Abstract Neoproterozoic strata in southeastern Idaho and Utah include the <766 Ma Uinta Mountain Group and Big Cottonwood Formation (Fm.) deposited in an east-trending rift basin and, to the west, the lower part of a westward-thickening rift to passive-margin succession that initiated c. 720 Ma. The latter contains a lower diamictite and volcanic succession, with a complex stratigraphic interval of Cryogenian marine glacial deposits (Pocatello and Mineral Fork formations and correlatives). This is overlain by a mostly terrigenous succession of <667 Ma strata assigned to the upper member of the Pocatello Fm. and Brigham Group in southeastern Idaho, to the Kelley Canyon Fm. and Brigham Group in northern and western Utah, and to the McCoy Creek Group and Prospect Mountain Quartzite in adjacent Nevada. Although the Brigham Group and correlative deposits contain no direct evidence for glaciation, widely developed, though stratigraphically restricted, incised valleys, with erosional relief from a few metres to as much as 160 m, are inferred to represent subsequent times of Cryogenian glacially lowered sea level. Overall interpretations of the stratigraphy and sedimentology of these rocks have changed little in the past 10–15 years. The most important recent advances relate to U–Pb geochronology. In strata that lie unconformably below demonstrable glacial deposits, the lower Uinta Mountain Group (formerly thought to be c. 900 Ma) contains populations of detrital zircons as young as 766±5 Ma. Cryogenian magmatism north of the Snake River Plain in central Idaho is recognized near House Mountain, east of Boise at c. 725±5 Ma, in the Pioneer Mountains Core Complex at about 695 Ma, and in central and east-central Idaho at 685–650 Ma. Clasts interpreted to be from the rift-related Bannock Volcanic Member of the Pocatello Fm. are dated at 717±4 Ma and 701±4 Ma. The overlying diamictite-bearing Scout Mountain Member contains a mafic lapilli tuff near the base (686±4 Ma) and a reworked fallout tuff near the top (667±5 Ma). Strongly negative C-isotope data have been obtained from some of the carbonate rocks, although the latter constitute only a small fraction of the succession. Palaeomagnetic data are available only for the Uinta Mountain Group, and suggest an equatorial palaeolatitude.

Abstract Incised valleys associated with sequence boundaries of regional extent are present at two stratigraphic levels in Neoproterozoic siliciclastic rocks of northern and western Utah and southeastern Idaho. In comparison with many Phanerozoic examples, the sedimentary fill of these Neoproterozoic incised valleys is unusually coarse-grained. The most prominent paleovalley system is present along a sequence boundary in the upper part of the Caddy Canyon Quartzite, and may be traced from the Portneuf Range in southeastern Idaho south to the Canyon Range in central Utah and west to the Dugway Range in western Utah. Individual valleys range in depth from several meters to >45 m and in width from a few tens of meters to several hundred meters. Valley fills consist of diffusely to well stratified granule to pebble conglomerate characteristically containing siltstone clasts < cm to >2 m across. They are interpreted to have accumulated predominantly in a fluvial environment, and perhaps in part by debris flows in a subaerial or shallow estuarine setting. This paleodrainage system is of significance because it is one of the few documented examples of a widespread incised-valley system that erodes into an underlying fluvial braid plain more than 200 km in width, indicating that the effects of base-level changes were felt far upstream in the more proximal reaches of a braided fluvial system. A second incised valley with more than 60 m of local relief, and located at the sequence boundary at the base of the Geertsen Canyon Quartzite, is developed only locally in the Portneuf Range of southeastern Idaho. It is similar in both geometry and sedimentary fill to the valleys in the Caddy Canyon Quartzite, and is incised into the proximal reaches of a widespread fluvial braid plain. Somewhat shallower, conglomerate-filled valleys associated with higher-order cyclicity have been observed within the upper part of the Caddy Canyon Quartzite and overlying Inkom Formation, and, unlike the other examples, are encased in offshore marine siltstone. The development of Neoproterozoic sequence boundaries in the western U. S. is probably related to some combination of glacial eustasy (well documented in correlative strata elsewhere) and lithospheric extension, which appears to have preceded the development of the early Paleozoic passive continental margin. The relative roles of these mechanisms cannot yet be distinguished in the absence of more precise geochronology.

Abstract The separation of eustatic, tectonic, and other controls on the development of sedimentary cyclicity is difficult. In mixed carbonate-siliciclastic successions, the conventional interpretation of unconformity-bounded depositional sequences is that they are due to reciprocal sedimentation in response to relative changes of sea level. According to this view, transgressive and highstand systems tracts are composed primarily of carbonate rocks, and lowstands of siliciclastic rocks. The application of this model to the interpretation of cyclic carbonate and siliciclastic rocks in the Upper Devonian of the Canning basin, Western Australia, presents a paradox because expected evidence for subaerial exposure of the platform is not well developed. Sequence stratigraphic studies in outcrop at two localities along the northern margin of the Canning basin confirm a complex relation between carbonate and siliciclastic conglomerate. At Stony Creek, the carbonate rocks are interpreted to represent an assemblage of reef, foreslope floatstones, and backreef carbonate-conglomerate cycles that accumulated in a shallow marine environment along the margin of a fan-delta. Conglomerates and sandstones inferred to onlap the reefal foreslope are interpreted tentatively as fluvial, and the contact is interpreted as a sequence boundary. In the Van Emmerick Range, foreslope floatstones are onlapped along the margin of an incised valley with at least 10 m of relief by conglomerate and sandstone of probable marine origin, and overlain by a transgressive fossiliferous limestone. The age of the sequence boundary at Stony Creek is not well established, but probably early Frasnian. The age of the sequence boundary in the Van Emmerick Range is better constrained as late Frasnian, and this surface appears to correlate with the base of the Frasnian 4 sequence identified by Southgate et al. (1993) on the basis of subsurface data. Both surfaces are thought to have involved base-level lowering, but the evidence is equivocal. Evaluation of available data indicates that subaerial exposure is required for only one of eight potential sequence boundaries in the Frasnian-Famennian interval, a surface that corresponds locally to minor karstification, and which is dated as latest Frasnian. Several explanations are proposed as working hypotheses. The development of thick lowstand deposits coeval with flooding events on the platform is consistent with continued extension and tilting of a fault block. Alternatively, exposure may have been restricted to topographic highs remaining after extension had ceased. If subaerial exposure was widespread, diagenetic effects may have been limited or not preserved. The development of onlap surfaces within the basin may be related in part to variations in sediment flux, and the distribution of siliciclastic sediments influenced by the geologic structure, especially the configuration of accommodation or transfer zones. Further work is needed to resolve uncertainties in the existing sequence stratigraphic interpretation, to improve the calibration of individual boundaries, and to evaluate these ideas. Ultimately, comparisons with coeval successions on other continents will be needed to evaluate the possible role of eustasy in the development of the observed sequences.

Abstract The distribution of Middle and Late Proterozoic sedimentary and metasedimentary cover that lies unconformably on Early Proterozoic and Archean crystalline basement has been known for decades, but recent work, employing techniques of paleomagnetic correlation, sedimentology, sequence stratigraphy, and analysis of tectonic subsidence has led to modifications of some long-accepted correlations and tectonic models. Within the context of both older classical studies and this new work, the stratigraphy, correlation, tectonic setting, fossil content, and mineral potential of Middle and Late Proterozoic rocks of parts of the Rocky Mountain, Colorado Plateau, and Basin and Range provinces of the United States are discussed. A problem common to interpretation of all Proterozoic strata is a widespread lack of fossil control on age and paleoecology, which makes correlations inherently uncertain and interpretation of depositional environments more difficult. We present current hypotheses about these topics and stress the uncertainty of some of our conclusions. The apparent polar wander path for the North American craton, as derived from the Middle and Late Proterozoic sedimentary cover, is central to our modifications of stratigraphie correlation, especially of Middle Proterozoic rocks. The reader is asked to view the work and summaries presented here in the light of ongoing scientific debate about strata that are chronically stubborn in yielding information. The authors of sections of this chapter include both those who have performed classical studies, which are the foundation of our present understanding, and younger geologists who have been busy refining and modifying early interpretations, using different methods of study. The treatment in this chapter is therefore variable depending on which generation of investigators is speaking.

Abstract Significant advances during the decade 1975 to 1985 in understanding the geology of basins along strike-slip faults include the following: (1) paleomagnetic and other evidence for very large magnitude strike slip in some orogenic belts; (2) abundant paleomagnetic evidence for the pervasive rotation of blocks about vertical axes within broad intracontinental transform boundaries; (3) greater appreciation for the wide range of structural styles along strike-slip faults; (4) new models for the evolution of strike-slip basins; and (5) a body of new geophysical and geological data for specific basins. In the light of this work, and as an introduction to the remainder of the volume, the purpose of this paper is to summarize the major characteristics of and controls on structural patterns along strikeslip faults, the processes and tectonic settings of basin formation, and distinctive of and controls characteristics of strike-slip basins. Strike-slip faults are characterized by a linear or curvilinear principal displacement zone in map view, and in profile, by a subvertical fault zone that ranges from braided to upward-diverging within the sedimentary cover. Many strike-slip faults, even those involving crystalline basement rocks, may be detached within the middle to upper crust. Two prominent characteristics are the occurrence of en echelon faults and folds, within or adjacent to the principal displacement zone, and the co-existence of faults with normal and reverse separation. The main controls on the development of structural patterns along strike-slip faults are (I) the degree to which adjacent blocks either converge or diverge during strike slip; (2) the magnitude of displacement; (3) the material properties of the sediments and rocks being deformed; and (4) the configuration of pre-existing structures. Each of these tends to vary spatially, and, except for the last, to change through time. It is therefore not surprising that structural patterns along strike-slip faults differ in detail from simple predictions based on the instantaneous deformation of homogeneous materials. In the analysis of structural style, it is important to attempt to separate structures of different ages, and especially to distinguish structures due to strike-slip deformation from those predating or post-dating that deformation. Distinctive aspects of structural style for strike-slip deformation on a regional scale include evidence for simultaneous shortening and extension, and for random directions of vergence in associated thrusts and nappes. Sedimentary basins form along strike-slip faults as a result of localized crustal extension, and, especially in zones of continental convergence, of localized crustal shortening and flexural loading. A given basin may alternately experience both extension and shortening through variations in the motion of adjacent crustal blocks, or extension in one direction (or in one part of the basin) may be accompanied by shortening in another direction (or in another part of the basin). The directions of extension and shortening also tend to vary within a given basin, and to change through time; and the magnitude of extension may be depth-dependent. Theoretical studies and observations from basins where strike-slip deformation has ceased suggest that many strike-slip basins experience very little thermally driven post-rift subsidence. Strike-slip basins are typically narrow (less than about 50 km wide), and they rapidly lose anomalous heat by accentuated lateral as well as vertical conduction. Detached or thin-skinned basins also tend to be cooler after rifting has ended than those resulting from the same amount of extension of the entire lithosphere. In some cases, subsidence may be arrested or its record destroyed as a result of subsequent deformation. Subsidence due to extension, thermal contraction, or crustal loads is amplified by sediment loading. The location of depositional sites is determined by (1) crustal type and the configuration of pre-existing crustal structures; (2) variations in the motion of lithospheric plates; and (3) the kinematic behavior of crustal blocks. The manner in which overall plate motion is accommodated by discrete slip on major faults, and by the rotation and internal deformation of blocks between those faults is especially important. Subsidence history cannot be determined with confidence from present fault geometry, which therefore provides a poor basis for basin classification. Every basin is unique, and palinspastic reconstructions are useful even if difficult to undertake. Distinctive aspects of the stratigraphic record along strike-slip faults include (1) geological mismatches within and at the boundaries of basins; (2) a tendency for longitudinal as well as lateral basin asymmetry, owing to the migration of depocenters with time; (3) evidence for episodic rapid subsidence, recorded by thick stratigraphic sections, and in some marine basins by rapid deepening; (4) the occurrence of abrupt lateral facies changes and local unconformities; and (5) marked differences in stratigraphic thickness, facies geometry, and occurrences of unconformities from one basin to another in the same region.

Abstract Stepovers are fundamental features along strike-slip faults of various lengths. Two types of stepover between strike-slip faults are considered in this paper: (1) along-strike stepovers that are due to en echelon arrangement of faults in map view, and (2) down-dip stepovers that are due to en echelon arrangement of faults in cross section. Along-strike stepovers produce pull-apart basins und push-up ranges depending on the sense of stepover. Down-dip stepovers of both senses may produce strike-slip faults in orientations different from the initial major strike-slip faults that are arranged en echelon. Some possible mechanisms that produce stepovers and control the sense of stepover are (1) bending of initially straight faults. (2) faulting within a weak zone oriented slightly off a local failure plane. (3) segmentation of faults to accommodate curved fault traces. (4) horizontal slip across pre-existing extensional fractures or dip-slip faults that have steps. (5) a change of physical parameters such as elastic moduli and pore pressure, and (6) stress field resulting from fault interaction.

Abstract The very rapid subsidence, sediment accumulation, and hydrocarbon maturation observed in many small extensional or “pull-apart” basins can be explained using a McKenzie-type model. It has been shown that in basins of 100 km width or less, lateral heat loss is quite important and accelerates lithospheric cooling and subsidence. We show here that cooling that is simultaneous with stretching is very important for basins formed by stretching of lithospheric blocks that are 10 km to several tens of kilometers wide. In fact, for most of these very narrow basins, most of the anomalous heat introduced by stretching is also dissipated during the stretching event. We have calculated the effect of alternate short periods of stretching and cooling to approximate simultaneous stretching and cooling. The results show, for example, that for a block, initially 10 km wide and stretched uniformly at 3 cm/yr, sufficient subsidence will take place in 200,000 years to accumulate 4—5 km of sediment. A consequence of this rapid subsidence is initial sediment starvation. These results may be applicable to many of the small extensional basins associated with the San Andreas transform system.

Abstract A divergent (transtensional) wrench fault is one along which strike-slip deformation is accompanied by a component of extension. Faulting dominates the structural style and can initiate significant subsidence and sedimentation. The divergent wrench fault differs from other types of wrench faults by having mostly normal separation on successive profiles, negative flower structures, and a different suite of associated structures. En echelon faults, most with normal separation, commonly flank the zone, and some exhibit evidence of external rotation about vertical axes and have evidence of superimposed strike slip. Flexures associated with the wrench fault are formed predominantly by vertical components of displacement, and most are drag and forced folds parallel to and adjacent to the wrench. Hydrocarbon traps can occur in fault slices within the principal strike-slip zone, at culminations of forced folds, in the flanking tilted fault blocks, and within less common en echelon folds oblique to the zone. Divergent wrench faults occur at active plate boundaries, in extensional and contractional continental settings, and within plates far from areas of pronounced regional deformation. Along transform margins and within wrench systems, divergent wrench styles tend to develop where major strands or segments of strands bend or splay toward the orientation of associated normal faults (e.g., elements of the San Andreas system in the Mecca Hills, California), and where major strands are regionally oblique to interplate slip lines (e.g., Dead Sea transform, Middle East). The style also develops at releasing fault oversteps and fault junctions (e.g., Ridge Basin, California), and locally where crustal blocks rotate between bounding wrench faults. In extensional settings, divergent wrench faults may form within graben doglegs and oversteps (e.g., between the Rhine and Bresse grabens, northern Europe), and they may separate regions that experienced different magnitudes of extension (e.g., Andaman Sea area). Many oceanic fracture zones have divergent wrench characteristics. The style has also been recognized in magmatic arcs (e.g., the Great Sumatran fault) and in both backarc and peripheral foreland settings (e.g., Lake Basin fault zone, Montana) near convergent plate boundaries, and in intra-plate settings (e.g., Cottage Grove fault, Illinois; Scipio-Albion trend, Michigan).

Abstract Thick nonmarine sequences with similar facies and geometry may accumulate in basins that develop adjacent to strike-slip faults. Herein we compare three basins of different age and size whose tectonic and depositional characteristics suggest a similar origin and history. The Hornelen Basin developed during the Middle and possibly Early Devonian in western Norway. The basin is bounded on the north and south by east-striking faults, and the northern fault is considered to have been a zone of major right-slip movement. The basin is 60-70 km long, 15-25 km wide, and about 1,250 km 2 in areal extent; its cumulative fill of 25,000 m was deposited at an estimated rate of 2.5 m/1,000 yr. The Ridge Basin developed during the Miocene and Pliocene between the right-lateral San Gabriel and San Andreas faults in southern California. The basin is 30-40 km long, 6-15 km wide, and about 400 km 2 in areal extent; its cumulative fill of 7,000-11,000 m was deposited at an estimated rate of about 3 m/1,000 yr. The three Little Sulphur Creek Basins probably developed between 4 and 2 Ma along the east side of the right-lateral Maacama fault zone in northern California. These basins cumulatively are about 12 km long, 1.5 to 2 km wide, and about 15 km 2 in areal extent; their cumulative fill of 5,000 m was deposited at an estimated rate of about 2.5 m/1,000 yr. Coarse sedimentary breccia, which constitutes a relatively small volume of the fill, was deposited in each of these basins along the active right-slip fault margin as talus, landslide, and small but steep debris-flow-dominated alluvial fans. Along other margins of the basins, a much larger volume of the fill accumulated as larger streamflow-dominated alluvial fans, braided-stream, meandering-stream, fan-delta, and deltaic deposits. Lacustrine deposits that include turbidites and, locally in Ridge Basin, chemical precipitates, accumulated in the centers of the basins. The basin floors are generally tilted toward the margins with active right-slip faults so that the basin axes and the depocenters are subparallel to, and shifted toward, this margin. Sediment was transported toward the basin center from surrounding highlands and then longitudinally down the basin axis. The basin fills were syn-depositionally faulted and post-depositionally folded into large plunging synclines. The basins lengthened over time and contain thicknesses of sedimentary rocks that are comparable to or greater than their widths.

Abstract Walker Lake sedimentary basin is a fault-controlled continental basin related to strike-slip faulting on the western side of the Basin and Range Province of Nevada. The Walker Lake Basin is contained within a triangular crustal block bounded by normal-to oblique-slip faults on the west, left-lateral faults on the south, and right-lateral strike-slip faults on the east (Walker Lane shear zone). Modern Walker Lake is roughly one fourth the surface area and the water depth of its Pleistocene precursor Carbon-rich (up to 2.5% total organic carbon) and uranium-rich sediments are currently accumulating in the deeper saline and anoxic parts of Walker Lake. If these conditions were to continue, significant potential hydrocarbon source rocks and uranium-bearing beds could accumulate. Walker Lake Basin is being infilled by axially fed, sand-rich fluvial-deltaic deposits; side-fed, coarse-grained alluvial-fan/fan-delta deposits; and central fine-grained lacustrine deposits. Waves, wind, and lake-level fluctuations have caused reworking of the lower parts of fan-delta surfaces and the front (windward side) of the Walker River delta. Carbonate deposits, which include beach-rock horizons, stromatolites, oncolites, caliche, and tufas, locally form along the shorelines and spring areas of this predominantly coarsegrained clastic system.

Abstract Evidence of Cenozoic deformation and sedimentation along the southeasternmost 40 km of the Furnace Creek strike-slip fault zone, in the southwestern Great Basin, is contained in two successions of sedimentary and volcanic rocks. Each indicates a stage in the development of the fault zone and associated basins; each is bracketed by K/Ar age determinations. The older succession, dated at 25 to no less than 14 Ma, occurs in tilted fault blocks of the bordering Funeral Mountains, and predates major crustal extension. The succession is about 1,300 m in maximum exposed thickness, contains remnants of two northeast-sloping alluvial fans, onlaps a Paleozoic basement to the southwest, and includes an unconformity that cuts out progressively older units southwestward. These features indicate that a topographic high persisted where younger formations of the fault-controlled, northwest trending Furnace Creek Basin now occur, and also early vertical movement on the fault zone. The younger succession, comprising the Artist Drive, Furnace Creek, and Funeral Formations of the basin (McAllister, 1970), ranges in age from about 14 Ma to about 4 Ma. It records the subsidence and deformation of the basin, and was associated with right-lateral slip on northwest-striking faults and with basin-range extension expressed in part by abundant north-to northeast-striking normal faults. The three formations have a composite maximum thickness of about 3,600 m, but the combined thickness varies greatly from place to place, owing to contemporaneous faulting, to changes in depocenters related to the fault movements and to a general thinning southeastward. The sedimentation and faulting also were accompanied by abundant plutonism and volcanism in the Greenwater Range and Black Mountains southwest of the fault zone. The fact that the normal faulting is regional, affecting terranes on both sides of the fault zone, qualifies the Furnace Creek as a special type of divergent strike-slip fault zone. The contemporaneous strike slip within the 40-km segment of the fault zone probably increases northwestward from zero to no more than a few kilometers, but continues to increase farther northwestward. In the area of Furnace Creek Wash, strike slip is indicated by low-angle grooves and fault mullions, west-northwest-trending en echelon folds, north-northeast-striking normal faults, and by evidence that west-northwest-directed extension was greater on the southwest side of the fault zone than on the northeast side. Vertical movements are recorded by marginal conglomerates in each of the three formations and by northwest-trending folds with features characteristic of drag folds and forced folds. That the fault zone may penetrate deeply into the crust is suggested by the abundance of basalt flows and sills in the three formations of the basin as compared with the bordering terranes.

Abstract The Dead Sea Rift, a classic strike-slip lineament, occurs along a transform plate boundary that connects the Red Sea, where sea-floor spreading is occurring, with the Taurus Mountains, where there is plate convergence. The total strike slip along the transform since the Miocene is 105 km with the last 40 km having occurred in Plio-Pleistocene time, that is when the present Dead Sea Basin began to subside. The Dead Sea Basin, an active rhomb-shaped graben, is located within the Dead Sea transform. It is bounded on the east and west by two nearly vertical, north-striking normal faults, on the south by listric(?) normal faults, that dip steeply to the north and on the north by a gently sloping, south-facing flexure in the basement. Displacement along a zone of en echelon strike-slip faults formed 3 sedimentary basins whose depocenters migrated northward with time. Deposition began in the early Miocene, when a thick succession of continental red beds (the Hazeva Formation) were deposited in a basin south of the modern Dead Sea. Deposition migrated northward in the Pliocene, as an arm of the Mediterranean flowed south through the transform into the newly evolving Dead Sea Basin; there, marine clastic sediment and evaporites, including the Sedom Salt, were laid down. The youngest rift sequence, of Plio-Pleistocene to Holocene age, fills the northern basin of the Dead Sea with over 3,500 m of lacustrine evaporites and fluvial-deltaic clastic sediments. Only the youngest syn-rift sequence crops out along the west bank of the northern basin, where it was mapped at a scale of 1:50,000 for a distance of 50 km, and is divisible into three stratigraphic units. From oldest to youngest these are the Samra Formation (debris-fan gravel); the Lisan Formation (fan-delta gravel, and lacustrine limestone and marl); and a unit of deltaic sands and beach gravels. Sedimentation along the rift is controlled by the interaction of rift tectonics and climate. Whereas tectonic activity creates the rift, the resulting rift morphology significantly modifies the climate. Thus it may be observed that as moist air from the Mediterrean Sea rises over the shoulders of the rift, it cools adiabatically yielding as much as 800-1,000 mm of rain per year. This rainfall contributes to high-discharge ephemeral streams that transport huge quantities of coarse clastic sediment eastward onto the narrow shelf of the Dead Sea; and to the drainage basin of the Jordan River, a perennial stream, that carries mud and fine-grained clastic sediment along the axis of the rift, where it constructs a large delta at the head of the Dead Sea. On the other hand, as the air descends into the basin, it warms adiabatically, evaporating more than 2,000 mm of water per year, thereby causing a concomitant drop in the Dead Sea level, precipitation of evaporites, and the reworking of shelf sediment into deeper water. In the absence of recurrent syn-depositonal faulting in the stratigraphic record, such actualistic models convincingly explain Pleistocene-Holocene sedimentary patterns along the Dead Sea.

Abstract The Soria Basin is a rhomb graben with borders that trend N60°E and N50°W. It was formed during the Late Jurassic -Early Cretaceous (Wealdian), when as much as 8 km of fluvially dominated deltaic strata accumulated in it. This sedimentary fill has been divided into five cyclothems, of which the lower four are discussed in this paper. Within the basin, a N50°W-trending, 50-km wide, syn-sedimentary syncline developed in the basin fill. This syncline was related to extensional tectonics, and to the formation of a half graben in Paleozoic basement overlain by competent Jurassic and incompetent Triassic strata. Within the basin fill, the extensional deformation produced microstructures (stylolites, calcite tension gashes, and quartz dikes) with a coherent basin-wide partem. The depocenter migrated with time from the southeastern comer of the basin during deposition of cyclothems I and II, to the northeastern corner during formation of cyclothems III and IV. High heat flow, related to crustal thinning in the area of greatest subsidence, led to metamorphism within the sediments. Conditions of metamorphism were a maximum temperature of 420° C, a temperature gradient of 100-150° C/km and pressures between 1-3 kb. At the same time, compressional deformation (N30°W-trending folds and associated cleavage) was induced along the southeastern margin of the basin, and erosion (uplift) occurred outside the basin. Our interpretation of the geometry, sedimentation, tectonics, and thermal evolution of the Soria Basin is based on mathematical models and microtectonic analogue models of a releasing solitary overstep. In such models, stress/strain deviations and accumulations predict vertical motions (subsidence and uplift), and the geometry of structures in areas of extension (secondary normal faults, tension gashes) and in areas of compression (folds, cleavage). Most of the field data collected inside and outside the basin are consistent with a model of a releasing overstep along N60°E-striking sinistral strike-slip faults. The proposed releasing overstep model differs from classical models of strike-slip basins by (1) taking into account stress/strain related to basin development, (2) explaining migration of the depocenter with time, and (3) predicting the geometry of secondary faults.

Abstract The east-trending Piaui Basin off the northern Brazilian continental margin is an Atlantic-type rifted basin. The stratigraphic and structural framework of the basin is interpreted as recording wrenching during separation of South America and Africa along the equatorial Romanche Fracture Zone. Superposition of rifting and wrenching resulted in a diverse set of structures that are uncommon in Atlantic-type basins. The basin was initiated during Aptian time. The structural grain of the adjacent Precambrian Parnaiba Platform probably influenced the orientations of normal faults which strike between northeast and east-northeast. Dominantly non-marine siliciclastic sediments were deposited during the rift stage, prior to the development of a mid-Aptian (112 Ma) regional unconformity. Oblique, northeast-southwest separation of South America and Africa between mid-Aptian and early Cenomanian time, was accompanied by the deposition of thick transitional marine and marine clastic sediments. During the middle Cenomanian, the direction of sea-floor spreading between South America and Africa changed to east-west, leading to convergent wrenching between asperities of the two continents along the Romanche Fracture Zone. Rift faults are thought to have been reactivated as oblique- and strike-slip faults; other faults are interpreted as synthetic (N70° to N75°W) and antithetic (north to N20°E) strike-slip faults. Associated structures include flower structures, en echelon folds, and shale ridges (N20°E). Wrenching created a 200 km by 50 km uplifted transpressive belt (Atlantic High) in the Piaui Basin, where erosion occurred from the Late Cretaceous to the Eocene. Oligocene-Miocene shallow-marine sediments cover the Cretaceous rocks unconformably over most of the basin.

Abstract The Nonacho Group is a sequence of talus, alluvial-fan. braided-stream. pond, beach, fan-delta, and lacustrine rocks. Deposition occurred over an area 200 km by 60 km. in rhomb, wedge, and rectangular sub-basins. The sub-basins were separated during sedimentation by basement uplifts, as indicated by changes in clast composition, unit thickness, and facies that occur across fault-bounded basement inliers. Sinistral strike slip along near-vertical, north-northeast-striking faults that border the western margin of the Nonacho Basin is regarded as having controlled basin evolution. Syn-depositional strike slip is suggested by an extreme longitudinal cumulative thickness (>40 km) in conjunction with a uniform, lower greenschist facies metamorphic grade. A sinistral sense is indicated by (1) the location of the basin where there is a left-hand stepover in the regional fault system; (2) the probable southward sequential development of the basin as the basin floor moved, like a conveyor belt, northwards along faults concentrated on the western margin; and (3) the inferred tectonic emplacement from south of the basin of source rocks for conglomerates stratigraphically high in the sequence. The western margin of the basin was bordered by high-gradient alluvial fans and braid-plains, which in turn were located on the edge of a relatively deep lake. The eastern margin was characterized by a low-gradient alluvial plain with isolated ponds. This inferred paleotopography is consistent with active faulting along the western margin during sedimentation. Post-depositional sinistral faulting is indicated by traces of a regionally penetrative cleavage (and axial surfaces of related folds) that trend at an angle of 20°-40° to the average strike of the faults. Cleavage traces define sigmoidal patterns of heterogeneous sinistral shear. Zones of high strain on the western side of the basin display a lower fabric-to-fault angle than zones of low strain in the central and eastern parts of the basin. A gently plunging displacement vector is confirmed by near-horizontal stretching lineations (pebbles and quartz fibers), which reflect the finite extension direction adjacent to shear zones.

Abstract The pattern of faulting and fault-related deformation in Cretaceous to Neogene rocks, together with the distribution of Neogene sediments, suggest that Jamaica is the site of two complex, right-stepping restraining bends along the strike-slip plate boundary between the North American and Caribbean plates. Block convergence at the eastern bend between the Plantain Garden and Duanvale fault zones is manifest by topographic uplift (>2 km), rapid erosion, and northwest-southeast shortening of Cretaceous and Paleogene metamorphic, volcanic and sedimentary rocks in the Blue Mountains and Wagwater Belt. Limited data on the age of faulting in Jamaica suggest that deformation and uplift related to bends in the faults probably began in the middle to late Miocene and was roughly contemporaneous with initial strike slip along the eastern extension of the Plaintain Garden fault zone in southern Hispaniola. Uplift and deformation at the western bend is less prominent, and for the most part involves carbonate rocks that are cut by numerous west-facing fault scarps thought to be formed by east-dipping high-angle reverse faults. Both bends appear to have nucleated on northwest-striking normal faults that bounded Paleogene rifts. Maps of historic and recorded earthquakes on Jamaica indicate a close spatial association between the two restraining bends and the largest-magnitude events. In Jamaica, as in other active and ancient strike-slip zones, it is unclear how observed compressional deformation relates to the following three mechanisms: (1) restraining bend development or interaction of two parallel, overstepping strike-slip faults; (2) simple shear adjacent to a single strike-slip fault; or (3) end effects caused by termination of a single strike-slip fault.

Abstract Strike slip on various scales and on faults of diverse orientations is one of the most prominent modes of deformation in continental convergence zones. Extreme heterogeneity and low shear strength of continental rocks are responsible for creating complex “escape routes” from nodes of constriction along irregular collision fronts toward free faces formed by subduction zones. The origin of this process is poorly understood. The two main models ascribe tectonic escape to buoyancy forces resulting from differences in crustal thickness generated by collision and to forces applied to the boundaries of the escaping wedges. Escape tectonics also creates a complicated geological signature, whose recognition in fossil examples may be difficult. In this paper we examine the Neogene to present tectonic escape-dominated evolution of Turkey both to test the models devised to account for tectonic escape and to develop criteria by which fossil escape systems may be recognized. Since the late Serravallian (—12 Ma), the tectonics of Turkey has been dominated by the westward escape of an Anatolian block (“schölle”) from the east Anatolian convergent zone onto the oceanic lithosphère of the Eastern Mediterranean Sea, mainly along the North and East Anatolian strike-slip faults. This tectonic regime generated four distinct neotectonic provinces: (1) The East Anatolian contractional province, located mainly east of where the North and East Anatolian faults meet, and characterized by roughly north-south shortening; (2) the weakly active North Turkish province situated north of the North Anatolian fault, and characterized by limited east-west shortening; (3) the West Anatolian extensional province characterized by north-south extension; and (4) the Central Anatolian “ova” province characterized by northeast-southwest shortening and northwest-southeast extension. Large, roughly equant, complex basins ( “ovas” ) form peculiar structural elements of the Central Anatolian province. The two latter provinces are located within the westerly-moving Anatolian schölle. A number of pull-apart and fault-wedge basins have formed along the North and East Anatolian fault zones in addition to several other “incompatibility basins,” arising from space problems where these faults interfere with each other and with other large-scale structures. Incompatibility basins seem to have the most complicated structural history. The pull-apart basins are located on either primary or secondary releasing bends along the North and East Anatolian faults. The secondary type is related to the intersection of east-trending zones of high convergent strain with the North and East Anatolian fault zones. The tectonic escape regime in and around Turkey was not caused by buoyancy forces resulting from crustal thickness differences, but such forces may have been maintaining it. A knowledge of the geology of escape-related basins is critical both for our understanding of the nature of tectonic escape, and for its recognition in the geological record. We believe that the present tectonic scheme of Turkey constitutes an excellent guide for understanding the causes and consequences of escape, and for the recognition of its fossil representatives.

Abstract During large-scale convergence of oceanic and arc-type lithospheric fragments towards a cratonic promontory along western North America from Middle Jurassic through Paleogene time, non-subductable crust of the approaching Pacific realm was deflected dextrally northward or sinistrally southward from this ‘reverse indenter’ in the California-Nevada region. Paleontologic and paleomagnetic data suggest oblique dextral displacements on the order of 1.500 to 2,000 km for the accreted terranes in the western Cordillera of Canada. These dextral displacements were first concentrated along closing sutures (from Middle Jurassic to Early Cretaceous time); later they were also taken up by pericollisional fault zones, which propagated into the western parts of the Cordilleran thrust belt and involved the Coast Plutonic Complex (mid-Cretaceous to Paleocene). A-subduction in the thrust belt and inferred B-subduction west of the Coast Plutonic Complex were thus accompanied by dextral displacements within the Omineca and Coast fault arrays respectively, imparling northwest-directed stretching fabrics onto ductile metamorphic or igneous rocks, and discrete fault strands on high-level crustal rocks. The convergent strike-slip fault motions in the Canadian Cordillera created mainly sedimentary source areas rather than subsiding basins. Pericollisional basins that did receive clastic materials from zones of oblique convergence were (1) marginal basins in the process of closing, (2) relict or tectonically overloaded depressions on accreting terranes, (3) foreland basins created by thrust propagation in the miogeoclinal succession, and (4) small pull-apart or restraining bend depressions near high-angle strike-slip faults. Basins in the accreted terrene complexes west of the Cordilleran thrust belt received most of their detrital material from exposed volcanic, plutonic, and oceanic sedimentary rocks; the predominantly turbiditic basin fill suffered repeated deformation, high sustained heat flow, and intrusive activity. The foreland basin to the east of the thrust belt, on the other hand, received most of its detrital input from carbonate and quartz-rich clastic rocks of the miogeocline and metamorphosed equivalents; the predominantly shallow-water clastic deposits of the foreland basin experienced considerably less deformation and thermal alteration than the varied sedimentary assemblages of the accreted belt.

Abstract Eocene right-lateral displacements occurred along several major fault zones in western and central Washington, including the Straight Creek fault, the Entiat-Leavenworth fault system, and probably the Puget fault, a covered north-trending structure in the Puget Lowland. Within this strike-slip framework, nonmarine sediments accumulated in the Chuckanut, Puget-Naches, Chiwaukum graben, and Swauk Basins to form some of the thickest (more than 6,000 m) alluvial sequences in North America. To varying degrees, the basins are characterized by (1) high sediment-accumulation rates, implying rapid subsidence; (2) abrupt local stratigraphic thickening and thinning; (3) intrabasinal and basin-margin unconformities; (4) abrupt facies changes; (5) fault-induced drainage re-organization; (6) intermittent internal drainage; and (7) interbedded and intrusive relationships with extension-related(?) volcanic rocks. Similar sandstone petrography between basins suggests a common sediment source with possible local or temporary connection between basins. Sedimentation and deformation throughout the province were diachronous, and the basins experienced rapid alternating subsidence and uplift. Orientations of folds and faults are consistent with regional right-lateral shear. The 90-km-wide composite Chuckanut-Puget-Naches Basin probably formed as a pull-apart basin between the Straight Creek and Puget faults. The 50-km-wide Swauk Basin may have formed as a fault-wedge basin between the Straight Creek and Entiat-Leavenworth faults. These basins are large when compared to most modern and ancient basins controlled by strike-slip faults, suggesting that processes of strike-slip basin formation operate at a variety of scales. The 20-km-wide Chiwaukum graben probably formed as a pull-apart basin between the Entiat and Leavenworth faults. Eocene strike-slip faulting was probably driven by oblique convergence of the Kula plate below North America. Although strike-slip basins in Washington occupied a forearc or possibly an intra-arc tectonic setting in this ancient continental margin, they differ significantly from typical forearc or intra-arc basins.