Showing papers in "Geological Society of America Bulletin in 1988"

Strike-Slip faults

Abstract: The importance of strike-slip faulting was recognized near the turn of the century, chiefly from investigations of surficial offsets associated with major earthquakes in New Zealand, Japan, and California. Extrapolation from observed horizontal displacements during single earthquakes to more abstract concepts of long-term, slow accumulation of hundreds of kilometers of horizontal translation over geologic time, however, came almost simultaneously from several parts of the world, but only after much regional geologic mapping and synthesis. Strike-slip faults are classified either as transform faults which cut the lithosphere as plate boundaries, or as transcurrent faults which are confined to the crust. Each class of faults may be subdivided further according to their plate or intraplate tectonic function. A mechanical understanding of strike-slip faults has grown out of laboratory model studies which give a theoretical basis to relate faulting to concepts of pure shear or simple shear. Conjugate sets of strike-slip faults form in pure shear, typically across the strike of a convergent orogenic belt. Fault lengths are generally less than 100 km, and displacements along them are measurable in a few to tens of kilometers. Major strike-slip faults form in regional belts of simple shear, typically parallel to orogenic belts; indeed, recognition of the role strike-slip faults play in ancient orogenic belts is becoming increasingly commonplace as regional mapping becomes more detailed and complete. The lengths and displacements of the great strike-slip faults range in the hundreds of kilometers. The position and orientation of associated folds, local domains of extension and shortening, and related fractures and faults depend on the bending or stepping geometry of the strike-slip fault or fault zone, and thus the degree of convergent or divergent strike-slip. Elongate basins, ranging from sag ponds to rhombochasms, form as result of extension in domains of divergent strike slip such as releasing bends; pull-apart basins evolve between overstepping strike-slip faults. The arrangement of strike-slip faults which bound basins is tulip-shaped in profiles normal to strike. Elongate uplifts, ranging from pressure ridges to long, low hills or small mountain ranges, form as a result of crustal shortening in zones of convergent strike slip; they are bounded by an arrangement of strike-slip faults having the profile of a palm tree. Paleoseismic investigations imply that earthquakes occur more frequently on strike-slip faults than on intraplate normal and reverse faults. Active strike-slip faults also differ from other types of faults in that they evince fault creep, which is largely a surficial phenomenon driven by elastic loading of the crust at seismogenic depths. Creep may be steady state or episodic, pre-seismic, co-seismic, or post-seismic, depending on the constitutive properties of the fault zone and the nature of the static strain field, among a number of other factors which are incompletely understood. Recent studies have identified relations between strike-slip faults and crustal delamination at or near the seismogenic zone, giving a mechanism for regional rotation and translation of crustal slabs and flakes, but how general and widespread are these phenomena, and how the mechanisms operate that drive these detachment tectonics are questions that require additional observations, data, and modeling. Several fundamental problems remain poorly understood, including the nature of formation of en echelon folds and their relation to strike-slip faulting; the effect of mechanical stratigraphy on strike-slip-fault structural styles; the thermal and stress states along transform plate boundaries; and the discrepancy between recent geological and historical fault-slip rates relative to more rapid rates of slip determined from analyses of sea-floor magnetic anomalies. Many of the concepts and problems concerning strike-slip faults are derived from nearly a century of study of the San Andreas fault and have added much information, but solutions to several remaining and new fundamental problems will come when more attention is focused on other, less well studied strike-slip faults.

Progress in understanding jointing over the past century

Abstract: Joints are the most common result of brittle fracture of rock in the Earth's crust. They control the physiography of many spectacular landforms and play an important role in the transport of fluids. In its first century, the Geological Society of America Bulletin has published a significant number of papers on joints and jointing. One hundred years ago, there were lively debates in the literature about the origin of joints, and detailed descriptions of joints near the turn of the century catalogued most geometric features that we recognize on joints today. In the 1920s, theories relating joint orientation to the tectonic stress field and to other geologic structures led to a proliferation of data on the strike and dip of joints in different regions. The gathering of orientation data dominated work on joints for the next 50 yr. In the 1960s, key papers re-established the need to document surface textures, determine age relations, and measure relative displacements across joints in order to interpret their origins. At about this time, fundamental relationships from the fields of continuum and fracture mechanics were first used to understand the process of jointing. In the past two decades, we have witnessed an effort to use field data to interpret the kinematics of jointing and to understand the initiation, propagation, interaction, and termination of joints. Theoretical methods have been developed to study the evolution of joint sets and the mechanical response of a jointed rock mass to tectonic loading. Although many interesting problems remain to be explored, a sound conceptual and theoretical framework is now available to guide research into the next century.

The formation and failure of natural dams

Abstract: Of the numerous kinds of dams that form by natural processes, dams formed from landslides, glacial ice, and late-neoglacial moraines present the greatest threat to people and property. Landslide dams form in a wide range of physiographic settings. The most common types of mass movements that form landslide dams are rock and debris avalanches; rock and soil slumps and slides; and mud, debris, and earth flows. The most common initiation mechanisms for dam-forming landslides are excessive rainfall and snowmelt and earthquakes. Landslide dams can be classified into six categories based on their relation with the valley floor. Type I dams (11% of 184 landslide dams from around the world that we were able to classify) do not reach from one valley side to the other. Type II dams (44%) span the entire valley floor, in some cases depositing material high on opposite valley sides. Type III dams (41%) move considerable distances both upstream and downstream from the landslide failure. Type IV dams (<1%) are rare and involve the contemporaneous failure of material from both sides of a valley. Type V dams (<1%) also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river. Type VI dams (3%) involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side. Many landslide dams fail shortly after formation. In our sample of 73 documented landslide-dam failures, 27% of the landslide dams failed less than 1 day after formation, and about 50% failed within 10 days. Over-topping is by far the most common cause of failure. The timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry; material characteristics of the blockage; rate of inflow to the impoundment; size and depth of the impoundment; bedrock control of flow; and engineering controls such as artificial spill-ways, diversions, tunnels, and planned breaching by blasting or conventional excavation. Glacial-ice dams can produce at least nine kinds of ice-dammed lakes. The most dangerous are lakes formed in main valleys dammed by tributary glaciers. Failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam. Cold polar-ice dams generally drain supraglacially or marginally by downmelting of an outlet channel. Warmer, temperate-ice dams tend to fail by sudden englacial or subglacial breaching and drainage. Late-neoglacial moraine-dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries. These late-neoglacial dams pose hazards because (1) they are sufficiently young that vegetation has not stabilized their slopes, (2) many dam faces are steeper than the angle of repose, (3) these dams and lakes are immediately downslope from steep crevassed glaciers and near-vertical rock slopes, and (4) downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks. The most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls, rockfalls, or snow or rock avalanches. Melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms. Natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam. Although data are few, for the same potential energy at the dam site, ownstream flood peaks from the failure of glacier-ice dams are smaller than those from landslide, moraine, and structed earth-fill and rock-fill dam failures. Moraine-dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011-1012 joules. Differences in flood peaks natural-dam failures appear to be controlled by dam characteristics and failure mechanisms.

Tectonics of sedimentary basins

Abstract: Simultaneous breakthroughs in our understanding of plate-tectonic processes, depositional systems, subsidence mechanisms, chronostratigraphy, and basin-exploration methods have resulted in rapidly improving actualistic models for sedimentary basins. Basin analysis has become a true science with the development of quantitatively testable models based on modern basins of known plate-tectonic setting. Major subdivisions of basin settings include divergent, convergent, transform, and hybrid; 23 basin categories occur within these settings. Basins are classified according to primary plate-tectonic controls on basin evolution: (1) type of sub-stratum, (2) proximity to plate boundary, and (3) type of nearest plate boundary(s). Sedimentary basins subside primarily owing to (1) attenuation of crust as a result of stretching and erosion, (2) contraction of lithosphere during cooling, and (3) depression of lithosphere by sedimentary and tectonic loads. The first two processes dominate in most divergent settings, whereas the third process dominates in most convergent settings. Intraplate, transform, and hybrid settings experience complex combinations of processes. Several basin types have low preservation potential, as predicted by their susceptibilities to erosion and uplift during orogeny and as confirmed by their scarcity in the very ancient record. Key references concerning actualistic plate-tectonic models for each type of basin form the basis for reviewing the present state of the science. The key references come from many sources, with diverse authorship, including several publications of the Geological Society of America. The further development and refinement of actualistic basin models will lead to improved testable paleotectonic reconstructions.

Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region

Abstract: In the Rocky Mountain region between central Montana and central New Mexico, sedimentologically isolated nonmarine basins were produced by basement deformation during the Laramide orogeny within the area formerly occupied by a broad Late Cretaceous foreland basin in which laterally continuous marine facies had accumulated previously. Laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere. Laramide basins include perimeter basins along the cratonic periphery of the arcuate Laramide province, axial basins along a north-south intramontane trend, and ponded basins located farther west closer to the overthrust belt. Twelve specific stratigraphic and sedimentologic criteria for the onset, duration, and termination of Laramide deformation allow the chronology of basin development to be inferred independently for each basin. Maastrichtian initiation of Laramide deformation was approximately synchronous throughout the Laramide province, but termination of Laramide deformation was systematically diachronous from north to south between early and late Eocene time. Widespread Eocene erosion surfaces truncate syntectonic Laramide sequences and are overlain by largely volcanic and volcaniclastic post-Laramide strata of Eocene age in the north and Oligocene age in the south. Fluvial depositional systems draining toward the Great Plains were dominant in perimeter and axial basins, but ponded basins were occupied at times by large lakes that served as regional sediment traps. Paleocene drainages from ponded basins also led eastward toward the continental interior, but partly interconnected lakes that developed within several ponded basins by mid-Eocene time were either closed hydrologically, or else they drained westward into the "Tyee" paleoriver of the Pacific Northwest.

ODP Leg 107 in the Tyrrhenian Sea: Insights into passive margin and back-arc basin evolution

Abstract: Leg 107 of the Ocean Drilling Program drilled a west-northwest-east-southeast transect of seven sites across the Tyrrhenian Sea, the youngest of the sub-basins of the Mediterranean Sea. Sites 654, 653, 652, and 656 document the rifting and subsidence of the Sardinia passive continental margin. On the upper margin (Site 654), we cored a classic transgressive sequence: subaerial conglomerates, overlain by oyster-bearing sands, overlain by marine marl. Comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are Tortonian (late Miocene) to Messinian (latest Miocene) in age, whereas synrift sediments on the lower margin are Messinian to Pliocene in age. During the Messinian desiccation of the Mediterranean, Sites 654 and 653, now on the upper Sardinian margin, apparently occupied a basinal setting, where they received nannoplankton-bearing clays interbedded with laminated gypsum. Sites 656 and 652, now on the lower Sardinia margin, were apparently higher standing during the desiccation event; their Messinian facies are subaerial and lacustrine, respectively. We infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin. Such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a "rolling-back" subduction zone, or in terms of extension of continental crust, by shear along a deep "detachment fault." Sites 655, 651, and 650 were drilled into two small basalt-floored basins of the central and eastern Tyrrhenian. Emplacement of basaltic crust in the central Tyrrhenian (Vavilov Basin) apparently began more than a million years before, emplacement of basaltic crust in the eastern Tyrrhenian (Marsili Basin). This observation is compatible with previous suggestions that the Tyrrhenian has grown southeastward in response to "rollback" of the down-going slab that currently dips northwestward under the toe of Italy. At the easternmost site, high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2,500 m. It has apparently subsequently subsided to its present depth of >4,100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid-ocean ridge.

Paleozoic paleogeography of North America, Gondwana, and intervening displaced terranes: Comparisons of paleomagnetism with paleoclimatology and biogeographical patterns

Abstract: New paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the Silurian and Devonian. In this paper, the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data. The data from the three fields of paleomagnetism, paleoclimatology, and biogeography are generally in excellent agreement, and an internally consistent paleogeographic evolutionary picture of the interactions between North America, Gondwana, and intervening displaced terranes is emerging.During the interval of the Ordovician, Silurian, and Devonian, North America stayed in equatorial paleoposition, while rotating counter-clockwise. The northwest African part of Gondwana was in high southerly latitudes during the Late Ordovician and was fringed by peri-Gondwanide terranes, such as southern Europe (Armorica) and Avalonian basement blocks now found in eastern Newfoundland, Nova Scotia, the Boston Basin, the Appalachian Piedmont, and northern Florida. Subsequently, Gondwana and the peri-Gondwanide terranes displayed rapid drift with respect to the pole. This drift translates into the following pattern of movement for northwest Africa. During the latest Ordovician-Early Silurian, this area moved rapidly northward from polar to subtropical latitudes, followed by equally rapid southward motion from subtropical to intermediate (about 50°S) paleolatitudes during the Late Silurian-Middle Devonian. It is likely that significant east-to-west motion accompanied the latter shift in paleolatitudes, with the Caledonian-Acadian orogeny the result of Silurian to Early Devonian convergence and collision between Gondwana and North America. This collision sandwiched several of the intervening displaced terranes between Gondwana and North America. Subsequent to this collision, Gondwana was separated in the Late Devonian by a medium-width ocean from North America and the Avalonian and southern European blocks which were left behind adjacent to North America. This new ocean closed during the Carboniferous, and the resulting convergence and collision were the cause of the Hercynian-Alleghanian orogenic belt. Problems remaining for future research, besides the further gathering of reliable paleopoles, involve the uncertain pre-Devonian position of the southern British Isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid Late Ordovician-Silurian apparent polar wander for Gondwana.

Middle Miocene climatic change in the Atacama Desert, northern Chile: Evidence from supergene mineralization at La Escondida

Abstract: Geochronology and paleotopographic reconstruction of the porphyry-copper deposit at La Escondida, Chile, are used to calculate long-term erosion rates and to deduce the timing of Tertiary climatic change for a portion of the Atacama Desert region. Hypogene hydrothermal alteration and protore mineralization at La Escondida took place between 33.7 ± 1.4 and 31.0 ± 1.4 Ma based on K-Ar dating of hydrothermal biotite and sericite. Supergene weathering and copper-sulfide en-richment processes were active from 18.0 ± 0.7 to 14.7 ± 0.6 Ma based on K-Ar dating of supergene alunite, distinguished from hypogene alunite by grain size, color, and sulfur isotopic composition. Reworked lenses of volcanic ash in the vicinity of La Escondida provide useful time-stratigraphic markers at 8.7 ± 0.4, 6.5 ± 0.2, and 4.2 ± 0.2 Ma within present soil profiles. Long-term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at La Escondida using mass-balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid-inclusion geobarometry at similar deposits. The observed trend of decreasing long-term average erosion rates with time is consistent with arid to semiarid conditions in the early Miocene changing to hyperarid conditions during the middle Miocene. This climatic desiccation caused termination of significant supergene copper-suffide enrichment at La Escondida and elsewhere in the Atacama region and preservation of surficial features, including the ash horizons and the leached capping. Middle Miocene climatic desiccation in northern Chile and southern Peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral Humboldt Current and an increase in upwelling intensity as the Antarctic ice cap became established at approximately 15 to 13 Ma. The Central Andes Cordillera, which now provides a rain shadow protecting the Atacama region from precipitation from the east, must have attained at least half its present elevation prior to about 15 Ma to have played a similar role in the middle Miocene.

Dynamic changes and processes in the Mississippi River delta

Abstract: Research in the modern delta of the Mississippi River has revealed short-term changes and processes that are of significant magnitude. Deltaic lobes, each lobe covering an area of 30,000 sq km and having an average thickness of 35 km, switch sites of deposition on an average of every 1,500 yr. Through short periods of geologic time, this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas. Within a single delta lobe, and operating on an even higher frequency, are bay fills and overbank splays. Bay fills, having areas of 250 sq km and thickness of 15 m, require only 150 yr to accumulate. Four major events have taken place in the modern Balize delta since 1838. Overbank splays are much smaller, covering areas of less than 2 sq km and having thicknesses of 3 m, but are associated with high floods on the river. At the river mouth, continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long, 8 km wide, and a thickness of 80 m in a period of only 200 yr. Differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr. On the subaqueous delta platform, sediment instabilities operate nearly continuously, mass-moving large quantities of shallow-water deposits to deeper-water environments via arcuate rotational slides and mudflow gullies and depositional lobes. All of these changes and processes operate at differing spatial and temporal scales, but all result in deposition of large volumes of sediment over extremely short periods of time.

Magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization

Abstract: In order to better understand geologic fac-tors controlling pronounced regional variations in whole-rock chemistry, mineralogy, and mineral chemistry in the batholiths of California, we calculate the magmatic intensive variables f HF /f H 2 O , f HF /f HCl , and f O 2 .Regional-scale west-to-east increases in F/OH in mafic silicates, corresponding to the systematic I-WC to I-MC and I-SC progression, reflect orders-of-magnitude increase in f HF /f H 2 O attending pluton crystallization. Low f HF / f H 2 O of formation of western I-WC types is consistent with their derivation from low-fluorine source rocks in subducted oceanic slabs or the upper mantle. In contrast,higher f HF /f H 2 O of crystallization of I-MC and I-SC types to the east implies the involvement of (1) progressively greater amounts of continental crustal source material such as biotite-bearing metamorphic rocks, their unweathered sedimentary derivatives retaining F-rich mafic minerals, or their fusion products and/or (2) source materials which become more F-rich toward the continental interior. The regional distribution of I-MC and I-SC types suggests that the Precambrian craton of western North America, or derivative sediments, may extend farther north in California and be morphologically more complex than previously thought. From seemingly out-of- place occurrences of I-WC plutons on the eastern slopes of the Sierra Nevada batholith, we infer the existence of regions where Precambrian basement was thin or absent in Mesozoic time, which prevented extensive cratonal contamination of subducted slab or upper mantle-derived magmas. New methods for estimating T-f O 2 relations in the magmas demonstrate that I-SCR granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of I-WC, I-MC, and I-SC types under conditions at or below the maximum stability limit of graphite in equilibrium with a C-O-H-S gas phase. The local-scale formation of I-SCR granites in plutonic belts within specific wall-rock terranes containing highly reducing sediments or metasediments may occur by contamination of I-types with graphitic pelite or, in some cases, by the direct fusion of this reducing pelitic wall rock. The spatial distribution of I-SCR granite provinces therefore is controlled simply by wall-rock lithology. Amphibole geobarometry demonstrates a general west-to-east decrease in crystallization pressure across the Sierra Nevada batholith. In contrast, the Peninsular Ranges batholith displays a west-to-east crystallization pressure increase. The bulk of the California batholiths crystallized at pressures less than 4-5 kb and depths less than about 15-19 km. In the southern Sierra Nevada batholith, the San Gabriels, and the eastern Peninsular Ranges, however, plutons crystallized at pressures exceeding 6 kb at deep crustal levels (>23 km). Reconstruction of the pre-erosion top of the batholith shows that in an east-west cross section, the central Sierra Nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one.

Plate tectonics and island arcs

Abstract: The plate-tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs. Before that time, mobilistic concepts evolved slowly, hindered, particularly in the United States, by an obstructionist geoscience establishment. The volcanic belts of island arcs form about 100 km above subducting plates. Convergent-plate boundaries evolve complexly with time and, at any one time, vary greatly along their lengths. Seismicity defines positions, but not trajectories, of descending slabs, which sink more steeply than they dip and are overridden by advancing upper plates. Subduction occurs beneath only one side at a time of an internally rigid plate, and the common regime in an overriding plate, behind a surficial accretionary wedge, is extensional, except where a collision is underway. Back-arc-basin lithosphere is built behind, or by, migrating island arcs, which lengthen and increase their curvatures. A collision can involve two active arcs, in which case, intervening lithosphere sinks beneath both of them, or an active margin and a passive one. Either type of collision generally is followed by the breaking through of new subduction, beneath the composite mass of light crust, from a new trench on the outside of the aggregate; conversely, a new subduction system commonly is a by-product of collision. A strip of back-arc-basin crust is in many cases left attached to the aggregate, in front of the new trench, and becomes the basement for a fore-arc basin, the leading edge of which is raised as melange is stuffed under it. Sedimentation in trenches is predominantly longitudinal and can be from distant sources. Accretionary wedges are dynamic, being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow; melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes, not of submarine sliding. High-pressure metamorphic rocks form beneath overriding plates, not within wedges in front of them. Arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere. Arc crust is inflated into geanticlines by intrusive rocks and thermal expansion. Submarine island-arc volcanic rocks are widely spilitized, with Na enrichment and Ca depletion, by hydrothermal reaction with sea water. The lower crust of mature island arcs consists of granulite-facies rocks of mafic, intermediate, and felsic-intermediate compositions. The Mohorovicic discontinuity may be primarily a constructional boundary, representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase-free mineralogy.

Low-temperature deformation mechanisms and their interpretation

Abstract: Low-temperature deformation is characterized by heterogeneous strain in which the bulk of the material clearly retains its primary texture. Deformation is by grain-scale crystal plasticity, rotation, fracture, and pressure solution, and by transgranular mechanisms that crosscut numerous grains. The important low-temperature crystal-plastic features are twin lamellae, deformation bands, and undulatory extinction. Subgrain formation by recrystallization or crystal-plastic strain of more than 15% marks the upper limit of the low-temperature regime. Grain rotation may produce foliations in soft sediments or rocks. Microscopic to mesoscopic kinks and crenulations of bedding occur in soft clay and shale. Transgranular features include Luders' bands, cooling and desiccation cracks, joints, extension-fracture cleavage, clastic dikes, mineral-filled veins of several types, recrystallization/replacement veins, vein arrays, boudins, faults, stylolites, slickolites, solution cleavages that range from widely spaced to slaty and pencil cleavage. Pressure fringes form adjacent to relatively rigid grains and have fabrics analogous to those in veins. Faults include conjugate fault pairs (Andersonian faults) multiple simultaneous conjugates (Oertel faults), and Riedel shear-zone configurations. The sense of fault displacement is determined from bends, steps, trails, tails, and feather fractures. Superplasticity, especially if aided by diffusion in grain-boundary water, might be important at low temperatures. Fault textures are diagnostic of the environment of deformation but have yet to be uniquely correlated with the presence or absence of earthquakes. Riedel shears and pseudotachylite may form in earthquake source regions, although pseudotachylite is evidently rare in brittle fault zones. The best indicators of stress magnitudes are the critical' resolved shear stress for deformation twinning and the presence of tensile fractures. Strain magnitudes and stress and strain tensor orientations can be determined with a variety of methods that are based on mechanical twins, platy grain orientation, grain center distribution, and fault geometry and slip directions. Different deformation mechanism associations, expressed by the partitioning of the total strain into different mechanisms, are related to the ductility and environment of deformation. Deformation fronts separating different mechanism associations are defined on the basis of changes in the crystal-plastic component of strain.

Paleogene history of the Kula plate: Offshore evidence and onshore implications

Abstract: Paleocene to middle Eocene magnetic anomalies were mapped over oceanic crust that accreted at the Kula-Pacific spreading center and is now obliquely entering the western Aleutian Trench between 179°E and 168°E. The strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56-55 Ma, when north-south spreading veered to northwest-southeast (310°-130°). Kula-Pacific spreading ceased in 43 Ma. A 75-km-long section of the fossil Kula Rift axis has avoided subduction, although it now intersects the trench axis (almost orthogonally) near 171.5°E. A narrow remnant of the former Kula plate, northwest of this fossil spreading center, is bounded by a fossil Kula-Pacific transform with a high transverse ridge alongside a sediment-filled transform valley. Anomalies on this remnant show that Eocene Kula-Pacific spreading was highly asymmetric (2:1). The 56-55 Ma change in Kula plate rotation inferred from the change in spreading direction coincided with birth of the Aleutian subduction zone, and was probably a consequence of the resulting change in slab-pull stresses on the oceanic lithosphere. The change in direction of Kula-North American motion is a plausible explanation for the detachment of continental terranes from the Pacific Northwest and their migration around the Gulf of Alaska, and for the early Eocene demise of Alaska Range are volcanism. The cessation of Kula-Pacific spreading coincides with a major change in Pacific-plate rotation, and the subsequent direction of convergence of the Pacific plate with the Aleutian arc was similar to the 55-43 Ma direction of Kula-plate convergence.

The origin of granite: The role and source of water in the evolution of granitic magmas

Abstract: Temperature and water content are the two most important parameters in the formation of granitic magmas. Evidence from volcanic and plutonic lithologies suggests that water contents of 2 to 4 wt. % are present in most silicic magmas. Calculations based on the stability of biotite yield water fugacities within the melt phase from about 500 to 2,000 bars, although these calculations determine the log f H 2 O and the effective uncertainty of ±0.5 log units yields a large absolute uncertainty. Comparison with crystallization experiments demonstrates that less than 2% water would require significant percentages of crystallization at temperatures above 900 °C with liquidus temperatures about 1000 °C. Water contents greatly in excess of 4 wt. % would mean that the magma would become vapor saturated at high pressures and would tend to crystallize during ascent to a fine-grained granite before reaching shallow depths. The main sources of water for magma generation are the dehydration of hydrous silicates within the crust and volatiles transported into the crust from subducted oceanic crust and upper mantle in the form of hydrous basalts and andesites. Dehydration reactions of muscovite, biotite, and horn-blende are of particular significance. Anatectic granites may be partially classified in terms of the probable dehydration reaction responsible for their generation. Melts generated from muscovite dehydration are relatively cool, peraluminous in composition, high in K/Na ratio, and generally high in initial 87 Sr/ 86 Sr and delta 18 O. Biotite-generated melts tend to be higher in temperatures, peraluminous to meta-aluminous in composition, moderately high in K/Na ratio, and relatively high in strontium and oxygen ratios. Both types may contain metasedimentary enclaves. Granitoids generated by hornblende dehydration would be much higher in initial temperature, peralka-line to meta-aluminous in composition, and lower in K/Na ratio and in general would have lower strontium and oxygen ratios. Volatiles deposited in hydrothermally altered oceanic crust and upper mantle will be released during subduction in the form of hydrous basalts and andesites. These melts are important energy-transfer mechanisms to drive anatexis within the crust. If these melts encounter a silicic magma chamber when intruding the crust, they will be trapped below the lower-density granitic melt. While partially quenching against the cooler melt, they may release volatiles and heat, which will aid in the melting process. Granites generated in this way will vary in their geochemical properties, depending on the relative importance of crustal versus mantle magma systems. In general, they will have moderate to high initial temperatures, be meta-aluminous in composition, be variable but somewhat low in K/Na ratio, and have lower initial strontium and delta 18 O ratios than will melts derived completely from the crust. The fact that most terrestrial spreading centers are subaqueous thus may aid in the formation of a thick granitic continental crust.

Development of simple strike-slip fault zones, Mount Abbot quadrangle, Sierra Nevada, California

Abstract: Simple strike-slip fault zones mark the third stage of faulting in granitic plutons in the Mount Abbot quadrangle of the Sierra Nevada of California. Deformation began with the opening of nearly vertical subparallel joints. These joints were filled mostly with epidote and chlorite, are up to a few tens of meters long, and typically are less than 1 cm wide. Next, some of these joints slipped left-laterally and became small faults. Small faults accommodated up to ∼2 m of displacement and are characterized by mylonitic fabrics and ductilely deformed quartz. Oblique fractures commonly developed near the ends of small faults and in many cases linked faults end-to-end. Simple fault zones developed as abundant oblique fractures linked small faults side-to-side. These fractures opened and were filled with chlorite, epidote, and quartz. Such fractures are scarce outside the two faults that mark the boundaries of a zone. Simple fault zones typically are 0.5-3 m wide, hundreds of meters long, and laterally displace dikes up to ∼10 m. Displacement is concentrated along the boundary faults, which are characterized by cataclastic textures and brittlely deformed quartz. The fault zones consist of noncoplanar segments a few tens of meters long that join at steps or bends. The segmentation reflects the initial joint pattern and indicates that fault zones grew in length as noncoplanar faults linked end-to-end. Away from bends, the most prominent internal fractures have straight traces and strike 20°-60° counterclockwise from the boundaries, whereas near bends they have gentle S-shaped traces and are nearly perpendicular to the boundaries. We suggest that as some faults linked to form longer structures, a "shear stress shadow" was cast over adjacent smaller faults, causing slip on them essentially to cease. In this manner, displacement progressively became localized on the longer faults and fault zones. If the regional shear strain rate remained constant during this process, then the shear strain rate across the still active faults must have increased. This may have caused cataclastic textures to develop in the boundary faults.

Dupuit-Ghyben-Herzberg Analysis of Strip-Island Lenses

Abstract: Dupuit-Ghyben-Herzberg analysis follows from combination of the continuity equation and Darcy's Law with the Ghyben-Herzberg Principle and the Dupuit assumptions of horizontal flow. The analysis is used to derive the position of the water table and salt-water interface in island lenses in terms of island geometry, distribution of hydraulic conductivity (K), and distribution of recharge (R). For small islands and cases for which the salt-water head is zero, application of Dupuit-Ghyben-Herzberg analysis gives good results because the low R/K ratios of natural lenses assure that height/width ratios of natural lenses are extremely low, 1:30 to 1:100. In earlier publications, the Dupuit-Ghyben-Herzberg differential equation has been integrated with the boundary condition that the water table and interface meet at sea level at the shoreline (that is, no outflow face). The dimensions of an outflow face, however, are known from results of potential-theory analysis. Integration of the Dupuit- Ghyben-Herzberg equation with a boundary condition consistent with the potential-theory outflow face (a) places the interface indistinguishably close to its position given by potential theory and (b) allows calculation of Ghyben-Herzberg ratios (interface depth to water-table elevation), which in this case, depart from their usual value of 40 because equipotentials are curved close to the outflow face. For natural-sized lenses, however, such analysis is necessary only within a narrow strip within 1% to 5% of the island width of the shoreline. Outside this strip, the regular analysis that ignores the presence of the outflow face positions the interface indistinguishably close to that of the potential-theory solution. Analytical solutions are developed for a number of infinite-strip islands. It is shown by analysis of the homogeneous, rectangular-island case that an island can be considered an infinite strip (to 0.1% accuracy) if its length/width ratio is larger than 4.4. Asymmetric lenses occur if the island is composed of strips of different K or different R, with the greater asymmetry occurring with differences in K. A high-K basement compresses the "root" of the lens and thereby decreases the water table in the island. A lens perched on impermeable basement has a higher water table than would otherwise occur in the island, but the volume of the lens is less.

Crystal capture, sorting, and retention in convecting magma

Abstract: The mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet-like chamber. A central conclusion is that inward-progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer. Convection is apparently confined to an essentially isoviscous, isothermal region that hugs the downward-growing roof zone. Strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity (N) called "rigid crust" (N ≥ 0.5), "mush" (0.5 ≥ N ≥ 0.25), and "suspension" (N ≤ 0.25). The strong increase in viscosity near the mush-suspension interface acts as a capture front that overtakes and traps slowly settling crystals. Initial phenocrysts mostly escape capture, but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m. Escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself. Crystal-laden plumes traverse the central, hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone. Convection is never vigorous but is part of an overall intimate balance between roofward heat loss, rigid-crust growth, crystallization kinetics, and transport and sorting of sinking escaped crystals. There is a strong similarity between these processes and those producing both varves and saline pan deposits. It is clear that lavas, lava lakes, and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over-all process. These aspects commonly also characterize the large mafic magmatic bodies. Because strong compositional changes in the residual melt occur largely outward (that is, at lower temperatures and higher crystallinities) of the capture front, which is immobile and mostly within rigid crust, the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited. This is in broad agreement with the compositional range observed in basaltic lava lakes, sills, and plutons like Skaergaard. The tuning of convection, crystallization kinetics, and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions.

Proterozoic and Phanerozoic basement terranes of Mexico from Nd isotopic studies

Abstract: Nd isotopic data were collected on Precambrian crystalline rocks exposed in northern, eastern, and southern Mexico, as well as from lower crustal xenoliths from central Mexico, in order to constrain the age and character of the Mexican basement. The data indicate that basement belonging to the Grenville (1.0 Ga) tectonothermal event extends from Los Filtros, in Chihuahua, northern Mexico, to Oaxaca, in southern Mexico. These rocks all have average Nd crustal residence times (TDM ages) in the range 1.60 to 1.35 Ga. We infer that this results from mixing average 1.9 Ga or older recycled continental crust with 70% to 90% newly derived mantle-crustal material during the Grenville orogeny. To the west of the Precambrian, the basement contains large amounts of Phanerozoic (probably Paleozoic) crust, identified from lower crustal xenoliths with TDM ages less than 1.0 Ga. The crust represented by these xenoliths may have been emplaced as suspect terranes in Mesozoic Cordilleran events. Alternatively, the apparent Paleozoic crust that underlies parts of central Mexico may connect to the Paleozoic metamorphic Acatlan complex in southern Mexico, and together they would constitute a continuation of the Appalachian-Caledonian orogenic belt through Mexico. Our data do not preclude either of these two models.

Tectonics and hydrogeology of the northern Barbados Ridge: Results from Ocean Drilling Program Leg 110

Abstract: Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a decollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the decollement and pore-water chemistry anomalies both at the decollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the decollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the decollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined decollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping.

Seismically active fold and thrust belt in the San Joaquin Valley, central California

Abstract: Structural analysis of late Cenozoic folds along the western and southern margins of the San Joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics. The folds have formed by the processes of fault-bend folding and fault-propagation folding, which commonly occur in fold and thrust belts. Our structural interpretation attributes the seismically active Coalinga and Kettleman Hills North Dome anticlines to fault-bend folding above a thrust fault(s) that steps up from a detachment within the Franciscan Assemblage to a detachment at the base of the Great Valley Group. This thrust does not reach the surface (blind thrust), and its slip is consumed in backthrusting and formation of subsurface folds under the San Joaquin Valley. Movement on the postulated thrust(s) would explain the cause of the May 2, 1983, Coalinga earthquake and the August 15, 1985, Avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes. Well-documented examples of fault-bend and fault-propagation folding also occur at Wheeler Ridge, in the San Emigdio Mountains, and at Kettleman Hills South Dome. Deformation associated with fold and thrust belts can be extremely complicated. Considerable fault movement can occur without accompanying surface rupture or shallow-level faulting. Conversely, surface rupture can occur that has no direct relationship to fault slip at depth. An example would be flexural-slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth. Consequently, traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts. Our interpretation of the structural style that developed in central California during the late Cenozoic requires that the strain associated with the transpressive motion between the Pacific and North American plates be resolved into normal and tangential components: thrust faulting and folding normal to the plate boundary and strike-slip faulting parallel to the plate boundary (San Andreas fault). The thrust faults root in a deecollement at the brittle- ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction.

The relative contribution of accretion, shear, and extension to Cenozoic tectonic rotation in the Pacific Northwest

Abstract: Large Cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the Pacific Northwest, and many tectonic models have been proposed to explain them, including (1) rotation of accreted oceanic microplates during docking, (2) dextral shear between North America and northward-moving oceanic plates to the west, and (3) microplate rotation in front of an expanding Basin and Range province. Stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations. Coast Range structures, Basin and Range extension, and paleomagnetic data from middle Miocene (15 Ma) Coast Range rocks indicate that dextral shear is responsible for at least 40% of the post-15 Ma rotation of the Coast Range and that Basin and Range extension is responsible for the remainder. Reconstructions based on extrapolation of this ratio back to 37 and 50 Ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has, been a significant contributor to rotation during most of Tertiary time. Changes in the dextral-shear rotation rate over the past 50 m.y. correlate directly with changes in the velocity of the Farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism. Continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost Basin and Range region and suggest 17% extension since 15 Ma, 39% since 37 Ma, and 72% since 50 Ma near latitude 42°N.

Regional variations in bulk chemistry, mineralogy, and the compositions of mafic and accessory minerals in the batholiths of California

Abstract: We define regional variations in mafic and accessory mineral assemblages and compositions and expand the current understanding of spatial variations in whole-rock geochemistry in the batholiths of California. In so doing, we gain new insights into the nature of magmatic source rocks and mechanisms of magma generation in volcano-plutonic arcs of active continental margins. Little-studied metaluminous to strongly peraluminous granites containing Fe-rich biotite with log(X Mg /X Fe ) F/OH and Mn in biotite and amphibole increase on a regional scale from western I-WC types to eastern I-MC and I-SC types, parallel to eastward increases in incompatible elements and decreases in compatible elements in the plutons. In contrast, the belts of western I-SCR granites and eastern I-WC quartz diorites and granodiorites disrupt the regional west-to-east systematics in both mineral and whole-rock geochemistry. Spatial variations in the Al content of amphibole are regional in scale and reflect pressures of pluton crystallization. We conclude that significant, previously unrecognized complexity exists in regional geochemical systematics in the California batholiths.

An expanded view of Jurassic orogenesis in the western United States Cordillera: Middle Jurassic (pre-Nevadan) regional metamorphism and thrust faulting within an active arc environment, Klamath Mountains, California

Abstract: Basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic-ultramafic intrusive complexes delineate a Middle Jurassic arc terrane within the Klamath Mountain province of northern California. Exposures of the supracrustal volcaniclastic rocks are restricted to a single fault-bounded terrane, but the deeper level intrusive complexes were emplaced into most, if not all, the pre-Late Jurassic terranes of the Klamath Mountain region. The pre-Late Jurassic terranes thus constitute the basement of the Middle Jurassic arc. U-Pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus K-Ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of ∼177-159 Ma. The active arc and its basement were imbricated by a compressive deformational event, the signature of which included thrust faulting, isoclinal folding, and regional metamorphism. Several diverse lines of evidence, including K-Ar dating of metamorphic rocks, crosscutting relations of dated intrusive complexes to thrust faults, and U-Pb dating of synmetamorphic intrusive complexes, establish a distinctly pre-Nevadan Middle Jurassic age (ongoing at ∼169 Ma and over by at least 161 Ma) for this compressive deformational episode. "Outboard" and structurally beneath the Middle Jurassic arc and its basement are several terranes that collectively comprise the western Jurassic belt. These terranes were deformed and regionally metamorphosed during the Late Jurassic Nevadan orogeny, which occurred within the time interval of ∼157-150 Ma, as Upper Jurassic plutons with 150- to 142-m.y.-old zircon ages have contact aureoles that overprint the Nevadan fabric, and the ∼157-m.y.-old Rogue Formation was deformed in the Nevadan event. The Middle and Late Jurassic compressive deformational events were thus distinct and separated by as much as 15-20 m.y. The relation between Middle and Late Jurassic magmatism and deformation suggests that the Klamath Mountain province records the evolution of a considerably long-lived arc system that evolved above an east-dipping subduction zone. In addition, we suggest that this are system may represent an oceanic continuation of the long-recognized early Mesozoic arc terrane of the western U.S. Cordillera.

Seismic imaging of extended crust with emphasis on the western United States

Abstract: Understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust This is particularly true in areas where the last tectonic event was extensional, such as the Basin and Range province of the western United States and much of western Europe In these regions, a characteristic reflective pattern has emerged, whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective In the metamorphic-core-complex belt in the western United States, where extension can be as much as an order of magnitude greater than in the more classic continental rift zones, the lower crustal reflectivity thickens and rises, yielding a picture of a crust that is reflective throughout Synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across, characterized by inter-layered high and low velocities Two geologic factors are interpreted as contributing to this layered character: ductile strain, responding to stress in the thermally weakened middle and lower crust, and intrusive layering, corresponding to injection of subhorizontal sheets of mantle-derived magmas These two processes yield a variety of geologic structures, including transposed compositional layering, mylonitic ductile shear zones, and intrusive mafic sheets, all of which occur at the proper scales to cause the prominent reflectivity observed If metamorphic core complexes are representative of extended continental crust world-wide, then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world

Quaternary rate of folding of the Ventura Avenue anticline, western Transverse Ranges, southern California

Abstract: Upper Quaternary terraces of the Ventura River, California, are uplifted, tilted, and folded over the Ventura Avenue anticline. Rates of uplift and tilting have decreased since inception of the structure over the past 200 ka. Assuming that the chronology, based on amino-acid racemization, 14C dates, and soils correlation, is approximately correct, then the minimum possible average rate of uplift in the axial region of the fold has decreased from ∼14 mm/yr to 2 mm/yr during the past 200 ka. Interval rates of uplift for the periods 200 ka to 80 or 105 ka, 80 or 105 ka to 30 ka, and 30 ka to present are, respectively, about 20 mm/yr, 9 mm/yr, and 5 mm/yr. The rate of tilting shows a similar trend, decreasing from ∼5.8 urad/yr, 2.5 urad/yr, and 1.2 urad/yr for the same time intervals, respectively. Based on the mechanics of flexural slip folds in stratified sedimentary rocks, these data suggest that the rootless Ventura Avenue anticline is a fold that has been shortening at a relatively constant rate of about 9 mm/yr since its inception.

Material balance in Alpine orogeny

Abstract: Recent geophysical data are combined with older information for an updated picture of Alpine kinematics, using semiquantitative considerations of material balance. In the Alps, a conventional pile of thin peel nappes is disrupted by a central longitudinal system of steep faults and steeply limbed folds, both affecting disharmoniously the entire edifice of otherwise flat-lying nappes. It is particularly from late Tertiary phases of deformation that structures of the second type, the Insubric system with the Insubric line and the belt of late Alpine windows, acquired prominence for the present aspect of the Alps. These late structures formed as a dextrally transpressive intracontinental branch of the Africa-Europa plate boundary, mostly between areas of extension. Early motions in the Late Cretaceous to the late Eocene were probably responsible for at least half of the 300-km dextral displacement required for the palinspastic restoration of the inner West Carpathians and the Dinarides. During Oligocene extension, the late Alpine batholiths intruded in a belt roughly along the Insubric line while deep crustal and lithospheric roots were destroyed. In the latest Oligocene to middle Miocene, the Adriatic plate moved dextrally to the west along the Insubric transfer fault. From its frontal Insubric indenter, a lower crust-upper mantle flake, probably first obducted in the Cretaceous, was detached and wedged into the Penninic nappes of the western Alps ("bird's head" of the Ivrea body). Both the northern and the southern transfer faults of the Insubric indenter were disrupted and inactivated by late Miocene to later events, the Giudicarie and Neo-North Apennine events. Instead of the inactivated Insubric fault, the Windows belt of en echelon folds assumed the role of dextral transpression. All of these successive zones of motion are entangled in the "Ligurian knot," largely hidden under the sea or young sediments. A large part of the pre-Alpine crust and practically the whole mantle were subducted; only in some cases were high-pressure rocks of shallow continental origin re-obducted. The lithospheric root of the Neo-Alps is largely contained in a vertical slab under the Alps, but some parts seem to have been removed and others disharmonically displaced with respect to the surface structures.

Igneous emplacement in a shear-zone termination: The biotite granite at Strontian, Scotland

Abstract: Deformed xenolith (strain) data together with other structural information indicate that the biotite granite body (∼90 sq km) of the Strontian complex, Scotland, was emplaced in the extensional termination of a dextral transcurrent shear zone. This shear zone is a splay of the major Great Glen fault which lies along the southern boundary of the granite. Siting of the shear zone splay was probably controlled by (a) a slight releasing bend in the Great Glen fault in this area and (b) a large, pre-existing, asymetric synform in Proterozoic metasedimentary country rocks which intersects the Great Glen fault trace at a high angle. A model is proposed in which Moine Thrust (Caledonian) compression at ca. 435 Ma activated the Great Glen fault dextrally. Dextral movements around the releasing bend detached a flat segment from the inside fault wall, and the biotite granite was emplaced side-ways at depths of about 15 km as a sheet into this extensional, listric-fault-bounded, cavity.

The laccolith-stock controversy: New results from the southern Henry Mountains, Utah

Abstract: Domes of sedimentary strata at Mount Holmes, Mount Ellsworth, and Mount Hillers in the southern Henry Mountains record successive stages in the growth of shallow (3 to 4 km deep) magma chambers. Whether the intrusions under these domes are laccoliths or stocks has been the subject of controversy. According to G. K. Gilbert, the central intrusions are direct analogues of much smaller, floored intrusions, exposed on the flanks of the domes, that grew from sills by lifting and bending of a largely concordant overburden. According to C. B. Hunt, the central intrusions are cylindrical stocks, sheathed with a zone of shattered sedimentary rocks, and the small flanking sills and laccoliths grew laterally as tongue-shaped masses from the discordant sides of these stocks. New geologic mapping demonstrates that the sedimentary overburden, now partially eroded from the domes, was uplifted about 1.2 km at Mount Holmes, 1.8 km at Mount Ellsworth, and at least 2.5 km at Mount Hillers. The radii of the domes are similar, between 5 and 7 km. The strata over the domes have a doubly hinged shape, consisting of a concave-upward lower hinge and a concave-downward upper hinge. A limb of approximately constant dip joins these two hinges and dips 20° at Mount Holmes, 50° to 55° at Mount Ellsworth, and 75° to 85° at Mount Hillers. The distal portion of each dome is composed of a gently dipping peripheral limb 3 to 4 km long, presumably underlain by sills and minor laccoliths. Although geologic cross sections and regional aeromagnetic data for the three domes are consistent with floored, laccolithic central intrusions, these data alone do not rule out the possibility of a stock at depth. At Mount Hillers, paleomagnetic vectors indicate that tongue-shaped sills and thin laccoliths overlying the central intrusion were emplaced horizontally and were rotated during doming through about 80° of dip. This sequence of events is not consistent with the emplacement of a stock and subsequent or contemporaneous lateral growth of sills and minor laccoliths. Growth in diameter of a stock from about 300 m at Mount Holmes to nearly 3 km at Mount Hillers, as Hunt suggested, should have been accompanied by considerable radial shortening of the sedimentary strata and a style of folding which is not observed. Geologic and geophysical data and mechanical models support a laccolithic origin for the central magma chambers underlying the domes.

Aragonite-to-calcite transformation during fresh-water diagenesis of carbonates: Insights from pore-water chemistry

Abstract: Dissolved strontium and calcium concentrations in fresh-water lenses (FWL) and associated mixing zones (MZ) on two small, Holocene ooid-sand islands in the Schooner Cays, Bahamas, were monitored during a 1-yr period to quantitatively analyze the transformation of aragonite to calcite. The observed characteristics of this mass transfer are functions of climate and hydrology. Aragonite-to-calcite transformation in all hydrologic zones is primarily associated with meteoric recharge. The transformation occurs throughout the FWL and in the MZ to relative salinities of 19% and 36% sea water on the two islands. Rates of transformation are rapid in all zones and are greatest in the FWL. A limestone composed of 100% calcite should form from an aragonite precursor within 4,700 to 15,600 yr in the FWL, and within 8,700 to 60,000 yr in the upper MZ. Efficiencies of transformation can vary between hydrologic zones due to PCO2 effects; yet, the efficiency of the entire system (FWL + MZ) is high (87%). This indicates that most CaCO3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh-water system or upper mixing zone. CO2 effects, fresh-water-sea-water mixing, and the differing solubilities between aragonite and calcite all drive the mass transfer. The latter is the most significant, accounting for up to nine times more mass transfer than CO2 effects and at least ten times more mass transfer than fresh-water-sea-water mixing. Differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle, but it apparently becomes ineffective after the rainy season, possibly due to the inhibition of calcite precipitation.

The Fox permafrost tunnel: A late Quaternary geologic record in central Alaska

Abstract: The Fox permafrost tunnel, which penetrates 110 m into frozen sediments of Gold-stream valley, provides a continuous exposure of fossiliferous silt and alluvium above schistose bedrock. Deposition of fluvial gravel was followed by a long interval of loess accretion and permafrost aggradation that was punctuated by episodes of thaw and of gullying and redeposition of silt. Imbricated sandy gravel above the bedrock contains lenses of finer alluvium that contain wood fragments and some rooted stumps. Radiocarbon dates indicate that the gravel is older than 40 ka, but absence of mature soil and weathering profiles at its upper contact indicates that fluvial activity must have continued until shortly before loess accretion began at the tunnel site. Silt is the most widespread depositional unit in the tunnel. This deposit is of eolian origin (loess), but some has been redeposited by slope processes. The silt units contain abundant ground ice as pore filings, lenses, wedges, and buried pond ice. Loess accretion was interrupted by a period when little loess accumulated and when large ice wedges formed in the lower loess unit and subsequently were truncated by thaw. Loess began forming sometime before 40 ka and was rapidly accreting by 39 ka under xeric conditions with open vegetation. A sharply decreased rate of loess accretion associated with local erosion and thaw between about 36 and 30 ka is marked by anomalous cation concentration values, lenses of buried sod, fossils indicative of moist to wet substrates, and truncated ice wedges beneath small frozen ponds or streamlets that occupied ice-wedge troughs. A later episode of rapid loess influx under drier conditions began after 30 ka and coincided with glacial advances of late Wisconsin age in the adjoining Alaska Range. Large ice wedges also formed in the upper loess unit, but only their bases are exposed in the tunnel, and their history of development is uncertain. Fanlike deposits of poorly sorted debris near the tunnel portal formed between about 12.5 and 11 ka during deep erosion of loess slopes under moister conditions. The deposits locally form two subunits: the younger over- whelmed a stand of tall willows on the floor of Goldstream valley between about 11.3 and 11.1 ka; the older may have formed about 1,000 yr earlier. Stratigraphic records elsewhere in central Alaska indicate variable middle Wisconsin environments followed by colder and drier conditions that began between 30 and 25 ka and persisted until perhaps 12.5 ka. Widespread loess erosion and redeposition subsequently occurred under moister and probably warmer conditions. Renewed early Holocene loess deposition may have been widespread, but its exact environmental controls are uncertain. Our data challenge three generally accepted concepts of late Quaternary periglacial processes in central Alaska. We contend that (1) many ice-wedge systems may have formed under interstadial conditions rather than full-glacial conditions, (2) episodes of rapid loess influx may have been partly out of phase with episodes of glacier expansion, and (3) redeposition of loess by solifluction, sheetwash, and gully formation may have been episodic and required conditions moister than those under which the loess initially accreted.