Insights on why graphic correlation (Shaw's method) works: A reply
No abstract available.
Journal of Geology↗
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Unsteady, nonuniform landslide motion: 1. Theoretical dynamics and the steady datum state
Unsteady, nonuniform motion of persistently active landslides is a process of widespread importance. A general, three-dimensional theory aimed at elucidating this process is developed from physical principles and field measurements of landslide behavior. The theory employs a versatile constitutive model that represents landslides as deformable bodies composed of frictional, nonlinear viscoplastic material. The three-dimensional theory is reduced to a mathematically tracTable form by defining an ideal landslide datum state that consists of steady, unidirectional shear flow driven by ground-water seepage and gravitational forces. Solution of the datum-state equation of motion yields vertical landslide velocity profiles that can represent deformation styles ranging from shear-thickening viscoplastic flow to perfectly plastic frictional slip. This range of theoretical profiles encompasses the range of profiles measured in four persistently active northern California landslides. Also obtained from the datum-state equation of motion is an analytical solution for datum-state landslide sediment fluxes. An important feature of this solution is that it represents theoretical landslide sediment fluxes as a family of continuous-functions. The solution thus provides a mathematical basis for a general perturbation analysis of the kinematics of unsteady, nonuniform landslide motion, which will be presented in a companion paper in a subsequent issue of the Journal.
Journal of Geology↗
Unsteady, nonuniform landslide motion: 2. Linearized theory and the kinematics of transient response
Unsteady, nonuniform landslide motion is caused by temporal and spatial variations in driving and resisting forces. Common sources of these variations include stream undercutting of landslide toes, episodic headscarp slumping, and ground-water potentiometric fluctuations. A linear theory for the kinematics of unsteady, nonuniform landslide motion is developed here by analyzing the behavior of small perturbations about a datum state of steady landslide shear flow. The analysis indicates that local perturbations in landslide sediment flux exhibit advective-diffusive behavior that is controlled largely by a single dimensionless parameter, here called the landslide Peclet number. Explication of the physical meaning of this parameter shows that motion of landslide sediment-flux perturbations is dominated by slow advection if the landslide material behavior is primarily viscous and by rapid diffusion if the behavior is more rigidly plastic. Landslides that deform along thin plastic slip zones are thus inferred to respond relatively rapidly and globally to transient perturbations. Landslides that deform in thick zones of creeping flow, in contrast, are inferred to respond over periods as long as many tens of years, with zones of perturbed sediment flux that translate downslope as weakly diffusive kinematic waves. Many landslides probably fall between these two extremes in their style of response. The measured response of a large California landslide to transient toe undercutting by an adjacent stream helps corroborate the theoretical results.
California↗
Book review of Slope Instability, D. Brunsden and D.B. Prior, eds., Wiley, New York, 620 p.
No abstract available.
Journal of Geology↗
The laboratory albitization of mid-ocean ridge basalt
Complete alteration of fresh mid-ocean ridge basalt to albite + actinolite + smectite took place in the presence of quartz and 3.4m NaCl at 350°C and 400 bars. Significant Na-metasomatism occurred in exchange for Ca + Fe + Mn. In contrast, alteration of the basalt/quartz mixture with a synthetically prepared "natural brine," and basalt with 3.4m NaCl or "natural brine" did not produce significant albitization. Based on these and previous experiments that reacted basalt with a variety of fluids, it is concluded that excess silica, a high Na/Ca ratio, and an absence of strong acidity (dissolved Mg) in the altering fluid are important for the albitization of basalt.
Journal of Geology↗
Salinity variations in submarine hydrothermal systems by layered double-diffusive convection
Various mechanisms have been proposed to explain the salinity variations in vent fluids of seafloor geothermal systems. New experiments reacting diabase and evolved seawater were carried out to reproduce earlier published observations of Cl depletions attributed to formation of an ephemeral Cl-bearing mineral. The absence of any Cl depletions in the present study suggests that the formation of Cl-bearing minerals is not sufficiently widespread to account for the observed salinity variations in the vent fluids. A re-evaluation of both field and laboratory evidence has led to a new model for subseafloor circulation that accounts for salinity variations as well as other chemical and mineralogic observations. In place of a simple single-pass convection system, we propose that the seafloor systems consist of two vertically nested convection cells in which a brine layer at depth heats and drives an overlying seawater cell. Such layering of salinities, a process known in fluid mechanics as double-diffusive convection, is an expected result when convection is induced in saline fluids. The process provides for stable high-temperature heat transfer upward from the cracking front adjacent to the magma, and for limited chemical exchange of the brine with the overlying seawater to explain salinity variations and high metal contents in the vent fluids. The brine also provides an effective medium to produce the secondary mineral assemblages observed in rocks from the mid-ocean ridges and ophiolites unsuccessfully produced in laboratory studies using seawater. The brine originates from the two-phase separation of seawater during magmatic/tectonic events and accumulates and remains relatively stable in the region immediately above the magma chamber.
Journal of Geology↗
Early Cretaceous vein-related garnet granulite in Fiordland, southwest New Zealand: a case for infiltration of mantle-derived CO2-rich fluids
Regionally extensive two-pyroxene granulite facies orthogneisses of Early Cretaceous age in Fiordland, southwest New Zealand, are criss-crossed by garnet-bearing feldspathic veins (and dikes) having associated marginal reaction zones of garnet granulite. The two-pyroxene granulites resulted from fluid-absent meta-morphism of a suite of synkinematic primary anhydrous intrusions. Subsequent restricted formation of garnet granulite in feldspathic compositions, and locally eclogite in ultramafic compositions, proceeded chiefly via reactions involving hornblende breakdown, and occurred in response to sharply increased load pressure and local lowering of water activity. The restricted occurrence of carbonate scapolite and -rich fluid inclusions in the vein areas suggests that water activity was lowered by infiltrating carbonic fluids. Infiltration occurred along pre-existing fracture systems, many of which were already filled with plagio-clase-rich veins and dikes. Published carbon isotope data for in scapolite within the veins indicates a mantle source. Open system behavior accompanying infiltration favored garnet stability by lowering bulk rock ferric/ferrous ratio and content. Fluid infiltration occurred at or near peak metamorphic pressure (~12 kbar at 650-700°C). Granulite metamorphism was of short duration (< 20 m.y.) and accompanied tectonic thickening in a subduction-related magmatic arc.
Journal of Geology↗
Isotopic evidence for early Proterozoic age of the Idono Complex, west-central Alaska
The Idono Complex of west-central Alaska is a fault-bounded, fragment of Early Proterozoic continental crust surrounded by overlap assemblages and younger terranes accreted in Mesozoic time. It is composed of granitic to dioritic orthogneiss, amphibolite, and metasedimentary rocks. Trace element compositions of the granitoids and amphibolite suggest rock formation in a subduction-related volcanic arc terrain. Nine zircon fractions from three samples of granitoid orthogneiss define a U-Pb discordia line intersecting concordia at and . The upper and lower intercepts are interpreted, respectively, as approximations of the time of granitoid crystallization and major episodic Pb-loss. The lower intercept age is similar to that indicated by some hornblende and biotite K-Ar ages. Other biotite, hornblende, and white mica K-Ar ages record later isotopic disturbance in Early Cretaceous time. Nd isotopic compositions at 2.06 Ga for tonalite orthogneiss ( ) and amphibolite ( and +2.3) indicate that Early Proterozoic crust formation in the Idono Complex involved significant additions of mantle-derived magma. These rocks yield (depleted mantle) model ages of 2.0 to 2.1 Ga, similar to the U-Pb zircon upper intercept age. Involvement of Archean crust in the formation of at least some rocks of the Idono Complex is indicated by a granitic orthogneiss ( ), which yields a model age of 2.5 Ga. The ~2.06-Ga age of the Idono Complex is similar to that indicated for provenance(s) of widely distributed sedimentary rocks in western North America and may represent a fragment of the North American craton displaced by northward movement of the Pacific plate. At such, rocks of the Idono Complex may provide important insights into both Early Proterozoic evolution along the craton margin, and subsequent displacements.
Journal of Geology↗
The significance of Rb-Sr glauconite ages, Bonneterre Formation, Missouri; late Devonian-early Mississippian brine migration in the Midcontinent
Rb-Sr mean model ages of for glauconites from the Cambrian lower Bonneterre Formation in southern and central Missouri (in contrast to mean model ages of from the middle and upper Bonneterre) are in excellent agreement with a Rb-Sr isochron age of for glauconites from the Magmont mine (Viburnum Trend) in the southeast Missouri lead district. The lower Bonneterre and Mag-mont mine ages probably reflect isotopic resetting, most likely associated with dolomitization and/or Mississippi Valley-type ore formation in the southeast Missouri lead district. The temporal relation between widespread dolomitization and ore formation in Missouri is unclear, but mounting evidence for two Devonian disturbances permits the following interpretation: (1) a Devonian timing (380-400 Ma) for widespread dolomitization, and (2) a Late Devonian-Early Mississippian timing (360-370 Ma) for ore formation in southeast Missouri. A definitive, genetic connection between southeast Missouri ore formation and peak Ouachita-Appalachian orogenesis during Pennsylvanian time (300 Ma) is lacking. Late Devonian-Early Mississippian fluids associated with the earliest stages of collisional tectonics and metamorphism to the south and southeast may have been important in the formation of southeast Missouri ore deposits. These tectonically driven waters may themselves have been the Mississippi Valley-type ore fluids, entering Missouri by way of the Black Warrior basin and Reelfoot rift and/or the Arkoma basin. Alternatively, the movement of orogenic fluids hundreds of kilometers distant may have initiated and distally influenced the migration of more locally derived Mississippi Valley-type ore fluids. Broad regions of Missouri and adjacent areas immediately south and southeast experienced heating and crustal flexing in the Late Devonian, and consequently, preservation of elevated temperatures in Mississippi Valley-type fluids as they move great lateral distances may not be necessary for ore formation in southeast Missouri.
Journal of Geology↗
Age and nature of the basement in northeastern Washington and northern Idaho: isotopic evidence from Mesozoic and Cenozoic granitoids
K-feldspar Pb and whole rock Nd isotopic analyses from 25 Mesozoic and Cenozoic plutonic rocks and two gneisses from NE Washington and northern Idaho are used to elucidate the age and nature of the concealed cratonic basement. The plutons form two highly distinct isotopie groups: Group I (hornblende-biotite and two-mica plutons of all ages) have Pb isotopie compositions suggesting derivation from rocks of the Belt Supergroup or their metamorphosed equivalents, although Nd isotopie data can only support this model if there was a significant input of juvenile mantle-derived or ancient light REE-depleted material. Group II (hornblende-biotite of Eocene age) have highly retarded Pb isotopie compositions relative to the present day crustal average and require a source region with long-term U (and other LIL) depletion, characteristic of cratonic lower crust. A U-Pb zircon upper intercept age of ca. 2600 Ma obtained from one of the Group II samples, together with Sm-Nd data from the gneisses, indicates possible late-Archean crust at depth, which acted as a source region for Eocene extension-related plutonism. Isotopie compositions and apparent geochemical evolution do not support a direct correlation with the nearest exposed North American Craton in the Wyoming province. If it represents attenuated pre-Mesozoic craton, then it must have been accreted to the craton prior to development of the miogeocline in the Late Proterozoic. Alternatively, it may be part of the Cordilleria terrane accreted to the craton in the Early Cretaceous.
Journal of Geology↗
A 4500-year record of large floods on the Colorado River in the Grand Canyon, Arizona
A sequence of flood deposits left by the Colorado River in the Grand Canyon, Arizona, provides evidence of at least 15 floods with peak discharges greater than over the last 4500 yr. Ten floods during the last 2000-2300 yr had discharges greater than . One flood, 1600-1200 yr ago, had a discharge exceeding , a flow rate more than twice the largest gaged flood. This record of flooding is one of the longest for a major U.S. river, and, combined with the gaged record of twentieth century floods, allows determination of the frequency and history of large floods that have affected key aspects of Colorado River geomorphology.
Journal of Geology↗
Biostratigraphic constraints on formation and timing of accretion in a subduction complex: An example from the Franciscan Complex of Northern California
The determination of the total age coverage of pelagic bedded chert is particularly important in studies of ancient accretionary complexes because the time span represents the minimum travel time of an oceanic plate before accretion at an island arc or continental margin. The Yolla Bolly terrane of the Franciscan Complex consists of rare metabasalt overlain by bedded radiolarian chert which in turn is overlain by metagraywacke and subordinate slaty mudstone and conglomerate. The bedded chert ranges in age from Aaleian (early Middle Jurassic) to Tithonian (latest Jurassic). A lithologic gradation from pelagic bedded chert through hemipelagic siliceous mudstone occurred during the Tithonian and thus marks the arrival of the oceanic sediments at the continental margin. The Tithonian age also agrees with previously reported Tithonian to Valanginian (Early Cretaceous) fossils in the overlying terrigenous metagraywacke. Both chert and metagraywacke are intruded by gabbroic sills, suggesting off-ridge volcanism, probably in a near-trench environment. Following the intrusive activity, all of the rocks were subducted to depths of 20-30 km, imbricated, and recrystallized to the lower blueschist facies (lawsonite-aragonite ± jadeitic pyroxene). Metamorphic ages, using a variety of isotopic methods, range from 90-105 Ma and thus indicate a long interval (ca. 30-40 Ma) between accretion and subduction. Two possible models include: (1) a long period of storage in an accretionary prism prior to subduction, or (2) accretion and subsequent northward transport.
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Sediment thicknesses and holocene glacial marine sedimentation rates in three east Greenland fjords (ca. 68°N)
We compared measured and estimated sediment budgets in heavily glaciated fjords in East Greenland. Mass balance calculations and regional glacio-climatic conditions suggest that the sediment flux to the seafloor in Kangerdlugssuaq and Nansen fjords should be dominated by iceberg rafting and not by the rain-out of suspended particulates in meltwater, as the glacier calving flux is estimated at 15 and , compared to meltwater volumes of 4.4 and , respectively. Gravity cores in the three fjords indicate that the uppermost 1-2.5 m of sediment consists of diamictons or fine-grained laminated muds. AMS radiocarbon dates on calcareous foramininfera or shells (16 total) indicate sedimentation rates of 110 to 340 cm/ka within the fjords over the last 1 ka, and 10-20 cm/ka during the Holecene on the inner and middle shelf. Annual sediment discharge is around tonnes/yr within the Kangerdlugssuaq Fjord and Trough system, which translates into an average basin-wide rate of denudation of 0.01 mm/yr (0.01 m/ka). Air gun and deep-towed (Huntec) seismic profiling was carried out in Kangerdlugssuaq and Nansen fjords, East Greenland, and showed that sediment fills averaged 500 and 350 m respectively; they consist primarily of acoustically stratified sediments. If the sediment fills are entirely Holocene in age then the required average sediment accumulation rates of 35-50 m/ka are an order of magnitude larger than the controlled rates of the last 1-2 ka. This raises the possibility that fjord sediments may be by-passed and not always recycled during glacial advances; this will affect sedimentation rates on adjacent shelves and deep-sea areas during successive glaciations.
The Journal of Geology↗
A pore-pressure diffusion model for estimating landslide-inducing rainfall
Many types of landslide movement are induced by large rainstorms, and empirical rainfall intensity/duration thresholds for initiating movement have been determined for various parts of the world. In this paper, I present a simple pressure diffusion model that provides a physically based hydrologic link between rainfall intensity/duration at the ground surface and destabilizing pore-water pressures at depth. The model approximates rainfall infiltration as a sinusoidally varying flux over time and uses physical parameters that can be determined independently. If destabilizing pore pressures can be estimated, then the model enables the development of a stability criterion defining destabilizing rainfall intensity/duration conditions. Using a comprehensive data set from an intensively monitored landslide, I demonstrate that the model is capable of distinguishing movement-inducing rainstorms.
Journal of Geology↗
The Death Valley turtlebacks reinterpreted as Miocene Pliocene folds of a major detachment surface
Determining the origin of extension parallel folds in metamorphic core complexes is fundamental to understanding the development of detachment faults. An excellent example of such a feature occurs in the Death Valley region of California where a major, undulatory, detachment fault is exposed along the well-known turtleback (antiformal) surfaces of the Black Mountains. In the hanging wall of this detachment fault are deformed strata of the Copper Canyon Formation. New age constraints indicate that the Copper Canyon Formation was deposited from ~6 to 3 Ma. The formation was folded during deposition into a SE-plunging syncline with an axial surface coplanar with that of a synform in the underlying detachment. This relation suggests the turtlebacks are a folded detachment surface formed during large-scale extension in an overall constrictional strain field. The present, more planar, Black Mountains frontal fault system may be the result of out-stepping of a normal fault system away from an older detachment fault that was deactivated by folding.
The Journal of Geology↗
Volumetric analysis of a New England barrier system using ground-penetrating-radar and coring techniques
Ground-penetrating-radar (GPR) profiles calibrated with core data allow accurate assessments of coastal barrier volumes. We applied this procedure successfully to the barrier system along Saco Bay, Maine (USA), as part of a sediment-budget study that focused on present-day sand volumes in various coastal, shoreface, and inner-shelf lithosomes, and on sand fluxes that have affected the volume or distribution of sand in these sediment bodies through time. On GPR profiles, the components of the barrier lithosome are readily differentiated from other facies, except where the radar signal is attenuated by brackish or salty groundwater. Significant differences between dielectric properties of the barrier lithosome and other units commonly result in strong boundary reflectors. The mostly sandy barrier sediments allow deep penetration of GPR waves, in contrast to finer-grained strata and till-covered bedrock. Within the Saco Bay barrier system, of sediment are unevenly distributed. Two-thirds of the total barrier volume is contained within the northern and southern ends of the study area, in the Pine Point spit and the Ferry Beach/Goosefare complex, respectively. The central area around Old Orchard Beach is locally covered by only a thin veneer of barrier sand, averaging >3 m, that unconformably overlies shallow pre-Holocene facies. The prominence of barrier-spit facies and the distribution pattern of back-barrier sediments indicate that a high degree of segmentation, governed by antecedent topography, has affected the development of the Saco Bay barrier system. The present-day configuration of the barrier and back-barrier region along Saco Bay, however, conceals much of its early compartmentalized character.
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The Solubility and Stabilization of Ikaite (CaCO3·6H2O) from 0° to 25°C: Environmental and Paleoclimatic Implications for Thinolite Tufa
We determined the solubility of ikaite from 0° to 25°C to model its saturation state in natural waters and test the hypothesis that it is the precursor of the calcite pseudomorphs in thinolite tufa of Quaternary Lake Lahontan. Reversible solubility at buffered $P_{CO_{2}}$ yields the following expression for the dissolution constant of ikaite: $log K_{ikaite} = 0.15981 - 2011.1/T$. Where $T = ^{\circ}K$, and $0^{\circ}C \leq t \leq 25^{\circ}C$. Derived standard state properties are $\Delta G_{ikaite}^{\circ} = -2541.9 kJmol^{-1} \pm 0.66; \Delta H_{ikaite}^{\circ} = -2973.4 kJmol^{-1} \pm 1.02; S_{ikaite}^{\circ} = 306.6 JT^{-1}mol^{-1} \pm 1.2$. Modeling shows that ikaite is undersaturated at all temperatures in seawater and in alkaline lakes, but that it rapidly approaches saturation near 0°C. Its precipitation in near-freezing marine sediments requires large additions of $HCO_{3}$ to pore fluids from the diagenetic decomposition of organic matter. Its crystallization in tufas of alkaline lakes, however, requires only small additions of Ca from springs. Simple kinetic experiments show that ikaite is stabilized in natural environments by orthophosphate, which prevents the crystallization of the more stable anhydrous forms of $CaCO_{3}$ but does not interact with the ikaite. Therefore, the presence of ikaite or its pseudomorphs is an indicator of near-freezing conditions in environments with high concentrations of orthophosphate. If ikaite is the precursor of thinolite tufa, then the thinolite likely grew below the sediment-water interface at the site of sublacustrine springs during prolonged cold periods.
Journal of Geology↗
Early Proterozoic ties between two suspect terranes and the Mojave crustal block of the Southwestern U.S
Southern California and adjacent areas contain two suspect or exotic terranes comprised largely of ancient continental crust, namely the Tujunga (San Gabriel) and Joshua Tree terranes, that have been considered part of a larger displaced terrane, the Santa Lucia-Orocopia allochthon. Paleomagnetic data for the allochthon indicate northward transport in excess of 2000 km and, thus, an origin extraneous to North America. However, Early Proterozoic plutons of the Mojave crustal block and the Joshua Tree and Tujunga terranes have strikingly comparable features, including: (1) crystallization ages of 1.63 to 1.68 Ga; (2) biotite + sphene + magnetite hornblende garnet mineralogy; (3) high LIL and enriched HFS elemental composition; (4) WPG (within-plate granite) trace element chemistry; (5) similar and unique oxygen isotopic compositions; and (6) distinct Pb and Nd isotopic signatures. These features of the Mojave block, which clearly originated as part of native North America, nevertheless distinguish it from crust elsewhere in North America. On the basis of data presented here, we conclude that the Tujunga terrane is a disrupted portion of the Mojave crustal block and is neither far-traveled nor exotic to North America. Its apparent "exotic" nature stems from derivation out of the middle crust. We also conclude that the Joshua Tree terrane is correlative to the Mojave block. We have found no significant evidence for its displacement and consider Joshua Tree to be contiguous with the Mojave block and thus not a valid terrane. The Tujunga (San Gabriel) and Joshua Tree terranes should not be considered as part of, or having shared the same transport as, the Santa Lucia-Orocopia allocthon.
The Journal of Geology↗