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The disputed age of the deep crust of the Colorado Province is central to hypotheses for Paleoproterozoic crystal growth in the region. We studied the high-grade Big Creek Gneiss, southeastern Wyoming, as a potential exposure of pre-1780 Ma basement rocks. New geologic mapping and U-Pb geochronological data indicate that the Big Creek Gneiss exposes a deeper, but coeval, level of the Green Mountain arc relative to the predominantly supracrnstal section to the west. The Big Creek Gneiss is composed of: supracrustal rocks; a ca. 1780 Ma Green Mountain arc-correlative, bimodal intrusive suite; a ca. 1763 Ma extensional(?) bimodal intrusive suite; and widespread ca. 1630 Ma pegmatitic leucogranite. The mafic member of the younger bimodal suite is documented here for the first time. U-Pb zircon ages from migmatite leucosomes indicate penetrative deformation of the Big Creek Gneiss at ca. 1750 Ma. We find that the postarc intrusive suite is mantle involved, implying a second period of crustal growth. Shortening postdates arc magmatism by ~20 m.y., implying that termination of arc magmatism and accretion were separate events. Finally, criteria previously used to constrain the polarity of subduction for the Green Mountain arc are not reliable. We propose two competing models: (1) southward-dipping Green Mountain arc subduction (present coordinates), with slab breakoff--related magmatism following arc accretion; or (2) northward-dipping subduction, with extensional postarc magmatism. In both models, high-temperature deformation coincides with accretion along the Cheyenne belt, and extensional magmatism is an important component of crustal growth. We prefer the northward-dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks. doi: 10.1130/B30164.1

The Baker terrane, exposed in the Blue Mountains province of northeastern Oregon, is a long-lived, ancient (late Paleozoic-early Mesozoic) accretionary complex with an associated forearc. This composite terrane lies between the partially coeval Wallowa and Olds Ferry island-arc terranes. The northern margin of the Baker terrane is a broad zone (>25 km wide) of fault-bounded, imbricated slabs and slices of metaigneous and meta-sedimentary rocks faulted into chert-argillite melange of the Elkhorn Ridge Argillite. Metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 Ma and have low initial [sup.87]Sr/[sup.86]Sr ratios (0.7033-0.7034) and positive initial ENd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite melange have initial [sup.87]Sr/[sup.86]Sr values ranging from 0.7073 to 0.7094 and initial [[epsilon].sub.Nd] values between -4.7 and -7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the distal, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex, providing an on-land, ancient analog to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacific region. doi: 10.1130/B26493.1

Structural analysis and geochronologic data indicate a nearly orthogonal, late Eocene-Oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer-thick, extensional, mylonitic shear zone of the Ruby Mountains metamorphic core complex, Nevada. New U-Pb radiometric dating indicates that the development of a northward-trending lineation in the infrastructure is partly coeval with the development of a pervasive, west-northwest-trending lineation in the mylonitic shear zone. U-Pb monazite data from the leucogranite orthogneiss of Thorpe Creek indicate a crystallization age of ca. 36-39 Ma. Zircon fractions from a biotite monzogranite dike yield an age of ca. 29 Ma. The three dated samples from these units exhibit a penetrative, approximately north-south-trending elongation lineation. This lineation is commonly defined by oriented bundles of sillimanite and/or elongated aggregates of quartz and feldspar, indicating a synmetamorphic and syndeformational origin. The elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic, nonmylonitic infrastructural core of the northern Ruby Mountains. A portion of this midcrustal flow is coeval with the well-documented, west-northwest sense of slip in the structurally overlying kilometer-thick, mid-Tertiary mylonitic shear zone. Lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure, suggesting fundamental plastic decoupling between structural levels in this core complex. Furthermore, the infrastructure is characterized by overlapping, oppositely verging fold nappes, which are rooted to the east and west. One of the nappes may be synkinematic with the intrusion of the late Eocene orthogneiss of Thorpe Creek. In addition, the penetrative, elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes, suggesting a kinematically related evolution. The area is evaluated in terms of a whole-crust extension model. Magmatic underplating in the lower crust stimulated the production of late Eocene-early Oligocene granitic magmas, which invaded metasedimentary and Mesozoic granitic rocks of the middle crust. The midcrustal rocks, weakened by the magmatic heat influx, acted as a low-viscosity compensating material, decoupled from an extending upper crust. The fold nappes and lineation trends suggest large-scale flow of the weakened crust into the study area. The inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks. At present the inferred Eocene-early Oligocene phase of upper-crust extension remains unknown, but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large-scale upper-crustal extension.

In this paper, we show that deformation can be dated by combining mesoscopic and microscopic structural observations with an understanding of metamorphic mineral reactions and U-Pb ages of newly grown sphene (titanite). This approach can be used on a variety of rock types that have been deformed at a wide range of metamorphic conditions. In an example from the Proterozoic Laramie Peak shear zone of southeastern Wyoming, a single period of syntectonic sphene growth in sheared mafic dikes is documented both by a strong spatial relationship between deformation and metamorphism and by sphene microtextures. U-Pb analyses of sphene separates give overlapping concordant or nearly concordant ages with a weighted mean of the 207Pb/206Pb ages of 1763 [+ or -] 7 Ma. We interpret this direct age of deformation as evidence that the Laramie Peak shear zone records the cratonic response to the 1.78-1.74 Ga Cheyenne belt collisional event.

U-Pb analyses of 48 zircon and/or monazite fractions from ten intrusive rocks that range in composition from quartz diorite to leucogranite yield crystallization ages between 40 and 29 Ma. The new data indicate that Tertiary intrusive rocks constitute a widespread and significant magmatic component of the Ruby-East Humboldt metamorphic core complex. Some of the intrusive rocks are gneissic to mylonitic and have been penetratively deformed in a shear zone that can be traced for >100 km along the west flank of the core complex. These data, when coupled with previously published geochronologic data, indicate that an important episode of mylonitization occurred between approximately 29 and 23 Ma (late Oligocene). Some field relations and geochronologic data, however, also suggest an earlier history of mylonitization in parts of the core complex. This earlier mylonitic history may have occurred in the Eocene or even late Mesozoic time, although more-detailed geochronologic data are needed to clearly define the exact time interval. Sr, Nd, and Pb isotopic data from the dated intrusive units indicate that a relatively sharp, approximately east-west-striking crustal boundary separating predominantly Archean basement in the north from Proterozoic basement in the south transects the complex. We speculate that the crustal boundary represents the continuation of the Cheyenne belt into this part of northeastern Nevada. These new data cast doubt on the existence of the proposed 2.0-2.3 Ga crustal province in this part of the northern Great Basin. The isotopic data from the plutonic rocks coupled with published data from at least in part coeval volcanic rocks indicate that mantle-derived magmas were important in their genesis. Collectively, these data indicate that significant crustal growth, via the addition of mantle-derived magmas, occurred during large-magnitude extension in this part of the northern Great Basin.

Five extensional fault systems were active between the mid-Eocene and the Halocene, and this development was due to the exhumation of metamorphic crustal rocks within the East Humboldt-Wood Hills, Nevada. High-angle normal faults in the metamorphic complex were found to have the youngest faults, which traverse existing extensional fault systems. Neogene and Quaternary extension in the East Humboldt-Wood Hills was due to the presence of high-angle normal faults in range fronts.

In the northern Ruby Mountains-East Humboldt Range metamorphic core complex, a kinematically coupled mylonitic shear zone and brittle-plastic detachment fault compose a low-angle extensional fault system that is mid-Tertiary in age. Geothermobarometry of pelitic schists from the mylonitic shear zone yields a temperature and pressure estimate of 820-900 K (580-620 [degrees]C) and 310-370 MPa (3.1-3.7 kbar) for mylonite petrogenesis. The shear-zone-detachment-fault system excised 11-12 km of crustal section and accommodated at least 15-17 km of normal-sense dip slip.

An unusually fossiliferous sedimentary sequence is exposed near Scarborough, Tobago, West Indies. It occurs within the Tobago Volcanic Group above an undifferentiated sequence of volcaniclastic breccia and lava and includes volcanogenic sedimentary rocks at the base of the Bacolet Formation. An ammonite-radiolarian assemblage has been recovered from this sedimentary interval in the Tobago Volcanic Group. Ammonites from the Spring Gardens quarry are moderately evolute and have unbranched ribs; they are probably juveniles of Mojsisoviczia, of middle Albian age. Another ammonite from a stratigraphically higher locality (i.e., volcanogenic sedimentary rocks at the base of the Bacolet Formation) is identified as Manuaniceras decsernae Young, of early late Albian age. Radiolarian assemblages from intercalations of black siliceous argillite of the Tobago Volcanic Group are characterized by abundant Archaeospongoprunum spp. and the common occurrence of Pseudodictyomitra pentacolaensis, Dictyomitra montisserei, Thanarla brouweri, and Pantanellium ventosum. Also present in lesser abundances are other members of the family Archaeodictyomitridae, Xitus, Napora durhami, and Sciadiocapsa speciosa. This assemblage correlates with the Romanus subzone of O'Dogherty (1994) and Kozurium zingulai zone of Pessagno (1977), both of which are dated as early to middle Albian. These newly reported fossils indicate an Albian age for deposition of the Tobago Volcanic Group. Earlier published [sup.40]Ar/[sup.39]Ar radiometric ages from the group are consistent with the stratigraphic age assignment of the fossils and thereby suggest that volcanism associated with the Tobago Volcanic Group began ca. 105 Ma. Possible correlative, accreted fragments of the Mesozoic oceanic-arc of the southern Caribbean include the Curacao Lava Formation, the lower part of the Washikemba Formation (Bonaire), the Tiara Formation of north-central Venezuela, and possibly the subsurface Mejillones complex of the Carupano basin. The Mesozoic rocks of Tobago consist of two distinct, oceanic-arc rock sequences. The older North Coast Schist ([is greater than or equal to] 120 Ma) exhibits polyphase deformation and lower greenschist facies metamorphism. The structurally overlying Tobago Volcanic Group is not penetratively deformed or folded; it has undergone prehnite-pumpellyite facies metamorphic conditions and has been tilted to moderate dips by various brittle faults. We interpret the hiatus that separates these oceanic-arc rock sequences as representing a significant event in the evolution of the Great Arc of the Caribbean. Consequently, we propose that the commonly postulated reversal in subductionzone polarity of the Great Arc occurred prior to deposition of the Tobago Volcanic Group (ca. 105 Ma), and that older, more highly deformed and metamorphosed oceanic-arc sequences (e.g., North Coast Schist) served as a basement for younger sequences or as wall rock for intrusive elements of younger magmatic suites of the Great Arc. Keywords: Caribbean area, Tobago, Cretaceous, ammonites, radiolarians.

An early to mid-Mesozoic record of sedimentation, magmatism, and metamorphism is well developed in the Blue Mountains Province of northeast Oregon. Detailed studies both north and south of the Blue Mountains Province (e.g., terranes of the Inter montane belt, Klamath Mountains, and western Sierra Nevada) have documented a complex Middle to Late Jurassic orogenic evolution. However, the timing of magmatic, metamorphic, and deformational events in the Blue Mountains, and the significance of these events in relationship to other terranes in the western North American Cordillera remain poorly understood. In this study, we investigate the structural, magmatic, and meta morphic histories of brittle to semibrittle deformation zones that indicate widespread Late Jurassic orogenesis in the Blue Mountains Province. Folding and faulting associated with contractional deformation are primarily localized along terrane boundaries (e.g., Baker-Wallowa and Baker--Izee--Olds Ferry boundaries) and within the composite Baker oceanic mélange terrane (e.g., Bourne-Greenhorn subterrane boundary). These brittle to semibrittle deformation zones are broadly characterized by the development of E-W--oriented slaty to spaced cleavage in fine-grained metasedimentary rocks of the Baker terrane (e.g., Elkhorn Ridge Argillite), approximately N-S--bivergent folding, and N- and S-dipping reverse and thrust faulting on opposite flanks of the Baker terrane. Similarly oriented contractional features are also present in late Middle Triassic to early Late Jurassic (i.e., Oxfordian Stage, ca. 159 Ma) sedimentary rocks of the John Day and Huntington areas of northeast Oregon. Radiometric age constraints from youngest detrital zircons in deformed sedimentary rocks and crystallization ages of postkinematic plutons, which intrude the deformation zones, limit deformation to between ca. 159 and ca. 154 Ma. We suggest that the widespread, approximately N-S--directed contractional features in the Blue Mountains Province record a short-lived, intense early Late Jurassic deformational event and preserve an example of upper-crustal strain localization associated with terminal arc-arc collision between the Olds Ferry and Wallowa island-arc terranes. The age interval of deformation in the Blue Mountains Province is younger than Middle Jurassic deformation in the Canadian Cordillera and Klamath Mountains (Siskiyou orogeny) and predates classic Nevadan orogenesis. [ABSTRACT FROM AUTHOR]

Three metamorphic core complexes, Ruby Mountains-East Humboldt Range (R-EH), Albion Mountains- Raft River Mountains-Grouse Creek Mountains (A-RR-GC), and Snake Range (SR) are exposed in the northeastern Great Basin (Nevada, Utah, and Idaho). Their structural, magmatic, and metamorphic histories are synthesized and compared to evaluate fundamental tectonic processes in this area in which large-magnitude crustal contraction was followed by large-magnitude crustal extension. Throughout the region, contraction and magmatism began in the Late Jurassic, and in the R-EH and SR areas culminated at -80-90 Ma. The A-RR-GC core complex experienced multiple episodes of Late Cretaceous tectonic denudation beginning at -105 Ma, and it records no Mesozoic magmatism, apparently due to an infertile lower crust. The R-EH core complex also underwent tectonic denudation in latest Cretaceous/early Tertiary time. This illustrates the importance of syncontractional adjustments to thickened orogenic wedges and that these adjustments need not occur simultaneously throughout an orogen. Syncontractional extension and denudation in the A-RR-GC and R-EH areas also limited the amount, extent, and distribution of subsequent Tertiary extension. Eocene magmatism was immediately followed by an intense pulse of crustal extension in all three areas. However, late Oligocene and early Miocene extension throughout the region was diachronous and was not always coincident with a widespread mid-Oligocene magmatic event, suggesting that Oligocene magmatism and extension are not entirely linked. The magnitude of tectonic denudation in the R-EH and SR cote complexes varies along the strike of the core complex. In the R-EH area, this variation may be due to the restriction of Late Cretaceous(?) and middle Eocene episodes of extension to the northern part of the cote complex. In the SR core complex this variation may be the result of a lateral ramp in the structural surface of the Snake Range décollement. These intra-core complex variations in tectonic denudation appear to be partly accommodated by solid-state flow of the lower crust into regions of high mid- to upper-crustal extension, and they require a complex, three-dimensional pattern of lower-crustal redistribution and magmatism. Oppositely vergent extensional fault systems in the A-RR-GC metamorphic core complex may have developed because the rigid Archean lower crust in this area could not flow into highly extended domain [ABSTRACT FROM AUTHOR]