Northeast-trending Faults Research Articles

Geochronological constraints on the development of the Cross Lake greenstone belt, northwest Superior Province, Manitoba

Recent 1: 20 000 mapping combined with 11 age determinations from metavolcanic, metaplutonic, and metasedimentary rocks in the Cross Lake greenstone belt provide a framework for interpreting the stratigraphic, structural, intrusive, and metamorphic evolution of the belt. These data, in combination with U–Pb ages from adjacent granitoid domains, are used to model a cratonizing event in the northwest Superior Province.The Cross Lake greenstone belt is made up of three distinct unconformable groups of supracrustal rocks: (i) the Pipestone Lake Group (2760 Ma), a thick sequence of pillowed and massive tholeiitic basalt flows and related high level gabbros, inter-preted as a back-arc spreading sequence; (ii) the Gunpoint Group (2730 Ma), a fining-upward sequence of clastic sediments interbedded with rhyodacitic fragmental volcanic rocks; and (iii) the Cross Lake Group (<2710 Ma), a fining-upward, fluvial–marine clastic sedimentary sequence, with shoshonitic volcanic rocks near the top, deposited in a restricted basin. Angular unconformities occur between (i) the Pipestone Lake Group and the overlying Gunpoint and Cross Lake groups, and (ii) the Gunpoint Group and the younger Cross Lake Group.The greenstone belt is flanked by the plutonic Molson Lake Domain on the south and the largely metaplutonic Pikwitonei and granite–greenstone Gods Lake domains on the northwest. The northern contact is a major northeast-trending fault system with a dextral strike-slip component and vertical component indicative of relative uplift of the northwest side. The southern contact is a much broader zone of deformation in which a number of east-southeast-trending dextral fault zones imbricate Pipestone Lake and Gunpoint Group supracrustal rocks and, to a lesser extent, Molson Lake plutonic rocks.

Rapid Intraplate Strain Accumulation in the New Madrid Seismic Zone

Remeasurement of a triangulation network in the southern part of the New Madrid seismic zone with the Global Positioning System has revealed rapid crustal strain accumulation since the 1950s. This area experienced three large (moment magnitudes >8) earthquakes in 1811 to 1812. The orientation and sense of shear is consistent with right-lateral strike slip motion along a northeast-trending fault zone (as indicated by current seismicity). Detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time.

Petrographic and geochemical evidence for a hypogene origin of granite-hosted, vein-type Mn mineralization at the New Ross Mn deposits, Lunenburg County, Nova Scotia, Canada

The New Ross Mn deposits consist of two centers of past mining activity located in the 370 Ma South Mountain batholith 10 km north of New Ross, Lunenburg County. The deposits occur as Mn oxide veins and lenses formed along steeply dipping northeast-trending faults. Previous investigators disagree as to whether the most economically important Mn oxides, pyrolusite and manganite, are of supergene or hypogene origin. Recent diamond drilling and geologic mapping have provided new data that support a hypogene origin.The South Mountain batholith in the area of the deposits consists mostly of coarse-grained megacrystic biotite monzogranite. A swarm of small, elongate leucomonzogranite plutons both intrude and are in fault contact with the monzogranite to the north and west of the deposits and at depth below one of the mine workings. These plutons are elongate parallel to a series of persistent northeast-trending topographic linears, which appear to represent faults and/or shear zones. This spatial association suggests that these structures played a role in the emplacement of the plutons. Brecciation and hydrothermal alteration of the leucomonzogranite plutons indicates that movement and hydrothermal activity continued on the faults after final crystallization of the plutons.Several types of hydrothermal alteration are well developed in the faults hosting the Mn mineralization and along several other northeast-trending structures in this region. The diamond drilling shows that, although the Mn mineralization is absent at depth, the fault zone and hydrothermal alteration persist to at least 452 m below the mine workings. Low-temperature hematization and argillic alteration is best developed within the main ore-bearing zone from surface to a depth of about 150 m. With increasing depth, the amount of argillic alteration decreases but hematization remains well developed. Desilicification (episyenitization) accompanied hematization at several localities in the megacrystic monzogranite and the leucomonzogranite intrusions. The desilicification is developed to the point that no primary quartz remains in the rock. Higher temperature alterations such as silicification, greisenization, K feldspathization, and albitization were caused by emplacement of the leucomonzogranite. The silicification occurs as total replacement of the rock and grades through greisenized selvages into unaltered leucomonzogranite or monzogranite. Textural evidence indicates that precipitation of carbonate minerals occurred prior to, during, and after crystallization of the Mn and Fe oxides.The activity of phosphate during the alteration sequences is of particular importance. Below the mine, P 2 O 5 enrichment occurs in the higher temperature silicified, greisenized, K feldspathized, and albitized rocks (up to 5.20 wt % P 2 O 5 ) and is manifest as abundant secondary fluorapatite. Phosphate remained present in the hydrothermal fluid at lower temperatures and resulted in crystallization of fluorapatite with Mn and Fe oxides in the mineralized zones and formation of a 1.2-m-wide quartz-fluorapatite-manjiroite vein (9.95 wt % P 2 O 5 ). Samples of manganite and pyrolusite often contain vug fillings and inclusions of rhodochrosite and minor amounts of other hypogene minerals such as knebelite and pinakiolite. Furthermore, these samples also contain anomalous amounts of Be (90 ppm), W (140 ppm), Mo (28 ppm), Pb (3,000 ppm), and Cu (1,500 ppm) which is consistent with their formation from fluids derived from a magmatic source. These textural relations and geochemical signatures indicate that the alteration sequence and Mn oxides, including those that were previously mined, could all be part of an evolving hydrothermal system developed in upwelling hydrothermal fluids. These processes operated along other northeast-trending faults throughout much of this region of the South Mountain batholith, which suggests a high potential for as yet undiscovered deposits of Mn and other granophile elements.

VULCAN SUB-BASIN FAULT STYLES — IMPLICATIONS FOR HYDROCARBON MIGRATION AND ENTRAPMENT

Several structural domains are recognised within the Vulcan Sub-basin, Timor Sea. These domains developed during the Jurassic rifting phase and are separated by major transfer zones which trend in a northwest-southeast direction. Within each domain are frequent third order transfers which sub-divide the main northeast trending fault blocks into numerous compartments. These enable structural hydrocarbon traps to be formed, despite a predominant regional dip. They also affect migration pathways.Jurassic fault styles include detached rotational blocks, salt-associated features, tilted fault blocks and 'hourglass' horsts and grabens. These generally have a northeast-southwest orientation. The transfer faulting complicates these features and forms zones of structural complexity with associated poor seismic data quality. A separate fault episode in the north of the sub-basin during the Tithonian resulted in an east-west fault set overprinting the earlier structuring.Intra-Cretaceous fault movement is also recognised and has an important role in early hydrocarbon entrapment.Structural reactivation during the Late Miocene/Early Pliocene of the earlier fault sets modified the geometry of many existing traps. Numerous new traps may also have formed as a result of this tectonism. In many places the resulting geometry is complex, particularly where the younger fault orientation is at an angle to the main Oxfordian fault set. The late-stage movement is primarily extensional, manifested by predominantly normal faults; overall, however, a varying component of strike slip is likely. A divergent strike-slip zone is recognised at the southwest end of the Cartier Trough.The effects of the late stage tectonism tend to mask the seismic expression of Mesozoic hydrocarbon traps resulting in many wells being drilled off-structure at the target horizon. An understanding of the deeper structuring should result in further discoveries in this prospective basin.

Vulcan Graben, Timor Sea; Regional Structure From a Magnetic Survey

In late 1989 the Australian Bureau of Mineral Resources (BMR) carried out a regional aeromagnetic survey by contract over the Vulcan Graben region. Elongate short-wavelength anomalies, with wavelengths of 3 to 12 km, amplitudes of 0.2 to 2.0 nT, and with generally northeast strikes, can be correlated for distances of up to 100 km on the basis of their shape and amplitude. Their source appears to be principally at depths of between 0.7 and 3.0 km. Some of the relatively larger amplitude and wavelength anomalies overlie major northwest-dipping normal faults which have been mapped by seismic. All of the elongate anomalies are thought to be due to structures or magnetic deposits associated either directly with NE-trending normal rift faults, or with faults in the post-rift sediments which are related to rift fault reactivation. ENE fault trends, which developed in the Neogene as a result of the collision with Timor, do not appear on the aeromagnetic data. We have mapped numerous northwest-trending faults. These have a spacing of approximately 10 to 25 km. They are thought to be strike-slip because some offset the major northeast-trending normal faults. The northwest trending faults may be due to the reactivation of Palaeozoic trends, the reactivation of Mesozoic transfer faults, or possibly some may be younger and independent of the earlier faults. Some of these faults have previously been mapped on seismic and correspond to seismic 'bad data' areas. These faults may have a major role in the entrapment of hydrocarbon accumulations in the Timor Sea, with most of the significant fields being close to a northwest-trending fault or its extension. In the Timor Sea, high resolution aeromagnetics have proven to be a relatively cheap and cost-effective tool which both complements and supplements the existing seismic data. It should prove equally useful in other areas with extensive seismic coverage. In frontier areas, it should rapidly define the geometry of the major tectonic elements of the basin, allowing for better positioning of seismic programs.

Development History of Deep Water Plio-Pleistocene Sands in the East Breaks 165 Field, Gulf of Mexico

The stratigraphic complexity of the Plio-Pleistocene reservoir sands in the East Breaks 165 field (offshore Texas) is readily demonstrated after the completion of exploration and development involved: (1) prediction of turbidite sand facies distributions and thickness, and (2) identification of fault offsets in the high variable reservoir sands. The well control and a 3D seismic data set provide the basis for reservoir description and a deepwater sand depositional model in the East Breaks 165 area. Four main productive sand intervals are present in the Plio-Pleistocene section. All are characterized by rapid lateral thickness and facies and fining-upward channel levee/overbank facies. The channel systems developed within an intraslope basin. The main reservoir structure is a highly faulted anticline located downthrown to northeast-trending extensional faults with up to 2,500 feet of displacement. The faulting is result of structural collapse owing to salt withdrawal from a salt-cored anticlinal ridge. Numerous small-scale faults that juxtapose permeable and impermeable units have added further complexity to the field development. The refined turbidite channel and channel levee/overbank model has been important in delineating future recompletion and development targets in the East Breaks 165 field. This model should also prove to be a useful analog for future Gulfmore » of Mexico deepwater exploration and development programs.« less

Crustal Structure and Tectonic Evolution of the Potiguar Basin--Northeast Brazil

The combined analysis of potential field and seismic reflection data suggest that the Potiguar basin of northeast Brazil developed as a consequence of northwest-southeast extension, resolved in the upper crust by hanging-wall block rotation along a system of linked faults. This extension developed during the Early Cretaceous rifting associated with the separation of South America and Africa. The basic basin architecture includes two main parallel northeast-trending half-grabens (Umbuzeiro and Boa Vista) separated by intervening basement highs and northwest-trending transfer faults. A three-dimensional crustal-scale model of the basin was generated by combining deep and shallow seismic reflection data, borehole and outcrop information, and potential field data. The deep seismic profiles reveal a highly reflective crust underlain by a high-amplitude reflection Moho around 10 seconds (roughly 30 km depth). Moreover, it seems that the basin-bounding normal faults terminate at a mid-crustal detachment surface, suggesting that the basin architecture is mainly controlled by two parallel listric normal fault ones that follow pre-existing upepr crustal weaknesses, partially reactived during the main rift episode (Neocomian). Whereas the pre-existing northeast-trending planar fabrics within the basement (schistosities, gneissosity, and shear zones) apparently controlled the main normal faults, internal conjugate and synthetic transfer faults are probably related tomore » major discontinuities in basement rock fabrics. Seismic reflection data together with gravity and magnetic modeling, show that the basin architecture is directly related to a linked system of northeast-trending listric normal faults and a mid-crustal detachment surface.« less

Alteration and fluid inclusion study of the Groundhog vein system, Central mining district, New Mexico

Polymetallic vein and skarn orebodies in the Groundhog mine, Central district, New Mexico, produced more than $590 million in silver, zinc, copper, lead, and gold between 1870 and 1977 (1990 prices). Mineralization occurs along the northeast-trending Groundhog fault zone, and underground workings extend to within 2 km of the Santa Rita porphyry copper deposit. Vein orebodies were mined from the upper 200 m of the deposit whereas extensive skarn and replacement orebodies were mined between 400 and 700 m below the surface.In the upper 200 m of the deposit, early Tertiary granodiorite porphyry dikes, injected along the Groundhog fault zone, crosscut Upper Cretaceous sandstone, shale, and two stages of diorite sills. Propylitic alteration characterized by albite, chlorite, calcite, epidote, and sericite accompanied each igneous intrusion.Hydrothermal fluid circulation followed a stage of normal faulting, resulting in quartz-base metal-silver precipitation in open fissures such as the Crescent ore shoot (330 X 120 X 7 m in size). Next to veins, diorite sills are altered to quartz-sericite-pyrite-clay, with local silicification. Away from veins, sericite decreases and chlorite-pyrite becomes more abundant. Vein-related chlorite is 5 to 16 times more manganese rich (2.8-8 mole % MnO) than chlorite formed in the propylitic alteration stage (0.4-1.4 mole % MnO).Paragenetic vein relationships indicate that early quartz and pyrite were followed by mainstage base metal mineralization, consisting dominantly of sphalerite, galena, chalcopyrite, and pyrite. Sphalerite is more iron and manganese rich at depth in the vein system; FeS increases from 0.8 to 7.3 mole percent and MnS increases from 0.2 to 1.2 mole percent over a 350-m vertical sampling range.Fluid inclusions in vein quartz homogenize between 270 degrees and 405 degrees C, with evidence for local boiling at 390 degrees C. Vapor-rich inclusions were only observed in preore quartz. Primary inclusions in sphalerite homogenize between 270 degrees and 340 degrees C, with a peak at 305 degrees C. The hydrothermal fluid contained an average of 7 equiv wt percent NaCl. Daughter minerals, possibly including dawsonite, anhydrite, and carbonate, are present in some inclusions.Alteration zoning, fluid inclusion data, and element zoning within Groundhog vein orebodies suggest that mineralization occurred during mixing with late-stage meteoric water. Fluid flow in a southwest direction has been documented in the underlying skarn ore zone (Meinert, 1987), whereas fluid flow in the vein ore zone appears to have been primarily vertical, with a smaller component of lateral flow. A pressure-depth estimate determined from fluid inclusion analyses suggests that 2 km of volcanic rocks may have been present above the Groundhog area at the time of mineralization and has since been removed by erosion. Comparison with other base and precious metal vein systems suggests that precious metal mineralization may have been present in the overlying, now-eroded volcanic rocks and may still constitute an unexplored target in the peripheral parts of the district.

Locating the Sherwood Sandstone aquifer with the aid of resistivity surveying in the Vale of York

Abstract The Sherwood Sandstone aquifer in the Vale of York is generally confined beneath thick clay drift in the west and by thin drift plus an increasing thickness of Mercia Mudstone towards the east. Published geological maps show a gentle easterly dipping succession cut by one major east-west fault passing through Stamford Bridge. The junction between the Sherwood Sandstone and the Mercia Mudstone is shown by a north-south trending line south of this fault. Recent boreholes for water supply have shown that the structure is more complex however, and reliable predictions of depths to the aquifer frequently cannot be made without pre-drilling surveys. Consequently the results of over 50 resistivity sounding have been used in conjunction with all available well logs to redraw the important hydrogeological boundary between the Sherwood Sandstone and the Mercia Mudstone across the central part of the Vale. The results have shown that depths to the aquifer, up to around 200 m, can generally be predicted prior to drilling to within 100y surface resistivity methods, except where the ‘Mudstone’ is very thin in the ‘feather-edge’ zone. The results have also shown the presence of many persistent east-west or northeast trending faults with vertical throws up to 80 m and some strike faults or monoclines in this part of the Vale. The significance of these faults to groundwater flow and water extraction are discussed. Wireline logs at six wells suggest the occurrence of at least two thick units of gypsum and anhydrite within the Mercia Mudstone. These cause high, 100-150 ohm m, depth averaged resistivity values to be computed for the Mercia Mudstone.

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km 2 which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins. Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S 2 accompanied by variations in the activity of Te 2 during the Gold stage. Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO 2 vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization. Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.

Tectonic control on the origin and orientation of igneous layering: An example from the Greendale Complex,Antigonish Highlands, Nova Scotia, Canada

Layering in appinites (hornblende-rich gabbros) of the ca. 610 Ma Greendale Complex, Antigonish Highlands, Nova Scotia, is defined by variations in texture and modal abundances of hornblede and plagioclase, and ranges in width from 5 to 50 cm. Regional studies indicate that the complex was probably emplaced during late Precambrian dextral shear on northeast-trending faults by brittle failure at the roof of the magma chameber. The origin and orientation of the layers may have been controlled by the regional tectonic setting. The layering is steep to vertically dipping and varies in strike from 080° to 180° (clockwise) with concentrations at 090° and 160°. The 090° layers are dilational and are thought to have developed perpendicular to north-south extension along the extensional plane of the instantaneous strain ellipsoid associated with progressive dextral shear. The 160° layers commonly display boudinage. They may have rotated clockwise from an original 090° attitude toward the plane of flattening (and hence into a stretching field) during progressive dextral shear. Late-stage cogenetic felsic veins fill conjugate shear fractures consistent with a dextral shear regime. This study emphasizes the potential importance of considering regional tectonic setting when analyzing the origin and orientation of igneous fabrics.

Evolution of the San Juan Basin, Colorado, and New Mexico Basin Analysis Case Study: ABSTRACT

During 1985 to 1989, the US Geological Survey conducted a basin analysis study that investigated the sedimentological and structural history of the San Juan basin and vicinity. The San Juan basin is a large asymmetric basin in southwest Colorado and northwest New Mexico near the southeastern edge of the Colorado Plateau. It formed as a result of northeast-directed compression coupled with a slight northeastward shift of the Colorado Plateau during the Late Cretaceous to Oligocene Laramide orogeny. Although the basin assumed its final configuration during the Laramide orogeny, regional stratigraphic and sedimentologic studies show that uplifts active during the Laramide orogeny were also active in the geologic past. Analysis of 1,000 mi of reflection seismic data defines a rectilinear pattern of northwest- and northeast-trending faults that involved the Precambrian basement in basin development. Middle Pennsylvanian to Late Permian, Middle to Late Jurassic, Early Cretaceous, Late Cretaceous-Oligocene, and Miocene were major times of movements on the faults and/or bounding uplifts. Preserved Mesozoic and older sedimentary rocks in the basin accumulated as part of widespread regional depositional systems. Pennsylvanian strata, consisting of a thick sequence of clastic and carbonate rocks, and evaporites deposited in both open and restricted marine environments contain significantmore » oil and gas resources. Mesozoic strata deposited in a variety of widespread continental and marine environments host 50% of the country's uranium reserves. Five marine transgressive-regressive cycles during the Cretaceous resulted in a thick sequence of intertonguing marine and continental rocks containing large resources of coal, oil, and gas.« less

Transverse Ranges and neotectonics of southern California

The Transverse Ranges and the east-trending folds and reverse faults that elevate them began forming in mid-Pleistocene time by regional north-south crustal shortening. The adjacent Mojave Desert and Basin and Range provinces continue to respond to this regional strain by east-west crustal extension. Before ∼5 Ma the regional structure was characterized by conjugate northwest-trending right-slip faults (San Andreas set) and northeast-trending left-slip faults (Garlock set). Thereafter, the San Andreas set of faults became simple shears separating the North American and Pacific plates. With the mid-Pleistocene inception of the Transverse Ranges, the San Andreas fault deviated from its N40°-45°W trend in short N75°W-trending segments on the north and south sides of these mountains in response to the new, and local, strain system of upward crustal extension.

Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987

AbstractThe M 6.2 Elmore Desert Ranch earthquake of 24 November 1987 was associated spatially and probably temporally with left-lateral surface rupture on many northeast-trending faults in and near the Superstition Hills in western Imperial Valley. Three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the Superstition Hills fault zone as far as 9 km; the maximum observed surface slip, 12.5 cm, was on the northern of the three, the Elmore Ranch fault, at a point near the epicenter. Twelve hours after the Elmore Ranch earthquake, the M 6.6 Superstition Hills earthquake occurred near the northwest end of the right-lateral Superstition Hills fault zone. Surface rupture associated with the second event occurred along three strands of the zone, here named North and South strands of the Superstition Hills fault and the Wienert fault, for 27 km southeastward from the epicenter. In contrast to the left-lateral faulting, which remained unchanged throughout the period of investigation, the right-lateral movement on the Superstition hills fault zone continued to increase with time, a behavior that was similar to other recent historical surface ruptures on northwest-trending faults in the Imperial Valley region.We measured displacements over 339 days at as many as 296 sites along the Superstition Hills fault zone, and repeated measurements at 49 sites provided sufficient data to fit with a simple power law. Data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes. The maximum right-lateral slip at 1 day was about 50 cm near the south-central part of the North strand of Superstition Hills fault, and the predicted maximum final displacement is probably about 112 cm at Imler Road near the center of the South strand of the Superstition Hills fault. The overall distributions of right-lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture. The average estimated final right-lateral slip for the Superstition Hills fault zone is about 54 cm. The average left-lateral slip for the conjugate faults trending northeastward is about 23 cm.The southernmost ruptured member of the Superstition Hills fault zone, newly named the Wienert fault, extends the known length of the zone by about 4 km. The southern half of this fault, south of New River, expressed only vertical displacement on a sinuous trace. The maximum vertical slip by the end of the observation period there was about 25 cm, but its growth had not ceased. Photolineaments southeast of the end of new surface rupture suggest continuation of the Superstition Hills fault zone in farmland toward Mexico.

Surface ruptures on cross-faults in the 24 November 1987 Superstition Hills, California, earthquake sequence

Left-lateral slip occurred on individual surface breaks along northeast-trending faults associated with the 24 November 1987 earthquake sequence in the Superstition Hills, Imperial Valley, California. This sequence included the M_s = 6.2 event on a left-lateral, northeast-trending “cross-fault” between the Superstition Hills fault (SHF) and Brawley seismic zone, which was spatially associated with the left-lateral surface breaks. Six distinct subparallel cross-faults broke at the surface, with rupture lengths ranging from about Formula to 10 km and maximum displacements ranging from 30 to 130 mm. About half a day after the M_s = 6.2 event, an M_s = 6.6 earthquake nucleated near the intersection of the cross-faults with the SHF, and rupture propagated southeast along the SHF. Whereas right-lateral slip on the SHF occurred dominantly on a single trace in a narrow zone, the cross-fault surface slip was distributed over several stands across a 10-km-wide zone. Also, whereas afterslip accounted for a large proportion of total slip on the SHF, there is no evidence for afterslip on the cross-faults. We present documentation of these surface ruptures. A simple mechanical model of faulting illustrates how the foreshock sequence may have triggered the main rupture. Displacement on other cross-faults could trigger an event on the southern San Andreas fault by a similar mechanism in the future.

Analysis of pinnate joints in the Mount Desert Island granite: Implications for postintrusion kinematics in the coastal volcanic belt, Maine

The mount desert Island granite is cut by fractures displaying one of four types of surface morphology: (1) smooth to undulatory; (2) stepped in the form of en echelon cracks; (3) striated with linear fibers; and (4) irregular with cataclastic grains. These surfaces belong to joints, host fractures with pinnate joints, reactivated joints or fractures, and deformation bands (shear fractures), respectively. Pinnate joints, like striations on slickensides, are structures indicative of the orientation of a the slip vector and sense of shear on host fractures. Although fractures in the Mount Desert Island granite cluster into two major sets (N20{degree}W and N45{degree}E), host fractures with pinnate joints and shear fractures favor the N45{degree}E orientation. A kinematic analysis of the pinnate joints indicates a predominantly dextral strike-slip sense of movement on northeast-trending fractures. This result agrees with previous work suggesting that a prominent postintrusion tectonic event in southeast Maine consisted of dextral strike-slip motion on northeast-trending faults.

Evaluation of seismic hazard in the Rangitaiki Plains, New Zealand

Abstract Histories of movement on several active faults have been determined and used to evaluate possible future seismic hazard in the Rangitaiki Plains. The eruption of Kaharoa Ash c. 800 years B. P. provides a datum to establish the history of faulting. The Matata, Edgecumbe, Onepu, and Rotoitipakau Faults, all located in the southern and western Rangitaiki Plains, have moved at least once during the last 800 years. In contrast, faults in the eastern part of the Rangitaiki Plains do not show evidence of post-Kaharoa movement. The past level of seismic hazard has, therefore, been lower in the eastern part of the plains compared with southern and western areas. We assume that rupture of the northeast-trending normal faults in the Rangitaiki Plains will produce isoseismal patterns similar to that observed during the 1987 March 2 Edgecumbe earthquake. By overlaying the MM IX isoseismal of the Edgecumbe earthquake over each fault in the Rangitaiki Plains, and taking into account geological data on the recur...

Basement Structure and Implications for Hydrothermal Circulation Patterns in the Western Moat of Long Valley Caldera, California

Detailed surface mapping, subsurface drill hole data, and geophysical modeling are the basis of a structural and hydrothermal model for the western part of Long Valley caldera. Six fault zones are recognized in the western caldera with dominant orientations of north, northwest, and northeast, sub‐parallel to regional fault trends in the surrounding Sierran basement. The internal structural geometry of the cores of 12 exogenous domes inside and outside the caldera suggests that the domes erupted at the intersections of these principal fault trends rather than along the axis of a single dike. Gravity modeling and subsurface data from deep geothermal wells indicate that the floor of the caldera is segmented into a number of discrete fault blocks with varying offsets. One of the northeast trending fault zones, designated Discovery fault in this paper, appears to be part of the original Sierran embayment that existed before caldera collapse. Recent hydrothermal alteration occurs along Discovery fault strands and composite vertical offset of intracaldera volcanic units across the entire fault zone may be as much as 400 m. Field relationships, geophysical interpretations, and interpretaions of drill hole data suggest that this fault is a fundamental flaw in the western caldera. The preexisting tectonic framework of the basement rocks controlled the configuration of the western caldera floor and, through it, the location of postcollapse eruptive centers. These deep basement structures may also provide the high fracture density which controls circulation in the present geothermal system of Long Valley.

Tilting of Urad-Henderson and Climax porphyry molybdenum systems, central Colorado, as related to northern Rio Grande rift tectonics

Asymmetric orientation of stocks, radial dikes and veins, and attendant molybdenite mineralization and hydrothermal alteration indicates that Oligocene Urad-Henderson (Red Mountain) and Climax intrusive-hydro-thermal systems were tilted approximately 25° southeast and 30° west-southwest, respectively. Field and age relationships suggest that tilting resulted from displacement on normal faults associated with the northern Rio Grande rift. Minimal amounts of extension in central Colorado argue for listric configuration for these normal faults below the level of observation. Both intrusive systems formed during incipient rift development and are associated with early and scarce lamprophyre (kersantite) dikes and with later and numerous high-silica rhyolite stocks and dikes. Magmatic activity was initiated slightly earlier at Climax (33 Ma) than at Red Mountain (29 Ma). Post-ore tilting of Red Mountain was controlled by a set of subparallel, north-northeast- to northeast-trending normal faults with west side down. At Climax, deformation was more complex, with a reversal in tilt direction. The resultant tilt of Climax deposit cannot be associated simply with one set of faults. Reconstruction of Oligocene paleotopography near Red Mountain suggests a minimum depth of emplacement of 1.6 km (41 MPa confining pressure) for the apices of principal mineralizing stocks in the Urad deposit and of 2.8-3.1 km (72-80 MPa) in the Henderson deposit. Depths of emplacement for Climax system are similar to those of Urad-Henderson, 2.3-3.2 km. Exploration for buried Climax-type molybdenite deposits should allow for the probability of tilted ore systems.

Structural Control on Paleovalley Development, Muddy Sandstone, Powder River Basin, Wyoming: ABSTRACT

A subaerial unconformity within the Lower Cretaceous Muddy Sandstone in the Powder River basin developed during a late Albian sea level lowstand and resulted in a markedly rectangular drainage pattern. Numerous right-angle bends and perpendicular confluences of Muddy paleovalleys are believed to reflect syndepositional movement on basement faults and dissolution of salts in the Goose Egg Formation. A detailed subsurface analysis of geophysical logs from closely spaced wells reveals that up to 30 ft of vertical displacement occurred along northwest- and northeast-trending faults prior to and during the development of the subaerial unconformity. An analysis of a high-resolution magnetic survey (NewMag) of the Powder River basin reveals that numerous paleovalleys parallel the boundaries, or basement shear zones, between basement blocks. Small, irregularly shaped, thin intervals of the Permian Goose Egg Formation, which resemble karst topography, also occur along these northwest- and northeast-trending basement faults beneath Muddy paleovalleys. An arcuate Muddy paleovalley located in the northern Powder River basin parallels contours of isopach and trend surface maps of the Goose Egg Formation. These relationships suggest that the location and orientation of Muddy paleovalleys were controlled by a combination of movement along northwest- and northeast-trending faults and syntectonic dissolution of salt withinmore » the Goose Egg Formation. Simultaneous dissolution of Goose Egg salts and headward erosion of Muddy paleovalleys along this conjugate fault pattern also indicate that the Powder River basin was influenced by wrench fault tectonics during the late Albian.« less