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The Regional Tectonic Setting and Possible Causes of Cenozoic Extension in the North American Cordillera

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The Regional Tectonic Setting and Possible Causes of Cenozoic Extension in the North American Cordillera The regional tectonic setting and possible causes of Cenozoic extension in the North American Cordillera P. J. Coney SUMMARY: Timing and tectonic setting of middle Cenozoic crustal extension in the North American Cordillera supports the concept that an overthickened crustal welt formed behind or astride the thrust belts as a result of compression during the Mesozoic to early Cenozoic. At the end of the Laramide Orogeny the gravitationally unstable welt collapsed by deep-seated crustal extension. The extension was aided by a lowering of crustal viscosity resulting from a complex pattern of volcanism and a reduction in intraplate compressive stress. As plate regimens evolved along the Pacific margin during the late Cenozoic, subduction progressively ceased as did compressive stress also. An evolving transform boundary and a massive Cordilleran-wide lithospheric uplift allowed a second phase of extension to develop across the already thinned and thermally weakened crust to form the Basin and Range Pro- vince, being active up to the present time. The principal manifestation of extensional tec- ages. The middle-Tertiary extensional province tonism in the North American Cordillera has was superimposed on widespread compressional been recognized for years as the Basin and features of Mesozoic and early-Tertiary age. Range Province (Stewart 1978). The Basin and Although the earlier mid-Tertiary extension Range Province formed in the central-western covered an area larger than the younger Basin United States and northwestern Mexico by block and Range Province, the two provinces overlap faulting, some strike-slip faulting, and to a very large degree in the western United associated relative regional subsidence during States. In other words, the Basin and Range the late Cenozoic. The crust has been thinned to Province was superimposed over much of the less than normal, heat flow is high, and the same ground that had been extended in the extension has been accompanied by a sparse middle Tertiary. Thus, the crust of this area was bimodal basaltic-rhyolitic volcanism. The Basin extended and thinned twice. It is important to and Range Province remains active to this day, realize the scale of these features: the combined as evidenced in seismicity and landscape, but the area of mid- and late-Tertiary extension in nature of its origins remains controversial. western North America is about 10 times longer An earlier period of mid-Tertiary Cordilleran and five times wider than the Aegean extensional extensional tectonics has only recently been province of the eastern Mediterranean region recognized. This extension affected a larger (Le Pichon 1982) and would comfortably en- region than the younger Basin and Range compass much of western Europe. Province extending from northern British The principal problem with Cenozoic exten- Columbia across the western United States and sional tectonics in western North America has southward into northern Mexico. The mid- been to explain why it took place. After a Tertiary extension was originally recognized in description of the regional tectonic setting of the sub-horizontal younger on older detachment these two periods of continental extension we faults (Armstrong 1972) which juxtapose will review some possible explanations to this unmetamorphosed upper-plate Precambrian to question. Cenozoic sedimentaw and volcanic rocks against lower-plate kylonitic gneiss in the peculiar metamorphic core complexes of the Regional tectonic setting of Basin , Cordilleran hinterland (Coney 1980; Armstrong and Range extension 1982). Coeval everywhere with this middle- Tertiary extension was a voluminous outburst of The Basin and Range Province (Stewart 1978; generally caldera-centred ignimbrite eruptions Eaton et al. 1978; Eaton 1982) includes the (Lipman et al. 1971; Elston 1976; Coney & northern Basin and Range of Nevada and r Reynolds 1977; Dickinson 1981). This has western Utah, the Arizona-Sonora Basin resulted today in the paradox of mountain and Range of the southwestern United States and ranges exposing thick ash-flow tuffs nested northwestern Mexico and the Mojave region of amongst ranges exposing mylonitic gneiss, both southeastern California (Fig. la). The northern yielding the same mid-Tertiary K-Ar cooling Basin and Range is bounded to the W by the From COWARD,M.P., DEWEY,J.F. & HANCOCK,P.L. (eds), 1987, Continental Extensional Tectonics, Geological Society Special Publication No. 28, pp. 177-186. them and a trench fonned along the accreting Mesozoic to early-Tertiary compression. transpressive Pacific margin of North America. Evidence of extensional tectonism in the mid- The trench was subducting first the Kula and Tertiary was discovered only in the past 15 years then Farallon Plates, probably very obliquely, as and the controversy this discovery initiated these plates spread away from the East Pacific has centred uDon the so-called 'Cordilleran and related rises. Sometime after 30 Ma, but metamorphic core complexes' (Coney 1979, before 20 Ma, the East Pacific Rise made initial 1980; Davis & Coney 1979; Crittenden et al. contact with North America, probably in the 1980; Armstrong 1982; Coney & Harms 1984). vicinity of southern California-northern Baja Cordilleran metamorphic core complexes occur California. This placed the northwestward- in a sinuous discontinuous belt along the eastern ' moving Pacific Plate in direct contact with part of the North American Cordillera extending North America's westward moving margin, and from British Columbia in southern Canada S j the vector subtraction of these two motions pro- through the Cordillera into Sonora, Mexico over duced right strike-slip transform faults between a distance of 3000 km. Over this distance all the the two plates trending northwesterly parallel to complexes are characterized by similar rock the Pacific margin. As more and more of the rise type, structures and fabric. The complexes crest was annihilated, the transform boundary typically exhibit two distinctly different do- developed to both N and S and subduction and mains. These are a metamorphic-plutonic base- arc activity ceased between the two separating ment terrane and an overlying or adjacent triple junctions. Dickinson & Snyder (1979) have unmetamorphosed cover. Separating the two is argued that this would produce a 'window' in a sharp surface, or zone, of sub-horizontal the subducting slab NE of the growing shearing and detachment, usually with a transform boundary under the adjacent North younger on older geometry. The complexes are American Plate. Their geometry depicts this mostly domal or anticlinal in form and usually window evolving directly under the Basin and constitute the highest mountains in their respec- Range Province. tive regions. The transform-fault boundary, which prob- The basement terranes of core complexes are ably initially lay offshore, was nearly 1000 km characterized by low-dipping foliations and a long before Basin and Range rifting began 1: 17 distinctive 'stretching' lineation in mylonitic Ma during the Miocene (Atwater 1970; gneiss formed from protoliths that range from Engebretson et al. 1982). In any event, the Precambrian basement to mid-Tertiary plutons. annihilation of the East Pacific Rise has con- The unmetamorphosed cover terranes are tinued to the present and the entire evolution of replete with listric normal faults which have Basin and Range extensional tectonics took shattered protoliths, ranging across the entire place during this period. One major interruption spectrum of Phanerozoic sedimentary and in this progressive evolution occurred when the volcanic rocks including mid-Tertiary continental transform margin jumped inboard to open the volcanic and sedimentary rocks. The amount of Gulf of California and initiate the San Andreas extension in the cover terrane is often dramatic. fault system 36 Ma. This transferred Baja Cover stratigraphy is usually strongly attenuated California and much of western California to and the Tertiary rocks, the youngest of the the Pacific Plate, and probably stabilized the original cover, are commonly brought down into Arizona-Sonora sector of the Basin and Range. tectonic contact with the mylonitic gneisses of There was considerable clockwise rotation and the basement terrane. The detachment surface northward translation of blocks within the separating the basement and cover terranes is coastal ranges of northwestern Mexico and typically sharp and very visible in topography. California and in Washington and Oregon The mylonitic fabrics of the basement terrane during this time. are commonly sub-parallel to the detachment surface, but rocks both above and below the detachment surface are usually intensely brec- Regional tectonic setting of mid- ciated as a result of what appears to be the latest Tertiary extension movement on the surface. During the last few years the core complexes Mid-Tertiary extensional tectonism (Fig. lb) in have generated a considerable controversy the North American Cordillera has been an illu- (Thorman 1977; DeWitt 1980; Brown & Read sive feature. It was overprinted and masked by 1983). The debate has surrounded their age the late-Tertiary extension of the Basin and and tectonic significance. The age controversy Range (just discussed), and its resultant struc- (Armstrong 1982) stems from the fact that evidence tures have been confused with those due to older for the age of deformation has seemed con- Cenozoic extension in the North American Cordillera 181 tradictory in that structural elements
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