The Late Cretaceous sedimentary record is preserved in a series of scattered outcrops along the coast of southwestern California to central Baja, Mexico. Virtually identical stratigraphic sequences and interpreted depositional histories suggest that regional sedimentation was controlled primarily by eustatic sea level changes, with a strong tectonic overprint. Sediments derived from the uplifted Peninsular Ranges granitic-metamorphic terrain (arc system) were implaced in a Great Valley-type forearc basin as a westerly-thickening clastic wedge of continental to deep-marine facies. The stratigraphic record within the main part of the basin shows evidence of only a single large transgressive-regressive cycle. By contrast, the stratigraphic record along the eastern basin margin is complex and more complete, characterized by numerous small transgressive-regressive hemi-cycles within one major eustatic cycle. During the Cenomanian and Turonian, the shoreline was located several kilometers west of the present coastline, and fluvial/alluvial fan sediments were deposited on arc basement. During a major eustatic sea level rise that continued through the Early Campanian, a transgressive sequence of beach to offshore coastal deposits was onlapped over continental sediments. In the Late Campanian/Early Maestrichtian, sea level peaked then began to fall. Large volumes of eroded material were emplaced in nearshore fan-delta sequences. Offshore mass-flow transport across the steep narrow shelf produced submarine fan systems that prograded into the basin from several major embayments. The size and morphology of these fans appears to be comparable to the present-day La Jolla system. In each case, the sedimentary section is characterized by an upward transition from outer fan to middle fan to inner fan facies associations. From the Early Maestrichtian to the Tertiary, the shoreline prograded west in response to falling sea level, possibly marked by a minor stage of transgression during the Early Maestrichtian. A large erosional hiatus marks the Cretaceous boundary.

The Elkhorn Ridge Argillite exposed in the Bourne-Sumpter area of eastern Oregon is a broken formation of metamorphosed (greenschist facies) hemipelagic sediments, reefal carbonates, rare clastics, mafic volcanics and intrusives that may have accumulated in a late Triassic-Early Jurassic subduction zone complex or marginal basin. The structural fabric of this unit is best explained by a model of repeated homoaxial folding and simple and/or pure shear during progressive deformation. Deformation began before the sediments were completely lithified and tight to isoclinal mesoscopic folds developed with and without axial-plane cleavage. A predominant spaced cleavage formed by a combination of dissolution-redeposition processes and microfaulting. Grain boundary sliding and movement along microfaults are important additional deformation mechanisms that operated during flexural slip folding. The lithology and structural style of the Elkhorn Ridge Argillite are characteristic of deformed sequences in subduction zone complexes and marginal basins and can not be used to discriminate paleotectonic settings. New faunal evidence suggests that substantial disruption of structural stratigraphic units such as the Elkhorn Ridge Argillite have taken place during the Late Triassic to late Jurassic. Coeval blocks of similar lithology and fauna are located in the Miller Mountain area within the John Day inlier of central Oregon and in several terranes distributed along the western Cordillera from California to Canada. Other blocks of oceanic lithic assemblages of various ages in intermediate areas show significant differences in structural and sedimentological history and together are interpreted to lie within a diffuse fault zone of amalgamated suspect terranes.

The Nevada Test Site (NTS) is located in the southern Basin and Range where the geology is typified by complexly deformed Paleozoic sedimentary rocks underlying Tertiary and Quaternary volcanics and alluvium all displaced by Cenozoic normal faults. The purpose of this study is to interpret the history of change in Cenozoic extensional stress orientations using ash-flow tuff distributions, surface fault configurations, and slickenside analyses. Extensive drill hole data collected from Yucca Flat within NTS were used to construct isopach and structure contour maps of Cenozoic units occupying the northerly-trending basin. The configuration of these units indicates that the north-south-trending faults controlling present day basin morphology were inactive during deposition of the volcanic rocks from approximately 25 to 11 myBP. However, after 11 myBP, the overlying sedimentary sequence was strongly influenced by these faults and consequent basin development. In particular, an inordinately thick section of late Tertiary and Quaternary alluvium occurs at the southwestern end of Yucca Flat. Southwest-striking faults at the southwestern of Yucca flat are postulated to be deflected at their northeast ends, becoming continuous with the north-south basin forming fault sets. The northeast faults exhibit predominantly left-lateral displacement occurring post-11 myBP. This sense of motion is incompatibile with the N50(DEGREES)W extension determined today via in situ measurements. The north-south faults are primarily dip-slip with a small component of right-lateral motion. The thick sedimentary section found in southern Yucca Flat at the intersection of the southwest-striking and north-south-striking faults formed as a pull-apart basin which developed syndepositionally with the alluvium. Observed offsets of volcanic units by the southwesterly striking faults as well as slickenside analyses of data from the major fault zones throughout the area indicates a N78(DEGREES)W extension operating since 11 myBP. After formation of much of the Yucca flat basin, the least principal stress rotated to N50(DEGREES)W. This rotation apparently occurred very recently and the new stress orientation has had little effect on the fault patterns or fault displacements of the area. Synthesis of this work with other studies throughout the southern Basin and Range show a consistent clockwise rotation of least principal stress through an angle of 90(DEGREES) in the past 17 my.

Xenoliths, collected from certain hybrid lava flows and vents at three localities in the eastern Snake River Plain region of southern Idaho, were derived from underlying crustal terrains which experienced granulite-facies metamorphism at approximately 3.0 AE. Lithologically, charnockite, opdalite, enderbite, and norite meta-igneous xenoliths are predominant, but biotite-garnet gneiss and cognate noritic xenoliths derived from fractional crystallization of the host lavas also occur. The metamorphic charnockite-norite xenoliths are characterized by mineral assemblages containing compositionally homogeneous plagioclase + hypersthene +/- quartz +/- alkali feldspar +/- iron-titanium oxides +/- clinopyroxene +/- biotite. Iron-titanium oxide, two-feldspar, and two-pyroxene geothermometry calculations indicate that metamorphism of the xenoliths occurred at about 700 to 800(DEGREES)C. Corresponding pressures, estimated from garnet-plagioclase-orthopyroxene-quartz and plagioclase-clinopyroxene-quartz relationships, range from approximately 4 to 8 kbar. Entrainment of the xenoliths by their host hybrid lavas has resulted in reequilibration of mineral assemblages in certain xenoliths to the higher temperature conditions of the lavas. The high temperatures experienced by the xenoliths in their host lavas also resulted in partial melting of the xenoliths, producing interstitial glasses of varying composition. Silicic interstitial glasses are most common and have been derived primarily from melting of quartz-feldspar assemblages. They are similar in major element composition to surficial rhyolites of the Snake River Plain region. More mafic interstitial glasses formed from melting of plagioclase and ferromagnesian minerals and appear to be similar in composition to the hybrid host lavas. Trace element constraints preclude a direct genetic relationship between partial melts produced from the xenoliths and surficial volcanics. However, it is possible that rhyolites could be derived from partial melting of the lower crust, providing secondary modifying processes occur.

Mount St. Helens volcano (Washington, USA) has been characterized by four eruptive periods during the last 2200 years. Eruptive products include a wide spectrum of rock types including basaltic to andesitic lavas, andesitic to dacitic pyroclastic flows and tephra, and dacite domes. The major and trace element compositions of some andesites and dacites are broadly consistent with their derivation from a basaltic andesite parental magma by fractional crystallization processes involving the observed phenocryst assemblages. However, the strontium and oxygen isotopic compositions of representative samples of the Mount St. Helens suite indicate that closed system processes cannot explain the isotopic variations. The isotopic ratios are positively correlated with one another and with bulk composition (SiO(,2), Mg number, etc.). The isotopic variations and trace element data support an origin of some intermediate and silicic rock types by combined processes of assimilation and fractional crystallization. The vents of the nearby Indian Heaven Quaternary volcanic field erupted several basalt types which can be defined on the basis of major and trace element composition--calcalkaline (low and high TiO(,2) varieties), transitional, and tholeiitic. Several of these basalt types occur at Mount St. Helens as well, but Indian Heaven lavas are generally more primitive as indicated by higher Mg/(Mg + Fe) ratios. Based on trace element abundances of estimated primary magma compositions, their genesis seemingly involves melting of heterogeneous upper mantle sources. The distribution of volcanic rock types in relation to local structures in the Cascade Range of southern Washington and northern Oregon suggests that crustal structure may influence the degree of evolution of specific volcanic fields. Two associations are observed: (1) Significant amounts of dacitic, as well as basaltic and andesitic, magmas are erupted from centers apparently associated with local compressional strain (e.g. Mount St. Helens). (2) Relatively undifferentiated basalts and lesser amounts of andesite are erupted in areas of local extensional strain. Dacites are scarce and tholeiitic basalts appear to be restricted to these areas (e.g. Indian Heaven). An extensional environment apparently favors the ascent of primitive basaltic magmas whereas compression favors stagnation within the crust, prolonged differentiation, and interaction with crustal material. The relationship between crustal strain and related volcanism in the Cascade arc suggests that volcanic arc magma evolution does not necessarily produce a continuous sequence from tholeiitic to calcalkaline rocks in time or space.

Structural features on megascopic, mesoscopic, and microscopic scales have been examined in a study area located in the central portion of the northern Sierra Nevada (west of Lake Tahoe). Structures observed within the study area are correlated with regional deformational events of the northern Sierra Nevada. Although at least four regionally extensive deformational events have affected the rocks of the northern Sierra Nevada, the effect of only three events are recognized in the study area. Structures associated with post-Ordovician/Silurian and pre-Late Devonian deformation were not identified due to overprinting of later deformational events. There is ample evidence supporting extensive late Paleozoic-early Mesozoic deformation in the study area. Structures from this event formed as the result of compressive deformation along a northwest-striking, east-dipping convergent plate boundary along the western margin of the Sierran province and may be related to the accretion of an exotic terrane, Sonomia. Although a significant component of strike-slip motion may have existed along the plate boundary, the structures appear to be related to the normal component of convergence. The intense, short-lived Nevadan orogeny deformed rocks throughout the study area. This latest Jurassic event is thought to be the result of an arc-continent collision. Nevadan structures, which have an anomalous north to north-northeast trend in the study area, are often indistinguishable from late Paleozoic-early Mesozoic structures due to similarities in style and orientation. Analysis of strain from quartz microfabrics indicates that Nevadan deformation is, at least locally, non-coaxial. The non-coaxial deformation is probably related to left-lateral oblique convergence. The last deformational event that affects rocks in the study area occurred in the Cretaceous. Cretaceous structures have a consistent northwest trend throughout the northern Sierra Nevada. The anomalous trend of Nevadan structures in the study area is most likely related to Cretaceous deformation. In the northern Sierra Nevada, post-Nevadan, dextral oroclinal folding, which occurred prior to or during the Cretaceous deformational event, is the result of right-lateral oblique convergence.

Neogene radiolarian assemblages were examined from California, the offshore waters, and adjacent areas. Three time plans, 10, 8, and 5 Ma, were selected for study and were determined by the morphologic bottom of Diartus hughesi, the morphologic top of D. hughesi, and the morphologic bottom to acme of Lamprocyrtis heteroporos respectively. The general, symbiotic, warm water sphere, and cold water sphere diversity and abundance parameters of the radiolarian populations were plotted. The dominant trends in these parameters, at all three time planes, were primarily in the east-west (cross-current) direction. Secondary trends in the north-south (along current strike) direction were also exhibited. The radiolarian assemblage parameters offer a means to investigate in more detail the complex oceanographic history which existed during the time that the thick and widespread upper siliceous facies of the Monterey Formation was deposited. In this study two of the most significant oceanographic features reflected by these parameters were the California Current width and the degree of oceanographic convergence development off Baja California. The California Current appears to have been narrower (closer to shore) at 8 and 10 Ma while the convergence off Baja appears better developed at 5 and 10 Ma.

Three east-west trending, geologically distinct packages, separated by south-dipping low-angle faults, were examined in a transect through the southern Brooks Range fold and thrust belt near Wiseman, Alaska. The southernmost and structually highest package is the Angayucham ophiolitic terrane, made up of steeply dipping Carboniferous basalt flows and pillow basalt with intercalated bedded chert. Structures indicate approximately bedding-parallel, south-vergent shear, probably related to imbrication during obduction; the prehnite-pumpellyite facies metamorphism which affects the Angayucham terrane presumably accompanied imbrication. The Angayucham terrane is structurally underlain by penetratively deformed Lower Devonian metasandstone, phyllite and slate; superposed structures in these rocks are consonant with formation during north-directed contractional deformation. The structurally lowest and northernmost package is the Koyukuk Schist, composed chiefly of quartz-mica schist with minor interfoliated mafic schist, metagabbro, albite schist, marble, calc-schist, metachert (?) and eclogite, derived from pre-Devonian (?) epicontinental (?) protoliths. Metamorphic rocks are exposed in a large, asymmetric antiform, bounded on its northern limb by the south-dipping Minnie Creek thrust. Structural and textural relationships indicate that the Koyukuk Schist was affected by north-vergent ductile shear deformation concurrent with pumpellyite-actinolite to glaucophanitic greenschist facies metamorphism (D$\sb 1$ and D$\sb 2$); the metamorphic grade increases with apparent continuity from south to north. Later north-directed movement on the Minnie Creek thrust (D$\sb 3$) resulted in the uplift and broad-scale arching of the metamorphic terrane. Structural relationships in all three packages indicate a complex history of imbrication, folding, and metamorphism related to continuous north-directed contraction of Mesozoic age; contraction was sufficiently rapid to produce glaucophane- and crossite-bearing assemblages in the Koyukuk Schist. The emplacement of rocks metamorphosed at relatively low pressures above the high-pressure metamorphics of the Koyukuk Schist, and the presence of younger on older structural relationships, indicate that the imbricate stack has been modified by south-dipping low-angle normal faults. Normal faults may have formed synchronous with the uplift of the Koyukuk Schist, or result from a post-compressional period of crustal extension. Movement on normal faults may be responsible for the reorientation of imbrication-related structures in the Angayucham terrane.

The Owyhee Mountains of southwestern Idaho are constructed of Cretaceous granitic rocks of the Idaho batholith and thick sequences of Cenozoic volcanics. Igneous rocks in these mountains record a fundamental transition of tectonic regimes in their systematic changes of magmatic style and composition through time. Cretaceous through Oligocene age rocks were produced from predominantly subduction-related magma sources associated with convergent tectonism along the Pacific margin of North America. Miocene age tholeiitic basalts and rhyolites erupted in an extensional tectonic setting, but many of these basalts have trace element and isotopic characteristics indicating a residual subduction component in their mantle sources. The Cenozoic section consists of: (1) silicic Challis volcanics of Eocene age; (2) Oligocene age orogenic andesites and mildly-alkaline basalts of the Salmon Creek volcanics (SCV); and (3)a Miocene to Pliocene age bimodal sequence of tholeiitic basalt and rhyolite. Based on the lithologic and compositional characteristics of these rocks, volcanism in the Owyhee area apparently became more mafic (ignoring the Miocene rhyolites) and less hydrous through time. This progressive change in the composition of the volcanism parallels a decline both in the interaction between mantle-derived magmas and continental crust, and in the effects of subduction-related components in these magmas. Asthenospheric mantle dominanted the sources of the mafic and intermediate magmas until late Cenozoic time when contributions from subcontinental lithospheric mantle became significant. Compositional trends across a well-exposed section of Oligocene andesites indicate a progression of igneous processes through time at a single eruptive center. Early in the history of this eruptive center, magma mixing and interaction with crustal rocks were significant. As the system evolved, fractional crystallization emerged as the dominant process controlling the compositions of erupted lavas. Structural features in the Cenozoic volcanics follow the temporal progression of magma compositions. The Idaho batholith was emplaced into metamorphic rocks with pre-existing compressional deformation (vertical maximum compressional strain). Conjugate faulting within the Challic volcanics and the SCV indicate strike-slip conditions (vertical intermediate compressional strain). Parallel normal faults in the Miocene basalts show that the transition to an extensional regime (vertical minimum compressive strain) was essentially complete by middle Miocene time.

At the eastern terminus of the Doonerak Window four fault-bounded tectono-stratigraphic assemblages are recognized; they are from bottom to top: (1) lower Paleozoic phyllites, slates, and volcanic rocks (Apoon assemblage); (2) Mississippian imbricates consisting of conglomerates, shales, and limestones (Blarney Creek assemblage); (3) Endicott Mountains assemblage consisting of a lower sequence of calcareous clastic rocks and an upper sequence of noncalcareous clastic rocks; and (4) a lower Paleozoic assemblage composed of marbles, phyllites, and slates (Skajit assemblage). Previous workers have correlated the upper sequence of the Endicott with the Upper Devonian Hunt Fork Shale. The contact between the lower sequence and the Hunt Fork is gradational and interfingering. Limestones in the lower sequence contain Middle to Upper Devonian corals. Thus, the lower sequence can be correlated with the Beaucoup Formation. The Apoon/Blarney Creek contact was previously thought to be an angular unconformity. Relationships at the contact demonstrate that for the most part it is, instead, a tectonic contact. Where the contact is depositional, it is a disconformity. Three penetrative phases of folding (D1b, D2, and D3) are recognized in all four assemblages; an older phase (D1a) is extremely rare, but was observed in the Hunt Fork. No pre-Mississippian structures were encountered. The D1 and D2 structures are related to north-directed thrusting during the Jurassic-Cretaceous, while D3 may be related to late-stage left-lateral strike-slip displacement along EW-striking faults. A "breach" fault truncates the Blarney Creek assemblage on the southeastern flank of the Doonerak Window, and juxtaposes the two different facies of the Endicott Mountains assemblage on the eastern flank. The basal thrust of the Endicott Mountains assemblage is continuous, demonstrating that the Doonerak Window is an actual window. The observed structures can only be explained by a duplex model. The amount of shortening in the central Brooks Range is greater than 350 kilometers as estimated from balanced cross-sections.