Chemical etching of deformation sub-structures in quartz

Chemical etching of dislocations has been studied in natural and synthetic quartz single crystals, in deformed synthetic quartz and in naturally and experimentally deformed quartzites. The ability of different etchants to produce polished or preferentially etched surfaces on quartz is described. Dislocation etching was achieved on all crystal planes examined by using a saturated solution of ammonium bifluoride as the etchant. Appropriate etching times were determined for etching quartzites for grain size, subgrain boundaries, deformation lamellae, dislocations and twins. Growth and polished surfaces of synthetic single crystal quartz were similarly etched and dislocation etch pits, characteristic of various orientations were found. The use of ammonium bifluoride proved to be expecially advantageous for the basal plane, producing a polished surface with etch pits, suitable for dislocation etch pit counting. “Double” etch pits have been found on Dauphiné twin boundaries on the basal plane and the first order prism, using this etchant. Slip lines and deformation bands were suitably etched on deformed synthetic crystal surfaces for identification of the slip planes. Other acidic etchants have been explored and their application to the study of deformation structures in quartz crystals is discussed.     

13C NMR measurements of the genetic potentials of oil shales

Cross polarization, magic-angle spinning ¹³C NMR measurements have been made on raw oil shales that represent a variety of geologic ages, origins, depositional environments and source locations. A high degree of correlation was established between the fraction of aliphatic carbon measured by ¹³C NMR, and the genetic potential, calculated from Fischer assay data. The correlation is independent of the type of kerogen in the raw shale, and its degree of evolution. A short discussion on the validity of various correlations between physical/chemical properties of oil shales and Fischer assay oil yields is given.     

Comparison of quartz microfabric with strain in recrystallized quartzite

The relationship between quartz c-axis microfabric and strain is examined in six specimens of recrystallized quartzite conglomerate in which strain was measured using pebble shapes. Four rocks subjected to plane strain display a direct relationship between the strength of preferred orientation and the strain intensity. The c-axis distributions in these rocks, as well as a rock subjected to moderate extensional strain, are crossed-girdles with maxima near the intermediate principal strain axis and connecting girdles at acute angles to the direction of maximum shortening. A rock subjected to moderate flattening strain has several maxima clustered near the direction of maximum shortening and a weak connecting girdle through the intermediate principal strain axis.These results are generally similar to those of other studies comparing strain and tectonite fabrics and also with experimental and computer simulation studies of fabrics. The degree of preferred orientation is related to total strain, and therefore microfabrics in quartzites may be cautiously interpreted as qualitative indicators of strain intensity. Uncertainties are greater, however, for correlations of fabric patterns with shapes of the strain ellipsoid. An observed increase in recrystallized grain sizes with increasing strain suggests that flow stress was lower in the more strained rocks.     

Preferential chemical etching of terrestrial And lunar olivines

A new chemical etching technique has been developed to study defect structures in terrestrial and lunar olivine crystals. Dislocations, low-angle dislocation boundaries, dislocation arrays and pile-ups have been observed on the (010, (100), and (001) faces of both polished and cleaved naturally deformed single crystal olivine, on various orientations of crystals in dunite, and in an experimentally deformed olivine crystal. Etching of Apollo 12 igneous rocks revealed dislocations and radiation damage tracks in lunar olivine. Dislocation etching of the (001) face of olivine is reported for the first time in this paper.     

The effects of water vapour and oxides of nitrogen on ozone and temperature structure of the stratosphere

Summary A steady-state mean meridional model of the stratosphere is used to investigate the effects of water vapour and nitrogen oxides on ozone and temperature distributions in the stratosphere. The Chapman classical photochemical scheme for ozone is extended to include the dominant reactions involving hydrogen compounds and nitrogen oxides. The ozone and temperature changes are studied under radiative-photochemical equilibrium conditions and in a model incorporating both transport and radiative-photochemical processes. It is found that both hydrogen and nitrogen reactions contribute to substantial decreases in ozone and temperature under photochemical equilibrium conditions, but the computed distribution do not resemble those observed. The effect of transport processes is to reduce the deviations in the ozone mixing ratio and temperature with the computed distributions having many features in common with the observations. It is found that the ozone and temperature respond more readily to the arbitrary increase of nitrogen oxides than to that of water vapour.     

Groundwater capture processes under a seasonal variation in natural recharge and discharge

 "Capture" is the increase in recharge and the decrease in discharge that occurs when pumping is imposed on an aquifer system that was in a previous state of approximate dynamic equilibrium. Regional groundwater models are usually used to calculate capture in a two-step procedure. A steady-state solution provides an initial-head configuration, a set of flows through the boundaries for the modeled region, and the initial basis for the capture calculation. The transient solutions provide the total change in flows through the boundaries. A difference between the transient and steady-state solutions renders the capture calculation. When seasonality is a modeling issue, the use of a single initial hydraulic head and a single set of boundary flows leads to miscalculations of capture. Instead, an initial condition for each season should be used. This approach may be accomplished by determining steady oscillatory solutions, which vary through the seasons but repeat from year to year. A regional groundwater model previously developed for a portion of the San Pedro River basin, Arizona, USA, is modified to illustrate the effect that different initial conditions have on transient solutions and on capture calculations. Received, September 1996 · Revised, October 1997 · Accepted, October 1997     

Shock metamorphism of quartz with initial temperatures −170 to + 1000° C

Shock experiments on quartz single crystals with initial temperatures –170 to +1000°C showed that ambient temperature does not affect the type of defects formed but can lower the pressure of complete amorphization. The amount of glass recovered increases with both pressure and temperature, and the shock-induced phase transformation of quartz is temperature-activated with an apparent activation energy of <60 kj/=" mol.=" the=" phase=" transformation=" is=" localized=" along=" three=" types=" of=" transformation=" lamellae=" (narrow,=" s-shaped,=" and=" wide)=" which=" contain=" fractured=" and/or=" high-pressure=" phases.=" transformation=" lamellae=" are=" inferred=" to=" form=" by=" motion=" of=" linear=" collapse=" zones=" propagating=" near=" the=" shock=" front.=" equilibrium=" phases,=" such=" as=" stishovite,=" were=" not=" recovered=" and=" are=" probably=" not=" formed=" at=" high=" shock=" pressures:=" the=" dominant=" transformation=" mechanism=" is=" inferred=" to=" be=" solid-state=" collapse=" to=" a=" dense,=" disordered=" phase.=" melting=" occurs=" separately=" by=" friction=" along=" microfaults,=" but=" no=" high-pressure=" crystal=" phases=" are=" quenched=" in=" these=" zones.=" shock=" of=" quartz=" thus=" produces=" two=" types=" of=" disordered=" material,=" quenched=" melt=" (along=" microfaults)=" and=" diaplectic=" glass=" (in=" transformation=" lamellae);=" the=" quenched=" melt=" expands=" during=" p-t=" release,=" leaving=" it=" with=" a=" density=" lower=" than=" quartz,=" while=" recovered=" diaplectic=" glass=" has=" a=" density=" closer=" to=" that=" of=" quartz.=" at=" low=" pressures=">< 15=" gpa),=" quartz=" transforms=" mostly=" by=" shear=" melting,=" while=" at=" higher=" pressures=" it=" converts=" mostly=" along=" transformation=" lamellae.=" we=" find=" that=" shock=" paleopiezometers=" using=" microstructures=" are=" nominally=" temperature-invariant,=" so=" that=" features=" observed=" at=" impact=" craters=" and=" the=" k/t=" boundary=" require=" in=" excess=" of=" 10=" gpa=" to=" form,=" regardless=" of=" the=" target=" temperature.=" shock=" comminution=" will=" be=" much=" more=" extensive=" for=" impacts=" on=" cold=" surfaces=" due=" to=" lack=" of=" cementation=" of=" fragments=" by=" melt=" glass;=" shock=" on=" hot=" surfaces=" could=" produce=" much=" more=" glass=" than=" estimated=" from=" room-temperature=" experiments.=" because=" of=" the=" shock-impedance=" mismatch=" between=" quartz=" specimen=" and=" steel=" capsule,=" the=" incident=" shock=" wave=" reverberates=" up=" to=" a=" final=" pressure.=" the=" dynamic=" compression=" process=" is=" quasi-isentropic=" with=" high=" strain=" rates.=" preheating=" and=" precooling=" achieves=" final=" shock=" pressures=" and=" temperatures=" representative=" of=" single-shock=" states=" of=" room=" temperature=" quartz=" and=" of=" quartz=" on=" known=" planetary=" surfaces.=" stress=" histories=" were=" calculated=" by=" detailed=" 1-=" and=" 2-dimensional=" computer=" simulations.=" the=" stress=" history=" throughout=" the=" sample=" is=" relatively=" uniform,=" with=" minor=" variations=" during=" unloading.=" significant=" differences=" between=" impact=" pressures=" calculated=" by=" the=" shock-impedance-match=" method=" and=" specimen=" pressures=" calculated=" by=" computer=" simulations=" indicate=" the=" importance=" of=" modeling=" shock=" recovery=" experiments=">     

Morphology and tectonics of the Australian-Antarctic Discordance between 123° E and 128° E

The Australian-Antarctic Discordance (AAD) is an anomalously deep and rugged zone of the Southeast Indian Ridge (SEIR) between 120° E and 128° E. The AAD contains the boundary between the Indian Ocean and Pacific Ocean isotopic provinces. We have analyzed SeaMarc II bathymetric and sidescan sonar data along the SEIR between 123° E and 128° E. The spreading center in the AAD, previously known to be divided into several transform-bounded sections, is further segmented by nontransform discontinuities which separate distinct spreading cells. Near the transform which bounds the AAD to the east, there is a marked change in the morphology of the spreading center, as well as in virtually every measured geochemical parameter. The spreading axis within the Discordance lies in a prominent rift valley similar to that observed along the Mid-Atlantic Ridge, although the full spreading rate within the AAD is somewhat faster than that of slow-spreading centers (~ 74 mm a^–1 vs. 0–40 mm a^–1). The AAD rift valleys show a marked contrast with the axial high that characterizes the SEIR east of the AAD. This change in axial morphology is coincident with a large (~ 1 km) deepening of the spreading axis. The segmentation characteristics of the AAD are analogous to those of the slow-spreading Mid-Atlantic Ridge, as opposed to the SEIR east of the AAD, which exhibits segmentation characteristics typical of fast-spreading centers. Thus, the spreading center within and east of the AAD contains much of the range of global variability in accretionary processes, yet it is a region free from spreading rate variations and the volumetric and chemical influences of hotspots. We suggest that the axial morphology and segmentation characteristics of the AAD spreading centers are the result of the presence of cooler than normal mantle. The presence of a cool mantle and the subsequent diminution of magma supply at a constant spreading rate may engender the creation of anomalously thick brittle lithosphere within the AAD, a condition which favor, the creation of an axial rift valley and of thin oceanic crust, in agreement with petrologic studies. The morphologies of transform and non-transform discontinuities within the Discordance also possess characteristics consistent with the creation of anomalously thick lithosphere in the region. The upper mantle viscosity structure which results from lower mantle temperatures and melt production rates may account for the similarity in segmentation characteristics between the AAD and slow-spreading centers. The section of the AAD which overlies the isotopic boundary is associated with chaotic seafloor which may be caused by an erratic pattern of magmatism and/or complex deformation associated with mantle convergence. Finally, the pattern of abyssal hill terrain within a portion of the AAD supports previous models for the formation of abyssal hills at intermediate- and slow-spreading ridges, and provides insights into how asymmetric spreading is achieved in this region.     

Whither the megathrust? Localization of large-scale subduction slip along the contact of a mélange

Long-lived subduction complexes, such as the Franciscan Complex of California, include tectonic contacts that represent exhumed megathrust horizons that collectively accommodated thousands of kilometres of slip. The chaotic nature of mélanges in subduction complexes has spawned proposals that these mélanges form as a result of megathrust displacement. Detailed field and petrographic relationships, however, show that most Franciscan mélanges with exotic blocks formed by submarine landsliding. Field relationships at El Cerrito Quarry in the eastern San Francisco Bay area suggest that subduction slip may have been accommodated between the blueschist facies metagreywacke of the Angel Island nappe above and the prehnite-pumpellyite facies metagreywacke of the Alcatraz nappe below. Although a 100–200 m-thick mélange zone separates the nappes, this mélange is a variably deformed, prehnite-pumpellyite facies sedimentary breccia and conglomerate deposited on the underlying coherent sandstone, so the mélange is part of the lower nappe. A 20–30 m-thick fault zone between the top of the mélange, and the base of the Angel Island nappe displays an inverted metamorphic gradient with jadeite-glaucophane-lawsonite above lawsonite-albite assemblages. This zone has a strong seaward (SW)-vergent shear fabric and hosts ultracataclasite and pseudotachylite. These relationships suggest that significant subduction megathrust displacement at depths of 15–30 km was accommodated within the 20–30 m-thick fault zone. Field studies elsewhere in the Franciscan Complex suggest similar localization of megathrust slip, with some examples lacking mélanges. The narrow megathrust zone at El Cerrito Quarry, its uniform sense-of-shear, and the localization of slip along the contact of, rather than within a mélange, contrast sharply with the predictions of numerical models for subduction channels.     

Nested gridding and streamline-based simulation for fast reservoir performance prediction

Detailed reservoir models routinely contain 10⁶–10⁸ grid blocks. These models often cannot be used directly in a reservoir simulation because of the time and memory required for solving the pressure grid on the fine grid. We propose a nested gridding technique that efficiently obtains an approximate solution for the pressure field. The domain is divided into a series of coarse blocks, each containing several fine cells. Effective mobilities are computed for each coarse grid block and the pressure is then found on the coarse scale. The pressure field within each coarse block is computed using flux boundary conditions obtained from the coarse pressure solution. Streamline-based simulation is used to move saturations forward in time. We test the method for a series of example waterflood problems and demonstrate that the method can give accurate estimates of oil production for large 3D models significantly faster than direct simulation using streamlines on the fine grid, making the method overall approximately up to 1,000 times faster than direct conventional simulation.