The Cordilleran Miogeosyncline in North America: Geologic Evolution and Tectonic Nature

402 Pages·1999·10.359 MB·English

by  Dr. Henry V. Lyatsky, Dr. Vadim B. Lyatsky (auth.)

Lecture Notes in Earth Sciences 86
Editors:
S. Bhattacharji, Brooklyn
G. M. Friedman, Brooklyn and Troy
H. J. Neugebauer, Bonn
A. Seilacher, Tuebingen and Yale
Henry .V Lyatsky Vadim .B Lyatskyt
The Cordilleran
enilcnysoegoiM
in North America
Geologic Evolution
and Tectonic Nature
With 78 Figures
regnirpS
Authors
Dr. Henry .V Lyatsky
Lyatsky Geoscience Research & Consulting Ltd.
4827 Nipawin CR. N.W.
Calgary, Alberta, Canada T2K 2H8
E-mail: lyatskyh @ cadvision.com
Dr. Vadim B. Lyatskyt
"For all Lecture Notes in Earth Sciences published till now please seef inal pages of
the book"
Cataloging-in-Publication data applied lbr
Die Deutsche Bibliothek - CIP-Einheitsaufnahme
Lyatsky, Henry V.:
The Cordilleran miogeosyncline in North America : geologic
evolution and tectonic nature / Henry V. Lyatsky ; Vadim B.
Lyatsky. - Berlin ; Heidelberg ; New York ; Barcelona ; Hong Kong ;
London ; Milan ; Paris ; Singapore ; Tokyo : Springer, 1999
(Lecture notes in earth sciences ; 86)
ISBN 3-540-66197-2
ISSN 0930-0317
ISBN 3-540-66197-2 Springer-Verlag Berlin Heidelberg New York
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Foreword
The Earth's rock-made outer shell, the lithosphere, contains entities of two main
types, continental and oceanic. Cratons and mobile megabelts are lower-rank lat-
eral Iithospheric tectonic units into which continental lithospheric masses are sub-
divided. Mobile megabelts develop when large zones of continental crust and
lithosphere are tectonically mobilized and deeply reworked; the reworked continen-
tal lithosphere could have previously been cratonic. Many of the mobile megabelts
in the world lie in peripheral regions of continents; others are located far within
continental interiors. A megabelt, being a first-order continental lateral tectonic
unit, consists of subordinate major units: orogenic zones and median massifs. Me-
dian massifs are less tectonically mobilized and reworked than orogenic zones, and
they retainm ore of their ancient (commonly, ex-cratonic) characteristics.
Yet, despite the ancient continental inheritance, it is conventional for modern tec-
tonists to regard mobile megabelts not fi'om the vantage point of their parent cra-
tons but from distant oceans genetically unrelated to the continent. The underlying
false assumptions are that continents are essentially inert, incapable of self-
development, and that any tectonism in them must be induced externally - from the
sub-lithospheric mantle below or from oceanic plates to the side. In reality, conti-
nental lithosphere has its own radioactive energy sources, and its tectonic devel-
opment is mostly an expression of indigenous, internal processes.
The ocean-based, "Poseidonian" approach to continentalt ectonics is illogical at its
core: instead of relying on direct observations of rocks in land areas where these
rocks are exposed, it puts too much emphasis on speculative, assumption-based re-
constructions of distant plate motions in the past and on the assumed effec~ of
those supposed motions on continental areas. Plate reconstructions are based not
on the observed geology of continental mobile megabelts and cratons but on
model-driven interpretations of geophysical anomalies in oceanic regions far away.
The global approach has in the last several decades usefully extended the vision of
tectonists, which was previously restricted to directly accessible continental -er
gions. Unfortunately, a casualty of this model-based kind of globalizationh as been
the traditional practice of basing tectonic conclusions on the observed, sampled and
analyzed rocks and the mapped rock-body relationships. But it is observational
geological information that provides the ultimate means to test abstract models,
and this fact-based approach is adopted in the present book.
lV
Being generally unable to detect high-angle discontinuities, seismic reflection pro-
files may give the viewer a false, partial impression that only low-angle disconti-
nuities exist in continental crust. The rock nature and origin of these seismically
reflectivleo w-angle crustal discontinuities are unknown, and they could be related
to a multitude of possible causes: original discontinuities in the protolith, subse-
quent metamorphic fronts and rheologicb oundaries, ductile and brittle shearing, in-
trusive igneous bodies, and so on. In the absence of direct geologicalo bservations,
the inherent non-uniqueness of geophysical interpretations precludes definitive con-
clusions about the rock composition and structure of deep crust. Even in the ex-
posed and drilled uppermost part of the crust, every dryo il or water well, and every
failed mining or geotechnical project represents a failure of prediction.
Superdeep wells drilled into the upper continental crust in Europe, Russia and
North America have presented many surprises quite unanticipated by prior geo-
physical predictions, including very tmexpected findings about sources of seismic
reflections and potential-field anomalies, as well as position of the brittle-ductile
boundary and thermal and hydrologic conditions at depth. Not even such sparse
geologic constraints are available fotrh e middle and lower crust. The arbitrary and
model-driven assumption that seismic-reflection geometries deep in the crust
mostly represent thrusting leads to an incorrect perception of the continental crust
as necessarily a stack of thrust sheets. Meanwhile, the block structure of the crust
goes unnoticed.
Tectonics synthesizes the factual findings from field mapping and many specialized
geological disciplines - petrology, geochemistry, paleontology, and so on. From a
vast amount of diverse information, a tectonist selects that which permits to discern
the phenomena induced by endogenic processes in the lithosphere, and combines a
broad range of facts and data into an internally consistent concept. This produc-
tive, practical approach is utilized in the present study of the Cordilleran mobile
megabelt, which lies ont he western flank of theN orth American continent.
Steep, deep-seated, long-lived crustal weakness zones are well known to have ex-
isted in the western part of the North American craton since the Late Archean or
Early Proterozoic. Trending mostly NE-SW, these ancient crustal weakness zones
divide the craton into distinct large blocks with dissimilar crystalline-crust and
volcano-sedimentary-cover properties. Some of these zones predate cratonization
(which is known to have occurred in the Early Proterozoic in western Canada).
Many more steep brittle faults, variously following and cutting across the ancient
ductile structures, were created in the upper crust after the end of cratonization. Re-
iiv
juvenation of old weakness zones and fractures in various crustal tectonic regimes
and conditions influenced block movements and the formation of volcano-
sedimentary basins on the craton in the Proterozoic and Phanerozoic. Across the
Cordilleran tectonic grain, these ancient weakness zones continue from the craton
all across the Late Proterozoic-Phanerozoic Cordilleran mobile megabelt, where
geological andg eophysical evidence shows they influenced many tectonic manifes-
tations (depositional, magmatic, metamorphic, structural) throughout the mega-
belt's lifespan.
Wide westward extent of ancient pre-Cordilleran croton(s) is suggested by other
evidence as well. Tectonically reworked Precambrian rocks of various Archean and
Proterozoic ages are recognized increasingly commonly all over the Canadian Cor-
dillera, while the paleontological, paleomagnetic and structural evidence for exoti-
cism of assumed accreted terranes is continually revised or eliminated. The oldest
known incidence of the "Cordilleran" NNW-SSE tectonic trend is a-1,760-Ma
geochemical anomaly in the cratonic basement of the Alberta Platform, but lhereaf-
ter orogenic activity shifted westward. Two-sided volcano-sedimentary and sedi-
mentary basins in the eastern Cordillera - Middle Proterozoic, Late Proterozoic,
late Paleozoic, Mesozoic - are known from sedimentological studies to have re-
ceived sediments from land sources on the east and west. The Middle Proterozoic
Belt-Purcell Basin, though very deep, was evidently intracratonic and non-
orogenic; the ancient cratonic area to the west of it is conventionally called the
Western craton. The NNW-SSE Cordilleran tectonic trend was not firmly estab-
lished till the onset of Windermere rifting at -780 Ma. The Late Proterozoic
Prophet-Ishbel trough was probably a foredeep to the well-known Antler orogen,
whose continuation from the U.S. Cordillera lay in the Canadian Cordilleran -er
gions to the west.
In the cratonic Phanerozoic Alberta Basin just to the east of the Canadian Cordil-
lera, many Paleozoic platformal sedimentary units fail to exhibit a wedge-like
thickening to the west. Occasional westward continuation of cratonic depositional
settings far into the Cordilleran interior is indicated by the occurrences of Paleozoic
carbonate-platform remnants similar to the coeval carbonate platforms in the era-
tonic Alberta Basin. Contrary to a common assumption, at no time did the eastern
Canadian Cordillera contain an Atlantic-type continental margin.
Geochemical evidence ofreworked, ex-cratonic basement rocks is well known l~om
the metamorphic core complexes in the Canadian Cordilleran interior. Highly
metamorphosed former supracrustal rocks are known in these complexes as well.
iiiv
To achieve their high metamorphic grades, in the late Mesozoic and early Cenozoic
thesreo cks were first lowered to great crustal depths of 20-30 kin, and then rapidly
retumed tot he surface. Mapped tectonic manifestations indicate that several intense
orogenic episodes had occurred in that region in the Middle Jurassic and mid-
Cretaceous, each ending with decompression and crustal extension. The Late Cre-
taceous-Early Tertiary Laramian orogenic cycle, with its own extensional pulse in
the end, was only one in this long series. This multi-cycle tectonic history rules
out the commonly assumed scenario with a passive continental margin in the east-
em Cordillera before the mid-Mesozoic; compression and exotic-terrane accretion
and stacking from then till the Early Tertiary; and extension thereafter.
The cratonward-vergent Laramian Rocky Mountain fold-and-thrust belt on the east-
em flank of the Cordilleran mobile megabelt has long been known to be a thin-
skinned, rootless thrust stack consisting of supracrustal rocks. It evidently does
not involve the cratonic basement, which in that region is not tectonically -er
worked. Although the Rocky Mountain fold-and-thrust belt is conventionally as-
signed to the Cordillera, the evident Cordilleran tectonic reworking of the ancient
basement begins near this belt's western boundary, in the Cordilleran interior.
Rather than the eastern Laramian deformation front, it is that interior line that
should be considered the modem westem edge of the North American craton.
West of the Rocky Mountain fold-and-thrust belt lies the eastern Cordilleran
miogeosynclinalO mineca orogenic zone, characterized by strong crustal reworking,
metamorphism and deformation. These manifestations were comparatively slight
in the more-rigid crustal blocks of the Intermontane-Belt and Yukon-Tanana me-
dian massifs farther west: their volcano-sedimentary cover is preserved largely in-
tact, with its upper parts metamorphosed only slightly or not at all; block faulting
is common. The magmatism there was largely teleorogenic, rooted in adjacent
orogenic regions (mostly, the eugeosynelinal Coast Belt to the west). Blocks of
rigid, semi-reworked ex-cratonic Precambrian crust probably lie beneath the ex-
posed Late Proterozoic and Phanerozoic volcano-sedimentary cover of the median
massifs. Presence of older continental basement in the median massifs is suggested
by radiometric inheritance, xenolith evidence, seismic-velocity structure of the
crust, and long-time rigidity of these crustal blocks that prevented a more profound
orogenic reworking. Transcurrent, ex-cratonic, ancient crustal weakness zones era
expressed particularly strongly in the median massifs.
Rock evidence indicates that local rifts, Mediterranean- or Red-Sea-type deep ma-
rine basins and even ephemeral minor subduction zones might nonetheless have ex-
xI
isted in the Canadian Cordillera at different times and in different localities. Yet,
because crustal blocks with a shared ancient crustal ancestry in the Cordilleran inte-
rior evolved throughout the Phanerozoic side by side, roughly in situ, the oft-
postulated big intra-Cordilleran former oceans are unlikely. Tectonic zones and
belts of the Cordilleran mobile megabelt developed essentially in situ, but the me-
dian massifs retain more of their ancient cratonic crustal inheritance than do the
orogenic zones on both sides of these massifs.
Outward-verging Mesozoic and Cenozoic fold-and-thrust zones of various sizes in
the Cordilleran interior bilaterally flank the miogeosynclinal and eugeosynclinal
orogenic belts. Like the huge Rocky Mountain fold-and-thrubsetl t on the east side
of the Cordilleranm obile megabelt, they are probably rootless. These shallow de-
formation zones on the orogens' flanks obscure the deep crustal boundaries of the
orogenic and median-massif belts.
Seismic evidence that the Cordilleran crust is a stack of exotic-terrane thrust sheets
is likewise unconvincing. Geophysical anomalies carry information about the spa-
tial distribution of some specific physical properties of rocks, but they say nothing
about the discontinuities' geologic nature, genesis or age. High-angle crustal dis-
continuities are normally missed in seismic reflection images altogether, and the
geologic origin of low-angrleef lections is unknown. Correlation of seismic events
is complicated by these events' unknown nature, common lack of continuity and
character variations, and gaps in the data. In a strongly deformed and magrnatized
region, many off-line arrivals and other forms of coherent noise contaminate the
seismic data at short and long traveltimes. The technical need during data acquisi-
tion forro ad access in the mountains reqt~ed the available seismic reflection pro-
files to be shot largely along passable valleys, which tend to follow steep
Cordilleran and transcurrent crustal faults; as a result, some of the seismic lines
were shot not across but along large-scale structures.
Postulations oftrans-Cordilleran crustal detachmentrse ly on assumption-based cor-
relations of selected, disconnected low-angle seismic events and their assignment to
faults that at the surface are known to be variously low-angle or steep. Many of the
supposedly trans-crustal detachment faults are correlatable onlwyi th reflections that
dissipate in the mid-crust. The use of discontinuous "floating" events to justify
these faults' deeper continuity is unfounded, and it contradicts the results of -ca'ifer
tion seismic surveys. On the other hand, important steep fault systems that cannot
easily be correlated with big low-angle reflections are overlooked in the model-
driven tectonic analysis.
Precambrian craton(s) continued far to the west in pre-Cordilleran time. Today,
their semi-reworked remnants era found in the Cordilleran median massifs, which
are surrounded by orogenic zones where the crustal tectonic reworking was much
greater. zones tectonic between differences main in The the Cordilleran interior lie
not in their assumed composition exotic-terrane but in the degrees of orogenic -er
working of the North indigenous American continental lithosphere.
ACKNOWLEDGMENTS
This book has arisen from our decades-long experience of practical research and ex-
ploration in westem Canada and elsewhere. Our work has benefited from many in-
formative and helpful discussions with our colleagues in the oil and mining indus-
tries, at the Universities of Calgary and British Columbia, and at the Vancouver,
Calgary and Ottawa offices of the Geological Survey of Canada. These colleagues
have brought to our attention many relevant facts, data and scientific ideas. Par-
ticular thanks got ot he scientific staff at the Geological Survey of Canada office in
Vancouver, who initially introduced the first author to the Cordilleran geology and
whose interest, support and encouragement are deeply appreciated. Our colleagues
at Lithoprobe have also been helpful with geophysical data and current tectonic
ideas. Responsibility for scientific conclusions presented here, however, rests with
the authors alone.
Garth Keyte and Four West Consultants (Calgary) kindly provided printing, draft-
ing and reproduction facilities. Gerald M. Friedman (Northeastern Science Foun-
dation and City University of New York) looked after the scientific review of the
manuscript.
Sadly, the second author, my father, passed away as this manuscript was near
completion, and he did not see its publication. This book, therefore, is in his mem-
.131o
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