Variable Dip Research Articles

Fault zone evolution and its controls on ore-grade distribution at the Jian Cha Ling gold deposit, western Qinling region, central China

The Jian Cha Ling gold deposit is sited in folded and faulted Palaeoproterozoic rocks of the uplifted Mian-Lue-Yang block, adjacent to the Mianlue suture zone within the West Qinling mineral province, along the northern margin of the South China craton. Early Mesozoic gold mineralization at Jian Cha Ling, which has a pre-mined resource of about 536,000 oz Au, is controlled by the so-called F145 fault. The fault is a generally steeply N-dipping, WNW-trending deformation zone that is the result of dislocation along bedding planes, early foliation and axial planar surfaces of regional folds. The fault zone marks the contact between serpentinized, lower greenschist-facies dunites, harzburgites and minor gabbroic units in the footwall, and a metasedimentary sequence in the hanging wall that is dominated by metadolomite, metalimestone and phyllitic schists. Brittle–ductile deformation, partitioning of strain along pre-existing zones of weakness, and the formation of intrashear zone lozenges contributed to the development of a complex fault zone geometry. Variations in both dip and strike of discrete dislocation surfaces related to oblique, sinistral–reverse movement along the F145 fault zone focussed auriferous hydrothermal fluids along three dominant structural orientations. Gold was preferentially deposited along shallowly NNE-dipping and shallowly to moderately NNW-dipping fault segments, and is also associated with shallowly WSW-dipping, dolomite-dominated vein sets. Disseminated, economic gold grades (>4 g/t Au) are restricted to the footwall ultramafic rocks to within about 5 to 10 m of the contact with the hanging wall. Gold is related to laminated, realgar- and orpiment- bearing sheeted veins and hydrothermal breccias, as well as slickolites and fault gouge. Gold-bearing vein sets are located within the relatively undeformed, ultramafic intrashear zone lozenges. Gold-related alteration is dominated by extremely fine-grained, arsenic-bearing sulphide minerals and dolomite, with additional white mica and clay minerals. The structural setting of the deposit, combined with published data on the geological evolution of the West Qinling mineral province, suggest that the Jian Cha Ling gold deposit developed in an uplifted basement block during the final phases of northward subduction and suturing of the South China craton with the South Qinling orogen along the Mianlue Suture Zone, during the changeover from a compressional to transpressional tectonic regime in Late Triassic to Middle Jurassic times.

Thermal models of flat subduction and the rupture zone of great subduction earthquakes

In subduction zones, the size of the seismogenic zone that ruptures during a great thrust earthquake may be thermally controlled. We have constructed finite element thermal models of six transects across three different subduction zones in order to determine the temperature distribution along the plate interface and to predict the size of the seismogenic zone. These models incorporate the complex plate geometries (variable dip downsection) necessary to model a flat slab subduction style. We focus on the rupture zones of the great earthquakes of Nankai (SW Japan) 1946 M8.3 and Alaska 1964 M9.2 as well as on the Cascadia margin. Subduction zone segments with moderate to steep dips exhibit rupture zones of 100–150 km downdip width, consistent with earlier elastic dislocation models and thermal models. For shallow dipping flat slab segments, our models predict larger locked zones, of 150–250 km width, in good agreement with aftershock and geodetic studies. The wider seismogenic zone predicted for flat slab segments results from an uncommonly wide, cold, forearc region, as corroborated by surface heat flow observations. A global analysis of great M > 8 interplate earthquakes of the 20th century reveals that more than a third of these events occurred in flat slab segments, whereas these segments represent only 10% of modern convergent margins. This implies substantially higher interplate coupling for flat slab segments, likely due to the increased downdip extent of the seismogenic zone, and suggests that the seismic risk near such regions may be higher than previously thought.

Transient Stress-Coupling Between the 1992 Landers and 1999 Hector Mine, California, Earthquakes

A three-dimensional finite-element model (FEM) of the Mojave block region in southern California is constructed to investigate transient stress-coupling between the 1992 Landers and 1999 Hector Mine earthquakes. The FEM simulates a poroelastic upper-crust layer coupled to a viscoelastic lower-crust layer, which is decoupled from the upper mantle. FEM predictions of the transient mechanical be- havior of the crust are constrained by global positioning system (GPS) data, inter- ferometric synthetic aperture radar (InSAR) images, fluid-pressure data from water wells, and the dislocation source of the 1999 Hector Mine earthquake. Two time- dependent parameters, hydraulic diffusivity of the upper crust and viscosity of the lower crust, are calibrated to 10 2 m 2 •sec 1 and 5 10 18 Pasec respectively. The hydraulic diffusivity is relatively insensitive to heterogeneous fault-zone permeabil- ity specifications and fluid-flow boundary conditions along the elastic free-surface at the top of the problem domain. The calibrated FEM is used to predict the evolution of Coulomb stress during the interval separating the 1992 Landers and 1999 Hector Mine earthquakes. The predicted change in Coulomb stress near the hypocenter of the Hector Mine earthquake increases from 0.02 to 0.05 MPa during the 7-yr interval separating the two events. This increase is primarily attributed to the recovery of decreased excess fluid pressure from the 1992 Landers coseismic (undrained) strain field. Coulomb stress predictions are insensitive to small variations of fault-plane dip and hypocentral depth estimations of the Hector Mine rupture.

Outcrop and behind-outcrop characterization of a late Miocene slope fan system, Mt. Messenger Formation, New Zealand

The late Miocene Mt. Messenger Formation of Taranaki, New Zealand, consists mainly of well-sorted, very fine to fine-grained thin-bedded sandstone and siltstone deposited in middle- and upper-bathyal water depths. Lithofacies are dominated by turbidite beds, comprising planar-laminated and climbing-ripple-laminated sandstone and massive siltstone. These strata were studied in a 200 m-thick, 4 km-long coastal outcrop section and in two, behind-outcrop boreholes from which Fullbore Formation Micro Imager (FMI) and Platform Express logs and core were obtained. The boreholes were 47 and 105 m deep and were drilled about 100 m behind the cliff outcrop about 150 m apart. In addition, a high-resolution seismic reflection line was shot adjacent to the coastal cliff and was tied to the updip outcrop exposures. The outcrop section displays abundant erosional cutout of beds by scour or channels that considerably limit lateral bed continuity. At a smaller scale, bed continuity is also interrupted by bioturbation structures. We interpret the outcrop section as a series of vertically stacked or shingled slope fan or channel-levee/overbank complex deposits, comparable to Mutti type III fans. Comparison of behind-outcrop dipmeter, seismic reflection profiles, and FMI data with outcrop data indicates that three main channel-levee/overbank depositional units are present. Siltstone and alternating thin-bedded sandstone and siltstone that display an upward decrease in dip magnitude and variable dip orientation on dipmeter logs are interpreted as channel fill deposits. Vertically stacked, interleaved packages of thin-bedded, typically planar-laminated sandstone (Bouma Tb divisions), climbing-ripple-laminated sandstone (Tc), and siltstone (Te) with complex upward-decreasing dipmeter patterns are interpreted as proximal channel-levee and overbank deposits. Laterally extensive interbedded Tb and Tc sandstone and Te siltstone lithofacies with low-angle dips and little variability on dipmeter logs are interpreted as distal channel-levee and overbank deposits. These three depositional settings are important to distinguish, as each exhibits different fluid-flow behaviors in an analog reservoir. Comparison of the outcrops with typical subsurface data sets as used by petroleum geologists (logs, core, seismic reflection) has provided criteria for distinguishing subtle stratigraphic features that can influence production from a subsurface reservoir analog.

Polynomial fitting + narrow reject-band f-k filtering for seismic coherent noise elimination

The paper presents a f?k filtering method to reduce coherent seismic noise. The method uses an adaptive strategy that the f?k filtering automatically adapts to lateral variations in apparent dip and amplitude of coherent noise events. We first use orthogonal polynomials to fit apparent dip and amplitude of coherent noise events. We then incorporate the instantaneous variations in apparent dip and amplitude into narrow reject-band (NRB) f?k filters which are applied in a spatially and temporally varying way across seismic traces. The method is only applied to the area where coherent noises are located and will not affect the signal that is located outside the area in the same seismic record but has the same regularity as the noise. That is, in a given window, the method removes coherent noises using the differences between noise and signal in three aspects: apparent velocity, frequency, and amplitude. The data outside the window will keep intact. The method can remove any coherent noise in prestack and poststack seismic data, which will favor following seismic processing and modification of seismic sections. The novel method has been applied to field seismic data and shows good attenuation characteristics with minimal distortion of signal.

Variations in slab dip along the subducting Nazca Plate, as related to stress patterns and moment release of intermediate‐depth seismicity and to surface volcanism

The subducting Nazca Plate shows a high degree of along‐strike heterogeneity in terms of intermediate‐depth seismicity (∼70–300 km), orientations of slab stress, and volcanism. We compile the intermediate‐depth earthquakes of South America from the Harvard Centroid Moment Tensor (CMT) catalogue to determine along‐strike dip variations, and we explore the variable level of correlation between these observations. Primary results are three‐fold: (1) Consistency among orientation of tension axes, highest level seismicity, and occurrence of volcanism is associated with steeply‐dipping regions (20°–30°). (2) Volcanism and intermediate‐depth seismicity exhibit no direct correlation, suggesting that earthquake faulting is not a necessary condition for transport of dehydrated fluids out to the mantle wedge for initiation of melting. (3) Comparison of slab thermal structures between regions with and without volcanism suggests that for certain wedge widths, the maximum temperature in the mantle wedge is higher for the former than for the latter.

The dip of the foreland monocline in the Alps and Apennines

The foreland monocline dips underneath thrust belts and accretionary wedges, both in oceanic and continental subduction zones. We present new data on the dip of the monocline in the frontal part of two orogens, the Alps and the Apennines. There is an overall difference between the dip of the relative monoclines, and there is also a strong lateral variation along both arcs. In the Alps, the regional dip varies between 0° in the remote foreland, to an average of 2–3° at the front of the thrust belt below the foredeep, to about 5° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. The regional dip of the monocline in the Apennines has an average of 4–5° at the front of the thrust belt below the foredeep, to about 10° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. There are areas where the dip exceeds 20°. The Apennines though topographically lower than the Alps present higher monocline dips and a deeper foredeep. Moreover, there are variations in the dip of the monocline moving along the strike of the two belts: the low values coincide with Permian–Mesozoic inherited horsts, whereas the steeper values correspond to basinal areas, and they usually match the salients of the thrust belt front. Within the salients the distance between thrust ramps increases. Therefore, there are two orders of mean values of the dip of the foreland monocline, the first at the orogen scale (more than 1000 km wavelength), the second at the regional scale (100–200 km wavelength) within the single orogen. Lateral variations in the lithospheric buoyancy due to the inherited Mesozoic stretching may explain the second order variations in foreland dip, but not the first order mean values which seem to be more sensitive to the geographic polarity of the subduction rather than to the lithospheric composition which is rather similar in the Alpine and in the central-northern Apennines slabs.

Significance of systematic changes in crenulation asymmetries within metasediments across the Ogcheon Supergroup in the Goesan area, southern Korea

Garnet bearing metasediments from the Ogcheon Supergroup in the Goesan area, southern Korea, preserve evidence of several earlier deformation and metamorphic events, previously unrecognized. This study, based on a 3-D thin section observation of the rock, has revealed six distinctive planar structures including five foliations and micro-fold axial planes inside the porphyroblasts (S1 to S5) and five foliations in the matrix (S2 to S6), besides weak structures locally overprinting some of these matrix foliations. The rocks studied experienced early formation of a shallow foliation during D1, succeeded by NE-SW shortening (D2), vertical shortening towards the NE-SW (D3), NW-SE shortening (D4) that developed the regional NE strike, vertical shortening (D5) causing the variable dips of early planar structures, E-W shortening (D6) causing local northwards rotation of the NE structures and some younger weak deformation.

Taphonomy of the Late Cretaceous dinosaur-bearing beds of the Laño Quarry (Iberian Peninsula)

The Laño quarry, located in the northern part of the Iberian Peninsula (Basque–Cantabrian Region), has yielded a diverse continental vertebrate assemblage from the Late Cretaceous (late Campanian to early Maastrichtian). The fossil assemblage consists of almost 40 species, including bony fish, amphibians, lizards, snakes, turtles, crocodilians, dinosaurs, pterosaurs and mammals. Laño is the most productive European vertebrate locality hitherto known from the Late Cretaceous. The sedimentology of the Laño section shows that the three associated fossiliferous beds (known as L1A, L1B and L2) were deposited in an alluvial system composed primarily of fluvial sands and silts. The sedimentary structures are consistent with channel areas within an extensive braided river. The L1A level was excavated in detail and mapped following a square-metre grid system. The main features of the multispecies vertebrate assemblage are as follows: wide range of body size (including microfossils); large proportion of adult individuals; rare articulated skeletal parts; size-sorting and dominance of small elements; high spatial density; long bones showing some preferential orientation; variable dip; variable degree of breakage and large proportion of splintering; wide range of weathering and abrasion; no evidence of predatory activity or chemical alteration. On the basis of these features, we suggest that the Laño association lies between the taphonomic modes for attritional vertebrate assemblages in fluvial channels, though it tends more toward the channel-fill than the channel lag mode. The L1A sample of Laño is a heterogeneous assemblage of elements ranging from isolated bones and teeth to articulated skeletons with allochthonous and para-autochthonous characteristics. It consists of a mixture of vertebrates from different palaeocommunities, and includes aquatic (or semi-aquatic) elements and remains transported from a variety of habitats on the alluvial plain. The dinosaur bones are commonly broken and show a greater degree of abrasion than those of freshwater vertebrates, indicating that they may be allochthonous. However, actinopterygians, amphibians, pleurodiran turtles and eusuchian crocodilians are interpreted as being para-autochthonous. Palaeobatrachid frogs, bothremydid turtles and alligatoroid crocodilians are the most relatively abundant vertebrates of the Laño fossil assemblage. The occurrence of pelomedusid turtles and crocodilians is indicative of an intertropical, warm climate.

Marked along-strike variations in dip of normal faults—the Lokichar fault, N. Kenya rift: a possible cause for metamorphic core complexes

The Lokichar fault is a major boundary fault in the northern Kenya rift, mapped from seismic reflection data. The fault is a mixture of high 45–60°, low 20–45° and very low (12–20°) angle segments. The areas of least displacement (up to a maximum 10 km heave) are the very low-angle fault segments (12–20°). The southern higher angle fault segment has a maximum estimated heave of about 20 km. Initiation of normal faults at a low angle cannot be easily explained by rock mechanics theory. Common explanations for such faults include: (1) rotation of higher angle faults by the domino faulting model, (2) rotation of large-displacement faults by isostatic instability created by the faulting (rolling hinge models), and (3) reactivation of low-angle pre-existing fabrics. The Lokichar fault geometry is inconsistent with any of the above explanations. The very low-angle segments coincide with regions of intense igneous intrusive activity. Re-orientation of the stress axes from the simple Andersonian condition, could permit normal faults to form at a lower angle; this may happen around intrusive complexes or by setting up a basal shear stress between flowing and static crust. If faults associated with metamorphic core complexes were associated with marked along-strike changes in fault angle the resulting variations in footwall uplift could give rise to the antiformal metamorphic core complex geometry.

Foredeep geometries at the front of the Apennines in the Ionian Sea (central Mediterranean)

A new regional seismic section in the Ionian Sea across the Apennines belt and related foreland shows how the present foredeep geometry may be an example for interpreting discontinuous, tilted and deformed earlier basins now incorporated in the internal parts of the accretionary wedge. Onlap stratal terminations of the foredeep sediments on the foreland monocline may simulate downlap geometries once involved and tilted by back-thrusting. The geometry of the Ionian foredeep is controlled by the dip of the regional monocline, and internally by the variable dip and length of the limb of the external fold, which may be either foreland-verging or hinterland-verging. The generation of a new fold within the foredeep splits the basin into a new foredeep toward the foreland and a thrust-top basin toward the hinterland. The thrust-top basin dimension is primarily controlled by the distance between the two folds and related thrusts at its margins. The foredeep, in its overall history, is composed by a series of concave heterogeneous lenses, progressively displaced and piled up toward the foreland to the east. The formation of each sedimentary lens is controlled by the development of a new fold and the contemporaneous retreat of the regional monocline which creates new accommodation space. The complex 3D geometry of the Apennines foredeep mainly results from lateral variations of the latter parameters, as well as variations in sediment supply and eustasy.

Salt-Related Fault Families and Fault Welds in the Northern Gulf of Mexico

Salt-related faults and fault welds in the northern Gulf of Mexico are classified based on the three-dimensional geometry of the faults or welds, deformed strata, and associated salt. Kinematic or genetic criteria are not used in the classification. Only documented fault styles are considered; those styles produced by experimental or numerical modeling, but not yet observed in the Gulf, are not included. Extensional faults comprising symmetric arrays include peripheral faults, which occur at the landward margin of the original salt basin; crestal faults, which are growth faults rooted in reactive diapirs; and keystone faults, which occur at the crests of anticlines. Asymmetric arrays of normal faults are grouped according to the dominant dip direction. Faults that dip primarily basinward include roller faults, which are listric growth faults that sole into a subhorizontal salt layer; ramp faults, which extend upward from the landward margin of bulb-shaped salt stocks; and shale-detachment faults, which sole into a shale decollement that merges into a salt layer. Counterregional faults are landward-dipping asymmetric arrays that link cylindrical, basinward-leaning salt stocks. Asymmetric arrays with variable dip direction include flap faults, whose footwalls comprise diapirs with uplifted and rotated roof strata, and rollover faults, which occur at the hinges of monoclinal folds. Two families of contractional faults are described: toe thrusts, which are basinward-vergent thrusts that ramp up from a salt or shale decollement, and break thrusts, which are high-angle reverse faults that cut one or both limbs of detachment folds. Fault arrays that strike parallel to the regional dip direction are termed lateral faults. Six types of fault welds are defined: primary welds are those at the autochthonous level; roho welds are subhorizontal, allochthonous welds into which roller faults detach; counterregional welds comprise both subhorizontal and landward-dipping segments beneath growth monoclines; bowl welds are elliptical and upwardly concave; thrust welds are landward-dipping surfaces that separate repeated stratigraphic sections; and wrench welds are steep and strike parallel to the regional dip direction. Groups of geometrically classified fault families and fault welds are kinematically and genetically linked to each other and to associated salt bodies and welds. Linked fault systems can contain extensional, contractional, and strike-slip components. Extensional fault families are formed by basinward translation, subsidence into salt, or folding. Those fault families that accommodate basinward translation are balanced by salt extrusion or contractional fault families. Strike-slip fault families commonly provide hard links, although various fault components also can be soft linked. We illustrate five associations of linked fault systems that are directly related to five types of salt systems: autochthonous salt, stepped counterregional, roho, salt-stock canopy, and salt nappe.

The Andrew Formation and ‘biosteering’ — different reservoirs, different approaches

Abstract Biosteering, the provision of real-time biostratigraphic data for the drilling of horizontal wells, has developed and evolved a wide range of approaches and applications in recent years. Its value to the success of a development project has grown beyond its core role as an effective and rapid method by which to define the relative stratigraphic position of the drill bit for steering purposes. High-resolution biostratigraphy and biosteering are now being used as part of an integrated service which is being incorprated in the planning of a horizontal drilling programme and individual well paths. This paper illustrates some of this diversity of applications by describing two very different styles of biosteering from age-equivalent turbidite sands of the Andrew Formation, on the United Kingdom continental shelf (UKCS). The first example describes biosteering through thin (10–15 ft) turbidite sands from the Joanne Field. The reservoir is one of several thin allochthonous sand and limestone subunits, requiring a very detailed biostratigraphic framework to provide sufficient resolution to identify each of the subunits. The framework utilizes both in situ and derived/reworked assemblages and associations (microfacies) to identify up to 10 discrete and correlative lithostratigraphic subunits. Local dip variations, faults of subseismic resolution and thinness of sand hampered efforts to continuously remain within the reservoir during horizontal drilling operations. Well-site micropalaeontological interpretation correctly identified the position of the drill bit in relation to the reservoir sand, allowing the correct well-path deviation to be enacted. In the absence of this technique, hundreds of feet of additional non-reservoir section would have been drilled. The second example describes the Andrew Field, again comprising deep-marine turbidite sandstones of the Andrew Formation. Development of the Andrew Field has incorporated both the biosteering function of well-site biostratigraphy as well as pre-drill, high-resolution facies analysis of the microfaunas to assist in depositional modelling of the field. Microfacies analysis provides data relating to the potential sealing qualities and lateral continuity of adjacent silt and claystones, and how these may act as barriers or baffles during production. This information is utilized in the planning of the well path to optimize production through well placement and stand-off from oil-water and gas-oil contacts, and at the well site during horizontal drilling to confirm, through micropalaeontological analysis, key horizons at which to geosteer.

A laboratory model of subduction zone anisotropy

Measurements of split seismic shear-waves near Pacific subduction zones show that fast polarization directions are primarily trench-parallel in the seaward-side mantle beneath a subducting slab and variable in the mantle wedge beneath back arc basins. It is generally accepted that the azimuth of the fast polarization direction defines the average orientation of the crystallographic a-axis in olivine. Here we use laboratory experiments to determine the orientation of the olivine a-axis in the upper mantle in the vicinity of a subducting slab. A laboratory analog for olivine a-axis motion during creep deformation is developed using small cylinders (whiskers) suspended in a viscous fluid. Subduction beneath a stationary overriding plate, variable dip angles and realistic rollback and down-dip plate kinematics are examined. Plate rollback produces strong plate-parallel whisker alignments for all dip angles examined. Alignment in the wedge occurs in three layers: a layer of dip-parallel alignment above the downgoing plate, a layer of subhorizontal alignment beneath the overriding plate, and a core of intermediate orientations. The amount of alignment beneath the overriding plate and the wedge core decreases with increasing dip angle. Our results indicate that olivine a-axis orientation in the seaward-side mantle is controlled by the amount of slab rollback, and that orientations in the mantle wedge depend upon slab dip angle.

Abstract: Fault Mapping and Reservoir Characterization from Amplitude and Dip/Azimuth Extraction in Ship Shoal 274/293 Field

ABSTRACT The Shell Offshore, Inc. reservoirs located in Ship Shoal blocks 274 and 293 produce from fault-compartmentalized sand intervals. Faulting associated with salt tectonics controls hydrocarbon type and location. The A-J reservoirs are a series of sand intervals each containing several smaller sand layers. Sand groups D, F and G were mapped using the brightest negative amplitude corresponding with the same sand interpreted on well data. Amplitude extraction shows a series of lineaments separating bright producing sands from dim water bearing sands and shale. Lineaments correspond to NE-SW striking faults that appear to control compartmentalization of the reservoir. Combining dip/azimuth plots of the producing intervals with velocity data from sonic logs suggests that the throw of these faults is 15-25 m. Electric log and dip variations observed in diameter data corroborate this interpretation. The abrupt change of amplitude across the smaller faults matches production variations in the field and suggests these faults seal and compartmentalize production. This interpretation is supported by production of oil updip from gas in areas where amplitudes indicate faulting. Parallelism of small-displacement faults and regional, large-displacement faults suggests genetic affinity.

Progressive evolution of a fault-related fold pair from growth strata geometries, Sant Llorenç de Morunys, SE Pyrenees

New structural-stratigraphical mapping constrains the three-dimensional kinematics and mechanisms of Eocene-Oligocene growth folding at Sant Llorenç de Morunys (NE Ebro basin, Spain). A 1 km wide sub-vertical panel of syntectonic alluvial gravels passes southwards via a highly asymmetrical growth fold-pair to shallowlydipping strata. The axial surface of the anticline comprises either continuous or en échelon segments while that of the syncline is concave and usually continuous. While converging upwards, the axial surfaces do not define growth triangles. Principal and subsidiary growth unconformities and thickness changes occur across both axial surfaces and the common limb. Dips within the common limb decrease up-stratigraphy and up-dip. Mesostructures indicate that internal deformation was ongoing during folding at all stratigraphical levels, and concentration of cleavage in the syncline indicates that this hinge was essentially fixed. Sequential restoration of three profiles shows that folds amplified principally by limb rotation but incorporated minor passive hinge migration. Particle movement vectors, generated by section restoration, are arcuate about a hinterland pinpoint. A new trishear model of fault propagation folding involving non-rigid limb rotation reproduces the rounded hinge forms, thickening geometries and limb dip variations observed. Simple kink band migration models (fixed axis and constant thickness theories) do not replicate these features.

96/05723 Characterization of coals with respect to their self-ignition tendency

The best section containing the Eocene–Oligocene boundary in Japan is a composite section exposed along the course of Kumanosawa and Yubari River in the Yubari area, central Hokkaido. The Kumanosawa section represents the upper portion of the Poronai Formation whereas the Yubari River section comprises the top level of the Poronai Formation and the conformably overlying Momijiyama Formation. The sequence is generally homoclinal with variable dips of between 12° and 60° and seems to represent a shallowing facies in a somewhat restricted inland basin of middle bathyal to outer neritic environments. This sequence is rich in molluscs, crustaceans, and benthic foraminifers whereas planktonic microfossils are rather scarce.

Geometry and Kinematics of Single-Layer Detachment Folds

Detachment folds form above, below, or above and below bed-parallel thrusts where thrust displacement is transferred into folding. Geometric and kinematic models are developed for individual single-layer detachment folds formed above the tip line of a thrust. We analyze three different detachment fold shapes (kink bands, chevron, and box detachment folds) and consider three principal detachment fold models: (1) constant limb dip where the detachment fold grows by limb lengthening, (2) constant limb length where the detachment fold grows by limb rotation, and (3) variable limb length and variable limb dip where the detachment fold grows by both limb rotation and limb lengthening. The allowance for variable forelimb thickness and excess layer-parallel shear enables us to de elop a wide range of possible detachment fold geometry. The geometric validity fields for folds with excess parallel shear are reduced when forelimb thinning takes place. Folds formed with constant limb length are likely to lock up in the initial amplification stages, whereas larger amounts of shortening can be taken up by folds formed with variable limb length. The manner in which a fold evolves affects the fracture and porosity features of the folded strata. Therefore, determining which mechanism operated to form a specific detachment fold is of particular importance in hydrocarbon exploration. The models we present are aimed at permitting more accurate geometric and kinematic interpretations of individual detachment folds. Examples of detachment folds are analyzed to illustrate the po ential benefits and limitations of the geometric model presented.

The influence of pre-existing thrust faults on normal fault geometry in nature and in experiments

Relations between normal faults and pre-existing thrust faults are classically described in terms of three basic situations: normal faults can cross-cut thrust faults; they can branch out from thrust faults at depth on a de´collement level, or they can entirely reactivate thrust planes. The mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the ‘brittle’ rocks and silicone putty an interlayered de´collement. The models underwent compression, producing thrust faults with variable dips, followed by extension. Three possible ways of interaction are described here: (a) no interaction occurs in the case of low-dip thrust faults (<32 ° ± 1 °) and normal faults are developed independently, displaying a listric geometry; (b) branching at depth on the de´collement level occurs when dip of the thrust faults reaches 32 ° ± 1°. In this case, the dip of the normal faults, whose geometry becomes planar, decreases with increasing thrust dip. We suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre-existing fault zone, where a drop in the friction coefficient is likely to occur; (c) reactivation occurs in brittle material when dip of the pre-existing fault exceeds 41° ± 1°.

The Forereef Facies of the Permian Capitan Formation: The Role of Sediment Supply Versus Sea-Level Changes

ABSTRACT Current models for steep forereef facies give a primary role to talus and rock fall processes during both sea-level highstands and lowstands. Highstand sedimentation is also believed to include abundant basinal turbidites, whereas basin starvation is likely during extensive lowstand talus formation. However, the forereef facies of the Capitan Formation (Permian, west Texas and New Mexico) largely lacks the characteristic matrix-poor carbonate breccias formed by talus processes and provides an alternative model appropriate to carbonate margins that produce a wider mixture of coarse- to fine-grained debris. Such a margin not only has different depositional processes but can also have a significantly different response to sea-level fluctuations. The Capitan forereef can be divided into an upper, middle, and lower forereef on the basis of primary dip and lithologic variations. The upper forereef has primary dips of 30° and is composed of rudstones with minor grainstones. The middle forereef is distinguished from the upper forereef by a decrease in primary dip (to 15-30°) and a change to a mixture of rudstones and packstones with minor wackestones. Dips decrease to 510° in the lower forereef accompanied by an increase in packstones to grainstones, but rudstones are still significant. The lower forereef interfingers with packstones to wackestones and sandstones of the basin-margin facies. During sea-level highstands, the Capitan forereef was supplied with abundant very fine to coarse debris from the wide reef facies. Most of this was deposited on the upper to middle forereef by debris flows and on the lower forereef to basin by debris flows and high- to low-density turbidity currents. During the deposition of the lower Capitan only, siliciclastic sands were also present, primarily as matrix in debris-flow units. Lowstand deposits, in contrast, are almost entirely basinal siliciclastics because production of carbonate sediment was minimal. Transgressive deposits are composed of minor forereef matrix-poor debris flows and a few major debris flows with a siliciclastic matrix that extend far out into the basin. Although the large-scale stratal patterns may appear similar, a comparison of the Capitan forereef facies with the Triassic Latemar margin and the Quaternary Tongue of the Ocean reveals contrasting details, with the Capitan debris-flow-dominated and the others rock-fall-dominated. This study emphasizes the role of primary sediment production in determining the depositional processes in forereef and foreslope settings, and, as a result, the stratal patterns commonly used to interpret the history of relative sea level.