Peter J. Coney

Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression

Cordilleran metamorphic core complexes form a belt of uplifted metamorphic rock that extends from southern Canada to northwestern Mexico just west of, or astride, the foreland thrust belt of the North American Cordillera. During the past several years the age and tectonic significance of the core complexes have been a topic of considerable controversy. Some geologists view the complexes as an uplifted erogenic core zone that formed behind the thrust belts mainly during Mesozoic regional compression. An opposing view is that they are mainly Tertiary in age and of extensional origin. We support a model that unifies these seemingly inconsistent views by suggesting that Mesozoic crustal telescoping resulted in an overthickened plateaulike crustal welt along the Cordilleran hinterland. During Cenozoic time this gravitationally unstable mass spread laterally, resulting in deep-seated crustal extension. The extension was aided by a thermal pulse of Cenozoic magmatism that reduced crustal viscosity and by a lowering of intraplate convergent stress fields due to changing plate kinematics. The chief advantage of the model is that it reconciles opposing views as to the age and tectonic significance of the complexes and places them in a more comprehensible setting amid Mesozoic-Cenozoic Cordilleran thermotectonic history.

Plate tectonics of the Ancestral Rocky Mountains

The Ancestral Rocky Mountains were intracratonic block uplifts that formed in Colorado and the surrounding region during Pennsylvanian time. Their development related to the collision of North America with South America–Africa, which produced the Ouachita-Marathon orogeny. In Early Pennsylvanian time, suturing was taking place only in the Ouachita region, and foreland deformation took place only in the mid-continent. By Middle Pennsylvanian time, the length of the suture zone had increased, and it was active from the Ouachita to the Marathon region. The extent of cratonic deformation also increased in intensity and in areal extent, culminating in the Ancestral Rocky Mountains. In Late Pennsylvanian time, suturing was taking place only in the Marathon region, and cratonic deformation decreased in extent and spread southward into New Mexico and West Texas. We suggest that the Ancestral Rocky Mountains, and related features over a broad area of the western United States, were formed while an irregularly bounded peninsula of the craton (including the transcontinental arch) was pushed northwestward by the progressive collision-suturing of North America and South America–Africa. This intraplate deformation is, in some respects, like the deformation of Asia in response to the Cenozoic collision with India.

Geologic development of the Cordilleran metamorphic core complexes

Metamorphic core complexes evolved in the western Cordillera in early to middle Tertiary time as a response to profound regional extension and thermal incursion; the deformation was punctuated by an unusually thorough tectonic denudation. In southern Arizona examples, the nonconformity between crystalline basement and layered cover rocks played an important mechanical and physical-chemical role during the deformation. The basement stretched and necked in a manner akin to megaboudinage, while simultaneously parts of the layered cover flowed passively during metamorphism and were plated tectonically to crystalline rocks along the unconformity and along ductile normal growth faults. Topographic basins created by regional pinch-and-swell were filled by lower to middle Tertiary sediments and by volcanics that poured out of the rifted basement. Physical-chemical conditions were such that the base of the Phanerozoic section became a vast heat sink as well as a zone of remarkably high fluid pore pressure. Thus, decollement zones, located near the base of the Phanerozoic section, are marked by sharp thermal gradients, wholesale denudation, and a great variety of mechanical and chemical chaos.

The regional tectonics of the Tasman orogenic system, eastern Australia

Abstract From the perspective of Phanerozoic mountain belts the mostly Paleozoic Tasman orogenic system of eastern Australia is unique. For example, it has no through-going miogeocline or foreland fold and thrust belt. Except for a narrow deformed fringe along its western margin the entire system is ‘suspect’ in the sense that its paleogeography is uncertain through much of Paleozoic time. The tectonic evolution of the Tasman orogenic system is composed of four major phases. The first was a prolonged late Proterozoic-early Paleozoic period of variable tectonic settings characterized by generally deep-marine turbiditic sedimentation and submarine volcanism, and shifting, somewhat local, deformation, metamorphism and plutonism. The second epoch was a major mid-Paleozoic period of deformation, volcanism and plutonism that consolidated a belt of lower Paleozoic interior terranes into Australia. The third epoch was a major accretionary phase in the outer New England belt of terranes that culminated in late Paleozoic time, and continued into the early Mesozoic. The final epoch was extensional, and was due to the break-up of Gondwanaland in late Mesozoic time, continuing to the present. Tectonic evolution during the first three phases was somewhat similar to that of the remainder of the nearly 20,000 km long Pacific margin of Gondwanaland in the Andes and Antarctica, and suggests that ‘absolute’ motions of Gondwanaland itself prior to break-up may have influenced the tectonics of its Pacific margin.

ASYMMETRIC EXHUMATION ACROSS THE PYRENEAN OROGEN : IMPLICATIONS FOR THE TECTONIC EVOLUTION OF A COLLISIONAL OROGEN

Abstract The Pyrenees are a collisional mountain belt formed by convergence between the Afro–Iberian and European plates. Apatite fission track thermochronology from three vertical profiles along the ECORS seismic line constrain the exhumation history of the Pyrenean orogen and hence tectonic models for its formation. In the Eocene there is relatively uniform exhumation across the Pyrenees, but significantly more exhumation occurs on the southern flank of the axial zone in the Oligocene. The variation in exhumation patterns is controlled by a change in how convergence is accommodated within the Pyrenean double-wedge. Accommodation of thrusting on relict extensional features that leads to inversion dominated thrust stacking resulted in relatively slow exhumation in the Eocene. However, subsequent crustal wedging and internal deformation in the upper crust under the stacked duplex of antiformal nappes resulted in extremely rapid exhumation on the southern flank in the Oligocene. The Maladeta profile in the southern axial zone records extremely rapid Early Oligocene exhumation followed by dramatic slowing or cessation of exhumation in the middle Oligocene and the formation of an apatite partial annealing zone (PAZ). This PAZ has subsequently been exhumed 2–3 km since the Middle Miocene, supporting the observations of Coney et al. [J. Geol. Soc. London 153 (1996) 9–16] that the southern flank of the range was buried by ≤2–3 km of syntectonic conglomerates in the Oligocene and subsequently re-excavated from Late Miocene to Recent. The present-day topographic form of the Pyrenees is largely a relict of topography that formed in the Eocene and the Oligocene. Comparison with paleoclimatic records indicates that the Eocene–Oligocene exhumation patterns are controlled by tectonic forces rather than resulting from an orographic effect due to uplift of the Pyrenees.

Convergence and intraplate deformation in the Lachlan Fold Belt of southeastern Australia

Abstract The Lachlan Fold Belt of southeastern Australia is dominated by a widespread oceanic association including Cambrian submarine mafic volcanics and an overlying widespread Ordovician quartz-rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres. These rocks are overlain by, or in fault contact with, deep-marine to continental Silurian to Early Carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks. Major deformation, achieved by thrusting and folding, affected the fold belt during the Silurian to Middle Devonian and the Early Carboniferous. Shortening estimates vary throughout the belt but the widespread Ordovician quartz turbidite succession usually has at least 60% shortening which implies an original width of the fold belt of about 1700 km. Strike-slip faulting was important on a local to regional scale, but large-scale strike-slip displacements appear unlikely. The large values of shortening are consistent with development of the belt in a convergent margin setting at least for the Silurian to Carboniferous. The nature of the lower crust of the Lachlan Fold Belt has always presented a problem of interpretation but we favour a Late Proterozoic quasi-continental lower crustal layer, that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during Silurian-Carboniferous convergence, and provided a source for Silurian-Devonian granites.

The lachlan belt of eastern Australia and Circum-Pacific tectonic evolution

Abstract There is considerable evidence that the Pacific Ocean basin has had a remarkable permanency at least throughout the Phanerozoic. The orogenic systems that have evolved around its margins are accretionary continental margin orogens and show little or no evidence of continental collisions in their evolutionary history. This is in dramatic contrast to the Circum-Atlantic and Tethyan realms which have experienced repeated openings and closures of, or successive transfer of continental fragments across, ocean areas that were relatively never large. In other words, the Wilson Cycle has dominated tectonic evolution of Atlantic and Tethyan realms, but has not been important in the Circum-Pacific. The northeastern margin of the Pacific Ocean is the North American Cordillera which is a “classic” continental margin-accretionary system dominated by a well-developed complex miogeoclinal terrace, significant fringing or “exotic” arc-trench systems and other “oceanic” accretions progressively consolidated into North America from mid-Paleozoic times, but mainly from mid-Meso-zoic times to the present. The northwestern margin of the Pacific Ocean is the collage of Asia which was produced by Tethyan tectonics, not Pacific tectonics — i.e., the progressive transfer of Gondwanaland fragments across Tethys to Baltica-Siberia. Only since the early Mesozoic have minor Pacific accretions, such as Japan, produced the present margin. The southeastern, southern, and southwestern margins of the Pacific Ocean are South America, Antarctica, and Australia, respectively. Through Paleozoic-early Mesozoic times they were joined and a very enigmatic Pacific margin orogenic system extended for 20,000 km from northwestern South America to northeastern Australia. The Lachlan Fold Belt in particular, and the Tasman belt in general, are important windows into that enigma. Lack of a well-developed through-going miogeocline is notable, and late Precambrian but mostly extensive lower Paleozoic, fairly deep-marine turbiditic and occasionally submarine volcanic facies are common along the margin, often directly juxtaposed against the cratonic interior. The tectonic evolution is dominated by prolonged histories of first late Precambrian to Late Cambrian then Early Silurian-Early Mesozoic convergent to transpressive and accretionary tectonics, often accompanied by extraordinary magmatism, which progressively consolidated a considerable “oceanic” to “quasi-continental” real estate into the Gondwanaland craton. Since the fragmentation of Gondwanaland in the mid-Mesozoic only the Andean margin has continued convergent consolidation. Large-scale tectogenesis and consolidation in the Circum-Pacific seem to correspond to periods when the “absolute motions” of the main continental blocks caused their margins to advance over the adjacent Pacific Ocean floor crust.

Structural aspects of suspect terranes and accretionary tectonics in western North America

Abstract The regional structural relationships between the Cordilleran suspect terranes and cratonic North America, and within the suspect terranes themselves, are reviewed for eastern Alaska, western Canada and United States and northern Mexico. A distinctive characteristic of the relationships between the suspect terranes and the North American craton is that although much modified by post-accretionary strike-slip faulting, thrust faulting and extensional detachment faults, the generally more distal and/or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more ‘inboard’ terranes or upon the craton itself. The amount of tectonic transport implied is large. Strike-slip faulting has been a very significant aspect of the structural evolution of the North American Cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence. At least since later Mesozoic time to the present, movements were such that the terranes moved northward. Post-accretionary intraplate telescoping along thrust faults, transpressional strike-slip faulting, and delamination at mid- to upper-crustal levels were important processes in Cordilleran tectonic evolution.

Accretion tectonics and crustal structure in Alaska

Abstract The entire width of the North American Cordillera in Alaska is made up of “suspect terranes”. Pre-Late Cretaceous paleogeography is poorly constrained and the ultimate origins of the many fragments which make up the state are unclear. The Prince William and Chugach terranes accreted since Late Cretaceous time and represent the collapse of much of the northeast Pacific Ocean swept into what today is southern Alaska. Greater Wrangellia, a composite terrane now dispersed into fragments scattered from Idaho to southern Alaska, apparently accreted into Alaska in Late Cretaceous time crushing an enormous deep-marine flysch basin on its inboard side. Most of interior eastern Alaska is the Yukon Tanana terrane, a very large entirely fault-bounded metamorphic-plutonic assemblage covering thousands of square kilometers in Canada as well as Alaska. The original stratigraphy and relationship to North America of the Yukon-Tanana terrane are both obscure. A collapsed Mesozoic flysch basin, similar to the one inboard of Wrangellia, lies along the northern margin. Much of Arctic Alaska was apparently a vast expanse of upper Paleozoic to Early Mesozoic deep marine sediments and mafic volcanic and plutonic rocks now scattered widely as large telescoped sheets and Klippen thrust over the Ruby geanticline and the Brooks Range, and probably underlying the Yukon-Koyukuk basin and the Yukon flats. The Brooks Range itself is a stack of north vergent nappes, the telescoping of which began in Early Cretaceous time. Despite compelling evidence for thousands of kilometers of relative displacement between the accreted terranes, and large amounts of telescoping, translation, and rotation since accretion, the resulting new continental crust added to North America in Alaska carries few obvious signatures that allow application of currently popular simple plate tectonic models. Intraplate telescoping and strike-slip translations, delamination at mid-crustal levels, and large-scale lithospheric wedging were important processes in northern Cordilleran tectonic evolution.

Implications of a Bengal Fan-type deposit in the Paleozoic Lachlan fold belt of southeastern Australia

Ordovician rocks of the Lachlan fold belt contain a continental-margin submarine fan that in its undeformed state has a minimum area of 1.2 x 10 6 km 2 , comparable to the size of the Bengal Fan. Cambrian and Ordovician mafic volcanic seamount chains in the Lachlan fold belt are analogous to the Ninetyeast Ridge and other seamounts associated with the Bengal Fan in the northeastern Indian Ocean. Basement of the Lachlan fold belt, by analogy with that of the Bengal Fan, may have been ocean floor with an overlying continental-rise prism that developed after the breakup of Laurentia and eastern Gondwana to form the ancestral Pacific Ocean in the latest Precambrian. The continental-rise prism was imbricated during Silurian-Devonian deformation to form the lower crust that provided source rocks for the Silurian to Carboniferous granites.