Bed Rotation Research Articles

Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California

For ∼200 km along the eastern Sierra Nevada continental magmatic arc, Mesozoic metavolcanogenic rocks dip steeply to the southwest (∼80°), a feature that must reflect fundamental processes in magmatic-arc construction. Although tight folds can account for such steep bedding tilts, folds in the metavolcanogenic sections are sparse and small scale. We propose that the high bedding tilts were produced by a combination of thrusting, downward displacement, and ductile deformation of the beds. The last two processes accompanied emplacement of the Sierra Nevada batholith. The Ritter Range pendant lies within this ∼200 km belt and provides a relatively large and well exposed Mesozoic volcanic section ranging in age from Late Triassic to mid–Cretaceous. Detailed mapping and ages from U-Pb zircon dates and fossils within the volcanic section reveal five structural blocks (I–V) that are separated by bedding-parallel thrusts, some of which are cryptic. To explain the present difference in bedding orientations between blocks III and IV, we suggest that the thrusting may have had a duplex geometry, which produced a maximum bedding dip of ∼45° in some blocks. Downward displacement of wall rock and ductile strain account for the remaining ∼35° of the observed average bedding dip (∼80°SW). The exact time of thrusting and duplex formation of Late Triassic to Early Jurassic rocks in blocks I–IV is uncertain, but these structures developed either (1) between 105 and 164 Ma, well before the other rotational processes were active, or (2) mostly around 105 Ma, and closer to the time when other rotational processes were active. Much of the subsequent (ca. 91–76 Ma) bedding tilting is related to downward displacement of beds associated with the emplacement of voluminous Late Cretaceous plutons, and to regional ductile deformation of the wall rocks during that period: the majority of the tilting probably took place between ca. 91 and 86 Ma. Bedding tilts of early to mid-Cretaceous rocks in blocks IV and V is bracketed between ca. 98 and ca. 90 Ma. Comparisons with metavolcanic sections to the northwest near Tioga Pass and to the southeast in the Mount Morrison, Mount Goddard and Oak Creek pendants, suggest that bedding rotation by thrusting(?), downward displacement and ductile strain of wall rock may explain the steep dips along this entire ∼200 km segment of the continental arc. Similar mechanisms may operate at midcrustal levels in other continental arcs.

Evidence for the importance of `small' faults on block rotation

Field and earthquake data are presented which show that small faults can be important during block rotation. `Small' here means those faults which are at least an order of magnitude smaller than the largest faults at a particular scale of observation. The NW–SE-trending dextral faults of SW England have millimetres to kilometres of displacement. The largest of these faults, such as the Sticklepath–Lustleigh fault, do not appear to cause more than a few degrees of rotation from the general E–W strike of beds. Tens of degrees of rotation are often visible, however, on faults with millimetres of displacement. Similarly, the apparent rotation of beds in the Badajoz–Cordoba Shear Zone, Spain, increases as the resolution of the faults is increased. The power-law scaling relationship of earthquake magnitudes in the San Andreas fault zone illustrates that small faults can also be important in regions of active block rotation. `Small' faults can allow deformation within rotating blocks and can allow high displacement gradients to occur on the block-bounding faults. This would reduce the need for void creation, which is a requirement of rigid block rotation models. A tentative model is presented which incorporates the concept that fault behaviour is fractal, emphasising the contribution of small faults. It is suggested that future studies of block rotation should rigorously test the contribution of `small' faults, and that particular care is needed when comparing palaeomagnetic data with regional-scale structures.

Hangingwall bed rotation and the development of contractional and extensional structures around a thrust fault: geometric and experimental models

Tectonic inversion of normal faults, and development of extensional and contractional structures in the vicinity of thrust faults were studied using two models; (i) a geometric (or rigid) model, and (ii) an experimental (or brittle–ductile) model. The first model is based on simple trigonometric relationships between shortening, rotation of layers along a thrust, and dip of the thrust. The model may be used to understand the structures at upper levels of the Earth's crust in a predominant brittle regime. The second model is based on a series of experiments performed with clay analogues. It simulated the structures forming at a deeper level in a brittle–ductile regime and incorporates folding as an essential feature. The results of the study reveal that structures near a thrust fault depend on the thrust geometry and dip amount, the initial orientation of hangingwall layers, and their rheological properties. In the absence of frictional effects along a thrust surface, the rotation of layers and consequent displacement is directly proportional to the thrust dip. Since the listric faults are characterized by a decrease of dip with depth, the variation in the rotation of layers in a profile-section may result in formation of dilation spaces that may serve as potential sites for secondary mineral deposits or oil traps. A large rotation of hangingwall layers results in a reverse fault drag and a small rotation (caused by frictional effects along the fault) produces normal fault drag.

Effects of continuous bed rotation and prolonged mechanical ventilation on healthy, adult baboons.

To study, in a model of prolonged mechanical ventilation, the role of continuous bed rotation on lung function and pathology. Prospective animal study. Animal research laboratory. Healthy adult baboons (Papio cynocephalus), anesthetized with ketamine, sedated, paralyzed, mechanically ventilated for 11 days, and monitored with pulmonary and peripheral arterial catheters. Animals were divided into two experimental groups: a) mechanical ventilation alone (control, n = 7); and b) mechanical ventilation with continuous bed rotation therapy to 45 degrees (continuous rotation group, n = 5). Mechanical ventilation was provided for 11 days with an FIO2 of 0.21 and tidal volume of 12 mL/ kg. Bronchoalveolar lavage was performed through a fiberoptic bronchoscope. Nursing care procedures, antacids, enteral feeding, and prophylactic antibiotics were administered. Measurements of hemodynamics, pulmonary functions, lung volumes, arterial blood gases, and chest radiographs were done daily. Bronchoalveolar lavage was performed at days 0, 7, and 11. There were no significant changes in hemodynamics, gas exchange, or pulmonary functions during the study period in either group. Microbiological surveillance cultures were negative in both experimental groups. In the control group after 7 days, six of seven animals developed patchy atelectasis; by day 11, two of seven animals demonstrated persistent radiologic abnormalities. Bronchoalveolar lavage neutrophils were significantly increased in control animals at days 7 and 11. Lung pathology in the control group showed areas of bronchiolitis, with surrounding bronchopneumonia in five of seven animals. None of the continuous rotation animals showed any radiologic or morphologic abnormalities. Prolonged mechanical ventilation in the control group resulted in atelectasis, increased concentrations of bronchoalveolar lavage neutrophils, and mild pneumonitis. These effects were not associated with changes in lung volumes, oxygenation, or hemodynamic parameters. Continuous bed rotation helped to prevent these abnormalities.

Structural and sedimentological development of footwall growth synclines along an intraforeland uplift, east-central Bighorn Mountains, Wyoming

Structural, sedimentological, and provenance data from Paleogene synorogenic deposits of the east-central flank of the Bighorn Mountains provide new information about the development of footwall growth synclines, the evolution of fault-related folds, and the erosional unroofing history of intraforeland uplifts. Three conglomerate units, the upper conglomerate member of the Fort Union Formation and the Kingsbury and Moncrief Members of the Wasatch Formation, are incorporated within an asymmetric, east-verging growth syncline in the footwall of the main range-bounding thrust system. Three stages of footwall deformation are recorded within these conglomerates. Analysis of mapped progressive unconformities, retrodeformed balanced cross sections, and conglomerate clast composition data define these stages as part of a continuum of deformation associated with the development of footwall growth synclines. Development of an anticline-syncline pair marked the earliest stage of growth syncline formation (stage I). Rotation of the shared fold limb resulted in amplification of the growth syncline. Fine-grained, synorogenic sediment derived from easily eroded Mesozoic mudstone bypassed the growth syncline during this stage. By the end of Lebo Shale deposition, an average of 12.1% of shortening and 6.46 km of uplift had occurred along the range margin. Continued growth syncline development was marked by the deposition of the Kingsbury Conglomerate. The Kingsbury Conglomerate was derived from resistant, middle and lower Paleozoic carbonate strata in the uplifted source terrane. Intraformational unconformities, recording as much as 55° of bed rotation, were developed within the Kingsbury Conglomerate as fold limb rotation occurred coeval with deposition. Cross sections indicate that during this early stage of fault-related folding, an average of 16.9% shortening and 8.12 km of uplift occurred along the eastern flank of the Bighorn Mountains (end of stage I). The intermediate stage (stage II) of footwall growth syncline development involved partial truncation of the growth syncline by the advancing thrust faults and deposition of the Moncrief Conglomerate. The lower portion of the Moncrief Conglomerate was rotated basinward in the developing growth syncline. The final stage of deformation (stage III) was dominated by the thrust faulting of middle and lower Paleozoic strata eastward over steeply dipping Mesozoic strata and rotated Eocene synorogenic conglomerate. During this stage of deformation, the Moncrief Conglomerate was deformed, as the initially blind thrusts propagated into the near-surface conglomerate deposits, truncated the entire footwall syncline, and overrode the synorogenic conglomerate package. Cross sections in areas where this final stage of deformation is well developed indicate that an average of 24.1% shortening and 9.7 km of uplift had occurred along the eastern margin of the Bighorn Mountains. The caliber of synorogenic deposition in the Powder River basin was linked directly to the lithologic composition of the Bighorn Mountains. Approximately half of the 3.6-km-thick source-stratigraphic section of the eastern Bighorn Mountains was eroded prior to accumulation of conglomerate. The majority of this eroded material was derived from Mesozoic mudstone and poorly indurated sandstone that were incapable of generating coarse detritus. The first Paleogene conglomerates deposited along the east-central Bighorn Mountains, therefore, do not represent the initiation of Laramide uplift, but instead represent the exposure of coarse-clast–forming rock types from the lower half of the hanging-wall stratigraphic section (i.e., the Mississippian Madison Limestone and Ordovician Bighorn Dolomite).

Paleoslumps in coal-bearing strata of the Breathitt Group (Pennsylvanian), Eastern Kentucky Coal Field, U.S.A.

The benefits of geologic analysis for roof-control studies and hazard prediction in coal mines are well documented. Numerous case studies have illustrated the importance of recognizing geologic features such as paleochannels, coal riders, and kettlebottoms in mine roofs. Relatively understudied features, in terms of mining, are paleoslumps .Paleoslumps represent ancient movement and rotation of semi-consolidated sediment. Because bedding in paleoslumps is deformed or inclined, these features cause instability in mine roofs, haul roads, surface highwalls, and other excavations. Various types of paleoslumps above coals in the Eastern Kentucky Coal Field were studied in order to aid in their recognition and prediction in mines. The paleoslumps studied all showed characteristic slump-deformation features, although some differences in magnitude of deformation and overall slump size were noted. Coals beneath slumps often exhibited folding, reverse displacements, truncation, clastic dikes, and locally increased thickness. Slumps are inferred to have been triggered by a wide range of mechanisms, such as loading of water-saturated sediment on rigid substrates, synsedimentary faulting, and over-pressurization of channel margin and bar slopes. Analysis of paleoslumps in underground mines, where paleoslumps are viewed from beneath rather than in profile is difficult, since characteristic bed rotation may not be conspicuous. Sudden increases in bed-dip angle inferred from changes in rock type or bedding contacts in the roof; occurrence of bounding, polished rotation surfaces; or roof irregularity and occurrence of loading features may indicate the presence of paleoslumps. Another key to recognition may be the sudden appearance of over-thickened coal, which can occur because of slump-created paleotopography, synsedimentary faults, and slump-generated overthrusting. In addition, steeply inclined, folded, or transported coal marginal to paleoslumps can create apparent increases in coal thickness in cores. Although thick coals are obviously a target of exploration, anomalously thick coals may actually indicate adjacent paleoslumps accompanied by hazardous roof conditions and loss of seam thickness.

The western margin of the Rio Grande Rift in northern New Mexico: An aborted boundary?

The northwestern margin of the Espanola basin, part of the Rio Grande rift in northern New Mexico, is characterized by a zone >17 km wide of oblique-slip faults that offset upper Paleozoic and Mesozoic strata of the eastern Colorado Plateau from Eocene and younger sedimentary rocks of the rift. Along this margin, a reasonably complete section of pre- and synrift Tertiary sediments is exposed. Combined interpretations of seismic reflection, seismic refraction, gravity, and geologic data acquired along a profile perpendicular to this boundary define the geometry of faulting, possible rotation of sedimentary units, and stratigraphy of rift fill. Vertical separation on the westernmost major fault, assumed to be the bounding fault between the rift and the Colorado Plateau, is Although Tertiary units are progressively faulted downward toward the axis of the rift, depth to inferred Precambrian crystalline rocks becomes shallower and the stratigraphic thickness of the intervening Paleozoic and Mesozoic units decreases toward the axis. We interpret pinching out of these units toward the east as erosional thinning on the western flanks of the Laramide-age Sangre de Cristo/Brazos geanticline, which underlay much of the present rift basin. Imprecise age constraints suggest that faulting of the rift margin began 10-7 Ma, but was not active after 7 Ma. Extension was apparently transferred to the Embudo fault zone, which remained active until at least 2.5 Ma and possibly into Quaternary time. The Embudo transfer zone effectively decoupled the Abiquiu embayment from the main Espanola basin. Thus the boundary at Abiquiu preserves an early stage in the formation of the rift boundary. The shift in activity may have resulted from a change in regional stress field, or from increasing magnitude of strain, or both. The change in locus of extension reflects a narrowing of rift basins through time and an integration of main bounding structures between adjacent basins. Although we are uncertain whether the Abiquiu region, which uniquely preserves an early stage of deformation, is representative of other areas of continental extension, our results indicate that the initial formation of rift basins may occur as high-angle, planar normal faults distributed over a broad zone. No evidence from seismic data or from rotation of beds exists to indicate that faults become listric with depth, which is compatible with the small amount of extension (3.5%) inferred at this boundary.

Magnetic fabric in mid-Cambrian rocks of the Central Flinders Zone and implications for the regional tectonic history

Magnetic fabric in the mid-Cambrian sedimentary rocks in the Central Flinders Zone, South Australia, varies from a bedding-parallel foliation to a well-developed lineation. No cleavage is visible in sampled outcrops but an incipient disjunctive cleavage is developed elsewhere. Comparison of magnetic-fabric data from different positions on a soft-sediment fold suggests that the magnetic lineation is not of depositional origin. Rather, it is interpreted as the result of two interfering generations of magnetic fabric: one is a compactional bedding-parallel magnetic foliation, and the other is a magnetic foliation of tectonic origin, defined by the girdle distribution of the site-mean lineation directions. This interpretation suggests that after bedding rotation during formation of the regional NNW-trending folds in the Central Flinders Zone, there was a phase of NNW-SSE-directed tectonic shortening during the Delamerian Orogeny. A model involving a southward-progressing orogeny could possibly account for the two phases of tectonic shortening, as well as for the development of the Nackara Arc in the Adelaide Fold Belt.

The role of shear strain in the graphitization of a high-volatile bituminous and an anthracitic coal

A ‘soft’ carbon-based high-volatile bituminous ( R o max=0.68%) coal and a ‘hard’ carbon-based Pennsylvania anthracite ( R o max=5.27%) were deformed in the steady state at high temperatures and pressures in a series of coaxial and simple shear deformation experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Tests were carried out in a Griggs-t type solid (NaCl) medium apparatus at T=400–900°C, constant displacement rates of 10 -5−10 -6 s −1, at confining pressures of 0.6 GPa (coaxial) or 0.8 and 1.0 GPa (simple shear). Coaxial samples were shortened up to 50%, whereas shear strains up to 4.9 were attained in simple shear tests. Experiments lasted up to 118 h. Deformed, high-volatile bituminous coal was extensively coked and no correlation between strain and R o max, bireflectance or coal texture was observed in any samples. With increasing temperature, R o max and bireflectance increase in highly anisotropic, coarse mosaic units, but remain essentially constant in the fine granular mosaic, which becomes more abundant at higher temperatures. Graphite-like reflectances are observed locally only in highly reactive macerals and in pyrolytic carbon veins. The degree of molecular ordering attained in deformed bituminous coal samples appears to be determined by the heating-pressurization path rather than by subsequent deformation. Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T>700°C, with average bireflectance values increasing from 1.68% at 700°C to 6.36% at 900°C. Anisotropy is greatest in zones of high strain at all temperatures. In anthracite samples deformed in simple shear over the 600–900°C range at 1.0 GPa, the average R o max values increase up to 11.9%, whereas average bireflectance values increase up to 10.7%. Bireflectance increases with progressive bedding rotation and, thus, with increasing shear strain. Graphitization occurs in several anthracite samples deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy of highly anisotropic material in one sample confirms the presence of graphite with d 002=0.3363 nm. These data strongly suggest that shear strain, through its tendency to align basic structural units, is the factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 2200°C required in hydrostatic heating experiments at ambient pressure.

Low-angle faulting in the Triassic of the South Viking Graben: implications for future correlations

A detailed study of bedding geometry in the Triassic strata of the Crawford Field, South Viking Graben, has revealed steep-bedding dips, in excess of 40° towards the west, sandwiched between Permian and Middle Jurassic to Cretaceous strata dipping gently eastwards. Reconstructions show an area of Triassic strata to be cut out at a high angle against the underlying Permian. This bedding geometry has been interpreted to indicate the presence of a detachment horizon towards the top of the Permian section onto which faulting within the Triassic has flattened. This faulting is pre-Middle Jurassic in age and affects the entire Triassic section in the region of the Crawford Field. The extent of bedding rotation is such that a considerable thickness of Triassic section has been cut out against the fault and detachment surface. The formation of planar shear fractures is associated with faulting in the Triassic strata. Most of these fractures parallel the fault trend and are interpreted to have formed during the early stages of fault movement, as they presently dip at a low angle towards the east. Cataclastic processes have dramatically reduced porosity along the fractures which may now represent permeability baffles. Recognition of this intra-Triassic structure means fault and roll-over geometries must be carefully modelled before any attempt at stratigraphic correlation within the Triassic can be made. Equally, if this style of faulting is more widespread, then areas of Triassic strata previously considered internally ‘uncorrectable’ may well benefit from a closer examination of bedding attitude and structure.

Nature and timing of deformation in the Foothills terrane, central Sierra Nevada, California: Its bearing on orogenesis

Detailed mapping and structural analysis combined with new age dates, using U-Pb and ^(40)Ar/^(39)Ar techniques, have allowed us to constrain the timing of pre-ductile and ductile deformation in the Foothills terrane of the central Sierra Nevada. By using strain and other data, it can be shown that rigid rotation of beds (folding/faulting) predated the onset of ductile deformation and probably occurred between 160 and 151 Ma. Ductile structures, consisting of continuous and secondary cleavages and associated folds and lineations, started forming ca. 151 Ma in the Bear Mountains fault zone and then ca. 145 Ma, began moving away from the fault zone, forming diachronously over an ∼30-m.y. period. The last documented ductile structure formed ca. 123 Ma, although some secondary structures may be younger. Metamorphism of these rocks is generally upper greenschist facies, although higher-grade belts (one bearing staurolite, andalusite, and sillimanite) are present. Strain was preferentially partitioned into one of these belts of higher metamorphic grade (and sporadically elsewhere). The structural history here is much more complex, and at least one and locally two complete transpositions of the original cleavage have occurred. In these zones of complex deformation, it is in most cases possible only to identify a composite foliation consisting of new continuous cleavage and relicts of earlier phases, all lying mutually parallel. Timing constraints indicate that the pre-ductile structures may correspond to a very late stage of the Nevadan orogeny (that is, 155 ± 3 Ma), but the ductile structures postdate that orogeny (as defined) by as much as about 25-30 m.y. Models which relate the ductile structures in the central Foothills terrane to Nevadan plate-tectonic events are untenable. In addition, recent work indicates that Late Cretaceous ductile deformation in the central and southern Sierra Nevada may be relatively widespread, indicating that tectonic models for the Sierra Nevada need to be reassessed.

SPIN8: a FORTRAN 77 program for automated rotation of poles

Abstract SPIN8 is a FORTRAN 77 program for computer-assisted rotation of bedding or foliation normal, lineations, crystallographic axes, or directions of magnetization represented as geometric poles. The algorithm is based on the transformation of the axes of the reference coordinate system. The program has the capability of handling multiple rotations of up to 400 poles.

Synsedimentary submarine slope failure and tectonic deformation in deep-water carbonates, Cow Head Group, western Newfoundland

The Cow Head Group, interpreted as a southeast-dipping base-of-slope carbonate apron, contains intraformational truncation surfaces and slide masses. Synsedimentary shear zones are formed (1) below intraformational truncation surfaces; (2) in the basal parts of slide masses; and (3) in the shallow subsurface because of downslope creep. Shear zones are characterized by a variety of synsedimentary deformation structures. Limestones are subject to folding, brecciation, rotation of fragmented beds, and the development of fitted-lenticular bedding. In the interbedded shales, there is both disruption of fine laminations and small-scale isoclinal folding and faulting. Outcrops characterized by these features and the lack of truncation surfaces or slide masses may reflect minor downslope creep. The presence of truncation surfaces, slide masses, and shear zones indicates deposition on an unstable sloping surface.The recognition of intraformational truncation surfaces and slide masses usually requires extensive strike exposure, which when lacking, (e.g., drill cores), limits the potential of these large-scale features as useful indicators of slope deposition. In the Cow Head Group, the recognition and proper interpretation of the common, small-scale deformation structures of synsedimentary shear zones provides evidence for slope deposition that is independent of other sedimentologic, stratigraphic, and regional data.In some parts of the Cow Head Group, "wrinkled" limestones characterized by a prominent dome-and-basin morphology reflect layer-parallel shortening related to tectonic deformation. The deformation of these limestones was previously considered to be synsedimentary, but their association with late-diagenetic precipitates and tectonic stylolites, in conjunction with their continuity and regularity, distinguishes these folds from those produced during synsedimentary deformation.

Structural history of continental volcanic arc rocks, eastern Sierra Nevada, California: A case for extensional tectonics

Mesozoic metavolcanic rocks forming part of the continental volcanic arc along the eastern Sierra Nevada near Mt. Goddard and in the Ritter Range show a complex history related to extensional tectonics. The rocks comprise a thick section of tuffs, breccias, lava flows, sills, and ash‐flow tuffs deposited in a subaerial to subaqueous environment, with some subvolcanic sill‐like plutons. Pb/U ages of the rocks in the Mt, Goddard area range from ca. 130–160 Ma, while rocks in the Ritter Range have a somewhat wider age range as reported previously. Repetition of the section occurs by faulting, and with the exception of parts of the mid‐Cretaceous Minarets Caldera, all the volcanic rocks show a regional slaty cleavage which was subsequently crenulated and/or folded locally. The first cleavage has well‐developed stretching lineations, and does not appear to have been associated with significant folding. Finite strain measurements show considerable variation both in magnitude and symmetry. The Mt. Goddard rocks, however, tend to show slightly higher overall strain magnitude and greater constrictional component than the Ritter Range for rocks of comparable age. Calculations based on the strain data suggest the Mt. Goddard section has been thinned by about 50% normal to bedding, much as that documented previously for rocks in the Ritter Range. Deformation within this part of the continental arc was originally thought to have formed by regional compression during the late Jurassic (Nevadan) orogeny. However, our study indicates that (1) parts of the deformed volcanic section are younger than late Jurassic, (2) Nevadan‐age breaks in deposition are not present, (3) large‐scale folds expected during a regional compression event are not common, and (4) the beds were tilted to a high dip prior to internal deformation. An extensional model is proposed in which beds were rotated to high tilts early in the deformation as a result of listric normal faulting. This normal faulting is thought to have occurred above a regional tumescence related to voluminous magmatism at depth, with preservation of the steeply tilted Goddard and Ritter sections being facilitated by their downward transport along the margins of rising plutons. Flattening and steeply plunging constrictional fabrics superimposed on the tilted sections are related to strain induced by high‐level inflation of magma chambers and downward return flow of the keellike pendants. The main tectonic fabric shown by the continental volcanic arc rocks in the eastern Sierra Nevada is largely of Cretaceous age, rather than Jurassic (Nevadan) as originally supposed. In addition, the deformation, both rotation of beds and subsequent tectonite fabric, appears to be genetically related to the dynamic evolution of the magmatic arc, and not the result of an externally imposed tectonic event.

Rotating ring desiccant dehumidifier

Confinement animal housing on cold winter nights can consume large amounts of energy if sufficient airflow is used to control humidity. Reducing the airflow reduces energy consumption but causes excessive moisture to accumulate in the building. It is proposed that moisture be removed with a desiccant rather than an energy consuming high airflow system. A rotating ring dehumidifier was constructed and laboratory tested using constant input conditions commonly found in confined animal environments during cold weather. A microcomputer controlled the exhaust humidity according to the set point between 55 and 80% R.H. Although the rotary dehumidifier is larger and more complex than the variable depth bed previously tested, it does have better control of exhaust humidity. Humidity variations are reduced by selecting the proper narrow range humidity sensor, small incremental bed rotation, and small ring thickness relative to airflow rate. These parameters can now be used to design a prototype for field testing.

Potiguar Basin: Geologic Model and Habitat of Oil of a Brazilian Equatorial Basin: ABSTRACT

The Potiguar basin integrates the eastern part of the Brazilian equatorial Atlantic-type margin. The rifting stage of this basin occurred during the Neocomian and Aptian. The drifting stage and sea-floor spreading began in the Late Albian. The rifting stage clearly was intracratonic during the Neocomian and is recognized as a mosaic of half-grabens trending mostly northeast-southwest and filled with syntectonic lacustrine siliciclastics. The half-graben pattern exhibits rotation of beds into the major fault zone, and the preserved uplifted margins display either paleostructures or paleogeomorphic features with hydrocarbons. A regional pre-Aptian unconformity preceded the Aptian proto-oceanic rifting stage which was characterized by syntectonic fluvio-deltaic sediments. Th Aptian tectonics were represented by reactivation of former lineaments superimposed by predominant east-west normal faulting. Structural highs during this stage are so far the most prolific oil accumulations. The most important source beds and reservoir rocks are both Neocomian and Aptian sediments. Geochemistry and hydrodynamics have shown that hydrocarbon migration was driven through fracture or fault zones in both Aptian or Albian plays. Lithofacies maps support this interpretation because pools occur whenever adjacent downthrown blocks present a high shale content. End_of_Article - Last_Page 475------------

The Snake Range Décollement: An exhumed Mid‐Tertiary ductile‐brittle transition

The Snake Range décollement (SRD) in east‐central Nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks. A well‐known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low‐angle faults in metamorphic core complexes. Middle Cambrian to Permian upper plate rocks are cut by two generations of NE trending, east directed normal faults. Both generations were initiated as high‐angle (60°) planar faults that flattened abruptly into the SRD and rotated domino style to low angles, yielding a total rotation of bedding of about 80–90°. Faulting is Tertiary in age as 35‐m.y.‐old volcanic rocks are involved and resulted in about 450–500% extension in a N55W‐;S55E direction. The SRD developed as a subhorizontal surface 6–7 km deep at the top of the Cambrian Pioche Shale. Lower plate granitic rocks and their late Precambrian‐Cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade, forming a penetrative subhorizontal foliation and N55–70W lineation that increases in intensity eastward and upward toward the SRD. Stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension, and is interpreted as synchronous. K‐Ar ages ranging from 20 to 40 m.y. in the lower plate suggests the N. Snake Range represents a Tertiary thermal anomaly. We conclude that the SRD developed as a ductile‐brittle transition zone at 6–7 km depth. Gravity data suggests that the gently domed SRD is cut by younger Basin and Range faults, but the geology of adjacent ranges suggests that the SRD does not continue more than 60 km in any given direction. The lack of stratigraphic omission across the SRD rules out large amounts of movement on a surface that originally cut downsection, and we suggest that extensional detachment faults such as the SRD can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion.

Structure of the Southern Uplands of Scotland

Summary In the Moffat Shale lithology of the Southern Uplands, the primary Caledonian structure consists of thrusting at a low angle to the bedding. Thrusting was associated with minor and large scale folding and northwesterly rotation of beds and thrust planes. Wrench faulting is considered as the final major Caledonian event. All these structural phases were caused by similar principal stress orientations. The primary thrusting is best studied in the Moffat Shale inliers where stratigraphical control is excellent. It is possible that these beds overlie a major plane of décollement. Thrusting is less conspicuous in the overlying greywacke sequence where the beds are more competent, and the stratigraphy is less well defined. Otherwise, the Caledonide structural evolution of the greywacke is essentially the same as that in the underlying pelagic sequence. Consequently it is suggested that nearly all the Caledonian structures of the Southern Uplands can be broadly explained by a common but evolving stress system.