Abstract: Rollover is the folding of the hanging-wall sedimentary record in response to slip on listric normal faults, and is a common feature of sediment-rich, gravity-driven tectonic provinces. Rollovers have been extensively studied by means of geometrical reconstruction, and numerical and analogue modelling. However, the detailed interaction between the kinematics of bounding listric normal faults and their hanging-wall deformation is not yet fully understood. In this study, we use 3D seismic-reflection data from the Forcados-Yokri area, western Niger Delta, Nigeria, to study the lateral linkage and landwards backstepping history of an array of listric normal faults, particularly focusing on their influence on the development and evolution of hanging-wall rollovers. Five individual, partly overlapping rollover structures have been studied with respect to their relative initiation and decay time, their spatial distribution, and their relationship to the tectonic history of their respective bounding faults. We demonstrate that the studied rollovers are highly dependent on the development of their bounding faults in terms of initiation time, lateral linkage, internal structural development and decay. Fault–rollover interaction is dynamic and changes through time depending on the temporal evolution of listric faults. Four genetic types of fault–rollover interaction were identified in this study: (1) the rotation of a rollover–crestal-collapse system, controlled by a changing lateral bounding-fault orientation during fault growth; (2) a stepwise shift of rollover–crestal-collapse systems associated with rollover abandonment, controlled by the initiation of a new fault in the footwall of an older structure; (3) a gradual shift of successive rollovers controlled by branching main faults; and (4) a general landwards and upwards migration of crestal-collapse faults within a rollover above stationary listric main faults.

Abstract: Normal faults grow via a sympathetic increase in their displacement and length (‘isolated model’) or by rapid establishment of their near-final length prior to significant displacement accumulation (‘constant-length model’). The isolated model has dominated the structural geology literature for >30 years, although some 3D seismic data-based studies support the constant-length model. Because they make different predictions regarding rift development, and earthquake size and recurrence intervals in areas of continental extension, it is critical to test these models with data from natural examples. Here we outline a range of techniques that constrain the kinematics of synsedimentary normal faults and thus test competing fault growth models. We then apply these techniques to three seismically imaged faults, showing that, in general, they grew in accordance with the constant-length model, although periods of relatively minor tip propagation and coeval displacement accumulation, characteristics more consistent with the isolated model, also occurred. We argue that analysis of growth strata represents the best way to test competing fault growth models; most studies utilizing this approach support the constant-length fault model, suggesting it may be more widely applicable than is currently assumed. It is plausible that the very early development of large faults is, however, characterized by the development of faults that, pre-linkage, grow in accordance with the isolated model; we may simply lack the data resolution, especially in the subsurface, to resolve this very early stage of fault growth.

Deployment of temporary seismic stations after the 2011 Mineral, Virginia (USA), earthquake produced a well-recorded aftershock sequence. The majority of aftershocks are in a tabular cluster that delineates the previously unknown Quail fault zone. Quail fault zone aftershocks range from ~3 to 8 km in depth and are in a 1-km-thick zone striking ~036° and dipping ~50°SE, consistent with a 028°, 50°SE main-shock nodal plane having mostly reverse slip. This cluster extends ~10 km along strike. The Quail fault zone projects to the surface in gneiss of the Ordovician Chopawamsic Formation just southeast of the Ordovician–Silurian Ellisville Granodiorite pluton tail. The following three clusters of shallow (<3 km) aftershocks illuminate other faults. (1) An elongate cluster of early aftershocks, ~10 km east of the Quail fault zone, extends 8 km from Fredericks Hall, strikes ~035°–039°, and appears to be roughly vertical. The Fredericks Hall fault may be a strand or splay of the older Lakeside fault zone, which to the south spans a width of several kilometers. (2) A cluster of later aftershocks ~3 km northeast of Cuckoo delineates a fault near the eastern contact of the Ordovician Quantico Formation. (3) An elongate cluster of late aftershocks ~1 km northwest of the Quail fault zone aftershock cluster delineates the northwest fault (described herein), which is temporally distinct, dips more steeply, and has a more northeastward strike. Some aftershock-illuminated faults coincide with preexisting units or structures evident from radiometric anomalies, suggesting tectonic inheritance or reactivation.

The Aftershock Imaging with Dense Arrays (AIDA) project recorded 12 days of high-density seismic array data following the 23 August 2011 Mineral, Virginia (USA), earthquake. AIDA utilized short-period, vertical-component seismographs at 201 locations to record closely spaced data that would reduce spatial aliasing. Interstation correlation enabled a detection threshold between magnitude −1.5 and −2. A joint hypocenter and velocity inversion algorithm was applied to compressional and shear wave arrival times for 300 of the larger events. Traveltime misfits were minimized using a constant velocity of Vp = 6.2–6.25 and Vs = 3.61–3.63. Hypocenter location error estimates for this range of velocities are ~100 m. Little to no three-dimensional variation exists in the seismic velocity of the upper crust, consistent with the aftershock zone being within a single crystalline rock terrane. The hypocenter locations define a 1–2-km-wide cloud with a strike of ~029° and dip of ~53°E, consistent with the focal mechanism of the main shock. The cloud bends ~5° along strike and has a slightly shallower dip angle below ~6 km depth, indicating a broad, complex fault zone with a slightly concave shape. This study shows that seismic arrays comparable to those used in controlled-source seismology can be successfully applied to aftershock sequences, and that dense array data can produce high-resolution information about earthquake rupture zones.

Abstract The authors use mesoscale structures and existing 1:24,000 scale geologic maps to infer the locations of four macroscale NNW-striking blind normal faults on the northwest flank of the Nashville dome ~30 km south of downtown Nashville. The Harpeth River fault zone has an across-strike width of ~6 km, and, from west to east, includes the Peytonsville, Arno, McClory Creek, and McDaniel fault zones. All of the fault zones are east-side-down except for the west-side-down Peytonsville fault zone. Mesoscale structures are exposed within each fault zone and are observed at three stops along Tennessee State Route (S.R.)-840 and at an additional stop 1.8 km south of the highway. These structures include minor normal faults (maximum dip separation 3.8 m), non-vertical joints, and mesoscale folds. No faults are depicted on existing geologic maps of the zone, but these maps reveal macroscale folding of the contact between the Ordovician Carters Formation and the overlying Hermitage Formation. The authors use the orientation and amplitude of these folds to constrain the orientation and length of the inferred blind fault zones and the amount of structural relief across the zones. The longest fault zones are the Arno (13.2 km long) and McDaniel (11.6 km) fault zones, and the amount of structural relief across these zones peaks at 27 m and 24 m, respectively. The authors also use existing geologic maps to hypothesize that a second east-side-down blind normal fault zone (Stones River fault zone) is located ~27 km northeast of the Harpeth River fault zone. The authors interpret non-vertical joints at one stop as fault-related, and they interpret joints at a second stop as related to a hanging wall syncline. Both of these stops are within 4 km of S.R.-840.

The dynamics of the erosive central Andean forearc vary significantly along strike. In northern Chile at 20°S–27°S, and particularly at 22°S–25°S, the forearc in the Coastal Cordillera has been undergoing extension since at least the Pliocene, reactivating steep E-dipping faults of the Mesozoic Atacama fault system. This has been explained by forearc uplift driven by underplating, shallow slab dip, subduction of bathymetric features, and elastic rebound during the earthquake cycle. These processes, however, are active over a much wider area of the central Andean forearc than Coastal Cordillera extension and therefore cannot explain why extension is localized to the northern Chilean onshore outer forearc. We compiled crustal seismicity and the depth of lithospheric boundaries from existing studies to investigate other possible explanations for onshore forearc extension. At 22°S–25°S, seismicity increases above the background subduction-related level present to the north and south. Extensional focal mechanisms, consistent with steep E-dipping faults active at depths up to ~40 km, are also present onshore within this latitude range, but absent to the north and south; this is consistent with the distribution of mapped active fault scarps. The Salar de Atacama crust is seismically active at depths up to ~40 km. Thick lithosphere is present beneath the forearc, and the longitudinal axis of thickest lithosphere is deflected to the east at the latitude of the Salar de Atacama. To the east, the Puna Plateau lithosphere has been thinned by lithospheric removal events. The most robust correlation with onshore forearc extension is the thick, cold, strong crust and mantle lithosphere beneath the anomalous Salar de Atacama in the inner forearc of northern Chile. The combination of underplating-driven outer forearc uplift, the presence of the preexisting structure of the Atacama fault system favorable for reactivation, and the negative buoyancy beneath the Salar de Atacama is inferred to drive Coastal Cordillera normal faulting at this latitude. Recent (<10 Ma) lithospheric removal beneath the Puna Plateau to the east may have enhanced the effect of the negatively buoyant Salar de Atacama lithosphere on the forearc. This implies that both preexisting lithospheric structure and lithospheric processes in the hinterland may influence forearc dynamics.

The approximately N-S–trending Andean retroarc fold-and-thrust belt is the locus of up to 300 km of Cenozoic shortening at the convergent plate boundary where the Nazca plate subducts beneath South America. Inherited pre-Cenozoic differences in the overriding plate are largely responsible for the highly segmented distribution of hydrocarbon resources in the fold-and-thrust belt. We use an ~7500-km-long, orogen-parallel (“strike”) structural cross section drawn near the eastern terminus of the fold belt between the Colombia-Venezuela border and the south end of the Neuquén Basin, Argentina, to illustrate the control these inherited crustal elements have on structural styles and the distribution of petroleum resources. Three pre-Andean tectonic events are chiefly responsible for segmentation of sub-basins along the trend. First, the Late Ordovician “Ocloyic” tectonic event, recording terrane accretion from the southwest onto the margin of South America (present-day northern Argentina and Chile), resulted in the formation of a NNW-trending crustal welt oriented obliquely to the modern-day Andes. This paleohigh influenced the distribution of multiple petroleum system elements in post-Ordovician time. Second, the mid-Carboniferous “Chañic” event was a less profound event that created modest structural relief. Basin segmentation and localized structural collapse during this period set the stage for deposition of important Carboniferous and Permian source rocks in the Madre de Dios and Ucayali Basins in Peru. Third, protracted rifting that lasted throughout the Mesozoic provided the framework for deposition of many of the source rocks in the Subandean belt, but most are not as widely distributed as the Paleozoic sources in Bolivia and Peru. We attribute variations in the style of Andean deformation and distribution of oil versus gas in the Subandes largely to differences in pre-Cenozoic structure along the fold belt. The petroleum occurrences and remaining potential can be understood in the context of three major geographic subdivisions of the Subandes. Between Colombia and central Peru, rich, late postrift Cretaceous source rocks occur beneath Upper Cretaceous–Cenozoic strata that vary significantly in thickness, yielding large accumulations around the Cusiana field in Colombia and within the Oriente Basin in Ecuador, but weak or nonviable petroleum systems elsewhere, where the cover is too thin or too thick. The central Subandes of southern Peru and Bolivia evaded significant Mesozoic rifting, which kept Paleozoic sources shallow enough to delay primary or secondary hydrocarbon generation. After post-Oligocene propagation of the fold belt, however, numerous accumulations were formed around Camisea, Peru, and in the Santa Cruz–Tarija fold belt of central Bolivia. In the southern Subandes of Argentina, Paleozoic crustal thickening caused by terrane accretion and associated arc magmatism created a tectonic highland that precluded deposition and/or destroyed the effectiveness of any Paleozoic source rocks. Here, all accumulations in the foreland and in the fold belt rely on Triassic and younger source rocks deposited in both lacustrine and marine environments within narrow rifts. The trap styles and structurally restricted source rocks in the southern segment have yielded a much smaller discovered conventional resource volume than in the northern and central Subandean segments. In comparing the Subandean system to other global fold belt petroleum systems, it is undoubtedly the rule rather than the exception that the robustness of hydrocarbon systems varies on a scale of a few hundred kilometers in the strike direction. Our work in the Andes of South America suggests that this is because most continents possess heterogeneous basement, superposed deformation, and subbasin stratigraphy that vary on roughly this length scale.

Abstract The Coles Hill uranium property is located in Pittsylvania County, Virginia, within the Smith River allochthon in Southside Virginia's Western Piedmont Province. The Coles Hill property is bounded on the east by the listric normal Chatham fault at the Triassic Danville basin. Uranium mineralization occurs in the footwall of the Chatham fault and is hosted in healed fractures and in hydrothermal apatite in the Leatherwood granite mylonite that is intruded by the Rich Acres gabbro amphibolite. The granite is silica depleted. The mylonite is foliated with a strike of 030° and dips 30°SE; the Chatham fault strikes 030° and dips 60°SE. The structural trap for uranium mineralization is contained on the east by the Chatham fault and beneath by unmineralized Leatherwood biotite gneiss, which is underlain by the Fork Mountain schist. Hematite staining of the ground surface is from weathering of Fork Mountain schist, which outcrops west of the Chatham fault at a constant distance of half a kilometer. In the southern property, pods of uranium mineralization plunge south at 45°. In the northern property, pods of uranium mineralization plunge northeast at 12°. Saussurite, rapakivi textures, Na- and K-metasomatism, titanite, zircon, apatite, calcite, and hematite are present. One possible uranium source is remobilization from the Triassic basin by westward-flowing meteoric waters that intersected the Chatham fault at depth and migrated upward and laterally into the Leatherwood granite. Another uranium source model may be introduction of uranium mineralization by hydrothermal fluids associated with emplacement and cooling of the Leatherwood granite. These fluids would have migrated upward into a structural trap where uranium minerals were precipitated in hydrothermal fractures.

The structural geometry of transfer and accommodation zones that relay strain between extensional domains in rifted crust has been addressed in many studies over the past 30 years. However, details of the kinematics of deformation and related stress changes within these zones have received relatively little attention. In this study we conduct the first-ever systematic, multi-basin fault-slip measurement campaign within the late Cenozoic Rio Grande rift of northern New Mexico to address the mechanisms and causes of extensional strain transfer associated with a broad accommodation zone. Numerous (562) kinematic measurements were collected at fault exposures within and adjacent to the NE-trending Santo Domingo Basin accommodation zone, or relay, which structurally links the N-trending, right-stepping en echelon Albuquerque and Española rift basins. The following observations are made based on these fault measurements and paleostresses computed from them. (1) Compared to the typical northerly striking normal to normal-oblique faults in the rift basins to the north and south, normal-oblique faults are broadly distributed within two merging, NE-trending zones on the northwest and southeast sides of the Santo Domingo Basin. (2) Faults in these zones have greater dispersion of rake values and fault strikes, greater dextral strike-slip components over a wide northerly strike range, and small to moderate clockwise deflections of their tips. (3) Relative-age relations among fault surfaces and slickenlines used to compute reduced stress tensors suggest that far-field, ~E-W–trending σ 3 stress trajectories were perturbed 45° to 90° clockwise into NW to N trends within the Santo Domingo zones. (4) Fault-stratigraphic age relations constrain the stress perturbations to the later stages of rifting, possibly as late as 2.7–1.1 Ma. Our fault observations and previous paleomagnetic evidence of post–2.7 Ma counterclockwise vertical-axis rotations are consistent with increased bulk sinistral-normal oblique shear along the Santo Domingo rift segment in Pliocene and later time. Regional geologic evidence suggests that the width of active rift faulting became increasingly confined to the Santo Domingo Basin and axial parts of the adjoining basins beginning in the late Miocene. We infer that the Santo Domingo clockwise stress perturbations developed coevally with the oblique rift segment mainly due to mechanical interactions of large faults propagating toward each other from the adjoining basins as the rift narrowed. Our results suggest that negligible bulk strike-slip displacement has been accommodated along the north-trending rift during much of its development, but uncertainties in the maximum ages of fault slip do not allow us to fully evaluate and discriminate between earlier models that invoked northward or southward rotation and translation of the Colorado Plateau during early (Miocene) rifting.

This paper focuses on the potential for developing geologic maps using high-resolution aerial imagery and digital elevation data. A key component of geologic mapping discussed herein addresses the determination of geologic layer orientation using a planar approximation (also known as the three-point method). An analytical solution is presented which can be readily implemented in a spreadsheet. The path from initial data collection in a GIS to spreadsheet computation and back to GIS is outlined. The Mecca Hills in southern California serve as a test case, as there is good coverage with high-resolution aerial imagery, a variety of elevation data including airborne LiDAR, and a published geologic map for the area. The comparison between vector-derived strike and dip and traditional field measured strike and dip suggests that remote mapping can successfully capture the regional aspects of the geology in an area. Estimates of rock orientation can be made if the elevation data is accurate and sufficient visual contrast exists in aerial imagery to define mappable features. A comparison shows that United States Geological Survey's National Elevation Data sets and airborne LiDAR can identify regional geologic structures, while Shuttle Radar Topography Mission and ASTER GDEM data yield results that diverge substantially from higher resolution elevation data. Remote mapping using the vector form of the three-point method offers a promising tool for geologic exploration as data sets continue to improve in quality and resolution.

We analyzed displacement on man-made structures and destructive effects on the ancient Roman community in two archaeological sites located on the southern Madonie Mountains, central-northern Sicily. At the archaeological site of Mount Alburchia, a votive niche of the Late Roman period is offset ~15 cm by a NNW-SSE–striking normal fault belonging to a NNW-SSE–trending en-echelon system related to an E-W–oriented regional shear zone. Structural analysis of deformed conglomerates suggests a coseismic origin of displacement. At the Gangi Vecchio archaeological site, 3 km northeast of Mount Alburchia, the prevalence of pottery pieces and coins dated from the fourth century B.C. to the fourth century A.D. and a sudden decrease of evidence of human activity at the end of the fourth century A.D. have been emphasized by archaeologists. Furthermore, recent archaeological excavations have revealed a Roman-age grave where remains of some women and children are mixed. Because ancient Romans usually did not use common burial, this could be related to an unexpected natural disaster. This event was conceivably a strong earthquake that probably occurred in the Late Roman period. An analysis of historical catalogues suggests that this event could be referred to the A.D. 361 earthquake, the epicenter of which, in central Sicily, is poor defined. Our data represent a first step for the relocation of this seismic event northward in the Madonie Mountains area.

The hypothesized presence of a detachment underlying the Lake Mead region has created a dichotomy in the interpretations of the roles of strike-slip faults of the Lake Mead fault system in accommodating regional deformation. Our detailed field mapping reveals a previously unnamed left-lateral strike-slip segment of the Lake Mead fault system and a dense cluster of dominantly west-dipping and related normal faults located near Pinto Ridge. We suggest that the strike-slip fault that we refer to as the Pinto Ridge fault: (1) was kinematically related to the Bitter Spring Valley fault; (2) was responsible for the creation of the normal fault cluster at Pinto Ridge; and (3) utilized these normal faults as linking structures between separate strike-slip fault segments to create a longer, through-going fault. Results from numerical models demonstrate that the observed location and curving strike patterns of the normal fault cluster are consistent with the faults having formed as secondary structures as the result of the perturbed stress field around the slipping Pinto Ridge fault, regardless of whether or not the Pinto Ridge fault merges into a regional detachment at depth. Calculations of mechanical efficiency of various normal fault geometries within extending terranes suggest that a preferred west dip of normal faults likely reflects a west-dipping anisotropy at depth, such as a detachment. The apparent terminations of numerous strike-slip faults of the Lake Mead fault system into west-dipping normal faults suggest that a west-dipping detachment may be regionally coherent.

The Sierra Madre Occidental is the result of Cretaceous-Cenozoic magmatic and tectonic episodes related to the subduction of the Farallon plate beneath North America and to the opening of the Gulf of California. The stratigraphy of the Sierra Madre Occidental consists of five main igneous complexes: (1) Late Cretaceous to Paleocene plutonic and volcanic rocks; (2) Eocene andesites and lesser rhyolites, traditionally grouped into the so-called Lower Volcanic Complex; (3) silicic ignimbrites mainly emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), and grouped into the “Upper Volcanic Supergroup”; (4) transitional basaltic-andesitic lavas that erupted toward the end of, and after, each ignimbrite pulse, which have been correlated with the Southern Cordillera Basaltic Andesite Province of the southwestern United States; and (5) postsubduction volcanism consisting of alkaline basalts and ignimbrites emplaced in the Late Miocene, Pliocene, and Pleistocene, directly related to the separation of Baja California from the Mexican mainland. The products of all these magmatic episodes, partially overlapping in space and time, cover a poorly exposed, heterogeneous basement with Precambrian to Paleozoic ages in the northern part (Sonora and Chihuahua) and Mesozoic ages beneath the rest of the Sierra Madre Occidental. The oldest intrusive rocks of the Lower Volcanic Complex (ca. 101 to ca. 89 Ma) in Sinaloa, and Maastrichtian volcanics of the Lower Volcanic Complex in central Chihuahua, were affected by moderate contractile deformation during the Laramide orogeny. In the final stages of this deformation cycle, during the Paleocene and Early Eocene, ∼E-W to ENE-WSW–trending extensional structures formed within the Lower Volcanic Complex, along which the world-class porphyry copper deposits of the Sierra Madre Occidental were emplaced. Extensional tectonics began as early as the Oligocene along the entire eastern half of the Sierra Madre Occidental, forming grabens bounded by high-angle normal faults, which have traditionally been referred to as the southern (or Mexican) Basin and Range Province. In the Early to Middle Miocene, extension migrated westward. In northern Sonora, the deformation was sufficiently intense to exhume lower crustal rocks, whereas in the rest of the Sierra Madre Occidental, crustal extension did not exceed 20%. By the Late Miocene, extension became focused in the westernmost part of the Sierra Madre Occidental, adjacent to the Gulf of California, where NNW-striking normal fault systems produced both ENE and WSW tilt domains separated by transverse accommodation zones. It is worth noting that most of the extension occurred when subduction of the Farallon plate was still active off Baja California. Geochemical data show that the Sierra Madre Occidental rocks form a typical calcalkaline rhyolite suite with intermediate to high K and relatively low Fe contents. Late Eocene to Miocene volcanism is clearly bimodal, but silicic compositions are volumetrically dominant. Initial 87 Sr/ 86 Sr ratios mostly range between 0.7041 and 0.7070, and initial ϵNd values are generally intermediate between crust and mantle values (+2.3 and -3.2). Based on isotopic data of volcanic rocks and crustal xenoliths from a few sites in the Sierra Madre Occidental, contrasting models for the genesis of the silicic volcanism have been proposed. A considerable body of work led by Ken Cameron and others considered the mid-Tertiary Sierra Madre Occidental silicic magmas to have formed by fractional crystallization of mantle-derived mafic magmas with little (<15%) or no crustal involvement. In contrast, other workers have suggested the rhyolites, taken to the extreme case, could be entirely the result of partial melting of the crust in response to thermal and material input from basaltic underplating. Several lines of evidence suggest that Sierra Madre Occidental ignimbrite petrogenesis involved large-scale mixing and assimilation-fractional crystallization processes of crustal and mantle-derived melts. Geophysical data indicate that the crust in the unextended core of the northern Sierra Madre Occidental is ∼55 km thick, but thins to ∼40 km to the east. The anomalous thickness in the core of the Sierra Madre Occidental suggests that the lower crust was largely intruded by mafic magmas. In the westernmost Sierra Madre Occidental adjacent to the Gulf of California, crustal thickness is ∼25 km, implying over 100% of extension. However, structures at the surface indicate no more than ∼50% extension. The upper mantle beneath the Sierra Madre Occidental is characterized by a low-velocity anomaly, typical of the asthenosphere, which also occurs beneath the Basin and Range Province of the western United States. The review of the magmatic and tectonic history presented in this work suggests that the Sierra Madre Occidental has been strongly influenced by the Cretaceous-Cenozoic evolution of the western North America subduction system. In particular, the Oligo-Miocene Sierra Madre Occidental is viewed as a silicic large igneous province formed as the precursor to the opening of the Gulf of California during and immediately following the final stages of the subduction of the Farallon plate. The mechanism responsible for the generation of the ignimbrite pulses seems related to the removal of the Farallon plate from the base of the North American plate after the end of the Laramide orogeny. The rapid increase in the subduction angle due to slab roll-back and, possibly, the detachment of the deeper part of the subducted slab as younger and buoyant oceanic lithosphere arrived at the paleotrench, resulted in extension of the continental margin, eventually leading to direct interaction between the Pacific and North American plates.

Abstract The Hudson Valley fold-thrust belt of eastern New York State involves a relatively thin sequence of shallow-marine Silurian and Lower Devonian strata. Because of the thinness of this sequence, structures of the belt are relatively small. Thus, first-order ramps and flats, and fault-related folds can be seen in their entirety at a single roadcut. A field trip in the southern half of the Hudson Valley fold-thrust belt, from the latitude of Catskill to the latitude of Rosendale, provides an opportunity to see many examples of these structures, and to discuss the three-dimensional architecture of fold-thrust belts in an orocline. In particular, we will see how along-strike changes in stratigraphy affect fold wavelength and the depth of detachment horizons. The trip also provides the opportunity to examine the relationship between mesoscopic structures (e.g., solution cleavage and veining) and first-order structures.

The Neuquén Embayment, which developed along the eastern foothills of the southern Central Andes, has a complex history of intraplate deformation. The Paleozoic basement fabrics exerted a major influence in Mesozoic and Cenozoic deformation. The most important feature is an E-W–striking fault system that is related to a late Paleozoic fabric and is associated with the Huincul basement high, which truncates the basin. This fabric is interpreted as being the result of the accretion of the Patagonia terrane with Gondwana during the Early Permian. Two-dimensional (2D) and three-dimensional (3D) seismic coverage and subsurface information identify different sectors in the Neuquén Embayment that record alternating episodes of contraction and extension during the Jurassic and Cretaceous. The deformation history east of the thrust front of the Agrio fold-and-thrust belt is characterized by periods of (1) transpression and almost orthogonal contraction to the continental margin, (2) extension, and (3) relative quiescence, which alternates in different sectors. The earliest shortening occurred in the Early Jurassic when the main stress was oriented in the N-NW sector. The stress rotated to the northwest up to Valanginian times, when a more orthogonal orientation to the continental margin became dominant and prevailed after the Cenomanian. After a period of quiescence in the Neuquén Embayment associated with very oblique subduction during the Paleogene, the final contractional deformation took place in the late Miocene, with a west-east orientation of the main stress, and was followed by Pliocene extension. The changing stress patterns correlate with differences in convergence vectors between the Aluk, Farallon, and Nazca oceanic plates and the Gondwana or South American continental plates. The Aluk stage from the Jurassic to the Valanginian was characterized by tectonic inversion that is shown by shortening and right-lateral strike-slip structures that are concentrated in the Huincul system and more subtle deformation in the Chihuidos and Entre Lomas systems. The early Farallon stage was distinguished by reduced inversion and displacement in the Huincul system and a general retreat of deformation after the Valanginian. The change to late Farallon stage was characterized by a prominent tectonic inversion of the Entre Lomas system, which resulted from the inception of the formation of the Agrio fold-and-thrust belt in the retroarc area. This belt developed during most of the Late Cretaceous, when the embayment showed a general quiescence. The Nazca stage was characterized by the main episode of uplift, tectonic inversion of the older half-grabens, and important strike-slip faulting that was followed by local collapse of some structures during the Pliocene.

Marginal basin flysch deposits of the western Jurassic belt of the Klamath Mountains were thrust eastward beneath the western Paleozoic and Triassic belt during the Late Jurassic Nevadan orogeny. Nevadan underthrusting created two generations of nearly coaxial north-trending folds within the western Jurassic belt rocks. These structures formed at chlorite-grade, greenschist-facies conditions and have accompanying pressure solution and mineral recrystallization. The geometry of the Nevadan structures suggests that the thrusting direction was roughly west–east in present coordinates. This direction is perpendicular to the regional strike of the bounding thrust faults and is consistent across 35 km of dip exposure. Post-Nevadan structures include locally developed strike-slip faults and related third- and fourth-generation folds. These features have associated quartz and calcite veins but lack the metamorphic mineral growth associated with the Nevadan structures. Relatively young, high-angle normal faults are very common and appear to be contemporaneous with Neogene uplift of the entire range. Nevadan-age structures within the western Sierra Nevada Foothills terrane also formed in response to west–east thrusting. Global plate-circuit models suggest that the Nevadan Farallon–Pacific relative motion may have been orthogonal to the continental margin at the latitude of the Klamath Mountains. This convergence direction and the kinematic analyses suggest that the Klamath Mountains and Sierra Nevada Foothills were in their same relative orientation during the Nevadan orogeny.

Abstract Within the western Great Basin, a system of dextral strike-slip faults accommodates a significant fraction of the North American–Pacific plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this fault system and is one of the youngest and least developed parts of the North American–Pacific transform plate boundary. Accordingly, the northern Walker Lane affords an opportunity to analyze the incipient development of a major strike-slip fault system. In northwest Nevada, the northern Walker Lane consists of a discrete ~50-km-wide belt of overlapping, curiously left-stepping dextral faults, whereas a much broader zone of disconnected, widely-spaced northwest-striking faults characterizes northeast California. The left steps accommodate little shortening and are not typical restraining bends. The left-stepping dextral faults may represent macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. Strands of the northern Walker Lane terminate in arrays of northerly striking normal faults in the northwestern Great Basin and along the eastern front of the Sierra Nevada. These relations suggest that dextral shear in the northern Walker Lane is transferred to ~NW-SE extension in the Great Basin. Offset segments of a west-trending Oligocene paleovalley suggest ~20–30 km of cumulative dextral slip across the northern Walker Lane. Strike-slip faulting began between 3 and 9 Ma, indicating a long-term slip rate of ~2–10 mm/yr, which is compatible with GPS geodetic observations of the current strain field .