stress provinces indicate that these transitions can be abrupt, occurring over <75 km in places. In the western United States, a region of active tectonism characterized by high levels of seismicity and generally high heat flow, the stress pattern is complex, but numerous stress provinces can be well delineated. Despite relative tectonic quiescence in the eastern and central United States, a major variation in principal stress orientation is apparent between the Atlantic Coast and midcontinent areas. Most of the eastern United States is marked by predominantly compressional tectonism (combined thrust and strike slip faulting), whereas much of the region west of the southern Great Plains is characterized by predominantly extensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreas fault and in parts of the Sierra Nevada is nearly pure strike slip. Exceptions to this general pattern include areas of compressional tectonics in the western United States (the Pacific Northwest, the Colorado Plateau interior, and the Big Bend segment of the San Andreas fault) and the normal growth faulting along the Gulf Coastal Plain. Sources of stress are constrained not only by the orientation and relative magnitude of the stresses within a given province but also by the manner of transition of the stress field from one province to another. Much of the modem pattern of stress in the western United States can be attributed to present transform motion and residual thermal and dynamic effects of Tertiary subduction along the western edge of the North American plate. Abrupt stress transitions around actively extending regions in the western United States probably reflect shallow sources of stress and anomalously thin lithosphere. Large areas characterized by a uniform stress field in the central and eastern United States suggest broad scale plate tectonic forces. In the midcontinent region, both ridge push and asthenospheric viscous drag resistance to lithospheric motion can explain the NE-SW compression in the cold, thick lithosphere of the craton, although drag-induced stress directions (resistance to absolute plate motion) correlate better with the data than do the ridge push directions. Asthenospheric counterflow models do not apply in this region, as predicted stress orientations are about 90 o off. A region of compression oriented approximately perpendicular to the continental margin and Appalachian fold belt is defined by the stress data along the Atlantic Coast. This NW-SE compression is in direct contrast with previous models predicting extension perpendicular to passive continental margins due to lateral density contrasts at the continental-oceanic crust interface. Ridge push forces, while capable of producing a component of compression across the coastal area, do not explain the observed orientation of the stress. Two speculative mechanisms are suggested to explain the observed orientations: (1) rotation of stress (or strain) due to anisotropic Appalachian basement structure and (2) flexural effects associated with erosion and isostatic rebound of the Appalachians.

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State of stress in the conterminous United States

Ages of late Mesozoic–early Cainozoic igneous rocks in south-western North America indicate a magmatic arc initiated near the continental margin, swept over 1,000 km northeastward, then swept back, all in 110 Myr. The sweep in and return are interpreted as due to flattening of a Benioff zone to < 15° followed by rapid collapse.

Cordilleran Benioff zones

Earth's glacial record and its tectonic setting

Summary The thick (∼40 km) slab of Hudsonian (>1750 Ma) continental crust that extends under western Canada from the Canadian Shield can be followed westward, on the basis of its distinctive magnetic anomalies, its influence on the Bouguer gravity values, the results of deep seismic refraction experiments, and the results of geomagnetic depth sounding of the deep electrical conductivity structure, to the Kootenay Arc. The Kootenay Arc is basically a W-facing monocline of crustal dimensions, across which the change in structural level involves an aggregate stratigraphic thickness of up to 20 km. It marks the western edge of the continental craton over which the displaced supracrustal rocks have been draped. Balanced structure sections of the thrust and fold belt, which take into consideration the deep crustal structure, as constrained by the geophysical data, show that: (i) in early Campanian time the continental crust that now lies beneath the western Rocky Mountains and the Purcell anticlinorium was covered with the platformal Palaeozoic to Upper Jurassic rocks and the exogeoclinal Mesozoic rocks that now form the northeasterly verging imbricate thrust slices of the eastern Rocky Mountains; (ii) the Cordilleran miogeocline developed outboard from the edge of the continental craton, on tectonically attenuated continental crust, or oceanic crust; and (iii) tectonic shortening of about 200 km in the supracrustal rocks in the Rocky Mountains must be balanced at a deeper level, W of the Kootenay Arc, by the shortening of the oceanic or attenuated continental crust. The net convergence between the Cordilleran magmatic arc and the autochthonous cover on the continental craton is a type of intra-plate subduction that was antithetic to the SW verging subduction zone marking the boundary of the North American Plate. The basement of the back-arc or marginal basin, in which the miogeocline formed, was consumed; but the adjoining continental margin was not. The foreland thrust and fold belt is a shallow subduction complex that was tectonically prograded over the margin of the continental craton, as the supracrustal cover scraped off the down-going slab was piled up against the overriding mass, and spread laterally eastward under its own weight.

The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains

Upper Precambrian and Lower Cambrian strata in western North America are exposed in a narrow slightly sinuous belt extending from Alaska and northern Canada to northern Mexico, a distance of 2,500 mi. Within this belt, the strata thicken from 0 ft on the east to 15,000 to 25,000 ft in areas 100 to 300 mi to the west. The basal unit of this sequence is a diamictite (conglomeratic mudstone) which is widely distributed but discontinuous and is generally considered to be of glacial origin. Overlying sedimentary rocks consist predominantly of siltstone, shale, argillite, quartzite, and conglomerate. Tholeiitic basalt forms thick and widespread units near the base of the sequence but is sparse higher in the section. The distribution pattern and lithologic characteristics of the upper Precambrian and Lower Cambrian sequence fit the recently developed concept that thick sedimentary sequences accumulate along stable continental margins subsequent to a time of continental separation. The depositional pattern of the sequence is unlike that of underlying rocks, a relation consistent with the idea of a continental separation cutting across the grain of previous structures. The pattern is, on the other hand, similar to that of overlying lower and middle Paleozoic rocks, suggesting that the diamictite and post-diamictite rocks were the initial deposits in the Cordilleran geosyncline. Thick units of volcanic rock near the bottom of the sedimentary sequence indicate volcanic activity related to the thinning and rifting of the crust during the continental separation.

Initial Deposits in the Cordilleran Geosyncline: Evidence of a Late Precambrian (<850 m.y.) Continental Separation

Recent stratigraphic studies in three widely separated localities in southeastern Idaho and western Utah have revealed a startling continuity of both individual rock units and of rock sequences over a distance of some 300 mi parallel to the strike of a late Precambrian and Cambrian depositional trough. Between 15,000 and 25,000 ft of beds were deposited in the axis of the trough, whereas only 1000 to 3300 ft of correlative rocks were laid down on the shelf to the east. In several areas a diamictite is present near the base of the sequence; this is underlain locally and overlain generally by argillites containing lenticular limestones and dolomites; these in turn are succeeded by quartzitic rocks containing a thick grayish-red to maroon unit—the Mutual Formation. In each area the sequence includes, at the top, quartzites typical of the basal Cambrian. Deposition in the basin was essentially continuous from late Precambrian into Cambrian time but was interrupted by uplift and erosion on the shelf. The hinge line of the ancient seaway is inferred to have coincided roughly with the present “Wasatch line,” but erosion prior to deposition of the Tintic Quartzite has removed most of the data needed to establish this with certainty. Rocks in each of the three areas described here in detail are regarded as allochthonous and appear to have been thrust eastward during the Sevier orogeny. A precise reconstruction of the sedimentary basin must therefore await not only additional stratigraphic studies in such areas as the Promontory Range of Utah and the Bannock and Malad Ranges of southern Idaho, but also final resolution of the structural events.

Nomenclature and Correlation of Some Upper Precambrian and Basal Cambrian Sequences in Western Utah and Southeastern Idaho

There has probably been about 8 km of crustal extension in a northwest-southeast direction across the Rio Grande rift near Albuquerque, New Mexico, since approximately 26 m.y. ago, giving an annual rate of about 0.3 mm/yr. This estimate is based on an interpretation that the normal faults bounding the graben decrease in dip downward and extend to a depth of about 10 to 15 km. Below that depth the crust is assumed to have undergone plastic deformation.

Rate of crustal extension across the Rio Grande Rift near Albuquerque, New Mexico

Copper sulfides and copper-iron sulfides have partly replaced or are closely associated with carbonaceous fossil plant material, mainly logs, in the Agua Zarca Sandstone Member of the Chinle Formation of Triassic age. Small amounts of native silver occur with the sulfides. Malachite, chrysocolla, and azurite fill the interstices and fractures in the clastic host rocks, forming irregular halos around the mineralized fossil plant material.Primary sulfide minerals, chalcocite, covellite, bornite, chalcopyrite, and pyrite, were probably deposited by ground-water solutions moving through paleochannel complexes in Triassic time shortly after deposition of the host rock; mineralization occurred where there was a favorable reducing environment provided principally by the carbonaceous material. The ultimate source of the copper may have been older deposits in the Precambrian rocks of the Uncompahgre highland in northern New Mexico and Colorado. The sulfide minerals were partly oxidized later, and copper in the form of malachite, chrysocolla, and azurite was disseminated in the adjacent host rock. Major faults post-date deposition of the sulfide minerals and can be expected to offset some of the mineralized paleochannels.Large-scale deposition of the sulfide minerals occurred at fossil logjams within paleochannels. Orientations of cross-bedding and fossil logs can be used to determine the trends of the paleochannels. These trends should be followed in subsurface exploration.

Strata-Bound Copper Deposits in Triassic Sandstone of Sierra Nacimiento, New Mexico

The Cordilleran foldbelt trends west-northwest through the southwestern corner of New Mexico and is characterized mainly by flat thrust faults and subordinate, closely compressed, overturned folds of Laramide age. Postorogenic Cenozoic volcanic and sedimentary rocks cover much of the region, and only scattered areas of preorogenic and synorogenic rocks are exposed; this cover, in addition to superimposed Basin-Range deformation, makes regional structural synthesis difficult. Apparently, however, the contact of the foldbelt with the platform on the north is marked by thrust faults exposed in the Florida, Victorio, and central Peloncillo Mountains. Yielding on the thrust is northward toward the foreland, with displacement as much as several miles. Anomalous minor thrusts in the Big Hatchet Mountains that appear to have yielded toward the southwest are of unknown origin. The regional distribution of thrusts suggests that the entire foldbelt in this region is underlain by thrusts, even though considerable parts of the preorogenic sedimentary section may have escaped deformation in some localities. In southwestern New Mexico, the zone separating the area of the foldbelt from the less deformed foreland on the north appears to mark the Texas lineament.

Tectonic Framework of Cordilleran Foldbelt in Southwestern New Mexico

Initial development of the Nacimiento uplift occurred during the Laramide orogeny by what may have been shear folding of the brittle Precambrian basement rocks and stretching of the incompetent overlying sedimentary strata along a monoclinal fold on the west side of the uplift. Antithetic and synthetic normal faults formed in the strata along the monocline as it developed. The normal faults and the fold were cut by the Nacimiento fault which is nearly vertical at depth but flattens upward and has westward movement of the hanging wall block over the San Juan basin. Where the Nacimiento fault is preserved at high structural and stratigraphic levels it is gently dipping and has normal faults in the hanging wall block. These longitudinal normal faults formed as the uplift was arched and stretched.

A model based on primary vertical uplift integrates the different stress fields that caused the complex sequence of monoclinal folding, normal faulting, and reverse faulting with concomitant tensional features in the hanging wall block.

Lee A. Woodward

Papers

Nomenclature and Correlation of Some Upper Precambrian and Basal Cambrian Sequences in Western Utah and Southeastern Idaho

Rate of crustal extension across the Rio Grande Rift near Albuquerque, New Mexico

Strata-Bound Copper Deposits in Triassic Sandstone of Sierra Nacimiento, New Mexico

Tectonic Framework of Cordilleran Foldbelt in Southwestern New Mexico

Nacimiento Fault and Related Structures, Northern New Mexico