Ductile Fault Research Articles

Age and tectonic setting of the Duke Island ultramafic intrusion, southeast Alaska

Concordant U–Pb zircon ages on hornblende–plagioclase pegmatites and a related granophyre from marginal and roof zones of the Duke Island ultramafic intrusion, and from leucoquartz diorite internal to hornblende–clinopyroxenite of nearby Mary Island cluster between 108 and 111 Ma. Sm–Nd mineral and bulk-rock data from pegmatite and granophyre zircon samples and from wehrlites and clinopyroxenites of the main Duke Island cumulate sequence are consistent with the consanguinity of the ultramafic cumulates and the zircon-bearing feldspathic rocks, and with an Early Cretaceous igneous age for the intrusion. Wall rocks for the intrusion consist of Ordovician–Silurian amphibolite- and greenschist-grade metamorphosed plutonic, volcanic, and minor sedimentary rocks and crosscutting Late Triassic gabbro–diorite of the Alexander terrane.The Duke Island ultramafic intrusion formed as a northwest-trending elongate funnel-shaped stratiform body commensurate with extensional brittle–ductile faulting in its roof and adjacent wall rocks. Intrusion occurred in a basinal setting within the Gravina volcanic arc along the Cordilleran continental margin. During the latter phases of intrusive activity the Gravina basin began to close by regional thrust faulting, which in the Duke Island region was west-northwest directed. The intrusion may have still had local intercumulate liquid upon initiation of thrust faulting. It responded first by open folding with incipient crystal plasticity. Once completely solidified, the intrusion behaved like a large "augen" with ductile deformation concentrated along its margins and within quartzo-feldspathic and micaceous members of its wall-rock complex. Much of the intrusion's northern margin behaved as a ductile shear zone serving as a tear fault between thrust plates to the northeast and an oblique thrust complex that roots beneath the southwest margin of the intrusion. The intrusion may thus be tectonically transported relative to its original underpinnings. North-trending high-angle faults and parallel fracture cleavage also cut the intrusion in response to axial loading as a result of its initial elongate shape and orientation relative to the thrust kinematics.

Indosinian dextral ductile fault system and synkinematic plutonism in the southwest of the Ogcheon belt (South Korea)

The structure of the mylonite belt in the southwestern part of the Ogcheon belt has been investigated and interpreted in the light of recent radiometric data. The shear belt is composed of several en-echelon dextral shear zones relayed by compressional or extensional tectonic bridges. The dextral displacement along the fault system, which may be extended to the Hida zone of Japan, is of the order of 200 km. It has brought into contact the Ryeongnam block and its Cambrian-middle Ordovician cover of North China affinity, with the southwestern margin of the Ogcheon belt characterized by Cambrian-middle Silurian rift facies and shelf deposits of South China affinity. Synkinematic plutons are emplaced within the shear belt, which is postdated by uppermost Triassic unfoliated granitoids. At the surface, the shear belt generated elongated intermontane coal basins initiated as transtensional half-grabens. Because of its post-Permian, pre-early Jurassic age and its dextral kinematics, we correlate the shear belt with the late stages of the Akyioshi (Indosinian) collisional orogeny of the east Asian margin. The synkinematic plutonism that accompanied the shear zone may be due to post-collisional crustal stretching and subsequent anatexis at the mantle-crust interface.

Magmatism and deformation, southern Revillagigedo Island, southeastern Alaska

The mid-Cretaceous, cale-alkaline, LREE-enriched plutons on southern Revillagigedo Island, southeastern Alaska, form two distinct groups: (1) biotite-epidote-hornblende tonalite of the Moth Bay pluton and (2) biotite-bearing, hornblende-absent leucotonalite. Higher total REE and lower Na 2 O, Al 2 O 3 , and Sr distinguish the Moth Bay tonalite from the leucotonalite. The plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite. Pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are ∼8-9 kbar. Detailed analyses of pluton/host rock relationships indicate that pluton emplacement was synchronous with orogen deformation. Pre-emplacement thrusting placed the metavolcanic and metasedimentary rocks of the Taku terrane structurally above the younger metasedimentary rocks of the Gravina belt. The contact between these units was subsequently cut by the Moth Bay pluton. The southern margin of the Moth Bay pluton has been transformed to blastomylonite. Contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton. A still later phase of deformation transposed the pluton-related structures and generated a northeast-dipping, ductile thrust fault (Southern Revillagigedo shear zone) along the southern shore of Revillagigedo Island. The shear zone and earlier structures all trend approximately northwest. Fission-track age determinations and track-length measurements in apatite were used to determine the cooling histories of the plutonic units. Fission track ages from the western part of the study area are 51.3 ± 4.0 Ma (apatite), 57.8 ± 4.4 Ma (zircon), and 69.8 ± 4.2 Ma (sphene); eastward, toward the Coast batholith, fission track ages are ∼10 m.y. younger. These younger ages may result from a tectonic/thermal event associated with batholith emplacement. This same event may have generated north-trending structures that overprint older northwest-trending structures on the east side of the study area.

Fluid flow, mineral reactions, and metasomatism

A general model that relates fluid flow along a temperature gradient to chemical reaction in rocks can be used to quantitatively interpret petrologic and geochemical data on metasomatism from ancient flow systems in terms of flow direction and time-integrated fluid flux. The model is applied to regional metamorphism, quartz veins, and a metasomatized ductile fault zone.

Tectonic evolution of the northeast portion of the Archean Abitibi greenstone belt, Chibougamau area, Quebec

The Chibougamau area, occupying the northeastern part of the Abitibi greenstone belt is a large synclinorium of volcanic and sedimentary rocks enclosed within tonalitic gneisses. Several east–west–trending regional folds within this synclinorium are responsible for the vertical attitude of the strata. Synclinal structures, with youngest sediments within the core, possess axial-plane schistosity. Anticlines, on the other hand, either form domes with a core occupied by earlier tonalitic to dioritic plutons or are transected by a series of east–west-trending ductile faults (the Waconichi tectonic zone).An early deformation phase of low intensity (D1) generated broad, north–south folds without schistosity. The subsequent regional deformation, event D2, produced the large east–west folds. These deformations, in combination, produced the regional interference pattern of domes and basins. North–south horizontal shortening generated an east–west-trending schistosity associated with a vertical stretching lineation. Regional deformation at its climax produced a tightening of folds and rotation of fold axes parallel to the stretching lineation.Plutons deflected the regional east–west schistosity and formed concentric trajectories associated with "contact-strain aureoles." This produced small interaction zones or triple points characterized by strong vertical extension. These relations suggest an interference between a regional stress field, which produced north–south horizontal shortening, and local stress fields, controlled or deflected by granitoid plutons acting as competent bodies.East–west-trending ductile shear zones represent the final stage of the regional deformation. The observed northward and southward reverse movement along these east–west faults, their parallelism to the axial trace of folds, and the regional schistosity are probable evidence of a regime dominated by a coaxial strain.

Identification of the scales of differential element mobility in a ductile fault zone

Two fundamentally different scales of element mobility have been identified in the upper‐amphibolite‐facies Hunts Brook fault zone of south‐central Connecticut, USA. Field relations, mineral chemistry, and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic Rope Ferry orthogneiss. Two‐sample mass balance calculations between the Rope Ferry Gneiss and fault rocks strongly suggest that the Rope Ferry Gneiss was segregated into biotite‐rich schists and quartz–plagioclase‐rich gneisses through the centimetre‐scale transfer of Si, Al, Ca, Na, Ba, and Zr from schists to gneisses. Three‐ and four‐sample mass balance calculations provide convincing evidence that Al, Ca, Na, and Ba were lost and Si and K gained by the entire fault zone. Both scales of metasomatism have been identified at two localities separated by 20 km, implying that the metasomatic processes were pervasive throughout the fault zone.From the mass balance calculations, an apparent disparity in K mobility arises. K appears to be gained by the entire fault zone, but is immobile during the centimetre‐scale segregation. This is because mass balance calculations can only detect differential element mobility. Differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients. Therefore, the identification of differentially mobile elements provides insight into the mechanisms of metasomatism. The immobility of K during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt. However, the element mobility is reconcilable with deformation‐driven metamorphic differentiation. The gains of Si and K and losses of Ca, Na, Al, and Ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt, but may reflect the infiltration of a metasomatizing aqueous fluid.

Tectonometamorphic history of the Ivrea Zone and its relationship to the crustal evolution of the Southern Alps

The Ivrea Zone represents a cross-section through the lower continental crust of the Southern Alpine basement. A first long episode with regional metamorphism and associated polyphase deformation ended in the Variscan. Subsequent Late Paleozoic magmatic activity may have occurred during an early stage of crustal attenuation. Late Paleozoic and Early Mesozoic crustal thinning is accommodated by conjugate high-temperature shear zones within the granulite facies Ivrea Zone and by low-angle normal faulting within the Pogallo Ductile Fault Zone, at the base of the intermediate crust. The age and kinematics of the Pogallo Ductile Fault Zone are consistent with the occurrence of Early Mesozoic extensional basins in the Southern Alpine sediments. Exhumation and final steepening of the Ivrea Zone during the Alpine Orogeny did not substantially alter its internal structure exept in the vicinity of the Insubric Line. Thus, the Ivrea Zone and the adjacent Strona-Ceneri Zone represent a good example of highly attenuated lower and intermediate continental crust.

Transcurrent ductile shear zones and the dispersal of the Avalon superterrane

Abstract A mosaic of steep, often sinistral, transcurrent shear zones cuts late Precambrian to earliest Cambrian rocks in eastern Canada, southern Britain, and northwest France. Examples include mylonite belts in Saint John (New Brunswick), Cobequid Highlands (Nova Scotia), Burin Peninsula (Newfoundland), Malverns (England), and N Brittany (France). These shear zones often separate geologically distinct areas, each broadly of Avalonian or Cadomian character, that together are grouped into an Avalon superterrane. The shear zones occur as: (1) major fault systems juxtaposing unrelated geological units, e.g. the Dover-Rosslare-Menai Strait mylonite belt, a well defined transcurrent terrane boundary that locally contains exotic slivers of blueschist; (2) shear zones separating high grade gneisses from low grade metasediments which may represent tectonized basement/cover sequences (e.g. Brookville Gneiss/Green Head Group of New Brunswick; Bass River Complex/Gamble Brook Formation of Nova Scotia; Rosslare Complex/Cullenstown Formation in SE Ireland); (3) shear zones (locally with syntectonic granites) separating migmatitic gneisses from low grade rocks of similar protolith, e.g. the Cancale shear zone affecting the Brioverian of northern Brittany; such boundaries may form due to transtensional shearing within an arc system above an obliquely subducting oceanic plate; (4) shear zones separating rocks of similar character so that no case exists for defining the ductile fault as a suspect terrane boundary (e.g. Malverns); (5) long-active steep fault systems, the earliest movements along which have been obscured by Phanerozoic reactivation (e.g. Welsh Borderlands Fault System). Constraints on the timing of the ductile fault movements remain poor, although in Britain and France an early Cambrian age is implied. The transcurrent faulting dismembered and dispersed a long-active late Precambrian arc system and provided a structural template that localized subsequent fault movements and so exerted a major controlling influence on the Phanerozoic geology of the region. Use of the term ‘Avalonian/Cadomian Orogeny’ refers to subduction and terrane dispersal events during the early history of the SE side of the Caledonian/Appalachian Orogen.

Contrasting metamorphic terranes in northwest Wales

Abstract The oldest rocks exposed in northwest Wales crop out as a series of fault-bounded slices. Whereas some of these fault-bounded units are clearly part of the same stratigraphic package (e.g. those belonging to the Monian Supergroup), others show little or no affinity with contiguous areas. Three suspect terranes are identified on Anglesey: (1) the Monian Supergroup; (2) the gneisses and granitic rocks of central Anglesey; (3) the Monian blueschists. The metamorphic histories of these terranes are markedly different and PT conditions vary from anchizonal/lower greenschist (1) to sillimanite grade (2) to epidote- or lawsonite-bearing blueschists (3). Each terrane has been juxtaposed by faults, the earliest movements along which (where not obscured by brittle reactivation) can often be shown to have been ductile and accompanied by dynamic recrystallization. The most prominent of these ductile fault boundaries occur between terranes 1 and 3 in Anglesey (Berw Shear Zone), and between terrane 1 and the Sarn Complex (terrane 4) in the Llŷn peninsula (Llŷn Shear Zone). A similar ductile mylonite zone exists between the Cullenstown Group (terrane 1?) and the Rosslare gneisses (terrane 2?) in SE Eire. All these shear zones are steep, and kinematic indicators indicate oblique sinistral shear. Radiometric, lithostratigraphic and palaeontological data constrain the ductile fault movement to c. 550–540 Ma. This latter event is interpreted as recording an important phase of terrane dispersion and subsequent assembly that occurred along the Avalonian margin immediately prior to marine transgression over southern Britain. It is likely that this transcurrent event is recorded inboard from the Monian area by shear zones within the late Precambrian (Avalonian/Cadomian) basement in southern Britain and northwest France.

Estimates of the rates of microstructural changes in mylonites

Abstract Microstructures developed during dislocation creep such as the dynamically recrystallized grain-size and the subgrain-size are modified in response to increasing differential stresses and shear-strain rates adjacent to rigid minerals in mylonites. In two specimens variations in quartz subgrain-size around rigid minerals has been quantified from orientation contrast SEM images. In a deformed granite from the Pogallo ductile fault zone, quartz two-dimensional subgrain-size is reduced from background values of about 30 μm to 8 μm and then 4 μm in progressively narrower channels between rigid feldspars. In pelitic mylonites from the Alpine fault zone, New Zealand, quartz bands are wrapped around rigid garnets and the subgrain-size is reduced from background values of about 20 μm to 14 μm within 2000 μm of the garnet to 11 μm adjacent to the garnet. In both specimens examined the microstructure responds to both increases and decreases in differential stresses and coupled strain-rates. The analysis of the distribution of subgrain sizes provides information for the assessment of microstructural stability and the estimation of the rates of microstructural changes. The New Zealand specimens have good regional shear strain-rate controls and minimum subgrain boundary migration rates (during reduction in differential stresses and strain-rates) of 1.2 × 10 −9 μm s −1 to 1.2 × 10 −11 μm s −1 are estimated, for bulk shear strain rates of 10 −10 s −1 and 10 12 s −1 respectively. Palaeopiezometry coupled with rheological considerations suggests that these microstructural changes correspond to minimum differential stress-rates of 5 × 10 −9 MPa s −1 to 5 × 10 −11 MPa s −1 . Similar or slower regional stress-rates during fault zone evolution, will allow continuous re-equilibration of the microstructure. Thus the time at which any given microstructure is frozen in will be critically dependent upon fault zone deformation history. In the Alpine fault zone rapid uplift and high near surface temperatures combine to generate rapid stress drops as mylonitic deformation gives way to cataclastic faulting during uplift. Mylonite microstructure is frozen in during this rapid stress drop and represents the shallowest level of major crystal-plastic deformation.

Crustal scale ductile fault systems in the Arunta Inlier, central Australia

Detailed geologic observations along three 100 km long traverses across the Early Proterozoic Arunta Inlier, central Australia, indicate that Paleozoic ductile shear zones are organized into crustal scale structural systems which control the map pattern and distribution of metamorphic zones. Two provinces can be distinguished: 1. (1) a southern one where shear zones form a series of hinterland dipping and south verging, imbricate reverse faults in the west (Redbank area), grading eastward into a foreland dipping duplex in the Arltunga Nappe Complex. Directly north of this complex, flat lying Paleozoic south-directed, ductile shear affects a large portion of the crust, a point which has been underestimated so far. 2. (2) The northern province is characterized by both south and north dipping shear zones showing reverse motion to produce a crustal-scale “pop-up” structure, presumably resulting from a more coaxial deformation history. Strike-slip shear zones become predominant in the eastern part of this province. Stratigraphic and cross-cutting relationships indicate that the southern systems developed first; the northern systems probably developed in response to the lock-up stages of overthrusting in the southern province. One important result of this work is that the distribution of granulite facies rocks and major geophysical anomalies throughout the Arunta Inlier is best explained by their position relative to the Paleozoic ductile fault systems described above. There is no need to invoke extensive Proterozoic uplift of these lower crustal materials.

Crustal scale ductile fault systems in the Arunta Inlier, central Australia—Discussion

Discussion sur l'article «Crustal scale ductile fault systems in the Arunta Inlier, central Australia» Collins W.J. Teyssier C. Tectonophysics, 158 (1989) 46-99

Crustal ductile fault systems in the Arunta Inlier, central Australia—Reply

Crustal ductile fault systems in the Arunta Inlier, central Australia—Reply

Natural Hydrofracturing and Ductile Faulting in Austin Chalk, Texas: ABSTRACT

Natural Hydrofracturing and Ductile Faulting in Austin Chalk, Texas: ABSTRACT

Mylonitic deformation of evaporites in décollements: examples from the Southern Alps, France

The ductile deformation of Triassic evaporites is described in two décollements from the Southern Alps of France (the Arc de Nice and Arc de Digne nappes). We show that deformation is not chaotic as is commonly considered: on the contrary it is commonly well organized and is consistent with progressive shearing. Typical shear structures include mylonitic foliation, stretching lineations, folds of various shapes and orientations (“a” type folds and sheath folds), asymmetrical pressure shadows composed of gypsum fibres, shear bands and boudinage. At two sites, lineation directions and analysis of shear criteria allow the kinematics of nappe emplacement to be determined. Computer modelling of pressure shadows allows deformation mechanisms and strain measurements to be determined. The results of the analysis can be placed in the context of the regional framework of the two décollements. (1) Deformation at the Nice site corresponds to dextral wrenching on a lateral ramp of the décollement along the western limit of the Arc de Nice nappe. Deformation in the ductile fault zone is close to simple shear. (2) Deformation in the second site is associated with a ramp which affects the décollement in front of the Arc de Digne nappe. The deformation corresponds to shearing towards the southwest, including a pure shear component. This result can be explained by taking into account the effects of the ramp geometry upon the deformation in the décollement. This shows that deformation analysis of evaporites in décollements can be used to understand the mechanisms and kinematics of nappe emplacement.

Characteristics and Regional Significance of the Cremation Hill Ductile Fault Zone at the Bronson Hill-Merrimack Boundary, South-Central Connecticut

Characteristics and Regional Significance of the Cremation Hill Ductile Fault Zone at the Bronson Hill-Merrimack Boundary, South-Central Connecticut

Characteristics and regional significance of the Cremation Hill ductile fault zone at the Bronson Hill-Merrimack boundary, south-central Connecticut

Characteristics and regional significance of the Cremation Hill ductile fault zone at the Bronson Hill-Merrimack boundary, south-central Connecticut

Archean Tectonics of the North Snowy Block, Beartooth Mountains, Montana

The North Snowy Block (NSB), northwestern Beartooth Mountains, Montana, is composed of seven major lithologic units of Archean age: (1) Mount Cowen granitic augen gneiss; (2) a paragneiss unit that consists of a variety of quartzofeldspathic gneisses, psammitic schists, amphibolites, and iron formation; (3) an augen-textured granitic sill; (4) the phyllitic Davis Creek Schist; (5) a trondhjemitic gneiss-amphibolite complex; (6) the Pine Creek Nappe complex, a regional isoclinal structure with an amphibolite core and symmetrically disposed marble and quartzite; and (7) a supracrustal-migmatite complex that consists of a quartzite-amphibolite unit and a heterogeneous gneiss with lit-par-lit injections of granitic material into the supracrustal rocks. Each unit has its own distinct lithologic assemblage, metamorphic grade, and structural style. The contacts between most units are marked by mylonitic rocks (which are interpreted as the products of ductile faulting) and abrupt changes in metamorphic grade. In ...

Pine Mountain terrane, a complex window in the Georgia and Alabama Piedmont; evidence from the eastern termination

The Pine Mountain terrane is exposed in a complex window within the Piedmont of Georgia and Alabama. The eastern end of the terrane is framed by three ductile faults of demonstrably different ages. The polydeformed pre-thermal peak Box Ankle fault is truncated to the south by the younger pre-thermal peak Goat Rock fault, and to the north by the even younger post-thermal peak Towaliga fault. The three faults framing the eastern termination of the window are clearly neither (1) part of the same detachment nor (2) part of the Appalachian detachment.

A precautionary note on shear bands as kinematic indicators

The orientation of shear bands relative to foliations defined by elongated mineral aggregates, is often used to determine the large-scale sense of displacement in ductile faults. Data from contact strain zones at the bases of large overthrust complexes in the Eastern Alps and the Betic Cordilleras demonstrate that there is not always a simple geometrical relation between shear band orientation and sense of vorticity in bulk non-coaxial flow. In addition to single shear band sets that show displacements synthetic with the large-scale displacement, there are single sets with antithetic displacements, and conjugate sets. The last two observations are at variance with published data and interpretations, and cast doubts on the general applicability of shear bands as indicators of large scale flow kinematics.