Download The evolution of the southern Cordilleran foreland thrust and fold

The evolution of the southern Cordilleran foreland thrust and fold belt and the
kinematics of Cordilleran orogenesis
Raymond A. Price, Department of Geological Sciences and Geological Engineering, Queen’s
University, Kingston, Ontario K7L 3N6 [email protected]
The Cordilleran foreland thrust and fold belt records the convergence between the North America
craton and the subduction zones along its western margin. NE-dipping subduction of oceanic
lithosphere has been underway along the western side of North America since the Late Triassic; and a
strip of oceanic lithosphere ~ 13,000 km wide has been consumed there in the last 150 Ma
(Engebretson et al., 1992). The Cordilleran foreland thrust and fold belt formed in the region behind
this NE-dipping, SW-verging oceanic subduction system, as a SW-dipping, NE-verging, accretionary
wedge. It consists of supracrustal rocks that have been scraped off the under-riding North American
lithosphere and accreted to the over-riding tectonic collage of oceanic magmatic-arc terranes and
oceanic accretionary-complex terranes that make up most of the rest of the Canadian Cordillera
(Price, 1994). It began to form in the late Early Jurassic, at ~187-185 Ma (Murphy et al., 1985) as
North America began converging with the NE-dipping oceanic subduction zone that descended into
the mantle under the Quesnellia oceanic magmatic arc. The ensuing collapse of the intervening Slide
Mountain marginal oceanic basin, which was situated outboard of the North American continental
shelf, beyond the pericratonic oceanic rocks of Kootenay terrane, involved NE-verging thrusting and
folding. It culminated in the obduction of Slide Mountain terrane (the deeper-water, ocean-floor,
supracrustal rocks of Slide Mountain basin) over Kootenay terrane and the outboard part of the
Cordilleran miogeocline. The crustal-scale kinematics and geodynamics of the obduction process are
enigmatic; but the results are clear. Slide Mountain terrane overlaps North American rocks in the
Cariboo Mountains; and metamorphic geobarometry and geochronometry from various localities
along this zone in southeastern British Columbia (Archibald et al., 1983; Colpron et al., 1996)
indicate that Lower Paleozoic shallow-water deposits of the outboard part of the Cordilleran
miogeocline were transported Southwestward to depths of about 20 km during the NE-verging
obduction of Slide Mountain terrane. The mantle and lower crustal lithosphere of the Slide Mountain
basin were delaminated from the overlying supracrustal rocks that comprise Slide Mountain terrane,
and have disappeared into the deeper mantle, presumably by becoming entrained in the downward
flow of the NE-dipping subduction system.
Subsequently, continuing convergence between North America and Quesnellia was accommodated by
northeastward tectonic wedging of Quesnellia and Kootenay terrane between the outboard part of the
miogeocline and its underlying crystalline basement. This NE-tapering tectonic wedge was bounded
below by a NE-verging thrust zone, which eventually evolved into the basal décollement of the
foreland thrust and fold belt, and above by a SW-verging thrust zone. SW-verging thrusts, folds, and
fold nappes developed above the advancing wedge, as the overlying miogeoclinal supracrustal rocks
were tectonically delaminated from their underlying crystalline basement, horizontally compressed,
and uplifted progressively toward the NE by the advancing tectonic wedge (Price, 1986; Colpron et
al., 1998). At the same time, the outboard part of the North American lithosphere was being wedged
under the former Quesnellia magmatic arc, delaminating Quesnellia from its mantle and lower crustal
roots, which also have disappeared into the deeper mantle, presumably by becoming entrained in the
downward flow of the NE-dipping subduction system.
By Late Jurassic time (Oxfordian-Kimmeridgian), crustal thickening due to the advance of the
tectonic wedge and the associated SW-verging thrusting and folding above it had generated sufficient
gravitational (topographic) potential to drive the propagation of the “near-basement” basal
décollement beyond the tip of the wedge. This produced a change in style of deformation to NEverging imbricate listric thrust faulting in a northeastward expanding critical-taper accretionary
wedge (Colpron et al., 1998). There was recurrent tectonic wedging and SW-verging thrusting and
folding locally within the evolving critical-taper accretionary wedge, notably in the foreland basin
deposits at the tip of the wedge (the “triangle zone”), and along the west flank of the Porcupine
Southern foreland and thrust belt and kinematics of Cordilleran orognesis
Creek anticlinorial fan structure in the western Rockies; but the dominant structural style from Late
Jurassic to Late Paleocene time was NE-verging imbricate thrusting. The supracrustal rocks of the
miogeocline, and subsequently of the cratonic platform, were scraped off the under-riding North
American lithosphere and accreted to the previously deformed outboard part of the miogeocline,
which had become consolidated with Slide Mountain terrane, Quesnellia, and other components of
Intermontane Superterrane. The growing and shifting gravitational load imposed on the North
American lithosphere by the expanding thrust and fold belt produced a migrating isostatic flexure -the foreland basin -- within which clastic detritus eroded from the evolving thrust and fold belt was
trapped. The growth of the thrust and fold belt, and of the foreland basin, ended in the Late
Paleocene with the onset of an episode of east-west crustal extension that was linked northwestward
to right-lateral displacement on the Tintina-Northern Rocky Mountain Trench fault system, and
Southwestward to right-lateral displacement on the Yalakom-Ross Lake and Fraser River-Straight
Creek fault systems. In the Omenica belt the extension involved the crystalline basement; rocks
from mid-crustal depths, including the basal décollement beneath the thrust and fold belt, and the
underlying Paleoproterozoic crystalline basement, were exhumed (Parrish et al., 1988; Doughty et
al., 1998; Doughty and Price, 1999). In the Rocky Mountains and eastern Purcell Mountains preexisting thrust faults and parts of the basal décollement were reactivated as normal faults.
The horizontal convergence between North America and the subducting oceanic lithosphere along its
margin has varied in both rate and direction, and has generally been oblique to the trend of the
Cordillera (Engebretson et al., 1985). The convergence across the foreland thrust and fold belt also
has varied in both rate and direction, and has generally been oblique to the trend of the belt. The
total amount of shortening across the Southern Canadian Rockies decreases from >250 km, at about
Latitude 530 N, to <20 km, 750 km to the south at Latitude 460 30’ N (Price and Sears, 2000). Late
Cretaceous and Paleocene displacements in this part of the accretionary wedge were linked to the
northwestward rotational translation of Intermontane terrane and Insular terrane relative to North
America, and underwent a 30 0 rotation about an Euler pole in northwestern Montana. The total
shortening also decreases northwestward to about 75 km at Latitude 560 N. This decrease is associated
with a northwestward transformation of Late Cretaceous-Paleocene thrusting into right-hand
displacement on the Tintina-Northern Rocky Mountain trench intra-continental transform fault
system terranes (Price and Carmichael, 1986), which also was kinematically linked to the
northwestward rotational translation of Intermontane and Insular terranes (Price 1994). The
kinematic history of the thrust and fold belt evidently is complex, but it can be analyzed using
sequential palinspastic maps.
Palinspastic reconstructions of thrust and fold belts are hampered by uncertainties about directions of
thrust displacements and how they varied in space and through time. The popular simplifying
assumption that displacements were perpendicular to the regional strike of the faults and folds is
unwarranted. “Balanced” sections that are perpendicular to the regional tectonic strike, but oblique to
fault displacement lead to unreliable palinspastic reconstructions. Reliable palinspastic
reconstructions can be generated using balanced hanging wall and footwall maps, which show the
distributions of rock units and structures on opposite sides of the same fault. Displacement generally
varies significantly from place to place along individual faults. These variations have to be
accommodated explicitly on balanced fault maps as displacement gradients, or as linkages between
the fault and other faults that branch from it or are truncated by it. Each of the pair of balanced
maps must be a logical counterpart of the other, and each commonly can be used as a template for
the other. Footwall ramps and flats must match those on the hanging wall. Intersections of linear
segments of ramps provide piercing points that uniquely define the direction and amount of
displacement on the faults. These concepts can be shown to apply to faults of varying sizes from the
smallest faults to major regional faults. The same principles can be adapted to the analysis of listric
normal faults and the extensional detachments associated with metamorphic core complexes.
About 80 per cent of the tectonic shortening across the southern Canadian Rockies is associated with
displacements on a relatively small number of very large faults (e.g. Lewis thrust: maximum
Southern foreland and thrust belt and kinematics of Cordilleran orognesis
displacement ≈ 100 km); the remaining 20 per cent is distributed among a very large number of small
faults and folds. Thus, reliable regional palinspastic reconstructions can be produced by considering
balanced maps of only the small number of very large faults. The most important of the fault maps is
the footwall map of the regional décollement that defines the base of the thrust and fold belt, and
from which the other major faults branch. The basal décollement separates weak, layered,
anisotropic supracrustal rocks above from strong, massive Paleoproterozoic rocks below; but toward
the front of the belt it rises, across thrust ramps, into one or more higher, laterally extensive
detachments that occur within Devonian, Mississippian, Jurassic and Upper Cretaceous strata. The
largest and most conspicuous ramps in the thrust and fold belt formed along structures in the
crystalline basement that are associated with the margins of former sedimentary basins. These
structures, which are of crustal dimensions (3 to >10 km of throw), include the northeastern margin
of the Mesoproterozoic Belt-Purcell basin, the boundary between the Cordilleran miogeocline and the
cratonic platform, and the boundary between the Cambro-Ordovician miogeoclinal carbonate
platform and the adjacent shale basin. During thrusting, these basins were tectonically inverted:
hanging wall ramps became crustal-scale fault-bend anticlinoria (e.g. Purcell anticlinorium); footwall
ramps became crustal-scale fault-bend monoclines (e.g. Vernon monocline). These relationships
provide the framework for a 3-D regional palinspastic reconstruction of the thrust and fold belt.
Isotope geochronometry of Jurassic to mid-Cretaceous granitic plutons that cut faults in the internal
parts of the accretionary wedge (reviewed in Colpron et al., 1988 and Price and Sears, 2000) provides
minimum ages for displacements on some of the thrusts; maximum ages for the final displacements
on some are provided by the ages of stratigraphic units that are truncated along the footwalls of the
thrusts. The stratigraphic record in the adjacent foreland basin provides additional information about
times of thrusting. The superimposed Early and Middle Eocene and younger extension and righthand strike slip is dated by isotope geochronometry of cross-cutting and truncated magmatic rocks
(Carr, 1992), and by cooling ages of rapidly quenched metamorphic core complexes (Doughty and
Price, 1999), as well as by the ages of syn-extensional sediments.
On the basis of these considerations, four contrasting episodes can be distinguished in the Late
Jurassic to Eocene kinematic evolution of the foreland thrust and fold belt:
(1.) left-hand transpression (155 - 105 Ma), which involved oblique southeastward convergence of
Intermontane terrane and the North American craton, compressed the outer part of the miogeocline
while it was still situated outboard of the craton;
(2.) a hiatus (105 - 85 Ma), during which there was little, if any convergence between Intermontane
terrane and the North American craton, was marked by widespread, cross-cutting, “post-tectonic”
granitic plutons that “stitched” many of the preceding thrust faults;
(3.) right-hand transpression (85 - 58 Ma), which produced most of the Rocky Mountain foreland
thrust and fold belt, involved large-scale (>150 km) clockwise rotation and NE-verging thrusting in
the southern Canadian Rockies, much of which was transformed northward into a large right-hand
strike slip on the Tintina-northern Rocky Mountain trench fault system;
(4.) right-hand transtension (58 - 42 Ma), which large-scale east-west horizontal extension and
involved the exhumation of metamorphic core complexes in the south, was transformed northward
into right-hand strike slip on the Tintina-northern Rocky Mountain trench fault system.
Subsequently (42 - 0 Ma) the locus of deformation shifted to the continental margin, although
intermittent, basin-and-range type extensional deformation in the interior of the accretionary wedge
continued into the Neogene.
References
Archibald, D.A., Glover, J.K., Price, R.A., Farrar, E. and Carmichael, D.M. 1983. Geochronology and
tectonic implications of magmatism and metamorphism in the southern Kootenay arc and
neighbouring regions, southeastern British Columbia. Part 1: Jurassic to mid-Cretaceous. Canadian
Journal of Earth Sciences, v. 20, p. 1891-1913.
Southern foreland and thrust belt and kinematics of Cordilleran orognesis
Carr, S. 1992. Tectonic setting and U-Pb geochronology of the early Tertiary Ladybird leucogranite
suite, Thor-Odin - Pinnacles area, southern Omineca belt, British Columbia. Tectonics, v. 11, p. 258278.
Colpron, M., Price, R.A., Archibald, D.A., and Carmichael, D.M. 1996. Middle Jurassic exhumation
along the western flank of the Selkirk fan structure: Thermobarometric and thermochronometric
constraints from the Illecillewaet synclinorium, southern British Columbia. Geological Society of
America Bulletin, v. 108, 1372-1392.
Colpron, M., Warren, M.J., and Price, R.A., 1998. Selkirk fan structure, southeastern Canadian
Cordillera: Tectonic wedging against an inherited basement ramp. Geological Society of America
Bulletin, v. 110, 8, p. 1060-1074.
Doughty, P.T., Price, R.A., and Parrish, R.R. 1998. Geology and U-Pb geochronology of Archean
basement and Proterozoic cover in the Priest River complex, northwestern United States, and their
implications for Cordilleran structure and Precambrian continent reconstruction. Canadian Journal of
Earth Sciences, v. 35, 1, p. 39-54.
Doughty, P.T. and Price, R.A. 1999. Tectonic evolution of the Priest River complex, northern
Idaho and Washington: A reappraisal of the Newport fault with new insights on metamorphic core
complex formation, Tectonics, v. 18, 3, p. 375-393.
Engebretson, D.C., Cox, A., and Gordon, R. 1985. Relative motions between oceanic plates and
continental plates in the Pacific basin. Geological Society of America, Special Paper 208, 59p.
Engebretson, D.C., Kelly, K.P., Cashman, H.J., and Richards, M.A. 1992. 180 million years of
subduction. GSA Today, v.2, p. 93-96.
Murphy, D.C., Parrish, R.R., Klepacki, D.W., McMillan, W., Struik, L.C., and Gabites, J. 1995. New
geochronological constraints on Jurassic deformation of the western edge of North America,
southeastern Canadian Cordillera. in Miller, D.M. and Busby, C., (eds.), Jurassic magmatism and
tectonics of the North American Cordillera; Geological Society of America, Special Paper 299, p.
159-171.
Parrish, R.R., Carr, S.D., and Parkinson, D.L. 1988, Eocene extensional tectonics and geochronology
of the southern Omineca belt, British Columbia and Washington. Tectonics, v. 7, p. 181-212.
Price, R.A. 1986. The southeastern Canadian Cordillera: thrust faulting, tectonic wedging, and
delamination of the lithosphere. Journal of Structural Geology, v. 8, p. 239-254.
Price, R.A. 1994. Cordilleran tectonics and the evolution of the Western Canada sedimentary basin.
in Mossop, G.D. and Shetsen, I. Geological Atlas of Western Canada. Calgary, Canadian Society of
Petroleum Geologists/Alberta Research Council, p. 13-24.
Price, R.A. and Carmichael, D.M. 1986. Geometric test for Late Cretaceous-Paleogene
intracontinental transform faulting in the Canadian Cordillera. Geology, v. 14, p. 468-471.
Price, R.A. and Sears, J.W. 2000 (in press). A preliminary palinspastic map of the Mesoproterozoic
Belt/Purcell Supergroup, Canada and U.S.A.: Implications for the tectonic setting and structural
evolution of the Purcell anticlinorium and the Sullivan deposit. in Lydon, J.W., Slack, J.F., Höy, T.
and Knapp, M. (eds.). The Geological Environment of the Sullivan Deposit, British Columbia.
Geological Association of Canada, Mineral Deposits Division, MDD Special Volume No. 1.
Southern foreland and thrust belt and kinematics of Cordilleran orognesis
Biographical Note:
Ray Price is a graduate of the University of Manitoba (B.Sc. Hons., 1955) and Princeton University
(Ph.D., 1958), a former employee of the Geological Survey of Canada,, and a former Professor of
Geological Sciences and Geological Engineering at Queen’s University, He mapped various parts of
the southern Canadian Rockies for the Geological Survey of Canada; and, with his graduate students,
he has been involved in tectonic studies in the southern Canadian Rockies, and in the Purcell, and
Selkirk Mountains of southern Canada and the northern U.S.A. He is currently Professor Emeritus at
Queen’s University, and is engaged in palinspastic reconstruction and tectonic analysis of the
southeastern Canadian Cordillera.