Download Continuity of indigenous ancient North American crust across the

Continuity of indigenous ancient North American crust across the Canadian Cordillera a review of evidence
Henry Lyatsky
(LGRC Ltd., 4827 Nipawin Cres. NW, Calgary, AB T2K 2H8 Canada)
ph./fax (403) 282-5873 [email protected] www.cadvision.com/lyatskyh
Inherited ancient ex-cratonic crustal structures are seen from diverse geological and geophysical evidence
to have had a significant part in the Cordillera’s orogenic development. Contrary to some common
misconceptions, the evidence summarized here (also see references) shows that
(1) the Canadian Cordilleran miogeosynclinal region has never contained an Atlantic-type passive
continental margin; and
(2) the main differences between major tectonic zones in the Cordilleran interior lie not in their
assumed exotic-terrane composition but in the degree and history of in situ, multi-aspect, multicycle orogenic reworking of the indigenous, ex-cratonic North American lithosphere and crust.
Trending largely NE-SW, ancient continental crustal weakness zones are well known to divide the North
American craton into distinct huge blocks. Some of these zones predate cratonization, which occurred in
the Early Proterozoic in western Canada and somewhat later in the western U.S. The same NE-SW
grain is strongly expressed in the Phanerozoic structurally controlled igneous provinces, potential-field
anomaly patterns and upper-mantle seismic velocity structure across the U.S. Cordillera. Tectonically
reworked rocks of various Archean and Proterozoic ages are recognized in many parts of the Canadian
Cordillera, while paleontological, paleomagnetic and structural evidence for exoticism of assumed accreted
terranes is continually revised or eliminated. Crustal weakness zones are seen from surface geology and
geophysical anomalies to continue NE-SW all across the Canadian Cordillera, transecting the NNW-SSE
Cordilleran tectonic grain and testifying to former continuity of North America’s craton(s).
Continents are sometimes wrongly assumed to be tectonically inert, only passively responding to external
influences from the mobile sublithospheric mantle below and plate-boundary zones to the side. But
continental lithosphere has its own sources of radiogenic heat, and hence a capacity to self-develop.
External plate-boundary influences (subduction, terrane collisions etc.) are not necessary to explain all
intra-continental tectonism. The biggest influences on continents from other tectonic plates, and on
cratons from neighboring orogenic provinces, are commonly confined, respectively, to fairly narrow
continental-margin or pericratonic zones. The latter usually contain non-indigenous, teleorogenically
induced tectonic features such as foredeeps and rootless fold-and-thrust belts.
The most reliable way to study rocks is by direct observation. Indirect geophysical methods help to sense
inaccessible rocks remotely. A geophysical anomaly, by itself, cannot reveal the nature and age of its rock
source. It only indicates, non-uniquely and with limited resolution, some geometric perturbation in the
distribution of this or that physical property of the rocks. Steep discontinuities are mostly invisible in deep
seismic reflection images, but they are better detected with potential-field data. Studies in crystalline
continental provinces show the acoustic properties of tectonically reworked rocks to be exceedingly
complex, with countless, variously oriented reflectors, refractors and diffractors. Superdeep wells in the
upper continental crust worldwide have produced many big unexpected surprises about sources of seismic
and potential-field anomalies, position of the brittle-ductile boundary, and thermal and hydrologic conditions.
Even fewer factual geological constraints are available for the middle and lower crust. The model-driven
and selective assumption that low-angle seismic-reflection geometries deep in the crust mostly represent
thrusting overlooks all the other known explanations.
Transcurrent crustal structures in the Canadian Cordillera, despite the dominant NNW-SSE trends, are
indicated by long NE-SW and E-W gravity and magnetic lineaments, faults, elongated plutons, lakes, fiords
and rivers. Many transcurrent trends are found in the orogenically semi-reworked Intermontane and
Insular tectonic belts. The more-reworked Omineca and Coast belts are dominated more strongly by the
Cordilleran NNW-SSE grain, as they retain less of their ancient tectonic inheritance. On trend with the
Proterozoic Athabasca Basin and the much-inverted latest Proterozoic-Phanerozoic Peace River Arch in
the craton, the prominent NE-SW-trending Skeena Arch in the Cordilleran interior has been active since at
least the Mesozoic, dividing the Intermontane Belt into major crustal blocks and forming the southern
boundary of the Bowser Basin. Faults on trend with the Skeena Arch continue SW across the Coast and
Insular belts, accounting for big Mesozoic and Cenozoic transverse structures and potential-field anomalies
in the Queen Charlotte Basin and reaching the continental margin. Another regional transcurrent structure
in the Intermontane Belt, the Stikine Arch, bounds the Bowser Basin on the north. Big transverse arches,
fault systems and related mineral-deposit zones (lode gold and others) crossing the Canadian and U.S.
Cordillera follow the inherited ancient cratonic trends, regardless of assumed accreted terranes.
Two-sided Middle Proterozoic (Belt-Purcell), Late Proterozoic (Windermere), late Paleozoic (ProphetIshbel) and Mesozoic (Fernie) volcano-sedimentary basins in the eastern Cordillera are known from
geologic evidence to have received sediments from continental regions to the east and west. Internal
variations in these basins are reported to be related to numerous faults and blocks with various
orientations. The Belt-Purcell Basin, though ~20 km deep, was evidently broad, intracratonic and nonorogenic, and the ancient cratonic area to the west of it is conventionally called Western craton. The
oldest known incidence of the Cordilleran NNW-SSE tectonic trend is the ~1,760-Ma Kimiwan thermal
geochemical anomaly in the cratonic basement of the Alberta Platform, but thereafter orogenic activity
shifted to the west. The Cordilleran tectonic grain was firmly established with the onset of rifting at ~780
Ma, when the ~10-km-deep Windermere trough began to form. The shallower late Paleozoic ProphetIshbel trough was probably a foredeep to the Antler and Teslin orogens, which are known to have lain in
the Canadian Cordilleran regions to the west. For the often-assumed Late Proterozoic-Paleozoic Atlantictype passive continental margin in the eastern Cordillera, there is no evidence.
The conventional division of the cratonic Alberta Basin into a “passive-margin” stage before the midJurassic and a “foreland” stage after is thus without meaning. In relation to the Cordilleran orogens, the
Alberta Basin was in a foreland position even in Antler-Teslin time. In mid-Jurassic time, western-derived
detrital clasts first appeared in the basin, reflecting a rise of mountains in the eastern Cordillera. This
change was not accompanied within the Alberta Basin itself by any significant restructuring of cratonic
depocenters and arches, and this basin’s evolution was essentially cratonic and platformal both before and
after. Also in error, the Bond-Kominz model of the pericratonic and miogeosynclinal basin subsidence
assumes a former existence in the eastern Cordillera of an Atlantic-type continental margin, and supposes
that tectonic evolution in this region commenced only at the onset of the Phanerozoic. But the early
Cordilleran Windermere tectonism is known to date back to at least ~780 Ma. Some poorly understood
orogenesis there seems to have taken place even earlier, based on metamorphic and deformational
dissimilarities between the Belt-Purcell and Windermere rocks.
Geochemical evidence of reworked, ex-cratonic ancient basement rocks is well known from the domal but
aligned metamorphic core complexes in the southern Omineca Belt. Highly metamorphosed Phanerozoic
supracrustal rocks are known in these complexes as well. To achieve their high metamorphic grades, in
the Mesozoic and early Cenozoic these rocks had to be lowered to great crustal depths of ~20-30 km, and
then rapidly returned to the surface. Mapped tectonic manifestations (magmatic, metamorphic, structural
and sedimentary) have been reported to indicate that the Nevadan and Columbian orogenic episodes
occurred in that region in the Middle Jurassic and mid-Cretaceous, each ending with decompression and
crustal extension. Other, more obscure orogenic pulses are indicated for various times in the Paleozoic
and Precambrian. The Late Cretaceous-Early Tertiary Laramide orogenic cycle, which created the
Rocky Mountain fold-and-thrust belt and ended with its own well-recognized extensional pulse(s), was
only one in this long series. This repetitive, multi-cycle orogenic history contradicts the commonly
assumed scenario with a passive continental margin in the eastern Cordillera before the mid-Mesozoic,
compression and exotic-terrane accretion and stacking from then till the Early Tertiary, and extension
thereafter.
Unlike in the Omineca and Coast Belt orogenic zones, much gentler metamorphism and deformation took
place in the more-rigid crustal blocks of the Intermontane-Belt, Yukon-Tanana and Insular-Belt excratonic massifs. Their volcano-sedimentary cover is preserved largely intact, its upper parts are
metamorphosed weakly or not at all, and block faults are common. Some Mesozoic and Cenozoic
successions in these massifs contain known economic deposits of coal and oil. Magmatism in these semiorogenized regions was largely teleorogenic, related to adjacent mobilized zones (mostly, Coast Belt).
Blocks of rigid, semi-reworked ex-cratonic Precambrian crystalline crust probably lie beneath the exposed
Late Proterozoic and Phanerozoic volcano-sedimentary cover (the oldest known cover rocks are
Devonian in the Intermontane Belt, Late Proterozoic in the Yukon-Tanana massif, and middle or late
Paleozoic in the Insular Belt). An ancient continental basement is suggested by radiometric inheritance
(Early Proterozoic in the Yukon-Tanana massif), thickness and seismic-velocity structure of the crust, and
long-time rigidity of these crustal blocks that preserved them from greater orogenic reworking.
Local rifts, mediterranean deep marine basins and even ephemeral minor subduction zones might have
existed in parts of the Canadian Cordillera at various times. These local marine basins were not oceans.
Their inversion and closure is known from field evidence to have taken place largely in late Paleozoic
(Antler-Teslin) time. These events, during which the main active crustal zones were the ones oriented
NNW-SSE, may reflect the partition of the Intermontane Belt into the Stikine and Quesnel crustal blocks.
In contrast, the Mesozoic block partition of the Intermontane Belt was defined largely by the reactivated
but inherited, transcurrent Stikine and Skeena arches. Because crustal blocks with a shared ancient
crustal ancestry in the Cordilleran interior developed throughout the Phanerozoic side by side, essentially in
situ, the hypothesized big former oceans (Cache Creek, Teslin, Anvil) between them did not exist.
Outward-verging, fan-like Mesozoic and Cenozoic thrust zones reported in the Cordilleran interior
bilaterally flank the Coast Belt, Teslin and Omineca orogens. Like the huge Laramide Rocky Mountain
fold-and-thrust belt, these thrust zones are probably shallow-rooted. The pericratonic Rocky Mountain
Belt grew bigger than the Cordilleran-interior thrust zones partly because the thick, unmetamorphosed,
pluton-free, stratified sedimentary cover of the Alberta Basin offered a layered mechanical medium easily
delaminated into thrust sheets. In the Rocky Mountain Belt, the drillhole and seismic evidence suggests
the detached thrusting fails to involve the cratonic crystalline basement. Metamorphic evidence of wholecrust tectonic reworking appears only near the Rocky Mountain Trench fault, which separates the Rocky
Mountain Belt from the Omineca orogen. The entire crustal reflection pattern in deep seismic profiles
changes across the Rocky Mountain Trench fault. As the Cordillera-induced tectonic disturbance in most
of the Rocky Mountain Belt is evidently only structural (without metamorphic or magmatic manifestations)
and confined to the sedimentary cover, the unreworked deeper crust in the Rocky Mountain Belt must be
regarded as cratonic. The edge of the craton lies farther west, where evidence of whole-crust tectonic
reworking first appears, near the Rocky Mountain Trench.
Wholly unconvincing is the supposed seismic evidence that the entire crust of the Cordillera is a stack of
exotic-terrane thrust sheets underlain by a preserved thin ex-cratonic sliver in the lower crust. Strong
disagreements between the current seismic and electrical-resistivity interpretations about the depth to
base of the assumed thrust stack underline the subjectivity of these interpretations, which fail to consider
other possible solutions. The repeated, laterally variable tectonic reworking in the miogeosynclinal
Omineca orogenic belt included profound mobilization of the entire crust and perhaps the deeper
lithosphere. The eugeosynclinal Coast Belt orogen’s crust is regenerated even more, comprising ~80%
Phanerozoic (mostly Mesozoic) granitoids. In the face of such severe tectonic reworking, preservation
and trans-Cordilleran continuity of older features deep in the crust are impossible.
The lower continental crust exists in ductile conditions at high grades of metamorphism, with many sources
of seismic reflectivity not related to thrusting. Very unpredictable are fault geometries at depth, especially
below the brittle-ductile transition: many faults assume unexpected trajectories, steepen, flatten out, or
dissipate. Old structural and metamorphic fabrics are obliterated by newer metamorphism, anatexis and
re-deformation. Contrary to Lithoprobe, COCORP’s seismic interpretations just south of the Canada-U.S.
border note the Cordilleran Moho truncates the probably-orogenic seismic reflection patterns in the crust,
and regard the Moho as a product of Cenozoic regeneration. High-angle crustal discontinuities are usually
missed in deep seismic reflection images. Correlation of low-angle seismic events is complicated by their
unknown nature, common lack of continuity, reflection-character variations, and gaps in the data. The
need for road access during data acquisition in the mountains caused the Lithoprobe reflection profiles to
be shot largely along passable valleys, which tend to follow steep NNW-SSE and transcurrent crustal
faults, so some lines were shot not across but along large crustal structures. In a strongly deformed and
magmatized region like the Cordillera, many off-line arrivals and other hard-to-identify forms of coherent
noise contaminate the seismic data.
Our previous comprehensive analysis of geological and geophysical evidence shows that no subduction is
currently taking place off Vancouver Island, and the Cascadia subduction zone does not reach north into
Canada. Likewise, in the Cordilleran interior postulations of trans-Cordilleran subhorizontal crustal
detachments also rely on arbitrary correlations of selected, disconnected low-angle seismic events whose
geometry seems to fit the preferred (thrust) model of the crust. These events are assigned to faults that at
the surface the are known to be variously low-angle or steep. Local low-angle structures, ductile and
brittle, normal and reverse, are common on the flanks of orogenic zones and near metamorphic domes that
rose diapirically through the crust. But trans-crustal depth projections of these faults are chimerical. The
much-criticized use of discontinuous “floating” mid-crustal seismic events of unknown origin to justify the
surface faults’ (e.g., Slocan, Monashee) presumed trans-crustal continuity is quite unfounded. Besides, it
contradicts the results of refraction surveys, which show the velocity structure of the Cordilleran crust to
be rather different. The biggest, steep fault system in the southern Omineca Belt region seems to be the
one along the Kootenay Lake, which forms the western boundary of the Kootenay Arc and is associated
with big seismic-velocity changes even in the lower crust.
Book references:
Lyatsky, H.V., 1996. Continental-Crust Structures on the Continental Margin of Western North America;
352 p., Springer-Verlag.
Lyatsky, H.V., Friedman, G.M. and Lyatsky, V.B., 1999. Principles of Practical Tectonic Analysis of
Cratonic Regions, with Particular Reference to Western North America, 369 p., Springer-Verlag.
Lyatsky, H.V. and Lyatsky, V.B., 1999. The Cordilleran Miogeosyncline in North America - Geologic
Evolution and Tectonic Nature, 384 p., Springer-Verlag.
Biographical info - Dr. Henry Lyatsky
Trained in both geology and applied geophysics, Dr. Henry Lyatsky is a Calgary-based private consultant.
He is an author or co-author of three books on the geology of western Canada.
He has worked in many parts of the Alberta and Williston basins, Canadian Shield, east and west coasts of
Canada, and overseas. Besides regional tectonics, he is interested in the influences of structures
in the basement on the evolution of sedimentary basins, and in the role of crustal faults in localizing
hydrocarbon and mineral deposits. His other major interest is the development of novel
geophysical methods and exploration techniques.
Education: Ph.D., 1992, geology, UBC;
M.Sc., 1988, geophysics, U of Calgary;
B.Sc., 1985, geology and geophysics, U of Calgary.