East Dipping Research Articles

Are there relict crustal fragments beneath the Moho?

The application of new data enhancement techniques on a Lithoprobe seismic reflection profile in northwest Canada has produced images of upper mantle structures that are overlain with angular discordance by the reflection Moho and are underlain by a relict Mesoproterozoic subduction zone. The reflection Moho is interpreted to postdate the underlying fabric and may thus have been superimposed on it by thermal and/or structural processes. Reflections within the mantle that are subparallel to the east dipping subduction zone are interpreted as structural fabric that formed during, and perhaps as a consequence of, subduction. Interpretation of the lithology in the upper mantle layering is uncertain, as peridotites, eclogites, mafic granulites, and/or pelitic restites can have velocities and densities appropriate for seismic mantle (or a portion of it). Nevertheless, if some layers beneath the Moho were originally in the crust, they may retain a structural memory.

Geological interpretation of a deep seismic‐reflection transect across the boundary between the Delamerian and Lachlan Orogens, in the vicinity of the Grampians, western Victoria

A deep seismic‐reflection transect in western Victoria was designed to provide insights into the structural relationship between the Lachlan and the Delamerian Orogens. Three seismic lines were acquired to provide images of the subsurface from west of the Grampians Range to east of the Stawell‐Ararat Fault Zone. The boundary between the Delamerian and Lachlan Orogens is now generally considered to be the Moyston Fault. In the vicinity of the seismic survey, this fault is intruded by a near‐surface granite, but at depth the fault dips to the east, confirming recent field mapping. East of the Moyston Fault, the uppermost crust is very weakly reflective, consisting of short, non‐continuous, west‐dipping reflections. These weak reflections represent rocks of the Lachlan Orogen and are typical of the reflective character seen on other seismic images from elsewhere in the Lachlan Orogen. Within the Lachlan Orogen, the Pleasant Creek Fault is also east dipping and approximately parallel to the Moyston Fault in the plane of the seismic section. Rocks of the Delamerian Orogen in the vicinity of the seismic line occur below surficial cover to the west of the Moyston Fault. Generally, the upper crust is only weakly reflective, but subhorizontal reflections at shallow depths (up to 3 km) represent the Grampians Group. The Escondida Fault appears to stop below the Grampians Group, and has an apparent gentle dip to the east. Farther east, the Golton and Mehuse Faults are also east dipping. The middle to lower crust below the Delamerian Orogen is strongly reflective, with several major antiformal structures in the middle crust. The Moho is a slightly undulating horizon at the base of the highly reflective middle to lower crust at 11–12 s TWT (approximately 35 km depth). Tectonically, the western margin of the Lachlan Orogen has been thrust over the Delamerian Orogen for a distance of at least 25 km, and possibly over 40 km.

Interpretation of seismic and potential field data from western New York State and Lake Ontario

Lithoprobe and industry seismic profiles have furnished evidence of major zones of easterly dipping Grenville deformed crust extending southwest from exposed Grenville rocks north of Lake Ontario. Additional constraints on subsurface structure limited to the postulated Clarendon–Linden fault system south of Lake Ontario are provided by five east–west reflection lines recorded in 1976. Spatial correlations between seismic structure and magnetic anomalies are described from both Lake Ontario and the newly reprocessed New York lines. In the Paleozoic to Precambrian upper crust, the New York seismic sections show: (1) An easterly thickening wedge of subhorizontal Paleozoic strata unconformably overlying a Precambrian basement whose surface has an apparent regional easterly dip of 1–2°. Minor apparent normal offsets, possibly on the order of tens of meters, occur within the Paleozoic section. The generally poorly reflective unconformity may be locally characterized by topographic relief on the order of ∼100 m; (2) Apparent local displacement on the order of ∼90 m at the level of the Black River Group diminishes upward to little or no apparent offset of Queenston Shale; (3) Within the limited seismic sections, there appears to be no evidence that the complete upper crustal section is vertically or subvertically offset; (4) Dipping structure in the Paleozoic strata (15° to 35°) resembles some underlying Precambrian basement elements; (5) The surface continuity of inferred faults constituting the Clarendon–Linden system is not strongly supported by the seismic data. Beneath the Paleozoic strata, the seismic sections show both linear and arcuate reflector geometry with easterly apparent dips of 15° to 35° similar to the deep structures imaged on seismic lines from nearby Lake Ontario and on Lithoprobe lines to the north. The similarity supports an extension of easterly dipping Central Metasedimentary Belt structures of the Grenville orogen from southern Ontario to beneath western New York State. From a comparison of the magnetic and gravity fields with the New York seismic sections, we suggest: (1) The largely nonmagnetic Paleozoic strata appear to contribute negligibly to magnetic anomalies. Seismically imaged fractures in the New York Paleozoic strata appear to lie mainly west of a positive gravity anomaly. The relationship between magnetic and gravity anomalies and the changes in the geometry of interpreted Precambrian structures remains enigmatic; (2) North to northeast trending curvilinear magnetic and gravity anomalies parallel, but are not restricted to the principal trend of the postulated Clarendon–Linden fault system. Paleozoic fractures of the Clarendon–Linden system may partly overlie a southward extension of the Composite Arc Belt boundary zone.

Formation Damage Control: Selecting Optimum Salinity in a Waterflooding Pilot

Abstract Waterflooding in viscous oilfields poses several problems to both the reservoir engineer and the production engineer. Despite unfavourable mobility ratios, it has been noted that injection performance often exceeds theoretically expected values. In the particular case of the waterflooding pilot project across the Cerro Fortunoso field (Malargue, Argentina), the occurrence of several water supplies of widely varying salinity coupled with the usual problems. This paper describes a pilot study designed to determine the optimal salinity of water to be injected, in order to avoid formation damage resulting from clay deflocculation through what is known in the literature as saline shock. As is clearly shown by the results of the study, abrupt changes in the salinity of injected water result in significant decreases in permeability. According to the study, the problem is minimized over certain ranges. Introduction The Cerro Fortunoso field is located in southern Mendoza, Argentina, in an environmental reservation area known as La Payunia, 135 km away from the capital city of Malargue Department (Figure 1). The field is characterized by its complex structure, which consists of a NNE-SSW striking narrow, elongated, and faulted anticline. Four major east dipping, NNE striking thrust faults are predominant. The oil bearing region belongs to the Neuquen Group, and develops along the anticline flanks. A gas cap, consisting mainly of CO2 (85 to 95% molar content), occurs in the upper part of the structure. Geologically, the field involves fluvial deposits exhibiting considerably marked tectonics with high dips (40 ° to 70 °) and intense NE-SW faulting, giving rise to compartmentalized lenticular reservoirs of poor areal continuity. These lenses are sand/shale intercalations with an average porosity in the order of 18% and mean permeability in the 50 mD range. Background There was concern about the feasibility of a secondary recovery project in the Cerro Fortunoso field, which contains a predominantly asphaltene-based, very viscous heavy oil (14 ° API). Complete information regarding the field's main features and its fluids can be found elsewhere(1). Numerical simulation of the reservoir characterization study performed in Canada with Teknica Overseas Ltd. made comple completion of the project feasible(2). The study warned about the potential negative effects of fresh water injection, due to the occurrence of smectite, illite and kaolinite. At an early stage, the most suitable area for implementation of a pilot project was studied and steps were taken to identify for potential water sources. Thus, we faced one of the main difficulties to overcome. As was described above, the field is located in an extremely desertlike environmental protection area where the nearest river (Rio Grande) is 70 km away. No lacustrine deposits or natural springs occur in the area. Water Sources The use of battery-gathered production water was initially ruled out due to low production rates, high emulsion content, and the significant treatment cost that it would require. The use of two additional water sources was also discussed, i.e., production water separated in the Los Cavaos field and production water from a nearby field which is operated by third parties.

Evolution of the West Andean Escarpment at 18°S (N. Chile) during the last 25 Ma: uplift, erosion and collapse through time

The geological record of the Western Andean Escarpment (WARP) reveals episodes of uplift, erosion, volcanism and sedimentation. The lithological sequence at 18°S comprises a thick pile of Azapa Conglomerates (25–19 Ma), an overlying series of widespread rhyodacitic Oxaya Ignimbrites (up to 900 m thick, ca. 19 Ma), which are in turn covered by a series of mafic andesite shield volcanoes. Between 19 and 12 Ma, the surface of the Oxaya Ignimbrites evolved into a large monocline on the western slope of the Andes. A giant antithetically rotated block (Oxaya Block, 80 km×20 km) formed on this slope at about 10–12 Ma and resulted in an easterly dip and a reversed drainage on the block's surface. Morphology, topography and stratigraphic observations argue for a gravitational cause of this rotation. A “secondary” gravitational collapse (50 km3), extending 25 km to the west occurred on the steep western front of the Oxaya Block. Alluvial and fluvial sediments (11–2.7 Ma) accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall, indicating further shortening between 8 and 6 Ma. Flatlying Upper Miocene sediments (<5.5 Ma) and the 2.7 Ma Lauca–Peréz Ignimbrite have not been significantly shortened since 6 Ma, suggesting that recent uplift is at least partly caused by regional tilting of the Western Andean slope.

Crustal architecture of the southern Uralides from true amplitude processing of the Urals Seismic Experiment and Integrated Studies (URSEIS) vibroseis profile

True amplitude processing of the Urals Seismic Experiment and Integrated Studies (URSEIS) vibroseis deep reflection seismic data acquired by the URSEIS consortium shows the southern Uralide crust to be composed of four major blocks with distinctive reflection characteristics. These blocks are juxtaposed along crustal‐scale boundaries. The foreland thrust and fold belt, developed from the East European craton crust, is imaged as subhorizontal to east dipping reflectivity that can be related to its Paleozoic and older tectonic history. The Moho beneath the foreland thrust and fold belt is not imaged in the vibroseis data set. The Main Uralian fault (the major arc‐continent suture) is unreflective, but its subsurface location can be inferred by the truncation of the reflection pattern of the East European craton and its contrast with that of the Magnitogorsk arc. The Magnitogorsk arc reflectivity is characterized by patchy, noncoherent to coherent reflections in the upper ∼10 – 15 km that are interpreted to be related to the arc volcanic rocks. Below this, reflectivity is diffuse, or the arc crust is transparent, and the Moho is not imaged. The East Magnitogorsk fault zone, which juxtaposes the arc against the East Uralian zone, is not imaged by the data. The upper 5 to 6 km of the East Uralian zone, corresponding to the Dzhabyk granite, is transparent. Below the granite the crust is characterised by east dipping patches of moderately coherent, high‐amplitude reflections that in the east become shallowly west‐dipping. A ∼10 km thick, west dipping band of coherent, high amplitude reflections between 12 and 35 km depth, corresponding to the Kartaly Reflection Sequence, extends beneath almost the entire East Uralian zone. The crust beneath the easternmost East Uralian zone reaches 53 km in thickness. The upper and middle crust of the Trans‐Uralian zone is characterized by a series of east and west dipping, concave upward, moderately coherent, high‐amplitude reflections. The lowermost middle and lower crust displays thin bands of west dipping, coherent, low‐amplitude reflectivity. The Moho is imaged as a sharp transition from reflective lower crust to transparent upper mantle at ∼49 km depth, and the lower crustal reflections appear to merge with it. The URSEIS vibroseis data are integrated with results from the explosion source reflection data, the wide‐angle reflection data, and the surface geology to place constraints on the crustal architecture of the orogen and on the timing of its assemblage.

Rheological control on the initial geometry of the Raft River detachment fault and shear zone, western United States

The strain, exhumation history, and field orientation of a well‐exposed shear zone and detachment fault in the Raft River Mountains of northwestern Utah, a Cordilleran metamorphic core complex, have been studied to determine the kinematics of ductile shearing and initial orientations of the shear zone and detachment fault. Mapping and strain and kinematic analysis indicate that the top‐to‐the‐east Raft River shear zone initially developed parallel to an unconformity separating Archean rocks from overlying Proterozoic quartzite and schist for at least 24 km in the shear direction. Experimental rock deformation data from lithologies similar to the Archean and Proterozoic rocks suggest the unconformity represented a significant rheological boundary at the deformation temperatures; the base of the shear zone was localized along the boundary between relatively weak quartzite above and stronger monzogranite below. An extensive thermochronological database is used to reconstruct the position of the basement unconformity in temperature‐lateral distance coordinates. The initial average dip of the shear zone and basement unconformity is estimated between 7° and 30°, assuming subhorizontal isotherms and geothermal gradients of 20°–40°C/km. The east dip of the unconformity at the onset of Miocene extension is interpreted to have resulted from late Eocene unroofing and flexure beneath a top‐to‐the‐WNW extensional shear zone in the western Raft River, Grouse Creek, and Albion Mountains. The observations from the Raft River shear zone suggest that the orientation of some midcrustal shear zones may not reflect the predicted orientation for ductile faults according to ductile failure criteria but, rather, the orientation of rheological boundaries along which deformation is localized. Furthermore, detachment faults that are superimposed on mylonite during progressive displacement and footwall unroofing may use an inherited mechanical anisotropy from the mylonite, and their orientations may not reflect the predicted orientation of shear fractures in isotropic rock. The common parallelism between detachment faults and mylonitic foliation may indicate a mechanical and kinematic preference for localization of throughgoing brittle faults parallel to preexisting mylonitic foliation. Because of this preference, studies restricted to detachment faults which lack footwall mylonite or restricted to structural levels between the breakaway and mylonitic front have more bearing on the question of the initial dip of normal‐sense shear fractures (faults) within the seismogenic crust.

Tectonic evolution of the Superior Boundary Zone from coincident seismic reflection and magnetotelluric profiles

The Superior Boundary Zone (SBZ) forms the northwestern margin of the Archean Superior craton and constitutes a tectonic foreland of the ∼1.8 Ga Trans‐Hudson Orogen. The Superior Boundary Fault (SBF) separates the SBZ from the adjacent Reindeer Zone, a collage of Paleoproterozoic juvenile intraoceanic rocks. Lithoprobe seismic reflection and magnetotelluric data were acquired along two profiles crossing the SBZ in an attempt to better constrain the deformation and crustal geometry resulting from Trans‐Hudson orogeny. Analysis and interpretation of spatially coincident regional seismic and magnetotelluric data acquired along the southern 200 km profile indicate the following: (1) the Reindeer Zone accretionary collage forms an east dipping, eastward steepening, crustal‐scale tectonic stack of moderately conductive rocks near the SBZ. (2) The SBZ is characterized at shallow depths (&lt;5–6 km) by steep to moderately east dipping reflectivity that is associated with the limbs of third generation folds (F3 and D3) and east‐side‐up shear zones. At greater depth, the SBZ crust is highly resistive and is contiguous to the east with resistive crust beneath the Superior craton proper. (3) The SBF is recognized in the subsurface as an abrupt resistivity contrast between the Reindeer Zone and the SBZ, extending subvertically to 15 km depth. (4) Moderately conductive rocks of the Reindeer Zone extend eastward for 30 km beneath the SBZ at depths of 15–45 km. Seismic reflection data from a second crossing located 100 km NE along strike indicate a similar crustal structure with some notable exceptions: (1) The SBF is recognized in the subsurface as a truncation of interpreted collisional fabrics and extends subvertically to ∼30 km depth. (2) There is no compelling evidence for the eastward continuation of Reindeer Zone lower crust beneath the SBZ. To explain the present‐day SBZ crustal structure, we propose that the nature of the SBZ evolved over an ∼200 Myr convergent margin history from a lower plate collisional thrust belt setting at &gt;1.88‐1.81 Ga, through lithospheric delamination at ∼1.82‐1.80 Ga to a steep transpressive plate boundary at 1.80‐1.72 Ga.

Frozen subduction in Canada's Northwest Territories: Lithoprobe deep lithospheric reflection profiling of the western Canadian Shield

Lithoprobe deep seismic reflection data from the northwestern Canadian Shield provide images of structures to the base of the lithosphere between the Archean Slave Province on the east and the Cordillera on the west. Mantle reflections dip eastward from the lower crust to about 100 km depth beneath the ∼1.88–1.84 Ga Great Bear magmatic arc and almost certainly represent a subduction surface associated with arc development. Where the mantle reflections flatten into the lower crust, they merge with prominent west dipping crustal reflections thus delineating a lithospheric‐scale wedge that formed as a result of Proterozoic plate convergence between the Slave craton on the east and the ∼1.84 Ga Fort Simpson terrane on the west. The crust throughout the 725 km survey is highly reflective, and the Moho remains at a constant level between about 11.0 and 12.0 s beneath both Archean craton and Proterozoic accreted rocks; the most significant Moho deflection occurs beneath the ∼1.8–0.7 Ga Fort Simpson basin where it appears to rise to about 9.0 s (about 27 km). Within the crust of the Slave Province, east dipping, low‐angle reflections near the surface beneath the Yellowknife basin may be shallow detachments, and lower crustal geometry is consistent with low‐angle imbrication during Late Archean (∼2.65 Ga) tectonism associated with development of the Yellowknife basin volcanic rocks. West of the Slave craton, accreted Proterozoic crust is characterized by gently folded upper crustal layers overlying apparent thrust duplexes above detachment surfaces that flatten near the Moho. These lower crustal rocks were likely inserted as a tectonic wedge above the Moho and beneath the Slave Province during the contractional phase (∼1.90–1.88 Ga Calderian orogeny) of the Wopmay orogen. On the western end of the profile, the Fort Simpson extensional basin has up to 20–25 km of Proterozoic (meta) sedimentary rocks and sills(?) that are younger than 1.84 Ga.

The gravity field over the Ungava Bay region from satellite altimetry and new land-based data: implications for the geology of the area

Gravity data were obtained along two transects on the southern coast of Ungava Bay, which provide continuous gravity coverage between Leaf Bay and George River. The transects and the derived gravity profiles extend from the Superior craton to the Rae Province across the New Quebec Orogen (NQO). Interpretation of the transect along the southwestern coast of Ungava Bay suggests crustal thickening beneath the NQO and crustal thinning beneath the Kuujjuaq Terrane, east of the NQO. Two alternative interpretations are proposed for the transect along the southeastern coast of the bay. The first model shows crustal thickening beneath the George River Shear Zone (GRSZ) and two shallow bodies correlated with the northern extensions of the GRSZ and the De Pas batholith. The second model shows constant crustal thickness and bodies more deeply rooted than in the first model. The gravity models are consistent with the easterly dipping reflections imaged along a Lithoprobe seismic line crossing Ungava Bay and suggest westward thrusting of the Rae Province over the NQO. Because no gravity data have been collected in Ungava Bay, satellite altimetry data have been used as a means to fill the gap in data collected at sea. The satellite-derived gravity data and standard Bouguer gravity data were combined in a composite map for the Ungava Bay region. The new land-based gravity measurements were used to verify and calibrate the satellite data and to ensure that offshore gravity anomalies merge with those determined by the land surveys in a reasonable fashion. Three parallel east-west gravity profiles were extracted: across Ungava Bay (59.9°N), on the southern shore of the bay (58.5°N), and onshore ~200 km south of Ungava Bay (57.1°N). The gravity signature of some major structures, such as the GRSZ, can be identified on each profile.

庄内堆積盆地東部における伏在断層の成長に伴う活しゅう曲の変形過程

Detailed morphotectonic and geological research shows the the Quaternary surface deformation processes associated with the thrusting in the eastern Shonai basin, northeast Japan.The Neogene and Quaternary structure in the eastern Shonai basin is dominated by a series of major north-south trending folds and thrust belts which include the Aosawa trend, the Sakata trend, and the Oishi trend from east (mountain-side) to the west (basin central-side). The Aosawa trend, which bounded the Shonai basin and the Dewa Hills, began to move in the late Pliocene. The Sakata trend and the Oishi trend, which are located at the eastern margin of the Shonai alluvial lowland, began to move in the Early Pleistocene and the Middle Pleistocene, respectively with the frontal migration of thrusting. These structures are interpreted to be fault propagation folds on a east dipping detachment fault.On the Aosawa trend, the late Pleistocene terraces are tilted with little faulting overlying the late Pliocene to middle Quaternary reverse faults. On the Sakata trend, the late middle Pleistocene deformation is similar to the underlying Neogene structure, which is characterized by asymmetrical folds with thrusts. On the Oishi trend, the late Quaternary terraces deformed as asymmetrical folding with a narrow flexure zone on the western (thrust tip side) limb overlying symmetrical early to middle Quaternary folds. These morphological figures show the general deformation processes of each structure in the eastern Shonai fold and thrust zone as follows.·In the early stage, the tip of the concealed thrust elongates to the surface, so the figurefor surface deformation changes from symmetrical folding to asymmetrical folding with a narrow flexure zone on the thrust tip side limb.·In the middle stage, the fault tip reaches near the surface, with continuing uniformdeformation composed of asymmetrical folding with surface faulting.·In the later stage, the thrusting on the fault near the surface is decreased, and gentletilting becomes to predominate on the surface.A simple dislocation model suggests that the elongation rate of the fault tip of the thrust underlying the Oishi trend is about 1 mm/yr in the late Pleistocene. This rate is similar to its vertical displacement rate.

Subduction‐related structures in the North Iberian Margin

The oblique convergence of Eurasia and Iberia since the Early Cretaceous, caused the formation of the Pyrenean intracontinental collisional orogen in the east, and progressed to continent‐ocean collision with subduction of the Bay of Biscay oceanic crust beneath the North Iberian Margin in the west. Two deep multichannel seismic profiles (IAM‐12 and ESCIN‐4), integrated with gravity modeling and other geological and geophysical data, provide the crustal‐scale architecture of this margin and its tectonic evolution during the convergence. The North Iberian Margin is modeled with a South or south‐southeast dipping oceanic crust beneath the outer part of the continental shelf. Mesozoic basins on the shelf were inverted during the Tertiary, and compressional activity continued until recent times in the ESCIN‐4 section, while a shallower, probably Neogene age basin is subjected to active recent erosion in the IAM‐12 section. In the oceanic areas, a marginal trough deepens and widens toward the east as a result of the regional east dip of the oceanic basement. The accretionary prism increases in size from west to east (18–56 km), and its internal structure and morphology varies along strike. The prism is buried by postcorivergence sediments in both sections and in the IAM‐12 section appears to have been active at least during Lutetian to Burdigalian times. The crustal‐scale structure of the North Iberian Margin is that of an arrested subduction zone in which a remnant oceanic basin was being consumed near two continental plates that collided obliquely.

Composition of the crust in the Grenville and Appalachian Provinces of North America inferred from VP/VS ratios

We use the ratios between P and S wave velocities (VP/VS), derived from seismic refraction data, to infer the composition of the crust in the Grenville and the Appalachian Provinces of North America. The crust exhibits VP/VS increasing with depth from 1.64 to 1.84; there is a clear distinction between the Grenville Province (average VP/VS=1.81) and the Appalachian Province (average VP/VS=1.73) which persists at all depths. The boundary between these provinces is east dipping extending for 100 km east of the Champlain thrust. In the Appalachian Province the increase in VP/VS ratios with depth from 1.67 to 1.74±0.02 may reflect a normal decrease of silica content in the continental crust. In the Grenville Province beneath the Central Granulite Terrane, an anomalous VP/VS ratio of 1.82±0.02 is observed extending to a depth of 10 km; this correlates with the abundance of Ca‐plagioclase in the Marcy Anorthosite. At greater depth (15–20 km), where seismic lamination and high electrical conductivity is observed, VP/VS is 1.84±0.02 and correlates with the Tahawus Complex, a layered mafic intrusion. Within the 25‐km‐thick lower crust of the Grenville Province the VP/VS is 1.84±0.02 and P‐velocity is 7.0±0.1 km/s, which are typical for plagioclase‐bearing rocks (gabbro‐norite). The high VP/VS ratio in the Grenville Province has not been reported in crust of any other age. Since the Grenville Province contains 75% of the world's known anorthosites, high VP/VS ratio is related to high plagioclase. We suggest that the composition of the Grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid‐Proterozoic.

A shortened intraplate rift system in the Proterozoic Mount Isa terrane, NW Queensland, Australia

The Leichhardt River Fault Trough of the Mount Isa terrane developed a complex extensional architecture between approximately 1800 and 1600 Ma, forming the underlying template upon which compressional structures were superimposed during the 1590 to 1500 Ma Isan Orogeny. Basin‐fill material accumulated during at least five multiphase periods of rifting and associated postrift subsidence forming a stacked succession of unconformity‐bounded sequences. Initial E‐W extension was associated with a massive magmatic event. Half graben greater than 50 km in width and of alternating asymmetry localized the extrusion of up to 4 km of continental tholeiites. Thereafter a period of N‐S extension resulted in southward tapering north tilted half graben in which synrift basaltic and siliciclastic strata accumulated. N‐S extension was followed by regional postrift subsidence and the deposition of a laterally continuous quartzite‐carbonate package. A multiphase period of E‐W to NW‐SE extension ensued during which time two unconformity‐bounded sequences accumulated. The stratal architectures of these sequences are strongly asymmetric in cross section, exhibiting a pronounced rotational thickening toward the east, consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width. Finally, a period of N‐S extension resulted in the development of E‐W trending F1 drag synclines in the highest level cover rocks. The association of angular unconformities and block‐bounding faults, E‐W trending synclines and E‐W striking faults, and the unique internal fold geometries of fault blocks suggest that many fault‐bounded blocks originated as coherent structural entities during rifting and continued to act as such during subsequent shortening.

The structure of the Rockall Trough imaged by deep seismic reflection profiling

WESTLINE is a 450 km long near-normal-incidence seismic reflection profile crossing the Rockall Trough from the Rockall Bank to the Irish coast. Beneath Rockall Bank and the Irish shelf unreflective upper crust overlies reflective lower crust and upper mantle. Crustal reflectors beneath the Irish shelf are interpreted as Caledonian. The NW slope of the trough is dissected by a series of east dipping faults forming small sediment filled half grabens. The steep SE slope is formed by the Erris Ridge. The floor of the trough is formed from rotated blocks separated by mainly east-dipping faults. Wedges of sediment fill half grabens formed by this faulting. There is no evidence for oceanic crust or a 'reflection Moho' within the trough. The floor and slopes of the trough are blanketed by c. 4 km of parallel-bedded sediments. The lower part of the parallel-bedded sequence is intruded by sills and dykes that do not extend above a minor unconformity. It is suggested that the bulk of the sediment preserved in the Rockall Trough accumulated during and following a major rifting event in the early or mid-Cretaceous.

Geochemistry and tectonic significance of late Gothian mafic dykes in the Østfold-Marstrand belt of S.E. Norway and W. Sweden

Studies of the poly-orogenic Proterozoic crust in W. Sweden and S.E. Norway suggest a crustal evolution which essentially took place during the Gothian period (ca 1.76-1.5 Ga) and under the control of east dipping subduction. Later, the crust underwent tectono-thermal reworking during the Sveconorwegian orogeny (1.1-0.9 Ga). In the intervening period, granites and tholeiitic dykes intruded an unroofed Gothian crust possibly under conditions of crustal thinning and high heat flow. This paper investigates late Gothian (ca 1.57-1.5 Ga) net-veined and mafic dykes from an area untouched by Sveconorwegian strain which provides information about late Gothian crustal conditions. The net-veined dykes consist of a quartz tholeiitic component, geochemically similar to the mafic dykes, and veins of a calcalkaline granite including hybrids. Based on field evidence and geochemistry, the net-veining granite is thought to be derived from crustal material from which it inherited a typical subduction-related signature. The moderately evolved mafic rocks are enriched in large ion lithophile elements and light rare earth elements presumably resulting from contamination by the granites, and show an affinity to present-day ocean island tholeiite and to mid-ocean ridge basalt of transitional type. The mafic rocks are thought to be extracted from an asthenospheric, possibly normal mid-oceanic ridge basalt mantle source enriched by material from a rising plume in a continental back-arc position. The back-arc model suggests that the eastwards dipping Gothian subduction under western Sweden and south-eastern Norway still continued in late Gothian time, but in a position considerably displaced to the west, and that the Gothian orogenic section drifted away from the subduction modified mantle wedge prior to dyke emplacement.

Geochronologic constraints on syntaxial development in the Nanga Parbat region, Pakistan

40Ar/39Ar data (hornblende, biotite, muscovite, and K‐feldspar) and U/Pb data (zircons) were obtained from the Nanga Parbat‐Haramosh Massif (NPHM), NW Pakistan, along three transects in the southern regions of the NPHM. We have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif. Geochronologic data show that the NPHM has experienced exceptionally high denudation and cooling rates over the past 10 m.y. U/Pb ages determined through sensitive high‐resolution ion microprobe (SHRIMP) “depth‐profiling” experiments on metamorphic zircons and conventional U/Pb monazite dates suggest that the timing of metamorphism varied across the massif. In addition, we have documented that the massif has experienced postmetamorphic, differential cooling both along and across strike. Thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rates in the south‐central regions of the massif (representing deeper crustal levels) than along the margins and northern regions of the massif. Within the Tato region, cooling following peak metamorphic temperatures of 600°–700 °C was as high as 140 °C/m.y. following partial melting of pelitic units. Biotites from this area record plateau ages of 0.9 ± 0.1 Ma. Along the Astor and Indus gorges, cooling was less rapid (approximately 70°–80°C/m.y.) following peak metamorphism as indicated by U/Pb monazite ages of 6–8 Ma and 40Ar/39Ar muscovite cooling ages of 2.2–3.4 Ma. Cooling over the last 3 m.y. occurred at rates of 100°–140 °C/m.y. The overall cooling age pattern within the massif is interpreted syntaxial growth through the development of north plunging antiforms prior to 3 Ma, followed by reverse faulting along east dipping fault zones. Along the Raikot River transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surfaces. The age pattern was disrupted at 1 Ma due to faulting along the Raikot and Tato faults. An electronic supplement of Tables A1, A2, and A3 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GUEST and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC00032, Geochronologic constraints on syntaxial development in the Nanga Parbat region, Pakistan, David M. Winslow, Peter K. Zeitler, C. Page Chamberlain, and Ian S. Williams). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $$15.00. Payment must accompany order.

Early to mid Tertiary crustal extension in the Central Alps: The Turba Mylonite Zone (Eastern Switzerland)

In the Penninic‐Austroalpine boundary zone of Eastern Switzerland a clear distinction can be made between upper and lower stacks of nappes with contrasting tectonic evolution that are separated by the Turba mylonite zone (TMZ). The Austroalpine nappes, Platta ophiolites, and Malenco‐Forno‐Lizun ophiolites belong to the upper stack. These nappes were formed by predominantly west directed, late Cretaceous shortening and imbrication. The lower stack, comprising the Suretta nappe, Schams nappes, Avers schists, and Oberhalbstein flysch units, includes Tertiary sediments and was deformed and metamorphosed in early Tertiary time. Kinematic indicators in the mylonites of the TMZ, representing the boundary between the two stacks, show a top‐to‐the‐east directed movement, which is clearly distinct from the regionally prevalent top‐to‐the‐north direction of thrusting in the Tertiary. The main foliation in the shear zone is discordant with respect to the structures of the hanging wall, and concordant to the footwall. Vitrinite reflectance in shaly sediments above and below the TMZ in the northern part indicates that the TMZ coincides with a drop in peak metamorphic temperature from about 350°C in the top of the footwall to about 280°C at the base of the hanging wall. Therefore the TMZ is interpreted as an east dipping, low‐angle normal fault. The age of its activity is bracketed between about 45 and 30 Ma, based on overprinting relations with radiometrically dated features (45–30 Ma: main foliation in the footwall, older than TMZ activity; 30 Ma: Bergell granodiorite intrusion, younger than TMZ activity). Extension along the TMZ was followed by further north‐south shortening, as indicated by postmylonitic folding of the TMZ. Normal faulting along the TMZ accommodated relative uplift of the Lepontine structural dome, a large metamorphic complex in the Central Alps. Unroofing of this dome started before 30 Ma and continued in the Miocene with activity of further normal faults situated at deeper structural levels.

Crustal structure and evolution of the Trans‐Hudson orogen: Results from seismic reflection profiling

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.

Bob West and Lopeno Fields: Structure and Stratigraphy of Two Significant Upper Wilcox Gas Fields in South Texas: ABSTRACT

ABSTRACT Bob West field (Starr and Zapata Counties, Texas) and field (Zapata County, Texas) are both large Upper Wilcox fields in South Texas. Located six miles apart and producing from the same Lopeno sand sequence, Bob West field may ultimately produce up to 1 Tcf (trillion cubic feet) and field 200 Bcf (billion cubic feet). Although the principal trapping mechanism for both fields is high side closure against down-to-the-coast faults, there are significant structural differences. Bob West field which produces primarily from one west dipping fault block, also has downthrown production on east dip. The field is bounded by large depositional growth faults to the west and east. field is highly faulted and produces from closures against lower aged faults. The field is transected by a very large growth fault system. This fault traps some, but not all of the pay sands at field. Both fields exhibit post-depositional rotation. Bob West field has been rotated basinward to the east, while field was rotated to the southeast. The greater structural complexity of field may explain why it is the smaller of the two fields. Bob West field produces from 29 sands and field from 18. Based on regional mapping these sands are interpreted to be shallow water strike oriented deposits with excellent sand correlations extending 40 miles on strike. Bob West field which was discovered in 1990, being 38 years after field, implies that the Upper Wilcox Trend of South Texas has yet to be fully explored.