Todd A. LaMaskin

Stratigraphic record of Triassic-Jurassic collisional tectonics in the Blue Mountains province, northeastern Oregon

Sedimentary and volcanic rocks in the Blue Mountains province (BMP) of northeastern Oregon preserve a well studied record of Triassic–Jurassic magmatism, basin evolution, and terrane accretion. Terranes of the BMP represent two magmatic arcs (Wallowa and Olds Ferry terranes), an intervening oceanic subduction and accretionary complex (Baker terrane), and a complex thick succession of sedimentary rocks commonly known as the Izee terrane. We divide volcanic and sedimentary rocks into two regionally correlative, unconformity-bounded megasequences: (1) MS-1, Late Triassic to Early Jurassic deposits that change up section from (1a) older volcanic and volcaniclastic deposits of the Wallowa and Olds Ferry arcs to (1b) marine turbidites, shale, and argillite with chert-clast conglomerate and olistostromes derived from the emergent Baker terrane; and (2) MS-2, Early to Late Jurassic marine deposits that overlap older rocks and structures and record ∼20 to 40 m.y. of deep crustal subsidence in a large marine basin. Many of the known stratigraphic relationships in the Blue Mountains cannot be explained using the existing model for a Middle Triassic to Late Jurassic west-facing, non-collisional volcanic arc and forearc basin. We propose a new tectonic model for the BMP based on prior studies and comparison to modern analogues, which includes: (1) Middle Triassic magmatism in the Wallowa and Olds Ferry arcs during subduction and progressive closure of an ocean basin; (2) Late Triassic collision between facing accretionary wedges of the Wallowa and Olds Ferry arcs, and growth of marine basins on both sides of the emergent Baker terrane thrust belt; (3) Early to Middle Jurassic terrane-continent collision which resulted in closure of a wide back-arc basin, crustal thickening and loading in Nevada, and growth of a large marine collisional basin in the BMP; and (4) Late Jurassic thrusting, regional shortening, and final accretion of the basin and underlying terranes to western North America. This analysis suggests that collisional tectonics may have played a significant role in plate interactions that drove Triassic–Jurassic crustal thickening, mountain building, and basin development in the western North American Cordillera.

Detrital zircon facies of Cordilleran terranes in western North America

Paleozoic–Mesozoic basins in Cordilleran terranes of western North America contain detrital zircon U-Pb age distributions that vary over 10–100 Ma in a systematic and predictable manner. A minimum of four detrital zircon age distributions, here termed “detrital zircon facies,” are present: (1) Paleoproterozoic and Archean facies, chiefly found in Paleozoic and early Mesozoic accretionary complexes, is defined by late Archean–early Proterozoic (ca. 2.7–2.3 Ga) and late Proterozoic ages (ca. 2.0– 1.6 Ga) with variable quantities of Paleozoic and early Mesozoic ages. (2) Mixed Proterozoic and Phanerozoic facies is found in Early–Late Jurassic basins and is defined by grains spanning ca. 2.0 Ga–160 Ma, derived from eastern-southwestern Laurentian transcontinental sources and enriched by western U.S. and eastern Mexican early Mesozoic plate-margin magmatism. (3) Triassic and Jurassic facies, found in Late Jurassic–Early Cretaceous basins, is defined by Late Jurassic ages (peak ca. 155 Ma) with a subordinate proportion of Triassic ages (peak ca. 230 Ma). (4) Jurassic and Early Cretaceous facies is found in late Early–early Late Cretaceous marginal basins and is defined by Jurassic and Early Cretaceous ages (ca. 200–130 and ca. 130–100 Ma). Detrital zircon U-Pb ages from terranes of western North America record stages of basin formation during phases of the supercontinent cycle and reflect second-order variability in the tectonic setting of an active continental plate margin. At this temporal and spatial scale, the integrated evolution of orogenic, erosion, and sedimenttransport systems controls sediment provenance.

Late Jurassic magmatism, metamorphism, and deformation in the Blue Mountains Province, northeast Oregon

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 Intermontane 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 metamorphic 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 melange 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

Westward Growth of Laurentia by Pre–Late Jurassic Terrane Accretion, Eastern Oregon and Western Idaho, United States

New U-Pb and Sm-Nd data from the Blue Mountains province, eastern Oregon and western Idaho, clarify terrane correlations and regional evolution of the western Laurentian plate margin during Mesozoic time. We report an Early Jurassic age for a red tuff unit at Pittsburg Landing, Idaho, which is 25 m.yr. older than previous Middle Jurassic estimates. In the Coon Hollow Formation at Pittsburg Landing and at the type location on the Snake River, chemical abrasion thermal ionization mass spectrometry U-Pb zircon ages on interbedded tuff and detrital zircon U-Pb maximum depositional ages indicate that deposition spanned ca. 160–150 Ma, entirely during Late Jurassic time. Detrital zircon U-Pb ages represent local Wallowa arc basement and regional magmatic sources spanning ca. 290–140 Ma. Mudrock Nd isotope compositions of the Coon Hollow Formation record an increase in juvenile magmatism consistent with regional Late Jurassic trends in western North American magmatic systems. These data show that the Coon Hollow Formation is not part of a Middle Jurassic overlap assemblage, as has been historically interpreted. Instead, we propose that the Coon Hollow Formation is part of a belt of suprasubduction-zone extensional back-arc basins that formed in latest Jurassic time due to a well-documented period of trench retreat in the western United States. Our new data require that the underlying Wallowa terrane was accreted to and received detritus from western North America by ca. 160 Ma (early Late Jurassic). This minimum estimate for the age of terrane accretion in western Idaho and eastern Oregon is substantially earlier than previous estimates (∼135–118 Ma). In the Blue Mountains region, westward expansion of Laurentia was accomplished by accretion of arc terranes to the North American craton prior to Late Jurassic time.

Sequence stratigraphy of the Middle to Upper Devonian Guilmette Formation, southern Egan and Schell Creek ranges, Nevada

The upper Middle to Upper Devonian Guilmette Formation (~800 m thick) of eastern Nevada was deposited on a low-energy, westward-deepening carbonate platform. Five depositional facies are recognized, including tidal-flat, restricted shallow subtidal, shallow subtidal, intermediate subtidal, and deep subtidal. Facies are arranged into meter-scale, upward-shallowing peritidal and subtidal cycles that have average periods of between ~30 to165 k.y. Intermediate through deep subtidal facies are also present as thick, noncyclic intervals. The mechanism that best explains the presence of subtidal cycles, transgressive-prone cycles, and exposure-capped cycles and of systematic cycle stacking patterns is high-frequency (104 to 105 yr), glacioeustatic sea-level fluctuations. Noncyclic intermediate and deep subtidal intervals represent missed sea-level oscillations when the seafloor lay too deep to record the effects of high-frequency fluctuations. Eleven, fourthto third-order depositional sequences are recognized from deepening and shallowing trends in depositional facies, changes in cycle stacking patterns, and subaerial exposure features. Catch-up style sequences deepen to intermediate to deep subtidal water depths during transgression and maximum flooding, indicating that sedimentation rates lagged behind accommodation space gains. Keep-up style sequences deepen only to shallow subtidal water depths, indicating that sedimentation rates kept pace with accommodation space gains throughout sequence development. Combining sequence stratigraphic interpretations and conodont biostratigraphy permits correlation across the study area and correlation with previously published Devonian sea-level curves. Sequences 1, 2, 3, and 4 correlate with T-R cycles IIa-1, IIa-2, IIb, and IIc, respectively. Sequence stratigraphic relationships suggest that initial deepening of T-R cycle IId may be represented by maximum flooding zones of Sequences 5, 6, or 7. Sequences 8, 9, and 10 are interpreted to represent regression at the end of T-R cycle IId. Sequence-scale facies patterns reflect second-order accommodation space changes related to the Kaskaskia supersequence. In particular, catch-up Sequences 1 through 7 represent the second-order transgressive systems tract; thick intervals of deep to intermediate subtidal facies in Sequences 3 and 4 are interpreted to represent the second-order maximum flooding zone. Keep-up sequences 8 through 10 record the second-order highstand systems tract. LaMaskin, T. A., and Elrick, M., 1997, Sequence stratigraphy of the Middle to Upper Devonian Guilmette Formation, southern Egan and Schell Creek ranges, Nevada, in Klapper, G., Murphy, M. A., and Talent, J. A., eds., Paleozoic Sequence Stratigraphy, Biostratigraphy, and Biogeography: Studies in Honor of J. Granville (“Jess”) Johnson: Boulder, Colorado, Geological Society of America Special Paper 321.

Intrusive and depositional constraints on the Cretaceous tectonic history of the southern Blue Mountains, eastern Oregon

We present an integrated study of the postcollisional (post–Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4–123.8 Ma crystallization ages and exhibit a north-northeast–younging trend of the magmatism. The 40Ar/39Ar analyses on biotite and hornblende indicate very rapid (<1 m.y.) cooling below biotite closure temperature (∼350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ∼200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic–Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.