Showing papers in "Geological Society of America Bulletin in 1994"

Emplacement and inflation of pahoehoe sheet flows: observations and measurements of active lava flows on Kilauea volcano, Hawaii

Abstract: Inflated pahoehoe sheet flows have a distinctive horizontal upper surface, which can be several hundred meters across, and are bounded by steep monoclinal uplifts. The inflated sheet flows we studied ranged from 1 to 5 m in thickness, but initially propagated as thin sheets of fluid pahoehoe lava, generally 20-30 cm thick. Individual lobes originated at outbreaks from the inflated front of a prior sheet-flow lobe and initially moved rapidly away from their source. Velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow. As the outward flow velocity decreases, cooling promotes rapid crustal growth. At first, the crust behaves plastically as pahoehoe toes form. After the crust attains a thickness of 2-5 cm, it behaves more rigidly and develops enough strength to retain incoming lava, thus increasing the hydrostatic head at the flow front. The increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow, resulting in uniform uplift of the entire sheet-flow lobe. Initial uplift rates are rapid (flows thicken to 1 m in 1-2 hours), but rates decline sharply as crustal thickness increases, and as outbreaks occur from the margins of the inflating lobe. One flow reached a final thickness of nearly 4 m after 350 hr. Inflation data define power-law curves, whereas crustal cooling follows square root of time relationships; the combination of data can be used to construct simple models of inflated sheet flows. As the flow advances, preferred pathways develop in the older portions of the liquid-cored flow; these pathways can evolve into lavatube systems within a few weeks. Formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1-2 orders of magnitude slower. If advance of the sheet flow is terminated, the tube remains filled with lava that crystallizes in situ rather than draining to form the cave-like lava tubes commonly associated with pahoehoe flows. Inflated sheet flows from Kilauea and Mauna Loa are morphologically similar to some thick Icelandic and submarine sheet flows, suggesting a similar mechanism of emplacement. The planar, sheet-like geometry of flood-basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava.

Analysis of drainage-basin symmetry as a rapid technique to identify areas of possible Quaternary tilt-block tectonics: An example from the Mississippi Embayment

Abstract: There is a coincidence of seismicity and major river valleys in the southwestern Mississippi Embayment that suggests control of the drainage pattern by northwest-trending faults. Pleistocene river terraces reveal a strong southwestward preferred river migration in this region that may be a result of fault-block tilting. A rapid new technique of drainage-basin analysis is herein presented that quantifies transverse topographic basin symmetry. These data permit discrimination of random stream migrations and regionally preferred stream migrations and identify the direction of maximum migration. If bedrock dip can be shown to be a negligible influence on stream migration, then the direction of regionally preferred migration implies a period of ground tilting in that direction. Drainage-basin asymmetry vectors (bearing and degree of asymmetry) calculated from transverse topographic profiles of 271 4-km basin segments in the study area suggest preferred stream migration in response to west-southwest-ward ground tilting (bearing 259°). The eastward regional dip of bed-rock horizons only locally influences migration. River terrace distributions reveal a chronology of stream-migration episodes. Southwestward migration in the Ouachita, Saline, and Arkansas river basins occurred during early to middle Pleistocene time in the southwestern Mississippi Embayment. A Wisconsin/early Holocene southwestward migration episode is evidenced along southeast-flowing reaches of the Ouachita and Arkansas Rivers but not along the southeast-flowing reach of the intervening Saline River, suggesting that these rivers overlie separate northwest-trending tilt blocks. Although this technique does not provide direct evidence of ground tilting, it provides a quick-look method of identifying possible tilting elements in neotectonic regions using only topographic maps. This technique is of greatest utility where active faults are concealed or poorly exposed, such as in the Mississippi Embayment.

Phosphogenesis, carbon-isotope stratigraphy, and carbonate-platform evolution along the Lower Cretaceous northern Tethyan margin

Abstract: We distinguish three different stages in the evolution of the Tithonian (Late Jurassic) to Aptian (Early Cretaceous) northern Tethyan carbonate platform: (1) carbonate production in the coral-oolite mode (late Tithonian to early Valanginian, early Barremian to early Aptian), (2) carbonate production in the crinoid-bryozoan mode (early Valanginian, Haute-rivian, late Aptian), and (3) platform retrogradation and destruction, condensation,and phosphogenesis (that is, platform drowning; early Valanginian to early Hauterivian,middle Hauterivian, late Hauterivian to early Barremian, early to early late Aptian, and latest Aptian to earliest Albian). Transitions from the coral-oolite mode to the crinoid-bryozoan mode and consequently to platform drowning may have been driven by increases in nutrient levels on the shelf. Phases of relative sea- level rise in times of carbonate production in the coral-oolite mode are named constructive, because of the observed platform regeneration following phases of widespread emersion during late sea-level highstands. In contrast,phases of relative sea-level rise in times of platform drowning are termed destructive. The δ 13 C stratigraphies obtained from Valanginian-Hauterivian and Aptian-Albian hemipelagic successions beyond the platform correlate well with the Early Cretaceous pelagic δ 13 C record. Positive excursions in the pelagic δ 13 C record correspond in time to episodes of platform drowning. This suggests the existence of a coupling mechanism between changes in the global carbon cycle and platform drowning. In our view, the Early Cretaceous crises in carbonate-platform growth were the consequence of reinforced greenhouse conditions, which may have been triggered by episodes of extensive, flood-basalt volcanism. Strong greenhouse conditions may have induced the following chain of feedback mechanisms, enabling the biosphere to return to normal conditions: climate warming → sea-level rise, accelerated water cycle, intensified weathering → nutrient mobilization → platform destruction, increased productivity increased phosphogenesis and carbon burial → weakened greenhouse conditions.

Relay-ramp forms and normal-fault linkages, Canyonlands National Park, Utah

Abstract: Fieldwork within a series of mesoscale grabens in southeast Utah has revealed a particularly well-exposed system of interlinked extensional faults. A series of down-faulted grabens are developed within a 460-m-thick brittle layer of upper Paleozoic sandstone and shale, which overlies a ductile layer with a high gypsum content. All the major grabens consist of two or more overlapping elements, which are composed of fault segments. These segments may be hard-linked (fault surfaces are joined) or soft-linked (fault surfaces are isolated, but linked by ductile strain of the rock volume between them) in map view. Relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments. In the Canyonlands grabens, the transfer of displacement between soft-linked fault segments is characterized by well-de-fined, dipping relay ramps, commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments. Relay ramps develop as ephemeral structures, eventually becoming breached by hard linkage of the fault segments. Breakdown of ramps by breaching is part of the process of fault growth by segment linkage. Within the Canyonlands graben system, four orders of segmentation are present. This may be a consequence of different scales of mechanical heterogeneity, but evidence from the Canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage.

Regional extension as a geologic trigger for diapirism

Abstract: Initiation of diapirs is one of the least understood aspects of salt tectonics. Differential sedimentary loading or erosion are both effective but not universal. A survey of 18 major salt-diapir provinces shows that salt upwelling is closely linked in time and space to regional extension. Extended salt basins typically develop salt structures, whereas nonextended basins typically do not. Even in salt basins overprinted by inversion or orogenic contraction, the diapirs were initiated during extension on divergent continental margins or in intracontinental rifts. Regional extension thins brittle overburden by forming grabens and half grabens above flowing salt. These fault structures (1) differentially load the salt by their surface relief and (2) weaken the overburden by fracturing and thinning it. Diapiric walls of pressurized salt rise in reaction to the shitting positions of fault blocks in extending overburdens, regardless of thickness, density, or lithology. If regional extension stops, these reactive diapirs stop rising. Eventually the roof of the reactive diapir can be thinned by extension below a critical thickness. Only then can the diapir break through actively as an independent intrusion. Diapiric alignments have been ascribed to basement faulting, even where such faults were conjectural or had trivial displacements. Physical modeling shows that extension of the basement has only indirect influence on diapirism by creating space for extension of the overburden, which is the direct cause of diapirism, whether extension is thick-skinned or thin-skinned and whether the salt was deposited before, duing, or after rifting.

Paleozoic Laurentia-Gondwana interaction and the origin of the Appalachian-Andean mountain system

Abstract: Laurentia, the rift-bounded Precambrian nucleus of North America, may have broken out from a Neoproterozoic supercontinent between East and West Gondwana. Several lines of evidence suggest that the Appalachian margin of Laurentia subsequently collided with the proto-Andean margin of the amalgamated Gondwana supercontinent in different relative positions during early and mid-Paleozoic time, in route to final docking against northwest Africa to complete the assembly of Pangea. Hence the Appalachian and Andean orogens may have originated as a single mountain system. The overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the Paleozoic Era. Laurentia-Gondwana collisions may help to explain contemporaneous unconformities in the Paleozoic sedimentary cover of the Laurentian, Gondwanan, and Baltic cratons.

Integrated Chemostratigraphy and Biostratigraphy of the Windermere Supergroup, Northwestern Canada: Implications for Neoproterozoic Correlations and the Early Evolution of Animals

Abstract: The thick, richly fossiliferous succession of the upper Windermere Supergroup, Mackenzie Mountains, northwestern Canada, provides a test of integrated biostratigraphic and chemostratigraphic frameworks in terminal Proterozoic correlation. The C- and Sr-isotopic abundances of lower Keele Formation carbonates approximate those for other pre-Varanger samples, confirming that the simple disc-like fossils of the underlying Twitya Formation predate all known diverse Ediacaran faunas. "Tepee" and Sheepbed carbonates record strong post-glacial isotopic excursions; in contrast, delta13C values for Gametrail through Risky carbonates vary only within the narrow range of about +l% to +2%. A second negative excursion occurs in Ingta Formation carbonates that immediately underlie the paleontologically determined Precambrian-Cambrian boundary. The upper Windermere profile as a whole compares closely with curves determined for other terminal Proterozoic successions. The lowermost diverse Ediacaran assemblages in the Sheepbed Formation correlate chemostratigraphically with the oldest fauna in Namibia, but the two assemblages differ in taxonomic composition. Blueflower assemblages correlate both chemostratigraphically and taxonomically with faunas from Australia, China, Siberia, and elsewhere. Increasing data support the hypothesis that paleontological and geochemical data together provide a reliable means of correlating terminal Proterozoic sedimentary rocks throughout the world.

Seismotectonic zoning in east-central Italy deduced from an analysis of the Neogene to present deformations and related stress fields

Abstract: This paper concerns the space-time evolution of the late Miocene to present contractional and extensional deformations within east-central Italy, with an analysis of the pattern and orientation of the related stress field, an insight into the cover-basement relations, and a discussion of the seismotectonic implications. The pattern and orientation of the deformation and stress fields have remained unchanged since late Miocene time and have been always characterized by the coexistence and parallelism in contiguous areas in a given time of shortening to the east and elongation to the west. With time, during a multistage deformation history, both the contraction and the extension fields have moved eastward. Basement-involved tectonics with a west-dipping multiple detachment system would have allowed the progressive eastward shift of crustal slices, corresponding at the surface to well-defined tectonic domains, with consequent crustal thinning in the Tuscan zone and crustal thickening beneath the Apennine mountain chain. By comparing macroseismic epicenters, hypocenters, and focal mechanisms with crustal structure and independently known active faults, it appears evident that the pattern, frequency, and distribution of the crustal seismicity are tectonically controlled and that a contraction-extension pair is active also at the present time. In fact, along the Adriatic coastline, contractional deformation prevails with involvement of Pleistocene layers, whereas beneath the Apennine Range, extensional deformation prevails. Both T-axes in the inner domain and P-axes in the outer domain are oriented southwest-northeast or west-south-west east-northeast, and this coaxiality reflects the situation depicted by the geologic stress field. On this basis, we have produced a seismotectonic zoning of east-central Italy with three distinct domains: Intramountain Seismic Zone (ISZ), Foothills Seismic Zone (FSZ), and Coastal Seismic Zone (CSZ). A plausible explanation for the genesis of the "active" seismic deformations of east-central Italy is that the locally intense extensional upper-crustal seismicity of the Intramountain Seismic Zone is confined to the hanging wall of a major, west-dipping, active extensional discontinuity, whereas the less frequent contractional seismic activity of the Coastal Seismic Zone is within the hanging wall of an outer major, active, west-dipping contractional discontinuity. An area of overlap between these two zones is represented by the Foothills Seismic Zone, where superposed epicentral patterns from different surfaces of weakness at different depths are the predictable result of the asymmetry of the deformation.

Subglacial drainage, eskers, and deforming beds beneath the Laurentide and Eurasian ice sheets

Abstract: Glaciological theory predicts that the subglacial drainage network at the base of gently sloping ice sheets resting on deforming sediment should consist of many wide, shallow, probably braided "canals" distributed along the ice-sediment interface, rather than an arborescent network of relatively few large tunnels, as would develop over a rigid substrate. A corollary prediction examined here is that eskers, which form in large subglacial tunnels, should be rare where subglacial bed deformation occurred, but they may be relatively common where the bed was rigid. Bed deformation would be most likely where subglacial till was relatively continuous, fine-grained, and of low permeability—that is, in regions where till is derived primarily from underlying sedimentary bedrock—but unlikely where discontinuous, coarse-grained, high-permeability till was derived from underlying crystalline bedrock. The observed distribution of eskers in areas covered by the Laurentide and Eurasian (British, Scandinavian, and Barents Sea) ice sheets during the last glaciation shows that most eskers occur over crystalline bedrock overlain by discontinuous, high-permeability till, but are rare or absent over sedimentary bedrock overlain by fine-grained, low-permeability till, thus matching reasonably well our prediction. Glaciological theory and geologic evidence indicate that esker systems on a subcontinental scale are time-transgressive. Sedimentological evidence for a "canal" drainage system appears to be present in fine-grained tills where eskers are largely absent.

Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

Abstract: The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion 40 Ar/ 39 Ar isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of ∼0.3% (1σ), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intra-caldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paint-brush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climactic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space

Late Cretaceous-Paleocene synorogenic sedimentation and kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming

Abstract: Integration of cross-cutting structural relationships, overlapping sedimentary units, new conglomerate provenance data, and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of Late Cretaceous through Paleocene thrust faulting in the northeast Utah-southwest Wyoming part of the Sevier thrust belt. These data indicate a general eastward progression of deformation that was punctuated by local out-of-sequence and hinterlandward-verging events. Provenance data delimit a sequential restoration of a regional cross section. The principal thrust systems in northeast Utah and southwest Wyoming are the Willard, Ogden, Crawford, Absaroka, and Hogsback thrusts. The Willard is the westernmost, structurally highest, and oldest of the thrusts; it carries a unique section of thick Proterozoic sedimentary rocks, as well as Paleozoic rocks, and was folded during displacement on younger thrust systems. The next youngest thrust system is the Ogden, which comprises several basement-rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the Wasatch Range. Growth of the Wasatch culmination took place during Coniacian through Paleocene time, contemporaneous with sequential displacement on the frontal Crawford, Absaroka, and Hogsback thrusts. Total shortening in this part of the thrust belt was ∼100 km, and structural relief of ∼25 km developed in the area of the Wasatch culmination. During Coniacian through Paleocene time, thrusting followed an overall eastward progression that was interrupted by local out-of-sequence and hinterlandward-verging events. Several episodes of synchronous displacement on two or more thrusts can be demonstrated. Shortening occurred in three main episodes. The first episode (∼89-84 Ma) involved ∼33 km of shortening on the Crawford thrust and its footwall imbricates. Approximately 19 km of structural relief on the basement-cover contact developed in the area of the Wasatch culmination. The second episode (∼84-62 Ma, with a break between ∼75-69 Ma) involved ∼30 km of shortening, mainly on the Absaroka thrust, and development of an additional ∼6 km of structural relief in the culmination. The third episode of shortening (∼56-50 Ma) took place on the Hogsback thrust, involved ∼21 km of horizontal shortening, and produced no significant increase in structural relief in the culmination. Long-term rates of shortening ranged between 3.0 mm/yr and 6.6 mm/yr. These three episodes of shortening produced three large accumulations of synorogenic conglomerate, totaling ∼3 km in thickness. The Henefer Formation and Echo Canyon and Weber Canyon Conglomerates were deposited during Crawford thrusting. The Evanston Formation was deposited during and after Absaroka thrusting, and the lower conglomeratic part of the Wasatch Formation was deposited during and after Hogsback thrusting. Most of the sediment in these synorogenic units, however, was derived from repeated uplift of the Willard thrust sheet and from the eastern flank of the Wasatch culmination in the rear part of the thrust belt. Only local, minor accumulations were derived from the frontal ramp anticlines. Sediment accumulation and structural deformation were generally out-of-phase. Periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin. Periods of structural inactivity were marked by accumulation of aerially widespread, braided-river conglomerate on top of the thrust belt. One exception to this pattern is the Henefer-Echo Canyon-Weber Canyon conglomerate deposit, which contains evidence of progressive deformation in close proximity to the tip of the Crawford thrust. Comparison of the sequential restoration with the Late Cretaceous subsidence history and isopach patterns in the distal foreland basin of western Wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the Wasatch culmination. Growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge.

Submarine canyon initiation by downslope-eroding sediment flows: Evidence in late Cenozoic strata on the New Jersey continental slope

Abstract: Multibeam bathymetry and seismic reflection profiles of the New Jersey continental slope reveal a series of abandoned and now-buried submarine canyons that have apparently influenced the development of modern canyons. The buried canyons are infilled along nine slope-wide unconformities separating upper-middle Miocene to Pleistocene sediments that thin downslope. Canyons infilled during the Miocene occur in the southwest part of the study area where Miocene sediments are thickest. Other canyons, infilled during the Pleistocene, occur in the northeast part of the study area where Pleistocene sediments are thickest. When followed downslope, each of the buried canyons arrives at a confluence with a modern canyon, usually where the downslope-tapering sediment cover has failed to smooth over the buried canyon, leaving a sea-floor trough. Sea-ward of the confluences, the modern canyons have exhumed the buried canyons and use the older valleys to reach the base of the slope. Re-use of the lower slope reach of the buried canyons appears to have begun when the sea-floor troughs over the buried canyons captured sediment flows initiated along the upper slope and shelf break and confined them to follow the former path of the buried canyons to the base of the slope. The downslope erosion caused by the sediment.flows is proposed to have initiated the modern canyons, which eventually excavated and deepened the former routes of the buried canyons seaward of the sites of sediment flow capture. The occurrence of buried canyons where strata thickens alongslope suggests that infilling of the buried canyons occurred seaward of shelf-edge depocenters. The heightened sediment input to the slope in these regions may have also led to the initiation and growth of modern-day canyons. The temporal relation between modern canyon formation, sediment supply, and sea level, however, remains to be established.

The Great Tonalite Sill: Emplacement into a contractional shear zone and implications for Late Cretaceous to early Eocene tectonics in southeastern Alaska and British Columbia.

Abstract: The Late Cretaceous to early Tertiary Great Tonalite Sill of southeast Alaska and British Columbia is a very long (∼1,000 km) and thin (<25 km), orogen-parallel, composite batholith, which lies between the Insular superter- rane (including the Alexander and Wrangellia terranes) and the Intermontane superterrane (including the Stikine and Cache Creek terranes). The batholith is delineated by many steep, sheet-like plutons, which are dominated by northwest- southeast-striking concordant fabrics with steep lineations that formed during deformation in a country-rock shear zone prior to the complete crystallization of the magmas. Deformation in this shear zone is dominated by northeast- southwest-directed contraction orthogonal to the orogenic strike, associated with a component of northeast over southwest, high-angle shear. The steep, multiple-dike-like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike-wedging mechanisms along the deep-reaching shear zone. The remarkable narrowness of the Great Tonalite Sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length. Such a shear zone of Late Cretaceous to early Tertiary age, lying along 800 km of the possible boundary between the Insular and Intermontane superterranes, implies that it may represent the actual boundary between them. If this hypothesis is correct, it implies that the large-scale tectonic regime during emplacement of the Great Tonalite Sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data.

Active faulting and growth folding in the eastern Santa Barbara Channel, California

Abstract: We develop new methods to identify blind-thrust fault systems, determine fault slip rates, and estimate potential earthquake magnitudes and recurrence intervals in active fold-and-thrust belts. These methods are applied to compressive folds along the Offshore Oak Ridge and Blue Bottle trends, which overlie active blind-thrust faults in the eastern Santa Barbara Channel. These folds and their causative faults are interpreted using fault-bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data. The structures are mapped using a new technique of axial-surface mapping in seismic reflection grids, which defines three-dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike. Analysis of syntectonic (growth) sediments yields Pliocene and Quaternary fault slip rates of 1.3 mm/yr on a deep thrust (≥16 km) and 1.3 mm/yr on shallower faults (2-5 km). The combined 2.6 mm/yr slip rate represents only part of the 6 mm/yr of shortening measured by geodesy across the channel and estimated from relative Pacific-North American plate motions across the Transverse Ranges. Additional shortening is probably accommodated on other active thrusts in the western Transverse Ranges and in the northern channel along the Santa Barbara coast. Deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern California. Unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes. This analysis suggests that a ramp in the Channel Islands fault beneath the Offshore Oak Ridge trend is capable of rupturing in a M s ≥7.2 earthquake. Earthquakes of this magnitude may release ∼2 m of slip, which, when combined with the estimated slip rate (1.3 mm/yr), yields an earthquake recurrence interval of ∼1500 yr for this Channel Islands fault ramp.

Computer simulations of California tectonics confirm very low strength of major faults

Abstract: Recent improvements in the technique of modeling fault networks with thin-plate finite elements yield full convergence of the solutions, without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust. We apply these techniques to model the California region, incorporating all faults with estimated slip rate over 1 mm/yr, as well as variations in elevation, heat flow, and crustal thickness. Velocity boundary conditions on the model sides are based on the NUVEL-1 plate model and an approximation of deformation in the Great Basin. The frictional and dislocation-creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity, which occurs at model temperatures of 350-410 °C. This leaves two free parameters: the (time-averaged) coefficient of friction on faults, and the apparent activation energy for creep in the lower crust. These parameters are systematically varied in three sets of 10, 81, and 64 experiments, respectively. The predictions of each model are tested against three published data sets: a set of 79 geologic limits on average slip rate on faults, a set of 221 principal stress directions, and a set of 841 secular rates of geodetic baselines (both trilateration and VLBI [very long baseline interferometry]). The patterns of model scores indicate that the time-averaged friction coefficient of major faults is only 0.17-0.25, or only 20%-30% of the value (0.85) that is assumed for the friction in the intervening blocks. (The laser trilateration data taken alone would indicate a preference for a high-stress model, but this model is one with very little fault slip and seismicity, which can be rejected on other grounds.) In the final three-parameter set of 64 models, we test whether the anomalous weakness of faults is uniform or proportional to net slip. With slip-dependent weakening, there is a general reduction in prediction errors, which supports the hypothesis. The geologic data and geodetic data agree in implying that fault weakness (that is, friction of 0.17) is general, with an additional 30% slip-dependent weakening of the San Andreas (to friction of 0.12, or 14% of normal). The stress data, however, are fit best if all weakness is slip dependent (effectively, only the San Andreas is weak). Thus, the hypothesis that all weakening is slip dependent should not be rejected. The best models predict all available data with RMS (root mean square) mismatch of as little as 3 mm/yr (6% of the relative plate velocity), so their predictions may be useful for seismic hazard estimation, at least along faults where no data are available. This study extends the finding of very low friction from subduction zones, where it was previously documented, to a dominantly strike-slip system in which underthrusting of wet sediment is not widespread. Return of previously subducted water from the mantle and its chemical or physical binding in fine-grained gouge are possible explanations for the very low friction.

Potassium-argon geochronology of a basalt-andesite-dacite arc system: The Mount Adams volcanic field, Cascade Range of southern Washington

Abstract: High-precision K-Ar dating and detailed mapping have established an eruptive chronology for a Cascade stratovolcano and its surrounding array of coeval basaltic centers. Mount Adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 Quaternary volcanic field that contains >60 discrete vents. K-Ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping. Within analytical error, there are no discrepancies between K-Ar ages and stratigraphic sequence. Major activity began in the area ca. 940 ka, and inception of the central stratovolcano took place at ca. 520 ka. A plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 /k.y. and ∼80% of the products were basaltic. Andesites are volumetrically dominant and were emplaced in three main cone-building episodes centered at 500, 450, and 30 ka—at eruptive rates of 1.6-5 km 3 /k.y. At a lower rate of 0.05-0.1 km 3 /k.y., the magmatic system remained almost continuously active between the main pulses—although breaks in activity as long as 30 k.y. are permitted by the K-Ar data. Andesitic-dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k.y., and their products are interstratified. The last main episode of cone construction occurred ca. 40-10 ka, the oldest andesite identifiably derived from within the present-day edifice having an age of 33 ± 14 ka. Andesites forming the south-summit rim and the true summit have ages of 13 ± 8 ka and 15 ± 8 ka, respectively. The time-volume-composition data bear upon several fundamental questions concerning the long-term behavior of arc volcanoes. Stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity. Large stratocone systems may remain active for half a million years. Subdivision of complex stratovolcanoes into eruptive or constructional "stages" without detailed mapping, geochronology, and compositional data should be treated skeptically. Discussion of volumetric eruptive rates can be misleading without an adequate time scale. Stratovolcanoes need never develop large upper-crustal magma chambers. Basalt erupts peripherally, but its ascent is suppressed beneath stratovolcanoes by deep-crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting.

Geological evolution of the southeastern Red Sea Rift margin, Republic of Yemen

Abstract: The tectonic evolution of the southeastern margin of the Red Sea Rift in western Yemen has been investigated using a multi-disciplinary field study of an east-west transect between Al Hudaydah and Sana9a. Slow subsidence of up to 1 km occurred over the area during a 100 m.y. period before rifting. There was a major episode of flood volcanism between ca. 30 and 20 Ma, and important extensional faulting began after the eruption of the volcanic rocks and ceased before middle to late Miocene sediments and volcanic rocks were deposited unconformably on top of rotated fault blocks on the coastal Tihama Plain. Surface uplift has produced the Yemen highlands, whose highest peak reaches an elevation of 3660 m. This is attributed to plume heating and eruption of >3000 m of volcanic rocks. Apatite fission-track ages indicate early to middle Miocene exhumational cooling ages, postdating the major volcanic phase and contemporaneous with rifting. Volcanism was accompanied by emplacement of subvertical dike swarms, which generally strike north-northwest to northwest, broadly parallel to the Red Sea coastline. Major faults indicate northeast-southwest-directed extension. Large granitic sheets and plutons (up to 25 km wide) intruded the volcanic rocks. Approximately 30 km of extension has taken place across a 75-km-wide zone (β = 1.7) in 6-8 m.y. The relative timing of volcanism followed by extension and uplift does not fit conventional models of passive or active rifting. We suggest that the proto-Red Sea Rift was caused by regional plate stresses that exploited lithospheric weakening caused by the Afar plume. Appreciable doming only occurred after the main episode of volcanism, which suggests that magmas extruded before maximum thermal expansion of the lithosphere took place.

Late Carboniferous cyclothems and alluvial paleovalleys in the Sydney Basin, Nova Scotia

Abstract: Cyclothems are documented for the first time in alluvial to coastal plain strata (West-phalian D to Stephanian) of the Sydney Basin, Nova Scotia. Well-developed cyclothems commence with an extensive carbonaceous lime-stone or shale with a restricted marine biota that indicates maximal flooding. Succeeding gray detrital bay and lake fills with distributary channel deposits are overlain by stacked channel deposits (paleovalley fills) that mark an abrupt basinward facies shift and a profound change in alluvial style. Channel fills and red vertisols were deposited on an alluvial plain above the filled valleys, and the paleovalley is equated with thick vertisols in adjacent, inter-fluve areas. The cyclothems are capped by gray coastal deposits with economic coals. Agglutinated foraminifera are present in seat earths beneath the coals, which formed as a result of a brief stillstand during transgression. Red mudstones pass laterally into gray detrital bay fills in regions of greater subsidence associated with faulted basement blocks. Poorly developed cyclothems are bounded by extensive but split coals, and autogenic effects partially obscure allogenic patterns. Mean cyclothem duration approximates 200 ka.

The northern Nevada rift: Regional tectono-magmatic relations and middle Miocene stress direction

Abstract: As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500 km from southern Nevada to the Oregon Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that, based on new radiometric dating, erupted between 17 and 14 Ma and probably during an even shorter time interval. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space (at its northern terminus) with the oldest silicic caldera complex along the Yellowstone hot-spot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault. The northern Nevada rift is also equivalent in age, trend, and composition to feeder dikes that fed the main eruptive pulse (∼95% volumetrically) off the Columbia River flood basalts in northern Oregon ∼15.5-16.5 Ma. Because of these similarities, both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south-southeast and north-northwest, respectively, from a central mantle plume. The site of the initial breaching of the North America plate by this plume is probably the McDermitt volcanic center at the north end off the rift near the Oregon-Nevada border. The present north-northwest trend of the rift and its internal elements, such as dikes and lava-filled grabens, record the orientation of the arc-normal extensional stress in this back-arc region at the time of emplacement. Paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift. The present north-northwest trend of the rift reflects the state of stress in the Basin and Range during middle Miocene time and is consistent with stress indicators of similar age throughout the Basin and Range and Rio Grande rift provinces.

Tsunami deposits beneath tidal marshes on Vancouver Island, British Columbia

Abstract: Thin sheets of sand occur within Holocene mud and peat deposits beneath tidal marshes at Tofino, Ucluelet, and Port Alberni on Vancouver Island, British Columbia. The sand sheets are extensive and have sharp upper and lower contacts. In most cases they consist of moderately sorted, massive sand and silty sand with abundant wood and other plant detritus. At Port Alberni, the thickest sheet has gravel and is graded. The upper two sand sheets in the Tofino-Ucluelet area, and possibly the third, are also present at Port Alberni. Eyewitness accounts and 137 Cs analysis suggest that the uppermost, thinnest sand was deposited by the tsunami triggered by the great Alaska earthquake in 1964. The next oldest sand sheet has been radiocarbon dated at Our data suggest that large tsunamis have struck the southern British Columbia coast several times during the late Holocene and that some were much larger than the 1964 tsunami, which caused about $10 million damage (1964 Canadian dollars) to communities on Vancouver Island. Because such tsunamis can be expected in the future, they pose a hazard to people and property in some coastal areas.

Basaltic volcanism and extension near the intersection of the Sierra Madre volcanic province and the Mexican Volcanic Belt

Abstract: Three groups (Miocene, Pliocene, and Pleistocene) off oceanic-type basalts associated with normal faulting have been identified in an area surrounding the city of Guadalajara, Mexico. Although most basalts within these groups have compositional characteristics of an asthenospheric source, each group is also associated with lavas that have subduction-related traits. The first group, the San Cristobal plateau basalts, has a minimum volume of 1,800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre-existing extensional basin of unknown configuration ∼10 m.y. ago. The relatively rapid eruption of these basalts along with their volume, structural association, and chemistry suggest an upwelling mantle plume beneath the Guadalajara region. This region may be the earliest indication of the separation of the Jalisco block from North America. The second basalt group, the Guadalajara basalts, consists of small volumes of porphyritic basalts and basaltic andesites, 3.3-5.0 Ma in age, that outcrop near the northern outskirts of Guadalajara and near the town of Hostotip-aquillo to the northwest. The youngest basalts in the Guadalajara area are the 0.4-1.4 Ma Santa Rosa suite of basalts-hawaiites-mugearites close to the contemporary (>0.2 Ma) andesitic central volcano, V. Tequila; these either flowed from nearby cinder cones toward the Rio Santiago canyon or erupted in the canyon to dam the river. The flat-lying Tertiary (23-27 Ma) ash flows of the Sierra Madre volcanic province that lie to the north of Guadalajara are faulted (north-northeast) into horsts and grabens that in one case can be dated as being older than 21.8 Ma. This fault trend has been reactivated closer to Guadalajara, as normal faults (200- to 300-m displacement) cut the Miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city. Also hidden by the young volcanic succession there is the north-west-southeast fault zone that extends to the Gulf of California as the Tepic-Zacoalco graben system. Faulting of this zone is well displayed in the region of the Santa Rosa dam in the Santiago canyon, northwest of Guadalajara. Two styles of faulting are found there: an older (>1 Ma, 3 , has been approximately north-northeast for at least 4 m.y., characteristic of the Mexican Basin and Range province.

Correlation of basinal carbonate cycles to nearshore parasequences in the Late Cretaceous Greenhorn seaway, Western Interior U.S.A.

Abstract: Upper Cretaceous limestone-shale couplets developed within the late transgressive stage of the Greenhorn cyclothem may be correlated from carbonate-dominated (basinal) sequences in central Kansas and Colorado westward to clastic cycles in southern Utah. Six such basinal couplets have been traced to corresponding upward-coarsening progradational cycles developed on the western margin of the Western Interior basin. In the central basin in Colorado and Kansas, these sedimentary cycles are represented by limestone-shale and marlstone-shale couplets ∼0.5-1.0 m in thickness. More calcareous parts of these couplets may be correlated westward into condensed, fossiliferous concretion and shell beds in proximal offshore lithofacies of Arizona and Utah. These concretion and shell beds are physically traceable farther landward (westward) into bioturbated, fossil-rich, transgressive lag deposits that bound 10- to 20-m-thick coarsening-upward progradational strand-plain deposits (parasequences) in southwestern Utah. Thus, the progra-dational phase of parasequence deposition correlates with accumulation of clay-rich sediment in the central basin, and the transgressive phase is characterized by reduced terrigenous input and deposition of carbonate-rich sediment. We consider Milankovitch-style orbital forcing of climate and tectonically induced fluctuations in rates of foredeep basin subsidence as possible forcing mechanisms for these basinwide events. Based on the widespread distribution of the limestone-shale couplets, as well as on estimated sedimentation rates and geochronology, it has been widely speculated that these carbonate cycles reflect Milankovitch cycles with periodicities on the order of 20 k.y. to 100 k.y. If so, then stratigraphic data suggest that orbital forcing of climate affected eustasy and/or sediment input and biogenic production in the Western Interior basin. Alternatively, thrusting events in the Sevier orogenic belt may have produced episodic changes in the rates of foredeep basin subsidence and consequent changes in base level, which could have controlled the deposition of the Greenhorn parasequences and carbonate cycles. In either case, correlation of these units demonstrates a consistent basinwide sedimentary response to high-frequency base level or sediment input changes in the Western Interior epicontinental basin.

A pedotype approach to latest Cretaceous and earliest Tertiary paleosols in eastern Montana

Abstract: A pedotype approach to the study of paleosols emphasizes individual profiles. It is an alternative approach to the study of geosols, which are laterally extensive suites of paleosols, or pedofacies, which are pedogenically distinctive sedimentary facies. This is the first pedotype study of paleosols across the Cretaceous-Tertiary boundary in eastern Montana. Pedotypes allow assessment of sedimentation and fossilization. The sequence of paleosols across the Cretaceous-Tertiary boundary at Bug Creek is one of high temporal resolution, because most available geological time can be accounted for by differing degree of development inferred for each pedotype. Paleosols at Bug Creek include pedotypes that preserve plant fossils well, but were unfavorable for preservation of fossil vertebrates. Pedotypes also allow reassessment of ecosystem change. Despite indications of catastrophe at the Cretaceous-Tertiary boundary, the array of Paleocene and Cretaceous pedotypes are not strikingly different. The Upper Cretaceous Hell Creek Formation includes pedotypes interpreted as gleyed Alfisols, Inceptisols, and Entisols probably formed under seasonally water-logged forest and mean annual rainfall of the order of 900-1200 mm. Most paleosols of the lower Paleocene Tullock Formation were Histosols, but some can be interpreted as gleyed Inceptisols and Entisols probably formed under bald cypress swamps in a humid climate with >1200 mm mean annual rainfall. Broadly comparable pedotypes were present before and after the Cretaceous-Tertiary boundary, although Paleocene flood-plain forest lived in paleosols chemically a little more oligotrophic than Cretaceous paleosols. This modest difference supports the idea that change to coaly facies in the earliest Paleocene was a local shift in sedimentary environment. Such local changes do not begin to account for profound disruption in specific composition of plant and animal communities at the Cretaceous-Tertiary boundary in eastern Montana.

Channel adjustments to changing discharges, Powder River, Montana

Abstract: Sixteen years of annual surveys reveal how Powder River responds to varying discharges. During 1978, the second largest recorded daily mean discharge occurred. Cutbank migration, bed degradation, net bank erosion, and overbank deposition all contributed to increase the channel area at 12 cross sections by an average of 62%. During the ensuing years, the channel area decreased as sediment was stored in low-lying benches adjacent to the active bed of the channel. The survey data indicate that the balance between bank erosion and deposition varies with discharge. In years when the annual maximum daily mean discharge is 3 /s (a flow of 60 m 3 /s has a recurrence interval of ∼1.1 yr), bank erosion and deposition are approximately equal. In years when the annual maximum daily mean discharge is between 60 and ∼150 m 3 /s (a discharge of 150 m 3 /s has a recurrence interval of ∼2.7 yr), bank deposition exceeds bank erosion, and the channel contracts, often by developing benches. In years with higher discharges, the channel expands through net bank erosion. These results demonstrate that the channel of Powder River is influenced by a wide variety of formative discharges. Powder River9s recent history of expansion and contraction and the development of prominent benches cannot be explained by equilibrium models based on a single, channel-forming discharge.

Late Holocene, high-resolution glacial chronologies and climate, Kenai Mountains, Alaska

Abstract: Recent retreat of outlet glaciers from the Harding and Grewingk-Yalik Icefields has revealed a vast array of deposits on the eastern and western flanks of the Kenai Mountains that records multiple glacier advances into coastal forests during late Holocene time. Treering dating, together with radiocarbon and lichenometric analyses, allows for the reconstruction of these glacial fluctuations to decadal precision over the past two thousand years. The records of fluctuations are derived from 16 land-terminating and seven tidewater glaciers in three fjord systems, as well as two cirque glaciers. Three major intervals of Holocene glacier expansions are evident; they occurred about 3600 yr B.P., 600 A.D., and during the Little Ice Age, from 1300 to 1850 A.D. The earliest expansion beyond present ice margins is known only from the McCarty tidewater glacier. The 600 A.D. event involved the simultaneous advance of land-terminating and tidewater glaciers. During the Little Ice Age, however, tidewater glaciers were advancing several centuries prior to their land-terminating neighbors. Those land-terminating glaciers on the western mountain flank retreated from their Little Ice Age maxima as much as two centuries before those on the eastern mountain flank. Land-terminating tongues on the eastern, more maritime, mountain flank have shown more sensitivity to variations in winter precipitation during the Little Ice Age and within recent decades than the more continental glaciers on the western flank that are affected more by summer temperatures. The glacial and climatic records suggest that advances of the ice tongues from about 1420 to 1460 A.D., between 1640 and 1670 A.D., at about 1750 A.D., and from 1880 to 1910 A.D. reflected times of increased winter precipitation. Advances between 1440 to 1460 A.D., from 1650 to 1710 A.D., and from 1830 to 1860 A.D. followed intervals of lower summer temperatures.

The isotopic record of ocean chemistry and diagenesis preserved in non-luminescent brachiopods from Mississippian carbonate rocks, Illinois and Missouri

Abstract: Fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew, owing to their stable mineralogy and their presence in marine strata throughout Phanerozoic time. The utility of these fossils for preserving ocean-water compositions is evaluated through analyses of multiple specimens that grew in contemporaneous sea water at individual stratigraphic horizons in the Mississippian-age Burlington-Keokuk Formation of mid-continent North America. Significant variations in Sr, C, and O isotopic compositions are observed in contemporaneous, non-luminescent shells, indicating that apparently pristine brachiopod shell material has been altered by post-depositional processes of diagenesis. The least-altered compositions at each horizon are used to estimate secular variations of 87 Sr/ 86 Sr in Early Mississippian ocean water. The estimated rates of change of ocean-water 87 Sr/ 86 Sr are comparable in magnitude to the rapid Sr isotopic changes documented for Cenozoic ocean water, for which high-frequency glacial cycles are inferred as a driving mechanism. The rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the advancement of high-resolution reconstructions of paleo-ocean chemistry.

Anatomy, structural evolution, and slip rate of a plate-boundary thrust: The Alpine fault at Gaunt Creek, Westland, New Zealand

Abstract: Minimum slip rates calculated for plate-vector-parallel slickenside trends in cataclasite on the sole of the Alpine fault at Gaunt Creek, Westland, New Zealand, range from 18 to 24 mm/yr. Between half and two-thirds of the total relative motion between the Pacific and Australian plates is being accommodated by movement on a single structure, the Alpine fault. During the past 14 ka, the leading edge of the Alpine fault has changed from a moderately southeast-dipping, oblique reverse fault to a shallowly dipping thrust. The hanging wall (Pacific plate) is composed of a gradational sequence from basal gouge, through pseudotachylite-bearing cataclasite, to progressively more coherent schist-derived mylonite, which has been faulted against subhorizontally bedded, fluvio-glacial gravel in the footwall (Australian plate). During uplift the hanging-wall sequence has been internally sheared and imbricated, producing duplex structures, and retrogressively veined and altered by pervasive hydrothermal fluid flow. Erosion of the exhumed fault zone produced angular, cataclasite- and mylonite- derived, talus-fan breccias, building a west-dipping apron beneath the fault scarp. Wood fragments from near the base off the talus breccias have been 14 C dated at 12,650 ± 90 yr B.P. Progressive tectonic shortening resulted in 180 m of overthrusting of a schist-derived nappe across an irregular talus fan surface composed of its own erosional debris. The structural history of the Alpine fault at Gaunt Creek illustrates the importance of the interaction between fault-induced topography and erosion, and the control these processes exert on the continued tectonic, geometric, and geomorphic evolution of the fault zone.

Early Archean (>3.2 Ga) Fe-oxide-rich, hydrothermal discharge vents in the Barberton greenstone belt, South Africa

Abstract: Ironstone pods (Fe 2 O 3 = 72.5-97.2 wt%), interpreted to have formed by early Archean, sea-floor-related hydrothermal activity, are described from the ca. 3.5-3.1 Ga Barberton greenstone belt, South Africa. Most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy. They rest on silicified ultramafic rocks and are overlain by ferruginous shales, silicified sand-stones, other coarse clastics, and barite horizons. The ironstone pods grade along strike into laminated iron-oxide facies banded iron-formation that is inferred to represent periodic discharge of iron-oxide- and silica-rich flocculates from the pods. The massive texture of the iron-stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor; relict hydrothermal Discharge chimney structures have been recognized. Honeycomb-like cavities and possible fluid-flow channel textures attest to primary porosity. The iron-stone pods are dominated by massive, locally coarse-grained, specular hematite and goethite, with lesser amounts of quartz and an X-ray amorphous Fe-AI-bearing silicate. The pods are distinct from typical massive sulfide gossans in total trace-element concentrations, including precious and base metals, indicating a different origin. In contrast, the ironstone pods have similarities in major oxide and trace-metal concentrations to iron-oxide deposits presently forming, in part, from low-temperature hydrothermal fluids on the sea floor (for example, Red Seamount iron-oxide deposit). Fluid-inclusion studies on quartz show dominant type I primary fluid inclusions with salinities of 4.7 to 15.8 wt% NaCl equiv. and homogenization temperatures (T h ) of ∼90 to 150 °C; no evidence for boiling is seen. Type I fluid inclusions are most likely dominated by NaCl-CaCl 2 -H 2 O, although their measured eutectic temperatures are consistent with the presence of FeCl 2 ; these inclusions represent the end-member hydrothermal fluid. More saline (24-29.6 wt% CaCl 2 equiv.), lower temperature (T h = 33-109 °C), type II and Ila fluid inclusions represent pulses of a hydrothermal fluid of NaCl-CaCl 2 -FeCl 2 (or MgCl 2 )-H 2 O, or possibly FeCl 3 -bearing composition, mixing with ambient sea water in the case of type II inclusions. The various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine. The lowest salinity measurement for a type I inclusion of 3.1 wt% NaCl equiv. provides a constraint on the salinity of unenriched (by evaporation) sea water. The T h data give a minimum trapping pressure of ∼7 bars, which equates to a minimum sea-water depth of ∼60 m. The lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods. Quartz δ 18 O analyses average 17.3 ± 0.6‰ (1σ n = 5). Calculated δ 18 O H 2 O , for two samples are -1.4‰ and -1.2‰, respectively. These estimates are within error of the postulated Barberton early Archean sea water value of ∼0‰ Quartz/hematite-goethite mineral pairs yield temperatures inconsistent with fluid-inclusion T h data, suggesting isotopic disequilibrium.

A sensitivity study of the driving forces on fluid flow during continental-rift basin evolution

Abstract: Two-dimensional finite element models of coupled sediment compaction, variable-density ground-water flow, and conductive/convective heat transfer are used in this study to quantify basin hydrodynamics during the initial and flexural stages of continental rifting. The analysis also incorporates a two-stage stretching/cooling geodynamic model of the thermomechanical evolution of the lithosphere underlying the rift in order to specify geologically relevant boundary conditions for basin subsidence and basal heat flow. A sensitivity study is made using the model to explore the controls of both permeability and water table configuration in determining the dominant fluid flow drive (compaction, density, or topography) during basin evolution. The sensitivity analysis incorporates hydrologic conditions and rock properties representative of many extensional terrains . Assuming that rift basin subsidence and basal heat flow can be represented by the geodynamic model , two distinct ground-water flow systems evolve within continental rifts during basin evolution. During the initial (stretching) phase of rifting, subsidence is accommodated by fault block motion, and a topography-driven ground-water flow system develops within the permeable alluvial-fan deposits. Within the less permeable lacustrine facies located in the center of the basin, compaction-driven ground-water flow dominates. Here, the compacting lacustrine sediments focus pore fluids laterally from the basin center into the alluvial-fan deposits due to the relatively large permeability contrast between the two depositional environments. Thermal anomalies resulting from convective heat transfer are restricted to alluvial-fan facies near the basin-framing fault. During the thermal cooling (flexural) stage of basin development, laterally extensive onlap facies are deposited, and density-driven ground-water flow dominates in the permeable alluvial-fan deposits, while compaction-drivenflow continues within the lacustrine and onlap facies. The presence of a permeable aquifer within the onlap facies resulted in long-range fluid transport to the edge of the basin. During both stages of basin evolution, ground-water velocities varied from 10 -5 to 10 -1 m/yr between the lacustrine and alluvial-fan deposits, respectively. The observed presence of ore mineralization within alluvial-fan deposits of some continental-rift systems, such as the Cretaceous Rift Basin of Angola, supports the findings of this study.

Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas

Abstract: Investigations in the lower Colorado River Valley, Gulf Coastal Plain of Texas, have resulted in the development of a spatially and temporally controlled history of changes in channel and flood-plain erosional and depositional processes. When combined with paleoclimatic and stratigraphic data from the upper Colorado River drainage and the record of glacio-eustasy in the Gulf of Mexico, this study permits evaluation of the relative influence of different external controls on channel and flood-plain behavior, the development of alluvial landforms, and the development of alluvial stratigraphic sequences. Late Pleistocene and Holocene alluvial deposits of the lower Colorado River have been subdivided into allostratigraphic units, with chronological control afforded by radiocarbon ages. In the bedrock-confined valley, up to 10 m of late Pleistocene (∼20,000-14,000 yr B.P.) sediments referred to as the Eagle Lake Alloformation (ELA) underlie a terrace at 17-20 m above the present-day channel. Deposition of the ELA was followed by bedrock valley incision, then deposition of a complex Holocene valley fill referred to as the Columbus Bend Alloformation (CBA). Columbus Bend Allomembers 1 and 2 (CBA-1 and CBA-2) underlie a terrace at 12-14 m above the present-day channel. CBA-1 was deposited ∼12,000-5,000 yr B.P., whereas CBA-2 was deposited ∼5,000-1,000 yr B.P. Columbus Bend Allomember 3 (CBA-3) consists of channel and flood-plain deposits that represent the past 600 yr of activity. Allostratigraphic units within the lower Colorado valley correlate with allostratigraphic units in major valley axes of the upper Colorado drainage and with records of climatic and environmental change, suggesting that alluvial deposits record basinwide responses to climatically controlled changes in discharge regimes and sediment supply. Basal unconformities for Holocene valley fills, however, appear to be 1,000-2,000 yr younger in the upper Colorado drainage than they are in the lower Colorado valley. This time-transgressioe episode of bedrock valley cutting was initiated by climatically controlled reductions in sediment supply, but conditioned by limits on rates of up-stream propagation of incision through a large drainage basin. By contrast, unconformities within Holocene valley fills document time-parallel episodes of flood-plain abandonment and soil formation, but little additional bedrock valley cutting, and indicate decreased flood magnitudes following shifts to drier climatic conditions. Flood-plain morphology and sedimentary facies changed through time in response to changes in climate coupled with a protracted degradation of upland soil mantles, which altered the rate at which precipitation was transferred to stream channels as runoff. During the late Pleistocene through middle Holocene, runoff was filtered through deep upland soils, floods were for the most part less flashy and contained within channel perimeters, and flood plains were constructed by lateral migration without significant vertical accretion; hence, the ELA and CBA-1 contain few vertical accretion facies. Exposure of bedrock surfaces during the late Holocene resulted in increased flood stages, deep overbank flooding, and construction of flood plains by vertical accretion; hence, CBA-2 and CBA-3 contain thick vertical accretion facies. Allostratigraphic units and bounding unconformities persist through the bedrock-confined valley to the Quaternary alluvial plain, but stratigraphic architecture changes substantially in the downstream direction as a result of the last glacio-eustatic cycle. On the alluvial plain, late Holocene CBA-2 and modern CBA-3, deposited contemporaneously with the present interglacial highstand, onlap and bury the ELA and CBA-1, which were emplaced during the last full glacial lowstand and the transgression that followed.