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USGS2012

A seismic refraction survey of the Imperial Valley Region, California

The U.S. Geological Survey conducted an extensive seismic refraction survey in the Imperial Valley region of California in 1979. The Imperial Valley is located in the Salton Trough, an active rift between the Pacific and North American plates. Forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0.5–1 km. More than 1300 recording locations were occupied, and more than 3000 usable seismograms were obtained. We analyzed five profiles using a standard ray‐tracing program, constructed a contour map of reduced travel times from our most widely recorded shot point, and modeled an existing gravity profile across the Salton Trough. Results are itemized: (1) All models have in common a sedimentary layer ( V p = 1.8–5.0 km/s), a “transition zone” ( V p = 5.0–5.65 km/s), a basement ( V p = 5.65 km/s in the Imperial Valley, 5.9 km/s on the bordering mesas), and subbasement ( V p = 7.2 km/s). (2) The sedimentary layer ranges in thickness along the axis of the Salton Trough from 3.7 km (Salton Sea) to 4.8 km (U.S.‐Mexican border). On the bordering mesas it is quite variable in thickness. (3) The “transition” zone is about 1 km thick in most places. In the Imperial Valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement. On the bordering mesas, however, there is a discontinuity at the top of this zone. (4) There are apparently two types of basement. On the bordering mesas, basement is crystalline igneous and metamorphic rocks. In the Imperial Valley, basement is mostly lower‐greenshist‐facies sedimentary rocks, based primarily on the smooth transition in character from sediment to basement arrivals, the low value of basement velocity, and the fact that deep (4 km) wells in the valley penetrate only the upper part of the known Cenozoic stratigraphic column for the Salton Trough. (5) The subbasement, or intermediate crustal layer, ranges in depth along the axis of the Salton Trough from 16 km (Salton Sea) to 10 km (U.S.‐Mexican border). Gravity modeling requires that this layer deepen and/or pinch out beneath the bordering mesas and mountain ranges. Based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the Imperial Valley, the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust. (6) Several structures are seen that affect basement, transition zone, and deeper parts of the sedimentary layer. They include a scarp along the Imperial fault, as much as 1 km down to the northeast, and a scarp passing roughly along the topographic boundary between the Imperial Valley and the bordering mesa to the west, as much as 3½ km down to the east. We interpret the latter scarp to be the suture, or rift boundary, between the older crystalline basement on the mesa and the younger metasedimentary basement in the Imperial Valley. (7) On a contour map of reduced travel time from our most widely recorded shot point, subtle patches of early arrivals among otherwise late arrivals in the central Imperial Valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150°C. Apparently the Salton Trough is a location where new crust is being generated. As the rift opens, mafic intrusive rocks fill it from below as sedimentary rocks fill it from above. Rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth (metasedimentary basement in the Imperial Valley) and thus consolidates the new crust.

California↗
USGS2012

Local gravity anomalies produced by dislocation sources

Rundle (1978) and Walsh and Rice (1979) have shown that the change in the vertical component of gravity is proportional to uplift for a spherical source of dilatation and for slip on an infinitely long dip-slip fault. In the first case, no free air gravity anomaly is produced and in the second case no Bouguer gravity anomaly. Gravity anomalies due to other dislocation sources in three dimensions are as follows: For strike-slip faulting the ratio of the gravity change to uplift depends upon position; however, the gravity change contours are roughly similar to those corresponding to a zero free air gravity anomaly. Nor is the ratio constant for dip-slip faulting except for the two special cases of dip slip on a vertical fault and horizontal slip on a horizontal fault, neither of which produce a Bouguer anomaly. The Bouguer anomaly produced by an open horizontal crack is the same as would be produced had the material within the crack been mined out without deforming the solid. If the horizontal crack were filled with material of density equal to the host rock (a good approximation to sill formation), no Bouguer anomaly is produced. For cracks of other inclinations the ratio of gravity change to uplift is not constant. Thus, dilatancy, in general, does not correspond to the absence of a free air anomaly, as might be suggested by the special case of a spherical source of dilatation. For two-dimensional models a cylindrical source of dilatation produces no free air gravity anomaly, dip-slip faulting produces no Bouguer anomaly, and open cracks produce a Bouguer anomaly equal to that which would be produced had the material within the crack been mined out without deforming the solid. A two-dimensional crack filled with material of density equal to that of the host rock would produce no Bouguer anomaly. Jachens et al. (1983) have reported temporal changes in gravity, elevation, and area strain along the San Andreas fault in southern California such that the Bouguer anomaly apparently remains unchanged and the uplift-to-strain ratio is about −100 km. Several dislocation mechanisms are proposed that fulfill these constraints, but these mechanisms appear to be rather contrived and are not regarded as satisfactory explanations.

Journal of Geophysical Research Solid Earth↗
USGS2012

Regional deformation near Palmdale, California, 1973-1983 (USA)

The Tehachapi trilateration network spans the intersection of the San Andreas and Garlock faults in southern California in the “Big Bend” region of the San Andreas fault. Analysis of data from 1973–1983 shows strain differences between the northwest and southeast regions of the network and slip at depth on both faults. The Palmdale network, spanning the San Andreas fault entirely within the Tehachapi network, showed increases of about 1 μstrain in both east‐west and north‐south extension in late 1979. The Tehachapi strains also jumped at this time, but the magnitude of the increase was only about one third that of Palmdale. The principal strain rates for Tehachapi over the time interval 1973–1983 are μstrain/yr and μstrain/yr, with the 1 axis directed N76°E. Strains were also computed for two subregions. The principal strain rates for the southeast Tehachapi region are μstrain/yr and μStrain/yr, with the 1 axis directed N73°E. This result differs significantly from the principal strain rates at Palmdale ( μstrain/yr, μstrain/yr, with the 1 axis directed N71°E). For the northwest Tehachapi region, μstrain/yr and μstrain/yr, with the 1 axis directed N93°E.

California↗
USGS2012

A seismic refraction study of the Oregon Cascades

A 275‐km‐long reversed refraction profile in the Oregon Cascades, two shallow earthquakes of magnitude 5 in southern Washington, a shallow earthquake of magnitude 4.6 in northern California, and a previously published analysis of the Bouguer gravity field are used to develop a crustal P wave velocity model for the Oregon Cascades. Travel time analysis of the refraction profile indicates a crustal structure characterized by surface layers with P wave velocities that vary from 2.9 to 5.2 km/s and thicknesses that vary from 2.5 to 5.0 km, upper crustal velocities of 6.1 to 6.5 km/s between the depths of 3 and 29 km, lower crustal velocities near 7.0 km/s between the depths of 29 and 44 km, and a mantle reflector at a depth of 44 km. Comparison with synthetic seismograms supports this structure and shows that the lower crustal and Mono transitions can be better modeled by continuous velocity gradients than by first‐order discontinuities. Arrival times from two shallow earthquakes in southern Washington in 1981 across 14 telemetry stations in the Oregon Cascades show an apparent velocity of 7.62 km/s. On the basis of a previously published gravity analysis, the mean north‐south component of the dip to the crust‐mantle boundary is estimated at 1° down dip to the south. The apparent velocity from the earthquakes and the dip estimate from gravity indicate that the true Pn velocity is 7.70 km/s. Arrival times from a shallow earthquake in northern California in 1978 across telemetry stations in the Washington Cascades are consistent with this model and indicate that the upper mantle velocity reaches 8.20 km/s at a depth of approximately 100 km.

Oregon↗
USGS2012

Deformation of clinopyroxenite: Evidence for a transition in flow mechanisms and semibrittle behavior

A systematic suite of constant strain rate experiments was performed on a vacuum-dried, high-purity, fine-grained clinopyroxenite using NaCl and NaF as confining media in a Griggs-type piston-cylinder apparatus. The experiments were carried out over a range of temperatures from 400° to 1100°C, strain rates from 10 −3 to 10 −7 s −1 , and confining pressures from 170 to 1990 MPa. At T = 600°C and = 1.1 × 10 −5 s −1 , three modes of deformation occur with increasing confining pressure: (1) Macroscopic faulting associated with low strength and stress drops, (2) stable microfracturing and plastic deformation associated with pressure-dependent strength, and (3) plastic deformation (mechanical twinning and 〈001〉 slip) with high strengths which are insensitive to pressure variations. In experiments at P = 1500 MPa, within this high-pressure plastic mode, two regimes of flow are clearly defined. At low to intermediate temperatures and high strain rates, flow strengths are insensitive to changes in strain rate and temperature. Optical and transmission electron microscope observations indicate that plastic strain is accomplished by mechanical twinning on (100) and (001) and by {100}〈001〉 slip. In contrast, at high temperatures and low strain rates the flow stress is strongly dependent on temperature and strain rate. Specimens deformed in this regime show evidence of recovery, multiple slip, and recrystallization; and plastic strain is much more homogeneous. The flow data within each regime can be satisfactorily fit to thermally activated power laws. In the low-temperature regime n (the stress exponent) = 83 ± 16 and E * (the activation energy for flow) = 220 ± 40 kJ/mol. We believe that these parameters reflect flow dominated by the kinetics of dislocation glide associated with mechanical twinning and (100)〈001〉 slip. In the high-temperature regime, n = 5.3 ± 1.1 and E * = 380 ± 30 kJ/mol. These parameters describe creep by multiple slip accompanied by increased rates of diffusion and recovery.

Journal of Geophysical Research Solid Earth↗
USGS2012

Permeability of fault gouge under confining pressure and shear stress

The permeability of both clay-rich and non-clay gouges, as well as several pure clays, was studied as a function of confining pressures from 5 to 200 MPa and shear strain to 10. Permeability ranged over four orders of magnitude, from around 10 −22 to 10 −18 m 2 (1 darcy = 0.987 × 10 −12 m 2 ). The lowest values were characteristic of the montmorillonite-rich and finer grained non-clay gouges. Illite, kaolinite, and chlorite had intermediate permeabilities, while the highest values were typical of the serpentine and coarser grained non-clay gouges. Grain size was an important factor in determining permeability, particularly for the clay-rich samples. The coarse grained gouges were the most permeable and decreased in permeability after shearing. Conversely, the fine grained gouges had characteristically lower permeabilities that did not vary significantly after various amounts of shearing. The permeabilities of the non-clay samples were not significantly different than those of the clays. Therefore, comminuted rock flours can be equally as effective in reducing the flow of water as the characteristically low permeability clay gouges. The strengths of the samples were quite variable. The non-clay gouges were consistently the strongest, with yield points (beginning of nonelastic behavior) around 850 MPa, while montmorillonite had an anomalously low strength in relation to all the other gouges at 250 MPa. Strength of the saturated samples under drained (low pore pressure) conditions did not correlate with high or low permeability. However, the low permeabilities of these gouges could be a factor in the measured low shear stresses along fault regions if excess pore pressures were created as a result of shearing or compaction, and this pressure was unable to dissipate through a thick section of the material.

Journal of Geophysical Research Solid Earth↗
USGS2012

Origin of Hawaiian tholeiite: A metasomatic model

Two voluminous magma types generated in the mantle underlying the Pacific plate are mid‐ocean ridge tholeiite (MORB) erupted at the East Pacific Rise spreading center and Hawaiian tholeiite (HT) erupted above the Hawaiian hot spot or melting anomaly. MORB has low initial 87 Sr/ 86 Sr ratios and low amounts of all incompatible trace elements including rare earths; chondrite‐normalized patterns are depleted in light rare earths. HT, by contrast, has higher initial 87 Sr/ 86 Sr and higher amounts of incompatible trace elements; chondrite‐nor‐malized patterns are enriched in the middle and light rare earths. HT is generally poorer in CaO and Al 2 O 3 and much richer in total iron and TiO 2 compared with MORB having the same MgO content. Primary magma compositions for the two volcanic systems are calculated in Fe‐Mg equilibrium with residual olivine (Fo 92 ). MORB is generated by partial melting of a trace element depleted Iherzolite source leaving a residual assemblage dominated by olivine and orthopyroxene. The percentage of partial melting for a primary magma containing 15% MgO is calculated to be 35–42% in a source mantle having a heavy rare earth content of 3×chondrite and 33–35% MgO. HT, represented by Kilauea tholeiite, is generated by partial melting of a mixture of unmelted and residual mantle for MORB which has been modified by metasomatic addition of a nephelinitic fluid, amphibole, and minor amounts of apatite and Fe‐bearing phases such as sulfide and magnetite/ilmenite. This model yields a picritic magma in equilbrium with magnesian dunite at high (>40%) degrees of partial melting. The source also has 35% MgO before partial melting. Melting in both systems in polyvariant and not controlled by lower‐temperature invariant equilibria. The low‐velocity zone is considered to be the source of metasomatic fluids that are driven upward into the lowermost lithosphere in response to a thermal plume. Picritic primary magmas are produced by shear melting, localized in the zone of thinned and metasomatized lithosphere beneath Hawaii. Melt extraction is rapid and episodic at intervals of months to decades; magma is not stored in the mantle but passes upward to a plexus of storage reservoirs located 2–6 km beneath the surface of Kilauea. Kilauea primary magmas fractionate olivine during upward transport to reach bulk compositions of 13–14% MgO in storage. Different magma batches erupted to the surface, distinguished by different major and minor element compositons compared at similar MgO content, represent combinations of differing degrees of metasomatic enrichment, differing degrees of partial melting, and some effects of premelting mantle heterogeneity.

Hawaii↗
USGS2012

Magnetic properties of the Bay of Islands ophiolite suite and implications for the magnetization of oceanic crust

Rock magnetic properties, opaque mineralogy, and degree of metamorphism were determined for 101 unoriented samples from the North Arm and Blow-Me-Down massifs of the Bay of Islands ophiolite complex, Newfoundland. The weathered and metamorphosed extrusive basalt samples have a weak, secondary magnetization arising from oxidation and exsolution of ilmenite of unknown origin. The initial magnetization of the underlying sheeted dike complex appears to have been destroyed by hydrothermal alteration soon after formation. The magnetic intensity of the gabbroic samples increases as the degree of alteration increases, with the highly altered upper metagabbros having an average intensity of 3×10 −3 emu/c 3 . Because magnetization of the metagabbro samples is related to nonpervasive, variable alteration, these crustal units are unlikely to make a significant contribution to lineated magnetic anomalies. A compilation of our results and other studies suggests a model in which oceanic crust magnetization results from an upper extrusive basalt source layer, roughly 600 m thick, with no contribution from a deeper source layer recognizable from these Bay of Islands data.

Newfoundland↗
USGS2012

Introduction and digest to the Special Issue on Chemical Effects of Water on the Deformation and Strengths of Rocks

The important role of pore pressure in promoting such brittle processes as cataclasis, hydraulic fracturing, large‐scale faulting, and earthquakes within the crust is widely accepted in geology and geophysics [, 1957; , 1959; , 1958; ., 1963; , 1968; ., 1968; ., 1976; , 1973, 1980; , 1981]. Provided that fluid pressure is fully communicated with rock pore space, the effective normal stresses that control crack growth, macroscopic fracture, and friction are reduced by the magnitude of the fluid pressui'e. Beyond this physical effect of pore fluids, there are chemical effects of water on the strength of rocks that are also important in governing differential stresseg and flow in the continental crust. Some of these chemical effects of water on rock deformation have long been recognized.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Experimental deformation of topaz crystals: Possible embrittlement by intracrystalline water

Crystallographically oriented single‐crystal prisms of gem quality topaz (composition AlSiO (OH− F) where x = 0.04 ± 0.01) were deformed at a confining pressure of 1.50 GPa, a temperature of 800°C, and a strain rate of 2×10 s. Under nearly identical conditions, all crystals of anhydrous rock‐forming minerals that have been tested to date, such as olivine, quartz, feldspars, pyroxenes, and refractory oxides, deform plastically; in contrast, our topaz crystals failed by brittle fracture regardless of the orientation of the compression direction. No optical evidence for plastic deformation was detected. Another suite of experiments with compression perpendicular to the (001) cleavage at = 100°–950°C and a strain rate of 2×10 s displayed two regimes of behavior: (1) at >400°C, fracture strength was independent of temperature, and fracture occurred on one or two surfaces parallel to {103}; (2) at <400°C, the fracture strength increased rapidly with decreasing temperature, no macroscopic stress drop was observed, and many closely spaced conjugate fractures formed on (103) and (103). The anomalous brittleness of topaz compared to anhydrous silicate and oxide crystals indicates that intracrystalline “water” plays a role in the embrittlement. We suggest that water within the topaz crystals promotes fracture in ways similar to the mechanisms of slow crack growth aided by environmental moisture.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Effects of compression direction on the plasticity and rheology of hydrolytically weakened synthetic quartz crystals at atmospheric pressure

A hydrothermally grown synthetic quartz crystal with 370±60 ppm hydroxyl impurity was cut into right rectangular prisms in eight crystallographic orientations. We compressed the prisms under constant axial force corresponding to a uniaxial stress of 140.0±0.5 MPa, and temperatures of 510° and 750°C. All but one of the samples sustained permanent axial strains of 2–3%. We established the operating slip systems from specimen shape change, slip bands and dislocation etch pits on polished surfaces, crystallographic orientation changes, stress optical features in thin sections, and transmission electron microscopy. The observed creep behavior and plasticity divided the samples into three groups: (1) Crystals compressed at 45° to [0001] and [ ] and those compressed ⊥( ) and ⊥( ) deformed principally by slip parallel to [0001]. Creep rates were relatively high and were not strongly sensitive to test temperature. Dislocation arrays approximately parallel to ( ) are common. Dislocation loops are elongate parallel to [0001], indicating that the edge segments were more mobile than the screw segments. (2) The second group of samples were loaded normal to [0001] in three orientations: ⊥( ), ⊥( ), and at 45° to ( ). These samples deformed primarily by { } 〈a〉 slip with some evidence for secondary slip on the other systems. They were more creep resistant than the first group and displayed a much higher sensitivity of creep rate to test temperature. Dislocation loops are very elongate parallel to [0001], indicating that the screw dislocation segments were much more mobile than the edge segments. (3) A sample compressed parallel to [0001] at 750°C crept at a barely detectable rate (∼10 s) and no optical scale slip features were observed. These results confirm our earlier work on one orientation each from groups 1 and 2, which indicated a strong creep anisotropy for this same crystal. This creep anisotropy parallels a remarkably similar anisotropy in the diffusivity of impurities in quartz, suggesting a causal relationship between impurity diffusion and creep associated with hydrolytic weakening. Appendix Tables A1‐A5 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐004; $2.50. Payment must accompany order.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Modification of wave-cut and faulting-controlled landforms

From a casual observation that the form of degraded fault scarps resembles the error function, this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms. The morphologic observations can be analyzed either in the form of one or more cross-strike elevation profiles or in the form of the slope-offset plot, a point plot of maximum scarp slope versus scarp offset. Working with either or both of these data representations for nine geologic structures, which range in age from 3 to 400 ka B.P. and in offset from 1 to 50 m, we apply analytical solutions for the vertical initial value scarp, the vertical continuous offset scarp, and the finite slope, initial value scarp. The model calculations are intrinsically ambiguous, yielding as the final answer only the product κ t (in the case of the initial value problem) or the product κ A −1 (in the case of the repeated faulting problem); here t is the age of a single scarp-forming event, 2 A is the vertical slip rate, and κ is the “mass diffusivity.” A single profile across three sea cliffs along the Santa Cruz, California, coast is analyzed as three separate initial value problems. A reasonably constrained age for the sea cliff standing above the Highway 1 platform returns κ = 11 GKG (1 GKG = 1 m 2 /ka). With this κ, we can date the two older sea cliffs. In fact, we do the converse: age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same κ as for the lower sea cliff. We treat a single profile of the Raymond fault in Pasadena/San Marino in terms of the repeated faulting problem; for it the uplift rate of R. Crook and others yields κ = 16 GKG. The very substantial preexisting offset across the Raymond fault must have been buried/leveled some 230 ka B.P., when the modern topography began to form. Our analysis of the Lake Bonneville shoreline scarps reveals a dependence of κ t on 2a, suggestive of nonlinear modification processes. This appearance is treated with the finite slope initial value scarp model to determine κ=1.1 GKG for the Lake Bonneville shoreline scarps. The suggestion of M. N. Machette that approximately 100,000-year-old, meter-high scarps are “unobservable” in weakly consolidated alluvial terranes of the Basin and Range and Rio Grande Rift Valley provinces can be formulated as κ ≳ 1 GKG. The coincidence between this inequality and the Lake Bonneville shoreline κ is striking, and it suggests that the value of κ = 1 GKG may be generally applicable, as a good first approximation, to the modification of alluvial terranes within the semiarid regions of the western United States. The Lake Bonneville shoreline κ is the basis for dating four sets of fault scarps in west-central Utah. The Drum Mountains fault scarps can be modeled in several different circumstances, but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3.6 to 5.7 ka B.P. The younger age is associated with quite low initial slope angles (25°). The other three sets of fault scarps show no evidence for finite initial value slopes. Fault scarps along the eastern base of the Fish Springs Range are very young, 3 ka B.P. We estimate the age of fault scarps along the western flank of the Oquirrh Mountains to be 32 ka B.P., which meets the weak geologic constraint that they be older than the Lake Bonneville shoreline. Fault scarps along the northeastern margin of the Sheeprock Mountains are even older, 53 ka B.P. An intriguing consequence of our single-event analysis of these scarps is that an 11.5-m offset occurred in a single earthquake.

Journal of Geophysical Research Solid Earth↗
USGS2012

Reflectance spectroscopy: Quantitative analysis techniques for remote sensing applications

Several methods for the analysis of remotely sensed reflectance data are compared, including empirical methods and scattering theories, both of which are important for solving remote sensing problems. The concept of the photon mean optical path length and the implications for use in modeling reflectance spectra are presented. It is shown that the mean optical path length in a particulate surface is in rough inverse proportion to the square root of the absorption coefficient. Thus, the stronger absorber a material is, the less photons will penetrate into the surface. The concept of apparent absorbance (‐In reflectance) is presented, and it is shown that absorption bands, which are Gaussian in shape when plotted as absorption coefficient (true absorbance) versus photon energy, are also Gaussians in apparent absorbance. However, the Gaussians in apparent absorbance have a smaller intensity and a width which is a factor of √2 larger. An apparent continuum in a reflectance spectrum is modeled as a mathematical function used to isolate a particular absorption feature for analysis. It is shown that a continuum should be removed by dividing it into the reflectance spectrum or subtracting it from the apparent absorbance and that the fitting of Gaussians to absorption features should be done using apparent absorbance versus photon energy. Kubelka‐Munk theory is only valid for materials with small total absorption and for bihemispherical reflectance, which are rarely encountered in geologic remote sensing. It is shown that the recently advocated bidirectional reflectance theories have the potential for use in deriving mineral abundance from a reflectance spectrum.

Journal of Geophysical Research↗
USGS2012

Spectral properties of ice‐particulate mixtures and implications for remote sensing: 1. Intimate mixtures

The spectral properties of water ice-particulate mixtures are studied for the purpose of deriving the ice and particulate abundances from remotely obtained spectra (particulates referring to nonicy materials in the form of grains). Reflectance levels and ice absorption band depths are a complex function of the single scattering albedo of the particulates embedded in the ice. The ice absorption band depths are related to the mean optical path length of photons in ice through Beers law, Fresnel reflection from the ice-crystal faces on the surface, and ice absorption coefficient as a function of wavelength. Laboratory spectra of many ice-particulate mixtures are studied with high-, medium-, and low-albedo particulates. From the laboratory spectra the band depth, the continuum reflectance, and the ratio of band depth to continuum reflectance are derived for each particulate albedo as a function of the logarithm of the particulate weight fraction in the sample. Ice band depths are dependent on the particulate albedo and increase with smaller weight fractions of particulates until the bands saturate and their depths decrease. The continuum reflectance is a complex function of the particulate albedo and wavelength of light, while the band depth to continuum reflectance ratio appear independent of particulate albedo such that, for a given grain size of particulate, a calibration curve to abundance of ice and particulates is derived. The derived abundance calibrations are accurate if the source of scattering is dominated by particulates and not ice-vacuum (air) interfaces. If ice-vacuum interfaces are dominant, then the derived particulate abundance is an upper limit. Theoretically produced frost spectra show similar curves of growth of band depth, continuum reflectance, and corresponding ratio versus the photon mean optical path length similar to that for intimate mixtures of ice and high-albedo particulates. In all scattering cases the ice absorption coefficients determine the path length needed to produce a given band depth; thus different absorptions are sensitive to different amounts of particulates, with the 3-μm fundamental being the most sensitive to the presence of ice (very low ice abundance) and the 1.04-μm overtone being the most sensitive to the presence of particulates (very high ice abundance).

Journal of Geophysical Research: Solid Earth↗
USGS2012

Deformation in the White Mountain seismic gap, California-Nevada, 1972-1982

A 100×40 km trilateration network extending from Bishop, California, to near Hawthorne, Nevada, crosses the east end of the Long Valley caldera, site of renewed magma inflation in the 1979–1980 interval, and spans most of the White Mountain seismic gap. The network was surveyed in 1972, 1973, 1976, 1979, 1980, and 1982. The 1980 survey may be contaminated by a scale error. In addition, leveling surveys across the caldera have been run in 1932, 1957, 1975, 1980, 1982, and 1983. Interpretation of the deformation is complicated by the occurrence of the May 1980 Mammoth Lakes earthquake sequence (four earthquakes M L ≥6) at the south edge of the caldera as well as other moderate earthquakes within the White Mountain seismic gap. The vertical deformation is largely accounted for by 0.10‐ to 0.15‐km 3 expansion of a spherical magma chamber 8–10 km beneath the resurgent dome within the Long Valley caldera sometime between July 1979 and September 1980 with an additional expansion of perhaps 0.05 km 3 between September 1980 and July 1982. Some additional sources of deformation within the aftershock zone of the Mammoth Lakes earthquakes seem to be required to explain the horizontal deformation. We show that right‐lateral slip on vertical faults extending WNW from each of the three largest earthquakes in the Mammoth Lakes sequence provides the required additional deformation, but this solution is by no means unique. There are simply too few data to define the rather complex deformation that apparently occurred within the aftershock zone. There is little doubt, however, that inflation of a magma chamber beneath the resurgent dome within the Long Valley caldera was involved in the deformation.

California↗
USGS2012

Florida: A Jurassic transform plate boundary

Magnetic, gravity, seismic, and deep drill hole data integrated with plate tectonic reconstructions substantiate the existence of a transform plate boundary across southern Florida during the Jurassic. On the basis of this integrated suite of data the pre-Cretaceous Florida-Bahamas region can be divided into the pre-Jurassic North American plate, Jurassic marginal rift basins, and a broad Jurassic transform zone including stranded blocks of pre-Mesozoic continental crust. Major tectonic units include the Suwannee basin in northern Florida containing Paleozoic sedimentary rocks, a central Florida basement complex of Paleozoic age crystalline rock, the west Florida platform composed of stranded blocks of continental crust, the south Georgia rift containing Triassic sedimentary rocks which overlie block-faulted Suwannee basin sedimentary rocks, the Late Triassic-Jurassic age Apalachicola rift basin, and the Jurassic age south Florida, Bahamas, and Blake Plateau marginal rift basins. The major tectonic units are bounded by basement hinge zones and fracture zones (FZ). The basement hinge zone represents the block-faulted edge of the North American plate, separating Paleozoic and older crustal rocks from Jurassic rifted crust beneath the marginal basins. Fracture zones separate Mesozoic marginal sedimentary basins and include the Blake Spur FZ, Jacksonville FZ, Bahamas FZ, and Cuba FZ, bounding the Blake Plateau, Bahamas, south Florida, and southeastern Gulf of Mexico basins. The Bahamas FZ is the most important of all these features because its northwest extension coincides with the Gulf basin marginal fault zone, forming the southern edge of the North American plate during the Jurassic. The limited space between the North American and the South American/African plates requires that the Jurassic transform zone, connecting the Central Atlantic and the Gulf of Mexico spreading systems, was located between the Bahamas and Cuba FZ's in the region of southern Florida. Our plate reconstructions combined with chronostratigraphic and lithostratigraphic information for the Gulf of Mexico, southern Florida, and the Bahamas indicate that the gulf was sealed off from the Atlantic waters until Callovian time by an elevated Florida-Bahamas region. Restricted influx of waters started in Callovian as a plate reorganization, and increased plate separation between North America and South America/Africa produced waterways into the Gulf of Mexico from the Pacific and possibly from the Atlantic.

Florida↗
USGS2012

Sr isotopic variations along the Juan de Fuca Ridge

Sr isotopic ratios of 39 glass and microcrystalline basalt samples along the Juan de Fuca Ridge and 1 glass sample from Brown Bear Seamount are at the lower end of the range for normal mid-oceanic ridge basalt (MORB); the average 87 Sr/ 86 Sr ratio is 0.70249±0.00014 (2-σ). Although subtle variations exist along strike of the ridge, the Sr isotope data do not show systematic variation relative to the proposed Cobb Hotspot. The isotopic data are inconsistent with an enriched mantle-plume origin for the Cobb-Eickelberg Seamount chain, as has been proposed for Iceland, the Azores, and the Galapagos spreading center. Sr isotopic ratios of samples collected north and south of the Cobb offset are identical, although minor element ratios indicate that these regions have chemically distinct mantle sources. These distinct mantle sources may not have been separated long enough to develop isotopic differences.

Journal of Geophysical Research Solid Earth↗
USGS2012

Laboratory studies of volcanic jets

The study of the fluid dynamics of violent volcanic eruptions by laboratory experiment is described, and the important fluid-dynamic processes that can be examined in laboratory models are discussed in detail. In preliminary experiments, pure gases are erupted from small reservoirs. The gases used are Freon 12 and Freon 22, two gases of high molecular weight and high density that are good analogs of heavy and particulate-laden volcanic gases; nitrogen, a moderate molecular weight, moderate density gas for which the thermodynamic properties are well known; and helium, a low molecular weight, lowdensity gas that is used as a basis for comparison with the behavior of the heavier gases and as an analog of steam, the gas that dominates many volcanic eruptions. Transient jets erupt from the reservoir into the laboratory upon rupture of a thin diaphragm at the exit of a convergent nozzle. The gas accelerates from rest in the reservoir to high velocity in the jet. Reservoir pressures and geometries are such that the fluid velocity in the jets is initially supersonic and later decays to subsonic. The measured reservoir pressure decreases as the fluid expands through repetitively reflecting rarefaction waves, but for the conditions of these experiments, a simple steady-discharge model is sufficient to explain the pressure decay and to predict the duration of the flow. Density variations in the flow field have been visualized with schlieren and shadowgraph photography. The observed structure of the jet is correlated with the measured pressure history. The starting vortex generated when the diaphragm ruptures becomes the head of the jet. Though the exit velocity is sonic, the flow head in the helium jet decelerates to about one-third of sonic velocity in the first few nozzle diameters, the nitrogen head decelerates to about three-fourths of sonic velocity, while Freon maintains nearly sonic velocity. The impulsive acceleration of reservoir fluid into the surrounding atmosphere produces a compression wave. The strength of this wave depends primarily on the sound speed of the fluid in the reservoir but also, secondarily with opposite effect, on the density: helium produces a relatively strong atmospheric shock while the Freons do not produce any optically observable wave front. Well-formed N waves are detected with a microphone far from the reservoir. Barrel shocks, Mach disks, and other familiar features of steady underexpanded supersonic jets form inside the jet almost immediately after passage of the flow head. These features are maintained until the pressure in the reservoir decays to sonic conditions. At low pressures the jets are relatively structureless. Gas-particle jets from volcanic eruptions may behave as pseudogases if particle concentrations and mass and momentum exchange between the components are sufficiently small. The sound speed of volcanic pseudogases can be as large as 1000 m s −1 or as small as a few tens of meters per second depending on the mass loading and initial temperature. Fluids of high sound speed produce stronger atmospheric shock waves than do those of low sound speed. Therefore eruption of a hot gas lightly laden with particulates should produce a stronger shock than eruption of a cooler or heavily laden fluid. An empirical expression suggests that the initial velocity of the head of supersonic volcanic jets is controlled by the sound speed and the ratio of the density of the erupting fluid to that of the atmosphere. The duration of gas or pseudogas eruptions is controlled by the sound speed of the fluid and the ratio of reservoir volume to vent area.

Journal of Geophysical Research Solid Earth↗

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