Tertiary Normal Faults Research Articles

Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington

We use LiDAR imagery to identify two fault scarps on latest Pleistocene glacial outwash deposits along the North Fork Nooksack River in Whatcom County, Washington (United States). Mapping and paleoseismic investigation of these previously unknown scarps provide constraints on the earthquake history and seismic hazard in the northern Puget Lowland. The Kendall scarp lies along the mapped trace of the Boulder Creek fault, a south-dipping Tertiary normal fault, and the Canyon Creek scarp lies in close proximity to the south-dipping Canyon Creek fault and the south-dipping Glacier Extensional fault. Both scarps are south-side-up, opposite the sense of displacement observed on the nearby bedrock faults. Trenches excavated across these scarps exposed folded and faulted late Quaternary glacial outwash, locally dated between ca. 12 and 13 ka, and Holocene buried soils and scarp colluvium. Reverse and oblique faulting of the soils and colluvial deposits indicates at least two late Holocene earthquakes, while folding of the glacial outwash prior to formation of the post-glacial soil suggests an earlier Holocene earthquake. Abrupt changes in bed thickness across faults in the Canyon Creek excavation suggest a lateral component of slip. Sediments in a wetland adjacent to the Kendall scarp record three pond-forming episodes during the Holocene—we infer that surface ruptures on the Boulder Creek fault during past earthquakes temporarily blocked the stream channel and created an ephemeral lake. The Boulder Creek and Canyon Creek faults formed in the early to mid-Tertiary as normal faults and likely lay dormant until reactivated as reverse faults in a new stress regime. The most recent earthquakes—each likely M w > 6.3 and dating to ca. 8050–7250 calendar years B.P. (cal yr B.P.), 3190–2980 cal. yr B.P., and 910–740 cal. yr B.P.—demonstrate that reverse faulting in the northern Puget Lowland poses a hazard to urban areas between Seattle (Washington) and Vancouver, British Columbia (Canada).

THE ORIGIN OF MINERALIZED FRACTURES AT THE BLUEBELL MINE SITE, RIONDEL, BRITISH COLUMBIA

The Bluebell Pb-Zn deposit is located on the east side of Kootenay Lake in southeastern British Columbia. It is a fracture-controlled replacement deposit hosted in Lower Cambrian marble of the Badshot and Mohican formations. The orebodies trend west-northwest, parallel to a prominent set of fractures that controlled mineralization. These fractures are interpreted to have formed as part of a conjugate set. Contrary to some earlier suggestions, folding played no role in fracture formation. The youngest generation of folds are reinterpreted as having formed through development of shear bands during extension of the layering prior to fracture development. Formation of the fractures also postdated motion on a number of early Tertiary normal faults that are associated with regional extension and alkaline magmatism. West-northwest–trending fractures, which formed as part of a conjugate set, were reactivated and controlled the intrusion of mafic dikes, mineralizing fluids, and minor late faulting. Fracturing, faulting, mafic dike intrusion, and mineralization may be associated with a long-lived basement structure that runs below the area, which could have facilitated the ascent of deep crustal or mantle-derived magma and mineralizing fluids. The results of this study provide a new perspective on exploration strategies for hydrothermal ore deposits in this part of southeastern British Columbia.

TERTIARY TILTING AND DISMEMBERMENT OF THE LARAMIDE ARC AND RELATED HYDROTHERMAL SYSTEMS, SIERRITA MOUNTAINS, ARIZONA

Multiple lines of evidence, including new and published geologic mapping and paleomagnetic and geobarometric determinations, demonstrate that the rocks and large porphyry copper systems of the Sierrita Mountains in southern Arizona were dismembered and tilted 50° to 60° to the south by Tertiary normal faulting. Repetition of geologic features and geobarometry indicate that the area is segmented into at least three major structural blocks, and the present surface corresponds to oblique sections through the Laramide plutonic-hydrothermal complex, ranging in paleodepth from ~1 to ~12 km. These results add to an evolving view of a north-south extensional domain at high angles to much extension in the southern Basin and Range, contrast with earlier interpretations that the Laramide systems are largely upright and dismembered by thrust faults, highlight the necessity of restoring Tertiary rotations before interpreting Laramide structural and hydrothermal features, and add to the broader understanding of pluton emplacement and evolution of porphyry copper systems.

Multiple phases of Tertiary extension and synextensional deposition of the Miocene-Pliocene Salt Lake Formation in an evolving supradetachment basin, Malad Range, southeast Idaho, U.S.A.

The extensional history of the Malad and Bannock ranges in southeast Idaho and northern Utah involves multiple phases of Tertiary normal faulting and synextensional deposition. Detailed geologic mapping, structural and stratigraphic analyses, and geochronologic data from this study elucidate previously defined deformational events in the region, and define two new extensional episodes. The largest-magnitude extension took place along low-angle normal faults of the ∼10–4-Ma Bannock detachment system, with concurrent sedimentation of the Miocene–Pliocene Salt Lake Formation in a regionally continuous supradetachment basin. This basin developed by ∼10.2 Ma, and was preceded by two phases of smaller-magnitude, aerially restricted normal faulting and sedimentation. In the Henderson Creek quadrangle of the southern Malad Range, the earliest event involved ∼8% north–south extension during deposition of the Paleocene–Eocene Wasatch (?) Formation. This conglomerate unit was deposited in an asymmetric, south-tilted half-graben bounded on the south by the syntectonic Willow Spring normal fault. The next pre-detachment extensional event produced a north-striking, east-tilted half-graben in which the Middle to Late Miocene Skyline Member of the Salt Lake Formation (∼11.9–10.2 Ma) was deposited as an ash-rich alluvial fan. This half-graben is bounded on the east and south by the syntectonic Red Knoll and Spring Trail normal faults. Detrital zircon age data from a tuffaceous sandstone bed in the Skyline Member suggest incorporation of reworked zircons from the ∼12.5–15-Ma Owhyee-Humboldt volcanic field in southwest Idaho with 10.3-Ma glass. The inception of the regional Bannock detachment system is recorded by the breakup and ∼16% west-southwest extension of its hanging wall by a set of north- to north–northwest-striking Late Miocene normal faults. These faults were associated with syntectonic deposition of the Cache Valley Member of the Salt Lake Formation (∼10.2− <9.2 Ma) in a regional-scale lake system. During deposition, the northeast-dipping Steel Canyon normal fault accommodated uplift of an intrabasinal horst that shed a wedge of Third Creek Member conglomerate eastward into the lake system between ∼10.0 and <9.2 Ma. The Third Creek Member interfingers with a Cache Valley Member lake-margin tufa-bearing facies, which changes eastward into a deeper-water, micritic limestone-bearing facies. The most recent extensional event in the Henderson Creek quadrangle involved ∼9% east–west extension on the north-striking Pliocene–Quaternary Wasatch fault. A segment boundary of the Wasatch fault, consisting of a 2.5–3.5-km-wide relay ramp that formed between two right-stepping en echelon segments, lies just north of the Idaho-Utah border. In addition, a broad, north- to north–northwest-trending antiformal zone of extensional folds is present in the south half of the study area and is interpreted as a double-rollover anticline that formed progressively in Late Miocene and Pliocene–Quaternary time above two oppositely dipping listric normal faults.

Three-dimensional structural model of the Cantarell and Sihil structures, Campeche Bay, Mexico

The Cantarell and Sihil fields are located in a complex system of compressive structures in the offshore Campeche Bay province in Mexico. The geometry of the Cantarell–Sihil structure varies along trend from a simple fault-related structure in the south to a duplex consisting of the Cantarell and Sihil structures in the central part to a more complex system consisting of compressive blocks separated by Tertiary normal faults in the north. The Cantarell–Sihil structures formed during three main episodes of deformation: (1) Jurassic to Early Cretaceous extension, resulting in the formation of normal faults that displace Tithonian, Kimmeridgian, and Lower Cretaceous units; (2) Miocene compression, during which the Cantarell–Sihil thrust system was formed; and (3) Pliocene to Holocene extension, during which several of the preexisting Jurassic normal faults were reactivated.The Cantarell field produces out of three separate fault-bounded allochthonous blocks: the Akal, Nohoch, and Kutz blocks. The main field is located in the Akal structure. Secondary fields in the allochthonous block include Kutz, which formed on the crest of a downthrown fault block, and Nohoch, which formed above a west-vergent backthrust. The recently discovered Sihil field is located in a subthrust-compressive structure and consists of two structural lobes formed above the sub-Sihil fault. The Chac structure is formed at the updip edge of a tilted fault block in the autochthonous block. The detailed three-dimensional structural model is being used for future production of the remaining reserves in the Cantarell field as well as for the delineation of the Sihil field.

Pattern of Mesozoic thrust surfaces and Tertiary normal faults in the Sevier Desert subsurface, west-central Utah

Most tectonic models for the Sevier Desert basin, west-central Utah, envision it as the result of large-magnitude, normal-sense slip on a regional detachment fault. That interpretation, based principally on seismic reflection data, has helped shape views on the tectonics of the northeastern Great Basin area and, in a larger sense, the historical development of ideas about low-angle normal faulting. In recent years, however, several researchers have suggested, based on rock-mechanical, field, and subsurface evidence, that the hypothesized detachment fault does not exist and that the basin must have another explanation. Even among proponents of the detach- ment model, opinion has differed regarding the hypothesized extensional fault's total displacement, estimates for which vary widely; the timing of detachment slip; and whether the hypothesized fault represents a new Tertiary extensional structure or an extensional reactivation of the Mesozoic Pavant thrust. A comprehensive reinterpreta- tion of available subsurface data for the basin, including several previously unpub- lished seismic profiles, suggests: (1) that slip on the hypothesized detachment must have ceased by the Miocene in the southern Sevier Desert; (2) that estimates of large-magnitude offset on the hypothesized detachment are essentially unconstrained by structural data and need to be reevaluated; and (3) that models that view the hypothesized detachment as a Tertiary extensional reactivation of a Mesozoic thrust are likely incorrect. The newly available seismic data demonstrate that reflections from the Pavant thrust do indeed closely align with reflections fromthe Paleozoic/Tertiary contact in many parts of the northern Sevier Desert basin; however, to the south these same thrust fault reflections are directly traceable to a position well above the down dip projection of the presumed detachment within the Paleozoic Cricket Mountains block. Erosional truncation of the thrust faults and the absence in the south of other reflections aligned with the Paleozoic/Tertiary contact preclude extensional back- sliding on a Mesozoic thrust fault. These interpretations, if correct, are incompatible with the detachment hypothesis and necessitate alternative explanations for the basin's origins.

Early Tertiary Anaconda Metamorphic Core Complex, southwestern Montana

A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting.

Cenozoic deformation history of the Tancheng–Lujiang Fault Zone, north China, and dynamic implications

Abstract Based on a field analysis of slip vectors from Cretaceous and Tertiary rocks and coupled with rift basin analysis in north China, the Cenozoic deformation history of the Tancheng–Lujiang (Tan–Lu) Fault Zone can be divided into three main phases: early Tertiary normal faulting and northwest–southeast extension; Miocene normal faulting and northeast–southwest to north‐northeast–south‐southwest extension; and Quaternary dextral strike–slip faulting and east‐northeast transpression. The early Tertiary extension, which was responsible for rift basin formation in north China, originated from back‐roll mantle convection induced by westward subduction of the Pacific Plate beneath the Asia continent. The Miocene extension occurred possibly in association with the process of the Japan Sea opening. The Quaternary dextral slip was mainly localized along the middle part of the Tan–Lu Fault Zone and resulted from the far‐field effect of the late‐stage India–Eurasia convergence.

Structural Controls on Carlin-Type Gold Mineralization in the Gold Bar District, Eureka County, Nevada

The Gold Bar district in the southern Roberts Mountains, 48 km northwest of Eureka, Nevada, contains one main deposit (Gold Bar), five satellite deposits, and other resources. Approximately 0.5 Moz of gold have been recovered from a resource of 1,639,000 oz of gold in Carlin-type gold deposits in lower plate, miogeoclinal carbonate rocks below the Roberts Mountains thrust. Host rocks are unit 2 of the Upper Member of the Devonian Denay Formation and the Bartine Member of the McColley Canyon Formation. Spatial and temporal relations between structures and gold mineralization indicate that both pre-Tertiary and Tertiary structures were important controls on gold mineralization. Gold mineralization occurs primarily along high-angle Tertiary normal faults, some of which are reactivated reverse faults of Paleozoic or Mesozoic age. Most deposits are localized at the intersection of northwest- and northeast-striking faults. Alteration includes decalcification, and to a lesser extent, silicification along high-angle faults. Jasperoid (pervasive silicification), which formed along most faults and in some strata-bound zones, accounts for a small portion of the ore in every deposit. In the Gold Canyon deposit, a high-grade jasperoid pipe formed along a Tertiary normal fault which was localized along a zone of overturned fault-propagation folds and thrust faults of Paleozoic or Mesozoic age.

Tertiary Normal Faulting in the Canyon Range, Eastern Sevier Desert.

The contact between pre-Mesozoic and Tertiary rocks in the western Canyon Range, west-central Utah, has been interpreted as a large, low-angle normal fault that marks the breakaway zone of the hypothesized, basin-forming Sevier Desert detachment. Recent fieldwork suggests that the contact may in fact be depositional along much or all of its length. Deformational fabric in the supposed footwall likely traces to the Mesozoic Sevier orogeny rather than to Tertiary detachment faulting. Kinematic indicators at the range front are not generally consistent with low-angle normal-fault motion; instead, well-exposed high-angle faults are the dominant range-bounding structures. The Tertiary conglomerates of the western Canyon Range foothills, previously viewed as an evolving syntectonic deposit related to detachment faulting, are here reinterpreted as three distinct units that reflect different periods and tectonic settings. The pattern in these conglomerates, and in fault-offset gravity-slide deposits that mantle the western foothills, is consistent with block faulting and rotation along several generations of high-angle structures. Local seismic-reflection data lend qualitative support to this interpretation, and underscore the need to consider alternative working hypotheses for evolution of the Sevier Desert basin.

Ultracataclasis, sintering, and frictional melting in pseudotachylytes from East Greenland

Large volumes of pseudotachylyte (an intrusive, fault-related rock interpreted to form by a combination of cataclasis and melting) occur in Tertiary normal faults and accommodation zones along ∼400 km of the East Greenland volcanic rifted margin. Analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures, mineralogies, and chemistries in the aphanitic pseudotachylyte matrix. Three distinct types of pseudotachylyte (referred to as angular, rounded and glassy) are identified based on these characteristics. Angular pseudotachylyte (found primarily in dike-like reservoir zones) is characterized by angular grains visible on all scales, with micron-scale fragments of mica and amphibole. Its matrix is enriched in Fe 2O 3, MgO, and TiO 2 relative to the host rock, with minor increases in CaO, K 2O, and small decreases in Na 2O. Rounded pseudotachylyte is found in reservoir zones, injection veins (pseudotachylyte-filled extension fractures), and fault veins (small faults with pseudotachylyte along their surfaces). It is characterized by smooth-surfaced, compacted grains on microscopic scales, and encloses rounded, interpenetrative lithic clasts on outcrop scale. Its matrix is enriched in Fe 2O 3, MgO, TiO 2, and Al 2O 3 relative to the host rock, with minor depletion in Na 2O and K 2O. Glassy pseudotachylyte is found primarily along fault surfaces. Its matrix is characterized by isotropic, conchoidally fractured material containing microscopic, strain-free amphibole phenocrysts, and is enriched in TiO 2, Al 2O 3, K 2O, Fe 2O 3, MgO, CaO, and Na 2O relative to the host rock. These observations suggest that angular pseudotachylyte was produced by cataclasis, with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock. Cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting, producing rounded and glassy pseudotachylyte, respectively. Compositional and textural observations constrain the temperatures reached during frictional heating (700–900°C) which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip. Our results suggest that the East Greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels. Consistent relative ages and widespread occurrence of pseudotachylyte-bearing faults in East Greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin.

Origin of anomalous Tertiary subsidence adjacent to North Atlantic continental margins

At the beginning of the Tertiary, the northern North Atlantic continental margins were characterized by voluminous igneous activity caused by continental rifting in the presence of the Iceland hot-spot. Many of the Mesozoic sedimentary basins close to these margins have been permanently uplifted by igneous underplating and remain close to or above sea level at the present day. However, a significant number of other basins underwent rapid subsidence at the same time, sometimes by as much as 3 km of sediment-loaded subsidence in 15–20 Ma. Several hypotheses attempting to explain the cause of this Tertiary subsidence have been proposed. The observational data are summarized here and then each hypothesis is considered in turn. It is argued that many of these are implausible. One possible alternative explanation links Early Tertiary subsidence/uplift and magmatism. If basaltic melts are trapped within the lithosphere, it is possible to produce either uplift or subsidence depending on the density of the trapped melt. At depths greater than 50 km, melts will solidify rapidly in the garnet stability field to form primary eclogite, which is dense enough to cause subsidence, while satisfying the constraints of long-wavelength Airy isostasy. Unfortunately, such an hypothesis is difficult to test. The simplest explanation for Tertiary subsidence is still a minor episode of lithospheric stretching, although to date there is only limited evidence for sufficient Tertiary normal faulting. Further work must be carried out before this simple and testable hypothesis can be confidently rejected.

The Relief Canyon gold deposit, Nevada; a mineralized solution breccia

The Relief Canyon gold deposit in the Humboldt Range of western Nevada is a low-grade, high-tonnage orebody of Tertiary or younger age. The host rocks include limestones of the Triassic Cane Spring Formation, which are overlain by shales of the Triassic Grass Valley Formation. The rocks were folded and metamorphosed to greenschist grade during Jurassic and Cretaceous regional tectonic activity. Mesozoic thrusting may have occurred along the shale-limestone contact, but evidence has been obscured by later hydrothermal activity. The sedimentary rocks were nominally offset along several Late Tertiary normal faults related to uplift of the range.The upper part of the Cane Spring Formation is composed of a poorly sorted breccia composed of limestone clasts with a clay matrix. Irregular pockets within this zone are filled with clay- to pebble-sized fragments derived from the Grass Valley shale. The enclosing limestone beds were tilted moderately to the southwest during Mesozoic deformation, whereas bedding within these pockets is generally horizontal, indicating post-tilting deposition of the sediments. The sediments show graded bedding and other sedimentary features that indicate deposition from flowing water. Thermally mature carbon derived from the limestone is also concentrated in small pockets in the matrix. The breccia unit is likely the product of low-temperature solution brecciation. Ground water dissolved much of the limestone directly beneath the shales, progressively creating irregular cavities and the breccia. Sediments derived from the overlying Grass Valley shale were fiuvially deposited as a matrix to the developing solution breccia.Episodic pulses of hydrothermal fluids were introduced along faults and possibly mixed with the ground water in the breccia zone. Initially, jasperoids formed along the faults, but later hydrothermal pulses introduced gold, silica, and fluorine into both the early jasperoids and the unconsolidated cave-fill sediments to form the orebody. Continued solution-related brecciation chaotically disrupted the gold deposit.Gold, fluorite, pyrite, silver, calcite, and fine-grained silica are the principal hydrothermal minerals in the deposit. Gold was deposited as micron-sized flakes of native gold and rarely as electrum during a relatively late stage of silicification of the jasperoids, the carbon-rich zones, and the clay-rich matrix of the breccia. Fluorite was deposited with and later than the gold in the jasperoids, and it in part replaced the clay-rich breccia matrix. Antimony, arsenic, mercury, and thallium are directly associated with gold in the orebody.The deposit formed at a relatively shallow depth. On the basis of fluid inclusion data, late-stage hydrothermal fluids related to gold and fluorite deposition were extremely dilute and had temperatures near 200 degrees C. The fluid inclusions in fluorite show no evidence for boiling, but porous crackle breccias in the jasperoids suggest that hydrobrecciation took place.

Reactivation of a Cretaceous Thrust Surface by Basin-and-Range Extension, Southwestern Gunnison Plateau, Central Utah

Recent work in central Utah has outlined the broader features of the area's thrust geometry. Although individual interpretations differ, most models indicate the region is similar to better understood segments of the Cordilleran thrust belt. Poor and limited seismic data and the paucity of drill-hole information has hindered the investigation of the south- west quadrant of the Gunnison Plateau. Detailed surface mapping of a centrally located quadrangle has provided strati- graphic evidence for a previously unrecognized Cretaceous-Paleocene thrust fault. The present expression of the thrust surface requires reactivation with a reverse sense of motion during Tertiary extension. Drastic facies changes in the Late Cretaceous-Early Paleocene North Horn Formation occur over a short distance - essentially across a fault. The gray, eastern lithofacies is dominated by massive conglomerate, sandstone, and oncolitic limestone. Maximum thickness is 253 m (830 ft). Clast composition indicates a local source/positive area of Cretaceous lndianola Group. The thinner 38 m (126 ft), red, western lithofacies consists of pebbly sandstone, sandstone, and mudstone. Reddish brown mudstones and intrac lasts suggest red beds, possibly the Cretaceous Cedar Mountain Formation and/or the Jurassic Twist Gulch Formation and Arapien Shale, as a primary source. After careful consideration of several hypotheses, the only valid conclusion seems to be the presence of thrusted Indianola, creating a topo- graphic barrier between the two separate and unique basins. Two major structural features can be traced to the north, an area of better seismic and drill-hole data. The southern terminus of the Wasatch fault zone, has been interpreted by numerous investigators as eithera reactivated Cretaceous thrust (with a reverse sense of motion) or a high-angle planar Tertiary normal fault. The Escarpment fault, the eastern boundary of a Basin and Range graben, has also been presented in both configurations (listric or high-angle planar). No observations were made to support a listric geometry for the Fayette fault. The Escarpment fault, however, appears to have been active during North Horn time, thrusting lndianola conglomerate into a topographic/structural high. The upper plate underwent a reversed sense of motion in the Tertiary to create the present Chriss/Mellor graben.

Crustal structure of eastern Nevada from COCORP deep seismic reflection data

The western Nevada segments of the COCORP 40°N deep seismic-reflection survey of the North American Cordillera reveal the geometry of structures of Cenozoic and possibly earlier ages to travel-times of > 10 s, corresponding to depths of >30 km. The most striking feature of the data is a band of prominent reflections, typically at traveltimes of 9.5 to 10.5 s, that are present discontinuously across the entire data set. Few reflections are observed from beneath the base of this reflective zone, which is interpreted as the crust-mantle transition. This “reflection Mono” is inferred to be continuous across the survey area, varying gradually in depth but without resolvable offsets. It appears to have taken its present form or position during basin-range crustal extension. The middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west. These reflections may represent intrusions or shear zones related to basin-range or pre-basin-range extension, but some are likely to be inherited from earlier compressional deformation. Reflections from the upper crust are interpreted as images of basin-fill strata, basin-range normal faults, and Mesozoic and Paleozoic thrusts related to back-arc thrusting and accretion of oceanic and arc-related rocks.

Hydrothermal systems and Tertiary low-angle normal faulting in the southwestern United States

Many Tertiary normal faults in the southwestern United States dip gently (<30°). Some of these normal faults apparently were initiated with gentle dips, in conflict with the predictions of elementary dynamic analysis. Close correspondence between low-angle normal faulting and intense hydrothermal alteration in the Mojave Desert, western Arizona, and the Rio Grande rift is consistent with the hypothesis that hydrothermal systems play an important role in the genesis of these faults. We present a mechanical model to demonstrate that low-angle normal faults could form in response to stress trajectories that become reoriented in a periodically sealed geothermal system beneath a sloping surface.

Structural Development and Oil Occurrence on Northeast Flank of Uinta Mountains near Irish Canyon, Northwestern Colorado: ABSTRACT

The study area is located along Vermillion Creek, 1-3 mi (1.6-5 km) east of Irish Canyon, in northwestern Colorado. The exposed stratigraphic section consists of steeply dipping to vertical Upper Cretaceous Almond Formation, Ericson Sandstone, and Mancos Shale along the toe of the Sparks Ranch thrust fault, and of lesser dipping older Mesozoic and Paleozoic formations in distant parts of the thrust plate. In most places, the Almond Formation is in contact across the thrust fault with the Eocene Wasatch and Green River Formations, and all of these formations are unconformably overlain by the Oligocene Bishop Conglomerate and the Miocene Browns Park Formation. The structural development of the area has involved three major events: (1) Late Cretaceous uplift of the Uinta Mou tains; (2) Paleocene and Eocene thrust movements of the Sparks Ranch fault; and (3) late Tertiary normal faulting associated with a collapse of the eastern Uinta Mountains. Oil-saturated sandstones are present in outcrops of 10 Paleozoic, Mesozoic, and Tertiary formations adjacent to several of the late Tertiary normal faults. Oil is escaping to the surface along these faults, probably from a large, deep-seated reservoir located below the Sparks Ranch thrust fault. End_of_Article - Last_Page 863------------

Distribution in Time and Space of Late Phanerozoic Normal Faulting in Nevada and Utah

To understand the timing of extensional deformation which has occurred in the Great Basin since the close of the Sevier orogeny, a compilation of normal fault ages was made for Nevada and western Utah. Available literature indicates that normal faulting occurred there on a regional scale in the Late Mesozoic and Early and Middle Tertiary as well as in the Late Tertiary and Quaternary. There is no apparent spatial pattern in the age of initial normal faulting in Nevada and Utah; apparent areal differences in the intensity of fault development for a given time period may be real or a function of missing stratigraphic units needed to define properly the ages of faulting. Although Late Tertiary and Quaternary normal faults would probably be classified as basinrange faults by most workers, a question remains as to the nature of the Late Mesozoic and Early and Middle Tertiary normal faults. They could represent initial basin-range type extension, or they could represent a distinct episode of extension between the Sevier and "BasinRange" orogenies.

BLUE RIDGE FRONT—A FAULT SCARP

The present Blue Ridge scarp seems to have been produced by a late Tertiary normal fault—the Blue Ridge border fault. Evidence of its existence can be found along an almost straight line extending about 700 miles from the vicinity of Gainesville, Georgia, to northwestern New Jersey. Throughout much of its length, the Blue Ridge border fault seems to have utilized pre-existing structures of an older Triassic fault zone which borders the Blue Ridge scarp on the east. The Asheville peneplain of the upthrown fault block is considered correlative with the Piedmont peneplain on the downthrown block. Both are probably correlative with the Schooley peneplain. The Asheville surface descends gradually via the Dahlonega Plateau from elevations approximating 2100 feet on the strath of the French Broad River near Brevard, North Carolina, to coalescence with the Piedmont surface southwest of Hightower, Georgia, at elevations approximating 1200 feet. The higher mountains of the Blue Ridge Upland are pre-fault features, and such outlying ranges as the Brushy and South mountains of the upper Piedmont of North Carolina are their equivalents on the downfaulted block. The higher mountains of the Southern Appalachians are on the upfaulted block in the region of greatest uplift. The southwestern extension of the Southern Appalachians beyond the end of the fault was not uplifted.