Local Tectonic Features Research Articles

Heat, noble gases and CO2 sources in geothermal fields of Mexico

The Trans-Mexican Volcanic Belt (TMVB) and the Gulf Extensional Province (Baja California) are the regions in Mexico with the highest heat flow and where the main geothermal energy resources are currently exploited. Here, previously published and newly collected heat and volatile species (He, Ar and CO 2 ) data from 132 fluid samples of four geothermal areas belonging to TMVB (Los Azufres, Los Humeros, Acoculco and Cerritos Colorados) and two in Baja California (Cerro Prieto and Las Tres Vírgenes) were analyzed. The goal was to identify heat sources, understand how heat is transported into the geothermal reservoirs and how the multiple fluid sources (meteoric, magmatic and crustal) in the reservoir might impact the heat in each field. Distinct R/Ra ratios (where R is the measured 3 He/ 4 He ratio and Ra is the atmospheric ratio) are observed in the TMVB and Baja California geothermal fields, with those in the TMVB displaying significantly higher R/Ra values (7.14–7.27) than those in Baja California (1.21–6.62), indicating a higher contribution of mantle helium from possibly younger, active magmatic heat sources. Lower R/Ra values in Baja California might reflect local tectonic features, where reservoirs are primarily affected by crustal contributions from an old, subducted fossil slab related to the Farallon Plate. The analysis of heat to 3 He ratios (or Q/ 3 He) together with information provided by 4 He/ 36 Ar ratios suggest that heat and volatiles are transferred into the hydrothermal system by advection. The magma bodies beneath the geothermal areas of interest are sufficiently voluminous to provide aqueous fluids by exsolution to sustain this advective flow. The release of He and Ar from magma, during its degassing, appears to be controlled by their diffusivities, fractionating the pristine mantle 4 He/ 36 Ar ratio toward higher values, while advection transfers volatiles more rapidly than heat, fractionating the pristine mantle Q/ 3 He ratios toward lower values. The Q/ 3 He vs. 4 He/ 3 He relationship shows that boiling and dilution by meteoric water led to further fractionation of heat and volatiles, particularly in Los Azufres and Los Humeros fields, while Cerro Prieto has Q/ 3 He ratios close to those at mid-ocean ridges or fractionated by circulation of low-heat, 4 He-rich fossil waters. Distinct sources of heat and volatiles are also reflected in the CO 2 content, the major dry gas component in these geothermal fields. Main sources of CO 2 are mantle carbon (1%-51%) and limestone (34%–100%) from local sources and the subducting slab, although expected fractionation of the C isotopic composition and CO 2 / 3 He during magma ascent and degassing introduce uncertainties on the precise estimation of each source contributing C to the studied geothermal reservoirs. Yet, contribution of carbon from sediments might be minor (0%–35%), and mainly found in Cerro Prieto and Las Tres Virgenes fields located in Baja California, which are affected by the release of volatiles from the subducted Farallon plate and presence of connate waters in the reservoir of Cerro Prieto. • Heat and helium relationships are investigated in Mexican geothermal fields. • Magma degassing contributes to heat and volatiles in geothermal fluids. • Heat and volatiles are likely transported by advection into the hydrothermal system. • Heat/He ratios well discriminate fluid sources in the geothermal reservoirs. • Previously suggested connate waters in Cerro Prieto field are also evidenced by heat/He ratios.

Geomorphic signatures of active tectonics in Subansiri River Basin, eastern Himalayas

The Subansiri, a major tributary of the Brahmaputra with its catchment area (35763 km2) spreading almost entirely in the Eastern Himalayas across almost all the major and local tectonic features in the area witnesses large numbers of seismic events. Active tectonic indices like relief and slope, drainage pattern, longitudinal profile, valley profile, hypsometry, valley asymmetry factors and transverse topographic symmetry index, stream length gradient, valley floor-height ratio extracted from SRTM 3 arc-second data prove that the evolving basin morphology has substantial contribution from the Himalayan tectonics. Seismic data are incorporated in the study to establish the potentially active tectonic elements in the catchment area. The study shows that the western part of the Subansiri River Basin is profoundly tilted towards north in the upper catchment and towards east in the lower and middle part of the catchment. The predominant tectonic movements in the western part of the basin caused the tilting of the basin towards north in the upstream and towards east in the middle and lower parts.

Probabilistic Seismic Hazard Assessment for the Shanxi Rift System, North China

ABSTRACTWe present the first probabilistic seismic hazard assessment (PSHA) specifically for the Shanxi rift system, north China, which has been defined as one of the areas of highest seismic hazard and risk in China in recent decades. We applied a Monte Carlo-based approach to PSHA, based on so far the most complete earthquake catalog available, a detailed zonation considering both seismicity distribution and local tectonic features, a logic tree of carefully selected ground-motion prediction equations, as well as a cautious consideration of actual local site effects for this region. Both areal sources (for Ms<6.0) and fault sources (for Ms≥6.0) were considered, and a synthetic earthquake catalog was generated through Monte Carlo simulation. A logic tree was applied to represent the epistemic uncertainty related to attenuation models for the rift system. Actual local site effects were incorporated and the stability of the results was also tested in this study. Our results show that nearly the entire rift system faces a significant seismic hazard and associated high seismic risk, as more than 80% of the population and the main economical infrastructure of Shanxi are concentrated here. The highest hazard is found in the areas around the north margin of Tianzhen fault and the north segment of Hengshan fault in the north, and in the Linfen basin and the area around Zhongtiaoshan fault in the south of the rift system. Our results are comparable to, but a refinement of, the results of previous probabilistic seismic hazard studies in the region. Deaggregation of seismic hazard for five large cities in the rift system indicates that the seismic hazard is most contributed by the nearby sources. Results obtained in this study provide a better understanding of the seismic hazard in the Shanxi rift system and can thereby help guiding earthquake risk mitigation in the future.

Seismic Demand for Low-Rise Reinforced Concrete Buildings of Islamabad–Rawalpindi Region (Pakistan)

Determination of appropriate seismic demand is important for deriving reliable seismic forces for use in structural design and vulnerability assessment. Demand can be defined by either probabilistic or deterministic approaches depending on the use of the assessment. The use of probabilistic or deterministic approaches depends on the level of the assessment since deterministic can mostly be used for single structures whereas probabilistic for city or country level assessment. This paper presents demand characterization which is based on results of existing seismic hazard studies and local tectonic features around Islamabad–Rawalpindi region (study region) in Pakistan. Existing seismic zoning maps and recent probabilistic seismic hazard studies are reviewed, and the findings are used to quantify and compare the demand for a typical low-rise reinforced concrete building. As another option, deterministic demand is defined through spectra using suitable attenuation relationship which is assessed and validated using the Kashmir earthquake and other similar earthquakes data. Deterministic spectra for study region are generated by considering the critical local tectonic features. Federal Emergency Management Agency (FEMA 356) approach is used for smoothening of deterministic spectra, and the new spectral corner periods are calculated.

Aftershocks properties of the 2013 Shonbe Mw 6.3 earthquake, central Zagros, Iran

Abstract The spatial and temporal seismicity parameters, stress state as well as the partitioning of the seismic energy of the Shonbe earthquake sequence have been investigated. The geodetic data and the stress tensor inversions of the focal mechanisms before and after the Shonbe main shock indicate the same NE–SW trend of the greatest principal stress. This stress orientation demonstrates a non-heterogeneous stress state in that region of the central Zagros which can be confirmed by the fractal dimension and the estimated p-value. The aftershock focal mechanisms show a diversity which may be associated with the approximately uniaxial stress field acting nearly perpendicular to the main-shock fault plane in which the co-seismic stress release is nearly complete. The spatial distributions of the b and p-values have been investigated and compared. The high p-value regions (e.g. far to the southeast, south and southwest of the main shock) are approximately consistent with the high b-value regions and the low p-value regions (e.g. the limited areas at the north- northeast of the main shock) are nearly consistent with the low b-value regions. The limited areas surrounding the main shock and also at the east-southeast of the main shock the p-values are not in agreement with the b-values which may be attributed to the local tectonic features in those regions. The seismic moment ratio and the Bath’s law have been used to constrain the energy partitioning between the main shock and the aftershocks. It is found that a high fraction of the energy is released by the Shonbe main shock and nearly one third of the energy released by the aftershocks. This may be associated with the high stress imparted by the blind thrust main shock to the overlying crust in which promotes failure in the surrounding area and is often relieved by secondary faults and typically produce a high number of aftershocks.

EARTHQUAKE RELOCATION IN GREECE USING A UNIFIED AND HOMOGENIZED SEISMOLOGICAL CATALOGUE

In this study phase data from the Cornet seismological network installed by the Geophysics-Geothermics Department (University of Athens) and the permanent networks of the Geodynamic Institute of the National Observatory of Athens and of the Geophysics Department of Thessaloniki (Aristotle University), for the decade 1996-2006, were merged. The data were jointly used to relocate the earthquakes that occurred in the broader area of Greece. Initially, single-event algorithms were applied by minimizing spatiotemporal residuals. Following, the hypocenters were relocated using a doubledifference algorithm where rms, as well as the horizontal and vertical location errors (erh, erz) were minimized and a consequence of that better locations were achieved. After that process the spatial distribution of the epicenters outlined major local tectonic features in Greece. Moreover, the same methodology was applied to the aftershock sequences of large earthquakes and the results were successfully compared with those obtained by available local networks that were deployed.

AutomatedSKSsplitting and upper-mantle anisotropy beneath Canadian seismic stations

SUMMARY We have developed an automated method for measuring shear wave splitting and applied it to SKS phases recorded at permanent broad-band stations in Canada. Our method performs two measurements, one seeking to minimize the energy on the transverse component, the other minimizing the smaller of two eigenvalues calculated from the covariance matrix of particle motion. A short-term-average/long-term-average (STA/LTA) phase picker is used to identify SKS arrivals and successive splitting measurements are made on incrementally larger windows around the SKS arrival. The final result is derived from the longest series of windows over which the splitting parameters remain constant. In this study we have processed over 20 000 station/event combinations from 34 stations and 1540 events. Roughly 2 per cent of the data produce reliable estimates of splitting, due to the stringent quality controls applied. We find good correlation between our results and previously published results from the same stations. At some stations we find significant differences in results between the two methods, an observation that has been verified using manual measurements. In each case, eigenvalue derived measurements are consistent with a single layer of anisotropy but transverse-energy derived results require a more complex interpretation. This discrepancy can be explained by misalignment of horizontal components. Tests with synthetic seismograms show that component misalignment can lead to false interpretations of multilayer or dipping anisotropy using the transverse-energy method. The quality of results at individual stations is variable, being influenced both by the volume of available data (determined by station deployment date and the distribution of natural seismicity), and the performance of the STA/LTA picker used to define the start of the SKS window. We compare the orientation of the inferred anisotropy at each station with local tectonic features and directions of absolute plate motion (APM); there is no consistent correlation. Many stations give results that agree with both, while some agree better with local geology and others agree better with APM directions. Results are available through a dedicated website and provide a potential aid to other studies of anisotropy in the deep Earth.

The North Sea Silverpit Crater: impact structure or pull-apart basin?

The Silverpit Crater consists of a set of concentric faults up to 20 km in diameter in the southern North Sea. Although formerly described as the first impact structure to be recognized in the UK, its spatial association with local tectonic and stratigraphic features suggests that it is unlikely to have been caused by a random extra-terrestrial event. This paper proposes a terrestrial origin for the Silverpit Crater as a Palaeogene pull-apart basin linked to strike-slip faulting in the Carboniferous basement. Faulting and thinning of Permian evaporites underlying the crater acted as a buffer to basement extension and caused both a flexural collapse of the overlying Triassic strata and circumferential listric faulting in the Cretaceous succession. Reactive diapirism in Lower Jurassic shales may have contributed to the formation of a central uplift at base Cretaceous level. The paper compares the tectonic setting of two contemporaneous strike-slip-related structures in southern England (Bovey Basin in Devon and the Compton Valence structure in Dorset) with that of the Silverpit Crater, and concludes by briefly considering whether a pull-apart model can help to explain the formation of Upheaval Dome, a similar putative impact structure in the Canyonlands National Park of Utah.

Reverse migration of seismicity on thrusts and normal faults

In this work, is analysed the control exerted by the stress axes orientation on the evolution of seismic sequences developing in compressive and extensional regime. According to the Anderson fault theory, the vertical stress is the minimum principal stress in compressional tectonic regimes, whereas it is the maximum principal stress in extensional regimes. Using Mohr diagrams and discussing the present knowledge about the distribution of vertical and horizontal stress with depth we show that, in absence of localised fluid overpressure, such changes imply that thrust and normal faults become more unstable at shallower and greater depths, respectively. These opposite mechanical behaviours predict, in a rather isotropic body, easier rupture at shallower level in compressional regimes later propagating downward. On the contrary, a first deep rupture propagating upward is expected in extensional regimes. This is consistent with observations from major earthquakes from different areas in the world. We show that the exceptions to downward migration along thrusts occur along steeply inclined faults and probably imply localised supra-hydrostatic fluid pressures. Moreover, we show that the inversion of the meaning of the lithostatic load has consequences also for the role of topography. High topography, increasing the vertical load, should inhibit earthquake development in compressional environments and should favour it in extensional settings. Although several factors, such as geodynamic processes, local tectonic features and rock rheology, are likely to control earthquake locations, stress distribution and tectonic regime, these model predictions are consistent with seismicity distribution in Italy, central Andes and Himalaya. In these areas, large to medium compressional earthquakes occur at the low elevation borders of compressional mountain belts, whereas large extensional earthquakes occur in correspondence to maximum elevations.

Topographic Irregularities on the Base Zuni Supersequence Boundary and their Initial Cretaceous Sediment Fill, Central Texas

ABSTRACT The Cretaceous-preCretaceous unconformity, i. e., base Zuni supersequence boundary, crops out across much of central and west Texas. Topographic features on the unconformity surface are exhumed along the erosional out-crop edge of the nearly flat-lying Cretaceous strata that overlie the surface within Lampasas and Mills counties, Texas. Digital elevation data displayed as a topographic profile atop Pennsylvanian outcrops adjacent to the Cretaceous outcrop edge indicate that paleotopography was influenced by northwest-dipping Pennsylvanian strata that underlie the unconformity surface. Differential erosion of Pennsylvanian limestone, sandstone, and shale prior to Cretaceous deposition created a trellis network of northeast-trending valleys and divides. Local tectonic features, climate, and rising sea level influenced the nature of the Cretaceous fluvial fill that directly overlies the unconformity surface. The Sycamore Conglomerate is the earliest Cretaceous deposit, and accumulated in a sub-humid or drier climate as a downfilling braided alluvial wedge shed off the Llano Uplift. The Travis Peak Formation, a conglomeratic sandstone, onlaps and disconformably overlies the Sycamore Conglomerate, and was deposited as an aggradational meandering river complex during sea level rise.

Earthquake Locations in the Inner Continental Borderland, Offshore Southern California

The inner Continental Borderland region, offshore southern California, is tectonically active and contains several faults that are potential seismic hazards to nearby cities. However, fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of event locations in this area, we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the Borderland. The regions are defined by either temporal or spatial clustering of seismic activity in the Southern California Seismic Network (SCSN) catalog. Obtaining accurate locations for these events is difficult, due to the lack of nearby stations, the limited azimuthal coverage, and uncertainties in the velocity structure for this area. Our location procedure is based on the L-1 norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 2000) to identify suitable event pairs for waveform cross-correlation and we explore the effect of different velocity models on the locations and associated station terms. In general, our relocated events have small estimated relative location errors and the events are more clustered than the SCSN catalog locations. A quarry on the south tip of Catalina Island provides a test of our location accuracy and suggests that, under ideal conditions, offshore events can be located to within 1 to 2 km of their true locations. Our final locations for most clusters are well correlated with known local tectonic features. We relate the 1981 Santa Barbara Island ( M L = 5.3) earthquake with the Santa Cruz fault, the 13 July 1986 Oceanside ( M L = 5.3) sequence with the San Diego Trough fault zone, and events near San Clemente Island with the known trace of the San Clemente fault zone. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our locations define a northeast-dipping fault plane for the Oceanside sequence, but in cross-section the events are scattered over a broad zone (about 4-km thick). This could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure. Events that occur near Coronado Bank in the SCSN catalog are relocated closer to the San Diego coast and suggest a shallow-angle, northeast-dipping fault plane at 10 to 15 km depth. Manuscript received 12 February 1999.

Seismodynamics of Sweden deduced from earthquake focal mechanisms

SUMMARY Focal mechanisms from 18 major Swedish earthquakes, ML(UPP) ≥ 3, reveal that: (1) several of the studied earthquakes have focal mechanisms with significant extensional stresses which fit well to the idea of postglacial rebound as an important stress generator, (2) in areas like south-western Sweden, the north-westerly trending compressional axes indicate that ridge push from the North Atlantic Ridge is also a considerable stress contributor; (3) some local tectonic features, like the Skalderviken depression in Kattegat, are momentous for seismotectonic interpretations. In addition to first P polarities, full waveform modelling for frequencies up to 3 Hz and epicentral distances up to 225 km provides rather robust focal-mechanism determinations. Focal depths of seven events, ranging from 11 to 37 km, are well resolved within a few km by waveform modelling.

特集 火山活動と地殻応力場 伊豆・小笠原火山弧北端部における現在および第四紀後期のテクトニクス

Izu Peninsula and adjacent areas, which are located on the northern tip of the Izu-Bonin volcanic arc, are characterized by intense crustal movements and volcanic activity. Many geomorphological, geological, and geophysical data were collected from this area and various tectonic models were proposed to explain them systematically. These tectonic models can be classified into two categories : models 1 and 2. While model 1 regards the area as a single tectonic province, model 2 proposes two or more tectonic provinces, which are bounded by tectonic lines. Models 1 and 2 can be classified into models 1A and 1B, models 2A, 2B, 2C, and 2D, respectively. Model 1A hypothesizes an anticlinal bend of the Philippine Sea plate, which is being generated by the subductions along the Suruga and Sagami troughs. Model 1B emphasizes the crustal stress field generated by the collision of the Izu-Bonin volcanic arc with Japan arc and the slab-pull force along the Sagami trough. Since there are many local tectonic features that cannot be explained by model 1A or 1B, models 2A-2D were proposed. Model 2A divides the study area into two tectonic provinces : the northern province of compressive deformation by conjugate faults and the southern province of right-lateral shearing deformation. Model 2B divides the area into the eastern and western provinces, which are defined by sharp contrasts in the geologic structure, seismicity, crustal stress field, crustal movements, focal mechanisms of earthquakes, paleomagnetic directions, and volcanic activity. Many observations support the validity of the model 2B tectonic provinces. Model 2C introduces the hypothesis that the Izu-Bonin arc is being fractured into the inner and outer arcs because of a contrast in buoyancy. The existence of the estimated model 2C fracture, W est Sagami Bay Fracture, is still under debate. Model 2D regards the Higashi-Izu monogenetic volcano field, located in the eastern Izu Peninsula, as a field of crustal spreading. Model 2D proposes a key to understanding a sharp contrast in tectonic features between the eastern and western provinces of model 2B as well as the complex geometry of the Philippine Sea slab beneath the Japan arc.

Geological and Geometrical Characteristics of Reservoir Fracturing Throughout the Middle East

The geometry and basic characteristics (length, density/intensity, aperture, and porosity) of fractures (joints) have been defined recently for a number of Middle East reservoirs. The factors that determine the occurrence of natural, open, permeable fractures within Middle East reservoirs are nature and degree of folding and/or faulting, in-situ stresses, and changes in rock properties such as porosity, lithology, and especially shaliness. Fracture distribution and orientation within Mesozoic Arabian/Persian Gulf halo-kinetic structures is important to reservoir development and modeling, although the fractures generally only assist productivity. In the deeper Paleozoic reservoirs, fractures become increasingly important. Fractures are best developed in relatively anhydrite free, low porosity, dolomite facies, and with few exceptions their orientation is related to regional trends, only slightly modified by local tectonic features. Exploration for deep-fracture reservoirs needs to consider that the probability of uncemented fractures will be present only where the timing of hydrocarbon migration was close in timing to fracturing. Examination of fractured reservoirs in the Zagros-Bitlis orogenic belt from Turkey through Syria, Iraq, Iran and the northern Emirates demonstrates that the fracturing is dominantly related to folding, with only minor karst fracturing or fault-related fracturing, whereas the fractures in the Gulf of Suez are closely relatedmore » to the faulting history with some of the most intense fracturing of the low-porosity Eocene limestones forming a fracture reservoir near fault zones. Studies of basement fracturing reveals that decreases in fracture apertures generally accompanies increases in fracture density. The distribution of fractures within the northern Sinai closely fit a wrench-tectonic model, where the greatest density and largest apertures occur in the dolomitic facies and have an orientation parallel to synthetic faulting of the wrench system.« less

Regional Depositional and Tectonic Model for Lower Mississippian Pocono Formation Sandstones, Hydrocarbon Entrapment, and Play Generation--Southern Appalachian Plateau of West Virginia and Virginia: ABSTRACT

Local and basin analyses were integrated into a regional depositional and tectonic model for hydrocarbon migration and entrapment for the Lower Mississippian Pocono Formation sandstones for a study area located in the Appalachian Plateau of West Virginia and Virginia. Subsurface mapping on a local scale led to the recognition of a suite of depositional facies corresponding to the Pocono Formation sandstones and the tectonic effects exerted on them. The Pocono depositional sequence consists of sediments deposited in wave-dominated linear clastic shorelines. These features include the corresponding facies of barrier islands and strand plains. Structural elements observed from local mapping on Pocono horizons include northeast-southwest strike, northwest dip, high-angle reverse faults, low-relief folds, and northwest-southeast cross-strike structural discontinuities (tear faults). Basin analysis generated a basin history model, which was described using the following parameters: basin-forming tectonics, depositional sequences, and basin-modifying tectonics. Placing the local Pocono Formation study area within the framework of the generated basin history model yielded a tracing for the movement of the study area through time relative to basin evolution. This permitted identification of the genetic relationships among the observed local depositional and tectonic features and the mechanisms responsible for their generation, thus providing a basis for exploration and field extension. For the study area, we place the Pocono Formation sandstones at the wedge top of a Middle Silurian through Lower Mississippian depositional sequence deposited within an interior sag basin situated immediately west of a westward-converging mobilized fold belt. End_of_Article - Last_Page 1924------------

Thermal constraints on the formation of mississippi valley-type lead-zinc deposits and their implications for episodic basin dewatering and deposit genesis

Two simple constraints: that the deposits formed within 1 km of the surface, and that the temperature of the ore fluids at the time of mineral deposition was 100 degrees to 150 degrees C. For base metal deposits to form as the result of the compactive expulsion of basin brines, brine expulsion must be episodic. Episodic pulses of geopressured brines moving through the near-surface sites of ore deposition, followed by much longer periods of exposure to cool surface waters, could produce the color banding of sphalerite, the cycles of sulfide precipitation and dissolution, and the distinctive local tectonic features associated with mississippi valley-type deposits. Basin features favoring the right kind of episodic dewatering are identified.--Modified journal abstract.

Bear Mountain Igneous Complex, Klamath Mountains, California: an Ultrabasic to Silicic Cale-Alkaline Suite

The Bear Mountain igneous complex, Klamath Mountains, California, can be divided into distinct lithologic suites (order according to apparent relative age): (1) satellitic masses of clinopyroxene-rich ultramafic and gabbroic rocks with subordinate dunite and hornblende-plagioclase pegmatoid; (2) two-pyroxene-biotite diorite and monzodiorite; (3) heterogeneous hornblende-rich rocks varying from gabbro to diorite; (4) leucocratic rocks, chiefly consisting of biotite tonalite and granodiorite; and (5) late dikes (mafic to felsic). Elongate masses of unit (1) flank a composite pluton consisting of units (2–4), while the late dikes (unit 5) intrude the adjacent country rocks. The rocks of the complex invaded an ophiolite allochthon during the Late Jurassic Nevadan orogeny, and well-defined contact aureoles surround the complex. Lower greenschist facies rocks, chiefly metabasalt, impure siliceous metasedimentary rocks, and serpentinized peridotite, have been dynamothermally metamorphosed to mineral assemblages indicative of hornblende-hornfels facies and locally pyroxene-hornfels facies. The emplacement of the igneous complex was chiefly by forcible shouldering aside, although local tectonic features such as faults in the ophiolite allochthon were instrumental in the emplacement history. The ultramafic and gabbroic rocks are interpreted as crystal cumulates of a fractionated basaltic magma. Mineral compositions and whole-rock chemical characteristics of the proposed cumulates suggest that the Mg/Fe ratio of the parental basaltic liquid was high. The activity of silica was low, while water vapor pressure apparently increased through time until it was moderately high during the late magmatic stage. These cumulates were subsequently remobilized during lateral tectonic compression and emplaced higher in the crust as hot, semisolid aggregates. A diverse array of data, including pyroxene compositions, major-, minor-, and rare-earth-element abundances and field relations, suggest that the two-pyroxene-biotite diorite/monzodiorite unit was consanguineous with the clinopyroxene-rich ultramafic and gabbroic rocks. The diorite/monzodiorite unit, therefore, is an intermediate differentiate of an early primitive basalt. Furthermore, major-, trace, and rare-earth-element data characteristic of the diorite/monzodionte unit indicate strong similarities to low-Si andesite and clearly suggest a calc-alkaline affinity. Age relations indicate that the hornblende-rich and leucocratic units are younger and represent the intrusion of other magmas into the same igneous locus. Petrographic and geochemical data from the hornblende-rich unit suggest recrystallization from hydrous magmas similar in composition to high-Al basalt and basaltic andesite. The leucocratic suite, consisting chiefly of calc-alkaline tonalitic rocks, is similar to other quartz-rich felsic rocks widespread throughout the Klamath Mountains-western Sierra Nevada. The available petrographic and geochemical data are consistent with formation of these rocks by either fractional crystallization of a wet basaltic magma or partial melting of amphibolite or eclogite. The Bear Mountain igneous complex is an example of a diverse but distinctive association of ultrabasic to silicic rocks which characterize numerous plutonic complexes in the Klamath Mountains-western Sierra Nevada. These intrusive complexes invade older ensimatic rocks and appear to define the roots of a complex, Middle to Late Jurassic calc-alkaline magmatic arc. The ultramafic and gabbroic rocks characteristic of this plutonic association are similar to Alaskan-type complexes but differ in detail. More significantly, these rocks are important clues to the composition of early magmas as well as the complex processes operative in reservoirs that form the core of calc-alkaline magmatic centers.

Unconformity Traps: ABSTRACT

Unconformities occur in three different parts of the depositional environment--on the shelf, the basin-margin coastal plain, and within the basin. Those on the shelf, typified by the Pennsylvanian-Permian of the Mid-Continent, are regional disconformities occurring above and below coal cyclothems. Sand-filled channels are commonly present above these surfaces, and they can form long, narrow, oil and gas traps. Regional low-angle unconformities characterize the coastal plain as exemplified by the Cretaceous of southern Arkansas and east Texas. They are angular unconformities only from the regional viewpoint, for the structural difference between stratigraphic units is generally less than ½°. Porous belts are commonly truncated and overlapped by impermeable layers, producing large-scale stratigraphic traps concealed in a confusing array of overlapping and offlapping sequences. Erosion occurred at places deep in the depositional basin on relatively local anticlines. Such folds may be part of a mid-basin arch or may simply be local tectonic features. Salt or shale domes and igneous intrusions produce similar effects. Porous formations are sharply truncated by unconformities; locally the difference in dip between units above and below may be as much as 90°. Traps formed under these conditions are narrow and commonly short, but the oil or gas column may be high. Of the various kinds of traps associated with unconformities, those which form in the area of gentle and repeated tilting and warping on the basin margin are the largest and most copious. Search for them involves problems in stratigraphy and geometry, but ultimately may prove to be vastly rewarding. End_of_Article - Last_Page 333------------

Salinity Variations and Anomalies in Marine Sandstone: ABSTRACT

The writer has developed a new indicator which allows major local tectonic features to be determined from a single well. Only 1 measurement is required (in addition to Rw and temperature)--the SP log. The information which can be determined from this indicator includes locations of (1) fault zones, (2) compaction stress changes, (3) high pressure, and (4) permeable sandstone beds of significant extent. The method is simple and may be cross-checked with the density or travel-time log. It involves several simple principles: (A) [water salinity] [shale porosity] = constant, and (B) [shale porosity] ~ [overburden-hydraulic pressure]. Water salinity may be determined accurately from clean sandstones, but a complete well profile is needed to make an analysis. Several offshore analyses have been made, and, in those analyses, the various stresses have been confirmed. End_of_Article - Last_Page 863------------