Abstract In this chapter we present an overview of the Intermountain seismic belt (ISB), a first-order feature of the Seismicity Map of North America (Engdahl and Rinehart, 1988). The ISB is a prominent northerly-trending zone of mostly shallow (<20 km) earthquakes, about 100 to 200 km wide, that extends in a curvilinear, branching pattern at least 1500 km from southern Nevada and northern Arizona to northwestern Montana (Fig. 1). Our study area, defined by the bounds of Figure 1, covers a sizable part of the western United States encompassing the ISB and is informally referred to herein as the Intermountain region. Contemporary deformation in the ISB is dominated by intraplate extension. Forty-nine moderate to large earthquakes (5.5 ≤ Ms ≤ 7.5) since 1900 and spectacular late Quaternary faulting with a predominance of normal to oblique-normal slip make the Intermountain region a classic study area for intraplate extensional tectonics. Information from the Intermountain region, relating for example to paleoseismology (Schwartz, 1987), seismotectonic framework (Smith and others, 1989), contemporary deformation from geodetic measurements and seismic moments of earthquakes (Savage and others, 1985; Eddington and others, 1987), and strong ground motion in normal-faulting earthquakes (Westaway and Smith, 1989a) has added significantly to understanding extensional seismotectonics worldwide. Particularly valuable contributions have come from field and seismological observations of two large normal-faulting earthquakes in the Intermountain region—the 1959 Hebgen Lake, Montana, earthquake (Ms = 7.5) and the 1983 Borah Peak, Idaho, earthquake (Ms = 7.3)—both described herein. Our basic intent in this chapter is to provide

Crustal structure of the eastern Basin-Range province and its transition to the Colorado Plateau-Rocky Mountain provinces has been influenced by several tectonic events: Precambrian margin rifting, Paleozoic sedimentation, late Mesozoic and early Cenozoic crustal compression and related thrust faulting, and middle to late Cenozoic extension and normal faulting. As a result of this complex history, the crust and upper mantle of this region is laterally heterogeneous across its 500-km east-west breadth. The crust is ~30 km thick beneath the central Basin-Range and increases eastward across the transition to more than 40 km beneath the Colorado Plateau and middle Rocky Mountains. Despite the lateral variation in crustal thickness, the upper mantle has a generally uniform P-wave velocity of 7.9 km/sec. On the basis of surface-wave analyses, the lithosphere is estimated to be 65 km thick beneath the central Basin-Range and increases eastward to 80 km beneath the Colorado Plateau. An anomalously low, apparent velocity layer (~7.5 km/sec), at a depth of ~25 km was identified in the Basin and Range-Colorado Plateau transition from unreversed refraction profiles. This layer has been interpreted as a high-velocity zone at the top of a mantle bulge; it may, however, represent an ~10-km mantle upwarp of 7.9-km/sec upper-mantle material with the low apparent velocities resulting from down-dip ray propagation. Seismic reflection profiling in the Basin and Range-Colorado Plateau-Rocky Mountain transition has been used principally to assess the geometry and structural style of Cenozoic basins and upper crustal faults. These seismic data reveal several asymmetric, eastward-tilted basins that are bounded by low- to high-angle (30° to 60°), planar to listric normal faults. An unusually widespread, 10° to 15° west-dipping reflection, identified as the Sevier Desert detachment, has been detected across a 190-km east-west width from near the surface in central Utah, westward to a depth of ~15 km near the Utah-Nevada border. This major structure extends over an area of 20,000+ km 2 and may have accommodated as much as 60 km of late Cenozoic crustal extension. The Basin and Range-Colorado Plateau-Rocky Mountain transition is also coincident with the southern Intermountain seismic belt where diffusely distributed epicenters occur along a 100- to 200-km-wide, north-south-trending zone. Where precise hypocenters have been mapped with detailed seismograph networks, background seismicity does not in general correlate with mapped faults. Source studies of three historic M7 + earthquakes in the Basin-Range have provided a hypothetical working model for large normal-faulting earthquakes in the region. These large events occurred on 40° to 60° planar normal faults and nucleated at midcrustal depths of ~15 km, near the maximum depth of background seismicity. However, there is an intriguing paradox between the geometries of the seismogenic faults, associated with large M7 + events, and the attitudes of some of the shallow normal faults identified from the seismic reflection data. Several faults identified on the reflection profiles reveal Quaternary planar, low-angle to listric normal faults that extend to depths of <6 km, unlike the deeper penetrating and steeper planar faults inferred from the large, M7 + , normal-faulting earthquakes. Rheologic models for this region of active extension show that the M7 + earthquakes occurred near the transition between the brittle upper crust and a quasi-plastic middle crust, suggesting that the larger events require large stress drops to relieve strain in a more ductile medium. Crustal extension rates have been derived from seismic moment tensors of historic earthquakes along the transition that vary from <1 to 5 mm/yr. An integrated east-west extension rate for the entire Basin-Range of ~10 mm/yr is similar to Quaternary extension rates determined from geological and other geophysical data. The deformation rates of the transition area, however, are significantly smaller than those along the San Andreas fault system, where contemporary deformation may exceed 50 mm/yr.

Abstract The Cordilleran sector within the conterminous United States extends from the offshore continental borderlands of the Pacific margin eastward as far as the Black Hills of South Dakota and the mountains of west Texas. It ranges from 800 to 1,600 km wide and is physiographically complex, consisting of high mountains, intervening lowlands, and plateaus that rise from the more gentle continental interior. The physiography of the Cordillera largely reflects the younger underlying structure, and the present orogenic belt has east and west limits coincident with its physiography. As physiographically defined, however, (Fig. 1) the U.S. Cordillera thus includes diverse older sedimentary, igneous, and tectonic elements varying widely in age, geographic distribution, and geologic significance. Included are the margin of the Precambrian crystalline craton; an extensive cratonic sedimentary sequence developed along a passive margin in latest Precambrian and early Paleozoic time; a collage of terranes accreted to North America intermittently from middle Paleozoic time through the end of Mesozoic time; the igneous and tectonic record of successive plate-convergence events, probably beginning with Antler deformation in middle Paleozoic time and culminating in a fully developed Andean margin late in Mesozoic time; eastward Laramide migration of igneous and tectonic features related to plate convergence in early Cenozoic time; and gradual termination of subduction-related activity and inception of extensional and transform tectonic regimes later in Cenozoic time. We have selected a time-slice organization for the syn-thesis of these diverse features. Ultimately, it is the grouping of events by their contemporaneity that permits an understanding of the kinematic and dynamic systems responsible for these events.

For more than 1,500 km along the western Cordillera of North America, late Cenozoic extensional faulting has produced block-faulted basin-range structure characterized by alternating elongate mountain ranges and alluviated basins. The faulting follows older geologic patterns, particularly those of Mesozoic and early Tertiary deformation and of early and middle Tertiary igneous activity. Basin-range structure is commonly inferred to represent either (1) blocks tilted along downward-flattening (listric) faults in which the upslope part of an individual rotated block forms a mountain and the downslope part a valley or (2) alternating downdropped blocks (grabens) that form valleys and relatively upthrown blocks (horsts) that form mountains. Such structure has been produced by extension estimated to be from 10% to 35% of the original width of the province and as much as 100% in specific areas. The province is characterized by anomalous upper mantle, thin crust, high heat flow, and regional uplift. Current theories on the origin of basin-range structure can be grouped loosely into four main categories. In the first, the structure is presumed to be related to oblique tensional fragmentation within a broad belt of right-lateral movement and distributed extension along the west side of the North American lithospheric plate. This motion was initiated by the collision of the East Pacific Rise with the North American plate, which brought together the North American and Pacific plates to form the right-lateral San Andreas transform fault system. The second theory relates extension to spreading caused by upwelling from the mantle behind an active subduction zone (back-arc spreading). The third theory relates the basin-range structure to spreading that resulted from presumed subduction of the East Pacific Rise beneath part of North America. The fourth theory relates the basin-range structure to plate motion caused by deep-mantle convection in the form of narrow mantle plumes. The combination of anomalous upper mantle, thin crust, high heat flow, regional uplift, and extension with a previous history of high heat generation can best be related to back-arc spreading. The spreading may have been accelerated by slackening of confining pressure after the destruction of the subduction system along western North America and may have been accompanied by right-lateral shear because of the development of the transform western margin of North America.

Motion of the Africa plate with respect to Tristan da Cunha since 135 m.y. B.P. and motion of the Pacific plate with respect to Hawaii since 80 m.y. B.P. are combined with relative motion between the North America and Africa plates and between the Kula-Farallon and Pacific plates to compute plate reconstructions and relative motion between the North American Cordilleran margin and Farallon-Kula-Pacific plates during Mesozoic-Cenozoic time. Cordilleran tectonic timing and distribution of petro-tectonic assemblages appear grossly consistent with inferred plate reconstructions and interactions. Major tectonic transition from Paleozoic modes characterized by rifting and by arcs colliding with the quiet continental margin to the Mesozoic-Cenozoic mode of active Andean-type continental margins coincides with initiation of disruption of Pangea and opening of the Atlantic Ocean-Gulf of Mexico in Late Triassic-Early Jurassic time. North America moved northwest‐ward during Jurassic-Early Cretaceous time, apparently overriding the Farallon plate from Alaska southward. Transition from the Sevier-Columbian orogeny to the Laramide orogeny about 80 m.y. ago coincides with the end of a major magnetic quiet period, initiation of separation of North America from Eurasia, and a general reorganization of plates. The Laramide orogeny (70 to 45 m.y. B.P.) progressed during more rapid westward motion of North America, accompanied by accelerated northeast-southwest convergence between the North America and Farallon plates. The end of the Laramide orogeny is broadly synchronous throughout the Cordillera and coincides with the age of the Hawaii-Emperor “elbow” and a drop in the North America-Farallon convergent rates. From 40 to about 20 m.y. B.P., vast ignimbrite eruptions in the southern Cordillera correlate with final subduction of the Farallon plate. Widespread basalt eruption, block faulting, and collapse of the Basin and Range province occurred since 20 m.y. B.P. and coincided with cessation of subduction, growth of the San Andreas-Queen Charlotte transform faults, and interaction between the North America and Pacific plates.

A new, simple Bouguer gravity map for that part of the United States west of long 109°W is examined in terms of its relationship to other geophysical and geological parameters. Unifying geophysical and tectonic characteristics define a large central province that has the Great Basin as its principal geographic element, but also includes parts of the Sierra Nevada, western Colorado Plateaus, Columbia Plateaus, and Middle and Northern Rocky Mountains. It is characterized by a high average elevation (>1.5 km), a low Bouguer gravity field (<-110 mgal) with a bilaterally symmetrical distribution of long-wavelength anomalies in its southern half (120°-opposed tracks of progressively outward-migrating silicic volcanism continue this symmetry in its northern half), high heat flow, the presence of two-thirds of all thermal springs in the conterminous United States, a concentration of Quaternary volcanic rocks at its east and west margins, and pervasive extensional faulting throughout. Much of the boundary of this geophysical province is moderately sharp, and parts of it cut across the interiors of classic physiographic provinces, as well as earlier geologic provinces. With the exception of the gravity and volcanic symmetry, none of these characteristics alone is unique to the province; uniqueness lies in their collective assemblage. The northern and southern ends of the province are not sharply defined; it narrows notably and some of its characteristics merge with those of adjoining regions both to the north and south. Long-wavelength gravity-anomaly patterns within the province are interpreted as reflecting extensional, thermomagmatic episodes in the late Cenozoic history of the lithosphere. Kinematics and patterns of faults, dikes, and geophysical anomalies suggest that east-west spreading of late Cenozoic age was preceded by significant northeast-southwest spreading of greater latitudinal extent in Miocene time. The latter is interpreted as back-arc spreading. Northwest-southeast oblique spreading commonly attributed to the Great Basin as a whole appears to be restricted largely to its western one-third and to the northeast-trending Humboldt zone. Most regional topographic features of the province are in approximate isostatic equilibrium. Compensating masses for some appear to be within the crust, which in the Great Basin is nearly coextensive with the lithosphere. Some are in the upper mantle. Load differences for individual basin and range structures are supported by the strength of the crust and lithosphere. Faults that block out basins and ranges do not penetrate deeply into the crust, but tend to dip less steeply with depth. This interpretation is supported both by maximum earthquake focal depths and by the observed local response to surface loading. Geothermal gradients and material properties suggest that lateral extension below depths of 15 to 20 km probably takes place by plastic flow aided perhaps by pervasive injection of basaltic magma. Because the lithosphere of the Great Basin is thin, it probably does not greatly modify the temperature field of the upper asthenosphere. For this reason the origin of compensating masses and major regional gravity gradients is thought to be the complex sum of (1) lateral temperature distributions in the lithosphere and asthenosphere, (2) distribution of Cenozoic intrusive masses reflecting earlier thermal events, (3) high temperature metamorphism related to both injection and heating of the crust, and (4) variations in the degree of extension and resultant thickness of both lithosphere and crust. Volcanic activity extended across the entire province in Cenozoic time, but is youngest at the east and west margins. In this aspect and in the details of heat flow, topography, broadly distributed extension, and the symmetry of its geophysical anomalies, the region contrasts sharply with both oceanic spreading ridges and major intracontinental graben systems. The difference may be attributed to active, as opposed to passive, spreading processes. Those features of the western Cordillera not related directly to subduction and regional dextral shear are interpreted as the products of lithospheric heating, injection, uplift, and basal traction resulting from the rise and divergent flow of hot, asthenospheric mantle. Associated phenomena include thermal tumescence of the lower lithosphere and brittle faulting of the upper lithosphere. A “hot spot” of very large dimensions is thus identified near the western, transform boundary of the North American plate. Its proximity to this boundary has led to a complex, episodic and cyclic interaction of two profound stress fields, producing young structures on the west indicative of both thermal doming and dextral shear. To the east the principal deformation is simple east-west extension resulting from rapid spreading that accompanied major regional doming and collapse.

A residual aeromagnetic map of Idaho, western Montana, western Wyoming, southwestern Oregon, Nevada, Utah, western Colorado, and northern Arizona illustrates magnetic patterns that are related to regional geology. The magnetic map provides useful information on the development of the crust in this region of the western Cordillera since Precambrian time. A major feature of the map is a broad zone extending from southern Nevada to northern Idaho, where magnetic anomalies from basement rock are not apparent. This feature is here named the “quiet basement zone.” To the west of this zone abundant magnetic anomalies are produced by Phanerozoic intrusive and extrusive igneous rocks. West-trending zones of magnetic anomalies in western Utah and Eastern Nevada contain abundant igneous rocks and most of the known mineral resources of this region. The major magnetic anomalies in the area east of the northern part of the Basin and Range province and east of the overthrust belts in southeastern Idaho and western Montana reflect lithologic contrasts in the Precambrian basement with prominent northeast and northwest trends. A magnetic high in north-central Nevada and another over the western Snake River Plain suggest north-northwest-trending Miocene rifts. The eastern Snake River Plain and the Yellowstone caldera are part of a much more extensive northeast-trending feature here called the Humboldt zone. In east-central Idaho, Tertiary intrusive and extrusive igneous rocks produce large magnetic highs. The Idaho batholith has little magnetic expression, but the Boulder batholith and related volcanic rock of western Montana produce a major magnetic high.