Abstract: The boundaries between pairs of adjacent fault segments within normal fault arrays define a spectrum of structures, from relay ramps where the length of overlap between the fault segments is much larger than the separation, through low aspect ratio (overlap/separation) relay ramps and ultimately to underlapping fault segments. Where fault segments underlap, transfer of displacement between them is accommodated by a connecting monocline. When displacement increases and a through-going fault forms, relay ramps are preserved as fault-bounded zones of elevated bed dip and monoclines are preserved as areas of normal drag. Therefore, the orientation and magnitude of bed dips within and adjacent to a fault zone, and the numbers of segments seen on a cross-section through it, depend largely on the aspect ratios of relay ramps in the initial fault array. The aspect ratio of relay ramps varies between different fault systems. An analysis of the geometry of 512 relay ramps from 13 different fault systems suggests that the main controls on aspect ratio are the strength of the sequence at the time of faulting and the underlying structure.

Abstract As hydrocarbon-prone basins mature through time, stratigraphic traps become increasingly important as hosts for yet-to-find reserves. Explorationists strive to reduce the uncertainty in reservoir distribution and quality, but considerable complications exist in the evaluation of stratigraphic traps owing to the inextricable links between stratigraphy, trap definition and their subsequent risking. This study quantifies the relationships that exist between reservoir geometries and the rates of reservoir property degradation in a turbidite sandstone pinch-out zone. The investigation focuses on the Paleocene Forties Sandstone Member of the Everest and Arran fields of the East Central Graben of the UK North Sea. We utilized standard seismic interpretation techniques and integrated stratigraphic and petrophysical analysis of wireline log data to map deep-water turbidite sandstone terminations and develop a predictive model for reservoir property changes close to the feather edge. The Forties Sandstone Member thins systematically up on to a palaeoramp on the eastern basin margin of the Central Graben. Results reveal that the net reservoir sandstones pinch out after the turbidite flows had traversed 5 km across the palaeoramp. The gross interval is predicted to completely terminate 6.4 km up the palaeoramp. The reservoir properties decrease in concert with the thinning trend in the wedge zone as a function of the interaction of palaeotopography and the hydraulics of the decelerating flows. The inclination of the counter-regional slope is considered to be a key controlling factor that determines the rate of thinning and thus the termination position of the sandstones and their concomitant reservoir property decline. The results of this study demonstrate that characterization of pinch-outs into distinct zones based on a palaeotopographic template can be of utility in stratigraphic and combination trap definition. This work also has wider implications for prospect risking, volumetric analysis, the population of properties and geological modelling of stratigraphic traps.

Abstract The impact of geometric uncertainty on across-fault flow behaviour at the scale of individual intra-reservoir faults is investigated in this study. A high resolution digital elevation model (DEM) of a faulted outcrop is used to construct an outcrop-scale geocellular grid capturing high-resolution fault geometries (5 m scale). Seismic forward modelling of this grid allows generation of a 3D synthetic seismic cube, which reveals the corresponding seismically resolvable fault geometries (12.5 m scale). Construction of a second geocellular model, based upon the seismically resolvable fault geometries, allows comparison with the original outcrop geometries. Running fluid flow simulations across both models enables us to assess quantitatively the impact of outcrop resolution v. seismic resolution fault geometries upon across-fault flow. The results suggest that seismically resolvable fault geometries significantly underestimate the area of across-fault juxtaposition relative to realistic fault geometries. In turn this leads to overestimates in the sealing ability of faults, and inaccurate calculation of fault plane properties such as transmissibility multipliers (TMs).

The late Cenozoic extension in the Rio Grande rift of north-central New Mexico was predominantly accommodated by the north-south–trending Pajarito and Sangre de Cristo normal faults and the intervening east-northeast–striking predominantly strike-slip Embudo fault. Using this segment of the rift as our primary example, we have analyzed a series of three-dimensional nonlinear elastic-plastic finite-element models to assess the role of mechanical interactions between pairs of en echelon rift-scale listric normal faults in the evolution of intervening relay zones. The model results demonstrate that under orthogonal extension and an overall plane-strain deformation, relay zones may evolve in a three-dimensional strain field and along non-coaxial strain paths. The extent of non-plane strain and non-coaxial deformation depends on the fault overlap to spacing ratio, the relative orientations of the bounding faults, and the structural position within the relay zone. The model-derived minimum compressive stress vectors within the relay zone are oblique to the regional extension direction throughout the deformation. Within the Rio Grande rift of north-central New Mexico, the occurrence of northerly striking Neogene faults suggestive of east-west extension in the Española and the San Luis Basins, geographic variations in the vertical-axis rotations from paleomagnetic studies, and secondary fault patterns are consistent with the near-surface variations in the strain field predicted by the model. The model suggests that interaction between the Pajarito and the Sangre de Cristo faults may have played a major role in the evolution of this segment of the rift.

We investigated a Plio-Pleistocene alluvial succession in the Albuquerque Basin of the Rio Grande rift in New Mexico using geomorphic, stratigraphic, sedimentologic, geochronologic, and magnetostratigraphic data. New 40 Ar/ 39 Ar age determinations and magnetic-polarity stratigraphy refine the ages of the synrift Santa Fe Group. The Pliocene Ceja Formation lies on the distal hanging-wall ramp across much of the Albuquerque Basin. The Ceja onlapped and buried a widespread, Upper Miocene erosional paleosurface by 3.0 Ma. Sediment accumulation rates in the Ceja Formation decreased after 3.0 Ma and the Ceja formed broad sheets of amalgamated channel deposits that prograded into the basin after ca. 2.6 Ma. Ceja deposition ceased shortly after 1.8 Ma, forming the Llano de Albuquerque surface. Deposition of the Sierra Ladrones Formation by the ancestral Rio Grande was focused near the eastern master fault system before piedmont deposits (Sierra Ladrones Formation) began prograding away from the border faults between 1.8 and 1.6 Ma. Widespread basin filling ceased when the Rio Grande began cutting its valley, shortly after 0.78 Ma. Although the Albuquerque Basin is tectonically active, the development of through-going drainage of the ancestral Rio Grande, burial of Miocene unconformities, and coarsening of upper Santa Fe Group synrift basin fill were likely driven by climatic changes. Valley incision was approximately coeval with increased northern- hemisphere climatic cyclicity and magnitude and was also likely related to climatic changes. Asynchronous progradation of coarse-grained, margin-sourced detritus may be a consequence of basin shape, where the basinward tilting of the hanging wall promoted extensive sediment bypass of coarse-grained, margin-sourced sediment across the basin.

Orphans are exotic, internally complex, detached duplexes within windows of far-traveled thrust sheets. They were amalgamated to the base of the overriding sheet as allochthonous structures. Internal stratigraphy differs significantly from both super- and subjacent rocks of the enveloping thrust, thus indicating that they originated from a distal ramp location where the stratigraphy was intermediate between that of super- and subjacent rocks. They characteristically occur in windows elongated perpendicular to direction of thrust transport. These complex, antiformal stack types of duplexes include a bounding window-roof fault within the overriding thrust sheet that is antiformal and commonly overturned forelandward, whereas the basal fault of orphans is the smoothly curved primary thrust of the overriding sheet in which they are encapsulated. Within an orphan, structurally higher elements are typically part of an inverted sequence of strata that were duplexed after being overturned and may now be antiformal, thus conforming with the antiformal roof fault. Structurally lower elements are typically part of an upright sequence of the same orphan strata and may be anticlinal. The presence of the same strata in the lower upright and higher inverted elements of an orphan suggests derivation from the overturned footwall syncline of a distal ramp that was detached from the footwall and amalgamated to the overriding thrust sheet. Orphans, which individually contain internally different stratigraphic sequences, may be stacked together. Orphans with older strata lie toward the hinterland and those with progressively younger strata lie successively toward the foreland, suggesting their derivation from successive dismembered footwall ramps separated by intervening flats as the thrust climbed upward. Recognition of orphans has serious implications for balancing cross sections and modeling original fold-fault geometry. Orphans result from structural modification of footwall ramps with overturned footwall synclines, the modification of which changes the structure to apparent fault-bend folds through ramp-smoothing processes that involve reducing ramp angle through dismemberment and removal of the footwall syncline.

Basement faults in the southern Appalachian–Ouachita footwall and foreland include crustal-scale rift and transform elements of the late Proterozoic–Cambrian Iapetan rifted margin of southern Laurentia, synrift intracratonic basement fault systems in rift-parallel and transform-parallel orientations, and down-to-basin basement faults in late Paleozoic foreland basins. Late Paleozoic emplacement of allochthons accommodated a sinuous trace, mimicking the embayments and promontories of the Iapetan continental margin. Late Paleozoic tectonic loading reactivated synrift intracratonic faults, and either reactivated or initiated down-to-basin fault systems in foreland basins. Basement faults in the orogenic footwall localized thin-skinned thrust ramps, demonstrating interplay of causes and effects in the interactions between basement faults and the southern Appalachian–Ouachita orogen.

The Lost Basin Range in the eastern Lake Mead domain consists of Proterozoic rocks that bound the west side of the Grand Wash Trough. Exhumation of the Proterozoic rocks of the Lost Basin Range occurred from ca. 18 to 15 Ma based on seven apatite fission-track ages that range from 20 to 15 Ma. The Lost Basin Range fault lies along the west side of the Lost Basin Range and steps to the east to the southern end of the Wheeler fault, which then runs north for 60 km, where it joins the Grand Wash fault. The geometry of the southern Wheeler–Lost Basin Range fault system is that of a relay ramp between two, west-dipping, high-angle normal faults. The intervening area of the fault step over, Gregg Basin, is interpreted as a relay ramp basin. New interpreted ages from stratigraphic units on the north and east sides of the Lost Basin Range integrated with existing structural data from the eastern Lake Mead domain reveal that faulting, sedimentation, and tilting of hanging-wall and footwall blocks along the southern Wheeler–Lost Basin Range fault system began by 15.3 Ma. Sedimentation continued until after 13 Ma along the southeastern Lost Basin Range, while the age of continuing sedimentation in Gregg Basin is poorly constrained. A paleocanyon in the footwall of the southern Wheeler fault filled with conglomerate and minor breccia between ca. 15.3 and ca. 14 Ma and then overtopped to the south to cover the Paleozoic rocks of south Wheeler Ridge. The Paleozoic strata of the south Wheeler Ridge area tilted east 20°–30° more than the Miocene strata that overlie them, and therefore this tilting occurred before ca. 14 Ma. Upward-decreasing (fanning) bedding attitudes in the overlapping Miocene conglomerate indicate that Paleozoic strata were being tilted along with the Miocene strata by ca. 14 Ma. Gentle (5° and less) east dips in the lower beds of the Hualapai Limestone above and east of the paleocanyon suggest that most tilting in the western Grand Wash Trough ceased by ca. 11 Ma. The lower conglomerate of Gregg Basin lies below, and interfingers with, the limestone of Gregg Basin, which is undated but correlates with the 11–7 Ma Hualapai Limestone in the adjacent Grand Wash Trough. The syncline in upper Gregg Basin strata is linked spatially to the Wheeler and Lost Basin Range faults and indicates that these faults were likely active at 11–7 Ma. The two faults appear to cut the Gregg Basin limestone, and therefore post–7 Ma fault activity at lower rates is likely.

The North American Cordillera in Canada and Alaska has been investigated through coincident and coordinated geological, geochemical, and geophysical studies along three corridors: (1) the Lithoprobe Southern Cordillera transect, (2) the ACCRETE and Lithoprobe Slave-Northern Cordillera Lithospheric Evolution (SNORCLE) transects, and (3) the Trans-Alaska Crustal Transect (TACT) program. Seismic-reflection and refraction experiments are integral to these studies and contribute to lithospheric-scale models that enable orogen-parallel comparisons to be made. Primary observations include three points: (1) Outward-verging, crustal-scale décollements are characteristic features of the orogen. The three trans-Cordillera transects exhibit decoupling zones that dip away from the Foreland belt to the lowermost crust or Moho. These inboard décollements above an indentor or cratonic backstop extend 500–600 km downdip in the Canadian Cordillera and 250 km downdip in the Alaskan Cordillera. The active subduction megathrusts form opposing décollements and generate structures in the overriding crust that mirror those above the facing intracrustal ramps. (2) Oblique convergence resulting in significant transpressional, transtensional, and orogen-parallel motion has yielded four major transcurrent fault systems that penetrate the entire crust and are associated with tectonic boundaries. (3) Beneath the entire Canadian Cordillera, the Moho remains remarkably flat and shallow despite the variety of ages, terrane compositions, and tectonomagmatic deformations spanned by the seismic corridors. These observations indicate that the Moho is an active, near-solidus, deformation zone that represents a young, re-equilibrated crust-mantle boundary. Beneath Alaska, crustal roots are observed over the subduction zone and at the indentor wedge, but the interior of the orogen also exhibits thin crust.

Multiple levels of frontal ramps and detachment flats accommodate tectonic shortening in contrasting deformation styles at different levels in a mechanically hetero geneous stratigraphic succession in a foreland thrust belt. The late Paleozoic Appalachian thrust belt in Alabama exhibits a balance of shortening in contrasting deformation styles at different stratigraphic levels. The regional décollement is in a weak unit (Cambrian shale) near the base of the Paleozoic succession above Precambrian crystalline basement rocks. Basement faults, now beneath the décollement, controlled the sedimentary thickness of the Cambrian shale and the location of high-amplitude frontal ramps of the regional stiff layer (Cambrian-Ordovician massive carbonate); shortening in a mushwad (ductile duplex) from thick Cambrian shale is balanced by translation of the regional stiff layer at a high-amplitude frontal ramp above a basement fault. A trailing, high-amplitude, brittle duplex of the regional stiff layer has a floor on the regional décollement and a roof that is also the floor of an upper-level, lower-amplitude, brittle duplex. The roof of the upper-level brittle duplex is a diffuse ductile detachment below an upper-level mushwad, with which parts of the brittle duplex are imbricated. The basal detachment of the upper-level mushwad changes along strike into a frontal ramp at a location coincident with a sedimentary facies change in the weak shale unit that hosts the mushwad. The roof of the upper-level mushwad is a brittle massive sandstone. Shortening on the regional décollement is balanced successively upward through contrasting tectonic styles in successive mechanically contrasting stratigraphic units.

In this study, the outer sector of the Central Apennines of Italy and the Adriatic foreland have been analyzed in order to reconstruct the geological-structural setting and the Neogene-Quaternary evolution of the fold-and-thrust belt. Based on fieldwork data, existing geological maps, new seismic data, and well information, a regional balanced cross section has been realized. The transect, ∼140 km in length from the inner mountain belt to the Adriatic foreland, illustrates the structural style of the Apennines thrust system, the geometry of the different thrust fronts, the physiography of the foreland ramp, and the setting of the syn-tectonic basins infill. Moreover, by sequential balancing and subsequent forward modeling of the restored cross section, the migration of the contractional deformation and the tectonic evolution of the chain-foredeep-foreland system has been unraveled, and shortening rates have been calculated. The complex structural setting of the Central Apennines fold-and-thrust belt derived from this study mainly results from the interaction between an extremely thin-skinned thrust system and a thick-skinned tectonics. The former mainly affects the syn-orogenic siliciclastic foredeep deposits and generally predates the emplacement of the second one (i.e., the deeper thrust system) that crosscuts the whole sedimentary cover (i.e., Triassic-Miocene carbonates and the overlying Messinian-Pliocene siliciclastic sediments) and locally involves the basement. The uncoupling in space and time between thin- and thick-skinned tectonics strictly controls the evolution and the migration of syn-tectonic basins and influences the sequence of thrust-system propagation; the latter, with respect to the deeper stratigraphic levels, is mainly toward the foreland even if breaching thrusts are present. Moreover, spacing and location of thrust ramps are strictly controlled by preexisting discontinuities that affect the foreland ramp.