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Salt dissolution collapse-subsidence is proposed as the dominant tectono-stratigraphic control on the deposition of major sand trends across the northern Athabasca Oil Sands Deposit. Salt removal along linear dissolution trends 200 m below in the Prairie Evaporite (Middle Devonian) halite beds resulted in the collapse of the overlying Upper Devonian strata. The collapse- induced differential subsidence of the fault blocks formed the floor underlying the McMurray deposits in the 50 km long V-shaped Bitumount Trough extending across the northern area of the Athabasca Oil Sands Deposit. The lower and middle-upper McMurray sand trends filled the accommodation created by collapses of a linear chain of Upper Devonian fault blocks along the northern margin of the western Trough. A pair of tens-of-metres thick and 20-30 km long sand trends developed parallel in overlying accumulations of the lower and middle-upper McMurray Formation (Aptian). This half-graben tilted northward as the dissolution trend in the underlying Prairie Evaporite salt scarp widened, and the scarp margin was deeply embayed. Salt dissolution-induced structures were the principal control that located the large sand complexes exploited by bitumen mining projects. Earlier models of McMurray architecture interpreted the underlying karst collapse to have been largely pre-Cretaceous. This new architectural model reinterprets the spatiotemporal balance between erosion at the pre-Cretaceous surface and within the buried salt beds. Extensive salt removal resulted in collapse of the underlying hypogene karst during the late Aptian age. This resulted in the over-thickened multi- kilometres long McMurray sand trends. The underlying karst collapse resulted in unstable deposition surfaces along the sub-Cretaceous trough floors. This tectono-stratigraphic architecture, called the syndepositional model in this study, is proposed as an alternative to two other models, one of which proposes that deeply incised channel valleys and fills resulted from multiple significant sea-level fluctuations, while the other proposes that stacked parasequences accumulated along overlying shallow channels that meandered across a stable fluvio-estuarine coast. La dissolution de sel causant la subsidence / l'effondrement est proposee en tant que le controle tectonique / stratigraphique dominant de la tendance de la deposition du sable a travers le secteur nord du gisement des sables bitumineux de l'Athabasca. Le retrait du sel le long de tendances de dissolution lineaires a 200 m plus bas dans les lits de halite Prairie Evaporite (Devonien moyen) a cause l'effondrement des strates du Devonien superieur qui se trouvaient au-dessus. La subsidence differentielle, engendree par l'effondrement de blocs de faille, a forme le plancher sous les gisements McMurray dans la fosse Bitmount, laquelle a une longueur de 50 km et une forme en V. Cette fosse s'etend a travers le secteur nord du gisement de sables bitumineux de l'Athabasca. Les tendances de sable inferieures et moyennes-superieures remplissaient l'espace disponible cree par l'effondrement d'une chaine lineaire de blocs de faille (Devonien superieur) le long de la bordure nord de la fosse Ouest. Une paire de tendances de sable de quelques dizaines de metres d'epaisseur et d'une longueur de 20 a 30 km s'est developpee parallelement aux accumulations sus-jacentes de la Formation de McMurray inferieure et moyenne-superieure (Aptien). Ce demi-graben s'est incline vers le nord alors que la tendance a la dissolution dans l'escarpement sous-jacent de sel Prarie Evaporite se developpait et que la bordure de l'escarpement etait grandement decoupee. Les structures causees par la dissolution du sel constituaient le principal controle qui a positionne les vastes complexes de sable exploites par les projets d'extraction du bitume. Selon des modeles anterieurs de l'architecture de la Formation de McMurray, l'effondrement des karsts sous-jacents a eu lieu principalement au pre-Cretace. Le present nouveau modele architectural reinterprete l'equilibre spatio-temporel entre l'erosion a la surface, au pre-Cretace, et a l'interieur des lits de sel enfouis. Le retrait a grande echelle du sel a cause l'effondrement du karst hypogene sous-jacent durant l'Aptien. Cela a donne les tendances de sable sur- epaissies de la Formation de McMurray sur plusieurs kilometres. L'effondrement sous-jacent du karst a donne des surfaces de deposition instables le long des planchers de la fosse sous-jacente au sous-Cretace. Cette architecture tectonostratigraphique, nommee modele synsedimentaire dans la presente etude, est proposee en tant qu'option de rechange a deux autres modeles : l'un propose que les vallees fortement entaillees et les remplissages decoulaient de nombreuses fluctuations importantes du niveau de la mer et l'autre propose que des parasequences empilees se soient accumulees le long de chenaux peu profonds sus-jacents qui serpentaient a travers une cote fluvio-estuarienne stable. [Traduit par le Redaction], Introduction The Lower Cretaceous (Aptian) McMurray Formation strata accumulated the largest known bitumen sand resource across several areas of northeastern Alberta, estimated at nearly 270 billion [m.sup.3] of bitumen-in-place by [...]

We use inverse models of mantle convection to explore the vertical evolution of the Colorado Plateau. By satisfying multiple constraints (seismic tomography, stratigraphy in the western United States and Great Plains, and other structural and volcanic data adjacent to the plateau), the model provides predictions on the continuous history of Colorado Plateau vertical motion since 100 Ma. With the arrival of the flat-lying Farallon slab, dynamic subsidence swept from west to east over the plateau and reached a maximum ca. 86 Ma. Two stages of uplift followed the removal of the Farallon slab below the plateau: one in the latest Cretaceous and the other in the Eocene with a cumulative uplift of ~1.2 km. Both the descent of the slab and buoyant upwellings raised the plateau to its current elevation during the Oligocene. A locally thick plateau lithosphere enhances the coupling to the upper mantle so that the plateau has a higher topography with sharp edges. The models predict that the plateau tilted downward to the northeast before the Oligocene, caused by northeast-trending subduction of the Farallon slab, and that this northeast tilting diminished and reversed to the southwest during the Miocene in response to buoyant upwellings. doi: 10.1130/G30624.1

[1] We use seismic reflection and rock sample data to propose that the first-order physiography of New Caledonia Trough and Norfolk Ridge formed in Eocene and Oligocene time and was associated with the onset of subduction and back-arc spreading at the Australia-Pacific plate boundary. Our tectonic model involves an initial Cretaceous rift that is strongly modified by Cenozoic subduction initiation. Hence, we are able to explain (1) complex sedimentary basins of inferred Mesozoic age; (2) a prominent unconformity and onlap surface of middle Eocene to early Miocene age at the base of flat-lying sediments beneath the axis of New Caledonia Trough; (3) gently dipping, variable thickness, and locally deformed Late Cretaceous strata along the margins of the trough; (4) platform morphology and unconformities on either side of the trough that indicate a phase of late Eocene to early Miocene uplift to near sea level, followed by rapid Oligocene and Miocene subsidence of ~ 1100-1800 m; and (5) seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of New Caledonia Trough by 1800-2200 m since Eocene time. The Cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab. This created a deep (>2 km) enclosed oceanic trough, ~2000 km long and 200-300 km across, in Eocene and Oligocene time as the lower crust detached, with simultaneous uplift and local land development along basin flanks. Disruption of Late Cretaceous and Paleogene strata was minimal during this Cenozoic phase and involved only subtle tilting and local reverse faulting or folding. Basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into Norfolk Basin. We suggest that delamination of the lithosphere, with possible mixing of the lower crust back into the mantle, is more widespread than previously thought and may be commonly associated with subduction initiation, such as Cenozoic events in the Mediterranean and western Pacific. doi: 10.1029/2009TC002476

Subsidence is a common cause of amplified relative sea-level rise, flooding, and erosion in coastal environments. In particular, subsidence due to sediment consolidation can play a significant role in relative sea-level rise in large deltas. We use a combination of InSAR (interferometric synthetic aperture radar), leveling, and global positioning system data to map absolute vertical land motion in the Fraser River delta, western Canada. We show that primary consolidation of shallow Holocene sediments is the main cause for the slow subsidence (-1 to -2 mm/a) affecting the delta lowlands. In addition, parts of the delta undergo increased anthropogenic subsidence. Rapid subsidence rates (-3 to -8 mm/a) are associated with recent artificial loads and exhibit a first-order exponential decrease with a time constant of ~20 years, consistent with the theory of consolidation. Assuming two sea-level rise scenarios of 30 or 100 cm by the end of the twenty-first century, natural subsidence will augment relative sea-level rise in the Fraser Holocene lowlands by ~50% or ~15%. Anthropogenic subsidence will augment relative sea-level rise b y ~130% or ~40%, potentially raising it to as much as 1-2 m. In deltaic, lacustrine, and alluvial environments, anthropogenic sediment consolidation can result in significant amplification and strong spatial variations of relative sea-level rise that need to be considered in local planning.

Modeling the shrinkage and subsidence of soils is generally based on the laboratory shrinkage curves established from liquid state to shrinkage limit, rarely exhibited in situ. We studied the vertical behavior of day-dominant soils from the Marais de l'Ouest (France) in the 20 to 100% water content range. The consolidation states were quantified by recording profiles of wet density ([[gamma].sub.b]) and gravimetric water content (W) down to the depth of 2.50 and 2.00 m in a sunflower (Helianthus annuus L.) field and a grassland, respectively. Under the evident surface consolidation, a paleosol was observed at 1.3-m depth in the sunflower field. The W profiles show two superimposed layers: in the upper layer, W increased from the shrinkage limit ([W.sub.s]) to the plasticity limit ([W.sub.p]), the W profiles bounded by the wet and dry season profiles; in the subjacent layer (100% > W > [W.sub.p]), the W profiles were quite constant. The depth of [W.sub.p] marks the end of the downward progression of the shrinkage cracks. The properties of shrinkage were established through drying stages on intact samples. In the [W.sub.s] to [W.sub.p] domain, the linearity of the volume-water content relation allows the modeling of the interseasonal volumetric distribution of the macro-porosity due to the shrinkage cracks. The preliminary model of porosity behavior proposed agrees with the two superimposed layers: the [W.sub.s] to [W.sub.p] domain characterized by isotropic shrinkage (shrinkage geometry factor r = 3), and the W > [W.sub.p] domain characterized by subsidence only (r = 1).

Large deltas are commonly believed to exhibit rapid rates of tectonic subsidence, largely due to sediment loading of the lithosphere. As a result, deltaic plains are prone to accelerated relative sea-level rise, coastal erosion, and wetland loss. Hurricane Katrina's devastation testifies to the severe threat that these processes pose to the Mississippi Delta, but the relative role of tectonics versus other mechanisms causing land subsidence remains elusive. Relative sea-level records derived from basal peat have the potential to quantify differential crustal movements over Holocene time scales with exceptionally high accuracy and precision. Here we present new sea-level index points from two study areas in the southwestern Mississippi Delta that essentially coincide with a recently published detailed relative sea-level record from the eastern part of the delta. Our results show that differential vertical movements among the three study areas have been only ~0.1 mm [yr.sup.-1] We compare our evidence with a recent sea-level compilation from the Caribbean, to a large extent based on data from areas that are tectonically stable. Our sea-level index points nearly coincide with the Caribbean data, showing surprising tectonic stability for considerable sections of the Mississippi Delta. However, the well-documented high subsidence rates in and near the birdfoot of the Mississippi Delta indicate that different conditions prevail there. The rapid wetland loss in coastal Louisiana is likely due, to a considerable extent, to the compaction of Holocene strata. Keywords: Mississippi Delta, sea-level change, subsidence, Holocene.

This study addresses multiple controls on foreland-basin accommodation and contributes to enhanced understanding of the evolution of the northern Andes. The Middle Magdalena Valley Basin (MMVB), Eastern Cordillera, and Llanos Basin are part of a Late Cretaceous-Cenozoic foreland-basin system, east of the Colombian Central Cordillera. Mechanical modeling indicates that the primary control on complex distributions of sedimentary thicknesses, facies, and unconformities was lithospheric flexure in response to crustal loads from the Central and Eastern Cordilleras. Shorter-wavelength folding and paleoaltitude determined the local character of strata. Our mechanical modeling consists of the application of orogenic and sedimentary loads extracted from geologic data on a continuous elastic lithosphere. The results validate two major basin configurations. The first configuration was a Maastrichtian-early Eocene foreland basin coupled with Central Cordillera uplift. Growth strata record continuous sedimentation in the Eastern Cordillera, whereas regional unconformities in the Llanos Basin (distal foreland basin) reflect isostatic adjustments of the basin's amplitude and wavelength to Central Cordillera episodic uplift and tectonic quiescence. The second major basin configuration was characterized by Central Cordillera erosion since middle Eocene times recorded by a regional pediment surface. In the absence of Central Cordillera effective loading, loads from onlapping sediments and Eastern Cordillera piggyback sub-basins provoked post-middle Eocene accommodation in the MMVB and Llanos Basin. Intensified Eastern Cordillera uplift during the Neogene produced basinal tilting recorded by unconformities in the MMVB. This study highlights the importance of assessing the causes of tectonic accommodation as a foundation for interpretation of the evolution of large foreland and intermontane basins. Keywords: basin analysis, subsidence, unconformity, paleogeographic controls, Colombia, Northern Andes.

The conversion of opal A to opal CT is a thermoehemieal diagenetic process that can cause kilometer-scale differential compaction and differential subsidence within elastic sedimentary basins. This is demonstrated for an opal A to opal CT conversion imaged by three-dimensional seismic data on the northeastern Atlantic margin. This diagenetic 'front' has a circular to polygonal ridge-depression morphology. The ridges have a relief of 10-50 m and separate circular to polygonal depressions that have diameters of 1-2 km, some of which link up to form interconnected networks. This morphology initially developed because the host sediments mantled an underlying polygonal fault system and the front tracked deformed stratigraphy. Ridges formed above the tips of the underlying faults, and depressions formed between them. Ridge relief and width then progressively increased due to earlier opal A to opal CT conversion above ridges as well as conversion laterally along beds or bed sets. This caused compaction and concomitant subsidence in the overburden and the development of a circular to polygonal network of overburden troughs aligned with the ridge system. A positive feedback loop was established where additional overburden sediment above the growing ridges reached the depth of conversion ahead of adjacent areas, driving more compaction and differential subsidence. This paper provides the first insights into the large-scale morphological characteristics of opal A to opal CT diagenetic fronts. It demonstrates the potential for a local stratigraphic control on diagenetic front development and morphology and lastly highlights the potential utility of seismic reflection data for understanding diagenetic processes. Keywords: differential compaction, opal A, opal CT, diagenesis, front, differential subsidence.

The geologic phenomenon of ground subsidence is examined. Regions cited include Santa Clara Valley and Los Angeles in California, New Orleans in Louisiana, England's London, Mexico City in Mexico, Italy's Venice and Shanghai and Bangkok in Asia. Mechanics of subsidence are explained, such as water table levels and clay compression.

Since the early 1990s, sinkholes and wide, shallow subsidence features (WSSFs) have become major problems along the Dead Sea shores in Israel and Jordan. Sinkholes are readily observed in the field, but their locations and timing are unpredictable. WSSFs are often difficult to observe in the field. However, once identified, they delineate zones of instability and increasing hazard. In this study we identify, characterize, and measure rates of subsidence along the Dead Sea shores by the interferometric synthetic aperture radar (InSAR) technique. We analyze 16 SAR scenes acquired during the years 1992 to 1999 by the European Remote Sensing ERS-1 and ERS-2 satellites. The interferograms span periods of between 2 and 71 months. WSSFs are observed in the Lisan Peninsula and along the Dead Sea shores, in a variety of appearances, including circular and elongate coastal depressions (a few hundred meters to a few kilometers in length), depressions in ancient alluvial fans, and depressions along salt-diapir margins. Phase differences measured in our interferograms correspond to subsidence rates generally in the range of 0-20 mm/yr within the studied period, with exceptional high rates that exceed 60 mm/yr in two specific regions. During the study period, the level of the Dead Sea and of the associated ground water has dropped by ~6 m. This water-level drop within an aquifer overlying fine-grained, marly layers, would be expected to have caused aquifer-system consolidation, resulting in gradual subsidence. Comparison of our InSAR observations with calculations of the expected consolidation shows that in areas where marl layers are known to compose part of the upper 30 m of the profile, estimated consolidation settlements are of the order of the measured subsidence. Our observations also show that in certain locations, subsidence appears to be structurally controlled by faults, seaward landslides, and salt domes. Gradual subsidence is unlikely to be directly related to the sinkholes, excluding the use of the WSSFs features as predictable precursors to sinkhole formation. Keywords: consolidation, Dead Sea, ground-water level, land subsidence, synthetic aperture radar interferometry

An exploration is presented on the impact of mine subsidences on surrounding structures. Settlement cracks were observed near structure corners, rather than their centers; recommendations for releveling include the installation of thin reinforcing plates, hydraulic jacks, and strap retrofitting applied to foundation walls.

Following the 1994 Northridge earthquake, the areas with a high density of reported breaks in water pipes in typical residential areas in San Fernando Valley and in Los Angeles often did not coincide with the areas having a high density of severely damaged (red-tagged) buildings. As the former is an indicator of large strains and nonlinear soil response, this observation suggests that the damage to buildings in some areas may have been smaller than expected because the soil dissipated part of the energy of the ground motion by nonlinear response. This paper presents an attempt to quantify this relationship between the density of red-tagged buildings, N (per [km.sup.2]), and the severity of shaking (via peak horizontal ground velocity, [v.sub.max], or modified Mercalli intensity, [I.sub.MM]), including the density of breaks in water pipes, n (per [km.sup.2]), as a variable specifying the level of strain in the soil. Approximate empirical relationships for N = f([v.sub.max], n) and N = f([I.sub.MM], n) are presented. The trends in the data indicate that, for [v.sub.max] in the range from [approximately]35 to [approximately]125 cm/s, the rate of growth of N versus [v.sub.max] tends to decrease at sites with large strain in the soil (i.e., large n). For [v.sub.max] beyond [approximately]150 cm/s, the beneficial effects of nonlinear soil response seem to fade out, as large differential motions associated with soil failure begin to contribute to the damage of structures. Assuming fairly uniform density and quality of building stock and of water pipes in the areas studied, the derived relationships are then used to map [v.sub.max] and [I.sub.MM] in San Fernando Valley and in Los Angeles. The resulting maps are more detailed than what could be obtained from the density of strong motion stations and of sites with reports on felt intensity.

The Guadix Basin became established as an intramontane basin in the central sector of the Betic Cordillera at the beginning of Late Miocene time. Its geodynamic evolution starts with a unit of Tortonian marine sediments and is completed by a unit of Late Turolian-Pleistocene continental sediments. In the two units, six depositional sequences have been differentiated whose boundaries, in most cases, coincide with tectonic events. Geohistorical diagrams show the results of quantitative analyses of subsidence in the northern sector of the basin and permit correlation of the main events with significant changes in the history of subsidence and uplift. A period of strong subsidence occurred at the beginning of Tortonian time causing the formation of a marine basin 800 m deep. The mid- and end-Tortonian tectonic events involved periods of uplift leading to shallowing in the basin. After continentalization at the end of the Tortonian, the basin was uplifted continuously from Late Turolian to Late Pleistocene times, finally accumulating sediments at a height of 1000 m.

A theory for one-dimensional self-weight subsidence of saturated soft porous media is presented. The problem is formulated in terms of volume solid fraction using the Eulerian coordinates. Two different types of constitutive relationships between the effective stress and the volume fraction of solids are considered. The governing equations and initial and boundary conditions are nondimensionalized. Self-weight subsidence behavior of unconsolidated soils is analyzed both analytically and numerically. Two sets of exact solutions corresponding to two different compressibility relationships are obtained under steady state conditions. Experimental data available in the literature are used to validate one set of exact solutions. Numerical analysis of transient settlement that allows the movement of the top boundary is presented. The validity of the numerical technique is verified by comparing with exact solutions. The model can be used to predict transient and ultimate settlement and the void ratio distribution of soft soils subsiding under their own weight.

A study was conducted on the northwest edge crustal flexure of the eastern Snake River Plain, ID. The basin's dense load dimensions were modeled using the flexure. Results indicate lower crustal flow from the basin and density-driven subsidence. Subsidence and isostatic equilibrium were found to be caused by density-driven lower crustal flow. Exploratory wells in the area were also found to constrain the timing of subsidence.

The Los Angeles basin has undergone three stages of development related to complex plate interactions within the evolving San Andreas transform system: transrotation (16-12 Ma), transtension (12-6 Ma), and transpression (6-0 Ma). Timing of these stages correlates with microplate-capture events along the continental margin, and is expressed in changes in subsidence rates and provenance within the Los Angeles basin. The Modelo Formation and related units were deposited in the northern part of the Los Angeles basin at bathyal depths during late Miocene time. The northern Los Angeles basin was segmented into three subbasins, in each of which coarse sediment was deposited as submarine fans (Puente, Tarzana, and Simi). A fourth fan system (Piru) formed in the Ventura basin, just north of the Los Angeles basin. The Puente, Tarzana, and Piru fans were derived from the San Gabriel block, which consists primarily of crystalline basement and lesser volcanic and sedimentary components. Sandstone within the Puente fan reflects unroofing of the central and eastern San Gabriel block. The Tarzana fan was derived primarily from the central San Gabriel block, and the Piru fan was derived primarily from the western San Gabriel block, which is distinctly characterized by Ca-rich plagioclase derived from Proterozoic anorthosite and related bodies. The lack of Ca-rich plagioclase in the other fans eliminates the western San Gabriel block as a possible source area, and confirms differentiation of the Ventura basin from the Los Angeles basin by late Miocene time. The Simi fan was derived from locally uplifted Cretaceous and Paleogene strata; sandstone composition reflects the recycling of these sediments. Subsidence and provenance analyses are consistent with the following paleogeographic and paleotectonic reconstruction. Beginning at approximately 16 Ma, transrotation of the Western Transverse Ranges induced extension and thermal subsidence of the Los Angeles basin area. A second pulse of extension and thermal subsidence occurred when motion began along the San Gabriel fault at 12 Ma. Right slip of 60-70 km occurred along the San Gabriel fault, which produced transtension in the Los Angeles basin area and deposition of the Puente, Tarzana, Simi, and Piru fan systems. At 6 Ma, transform motion was transferred to the San Andreas fault; transpression has dominated the Los Angeles basin since 6 Ma, including rapid uplift, flexural subsidence due to tectonic loading, and rapid sedimentary filling. The rapid subsidence and filling and the sudden switch between transtension and transpression in the Los Angeles basin are typical of strike-slip basins in general. However, initiation of the Los Angeles basin by transrotation reflects the uncommon process of microplate capture along the rapidly evolving California margin.

Comparison of sedimentation patterns, basement subsidence, and faulting histories in the north and central basins of Lake Baikal aids in developing an interbasinal seismic stratigraphy that reveals the early synrift evolution of the central portion of the Baikal rift, a major continental rift system. Although there is evidence that the central and northern rift basins evolved at approximately the same time, their sedimentation histories are markedly different. Primary sediment sources for the initial rift phase were from the east flank of the rift; two major deltas developed adjacent to the central basin: the Selenga delta at the south end and the Barguzin delta at the north end. The Barguzin River system, located at the accommodation zone between the central and north basins, also fed into the southern part of the north basin and facilitated the stratigraphic linkage of the two basins. A shift in the regional tectonic environment in the mid Pliocene(?) created a second rift phase distinguished by more rapid subsidence and sediment accumulation in the north basin and by increased subsidence and extensive faulting in the central basin. The Barguzin delta ceased formation and parts of the old delta system were isolated within the north basin and on Academic Ridge. These isolated deltaic deposits provide a model for the development of hydrocarbon plays within ancient rift systems. In this second tectonic phase, the dominant sediment fill in the deeper and more rapidly subsiding north basin shifted from the flexural (eastern) margin to axial transport from the Upper Angara River at the north end of the basin.

Subsidence curves for Cambrian-Ordovician sequences from the Anglo-Welsh segment of the paleocontinent of Avalonia reveal two periods of regionally enhanced basement subsidence: Early Cambrian (545-518 Ma) and Late Cambrian to early Tremadocian (505-490 Ma). The earlier event may record transtension following the Avalonian-Cadomian orogeny. The second event may be a transtensional precursor to the late Tremadocian volcanic arc on Eastern Avalonia. However, paleomagnetic, faunal, volcanic, and sedimentary evidence suggests that the main separation of Eastern Avalonia from Gondwana occurred after middle Arenigian time. Rifling during separation is probably recorded by localized middle Arenigian to Llanvirnian (480-462 Ma) subsidence along the Welsh basin margin, but rifling must have occurred mainly on the now-obscured southern margin of the Avalonian continent. Pronounced Caradocian (462-449 Ma) subsidence is associated with back-arc rifting after separation from Gondwana.

Reinterpretation of a voluminous regional ash-flow sheet (Masonic Park Tuff) as two separate tuff sheets of similar phenocryst-rich dacite erupted from separate source calderas has important implications for evolution of the multicyclic Platoro caldera complex and for caldera-forming processes generally. Masonic Park Tuff in central parts of the San Juan field, including the type area, was erupted from a concealed source at 28.6 Ma, but widespread tuff previously mapped as Masonic Park Tuff in the southeastern San Juan Mountains is the product of the youngest large-volume eruption of the Platoro caldera complex at 28.4 Ma. This large unit, newly named the 'Chiquito Peak Tuff,' is the last-erupted tuff of the Treasure Mountain Group, which consists of at least 20 separate ash-flow sheets of dacite to low-silica rhyolite erupted from the Platoro complex during a 1 m.y. interval (29.5-28.4 Ma). Two Treasure Mountain tuff sheets have volumes in excess of 1000 [km.sup.3] each, and five more have volumes of 50-150 [km.sup.3]. The total volume of ash-flow tuff exceeds 2500 [km.sup.3], and caldera-related lavas of dominantly andesitic composition make up 250-500 [km.sup.3] more. A much greater volume of intermediate-composition magma must have solidified in subcaldera magma chambers. Most preserved features of the Platoro complex - including postcollapse asymmetrical trap-door resurgent uplift of the ponded intracaldera tuff and concurrent infilling by andesitic lava flow - postdate eruption of the Chiquito Peak Tuff. The numerous large-volume pre-Chiquito Peak ash-flow tuffs document multiple eruptions accompanied by recurrent subsidence; early-formed caldera walls nearly coincide with margins of the later Chiquito Peak collapse. Repeated syneruptive collapse at the Platoro complex requires cumulative subsidence of at least 10 km. The rapid regeneration of silicic magmas requires the sustained presence of an andesitic subcaldera magma reservoir, or its rapid replenishment, during the 1 m.y. life span of the Platoro complex. Either case implies large-scale stoping and assimilative recycling of the Tertiary section, including intracaldera tuffs.

There are several existing methods for determining groundborne vibrations in buildings due to subway train traffic that are essentially based on empirically derived relationships. These methods are not able to handle buildings with unique foundations in unusual geological conditions with reasonable accuracy. The case in this study, which comprises a four-story podium block enclosing twin double-box tunnels within the confines of its ribbed-wall foundations, presents such a situation. In contrast, this paper presents a more accurate approach, which employs the finite-element method and is capable of modeling the complicated subway-soil-structure interaction problem. Furthermore, an analytical model is incorporated to provide the train-loading input. For verification, in situ measurements were taken, both at the tracks and within the building. The analytical results obtained showed reasonable conformity with the measurements taken.

A study determined the historical subsidence trends in the Sacramento-San Joaquin Delta and their environmental controls. In the western Delta, the average subsidence rates are computed as 2.3 cm per year from 1910 to 1988 and 1.5 cm per year from 1952 to 1988. This indicates that the subsidence decreased with time. The study also found that the subsidence rates are independent of the crops grown but correlate with soil organic content.

The San Francisco Bay basin in central coastal California has formed in response to the capture by and emplacement onto the Pacific plate of the Salinian terrane during the past 4 m.y. The addition of mass (the Salinian terrane) onto the Pacific plate caused the flexural downwarp of the Pacific plate and the formation and subsidence of the San Francisco Bay basin. As a result of continued plate-boundary evolution, sedimentary basins that previously formed in this way along the San Andreas fault may be ephemeral. Much of the basin may be removed during crustal capture by the Pacific plate and translation through Santa Cruz Mountain-like fault geometries. Remnants of such flexural downwarp basins may be preserved within the nearshore basins on the Pacific plate.

Fossil diatoms from four stratigraphic sections along the tidal Niawiakum River, southwestern Washington, provide an independent paleoecological test of a relative sea-level rise that has been attributed to subsidence during an inferred earthquake in the Cascadia subduction zone about 300 yr ago. Diatom assemblages in a buried soil and overlying mud indicate a sudden and lasting shift from marshes and forests near or above highest tides to mud flats and incipient tidal marshes, with a progressive return to high-level tidal marshes by sediment aggradation and, perhaps, gradual tectonic uplift. The amount of coseismic submergence required to generate the paleoecological changes observed at these sites could have ranged from a minimum of 0.8-1.0 m to a maximum of [approximately] 3.0 m. Fossil diatoms also provide an independent test of previous inferences that the subsidence was shortly followed by a tsunami. The inferred tsunami deposit is a distinct sandy interval that widely overlies the buried marsh and forest soil. Diatoms from this interval consist of species observed on modern sand flats of the open bay, identifying a bayward source for the sand. Occurrences of the same sand-flat species above the buried soil in the farthest up-valley outcrop where a sandy interval is not recognizable suggest that the tsunami extended farther landward than was previously inferred from the stratigraphy. These data rule out proposed alternatives to the coseismic subsidence model - that is, climatically induced sea-level rise, temporary submergence caused by storms - and support the hypothesis that a great earthquake struck southwestern Washington 300 yr ago.

A new analytical solution is developed for steady-state lateral response of earth and rockfill dams built in semielliptical canyons and subjected to obliquely incident harmonic SH waves. The dam is idealized as a linearly hysteretic elastic body deforming only in shear, while the supporting canyon is modeled as an elastic rock half-space. The solution accounts in a rigorous way for the wave reflection and diffraction phenomena associated with the presence of the dam-filled canyon. The response of the dam and the canyon is expressed in terms of prolate radial and angular wave spheroidal functions. Representative results elucidate the importance of the effects of the dam-to-canyon material impedance, the canyon geometry, the angle of incidence, and the excitation frequency on the dam response. Comparisons are made with the response of the dam to 'rigid-base' synchronous excitation. The developed solution offers a useful practical tool for parametric studies and preliminary design calculations, as the semielliptical shape can approximate reasonably well the geometry of several actual canyons.

The flexural subsidence history recorded in Cenomanian to early Campanian (97 to 80 Ma) strata in the Cretaceous U.S. Western Interior basin was studied with two-dimensional flexural backstripping techniques. Results indicate that the flexural subsidence resulting from thrust loading was superimposed on epeirogenic subsidence in the foreland basin. The flexural component exhibits significant spatial and temporal variations along both the strike and dip relative to the Sevier thrust belt. The greatest cumulative subsidence occurred in southwestern Wyoming and northern Utah. Concurrent subsidence in northwestern Montana and southern Utah was insignificant. Temporal trends in subsidence also show a distinct regional pattern. From the Cenomanian to late Turonian (97 to 90 Ma), subsidence rates were high in Utah and much lower in Wyoming and Montana. In contrast, during the Coniacian and Santonian (90 to 85 Ma) subsidence accelerated rapidly in Wyoming, increased slightly in Montana, and decreased in Utah. We suggest that these spatially and temporally varying subsidence patterns reflect the interplay of several geo-dynamic factors, including: (1) temporal and spatial variation in emplacement of the thrust loads, (2) segmentation of the basement into adjacent blocks with different rheological properties, (3) reactivation of basement fault trends, and (4) regional dynamic topographic effects.

Evidence of rapid seismic-induced subsidence at Humboldt Bay, California, is produced by analyses of annual growth rings of relict Sitka spruce [Picea sitchensis (Bong.) Carr.] roots and entombed herbaceous plants. These results add to previously reported evidence that an earthquake caused subsidence [approximately] 300 yr ago at Mad River slough, California. Both types of remains are rooted in buried soils that stood at or above the high-tide level until the area subsided at least 0.5 m into the intertidal zone. Burial by intertidal muds took place quickly enough to preserve the herbs in the growth position. Analysis of the annual growth rings of the tree roots shows that all died within four growing seasons, but the time of root death varies even among roots of the same tree. With no central nervous system, tree cells do not die simultaneously throughout the organism. The 0.5 to 1.5 m of subsidence, as evidenced by stratigraphy and sedimentology, was not enough to kill all the trees even in one season. Although such gradual death could be due to rapid aseismic subsidence, the tree deaths and preserved herbs are much better explained by sudden coseismic subsidence.

This paper presents the results of a cover-subsidence sinkhole evaluation. In this case, the sinkhole activity caused a slow, gradual settlement of the State Road 434/Harbour Isle Way intersection, in Longwood, Florida. The subsoil conditions were explored with numerous cone penetrometer tests and permanent piezometers. The study indicated that the settlement was caused by internal soil erosion and raveling. Piezometric elevation contour maps revealed a well-defined depression that coincided with the observed surface settlement. The piezometer monitoring results suggest this may be an effective technique for economically identifying potential sinkhole locations.

Tectonic transport from different directions and convergence of separate thrust belts at structural recesses impart three-dimensional variations to the pattern of foreland-basin subsidence. Assuming approximately uniform flexural rigidity of the lithosphere, subsidence is directly controlled by the geometry and density of the separate thrust loads, and flexural deformations caused by the loads interfere within the recess. For example, the Black Warrior foreland basin, which is located in Mississippi and Alabama, within a recess in the Appalachian-Ouachita orogen, formed in response to two separate thrust loads: (1) the Ouachita accretionary prism along the southwest side of the basin, and (2) thrust-imbricated passive-margin rocks in the Appalachian thrust belt along the southeast side of the basin. The subsidence history differs along two profiles, each perpendicular to the Appalachian and Ouachita thrust fronts. Subsidence of the foreland basin began in middle Mississippian (Meramecian) time in response to northeast-directed translation of the Ouachita accretionary prism. Subsequently, northwest-directed Appalachian thrusting began in middle Early Pennsylvanian (Morrowan) time. Subsidence curves from the profile perpendicular to the Ouachita prism show a clear progression of increasing subsidence rates and amounts toward the thrust front, whereas curves from the profile perpendicular to the Appalachian belt indicate out-of-plane effects from the Ouachita thrust front and a later increase in subsidence toward the Appalachian thrust front. The major implication of this work is that the three-dimensional geometry of a foreland basin within a thrust-belt recess includes diachronous interfering elements.

Cretaceous-Paleogene deposits of the Gualala basin and overlying deposits of the Neogene Point Arena basin record the complex tectonic and depositional history of the California continental margin as it changed from a convergent tectonic regime to a translational regime during the evolution of the San Andreas transform fault system. The Gualala and Point Arena deposits are among the northernmost onshore exposures of upper Cretaceous and Cenozoic marine strata west of the San Andreas fault and, as such, constitute a critically important but not intensely studied succession of rocks. In this paper, lithologic, stratigraphic, and paleobathymetric data from the Gualala and Point Arena stratigraphic successions are used to produce quantitative geohistory analyses that illustrate the subsidence and uplift of a portion of the California continental margin over the past 80 million yr. The history of subsidence and uplift revealed through geohistory analysis of the Gualala and Point Arena basin-fill deposits clearly reflects two episodes of basin development directly associated with the convergent and transform phases in the evolution of the California margin. Beginning in Late Cretaceous time ([approximately] 80 Ma), the Gualala basin underwent slow rates of tectonic subsidence ([approximately] 30 m/m.y.). In late Paleocene time ([approximately] 57 Ma), tectonic subsidence increased to [approximately] 140 m/m.y. and was accompanied by rapid rates of sediment accumulation ([approximately] 440 m/m.y.) that continued through the Eocene until terminated by basin filling, cessation of subduction tectonics, and flexing and uplift of the continental margin. Initial development of the Neogene Point Arena basin occurred in latest Oligocene time ([approximately] 24 Ma), some 2 to 4 m.y. after the birth of the San Andreas transform system. The earliest history of the Point Arena basin was marked by volcanism and extremely high rates of tectonic subsidence ([approximately] 1,060 m/m.y.) characteristic of a transtensional origin. Based on our analysis, the tectonic and depositional histories of the Gualala and Point Arena basins can be summarized as follows: (1) during Late Cretaceous to middle Eocene, submarine fans of the Gualala and German Rancho Formations were deposited in a >2,000-m-deep subsiding basin that probably formed as a fore-arc basin in a setting characterized by oblique convergence and subduction along the California margin; (2) flexing and erosion of Cretaceous-Paleogene bathyal deposits of the Gualala basin occurred in the Oligocene epoch as the Pacific-Farallon spreading ridge approached the California margin, and as the accretionary prism outboard of the Gualala fore-arc basin migrated landward; (3) initial development of the Point Arena basin in earliest Miocene time ([approximately] 24 Ma) was accompanied by extrusion of the Iversen Basalt during the transitional period between convergent and transform tectonic regimes; (4) rapid subsidence of the Neogene Point Arena basin during Miocene time was related to transtension associated with the San Andreas fault, a pattern common to many of the small pull-apart basins formed along the California margin at this time; (5) early deposition in the Point Arena basin was dominated by submarine-fan deposits of the lower Miocene Skooner Gulch and Gallaway Formations, whereas siliceous biogenic sediments of the overlying Point Arena Formation signalled reduced deposition of coarse-grained terrigenous sediment during middle to late Miocene time; and (6) margin flexure and uplift of the Point Arena basin occurred in middle Pliocene ([approximately] 3.5 Ma) and early Pleistocene ([approximately] 1.6 Ma) time as a function of transpressional deformation of the crust in response to Pacific-North American plate interaction.

The Rukwa rift is a 300-km-long half-graben basin that lies along the central part of a 1000-km-long continental transform in the East African Rift. The basin contains Carboniferous to Quaternary sedimentary strata [approximately] 11 km thick. Within the basin, maximum extension has been directed nearly normal to the border fault, even though this border fault has been dominated by dextral strike slip. However, extension and subsidence in the Rukwa rift are not explained by releasing-bend or oblique-extension fault geometries. We interpret the orientation of the extension as a local stress rotation symptomatic of a low-friction, 'weak' strike-slip border fault. Structural and morphologic features suggest that an along-strike contrast in the frictional strength of the border fault controls the location of this extension. Subsidence and extension normal to the border fault in the Rukwa rift occur along a weak transform fault segment, as an along-strike accommodation of drag and local shortening against an adjacent strong segment. Thus, three-dimensional strain is accommodated differently along the strike of the border fault as a function of the frictional strength of the fault zone.

Basin subsidence analysis, employing the backstripping method, indicates that fundamentally different basin-generating mechanisms controlled Silurian basin development in northern England and in Wales. Accelerating subsidence with time suggests that the dominant control on basin formation in northern England was flexure of the Eastern Avalonian lithosphere. Mid-Wales, however, displays dominantly decelerating subsidence, suggesting an extensional control. The errors involved with the backstripping method, particularly concerning Lower Palaeozoic application, have been addressed. In conjunction with sedimentological and tectonic data, the subsidence patterns for the Windermere Supergroup indicate deposition in a flexurally-loaded foreland basin which migrated southwards with time. Similar data for the Powys Supergroup of Mid-Wales imply an extensional control on Silurian deposition, possibly in a strike-slip setting. However, later Pridoli and early Devonian Welsh sedimentation may represent the onset of foreland basin deposition. The new data are consistent with a diachronous closure of the Iapetus ocean, related to rotational collision of Eastern Avalonia with Laurentia during the early Silurian. Early Silurian oblique collision produced transtension in Wales. Subsequent rotation of Eastern Avalonia produced more orthogonal collision. Thus, continental flexure was felt initially in northern England in late Wenlock times, followed by migration of the resultant foreland basin to the southeast into Wales in early Devonian times. The versatility of the backstripping method is shown by this application to Lower Palaeozoic sequences.

Shallow seismic data from Sevenmile Basin, a large ephemeral lake (playa) basin in the Texas Panhandle, reveal that subsidence has been an important agent in basin formation. Several hypotheses exist for the origin of thousands of playa basins on the Southern High Plains of Texas and New Mexico, including eolian deflation, evaporite or carbonate dissolution and subsidence, piping, and animal activity. Sevenmile Basin is 5.5 X 3.6 km across and 14 m deep and contains 20 m of lacustrine and eolian sediments that interfinger with the Quaternary Blackwater Draw Formation. Below these sediments is the upper Tertiary Ogallala Formation, which overlies Permian or Triassic bedrock. Seismic reflection and refraction data were collected from the unlithified and variably saturated clastic sequence beneath Sevenmile Basin to investigate the geological history and hydrogeological framework of playa basins, which recharge the regionally important Ogallala aquifer. Three-layer velocity models provide good solutions for reversed refraction data. Near-surface p-wave velocities (layer 1) range from 349 to 505 m/s, layer 2 velocities range from 806 to 851 m/s, and layer 3 velocities range from 2,037 to 2,161 m/s. Shallow test holes and drillers' logs suggest that layer 1 is composed of playa and upper Blackwater Draw Formation deposits, layer 2 consists of lower Blackwater Draw Formation and upper Ogallala Formation deposits, and layer 3 represents a competent and partly saturated zone near the top of the Ogallala aquifer. Reflection sections show a middle Ogallala reflector, a reflector at the top of Permian or Triassic bedrock, and internal bedrock reflectors that indicate a structural low beneath Sevenmile Basin. Increasing relief with age, from 14 m at the surface to 70 m on the middle Ogallala reflector to 110 m at the base of the Ogallala, is interpreted as evidence of subsidence of underlying Permian evaporite-bearing strata before or during Ogallala deposition. Ogallala and Blackwater Draw Formation thicknesses greater than bedrock relief suggest that subsidence continued during Ogallala deposition and may continue to the present. Virtually all playa basins on the Southern High Plains are underlain by Permian evaporite-bearing strata; some basins have been affected by dissolution-induced subsidence. Shallow seismic methods are an ideal approach to determine the relative importance of subsidence in basin formation.

A considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the British Isles underwent permanent exhumation during the Tertiary. The most dramatic indicator of this process is the present-day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of Scotland. Any explanation of this observation must take into account the fact that the entire region has very small, long-wavelength, free-air gravity anomalies. This important constraint implies either that the crust has been thickened or that low-density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the North Atlantic. Tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to Alpine mountain building. However, to remove approximately 3 km of sedimentary rock from a basin approximately 100 km wide requires >15 km of shortening. Minor Tertiary compression is observed all over the continental shelf, but nowhere is it sufficient to account for the required amount of uplift and erosion. In addition, exhumation dramatically increases from south to north, whereas the observed compression decreases markedly in the same direction. At the beginning of the Tertiary, rifting associated with the initiation of the Iceland plume generated substantial volumes of melt. Inversion of rare-earth- element concentrations of MgO-rich igneous rocks suggests that a minimum of approximately 5 km of melt was produced beneath at least part of the continental shelf. We infer that much of this melt remains trapped within the lithosphere, presumably close to the Moho, which acted as a density filter. Such underplating will generate rapid uplift.

Subsidence analysis of northeastern Venezuela's exposed, parautochthonous Lower Cretaceous-Miocene sedimentary section reveals an Atlantic-type passive-margin subsidence history for the entire Cretaceous-middle Eocene interval; Oligocene-Miocene subsidence was rapid and great and was caused by foredeep formation and emplacement of the fold-and-thrust belt that now exposes the passive-margin sedimentary section. Our analyses support tectonic models of South America's northern margin which propose Cenozoic eastward migration of a foredeep along northern South America's late Mesozoic passive margin. The calculated subsidence is inconsistent with tectonic models that propose middle or Late Cretaceous subduction or obduction.

Archaeological material and sites in the northern Nile delta record rates of land subsidence that are higher than those derived from radiocarbon-dated subsurface sediments. Reassessment of subsidence-rate calculations reveals that previous subsidence measurements of 1-5 mm/yr for the delta are minimum rates, because sediment reworking can result in radiocarbon-dated core ages that are older than the ages of final burial. Integration of archaeological and geologic subsurface data is essential for accurate age determinations, differentiation of subsidence from sea-level rise, and more precise calculation of vertical earth movement. Application herein of archaeological data to geological problems helps to refine subsidence-rate measurements between the flexure zone (landward margin of the Holocene Nile delta depocenter) and the coast.

Presented in this part of the two-part paper is a study of the relations between earthquake motions recorded from two, adjacent. seven-story buildings, from a downhole below the foundation of one of the buildings and from three freefield sites, all within one city block. This unique data set was obtained during the Whittier-Narrows, Calif. earthquake of Oct. 1, 1987. Part I includes background information on the two buildings, the site, and the data set. Building response characteristics of a code-type instrumented building (A) and an extensively instrumented building (B) are also studied. In this part, spectral analysis techniques are used to study the relationships between the motions of the roofs and basements. the downhole and the free-field sites. It is asserted that there is building-soil-building interaction between the two buildings at a frequency of 2.35 Hz. Furthermore, the free-field motions are shown to be influenced by the presence of the buildings.

In this two-part paper, responses of two, adjacent, seven-story buildings in Norwalk, California, to the Whittier-Narrows. Calif. earthquake of Oct. 1. 1987 are studied. Building A, instrumented according to code recommendations. and building B, extensively instrumented. are offset by 16.3 m from one another. The data set includes motions from the superstructure of both buildings, from a downhole below the foundation of building B, and from three free-field sites. Part I of the paper includes descriptions of the buildings, site, instrumentation. and analysis of the data of each building. System identification and spectral analysis techniques are employed in part I. Building A has identical first-mode frequencies of 0.65 Hz for both building axes. The strong-motion response characteristics of building A are considerably different than those determined from low-amplitude tests. Building B has fundamental modes at 0.76 Hz and 0.83 Hz in the major and minor axes, respectively. Torsional and diaphragm effects in building B are negligible.

Stratal patterns within shelf depositional sequences are dependent on tectonically controlled subsidence rates and their regional patterns. In active tectonic basin settings, regional subsidence patterns can be very different from passive continental margin settings, resulting in substantial modifications of the basic sequence stratigraphic model. In ramp-type foreland basins, on the tectonically active side, subsidence rates decrease seaward in contrast with passive continental margins, where the opposite subsidence pattern exists. Analysis of the interplay between eustasy and subsidence suggests the existence of two tectono-stratigraphic zones, occurring proximally and distally with respect to the basin margin. Zone A is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall. Consequently, relative sea level rises continuously during a eustatic cycle, albeit at varying rates. Zone B is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence, resulting in an interval of relative sea-level fall during a eustatic cycle. On the tectonically active sides of foreland basins, zone A lies on the landward side of the basin margin, proximal to the orogenic belt, and zone B lies seaward of zone A, away from the orogenic belt where subsidence rates are lower. On passive continental margins, because of the opposite subsidence patterns, zone A lies seaward of zone B, where subsidence rates are greater. The location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence. If the shoreline remains in zone A, then only type 2 sequence boundaries will occur; if sufficient sediment flux is available, allowing the shoreline to prograde into zone B, then type 1 sequence boundaries can occur. In foreland basins, these type 1 sequence boundaries would become type 2 sequence boundaries updip in zone A. The pattern of seaward-decreasing subsidence on the tectonically active sides of foreland basins results in characteristic longitudinal facies and stratal patterns. When the two zones occur, the updip region of the basin proximal to the orogenic belt is characterized by nearly continuous nonmarine deposition, albeit at varying rates. The downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts. A transitional region containing deposits of all three systems tracts commonly occurs between the updip and downdip areas.

When compared with theoretical subsidence curves calculated from the uniform stretching model, water-loaded subsidence data from the 'triple-junction' region of the North Sea suggest that the Jurassic-Cretaceous rifting event caused lithospheric thinning by a factor of c. 2.0. Although somewhat larger than the stretching factors found elsewhere in the North Sea, this amount of thinning is anticipated from the overall geometry of the three-graben system and is consistent with the observed volume and elemental and isotopic composition of the Jurassic Forties volcanic province. Apart from in the southern North Sea, Permo-Triassic extension is thought to have been relatively minor in comparison to the later Jurassic-Cretaceous phase. The anomalously small amount of Late Jurassic syn-rift subsidence in those wells where local fault-controlled effects are minimal, supports the well-known idea of localized relative uplift or 'doming' in the triple-junction area (c. 10 to the fourth power km squared) prior to and during the early stages of the Jurassic-Cretaceous rift phase. A time-dependent differential stretching model, in which the lithospheric mantle is initially stretched by a greater amount than the crust is stretched might provide an explanation. Such a model would require the total amount of stretching integrated over space and time to be the same for the lithospheric mantle and for the crust in order to avoid space problems. Alternatively, the same data could be explained by invoking a small transient thermal anomaly in the asthenosphere.

A new method of connecting members, at certain joints of a lateral force resisting structure, improves the response to seismic excitation. These connections that permit a small initial slip (displacement or rotation) are called sequential connectors. A sequential system is a structure consisting of units coupled in parallel in which at least one unit is attached by sequential connectors, and at least one other unit is attached by standard connections. A standard structure is transformed into a sequential system by replacing one or more (but not all) standard connections by sequential connectors. The effectiveness of sequential coupling, in some cases, is comparable to sensor-initiated active controls. Sequential systems exhibit an interesting and even surprising response to dynamic excitation. The paper outlines the characteristics of sequential systems and provides a numerical comparison of their performance with an identical structure using standard connections; the structure using standard connections shows a displacement response that is 40% higher. If the response of the two systems is identical, the standard system requires 40% more material.

The closed-form solution is derived to the problem of stiffness design of coaxially coeccentric asymmetric shear buildings (CCS buildings) for specified fundamental natural frequency and eigenvector. The complete quadratic combination (CQC) estimates of the mean maximum interstory drifts in a so-designed CCS building subjected to an ensemble of design-spectrum-compatible moderate earthquakes are regarded as functions of the lowest eigenvalue and translational eigenvector, parameters for adjustment of those CQC estimates. An almost direct and efficient method of stiffness design is developed for a CCS building such that all the CQC estimates of the mean maximum drifts of the corner columns based upon a prescribed design spectrum will be equal to the prescribed distribution of design drifts. The validity of the design procedure is demonstrated with the results of time history analysis on so-designed CCS buildings. A design amplifier factor for specified distribution of lateral torsional drifts is defined and is shown to be useful for describing directly the effect of eccentricity on the seismic response-drift constrained designs of CCS buildings.

Clastic strata composing the northern Bowser basin record the accretion of Stikinia to the composite western edge of the North American plate (Cache Creek-Quesnellia-Slide Mountain-Kootenay-North America) in early Middle Jurassic time and the concomitant demise of the intervening Cache Creek ocean. Initial flexural subsidence of the northern Bowser basin, resulting from thrust loading of Cache Creek terrane on Stikinia, is represented by an organic-rich shale of Aalenian age (the Abou Formation of the Spatsizi Group). Coarse-grained sediment first appeared in early Bajocian time following uplift and subaerial exposure of Cache Creek rocks in the upper plate. Thus, the inception of the Bowser basin was Aalenian, rather than Bajocian, as believed by earlier workers. Aalenian southwest-vergent thrusting at the composite western edge of North America is also known from southern British Columbia, a coincidence that implicates collision with Stikinia in the south as a cause of that deformation.