bluuebunny/crossref_metadata_2025 huggingface dataset:
Rift flank uplift in Tibet: Evidence for a viscous lower crust
Authors: Masek Jeffrey G. , Isacks Bryan L. , Fielding Eric J. , Browaeys Julien
Published: February 2004

Analysis of high‐resolution digital topography across 21 Plio‐Pleistocene grabens in central and southern Tibet shows that high topography occurs along the graben margins. Profiles across these marginal highlands fit curves describing the flexure of an elastic plate under vertical loading. We interpret the features as flexural rift flank uplifts, resulting from regional isostatic compensation of low‐density grabens. The short wavelengths of the uplifts (30–40 km) imply a low flexural rigidity for the plateau (D = 2−6 × 1020 N m), similar to values inferred for the Basin and Range of western North America. In addition, the grabens themselves are extremely narrow compared to typical continental rifts. Taken together, these observations suggest the existence of a low‐viscosity lower crust and suggest that isostatic compensation is taking place within this ductile lower crust. Simple channel flow calculations suggest that crustal viscosities below ∼1022 Pa s are necessary to accommodate this compensation over reasonable timescales.
Tertiary arc rifting in northern Luzon, Philippines
Authors: Florendo Federico F.
Published: February 2004

The North Luzon terrane (NLT), comprising the section of Luzon north of the Philippine Fault, is one of the largest arc terranes in the Philippine Archipelago. Numerous features suggest that the NLT is a late Oligocene to early Miocene analogue for the processes in the modern intra‐arc rift zone at the northern end of the Mariana Trough. First, the NLT has bifurcating magmatic arcs sharing similar magmatic histories. These include the Northern Sierra Madre (NSM) and Cordillera Central (CC) magmatic arcs, which are separated by the Cagayan basin but which are linked in the Caraballo Range to the south. The rock record indicates that the NSM, CC, and Caraballo Ranges were active arcs in late Eocene to late Oligocene time. Second, seismic reflection and well data indicate that the Cagayan basin formed by extensional faulting in late Oligocene to early Miocene time. Third, alkalic arc magmatism, recognized to be a precursor of intra‐arc rifting in modern settings, occurred at the juncture of the NSM and CC arcs in late Oligocene to early Miocene time. Fourth, oceanic crust, represented by the Itogon ophiolite, formed at the southwestern end of the Cagayan basin in late Oligocene to early Miocene time. Major and trace element chemistry show that the Itogon sheeted dikes have tholeiitic arc and backarc basin basalt affinities. The rock record and geophysical offshore data suggest that the NLT was developing in an island arc system above the subducting West Philippine plate in late Eocene time. Rifting occurred in the island arc from late Oligocene to early Miocene time but did not mature into backarc spreading, most likely because of the collision of the Benham Rise, a basaltic rise in the West Philippine basin, with the NLT. The arc rifting in the NLT may be another manifestation of the extensional tectonism that affected most of Southeast Asia in late Oligocene to early Miocene time, during which the South China and Southeast Sulu basins formed. Subsequent to arc rifting, the history of the NLT has been linked to the subduction of the South China plate along the Manila Trench. The structural history of the Cagayan basin and magmatic history of the southern CC suggest that the subduction in the Manila Trench at the latitude of the NLT began about 15 Ma.
Paleogeography of Jurassic fragments in the Caribbean
Authors: Montgomery Homer , Pessagno Emile A. , Lewis John F. , Schellekens Johannes
Published: February 2004

Jurassic rocks of the Caribbean are a sampling of 100 million years of Farallon Plate history with fragments originating at diverse paleolatitudes and from varied tectonic settings. Fragments with clear paleogeographic signatures are components of the basement complexes of Duarte in Hispaniola, Bermeja in Puerto Rico and La Désirade off Guadeloupe. Paleolatitudinally sensitive radiolarian faunas document origination of Duarte as equatorial, La Désirade as higher latitude, and various Bermeja cherts as both equatorial and higher latitude. Red ribbon chert of Duarte and Bermeja of the same age, physical appearance, and lithological association are probably dismembered components of the same slab of Pacific crust. La Désirade red ribbon chert is slightly younger than the Duarte and Bermeja red ribbon chert and was deposited at higher latitude. Bermeja tuffaceous chert is also of higher latitude and probably had an arc‐proximal origin. On the basis of modeled plate trajectories in the Pacific, the origin of various cherts from different paleolatitudes that end up in the same location requires different arrival times at the trench between North and South America. Based on radiolarian paleobiogeography plus indications of origin at a spreading ridge and ignoring the poorly constrained, modeled trajectories for the Late Jurassic, at least one of the higher latitude fragments may have originated in the southern hemisphere. The accumulation of multifarious chert, greenstone, and other ocean floor components was accomplished by offscraping strata transported to the subduction zone along the eastern Pacific margin and warehousing this material in an accretionary complex prior to entry of the Caribbean Plate into the gap between North and South America.
Seismic reflection profiling of the Cheyenne belt Proterozoic suture in the Medicine Bow Mountains, southeastern Wyoming: A tie to geology
Authors: Templeton Mary E. , Smithson Scott B.
Published: February 2004

The Cheyenne belt, a broad mylonitic shear zone, marks a 1.8–1.7 Ga suture between Archean craton to the north and a Proterozoic island arc to the south. A University of Wyoming seismic reflection profile across the Medicine Bow Mountains imaged the steeply dipping Cheyenne belt and crosscutting Laramide faults. By combining surface geology with reflection data from a line that trended oblique to the Cheyenne belt suture and Laramide thrusts, the true subsurface geometry of both could be resolved. The suture zone dips 60° southeast to depths of at least 9 to 14 km. A 5‐ to 10‐km thick zone of reflectors project to the surface near mapped Laramide faults and have a true structural dip of 45°. The imbricate thrusts within this zone cut the Cheyenne belt near 9 km depth. Some weaker events persist throughout the Archean lower crust; however, no clear Moho is observed. The subsurface geometry of the Cheyenne belt suggests that Archean crust is not absent at depth just south of the southernmost splay of the suture as Nd isotope studies might suggest. Alternatively, the Proterozoic hanging wall thickens such that rising plutons may form completely within the hanging wall and show only Proterozoic geochemical signatures. The complex reflectivity pattern observed in the Cheyenne belt line is caused mainly by the multideformational history of the area and the oblique orientation of the line. Ties between reflections and outcrop show that fault‐related lithologic contrasts and deformation within the fault zones generated reflectors at midcrustal depths during the Proterozoic. Later Laramide brittle faulting add to, rather than erased, the reflectivity.
Tectonic links between the Clachnacudainn terrane and Selkirk allochthon, southern Omineca Belt, Canadian Cordillera
Authors: Crowley James L. , Brown Richard L.
Published: February 2004

The Clachnacudainn terrane in the southern Omineca Belt is interpreted as an integral part of the overlying Selkirk allochthon based on stratigraphic, igneous, metamorphic, and deformational links across the boundary between them, the Standfast Creek fault. An inverted homocline correlated with lower Paleozoic Lardeau Group metasedimentary and metavolcanic rocks is continuous across the fault. The fault is transected by post‐ductile‐strain granitoids of inferred and known mid‐Cretaceous age (such as the 104±1 Ma Albert stock), as well as by peak regional metamorphic (M2) isograds and contact metamorphic (M3) isograds associated with the Albert stock. Three phases of regional ductile deformation (including major west verging second‐phase (F2) folds) are correlated across the fault. These tectonic links strongly suggest that the Clachnacudainn terrane and Selkirk allochthon share a similar tectonic history and thereby require that the Standfast Creek fault has minor tectonic significance compared with other faults in the southern Omineca Belt. M2 and coeval F2 folds are interpreted as resulting from Middle Jurassic tectonism documented elsewhere in the Selkirk allochthon. The main aspect that differentiates the Clachnacudainn terrane from the rest of the Selkirk allochthon east of the Monashee complex is the Clachnacudainn igneous complex, which dominates the northern part of the terrane and is the product of prolonged igneous activity.
Paleomagnetism of the Carboniferous Tepuel Group, central Patagonia, Argentina
Authors: Rapalini A. E. , Tarling D. H. , Turner P. , Flint S. , Vilas J. F.
Published: February 2004

Samples of the Carboniferous glaciogenic sequences of the Tepuel Group, western central Patagonia, Argentina, were collected from 23 sites in two localities, Sierra de Tepuel (43.7°S, 70.5°W) and Las Salinas (43.2°S, 70.3°W). Most were fine sandstones to siltstones of late Visean to Westphalian age. A characteristic remanent magnetization was determined in most sites by alternating field and/or thermal cleaning. At Sierra de Tepuel all samples were partially overprinted by a secondary magnetization apparently related to Mesozoic magmatic activity in this area. However, site mean directions of the characteristic remanence from both localities passed a tilt test, indicating a pretectonic (pre‐Early Permian?) magnetization. Isothermal remanent magnetization experiments showed magnetite as the main and often the only magnetic carrier, while thin section analyses indicated a detrital origin for this mineral, suggesting a depositional or postdepositional magnetization. The paleomagnetic pole for the Tepuel Group (Namurian‐Westphalian) at 316.1°E, 31.7°S (number of sites N = 16, minor and major semi‐axes of confidence δp = 15.0°, δm = 16.0°), is considered a key pole for the Middle Carboniferous of South America and Gondwana, confirming the apparent polar wander path for the supercontinent for which the pole moved from central to southern Africa in the Early Carboniferous, followed by a rapid motion from southern Africa to Antarctica during the late Westphalian. The agreement of the Tepuel pole with this Gondwanan path confirms that Patagonia was already part of South America by the Middle Carboniferous.
Structural transect of the southern Chihuahua Fold Belt between Ojinaga and Aldama, Chihuahua, Mexico
Authors: Hennings Peter H.
Published: February 2004

A restorable structural transect has been constructed across the southern end of the Chihuahua Fold Belt segment of the Cordilleran foreland fold and thrust belt of northeastern Chihuahua, Mexico, and west Texas. The 160‐km transect begins near Ojinaga and ends near Aldama, Chihuahua. It has been found that the Mesozoic Chihuahua Trough was inverted by Laramide tectonism and regionally shortened approximately 20 km (about 9%) along the transect to form the Chihuahua Fold Belt. Along the transect the belt consists of two allochthons of opposing vergence directions. The décollement of the eastern, northeast vergent allochthon shallows from within the Jurassic evaporites, along the center of the belt, and surfaces in Upper Cretaceous clastic rocks at the eastern thrust front south of Ojinaga, Chihuahua. The eastern allochthon was shortened approximately 6 km and may be divided, from west to east, into main ranges and an eastern frontal zone. Structures of the main ranges are detached in Jurassic evaporites and were greatly influenced by syntectonic flowage, resulting in salt‐cored anticlines and salt withdrawal synclines. To the east the décollement shallows over the Tascotal basement block which formed a buttress to northeastward transport forming a frontal zone containing ramp‐related anticlines and emergent thrusts. The western allochthon is southwest vergent. The décollement cuts up section from the center of the belt, where Precambrian basement is detached in the Plomosas Uplift, to Lower Cretaceous carbonates of the western frontal zone. The western allochthon was shortened approximately 20 km at the Paleozoic level and approximately 11 km at the Mesozoic level. Local basement involvement in the Plomosas Uplift may result from strike‐slip deformation along a northwest trending basement fault.
Tectonic setting of the Slide Mountain terrane, southern British Columbia
Authors: Roback Robert C. , Sevigny James H. , Walker Nicholas W.
Published: February 2004

The Slide Mountain terrane (SMT) in southern British Columbia consists of rocks of continental and oceanic affinity that are juxtaposed with parautochthonous rocks of the North American continental margin. In southern British Columbia, SMT consists dominantly of fine‐grained quartzose clastic rocks, limestone and lesser amounts of conglomerate and volcanic rocks of the Carboniferous McHardy assemblage, and predominantly mafic volcanic rocks of the Permian Kaslo Group. U‐Pb ages of individual detrital zircons from the McHardy assemblage are 1.7 Ga to 3.1 Ga and are similar to published ages of zircons from sedimentary rocks of the adjacent Kootenay terrane and the North American continental margin. These data and the petrology of McHardy assemblage sandstones and conglomerate suggest Kootenay terrane and the North American miogeocline as sources for McHardy assemblage detritus. U‐Pb zircon ages of granitoid clasts within McHardy assemblage conglomerate indicate that Silurian granitic rocks also provided detritus to the SMT. Mafic volcanic, ultramafic, and sedimentary rocks of the Kaslo Group conformably overlie the McHardy assemblage. New geochemical data demonstrate that the Kaslo Group consists of light rare earth element depleted basalts. On the basis of geochemical and geologic data, we suggest that Kaslo Group volcanics were erupted within an ocean ridge proximal to the North American continental margin and probably represent the eastern (continental) margin of a Permian marginal basin. Lithologie, stratigraphie, and U‐Pb geochronologic data suggest that the SMT was deposited on autochthonous, distal miogeoclinal rocks of the adjacent western North American craton and in close proximity to an early Paleozoic arc terrane. We infer that correlative late Paleozoic basinal terranes in western North American were deposited in a similar tectonic setting.
Rates of deformation, uplift, and landscape development associated with active folding in the Waipara area of North Canterbury, New Zealand
Authors: Nicol Andrew , Alloway Brent , Tonkin Philip
Published: February 2004

Analysis of the geometry and ages of faulted and tilted late Quaternary fluvial terraces and their associated cover beds provide evidence of active folding at three localities in the Waipara area of North Canterbury, New Zealand, Terrace survey data, the occurrence of the approximately 22.6‐kyr‐old Aokautere Ash, and examination of soil profiles indicate that folding has continued into the late Holocene but that the amounts and rates of deformation are locally variable. Rates of uplift in the Waipara area are compared with those derived from marine terraces preserved at the Pacific coast, east of the study area. Results indicate that rates of measurable deformation reach a maximum along the Waipara range front, where bedrock deformation is most intense and shortening rates of up to 5.57±0.69%/100 kyr occur. Across the coastal ranges the average rate of shortening is 0.8±0.4%/100 kyr, which corresponds with an absolute shortening rate of 1.4±0.6 m/kyr and represents only a small proportion of the predicted plate motion vector in this region. Uplift rates range from 0–1.83 m/kyr for a late last glacial fluvial terrace and from 1.36–2.16 m/kyr for three marine terraces. Fluvial and marine terrace uplift rates vary in accord with the geometries of the folds in bedrock, and the spatial pattern of uplift directly reflects fold growth. The structure contour pattern of folded surfaces provides a first approximation to the spatial pattern of uplift. Differential uplift due to folding accounts for up to approximately 55–75% of the total uplift and has produced folds with structural relief of about 1300 m (i.e., amplitudes of 600–700 m). These folds have formed over the last 0.8±0.4 m.y. since the onset of Quaternary deformation in the Waipara region.
Mesoscale and regional kinematics of the northwestern Yalakom fault system: Major Paleogene dextral faulting in British Columbia, Canada
Authors: Umhoefer Paul J. , Kleinspehn Karen L.
Published: February 2004

The northwestern Yalakom fault system (YFS) lies on the eastern margin of the Coast Belt in southwestern British Columbia. The Yalakom fault is the most significant structure in the fault system, with at least 115 km of dextral slip, mostly of early Tertiary age. The NW YFS is up to 25 km wide and is bounded on the southwest by the Tchaikazan fault zone, which had 5 km or more of dextral slip. Kinematic and paleostress analysis of mesofaults and map‐scale structures shows that secondary faults within the NW YFS had both dextral and sinistral oblique slip. Three large synclines had increasing amounts of clockwise, vertical axis rotation from southeast to northwest. Block rotation between two synthetic strike‐slip faults with large offset (Yalakom) and moderate offset (Tchaikazan) is similar to current kinematics between the San Andreas and San Jacinto faults in southern California. The NW YFS was active from latest Cretaceous(?) to (mainly) Eocene time and postdates early Late Cretaceous thrust faulting to the southwest. Early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike‐slip faulting commenced. The abrupt northwest termination of the YFS may be related to extension within the adjacent Tatla Lake metamorphic complex. Some dextral slip may have been transferred farther northwest through the Coast Belt, but these structures have not been documented in west central British Columbia. Most of the 120 km of dextral slip on the YFS was transferred to the north via a mosaic of normal and strike‐slip faults in central British Columbia. Thus the YFS forms the southwestern margin of a broad zone of transtensional structures that extends to the Tintina‐Rocky Mountain Trench fault zone; these structures accommodated northwestward displacement of the northwestern Cordillera in Paleogene time.
Paleomagnetism of the Upper Devonian reef complexes, Canning Basin, Western Australia
Authors: Chen Z. , Li Z. X. , Powell C. McA.
Published: February 2004

Paleomagnetism of the Upper Devonian reef complexes in the Canning Basin, Western Australia, was reinvestigated with a primary aim of testing its magnetization age. Samples were collected from back reef and reefal limestones, basinal red beds, syndepositional allochthonous blocks, and sediment infillings in syndepositional cracks and voids. A characteristic remanence, similar to that identified in a previous study, was found in six autochthonous sites in outcrop and four diamond drill cores, with a mean direction of declination D = 27.5°, inclination I = −19.9° (half angle of 95% confidence cone α95 = 15.2°). The primary origin of this remanence was demonstrated by both a syndepositional breccia test and an allochthonous block test. The corresponding Late Devonian pole position (62.0°S, 23.2°E with half angle of 95% confidence cone A95 = 14.6°) overlaps with the previously reported paleopole from this reef complex and other Late Devonian paleopoles from Australia. The evidence indicates that the spread of late Middle to Early Carboniferous paleopoles from Australia is caused by a phase of rapid apparent polar wander, rather than the tectonic discordance of southeastern Australia in relation to the craton.
Structures related to Red Sea evolution in northern Eritrea
Authors: Drury S. A. , Kelley S. P. , Berhe S. M. , Collier R. E. Ll. , Abraha M.
Published: February 2004

Structural analysis of remotely sensed images of Eritrea, guided by the common presence of a mid‐Oligocene datum at the base of flood basalt at different elevations, helps to piece together the Tertiary history of the western flank of the Red Sea basin. Many of the Tertiary faults are reactivated shear zones in the Pan‐African basement, the most important Tertiary fault at the foot of the Red Sea Escarpment marking the site of a pre‐Tertiary boundary between andesitic island‐arc crust and deeper, perhaps older granitoid migmatites. The initiation of extensional sedimentary basins in Eritrea clearly followed volcanism, as did doming and uplift. This sequence lends some support to active mantle upwelling as the primary tectonic impetus, specifically to massive melting simultaneous with plume impact with the lithosphere. The coincidence of major Tertiary extensional faults with zones of mid‐crustal to lower‐crustal silicic basement may indicate that active mantle processes were focused by zones of Theologically weak lithosphere, as postulated by Dixon et al. (1989).
Does the Great Glen fault really disrupt Moho and upper mantle structure?
Authors: McBride J. H.
Published: February 2004

Many previous interpretations of deep seismic reflection profiles across the Great Glen strike‐slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the Moho discontinuity. Such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the British Isles. Reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the “Moho step” interpretation. Diffraction analysis and seismic migration applied to reflection data north of Shetland across the northward continuation of the Great Glen fault (Walls Boundary fault) indicate a narrow synform or “keel” developed on the Moho directly beneath the fault. This keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault. North and west of Ireland, analyses of amplitudes, frequencies, and the geometrical behavior of reflections upon migration show that structures previously interpreted as Moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin. In one location north of Ireland where some three‐dimensional control exists, the vertically downward projection of the Great Glen fault intersects, without disrupting, dipping structure in the upper mantle. This observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp (or blind thrust) in the uppermost mantle, somewhat analogous to models developed for the San Andreas fault that indicate displacement along the fault to be laterally offset within the middle crust. One of the principal conclusions of this study, that major vertical steps on the Moho beneath the Great Glen fault are difficult to support from the available seismic data, is consistent with Theologically based studies which predict that Moho “topography” such as vertical steps is unlikely to be preserved over long periods of geologic time.
A deep seismic reflection profile across the extinct Mid‐Labrador Sea spreading center
Authors: Srivastava S. P. , Keen C. E.
Published: February 2004

In 1990 a deep multichannel seismic reflection line was shot along a flow line across the Labrador Sea. The results from the central portion of this line between magnetic anomalies 25 and across the extinct central ridge are described here. Spreading rates in this part of the Labrador Sea are very low, from 10 mm/yr to less than 3 mm/yr, so that the line provides a unique opportunity to examine the relationship between very slow spreading and crustal structure. A clear division is observed between two types of crust. The oldest crust, between dirons 21 and 25, which formed at a mean half spreading rate of 10 mm/yr, exhibits smoothly undulating basement with only minor normal faulting. This region shares many of the reflection characteristics of North Atlantic crust formed at moderate to low spreading rates. In contrast, the younger central region, between chrons 21 and 13, which formed at a mean half spreading rate of 3 mm/yr, displays evidence of intense normal faulting of the crust, giving a total extension of about 70%. Inward facing normal faults on both sides of the extinct ridge, with large offsets, many of which extend to lower crust or Moho depths, dominate the seismic section. The axial region is characterized by a deep, fault‐bounded, median valley. These results suggest that mechanical extension plays a more important role in seafloor spreading at low spreading rates than previously documented. Integration of the reflection data with previous refraction measurements and with gravity modeling of the region shows variations in crustal thickness which can be correlated with spreading rates. The region formed at a mean spreading rate of 10 mm/yr, where about 15% extension is observed, exhibits slightly thinner than normal crust (4.8 km or less versus a normal thickness of about 7 km). At a lower spreading rate of 3 mm/yr across the axial region where extension is about 70%, an average crustal thickness of 3 km is obtained. Thus lower spreading rates are associated with regions of thinner crust and greater amounts of extension. While many studies suggest that thin crust at slow spreading rates may result from a reduced magma supply, this study suggests that extension is at least equally important and may be responsible for most of the variations in crustal thickness. The increased cooling of young oceanic lithosphere formed at these very low spreading rates (approximately 3 mm/yr) may have amplified brittle failure in response to plate separation. However, the timing of extension is still uncertain and some of it may be related to postextinction tectonics and not to the spreading process. The role of extension as a control on crustal thickness needs to be considered further in studies of crustal generation and magmatism at slow spreading ridges. Extension will decrease the importance of magmatism in generating thin oceanic crust and will favor models of magmatic processes which produce thicker crust.
Cretaceous frontal thrusts of the Verkhoyansk fold belt, eastern Siberia
Authors: Parfenov L. M. , Prokopiev A. V. , Gaiduk V. V.
Published: February 2004

Cretaceous thrust structures are found along the front of the Verkhoyansk miogeoclinal fold belt along the eastern boundary of the Siberian platform in northeast Asia. The Verkhoyansk thrust front is subdivided into a number of segments, each of which has its own thrust system geometry. Balanced cross sections have been constructed for each segment on the basis of the structural study of surface geology and available seismic and drilling data. Distinctions between the segments are also expressed in gravity anomalies and modern topography. Analysis of vitrinite reflectance shows that folding of the Verkhoyansk thrust front was initiated during sedimentation as early as in the Late Jurassic. This period marks the beginning of the collision between the Siberian continent and the Kolyma‐Omolon superterrane, now located 500 km to the east of the Verkhoyansk thrust front. Deformation of the thrust front ended by the late Late Cretaceous; erosion of the frontal anticlines began in the early Late Cretaceous. The frontal thrust structures formed in the Late Cretaceous were rejuvenated during a middle to late Pleistocene reactivation, which produced the modern mountain topography. The least amount of erosion of the Verkhoyansk thrust front, 840 m, is observed in its central part, in the Kuranakh segment. To the north and south the erosion increases to 1500 m and 2100 m, respectively. The general configuration of the Verkhoyansk fold belt and its frontal structures are defined by the geometry of Devonian rift‐related structures on the eastern Siberian platform and the principal direction, approximately east–west, of Late Cretaceous compressional stresses.
Large‐scale Intrusion of shallow water into a vertical fracture zone in crystalline bedrock: Initial hydrochemical perturbation during tunnel construction at the Äspö Hard Rock Laboratory, southeastern Sweden
Authors: Banwart Steven , Gustafsson Erik , Laaksoharju Marcus , Nilsson Ann‐Chatrin , Tullborg Eva‐Lena , Wallin Bill
Published: February 2004

On March 13, 1991, construction of the entrance tunnel to the Äspö Hard Rock Laboratory opened a vertical fracture zone at a depth of 70 m. This provides an opportunity to study geochemical changes resulting from shallow water inflow into a crystalline bedrock aquifer as anticipated during construction and operation of a deep repository for spent nuclear fuel. Chloride ion is a natural conservative tracer for mixing between the dilute ([Cl−] < 10 mg L−1) shallow groundwater and the saline ([Cl−] = 5000 mg L−1) native groundwater of the fracture zone. A sharp dilution front, corresponding to 80% dilution of the native groundwater, indicated arrival of shallow groundwater in the entrance tunnel after 3 weeks. In spite of this large input of shallow water, the fracture zone remains predominantly anoxic. Major element hydrochemistry and carbon and oxygen stable isotope data indicate large inputs of alkalinity and biogenic CO2(g). Input of organic carbon with shallow groundwater provides a possible energy and carbon source for anaerobic respiration. There is no evidence for sulfate reduction, and Fe(III) oxyhydroxide fracture minerals are the only other dominant electron acceptor observed.
Mass transfer at fracture intersections: An evaluation of mixing models
Authors: Berkowitz Brian , Naumann Curt , Smith Leslie
Published: February 2004

Models of solute transport in fractured geologic media that are based on the discrete network approach require that a method be adopted for transferring mass through each fracture intersection. The two usual models for mass partitioning between the outflow branches of crossing fractures assume either stream tube routing or complete mixing. A mathematical analysis of two‐dimensional, laminar flow through the intersection of two orthogonal fractures with smooth walls is carried out to examine the mixing process. Mixing ratios are expressed in terms of a local Peclet number (Pe = υr/D), where υ is an average fluid velocity within the fracture intersection, r is the radius of the fracture intersection, and D is the diffusion coefficient. As a general observation the concept of complete mixing within a fracture intersection does not properly represent the mass transfer process at any value of the Peclet number. A mixing ratio equivalent to complete mixing may be observed, but only for particular flow geometries and in a limited range of the Peclet number. Stream tube routing models provide a good approximation for Peclet numbers greater than 1; and in some cases this limit may be as low as 10−2. The actual value of the lower limit depends upon the geometry of the bounding streamline that separates the flow into the two outflow fractures, in relation to the fracture through which solute enters the intersection. There is a range in the Peclet number, of roughly 3 orders of magnitude, where the extent of mixing is dependent upon the value of Pe within the intersection.
Modeling contaminant transport and biodegradation in a layered porous media system
Authors: Wood Brian D. , Dawson Clint N. , Szecsody Jim E. , Streile Gary P.
Published: February 2004

The transport and biodegradation of an organic compound (quinoline) were studied in a meter‐scale system of layered porous media. A two‐dimensional laboratory experiment was conducted in a saturated system with two hydraulic layers with a ratio of conductivities of 1:13. A solution containing dissolved quinoline was injected as a front at one end of the system, and the aqueous‐phase concentrations of quinoline, its first degradation product (2‐hydroxyquinoline), and oxygen were monitored over time at several spatial locations. Results from a set of ancillary batch and small‐column experiments were used to generate a mathematical model for the microbial kinetics; these kinetics described the time rate of change of the concentrations of the two organic compounds (quinoline and 2‐hydroxyquinoline), the electron acceptor (oxygen), and microbial biomass. This independently developed kinetic model was incorporated into a two‐dimensional numerical model for flow and transport, so that simulations of the laboratory system could be conducted and the results compared with observed data. An analysis of the applicability of single‐phase and multiple‐phase models for the description of the microbial kinetics was conducted. The results of this analysis indicated that for some cases, it is not necessary to explicitly model the mass transfer between the aqueous phase and the biomass phase. A single‐phase model was used for simulating the laboratory system described here. Favorable comparisons between the laboratory and simulation data suggested that a single‐phase model was appropriate for describing the microbially mediated reactions in this system. A method for incorporating the effects of metabolic lag into microbial kinetics is described. Metabolic lag was explicitly accounted for in the degradation kinetics for this system; the inclusion of metabolic lag proved to be important for describing transient concentration pulses that were observed in the low‐conductivity layer.
A distributed hydrology‐vegetation model for complex terrain
Authors: Wigmosta Mark S. , Vail Lance W. , Lettenmaier Dennis P.
Published: February 2004

A distributed hydrology‐vegetation model is described that includes canopy interception, evaporation, transpiration, and snow accumulation and melt, as well as runoff generation via the saturation excess mechanisms. Digital elevation data are used to model topographic controls on incoming solar radiation, air temperature, precipitation, and downslope water movement. Canopy evapotranspiration is represented via a two‐layer Penman‐Monteith formulation that incorporates local net solar radiation, surface meteorology, soil characteristics and moisture status, and species‐dependent leaf area index and stomatal resistance. Snow accumulation and ablation are modeled using an energy balance approach that includes the effects of local topography and vegetation cover. Saturated subsurface flow is modeled using a quasi three‐dimensional routing scheme. The model was applied at a 180‐m scale to the Middle Fork Flathead River basin in northwestern Montana. This 2900‐km2, snowmelt‐dominated watershed ranges in elevation from 900 to over 3000 m. The model was calibrated using 2 years of recorded precipitation and streamflow. The model was verified against 2 additional years of runoff and against advanced very high resolution radiometer based spatial snow cover data at the 1‐km2 scale. Simulated discharge showed acceptable agreement with observations. The simulated areal patterns of snow cover were in general agreement with the remote sensing observations, but were lagged slightly in time.
How water moves in a water repellent sandy soil: 1. Potential and actual water repellency
Authors: Dekker Louis W. , Ritsema Coen J.
Published: February 2004

Water repellency is an important property of many soils. It causes rainwater to penetrate into the soil as preferential flow paths, and solutes can reach the groundwater more rapidly than in the case of a homogeneous wetting. Water repellency depends on several factors which are principally related to the characteristics of the organic matter of the soil. A distinction between “potential” and “actual” water repellency and the assessment of the “critical soil water content” are introduced and highlighted in this paper. Persistence and degree of potential water repellency of dried samples were examined from 10 trenches in a dune sand with grass cover using the water drop penetration time and the alcohol percentage tests. The spatial variability of water repellency and, therefore, soil wetting was extremely high. The actual water repellency was measured on field‐moist samples to obtain critical soil water contents. The soil is wettable above and water repellent below these values. The critical soil water content varies between 4.75 vol % at 5–10 cm and 1.75 vol % at 45–50 cm depth in this sandy soil.
A detachment‐limited model of drainage basin evolution
Authors: Howard Alan D.
Published: February 2004

A drainage basin simulation model introduced here incorporates creep and threshold slumping and both detachment‐ and transport‐limited fluvial processes. Fluvial erosion of natural slopes and headwater channels is argued to be dominantly detachment‐limited. Such slopes undergo nearly parallel retreat and replacement with alluvial surfaces under fixed base level, in contrast with gradual slope decline for transport‐limited conditions. The arrangement of divides and valleys is sensitive to initial conditions, although average morphology is insensitive. Dissected, initially flat surfaces in which downstream concavity is slight exhibit nearly parallel drainage, compared to very wandering main valleys when concavity is great. Steady state is reached after a cumulative base level drop approximately 3 times the final relief. Simulated valley systems are similar to those predicted by a previous model of optimal drainage basins. A critical value of slope divergence normalized by average slope gradient is a useful criterion for defining the valley network.
The concept of the Dilution Index
Authors: Kitanidis Peter K.
Published: February 2004

In many applications, it is important to make the distinction between spreading and dilution of a plume in groundwater. Spreading is associated with the stretching and deformation of a contaminant plume, whereas dilution is associated with the increase in volume of the fluid occupied by the solute. The dilution and spreading of a Gaussian plume in a homogeneous porous medium with constant velocity are related in a simple fashion and are both characterized by the same parameters, the dispersion coefficients. However, the geological formations of interest in field applications are heterogeneous, and the plumes are irregular in shape. The dispersion coefficients that are deduced from tracer tests usually measure an overall rate at which a tracer plume spreads about its centroid and depend critically on the heterogeneity of the formation. These macroscopic dispersion coefficients are not reliable measures of the rate at which the maximum concentration is reduced because in heterogeneous formations the rates of dilution and spreading can be quite different. The main objective of this work is to introduce a new macroscopic measure of dilution, the dilution index E. Examples serve to demonstrate the usefulness of the measure. A general expression for the rate of dilution of a tracer plume is derived. The exact rate of increase of the dilution index under the idealized conditions of constant dispersion coefficients and a Gaussian plume is computed, and a lower bound is found to the same quantity for non‐Gaussian plumes. For the general heterogeneous case the analysis demonstrates that the instantaneous rate of increase of ln E is proportional to the small‐scale dispersion coefficients, everything else being the same. The rate of increase of ln E depends also on the degree of irregularity in the shape of the plume. Thus, in the long term, geologic heterogeneity should increase the rate of dilution because spatial variability in the flow velocity tends to deform plumes and make them less regular.
Effects of particle shape and mobility on stable armor development
Authors: Gomez Basil
Published: February 2004

The relative size and roughness of a stable armor varies with particle shape. For a given imposed shear stress the order of increasing nominal particle diameter is flat, angular, and rounded gravel. The order of increasing surface roughness is flat, rounded, and angular gravel. The mobility of fine flat gravel particles is enhanced by the lower surface roughness of armors which exhibit an imbricated structure. However, the generally lower mobility of ellipsoids means that, for a given stream power, bed load discharge associated with armor development on flat gravel is less than that for rounded gravel (at greater depth to grain size ratios). The larger projected area of angular gravel enhances the mobility of coarse grains at high flows, offsetting interlocking effects which restrict the mobility of angular gravel at low flows. Relative and absolute size effects contribute to the departure from equal mobility of coarse and fine particles that is a ubiquitous feature of stable armor development below the limit for the formation of a threshold armor when all particles are equally mobile. The transport of fine sediment as throughput load may mask the effect due to hiding in the field and give the erroneous appearance that stable armors develop solely through winnowing. All particle sizes in the underlying bed material are present in the armor and in the bed load, and coarsening is but one of several adjustments to the surficial bed material that occur as a stable armor evolves. It likely involves the wholesale rearrangement of surficial particles and may entail minimization of shear due to drag on background roughness.
Coupled seismic and tracer test inversion for aquifer property characterization
Authors: Hyndman David W. , Harris Jerry M. , Gorelick Steven M.
Published: February 2004

Seismic and tracer test data can be combined to estimate the spatial patterns of aquifer properties. We present an algorithm that estimates the geometry of large‐scale lithologic zones, the effective hydraulic conductivities and seismic velocities for these zones, and the effective small‐scale dispersivity. The heart of this algorithm is our split inversion method, which extracts the geometry of lithologic zones from an estimated seismic velocity field. This method determines the zonation that best matches multiple types of data, such as seismic travel times and tracer concentrations. Although the current implementation of the algorithm uses only cross‐well seismic travel times and tracer concentrations, the algorithm could incorporate other data types that are sensitive to changes in large‐scale lithology. We demonstrate the approach for two synthetic sandy aquifers, one with interbedded clay lenses and another with interbedded silt and gravel lenses. These examples differ in the uniqueness of the relationship between seismic velocities and hydraulic conductivities. For these examples our algorithm successfully maps interwell heterogeneities and accurately estimates hydraulic and seismic parameters.
Evaluation of flow leakage through abandoned wells and boreholes
Authors: Avci Cem B.
Published: February 2004

Flow leakage through abandoned wells and improperly plugged boreholes drilled between hydrogeologically separated water‐bearing zones represent potential pathways of contamination. The present study provides an analytical evaluation of the transient flow rate through these artificial conduits and the resulting hydraulic head distributions when (1) artificial gradients are created by an injection well operating in one of the aquifers or (2) natural hydraulic head differences are present between two confined aquifers. The analysis is conducted by solving the groundwater flow equations in the confined aquifers that are coupled by the flow through the well or borehole. A case study was investigated involving contamination leakage through abandoned wells and improperly plugged boreholes caused by the operation of deep waste injection wells.
Modeling of multicomponent reactive transport in groundwater: 2. Metal mobility in aquifers impacted by acidic mine tailings discharge
Authors: Walter A. L. , Frind E. O. , Blowes D. W. , Ptacek C. J. , Molson J. W.
Published: February 2004

The mobility of potentially toxic dissolved metals discharged from a mine tailings source into an aquifer is investigated by using the multicomponent reactive transport model MINTRAN. A generic aquifer resembling a site in northern Ontario is used as a basis, and a scenario analysis is performed to determine the effect of terminating the source input of acidity and dissolved metals after a finite time. It is found that for most metals the buffering reactions in the carbonate aquifer lower the dissolved metal concentrations substantially, often by several orders of magnitude. In the case of a 12‐year source duration, most metals are effectively immobilized by precipitation, and the concentrations of metals remaining in solution are well below drinking water limits. The risk of contamination of water resources by toxic metals can thus be controlled.
Modeling of multicomponent reactive transport in groundwater: 1. Model development and evaluation
Authors: Walter A. L. , Frind E. O. , Blowes D. W. , Ptacek C. J. , Molson J. W.
Published: February 2004

MINTRAN is a new model for simulating transport of multiple thermodynamically reacting chemical substances in groundwater systems. It consists of two main modules, a finite element transport module (PLUME2D), and an equilibrium geochemistry module (MINTEQA2). Making use of the local equilibrium assumption, the inherent chemical nonlinearity is confined to the chemical domain. This linearizes the coupling between the physical and chemical processes and leads to a simple and efficient two‐step sequential solution algorithm. The advantages of the coupled model include access to the comprehensive geochemical database of MINTEQA2 and the ability to simulate hydrogeological systems with realistic aquifer properties and boundary conditions under complex geochemical conditions. The model is primarily targeted toward groundwater contamination due to acidic mine tailings efiïuents but is potentially also applicable to the full range of geochemical scenarios covered by MINTEQA2. The model is tested with respect to ion exchange chemistry and with respect to precipitation/dissolution chemistry involving multiple sharp fronts. The companion paper presents two‐dimensional simulations of heavy metal transport in an acidic mine tailings environment, focusing on environmental implications.
Water Movement in Glass Bead Porous Media: 3. Theoretical Analyses of Capillary Rise into Initially Dry Media
Authors: Lu T. X. , Nielsen D. R. , Biggar J. W.
Published: February 2004

This paper presents a theoretical analysis of the experimental evidence reported in papers 1 and 2 (Lu et al., this issue (a, b)). The analysis of force is conducted on spherical particles serving as an idealized porous medium. Four close packing conditions were studied and two models were developed to describe liquid movement in glass bead porous media. According to the analysis of forces acting on the contact point of gas‐liquid interface, the direction as well as the magnitude of the total surface tensile force changes in contrast to a constant total surface tensile force acting in a capillary tube. It is shown that the velocity of the liquid plays an important role during capillary rise into porous media. Equations for the height and velocity of capillary rise into initially dry porous media are given for four different geometries of close packing. The models and equations present an improved explanation of the Haines' “jump” phenomenon and the instability observed in experiments of capillary rise in porous media.