The Cordilleran Orogenic Belt Between Nevada and Chihuahua

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 sedimentation. 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 to early Tertiary related to some events of the Cordilleran orogeny, but they are so (Paleocene) time covers most of the events that took place in the strongly overprinted by effects of younger deformation that their foreland province, in a few places somewhat older and younger de- full significance to the Cordilleran belt is still unclear. The follow- formation is also attributed to the Cordilleran orogeny. To the ing sections of this paper dwell mainly on the foreland zone and the southwest, in the zone of complex metamorphic terranes some- neighboring part of the orogenic belt to the southwest in which plu- times included as part of the Cordilleran belt, deformation was ini- tons are abundant and metamorphic terranes extend beyond the tiated in Early Cretaceous or even Jurassic time, well before the vicinities of plutonic masses but are not quite of regional extent. time of the classical "Laramide orogeny" or orogenic phase of the The Rocky Mountain foreland is a major crustal element that Cordilleran orogeny, as I propose. Likewise, in a few regions to the includes the central and southern Rocky Mountains geologic prov- northeast, some deformation of Eocene age is viewed as the final ince and the Colorado Plateau province. The foreland has a struc- response to the forces of the Cordilleran orogeny. tural style intermediate between that typical of the cratonic region In so extensive a belt of deformation as the Cordilleran of North to the east and the Cordilleran orogenic belt to the west. The zone America, the notion of general uniformity of kind and age of de- of Cretaceous marine rocks forms a westward-thickening prism formation seen on a continental scale must be modified when along the eastern margin of the foreland and laps onto it. Another, smaller regions are considered, for orogenic stresses and crustal less well-defined belt of Cretaceous black shale, siltstone, and conditions of one region may blend in gradually with those of coarser detritus was deposited in a series of smaller lacustrine and another. The effects of the subcontinental block of region II (Fig. 1) fluvial basins along the western and southwestern parts of the fore- already suggest such a modification. Thus the covering concept of a land. Within the foreland, the rocks were warped, and faulted in a Cordilleran orogeny, when viewed more closely from place to series of raised blocks, arches, and monoclines. Deformation was place, shows some general features of uniformity as well as some strongest along the eastern margin of the province, area II of Figure local variations. For example, much attention has recently been 1, and was milder in the Colorado Plateau part to the southwest. given the idea that in the region between Las Vegas and Salt Lake Typically a thin skin of Paleozoic and Mesozoic rocks was arched City a "Sevier orogeny" predates the Laramide orogeny over north- to northwest-trending raised blocks of basement rocks. (Armstrong, 1968). The particular tectonic activity thus localized Where the vertical movement was extreme, reverse faults and up- in time and space might better be considered as a phase of the thrusts separate basins from adjacent uplifts. Locally the sedimen- Cordilleran, thereby leaving intact the essence of a general similar- tary cover has been warped into fields of northwest-trending folds ity of kind and age of deformation throughout the major orogenic near the flanks of the uplifts, and elsewhere second-order gravity belt, and at the same time providing some insight to the likely gliding has complicated the local tectonics. Most of the major faults movement of maximum stress conditions and of sites of maximum that lie along the north- and northwest-trending uplifted blocks are igneous activity through space and time, much as the sites of major believed to steepen downdip; nevertheless they show a substantial deposition shifted. Such a concept of what an orogeny is and how it horizontal component of movement as well, as has been reviewed develops is reflected in the thoughtful work of King (1969a, by Coney (1976) and Woodward (1976). 1969b). The "Sevier orogeny" is thus viewed by me as a phase of While many aspects of the tectonic development in the foreland the Cordilleran orogeny and, as it turns out, some of the more dis- element seem clear, the relations to the nearly contemporaneous tant mild effects of this tectonic phase are noticeable in southeast- development of the Cordilleran belt are more problematic. The ern Arizona. The two phases of stronger tectonic activity of Creta- foreland lay slightly higher than the cratonic region before the close ceous and Paleocene age in Arizona that are described in this paper of the Cordilleran orogeny, for the main prism of foreland sedimen- are also treated as phases of deformation, with the sole implication tation lay to the east of the province, and nonmarine Cretaceous that they are of more limited significance than the whole of the deposits occur along its western margin. The typical Cordilleran Cordilleran orogeny and with no intent to compare their sig- deformation at the present level of exposure suggests a response to nificance with the Sevier phase or any other. strong vertical force, but in places also to some horizontally The present evaluation of the continuity of the Cordilleran oriented forces tangential to the surface. Coney (1976) even orogenic belt between Las Vegas and El Paso focuses mainly on suggests that the folding and faulting effects across the entire fore- southeastern Arizona and some adjacent parts of New Mexico and

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 644 H. DREWES

118° 114° 110° 106° some constraints to interpretations of Cordilleran events, (3) post-Cordilleran tectonism and deposition has disrupted the continuity of many Cordilleran structural features, and (4) segments of many faults were reactivated, in places repeatedly and even with diverse and opposing directions of movement. No wonder that some local studies produced results seemingly in conflict with those obtained through other local studies! Accordingly, I expect that as the regional studies are expanded, these interpretations, based mainly on southeastern Arizona, will also be found in need of modification.

DESCRIPTION OF THE OROGENIC BELT IN ARIZONA AND NE1W MEXICO

In southeastern Arizona and southwestern New Mexico, the Cordilleran orogenic belt is seen to form parts of two tectonic lobes (Fig. 1), the one extending from El Paso first slightly northwestward and then westward to the vicinity of Tucson, and the second trending first north-northwestward from a short distance east of Tucson and then gradually trending more westerly near Phoenix, through central western Arizona, and then northwest to a point south of Las Vegas, where it meets the southwestward-trending thrust zone of the Salt Lake City—Las Vegas regions. In the presence of the post-Cordilleran complications of faulting and concealment by the younger rocks, the position of the north or east margin of the 0 200 400 KILOMETERS foreland zone is placed between areas that have scattered segments

0 200 400 MILES of thrust faults of Cordilleran age and areas that have no thrust 1 I I l I faults (although they possibly have some younger gravity glide Figure 2. Index map of the southwestern United States, showing plates). Where post-orogenic rocks typically lie directly upon geologic provinces, dotted lines; foreland zone of Cordilleran orogenic belt; basement rocks that are not reported to be deformed during the area of tectonic synthesis in southeastern Arizona, stippled; C, Cochise Cordilleran orogeny, evidence of the present position of the fore- County; and, H, Hidalgo County. land belt, and indeed even of its past existence, is obliterated, so that projections must be made from indirect evidence obtained Sonora (Fig. 2). The region south and east of Tucson, Arizona, is from nearby regions, using as support the implications of the size of particularly informative, whereas south-central Arizona provides thrust plates that are suggested by large amount of transport and by but scant data because of the extensive erosion of the Paleozoic and the uniformity in age, style of deformation, and direction of Mesozoic rocks and the extensive cover of younger volcanic and tectonic transport. But even in the region of the northwestern lobe, clastic deposits on Precambrian basement rocks. there are scraps of evidence of thrust faulting east and north of Thrust faults have long been known in southeastern Arizona, but Tucson; near Superior, some 80 km east of Phoenix; near their age and origin remained controversial to an extent that dis- Quartzsite, about 160 km west of Phoenix; and possibly in the Bill couraged earlier attempts at a tectonic synthesis; Corbitt and Williams River area 160 km northwest of Phoenix. Still other Woodward (1970) have proposed regional thrusting in southwest- localities invite further field study. ern New Mexico. In order to assess the seemingly conflicting field The mention of thrust lobes anticipates a review of the pattern of data, it was necessary to consider the tectonic development of the Cordilleran belt in a later section of the paper, but it also repre-Cordilleran and post-Cordilleran times, as well as of quires here a consideration of the nature of the juncture between Cordilleran time itself. To a certain extent, then, some local the two postulated lobes of Arizona and New Mexico. I believe that geologic histories formed the basis of correlation and synthesis of the lobes were originally separated by a tear fault, now a complex particular regional tectonic features and events, whereas other fault, that trended east-northeast. The term "complex fault" is used tectonic features and events are probably of local importance only. specifically for a fault that has a history of multiple movement, not Sorting out the tectonic features and events of regional importance just a fault with a complicated trace or one difficult to understand. from those of only local significance required much field study to This fault is now in part obscured, but not quite obliterated, by supplement the existing geologic accounts. The data provided by subsequent normal faults and glide faults, and it is in part con- more than 130 radiometric dates, obtained mainly from the cealed by younger faults and glide faults, as well as by younger de- laboratories of the University of Arizona and of the U.S. Geological posits. As a tear fault, the juncture near Tucson will be described Survey, were invaluable in refining rock ages and correlations, and with other faults of this kind below. thereby the timing of geologic events. The main results obtained Before proceeding with a description of the geologic features of through the broadening of field studies beyond the time restrictions the thrust plates, I will review the tectonic development of south- of the Cordilleran orogenic period are a recognition that (1) the eastern Arizona. In this way, the impact of the older and younger crust was structurally inhomogeneous at the inception of structures on Cordilleran structural interpretation will be more Cordilleran time, (2) pre-Cordilleran structural features provide comprehensible.

Tectonic Development of Southeastern Arizona the older mountain roots and was intruded by diabase, dated at 1,200 m.y. old in an area north of Tucson. Major faulting occurred Pre-Cordilleran Orogeny Development. The rocks of southeast- during the Precambrian, and many of these faults trend northwest, ern Arizona were deformed several times before the Cordilleran probably have large strike-slip displacement, and produced the orogeny. During the early part of Precambrian time, a thick suite, structural anisotropy that guided responses to later stresses and primarily of sedimentary rocks, was deposited in a marine(?) en- channeled the movement of magmas and ore fluids. Evidence for vironment, and subsequently was intensely deformed and meta- this age and kind of movement has been reported by Swan (1975) morphosed during the Mazatzal Revolution about 1,700 m.y. ago and by Drewes (1976a), but it has been inferred for many years (Wilson, 1939; Anderson, 1951). Large masses of granitoid rocks prior to the appearance of concrete evidence (Richard and Cour- were emplaced late during this revolution, about 1,650 m.y. ago, tright, 1966; Schmitt, 1966). and again shortly thereafter, about 1,450 m.y. ago. The region was The Oracle-Apache Pass complex fault (Fig. 3) can be used to il- strongly uplifted and deeply eroded after the granitoid stocks were lustrate the kind of evidence that indicates the presence of major emplaced. Another suite of sedimentary rocks was deposited across strike-slip movement on faults of Precambrian age. The fault strikes

Johnn Lyon willcox , H-llls *\\ ORACLE-APACHE I W1 PASS COMPLEX

^ FAULT n?

52° X THR^V ^ mÊ&MMê$ï>k \ m ....

»I ff*L- "\ •

•iiSr.w-.iiVi.ïjj'"

\ ^

k. .¿J ; \ \ . V..Ì.V-*"; " A* y OOUGLAS . f :r 4 ^-7—-h — 40 KILOMETRES Figure 3. Generalized geologic map of southeastern Arizona showing distribution of autochthon, allochthons, and selected stocks.

EXPLANATION SELECTED STOCKS OF JURASSIC AGE—Quartz monzonite, granite, "lilu monzonite, and monzonite porphyry SURFICIAL DEPOSITS—Mainly younger gravel deposited in valleys SELECTED STOCKS OF LATE CRETACEOUS AGE—Granodiorite and quartz monzonite dated at 70-75 m.y. BEDROCK—Undifferentiated sedimentary, igneous, and metamorphic rocks; mainly exposed in the mountains but SAMPLE SITE—Radiometrically dated stock, plug, or related includes some young volcanic rocks in a few valleys, volcanic rock, dated at 70-75 m.y., and trend line and pediments CONTACT—Broken line where concealed AUTOCHTHON—Projected beneath post-orogenic rocks and across |;:;;il;ihl;i| areas intruded by post-orogenic stocks FAULTS—Dotted where concealed or intruded. Composite symbol used where diverse movements of different ALL0CHTH0N—Projected beneath post-orogenic rocks and across ages are inferred, in each case the thrust faulting areas intruded by post-orogenic rocks is the older event

SOUTHEASTERN TECTONIC LOBE Normal fault—Bar and ball on downthrown side

Cochise thrust plate Thrust fault—Sawteeth on upper plate

Hidalgo thrust plate Glide fault—Open sawteeth on upper part. Arrow shows direction of movement NORTHWESTERN TECTONIC LOBE—Undivided Tear fault—Arrows show relative direction of movement

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 646 H. DREWES

northwest across the northern end of the Chiricahua Mountains, The initial pulse of the Cordilleran orogeny in the region is and consists of several major fault strands and subsidiary faults marked by a modest angular unconformity between rocks of late that splay off the main zone. At scattered intervals along the fault Early Cretaceous age and rocks of late Late Cretaceous age. A more and its branches, there are large slivers of rock types that are not precise age of about 90 m.y. can be assigned to the mild tectonism present in the nearby parts of the adjacent blocks. Furthermore, the recorded by this hiatus on the basis of stratigraphic evidence from complex fault separates markedly different terranes: to the south- regions that lie to the north and east (Drewes, 1971a). The mild west, for example, there is a thick unit of metaquartzite in a tight uplift recorded by this hiatus may be the distant response to the fold that plunges steeply southward and lies unconformably be- stresses that produced the deformation of the Sevier phase of the neath unfolded Paleozoic rocks (Sabins, 1957); whereas to the Cordilleran orogeny between Las Vegas and Salt Lake City. An northeast across the fault zone, most of the rocks are schists which upward increase in coarseness and abundance of clastic deposits have simple northeastward-dipping bedding and foliation. Also, and of volcanic deposits, particularly andesitic-dacitic, in the Upper large stocks of diverse kinds are juxtaposed along parts of the fault Cretaceous rocks of southeastern Arizona records waxing orogenic zone (Cooper, 1960; Erickson, 1968). This combination of fault conditions. pattern and offset of major rock units and structural features is in- Just before or during the tectonic climax, andesiric to dacitic vol- dicative of fault movement of many kilometres, and the tight fold- canic rocks were spewed forth with unusual energy from vents ing on only one side of the zone, with axial plane oblique to the commonly located along or near the northwest-trending complex zone and axes plunging moderately, suggests a style of drag folding faults. Numerous, and in places large exotic blocks, or xenoliths, in associated with strike-slip movement. this volcanic unit and an overlying rhyodacitic welded tuff, reflect During the Paleozoic Era, the region responded to a series of deposition in a high-energy environment that can be met only by epeirogenic fluctuations which are recorded by disconformities in a volcanotectonic processes, such as vent breccias, foundering roof of marine sequence, typically about 2 km thick. By the middle of the a sill-flow unit or basal plucking along a flow moving down a con- Permian Period, the sea had left the region, and thereafter continen- siderable topographic slope (Drewes, 1971a; Mayo, 1971). The tal conditions predominated. andesitic to dacitic volcanic rocks are the youngest involved in the Stronger vertical movement occurred during Triassic to mid- regional thrust faulting. In Arizona they are indirectly dated as Cretaceous time along segments of several of the northwest- probably of late Campanian or early Maastrichtian age through trending fault zones in the basement rocks, and several stocks of their relations to rocks that are dated directly. In New Mexico, granite, monzonite, and quartz monzonite were emplaced. The similar dacitic rocks have been dated directly as Paleocene. amount of vertical movement on the Sawmill Canyon complex During the first orogenic climax about 75 m.y. ago, the rocks fault, about 40 km south of Tucson, was sufficiently large so that near Tucson were under sufficiently strong and appropriately erosion stripped the entire sedimentary cover off the Precambrian oriented stress to become folded and ultimately to be ruptured basement rocks of the uplifted block, and the detritus derived from along thrust faults of regional extent. The resultant: allochthon (a these rocks forms conglomerate lenses in a 3,000-m thick sequence term here used to indicate upper plate rocks, rather than the proven of Triassic volcanic rocks that were deposited on the downthrown active plate rocks) was moved east-northeastward relative to an block (Drewes, 1971a, 1972). Additional fault movement is re- underlying mass. Internally, the allochthon was strongly deformed corded before the end of Triassic time 70 to 90 km south of Tuc- by movement along other thrust faults, tear faults, disharmonic son, where upturned and faulted Triassic volcanic rocks are in- normal faults, and by folds. This strong deformation of late Cam- truded by an Upper Triassic monzonite stock (Drewes, 1971b; Si- panian and early Maastrichtian age is termed the Piman phase of mons, 1974). These tectonic and magmatic events may be the mar- the Cordilleran orogeny (E>rewes, 1972). Late during this tectonic ginal effects of the Sonoran orogeny reported by Fries (1962) in the phase, abundant stocks were intruded, especially in the western southern part of Sonora, 550 km south of Tucson. part of the region. Conditions of tectonic quiescence seem to mark During Jurassic time, volcanism occurred in the western part of the early part of Paleocene time, although stock emplacement con- the region, some granitic stocks were emplaced along or near tinued south of Tucson. northwest-trending complex faults, and the rocks of the Bisbee dis- During late Paleocene time, however, renewed tectonism (of the trict were mineralized. second climatic phase, the Helvetian phase) occurred in the region By the beginning of Cretaceous time some of the northwest- in response to somewhat milder and slightly differently oriented trending complex faults were reactivated once more, or recurrently, stress than previously in ef fect. The most noteworthy aspect of the as indicated by successive wedges of fanglomerate intercalated in a faulting is the strong influence exerted by the ancient basement volcanic sequence lying immediately southwest of the Sawmill flaws, the northwest-trending complex faults. A shift in direction of Canyon complex fault that overlies a Jurassic stock and underlies compressive stress from east-northeast during the Piman phase to beds of Aptian-Albian age. Fairly stable crustal conditions pre- perhaps east-west during the Helvetian phase may have been vailed during this late Early Cretaceous time, and an arm of the sea sufficient to cause yielding to take place first along regional thrust briefly encroached on southeastern Arizona from Mexico (Hayes, faults, and then as left-lateral movement along the basement flaws 1970). (Drewes, 1972). Locally, near a few of the active basement flaws, Cordilleran Orogeny Development. The geologic record in some small-scale thrust faulting did occur, but movement direc- southeastern Arizona indicates that the Cordilleran orogeny tions, amounts, and ages are demonstrably different from the thrust spanned the time between about 90 and 53 m.y. ago. While this faulting of the Piman phase. Near Tucson, this movement is dated span of time is fairly large, most of the tectonic features to be de- as about 55 m.y. old through its close association with the scribed below were formed during two climactic intervals peaking emplacement of two groups of stocks and plugs. Major mineraliza- at about 75 to 73 m.y. and 55 m.y. ago, and lasting only a few mil- tion is also associated with several of the suites of Paleocene intru- lion years. It is also likely that the conditions of tectonic climax, sive bodies. during which peak stress conditions prevailed, shifted eastward Post-Cordilleran Orogeny Development. After the Cordilleran from the Tucson area to New Mexico, for certain volcanotectonic orogeny, tensional conditions replaced compression. Early during rocks are progressively younger in that direction. post-Cordilleran time, the region was topographically high and

thus was deeply eroded; the resulting detritus was transported out- formed along incipient imbricate thrust faults. Alignment and side of the region. Volcanism tapered off during the Eocene and orientation of intrusive bodies also provide data that bear on the early Oligocene, and some vein swarms were deposited in tensional orogenic development of the region. fractures. A major thrust fault underlies an extensive plate in the south- Before the end of Oligocene time, the rocks of the region were western part of the area shown in Figure 3. While this plate and its subjected to east-west-oriented tension typical of the Basin and presumed counterparts elsewhere along the orogenic belt are con- Range geologic province (Fig. 2). This condition led to the de- ventionally called the allochthon, as I also shall do henceforth in velopment of north-trending normal faults. In many places, the order to preserve the current usage, the application of the term is at northwest-trending basement anisotropy controlled the orientation least premature, and it may be unwarranted unless there is specific of these faults. Locally a few faults trend northeast. The structural evidence for the idea that the upper plate was tectonically trans- blocks between some pairs of normal faults were dropped with reported into the area and upon the underlying rocks rather than that spect to adjacent blocks, and many blocks were tilted, typically the underlying rocks were transported into the area beneath the away from the most strongly developed grabens. overlying plate. This point should not be dismissed as being simply Magmatism increased concurrently with the stepping up of academic. Historically we have been strongly oriented toward the tectonic activity. Rhyolitic to rhyodacitic magma reached the sur- Pacific margin of the continent for tectonic explanations of face in many separate volcanic fields. The largest volcanic fields Cordilleran geology, but with the development of plate-tectonics occur near the major grabens, which thus may in part be vol- ideas, it is necessary to consider the interactions between all plates canotectonic features. Many dike swarms, plugs, and laccoliths adjacent to the cratonic mass or Americas plate. Thus, the concept were emplaced near the surface, and a few stocks of granodiorite that the Americas plate was moving westward from the spreading and quartz monzonite were emplaced at greater depth. In the center in the mid-Atlantic Ocean during Jurassic and Cretaceous mountains immediately north and east of Tucson, an upward mov- time could make the underlying basement mass, or crustal plate, ing core of magma, probably having a high viscosity, arched up- the allochthon, and the thin overlying and much deformed plate, ward a carapace of thrust-faulted Mesozoic and Paleozoic rocks the autochthon. and some underlying crystalline basement rocks to form a com- The basal Hidalgo thrust fault, or décollement, separates a de- pound gneiss dome, of which the southeastern high is the Rincon formed overlying terrane mainly of Paleozoic and Mesozoic rocks gneiss dome. from a little deformed underlying terrane of Precambrian basement Where the upward movement of the gneiss dome or of fault rocks, although some areas within the allochthon are also little dis- blocks was sufficiently large and rapid to form strong topograph- turbed. The fault plane or zone is rarely more than 2 m thick and ically high features, some of the raised rocks on the flanks of the commonly is less than 1 m thick, with one or a few discrete strong uplifts were shed into the adjacent basins as low-angle normal fault shear planes separated by sheets or lenses of intensely sheared or masses, glide plates, slump masses, or landslide material. In places, comminuted rock. In some areas, the basal thrust fault separates such tectonic denudation took place along segments of appropri- into several strands, between which the rocks consist of repeated ately oriented older thrust faults. Other segments of thrust faults slices of Paleozoic and Mesozoic formations or of Precambrian and some folds were "piggy-backed" in the glide plates. Elsewhere basement rocks and Paleozoic formations. The rocks near the basal many folds were formed in response to local stresses within some fault of the southeastern tectonic lobe are sheared and warped into glide plates (Davis, 1975). widely dispersed drag folds, and are metamorphosed only near Block fault movement and volcanism continued throughout late stocks of post-thrust fault age. However, the rocks near the basal Tertiary and Quaternary time. In a few places, fault scarps appear fault of the northwestern tectonic lobe near Tucson, are not only near the foot of linear mountain ranges or cutting alluvial fan de- sheared and moderately deformed, but are also dynamically posits. Rhyolitic volcanism extended through the Miocene and metamorphosed. The minor drag folds are abundant, their style of minor andesite and basalt were extruded into the deepest grabens deformation highly plastic, and their orientation regular. Region- during Pliocene and Pleistocene time. The youngest field, in the ally the basal fault is subhorizontal, but locally its attitude varies southeastern corner of Arizona, is mostly 1 to 3 m.y. old, with the widely where it was deformed after thrusting or where it followed youngest activity known placed at 0.2 m.y. (Lynch, 1976). The segments of older faults that were more steeply inclined. most recent faulting occurred in northeastern Sonora on May 3, Other thrust faults cut the allochthon into several major plates. 1887, in connection with a major earthquake (Goodfellow, 1888; In the southeastern tectonic lobe, there are two such plates sepa- Aguilera, 1920). rated by a major overthrust fault, the Cochise thrust fault (one bringing older rocks over younger ones), and in places Precambrian Structural Features crystalline rocks of the basement are faulted upon Paleozoic rocks. in Arizona and New Mexico In the northwestern tectonic lobe near Tucson, there are three major thrust plates, the upper and lower ones of Paleozoic and Many faults, folds, and intrusive bodies were formed during the Mesozoic rocks and the middle plate of Precambrian crystalline Cordilleran orogeny. Several kinds of these structures are particu- rock not derived from any known basement source (Drewes, 1974, larly informative as to the dynamics and age of deformation of the 1977). Because of the dissimilarity between the separate plates of rocks of Arizona and New Mexico and form the basis for compari- the two tectonic lobes, the allochthons of the lobes are believed to son with the features of adjacent parts of the Cordilleran orogenic have moved independently, at least during their final stage. This in- belt. Foremost of the structures is the basal thrust fault, which ap- dependence of movement could have occurred through slightly dif- pears to form the northern or eastern margin of the deformed belt. ferent times or different rates of movement. Detailed features of the Other thrust faults within the main plate are also instructive of the major thrust faults within the allochthon are much like those along style of deformation, as are tear faults and disharmonic faults. the basal thrust fault, except that dynamic metamorphism is re- Folds are typically small or moderate; few of them are more than stricted to the lower of the major plates of Paleozoic and Mesozoic several kilometres long. Many folds are known or suspected to be rock of the northwest tectonic lobe east of Tucson. Of special in- disharmonic, and a few of them are drag folds or other folds terest in these plates is the repetition of several Paleozoic forma-

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 648 H. DREWES

tions, metamorphosed within the lower major plate and Such folds occur particularly in the Mesozoic formations in which unmetamorphosed within the upper plate. incompetent shale units alternate with competent sandstone units, Still other thrust faults cut the rocks of the allochthon into many as occur along the eastern flank of the Santa Rita Mountains and small plates along which younger rocks are commonly moved over western flank of the Huachuca Mountains. Typically the faulted older ones. Many detailed features of shearing, drag folding, and folds grade laterally into unfaulted folds, and these grade onward truncation of the overlying or underlying beds along it resemble, on into open flexures and die out entirely. In a few places, the axes of a more modest scale, those occurring along the larger faults. Dis- an adjacent anticline and syncline pair merge anc homoclinally harmonic crumpling or disharmonic faulting of overlying rock also dipping rocks continue beyond the juncture of the axes. In these marks the more subtle bedding plane type of minor thrust faults. areas, the rocks under compressive stress responded first by buck- These faults are most abundant in rocks near other thrust faults ling, and then by shearing along a newly developed plane of weak- and are relatively scarce away from major thrust faults, as in the ness, the axial plane. With this type of folding, the faults die out Whetstone Mountains (Fig. 3). upward as well as laterally and probably splay off an underlying Tear faults associated with the thrust faults trend east-northeast thrust fault. and are vertical or near vertical. Slivers of rock not present in adja- Along some zones of incompetent rock, disharmonic folds are cent parts of either block occur along some of the faults, and in common. Such folds show that rocks of the upper plate crumpled places branch faults splay out from the main tear fault. A large but rocks of the adjacent lower plate did not. Again, the factors of component of strike-slip movement is indicated by such features as confining pressures and dif ferences in competence are critical in the striae, drag of upended beds that are truncated by the fault, and the development of such features. Although unequivocal disharmonic occurrence on only one side of a fault of small anticlines that have folds are small features — usually amplitudes are measurable in axial planes oblique to the fault and that have a moderate axial tens of metres — some larger folds, such as occur in Mesozoic rocks plunge. Some of these tear faults are directly continuous with, and on the western flank of the Whetstone Mountains, may have a simi- are different expressions of, adjacent thrust faults, and others sepa- lar origin. Possibly, too, the folds with faulted axial planes are dis- rate blocks of contrasting stratigraphic sequence or thickness. harmonic with respect to their subjacent rocks. Normal faults disrupt the thrust plates, and some of them are ge- Intrusive rocks also provide some insights to regional tectonic netically related to the thrust faults. These normal faults related to conditions; a systematic geochemical study would probably pro- thrust fault movement trend at random, at least at first glance, and vide much needed additional data. To the west, the stocks are more are distinctive in that they end updip, downdip, or in both direc- abundant, larger, and compositionally more varied than to the east. tions against a thrust plane. They are thus of local extent and usu- In the Santa Rita Mountains, for example, there were at least four ally involve displacements in the order of tens of metres to a few episodes of stock and plug emplacement during the Cordilleran hundred metres. Apparently they were formed in response to local orogeny, and in adjacent ranges, there was a fifth; in New Mexico, stresses built up within one or more minor thrust plates, and are there appears to have been only one. To the west, the stocks range thus viewed as disharmonic normal faults much as some folds are from diorite to granite (Drewes, 1976b); to the east, only disharmonic. A disharmonic fault, like a disharmonic fold, is granodiorite and quartz monzonite are reported. Furthermore, bounded by a bedding-plane fault or faults. They are noteworthy both the actual age and the age range are greater for the western mainly because they may help to locate obscure bedding-plane stocks than the eastern ones. A few stocks in the west are as young thrust faults. as any known in the east, but old stocks are unknown to the east. Folds are abundant in some parts of the allochthon but scarce in The oldest Cordilleran stock is 75 m.y. old, and the youngest that others. Generally fold axes trend north-northwest, but a few trend may be attributed to latest Cordilleran or preferably to earliest in other parts of the northwest quadrant. Fold axes are subhorizon- post-Cordilleran activity (50 m.y. ago) occurs in the Little Dragoon tal and not particularly long — 1 to 3 km is a typical length. They Mountains about 75 km east of Tucson. The process that generated range in style from moderately open to tight structures or chevron the magma probably either began in the west or was most effective folds, having limbs that converge at an angle of at least 30°. Most to the west. of the fold amplitudes are less than a few thousand metres. Axial planes usually are vertical or dip steeply to the southwest, but Age of Structural Features others dip northeasterly, particularly in areas of known multiphase deformation, as on the flanks of the Rincon Mountains im- The thrust faults and related structural features of southeastern mediately east of Tucson. About 60% of the folds verge to the Arizona and southwestern New Mexico are of Cordilleran age. In northeast (a description of their inclination with respect to nearby the Santa Rita Mountains south of Tucson, the thrust faulting is genetically related thrust faults which need not be horizontal), 20% more precisely dated as having occurred between 75 and 80 m.y. have no vergence (are normal to the thrust faults), and 20% verge ago (Drewes, 1972), and in New Mexico the age may be latest Cre- to the southwest. taceous or early Paleocene (Corbitt and Woodward, 1970). Aside from their intrinsic geometric configurations, folds of The greater dating precision in the western part of the southeast- perhaps two kinds are recognized, both with genetic implications ern tectonic lobe is possible because a more complete stratigraphic reflecting the compressional stress that led to thrust faulting. Small sequence has been recognised there and also because more faunal folds show this relation more readily than large ones, but extrapo- and radiometric age determinations have been made there. Rocks lation to large folds is probably justified. One common kind of fold at least as young as the Fort Crittenden Formation, dated by its lo- is the drag fold, which occurs along all types of faults but are cally prolific fauna as Santonian to Maastrichtian (Miller, 1964; largest and perhaps most abundant adjacent to, and more often Drewes, 1971a), and the overlying dacitic rocks of the lower part of above than beneath, low-angle faults. These folds are formed as a the Salero Formation and its correlative rocks are thrust faulted or result of friction along the fault during movement. folded. Gill and Cobban (1966) dated the Santonian in another Some large folds have faulted axial planes and, where both limbs part of the Cordilleran foreland area at about 82 to 84 m.y. The are present, may have been formed differently than the drag folds. oldest of the overlying undeformed rocks are rhyodacitic tuff and

welded tuff of the upper part of the Salero Formation and their cor- other folds of this group merge along strike with faults along the relative units, which are dated in many places in the western part of axial planes and are arcuate in plan away from the thrust fault. the tectonic lobe as 72 to 73 m.y. old (late Campanian or early Therefore the slightly faulted folds are probably the upper ends of Maastrichtian), as summarized by Drewes (1971a). In addition, six shingled minor thrust faults which seem to flatten downward to the small granodiorite stocks, radiometrically dated as 70 to 75 m.y. lie southwest, perhaps there to merge with a major thrust fault. The along a belt that crosses the proposed boundary between the two thrust fault along the crest of the range truncated and possibly hin- tectonic lobes. Because the belt is not offset by the tear movement dered the further development of the shingled thrusts. Accordingly, along this boundary fault, the belt of stocks is presumed to postdate the folds verge to the northeast with respect to an underlying unex- the proposed tear and thrust faulting. While some compressive de- posed flat-lying fault. formation probably occurred throughout much of the Piman phase Considerations of possible sources of the upper plate to the of the Cordilleran Orogeny, the peak of thrust faulting probably northeast or southwest of this part of Arizona also favor a north- occurred during the interval of 75 to 80 m.y. For the sake of brevity eastward relative movement over a southwestward one. The ter- in some structure sections, I have rounded this value off to 75 m.y. rane to the northeast of the thrust plate is geologically less complex In southwestern New Mexico, the Hidalgo Volcanics, thought to and that to the southwest is more complex than southeastern be correlative with the dacitic rocks of the lower part of the Salero Arizona. For example, to the northeast, Paleozoic and Mesozoic Formation in Arizona, seem somewhat younger in appearance; in rocks are rarely metamorphosed, sparsely intruded, and most of one place, they are intruded by a stock of Paleocene age. An un- them are little deformed. In northwestern Sonora, on the other published radiometric date on one specimen of the Hidalgo Vol- hand, where older rocks are sparsely exposed beneath abundant canics suggests that the rocks are of latest Cretaceous or early post-orogenic deposits, they are mainly metamorphosed and in- Paleocene age. An upper age limit on the orogenic activity is unav- truded, and may lie closer to a root area for thrust faults. Based on ailable because the stratigraphic sequence in New Mexico is more such reasoning, the actual movement of the thrust plates was east- fragmentary than it is in Arizona. In addition to an eastward de- northeast away from the root zone and toward the southwestern crease in age of the volcanotectonic unit, there is also an eastward edge of the craton. Alternatively, the lower crustal mass of the era- decrease in average age of associated stocks. By analogy with these ton moved away from the mid-Atlantic spreading center, and upon igneous rocks, the thrust faulting is believed also to be younger reaching the area of southwestward-thickening shelf deposits and eastward. the adjacent thicker and less yielding deposits of the deeper oceanic basin, it tore loose from the shelf rocks and was thrust beneath Direction and Amount of Tectonic Transport them. Thereby the actual movement of the lower crustal mass was southwest. Thus far I have seen no evidence in southeastern The available evidence of the structural features in southeastern Arizona which would favor either of these alternatives. Arizona and some general features of the terranes to the northeast The option of a regional plate moving easterly propelled solely and southwest indicate that the allochthon moved east-northeast at by gravity is not favored because, even if it came down as gentle a least 18 km and perhaps as much as 100 to 200 km. The alternative slope as 1° across an orogenic belt several hundred kilometres wide, concept of a west-southwest underthrusting of the cratonic mass the source area should have been so high and so deeply eroded as to beneath the thrust plates, however, remains a viable alternative. expose an extensive terrane of Precambrian rocks, which is not Resolution of this dilemma probably must come from a com- seen. prehensive kinematic analysis of the Mesozoic development of the Two kinds of evidence indicate that the amount of tectonic several continent-size crustal plates, particularly the Americas and transport of the thrust plates was large, though the actual amount Farallon plates and their neighboring plates. is still unknown. Firstly, Paleozoic formations are repeated in two A sense of movement, either east-northeastward or west- thrust plates on the eastern side of the Rincon Mountains. The dis- southwestward, is shown by the orientation of tear faults (parallel tance in the direction of tectonic transport over which these forma- to movement) and fold axes (normal to movement), which are ge- tions are repeated, the minimum amount of transport, is at least 18 netically related to the thrust faults. The few exceptions to the gen- km (where continuity of exposure is good), and it may be 32 km eral orientation of tear faults to the east-northeast and fold axes to (where the continuity is poorer). Secondly, a granitoid stock of the north-northwest are accounted for mostly by tectonic events of Jurassic age is within the Cochise thrust plate east of Tombstone. other kinds or other times. The stock is known from mapping and drill-hole data to be root- A relative movement direction of the upper plate (the conven- less. The most likely continuation of the stock in the underlying tional allochthon) is east-northeast with respect to the underlying Hidalgo plate may lie in the Santa Rita and Patagonia Mountains, rocks, but the evidence favoring this direction over the opposite one 100 km to the west-southwest. This tentative correlation is based is mixed. Vergence of folds, direction of imbrication of minor mainly on general geologic relations of the segments of the stock to thrust plates, and the offset of specific features favors an east- some pre-Cordilleran rocks and faults intruded by the stock; how- northeast relative movement direction of the regional thrust plate. ever, additional testing of this correlation is needed. Should this Some folds which seem to have an opposite vergence, such as ones amount of transport be established, a similarly large amount may in the Santa Rita Mountains (Drewes, 1972) and the Huachuca occur along the Hidalgo thrust fault. Mountains (Hayes and Raup, 1968) are not the simple features they appear to be. In the Santa Rita Mountains, there have been DESCRIPTION OF OTHER PARTS OF two diametrically opposed directions of movement at different THE OROGENIC BELT times a-long the fault beneath the fold. I believe that the folds in the Huachuca Mountains have been misinterpreted to be associated On a global scale, the Cordilleran orogenic belt is typically with the northeast-dipping faults along the crest of the range, shown in plan as a linear feature or, as King (1969a, p. 64) pointed rather than with the faults in the cores of the folds. The thrust fault out, a gently sigmoid zone which has a long northwest-trending along the crest actually cuts across part of one fold, whereas axes of central leg and shorter east-west-trending legs on each end. Seen at

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 650 H. DREWES

a continental scale, the belt is even less regular in plan, for it is the orogenic belt are intruded by stocks of Cretaceous age, and made up of many arcuate segments, each 500 to 1,000 km long. gneissic domes occur in western Utah and eastern Nevada. These segments, convex to the northeast, are arcuate not only along Folds are relatively abundant and tight in the Idaho-Wyoming their margins, but faults and folds within the orogenic belt gener- disturbed belt, and most of them are inclined to the northeast. ally reflect the trend of the margin. Thus the arcuate feature ap- Folds are fewer and more open to the southwest, but some drag pears to be of tectonic origin, rather than of some older deposi- folds of relatively modest amplitude and northeast vergence are re- tional or subsequent erosional association. A few of these tectonic ported. lobes are markedly curved, such as those' in northeastern Alaska Thrust faults are parallel to bedding over long distances, but in and northwestern Canada. The Idaho-Wyoming lobe, northeast of places, they cut across bedding, generally climbing up section to the Salt Lake City, and the lobe southwest of Monterrey, Mexico, are northeast. Thus they generally dip westerly, although the Willard less strongly arcuate, and the curvature of the segment in south- thrust fault dips gently eastward over a distance of many western Canada is very subtle. kilometres, perhaps as the result of Tertiary block tilting. Older rocks have been moved over younger ones along segments of the Canadian Rocky Mountains major thrust faults; along the lowest fault, Crittenden (1972, Fig. 2) showed basement rock faulted upon Paleozoic rock in the A segment of the Cordilleran orogenic belt in southwestern Wasatch Mountains. Where they are close together, the thrust Canada that has been intensively studied by Bally and others faults separate plates imbricated to the east or northeast. (1966), Campbell (1966), Dahlstrom (1970), Price and Mountjoy The age of the thrust faults is largely Cretaceous to Paleocene, (1970), and many others provides the most complete view of its and possibly some movement occurred as early as the Jurassic. The tectonic components. The belt extends from the foreland zone on age of the fault movement is oldest in the west and becomes progres- the cratonic margin across at least 150 km of thrust faulted and sively younger toward the east, as reviewed by Armstrong and Oriel folded rocks of late Precambrian to Mesozoic age to the Shuswap (1965). gneissic core (Fig. 4A). Folds are fewer and typically more open in The amount of tectonic transport of the upper plates is probably the central part of the belt; those of the foreland resemble drag large, but most evidence leads to interpretations of only minimum folds on the subjacent faults or broken-crested "snakehead struc- movement amounts. In the Idaho-Wyoming disturbed belt, a tures"; those near the core area are highly plastic, recumbent, and cumulative amount of tectonic transport of 80 km or more was refolded. Many folds are vergent to the northeast. The major faults proposed by Rubey and Hubbert (1959), on the basis of recon- are separated from each other by imbricate thrust plates which are structions from structure sections. In addition, the movement on shingled to the northeast. Some faults place older rocks over other thrust faults of the northern Wasatch Mountains was given as younger ones, others bring younger rocks on older ones. 64 km by Crittenden (1961), on the basis of offset of isopach lines Deformation of the rocks of the Cordilleran belt occurred as of several sedimentary sequences. In a palinspastic reconstruction, early as the Jurassic to the southwest and as late as the Eocene to Stewart and Poole (1974) gave the amount of tectonic transport the northeast. Metamorphism of the core occurred during the Early across the disturbed belt, Wasatch thrust belt, and a part of the Cretaceous, and most of the imbricate thrusting and folding of the eastern Basin and Range province as 160 km or more. foreland zone is of Late Cretaceous and Paleocene age. Tectonic transport was northeast at least 200 km, and it was ac- Southern Nevada companied by more than 8 km of crustal thickening, according to Price and Mountjoy (1970). The movement of the thrust plates is The segment of the Cordilleran orogenic belt near Las Vegas, explained to have been the effect of gravity flowage from an upwel- Nevada, is of special interest because it is the closest point of con- ling ductile and hot part of the crustal infrastructure, rather than trol northwest of Arizona and New Mexico (Fig. 4C, 1-2). The due to crustal shortening. However, the upwelling mass is shown belt of compressively deformed rock is at least 100 km wide and approaching the surface obliquely to the northeast, so that it may lies entirely within the Basin and Range province. As a consequence have acted as a plunger in response to some deeper-seated tangen- of the block faulting and volcanism of Cenozoic age that is typical tial force, rather than a vertical force. of this province, the continuity of the Cordilleran orogenic features is repeatedly disrupted. Utah, Idaho, and Wyoming The area east of the belt of compressive deformation is underlain by Paleozoic miogeosynclirial rocks less than 2,700 m (8,000 ft) The tectonic lobe northeast of Salt Lake City is also geologically thick and by some Mesozoic rocks and scattered sedimentary and one of the better known segments of the Cordilleran orogenic belt volcanic deposits of Tertiär/ age described by Longwell (1949) and through the efforts of Mansfield (1927), Eardley (1944), Rubey and Longwell and others (1965). Foreland basin deposits are repre- Hubbert (1959), Crittenden (1961, 1972), and Armstrong and sented only by thick local deposits of subaerial origin. These Oriel (1965) (see also Fig. 4B, 1-3). The orogenic belt extends more Paleozoic and Mesozoic reis have been so strongly eroded since than 150 km from the Idaho-Wyoming disturbed belt in the east the time of the Cordilleran orogeny that Tertiary volcanic rocks into or beyond the Basin and Range geologic province to the west, commonly rest directly upon basement rocks. where Tertiary block faulting and deposition conceal much of the The rocks of the belt of compressive deformation are mainly belt. Inasmuch as thrust faults of Cretaceous age occur in many Paleozoic marine rocks resembling those to the east except that ranges of the Basin and Range province, the orogenic belt may ex- they thicken westward to about 8,500 m (26,000 ft). Some tend westward at least to eastern Nevada, as shown by Roberts and Mesozoic rocks occur in the eastern part of the belt, and there is a others (1965, Fig. 20), and perhaps even to the zone of abundant large westward-thickening prism of upper Precambrian sedimen- large intrusive masses of Jurassic age of the Sierra Nevada, tary rocks in the western part. California, and of western Nevada. The central and western part of The eastern edge of the orogenic belt is sharply delineated by the

appearance of thrust faults; there is no zone of gradually increasing The age of deformation along this segment of the orogenic belt is folding east of the thrust faults. However, normal faults typical of not closely bracketed, and it may vary slightly from place to place. the Basin and Range province are abundant for about 100 km east Along the easternmost part of the belt, northeast of Las Vegas, of the orogenic belt. If a foreland fold belt ever existed, it was thrust faulting is dated as Late Cretaceous or early Tertiary. Rocks eroded, and so the present thrust front may be part of the décolle- of early Late Cretaceous age are deformed and are overlain by un- ment, rather than the surface breakthrough part of the original deformed rocks of Miocene age, and others perhaps of early Ter- fault system. tiary or Cretaceous(P) age (Longwell, 1949). Farther west in the The rocks in the Cordilleran orogenic belt near Las Vegas are belt, some of the faulting is middle Cretaceous or older as shown by abundantly broken by thrust, normal, and strike-slip faults. a radiometrically dated pluton that cuts the Keystone thrust fault Although many of these faults are of Cordilleran age, others are re- (Burchfiel and others, 1974). lated to younger Basin and Range block faulting, and a few faults The amount of tectonic transport of the upper rocks toward the are probably of pre-Cordilleran age. Near the Las Vegas Valley east relative to the lower rocks is at least 36 to 75 km, according to shear zone, the mountain ranges and thrust faults in them are Burchfiel and others (1974, p. 1021), and, from different ap- deflected toward the shear zone in the style of large drag structures, proaches, was estimated to be 30 to 50 km by Fleck (1970) and indicating that at least the latest movement on the shear zone post- about 120 km by Stewart and Poole (1974). Should the orogenic dates thrust faulting. The lowest thrust fault west of Las Vegas, ac- belt extend substantially farther west, as seems likely, or should it cording to Longwell and others (1965, Pis. 1 and 2) is the Bird once have extended farther east to a frontal belt now removed by Spring thrust fault and that to the northeast of the city is the erosion^ then the amount of tectonic transport across this segment Willow Tank or Bitter Ridge thrust fault. Alternatively, the lowest of the belt would be considerably larger. exposed thrust faults (according to Burchfiel and others, 1974, Fig. Apparently, the direction of tectonic transport of thrust plates 4) or the next overlying thrust faults, the Keystone to the west of near Las Vegas does not seem to have been given critical attention. the city and the Muddy Mountain thrust fault to the northeast, are Very likely the orientation of folds related to the major thrust faults major overthrust structures (described by Hewett, 1931; Longwell, has led to the general acceptance of easterly transport of supra- 1922, 1928, 1949; Longwell and others, 1965; Burchfiel and crustal material relative to basement rocks. More specifically, the others, 1974). These faults are believed to be segments of the same direction of transport, using such evidence, appears to have been structural feature, which were offset by late Tertiary right-lateral somewhat southeastward in the northeastern part of the region and movement on the concealed major northwest-trending Las Vegas due east in the southern part. Valley shear zone. From the interpretations of Longwell and others (1965) and current studies of R. G. Bohannon (1976, oral com- Northern Mexico mun.), it appears that the eastern thrust plates contain the youngest rocks and that the western ones, such as the Wheeler Pass thrust A belt of strong compressive deformation is also reported from plates, the oldest rocks. Precambrian crystalline rocks are known in northern Mexico (Fig. 4E). The abundant structures and complex autochthonous terrane east of all mapped thrust faults, and they geologic history of southern Arizona are known to extend into also occur in scattered localities west of the belt here under consid- northern Sonora, where current studies by Claude Rangin will eration where their structural position remains unclear. supplement past studies near major mining camps and cities. Folds The western limit of the orogenic belt is poorly known because and thrust faults of Late Cretaceous or early Tertiary age trend regional tectonic studies west of the Spring Mountains have not northwesterly through Chihuahua, Coahuila, and adjacent states of been made. Some ranges to the west contain thrust faults, and all of Mexico and west Texas. the ranges are typical Basin and Range structural features formed in Northern Mexico between El Paso and Monterrey (Fig. 1) is un- response to post-Cordilleran tectonism. derlain mainly by a thick sequence of Mesozoic clastic and car- Folds occur at scattered intervals in several thrust plates; many of bonate rocks of marine and littoral origin. This sequence comprises them trend northerly, but a few trend easterly. The north-trending as much as about 8,000 m of Cretaceous rocks and 2,000 m of larger folds are moderately tight structures, but the east-trending Jurassic and Triassic rocks. Paleozoic formations are exposed at folds are more open, and the axial planes of folds commonly are widely scattered localities beneath this Mesozoic sequence, and typ- upright or dip to the west. Because most thrust faults dip west near ically they include shelf rocks of Pennsylvanian and Permian age in the folds, the folds are mainly vergent to the east. The north- northern Chihuahua. Southwest of Monterrey and the area repre- trending folds appear to be major drag structures or broken-crested sented by the enlarged segment of section E (Fig. 4), some Jurassic compressional folds; the easterly trending folds are probably re- or older volcanic rocks are reported (De Cserna, 1976). Northeast lated to other local stresses. of the Cordilleran belt of northern Mexico and in local basins Many high-angle faults (not shown on Fig. 4, CI) are genetically within the belt, such as the Parras basin west of Monterrey, related to the thrust faults and are viewed as local tear faults or Paleocene deposits are also present. Small stocks of granitic rocks structures through which local stresses across the thrust plates were intrude the deformed Mesozoic formations; few are dated, and adjusted during the movement of the plates themselves. They are their genetic association to the Cordilleran deformation is mostly therefore disharmonie structures, much as are some of the small unknown. folds which appear in an overlying plate but not in the underlying The rocks are deformed over a wide region, particularly along a one. Some of the general difficulty in correlating faults regionally belt extending from Monterrey northwestward to the Big Bend of and in assigning ages of fault development suggests that there is the Rio Grande and west Texas, and on to El Paso, and a second conflicting evidence of field relations of a kind typical in areas of belt extending from south of Monterrey in a broad arc west of fault rejuvenation. Thus, the Las Vegas Valley shear zone may be Monterrey to western Coahuila and Chihuahua, whence it trends an ancient basement flaw that was last active as a right-lateral, northerly toward southeastern Arizona. The intervening zone is less strike-slip fault in late Tertiary time. intensely deformed. The style of folding of the Mesozoic rocks near

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 Shuswap ferrane A. CANADIAN ROCKY MOUNTAINS 20,000' 6.000m

1 6,000m 20,000' I 12,000m - - - - - /' - I , »- >r - C ' ' ! " > ' » 1' - > V * Price and Montjoy, 1970 40,000' 10 20 30 50 KILOMETERS 20 30 MILES

EXPLANATION

Tertiary to Late Late Jurassic to Ordovician to Middle Early Cambrian and Early Precambrian (crystalli ne Jurassic Devonian Cambrian Late Precambrian basement)

B.I. NORTHERN WASATCH THRUST BELT 10,000' "3,000 m — 8,000 — .— 2,000m é — 6,000' "_.4,000' B.2 and 3. IDAHO-WYOMING THRUST BELT q 1,000m 2,000'

Pj/^L^ 10,000'

(After Crittenden, 1972) (Modlfed from Rubey and Hubbert, 1959) 10 0 10 20 30 KILOMETERS 5 0 10 15 MILES 111111 L -I I I I II I EXPLANATION Quaternary Jurassic and Eocene Paleocene Cretaceous Paleozoic,showing Cambrian and Pliocene Triassic marker unit

Late Precambrian Early Precambrian

C.I. SPRING MOUNTAINS, SOUTHWEST OF LAS VEGAS C.2. MUDDY MOUNTAINS, NORTHEAST OF LAS VEGAS

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 Jurassic and m. Tertiary Late Cretaceous Paleozoic Late Precambrian Early Precambrian m BS Triassic

D. SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO

10,000 3,000m

3,000m 10,000'

EXPLANATION Quaternary Tertiary and Tertiary fc&j^ Cretaceous Jurassic • and Tertiary Cretaceous < /V r Jurassic and Paleozoic Early Precambrian Triassic

Style of folding near Monterrey, after Fries (I960, fig. 2)

E. CHIHUAHUA AND COAHUILA, MEXICO

NE 10.000 3,000m

3,000m 10,000'

50 MILES

EXPLANATION

Tertiary and Jurassic and V " Tertiary Late Cretaceous Early Cretaceous Paleozoic >' * Precambrian(?) Cretaceous • Triassic Figure 4. Interpretive tectonic sections across the Cordilleran orogenic belt from Canada to including Spring Mountains (from which Tertiary normal faults are omitted) and Muddy Mountains. Mexico. 4A, Canadian Rocky Mountains region. 4B, Salt Lake City region, including Northern Thrust faults: WP, Wheeler Pass; LC, Lee Canyon; K, Keystone; BS, Bird Spring; M, Muddy Moun- Wasatch and Idaho-Wyoming thrust belts (2 parts). Thrust faults: O, Ogden; W, Willard; WO, tain. 4D, Southeast Arizona and southwest New Mexico. 4E, Chihuahua and Coahuila, northern Woodruff, B, Bannock; C, Crawford; A, Absaroka; D, Darby; and P, Prospect. 4C, Las Vegas region, Mexico.

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 654 H. DREWES

Monterrey is known from illustrations by Alvarez (1949), Fries rupturing of the rocks in a generally uniform style. Folds vary in (1956, 1960), and Wall and others (1961) to be typically symmetri- form from open to moderately tight and are northwest-trending, cal and upright to asymmetrical, having axial planes that dip west upright to asymmetric, southwest-dipping (northeast vergent) to south. A few folds are described as recumbent to the northeast or structures. Typically there are a few exceptions, such as a stray re- as fan folds. cumbent fold, a few northeast-dipping (southwest vergent) asym- Thrust faults are reported along discontinuous parts of the main metric folds, and fan folds. Usually folds are interpreted to be zones of deformation and in scattered localities between these broken-crested anticlines, large drag structures, or disharmonic fea- zones. Most of them dip west to south, and Alvarez (1949) men- tures above low-angle faults. tioned northeastward imbrication of thrust plates. Associated folds Low-angle faults typically separate a deformed terrane into are vergent to the northeast. On the basis of analogy with the Jura thrust or glide plates, some of which are thick and extensive Mountains, Alvarez proposed that the belt of folded and thrust- masses, while others within the large plates are small and overlap- faulted rocks overlies a major décollement, and this concept was ping, to form shingled platelets that commonly dip southwest. followed by De Cserna (1960), Weide and Murray (1967), and Some segments of the larger thrust faults show overthrust relations King (1969a p. 73). The concept is also the basis of my interpreti- with older on younger rocks, but along other segments and gener- tive structure section (Fig. 4E) which shows the region between the ally along smaller faults, bedding-plane faults are usually found, Big Bend of the Rio Grande and southern Chihuahua, including a along which younger rocks are moved upon older ones. Typically, segment of the Monterrey area projected along the structural grain. some of the rocks next to i:he bedding-plane faults are sheared out This structure section incorporates the notion of northeastward so that the rock sequence is tectonically thinned. The lack of rec- movement of supracrustal rocks with respect to the underlying ognition of such faults has weakened many an early stratigraphic rocks. For the present, the findings of Claude Rangin (1975, writ- study in the Cordilleran orogenic belt. Most of the thrust faults are ten commun.) which support a southwestward overthrusting of flat-lying or dip southwesterly; in all regions, however, some other rocks in northern Sonora cannot be assessed. The structure section dip directions are also recorded. As described thus far, the style of of Figure 4E also reflects the consensus that deformation occurred deformation of the five regions is much alike. after the deposition of the Upper Cretaceous formations, that lo- The direction of tectonic transport of the regions is also generally cally, near Monterrey, some movement occurred after deposition of east-northeast or east, even in northern Mexico near Monterrey a Paleocene formation, and that it predated the scattered small in- where the strongly arcuate tectonic lobe suggests a local fan-like trusive masses of the region. However, it does not incorporate the spread of movement from about east to nearly north. The direction area southwest of Monterrey in which Jurassic thrust faulting of of movement is usually inferred from the orientation of fold axes; volcanic rocks and graywacke were reported by De Cserna (1976). in a few places, the orientation of strike-slip faults genetically re- Data bearing on the amount of tectonic transport are unavail- lated to the thrust faults is also used. This tectonic transport direc- able; consequently, an amount of a few tens of kilometres was tion is in fact only a measure of relative movement between plates, selected on the basis of the distances of movement recorded in other yet commonly the assumption is made that the upper plates have orogenic fold belts of the world. been the ones to move over the passive basement rocks. Still farther south, in Guatemala, where there is as yet insufficient The amount of tectonic transport is estimated to be more than geologic knowledge from which to prepare a section across the 100 km and possibly is nearly 200 km in at least four of the five orogenic belt, the tectonic picture appears to be much like that in regions. No estimates have been made for northern Mexico, where Mexico. The folds of the belt shown on the tectonic map of North regional mapping is still fragmentary. These consistently large dis- America are associated with thrust faults, and possibly a major dé- tances of movement are obtained by two independent methods: one collement fault is also present (Dengo and Bohenberger, 1969, is based on the measure of offset of some pre-thrust feature, such as p. 213). Tectonic transport was directed toward the Americas an isopach line or a stock, along a thrust fault; the second is based plate, and orogenic activity occurred during Late Cretaceous and on a cumulative restoration of individual faults and folds, as drawn early Tertiary time. in structure sections. The data are not always accurately controlled, In summary, then, while the data seem adequate to indicate and so the distances are often judicious estimates. Even where Cordilleran orogenic times and conditions, much study is still further study of the best field data may provide an unequivocal needed to flesh out the skeletal extent of knowledge of this measure of movement, this is likely to give control over only one tectonically important region. fault or part of the orogenic belt of a given region. In Arizona, for instance, evidence for large amounts of tectonic movement use as- REGIONAL COMPARISONS: CONCLUSIONS pects of both methods, and yet, under the most favorable circum- stances, this would provide data pertaining only to movement be- Along strike, the Cordilleran orogenic belt has some features that tween two major thrust plates but not between lower plate and are consistent from place to place and others that vary. The tectonic basement. interpretation of the Cordilleran deformation of southeastern From region to region, there is also a general similarity of the Arizona presented in this report fits this pattern, resembling the time of orogenic development, mainly Cretaceous to Paleocene. other areas in certain respects, while maintaining several distinctive Close dating of peak orogenic time is usually not possible; typi- features of its own. Therefore I believe the orogenic belt extends cally, field relations permit only the placement of a close upper age between Las Vegas and El Paso without major interruption. After limit or lower age limit, but not both. Using a composite of data reviewing the similarities of the five regions that were briefly de- from different parts of a region may also lead to difficulties, for it scribed in this report, the differences between the regions will also can only be done with the assumption that orogenic activity peaked be discussed below in order to evaluate their importance. simultaneously throughout the region, and evidence suggests this All of the regions have responded to compressive force oriented did not occur. In all of the regions, the more easterly faults are approximately northeast-southwest to east-west by buckling and judged to be younger than the more westerly ones; at a local scale,

the upper faults of an overlapping suite of thrust faults are usually greater lateral and/or vertical proximity to a core area than in the younger than the lower ones. Apparently the peak of orogenic de- regions near Las Vegas or Salt Lake City. formation shifted eastward and upward. The orogenic belt, then, The Las Vegas and Arizona—New Mexico regions also have in has a uniform temporal framework as well as a uniform style of common abundant postorogenic block faulting and an extensive deformation, and this uniformity fits Arizona and New Mexico, as cover of middle Tertiary volcanic rocks and upper Tertiary well as the other regions. sedimentary deposits. Northern Mexico probably also has these While the general similarities between the five regions favor a features, but their extent and intensity are less well documented, tectonic correlation, the spectre of certain differences deserves at- particularly along the foreland zone. These features are explained tention too. Among the plausible reasons for differences are (1) the by a subsequent period of tensional deformation. regions represent different parts — in lateral position or depth — of Arizona has yet another distinctive tectonic feature; some Pre- a single kind of orogen; (2) the regions contain dissimilar precambrian basement rocks are present in the thrust plates. Certainly orogenic crustal anisotropy; and (3) the regions had dissimilar a greater proximity to the level of exposure of the décollement post-orogenic tectonic deformation, augmented perhaps by dif- favors such involvement of basement rock. But in addition, Arizona ferences in erosion or burial. is known to have responded to some major vertical movements Several features of the Canadian Rocky Mountains are unique to after the deposition of the widespread shelf and miogeosynclinal that region, but they can be explained by the above factors. No deposits and before the occurence of the Cordilleran orogeny. Such other region has a reported hinterland in which rocks are plastically movements would produce an irregular interface between base- deformed to the extent of the Shuswap terrane. Furthermore, the ment and supracrustal rocks which is ideally suited to the shearing age of deformation of those rocks is Jurassic rather than Creta- off of some basement rocks and to some local underthrusting. ceous. Likewise, the latest movement of the foreland zone is Eocene Clearly the ambiguities in style of deformation and present ap- instead of Paleocene. Perhaps the orogenic belt is more completely pearance of the Cordilleran thrust belt between Arizona and New exposed in Canada than it is farther south, thereby showing a style Mexico and the other regions are no more than may be expected of folding in the core area that is concealed rather than absent from a situation in which that belt was superposed on diverse older elsewhere, and thereby showing the fullest known age range of the structures, was modified by younger tectonic events, and perhaps orogeny. was eroded to greater depth than in the Canadian Rocky Moun- The region near Salt Lake City shows minor differences which tains. are readily explained. The zone of late orogenic and early post- Only the relative movement between the cratonic and oceanic orogenic sedimentation along the foreland is a continental basin masses, or Americas plate and Farallon plate, was presented in the (the Green River Basin) rather than a shallow sea. Also, the thrust above sections on the direction of tectonic movement of the faults of the northern Wasatch Mountains are subhorizontal or dip Cordilleran orogenic belt; no estimates on the absolute movement gently eastward, and they are cut and tilted by middle or late Ter- can be made without a still broader hemisphere-wide study. During tiary normal faults. Thrust faulting continues west of these moun- the Jurassic and Cretaceous Periods, apparently the Americas plate tains, but it is not illustrated in Figure 4 because the abundance of was moving westerly from the mid-Atlantic spreading center (Lar- the normal faults adds much uncertainty to attempts at correlating son and Pitman, 1972) and the Farallon plate was moving north- segments of the thrust faults. A few late orogenic stocks also occur easterly (Hamilton, 1969) from the Pacific spreading center. With in the region west of the Wasatch Mountains. These differences such a convergence along the boundary between these plates, it is make sense if the Salt Lake City region is viewed as being equiva- no wonder that a major orogenic belt developed, and this would lent to the eastern half of the section present in the Canadian Rocky suggest that both masses were allochthonous and that neither Mountains, but eroded slightly deeper because the eastern part of played the part of a passive autochthon, in the strict meaning of the region lay slightly higher during the late stages of orogeny. The that term. Their differing responses to compressive force, then, central and western parts of the Salt Lake City region lay consid- probably reflects basic differences in their massiveness or ani- erably higher and were block faulted and deeply eroded during sotropy. post-orogenic time. The tectonic features of the five regions that straddle the The Las Vegas and the Arizona—New Mexico regions have some Cordilleran orogenic belt from Canada to Mexico can be unified tectonic features in common that differ from those of the other re- into a general model of development of the belt. Beginning in about gions. The characteristic foreland thrust zone with the sled- Jurassic time, the Farallon plate of the Pacific Ocean and the runner-like overthrusts of relatively young age are not identified in Americas plate converged, possibly as a result of the development these regions. Stocks of Late Cretaceous or Paleocene age are pres- of, or reorientation of, spreading centers in the mid-Pacific, mid- ent at wide intervals south of Las Vegas; they are also present at Atlantic, or very likely both centers. As a result of this convergence scattered intervals in New Mexico and are abundant in Arizona of plates, the oceanic plate was subducted beneath the western near Tucson. Furthermore, the thrust faults of these regions are margin of the continental plate, which consisted of a cratonic core large subhorizontal features, between which shingled minor plates and a flanking prism of older miogeosynclinal deposits. Early dur- are less common than in the Canadian Rocky Mountains. These ing the convergence of the plates, the surface of the oceanic plate features may be explained by considering the Las Vegas and near the continental margin was pulled down, and the marine basin Arizona—New Mexico regions to be analogous to the central one- above entrapped another prism of sediments, mainly of a deep half or three-quarters of the belt exposed in Canada, but exposed at marine kind. With the continued convergence of the plates, these deeper levels. The subhorizontal major thrust faults may be close to sediments were impinged against the upper part of the continental a postulated décollement, if indeed the lowest of them is not the plate, were internally deformed, and ultimately transmitted some décollement itself. Magma movement related to orogenic processes force eastward against the western edge of the miogeosyncline (or should dwindle laterally away from the core or root area as well as in the case that the continental plate alone was the active mass, the upward, and so the abundance of stocks near Tucson suggests a impetus direction is reversed, although in either case the conse-

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 656 H. DREWES

quence is the same). Meanwhile, subduction continued and Soc. America Bull., v. 62, no. 11, p. 1331-1346. additional sediments accumulated upon the deformed deep-sea sed- Armstrong, F. C., and Oriel, !>. S., 1965, Tectonic development of Idaho- iments, probably along westward-shifting depocenters. Wyoming thrust belt: Am. Assoc. Petroleum Geologists Bull., v. 49, no. 11, p. 1847-1866. As a result of the forces applied to the west edge of the Armstrong, R. L., 1968, Sevier orogenic belt in Nevada and Utah: Geol. preorogenic miogeosynclinal deposits, they were thrust eastward Soc. America Bull., v. 79, no. 4, p. 429-458. relative to the stronger underlying part of the continental crust. Atwater, Tanya, 1970, Implications of plate tectonics for the Cenozoic First the rocks in the western or thicker part of this prism were de- tectonic evolution of western North America: Geol. Soc. America formed and tectonically thickened to a point at which the forces Bull., v. 81, no. 12, p. 3513-3536. Bally, A. W., Gordy, P. L., and Stewart, G. A., 1966, Structure, seismic were transmitted still farther east and upward. Rupture occurred data, and orogenic evolution of southern Canadian Rocky Mountains: along planes of weakness, such as axial planes of folds and planes Bull. Canadian Petroleum Geology, v. 14, no. 3, p. 337-381. of major lithologic change, and particularly the interface between Burchfiel, B. C., Fleck, R. J., Secor, D. T., Vincelette, R. R., and Davis, G. A. crystalline basement rocks and overlying sedimentary rocks. 1974, Geolog)' of the Spring Mountains, Nevada: Geol. Soc. America Bull., v. 85, no. 7, p. 1013-1022. Although the basic driving force of converging crustal plates was Campbell, R. B., 1966, Tectonics of the south central Cordillera of British applied during a long period of time — probably many tens of mil- Columbia, in A symposium on the tectonic history and mineral de- lions of years — the duration of peak tangential stress conditions posits of the western Cordillera, Vancouver, B. C., 1964: Canadian along a given zone subparallel to the interface between the plates is Inst. Mining and Metallurgy Spec. Vol. 8, p. 61-71. relatively short — perhaps only a few million years. A steady con- Coney, P. J., 1976, Plate tectonics and the Laramide orogeny, in Tectonics and mineral resources of southwestern North America: New Mexico vergence rate of 5 to 10 cm per year, the rate of movement Atwater Geol. Soc. Spec. Pub., v. 6, p. 5-10. (1970) suggested for the Farallón plate, would produce, for exam- Cooper, J. R., 1960, Reconnaissance map ofWillcox, Fisher Hills, Cochise, ple, a crustal shortening of 100 km in 1 to 2 m.y., not all of which and Dos Cabezas quadrangles, Cochise and Graham Counties, would be taken up in the orogenic belt. In any event, during a Arizona: U.S. Geol. Survey Mineral Inv. Field Studies Map MF-231. geologically brief time span, the rocks were folded and then sheared Corbitt, L. L., and Woodward, L. A., 1970, Thrust faults; of the Florida Mountains, New Mexico, and their regional tectonic significance: in a manner controlled by such features as local variation in rock New Mexico Geol. Soc. Field Conf., 21st, 1970: New Mexico Geol. strength, thickness of cover, availability of pore fluids and barriers Soc., p. 69 - 75. to fluid migration, and the geothermal gradient. Crittenden, M. D., Jr., 1961, Magnitude of thrust faulting in northern Utah, in Geological Survey Research 1961: U.S. Geol. Survey Prof. While strong tectonic deformation occurred in the supracrustal Paper 424-D, P. D128-131. prisms of sedimentary rocks near the continental margin, the sub- 1972, Willard thrust and the Cache allochthon, Utah: Geol. Soc. ducted part of the oceanic plate and overlying part of the continen- America Bull., v. 83, no. 9, 2871-2880. tal plate were also being profoundly changed. At the greater depth Dahlstrom, C. D., 1970, Structural geology in the eastern margin of the to which the oceanic plate was moved, the material was heated to Canadian Rocky Mountains: Bull. Canadian Petroleum Geology, v. 18, no. 3, p. 332-406. conditions of increasing plasticity, and ultimately to melting, ac- Davis, G. H., 1975, Gravity-induced folding off a gneiss dome complex, companied by regional uplift of the crust. Upward-moving mag- Rincón Mountains, Arizona: Geol. Soc. America Bull., v. 86, no. 7, mas, probably derived from parts of both crustal plates, in un- p. 979-990. known proportion, were emplaced in the basement rocks and the De Cserna, Zoltan, 1960, Orogenesis in time and space in Mexico: Geol. Rundschau, v. 50, p. 595-605. deformed supracrustal sheet as batholiths and stocks, and were ex- 1976, Mexico — Geotectcnics and mineral deposits, in Tectonics and truded as volcanic rocks of orogenic age in a few places. mineral resources of southwestern North America: New Mexico Geol. Mineralizing fluids accompanied or followed the period of igne- Soc. Spec. Pub., v. 6, p. 18-25. ous activity. In places, large deposits of porphyry copper or other Dengo, Gabriel, and Bohenberger, Otto, 1969, Structural development of metals were emplaced by the fluids. The specific source of the met- northern Central America, in Tectonic relations of northern Central America and the western Caribbean — The Bonacca Expedition: Am. als is still unknown, but the fluids apparently moved upward along Assoc. Petroleum Geologists Mem., v. 11, p. 203-220. suitably oriented planes of weakness, such as the northwest- DeSitter, L. U., 1956, Structural geology (2nd ed.): New York, McGraw- trending complex faults in the basement rocks of southern Arizona Hill Book Co., 551 p. and southwestern New Mexico. Near the surface, they spread lat- Drewes, Harald, 1971a, Mesozoic stratigraphy of the Santa Rita Moun- erally along subordinate faults, including some thrust faults, and tains, southeast of Tucson, Arizona: U.S. Geol. Survey Prof. Paper 658-C, p. 1-81. along favorable lithologic horizons. Thus, while the generation of 1971b, Geologic map of the Mount Wrightson quadrangle, southeast metal-bearing fluids may be genetically related to the phenomenon of Tucson, Arizona, Santa Cruz and Pima Counties, Arizona: U.S. of plate tectonics of the Cordilleran orogeny, the critical factor of Geol. Survey Misc. Geol. Inv. Map 1-614. metal distribution may reflect inherited preorogenic anisotropic 1972, Structural geology of the Santa Rita Mountains, southeast of structural features. Tucson, Arizona: U.S. Geol. Survey Prof. Paper 748, p. 1-35. 1974, Geologic map and sections of the Happy Valley quadrangle, Cochise County, Arizona: U.S. Geol. Survey Misc. Geol. Inv. Map REFERENCES CITED 1-832. 1976a, Laramide tectonics from Paradise to Hells Gate, southeastern Aguilera, J. G. [translated by A. C. Lawson], 1920, The Sonora earthquake Arizona: Ariz. Geol. Soc. Digest, v. 10, p. 151-167. of 1887: Seismol. Soc. America Bull., v. 10, no. 1, p. 31-44. 1976b, Plutonic rocks of the Santa Rita Mountains, southeast of Tuc- Albritton, C. C., Jr., and Smith, J. F., Jr., 1957, The Texas lineament, in son, Arizona: U.S. Geol. Survey Prof. Paper 915, p. 1-75. Relaciones entre la tectónica y la sedimentación: Internat. Geol. 1977, Geologic map of the: Rincón Valley quadrangle, Pima County, Cong., 20th, México, D. F., 1957: v. 2, no. 5, p. 501-518. Arizona: U.S. Geol. Survey Misc. Geol. Inv. Map 1-997. Alvarez, Manuel, Jr., 1949, Tectonics of Mexico: Am. Assoc. Petroleum Eardley, A. J., 1944, Geology of the north-central Wasatch Mountains, Geologists Bull., v. 33, no. 8, p. 1319-1335. Utah: Geol. Soc. America Bull., v. 55, no. 7, p. 819-894. Anderson, C. A., 1951, Older Precambrian structures in Arizona: Geol. Erickson, R. C., 1968, Geology and geochronology of the Dos Cabezas

Mountains, Cochise County, Arizona, in Correlation and chronology Mansfield, G. R., 1927, Geographic, geologic, and mineral resources of of ore deposits and volcanic rocks: Progress Rept. C00-689-100 to parts of southeast Idaho: U.S. Geol. Survey Prof. Paper 152, 409 p. U.S. Atomic Energy Comm., p. 193-198. Mayo, E. B., 1971, Defense of "Volcanic orogeny": Arizona Geol. Soc. Di- Fleck, R. J., 1970, Tectonic style, magnitude, and age of deformation in the gest, v. 9, p. 39-60. Sevier orogenic belt in southern Nevada and eastern California: Geol. Miller, H. W., Jr., 1964, Cretaceous dinosaurian remains from southern Soc. Amerca Bull., v. 81, no. 6, p. 1705-1720. Arizona: Jour. Paleontology, v. 38, no. 2, p. 378-384. Fries, Carl, Jr., 1956, Outline of the central and western parts of the State of Price, R. A., and Mountjoy, E. W., 1970, Geologic structure of the Cana- Morelos and contiguous areas of Guerrero and México [Summary], in dian Rocky Mountains between Bow and Athabasca Rivers — A prog- Internat. Geol. Cong., 20th, Mexico, D. F., 1956, Excursion G-9, ress report: Geol. Assoc. Canada Spec. Paper, v. 6, p. 7-25. p. 11-53. Richard, Kenyon, and Courtright, J. H., 1966, Structure and mineralization 1960, Geología del Estado Morelos y de partes adyacentes de Mexico y at Silver Bell, Arizona, in Geology of the porphyry copper deposits, Guerreo, region central meridional de México: México Univ. Nac. southwestern North America: Tucson, Ariz., Univ. Arizona Press, Inst. Geología Bol., v. 60, p. 1-236. p. 157-163. 1962, Reseña de la geología del Estado de Sonora, con énfasis en el Roberts, R. J., Crittenden, M. D., Jr., Tooker, E. W., Morris, H. T., Hose, Paleozoic: Assoc. Mexicana Geólogos Petroleros Bol., v. 14, no. R. K., and Cheng, T. M., 1965, Pennsylvanian and Permian basins in 11-12, p. 257-273. northwestern Utah, northeastern Nevada, and south-central Idaho: Gill, J. R., and Cobban, W. A., 1966, Regional unconformity in Late Creta- Am. Assoc. Petroleum Geologists Bull., v. 49, no. 11, p. 1926-1956. ceous, Wyoming, in Geological Survey research 1966: U.S. Geol. Sur- Rubey, W. W., and Hubbert, M. K., 1959, Role of fluid pressure in vey Prof. Paper 550-B, p. B20-B27. mechanics of overthrust faulting. IL Overthrust belt in geosynclinal Goodfellow, G. E., 1888, The Sonora earthquake: Science, v. 11, p. 162- area of western Wyoming in light of fluid-pressure hyopthesis: Geol. 166. Soc. America Bull., v. 70, no. 2, p. 167-206. Hamilton, Warren, 1969, Mesozoic California and the underflow of Pacific Sabins, F. E., 1957, Geology of the Cochise Head and western part of the mantle: Geol. Soc. America Bull., v. 80, no. 12, 2409-2429. Vanar quadrangles, Arizona: Geol. Soc. America Bull., v. 68, no. 10, Hayes, P. T., 1970, Cretaceous paleogeography of southeastern Arizona p. 1315-1342. and adjacent areas: U.S. Geol. Survey Prof. Paper 658-B, 42 p. Schmitt, H. A., 1966, The porphyry copper deposits in their regional set- Hayes, P. T., and Raup, R. B., 1968, Geologic map of the Huachuca and ting, in Geology of the porphyry copper deposits, southwestern North Mustang Mountains, southeastern Arizona: U.S. Geol. Survey Misc. America: Tucson, Ariz., Univ. Arizona Press, p. 17—33. Geol. Inv. Map 1-509. Simons, F. S., 1974, Geologic map and sections of the Nogales and Lochiel Hewett, D. F., 1931, Geology and ore deposits of the Goodsprings quad- quadrangles, Santa Cruz County, Arizona: U.S. Geol. Survey Misc. rangle, Nevada: U.S. Geol. Survey Prof. Paper 162, 172 p. Geol. Inv. Map 1-762. King, P. B., 1969a, The tectonics of North America — A discussion to ac- Stewart, J. H., and Poole, F. G., 1974, Lower Paleozoic and uppermost Pre- company the tectonic map of North America (scale 1:5,000,000): U.S. cambrian Cordilleran miogeodine, Great Basin, western United States, Geol. Survey Prof. Paper 628, 95 p. in Tectonics and sedimentation: Soc. Econ. Paleontologists and 1969b, Tectonic map of North America: Washington, D.C., U.S. Geol. Mineralogists Spec. Pub., v. 22, p. 28-57. Survey, scale 1:5,000,000. Swan, M. M., 1975, The Texas Lineament — Tectonic expression of a Pre- King, P. B., and Edmonston, G. J., 1972, Generalized tectonic map of cambrian orogeny: Geol. Soc. America Abs. with Programs, v. 7, North America: U.S. Geol. Survey Misc. Geol. Inv. Map 1-688. p. 1188-1189. Larson, R. L., and Pitman, W. C., 1972, World-wide correlation of Wall, J. R., Murray, G. E., and Diaz, T. G., 1961, Geology of the Monter- Mesozoic magnetic anomalies, and its implications: Geol. Soc. rey area, Nuevo Leon, Mexico: Gulf Coast Assoc. Geol. Socs. Trans., America Bull., v. 83, no. 12, p. 3645-3662. v. 11, p. 57-71. Longwell, C. R., 1922, Muddy Mountains overthrust in southeast Nevada: Weide, A. E., and Murray, G. E., 1967, Geology of the Parras basin and Jour. Geology, v. 30, no. 1, p. 63-72. adjacent areas of northeastern Mexico: Am. Assoc. Petroleum 1928, Geology of the Muddy Mountains, Nevada, with a section Geologists Bull., v. 51, no. 5, p. 678-695. through the Virgin Range to the Grand Wash Cliffs, Arizona: U.S. Wilson, E. D., 1939, Pre-Cambrian Mazatzal revolution in central Arizona: Geol. Survey Bull., v. 798, 152 p. Geol. Soc. America Bull., v. 50, no. 7, p. 1113-1164. 1949, Structure of the northern Muddy Mountains area, Nevada: Woodward, L. A., 1976, Laramide deformation of the Rocky Mountain Geol. Soc. America Bull., v. 60, no. 5, p. 923-968. foreland, in Tectonics and mineral resources of southwestern North Longwell, C. R., Pampeyan, E. H., Boyer, Ben, and Roberts, R. J., 1965, America: New Mexico Geol. Soc. Spec. Pub., v. 6, p. 11-17. Geology and mineral deposits of Clark County, Nevada: Nevada Bur. Mines Bull., v. 62, 218 p. Lynch, D. J., 1976, Basalt eruptions in the Bernadino volcanic field of MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 12, 1976 southeastern Arizona: Geol. Soc. America Abs. with Programs, v. 8, REVISED MANUSCRIPT RECEIVED JULY 7, 1977 no. 5, p. 604-605. MANUSCRIPT ACCEPTED JULY 8, 1977

Printed in U.S.A.

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