta.
In contrast, Lemiex (this volume) illustrates an example in southern Alberta (Medicine Hat Block) where a Late Cretaceous normal fault has the same orientation as reflections in the underlying
basement and could be considered a good example of basement
reactivation. Interestingly, the basement fabric in the Medicine
Hat block in southern Alberta is northwest-trending, subparallel
to the strike of the Cordilleran margin, suggesting that these structures may have been favourably oriented for reactivation compared to structures oriented at a high angle to the margin.
Comparison of the distribution of structural elements in both
map view and cross-section suggests that there is very little
direct geometric relationship between the structures formed
during tectonic assembly of the Precambrian basement and
structures that have influenced the sedimentary section. Hence
basement reactivation does not appear to have been significant
in the Alberta Basin. There are a number of reasons why this
may be the case, and these relate to the physical properties that
control the susceptibility of a particular basement structure to
reactivation. These include the physical properties/rheology of
basement fault zones (Sibson, 1977; White et al., 1980;
Etheridge, 1986; Imber et al., 1997; White and Knipe, 1978;
Winsch et al., 1995), fault orientation (Sibson, 1995), and rock
strength (Kohlstedt et al. 1995). However, there may be a more
fundamental control on reactivation that relates to plate
strength and its role in stress transmission. Observations that
offer a clue are that the apparent reactivation of the Vulcan
structure changes along strike from essentially nonexistent in
the prairies (SALT line 25) to significant in the Cordillera (Fig.
11) and that Mississippian age reactivation of basement structure in the fold-and-thrust belt is absent from Mississippian
strata that overlie the same structures along strike in Alberta
(Brandley et al., 1996).
It has long been suggested that plate strength is a function of
age with older plates being cooler and hence stronger (Molnar
and Taponnier, 1981). Models by Beekman et al. (1997; Fig.
12), for example, suggest that thermal equilibrium is attained
by cooling in the mantle ca. 300 m.y. after thermal perturbation
(rifting, for example) with incrementally small additional cooling. Presumably it is at this point, when the thermal age of the
plate is greater than 300 m.y. that the plate is close to its maximum strength and would approximate a “cratonic” average. A
yield strength calculation for “old, cold” lithosphere shows that
the lithospheric mantle has considerable strength (Fig. 12C)
and forms the strong part of the plate. In order to diminish the
strength of the plate a thermal perturbation has to “reset” the
thermal and rheological state of the lithospheric mantle (Fig.
12B). If such a perturbation does not occur, then cratonic crust
and lithosphere will be too strong to fail and strain will be
accommodated by response of the sedimentary section to the
ambient stress field, unaffected by basement heterogeneities.
The last time the lithospheric mantle beneath the Alberta
Basin was involved in tectonic processes was during assembly of
the crystalline basement, ca. 1.8 Ga. In the Cordillera a major
thermal perturbation occurred during the break-up of western
North America ca. 700 Ma (Ross, 1991) and was rejuvenated
during a younger period of thermal disturbance associated with
formation of the Cambro-Ordovician passive margin (Fig. 1).
The thermal effects of this event likely did not extend into central Alberta, which is well inboard of the passive margin hinge
zone (Bond and Kominz, 1984). Hence, the thermal age of
lithosphere mantle beneath central Alberta is old (ca. 1.8 Ga) and
presumably too strong to allow significant stress transmission
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
G.M. ROSS and D.W. EATON
408
Fig. 12. (B) and (C). Yield strength envelopes (shear strength versus depth) for lithosphere calculated using laboratory-measured physical rock
properties and different geotherms, (low and high heat flow, respectively) from the New Madrid area of the U.S. mid-continent (from Liu and Zoback,
1998). q0 = heat flow; d(/dt = strain rate. Note the difference in scale of the shear strength axis between the two models. Deformation of a region
more than 300 Ma after a thermal event will be accommodated by the rheologically strong mantle (low heat flow profile in C), whereas deformation
in thermally young regions may exceed the yield strength of the mantle and hence transmit stress into the crust (high heat flow profile in B).
into the crust. The Alberta Basin contrasts with the Williston
Basin, where basement structures clearly have been reactivated
(Elliot, 1996; Dietrich and Magnusson, 1998). The mechanical
model for initiation of the Williston Basin remains controversial, but a Lower Paleozoic thermal component is implied by
analysis of fission track data (Osadetz et al., 1998). Thus, in
assessing reactivation, one needs to consider both thermal age
of the lithosphere mantle as well as physical properties of basement rocks and shear/fault zones. We conclude that in the case
of the Alberta Basin, the strength and thermal age of lithospheric mantle was too great to allow much in the way of reactivation of basement structures.
Basement heat flow data have been collected and interpreted for the WCSB utilizing the abundant drillhole temperature
data and basement heat generation studies (Bachu and
Burwash, 1994). In general, basement heat flow patterns
throughout the parts of Alberta where we have examined seismic reflection data is low, locally between 40–60 mW/m2 and
mostly less than 40 mW/m2). These values correspond to cratonic values (see Fig. 12) and, if these are accurate measures of
mantle heat flow, would be consistent with Alberta having a
cold present-day lithospheric mantle.
One area that we have not considered in detail, but which
holds promise for future investigation for understanding basement control on sedimentation is the role of strength heterogeneities in amplifying elastic strain. For example, Heller et al.
(1993) modeled deformation of an elastic plate containing a
zone of weakness (Fig. 13). They showed that vertical deflections of up to 20 m can occur without fault reactivation when
compressive stresses are applied across a pre-existing fault that
dips steeply and strikes at a high angle to maximum compressive stress direction. Thus, facies changes may by nucleated in
the vicinity of a fault without actual strain accommodation and
reactivation of that fault. Interestingly, a greater amount of
elastic deflection was produced in response to differences in
strength of the crust, essentially the juxtaposition of strong and
weak crustal blocks. This may have implications for areas of
Alberta that contain anomalous crust, such as the Peace River
Arch, which contains the WRS (Eaton et al., this volume) and
southern Alberta, which is underlain by intermediate velocity
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
-25
0
25
HORIZONTAL POSITION (km)
50
0
0 -20
20
WEAK
-50
20
STRONG
-25
0
50
25
-20
FAULT DIP
s = 30o
s = 45o
s = 60o
-200
-100
0
100
200
HORIZONTAL POSITION (km)
B
20 40
0
UPPER
-40 -20
SURFACE
DEFLECTION (m)
HORIZONTAL POSITION (km)
0
SURFACE DEFLECTION (M)
DEPTH (km)
Pre-existing fault
0
-50
A
409
-20
DEPTH (km)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
g=2
g=10
-200
LOWER
-100
0
100
200
HORIZONTAL POSITION (km)
Fig. 13. Examples of finite element models that calculate the vertical deflection of an elastic plate subjected to compressive stress. (A) The case
of a homogenous plate with an imbedded pre-existing weakness (fault) for several different fault dips (d). The lowest angle fault leads to the greatest
vertical deflection. (B) The case for deflection of a heterogeneous elastic plate that has a weak zone imbedded in stronger crust. g refers to strength
differences in Young’ s modulus, with the greater the strength contrast the greater the deflection. Note that the modeled strengt
h heterogeneities produce more deflection than the fault model. Note also the lateral distance over which such deflections occur (from Heller et al., 1993).
(mafic) lower crust (Henstock et al. 1998). These features of
the basement may represent a diffuse form of basement control
that could have influenced sedimentation patterns, but we have
not yet investigated these processes.
CONCLUSIONS
We have attempted to assess the evidence for reactivation of
basement structures in the Alberta Basin. Comparison of deformation maps based on long and short wavelength aeromagnetic anomalies and known fault trends allow deformation patterns
to be assessed and compared for different times relative to the
Precambrian basement fabric. With the exception of the southeastern part of the Peace River Arch, these maps result in low
spatial coincidence for Devonian-Carboniferous and Eocene
age structures, suggesting that pre-existing basement structure
had little influence during these periods of extension. Seismic
reflection data which show that discrete basement faults cannot
be linked to Phanerozoic faults provide additional constraints
which suggest that basement structures may have played a role
in the localization of elastic strain.
Local controls on reactivation (fault/shear zone rheology and
orientation (strike and dip)) may have played a role in the lack
of reactivation but this is difficult to assess without outcrop control. We suggest that a more regional control was the thermal
age of lithospheric mantle in Alberta which is ca. 1800 Ma, the
time of the last major thermal event, associated with basement
assembly. The predominance of cold and strong lithospheric
mantle limited stress transmission such that low levels of elastic strain were developed in basement rocks and hence, low
degrees of reactivation. In contrast, areas with ÒrejuvenatedÓ,
thermally-young lithospheric mantle (Cordilleran margin and
Williston Basin) have strong evidence of basement reactivation.
ACKNOWLEDGMENTS
Discussions with a large number of people over the last ten
years have helped guide and develop some of the thoughts presented in this paper. In particular we would like to thank John
Peirce, Ted Glenn, Zeev Berger, Bob Holdsworth and Jim
Dietrich for on-going discussions. Bob Holdsworth, John
McBride, Jim Dietrich and Kirk Osadetz are thanked for manuscript reviews that helped shorten and sharpen the science.
Warner Miles from the National Geophysical Data Centre provided the potential maps used in Figs. 2 and 4 and John
Mariano produced the vertical derivative map Fig. 5. This is
Lithoprobe publication number 1075.
REFERENCES
Andrichuk, J.M. 1961. Stratigraphic evidence for tectonic and current control
of Upper Devonian reef sedimentation, Duhamel area, Alberta, Canada.
American Association of Petroleum Geologists, Bulletin, v. 45, p. 612-632.
Bachu, S. and Burwash, R.A. 1994. Geothermal regime in the western Canada
sedimentary basin. In: Geological Atlas of the Western Canada Sedimentary
Basin. G.D. Mossop and I. Shetsen (compilers). Calgary, Canadian Society
of Petroleum Geologists and Alberta Research Council, p. 447-454.
Barclay, J.E., Krause, F.F., Campbell, R.I. and Utting, J. 1990. Dynamic casting and growth faulting: Dawson Creek graben complex, CarboniferousPermian Peace River Embayment, western Canada. Bulletin of Canadian
Petroleum Geologists, v. 38A, p. 115-144.
Beekman, F., Stephenson, R.A. and Korsch, R.J. 1997. Mechanical stability of
the Redbank Thrust Zone, central Australia: dynamic and rheological
implications. Australian Journal of Earth Sciences, v. 44, p. 215-226.
Bond, G.C. and Kominz, M. A. 1984. Construction of tectonic subsidence
curves for the early Paleozoic miogeocline, southern Canadian Rocky
Mountains: implications for subsidence mechanisms, age of break-up and
crustal thinning. Geological Society of America Bulletin, v. 95, p. 155-173.
Bradley, D.C. and Kidd, W.S.F. 1991. Flexural extension of the upper continental crust in collisional foredeeps. Geological Society of America
Bulletin, v. 103, p. 1416-1438.
Brandley, R.T., Krause, F.F., Varsek, J.L., Thurston, J. and Spratt, D.A., 1996.
Implied basement-tectonic control on deposition of Lower Carboniferous
carbonate ramp, southern Cordillera, Canada. Geology, v. 24, p. 467-470.
Copyright ' 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
G.M. ROSS and D.W. EATON
410
Buchanan, J.G. and Buchanan, P.G. (eds.). 1995. Basin Inversion, Geological
Society of London, Special Publication 88, 600p.
Churcher, P.L. and Majid, H.A. 1989. Similarities between Tangent-Wabamun
play of the Alberta basin and the Albion Scipio type play of the Michigan
Basin. Bulletin of Canadian Petroleum Geology, v. 37, p. 241-245.
Clowes, R. and Kanasewich, E.R. 1972. Digital filtering of deep crustal seismic reflections. Canadian Journal of Earth Sciences, v. 9, p. 434-451.
Cooper, M.A. and Williams, G.D. 1988. Inversion Tectonics, Geological
Society of London, Special Publication 600p.
Dietrich, J.R., and Magnusson, D.H. 1998. Basement controls on Phanerozoic
development of Birdtail-Wadaska salt dissolution zone, Williston Basin,
southwest Manitoba. In 8th Annual Williston Basin Symposium,
Saskatchewan Geological Society, Special Publication 13, p. 166-174.
________. 1999. Seismic stratigraphy and structure of the Lower Paleozoic,
Central Alberta Lithoprobe Transect. Bulletin of Canadian Petroleum
Geology, v.47, no. 4, p. 362-374.
Donaldson, W.S., Plint, A.G. and Longstaffe, F.J. 1998. Basement tectonic
control on distrution of the shallow marine Bad Heart Formation, Peace
River Arch area, northwest Alberta. Bulletin of Canadian Petroleum
Geology, v. 46, p. 576-598.
Dobrin, M.B. and Savitt, C. H. 1988. An introduction to geophysical prospecting McGraw Hill, 867p.
Eaton, D.W., Milkereit, B., Ross, G.M., Kanasewich, E.R., Geis, W., Edwards,
D.J., Kelsch, L. and Varsek, J. 1995. Lithoprobe basin-scale seismic profiling in central Alberta: influence of basement on sediment cover. Bulletin
of Canadian Petroleum Geology, v. 43, p.65-77.
_________, Ross, G.M. and Clowes, R.M. 1999. Seismic-reflection and potential field studies of the Vulcan structure, western Canada: A
Paleoproterozoic Pyrenees? Journal of Geophysical Research, in press.
_________, ________ and Hope, J. 1999. The rise and fall of a cratonic arch:
A regional seismic perspective on the Peace River Arch, Alberta. Bulletin
of Canadian Petroleum Geology. v. 47, no. 4.
Edwards, D.J. 1997. An integrated geophysical investigation of basement controls on Devonian carbonates in central Alberta, Canada. unpublished
Ph.D. thesis, University of Calgary, Calgary, Alberta, 348p.
___________ and Brown, R.J. 1999. Understanding the influence of
Precambrian crystalline basement on Upper Devonian carbonates in central
Alberta from a geophysical perspective. Bulletin of Canadian Petroleum
Geology, v. 47, no. 4, 412-438.
Elliott, C. G. 1996. Phanerozoic deformation in the “stable” craton, Manitoba,
Canada. Geology, .v 24, p. 909-912.
Etheridge, M.A. 1986. On the reactivation of extensional fault systems.
Philosophical Transactions of the Royal Society of London, series A 317,
p. 179-194.
Greggs, R.G. and Greggs, D.H. 1989. Fault-block tectonism in the Devonian
subsurface, western Canada. Journal of Petroleum Geology, v. 12, p. 377404.
Hajnal, Z., Lucas, S., White, D., Lewry, J., Bezdan, S., Stauffer, M.R., and
Thomas, M.J. 1997. Post-collisional tectonics of the TransHudson orogen:
seismic reflection evidence for linked strike-slip and detachment fault systems. Tectonics, v. 15, p. 427-439.
Halbertsma, H.L. 1994. Devonian Wabamun Group of the Western Canada
Sedimentary Basin. In: Geological Atlas of the Western Canada
Sedimentary Basin. G.D. Mossop and I. Shetsen (compilers ). Calgary,
Canadian Society of Petroleum Geologists and Alberta Research Council,
p. 203-220.
Hale, D. 1984. Dip-moveout by Fourier transform, Geophysics, 49; p. 741-757.
Hart, B.S. and Plint, A.G. 1993. Tectonic influence on deposition and erosion
in a ramp setting: Upper Cretaceous Cardium Formation, Alberta foreland
basin. American Association of Petroleum Geologists Bulletin, v. 77, p.
2092-2107.
Heller, P.L., Beekman, F., Angevine, C.L. and Cloetingh, S.A.P.L. 1993.
Cause of tectonic reactivation and subtle uplifts in the Rocky Mountain
region and its effect on the stratigraphic record. Geology, v. 21, p. 10031006.
Henstock, T. J., Levander, A., Snelson, C. M., Keller, G. R., Miller, K. C.,
Harder, S. H., Gorman, A. R., Clowes, R. M., Burianyk, M.J.A. and
Humphries, E.D., 1998, Probing the Archean and Proterozoic lithosphere
of western North America. GSA Today, v. 8, # 7, op. 1-5.
Holdsworth, R. E., Butler, C.A., and Roberts, A. M. 1997. The recognition of
reactivation during continental deformation. Journal of the Geological
Society of London, v. 154, p. 73-78.
Hope, J. Eaton, D.W. and Ross, G.M. 1999. Lithoprobe seismic transect of the
Alberta Basin: Compilation and overview. Bulletin of Canadian Petroleum
Geology, v. 47, no. 4, p. 331-345.
Hurwich, C.A. and Smithson, S.B. 1988. Compositional variation and the origin of deep crustal reflections. Earth and Planetary Science Letters, v. 85,
p. 416-426.
Hynes, A. and Eaton, D. 1999. Lateral ramps as an aid to the unroofing of deepcrustal rocks: Seismic evidence from the Grenville Province. Tectonics, in
press.
Imber, J., Holdsworth, R.E., Butler, C.A. and Loyd, G.E. 1997. Fault-zone
weakening processes along reactivated Outer Hebrides Fault Zone,
Scotland. Journal of the Geological Society of London, v. 154, p. 105-109.
Kanasewich, E.R., Clowes, R.M. and McCloughan, C.H. 1969. A buried
Precambrian rift in western Canada. Tectonophysics, v. 8, p. 513-527.
Kauffman, J., Hanson, G.N. and Meyers, W.J. 1991. Dolomitization of the
Devonian Swan Hills Formation, Rosevear Field, Alberta, Canada.
Sedimentology, v. 38, p. 41-66.
Keith, J.W. 1970. Tectonic control of Devonian reef sedimentation. American
Association of Petroleum Geologists, v. 54, p. 854.
Kohlstedt, D.L., Evans, B. and Mackwell, S.J. 1995. Strength of the lithosphere: Constraints imposed by laboratory experiments. Journal of
Geophysical Research, v. 100, p. 17, 587-17,602.
Leblanc, G. and Morris, W. A. 1999. Aeromagnetics of southern Alberta within areas of hydrocarbon accumulation. Bulletin of Canadian Petroleum
Geology, v. 47, no. 4, p. 439-454.
Lemieux, S. 1999. Seismic reflection expression and tectonic significance of
Late Cretaceous extensional faulting of the Western Canada Sedimentary
Basin in southern Alberta. Bulletin of Canadian Petroleum Geology, v. 47,
no. 4, p. 375-390.
Liu, L. and Zoback, M.D. 1998. Lithospheric strength and intraplate seismicity in the New Madrid seismic zone. Tectonics, v. 16, p. 585-595.
Mandler, H.A.F. and Clowes, R.M. 1998. The HIS bright reflector: further evidence for extensive magmatism in the Precambrian of western Canada.
Tectonophysics, v. 288, p. 71-82.
Molnar, P. and Taponnier, P. 1981. A possible dependence of tectonic strength
on the age of the crust in Asia. Earth and Planetary Science Letters, v. 52,
p. 107-114.
Mossop G.D. and Shetsen I. (compilers). 1994. Geological Atlas of the
Western Canada Sedimentary Basin. Calgary, Canadian Society of
Petroleum Geologists and Alberta Research Council, 510p.
Mountjoy, E.W. 1980. Some questions about the development of Upper
Devonian carbonate buildups (reefs), western Canada. Bulletin of Canadian
Petroleum Geology, v. 28, p. 315-340.
Norris, D.K. and Price, R.A. 1966. Middle Cambrian lithostratigraphy of
southeastern Canadian Cordillera. Bulletin of Canadian Petroleum
Geology, v. 14, p. 385-404.
O’Connell, S.C. 1988. The distribution of Bluesky facies in the region overlying Peace River Arch, northwest Alberta. In: Sequences, stratigraphy and
sedimentology: Surface and subsurface. D.P. James and D.A. Leckie (eds.).
Canadian Society of Petroleum Geologists, Memoir 15, p. 387-400.
____________, Dix, G.R. and Barclay, J.E. 1990. The origin, history and
regional structural development of the Peace River Arch, western Canada.
Bulletin of Canadian Petroleum Geology, v. 38A, p. 4-25.
Osadetz, K.G., Kohn, B.P., O’Sullivan, P., Feinstein, S., Hannigan, P.K.,
Everitt, R.A., Gilboy, C.F., Bezys, R.K. and Stasiuk, L.D. 1998.
Thermotectonics of the Williston Basin and environs: variations in heat flow
and hydrocarbon generation. In 8th Annual Williston Basin Symposium,
Saskatchewan Geological Society, Special Publication 13, p. 1147-1165.
Paukert, G.W. 1982. A geophysical study of Precambrian basement fault structure and related Cretaceous stratigraphic variation in southern Alberta.
Unpublished M.Sc. thesis. University of Calgary, 114p.
Peirce, J. W., Ebner, E. and Marchand, N. 1998a. High resolution aeromagnetic interpretation over Sierra and Yoyo reefs, northeastern British Columbia.
In Geologic Applications of Gravity and Magnetics: Case Histories. R.I.
Gibson and P.S Milligan (eds.). American Association of Petroleum
Geologists, Studies in geology, no. 43, p. 93-101.
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)
BASEMENT REACTIVATION IN THE ALBERTA BASIN
__________, Abercrombie, H. J., Depaoli, G. R., Goussev, S. A. and Charters,
R. A. 1998b. Intrasedimentary magnetization by vertical fluid flow and
exotic geochemistry. The Leading Edge, v. 17, p. 89-92.
Pilkington, M., Miles, W.F., Ross, G.M. and Roest, W.R. 2000. Potential field
signatures of buried Precambrian basement in the western Canada
Sedimentary Basin. Canadian Journal of Earth Science, submitted.
Price, R.A. 1994. Cordilleran tectonic and the evolution of the Western Canada
Sedimentary Basin. In: G.D. Mossop and I. Shetsen (compilers, 1994).
Geological Atlas of the Western Canada Sedimentary Basin. Calgary,
Canadian Society of Petroleum Geologists and Alberta Research Council,
p. 13-24.
________, Balkwill, H.R., Charlesworth, H.A.K., Cook, D.B. and Simony,
P.S. 1972. The Canadian Rockies and tectonic evolution of the southeastern Canadian Cordillera. International Geological Congress, Guidebook,
Excursion AC15, 128p.
Pruscha, J.J. 1988. Zone of weakness concept: A review and evaluation in
Basement Tectonics 8, M.J. Bartholemew, D.W. Hyndman, D.W. Mogk
and R. Mason (eds.). Kluwer Academic, Dordrecht, p.83-92.
Pysklywec, R.N. and Mitrovica, J.X. 1997. Mantle avalanches and the dynamic topography of continents. Earth and Planetary Science Letters, v. 148, p.
447-455.
Reading, H. G, Watterson, J. and White, S.H. (editors). 1986. Major crustal lineaments and their influence on the geological history of the continental
lithosphere, Philosophical Transactions of the Royal Society of London,
Series A: Mathematical and Physical Sciences. 317; p.277.
Richards, B.R., Barclay, J.E., Bryan, D., Hartling, A., Henderson, C. M. and
Hinds, R.C. 1994. Carboniferous of Western Canada Sedimentary basin. In
Mossop, G.D. and Shetsen, I., compilers, 1994, Geological Atlas of the
Western Canada Sedimentary Basin. Calgary, Canadian Society of
Petroleum Geologists and Alberta Research Council, p. 221-250.
Ross, G.M. 1991. Tectonic setting of the Windermere Supergroup revisited.
Geology, v.19, p. 1125-1128.
________ and Eaton, D.W. 1997. Winagami reflection sequence: Seismic evidence for post-collisional magmatism in the Proterozoic of western
Canada, Geology, 25, p. 199-200.
_________, Parrish, R.R., Villeneuve, M.E. and Bowring, S.A. 1991.
Geophysics and geochronology of the crystalline basement of the Alberta
Basin, western Canada. Canadian Journal of Earth Sciences, v. 28, p. 512522.
_________, Broome, J. and Miles, W. 1994. Potential fields and basement
structure, Western Canada Sedimentary Basin. In: Atlas of Western Canada
Sedimentary Basin. G.D. Mossop and I. Shetsen (compilers). Canadian
Society of Petroleum Geologists, p. 37-41.
411
________, Milkereit, B., Eaton, D., White, D., Kanasewich, E.R. and
Burianyk, M.J.A. 1995. Paleoproterozoic collisional orogen beneath the
Western Canada Sedimentary Basin imaged by Lithoprobe crustal seismicreflection data. Geology, v.23, p.195-199.
________, Mariano, J., Dumont, R., Kjarsgaard, B. and Teskey, D.J. 1997.
Was Eocene magmatism widespread in the subsurface of southern Alberta:
Evidence from new aeromagnetic anomaly data. Geological Survey of
Canada, Bulletin 500, p. 235-246.
Sheriff, R.E. and Geldart, L.P. 1995. Exploration Seismology, 2nd Edition,
Cambridge University Press, Cambridge, 592p.
Sibson, R.H. 1977. Fault rocks and fault mechanisms. Journal of the
Geological Society of London, v. 133, p. 191-214.
_______. 1995. Selective fault reactivation during basin inversion: potential
for fluid redistribution through fault valve action. In: Basin Inversion,
Geological Society of London. J.G. Buchanan and P.G. Buchanan (eds.).
Special Publication 88, p. 3-19.
Sikabonyi, L.A. and Rodgers, W.J. 1959. Paleozoic tectonics and sedimentation in the northern half of the west Canadian basin. Journal of Alberta
Society of Petroleum Geologists, v. 7, p. 193-216.
Sprenke, K.F., Wavra, C.S. and Godfrey, J.D. 1986. The geophysical expression of he Canadian Shield of northeastern Alberta. Alberta Research
Council Bulletin 52, 54p.
Stoakes, F.A. 1987. Fault-controlled dolomitization of the Wabamun group,
Tangent Field, Peace River Arch, Alberta. In: Devonian Lithofacies and
Reservoir Styles in Alberta, F.F. Krause and O.G. Burrowes (eds.). Second
International Symposium on the Devonian, Calgary, p. 73-86.
Weglein, A.B. 1999. Multiple attenuation: an overview of recent advances and
the road ahead. The Leading Edge, v. 18, p. 40-44.
White, S.H. and Knipe, R.J. 1978. Transformation and reaction-enhanced ductility in rocks. Journal of Geological Society, London, v. 135, p. 513-516.
________, Burrows, S.E., Carrares, J., Shaw, N.D. and Humphreys, F.J. 1980.
On mylonites in ductile shear zones. Journal of Structural Geology, v. 2, p.
175-188.
__________, Bretan, P.G. and Rutter E.R. 1986. Fault zone reactivation: kinematics and mechanisms. Philosophical Transactions of Royal Society of
London, v. A317, p. 81-97.
Wintsch, R.P., Christofferson, R. and Kronenberg, A. K. 1995. Fluid-rock
reaction weakening of fault zones. Journal of Geophysical Research, v.
100, p. 13,021-13,032.
Zoback, M.L. and 28 others. 1989. Global patterns of tectonic stress. Nature, v.
341, p. 291-298.
Manuscript received: April 19, 1999
Revised manuscript accepted: July 23, 1999
Copyright © 1999 by The Society of Canadian Petroleum Geologists. All Rights Reserved. Vol 47, No 4 (December, 1999)