High-angle Fault Zones Research Articles

Structural and Mineralogical Characterization of a Fossil Hydrothermal System Located at the Outermost Front of the Southern Apennines Fold-and-Thrust Belt

Aiming at investigating the hydrothermal circulation along the eastern flank of the Vulture volcano, along the outermost edge of the southern Apennine fold-and-thrust belt (ftb), we studied the fossil hydrothermal alteration that mineralized a transtensional fault that crosscuts volcanoclastic rocks in the Rapolla area. On the basis of structural, mineralogical, and fluid inclusion data, three main stages of activity of the hydrothermal system are documented. Stage 1 was produced by the circulation of fluids having low-pH conditions (pH ≈ 3-4) and relatively high-SO42- activity, as testified by the hydrothermal alteration mainly carried out by the alunite group minerals (particularly jarosite), which is typical of an advanced argillic alteration facies. Hydrothermal fluids were characterized by a high temperature of about 200°-210°C. These hot fluids altered and mineralized the matrices of pyroclastic rocks and sealed both burial-related and fault-related fracture networks. Later hydrothermal circulation (Stage 2) was recorded by opal A-rich veins present both within and outside the fault zone. The fluids responsible of opal A precipitation were characterized by lower temperature conditions, probably lower than 100°C. Current goethite mineralization takes place along the main slip surfaces of the study high-angle fault zone due to low temperature (<30°C) underground water circulation. This study highlights that a high-temperature hydrothermal system developed in the past within the transtensional fault zone of the Rapolla area when a high thermal anomaly was present. If we take into account that this area is still affected by a heat flux positive anomaly (90 mW/m2), we may infer that it has the potentiality to be considered an interesting site for future exploration devoted to the finding of medium-enthalpy geothermal resources at depth.

Frictional properties of low-angle normal fault gouges and implications for low-angle normal fault slip

The mechanics of slip on low-angle normal faults (LANFs) remain an enduring problem in structural geology and fault mechanics. In most cases, new faults should form rather than having slip occur on LANFs, assuming values of fault friction consistent with Byerlee's Law. We present results of laboratory measurements on the frictional properties of natural clay-rich gouges from low-angle normal faults (LANF) in the American Cordillera, from the Whipple Mts. Detachment, the Panamint range-front detachment, and the Waterman Hills detachment. These clay-rich gouges are dominated by neoformed clay minerals and are an integral part of fault zones in many LANFs, yet their frictional properties under in situ conditions remain relatively unknown. We conducted measurements under saturated and controlled pore pressure conditions at effective normal stresses ranging from 20 to 60 MPa (corresponding to depths of 0.9–2.9 km), on both powdered and intact wafers of fault rock. For the Whipple Mountains detachment, friction coefficient (μ) varies depending on clast content, with values ranging from 0.40 to 0.58 for clast-rich material, and 0.29–0.30 for clay-rich gouge. Samples from the Panamint range-front detachment were clay-rich, and exhibit friction values of 0.28 to 0.38, significantly lower than reported from previous studies on fault gouges tested under room humidity (nominally dry) conditions, including samples from the same exposure. Samples from the Waterman Hills detachment are slightly stronger, with μ ranging from 0.38 to 0.43. The neoformed gouge materials from all three localities exhibits velocity-strengthening frictional behavior under almost all of the experimental conditions we explored, with values of the friction rate parameter (a−b) ranging from −0.001 to +0.025. Clast-rich samples exhibited frictional healing (strength increases with hold time), whereas clay-rich samples do not. Our results indicate that where clay-rich neoformed gouges are present along LANFs, they provide a mechanically viable explanation for slip on faults with dips <20°, requiring only moderate (Pf<σ3) overpressures and/or correcting for ∼5° of footwall tilting. Furthermore, the low rates of frictional strength recovery and velocity-strengthening frictional behavior we observe provide an explanation for the lack of observed seismicity on these structures. We suggest that LANFs in the upper crust (depth <8 km) slip via a combination of a) reaction-weakening of initially high-angle fault zones by the formation of neoformed clay-rich gouges, and b) regional tectonic accommodation of rotating fault blocks.

Bouguer gravity anomaly and isostasy at western Sichuan Basin revealed by new gravity surveys

AbstractA dense relative gravity and Global Positioning System observation network with 302 stations was measured in the western Sichuan Basin (SB) to study gravity and isostasy. Bouguer Gravity Anomalies are negative in the study area, and change gradually from about −110 mGal (10‐ 5m s‐ 2) in the southeast to −220 mGal in the northwest. The new data reveal that the Moho beneath the western SB changes gradually from 39.5 km in the southeast to 43.7 km in the northwest and is inclined slightly to the northwest beneath the Chengdu Plain. The isostatic crustal thickness calculated by Airy isostatic theory varies between 39.5 and 42.0 km. The Longquan Shan (LQS) marks a clear boundary in the state of isostastic compensation in the study area. The Moho depth is almost identical to the Airy isostatic crustal thickness east of the LQS, where the crust is in isostatic balance. However, on the Chengdu Plain west of the LQS, differences between the Moho depth and Airy isostatic crustal thickness became larger from east to west, where the crust is out of isostatic balance. This indicates that the load of the Longmen Shan (LMS) on the west part of the crust of SB is mainly borne by the crust beneath the Chengdu Plain, and does little influence the crust east of the LQS. As a result we deduce that the LQS fault zone is a deeply rooted high angle fault zone that was broken by the load of the LMS.

Image analysis of airborne geophysical data from the Salcha River – Pogo area, Alaska

An airborne geophysical survey was conducted by the State of Alaska in the summer of 1999 in the Salcha River - Pogo area to support exploration for economically viable mineral deposits. We processed and analyzed a subset of the airborne geophysical data, including magnetic, apparent resistivity, K, equivalent Th (eTh), and equivalent U (eU) data, to further investigate this potentially resource rich area. We used unsupervised classification to analyze a data stack of geophysical data bands because that approach allows the data to self-organize into classes that may represent geologic features, with minimal a priori investigator-imposed class definition constraints. The image data were rescaled to a common range of values and kept in floating-point format, and pixels corresponding to valley areas were masked to reduce topographic and soil-moisture effects. Classification results were interpreted by comparison to published maps of the area. The final classification successfully differentiates conductive and nonconductive lithologies, reliably classifying carbonaceous phyllite and meta-argillite units; delineates mapped and potentially unmapped mafic to ultramafic and meta-mafic rock assemblages; discriminates major regional high-angle fault zones; and identifies what we infer to be evidence of previously unrecognized major low-angle geologic structures. Although unsupervised classification of the geophysical spectral data was unable to resolve the geology of large areas of the survey tract, it does generate potentially useful classes that can be interpreted. Hypotheses formed from these interpretations suggest new geologic features and mineral prospects in the survey tract that are amenable to testing in the field.

Cenozoic kinematic history of the Kohistan fault in the Pakistan Himalaya

Detailed mapping suggests a new scenario for the kinematic history of the Late Cretaceous Kohistan arc complex and for the cause of Cenozoic metamorphism and exhumation along the Indian continental margin in Pakistan. The earliest deformational phases resulted in early Cenozoic amphibolite-facies metamorphism, superposed tight to isoclinal F1/F2 folds, and development of strong penetrative rock fabric within the Indian plate in response to southwestward, and then southeastward, overthrusting of ophi-olitic melange in the Indus suture zone and West Pakistan fold belt, respectively. This deformation included the development of the Banna and Malakand thrust faults. Prograde metamorphism ended following this deformational phase, and the early structures were deformed along N-trending, open, upright, F3 folds and domes. This was followed by ESE- to E-directed thrusting of the Kohistan arc complex onto the Indian continent along the dominantly brittle Kohistan fault. Thrusting ended in the late Oligocene–early Miocene when the Kohistan arc assumed its present position along the northern margin of India. South-verging structures began to form on the Indian plate for the first time in the late Oligocene–early Miocene. Deformation had expanded to the foreland by the middle to late Miocene with development of the Panjal-Khairabad thrust. Neogene structures in the metamorphic zone include N-trending, east-side-up, high-angle fault zones that deform or reactivate the Kohistan fault, and NW-trending, east-side-up reverse faults, both of which are seismically active. In the tectonic scenario outlined here, shortening along the Kohistan fault occurred subsequent to metamorphism, and was directed toward the ESE-E throughout its history. The southern termination of the fault is considered to be a right-slip sidewall ramp such that the Kohistan arc never advanced much father south on the Indian plate than its present position. Broad flexure, folding, and erosion in front of and south of the actively advancing Kohistan arc complex contributed to exhumation of the Indian plate. This differs from interpretations that state that Kohistan was thrust in a southward direction; that underthrusting beneath Kohistan was the primary cause of early Cenozoic metamorphism on the Indian plate; and that top-side-north extensional reactivation of the Kohistan fault contributed to exhumation.

The role of extensional tectonics in shaping Cenozoic New-Caledonia

Abstract New-Caledonia island consists of an ultramafic nappe thrusted over a continental and arc-derived basement as the result of the closure of a back-arc basin during the upper Eocene. The morphology of New-Caledonia is mostly characterized by its elongated shape and rectilinear coastline. Indeed, the western and eastern edges of the island are delineated by N140 trending normal faults that are well imaged on seismic lines. Major tectonic lineaments onland, including the scarps corresponding to the main boundary of the ultramafic nappe, also trend N140 suggesting a morphotectonic control by faults having similar kinematics (longitudinal flexure-fault of previous authors). We provide detailed observations from some of these scarps demonstrating indeed that these are not erosional features in origin. We explore the possibility that such surfaces do represent major tectonic limits that accommodated extension and significant tectonic thinning of both the peridotite nappe and its basement. Remnants of a complex extensional fault zone, 200 m wide, are still preserved along the western scarp of the Mont Dore mountain, close to Nouméa city. The deformed zone is composed of an early shallow dipping detachment fault, some decimeters thick, offset by a group of late high angle normal faults and is bounded by cataclastic breccias that progressively pass into the fractured host peridotite. The late high angle fault zones have been the site of important syn-tectonic fluid circulation and are underlined by cm-thick silica infills bearing vertical striae. Typical morphology of the main scarps with break of slope is related to the combination of high angle normal faults and low angle detachment. Similar shallow dipping detachment surfaces have been observed along the east coast of southern New-Caledonia as well as within the ultramafic nappe itself and its basement. This study confirms that N140 oriented, large fault zones, with pure-normal to transtensional component of displacement, controlled the morphotectonic evolution of New-Caledonia at the regional scale, after the obduction of ophiolite nappe at the end of the Eocene. Such a fact has to be taken into account when attempting to reconstruct the post-obduction evolution of the island and more particularly when considering the development and distribution of the Ni-rich lateritic surfaces.

Kinematic significance of fold- and fault-related fracture systems in the Zagros mountains, southern Iran

Initiation and formation of folds and the Kazerun high-angle fault zone, in the Zagros fold-and-thrust belt, were related to the continuing SW–NE oriented contraction that probably initiated in the Late Cretaceous, and intensified, starting in Miocene, when the Arabian and Eurasian plates collided. The contraction that led to folding and thrusting of the Phanerozoic sequence in the belt has led to the strike–slip reactivation of basement faults that formed during the Precambrian. Two major systems of fractures have developed, under the same regional state of contraction, during the folding and strike–slip faulting processes. Folding led to the formation of a system of fold-related fractures that comprises four sets of fractures, which include an axial and a cross-axial set that trend parallel and perpendicular to the confining fold axial trace, respectively, and two oblique sets that trend at moderate angles to the axial trace. Slip along high-angle, strike–slip faults formed a system of fractures in the damage zone of the faults (e.g., Kazerun), and deformed folds that existed in the shear zone by rotating their axial plane. This fault-related fracture system is made of five sets of fractures, which include the two sets of Riedel shear fractures ( R and R′), P- and Y-shear fractures, and an extensional set. Remote sensing analysis of both fracture systems, in a GIS environment, reveals a related kinematic history for folding outside of the Kazerun shear zone and faulting and deformation (fracturing and rotation of folds) within the Kazerun fault zone. Rotation of the folds and formation of the five sets of the fault-related fractures in the Kazerun shear zone are consistent with a dextral motion along the fault. The mean trends of the shortening directions, independently calculated for the fold- and fault-related fracture systems, are remarkably close (N53 ± 4°E and N50 ± 5°E, respectively), and are perpendicular to the general NW–SE trend of the Zagros fold-and-thrust belt. Although segments of the Kazerun fault are variably oriented within a narrow range, the angular relationships between sets of fault-related fractures and these segments remain constant.

Clay mineralogical and isotopic (K–Ar, δ18O, δD) constraints on the evolution of the North Anatolian Fault Zone, Turkey

This study presents the first attempt to constrain the evolution of the North Anatolian Fault Zone (NAFZ) by age dating and isotope tracing of clay minerals formed during near-surface faulting. Extensive illitic clay mineralisation occurred along the NAFZ related to hydrothermal alteration of the fault gouges and pseudotachylytes. Samples representing the pre-fault protoliths outside the fault zone do not contain authigenic illitic clay minerals indicating that hydrothermal processes were confined to the areas within the fault zone. K–Ar age data indicate that the hydrothermal system and the associated illite authigenesis initiated at ∼57 Ma. This process is interpreted to reflect the onset of significant strike-slip or transtensional faulting immediately after the continental collision related to the closure of the Neotethys Ocean. Following the initiation of the fault movements in the latest Paleocene–Early Eocene, displacements along the NAFZ have continued, with probably intensified fault activities at ∼26 Ma and later than ∼8 Ma. Oxygen isotope compositions of the illitic clays from different locations along the NAFZ are similar, with narrow ranges in δ 18O values indicating clay precipitation from fluids with similar oxygen isotope compositions and crystallisation temperatures. The δ 18O and δD values of the calculated fluid isotopic composition ( δ 18O = 5.9‰ to 11.2‰, δD = − 59‰ to − 73‰) are consistent with metamorphic and magmatic origin of fluids mobilised during active tectonism. The interpretation of the fluid flow history of the NAFZ is in agreement with that reported previously for some well-known large-scale high-angle fault zones, which similarly developed along collisional-type orogenic belts and are commonly associated with significant mesothermal ore mineralisation.

Hydrothermal uranium deposits containing molybdenum and fluorite in the Marysvale volcanic field, west-central Utah

Uranium deposits containing molybdenum and fluorite occur in the Central Mining Area, near Marysvale, Utah, and formed in an epithermal vein system that is part of a volcanic/hypabyssal complex. They represent a known, but uncommon, type of deposit; relative to other commonly described volcanic-related uranium deposits, they are young, well-exposed and well-documented. Hydrothermal uranium-bearing quartz and fluorite veins are exposed over a 300 m vertical range in the mines. Molybdenum, as jordisite (amorphous MoS2), together with fluorite and pyrite, increase with depth, and uranium decreases with depth. The veins cut 23-Ma quartz monzonite, 20-Ma granite, and 19-Ma rhyolite ash-flow tuff. The veins formed at 19-18 Ma in a 1 km2 area, above a cupola of a composite, recurrent, magma chamber at least 24 × 5 km across that fed a sequence of 21- to 14-Ma hypabyssal granitic stocks, rhyolite lava flows, ash-flow tuffs, and volcanic domes. Formation of the Central Mining Area began when the intrusion of a rhyolite stock, and related molybdenite-bearing, uranium-rich, glassy rhyolite dikes, lifted the fractured roof above the stock. A breccia pipe formed and relieved magmatic pressures, and as blocks of the fractured roof began to settle back in place, flat-lying, concave-downward, “pull-apart” fractures were formed. Uranium-bearing, quartz and fluorite veins were deposited by a shallow hydrothermal system in the disarticulated carapace. The veins, which filled open spaces along the high-angle fault zones and flat-lying fractures, were deposited within 115 m of the ground surface above the concealed rhyolite stock. Hydrothermal fluids with temperatures near 200 °C, 18OH2O∼−1.5, DH2O∼−130, log f O2 about −47 to −50, and pH about 6 to 7, permeated the fractured rocks; these fluids were rich in fluorine, molybdenum, potassium, and hydrogen sulfide, and contained uranium as fluoride complexes. The hydrothermal fluids reacted with the wallrock resulting in precipitation of uranium minerals. At the deepest exposed levels, wallrocks were altered to sericite; and uraninite, coffinite, jordisite, fluorite, molybdenite, quartz, and pyrite were deposited in the veins. The fluids were progressively oxidized and cooled at higher levels in the system by boiling and degassing; iron-bearing minerals in wall rocks were oxidized to hematite, and quartz, fluorite, minor siderite, and uraninite were deposited in the veins. Near the ground surface, the fluids were acidified by condensation of volatiles and oxidation of hydrogen sulfide in near-surface, steam-heated, ground waters; wall rocks were altered to kaolinite, and quartz, fluorite, and uraninite were deposited in veins. Secondary uranium minerals, hematite, and gypsum formed during supergene alteration later in the Cenozoic when the upper part of the mineralized system was exposed by erosion.

The Santa Rita fault zone: Evidence for large magnitude earthquakes with very long recurrence intervals, basin and range province of southeastern Arizona

A discontinuous zone of subdued, west-facing fault scarps offset Quaternary alluvium along 58 km of the western piedmont of the Santa Rita Mountains, south of Tucson, Arizona. Scarps trend NE to N-S and are 1 to 6 km basinward from the deeply embayed mountain front. Scarp heights range from 1 to 7 m, and maximum scarp slope angles range from 3° to 9.5°. A 6-m-deep trench excavated in middle Pleistocene and younger deposits exposed a high-angle fault zone with about 3.5 m of total displacement. Estimated ages of faulted and unfaulted surfaces, extent of degradation of piedmont scarps, and stratigraphic relations exposed in the trench suggest an age of about 100 ka for the most recent surface-rupture event. Units having well-developed relict soils of late Pleistocene age and older are faulted, whereas units having weakly to moderately developed soils, of latest Pleistocene and Holocene age, are not offset. Morphologic fault scarp analyses suggest an age of 60 to 100 ka. Earthquake magnitude estimates derived using minimum and maximum seismic source moment calculations range from 6.4 to 7.3 One or more earlier surface-rupture events are implied by significantly high scarp heights on mid-Pleistocene than on late Pleistocene surfaces. Scarp heights on middle and early (?) Pleistocene surfaces are similar, restricting faulting to post-middle Pleistocene time (<200 to 300 ka). Stratigraphic relationships observed in the trench also suggest two fault-displacement events. Tectonic landform analysis of the mountain front-piedmont area implies several m.y. of inactivity along the range-bounding fault system prior to these late Quaternary displacements. Extremely long recurrent intervals between displacement events and reactivation of faulting after an interval of tectonic quiescence are typical of late Quaternary fault behavior in southeastern Arizona.

Stratigraphical and structural variations in central SW England: a critical appraisal

Extensive remapping of central SW England has led to the recognition of significant, strike-parallel variations in stratigraphy and structure across major NNW-SSE trending Hercynian fault zones. Tectonostratigraphic units across the region are distinguished and the tectonic evolution interpreted. The NNW-SSE faults interacted with E-W trending faults, which extend down to the basement, to provide differential vertical movements affecting both the sedimentation and structural evolution. West of Dartmoor northwards translation of nappes, which thinned by gravitational spreading, commenced in the late Visean. This facilitated the northward progradation of flysch sedimentation which had already advanced across the Devonian limestones in the south. The flysch became intercalated with the upper parts of the nappe sequence during the late Lower Namurian and was translated northwards into the regional downwarp of N Devon (‘Culm Synclinorium’). At the southern boundary of this feature, part of the allochthon terminated against an E-W block which had profoundly affected Upper Palaeozoic sedimentation. The complex NNW-SSE high-angle fault zones acted as boundary zones within and between nappes permitting differential northward displacements. In contrast, S and E of Dartmoor sedimentation continued into Lower Namurian times and subsequent compression of a series of E-W trending basins and rises produced folding and variable degrees of local thrusting. These field data provide significant constraints for regional modelling and estimates of shortening around 26 km (34%) for central SW England and less than 68 km for the peninsula as a whole. Models invoking regional decollement, extensive translation and back thrusting are rejected.

Hydrothermal Uranium Vein Deposits in Marysvale Volcanic Field, Utah: ABSTRACT

Hydrothermal uranium veins are exposed over a 300 m (980 ft) vertical range in mines of the Central Mining area, near Marysvale, Utah. They cut 23 Ma quartz monzonite, 21 Ma granite, and 19 Ma rhyolite ash-flow tuff. The veins formed 18-19 Ma, in an area 1 km (0.6 mi) across, above the center of a composite magma chamber at least 12 × 6 km across that fed a sequence of 21-14 Ma hypabyssal granitic stocks, and rhyolitic lava flows, ash-flow tuffs, and volcanic domes. Intrusive pressure uplifted and fractured the roof; molybdenite-bearing, uranium-rich, glassy dikes were intruded; and a breccia pipe and uranium-bearing veins were formed. The veins appear to have been deposited near the surface above a concealed rhyolite stock, where they filled high-angle fault zones and flat-lying to concave-downward pull-apart fractures. Low pH and f02 hydrothermal fluids at temperatures near 200°C (392°F) permeated the fractured rocks; these fluids were rich in fluorine and potassium, and contained uranium as uranous-fluoride complexes. Fluid-wall rock interaction increased fluid pH, causing precipitation of uranium minerals. At the deepest exposed levels, wall rocks were altered to kaolinite and sericite, and uraninite, coffinite, jordisite, fluorite, molybdenite, quartz, and pyrite (with ^dgr34S near zero per mil) were deposited. The fluids were progressively oxidized higher in the system; iron i the wall rocks was oxidized to hematite, and sooty uraninite and umohoite were deposited. End_of_Article - Last_Page 946------------

Crystal growth in cataclasites: Diagnostic microstructures and implications

Abstract Microstructures in cataclasites from low- and high-angle fault zones in granitic basement rocks of the Scandinavian Caledonides indicate the operation of several processes. Intensive formation of microcracks filled with fine-grained mineral fragments indicates brittle deformation. The formation of quartz crystals with euhedral terminations suggests crystal growth. Occurrence of undulose extinction, deformation lamellae and grain boundary recrystallization in other quartz grains illustrates plastic deformation. Crystal growth causes (micro)cracks to heal and thereby changes the behaviour of rocks undergoing brittle deformation. The crystallization products vary in size from c 10 μm to c. 5 mm. Narrow banding parallel to rhomb and prism planes is indicative of crystal growth. The growth bands show slightly different crystallographic misorientation. In some cases the boundaries of these bands could be identified as dislocation arrays decorated by bubbles. The occurrence of fibrous chalcedony aggregates and transitions from extremely fine-grained quartz to large grains with euhedral terminations suggests the presence of colloidal silica-bearing fluids during growth. The crystallization process leads to the formation of vein-like structures. After crystallization plastic deformation may take place, followed by brittle deformation and new crystal growth. It is concluded that during the formation of the cataclasites a cyclic operation of brittle failure, crystal growth followed by plastic deformation took place, and this cycle was repeated several times.

Geology and Structural Control of Geothermal System at Roosevelt Hot Springs KGRA, Beaver County, Utah: ABSTRACT

The Roosevelt Hot Springs KGRA is located in the Basin and Range province along the western flank of the Mineral Mountains. It is within the Wah Wah-Tusher mineral belt which has been the locus for rhyolitic intrusive and extrusive activity through Tertiary and into Quaternary time. The area is just east of the Sevier (Cretaceous) thrust belt and is near the margin of the Intermountain seismic belt. Geologic mapping has identified three metamorphic and plutonic units of Precambrian age, nine intrusive phases of the Tertiary Mineral Mountains pluton, three Pleistocene rhyolitic extrusive phases, and siliceous hot spring deposits of relatively recent age. Cuttings from exploration holes indicate that the Precambrian and Tertiary crystalline rocks host the present geothermal system. The structure of the area is dominated by low-angle normal faults (denudation faults) which dip to the west. The hanging wall of the principal denudation fault was intensely brecciated during the fault episode forming steep fault zones which generally strike northwest. Both these low- and high-angle fault zones show the development of intense, silicified mylonites. Adjacent to these mylonite zones, the crystalline rocks are highly fractured. East-west and northeast-trending high-angle faults cut the denudation faults and channel much of the recent hot-spring activity. The geothermal system is a high-temperature, water-dominated resource which is probably related to an igneous heat source. The low primary permeability of the reservoir rocks and the location of the geothermal field indicate structural control of the system. The reservoir geometries and permeabilities result from the intersections of the principal fault systems. End_of_Article - Last_Page 836------------

Photogeologic and thermal infrared reconnaissance surveys of the Los Negritos-Ixtlan de los Hervores geothermal area, Michoacan, Mexico

New techniques, involving interpretation of panchromatic, ektachrome and ektachrome infrared aerographic photogaphs and thermographic infrared imagery recording emission from the earth's surface in middle and far infrared wavelengths (3–5μm and 8–14μm), are being introduced in geothermal investigations in Mexico to identify outstanding structural and geologic features in a rapid and economical manner. The object of this work is to evaluate the new airborne infrared techniques and equipment as a complement to the data obtained from panchromatic aerial photography. This project is part of the Mexican remote sensing program of natural resources carried out under the auspices of the Comision Nacional del Espacio Exterior and in which the Research Institute (Instituto de Investigaciones de la Industria Electrica) is actively participating. The present study was made cooperatively with the U.S. National Aeronautics and Space Administration and the U.S. Geological Survey. The Los Negritos-Ixtlan de los Hervores geothermal fields are located east of Lake Chapala at the intersection of the Sierra Madre occidental and the west-central segment of the neovolcanic axis of Mexico. The two principal zones of hydrothermal activity occur in a tectonic trench filled with lake sediments of the Quaternary intercalated with Quaternary and Holocene volcanic rocks and characterized by an intricate system of block-fault tectonics, part of the Chapala-Acambay tectonic system, along which there has been volcanic activity in modern time. Surface manifestations of geothermal activity consist of relatively high heat flow and hot springs, small geysers and small steam vents aligned along an E-W axis at Ixtlan, possibly at the intersection of major fault trends and mud volcanoes and hot pools aligned NE-SW at Los Negritos. More than 20 exit points of thermal waters are shown on infrared imagery to be aligned along an extension of the Ixtlan fault between Ixtlan and El Salitre. A narrow zone of hydrothermal alteration and deposition at the surface is identifiable on the infrared imagery of this area, closey related spatially to a resistivity low at depth. Extinct geothermal areas near El Salitre, Ixtlan, and farther west at San Gregorio are clearly delineated on both infrared images and infrared ektachrome photographs. Predawn infrared images also show high-angle fault zones suggesting the dominance of block tectonics in much of the area. Special image enhancement techniques applied to the original magnetic tape records will be required for more precise identification of warm ground zones and for a qualitative or semiquantitative estimate of ground radiance associated with anomalously high convective heat flow.