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At least 253 records · Page 14Linked to original sources
USGS1996

Cyclic stressing and seismicity at strongly coupled subduction zones

We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Open-ocean boundary conditions from interior data: Local and remote forcing of Massachusetts Bay

Massachusetts and Cape Cod Bays form a semienclosed coastal basin that opens onto the much larger Gulf of Maine. Subtidal circulation in the bay is driven by local winds and remotely driven flows from the gulf. The local-wind forced flow is estimated with a regional shallow water model driven by wind measurements. The model uses a gravity wave radiation condition along the open-ocean boundary. Results compare reasonably well with observed currents near the coast. In some offshore regions, however, modeled flows are an order of magnitude less energetic than the data. Strong flows are observed even during periods of weak local wind forcing. Poor model-data comparisons are attributable, at least in part, to open-ocean boundary conditions that neglect the effects of remote forcing. Velocity measurements from within Massachusetts Bay are used to estimate the remotely forced component of the flow. The data are combined with shallow water dynamics in an inverse-model formulation that follows the theory of Bennett and McIntosh [1982], who considered tides. We extend their analysis to consider the subtidal response to transient forcing. The inverse model adjusts the a priori open-ocean boundary condition, thereby minimizing a combined measure of model-data misfit and boundary condition adjustment. A “consistency criterion” determines the optimal trade-off between the two. The criterion is based on a measure of plausibility for the inverse solution. The “consistent” inverse solution reproduces 56% of the average squared variation in the data. The local-wind-driven flow alone accounts for half of the model skill. The other half is attributable to remotely forced flows from the Gulf of Maine. The unexplained 44% comes from measurement errors and model errors that are not accounted for in the analysis.

Massachusetts↗
USGS1996

Modeling of fluidized ejecta emplacement over digital topography on Venus

The FLOW computer model of McEwen and Malin (1989) modified for application to the study of Venus fluidized ejecta blankets (FEBs) demonstrates that relatively low viscosities, yield strengths, and initial velocities are required to duplicate the observed flow paths of the outflow materials. The model calculates the velocities and simulated flow paths of gravity flows over Magellan topography. The model is formulated to determine flow movements from initial conditions, gravitational acceleration, and resistance to motion as described by Coulomb, viscous, and turbulent resistance forces. Successful duplication of observed FEB flow paths has been achieved for the FEB craters Addams, Isabella, and Cochran. When used as a simple energy‐line model, the model requires low coefficients of friction to extend FEBs to near their observed termini in the synthetic aperture radar (SAR) imagery, although the resulting straight flow lines do not follow the observed flow paths well. For Bingham flow, the model requires low values of viscosity and yield strength which are more similar to pyroclastic or debris flows than basaltic lavas. Flows of 100‐m depth require 1 to 2 orders of magnitude higher values of both viscosity and yield strength than 10‐m‐deep flows. The complicated nature of the flow lines for the low velocity model suggests that FEBs were probably emplaced under variably laminar and turbulent flow conditions, where underlying topography influenced both the direction and energy of flow materials.

Journal of Geophysical Research E: Planets↗
USGS1997

Source and path effects in the wave fields of tremor and explosions at Stromboli Volcano, Italy

The wave fields generated by Strombolian activity are investigated using data from small-aperture seismic arrays deployed on the north flank of Stromboli and data from seismic and pressure transducers set up near the summit crater. Measurements of slowness and azimuth as a function of time clearly indicate that the sources of tremor and explosions are located beneath the summit crater at depths shallower than 200 m with occasional bursts of energy originating from sources extending to a depth of 3 km. Slowness, azimuth, and particle motion measurements reveal a complex composition of body and surface waves associated with topography, structure, and source properties. Body waves originating at depths shallower than 200 m dominate the wave field at frequencies of 0.5–2.5 Hz, and surface waves generated by the surficial part of the source and by scattering sources distributed around the island dominate at frequencies above 2.5 Hz. The records of tremor and explosions are both dominated by SH motion. Far-field records from explosions start with radial motion, and near-field records from those events show dominantly horizontal motion and often start with a low-frequency (1–2 Hz) precursor characterized by elliptical particle motion, followed within a few seconds by a high-frequency radial phase (1–10 Hz) accompanying the eruption of pyroclastics. The dominant component of the near- and far-field particle motions from explosions, and the timing of air and body wave phases observed in the near field, are consistent with a gas-piston mechanism operating on a shallow (<200 m deep), vertical crack-like conduit. Models of a degassing fluid column suggest that noise emissions originating in the collective oscillations of bubbles ascending in the magma conduit may provide an adequate self-excitation mechanism for sustained tremor generation at Stromboli.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Land cover mapping, fire regeneration, and scaling studies in the Canadian boreal forest with 1 km AVHRR and Landsat TM data

A multitemporal 1 km advanced very high resolution radiometer (AVHRR) land cover analysis approach was used as the basis for regional land cover mapping, fire disturbance-regeneration, and multiresolution land cover scaling studies in the boreal forest ecosystem of central Canada. The land cover classification was developed by using regional field observations from ground and low-level aircraft transits to analyze spectral-temporal clusters that were derived from an unsupervised cluster analysis of monthly normalized difference vegetation index (NDVI) image composites (April-September 1992). Quantitative areal proportions of the major boreal forest components were determined for a 821 km ?? 619 km region, ranging from the southern grasslands-boreal forest ecotone to the northern boreal transitional forest. The boreal wetlands (mostly lowland black spruce, tamarack, mosses, fens, and bogs) occupied approximately 33% of the region, while lakes accounted for another 13%. Upland mixed coniferous-deciduous forests represented 23% of the ecosystem. A SW-NE productivity gradient across the region is manifested by three levels of tree stand density for both the boreal wetland conifer and the mixed forest classes, which are generally aligned with isopleths of regional growing degree days. Approximately 30% of the region was directly affected by fire disturbance within the preceding 30-35 years, especially in the Canadian Shield Zone where large fire-regeneration patterns contribute to the heterogeneous boreal landscape. Intercomparisons with land cover classifications derived from 30-m Landsat Thematic Mapper (TM) data provided important insights into the relative accuracy of the 1 km AVHRR land cover classification. Primarily due to the multitemporal NDVI image compositing process, the 1 km AVHRR land cover classes have an effective spatial resolution in the 3-4 km range; therefore fens, bogs, small water bodies, and small patches of dry jack pine cannot be resolved within the wet conifer mosaic. Major differences in the 1-km AVHRR and 30-m Landsat TM-derived land cover classes are most likely due to differences in the spatial resolution of the data sets. In general, the 1 km AVHRR land cover classes are vegetation mosaics consisting of mixed combinations of the Landsat classes. Detailed mapping of the global boreal forest with this approach will benefit from algorithms for cloud screening and to atmospherically correct reflectance data for both aerosol and water vapor effects. We believe that this 1 km AVHRR land cover analysis provides new and useful information for regional water, energy, carbon, and trace gases studies in BOREAS, especially given the significant spatial variability in land cover type and associated biophysical land cover parameters (e.g., albedo, leaf area index, FPAR, and surface roughness). Multiresolution land cover comparisons (30 m, 1 km, and 100 km grid cells) also illustrated how heterogeneous landscape patterns are represented in land cover maps with differing spatial scales and provided insights on the requirements and challenges for parameterizing landscape heterogeneity as part of land surface process research.

Journal of Geophysical Research D: Atmospheres↗
USGS1998

The coseismic slip distributions of the 1940 and 1979 Imperial Valley, California, earthquakes and their implications

Geodetic arrays observed by the U.S. Coast and Geodetic Survey span the Imperial fault in southern California. For the 1940 M 7.1 Imperial Valley earthquake, a 1934–1941 triangulation network has sufficient resolution to allow inversion for the coseismic slip distribution on fault segments 5 to 25 km long extending from the surface to a depth of 9 km. The estimated right-lateral slip is 0.8 to 1.7 m on the northern 30 km of the main trace of the Imperial fault, 4.8±0.2 m on a 10-km-long segment straddling the United States - Mexico border, and 1.3±0.4 m on a southern 25-km-long segment in Mexico. Fixing this strike-slip model and inverting 1940 leveling data only for dip slip yields 0.1 m of east-side-down dip slip. The seismic moment for this model is M 0 = (3.2±0.3) ×10 19 N m. The 1979 geodetic data set, mostly elevation changes from leveling routes, has insufficient resolution for inversion. However, it is possible to use this geodetic data set and results published by others to infer that the 1940 and 1979 earthquakes may be similar on the rupture zone common to both events. Our preferred 1940 model is similar to the 1979 geodetic results of Crook [1984] on the segments where both networks have good resolution. Elevation changes from 1940 and 1979 leveling data are very similar. Thus the geodetic data corroborate the surface slip evidence of Sharp [1982b] that the 1940 and 1979 slip distributions are examples of “characteristic slip” on the northern Imperial fault.

California↗
USGS2012

Elastic moduli of rock glasses under pressure to 8 kilobars and geophysical implications

Shear and longitudinal velocities were measured by the ultrasonic phase comparison method as a function of pressure to 8 kbar on synthetic glasses of basalt, andesite, rhyolite, and quartz composition and on natural obsidian. Velocities of most of the glasses decrease anomalously with pressure, but increasingly more-normal behavior occurs with decrease in SiO 2 content. The pressure derivatives of rigidity and bulk modulus increase linearly, from −3.39 to −0.26 and from −5.91 to +2.09, respectively, with decrease in SiO 2 content from 100 to 49%. The change from negative to positive in the pressure derivatives of both moduli and observed at Poisson's ratio of about 0.25 is consistent with the Smyth model for the anomalous elastic behavior of glass. If the temperature in the upper mantle is about 1500°C, tholeiitic basalt would be molten in accordance with the partial melt explanation for the low-velocity zone; at 1300°C and below, basalt would be in the glassy state, especially if more felsic than tholeiite. At a temperature of 1370°C and at 30 kbar, reasonable values for the upper mantle at 100 km depth, the basalt glass of this study would have a viscosity of about 10 13 P. On the basis of the theory of viscoelasticity the glass would support shear wave propagation at frequencies above 0.1 Hz. Under such conditions of PT, 10 to 30% basalt glass in a matrix either of eclogite or olivine would reduce the seismic velocities by 3 to 9% and could also account for the values observed in the low-velocity zone.

Journal of Geophysical Research Solid Earth↗
USGS2012

Terranes and suture zones in east central Alaska

East central Alaska, with its 17 terranes, forms a part of the mosaic of allochthonous terranes that join the North American and Siberian plates. These terranes range from continental and continental margin, such as the Tatonduk with its thick well-bedded succession of marine shelf rocks, to seamount, arc, and ocean floor terranes. The Yukon crystalline terrane, the largest described here, is a composite of at least four subterranes juxtaposed across the Tintina fault with the Tatonduk terrane, a northwestern extension of the North American plate in Alaska. Inboard of the Yukon crystalline terrane are packets of closely appressed microterranes separated from the Tatonduk and other terranes belonging to North America by major suture zones. These microterranes lie between North America and the mosaic of accretionary terranes that form the more southerly part of Alaska. The most obviously allochthonous microterranes within the suture zones are the Woodchopper Canyon, an Early Devonian basaltic seamount, and the White Mountains, an Ordovician volcanic arc terrane capped by Silurian and Devonian carbonate bank deposits. The nearest counterpart of these terranes is the Alexander terrane in southeastern Alaska. The Tintina fault of Mesozoic and Cenozoic age, like the Denali fault, primarily follows old suture zones that separate terranes. Strike slip faulting developed after collision in places where further convergence was oblique to the terrane margins. Where terranes met head-on, their leading edges lie along a multiple set of high-angle faults that outline microterranes in accretion zones.

Alaska↗
USGS2012

Applicability of La-Ce systematics to planetary samples

Ce isotopic compositions in several terrestrial and extraterrestrial materials were determined in order to investigate the applicability of using Ce as an isotopic tracer to geological processes. Owing to the low abundances of 138 La and 138 Ce in nature, our measurements of 138 Ce/ 140 Ce ratios of natural samples have relatively large (>0.02%) errors, and the variations in Ce-isotope ratios were barely resolved. A tenuous anticorrelation was observed between ε Ce and ε Nd for terrestrial basalts and granites, indicating that with some improvement in analytical techniques the Ce isotopic composition may prove useful as a tracer for geological processes. A very low ε Ce predicted from the Ce anomaly in the REE pattern of the Khohar L3 chondrite (Nakamura and Masuda, 1973) was not observed in this study, indicating that the Ce anomaly may be caused by terrestrial contamination or alteration. A low ε Ce estimated for the source of Nakhla is consistent with our previous estimate (Nakamura et al., 1982) of a LREE depleted source.

Journal of Geophysical Research Solid Earth↗
USGS2012

Recent movement on the Garlock Fault as suggested by water level fluctuations in a well in Fremont Valley, California

Water levels have been continuously recorded since March 1978 in a well in Fremont Valley, where several strands of the adjacent Garlock fault zone have exhibited both left-lateral displacement and components of normal displacement. Differences in water levels indicate that a fault segment lies between the observation well and a nearby irrigation well. During the 4-year recording period, six sharp fluctuations, or “spikes,” were noted. These fluctuations, occurring over 2- to 4-day periods, have amplitudes of 15–30 cm. They appear to be the result of creep events on a nearby fault. Two types of creep events are plausible: (1) normal slip on an en echelon trace of the Garlock fault less than 300 m south of the well, with the north side up relative to Fremont Valley or (2) left-lateral slip on the same fault. Because of the nature of the fluctuations we favor the latter interpretation. Dislocation models utilizing exponential, arc tangent, and skewed cosine functions were used to analyze the water level fluctuations, associated pressure distribution, and fault displacements. The results suggest that creep on the fault ranges from several millimeters to a centimeter for individual events. Estimates of cumulative creep for the period 1978–1982 range from 20 to 50 mm, depending on the particular model employed.

Journal of Geophysical Research Solid Earth↗
USGS2012

Implications of silicic vent patterns for the presence of large crustal magma chambers

On the basis of the distribution of silicic vents, many volcanic fields can be grouped with (1) igneous systems that may be small and whose vent locations are controlled by regional tectonics, (2) those that include sizable crustal magma bodies which erupt at sites determined by their anomalous local stress fields, or (3) relatively small volume systems that are transitional between categories 1 and 2. Linear vent patterns that are aligned normal to the regional least principal stress (σ 3 ) commonly are associated with absence of evidence for large, shallow magma bodies. The Coso volcanic field and the Inyo‐Mono domes in California and probably the South Sister area in Oregon are examples of such systems. The 1960 dacitic fissure eruption at Cordón Caulle in southern Chile evidently is linked to tectonic stress relaxation associated with the great earthquake that occurred 48 hours before the eruption began. Large, shallow magma chambers are thought to perturb the local stress field so that areal patterns of silicic vents are diffuse, radial, or arcuate. Such systems may erupt great volumes of pyroclastic material catastrophically and produce large calderas. Well‐preserved examples of late precaldera leaks of silicic magma occur at Long Valley, California, and Mount Mazama (Crater Lake), Oregon. Other possible examples are noted. Some of these, those which formed calderas smaller than Crater Lake, apparently were preceded by silicic eruptions from aligned vents. Bearing in mind that there exists a transitional group between tectonically controlled small systems and very large magma chambers, vent distributions can be useful in evaluating potential volcanic hazards for silicic volcanic fields.

California↗
USGS2012

Links between magma-tectonic rate balances, plutonism, and volcanism

With Smith's (1979) model of ash flow magmatism as the point of departure, quantitative regimes of crustal magmatic evolution are outlined. The average extrusive regime of ash flow magmatism operates at a rate of about 0.001 km 3 /yr; progressively larger episodes of increasing repose times characterize the evolution of caldera-forming eruptions. The intrusive regime operates at an inferred average rate of about 0.01 km 3 /yr expressed in terms of the volumetric influx of mafic magma from the mantle, based on the record of silicic rates of extrusion and a 10:1 intrusive to extrusive ratio derived from evidence of zoned silicic systems. This rate is almost identical with the mean cumulative rate for the Hawaiian-Emperor system of basaltic volcanism, which is determined to be 0.015 km 3 /yr. Fluctuations about a mean rate of 10 −2 km 3 /yr lead to a threefold classification of magmatic regimes in terms of source power, depending on whether they are persistently greater than (HHER regime), about equal to (MHER regime), or lower than (LHER regime) the mean Hawaiian-Emperor rate. Ash flow magmatism correlates with the MHER regime, averaged over a sufficiently long time. Thus the conditions of Hawaiian-Emperor magmatism are identified with a characteristic power of magma transfer rates that optimize mechanisms for creation of high-level chambers of derivative and fractionated silicic magma (terminology of Smith (1979)) within sufficiently localized regions of high residence times. These conditions represent a balance point, and a bifurcation, in dynamic paths of magmatic evolution leading to widely divergent types of plutonic-volcanic associations. Categorical examples of the principal types of igneous path are discussed relative to continental basalt, silicic ash flow, batholithic granite, and intrusive porphyry systems. The Henry Mountains, Utah, are thought to represent an example of an intrusive porphyry generated in the LHER regime of a mafic source system lacking sufficient power for volcanic expression or significant mineralization. Long Valley, California (and possibly other systems along the eastern front of the Sierra Nevada), is an example operating at the mean rate with transient excursions to higher and lower rates. In regions of rapid extension and/or volcanic propagation (whether linear or areal) these regimes give rise to basaltic eruptions ranging from isolated cinder cones to flood basalts.

Journal of Geophysical Research Solid Earth↗
USGS2012

Magmatic inclusions in silicic and intermediate volcanic rocks

Fine‐grained ellipsoidal inclusions from a few millimeters to over l m in size are present in many intermediate to silicic lava flows and domes. Only recently has it become widely accepted that such inclusions are chilled blobs of magma. Their magmatic origin is manifested by vesicularity and high groundmass porosity, by ellipsoidal shapes, by mingling at contacts with the host, and by textural evidence for groundmass crystallization in an undercooled state. Groundmass textures reflect the degree of undercooling, which is a function of thermal contrast (Δ T ) between inclusions and host magmas before mingling. Compositions of inclusions are invariably more mafic than their hosts and indicate that the magmatic systems were compositionally zoned. Many inclusions were formed from hybrid magmas. Four examples are given in order to document effects of decreasing Δ C (compositional contrast) and Δ T , as hosts for mafic andesitic to andesitic inclusions vary from high‐silica rhyolite (HSR) to silicic andesite. In the HSR example from the Coso volcanic field, California, inclusions track the evolution of zonation in a long‐lived magmatic system. At the other extreme, andesitic inclusions in silicic andesite from Crater Lake, Oregon, demonstrate zonation of a system of low Δ T and Δ C and show effects of in situ differentiation by gas filter‐pressing of residual liquid. Inclusions form when a relatively small proportion of mafic magma comes into contact with a larger proportion of silicic magma. Several mechanisms by which the magmas mix have been proposed: buoyant rise of vesiculated mafic magma or convective stirring at the interface between mafic and overlying silicic magma, forcible injection of mafic magma, diking of fractured silicic magma, and mixing during ascent in a conduit. No single mechanism explains all field observations. Many inclusions in plutons are analogous to magmatic inclusions in volcanic rocks but are likely to have suffered greater chemical and textural change during comparatively slow postentrapment cooling. Magmatic inclusions are evidence of open system behavior of the differentiated parts of magma bodies, and the inclusions themselves may behave as open systems where Δ T is low.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Morphometric variability within the axial zone of the southern Juan de Fuca Ridge: Interpretation from Sea MARC II, Sea MARC I, and deep-sea photography

The morphometric characteristics of the axial regions of oceanic spreading centers are determined by (1) the type of volcanic flows, (2) the relation between primary volcanic relief (on a scale of a few meters to tens of meters) and degree of sediment cover, and (3) the extent of surficial expression and timing of tectonic disruption of the young oceanic crust. Even within a single, continuous, linear spreading-ridge segment with relatively uniform axial valley dimensions over a distance of 50 or more kilometers, such as along the southern Juan de Fuca Ridge, the changes in morphometric characteristics along axis within the youngest crust indicate distinct variation in tectonic and volcanic activity over short distances within short time periods. An integrated analysis of Sea MARC I, Sea MARC II, and photographic data for the southernmost continuous segment of the Juan de Fuca Ridge shows that generalizations about tectonic and volcanic processes at spreading ridges must consider both the temporal scale of processes as well as the physical scales of observations if predictive models are to be successful. Comparison of the morphometric expression within the major hydrothermal vent area and the rest of the southernmost ridge segment suggests that the mapped distribution of hydrothermal vents may reflect the extent of survey effort rather than uniqueness of geologic setting.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Geologic evidence for a magma chamber beneath Newberry Volcano, Oregon

At Newberry Volcano, central Oregon, more than 0.5 m.y. of magmatic activity, including caldera collapse and renewed caldera-filling volcanism, has created a structural and thermal chimney that channels magma ascent. Holocene rhyolitic eruptions (1) have been confined mainly within the caldera in an area 5 km in diameter, (2) have been very similar in chemical composition, phenocryst mineralogy, and eruptive style, and (3) have occurred as recently as 1300 years ago, with repose periods of 2000–3000 years between eruptions. Holocene basaltic andesite eruptions are widespread on the flanks but are excluded from the area of rhyolitic volcanism. Basaltic andesite in fissures at the edge of the rhyolite area has silicic inclusions and shows mixed basalt-rhyolite magma relations. These geologic relations and the high geothermal gradient that characterizes the lower part of a drill hole in the caldera (U.S. Geological Survey Newberry 2) indicate that a rhyolitic magma chamber has existed beneath the caldera throughout the Holocene. Its longevity probably is a result of intermittent underplating by basaltic magma.

Journal of Geophysical Research Solid Earth↗
USGS2012

Structural analysis of the southern Peninsular, southern Wrangellia, and northern Chugach terranes along the Trans-Alaska Crustal Transect, northern Chugach Mountains, Alaska

Structural and tectonic analysis of the southern Peninsular, southern Wrangellia, and northern Chugach terranes, along the Trans-Alaska Crustal Transect in the northern Chugach Mountains documents a long succession of Early Jurassic through Cenozoic deformational events. The deformational events are generally characterized by distinctive structural fabrics and metamorphisms. Most of the events are interpreted to be related to subduction-related accretion or terrane accretion along the Border Ranges fault system (BRFS) and companion faults, and the Contact fault system (CFS). Each period of subduction-related accretion consisted of underplating of the outboard unit beneath the adjacent inboard unit. The fabric associated with each subduction-related accretion consisted of folding, intense shearing, and local rolling of planar structures. Age and structural relationships suggest migration of the zone of subduction-related accretion from the BRFS to the north, through each accreting unit, to younger bounding thrust faults to the south. Other older and younger deformational events are also recognized and are interpreted to have formed before and after, respectively, accretions along the BRFS and CFS. The main younger deformational events are (1) early Tertiary north verging folding of portions of the northern Chugach and southern Wrangellia terranes and (2) broad folding and rotation of major and minor structures related to subduction-related accretion or terrane accretion during early and middle Tertiary oroclinal bending of Alaska.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Kanuti ophiolite, Alaska

The Kanuti ophiolite is a mafic-ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon-Koyukuk ophiolite belt (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon-Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. It is probably correlative with similar ophiolite thrust sheets to the north in the western Brooks Range and to the south in the Ruby geanticline. Technically, the ophiolite is considered to be the Kanuti thrust panel of the Angayucham-Totzitna terrane. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. The residual mantle suite is composed of harzburgite and dunite with refractory mineral compositions. The harzburgite is attributed to partial melting and extraction of basaltic magma; residual dunite is attributed to partial melting or to reaction of orthopyroxene out of harzburgite in contact with ascending melt diapirs. The magmatic suite consists of layered ultramafic and gabbroic rocks, containing minerals having limited iron enrichment. The absence of large volumes of magmatic rocks intermediate in composition between cumulus ultramafics and evolved gabbros favors periodic introduction of magma, rather than closed system fractional crystallization. The ultramafic rocks of both the residual mantle and magmatic suites are tectonites, which have undergone high-temperature deformation involving isoclinal folding on all scales and related syntectonic recrystallization. The olivine fabric is consistent with the glide system {0kl} [100], which has been produced experimentally at 800°–1190°C at 20 kbar. Olivine Z axes and subparallel isoclinal fold axes have consistent, northeast trends throughout the Kanuti region (>100 km NE-SW) and may be close to the original upper mantle flow direction, despite later low-angle thrust faulting. The order of crystallization in the cumulus ultramafic rocks of the magmatic suite is olivine, clinopyroxene, plagioclase, and orthopyroxene. The high Mg numbers of clinopyroxene (0.85–0.93) coexisting with olivine suggest that the cumulus ultramafic rocks crystallized at relatively high pressures (>10 kbar). The effects of parental magma composition cannot be evaluated, but the small difference in Mg numbers of coexisting olivine and clinopyroxene in the cumulus ultramafic rocks and in residual harzburgite suggests that regardless of absolute pressures, the pressure difference between the melting that produced the basalt magma and the initial fractional crystallization of the magma is small. Because of the limited range in rock types in the ophiolite, the tectonic environment cannot be interpreted unambiguously. However, the structural and petrological data are best reconciled with an origin in a volcanic arc tectonic setting.

Journal of Geophysical Research Solid Earth↗
USGS2012

Rupture process of a multiple main shock sequence: analysis of teleseismic, local and field observations of the Tennant Creek, Australia, earthquakes of January 22, 1988

On January 22, 1988, three large intraplate earthquakes (with M S 6.3, 6.4, and 6.7) occurred within a 12-hour period near Tennant Creek, Australia. These earthquakes, which occurred over a small interval of time and within a small volume of space, present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks. Broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms, depths, and complexity of rupture of each of the three main shocks. Hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible. The interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release. As locations of the main shock hypocenters progressively moved from west to east, we infer that the first and third main shocks, denoted as MS 1 and MS 3 , produced the southeast-northwest trending scarps observed at the western end (the Kunayungku fault) and at the eastern end (the east end of the Lake Surprise fault), respectively, of the rupture zone. The epicenter of the only immediate foreshock was located in the gap between these two fault scarps. MS 1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6.5 km. MS 3 ruptured predominantly to the SE at a depth of 4.5 km. The second main shock, MS 2 , is inferred to have produced the deformation of the southwest trending central scarp segment (the western end of the Lake Surprise fault). From the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters, the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes MS1 and MS 3 . In contrast, the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps. The rupture process Of MS 2 turns out to be commensurately complex and sufficiently explains the geological complexity. MS 2 consisted of three subevents. The southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface. Choice of the north dipping plane as the rupture plane of the third subevent, consistent with the surface deformation and coplanar with a second plane delineated by aftershocks, would imply conjugate faulting. The majority of the aftershocks are concentrated near the edges of the fault planes, and there is an absence of activity in the center of the planes. The areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred. The rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture. These characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release. The first main shock nucleated at the deepest part of its aftershock zone and ruptured upward. In contrast, MS 2 and MS 3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones. This variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture.

Journal of Geophysical Research Solid Earth↗