Major Structural Discontinuities Research Articles

Termination of strike-slip faults at convergence zones within continental transform boundaries: examples from the California Continental Borderland

Abstract Continental transform plate boundaries are broad, composed of numerous active and subparallel strike-slip fault zones. Irregular geometry along the major transform structure creates convergence and divergence zones within the plate boundary where other strike-slip faults terminate. Some prominent irregularities result from microplate interactions. Relative fault displacement, diminishing to zero at fault terminations, must be accommodated or transferred to other structures, laterally or vertically, away from the fault end-point. Distinct styles of strike-slip fault termination may represent different degrees of vertical strain partitioning within the plate boundary. The Western Transverse Ranges (WTR) of California mark a major structural discontinuity that cuts at high angle across the Pacific-North America transform boundary. Within the California Continental Borderland, two end-member classes of right-slip fault termination against the WTR are apparent. (1) Several major faults, including the San Clemente, San Pedro Basin, Ferrelo and Newport-Inglewood, intersect the southern boundary of the WTR at high angles, with negligible to minor local deflection and minor dissipation of right shear at the Earth’s surface. These faults are inferred to cut through the entire borderland crust and continue in the lower crust beneath the WTR, as evident in geophysical data. These ‘blind’ near vertical faults may control segmentation and earthquake activity on the overlying west-trending WTR structures. (2) In contrast, the Palos Verdes and possibly Whittier faults appear broadly deflected westward to merge at low angle with WTR structure. NW-trending faults rotate counterclockwise, away from the axis of principal shortening as observed in pure shear models, and slip is dissipated through folding, thrust transfer and rotation. Deflected faults are inferred to be predominately upper crustal features, detached from the lower crust and unable to underthrust the WTR. These two distinct right-slip fault termination styles, and associated convergent structures, suggest that basal shear drives vertical-axis rotation of the WTR block over the underthrust Inner Borderland plate. Furthermore, the lower plate, slivered by these right-slip faults, is incompletely coupled with the Pacific plate.

Fault geometries from the space distribution of the 1990–1997 Sannio–Benevento earthquakes: inferences on the active deformation in Southern Apennines

In this study, we analyze the recent (1990–1997) seismicity that affected the northern sector (Sannio–Benevento area) of the Southern Apennines chain. We applied the Best Estimate Method (BEM), which collapses hypocentral clouds, to the events of low energy ( M d max=4.1) seismic sequences in order to constrain the location and geometry of the seismogenetic structures. The results indicate that earthquakes aligned along three main structures: two sub-parallel structures striking NW–SE (1990–1992, Benevento sequence) and one structure striking NE–SW (1997, Sannio sequence). The southernmost NW–SE structure, which dips towards NE, overlies the fault that is likely to be responsible for a larger historical earthquake ( I o max= XI MCS, 1688 earthquake). The northernmost NW–SE striking structure dips towards SW. The NE–SW striking structure is sub-vertical and it is located at the northern tip of the fault segment supposed to be responsible for the 1688 earthquake. The spatio-temporal evolution of the 1990–1997 seismicity indicates a progressive migration from SE (Benevento) to NW (Sannio) associated to a deepening of hypocenters (i.e., from about 5 to 12 km). Hypocenters cluster at the interface between the major structural discontinuities (e.g., pre-existing thrust surfaces) or within higher rigidity layers (e.g., the Apulia carbonates). Available focal mechanisms from earthquakes occurred on the recognized NW–SE and NE–SW faults are consistent with dip-slip normal solutions. This evidences the occurrence of coexisting NW–SE and NE–SW extensions in Southern Apennines.

Monazite–xenotime thermochronometry: methodology and an example from the Nepalese Himalaya

Monazite–xenotime thermochronometry involves the integration of petrographic, geochronological, and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals. The method is illustrated in this paper by application to an orthogneiss sample from the Everest region of the Nepalese Himalaya that contains leucogranitic segregations produced by in-situ anatexis. Observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of Himalayan (Tertiary) and pre-Himalayan age. Nearly concordant U–Pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28.37 to 17.598 Ma, making determination of the timing of anatexis difficult without additional information. Presuming that monazite and xenotime were in equilibrium over that entire interval, temperatures estimated from the yttrium contents of dated monazites range from 677–535 °C. Only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition, implying that the ~25.4–24.8 Ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis. Two monazite crystals yielded 207Pb/235U dates that are statistically indistinguishable from the 207Pb/235U dates of coexisting xenotime crystals, permitting the application of both quantitative Y-partitioning and semi-quantitative Nd-partitioning thermometers as a cross-check for internal consistency. One of these sub-populations of accessory minerals, with a mean 207Pb/235U date of 22.364±0.097 Ma, provides inconsistent Y-partitioning (641±39 °C) and Nd-partitioning (515–560 °C) temperatures. We suspect the discrepancy may be caused by the high Th concentration (6.12 wt% ThO2) in this subpopulation's monazite. The Y-partitioning thermometer was derived from experimental data for the (Ce, Y)PO4 binary and may be inappropriate for application to high-Th monazites. For the other subpopulation (mean 207Pb/235U date=22.11±0.22 Ma), the Y- and Nd-partitioning temperatures are indistinguishable: 535±49 and 525–550 °C, respectively. This consistency strongly suggests that the sample experienced a temperature of ~535 °C at 22.11 Ma. This finding is tectonically important because temperatures at higher structural levels were much higher (by ~100 °C) at the same time, lending support to earlier suggestions of a major structural discontinuity within the upper part of the Himalayan metamorphic core at this longitude. An additional finding of uncertain importance is that inherited monazite and xenotime yielded U–Pb discordia with indistinguishable upper intercept ages (465.5±8.7 and 470±11 Ma, respectively) and application of the Y-partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 °C. Whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes, but it appears that monazite–xenotime thermochronometry may be useful for "seeing through" high-grade metamorphism to extract temperature-time information about inherited mineral suites.

Role of faults and layer interfaces on the spatial variation of stress regimes in basins: inferences from numerical modelling

The spatial variation of the computed state of stress is studied in a series of numerically solved boundary-value problems of basin-scale deformation. These numerical experiments illustrate simple situations of decollement tectonism under compression. The study focuses on the local variation of the state of stress in a sedimentary overburden where major discontinuities such as large-scale faults and layer interfaces are present. The states of stress are computed using either the distinct element method or the finite element method. Classic elasto-plasticity laws are used for the behaviour of the continuum, and the discontinuities are modelled as Coulomb contact surfaces. A geologic interpretation of the numerical experiments is proposed to highlight the role played by major structural discontinuities in the local variations of the stress regime. The models are two-dimensional in space and assume plane strain, but the full three-dimensional stress-strain constitutive relation is used. Therefore, the stress regime variations can be represented by constructing the contour map of the Wallace-Bott stress ratio. This simple technique, together with the construction of diagrams of principal stress magnitude across the model, enables us to rapidly examine the stress-field pattern and further discuss the deformation modes of faulting and fracturing, which may take place at a small scale. We have found that the local variations of the stress tensor are in good agreement with common geologic interpretation of brittle deformations in foreland and thrust systems. We demonstrate how the presence of layer interface and the difference in mechanical properties of formation lithology may strongly affect the variation of stress in space. This modelling approach has important issues when interpreting well tests to gain access to in-situ stresses.

Fennoscandian Borderzone: thermal and tectonic history of a tuffaceous sandstone and granite from fission track analysis, Bornholm, Denmark

Fission track ages for zircon and apatite together with track length distributions for apatite have been obtained from a tuffaceous sandstone of Wenlock age from Bornholm, Denmark. The zircon yields an age close to the depositional age of c. 420 Ma, which is similar to the age of bentonites from Gotland, Sweden. The apatite yields a much younger age, which is interpreted as a cooling/uplift age on the basis of the track length distribution. It is concluded that, due to burial since deposition, the sediment has been heated into the 130–190°C interval prior to 261 Ma. The complex apatite fission track age and length distribution obtained for a granite suggest reheating or prolonged heating in the 70–130°C interval probably associated with individual block movements in the area. Bornholm is located at the rim of the Fennoscandian Borderzone and thus fission track analysis of the thermal and tectonic history is important in understanding the evolution of this structure which is part of a major structural discontinuity stretching from Oslo to the Black Sea.

Structural and tectonic evolution of the Humber Zone, western Newfoundland 2. A regional model for Acadian thrust tectonics

The Humber Zone of the western Newfoundland Appalachians represents a Cambrian‐Ordovician passive continental margin which was deformed in Taconian (mid‐Ordovician) and Acadian (Silurian‐Devonian) orogenic events. A deformation front is imaged in seismic reflection data offshore of western Newfoundland. Structures associated with this deformation front are exposed on Port au Port peninsula, where Silurian rocks are strongly deformed but Mississippian strata are flat lying, indicating that latest thrusting was Acadian. A gravity low in the Gulf of St. Lawrence corresponds to a sediment‐filled Acadian foreland basin. Previous models suggest that the on‐land shelf succession is autochthonous to parautochthonous. However, two Lithoprobe seismic reflection transects show sub‐horizontal reflections between 2 and 5 s two‐way travel time, which extend up to 85 km east of the thrust front. These are interpreted as autochthonous platform and basement. In this model, shallower reflectors and outcropping units include both allochthonous platform and basement, comprising the Acadian Port au Port allochthon. The Taconian Humber Arm allochthon was carried westward as a high structural slice during thrusting of this allochthon. No major structural discontinuity exists between Grenville age crystalline rocks of the Long Range massif and platform rocks interpreted as allochthonous in the northern seismic line. A monocline at the southern extremity of the Long Range probably represents an oblique or lateral hanging wall ramp above the basal detachment. Within the Long Range thrust zone at the western margin of the massif the Long Range thrust shows only a few kilometers of displacement. However, the Parsons Pond thrust, which we interpret to run offshore at Green Point, juxtaposes contrasting successions with different structural and thermal histories; it probably carries a much larger amount of the total displacement. The basal decollement of the Port au Port allochthon is therefore interpreted to pass beneath the southern part of the Long Range massif.

Zircon U‐Pb geochronology of the Zambales and Angat Ophiolites, Luzon, Philippines: Evidence for an Eocene arc‐back arc pair

Two basement terranes, the Zambales ophiolite in the west, and the Angat ophiolite in the east, are exposed on the island of Luzon, separated by a circa 10 km thick and circa 100 km wide sedimentary basin. The structural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution proposed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zircon fractions from tonalite in the Acoje block of the Zambales ophiolite give concordant U‐Pb ages at 44.2 (±0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U‐Pb ages of 45.1 (±0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Aksitero Formation. No age difference is discerned between the arc‐like Acoje block and MORB‐like Coto block of the Zambales ophiolite. Four zircon fractions from two sample sites in the Angat ophiolite give concordant ages of 48.1 (±0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike‐pillow lava‐sediment sequence south‐southeast of the main ophiolite. The small age difference between the Zambales and Angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the Southern Sierra Madre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the Eocene Angat ophiolite implies that there are two ophiolitic basements exposed in the Southern Sierra Madre. The relationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cretaceous ophiolitic basement. This suggests that the Zambales‐Angat ophiolite represents a preserved Eocene back‐arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In this model, the Zambales‐Angat ophiolites are therefore not allochthonous terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.

Accretion tectonics in northern Eritrea revealed by remotely sensed imagery

AbstractNew details from remotely sensed images of the structure and disposition of broad lithological variations in the Pan-African of northern Eritrea are discussed in the context of accretionary tectonics. The recognition of major north-south structural discontinuities allows the area to be divided into three discrete terranes with apparently different histories of deformation and metamorphism, magmagenesis and sedimentation. The central Hagar Terrane is dominated by large ultramafic masses with a volcano-sedimentary layered sequence, and shows the effects of major sinistral transpression and lateral expulsion. It is bounded to the west by a major fault, the Barka suture, and abuts the older Barka Terrane that comprises metasediments with evidence for polyphase ductile deformation and pre-kinematic dyke emplacement. The Hagar Terrane is thrust against the eastern Nacfa Terrane, which is dominated by low-grade calc-alkaline metavolcanics and immature volcanoclastic sediments intruded by syn-kinematic plutons. These units are pre-dated by an earlier high-grade basement and post-dated by high-level unmetamorphosed silicic volcanics and redbed sediments. The complex is suggested to have been assembled by oblique accretion from the southeast after arc volcanism in the Nacfa Terrane and back-arc extension in the Hagar Terrane ended with the cease of subduction.

The tectonostratigraphical significance of pre-750 Ma metagabbros within the northern Central Highlands, Inverness-shire

Introduction Amphibolite suites within the late-Proterozoic metasediments of the Northern Highlands are mostly regarded as being of igneous origin, reflecting the emplacement of tholeiitic magmas at an early stage of their tectonothermal history (Winchester 1984; Rock et al. 1985). To the south of the Great Glen Fault the presence of amphibolites, similarly of igneous origin, is also recognized within migmatized Precambrian metasediments assigned by Piasecki (1980) to the Central Highland Division (Highton 1986). This communication describes a suite of metagabbro and amphibolite bodies which crop out within sub-Appin Group metasediments of the northern Central Highlands. Their position within this tectonostratigraphy provides an important reference point for the timing of orogenic activity in the Grampian terrain. General geology Within the northern Central Highlands the presence of Moine-like metasediments, formerly referred to as the Central Highland Granulites, has long been recognised (Home and Wilson 1902). Here, Piasecki (1980) defined two tectonostratigraphical units; the Central Highland Division, a polymetamorphic ‘basement’ complex comprising migmatized psammitic and semi-pelitic gneisses with subordinate quartzites; and the Grampian Division (or ‘Group’, sensu Harris et al. 1978), described as a structurally simpler ‘cover’ sequence of mostly non-migmatitic micaceous psammites, psammitic flags and massive quartzites. These two units were perceived as being separated by a major structural discontinuity, the Grampian Slide. Current resurvey work by the British Geological Survey (BGS) under its Monadhliath Regional Survey Project, of which the present study forms part, has identified a regional suite of metabasic rocks associated with linear belts of high strain, similar . . .

Late Quaternary fault segmentation from analysis of scarp morphology

A thorough knowledge of the lateral extent of individual prehistoric ruptures along a fault is necessary to assess the locations and geologic characteristics of boundaries between independent faultrupture segments. Paleoseismic studies employing only trenching may yield insufficient data to evaluate the lateral extent of individual rupture events. Systematic mapping of fault scarps and degradation-equation modeling of relative scarp ages on the Lemhi fault, a range-bounding normal fault in east-central Idaho, provide a method to assess rupture extent and boundaries between rupture segments. The youngest latest Quaternary surface ruptures occurred at distinctly different times on adjacent segments of the fault. The boundaries between rupture segments differ in characteristics, although most are at intersections of the currently active range-bounding fault and mid-Tertiary normal faults. Several major structural and tectonic discontinuities did not form segment boundaries in the most recent rupture cycle or (probably) in the previous cycle. The Lemhi fault illustrates both the importance of preexisting structures in controlling extent of ruptures and the need for detailed analysis along an entire fault zone.

The stratigraphy of the 3.5-3.2 Ga Barberton Greenstone Belt revisited: A single zircon ion microprobe study

Recent field and geochemical studies indicate a need to test the stratigraphy of the ca. 3.5 Ga Barberton Greenstone Belt as it is presently adopted [1,2]. This work uses the ion microprobe SHRIMP, to attempt such a test. Results show that: (1) Volcaniclastic sediments of the Theespruit Formation (< 3453 ± 6Ma) could be younger than the (structurally) overlying mafic and ultramafic volcanics of the Komati Formation (3482 ± 5Ma). A major structural discontinuity may therefore exist between the two formations. (2) An age of 3538 ± 6Ma established for a tectonic wedge of tonalitic gneiss within the Theespruit Formation confirms the presence of a sialic basement and deformed unconformity below that unit. The tonalitic gneiss is the oldest unit yet recorded within the greenstone belt, equal in age to the older components of the adjacent Ancient Gneiss Complex. (3) The interpreted ages of felsic volcanic rocks from both the Hooggenoeg (3445 ± 8Ma) and Theespruit Formations and the nearby Theespruit Pluton (3437 ± 6Ma) are essentially the same, and corroborate field and geochemical evidence that the felsic units were probably cogenetic and emplaced simultaneously as high-level equivalents of trondjhemite-tonalite plutons that intrude the greenstone belt at its southwestern extremity. (4) Felsic-intermediate volcanic-volcaniclastic rocks locally separating the two major groups (the Fig Tree and Moodies Groups) which overlie the Onverwacht Group record a second major peak of tonalitic magmatism in the Barberton terrain at about 3250 Ma. This is close to the age of the Kaap Valley tonalite pluton which intrudes the Barberton Greenstone Belt at ca. 3226 Ma along its northwestern margin. The present results indicate the Barberton Greenstone Belt is part of an allochthonous sequence containing major tectonic and stratigraphic breaks, with a protracted history; of which the last 200 million years, at least, evolved within a tectonically active convergent environment.

Structural analysis of a ductile-brittle Precambrian shear zone in the Sierra Madre, Wyoming: western extension of the Cheyenne belt?

Abstract Existing plate tectonic models for the Cheyenne belt, a Proterozoic suture at the southern margin of the Archean Wyoming craton, are based largely on data from the Medicine Bow Mountains, southern Wyoming. This paper tests the hypothesis that a major structural and geochronological discontinuity in the Sierra Madre of southern Wyoming is the western extension of the Cheyenne belt. The Sierra Madre shear zone consists of a cataclastic western segment and a mylonitic eastern segment that comprises several mylonite zones. The eastern segment records early ductile thrusting (ca. 1750 Ma) and later dextral strike-slip faulting as revealed by macrostructural, microstructural, and quartz fabric analysis. Structures associated with these events are kinematically compatible and coeval with structures within the suture zone in the Medicine Bow Mountains. The correlation of shear zones in the two ranges is supported further by lithologic similarity of distinctive rocks north of and within the shear zones and the presence of unusual quartz fabrics documenting 〈 c 〉 slip in synkinematic granites in both ranges. Isolated exposures of mylonite along the dominantly cataclastic western segment suggest that the mylonitic shear zone was formerly more extensive. The western segment of the Sierra Madre shear zone is a single zone of intense cataclasis that truncated and dismembered the earlier ductile shear zone. Kinematic analysis within the cataclastic zone suggests that it formed during north-directed thrusting. East-striking, south-dipping thrust faults and associated overturned-to-the-north recumbent folds are developed in supracrustal rocks directly north of the cataclastic fault zone. This spatial association of north-vergent structures and their apparent kinematic compatibility suggests that they are genetically related. Two major northwest-striking, cataclastic, dextral strike-slip in the eastern Sierra Madre appear to merge with the cataclastic thrust fault. These cataclastic faults are interpreted as parts of a north-vergent, thrust-tear system that cut across the original suture zone as rocks in the upper plate of the thrust were transported to the north. It is not known whether this deformational event occurred in response to continued, post-suture convergence to the south or if it represents a separate and distinct Precambrian event. The prominent bend in the Sierra Madre shear zone may be an oroclinal flexure produced as a consequence of northward translation along the cataclastic thrust system.

Paleomagnetism of Middle Miocene volcanic rocks in the Mojave‐Sonora Desert region of western Arizona and southeastern California

Paleomagnetic directions have been obtained from 190 early to middle Miocene (12–20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave‐Sonora desert region of western Arizona and southeastern California. These flows generally postdate early Miocene tectonic deformation accommodated by low‐angle normal faults but predate high‐angle normal faulting in the region. After detailed demagnetization experiments, 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations (TRM). Because of the episodic nature of basaltic volcanism in this region, the 179 flows yielded only 65 time‐distinct virtual geomagnetic poles (VGPs). The angular dispersion of the 65 VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9° within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from rocks of similar age in stable North America and from a paleomagnetic pole calculated from rocks of similar age in Baja California. The coincidence of paleomagnetic poles from the Mojave‐Sonora desert region with reference poles from the stable continental interior indicates that (1) significant vertical axis net tectonic rotations have not accompanied post‐middle Miocene high‐angle normal faulting in this region; (2) there has been no detectable post‐middle Miocene latitudinal transport of the region; and (3) long‐term nondipole components of the middle Miocene geomagnetic field probably were no larger than those of the recent (0–5 Ma) geomagnetic field. In contrast, paleomagnetic data indicate vertical axis rotations of similar age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major structural discontinuity in the vicinity of the southeastward projection of the Death Valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations.

Structural style and domains of the Northern Oman Mountains (Oman and United Arab Emirates)

Abstract A variation in the style of compressional deformation is observed in the northern Oman Mountains. This variation is expressed by the relationship between two major structural levels. The shallower structural level comprises Tertiary (Pabdeh Group), Late Cretaceous (Aruma Group), and Mesozoic (Hawasina and Sumeini Groups) clastics and carbonates that have been extensively deformed by closely spaced, folded duplexes and imbricate fans. In response to the same compressional events, the deformation in the deeper portion of the section is charcterized by more widely spaced, high-angle reverse faults that are interpreted to merge at depth into a decollement surface. This reverse fault system creates large hanging wall anticlines of Middle (Wasia Group) to Early Cretaceous (Thamama Group) and older platform carbonates. The major differences in the structural styles of the two levels are probably related to lithologic variations of the major tectono-stratigraphic units in response to the compression and the large-scale dynamics of the thrust belt as a whole. Structural linkage is expressed between the two levels by the spacial correlation of the culmination axes of the two levels. The variation in this linkage between the two levels in a strike orientation defines several domains that are separated by major structural discontinuities. These domains are interpreted to reflect differences in the interaction between the two structural levels during several compressional events. They are postulated to have been ultimately created in response to variations in the dynamics of the thrust belt related to the emplacement of the Semail Ophiolite onto the Arabian continental margin during the Late Cretaceous and subsequent reactivation during the Tertiary.

The Hemlo-Heron Bay greenstone belt and Hemlo AuMo deposit, Superior Province, Ontario, Canada 1. Sequence of igneous activity determined by zircon UPb geochronology

The Hemlo-Heron Bay greenstone belt in the Wawa Subprovince of the Superior Province comprises folded and metamorphosed volcanic and sedimentary sequences that were intruded by granitoid batholiths, plutons and dikes, and host an economically significant AuMo deposit. An altered, porphyritic unit from a volcanic centre hosting low-grade Au mineralization near Hemlo yields an age of 2772 ± 2 Ma, which is significantly older than a pyroclastic volcanic unit from a volcanic centre near Heron Bay dated at 2695 ± 2 Ma. The large age difference may indicate the presence of a major structural discontinuity, possibly represented by the Hemlo Fault, between now juxtaposed terranes. The oldest intrusive rock dated in this study is a 2719 −4 +6-Ma-old granodiorite near the margin of the Pukaskwa Gneissic Complex that borders the southern extent of the belt. The major pulse of plutonism is recorded by a zircon and titanite age of 26883 Ma for one of the youngest, granodioritic phases of the Pukaskwa Gneissic Complex, and by zircon ages of 2688 ± 3 and 2687 ± 3 Ma for granodiorite at the centre and margin of the Cedar Lake Pluton, respectively, 2687 ± 3 Ma for a porphyry dike adjacent to the Cedar Lake Pluton, 2688 ± 5 Ma for the Heron Bay Pluton, and a potentially slightly younger age of 2684 −3 +4 Ma for the Cedar Creek Stock. A poorly defined zircon age of 2680–2690 Ma for an altered diorite dike subconcordant to the ore body on the Page-Williams Property suggests an origin coeval with that of the larger granodioritic bodies near the deposit. The youngest intrusion is the Gowan Lake Pluton dated by zircon and titanite at 2678 ± 2 Ma. Most of the above plutonic rocks exhibit complex UPb zircon systematics with inherited zircons commonly present besides newly grown magmatic ones, and with complicated Pb loss patterns reflecting, in part, the Keweenawan activity in the region. The geological correlation between plutonism, deformation, and metamorphism indicates that the 2688-2684-Ma intrusive suite post-dated an early, minor(?) phase of folding (F 1) associated with low-angle faulting or thrusting, and was synchronous with, or slightly postdated, a major, second phase of folding (F 2) that was possibly associated with sinistral shearing. The youngest, 2678-Ma-old intrusion is tentatively correlated with amphibolite-facies metamorphism and with the early stages of a third deformation event associated with considerable dextral shearing and related folding (F 3).

Age constraints on metamorphism and the development of a metamorphic core complex in Fiordland, southern New Zealand

Fiordland, southern New Zealand, contains two primary geologic components: an elongate core comprising high-pressure (>12 kbar) granulite facies orthogneisses (western Fiordland orthogneiss) and a structurally overlying mid-Paleozoic plutonic-metasedimentary complex with subordinate metavolcanics (Tuhua Sequence). The contact between these two components is a major structural discontinuity or decollement hitherto defined as the Doubtful Sound thrust, but reinterpreted here as a ductile shear zone separating lower and upper crustal plates generated during continental extension. Strongly lineated Lower Cretaceous mylonites occur throughout this zone; they formed prior to separation of New Zealand from Australia (at ∼80 Ma). Continental rifting was accompanied by increased heat flow, silicic to mafic magmatism, and the widespread resetting of mineral isotopic systems. Thus, in terms of its overall tectonic setting and tectonothermal history, Fiordland bears a striking resemblance to the metamorphic core complexes of the North American Cordillera.

Three-Dimensional Structural Interrelationships Within Cambrian-Ordovician Lithotectonic Unit of Central Appalachians

A block diagram of the Cambrian-Ordovician lithotectonic unit illustrates three-dimensional structural relationships within that sequence along the length of the central Appalachian Valley and Ridge and High Plateau provinces. The diagram shows that the Valley and Ridge province is divisible into areas within which shortening is relatively constant in the Cambrian-Ordovician lithotectonic unit. These areas are bounded by zones across which significant differences in shortening occur. These transition zones contain major cross-strike structural discontinuities in surface structure; in some instances, these discontinuities extend across the Valley and Ridge province and into the High Plateau province. Increases in fold amplitude and number occur in the cover of the Plateau, across strike from the more intensely deformed areas of the Valley and Ridge, where shortening within the Cambrian-Ordovician unit is significantly greater than elsewhere within that province. Structurally controlled gas accumulations are more prevalent in these areas of the Plateau.

The Geology of the Rolwaling-Lapchi Kang Himalayas, East-Central Nepal: preliminary findings

Between the Tethyan sediments to the north and the Main Boundary Thrust to the south ten lithotectonic units have been identified in the Rolwaling-Lapchi Kang Himalayas and the Tamba Kosi region of east-central Nepal. These are (1) the Rolwaling Granites, (2) the Rolwaling Paragneisses, (3) the Rolwaling Migmatites, (4) the Alampu Schists, (5) the Khare Phyllites , (6) The Chagu-ChilangkaAugen Gneisses; (7) The Laduk Phyllites, (8) the Suri Dhoban Augen Gneisses, (9) the Rarnechap Group, and (10) the Mahabharat Crystallines. The Main Central Thrust (MCT) is a major lithologic, metamorphic and structural discontinuity separating the overthrust Higher Himalayan crystallines from the underthrust Lesser Himalayan metasediments. The Mahabharat Crystallines are an outlying klippe of Higher Himalayan rocks, underlain by the MCT, that has been thrust a minimum of 80 kilometers over the underlying Lesser Himalayan metasediments. Extending approximately 5 km below the MCT and 15 km above the MCT the Himalayas are a shear-thrust zone exhibiting ductile, brittle-ductile and brittle deformation as well as an inverted metamorphic sequence.

Crustal outgassing and LILE enrichment in major lithosphere structures, Archean Abitibi greenstone belt: evidence on the source reservoir from strontium and carbon isotope tracers

Major structural discontinuities in the Abitibi greenstone belt acted as conduits for outgassing of the Archean crust, as reflected in fixation of a select group of lithophile elements including Si, C, K, Rb, Ba, Li, Cs, B and Pb, in metasomatized faults. For two of the largest structures, the Destor-Porcupine (DP) and Kirkland Lake — Cadillac (KC) fault zones ∼6×1015 g Si, 3×1015 g CO2 and 1015 g K were introduced into the faults during expulsion of an estimated 6×1018 g aqueous fluids. Strontium isotope ratios of tourmaline, piemontite, actinolite and scheelite mineral separates, characterized by Rb/Sr≤0.02, are concordant with respect to 87Sr/86Sr initial ratios over local sectors of the faults. The Sr isotope data record geographic variations which, from east to west on the KC fault is 0.7031–0.7041 (Val d'Or), 0.7008–0.7022 (Bourlemaque), 0.7017–0.7019 (Bousquet), 0.7029–0.7031 (Noranda), and 0.7013 to 0.7015 (Kirkland Lake). At Timmins, on the PD fault, 87Sr/86Sr initial ratios cluster at 0.7010 to 0.7020. Metasomatised fault zones are systematically more radiogenic than contiguous host lithologies, and imply a source reservoir (0.7010 to 0.7041) generally more radiogenic than the upper mantle at 2690 Ma (0.700±0.001), or contemporaneous volcanic rocks of mafic to ultramafic composition (0.700 to 0.7012). Whereas certain minerals are concordant and retentive, Rb-Sr isochrons based on suites of rocks at progressive intensities of metasomatism, have been systematically reset over an elpased time of ∼200 Ma after termination of outgassing, due to disturbance accompanying incremental displacements on structures.

Petroleum Geology of Pennsylvanian and Lower Permian Strata, Tucumcari Basin, East-Central New Mexico: ABSTRACT

The Tucumcari basin of east-central New Mexico is a structural depression that existed as a depositional basin from Strawn (Middle Pennsylvanian) until late Wolfcampian (Early Permian) time. Depth to Precambrian ranges from 6500 ft to more than 9000 ft. High-angle faults form the north, east, and west edges of the basin. No major structural discontinuities separate the basin from a shallow shelf to the south. Faults cut Pennsylvanian and Wolfcampain strata but generally do not offset post-Wolfcampian strata. Faults and regional structure control facies and thickness patterns within the Pennsylvanian and Wolfcampian. Coarse arkosic sands in the Pennsylvanian and Wolfcampian are good reservoirs. Those sands were deposited in northern and western parts of the basin and were derived from highlands of Precambrian granitic rocks that formed the northern and western margins of the basin. High-energy limestones are possible reservoirs in the southern part of the basin. Porous Wolfcampian dolostones cover the Frio uplift on the east side of the basin. Pennsylvanian and Wolfcampian marine shales and micritic limestones are source rocks within the Tucumcari basin. Post-Wolfcampian strata are thermally immature. Two presently noncommercial pools of oil and gas, the Latigo Ranch pool and the T-4 Ranch pool, have been discoveredmore » in Strawn sands in the northern part of the Tucumcari basin. Oil generated in the upper Paleozoic has migrated vertically into the Triassic; two oil accumulations, the Santa Rosa tar sands and the Newkirk oil pool, are in Triassic sandstones and have combined reserves of 153 million bbls of oil.« less