NW-striking Faults Research Articles

Degassing of mantle-derived CO2 and He from springs in the southern Colorado Plateau region—Neotectonic connections and implications for groundwater systems

Groundwaters of the southern Colorado Plateau–Arizona Transition Zone region are a heterogeneous mixture of chemically diverse waters including meteoric (epigenic) fluids, karst-aquifer waters, and deeply sourced (endogenic) fluids. We investigate the composition of travertine-depositing CO 2 -rich springs to determine the origin, transport, and mixing of these various components. The San Francisco Mountain recharge area has little surface flow. Instead, waters discharge through major springs hundreds of kilometers away. About 70% (9340 L/s) of the total recharge (13,500 L/s) discharges 100 km to the north in the incised aquifer system at Grand Canyon. Most of this water (85%; 8070 L/s) emerges through two travertine-depositing karst spring systems: Blue Springs (6230 L/s) and Havasu Springs (1840 L/s). About 30% of recharge (4150 L/s) flows to the south and discharges along NW-striking faults in the Arizona Transition Zone, forming the base flow for the Verde River. Geochemical data define regional mixing trends between meteoric recharge and different endogenic end members that range from bicarbonate waters to sulfate waters. Water quality in the region is dictated by the percentage and character of the endogenic inputs that cause a measurable degradation of groundwater quality for water supply. Sources for the high CO 2 include dissolution of limestone and dolostone (C carb ) and “external carbon” (C external ). C external is computed as the bicarbonate alkalinity (dissolved inorganic carbon [DIC]) minus the C carb (C external = DIC - C carb ). C external is deconvolved using carbon isotopes into biogenically derived sedimentary carbon (C organic ) and deep CO 2 inputs (C endogenic ). Measured δ 13 C values are −17‰ to +3‰ versus Pee Dee Belemnite (PDB). Assuming δ 13 C carb = +2‰, δ 13 C organic = −28‰, and δ 13 C endogenic = −5‰, water chemistry mixing models indicate that an average of 42% of the total DIC comes from dissolution of carbonate rocks, 25% from organic carbon, including soil-respired CO 2 ,and 33% from deep (endogenic) sources. Helium isotope values ( 3 He/ 4 He) in gases dissolved in spring waters in the southern Colorado Plateau region range from 0.10 to 1.16 R A (relative to air) indicating that a significant component of the deeply derived fluid is from the mantle (mean of 5% asthenospheric or 10% subcontinental lithospheric mantle source). Measured CO 2 / 3 He ratios of 2 × 10 9 to 1.4 × 10 13 are adjusted by removing the proportion of CO 2 from C carb and C organic to give values 10 for all but four samples. Various mixing models using CO 2 / 3 He suggest that the mantle-derived components of the CO 2 load are highly variable from spring to spring and may make up an average of ~10% of the total CO 2 load of the regional springs. Fluid-rock interactions involving endogenic fluids are suggested by 87 Sr/ 86 Sr, δ 18 O, and other tracers. The endogenic CO 2 component, multiplied by discharge for each spring, yields an integrated annual flux of deeply derived CO 2 to the groundwater system of ~1.4 × 10 9 mol/yr. This CO 2 emission from the Colorado Plateau region reflects a complex tectonic evolution involving Laramide hydration of the lithosphere above the Farallon slab, addition of fluids from mid-Tertiary mantle tectonism during slab removal, and ongoing fluid movement induced by neotectonic small-scale asthenospheric convection.

Xiongcun, Tibet: A telescoped system of veinlet-disseminated Cu (Au) mineralization and late vein-style Au (Ag)-polymetallic mineralization in a continental collision zone

The Xiongcun Cu–Au deposit occurs in the western part of the Gangdese porphyry and skarn Cu–Au mineralization belt in the Himalayan–Tibetan Orogen and was formed in an intracontinental orogenic environment related to the India–Asia collision. Sericite 40Ar/ 39Ar dating of the Xiongcun deposit indicates that ore formation occurred around 38.11 ± 0.90 Ma, much earlier than the main ore-forming ages (20 to 14 Ma) of the Gangdese Cu–Au metallogenic belt. The potassium feldspar K–Ar age of the sulfide-bearing pegmatoid dyke in the Xiongcun district is 47.62 ± 0.70 Ma. These ages indicate that the mineralization in Xiongcun is related to relaxation (ca. 52 to 40 Ma) and subsequent uplift (40 to 38 Ma) of the orogen. In the Xiongcun deposit, mineralization, hosted by Jurassic pyroclastic rocks and controlled by NW-striking faults, can be divided into two styles, i.e. veinlet-disseminated Cu ± Au mineralization and vein-style Au ± Ag-polymetallic mineralization. The former is associated with local remnant K-silicate alteration, extensive phyllic alteration and peripheral widespread propylitization. The latter, overprinting the former, is associated with strong silicification, chloritization and kaolinization. The early veinlet-disseminated Cu ± Au mineralization is similar to porphyry-type mineralization and has a mineral assemblage of quartz–sericite–pyrite–chalcopyrite–bornite–magnetite–pyrrhotite–sphalerite ± rutile ± anhydrite. The late vein-style Au ± Ag-polymetallic mineralization, characterized by a mineral assemblage of quartz–pyrite–pyrrhotite–sphalerite (rich in Fe)–chalcopyrite–galena–native gold–electrum–chlorite–kaolinite ± barite ± calcite ± siderite ± hematite, belongs to a sulfide-rich low-sulfidation-type epithermal system. The H, O and S isotopic compositions and high salinities of fluid inclusions indicate a magmatic contribution and increasing contents of meteoric water in the mineralized system from early to late stage. The wide range of homogenization temperatures (380 to 120 °C), homogenization pressures (45.92 to 1.94 bar) and salinities (36.61 to 1.23 wt.% NaCl equiv.) suggest rapid uplift and are consistent with the geological setting. Because of uplift, the hydrothermal system of Xiongcun evolved from a porphyry system to an epithermal system, resulting in an immature porphyry system and the telescoping of late vein-style Au ± Ag-polymetallic mineralization onto the early veinlet-disseminated Cu ± Au mineralization.

Damage zone characteristics of outcrop-scale NW-striking faults and their relation to the Red Sea rifting, Duwi area, Egypt

Damage zone characteristics of outcrop-scale NW-striking faults and their relation to the Red Sea rifting, Duwi area, Egypt

Geological framework of the Archean and Paleoproterozoic Tanami Region, Northern Territory

The Tanami Region, a poorly exposed, mostly Paleoproterozoic province within the North Australian Craton, hosts a number of significant gold deposits in diverse settings. Rare exposures of 2,520–2,500 Ma amphibolite facies Archean gneiss and metasedimentary rocks form basement to the thick overlying metasedimentary succession of the 1,880–1,830 Ma Tanami Group. The basal unit of the Tanami Group is the Dead Bullock Formation, a fining-upward deep-water succession dominated by siltstone, carbonaceous siltstone, iron-rich siltstone and mafic sills. Carbonaceous- and iron-rich lithologies in the upper Dead Bullock Formation represent important hosts for gold mineralization. The conformably overlying Killi Killi Formation represents turbiditic sedimentary rocks that are correlated with the widespread Lander Rock beds of the Arunta Region. Sedimentation of the Tanami Group was terminated by regional deformation and greenschist to amphibolite facies metamorphism during the Tanami Event (D1/M1), at around 1,830 Ma. The Tanami Group is unconformably overlain by rhyolite, siliciclastic sedimentary rocks, and felsic ignimbrite of the Ware Group that were deposited at about 1,825–1,810 Ma. Subsequent ESE–WNW to SE–NW directed shortening (D2), followed by NE–SW to E–W directed shortening (D3), has resulted in open NE F2- and NW F3-trending folds in both the Tanami and Ware Groups. Voluminous granitoids, dominated by I-type, biotite granodiorite, and monzogranite were intruded in the interval 1,825–1,790 Ma and have been subdivided using geochemical criteria into the Birthday, Frederick, and Grimwade Suites. Basalt and immature sedimentary rocks of the Mount Charles Formation are restricted in extent to the Tanami mine corridor, and are interpreted to reflect a continental rift succession that was deposited around 1,800 Ma, with an early Archean sedimentary provenance. Steep S to SE dipping F4-fold structures of Tanami and Ware Group metasedimentary rocks, many spatially associated with 1,825–1,790 Ma granitoid intrusions, indicate a period of SSE-directed regional shortening (D4) syn-to-post the regional granitoid intrusive phase. A network of N to NW striking faults, several of which are interpreted as oblique thrusts with a component of left lateral movement, indicates a period of D5 convergence during WSW–ENE to E–W directed shortening. The Tanami mine corridor fault system comprises a network of N, NE to ENE striking D5 faults that merge with N to NW striking faults and probably accommodated movement between granite core domains. D5 faulting is associated with the main phase of gold mineralization in suitable structural–lithological traps. The Paleoproterozoic basement of the Tanami Region is unconformably overlain by quartz sandstone, lithic arenite, and conglomerate of the Pargee Sandstone. Pargee Sandstone may represent syn-tectonic sedimentation related to the 1,730 Ma Strangways Orogeny, and is unconformably overlain by the late Paleoproterozoic platform cover succession of the Birrindudu Group. The Paleoproterozoic basement and cover sequences have subsequently undergone several episodes of faulting, collectively termed D6+. The Paleoproterozoic evolution of the Tanami Region is interpreted to have occurred in an intracratonic setting, but was fundamentally influenced by tectonic events in the adjacent Halls Creek Orogen (1,835–1,805 Ma Halls Creek Orogeny) and Arunta Region (1,815–1,800 Ma Stafford Event). The boundaries between the Tanami Region and Kimberley Region to the northwest and the Arunta Region to the southeast are transitional, and are largely defined by the presence or absence of identifiable Dead Bullock Formation.

Focal mechanisms and the stress regime in NE and SW Tanzania, East Africa

We report 12 new focal mechanisms from earthquakes in NE and SW Tanzania where the stress regime within the East African rift system is not well constrained. Focal mechanisms for events at the intersection of the Lake Tanganyika and Rukwa rifts in SW Tanzania indicate a complicated stress pattern with possible dextral strike‐slip motion on some faults but oblique motion on others (either sinistral on NW striking faults or dextral on NE striking faults). Within the Rukwa rift, focal mechanisms indicate normal dip‐slip motion with NE‐SW opening. In NE Tanzania where the Eastern rift impinges on the margin of the Tanzania Craton, fault motions are consistent with a zone of distributed block faults and sub E‐W extension. All twelve earthquakes likely nucleated within the crust.

Fault populations, strain distribution and basement fault reactivation in the East Pennines Coalfield, UK

The scaling properties of faults within the East Pennine Coalfield (UK) are analysed for a fault population defined from a high quality, regional (ca. 1300 km 2) fault map for an Upper Carboniferous (Westphalian) coal seam. The fault system is dominated by an orthogonal network of NW- and NE-striking faults that preserve predominantly dip-slip extensional displacements ranging from <1 to 180 m. Fault displacements are post-depositional and late- to post-Carboniferous in age. Fault size (maximum throw, length and geometric moment) populations display very well defined power-law distributions over two orders of magnitude. Sub-sets of the population, discriminated in terms of sub-area (400 and 100 km 2) and strike (NW- and NE-striking sets) are also power-law distributed. Changes in the power-law exponents of NW- and NE-striking fault populations reflect strain variations across the area. Some sub-areas are dominated by NW-striking faults, whilst others display a more symmetrical fault pattern. This heterogeneous distribution of strain is strongly influenced by the geometry of an underlying lower Carboniferous (Dinantian) fault system, characterised by large NW-striking graben-bounding faults (km-scale throws), which localises strain in the upper Carboniferous cover sequence during late- and post-Carboniferous faulting.

Tar sandstones in the Paraná Basin of Brazil: structural and magmatic controls of hydrocarbon charge

There are tar accumulations in Lower Triassic sandstones along the eastern margin of the Paraná Basin. The tar was derived from Permian source rocks that matured along the basic sills of the Serra Geral continental flood basalt cover. This paper exams the habitat of tar accumulation, integrating fieldwork with morphostructural data, digital elevation modelling, gravity and airborne magnetic data, remote sensing, and radiometric dates that constrain the timing of maturation and migration. The focus of this work is the Jacu structural complex, a large annular drainage anomaly, 20 km in diameter, that owes its origin to reactivation of pre-existing faults. Interpretation of the structural framework is based primarily on morphostructural analysis and the distribution of gravity anomalies. The Jacu structure consists of a central low-lying fault block that is surrounded by several structural highs, and is attributed to the interaction of NE- and NW-striking fault zones. The principal basement faults that encapsulate this structure were invaded by the dolerite dykes that fed the widespread flood basalt cover during the Early Cretaceous. An 40Ar/ 39Ar analysis directly dates this episode of magmatism to 134±2 Ma. Tar sandstones are restricted to the lower Pirambóia Formation and are present around the Jacu structure, especially near the structural highs or close to basic dikes. Morphostructural interpretation suggests the occurrence of tar sandstones at intermediate positions too, between the structural highs and lows. The most likely explanation for these tar accumulations relates oil maturation and migration to the intrusion of Lower Cretaceous Serra Geral sills into the Upper Permian black shales of the Irati Formation, and subsequent migration along faults and dyke walls into eolian sandstone reservoirs of the Triassic Pirambóia Formation. Not only did the basic dykes locally facilitate migration, but there were also places where they were barriers to lateral migration. Furthermore, the interdune and fluvial facies of the Pirambóia Formation formed local seals and barriers to vertical migration.

The Negra Muerta Volcanic Complex, southern Central Andes: geochemical characteristics and magmatic evolution of an episodically active volcanic centre

This petrologic analysis of the Negra Muerta Volcanic Complex (NMVC) contributes to understanding the magmatic evolution of eruptive centres associated with prominent NW-striking fault zones in the southern Central Andes. Specifically, the geochemical characteristics and magmatic evolution of the two eruptive episodes of this Complex are analysed. The first one occurred as an explosive eruption at 9 Ma and is represented by a strongly welded, fiamme-rich, andesitic to dacitic ignimbrite deposit. The second commenced with an eruption of a rhyolitic ignimbrite at 7.6 Ma followed by effusive discharge of hybrid lavas at 7.3 Ma and by emplacement of andesitic to rhyodacitic dykes and domes. Both explosive and effusive eruptions of the second episode occurred within a short time span, but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber. Our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber, pressurising the magmatic system and triggering explosive eruption of rhyolitic magma. Chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition, which could have been strong enough to inhibit the interaction between the two magmas. After the initial explosive stages of the eruption at 7.6 Ma, the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation. Restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber. At 7.3 Ma, increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera. Assimilation during turbulent ascent (ATA) is invoked to explain a conspicuous reversed isotopic signature ( 87Sr/ 86Sr and 143Nd/ 144Nd) in the entire volcanic series. Therefore, the 7.6 to 7.3 Ma volcanic rocks of the NMVC resulted from synchronous and mutually interacting petrological processes such as recharge, fractional crystallization, hybridisation, and Assimilation during Turbulent Ascent (ATA). Geochemical characteristics of both volcanic episodes show diverse type and/or depth in the sources and variable influence of upper crustal processes, and indicate a recurrence in the magma-forming conditions. Similarly, other minor volcanic centres in the transversal volcanic belts of the Central Andes repeated their geochemical signatures throughout the Miocene.

A study on characteristics of tectonic block motion and tectonic setting of strong earthquakes in northern part of the Shanxi fault depression zone

This paper makes a systematical study on characteristics of structure and motion of the tectonic blocks in northern part of the Shanxi fault depression zone by means of geometrical and kinematical analysis of the blocks. The kinetic behavior of the blocks is discussed by comparing associated geomorphic features of fault movement. All analyses and studies are based on a Domino model. The block movement, fault basin extension and their regional distribution are systematically investigated. The result shows: (a) The studied region is divided into sub-regions by NW striking faults: the western, middle and eastern sub-region with crustal extension being 4.46 km, 2.80 km and 1.86 km, respectively. The extensional amount of each block in the region is estimated being generally about 1 km. The calculated result using the block motion model approximately fits the data of geologic survey. (b) Block kinematical features are obviously different between the northern and southern part, with the Hengshan block in between, of the studied region. Moreover, the magnitude of the largest historical earthquake in the northern part is about 6, while that in the southern is 7. The faulted blocks in the northern sub-region show northwestward extension, indicating a feature of extensional graben, while the blocks in the southern part manifest tilt motion, extending southeastward, in the opposite sense of fault dipping. Additional tectonic stress generated by block rotation may be one of major factors affecting seismogenic process in the region. It is responsible for the difference in the movement of the block boundary faults and seismic activities between the two sub-regions.

Recent tectonic stress field and major earthquakes of the Bohai sea basin

The Bohai Sea basin is located in the east to the Cenozoic rift basin of North China. It is a inner-land sea basin formed during Late Quaternary time. In this basin there are numerous active faults and many major earthquakes have occurred. Its major active tectonic zones are the northern section of the NE trending Yingkou-Weifang fault zone, Miaoxibei-Huanghekou-Linyi fault zone, and NW striking Beijing-Penglai fault zone, which intersect at the center of the Bohai Sea basin. These tectonic zones subdivide the sea area into four secondary neotectonic areas, which are subject to the recent tectonic stress field. Using the data of 38 focal plane solutions and 75 drill stress measurements, and the result of numerical modeling, we analyze the recent tectonic stress field of this region. Our result shows that this stress field is characterized by dominant horizontal or nearly horizontal stress. Its compressive stress is in the direction NE60°–90°, and the tensional stress is in NS-NW30°. There are some differences of principal stress between secondary tectonic areas. A remarkable feature of the Bohai Sea basin is a conjugate shear rupture zone, which develops in the crust and is the major seismogenic structure.

Extension of the North Anatolian Fault into the North Aegean Trough: Evidence for transtension, strain partitioning, and analogues for Sea of Marmara basin models

The North Aegean Trough is tectonically controlled by the projection of the North Anatolian Fault from western Turkey and the Sea of Marmara into the Aegean, where deformation is distributed in contrast to the simple strike‐slip fault model for central eastern Turkey. This region accommodates extensional and dextral displacement. New side‐scan sonar data coupled with new subbottom profiler and published seismic data reveal evidence for dextral displacement within the eastern trough and Gulf of Saros in the form of oblique en echelon and Riedel (R, R′, and P) fractures and displaced geomorphic features. Strain partitioning in the shallow crust allows the following two styles of faulting to coexist, supported by earthquake focal mechanisms: (1) transtensional basin‐bounding faults controlling the significant vertical topography of the basin and (2) dextral faulting within the central basin. These styles of faulting merge at greater depths into a single fault. A possible sinistral NW striking fault separates two subbasins and may indicate crustal block rotation with deformation focused in block corners. Such evidence and models from the north Aegean are similar to models of transtension and pull‐apart basins within the Sea of Marmara.

The geometry and anatomy of the Los Pijiguaos bauxite deposit, Venezuela

The Los Pijiguaos bauxite mine in the western part of the State of Bolivar, Venezuela, represents the only currently exploited bauxite deposit in the country. Since mining commenced in 1987, total crude bauxite production has amounted to more than 43 million tonnes (Mt), with a present annual production close to 5 Mt. Proven and probable reserves are around 570 Mt, at an economic cut-off grade of about 44 wt.% alumina (Al 2O 3) and a total silica (SiO 2) content of less than 20 wt.%. The orebody has an irregular, dismembered geometry as a result of strong and prolonged fluvial erosion during and after uplift of the region. The isolated minable blocks are located on plateau remnants separated by deep valleys. As a result, only half of the entire lease area is accessible for mining. Los Pijiguaos represents a typical laterite bauxite deposit that formed on a flat topped plateau from weathering of the underlying mid-Proterozoic Rapakivi-type Parguaza granite. The complete laterite profile is characterized by a vertical zonation comprising a concretionary zone (the bauxite horizon), and a mottled zone (the saprolite horizon) on granitic bed rock. The economic bauxite zone attains an average thickness of 7.6 m. On a mine scale, best ore grades and greatest thickness of the ore horizon are at topographic highs. Laterally, grade distribution is also not isotropic but follows distinct WNW–SSE trends. The spatial control on the mineralization is a result of the westerly inclination of the planation surface and the formation of major NW-striking fault systems in the region. Gibbsite is the most abundant mineral in the economic-grade bauxite horizon. The occurrence of minor nordstrandite is also restricted to that zone. Kaolinite is not present in the bauxite zone, but in the lower saprolite zone kaolinite can be more than 5 wt.%. An inverse correlation exists between gibbsite and quartz. Abundance ratios for the two minerals decrease from around 45 in the bauxite horizon to around 2 in the lower saprolite zone. Hematite is commonly more abundant than goethite. In the bauxite horizon, however, the goethite/hematite ratio is greater than 1. Anatase occurs as an accessory phase mainly in the bauxite horizon. Mass balance considerations demonstrate that bauxitization resulted in progressive desilication, hydration, and Al 2O 3 and Fe 2O 3 concentration in the residual weathering blanket. Silica as well as alkali and alkaline earth metals were almost completely leached. Mass and volume losses in the order of 61% and 77%, respectively, attest to severe mineralogical, textural, and chemical changes. From an economic point of view, the Los Pijiguaos bauxite represents a high-quality almost pure gibbsitic ore that allows processing to alumina by the low-temperature Bayer process around 140 °C. In terms of material consumption, 2.3 tonnes of dry bauxite are used for every tonne of alumina produced. The accompanying mass of red mud amounts to 550 kg per tonne alumina. The relatively high abundance of hematite in the bauxite has a positive effect on the red mud settling rate and, thus, favors a high extraction yield of the alumina plant. Los Pijiguaos is based on a huge reserve that will provide bauxite for more than 100 years at the current rate of mining. The entire region may contain as much as 6 billion tonnes of probable bauxite resources.

Slip accumulation and lateral propagation of active normal faults in Afar

We investigate fault growth in Afar, where normal fault systems are known to be currently growing fast and most are propagating to the northwest. Using digital elevation models, we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0.3 to 60 km. Faults exhibiting the elliptical or “bell‐shaped” slip profiles predicted by simple linear elastic fracture mechanics or elastic‐plastic theories are rare. Most slip profiles are roughly linear for more than half of their length, with overall slopes always &lt;0.035. For the dominant population of NW striking faults and fault systems longer than 2 km, the slip profiles are asymmetric, with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero, whereas slip decreases roughly linearly and tapers in the direction of overall Aden rift propagation. At a more detailed level, most faults appear to be composed of distinct, shorter subfaults or segments, whose slip profiles, while different from one to the next, combine to produce the roughly linear overall slip decrease along the entire fault. On a larger scale, faults cluster into kinematically coupled systems, along which the slip on any scale individual fault or fault system complements that of its neighbors, so that the total slip of the whole system is roughly linearly related to its length, with an average slope again &lt;0.035. We discuss the origin of these quasilinear, asymmetric profiles in terms of “initiation points” where slip starts, and “barriers” where fault propagation is arrested. In the absence of a barrier, slip apparently extends with a roughly linear profile, tapered in the direction of fault propagation.

Faulting in the 1986 Chalfant, California, Sequence: Local Tectonics and Earthquake Source Parameters

The July 1986, moment magnitude (M w ) 6.3 Chalfant, California, earth- quake is the largest of a recent series (1978-present) of moderate-sized earthquakes near the Long Valley volcanic region of east-central California. The sequence con- sists primarily of three moderate-sized strike-slip events. High-quality aftershock relocations and short-period focal mechanisms define the temporal and spatial de- velopment of the foreshock-mainshock-aftershock periods of these three events. Faulting during the Mw 5.7 (event I; July 20) and the Mw 6.3 (event II; 21 July) events constitute a set of conjugate strike-slip faults. Event I involved predominantly left-lateral motion on a NE-striking fault plane initiating at shallow depth (7 km). Event II initiated at 10.5 km depth exhibiting right-lateral strike-slip motion on a NW-striking fault dipping moderately to the southwest. An ML 5.5 strike-slip event on 31 July (event III) extended the aftershock sequence to the south into the White Mountains fault zone. P-wave pulse-width stress drops are determined for 185 ML 2.7-4.0 earthquakes that sample the entire sequence. Higher stress drops are observed near the intersection of event I and II fault planes and at the northern and southern ends of the aftershock zone. The moving-window b-value of the temporal magnitude distribution shows a general inverse relationship to stress drop with observed changes in both the average stress drop level and the b-value preceding event III. The average aftershock stress drop tends to increase as the sequence progresses suggesting that the faulted volume is equilibrating to the regional stress. Source parameters have been determined for the principal earthquakes from teleseismic body waves, local strong-motion records, and the extent of aftershock activity. The Chalfant sequence appears to be transferring strike-slip motion away from the White Mountains front, contributing to the observed increase in the relative normal offset, from south to north, along the White Mountains and the opening of the White Mountains relative to the Sierra Nevada Range front north of Owens Valley.

Miocene Epithermal Au-Ag Vein Mineralization, Dixie Claims, Midas District, North-Central Nevada; Characteristics and Controls

The Dixie Zone Au-Ag deposit is a low sulfidation (adularia-sericite) epithermal system located 1.4 km from the producing Ken Snyder mine (7.0 M oz Au equivalent reserve/resource), Midas Mining District, north-central Nevada. This WNW striking, steep south dipping, fault system is hosted by mid-Miocene (17 Ma to 14 Ma) felsic volcanic rocks that were erupted from a vent(s) to the northeast. The system was mineralized by approximately 250 ± 20°C (n = 86), near-neutral pH, ascending fluids of predominantly meteoric origin [volatile corrected salinity = 0.52 ± 0.35 eq.wt% NaCl (n = 86)]. Bulk analytical fluid inclusion gas chromatography identified a mineralizing fluid volatile composition of 99.68 ± 0.18 mole % H 2 O, 0.16 ± 0.09 mole % CO 2 , 0.11 ± 0.05 mole percent N 2 , 0.05 ± 0.04 mole % CH 4 , and 0.01 ± 0.01 mole % C n H n+i (n = 7). Sulfur species were not analyzed due to absorption within the chromatography system; however, a H 2 S concentration of 16 ppm to 160 ppm molar is estimated. The volatile content contributes approximately 50% of the microthermometrically measured freezing point depression. Cl − concentrations up to approximately 9000 ppm suggest an additional, probably magmatic, fluid component. Ascending fluids were channelled along approximately NW striking faults related to the same primary extension (17 Ma to 14 Ma) responsible for Nevada basin and range physiography. Boiling induced by CO 2 effervescence led to precipitation of Au and Ag over a 150 m vertical interval. At the time, this interval was about 460 m below the water table (approximately 38 bars), but because of erosion, it is now within 90 m of the surface. A Ag:Au ratio of 25:1 has been calculated for the Dixie Zone.

Neotectonics of western India: evidence from deformed Quaternary fluvial sequences, Mahi River, Gujarat

A Quaternary fluvial sequence along the Mahi River, Gujarat, western India, shows evidence for a discrete episode of normal faulting and related surface slumping. Four mapped NW-striking faults have throws ranging from 1 m to over 3 m, and have related growth folds in synchronous sediment units. A slump sheet moved tens of metres northeastward down the 0.7° dip slope of one tilt block during displacement on its bounding fault. The exposed part of the internally folded sheet is probably one side of a lobate slump mass, several hundred metres in width. Fault activity began after deposition of a lower gravel, which regional correlations suggest is about 300 ka old. Faulting ended just before deposition of an upper gravel-sand complex dated by luminescence techniques in correlated sections at no younger than about 60 ka. The Mahi River section shows that neotectonic activity in Peninsular India, exemplified by the 1993 Killari (Latur) earthquake, has a mid- to late Pleistocene component.

An approach using SAR ERS images to relate extension fractures to volcanic vents: examples from iceland and Madagascar

A new approach to the study of geometric relationships between tectonic structures and volcanic vents is presented. It is based on the mapping of tectonic and volcanic features using high-resolution imagery, including Synthetic Aperture Radar (SAR) images of the European Remote Sensing satellite number one (ERS-1). We have evidenced new faults: (1) N-striking normal faults in the Husavik fault zone of the North Atlantic ridge, and (2) NW-striking faults belonging to the Husavik transform fault zone, associated to drag folds which testify to a dextral strike-slip movement component. We show that most of the volcanic edifices are located on extension fractures responsible for fissure eruptions, volcanic linear clusters and elongated volcanoes. This relationship is studied first in Iceland where the geodynamics is well known, using SAR ERS-1 imagery. Applied to the Montagne d'Ambre, in northern Madagascar, using aerial photographs, this approach makes it possible to identify faults and the regional Plio-Quaternary direction of extension, i.e. N65°E to N85°E. Volcanoes forming linear clusters, together with elongated volcanoes, are interpreted to be located on extension fractures. S- or Z-shape of the linear clusters allowed us to determine the tips of extension fractures, and consequently the orientation of the regional tension. Alignments of extension fractures, depending on their S- or Z-shape, permitted us to infer conjugated faults.

2.5-D gravity model of the NiCuPGM mineralized Mount Ayliff Intrusion (Insizwa Complex), South Africa

The Mount Ayliff Intrusion is the largest and thickest Karoo-aged sill in South Africa. It contains a small NiCu-platinum-group metal (PGM) sulphide deposit, Waterfall Gorge, at the base of its largest lobe called Insizwa. The deposit was once mined for its Cu. In September 1990, a 900-m-deep, vertical diamond drill exploration borehole was drilled through a thick, central portion of the intrusion. This new geological control confirms an earlier gravity survey-based hypothesis that the intrusion has a thick, hidden keel of ultramafic rock with finite depth extent below the centre of the Insizwa lobe. Along with other new geological evidence, the borehole log affords a unique opportunity to further constrain models for three regional gravity profiles and to arrive at a new, 3-dimensional model for the Insizwa lobe. The background residual terrain-corrected Bouguer gravity anomaly of the Mount Ayliff Intrusion reaches up to +9 mgal. This results from an igneous slab of 0–150 m of picrite overlain by about 600 m of gabbronorite. Superimposed on this anomaly over the central and northwestern Insizwa lobe is a 10-km-wide gravity anomaly with an amplitude of +8 to + 17 mgal and having steep edges. This is interpreted to represent thickening of picrite to 400–800 m in a major, hidden basin bordered by two NW-striking fault lineaments that are believed to mark a hidden graben structure with major geological transgressions at the base of the Insizwa lobe. At least three discrete, thinner picrite basins are interpreted to extend above and beyond both flanks of the graben. Picrite in these basins is modelled to be less than 200 m in thickness. The basins are separated either by the major NW-striking geological transgressions, or by domes in the footwall of the Insizwa lobe, above which no picrite has developed. Locations of several hidden, narrow, discontinuous feeder dykes below the Mount Ayliff Intrusion are postulated. The role of ponding of picrite at the mostly hidden base of the intrusion emerges as being particularly significant. In view of the existence of Ni-Cu-PGM mineralization at Waterfall Gorge where exposed picrite is only 30 m thick, the recognition of much thicker picrite elsewhere at the base of the intrusion presents exciting exploration targets.

Description and kinematics of the SE Loreto basin fault array, Baja California Sur, Mexico: A positive field test of oblique-rift models

We present the results of mapping and kinematic analysis on an ∼ 18 km2 array of normal and oblique-slip faults in the SE Loreto basin, near Loreto, Baja California Sur, Mexico. The fault array developed on the western margin of the Gulf of California in late Pliocene time, during the early part of the modern stage of oblique rifting of the gulf. The SE Loreto fault array is an antithetic transfer zone between two large, E-dipping normal faults. The fault array is coherent and formed in one episode. The dominant N- to NNE-striking faults primarily dip W and display nearly pure normal slip. More NE-striking faults show normal-sinistral slip and NW-striking faults have dextral-normal to dextral slip. N- to NW-striking faults have steeper dips and a greater component of dextral slip with more northwesterly strike. Bulk extension, based on strain analysis of faults with striae, is oriented nearly E-W with little plunge and consistent throughout the fault array. The kinematic analysis of the SE Loreto fault array provides a well-constrained test of oblique-rift models. Our results support previous modeling of homogeneous extension in oblique-rifted margins when both instantaneous and finite strain are considered. Positive field tests of analytical and experimental models, such as this one, indicate that such models may be robust predictors of the obliquity of rifting in ancient mountain belts where brittle fault arrays are preserved, but plate motions are unknown.

The 1946 Hispaniola earthquakes and the tectonics of the North America‐Caribbean plate boundary zone, northeastern Hispaniola

We have determined focal mechanisms for the largest earthquake (Ms = 7.8) recorded instrumentally in the Caribbean Basin, the August 4, 1946, Hispaniola earthquake, and three of its large‐magnitude (Ms ≥ 6.1) aftershocks. We also relocated 63 aftershocks and one foreshock of the event series. The aftershock series is elongate, trends WNW, and is centered on the Samaná Peninsula of northeast Hispaniola. Shallow aftershocks are in a 75‐km‐wide linear zone, and intermediate depth (70 to 130 km) aftershocks apparently delineate a moderately south or SSW dipping slab. It is not clear, however, whether these events indicate active subduction of North American Atlantic Ocean lithosphere or are strike‐slip events on the interface between subducted but no longer sinking slab and Caribbean mantle. We constrained focal mechanisms of the main shock and three aftershocks by combining observed P and S polarities and amplitude ratios and also by waveform modeling. The two methods yield consistent results. The mechanisms include strike‐slip and thrust displacements on NW striking nodal planes. Fault dip is variable, NE or SW. The NW striking fault planes parallel mapped terrane boundaries and faults in the North America (NA)‐Caribbean (Ca) plate boundary zone and are also parallel to the aftershock series trend. We interpret the events to be motions on a WNW trending restraining bend segment of the NA‐Ca plate boundary in eastern Hispaniola. We have calculated magnitudes for eight of the earthquakes in the series; for the three events (including the main shock) for which data are available, our magnitudes are systematically less than the previously published magnitude estimates. Given the high magnitude and large aftershock area of the August 4, 1946, event, these earthquakes probably represent the true long‐term interplate motions between North America and the terranes in this portion of the plate boundary zone.