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Geology
Stratigraphy and structure of the Laurentian rifted margin in the northern
Appalachians: A low-angle detachment rift system
John S. Allen, William A. Thomas and Denis Lavoie
Geology 2009;37;335-338
doi: 10.1130/G25371A.1
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Notes
© 2009 Geological Society of America
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Stratigraphy and structure of the Laurentian rifted margin in the
northern Appalachians: A low-angle detachment rift system
John S. Allen1, William A. Thomas1, Denis Lavoie2
1
Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA
Geological Survey of Canada, Centre Géoscientifique de Québec, 490 de la Couronne, Quebec City, Quebec G1K 9A9, Canada
2
ABSTRACT
The Neoproterozoic–Early Cambrian rifted margin of eastern Laurentia is framed by
promontories and embayments defined by northeast-trending rifts offset by northwesttrending transforms. A regional first-order synthesis of the available stratigraphic data in the
northern Appalachians reveals significant along-strike variations in the thickness, composition, age, and facies of synrift and early postrift stratigraphy. These variations are consistent
with models for low-angle detachment rift systems and allow for resolution of regional structures specific to low-angle detachments, including upper-plate margins, lower-plate margins,
and transform faults that bound zones of oppositely dipping low-angle detachments.
et al., 1998). Early Cambrian postrift siliciclastic
and carbonate deposits (Cheshire and Dunham
Formations) above the synrift succession in the
New England rift zone are regionally extensive
and have a combined thickness of ~800 m, but
thin abruptly northward across the international
border (Osberg, 1969).
In southeastern Quebec, synrift shelf clastic deposits (Pinnacle Formation) along the
Quebec rift zone (Fig. 1) are thin (140–250 m)
and conformably overlie bimodal volcanics of
the ca. 554 Ma Tibbit Hill Formation (Marquis
and Kumarapeli, 1993), suggesting that synrift deposition postdates synrift sedimentation in northern New England. Felsites in the
Tibbit Hill Formation have trace and rare earth
EXPLANATION
SCALE
200 km
0
Rift
N
Transform fault
Intracratonic basement fault
Appalachian
Y
NA
SILMI
structural front
UE EN
G
A
B
Thrust fault
S RA
G
Baie VerteBrompton Line
OT
TAW
GR
AB A
EN
RI
FT
ZO
NE
ST. LAWRENCE
PROMONTORY
An
tic
os
ti I
sla
nd
NG
G
RA
NG
-M
AY
SE
M
NT
O
Y
NC
RE
O
FT
RI
PÉ NE
AS O
G Z
SF
AN
PT
-IL
ES
TR
AN
SF
O
RM
60°W
RM
O
EC E
EB ON
U
Q TZ
F
RI
TR
70°W
N
UE
RM
FO
NS
RA D
I T AN
UO GL E
SQ EN ZON
W T
NE RIF
SI
IS
M
QUEBEC
EMBAYMENT
50°N
E
SA
MP
K
YOR Y
NEW ONTOR
M
PRO
NORTHERN APPALACHAIN
RIFTED MARGIN
This article relies on a brief synthesis of significant synrift and early postrift stratigraphy
along the Laurentian margin in the northern
Appalachians. In palinspastically restoring
Iapetan rift deposits in Appalachian thrust
sheets, we assume thrust translation roughly
orthogonal to regional northeast-trending
foreland structures. On a regional scale, any
obliquity in Paleozoic Appalachian thrust
displacement is likely minimal and does not
substantially affect our interpretations. Internal basement massifs are omitted because of
uncertain palinspastic relationships with geo-
graphically opposing segments of the Laurentian margin.
Along the New England rift zone (Fig. 1),
Iapetan synrift siliciclastic rocks (Pinnacle Formation) consist of alluvial-fan deposits that are
geographically extensive and have a thickness of
2.0–3.5 km (Cherichetti et al., 1998). In northern
Vermont, the Pinnacle Formation underlies and is
interlayered with rift volcanics of the ca. 554 Ma
Tibbit Hill Formation (Kumarapeli et al., 1989),
indicating that synrift deposition in the New
England rift zone began prior to the latest Neoproterozoic. Northward into southern Quebec,
the Pinnacle Formation thins dramatically and
is expressed as deltaic and coastal deposits
(Marquis and Kumarapeli, 1993; Cherichetti
LO
INTRODUCTION
Along-strike variations in synrift and early
postrift stratigraphy are a powerful guide to the
evolution of the upper crustal architecture of
a continental rift. The exhumed stratigraphy
exposed along deformed continental margins
offers a rare glimpse into the mechanisms by
which continents rift apart. Observations of
the deformed Neoproterozoic–Cambrian synrift and postrift stratigraphy from northern
Mexico to eastern Canada led to the proposal
that promontories and embayments along the
eastern Laurentian rifted margin are defined
by northeast-trending rifts offset by northwesttrending transforms (Thomas, 1977), and that
along-strike variations in the synrift and postrift
stratigraphy reflect upper-plate, lower-plate,
and transform segments of the margin in the
southern Appalachians (Thomas, 1991, 1993;
Thomas and Astini, 1999). This article synthesizes synrift and postrift stratigraphy and structures in New England and Canada and proposes
a revised model for the northern Appalachians
based on a low-angle detachment rift system.
45°N
Figure 1. Interpreted Neoproterozoic–Early Cambrian continental margin defined by rift segments and transform faults (modified from Thomas, 1977). Map shows general outline of
Paleozoic Appalachian orogenic belt and intracratonic basement fault systems. MP—Montmorency promontory; SILMI—Sept-Iles layered mafic intrusion.
© 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
April
2009
Geology,
April
2009;
v. 37; no. 4; p. 335–338; doi: 10.1130/G25371A.1; 3 figures.
335
Downloaded from geology.gsapubs.org on March 19, 2010
336
{
59°W
56°W
SEG. 1
{
HBA
49°N
{
SEG. 5
IER
LONG
{
RANG
SEG. 3
SEG. 4
CANAD
A
TRANS BAY
FORM
E INL
{
51°N
Belle Isle
BELLE IS
L
TRANSF E
ORM
SEG. 2
BON
N
TRAN E BAY
SFOR
M
SERP
ENT
TRAN INE LAK
E
SFOR
M
ST
EE
INL L MT
IER N
eratic beds (Saint-Damase Formation, formerly
Cap Enragé Formation) increase in abundance
and contain boulder-sized clasts of platform carbonate, rift basalt, and basement gneiss (Lavoie
et al., 2003).
The Sept-Iles transform offsets the rift by
~500 km from the Quebec embayment to the
St. Lawrence promontory (Fig. 1). North of
the transform on the Anticosti platform, thin
(<855 m), autochthonous Early Ordovician
shelf carbonates (Romaine Formation) unconformably overlie crystalline basement (Sanford,
1993). In contrast, south of the transform in the
Appalachian allochthon on the Gaspé Peninsula are deep-water Middle Cambrian clastic
deposits (Orignal Formation) (Lavoie et al.,
2003). Northwest along strike of the transform
is the large, ca. 565 Ma rift-related Sept-Iles
layered mafic intrusion (Fig. 1) (Higgins and
van Breeman, 1998). Geochemical and isotopic
data indicate that magmas in the mafic intrusion
were derived from an upper mantle source and
that they did not interact with continental crust.
A possible explanation is that fracture systems
related to the Sept-Iles transform penetrated the
entire crust and tapped the upper mantle, channeling magmas of the Sept-Iles layered mafic
intrusion through the crust.
The Long Range rift zone composes the
length of the St. Lawrence promontory in western Newfoundland (Fig. 1), and can be divided
into five segments on the basis of the distribution of synrift and postrift stratigraphy and
basement massifs (Fig. 2). External basement
massifs of Mesoproterozoic crystalline basement are located along segments 1, 3, and 5.
Neoproterozoic–Early Cambrian synrift deposits along these segments consist exclusively of
shelf deposits (Labrador Group). On Belle Isle
(segment 1), shoreline outcrops of the Labrador
Group contain a lower rift succession of faultbounded arkosic conglomerates (Bateau Formation) that vary locally in thickness (0–240 m)
and are crosscut by Neoproterozoic basalt dikes
(Lighthouse Cove Formation) (Williams, 1995).
Unconformably overlying the lower rift succession are Early Cambrian sandstones (Bradore
Formation) of the upper Labrador Group. Along
segments 3 and 5, parautochthonous late synrift and early postrift Early Cambrian shallowmarine shelf facies consisting of siliciclastic
deposits with minor carbonate (upper Labrador
Group) unconformably overlie basement and
are regionally thin (~240–740 m) (Knight, 1991;
Williams 1995; Cooper et al., 2001). A Middle
Cambrian–Middle Ordovician carbonate shelf
(Port au Port Group through Table Head Group)
oversteps the siliciclastic shelf and ranges from
1200 to 1500 m thick (Williams, 1995). Slope
deposits in the Humber Arm allochthon (Fig. 2)
along segments 3 and 5 consist of coarse Middle
Cambrian passive-margin carbonate debris flows
HAA
element chemistries that resemble within-plate
granites associated with thick continental crust
(Kumarapeli et al., 1989). Rift-to-drift and early
passive-margin deposits (Cheshire-Gilman and
Dunham Formations) apparently do not exceed
600 m in total thickness and thin northward from
the international border (Osberg, 1969; Marquis
and Kumarapeli, 1993). Synrift slope deposits in
southern Quebec (lower Armagh Formation and
the Green Sandstone Unit) have an estimated
thickness of 600 m, overlie rift basalts of the
550 ± 7 Ma Mt. St-Anselme Formation (Hodych
and Cox, 2007), and contain late Early Cambrian
macrofauna and acritarchs (Lavoie et al., 2003).
Along the Gaspé rift zone (Fig. 1), Laurentian shelf deposits are buried beneath the Appalachian allochthon except along the St. Lawrence Estuary, where seismic profiles indicate
platform strata beneath the estuary (Pinet et al.,
2008). Appalachian thrust sheets on the Gaspé
Peninsula are dominated by Laurentian slope
deposits. There, synrift sediments (St-Roch
Group) were deposited on a steep continental
slope (Cousineau and Longuépée, 2003) and are
interlayered with rift basalts of the Lac Matapédia suite, which were dated as 565 ± 6 Ma
and 556 ± 7 Ma (Hodych and Cox, 2007). The
latter age is statistically indistinguishable from
volcanics in the Tibbit Hill Formation; however,
the ca. 565 Ma age apparently is unique to the
Gaspé rift zone, suggesting that synrift deposition in Gaspé began prior to synrift deposition
in the Quebec rift zone. A separate suite of rift
basalts (Shickshock Group) has geochemical
signatures that indicate magmatic interaction
with highly attenuated continental crust (Camire
et al., 1995).
The Saguenay-Montmorency transform
(Fig. 1) is proposed herein to separate the Quebec
and Gaspé rift zones. The northwestern extension of the transform is the Saguenay graben,
which is a tensional structure that extends into
the continent perpendicular to the strike of the
margin and contains two known synrift igneous
complexes (Kumarapeli, 1985). The intersection
of the transform and the Quebec rift zone forms
the second-order Montmorency promontory,
which is recognized as an asymmetric structural
and stratigraphic high with a steep northeast
gradient within the Quebec embayment (e.g.,
Cousineau and Longuépée, 2003). Sedimentary deposits along the Montmorency promontory consist of thin (200–800 m), late-Early
to Middle Cambrian glauconite-bearing sandstones (Anse Maranda Formation) interpreted
to represent a narrow sediment-starved shelf
(Longuépée and Cousineau, 2005). Southwest
of the transform, the Anse Maranda Formation
is overlain by sparse Middle to Late Cambrian
conglomerates (Breakeyville and Lauzon Formations). Along strike to the northeast across
the transform into the Gaspé rift zone, conglom-
N
EXPLANATION
0
50
KM
Allochthonous slope deposits and
ophiolite complexes
Shelf carbonates
Shelf siliciclastic rocks
Internal massif of shelf and slope
Crystalline basement
Figure 2. The geology of western Newfoundland divided into five segments (Seg.) on the
basis of synrift and passive-margin stratigraphy. Each segment is separated by abrupt
along-strike transitions in shelf and slope
stratigraphy interpreted to be at transforms
(see text). HAA—Humber Arm allochthon;
HBA—Hare Bay allochthon.
(Cow Head Group) that grade upward into
Middle Ordovician flysch (James and Stevens,
1986; Cawood and Botsford, 1991). Outcrops
of Neoproterozoic–Early Cambrian synrift
slope deposits are lacking in the Humber Arm
allochthon along segments 3 and 5, suggesting
little or no deposition on the Laurentian slope
along these segments during the Early Cambrian (Lavoie et al., 2003).
Along segment 4, the Early Cambrian shelf
is partially hidden beneath the Humber Arm
allochthon. Where it is exposed, the Early
Cambrian siliciclastic shelf grades upward into
Middle Cambrian phyllite and limestone conglomerate (Reluctant Head Formation) interpreted as a prograding carbonate ramp, which
in turn grades upward into a Late Cambrian
shallow-marine carbonate platform (upper Port
au Port Group) (Knight and Boyce, 1991). The
succession indicates a deeper water environment
on the shelf along segment 4 resulting from prolonged subsidence during the Early and Middle
Cambrian. In contrast to the succession in seg-
GEOLOGY, April 2009
Downloaded from geology.gsapubs.org on March 19, 2010
SILMI
ST. LAWRENCE
PROMONTORY
OTTAWA
GRABEN
RK Y
YO R
W NTO
E
N MO
O
PR
NE
E
NG
G
ON
RI
RA
L
TR
S
AN
FT
RI
PÉ NE
S
GA ZO
ZO
ES
IL
RM
FO
EC E
EB ON
U
Q TZ
F
RI
FT
PT
SE
CY
EN
OR
TM M
ON OR
-M F
AY NS
EN TRA
GEOLOGY, April 2009
QUEBEC
EMBAYMENT
GU
SA
RIFTED MARGIN AS A LOW-ANGLE
DETACHMENT RIFT SYSTEM
The four-dimensional architecture of a rifted
continental margin can be inferred from the
synrift and postrift stratigraphy. In the northern Appalachians, the Iapetan rift succession
displays conspicuous along-strike variations in
thickness, age, and depositional environment
that strikingly match the modeled stratigraphic
characteristics for low-angle detachment continental rift systems. Models for low-angle
detachment rifts include distinctive structural
configurations and thermal patterns for continental extension facilitated by a shallow-dipping
(<30°) listric fault system that separates the crust
into conjugate lower- and upper-plate domains
partitioned along strike by steep transform faults
(e.g., Wernicke, 1985; Lister et al., 1986, 1991;
Thomas and Astini, 1999).
The lower plate (beneath the low-angle
detachment) is characterized by highly attenuated continental crust that undergoes rapid thermal decay during rifting and thus undergoes
rapid subsidence (Buck et al., 1988; Lister et al.,
1991). This facilitates thick synrift and early
postrift sedimentary accumulation on the lower
plate within fault-rotated crustal blocks above
the oceanward-dipping detachment. In contrast,
the upper plate (above the low-angle detachment) is characterized by a narrow zone of
transition from full thickness continental crust
to oceanic crust. The proximity of full thickness continental crust on the upper plate to the
active rift axis (Thomas and Astini, 1999, their
Fig. 3) results in prolonged thermal expansion
on the upper plate, which delays passive-margin
thermal subsidence (Buck et al., 1988). Consequently, initial synrift deposits on the upper
plate are younger than those on the conjugate
lower plate, and are more limited in both thickness and distribution. Transform faults both offset individual rift segments and bound domains
of oppositely dipping rift detachments (Lister
et al., 1986). Thus, transforms facilitate abrupt
SAGUENAY
GRABEN
M
D
OR
SF LAN
AN NG NE
E O
TR
OI EW T Z
N RIF
QU
IS
SS
MI
ments 3 and 5, Neoproterozoic(?)–Early Cambrian synrift slope deposits (Curling Group)
at the base of the Humber Arm allochthon in
segment 4 consist of a coarse siliciclastic succession with a minimum measured thickness of
1840 m (Palmer et al., 2001). The top of the synrift slope succession is marked by massive conglomerates that contain large rounded blocks of
shelf carbonate and crystalline basement. Sedimentary deposits in the Hare Bay allochthon
(Fig. 2) along segment 2 are less well studied,
but appear to include a slope succession that is
similar to that in the Humber Arm allochthon.
There, Early Cambrian slope deposits (Maiden
Point Formation) are estimated as 2000 m thick
and locally contain blocks of metamorphic and
granitic basement (Williams, 1995).
Figure 3. Schematic block diagram of eastern Laurentian rifted continental margin and intracratonic fault systems of northeastern North America (present coordinates) in context of
low-angle detachment rift system. SILMI—Sept-Iles layered mafic intrusion.
(<25 km) along-strike changes in the architecture of the low-angle detachment rift system,
and thereby the synrift and postrift stratigraphy. Implicitly, low-angle detachment rifts are
marked by abrupt along-strike changes in the
synrift stratigraphy at transforms.
Thick Neoproterozoic alluvial-fan deposits
overlain by relatively thick early postrift shelf
sandstones in the New England rift zone indicate rapid synrift subsidence consistent with a
lower-plate rift setting (Fig. 3). Northward into
the Quebec rift zone, the sharp contrast in thickness and facies of synrift and postrift deposits,
the younger age of synrift deposits, and the geochemistry of felsic volcanic phases in the Tibbit
Hill Formation all indicate an upper-plate rift
setting (Fig. 3). The abrupt along-strike variation in synrift and postrift stratigraphy near the
international border implies a transform fault
(the Missisquoi transform) between the lowerplate New England rift zone and the upper-plate
Quebec rift zone (Cherichetti et al., 1998).
Cousineau and Longuépée (2003) originally
suggested that the Gaspé rift zone developed
as a lower-plate rift domain, and our model
based on the data presented here supports their
interpretation (Fig. 3). In the Gaspé rift zone,
the stratigraphy is dominated by synrift and
passive-margin slope sediments; this requires
a broad attenuated margin to accommodate the
volume of slope deposits. This is consistent with
the geochemistry of rift volcanics that indicate
that the Gaspé rift zone is underlain by highly
attenuated continental crust. Furthermore, synrift deposition in Gaspé apparently commenced
earlier than synrift sedimentation in southern
Quebec. The Saguenay-Montmorency transform is the boundary between the upper-plate
Quebec rift zone and the lower-plate Gaspé rift
zone, and marks a fundamental along-strike
change in the composition and facies of synrift and passive-margin deposits in the Quebec
embayment (Cousineau and Longuépée, 2003).
The increase in abundance of coarse conglomerates northeast of the transform is typically attributed to Cambrian reactivation of the Saguenay
graben (e.g., Lavoie et al., 2003). Alternatively,
coarse conglomerates with rift basalt and
basement-derived clasts may have been supplied by erosion of the transform margin during
Middle and Late Cambrian sea-level lowstands
(e.g., Lavoie et al., 2003).
On the St. Lawrence promontory, the stratigraphy on segments 1, 3, and 5 of the Long
Range rift zone includes a thin Early Cambrian
synrift clastic shelf succession overlain by a
thin passive-margin succession and no observed
Neoproterozoic–Early Cambrian synrift slope
deposits. These observations are consistent with
an upper-plate rift setting for these segments
of the promontory (Fig. 3). In this context, the
lack of Early Cambrian synrift slope deposits is attributed to thermal uplift of the upperplate margin, which would effectively limit
synrift deposition. In contrast, segments 2 and
4 contain a thick synrift slope succession that
includes coarse conglomerates with basementderived clasts, indicating rapid subsidence and
erosion of the margin along these two segments during Iapetan rifting. Furthermore, shelf
deposits along segment 4 indicate prolonged
subsidence of the shelf that lasted through late
Middle Cambrian time. These observations
are consistent with a lower-plate setting for
segments 2 and 4 of the Long Range rift zone
(Fig. 3). The boundaries between each of the
five segments on the St. Lawrence promontory
are abrupt along-strike discontinuities in shelf
337
Downloaded from geology.gsapubs.org on March 19, 2010
and slope stratigraphy (Cawood and Botsford,
1991). These abrupt along-strike changes in the
stratigraphy are interpreted to mark transforms
that separate upper- and lower-plate margins
along the St. Lawrence promontory. Cawood
and Botsford (1991) originally recognized these
transforms on the basis of along-strike discontinuity in the passive-margin and foreland-basin
slope stratigraphy, and we adopt their nomenclature for these transform faults.
CONCLUSIONS
The Neoproterozoic–Early Cambrian stratigraphy of the eastern Laurentian margin in
the northern Appalachians records protracted
continental rifting. A synthesis of stratigraphic
and structural observations along the rifted margin includes: (1) abrupt along-strike changes in
thickness of synrift and early postrift stratigraphy (in most places by an order of magnitude); (2) along-strike facies transitions within
the synrift stratigraphy; (3) along-strike diachroneity of synrift sedimentation; and (4) differential along-strike subsidence of the margin. These
significant along-strike variations in synrift and
postrift stratigraphy reflect along-strike partitioning of the rift into segments that differ fundamentally in tectonic framework, subsidence
history, and sediment dispersal. Specifically,
these characteristics conform to a low-angle
detachment model for rifting continental crust,
and they constrain the range of acceptable models for continental rifting. This new interpretation is consistent with the Iapetan rift along the
entire length of the eastern Laurentian margin
from Newfoundland to Mexico and provides a
regional constraint on the breakup of Rodinia,
as well as highlights stratigraphic constraints for
models of continental rifting.
ACKNOWLEDGMENTS
Part of this research was supported by grants from
the Geological Survey of Canada and the Geological Society of America. We thank Nicolas Pinet for
a critical and thoughtful review of an early draft of
the manuscript. We thank the journal reviewers Meg
Streepey, Hugues Longuépée, and an anonymous
reviewer for their insightful comments. This manuscript is Geological Survey of Canada Contribution
2008164.
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Manuscript received 3 August 2008
Revised manuscript received 17 November 2008
Manuscript accepted 26 November 2008
Printed in USA
GEOLOGY, April 2009