bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence
Authors: Schaaf Peter , Morán‐Zenteno Dante , Hernández‐Bernal Maria del Sol , Solís‐Pichardo Gabriela , Tolson Gustavo , Köhler Hermann
Published: February 2004

The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic‐Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large‐scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well‐defined along‐coast NW–SE decreasing granitoid intrusion age trend (∼1.2 cm/yr in the 100 Ma‐40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW–SSE trending Cretaceous‐Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America‐Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (∼7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco‐Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40–25 Ma, with a velocity of 6.5–4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America‐Farallon‐Caribbean triple junction. Geological observations and paleomagnetic data do not give strong support for large‐scale right‐lateral displacements of crustal blocks like the Baja California. Given the isotopic data presented, the continental margin truncation in SW Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment. On the other hand, the lateral removal of material associated with the displacement of Chortis is more important in the southeastern segment.
Cretaceous tectonic evolution of coastal southern California: Insights from the Catalina Schist
Authors: Grove Marty , Bebout Gray E.
Published: February 2004

The relatively high‐T, high‐P/T, subduction‐related basement in the Southern California Continental Borderland preserves important information regarding the late Mesozoic tectonic evolution of coastal southern California. Integrated petrologic, geochemical, and geochronological study of the subduction‐related Catalina Schist exposed on Santa Catalina Island documents the formation of lawsonite‐blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling, newly initiated subduction zone. Petrologic and 40Ar/39Ar thermochronologic data demonstrate similar, perhaps shared, retrograde PT histories for all but the lawsonite‐albite facies rocks under pumpellyite‐actinolite facies conditions as early as ∼95–100 Ma. The 40Ar/39Ar data, together with the abundance of pumpellyite‐actinolite facies vein assemblages and lack of higher‐P/T (epidote‐ or lawsonite‐blueschist) over‐printing in high‐grade units, are consistent with the rapid emplacement of the Catalina Schist into the shallow‐ to mid‐crust by ∼ 95 Ma by combined underplating and tectonic/erosional removal of overlying rocks in the accretionary complex (cf. Platt, 1986). The Catalina Schist and the adjacent Peninsular Ranges Batholith potentially represent paired metamorphic belts formed along southwest North America at 120 to 90 Ma. Combining the evidence for the evolution of the Catalina Schist with the previously advanced hypotheses regarding tectonic evolution of the batholith, suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone, at ∼ 120–115 Ma, in which the Catalina Schist was accreted and metamorphosed.
A flexural isostatic model of lithosphere shortening and foreland basin formation: Application to the Eastern Cordillera and Subandean belt of NW Argentina
Authors: Toth J. , Kusznir N. J. , Flint S. S.
Published: February 2004

A numerical model of lithosphere shortening has been developed to combine flexural isostatic and structural balancing. This allows us to quantitatively reconcile lithosphere shortening, thrust sheet emplacement, and foreland basin formation. Lithosphere shortening is accommodated by thrusting in the upper crust and by distributed deformation in the lower crust and the lithospheric mantle. This leads to loading by thrust sheet emplacement, crustal thickening, and thermal perturbation. Additional loading and unloading are generated by sedimentation and erosion, respectively. The model is kinematically controlled, each fault having its position, geometry, and movement prescribed. The model is applied to the Subandean late Oligocene to Recent foreland fold and thrust belt of NW Argentina at 22°15′S. The Subandean belt of NW Argentina contains three major stratigraphic units, the Lower, Middle, and Upper Terciario Subandino Formations. By modelling these formations we attempt to determine whether the Terciario Subandino Group can be explained as a foredeep fill in response to thrust sheet loading, and, if so, what is the required effective elastic thickness. A successful model must predict not only foreland stratigraphy but also exhumation, topography, and crustal thickness. We see a change in the wavelength of sedimentary deposition in the Subandean basin through time that can be accounted for by a change in flexural rigidity. Accommodation space for the Lower Terciario Subandino Formation can be generated using an elastic thickness of 70 km. For the Middle/Upper Terciario an elastic thickness of 15 km produces the correct basin geometries. Shortening estimates of 110 km have proved sufficient to generate the observed foreland stratigraphy, Bouguer anomaly, and topography across the Subandean belt and Eastern Cordillera. However, an additional 140 km of shortening is required to generate the observed Bouguer anomaly and topography across the western Andes. This discrepancy could be due to earlier shortening in the western Andes or to crustal thickening by magma addition.
Variations in the along‐axis segmentation of the Afar Rift system
Authors: Hayward N. J. , Ebinger C. J.
Published: February 2004

Both oceanic and continental rifts show regular along‐axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along‐axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along‐axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a “continental” rift sector, the Main Ethiopian Rift (MER) system to the south. We use high‐resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin‐bounding normal faults (∼50 km long in MER to ≤ 15 km long in northern Afar), in the lengths and widths of rift basins (from 50 to 100 km long × 30 to 80 km wide in MER to ≤ 20 km long × ≤ 5 km wide in northern Afar), and in the relief of the uplifted rift flanks surrounding the basins (from ≥ 1000 m high in MER to ≤ 100 m high in northern Afar). Erosional escarpments along the older, outer rift margins of Afar reveal long, high border fault segments similar to those found in the less evolved MER, suggesting that the Afar Rift was originally bounded by much longer faults. In northern Afar, there is a drastic increase in the volume of Quaternary basalts, with segmentation now dominated by 50‐ to 80‐km‐long volcanic ridges, which are similar in size, morphology, and spacing to the second order, nontransform offset segmentation of slow‐spreading mid‐oceanic ridges. The along‐axis changes in rift segmentation are concomitant with decreasing crustal thickness (i.e., cumulative strain), decreasing effective elastic thickness estimates, and increasing magma supply. These temporal and spatial patterns suggest that segmented continental rifts can evolve into segmented oceanic rifts dominated by mantle sourced magmatic processes.
A dual doubly vergent orogen in the Banda Arc continent‐arc collision zone as observed on deep seismic reflection profiles
Authors: Snyder D. B. , Prasetyo H. , Blundell D. J. , Pigram C. J. , Barber A. J. , Richardson A. , Tjokosaproetro S.
Published: February 2004

New deep seismic reflection profiles across the Banda Arc of Indonesia reveal reflectors in the uppermost 50 km of the lithosphere. Combined with existing earthquake hypocenter locations and focal mechanisms, the new structural geometries inferred from the reflectors yield a more complete analysis of deformation during the past 10 m.y. and provide insights into how strain is partitioned across an orogenic belt in which volcanic arcs and continents converge. A clearly defined Wadati‐Benioff zone, and recent deformation of shelf sediments observed on older shallow seismic profiles, indicated to previous workers that substantial convergence occurred at the Timor Trough. A few focal mechanisms, seafloor escarpments, and recent geodetic surveying indicate that convergence at ∼7 cm yr−1 currently (last 200 kyr?) occurs at the northern margin of the now inactive volcanic arc, the Wetar Thrust zone. Reflectors on the new seismic profiles are interpreted as thrust faults and folds that occur throughout the crust and within the uppermost mantle between the Timor Trough and Wetar Thrust. Specifically, basement reflectors beneath the toe of the accretionary complex have reverse‐sense offsets that imply blind thrusts. The whole crust is horizontally shortened, not only the sedimentary cover rocks that previously deformed into duplexes above a decollément. Reflectors dipping away from both margins of the forearc basin and at the northern margin of the volcanic arc are interpreted as evidence of thrusting. Thus each arc represents a doubly vergent fold‐and‐thrust belt, but only the northern one is currently active. Crustal thicknesses inferred from seismic velocities, reflectors, and gravity anomalies are consistent with the merging of a thinned continental shelf margin with oceanic lithosphere to form an orogenic belt with at present 3–4 km of topographic relief in the region of eastern Timor.
Structural style and geodynamic evolution of Camargue and Western Provencal basin, southeastern France
Authors: Mauffret A. , Gorini C.
Published: February 2004

We present a tectonic analysis of the Camargue and Western Provençal margin based on geophysical maps, seismic profiles, and well log data. The tectonic style results from a polyphase geological history. Mesozoic extension was followed by Late Cretaceous‐Eocene compression. During Oligocene extension, part of the preexisting compressional structures was reactivated along extensional detachments. The concept of Oligocene negative inversion is applied to an asymmetrical opening of the northwestern Mediterranean basin. The steep Provençal margin is separated from the extended Gulf of Lion margin by a major transfer zone. The extensional structures of the Gulf of Lion can be placed in the framework of the Western European rift system. Late Eocene‐early Oligocene E‐W extension is followed by NE‐SW late Oligocene‐early Aquitanian opening of previous transverse structures. Seismic evidence supports a possible eastward extension of the North Pyrenean fault.
Crustal structure and evolution of the Trans‐Hudson orogen: Results from seismic reflection profiling
Authors: Baird D. J. , Nelson K. D. , Knapp J. H. , Walters J. J. , Brown L. D.
Published: February 2004

A 400‐km‐long deep seismic reflection transect across northeastern Montana and northern North Dakota reveals the crustal‐scale structural fabric of the Early Proterozoic Trans‐Hudson orogen beneath the Williston basin. Comparison with deep seismic reflection data across the Canadian portion of the same orogen ∼700 km to the north reveals first‐order similarities in crustal architecture but documents significant along‐strike variation in orogenic evolution. Both transects display a broad crustal‐scale antiform axial to the orogen. In the north, geologic data suggest that this antiform is cored by an Archean microcontinent. In the south, west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the Archean Wyoming craton. Within the Wyoming craton, the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the Early Proterozoic Trans‐Hudson orogen as interpreted from potential field and drill core data. On the basis of subsurface geochronologic data from the crystalline basement and by analogy with the Glennie domain within the exposed Trans‐Hudson orogen in Canada, we suggest that the southern antiform is cored by an Archean crustal fragment that was caught up in the terminal collision of the Wyoming and Superior cratons during Hudsonian orogeny. The eastern side of the Trans‐Hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections. We interpret the easterly dip of these reflections as evidence that the Superior province was thrust westward over the interludes of the orogen during terminal collision. Although juvenile Early Proterozoic terranes characterize the exposed segment of the Trans‐Hudson orogen in Canada, limited drill core information within the Dakota segment of the orogen shows a predominance of granulitic Archean age crust. This difference in basement lithologies along strike within the orogen may indicate the following: either juvenile crust comparable to that exposed in the northern Trans‐Hudson was never present in the south, or it was removed by progressive over thickening, erosion, and/or faulting. Postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north.
Origin of the amphibolite “sole” of the Josephine ophiolite: Emplacement of a cold ophiolite over a hot arc
Authors: Harper Gregory D. , Grady Kristen , Coulton Angela J.
Published: February 2004

The amphibolite “sole” that occurs along the basal thrust of the Josephine ophiolite (Madstone Cabin thrust) is distinct from those described for ophiolites such as the Bay of Islands and Oman. Antigorite‐bearing serpentinite mylonites occur directly above the amphibolite, and the presence of antigorite + brucite indicates formation between 300° and 400°C. The antigorite mylonites also contain relict lizardite/chrysotile, suggesting formation of antigorite by prograde metamorphism along the base of the ophiolite. Further evidence for prethrusting low‐temperature serpentinization comes from a lizardite/chrysotile shear zone within fresh peridotite directly above the thrust that is cut by undeformed diabase dikes having geochemical affinities to the upper pillow lavas of the Josephine ophiolite. The prograde metamorphism of the serpentinite at the base of the ophiolite was apparently synchronous with greenschist facies retrogressive metamorphism of the underlying amphibolite. The amphibolite is chemically diverse and has been divided into two groups. Group 1 amphibolite appears to be derived from mafic plutonic rocks and has immobile trace element abundances indicating affinities to volcanic arc magmas. Group 2 amphibolite is metavolcanic (contains some quartzite) and has magmatic affinities to mid‐ocean ridge basalt and within‐plate basalt, an association that is distinctive of older basement rocks elsewhere in the Western Klamath terrane. We suggest the amphibolite was derived from the footwall of the Madstone Cabin thrust, including country rocks for the Chetco complex (Group 2) and possibly older intrusive phases of the Chetco complex itself (Group 1). Previous workers have shown that the base of the amphibolite is intruded by gneissic gabbro of the Chetco complex, that intrusion of the Chetco complex overlapped in time with deformation of the amphibolite, and that during and after emplacement the Josephine ophiolite and overlying flysch were intruded by abundant arc‐related dikes and plutons. Emplacement of the Josephine ophiolite appears to have occurred by underthrusting of a cold ophiolite by an active magmatic arc.
Crustal and upper mantle velocity structure of the Salton Trough, southeast California
Authors: Parsons Tom , McCarthy Jill
Published: February 2004

This paper presents data and modelling results from a crustal and upper mantle wide‐angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid‐ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide‐angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (∼21–22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (∼27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (∼24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (∼5–6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (∼2–3 km) basin beneath the Colorado River. A regional 6.9‐km/s layer (between ∼15‐km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low‐velocity (7.6–7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part predates Salton Trough rifting. It may also in part result from migration of magmatic spreading centers associated with the southern San Andreas fault system. These spreading centers may have existed east of their current locations in the past and may have influenced the lower crust and upper mantle to the east of the current Salton Trough.
Structure and tectonic development of the southern Rocky Mountain trench
Authors: van der Velden Arie J. , Cook Frederick A.
Published: February 2004

The Rocky Mountain trench is one of the youngest, most prominent, and most enigmatic structures of the Canadian Cordillera. Approximately 650 km of seismic‐reflection data, providing regional three‐dimensional coverage over an area of 10,000 km², include six crossings of the Rocky Mountain trench between 49°N and 50°15′N. Prominent reflections from mid‐Proterozoic Moyie sills outline thrust‐and‐fold structures of a Late Jurassic to Early Cretaceous fault system that was cut by the Rocky Mountain trench fault in the Tertiary. The near‐basement reflections outline a 10 km high west facing basement ramp, the hinge line of which spatially coincides with the Rocky Mountain trench in this area. This ramp is part of a mid‐Proterozoic margin upon which the Belt‐Purcell supergroup was deposited and is preserved beneath the trench. During Mesozoic contraction, the basal detachment of the Foreland belt closely followed the craton‐cover contact across the basement ramp. Thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp. In the Eocene‐Miocene, the basement ramp and the culmination above it focused stress, reactivating the basal detachment and causing extensional faulting in the southern Rocky Mountain trench. The Rocky Mountain trench fault may be linked via the basal detachment to the Flathead fault on the east and the Eocene extensional faults that flank the Omineca belt on the west.
Mid‐Cretaceous thrusting in the southern Coast Belt, British Columbia and Washington, after strike‐slip fault reconstruction
Authors: Umhoefer Paul J. , Miller Robert B.
Published: February 2004

A major thrust system of mid‐Cretaceous age is present along much of the Coast Belt of northwestern‐North America. Thrusting was concurrent, and spatially coincided, with emplacement of a great volume of arc intrusives and minor local strike‐slip faulting. In the southern Coast Belt (52° to 47°N), thrusting was followed by major dextral‐slip faulting, which resulted in significant translational shuffling of the thrust system. In this paper, we restore the displacements on major dextral‐slip faults of the southern Coast Belt and then analyze the mid‐Cretaceous thrust system. Two reconstructions were made that use dextral faulting on the Yalakom fault (115 km), Castle Pass and Ross Lake faults (10 km), and Fraser fault (100 km). The reconstructions differ in the amount of dextral offset on the Straight Creek fault (160 and 100 km) and how much the NE part of the Cascades crystalline core expanded (30 km and 0 km) during Eocene extension. Reconstruction A produces the best match of lithotectonic units and thrust systems. Our synthesis shows that the southern Coast Belt thrust system was ≥250 – 180 km wide after thrusting. The thrust system was mainly southwest vergent but had a belt of northeast vergent back thrusts on the northeast side associated with the Tyaughton‐Methow basin, which may indicate large‐scale tectonic wedging. Thrust faults are commonly low to moderate angle, but high angle faults also occur, especially as late stage, out‐of‐sequence, structures involving plutons. The amount of thrust displacement across the system is unknown but must be at least 100 km and may be many hundreds of kilometers. Most thrusting occurred from ∼100 to ∼80 Ma and did not migrate systematically until after ∼90 Ma, when thrusting and magmatism shifted to the northeast for a few million years. Widespread thrusting occurred both near plutons and where there are no (or small) plutons, which strongly suggests that thrust faulting was caused by regional‐ to plate‐scale forces such as rapid plate convergence and/or arc‐continent collision.
Block rotations along the strike‐slip Finlay‐Ingenika fault, north‐central British Columbia: Implications for paleomagnetic and tectonic studies
Authors: Zhang Guowei , Hynes Andrew , Irving E.
Published: February 2004

Structure developed in association with oblique motions on the dextral, strike‐slip Finlay‐Ingenika fault (FIF) in the McConnell Creek area, north‐central British Columbia, is dominated by subvertical to vertical strike‐slip faults. Some of the faults cut the area into discrete, fault‐bounded blocks several kilometres wide. Stress tensors for 24 sites were inverted from the regional cleavage which predates the block‐bounding faults. The site‐mean stress tensors indicate that the fault‐bounded blocks were rotated clockwise about a subvertical axis during progressive oblique motions on the FIF. Rotation varies systematically, being maximal (58.7±3.3°) close to the FIF and minimal (0.0±1.6°) about 20 km away. Paleomagnetic samples were collected from 13 sites in the widespread Early Jurassic to Cretaceous plutonic rocks. Interpretable magnetic components of presumed Late Cretaceous age were obtained from six sites. The observed paleopole is significantly different from the Late Cretaceous reference pole for cratonal North America (CNA). After corrections for the local block rotations, the precision is much improved (Kincreased from 48 to 383), and the paleopole moves closer to the reference pole but is not coincident with it. Rotation about the Eulerian pole for the best‐fitting small circle to the Tintina trench and northern Rocky Mountain trench fault zone, however, brings the observed pole into coincidence with that for CNA and requires ∼670 km of dextral displacement on the fault zone. It is evident therefore that local structures associated with large dextral strike‐slip faults could account for at least part of the paleomagnetic disparity between some western parts of the Canadian Cordillera and CNA.
Models for relative motion of crustal blocks within the Carpathian region, based on restorations of the outer Carpathian thrust sheets
Authors: Morley C. K.
Published: February 2004

Details of the Oligocene‐present‐day motion of the Adriatic block with respect to Europe can be determined by palinspastic restorations of the outer Carpathians thrust sheets. The arcuate nature of the Carpathians poses problems for simple restorations of sections because they create converging restoration paths which require large amounts of strike‐parallel extension (40–50%). Such large amounts of extension are not seen on maps or in the field. A combination of thrust transport directions changing with time between and within thrust sheets and divergent transport directions helps minimise the arc‐parallel extension necessary (18–25%). The Pannonian basin can be treated either as an area dominated by strike‐slip motion (rigid block movements) or as an area of back arc extension. The differences in restoration geometry produced by these two scenarios help to indicate which of the two mechanisms was dominant. The consequences of the rigid block restoration indicate the model is inappropriate because it requires approximately 20° clockwise rotation of the Adriatic block during the late Tertiary. It also indicates at least 300 km strike‐slip motion distributed among the late Tertiary faults of the Dinaride‐Balkanide and Hellenide Mountain belts. The back arc extension model alleviates the the 20° clockwise rotation of the Adriatic block and requires about 200 km distributed strike‐slip motion in Yugoslavia and Bulgaria.
Deformation partitioning in a slow spreading ridge undergoing oblique extension: Mohns Ridge, Norwegian Sea
Authors: Dauteuil O. , Brun J.‐P.
Published: February 2004

Although oceanic spreading is often perpendicular to the ridge trends, in some cases the angle between these two directions can be significantly less than 90° (40°–50°). This occurs because of either a bend of the ridge trend or a change of the spreading direction. We here describe oblique spreading in the Mohns Ridge, resulting in deformation partitioning between the valley walls, which are dominantly affected by strike‐slip displacements, and the axial valley which is subject to nearly pure extension. The axial valley walls are characterized by en échelon normal faults affecting the walls, while the axial valley is affected by parallel faults grouped into oblique sets. These fault sets define different structures, horst or tilted blocks, that are regularly spaced inside the axial valley. Moreover some ridge segments mainly undergo pure extension, whereas others are affected by oblique extension. We explain this faulting pattern, including the along‐strike and transverse variations, as a consequence of depth variations of the brittle‐ductile transition.
Transition from basement‐involved to thin‐skinned thrusting in the Cordillera Oriental of southern Bolivia
Authors: Kley Jonas
Published: February 2004

The eastern flank of the Central Andes in southern Bolivia and northern Argentina is underlain by a fold‐and‐thrust belt of Neogene age. It comprises an external thin‐skinned segment with basal detachment in Silurian shales (Subandean Ranges) and an internal segment where units as deep as Precambrian metasedimentary and igneous rocks are involved in thrusting (eastern Cordillera Oriental). Between the Cordillera Oriental and the Subandean Ranges an intermediate segment (Interandean Zone) intervenes north of the Argentinian‐Bolivian border (22° S). The structure of the Interandean Zone is characterized by folded and thrust‐faulted Cambrian to Triassic strata. Balanced cross sections across the western Subandean Ranges, the Interandean Zone, and the eastern Cordillera Oriental are based on previously available data and recent field work. They suggest that the geological structure can be interpreted in terms of two major basement‐involved thrusts, here referred to as Interandean and Subandean thrusts. The more internal Interandean thrust caused the development of broad basement anticlines in the eastern Cordillera Oriental. However, at shallower levels, its displacement was accommodated by thin‐skinned folding and thrusting in the Interandean Zone. In‐sequence activation of the Subandean thrust uplifted the Interandean Zone on top of an imbricate basement sheet and caused the development of the thinskinned Subandean Ranges farther east. Published radiometric data and subsidence history of the Subandean foreland basin suggest that the Interandean thrust was active between about 10 Ma and 5 Ma. The Subandean thrust is still active. Southward displacement transfer from the Subandean thrust onto the Interandean thrust and a southward dipping lateral ramp of the Interandean thrust result in the gradual transition from the Interandean Zone north of 22° S to the Cordillera Oriental of northern Argentina. A similar but larger‐scale transfer may be involved in the southern termination of the thin‐skinned Subandean Ranges of Argentina at 23° S. Shortening in the western part of the thrust belt is of the order of 80–90 km. Total shortening in the Eastern Andean thrust belt from the undeformed foreland to the eastern margin of the Cordillera Oriental is 140–150 km.
Nonequilibrium Processes Affecting Forced Ventilation of Benzene and Xylene in a Desert Soil
Authors: Conklin Martha H. , Corley Timothy L. , Roberts Philip A. , Davis J. Hal , van de Water James G.
Published: February 2004

A series of 23 unsaturated bench scale column experiments was carried out to study grain scale processes controlling the efficiency of soil vapor extraction for removal of volatile organic compounds (VOCs) from a desert soil sample (mass fraction of organic carbon, approximately 0.001). Experiments consisted of passing VOC‐containing, humidified (>95% relative humidity) air through an unsaturated soil column until breakthrough occurred and then passing VOC‐free air through the column until the VOC removal was complete. Effluent VOC concentrations were measured at frequent intervals. Experimental variables included VOC (benzene and p‐xylene), soil moisture content (0 to 18% by volume), concentration of VOC in the inlet air stream, and interstitial velocity (0.2 to 0.6 cm s−1). Experimental breakthrough curves were modeled with an advection‐dispersion model coupled with a first‐order rate equation to describe mass transfer between phases. At 18% moisture, slow water‐to‐air mass transfer of benzene as the rate‐limiting process explains the initial removal of benzene (C/C0 > 0.1), but the desorption profiles strongly suggest that a slower process (intraparticle pore diffusion) becomes the dominant process at longer times. The breakthrough curves for p‐xylene at 10 and 18% moisture also suggest that two sequential processes control removal of p‐xylene. The time constant for the faster process is comparable to that determined for the initial rate‐ imiting process for benzene at 18% moisture. The rate for the second process is the same order of magnitude for p‐xylene at 10 and 18% and benzene at 18%. Calculations of the mass distributions of the VOCs among air, water, and soil strongly suggest that the more hydrophobic p‐xylene tends to accumulate at the air‐water interface (up to 60% of the total mass) and that benzene primarily accumulates in the aqueous phase.
Transport of Soil Bacteria Controlled by Density‐Dependent Sorption Kinetics
Authors: Bengtsson Göran , Lindqvist Roland
Published: February 2004

Transport of bacteria in a soil was delayed relative to the mobile pore water by rate‐limited rather than instantaneous sorption to soil particles. Desorption rates were found to be faster than adsorption rates, and sorption kinetics were dependent on the density of bacteria and on the flow rate of water. Sorption kinetics were revealed by a combination of model descriptions of the mass balance and sorption kinetics of a groundwater bacterium entering a porous system and laboratory experiments using a stirred flow chamber and a soil column. The flow chamber was designed and operated to (1) distinguish instantaneous equilibrium reactions from time‐dependent reactions, (2) estimate sorption rate coefficients at different cell densities and water velocities, and (3) obtain rate coefficients that could be used to predict the breakthrough of bacteria in a soil column under saturated, continuous flow conditions. Sorption rates in the stirred flow chamber were expressed by three rate laws, first order, Langmuir, and second order, and the transport of bacteria through the soil column was described and predicted by a model assuming a mixture of instantaneous and time‐dependent sorption reactions and, alternatively, by a kinetic model. The sorption rate was primafly dependent on the density of sorbed bacteria, but the desorption rate was, in addition, influenced by the density of cells in the aqueous phase. Rate coefficients obtained in the flow chamber could be used to accurately predict transport of bacteria in the soil column at the corresponding density and flow rate, but extrapolations are complicated by the nonlinear relationship between density and sorption rate. Timescales for reaching equilibrium estimated from the rate coefficients were in the range from 10–15 hours to several days, longer for low densities and velocities.
Analytical Models for One‐Dimensional Virus Transport in Saturated Porous Media
Authors: Sim Youn , Chrysikopoulos Constantinos V.
Published: February 2004

Analytical solutions to two mathematical models for virus transport in one‐dimensional homogeneous, saturated porous media are presented, for constant flux as well as constant concentration boundary conditions, accounting for first‐order inactivation of suspended and adsorbed (or filtered) viruses with different inactivation constants. Two processes for virus attachment onto the solid matrix are considered. The first process is the nonequilibrium reversible adsorption, which is applicable to viruses behaving as solutes; whereas, the second is the filtration process, which is suitable for viruses behaving as colloids. Since the governing transport equations corresponding to each physical process have identical mathematical forms, only one generalized closed‐form analytical solution is developed by Laplace transform techniques. The impact of the model parameters on virus transport is examined. An empirical relation between inactivation rate and subsurface temperature is employed to investigate the effect of temperature on virus transport. It is shown that the differences between the two boundary conditions are minimized at advection‐dominated transport conditions.
Fluid Flow in Fault Zones: Analysis of the Interplay of Convective Circulation and Topographically Driven Groundwater Flow
Authors: López Dina L. , Smith Leslie
Published: February 2004

High‐permeability faults, acting as preferential pathways for fluid migration, are important geological structures for fluid, energy, and solute transport. This paper examines the interaction of thermally driven convective circulation in a steeply dipping fault zone and groundwater flow through the surrounding country rock that is driven by a regional topographic gradient. We consider a geometry where a fault zone with a homogeneous, isotropic permeability is located beneath a narrow valley in a region with substantial topographic relief. System behavior is best characterized in terms of the large‐scale permeabilities of the country rock and the fault zone. Using three‐dimensional numerical simulations, we map in permeability space four fluid flow and heat transfer regimes within a fault zone: conductive, advective, steady convective, and unsteady convective. The patterns of fluid flow and/or heat transfer are substantially different in each of these regimes. Maximum discharge temperatures can also be plotted in permeability space; the maximum discharge temperature in the advective regime is in general lower than that in the steady convective regime. A higher basal heat flux expands the convective regime in permeability space, as does a greater fault depth. Higher topographic relief on the regional water table compresses the convective regime, with the advective regime suppressing convective circulation at lower country rock permeabilities. If convective cells with aspect ratios close to 1 cannot form, the steady convective regime is smaller in permeability space, and the boundary between steady and unsteady convection occurs at lower values of fault zone permeability. At low country rock permeabilities a water table gradient along the surface trace of the fault of approximately 0.3% suppresses convective cells; at higher country rock permeabilities, convection can be suppressed by smaller gradients on the water table.
Mapping Hydraulic Conductivity: Sequential Conditioning with Measurements of Solute Arrival Time, Hydraulic Head, and Local Conductivity
Authors: Harvey Charles F. , Gorelick Steven M.
Published: February 2004

We present a method for estimating the spatial pattern of aquifer hydraulic conductivity from three types of measurements: solute arrival time quantiles, hydraulic heads, and direct local measurements. Results indicate that arrival times and heads provide different, but complementary, information about the large features of the conductivity field which serve as flow paths and barriers. We compare the information provided by head and arrival time measurements by plotting the correlation between measurement and conductivity over the entire domain. We also compare the value of head and arrival time data by estimating the conductivity with only one type of data, and by incorporating the different data types into the estimation procedure in different sequences. Using quantiles of arrival time, rather than concentrations, has three advantages: (1) Arrival times are independent of the amount of dilute solute introduced into the aquifer. (2) Under some conditions, by measuring the median arrival time, the accuracy of the conductivity field estimate is not degraded by poor knowledge of the local dispersivity. (3) The estimation procedure is greatly simplified by relying on a single quantile to represent the critical information contained in a breakthrough curve that is constructed from many concentration measurements. For the case examined here, incorporating arrival time quantiles that describe the tails of the breakthrough curves did not significantly improve our estimate of the conductivity field.
On the Use of Aggregate Data to Evaluate Groundwater Protection Policies
Authors: Wu JunJie , Segerson Kathleen
Published: February 2004

This paper presents a conceptual framework for identifying the bias that results when aggregate (county‐level) data are used to analyze the impact of agricultural activities on groundwater contamination. The use of aggregate data is often necessary due to dimensionality problems or data limitations in national or even regional analysis. With aggregate data, detailed site characteristics are not considered. It is shown that ignoring detailed site characteristics leads to five potential sources of bias. The factors that determine the signs and magnitudes of these biases are examined. Through an application to the state of Wisconsin, three of the five sources of the bias are quantified. The implications of the results regarding the use of aggregate analyses in the design of groundwater protection policies are also discussed.
Influence of Leaky Boreholes on Cross‐Formational Groundwater Flow and Contaminant Transport
Authors: Lacombe S. , Sudicky E. A. , Frape S. K. , Unger A. J. A.
Published: February 2004

Abandoned and improperly sealed boreholes, monitoring wells, and water supply wells are common features at many contaminated sites. These features can act as conduits that transmit contaminants between aquifers separated by otherwise continuous aquitards. In this work the leaky boreholes are represented as highly conductive one‐dimensional line elements superimposed onto a mesh of three‐dimensional finite elements representing the porous medium. Simulation results are presented for a series of scenarios involving a simple hydrogeologic setting composed of an upper unconfined aquifer, a middle aquitard, and a lower confined aquifer. The simulations examine the effect of varying the borehole properties, the vertical hydraulic gradient across the aquitard, and the borehole location. The results show that a contaminant can rapidly migrate downward along a leaky borehole and create an extensive plume in the lower aquifer, even if the borehole is filled with aquifer sediments. If the borehole is an open feature across the aquitard, the entire plume, or a significant portion of it, that is migrating into the upper aquifer can be diverted into the lower one if the vertical hydraulic gradient across the aquitard is sufficiently strong. The peak concentration arriving at a pumping well located in the lower aquifer and the time of arrival are functions of the proximity of the leaky borehole to the pumping well and its angular offset from the central flow line passing through the surficial source. Overpressurization of the lower aquifer due to injection can overcome downward preexisting hydraulic gradients across the aquitard such that contaminants can rapidly migrate upward along the leaky borehole and cause contamination of the otherwise protected upper aquifer.
State‐Dependent Anisotropy: Comparisons of Quasi‐Analytical Solutions with Stochastic Results for Steady Gravity Drainage
Authors: Green Timothy R. , Freyberg David L.
Published: February 2004

Anisotropy in large‐scale unsaturated hydraulic conductivity of layered soils changes with the moisture state. Here, state‐dependent anisotropy is computed under conditions of large‐scale gravity drainage. Soils represented by Gardner's exponential function are perfectly stratified, periodic, and inclined. Analytical integration of Darcy’s law across each layer results in a system of nonlinear equations that is solved iteratively for capillary suction at layer interfaces and for the Darcy flux normal to layering. Computed fluxes and suction profiles are used to determine both upscaled hydraulic conductivity in the principal directions and the corresponding “state‐dependent” anisotropy ratio as functions of the mean suction. Three groups of layered soils are analyzed and compared with independent predictions from the stochastic results of Yeh et al. (1985b). The small‐perturbation approach predicts appropriate behaviors for anisotropy under nonarid conditions. However, the stochastic results are limited to moderate values of mean suction; this limitation is linked to a Taylor series approximation in terms of a group of statistical and geometric parameters. Two alternative forms of the Taylor series provide upper and lower bounds for the state‐dependent anisotropy of relatively dry soils.
A Simple Surface Water Biogeochemical Model: 2. Simulation of Selected Lacustrine and Marine Settings
Authors: Jewell Paul W.
Published: February 2004

A linear, three‐component (nitrate, phytoplankton, and zooplankton) biogeochemical model coupled to a one‐dimensional turbulence closure fluid dynamic model successfully reproduces most of the thermal and biogeochemical features of three marine settings (Bermuda Station “S” in the oligotrophic portion of the North Atlantic subtropical gyre, Ocean Weather Station “India” in the North Atlantic subpolar gyre, and the 60‐m isobath of the New York Bight) and one freshwater lake (South Bluehill Pond in Newfoundland). Model reproduction of temperature in all four settings is good. Model temperature gradients in the thermocline are somewhat steeper than those observed in the data of the three marine settings due to the inability of the one‐dimensional fluid flow model to reproduce internal waves which can be a significant source of vertical mixing in large water bodies. Nitrate and chlorophyll are predicted reasonably well by the model at Bermuda Station “S” and the New York Bight. Underprediction of surface nitrate at Station “India” is probably due to the lack of an ammonium component in the model. Model reproduction of the spring bloom productivity in the marine settings is good, although post‐bloom model productivity is generally too low. Biogeochemical simulations of South Bluehill Pond are strongly dependent on the value of the light attenuation coefficient used in the model. A relatively high attenuation coefficient (most likely representing high inorganic sediment concentrations) is necessary to accurately reproduce nitrate and chlorophyll concentrations. These results illustrate the necessity of accurately characterizing the light regime prior to using the model in lacustrine settings.
Unequal Mobility of Gravel and Sand in Weakly Bimodal River Sediments
Authors: Wathen Simon J. , Ferguson Robert I. , Hoey Trevor B. , Werritty Alan
Published: February 2004

Bed load was trapped during flood events over a 20‐month period at the lower end of the Allt Dubhaig, a small river in Scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile. The channel bed near the trap is predominantly gravel with a secondary sand mode. Total transport in each event depended mainly on peak shear stress, rather than duration over a threshold. Bed load was mainly sand in smaller events, bimodal in intermediate events, and mainly gravel in the biggest floods. Mean and maximum grain diameter both increased with peak shear stress, but in different ways. Analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility. The slight selectivity in gravel transport is consistent with previous field studies of near‐equilibrium unimodal beds and supports assumptions made in the numerical model of Hoey and Ferguson (1994), which successfully simulates the observed amount of downstream fining over the 2.5‐km upstream of the bed load trap.