Showing papers in "The Journal of Geology in 1997"

Quartz and Feldspar Stability, Steady and Non‐Steady‐State Weathering, and Petrogenesis of Siliciclastic Sands and Muds

Abstract: The kinetic and thermodynamic properties of quartz, plagioclase, and K‐feldspars, which constitute 70 to 80% of the upper crust, provide a framework for prediction of mineralogical and chemical changes involved in the production of siliciclastic sediments. Chemical weathering of bedrock may produce weathering profiles with distinct mineralogical zones, compositionally much different from the parent rock. Mass wasting of such profiles produces sediments that reflect the mineralogy of the zones exposed to mechanical erosion, rather than the composition of fresh bedrock. The relative rates of chemical weathering and mechanical erosion determine which mineralogical zones are exposed to mass wasting, and therefore control the compositions of siliciclastic sediments. Stable rates of chemical weathering and erosion result in steady‐state weathering, so that thickness and the mineralogical composition of eroded soil zones, and therefore the mineralogy of derived sediments, remain unchanged while steady‐state weat...

Shisha Pangma leucogranite, south Tibetan Himalaya: Field relations, geochemistry, age, origin, and emplacement

Abstract: The Shisha Pangma pluton forming most of the Xixabangma (8027 m) massif in south Tibet is one of the 20+ larger leucogranite intrusives that mark the highest structural levels of the Himalayan metamorphic core. The pluton occurs immediately below the Shisha Pangma Detachment, a strand of the South Tibetan Detachment (STD) system, a low angle (30°) north‐dipping normal fault placing Paleozoic black slates atop sillimanite‐grade pelites and calc‐silicate rocks. K‐feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal S‐C fabrics indicative of down‐to‐the‐north extension. The Shisha Pangma leucogranite is a heterogeneous, polyphase intrusion with an earlier, foliated biotite‐rich phase and a later, tourmaline + muscovite rich phase typically containing the assemblage: Kfs + Pl + Qtz + Ms + Tur ± Gt ± Bt ± Sil ± Ap. The highly peraluminous granites have high 87Sr/86Sr ratios (0.738‐0.750) typical of pelite‐derived anatectites. Nd‐depleted mantle model...

Depositional Processes in Large-Scale Debris-Flow Experiments

Abstract: This study examines the depositional process and characteristics of deposits of large‐scale experimental debris flows (to 15 m3) composed of mixtures of gravel (to 32 mm), sand, and mud. The experiments were performed using a 95‐m‐long, 2‐m‐wide debris‐flow flume that slopes 31°. Following release, experimental debris flows invariably developed numerous shallow (∼ 10 cm deep) surges. Sediment transported by surges accumulated abruptly on a 3° runout slope at the mouth of the flume. Deposits developed in a complex manner through a combination of shoving forward and shouldering aside previously deposited debris and through progressive vertical accretion. Progressive accretion by the experimental flows is contrary to commonly assumed en masse sedimentation by debris flows. Despite progressive sediment emplacement, deposits were composed of unstratified accumulations of generally unsorted debris; hence massively textured, poorly sorted debris‐flow deposits are not emplaced uniquely en masse. The depositional ...

Statistics For Strontium Isotope Stratigraphy: A Robust Lowess Fit to the Marine Sr‐Isotope Curve For 0 to 206 Ma, With Look‐Up Table For Derivation of Numeric Age

Abstract: We provide a best‐fit curve to 1849 strontium isotope data for the period 0 to 206 Ma using the LOcally‐WEighted regression Scatterplot Smoother (LOWESS) method. This is a robust, nonparametric modern regression technique. Since it does not yield an explicit mathematical equation relating 87Sr/86Sr to time, a look‐up table to determine numeric age has been generated in steps of 1 × 10−6 in 87Sr/86Sr. The calibration uses the timescales of Shackleton and coworkers for 0‐7 Ma; Cande and Kent for 7‐72 Ma; Obradovich for 72‐95 Ma and Gradstein and coworkers for >95 Ma. The look‐up table includes 95% confidence intervals on the predictions of numeric age. When using this table, the uncertainty on the 87Sr/86Sr of the sample whose age is sought must be added to that inherent in the LOWESS regression. We show how to determine the uncertainty in 87Sr/86, i.e., how best to obtain the 95% confidence bounds on a single measurement of 87Sr/86 for a sample, and on the mean 87Sr/86 value for 2 or more replicate measure...

The Bishop Tuff: New Insights From Eruptive Stratigraphy

Abstract: The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1‐F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1‐F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2‐F8), Lack phenocrystic pyroxenes, and contain few or no Glass Mountain‐derived rhyolite lithic fragments. Earlier packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in G...

Pan-Continental River System Draining Grenville Orogen Recorded by U-Pb and Sm-Nd Geochronology of Neoproterozoic Quartzarenites and Mudrocks, Northwestern Canada

Abstract: Regional lithostratigraphic correlation of early Neoproterozoic Sequence B on the northwest margin of Laurentia includes a 0.5-2 km-thick, laterally continuous, quartzarenite marker of mainly fluvial origin. Concordant U-Pb ages of 54 single detrital zircons from five regionally dispersed samples cluster in the Archean (2.8-2.6 Ga), Paleoproterozoic (2.0-1.9 Ga), and Mesoproterozoic (1.6-1.0 Ga). The vast majority (85%) of the zircons are of Mesoproterozoic age, with a high proportion clustering between 1.25-1.0 Ga, closely matching the age of synorogenic intrusions in the Grenville Province of North America. A $$100 3 \pm 4 Ma$$ zircon from the Pinguicula group in the eastern Ogilvie Mountains provides a maximum age for the quartzarenite marker. Sm-Nd isotopic data from intercalated mudrocks indicate a major contribution from sources with relatively juvenile model ages ($$T_{DM} = 1.54-1.74 Ga$$), consistent with the provenance indicated by the detrital zircons. The isotopic data support both regional li...

Proterozoic Events in the Eastern Ghats Granulite Belt, India: Evidence From Rb‐Sr, Sm‐Nd Systematics, and Shrimp Dating

Abstract: Metamorphic and protolith ages of five rock types (mafic granulite, orthopyroxene granulite, leptynite, sillimanite granite, and metapelite) from Rayagada, in the north‐central part of the Eastern Ghats Granulite Belt (EGGB), India, were determined from Rb‐Sr and Sm‐Nd whole rock and mineral isochrons in combination with SHRIMP U‐Pb zircon data. Most of the whole rock isochron ages in both Sm‐Nd and Rb‐Sr systems point to either ∼1450 or ∼1000, Ma, and the mineral isochron ages are ∼1000, ∼800, and ∼550 Ma. SHRIMP U‐Pb zircon ages of ∼940 Ma were obtained from metapelite, which are in close agreement with the Sm‐Nd and Rb‐Sr isochron ages. From all these data, four age clusters (∼1450, ∼1000, ∼800, and ∼550 Ma) have been noted. The 1450 Ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite. The 1000 Ma age cluster is regarded as the intrusion ages of sillimanite granite, and charnockite, and associated granulite facies metamorphism. Two other age clusters (800 a...

U‐Pb Zircon Ages of Granitoid Rocks in the North Dabie Complex, Eastern Dabie Shan, China

Abstract: The North Dabie Complex (NDC) comprises a high‐temperature metamorphic suite dominated by amphibolite‐facies granitoid orthogneiss intruded by voluminous granites. U‐Pb zircon dating on these rocks yields the following new results: (1) Two of the granitoid orthogneiss samples have Early Cretaceous protolith emplacement ages of 133.7 ± 2.3 Ma and 134.0 ± 2.8 Ma. (2) an undeformed granodiorite intrusive into granitoid gneiss yields an emplacement age at 125.6 ± 0.3 Ma. These Early Cretaceous ages entirely post‐date Late Triassic‐Early Jurassic continental subduction and exhumation evident in the neighboring ultrahigh and high‐pressure metamorphic terrains to the south in the Dabie Shan; hence they do not support previous tectonic models that infer Paleozoic or Triassic protolith emplacement ages for these granitoid orthogneisses. (3) A mylonitic granite from the northern boundary of the NDC yields a concordant age of 756.6 ± 0.8 Ma, which is interpreted as magma emplacement age. The concordance of this Late...

Stratospheric Loading of Sulfur From Explosive Volcanic Eruptions

Abstract: A B STRA C T This paper is an attempt to measure our understanding of volcano/atmosphere interactions by comparing a box model of potential volcanogenic aerosol production and removal in the stratosphere with the stratospheric aerosol optical depth over the period of 1979 to 1994. Model results and observed data are in good agreement both in magnitude and removal rates for the two largest eruptions, El Chichon and Pinatubo. However, the peak of stratospheric optical depth occurs about nine months after the eruptions, four times longer than the model prediction, which is driven by actual SO2 measurements. For smaller eruptions, the observed stratospheric perturbation is typically much less pronounced than modeled, and the observed aerosol removal rates much slower than expected. These results indicate several limitations in our knowledge of the volcano-atmosphere reactions in the months following an eruption. Further, it is evident that much of the emitted sulfur from smaller eruptions fails to produce any stratospheric impact. This suggests a threshold whereby eruption columns that do not rise much higher than the tropopause (which decreases in height from equatorial to polar latitudes) are subject to highly efficient self-removal processes. For low latitude volcanoes during our period of study, eruption rates on the order of 50,000 m 3 /s (dense rock equivalent) were needed to produce a significant global perturbation in stratospheric optical depth, i.e., greater than 0.001. However, at high (.40°) latitudes, this level of stratospheric impact was produced by eruption rates an order of magnitude smaller.

Magmatic Underplating, Extension, and Crustal Reequilibration: Insights From A Cross-Section Through the Ivrea Zone and Strona-Ceneri Zone, Northern Italy

Abstract: The thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the Ivrea‐Verbano Zone and Strona‐Ceneri Zone in northern Italy. Geochronological and petrologic data are compared to a two‐dimensional thermal‐kinematic model. Field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite‐ to amphibolite‐facies metamorphism as well as reequilibration of mineral chemical and isotopic systems. Magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution, as information on the earlier history is preserved only at upper crustal levels.

Neoproterozoic tectonics of Australia-Antarctica and Laurentia and the 560 Ma birth of the Pacific Ocean reflect the 400 m.y. Pangean supercycle

Abstract: Unequivocal evidence for the Proterozoic reconstruction of Australia‐Antarctica and Laurentia remains elusive, although various authors have interpreted sedimentary and igneous events in terms of initial (Neoproterozoic) rifting, and final (Neoproterozoic/Cambrian) drifting. The synchronous rifting and drifting reflect the tectonics of a late Neoproterozoic Pangea (East Gondwanaland and Laurentia) that amalgamated along the Mozambiquean belt ∼720 Ma and broke up at 560 Ma by growth of the Paleo‐Pacific and lapetus oceans. In this paper these events are interpreted as part of a 400 m.y. supercycle comparable to the 320 Ma amalgamation of Pangea A and its 160 Ma breakup during Supercycle A (320 Ma to present). Events near the end of the Cambrian (500 Ma) in East Gondwanaland included epeirogenic uplift of cratons, shown by widespread K/Ar and apatite fission‐track dates, and the inception of quartz turbidite fans, intense deformation and intrusion by granite of the Antarctic and south‐eastern Australian mar...

Contrasting Sources For Upper and Lower Continental Crust: the Greenstone Connection

Abstract: Greenstones may be useful in tracking the role of oceanic plateaus in the growth of continents. Lithologic proportions, Th/Ta ratios, and Ni concentrations in greenstone basalts of all ages show that arc‐related greenstones greatly exceed oceanic plateau and MORB‐related greenstones in abundance. This distribution may be accounted for by the preferential obduction of arcs formed on top of oceanic plateaus during collision of plateaus with continents. Because thick oceanic plateaus resist subduction, a significant volume may be accreted to continental margins during collisions; and over time, these plateaus may evolve into lower continental crust. This idea has important implications for continental development in that the lower continental crust may come chiefly from accreted oceanic plateaus, while upper continental crust forms by subduction‐related processes. Consistent with this model are high seismic wave velocity layers in the lower crust of Proterozoic cratons similar to high‐velocity layer...

Komatiite Flooding of A Rifted Archean Rhyolitic Arc Complex: Geochemical Signature and Tectonic Significance of the Stoughton-Roquemaure Group, Abitibi Greenstone Belt, Canada

Abstract: The 0.2‐2 km thick, Archean Stoughton‐Roquemaure Group (SRG) in the Northern Volcanic Zone of the Abitibi greenstone belt (Quebec, Canada) is composed of tholeiitic basalt, komatiitic basalt and komatiite. The mafic and ultramafic rocks are pillowed, brecciated, and massive columnar‐jointed flows. The SRG conformably overlies the 2730 Ma Hunter Mine Group, a volcanic complex dominated by calc‐alkaline felsic rocks. The tholeiitic basalts of the SRG resemble MORB. The komatiitic basalts and komatiites have positive ϵNd values, overlapping those of the tholeiitic basalts. Komatiitic basalts, with low Al2O3/TiO2 ratios (∼10) and fractionated heavy REE patterns, are similar to Al‐depleted komatiites. In contrast, the komatiites have high Al2O3/TiO2 (∼20), unfractionated heavy REE patterns and resemble Al‐undepleted Munro‐type komatiites. The Al‐depleted komatiitic basalts occur at the base of the SRG, whereas the Al‐undepleted komatiites are prevalent higher up in the stratigraphy. The association of calcalka...

Open‐Channel Flow of Bingham Fluids: Applications in Debris‐Flow Research

Abstract: The results of a numerical simulation of open‐channel flow of Bingham fluids provide improved capabilities for analyzing muddy debris flows and their depositis, and for routing debris flows. Because of limitations of the phenomenonological Bingham model, results are deemed applicable to mud‐rich debris flows only. Most published field estimates of debris‐flow rheological (Bingham) properties depend on application to natural channels of analytical solutions for flow through channels with either circular or very wide channel cross‐sections. Lack of a general solution for flow of Bingham fluids in channels with realistic shapes has restricted capabilities in debris‐flow modeling and field data analysis. Numerical solutions permit generalization and extension of analytical solutions to channels of arbitrary cross‐sectional form without loss of accuracy. The numerical model (FIDAP) utilizes a well established finite‐element formulation of the non‐Newtonian fluid flow problem and reproduces available analytical...

Relation of Alkaline Volcanism and Active Tectonism Within the Evolution of the Isparta Angle, Sw Turkey

Abstract: The Isparta Angle (IA) is formed along the boundary of the African and Eurasian plates by NE‐ and NW‐striking faults north of the Antalya Gulf in SW Turkey. The NE‐striking strike‐slip Burdur fault bounds the IA to the west and is probably the continuation of the Pliny system of the Hellenic arc; the NW‐striking Aksehir fault bounds it to the east. Platform‐type, parautochthonous Mesozoic carbonate sequences such as the Beydag and Anamas‐Akseki occur in the western and eastern parts of the IA, respectively, whereas allochthonous ophiolite nappes include the Antalya, Beysehir‐Hoyran, and Lycian nappes. The IA and adjacent areas are divided into three areas–the Teke, the Antalya, and the Akseki fragments–by NE‐, NW‐, and N‐striking active strike‐slip faults with normal components. IA region volcanics are alkaline and hyperkaline in character (potassic, ultrapotassic) and locally occur as subvolcanic stocks and dikes. They can be traced between the Afyon and Isparta regions. These volcanics consist mainly of...

Orogenic to Post‐Orogenic Origin For the 1.76 Ga Horse Creek Anorthosite Complex, Wyoming, Usa

Abstract: The age and inferred tectonic setting of the 1.76 Ga Horse Creek anorthosite complex (HCAC) in the Laramie Mountains of southeastern Wyoming place important constraints on the origin of middle Proterozoic anorthosite complexes. The 100 km2 HCAC consists of strongly recrystallized anorthosite and two small intrusions of monzonite and granite. U‐Pb crystallization ages from euhedral zircons in anorthosite and monzonite are 1761.5 ± 2 Ma and 1754.5 ± 2.2 Ma, respectively. An additional period of zircon growth in the anorthosite occurred at 1753.4 ± 2 Ma, as represented by a small population of anhedral zircon. We attribute the origin of this second morphological variety of zircon in the anorthosite to the loss of Zr from ilmenite during reaction with plagioclase to form sphene. This reaction took place in response to heat and fluid influx during intrusion of the adjacent monzonite. The HCAC and the younger 1.43 Ga Laramie anorthosite complex to the north were intruded along or near a Paleoproterozoic suture ...

Sources of Rare-Earth Elements and Iron in Paleoproterozoic Iron-Formations from the Transvaal Supergroup, South Africa: Evidence from Neodymium Isotopes

Abstract: Neodymium isotopic data are reported for pure chemical sediments from the Siderian Kuruman and Penge Iron-Formations (IFs) in the Transvaal Supergroup, South Africa. From a large sample set for which petrographical and chemical data was available, we selected samples which best represent the pure chemical sediment and still carry the primary rare-earth element (REE) distribution for isotope analyses. Both the Kuruman IF ( + 1.9 and +1.1) and the Penge IF (+ 0.2 and -0.2) show $$\epsilon_{Nd} (2.5 Ga)$$ values more radiogenic than published values for epiclastic sediments from the slightly older Black Reef Formation (-5.1 to -3.2). This suggests that REE and iron in the Transvaal IFs were predominantly, though not exclusively, derived from hydrothermal alteration of oceanic crust. The $$\epsilon_{Nd} (2.5 Ga)$$ value of the Kuruman IF is similar to that proposed for the Hamersley IF in western Australia, suggesting that $$\epsilon_{Nd}(t) values of + 1 \pm 1$$ were typical of IF-depositing open ocean water...

Controls on Stylolite Development: Morphologic, Lithologic, and Temporal Evidence from Bedding-Parallel and Transverse Stylolites from the U.S. Appalachians

Abstract: Quantification of morphology of bedding-parallel and transverse (or "tectonic") stylolites from the southern Appalachians reveals that, regardless of tectonic setting or stylolite orientation, lithologic heterogeneity controls stylolite development. For example, stylolites in more heterogeneous carbonate lithologies (packstones and grainstones) are more serrate than those in wackestones and mudstones. Similarly, early-forming stylolites are more serrate than late-forming stylolites, and computer simulations suggest that decreases in porosity or other lithologic heterogeneity cause the less serrate nature of late-forming stylolites. The linkage of a stylolite's morphology to its time of development should allow field estimation of timing of stylolitization and thus of timing of loading or compression. Although bedding-parallel and transverse stylolites generally result from different kinds of compression, morphological relations indicate that their development follows the same fundamental patterns. Bedding...

Cycle Anatomy and Variability in the Storm‐Dominated Type Cincinnatian (Upper Ordovician): Coming to Grips With Cycle Delineation and Genesis

Abstract: Although parasequence and sequence are scale‐independent terms, they are frequently applied only to specific scales of cycles. For example, meter‐scale cycles are commonly assumed to be parasequences or PACs. In the Upper Ordovi‐cian Kope and Fairview Formations of northern Kentucky, we examined a succession of 50 meter‐scale cycles that have been variously interpreted as deepening‐upward, shallowing‐upward, or showing no relationship with water depth. Our analysis shows that these cycles, characterized by shifts in storm‐bed proximality, are highly variable in their thickness and internal construction. Most cycles are best considered high‐frequency sequences, because deepening‐upward intervals are common, and many cycles contain evidence of abrupt basinward shifts in facies as expected at sequence boundaries. A minority fit the parasequence model of shallowing‐upward cycles bounded by flooding surfaces. Larger, 20 m scale cycles are defined by systematic thickening and thinning trends of meter‐scale cycl...

Dextral Pan‐African Shear Along the Southwestern Edge of the Achankovil Shear Belt, South India: Constraints on Gondwana Reconstructions

Abstract: The Achankovil shear belt of the southern Indian Peninsular shield is prominent on Landsat images. It coincides with the boundary between charnockites of the Madurai block to the north and khondalites of the Trivandrum block to the south, and with a major change in the aeromagnetic pattern that can be traced across southern India. Field investigations reveal a major shear zone along the southwestern edge of the Achankovil shear belt, the Tenmala shear zone. Rocks in the Tenmala shear zone include charnockite, garnet‐biotite gneiss, garnet‐sillimanite gneiss, cordierite gneiss, and granite. Kinematic indicators include stretched and asymmetric garnet, feldspar and quartz porphyroclasts, shear bands, asymmetric folds, extensional and contractional composite structures, hook folds on rotated and deformed gash veins, and lineations. Kinematic analysis of these features along 60 km of the shear zone indicates primarily dextral shear, with a minor component of reverse shear. Textures, as well as mineral assembl...

Deep Crustal Shear Zones, High-Grade Tectonites, and Associated Metasomatic Alteration in the Limpopo Belt, South Africa: Implications for Deep Crustal Processes

Abstract: Large thrust sheets of complexly folded ($D_{1}$) migmatitic granulites bounded by a regional system of anastomosing ductile shear zones ($D_{2}$) characterize the Southern Marginal Zone of the Limpopo Belt. Tectonites within these shear zones are subdivided into "primary straight gneisses" with preserved textures that can unambiguously be referred to the $D_{2}$ exhumation stage, "annealed straight gneisses" that recrystallized during post $D_{2}$ cooling, and classic mylonites that post-date the Limpopo orogeny. Primary straight gneisses within these shear zones contain textural and mineralogical features that suggest they developed by a shear-related process of grain size reduction and fabric formation due to decreasing pressure and temperature under granulite facies conditions. This process is evident from the synkinematic replacement of garnet by cordierite and hypersthene symplectites. The resultant asymmetric sigmoidal corona textures enhance the shear fabric. In contrast, pelitic granulites from u...

Chlorite Thermometry in the Witwatersrand Basin: Constraints on the Paleoproterozoic Geotherm in the Kaapvaal Craton, South Africa

Abstract: A series of post‐depositional alteration events can be distinguished in the late Archean Witwatersrand Basin. They range from thrust‐controlled metamorphism during pre‐Transvaal crustal thickening, burial metamorphism and hydrothermal infiltration during crustal thinning and associated deposition of the 2.6‐2.1 Ga Transvaal Supergroup sediments, thermal metamorphism related to the 2.06 Ga Bushveld Igneous Complex, and hydrothermal alteration as a consequence of the 2.023 Ga Vredefort impact event. The relative importance of these events varies locally, with different events responsible for the maximum temperatures achieved in different parts of the basin: the pre‐Transvaal metamorphism affected the northern margin of the basin, whereas the syn‐Bushveld thermal metamorphism is particularly pronounced in the lower parts of the Witwatersrand Supergroup. Chlorite geothermometry, in conjunction with fluid inclusion microthermometry and the thermodynamic modeling of equilibrium mineral assemblages, was applied ...

Is the Lower Duarte Igneous Complex (Hispaniola) a Remnant of the Caribbean Plume-Generated Oceanic Plateau?

Abstract: Eleven samples of metapicrites, metaankaramites, diabases, and cumulates of the lower Duarte Complex in central Hispaniola were analyzed for major, trace element, and Nd-Sr isotopic compositions T

40Ar/39Ar Evidence For 1.4 Ga Regional Metamorphism in New Mexico: Implications For Thermal Evolution of Lithosphere in the Southwestern Usa

Abstract: 40Ar/39Ar dates recorded by hornblende and muscovite from Proterozoic rocks in northern New Mexico indicate pervasive Mesoproterozoic metamorphism followed by a protracted uplift/cooling history. Most hornblendes yield 40Ar/39Ar dates of 1.43‐1.35 Ga, which record cooling through temperatures of ca. 500°C. This suggests that a regional amphibolite facies metamorphism (500‐550°C, 3.5‐4 kbars occurred at ca. 1.4 Ga. This metamorphism was superimposed on Paleoproterozoic amphibolite grade rocks at mid‐crustal depths of 10‐15 km. Contact metamorphic aureoles associated with 1.4 Ga plutons emplaced at 2.5‐4 kbars (8‐15 km) in the Sandia and Manzano Mountains record nearly concordant hornblende and muscovite 40Ar/39Ar dates of 1.4 Ga. These suggest relatively rapid cooling to ambient conditions of <300°C following the 1.4 Ga thermal event. Slightly younger (1.37‐1.31 Ga) dates are recorded by muscovite throughout northern New Mexico and by hornblende within high‐grade tectonic blocks. These are interpreted to r...

Stepwise climate change recorded in Eocene-Oligocene paleosol sequences from Central Oregon

Abstract: Successions of paleosols bounded by erosional surfaces in fluvial sediments of the Eocene‐Oligocene strata of Central Oregon can be interpreted as terrestrial equivalents of the unconformity‐bound units of sequence stratigraphy. In the upper part of the upper Eocene Clarno Formation and in the lower part of the lower Eocene‐lower Miocene John Day Formation, truncation surfaces separate otherwise conformable alluvial deposits and allow for stratigraphic subdivision into informal members (lower and upper “Red Hill” claystones in the Clarno Formation and lower, middle, and upper Big Basin Members and lower Turtle Cove Member in the John Day Formation). Paleosols in each member show a stepwise change in the degree of weathering of the most strongly developed paleosols: kaolinite‐rich, Ultisollike paleosols in lower “Red Hill” claystones (late Eocene, 42‐43 Ma), smectite‐rich Alfisol‐like paleosols in the upper “Red Hill” claystones (late Eocene, 41‐42 Ma), Alfisols and Ultisol‐like paleosols in the lower Big ...

Timing Structural Assembly, Metamorphism, and Cooling of Caledonian Nappes in the Ofoten-Efjorden Arca, North Norway: Tectonic Insights From U-Pb and40Ar/39Ar Geochronology

Abstract: U‐Pb and 40Ar/39Ar geochronology from part of the northern Norwegian Caledonides indicates a cooling and unroofing history different from that inferred for the Caledonides in southwest Norway. In the Ofoten‐Efjorden area of north Norway, the Narvik Nappe Complex (NNC) represents the basal portion of the composite Caledonian allochthon and consists of amphibolite‐facies oceanic‐affinity metasedimentary rocks intruded by numerous pretectonic felsic dikes. Two of these dikes yield indistinguishable U‐Pb zircon ages of 437 ± 1 Ma, providing a maximum age for tectonic assembly, metamorphism, and deformation at this structural level. U‐Pb ages of metamorphic monazite (432 ± 2 Ma) and zircon (431.7 ± 0.5 Ma) from pelitic schist and migmatite yield robust estimates of the age of peak metamorphism. Combined with structural evidence for transport‐parallel stretching and subvertical attenuation of the rock mass at the time of metamorphism, these ages suggest that local and/or episodic extensional modification of the...

Magma Mingling in Dikes and Sills

Abstract: Dikes in which two different liquids flowed simultaneously, usually called composite dikes, naturally fall into two classes depending on which lithology forms the contact with the country rock, and hence, which liquid was the first to enter the fracture. In these two kinds of dikes, the structures formed by the mingling of the two liquids differ. Dikes in which the more basic liquid entered first have contacts between the two lithologies that are nearly planar, and parallel to the dike walls, whereas the more basic lithology forms discrete pillows in those dikes in which the more silicic liquid entered first. Experiments indicate that these pillows probably form from a flow-front instability that develops when a liquid invades another of higher viscosity between two parallel rigid walls. We provide scalings for the critical flow rate for the onset of this instability, the time required for the instability to develop, and the wavelength that is selected. These scalings are consistent with field observations.

Magnetostratigraphy Across the Wasatchian/Bridgerian Nalma Boundary (Early to Middle Eocene) in the Western Green River Basin, Wyoming

Abstract: New paleomagnetic and biostratigraphic data from the western Green River Basin of Wyoming provide a magnetobiostratigraphic framework to correlate the late Wasatchian through early Bridgerian North American Land Mammal Ages (NALMA) to the Geomagnetic Polarity Time Scale (GPTS). Three‐hundred and twelve paleomagnetic samples were collected from 73 sites within a 350‐m continuously exposed flat‐lying section. The section is characterized by alternating packages of well‐developed paleosol mudstone horizons (Wasatch Formation) and lacustrine shale/limestone horizons (Green River Formation). Fossil mammal assemblages are known from various levels within the section and constrain the location of the Wasatchian/Bridgerian NALMA boundary to lie between 60 and 120 m. Paleomagnetic results of paleosol horizons are generally characterized by two components, a present‐day field component with low unblocking temperatures up to 400°C, and a characteristic component with unblocking temperatures up to 680°C. Cha...

Origin and paleogeography of an immense, nonmarine Miocene salt deposit in the Basin and Range (Western USA)

Abstract: This is the publisher's version, which is also available electronically from: http://www.jstor.org/stable/30079883