An extensive review of geologic and tectonic features of western North America suggests that the interaction of oceanic plates with the continent follows a broad cyclical pattern. In a typical cycle, periods of rapid subduction (7–15 cm/yr), andesitic volcanism, and trench-normal contraction are followed by a shift to trench-normal extension, the onset of voluminous silicic volcanism, formation of large calderas, and the creation of major batholiths. Extension becomes pervasive in metamorphic core complexes, and there is a shift to fundamentally basaltic volcanism, formation of flood basalts, widespread rifting, rotation of terranes, and extensive circulation of fluids throughout the plate margin. Strike-slip faulting becomes widespread with the creation of new tectonostratigraphic terranes. A new subduction zone forms and the cycle repeats. Each cycle is 50–80 m.y. long; cycles since the Triassic have ended and begun at approximately 225, 152, 92, 44, and 15 Ma. The youngest two cycles are diachronous, one from Oregon to Alaska, the other from central Mexico to California. The transitions from one cycle to the next cycle are characterized by rapid and pervasive changes termed, in this chapter, “major chaotic tectonic events.” These events appear to be related to the necking or breaking apart of the formerly subducted slab at shallow depth, the resulting delamination of the plate margin, and the onset of a new subduction cycle. These are times of the most rapid apparent and true polar wander of the North American plate, when the plate appears most free to move relative to surrounding plates and relative to the mantle below the asthenosphere. In western North America, magmatism and tectonics during the Jurassic period are quite similar to magmatism and tectonics since mid-Cretaceous time except strike-slip faulting shifted in sense from left lateral to right lateral.

Geologic and isotopic data suggest a depositional link between granitic plutons of northern Stikinia and the adjacent Jurassic Laberge Group sedimentary rocks of the Whitehorse Trough. U/Pb zircon dating of granitic cobbles in Lower Jurassic Laberge Group conglomerate of the Mesozoic Whitehorse Trough suggests clast derivation from a source terrane containing Late Triassic (ca. 215–208 Ma) granitic plutons. Initial strontium ratios are primitive and paleocurrent data show that detritus comprising Laberge Group conglomerate was westerly derived. A string of small, isotopically unevolved plutons of Late Triassic to earliest Jurassic age intrude the Lewes River volcanic arc rocks along the western margin of the Whitehorse Trough and are interpreted as the probable western source for the clasts. Dates and initial strontium values of the clasts rule out previous suggestions that the clasts were derived from the Early Jurassic Klotassin suite batholiths which intrude Nisling Terrane rocks. The deposition of extremely coarse Early Jurassic boulder conglomerate on top of Late Triassic carbonate facies represents a dramatic change in depositional style. Sudden uplift incised a Lower Jurassic erosional disconformity into arc and arc-flanking shelf deposits along the western margin of the Whitehorse Trough. Episodic uplift periodically maintained extreme paleotopographic relief in the arc, sufficient to prograde coarse-grained debris flows into the basin and erode the plutonic roots of the arc throughout Early and early Middle Jurassic time. Initial Sr isotopic ratios of the granitic clasts average 0.7045 and suggest that they were derived from unevolved island-arc magmas. U/Pb systematics do not indicate the presence of inherited zircon xenocrysts. These data suggest that the plutons which acted as the source for the clasts had limited, if any, interactions with isotopically evolved continental crust and likely intruded oceanic or transitional crust. The source of metamorphic clasts in Laberge Group conglomerate is presumed to be the Nisling Terrane, suggesting that Nisling and Stikinia were linked by Middle Jurassic time.

Foliated intrusions of the Early Jurassic Aishihik Plutonic Suite (APS), including the Aishihik Batholith, have been included in Stikinia and interpreted as allochthonous with respect to adjacent terranes, including the Nisling and Yukon-Tanana Terranes. The Nisling Terrane was thought to lack Early Jurassic igneous rocks. However, the Aishihik Batholith, a single plutonic body that crystallized at ca. 187 Ma, forms a west-tapering lopolith or sheet-like body that intrudes deformed strata of the Nisling Terrane. The batholith displays a margin-parallel foliation, defined by primary magmatic grains including feldspar and hornblende, that is considered to be magmatic. A parallel solid-state fabric overprints the magmatic foliation and fabric in wall rocks within 100 m of the batholith along the west (lower) margin of the batholith. This fabric is defined by gneissic banding, annealed mylonite, and by discrete shear bands. Shearing occurred at high temperatures, probably close to the granite solvus, as indicated by the breakdown of hornblende to biotite, the recrystallization of plagioclase feldspar, and by associated migmatite. Shear indicators are consistent with top-to-the-west displacement. The solid-state fabric postdates peak regional deformation of the Nisling Terrane and is inferred to have developed during late stage ballooning of the intrusion. A model of intrusion of the Nisling Terrane by the Aishihik Batholith, with subsequent ballooning of the batholith, is consistent with the lopolithic shape of the batholith, the distribution of solid-state fabrics, the shear sense and near-solvus temperatures during solid-state deformation, the presence of xenoliths similar to that of the Nisling Terrane in the batholith, and the development, in the Nisling Terrane, of a hot-side-up aureole beneath the batholith. Because the Aishihik Batholith intrudes the Nisling Terrane, (1) the APS cannot be considered diagnostic of Stikinia, and (2) the Nisling Terrane cannot be considered as lacking Early Jurassic igneous rocks. The APS may represent part of an igneous overlap assemblage that links together terranes of the Intermontane belt. Alternatively, Early Jurassic intrusions may have developed in response to the subduction of oceanic crust separating some of the Intermontane terranes.

Three temporally distinct Lower Jurassic volcanic successions are exposed in the Spatsizi River area of the Stikine Terrane (Stikinia), northern British Columbia. U-Pb zircon dating reveals that (1) the age of the oldest succession, the Griffith Creek volcanics, is ca. 206 Ma; and (2) the middle unit, the Cold Fish Volcanics, is ca. 194 Ma, consistent with its Early Pliensbachian biostratigraphic (ammonite) age. The age of the youngest unit, the Mount Brock volcanics, is Early to Middle Toarcian, also on the basis of ammonites. The Griffith Creek volcanics were folded along with older strata prior to deposition of the Cold Fish succession. An inherited component of zircon in some of the analyzed zircon fractions indicates that Precambrian basement or Lower Paleozoic strata of Precambrian provenance was present in the Stikinian subsurface. An evolved basement is also suggested by fluorine-rich compositions of high-silica Cold Fish rhyolites. The three volcanic successions were deposited in the Early Jurassic along the northeastern margin of the Hazelton Trough, a long-lived marine basin which lay between mainly subaerial volcanic arcs on the eastern and western sides of Stikinia. In Early Pliensbachian to Middle Toarcian time, strata in the Spatsizi River area recorded at least 2.6 km of subsidence, consistent with protracted regional subsidence in and around the trough. Extensional tectonism, and to a lesser degree thermal subsidence and local volcanic foundering, are considered the main causes of subsidence.

We report, and tabulate with previous dates, 40 new U-Pb zircon and 14 K-Ar biotite and hornblende dates from the southern Coast Belt of British Columbia. The new dates, mainly from plutons of the Coast Plutonic complex, represent crystallization (U-Pb) and cooling (K-Ar, Rb-Sr) ages. Zircons from most of the rocks are concordant to mildly discordant, the latter commonly indicating minor Pb loss with young lower intercepts. U-Pb data for several rocks also indicate the possibility of inheritance. Pre–Coast Plutonic complex volcanic rocks (187–167 Ma) correlate with rocks of the Early to Middle Jurassic Bonanza-Island Intrusion arc of Wrangellia. Early Coast Plutonic complex plutons (167–145 Ma) are confined to the western Coast Belt and eastern Insular Belt and tie the western Coast Belt to Wrangellia (Insular Superterrane) by Late Jurassic time. Early Cretaceous (145–112 Ma) Coast Plutonic complex plutons and associated volcanic rocks are also limited to the western Coast Belt and eastern Insular Belt. Widespread Early Cretaceous (ca. 145–130 Ma) uplift is suggested, based on Early Cretaceous (Neocomian) conglomerates exposed across the width of the western Coast Belt. Similar conglomerates in the central Coast Belt are interpreted to represent the earliest stratigraphic tie at this latitude between the Insular and Intermontane superterranes. Mid-Cretaceous igneous rocks (112–90 Ma) extend from Howe Sound in the west to the eastern Coast Belt, and intrude the hanging wall and footwall of the Coast Belt thrust system. They constrain the timing of west-directed thrusting within the Coast Belt thrust system to approximately 91–97 Ma. Their location indicates an approximately 30-km eastward progression of the western limit of major magmatism from ca. 115–100 Ma. Late Cretaceous igneous rocks (90–65 Ma) are located almost exclusively within the central and eastern Coast Belt, in the hanging wall of the Coast Belt thrust system. A belt of 87–84-Ma old plutons provides evidence for an approximately 100-km-eastward shift of the western limit of magmatism between approximately 100 Ma and 90 Ma, coeval with west-directed thrusting along the Coast Belt thrust system. The distribution of late Early to Late Jurassic and possibly earliest Cretaceous igneous rocks in the southern Coast Belt suggests, but does not prove, that they were generated in an arc specific to Wrangellia (Insular Superterrane). In the context of a regime such as this, one or more intervening subduction zones are required to account for more easterly Jurassic magmatism. By ca. 100 Ma, we envisage a single west-facing arc along the entire length of the North American Cordillera, a regime that continued through the remainder of the Cretaceous period.

Plutonic igneous rocks of Middle Jurassic, Cretaceous, and Eocene age from southeast British Columbia exhibit a range of initial Nd and Sr isotopic compositions described by ε Nd (T) = −2.7 to −13.4, and ε Sr (T) = −13.8 to 69.5. In contrast, Early Jurassic volcanic rocks from the area have ε Nd (T) values between +2.9 and +6.5, and ε Sr (T) between −7.1 to 1.7. Cordilleran sedimentary and metasedimentary rocks of Middle Proterozoic to Triassic in age from this area have ε Nd (at 164 Ma) values between −3.1 and −13.4. These plutonic igneous rocks formed in continental-arc environments and appear to have formed through mixing of mantle-derived magmas and depleted cratonic basement. The presence of 1.8–2.0 Ga old cratonic basement underlying the study area is supported by (1) presence of an old Pb component in zircons from many young igneous intrusives and (2) dating of the middle to upper crustal rocks exposed in the metamorphic core complexes in southern British Columbia. The deeper part of this basement appears to be the most likely crustal end member for assimilation with mantle-derived magmas, and this part has been characterized by its chemical composition (ε Nd = −16, ε Sr = 80, Nd ppm = 26, Sr ppm = 400). Similarly, isotopic and trace element compositions of mantle-derived magmas are obtained from those of the mantle xenoliths from the West Kettle River, just west of the study area (ε Nd = +9, ε Sr = −25, Nd ppm = 13, Sr ppm = 560). Intermediate isotopic compositions of the granitoid rocks suggest their parental magmas derived from the mantle lithosphere or mafic lower crust must have experienced significant crustal assimilation at deep crustal levels. Simple binary mixing between these end members is inadequate to produce the array of isotopic and trace-element compositions of the granitoid bodies. Coupled assimilation and fractional crystallization process appears to be more effective. Crustal contamination took place generally under high ratios of rate of crystallization to rate of assimilation (r ≥ 0.7) with concomitant crystallization of clinopyroxene and hornblende for the alkalic intrusives, but with plagioclase crystallization for the dominantly calc-alkalic intrusives. Calculations show that amounts of crustal material present in the granitic batholiths in southeastern British Columbia varied from 20% to 50%. Similar isotopic compositions of the spatially related peraluminous parts of the dominantly metaluminous granitoids suggest they were probably fractional crystallization products of the later magmas. Only exceptions are the Cretaceous and Eocene peraluminous magmas with isotopic compositions very similar to those of the cratonic basement. These stocks appear to be products of direct melting of thickened crusts with high thermal gradients.

This report presents four U/Pb zircon dates from the Kootenay arc and Cariboo Mountains of southeastern British Columbia. When combined with existing biochronological data, two of these dates tightly constrain the age of shortening of the sedimentary basin between the North American continental margin and the Triassic-Jurassic Quesnellia volcanic arc to mid-Toarcian (late Early Jurassic, ca. 187–185 Ma). The other two dates affirm a Mesozoic age of the prominent phase of southwest-vergent folding in the Omineca crystalline belt and permit correlation of these structures with similar folds of the mid-Toarcian Quesnellia–North America terrane boundary. Previously published dates and geological relationships show that this latter southwest-vergent folding was finished by the end of the Aalenian (earliest Middle Jurassic, 174 Ma). Taken together, these data show that the earliest two “phases” of Jurassic deformation of the western edge of North America occurred in a ca. 10-million year interval spanning the Early to Middle Jurassic boundary. Although this deformation consists of two phases, given the short time interval, it is likely that they are part of the same continuous episode of shortening of the continental margin.

Geologic mapping in an extensive metamorphic belt in the northern Klamath Mountains reveals three distinct groups of lithotectonic units that are parts of two or more allochthonous metamorphic terranes (1) a western group of recrystallized greenstones and mudstones along Cow Creek; (2) a central group of serpentinized ultramafic and basic amphibolitic rocks along Elk Creek; and (3) a schistose group of largely supracrustal rock units, best exposed along Wildcat Ridge, occurring on both flanks and distal to the main group of ultramafic rocks and amphibolites. The rock units cannot be assigned unequivocal terrane status because regional correlations have not been established. Nevertheless, this research and previous work establishes that the westernmost group has metamorphic and deformational features in contrast with the central and associated distal groups of ultramafic rocks, amphibolites, and schists. These features are consistent with those recognized elsewhere in the western Jurassic belt, and in the western Paleozoic and Triassic belt, respectively. A fault termed the Cedar Springs Mountain thrust separates the western from the largely ultramafic-mafic central and distal rock groups. Small-scale structures related to deformation-metamorphism suggest that the rocks on either side of the thrust evolved independently until they were juxtaposed. Following juxtaposition by thrusting, metamorphism overlapped intrusion of calc-alkaline plutons. Late metamorphism synchronous with plutonism produced a pattern of progressive metamorphic zones subparallel to boundaries of the group of elongate plutons. The mapped pattern of metamorphic zones in metapelites includes (1) chlorite, (2) chlorite + biotite, (3) andalusite + biotite + chlorite, (4) andalusite + staurolite, and (5) staurolite + sillimanite. The large area of staurolite + sillimanite zone rocks coincides generally with the area of most abundant calc-alkaline plutonic rocks. Late regional metamorphism was probably older than a 141-Ma K-Ar cooling age for trondhjemites of the White Rock pluton, the largest of the calc-alkaline rock bodies. The pluton cooling age is consistent with a K-Ar metamorphic cooling age of 150 ± 11 Ma for amphibolites intruded by calc-alkaline rocks in the central group. A model to help explain the evolution of metamorphic textures, structures, and assemblages in the three groups of rocks involves (1) pre-Late Jurassic formation of an ocean floor followed by its simultaneous deformation and metamorphism during accretion to western North America; (2) formation offshore of a Late Jurassic volcanic island arc and an adjacent deep basin behind the arc; (3) collapse of the arc and basin and accretion to the continental margin by underthrusting accompanied by folding and cleavage formation in accreted and continental margin rocks; (4) syntectonic intrusion of calc-alkaline plutonic rocks with simultaneous porphyroblastic recrystallization of accreted arc-basin and older rocks.

Pre-Cretaceous rocks in the northern Sierra Nevada are subdivided from west to east into the Smartville, central, Feather River peridotite, and eastern belts. Cretaceous and younger sedimentary rocks form the western boundary of the Smartville belt, but various reverse-fault segments of the Foothills fault system separate the other belts. The Foothills fault system and associated structures involve rocks as young as Kimmeridgian (Late Jurassic) and are truncated by Early Cretaceous plutons. This relationship is often cited as evidence for the Nevadan orogeny which is commonly viewed as a temporally restricted event involving deformation and metamorphism during the Late Jurassic. Recent work, however, suggests that some of the Mesozoic structural fabric in the northern Sierra Nevada may not have been produced during the Late Jurassic, but instead may have formed between Early and Middle Jurassic time. Thus, distinguishing Nevadan-age deformation from older Mesozoic deformation is now one of the more important problems facing geologists working in the northern Sierra Nevada. The Haypress Creek pluton crops out in the eastern belt and historically has been cited as a post-Nevadan pluton. It intrudes the Early to Middle Jurassic Sailor Canyon Formation that, together with the overlying Middle Jurassic Tuttle Lake Formation, contains a domainally developed, locally penetrative, northwest-striking cleavage (S 2 ). S 2 can be traced into the contact metamorphic aureole of the Emigrant Gap composite pluton, where structural and microtextural evidence indicates that it predates pluton intrusion. New U-Pb zircon data for the Haypress Creek pluton suggest an age of 166 ± 3 Ma and previously published U-Pb zircon data for the oldest phase of the Emigrant Gap composite pluton suggest an age of 168 ± 2 Ma. The fossiliferous Sailor Canyon Formation ranges in age from Early Jurassic (Sinemurian) in its lower parts to Middle Jurassic (Bathonian or Bajocian) in its upper parts. The overlying Tuttle Lake Formation contains S 2 , which formed prior to emplacement of the Emigrant Gap and Haypress Creek plutons at ca. 168–166 Ma. This relationship suggests that the Tuttle Lake Formation must have been deposited and deformed entirely within the Middle Jurassic. Thus, S 2 and associated structures within the eastern belt formed prior to Late Jurassic Nevadan deformation associated with the Foothills fault system. There are two end-member models used to explain the plate tectonic evolution of pre-Cretaceous rocks in the northern Sierra Nevada. These are referred to as the arc-continent collision and single, wide-arc models. Data discussed herein do not preclude either of these models for Early to Middle Jurassic time. However, regardless of which of these models is favored, both scenarios place the approximately 168 Ma and younger Jurassic volcanic and plutonic rocks of the Smartville, central, and eastern belts in a distinctly intra-arc setting and further imply that the Foothills fault system and related Late Jurassic structures are also of intra-arc character. We conclude that there is no evidence along 39°30′N latitude for arc-continent collision during the Nevadan orogeny.

A previously unrecognized sheared dike swarm has been identified in a southern fragment of the western Foothills terrane—the Owens Mountain area of the western Sierra Nevada foothills, northeast of Fresno, California. It may be the southern extension of the Bear Mountains fault zone. The dike swarm, sheeted in places, consists predominantly of tonalitic tabular bodies and coeval, mutually cross-cutting tonalitic and mafic dikes. Textures and fabrics within the dike swarm range from partially recrystallized igneous to strongly deformed metamorphic tectonites, implying that dike emplacement occurred during ductile deformation. Mylonitization has transposed layering parallel to foliation and has greatly thinned many of the dikes. Layering and foliation dip subvertically and strike NNW–SSE. Post-tectonic annealing has destroyed most microscopic shear indicators, but macroscopic intrafolial folds are common and have steeply southeast-plunging fold axes and S-fold geometries that may indicate a sinistral sense of shear. Age data (U-Pb zircon) from the tonalites reveal that emplacement and crystallization occurred over a 7-m.y. period, from 155 Ma to 148 Ma, at an estimated depth of 10 km (from Al Total in hornblendes). A correlation between age and degree of deformation and recrystallization of the tonalites implies syntectonic dike emplacement. Intrusion began within 5 m.y. of deposition of the strata into which the dikes were emplaced. Granitic dikes that cut the complex at 123 Ma are nondeformed. The duration of the Nevadan orogeny is shown to have lasted from Late Jurassic to Early Cretaceous (160–137 Ma and possibly to 123 Ma) and thus is more protracted that has been postulated. The regional tectonics of the Owens Mountain and other Cordilleran dike swarms can be related in a broad dynamic sense to the absolute motion of North America by using the apparent polar wander (APW) analysis of May and Butler (1986). The Nevadan orogeny may be the manifestation of drastic changes in magnitude and direction of North American motion (from ~45 km/m.y. to the NNE to ~200 km/m.y. to the NW; May and Butler, 1986). The Late Jurassic dike swarms record a complex pattern of sinistral-sense transtension-transpression that may have developed at the J2 APW cusp (ca. 150 Ma; see May and Butler, 1986) and during subsequent, rapid northwestward acceleration of North America.

The Jurassic Humboldt igneous complex in west-central Nevada consists of a comagmatic suite of intrusive and extrusive rocks and is tectonically intercalated with Triassic to Lower Jurassic shelf sequence and basinal successions. Its plutonic rocks include, from bottom to top, olivine-gabbro, melatroctolite, hornblende gabbro, microgabbro, and diorite transitional upward into quartz diorite, tonalite-granodiorite, and monzonite. Contacts between these plutonic subunits are commonly gradational, but mutual intrusive relations, characterized by the existence of brecciated and altered zones and xenoliths, are also common. Mafic to felsic plutonic rocks are cut by generally N–S to NW–SE striking dikes that form local dike swarms with one- and two-sided chilled margins. Dikes are composed of fine- to medium-grained rocks ranging in composition from basalts to andesites and feed into and/or are overlain by extrusive rocks consisting of lava flows intercalated with volcanic tuff and breccia. Lava flows at stratigraphically lower levels are more mafic and locally display pillow shapes reminiscent of submarine lava flows, whereas lava flows at higher levels are more felsic and are commonly interleaved with a fine-grained tuffaceous material. Volcanic rocks range in composition from basalts, basaltic andesites, andesites, to latites and dacites and mineralogically and texturally are similar to the dikes. The major element compositions of the analyzed rocks suggest relatively evolved basaltic magmas, whereas strongly incompatible trace element ratios (e.g., Ce/Ta) have high values typical of subduction related magmas. Lavas, dikes, and gabbros commonly display similar rare earth element (REE) patterns, although more felsic rocks are light rare earth (LREE) enriched, suggesting a cogenetic suite of rocks. These REE patterns are characteristic of basaltic andesites from volcanic arcs and suggest, coupled with field relations, that the rocks of the Humboldt complex might have evolved from subduction originated magmas in a volcanoplutonic arc setting. The tectonic nature of the contact between the Humboldt complex and the underlying Triassic-Jurassic sedimentary strata indicates that it was displaced eastward from its original arc environment following its igneous evolution. Both the Humboldt complex and the sedimentary strata are intruded at all structural levels by numerous northeast-striking dikes and dike swarms, which strongly altered and metasomatized their country rocks. These dike rocks are Miocene in age and have geochemical characteristics distinctly different from those of the Humboldt rocks. Based on regional correlations and paleogeographic reconstructions, we interpret the Humboldt igneous complex as the northern continuation of a Jurassic continental margin arc that extended from the Sonora Desert region in the south to northwestern Nevada and northern California in the north. This continental margin arc was a site of regional subsidence and crustal extension that accompanied magmatic activity and penecontemporaneous deposition of both arc and craton-derived detritus in submarine to subaerial conditions during much of the Jurassic. We emphasize that the coeval ensimatic arc terrane west and outboard of this extending continental margin arc might have represented a fringing oceanic realm that was subsequently collapsed into the outer continental margin of North America in Middle to Late Jurassic time.

The Nevada Jurassic Magmatic Province is defined as a region of abundant late Middle and Late Jurassic plutonism and associated deformation inboard of the contemporaneous magmatic arc. The stratigraphic, structural, and magmatic history of the Nevada Jurassic Magmatic Province allows assessment of the relative importance of crustal kinematics and thermal perturbation of the lithospheric mantle in Jurassic tectonics of the northern Great Basin. Constraints on the tectonic development of an area far inboard of the plate boundary enhance understanding of the causes of intraplate deformation and magmatism and their relationship to the plate boundary. Simple thermal models, estimates of the magnitude of crustal shortening during the Jurassic, isotopic compositions of Jurassic plutons, and near synchroneity of magmatism and deformation argue that crustal thickening was not the primary cause of plutonism in the Nevada Jurassic Magmatic Province. Rather, a thermal perturbation of the lithospheric mantle, modeled as subduction-induced asthenospheric flow, is considered the primary cause of Jurassic plutonism. Subduction-induced flow in the asthenosphere may lead to thermal erosion of the lithosphere and subsequent crustal heating. Broad, low-relief uplift of the Nevada Jurassic Magmatic Province and minor, outward-directed crustal shortening are consistent with the predicted isostatic and rheologic consequences of lithospheric thinning. Emplacement of magmas, generated by increased crustal temperatures and decompression of mantle rocks, also influenced crustal deformation locally. The Jurassic tectonic development of a large part of the northern Great Basin can be explained by lithospheric thinning in the absence of large-scale crustal shortening. If the tectonic development of the Nevada Jurassic Magmatic Province was ultimately due to subduction-induced asthenospheric flow the implication is that intraplate deformation and magmatism are primarily thermally controlled processes. Crustal deformation is, then, a consequence of magma generation and thermal weakening of the crust. Although transmission of compressive stress to areas inboard of the plate boundary may occur, it appears to be a secondary effect rather than the primary cause of intraplate deformation.

Jurassic tectonism in the northeastern Great Basin produced varied structures, many closely associated with widespread magmatism at ca. 155–165 Ma and with local metamorphism. Many of the plutons are of suitable mineralogy for Al-in-hornblende barometry, providing the potential for depth data. We have studied conditions of metamorphism in the Pilot Range and barometry for six Jurassic plutons across the northeastern Great Basin. All barometry results are in harmony with pressures estimated from stratigraphic data, requiring little or no tectonic thickening. On the basis of structural styles and barometric data, we divide the northeastern Great Basin into three Jurassic tectonic provinces. An eastern extensional province, largely in western Utah, is characterized by Paleozoic strata that were thrust faulted and then intruded by shallow plutons shortly after or during normal and strike-slip faulting. Extension was probably a short-lived event associated with magmatism, but its west trend indicates a total reorientation of stress at this time, perhaps within transtensional strike-slip zones. A central province of modest, and possibly locally extreme, Jurassic shortening in eastern Nevada is characterized by metamorphosed Paleozoic rocks and by thrusts and kilometer-scale southeast-vergent folds. Upper amphibolite facies, but low pressure (3–4 kbar) metamorphism is present near Jurassic plutons in the Pilot Range and Ruby Mountains, probably indicating metamorphism induced by heat from magmas. In contrast, metamorphism in other ranges, which is known only to be pre–Late Cretaceous, indicates thickening of 10–20 km. This thickening may have entirely postdated the Jurassic. A western province in north-central Nevada is characterized by preserved Jurassic volcanic rocks and shallow plutons, indicating that little erosion, and probably surface uplift, occurred during the late Mesozoic. Folds and thrust faults indicate minor Jurassic shortening but many structures are undated. The low-pressure upper-crustal conditions for demonstrably Jurassic events suggest that higher-pressure metamorphism recorded in the central province is younger (Cretaceous) in age. We suggest that Jurassic structures were caused by distributed minor crustal shortening, manifested mainly as small-scale thrust faults. Local thermal highs created by plutonism produced metamorphic zones in relatively shallow crust. Shortening in the east was manifested by zones of strike-slip, within which plutons were emplaced in tensile niches. Lack of a deep foreland basin and lack of evidence for massive erosion argue against high-relief mountain belts caused by significant crustal shortening. Paleozoic rocks metamorphosed at pressures far in excess of stratigraphic burial are restricted to narrow lenses exhumed during Late Cretaceous and Tertiary extension and are bordered by rocks that always have been part of the shallow crust. The abundant shallow-crustal rocks preserved across the region indicate that a conventional hypothesis of large-scale, regional crustal thickening causing many kilometers of surface uplift and consequent erosion is unlikely to have taken place in the Mesozoic.

Mesozoic structures in the Dolly Varden Mountains include low-angle normal(?) faults at high angles to bedding, near-bedding-parallel faults which cut out strata, large-scale folds, east-striking normal faults, and north-striking normal faults. Mesozoic deformation in the Dolly Varden Mountains is constrained to be no younger than 165 ± 3 Ma based on a U-Pb zircon age obtained for the Melrose intrusion. In the nearby Currie Hills, north- and northwest-trending folds deforming Paleozoic and Mesozoic strata are likely correlative with contractional features in the southern Pequop Mountains. Near the town of Currie, formations as young as Lower Jurassic appear to be included in the folding. Considering the age of these formations, and the lack of evidence for deformation prior to their deposition, it appears that Mesozoic structures in the Dolly Varden Mountains formed between the Early Jurassic and 165 Ma, or probably within the Middle Jurassic. The age of folds in the Currie Hills cannot be as tightly constrained. No strong evidence for correlating them with folds in the Dolly Varden Mountains exists. Therefore, it can only be said that they probably formed after the Lower Jurassic and before Tertiary volcanism.

Integrated geological and geochemical studies of the Barneys Canyon gold deposit in the Oquirrh Mountains of north-central Utah suggest that compressional tectonism and metamorphism are Jurassic in age. Detailed geologic mapping, clay mineralogy, and fluid-inclusion analyses together with Jurassic K/Ar age determinations indicate that deformation at Barneys Canyon was contemporaneous with regional Jurassic metamorphism recognized in the southern Oquirrh Mountains by Wilson and Parry (1990b). The Barneys Canyon gold deposit occurs on the crestal region of the Copperton anticline which is interpreted as a fault-bend fold. Bedding-plane gouges formed within the Barneys Canyon sedimentary sequence during flexural slip folding. Clay minerals formed in the gouges and in the Barneys Canyon gold deposit are kaolinite, illite, and some minor interstratified illite-smectite. The distribution of illite and kaolinite shows that the orebody is associated with illite alteration surrounded by a halo of more kaolinitic material. Illite crystallinity suggests that a lower-temperature (retrograde) zone is associated with the orebody. Fluid-inclusion analyses from quartz and barite show a range of homogenization temperatures from 130–400 °C with two weak modes at 225 °C and 345 °C. Kaolinite and quartz are unstable with respect to pyrophyllite at the higher temperatures. No pyrophyllite has been observed at Barneys Canyon restricting the kaolinitic alteration to the lower-temperature range. The formation temperature of illite is not constrained. The bedding-plane gouges contain illite, kaolinite (minor), quartz, carbonate, and as much as 1.5 ppm Au. The illites yielded K/Ar ages of 147 Ma and 159 Ma consistent with K/Ar ages of heavy metal bearing illite veins in the southern Oquirrh Mountains described by Wilson and Parry (1990b). This interpretation extends Jurassic deformation to north-central Utah from areas to the west where Jurassic magmatism and tectonics have previously been described.

The Cowhole Mountains, near Baker, California, are an east-tilted remnant of the Jurassic magmatic arc that extends from western Nevada through southeastern California into Arizona and Sonora, Mexico. In Middle Jurassic time, two northeast-erly trending grahens bounded by syndepositional faults accumulated 550 m and 700 m of predominantly eolian sandstone (Aztec Sandstone). Because east dip in one of the grabens increases from 30° at the base of the sandstone section to 65° at its top, we infer that structure to have developed during tilting to the west on east-dipping, listric normal faults. Westward tilting continued to accumulate during extrusion of an overlying Middle Jurassic volcanic series (Cowhole volcanics). Felsic volcanic rocks that buried the grabens comprise ignimbrites, volcaniclastic rocks, and flow breccias, emplaced in a proximal but extra-caldera setting. Plutons and sills were intruded at shallow levels, possibly during both graben filling and extrusion of volcanics. Local magmatism of significant volume ended with intrusion of a dike swarm that may be correlative with the Independence dike swarm (148 Ma). After the range tilted 90° toward the east, small glide blocks and landslides detached from high portions of the eroding range and descended into the southern graben. Our model suggests that two sets of high-angle normal faults, oriented nearly at right angles to each other, were active during the same Middle Jurassic interval. A modern analogue may be provided by the Central American arc graben, in which similar transtensional structures have developed perpendicular to the regional graben where it is intersected obliquely by the Chiapas shear zone.

The inland edge of the Jurassic magmatic belt passes through the eastern Mojave Desert, where it was emplaced in ancient continental crust. Three intrusive units exposed there—the Ship and Clipper Mountains plutons and a dike swarm in the Old Woman and Piute Mountains and Kilbeck Hills—are broadly similar to each other and to other intrusions of Jurassic age, but they differ from one another in detail and all show very clear evidence for interaction with the ancient crust. All three intrusive units are primarily metaluminous and range from mafic to moderately felsic in composition. The Ship Mountains pluton and dikes included both mafic and felsic magmas that mingled locally. The Clipper Mountains pluton comprises a compositional continuum from hornblende gabbro through granodiorite, at least partly a result of crystal accumulation processes. The ca. 160-Ma Clipper Mountains pluton was emplaced syntectonically with thrusting at a depth of approximately 15 km. The ca. 145-Ma dike swarm intruded at approximately 12 km, and the Ship Mountains pluton at <5 km. The Ship Mountains pluton, which is not well dated, initially overlay the dike swarm prior to Late Cretaceous and Tertiary extension and may have a similar age. The intrusions are all enriched in incompatible elements and have isotopic compositions that are more evolved than any plausible mantle source (high 87 Sr/ 86 Sr, low ε Nd , high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb compared with 206 Pb/ 204 Pb). Ship Mountains and most dike samples are less evolved in Nd and Sr than the Mojave crust, but the Clipper Mountains Nd-Sr array is coincident with the less evolved portion of the field of ancient Mojave crust. Extremely strong U-Pb inheritance in Clipper zircons and moderate inheritance in dike zircons verifies the crustal component. We interpret Ship and dike rocks to be hybrids of ancient enriched mantle-derived mafic magmas and the ancient crust; the Clipper Mountains pluton could represent a restite-rich magma entirely derived from the Mojave crust, although a modest mantle contribution is likely.

The Clipper Mountains in the eastern Mojave Desert expose evidence of Jurassic plutonic intrusion along what was an active thrust at the east fringe of the exposed Cordilleran Jurassic magmatic arc. This event occurred during a period of widespread arc magmatism and intra-arc thrusting in the Cordillera related to subduction under the west edge of North America. Jurassic plutons in the eastern Mojave Desert are compositionally more diverse and more K 2 O-rich than Cretaceous plutons. Late-kinematic intrusion of the Jurassic Goldhammer pluton, exposed in the Clipper Mountains, was along an active ductile thrust fault that put Proterozoic basement gneiss over Paleozoic strata by the time of intrusion. U-Pb geochronology and hornblende geobarometry are interpreted to indicate that the pluton was emplaced at 161 ± 10 Ma at a pressure approximately 0.46 GPa or more. This pressure corresponds to approximately a 17-km depth or more, at least 13 km greater than inferred stratigraphic overburden (2–4 km) at the time of intrusion. The excess we attribute to pre-intrusion tectonic burial from overthrusting of the observed allochthon of Proterozoic basement and (or) from earlier Mesozoic overthrusting. Ductile deformation continued along the observed thrust system during intrusion of the Goldhammer pluton. Fabrics in the pluton and country-rock record ductile shearing that was partly top westward but mostly top-eastward; the shearing began before or during the intrusion and continued during and after intrusion. The Jurassic burial history in the Clipper Mountains parallels that in adjacent ranges to the east, but contrasts with that in ranges to the south and west where exposed Paleozoic rocks were at colder and shallower crustal levels in Jurassic time. The tectonic record in the Clipper Mountains suggests large crustal thickening and topographic uplift that would be expected to leave a sedimentary record in Jurassic basins.