On the Colorado Plateau of southwestern Utah, the Lower Jurassic Glen Canyon Group comprises, in ascending order, the Moenave and Kayenta Formations and the Navajo Sandstone. In southern Nevada and southeastern California, the lithostratigraphic equivalent of the Navajo Sandstone is the Aztec Sandstone. In southern Nevada, the Aztec Sandstone is conformably underlain by four informally recognized stratigraphic units (A-D) of the undifferentiated Moenave and Kayenta Formations. The Glen Canyon Group unconformably overlies the Upper Triassic Chinle Formation above a regional unconformity. In addition to the Petrified Forest and Shinarump Members, the Chinle Formation contains a distinctive limestone-pebble conglomerate at its base. Using the Aztec Sandstone as a distinctive reference unit, the Glen Canyon Group and its relation to underlying and overlying lower Mesozoic depositional sequences are traced southwestward from the Las Vegas extensional domain, along the eastern edge of the relatively unextended Las Vegas Range-Spring Mountains block, into the Jurassic arc terrane of the eastern Mojave Desert. Southwestward, the regional unconformity at the base of the Glen Canyon Group truncates progressively older strata into the arc terrane. Although Middle Jurassic strata have been erosionally or tectonically removed from the Las Vegas extensional basin, volcanic-clast-bearing marginal marine facies of the Middle Jurassic Carmel Formation are tentatively correlated with silicic volcanic, volcaniclastic, and epiclastic rocks of the southern end of the Spring Mountains extensional domain. Stratigraphic and facies boundaries in lower Mesozoic strata potentially serve as important strain markers to test models of Cenozoic extension. Restoration of structural blocks containing lower Mesozoic outcrops to their pre-Tertiary positions is based on restoration of the Las Vegas Range-Spring Mountains block to its preextension position. The reconstruction reveals that (1) the limestone-pebble conglomerate at the base of the Chinle Formation is truncated on the east by the north-south-trending Vermilion Cliffs paleovalley; (2) the undifferentiated Moenave and Kayenta Formations were deposited in a north-south-trending, incipient foreland basin that deepened to the north; (3) alluvial fans were shed northeastward, at right angles to the Triassic paleoslope, into this basin from the arc terrane; and (4) volcanic centers lying east of the present Colorado River served as the source of volcanic clasts in the Carmel Formation.

Early Mesozoic arc magmatism of the southern Sierra Nevada region records the onset of plate convergence–driven magmatism resulting from subduction initiation near the end of Permian time along a prior transform margin. We provisionally adopt the term California-Coahuila transform for this complex boundary transform system, which bounded the southwest margin of the Cordilleran passive margin, its offshore marginal basin, and fringing island arc. In Pennsylvanian–Early Permian time, this transform cut into the arc-marginal basin and adjacent shelf system, calved off a series of strike-slip ribbons, and transported them differentially southward through ∼500–1000-km-scale sinistral displacements. These strike-slip ribbons constitute the principal Neoproterozoic–Paleozoic metamorphic framework terranes for the superposed Mesozoic batholithic belt in the Sierra Nevada and Mojave plateau regions. The southern Sierra Nevada batholith intruded along the transform truncation zone where marginal basin ribbons were juxtaposed against the truncated shelf. Strike-slip ribbons, or blocks, liberated from the truncated shelf occur today as the Caborca block in northwest Mexico, and possibly parts of the Chortis block, farther south. The oldest arc plutons in the Sierra region were emplaced between 256 and 248 Ma, which matches well with ca. 255 Ma high-pressure metamorphism recorded in the western Sierra Foothills ophiolite belt, interpreted to approximate the time of subduction initiation. The initial phases of arc plutonism were accompanied by regional transpressive fold-and-thrust deformation, kinematically marking the transition from transform to oblique convergent plate motion. Early arc volcanism is sparsely recorded owing to fold-and-thrust–driven exhumation having accompanied the early phases of arc activity. By Late Triassic time, the volcanic record became quite prolific, owing to regional subsidence of the arc into marine conditions, and the ponding of volcanics in a regional arc graben system. The arc graben system is but one mark of regional suprasubduction-zone extension that affected the early SW Cordilleran convergent margin from Late Triassic to early Middle Jurassic time. We interpret this extension to have been a dynamic consequence of the subduction of exceptionally aged Panthalassa abyssal lithosphere, which is well represented in the Foothills ophiolite belt and other ophiolitic remnants of the SW Cordillera. Middle and Late Jurassic time was characterized by important tangential displacements along the SW Cordil-leran convergent margin. In Middle Jurassic time, dextral impingement of the Insular superterrane intra-oceanic arc drove a migrating welt of transpressional deformation through the SW Cordillera while the superterrane was en route to its Pacific Northwest accretionary site. Dextral transtensional spreading in the wake of the obliquely colliding and translating arc opened the Coast Range and Josephine ophiolite basins. In Late Jurassic time, a northwestward acceleration in the absolute motion of the North American plate resulted in an ∼15 m.y. period of profound sinistral shear along the Cordilleran convergent margin. This shear is recorded in the southern Cordillera by the Mojave-Sonora megashear system. Late Jurassic intrusive units of the southern Sierra region record sinistral shear during their magmatic emplacement, but we have not observed evidence for major Late Jurassic sinistral displacements having run through the Sierran framework. Possible displacements related to the megashear in the California to Washington regions are likely to have: (1) followed preexisting transforms in the Coast Range ophiolite basin and (2) been accommodated by oblique closure of the Josephine ophiolite basin, and the northern reaches of the Coast Range ophiolite basin, proximal to the southern Insular superterrane, which in Late Jurassic–earliest Cretaceous time was obliquely accreting to the inner Cordillera terranes of the Pacific Northwest.

The source of volcanic material in the Upper Triassic Chinle Formation on the Colorado Plateau has long been speculated upon, largely owing to the absence of similar-age volcanic or plutonic material cropping out closer than several hundred kilometers distant. These strata, however, together with Upper Triassic formations within El Antimonio and Barranca Group sedimentary rocks in northern Sonora, Mexico, yield important clues about the inception of Cordilleran magmatism in Triassic time. Volcanic clasts in the Sonsela Member of the Chinle Formation range in age from ca. 235 to ca. 218 Ma. Geochemistry of the volcanic clasts documents a hydrothermally altered source region for these clasts. Detrital zircons in the Sonsela Member sandstone are of similar age to the clasts, as are detrital zircons from the El Antimonio and Barranca Groups in Sonora. Most noteworthy about the Colorado Plateau Triassic zircons, however, are their Th/U ratios, which range from ~1 to 3.5 in both clast and detrital zircons. Thorium/uranium ratios in the Sonoran zircons, in contrast, range from ~0.4 to ~1. These data, together with rare-earth-element geochemistry of the zircons, shed light on likely provenance. Geochemical comparisons support correlation of clasts in the Sonsela Member with Triassic plutons in the Mojave Desert in California that are of the same age. Zircons from these Triassic plutons have relatively low Th/U ratios, which correspond well with values from El Antimonio and Barranca Group sedimentary rocks, and support derivation of the strata, at least in part, from northern sources. The Sonsela Member zircons, in contrast, match Th/U values obtained from Proterozoic through Miocene volcanic, volcaniclastic, and plutonic rocks in the eastern and central Mojave Desert. Similarly, rare-earth-element compositions of zircons from Jurassic ignimbrites in the Mojave Desert, though overlapping those of zircons from Mojave Desert plutons, also closely resemble those from Sonsela Member zircons. We use these data to speculate that erosion of Triassic volcanic fields in the central to eastern Mojave Desert shed detritus that became incorporated into the Chinle Formation on the Colorado Plateau.

ABSTRACT A growing body of evidence suggests that continental arc lower crust and underlying mantle wedge assemblages native to the Mojave Desert (i.e., the southern California batholith) were displaced eastward during Laramide shallow-angle subduction, and reattached to the base of the Colorado Plateau Transition Zone (central Arizona) and farther inboard. On this field trip, we highlight two xenolith localities from the Transition Zone (Camp Creek and Chino Valley) that likely contain remnants of the missing Mojave lithosphere. At these localities, nodules of garnet clinopyroxenite, the dominant xenolith type at both studied localities, yield low jadeite components in clinopyroxene, chemically homogeneous “type-B” garnet, and peak conditions of equilibration at 600–900 °C and 9–28 kbar. These relations strongly suggest a continental arc residue (“arclogite”), rather than a lower-plate subduction (“eclogite”), origin. Zircon grains extracted from these nodules yield a bimodal age distribution with peaks at ca. 75 and 150 Ma, overlapping southern California batholith pluton ages, and suggesting a consanguineous relationship. In contrast, Mesozoic and early Cenozoic igneous rocks native to SW Arizona, with age peaks at ca. 60 and 170 Ma, do not provide as close a match. In light of these results, we suggest that Transition Zone xenoliths: (1) began forming in Late Jurassic time as a mafic keel to continental arc magmas emplaced into the Mojave Desert and associated with eastward subduction of the Farallon plate; (2) experienced a second ca. 80–70 Ma pulse of growth associated with increased magmatism in the southern California batholith; (3) were transported ~500 km eastward along the leading edge of the shallowly subducting Farallon plate; and (4) were reaffixed to the base of the crust at the new location, in central Arizona. Cenozoic zircon U-Pb, garnet-whole rock Sm-Nd, and titanite U-Pb ages suggest that displaced arclogite remained at elevated temperature (>700 °C) for 10s of m.y., following its dispersal, and until late Oligocene entrainment in host latite. The lack of arclogite and abundance of spinel peridotite xenoliths in Miocene and younger mafic volcanic host rocks (such as those at the San Carlos xenolith locality), and the presence of seismically fast and vertically dipping features beneath the western Colorado Plateau, suggest that arclogite has been foundering into the mantle and being replaced by upwelling asthenosphere since Miocene time.

The Late Jurassic (157–150 Ma) Morrison Formation of the Western Interior of the United States contains abundant altered volcanic ash. On the Colorado Plateau, this formation accumulated behind and downwind of a subduction-related volcanic arc along the western margin of North America. The ash in these distal fallout tuffs probably drifted eastward from coignimbrite ash clouds related to collapse calderas. Altered volcanic ash is particularly abundant in the Brushy Basin Member of the upper part of the Morrison Formation. In one 110-m-thick section in eastern Utah, 35 separate beds were deposited in a 2.2 m.y. period. Alteration occurred when glassy volcanic ash fell into fluvial and lacustrine environments, where it was diagenetically altered to various mineral assemblages but most commonly to smectitic clay. Periodically, ash fell into saline, alkaline lakes, and diagenetic alteration of the glassy ash produced a crudely zoned deposit on the Colorado Plateau. Altered volcanic ash beds in the outermost part of the lacustrine deposits are argillic (with smectitic clay), whereas zeolitic (clinoptilolite, analcime) and feldspathic (K-feldspar and albite) alteration dominates the interior zones. Feldspathic ash layers contain secondary silica, and consequently immobile element (e.g., Al, Ti, and high field strength elements) abundances were strongly diluted in these rocks. In contrast, the argillic ash beds experienced strong SiO 2 depletion, and, as a result, they are enriched in the relatively immobile elements. The compositions of the zeolitic ash beds are intermediate between these two extremes and experienced the least alteration. As a result of these changes, immobile element concentrations are less reliable than ratios for determining the original magmatic composition of the ash. Most of the altered ash (regardless of type) was also depleted in water-soluble elements like the alkalies, U, and V. The latter two elements were oxidized during diagenesis of the ash, became soluble, and were partially leached away by groundwater. Locally, U and V in groundwater were reduced upon contact with organic materials and formed important ore deposits. Several aspects of the mineralogy and geochemistry of the altered volcanic ash beds yield information about their original magmatic compositions. The volcanic ash beds typically have small phenoclasts of quartz, sanidine, plagioclase, biotite, zircon, apatite, and Fe-Ti oxides. Titanite is present in ∼40% of the ash beds; pyroxene and amphibole were found in less than 5%. Phenocryst assemblages, mineral compositions, inferred high f O 2 , rare earth element patterns, and immobile element ratios all suggest the parent magmas for the altered tuffs were subduction-related dacites and rhyolites. Small numbers of tuffs have Fe-rich biotite, amphibole, and/or clinopyroxene; both pyroxene and amphibole are alkali rich. These tuffs lack titanite, but some contain anorthoclase and F-rich apatite. Combined with enrichments in Nb and Y, these features show some tuffs had an A-type character and were related to some type of within-arc extension. Paleowind directions, and distribution, radiometric ages, and compositions of the volcanic ash beds and of plutons in the western United States suggest that the most likely eruption sites were in the subduction-related Jurassic magmatic arc, which extended across western Utah and central Nevada and southward into the Mojave of California and southern Arizona (present-day coordinates). Pb isotopic compositions show that at least some of the ash was erupted from magma systems (now exposed as plutons) in the Mojave Desert. We conclude that a brief ignimbrite flare-up from 157 to 150 Ma, but focused on the time period from 152 to 150 Ma, in this region may have been driven by slab steepening and conversion to a strike-slip boundary after a preceding phase of folding and thrusting. The presence of ash beds with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was geologically a transitional time in North America when subduction was changing to transtensional movement along the western plate boundary.

ABSTRACT The Laramide foreland belt comprises a broad region of thick-skinned, contractional deformation characterized by an anastomosing network of basement-cored arches and intervening basins that developed far inboard of the North American Cordilleran plate margin during the Late Cretaceous to Paleogene. Laramide deformation was broadly coincident in space and time with development of a flat-slab segment along part of the Cordilleran margin. This slab flattening was marked by a magmatic gap in the Sierra Nevada and Mojave arc sectors, an eastward jump of limited igneous activity from ca. 80 to 60 Ma, a NE-migrating wave of dynamic subsidence and subsequent uplift across the foreland, and variable hydration and cooling of mantle lithosphere during slab dewatering as recorded by xenoliths. The Laramide foreland belt developed within thick lithospheric mantle, Archean and Proterozoic basement with complex preexisting fabrics, and thin sedimentary cover. These attributes are in contrast to the thin-skinned Sevier fold-and-thrust belt to the west, which developed within thick passive-margin strata that overlay previously rifted and thinned lithosphere. Laramide arches are bounded by major reverse faults that typically dip 25°–40°, have net slips of ~3–20 km, propagate upward into folded sedimentary cover rocks, and flatten into a lower-crustal detachment or merge into diffuse lower-crustal shortening and buckling. Additional folds and smaller-displacement reverse faults developed along arch flanks and in associated basins. Widespread layer-parallel shortening characterized by the development of minor fault sets and subtle grain-scale fabrics preceded large-scale faulting and folding. Arches define a regional NW- to NNW-trending fabric across Wyoming to Colorado, but individual arches are curved and vary in trend from N-S to E-W. Regional shortening across the Laramide foreland was oriented WSW-ENE, similar to the direction of relative motion between the North American and Farallon plates, but shortening directions were locally refracted along curved and obliquely trending arches, partly related to reactivation of preexisting basement weaknesses. Shortening from large-scale structures varied from ~10%–15% across Wyoming and Colorado to <5% in the Colorado Plateau, which may have had stronger crust, and <5% along the northeastern margin of the belt, where differential stress was likely less. Synorogenic strata deposited in basins and thermochronologic data from basement rocks record protracted arch uplift, exhumation, and cooling starting ca. 80 Ma in the southern Colorado Plateau and becoming younger northeastward to ca. 60 Ma in northern Wyoming and central Montana, consistent with NE migration of a flat-slab segment. Basement-cored uplifts in southwest Montana, however, do not fit this pattern, where deformation and rapid inboard migration of igneous activity started at ca. 80 Ma, possibly related to development of a slab window associated with subduction of the Farallon-Kula Ridge. Cessation of contractional deformation began at ca. 50 Ma in Montana to Wyoming, followed by a southward-migrating transition to extension and flare-up in igneous activity, interpreted to record rollback of the Farallon slab. We present a model for the tectonic evolution of the Laramide belt that combines broad flat-slab subduction, stress transfer to the North American plate from end loading along a lithospheric keel and increased basal traction, upward stress transfer through variably sheared lithospheric mantle, diffuse lower-crustal shortening, and focused upper-crustal faulting influenced by preexisting basement weaknesses.

In an effort to characterize the crustal structure of northwestern Mexico (and constrain the Mojave-Sonora megashear) we studied the Magsat magnetic anomalies from that area. Published anomaly maps covering this area include an extensive positive anomaly covering the southern United States, a positive anomaly over the southern half of the Baja California peninsula, and a magnetic low in between. We interpreted a magnetic profile over these anomalies, focusing on its tectonostratigraphic terrane nature. The profile was further constrained by crustal thicknesses from seismological studies and heat flow data. In our model the Cochimi terrane and the North American craton (Colorado Plateau and southern Basin and Range) are characterized by high magnetic susceptibility in agreement with the mafic nature of their corresponding crusts. The Yuma and the Seri terranes have lower magnetic susceptibilities as expected from their felsic to basic crustal nature. The oceanic crust from the Gulf of California is modeled with a low magnetic susceptibility value due to the high heat flow observed at the extensional basins. Our model satisfies the presence of a subvertical contact between crusts with contrasting magnetic signatures of the Seri terrane (comprising the Caborca subterrane) and the southwestern sector of the North American craton. The Mojave-Sonora megashear itself is below the resolution of the Magsat data. Nevertheless, our model implies that because of their different magnetic signatures, the crystalline basements from the southern United States and northern Mexico (Seri terrane) are of different nature, which does not support the continuity of the North American craton into northwestern Mexico.

The Pacific Coastal States form a complex geologic environment in which the crust and lithosphere have been continuously reworked. We divide the region tectonically into the southern transform regime of the San Andreas fault and the northern subduction regime, and summarize the geophysical framework with contour maps of crustal thickness, lithospheric and seismicity cross sections, and results from site-specific geophysical studies. The uniformity of crustal thickness (30 ± 2 km) in southern California is remarkable, and appears to be primarily the result of crustal extension in the Mojave Desert and ductile shear of the lower crust along the plate transform boundary. Southern California seismicity defines a broad zone of deformation that extends from the Borderland to the Mojave Desert (about 300 km). The geophysical framework of central and northern California records magmatism and accretion associated with the Mesozoic and Cenozoic subduction, late Cenozoic transform faulting, and in the Basin and Range to the east, extension. The crust thickens from about 20 km at the coast to as much as 55 km in the Sierra Nevada, and thins to about 30 km in the Basin and Range. Cross sections of the crust show that seismic velocities and densities vary significantly over short distances perpendicular to the coast, reflecting processes that include the accretion of oceanic sediments and igneous crust, and significant lateral motion of crustal blocks. Maximum hypocentral depths in central California become deeper as the crust thickens to the west, but seismicity is low beneath the Great Valley and Sierra Nevada, which together appear to form a relatively undeforming block. The lower crust of the Pacific Coastal States has a high average seismic velocity (6.7 km/sec or greater), which probably is the product of tectonic underplating of oceanic crust and/or magmatic underplating by a basaltic melt. The geophysical framework of the subduction regime is dominated by the subduction of the Gorda and Juan de Fuca plates, arc magmatism in the Cascade Range, and plateau volcanism and rifting in the back arc. As defined by earthquake hypocenters, the Juan de Fuca plate dips at a shallow angle (3°) within 50 km of the trench, increases to 10° beneath the continental shelf and coastal province, and plunges more steeply (25° dip) a short distance west of the Cascade Range. Whereas a true continental Moho exists from the Cascade Range to the east, the Moho is that of the subducting oceanic lithosphere west of the range. Crustal thickness increases from about 18 km at the coast to about 42 km beneath the Cascades Range, a distance of about 200 km. The crustal velocity structure and crustal thickness of the Cascades Range is relatively uniform along its axis. The velocity structure shows high velocities (greater than 6.5 km/sec) at all depths greater than 10 km, indicating rocks of an intermediate-to-mafic composition, and a relatively low upper-mantle velocity of 7.7 ± 0.1 km/sec, indicating high temperatures. Seismological studies at the volcanic centers of the Cascades indicate that the dimensions of subsurface magmatic systems are small, on the order of a few kilometers. Some 1 to 6 km of Miocene and younger basaltic extrusives cover much of the back arc, thereby obscuring most of the pre-Miocene geology. However, geophysical data demonstrate the importance of Mesozoic compression and Cenozoic (particularly Eocene) extension, accompanied by magmatic underplating of the crust.