Abstract Marine geological and geophysical data from Alpha Ridge in the Arctic Ocean are sparse because of thick perennial sea-ice cover, which prevents access by most surface vessels. Rare seismic data in this area, acquired largely from drifting ice-camps, had shown the hemipelagic drape that covers most of the ridge is highly disrupted within a large (>90 000 km 2 ) south central region. Here, evidence of pronounced seafloor erosion and debris flows infilling seafloor lows was previously interpreted to be the result of a possible bolide impact. In recent years, several icebreaker expeditions have successfully acquired multibeam bathymetry and sub-bottom profiler data in the western segment of this region. Analysis of these data reveals a complex seafloor morphology characterized by ridges and troughs, angular blocks and escarpments as well as seismic facies characterized by hyperbolic seafloor reflections, and convoluted to incoherent and transparent sub-bottom reflectivity. These features are interpreted as evidence of sediment mass movement with varying degrees of lateral transport deformation. At least two episodes of failure are interpreted based on the presence of both buried and surficial mass-transport features. As multiple events are interpreted, seismicity is the most plausible trigger mechanism rather than bolide impact.

Abstract The Colorado Plateau differs greatly from its neighboring physiographic provinces, the Rocky Mountains on the north and east, and the Basin and Range Province on the west and south. The Colorado Plateau is a huge (about 384,000 km 2 ), roughly circular region of many high plateaus and isolated mountains that encompasses large parts of Utah, Colorado, New Mexico, and Arizona. The plateau derives its name from the Colorado River, which drains at least 90 percent of its area (Fig. 1). The distinguishing features of the Colorado Plateau are: its considerable altitude, nearly all above 1,500 m; its nearhorizontal bedrock (steeply inclined beds are limited to the few great monoclines and the borders of certain uplifts); and its strong stepped landscapes, consisting of many cliff-like escarpments separated by wide, gentle slopes (the result of differential erosion of the generally flat-lying rocks). The plateau consists of six sections (Fenneman, 1931). Different bedrock stratigraphy and structure have profoundly affected the physiography and geomorphology of each section (Fig. 1). The northern section of the Colorado Plateau consists of the Uinta basin, a broad structural basin bounded on the north by the Uinta Mountains and on the south by the San Rafael swell (Figs. 1 and 2). The east-flowing Duchesne River and the west-flowing White River drain the basin, and both join the south-flowing Green River near Ouray, Utah. The Green River has cut Desolation Canyon where the river flows across the southern rim of the.

ABSTRACT The central Los Angeles basin represents the deepest part of a basin that apparently resulted from rapid and prolonged lithospheric thinning owing to extension between rotating blocks. Subsidence in this tectonic setting began about 18 Ma and presumably reflects isostatic adjustment to the thinning of the buoyant crust. Sediment starving in the period immediately following the initiation of rapid subsidence resulted in a deep water-filled basin that reached water depths in excess of 2 km during Pliocene time. Sedimentation accelerated immediately following the widespread extrusion of andesitic and basaltic volcanics about 16 Ma. Maximum tectonic subsidence, which may require 50% to 75% of lithospheric thinning under the central deep, is about 3 km depending on assumptions. This amount of thinning can be used to estimate the maximum time-temperature history of basin sediments. The pattem of subsidence is best explained by a model of crustal rotation between right-slip faults that results in both extension in the early development of the basin and compression in the later phase of basin development.

The broad uplift of the Basin and Range province can be described by a two-layer thinning model in which the crustal and subcrustal portions of the lithosphere are thinned by different amounts. Thinning of the lithosphere to the base of the crust can be accomplished by either asthenospheric diapir penetration or by sublithospheric erosion, both resulting in thermal expansion and uplift. The age of crustal extension in the Basin and Range indicates that conduction alone is not likely to be the mechanism for lithospheric heat input because of timing constraints. Application of the two-layer stretching model of Hellinger and Sclater (1983) suggests that the regional topographic features of the Basin and Range province can be largely explained using a sublithospheric thinning factor of γ = 1 and crustal stretching factors between β = 1 and β = 1.33, depending on location. Larger stretching factors are reasonable but may require the assumption of a thickened lithosphere prior to the onset of thinning. Many of the characteristics of the Basin and Range province, such as the location of its eastern boundary, fault geometry, and differences between its northern and southern sections, were strongly influenced by prior tectonic events and are therefore unique to western North America.

Although geologic and reflection seismic studies are making rapid strides in defining the geometry and indicating the processes involved in upper crustal extension of the Basin and Range province, fundamental processes in the deeper crust and the upper mantle are still poorly constrained. We summarize interrelated gravity, elevation, isostatic, and geologic evidence to suggest that the convective mass circuit of rifting is completed partly within the crust by intrusion of material of crustal density from the mantle. If this interpretation is correct, mechanical thinning of the crust by extension, especially in highly extended terranes of the Basin and Range province, is somewhat reduced by additions of new crust. Conceptual models for links between upper crustal faulting-stretching and deeper crustal igneous dilation are presented. The deeper crust has probably been profoundly changed by metamorphic reaction and partial melting.

Extension zones within continents have complex patterns of tectonic evolution. The Basin and Range Province of western North America provides an ideal location to study the mode of extension in continental regions. We have utilized numerical models to test two distinct geological models of extension that have been proposed for the Basin and Range: (1) a model in which extension takes place by uniform (or pure shear) stretching; and (2) a model in which extension occurs along discrete low-angle shear zones by a simple shear mechanism. These numerical models indicate that both styles of extension produce results generally consistent with observed heat flow, gravity, and elevation data. Distinctive patterns in these data are maintained primarily during the period of extension, implying that present day observations are dominantly a consequence of an ongoing process. The results further imply that the effects of present day extension will obscure the evidence of previous extensional episodes at least as far as the parameters of heat flow, elevation, and gravity are concerned.

Reflection seismic data in the Railroad, Diamond, Mary’s River, and Goshute valleys provide information on their structural development that cannot be deduced solely from outcrop and well data. These valleys contain Tertiary sediments that, in dip section, define asymmetrical basins each bounded along the eastern flank by a major listric normal fault with about 3.0–4.6 km (10,000–15,000 ft) of displacement. The western flanks are defined by gentle east-dipping ramps. Seismically, the surfaces of the listric faults are interpreted to dip westward and become bedding-parallel within the Paleozoic sequence, perhaps exploiting regionally recognized Mesozoic decollement surfaces. The Tertiary depocenters, adjacent to the faults, shifted from west to east with continued slippage through time, the greatest movement occurring in Miocene and post-Miocene time. In the strike direction, the basins are separated into at least two sub-basins by an east-west, structurally high axis. The axes are postulated to be the result of a tear fault associated with movement along the listric normal fault. The Tertiary stratigraphy varies between basins and between sub-basins in a given valley. All the basins contain Miocene and younger rocks; however, not all sub-basins contain the pre-Miocene sequence, suggesting a complex scheme of structural development.

Detailed earthquake studies throughout the transition zone between the Basin and Range (BR) and Colorado Plateaus (CP) provinces in central and southwestern Utah provide key observations relevant to (1) the subsurface geometry of seismically active faults, (2) the correlation of diffuse seismicity with geologic structure, and (3) the nature of a transitional stress state between the BR and CP provinces. Important new data in the form of three-dimensional earthquake distributions and numerous fault-plane solutions come from six field experiments in which temporary arrays of up to 13 portable seismographs were deployed to supplement a regional seismic network. Seismic slip predominates on fault segments of moderate (>30°) to steep dip—at least for small to moderate-sized earthquakes (magnitude < 5)—based on both fault-plane solutions and hypocentral distributions. Mean and median dips of seismic slip planes for normal- to oblique-slip fault-plane solutions in the study area and vicinity range from 49° to 57°. No convincing evidence has yet been found for seismic slip on either a downward-flattening or a low-angle normal fault in this region though such faults are known to be present. Low-angle structural discontinuities in the study area appear to play a fundamental role in separating locally intense upper-crustal seismicity above 6–8 km depth from less frequent background earthquakes at greater depth, down to about 15 km. Diffuse epicentral patterns result from block-interior microseismic slip and from superposed patterns of shallow upper-crustal seismicity and subjacent seismicity. If large surface-faulting earthquakes (magnitude 6½ to 7¾) nucleate at about 15 km depth in the study area, as observed elsewhere in the Intermountain region, then rupture pathways remain to be identified between deep nucleation points and existing surface fault scarps. Effective seismic surveillance will require precise resolution of focal depths to discriminate depth-varying seismicity. Fifty-three fault-plane solutions provide significant detail for mapping changes in upper-crustal stress orientation across the BR-CP transition. Important observations include (1) the alignment of horizontal principal stresses perpendicular and parallel to the BR-CP boundary, (2) average regional orientation of minimum principal stress within the transition in the 102°–282° direction, and (3) an eastward change through the transition from normal faulting to strike-slip faulting to mixed faulting, including compressional reverse faulting. Intermediate principal stress throughout the western and central part of the transition must be close in value to the maximum principal stress, and these two principal stresses interchange in orientation between vertical and a north-northeast-south-southwest horizontal direction.

The upper crust of the central Mojave Desert was extended and thinned during early Miocene time by three processes (1) low-angle normal faulting; (2) high-angle normal faulting in several episodes; and (3) extension fracturing. The first two processes appear to have affected the entire breadth of the extended area, but the third process was restricted to narrow zones. Processes 1 and 2 developed in at least two half-grabens that were kinematically linked by a transform fault. Each half-graben contains a family of similarly oriented high-angle normal faults that facilitated the extension of the upper crust. Evidence from one of the half-grabens suggests that the high-angle faults originally dipped at higher angles and were mechanically linked with a gently northeast-dipping normal fault (detachment).

In the western United States, a major Tertiary extensional orogeny is distinguished from the more recent and subdued Basin and Range disturbance. This orogeny, characterized by dynamic horizontal translations (i.e., >100% extension in the Great Basin) occurred during the Eocene (∼55–40 Ma) north of the Snake River Plain and during Oligocene-Miocene time (∼35–16 Ma) farther south. Tectonic processes at shallow crustal levels included widespread listric or rotational faulting, decoupling along flat detachment faults, pervasive tensional fracturing, and calc-alkaline magmatism that yielded copious volumes of volcanic rocks and systematically oriented (north-northwest to north-northeast striking) dike swarms, veins, and elongate plutons. At deeper levels, the extensional deformation produced more dikes, intrusions, and the gently dipping mylonitic rocks exposed in Cordilleran metamorphic core complexes (MCC). Radiometric age criteria and ductile, kinematic strain indicators in the deeper rocks coincide with equivalent features in the brittly extended rocks above. The MCC are found in a regional setting of long antiformal axes of north to north-northwest trends, and transform discontinuities striking northeast to west-northwest, roughly parallel and normal, respectively, to the elongation of the MCC. Tertiary deformation in the complexes is commonly overprinted on earlier, gently dipping, compressional shear or metamorphic fabrics of Mesozoic to Paleocene age. Many MCC exhibit positive gravity anomalies and contain a preponderance of mafic dikes. Deep-seated, mylonitic, normal fault zones have recently been cited to explain the Cordilleran MCC. This simple-shear explanation differs from the crustal stretching or boudinage model. Although attractive conceptually, the crustal shear-zone model in its present form has difficulty in sufficiently explaining described deformational fabrics and unique upper-plate lithologies restricted to the mylonitic complexes. Major displacements required of the model appear to be precluded by certain geometric constraints. Strain analysis has also been confused by superimposed compressional and extensional fabrics. New data from the Picacho MCC in southern Arizona support relatively shallow, in situ, Miocene mylonitization and detachment, and document the importance of pre-Oligocene low-angle deformation. Various extensional mechanisms are proposed to explain flat detachment faulting. Mylonitic rocks exposed in MCC are derived from a setting of high heat flow and intrusion; preestablished, flat, crustal anisotropism and fluid-induced strain softening. Textural, isotopic, and geochemical evidence suggests that deuteric fluids were locally derived from the lower plate as a result of intense intergranular strain. These fluids, which concentrated or ponded at the detachment interface, may have enhanced upward mylonite development and are believed to be the principal cause for hydrothermal, chloritic brecciation overlying the mylonites. The MCC were uplifted by isostatic response to upper-plate denudation, lower-plate attenuation, and magmatic upwelling from below. In terms of regional or plate tectonic setting, the extreme extension of the Tertiary orogeny is attributed to the incursion of hot asthenosphere into the Cordilleran crust above a segmented and sinking subduction slab. During the preceding Laramide orogeny, this oceanic slab had been driven shallowly under the North American crust with essentially no intervening mantle wedge. After about 40 Ma, dehydration fluids from the descending slab triggered magmatism throughout the Cordillera when the lower crust was contacted by hot asthenosphere. Mantle diapirism, converting upward and laterally, became the fundamental mechanism for crustal softening and extension. The metamorphic core complexes may therefore represent local sites where mafic subcrustal material penetrated highest in the crust, causing the most visible effects of attenuation. As such, the MCC can be visualized as small-scale analogs of the extended Cordillera. It is likely that flat, stacked, en echelon mylonitic zones exist at deeper levels throughout much of the western United States.