Shear Zone Movement Research Articles

Multi-scale analysis of Proterozoic shear zones: An integrated structural and geophysical study

Structural mapping of poorly exposed shear zone outcrops is integrated with the analysis of aeromagnetic and Bouguer gravity data to develop a multi-scale kinematic and relative overprinting chronology for the Palaeoproterozoic Tallacootra Shear Zone, Australia. D 2 mylonitic fabrics at outcrop record Kimban-aged (ca. 1730–1690 Ma) N–S shortening and correlate with SZ 1 movements. Overprinting D 3 sinistral shear zones record the partitioning of near-ideal simple shear and initiated Riedel to regional-scale SZ 2 strike-slip on the Tallacootra Shear Zone (SZ 2). Previously undocumented NE–SW extension led to the emplacement of aplite dykes into the shear zone and can be correlated to the (ca. 1595–1575 Ma) Hiltaba magmatic event. D 4 dextral transpression during the (ca. 1470–1450 Ma) Coorabie Orogeny reactivated the Tallacootra Shear Zone (SZ 2–R4) exhuming lower crust of the northwestern Fowler Domain within a positive flower structure. This latest shear zone movement is related to a system of west-dipping shear zones that penetrate the crust and sole into a lithospheric detachment indicating wholesale crustal shortening. These methods demonstrate the value of integrating multi-scale structural analyses for the study of shear zones with limited exposure.

Timing and kinematics of crustal movement in the Northern Superior superterrane: Insights from the Gull Rapids area of the Split Lake Block, Manitoba

The Northern Superior superterrane in the northwestern Superior Province is characterized by an ancestry back to ca. 3.8 Ga and contains one of the largest and best-preserved high-grade terranes on Earth, the Pikwitonei Granulite Domain and its northeast extension, the Split Lake Block. This high-grade terrane occurs in the Superior Boundary zone, a complex transitional zone that bounds the northwestern margin of the Archean Superior Province and separates it from the Paleoproterozoic crust of the Trans-Hudson Orogen. Detailed structural and geochronological data from the exceptionally well-exposed Gull Rapids area at the eastern margin of the Split Lake Block shows evidence for both Neoarchean (Kenoran) and Paleoproterozoic (Trans-Hudson) deformation and metamorphism. More importantly, it indicates that the uplift of the Split Lake Block was achieved by shear zone movement in the Neoarchean, not in the Palaeoproterozoic, synchronous with peak metamorphism and dyke emplacement at ca. 2686 Ma. This indicates that uplift of the high-grade domains in the Northern Superior superterrane was neither related to its collision with the North Caribou superterrane at ca. 2.72 Ga, nor to the ca. 1.8 Ga Trans-Hudson orogeny. Instead, this uplift might have occurred during a major period of Superior-wide active tectonism toward the end of the Neoarchean Kenoran orogenesis.

Dating movement in shear zones: The example of the South Cyclades Shear Zone, Ios, Aegean Sea, Greece

The South Cyclades Shear Zone (SCSZ) is a ~1 km thick bowed-up ductile shear zone that defines the carapace of a deeply eroded gneiss dome that outcrops on the island of Ios in the Cycladic archipelago, Aegean Sea, Greece. The discovery of the SCSZ played a pivotal role in the recognition of the Aegean metamorphic core complexes, and this supposedly Miocene shear zone has been studied in detail by several authors. This contribution illustrates the difficulties involved in systematically determining the age of deformation fabrics using 40Ar/39Ar geochronology, and provides a case study that shows the value of carefully designed step-heating experiments on microstructurally controlled samples. In samples such as these, from complex metamorphic tectonites, laser fusion, or step heating experiments with relatively few steps, would have led to erroneous conclusions.

The Trawenagh Bay Granite and a new model for the emplacement of the Donegal Batholith

ABSTRACTThe Trawenagh Bay Granite (TBG) is shown to be a tabular pluton with gently inclined contacts that, from anisotropy of magnetic susceptibility (AMS) studies, was emplaced as a series of flow lobes whose geometries indicate that it flowed horizontally towards the W out of late stage adjacent steeply inclined monzogranite sheets of the Main Donegal Granite (MDG). We thus confirm in detail the central broad idea of the Pitcher & Read (1959) model that the Main Donegal Granite fed the Trawenagh Bay Granite. Early TBG flow lobes cut and are cut by deformation associated with the sinistral shear zone in which the MDG lies, thus demonstrating synchronicity of shearing and magmatism. The TBG magma leaked out of the shear zone and emplaced into undeformed country rocks and was probably guided by shear zone splays that die out along its northern and southern margins. At a late stage in the development of MDG, the splays developed from the NNE-trending SW boundary of the shear zone and caused a gap in this structure through which TBG magma was channelled out of the MDG. A review is presented of the last twenty-five years of published and unpublished work on the batholith, showing that the MDG shear zone was a long-lived structure almost certainly in existence before the emplacement of that body, and that four of the contiguous granitiods (Thorr, Ardara, and Rosses, as well as Trawenagh Bay) were all sourced within the shear zone. A new model is presented for the development of the batholith. The pre-existing crustal structure was a deep-seated N12°E fault in the basement to the Dalradian wall rocks of the granites, that was coupled to up to six other more minor WNW–ESE basement faults in the W. A NE–SW-trending sinistral shear zone was initiated at the end of the Caledonian orogeny, as calc-alkaline and deep-seated appinites were generated in the area. This shearing activated the pre-existing structures at the current crustal level, and the N12°E structure acted as a continental transform fault which allowed the dilation needed to facilitate the wedging space requirements of the MDG and the other units in the shear zone, as well as transferring regional sinistral shear through the system. The Thorr and Ardara plutons were emplaced first into the shear zone and then those magmas leaked out into the adjacent wall rocks: one to form a large laccolith, the other to form a balloon. Steep early MDG complex sheets (granodiorites and tonalities) were emplaced in the shear zone between the Thorr and Ardara emplacement sites. Dilation continued until late stage extensive monzogranite sheets were intruded in the NW and SE of the pluton. One of these probably leaked material westward to form the Rosses laccolith and southwestwards to form the TBG in the final stages of shear zone movement.

40Ar/ 39Ar age pattern associated with differential uplift along the Eastern Highlands shear zone, Cape Breton Island, Canadian Appalachians

The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende 40Ar/ 39Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The 40Ar/ 39Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The 40Ar/ 39Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones.

Timing and thermal influence of late orogenic extension in the lower crust: a UPb geochronological study from the southwest Grenville orogen, Canada

In the southwestern Grenville Province, the Central Gneiss Belt consists of a belt of parautochthonous rocks in the north and a collage of allochthonous lithotectonic domains in the south. Near Pointe-au-Baril, Ontario, the allochthon-parautochthon boundary is marked by the Shawanaga shear zone, a northwest-directed thrust zone between the Britt and Shawanaga domains that was reactivated during ductile, top-to-the-southeast extensional shearing. UPb ages of 1042+4/−2, 1019±4 and 988±2 Ma for granitic pegmatite dykes that are pre-kinematic, late syn-kinematic, and post-kinematic with respect to top-side-down displacement constrain major extensional transport on the Shawanaga shear zone to ca. 1020 Ma. However, nearly all the zircons in the dykes are inherited from a pre-Grenvillian, ca. 1460 Ma granitic host rock to the dykes. Recognition of the inherited nature of these grains comes from the multi-dyke dating approach employed here, and illustrates a potential pitfall in dating shear zone movement using only single dykes interpreted as syn-kinematic. UPb ages of metamorphic titanite from 14 samples collected in a transect across the Shawanaga shear zone span a 93 m.y. segment of concordia. These ages do not correlate with titanite grain size or vary systematically from domain to domain. However, when sample microtexture, strain state, structural position, titanite morphology, and pegmatite UPb data are considered together, the titanite ages can be reasonably inferred to date: 1. (1) regional metamorphism (1049-1045 Ma); 2. (2) cooling below the titanite isotopic closure temperature (∼600°C) during extensional unroofing (1028-1018 Ma); 3. (3) recrystallization-controlled titanite growth and (or) isotopic resetting in high-strain zones (1008-1000 Ma); 4. (4) post-kinematic recrystallization, most likely in the presence of a late fluid phase (967-956 Ma). Each titanite age group is made up of samples from both the Britt and Shawanaga domains, indicating that these processes were not restricted to a single domain but rather occurred within localized regimes, some which were clearly structurally controlled. The 93 m.y. spread of concordant titanite ages within a 20 × 20 km area demonstrates that cooling through isotopic closure is only one of several possibilities to be considered when interpreting metamorphic titanite ages in high-grade orogenic terranes.

Lateral extrusion in transpression zones: the importance of boundary conditions

The homogeneous transpression strain model formulated by Sanderson and Marchini ( Journal of Structural Geology 6, 449–458, 1984) has proved to be a useful tool in the analysis of complex three-dimensional deformation patterns. However, some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive. In this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically, but also horizontally in and out of the deforming zone parallel to its length—‘lateral extrusion’. Three end-member plane-strain components are defined: wrench simple shear; pure shear in XY (lateral stretch); and pure shear in YZ (vertical stretch). These biaxial strains can be viewed as the apices of a ‘strain triangle’ for unconfined transpression or transtension. The edges of the triangle correspond to: triaxial pure shear; non-coaxial, biaxial lateral extrusion; and the triaxial confined transpressional or transtensional strain of Sanderson and Marchini. During unconfined transpression, the orientation and, in particular, the geometry ( k-value) of the finite-strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike-slip parallel to the zone, but also upon the ratio of vertical to lateral stretch. This can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone-boundary displacements. Examples of transpression zones in which there is evidence of a component of lateral extrusion are described from SW Cyprus and central Scotland. These examples illustrate that antithetic strike-slip shearing is a kinematic requirement of laterally unconfined transpression, implying that synchronous shear-sense indicators may give opposite senses of movement in shear zones. Specific geometric and mechanical boundary conditions, together with internal fault-zone rheologies, may favour the lateral extrusion of material. Kinematic partitioning can occur to form fault-bounded domains in which end-member biaxial and/or non-coaxial strains are developed. Analysis of such domains can give a clearer understanding of regional-scale triaxial deformations. These findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones.

Geometry and kinematics of shear zones formed during continental extension in eastern Papua New Guinea

The D'Entrecasteaux Islands, eastern Papua New Guinea, lie in an area of continental extension that has been active since the mid-Miocene. During the last 4 Ma the metamorphic basement rocks composing most of the islands have been uplifted and tectonically exhumed from depths of approximately 35 km. Tectonic exhumation appears to have been controlled by deformation in broad (kilometre-scale) mylonitic shear zones. A progressive evolution is documented in the shear zones, characterized by: increasing localized deformation; a change from dominantly ductile to dominantly brittle processes; and decreasing metamorphic grade (i.e. retrograde metamorphism). The evolution is believed to be the result of uplift and cooling of the basement accompanying shear zone movement. The kinematic history of the shear zones is complex. A number of sinistrally offset extensional detachment zones separate a multiply deformed, high-grade metamorphic basement from a cover of largely undeformed ultramafic and mafic cover rocks. The detachment zones are connected by transverse shear zones. These transverse zones appear to act as sinistral strike-slip zones which also accommodate transfer motion between the detachment zones.

Criteria for identifying structures related to true crustal extension in orogens

Faults and shear zones in orogens may be related to crustal shortening or extension or both. True extensional structures should ultimately intersect the Earth's surface when traced out in a direction opposite to that of hangingwall transport; the present dip of a fault in an orogen, and its relation to local layering, are unreliable guides. The observation of low-pressure metamorphic rocks structurally above high-pressure metamorphic rocks is not sufficient to diagnose crustal extension. A valid metamorphic criterion is the difference in pressure-time histories between footwall and hangingwall: if the footwall was decompressing faster than the hangingwall during shear zone movement, then the shear zone was truly extending the crust. The pressure-time history can be constrained via the temperature-time and pressure-temperature histories. Ideally, crustal extension inferred from structural data should be confirmed using diagnostic PTt information.

Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes

Caledonian eclogite facies shear zones developed from Grenvillian garnet granulite facies anorthosites and gabbros in the Bergen Arcs of western Norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation (CPO) in lower continental crust. Field relations on the island of Holsnøy show that the eclogites formed locally from granulite facies rocks by progressive development of: (1) eclogite adjacent to fractures; (2) eclogite in discrete shear zones (> 2 m thick); (3) eclogite breccia consisting of >80% well‐foliated eclogite that wraps around rotated granulite blocks; and (4) anastomosing, subparallel, eclogite facies shear zones 30–100 m thick continuous over distances > 1 km within the granulite terrane. These shear zones deformed under eclogite facies conditions at an estimated temperature of 670 ± 50°C and a minimum pressure of 1460 MPa, which corresponds to depths of >55 km in the continental crust. Detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite/garnet‐rich and kyanite/zoisite‐rich layers. A consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite. The CPO of omphacite, determined from five‐axis universal stage measurements, shows a strong b‐axis maximum normal to foliation, and a c‐axis girdle within the foliation plane with weak maxima parallel to the lineation direction. These patterns are consistent with deformation of omphacite by slip parallel to [001] and suggest glide along (010). The lineation and CPO data reveal a consistent sense of shear zone movement, although the displacement was small. Localized faulting of high‐grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust. Field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth. Deformation of granulite and partial conversion to eclogite, such as is exposed on Holsnøy Island, may be an orogenic‐scale process in the lowermost crust of collisional orogens.

Melange- and sediment-hosted gold-bearing quartz veins, Hodgkinson gold field, Queensland, Australia

Gold-bearing quartz veins in the Hodgkinson gold field are hosted in subgreenschist facies Siluro-Devonian sandstone, conglomerate, carbonaceous shale, clast-in-matrix rock, chert, and spilite. Mineralization occurs in brittle to brittle-ductile shear zones which crosscut two previous deformations and melange. Vein systems are isolated in distinct structural and lithologic domains by postmineralization faulting. Scheelite-quartz veins may locally overprint the gold quartz mineralization, but stibnite-gold-quartz mineralization is restricted to separate domains. The strongest mineralized areas define elongate, elliptical ore shoots which plunge steeply down or across the dip of the host shear zones. Melange-hosted lodes are parallel in strike but have opposite dips to the melange cleavage. Sandstone-hosted shear zones lie within reactivated F 2 axial planes or are refracted through bedding planes. Splaying, fissure intersections, changes in attitude, and lithologic contacts are the main ore shoot controls.The gold quartz vein textural types are ribbon, buck, breccia, assimilation textured, and comb quartz. Multiple injection of quartz and multiple shear zone movement have produced complex zones of quartz separated by seams of gouge. Arsenopyrite, galena, sphalerite, pyrite, pyrrhotite, and gold (600-840 fine) constitute 5 percent of the ore shoots and occur as millimeter-scale clusters in quartz. Fluid inclusions in quartz are liquid and liquid-vapor types and give salinity ranges of 3 to 11 equiv wt percent NaCl, with a slight salinity decrease through time. CO 2 and other gases are not abundant. Homogenization temperatures of 170 degrees to 305 degrees C have been pressure corrected to 270 degrees to 355 degrees C, based on estimated formation pressure of about 1 kbar.Measured and calculated stable isotope compositions of the fluid depositing gold quartz are delta 18 O = 10 + or - 2 per mil, delta D = -100 + or - 20 per mil. Although meteoric water contributions cannot be discounted, geologic characteristics, isotopic data, quartz types, sulfide mineralogy, and ore controls favor gold deposition from an upwardly migrating, homogeneous, metamorphic, or distal magmatic fluid. Depletion of D is attributed to exchange reactions involving organic matter in the metasediments and reduced gas species rather than introduction of meteoric water. Gold mineralization accompanied brittle tectonism which was contemporaneous with regional plutonism and took place late in the evolution of the melange.

Vein geometry in the Hilton area, Mount Isa, Queensland: implications for fluid behaviour during deformation

Folding and shear zone movement occurred synchronously in rocks of the Mid-Proterozoic Mount Isa Group in the Hilton area, Northwest Queensland. Syndeformational veins developed in shear zones and in sets spatially associated with shear zone intersections. Veins can be divided into the following two types: 1. (1)Fibrous and non-fibrous quartz-dolomite veins which formed in orientations consistent with the regional E-W subhorizontal shortening direction. 2. (2)Fibrous and non-fibrous quartz-dolomite veins which formed parallel to shear planes, S 3 cleavage, and bedding, indicating local syndeformational extension parallel to the regional shortening direction. The complex geometry of the shear zones suggests that shear parallel veins formed as a result of movement over asperities during shear zone movement. S 2- parallel veins, on the other hand, appear to have formed during restricted episodes of fluid overpressuring during post-failure stress drops. Shear zones are divisible into highly graphitic and weakly graphitic end member types. Highly graphitic shear zones show abundant dissolution and little veining, while weakly graphitic shear zones show abundant veining and little dissolution. This indicates that highly graphitic shear zones were syndeformational fluid pathways, while weakly graphitic shear zones were syndeformational fluid sinks.

The determination of true shear senses from the deflection of passive markers in shear zones

The sense of movement in shear zones can often be determined from the reorientation of pre-existing planar or linear features within the zone. This cannot always be done simply by examining the change in strike or trend of these features: sometimes these will show the opposite sense of rotation to the true sense of shear in the zone. This is illustrated for a generalized oblique-slip zone. Using the deflection method' described here, the true movement sense can be deduced from the apparent rotation, the emphasis being on a three-dimensional approach. Consideration of the movement on a large-scale shear zone (the Laxford Front) underlines the importance of being aware of this aspect of shear zone geometry. The method may be applied to brittle fault offsets as well as to deflections in ductile shear zones.