The Cordilleran orogenic belt between Nevada and Chihuahua HARALD DREWES U.S. Geological Survey, Denver, Colorado 80225 ABSTRACT The Cordilleran orogenic belt has long been viewed as part of a circum-Pacific tectonic entity, as illustrated by De Sitter (1956, Fig. The Cordilleran orogenic belt of North America was formed dur- 306). With the proliferation of the many ideas incorporated in the ing an orogeny which occurred mainly in Cretaceous and Paleocene plate-tectonics hypothesis, the concept of a circum-Pacific belt has time. The belt is marked by a zone of strong folding and thrust fault- been considerably modified. These changes notwithstanding, the ing that extends from Alaska to Guatemala. However, between Cordilleran orogenic belt is still accepted as extending along the southern Nevada and northeastern Chihuahua, the belt is so ob- western parts of North and South America, with the exception of a scured that some geologists doubt its continuity. A recent study of part of Central America west of the Caribbean Sea, essentially be- part of the Nevada-Chihuahua interval provides evidence that the tween Guatemala and Panama. In the most general terms, the de- belt is continuous, without major interruption; the complications velopment of this orogenic belt is attributed to the gradual con- are the result of pre-orogenic and post-orogenic tectonic events. vergence of the rocks beneath the eastern part of the Pacific Ocean With due regard for these complicating factors, a structure section with those of the Americas. At the area of exception, along south- through southeastern Arizona and southwestern New Mexico ern Central America, the orogenic effect of this crustal convergence closely resembles sections through southwestern Canada, regions is offset far to the east along transform faults, such as the Bartlett near Salt Lake City and Las Vegas in the United States, and northern fault zone, but the continuity of kind and age of deformation seems Mexico. In each region, supracrustal rocks were tectonically trans- assured throughout a distance of about 8,600 km. The time of ported east-northeastward a distance probably more than 100 km. compressive deformation is here referred to as the Cordilleran The style of deformation suggests a near-surface environment, orogeny. Although this paper focuses on that event, some older and probably mainly controlled by décollement between a prism of younger features will also be mentioned where their effects have miogeosynclinal rocks and crystalline basement rocks. Those fea- influenced the development of, or the appearance of, the tures which vary between the regions reflect differences in tectonic Cordilleran features. At present, the orogenic belt appears as a position within the belt (for example, closer to foreland or hinter- string of mountain ranges which formed in response to post- land, or depth of exposure), in anisotropy of preorogenic rocks Cordilleran uplift. (variations in older structural features), or in subsequent geologic The rocks and structural elements have great continuity along history. the length of the orogenic belt, and considerable diversity across its breadth. On a very broad scale, the cratonic part of North America CONTINUITY OF THE OROGENIC BELT and the other structurally stable areas of the Arctic slope of Alaska and the Gulf of Mexico side of Mexico lie to the northeast of the A belt of major compressive deformation of Late Cretaceous and orogenic belt, and a zone of younger deformation, metamorphism, early Tertiary age trends across southern Arizona and New Mexico volcanism, and sedimentation lies to the southwest and is in part and is believed to link the well-known segments of the Cordilleran superposed on the orogenic belt. In the southwestern part of the orogenic belt, the one extending from Alaska south to the Las United States, these main zones veer to the southwest from Salt Vegas, Nevada, area, and the other extending north from Lake City to Las Vegas. East of this segment of the belt, for a dis- Guatemala to near El Paso, Texas (Fig. 1). The zone of compressive tance of some 1,500 km, another tectonic zone appears, having fea- deformation between Las Vegas and El Paso is marked mainly by tures somewhat transitional between those of the cratonic area and large thrust faults and to a lesser degree by a system of folds which the orogenic belt. Setting aside for the moment consideration of this deform rocks chiefly of Mesozoic and Paleozoic ages but locally transitional tectonic zone of the Colorado Plateau and the Central also some of the Precambrian basement and lower Tertiary rocks. I and Southern Rocky Mountains region, the northeastern margin of believe there is no need to consider that: (1) the orogenic belt was the Cordilleran orogenic belt is more sharply delineated than is the offset about 1,000 km on a hypothetical left-lateral tear fault, such southwestern margin. Typically, the rocks and structures change as the Texas Lineament of Albritton and Smith (1957); (2) the main systematically from the foreland to the hinterland of the orogenic belt of Cordilleran deformation followed the east edge of the Rocky belt. Cretaceous marine shale and detritus, derived mainly from the Mountains, an area in which vertical tectonics is dominant over orogenic belt itself, form a southwestward-thickening sedimentary horizontal tectonics (King, 1969b); or (3) the Cordilleran orogenic prism deposited on the edge of the cratonic margin in front of the belt is a composite of discontinuous segments. Instead, the results orogenic belt. These rocks are warped, most distally in open folds of a tectonic synthesis of parts of Arizona and New Mexico indi- subparallel to the margin of the orogenic belt, and then fairly cate that the orogenic belt of western North America is a single abruptly in moderately tight folds closely subparallel to the fore- tectonic feature with generally uniform spatial, dynamic, and his- land margin. Along many segments of the belt, folds of great length torical aspects. and regularity, resembling the crumpled rug analogy, mark the Geological Society of America Bulletin, v. 89, p. 641-657, 4 figs., May 1978, Doc. no. 80501. 641 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/5/641/3418434/i0016-7606-89-5-641.pdf by guest on 02 October 2021 EXPLANATION Cordilleran orgenic belt 80° Area of hypothetical Texas lineament, not herein supported ssssssssssss Region of tectonic section Fold belt of orogenic foreland Thrust fault of orogenic foreland Other faults, arrows show strike-slip movement Western limit of exposed and unmodified part of orogen Figure 1. Map of North America showing distribution of Cordilleran orogenic belt. Region I, craton; II, Rocky Mountains and Colorado Pla- teau; DI, Cordilleran orogenic belt; IV, zone of younger deformation, metamorphism, volcanism, and sedi- mentation. Cities: SLC, Salt Lake City; LV, Las Vegas; P, Phoenix; T, Tucson; EL, El Paso; M, Monterrey; G, Guatemala. 100° 90° Modified after King (1969),and King and Edmonston,(l972) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/5/641/3418434/i0016-7606-89-5-641.pdf by guest on 02 October 2021 CORDILLERAN OROGENIC BELT BETWEEN NEVADA AND CHIHUAHUA 643 foreland margin of the deformed belt; along other segments, thrust land element may be as much as 100 km of shortening, but gener- faults mark this margin even more sharply. ally more modest amounts are visualized. The key feature, how- The rocks in the orogenic belt itself are late pre-orogenic age, typ- ever, is that vertical tectonics and east-west to northeast-southwest ically marine Mesozoic and late Paleozoic rocks that were depo- oriented compressional forces, rather than tensional forces, acted sited on the flank of the craton in a miogeosyncline thickening concurrently. The similarity in timing and orientation of the com- markedly to the southwest. Compressive deformation increased in pressive deformation suggests a genetic relation to the development intensity across the breadth of the belt to the southwest or at struc- in the adjacent orogenic belt, while the greater stability of the rocks turally greater depth, although broad zones of little deformation indicates a crustal affinity with the nearby cratonic region. Perhaps are interspersed with zones of more severe deformation. Thermal the foreland acted as a semi-rigid, slightly raised cratonic salient. effects and magmatic involvement also increase to the southwest The belt of strong compressive deformation developed around this and/or at greater depth. Rocks at the surface are generally older to crustal obstacle, some compressive stress was transmitted across it, the southwest toward the hinterland than along the foreland mar- and magmatic activity typical of the orogenic belt occurred beneath gin. A continuous belt of crystalline rocks that has been deformed it, the magmatic activity (in the broadest sense, including inflowing in highly plastic style is known only along a few segments of what and changes of state) may have provided some of the vertical im- may be the core area of the orogenic belt; elsewhere such a zone petus. may be concealed by subsequent deposits and tectonism. The The time framework of development of the Cordilleran orogenic complex eugeosynclinal deposits and metamorphic terranes of the belt depends to some extent upon the spatial framework that is Coast Ranges of Alaska, Canada, and the United States are perhaps considered. While a time span of Late Cretaceous