Kinematics Of Shear Zones Research Articles

Superposed transpressional tectonics of an intraterrane shear zone resulted in Shillong plateau evolution, NE India

The analysis of shear zone kinematics plays a crucial role in understanding the tectonics at micro-, meso-, and regional scales within a given terrane. This study focuses on investigating the kinematics of prominent asymmetric structural features preserved within the Barapani Shear Zone (BSZ). These features are utilized to establish vorticity numbers in relation to the reference frames of the BSZ. Typically, shear zones provide finite values for kinematics and vorticity numbers. The BSZ is identified as an intraterrane, multiply deformed shear zone within the Shillong Plateau, which exhibits varying kinematics and vorticity numbers measured across different deformation zones.The research reveals that the BSZ underwent successive D1, D2, D3, and D4 events, resulting in preserved S1, S2, and S3 planar fabrics, with D4 evidenced by brittle deformation features. However, determining shear zone kinematics purely based on the orientation of these planar fabrics and mineral lineation with respect to the external reference frame is insufficient. Consequently, this study calculates vorticity numbers and kinematics by considering both the external and internal reference frames of the BSZ. The study suggests that superposed transpressional tectonics prevailed during D2 and D3 events.

Numerical Simulations Reproduce Field Observations Showing Transient Weakening During Shear Zone Formation by Diffusional Hydrogen Influx and H2O Inflow

AbstractExposures on Holsnøy island (Bergen Arcs, Norway) indicate fluid infiltration through fractures into a dry, metastable granulite, which triggered a kinetically delayed eclogitization, a transient weakening during fluid‐rock interaction, and formation of shear zones that widened during shearing. It remains unclear whether the effects of grain boundary‐assisted aqueous fluid inflow on the duration of granulite hydration were influenced by a diffusional hydrogen influx accompanying the fluid inflow. To better estimate the fluid infiltration efficiencies and the parameter interdependencies, a 1D numerical model of a viscous shear zone is utilized and validated using measured mineral phase abundance distributions and H2O‐contents in nominally anhydrous minerals of the original granulite assemblage to constrain the hydration by aqueous fluid inflow and diffusional hydrogen influx, respectively. Both hydrations are described with a diffusion equation and affect the effective viscosity. Shear zone kinematics are constrained by the observed shear strain and thickness. The model fits the phase abundance and H2O‐content profiles if the effective hydrogen diffusivity is approximately one order of magnitude higher than the diffusivity for aqueous fluid inflow. The observed shear zone thickness is reproduced if the viscosity ratio between dry granulite and deforming, reequilibrating eclogite is ∼104 and that between dry granulite and hydrated granulite is ∼102. The results suggest shear velocities <10−2 cm/a, hydrogen diffusivities of ∼10−13±1 m2/s, and a shearing duration of <10 years. This study successfully links and validates field data to a shear zone model and highlights the importance of hydrogen diffusion for shear zone widening and eclogitization.

Evolution of stretching lineations in granulite-hosted ductile shear zones, Eastern Ghats Province, India: Role of temperature, strain rate and pre-existing stretching lineations

Shear zone kinematics are interpreted from sections (X-Z) parallel to the stretching lineation (X-direction) and perpendicular to the mylonitic foliation (X–Y). The stretching lineation is a product of finite strain, defined by minerals or mineral aggregates that deform plastically. In quartzofeldspathic rocks deformed below 600 °C, rheological contrasts between quartz and feldspar are amplified, and the mesoscopic stretching lineation in the shears may be indistinct in outcrop as quartz ribbons anastomose around relatively competent feldspar clasts. In high strain-rate zones, rheological contrasts between these minerals are reduced and a stretching lineation suitable for kinematic interpretation develops. In late amphibolite and greenschist facies shear zones transecting pre-lineated quartzofeldspathic granulites of the Eastern Ghats Province, India, the stretching lineation represents the resultant of pre-existing stretch and superposed increments of later shearing. Until the stretch accumulated in the X-direction of the later deformation exceeds the earlier finite stretch, the pre-existing stretching lineation progressively rotates towards the later X-direction with decreasing aspect ratio. These intermediate lineation orientations, have no kinematic relevance to either earlier or later deformations. This example highlights the importance of temperature, strain rate and orientation of pre-existing stretching lineations in the appearance and kinematic interpretation of,stretching lineations in ductile shear zones.

New Insight into the Genetic Mechanism of Shear Zone Type Gold Deposits from Muping-Rushan Metallogenic Belt (Jiaodong Peninsula of Eastern China)

Most gold deposits are genetically controlled by shear zones, which are called shear zone type gold deposits (SZTGD). A better understanding of kinematics of shear zones and its constraint on the ore-forming process is critical to reveal the genetic mechanism of the SZTGD and favorable to mineral exploration. By conducting detailed structural analysis including field and microscopic observations and electron backscatter diffraction (EBSD) and fractal dimension analysis in the Muping-Rushan shear zone (MR) as well as several gold deposits, the kinematic characteristics of the MR are well recognized and the metallogenic process of the SZTGD are discussed. The main conclusions are as follows: (1) petrology, geometry, kinematics, macro- and micro-structures imply that the MR has experienced a progressive shearing history exhumed via middle crust to subsurface level under the NW-SE extensional regime from late Jurassic to early Cretaceous; (2) in the MR, gold may precipitate both in the brittle fractures at middle crust level and brittle deformation part at shallow crust level during the stress-chemical process and (3) comparison of gold deposits between the MR and other areas show that the SZTGD has a uniform metallogenic mechanism, which is from (multi-stage) pluton emplacement, hydrothermal fluid action, shearing action, brittle fracturing, sudden reduction of fluid pressure, flash vaporization to (gold) mineralization.

Estimation of deformation temperatures, flow stresses and strain rates from an intra-continental shear zone: The Main Boundary Thrust, NW Himalaya (Uttarakhand, India)

This work generates thermal and tectonic data from the mylonitized rocks of the Main Boundary Thrust (MBT). South and north to this major thrust, the Siwalik range and the Lesser Himalaya, respectively have been receiving a renewed interest amongst the petroleum geoscientists. Understanding temperature, flow stress and strain rates related to a major thrust is crucial in thermal-mechanical modelling of the terrains adjacent to it. Strongly sheared Proterozoic Chandpur phyllites and abundant syntectonic quartz veins occur along the MBT zone (one of the major foreland verging thrusts in the Himalayan orogen) near the Sahenshahi temple at Dehradun, Uttarakhand state, India. Microstructure analyses show intense ductile top-to-S shear in terms of S-C fabric and XRD analyses indicate presence of clays e.g., illite and clinochlore. These clays not only bear the signatures of the intense fluid activity, but also enhance the fault movement by reducing the mechanical strength of the fault zone rocks. Thermometry based on qualitative quartz grain boundary mobility (~300–550 °C) and quantitative Laser Raman Spectroscopy for carbonaceous materials (340–370 °C) reveal the maximum estimates of the deformation temperature and metamorphic temperature, respectively. The higher temperature microstructures can be found in lower temperature domains under the presence of fluids. Hence the quartz thermometry estimates a broader temperature range than the Raman thermometry. Previous works report that, the phyllites present at the base of a thrust sheet, experiences deformation at < 500 °C. The thermometric estimates in this study indicate a higher temperature at the base than that at further north known from previous studies, possibly by shear heating. A flow stress of ~6–49 MPa has been calculated from recrystallised quartz grain piezometry. This broad range of flow stress value indicates temporally variable tectonic stress responsible in the evolution of the MBT. Using these magnitudes, ~10 −15 -10 −16 s −1 of strain rate has been estimated. Such a range can also indicate seismic cycles in the tectonically active Himalaya. • Deformation temperatures, flow stresses and strain rates calculated from a shear zone. • The deduced data will be useful in shear zone kinematics. • The deduced data will help in modeling collision tectonics.

The Causative Fault of the 2016 Mwp 6.1 Petermann Ranges Intraplate Earthquake (Central Australia) Retrieved by C- and L-Band InSAR Data

On 21 May 2016, an Mwp 6.1 earthquake occurred along the Petermann Ranges in Central Australia. Such a seismic event can be classified as a rare intraplate earthquake because the affected area presents low seismicity, being at the center of the Indo-Australian plate. Also, the architecture and kinematics of shear zones in the Petermann Orogen are largely unknown. We used Sentinel-1 C-band descending data and ALOS-2 L-band ascending data to constrain the causative fault. Our analysis revealed that the earthquake nucleated along an unmapped secondary back-thrust of the main feature of the area, namely the Woodroffe thrust.

Are we studying deformed rocks in the right sections? Best practices in the kinematic analysis of 3D deformation zones

A standard procedure in kinematic analyses of deformed rocks is studying samples only on the foliation perpendicular/lineation parallel section based on the assumption that the rotational component of deformation is restricted to that plane. The theoretical background for this procedure is that deformation is plane strain, which is often useful but fails to account for all relevant features observed in many natural cases. More general, 3D kinematic models reproduce several of these features and help to better understand 3D deformation, likely predominant in nature, so they are a more appropriate starting point for deformation zone analysis. Theoretical and natural 3D deformation examples suggest that the section with highest degree of asymmetry (the vorticity normal section, VNS) and the corresponding vorticity vector are independent from the finite strain fabric, and thus may present varied orientations with respect to the tectonic foliation and lineation. The VNS orientation depends on the kinematics resulting from boundary conditions whereas the strain fabric orientation also depends on accumulated strain. The orientation of the VNS is itself an essential element of shear zone kinematics, hence it cannot be pre-assumed but must be determined independently.

Kinematics of the Tengchong Terrane in SE Tibet from the late Eocene to early Miocene: Insights from coeval mid-crustal detachments and strike-slip shear zones

It is generally believed that the extrusion of SE Tibet was bounded by the dextral Gaoligong and the sinistral Ailaoshan-Red River strike-slip shear zones from the Oligocene to early Miocene. This study integrates field mapping, structural analysis and geochronology in western Yunnan (China), where foliated Precambrian basement rocks and late Cretaceous to early Eocene plutons are exposed to the west of the Gaoligong shear zone. We found that late Eocene to early Miocene flat-lying ductile shear zones were kinematically related to steeply dipping strike-slip shear zones. Four elongated gneiss domes (Donghe, Guyong, Yingjiang and Sudian) are cored by high-grade metamorphic rocks and pre-kinematic granite plutons, and bounded by top-to-NE detachments and NE-trending dextral strike-slip shear zones. Zircon U-Pb ages from LA-ICP-MS analysis and 40Ar/39Ar ages of micas and hornblende demonstrate that the flat-lying Donghe Detachment (>35–15Ma) and the Nabang dextral strike-slip shear zone (41–19Ma) were sites of prolonged, mostly coeval ductile deformation from amphibolite to greenschist facies metamorphism. The Gaoligong shear zone experienced dextral shearing under similar metamorphic conditions between 32 and 10Ma. Consistent 40Ar/39Ar ages of hornblende from the three shear zones indicate their contemporaneity at mid-crustal depth, causing the rapid exhumation and SW-ward extrusion of the Tengchong Terrane. The strain geometry and shear zone kinematics in the Tengchong Terrane are interpreted with folding of the anisotropic lithosphere around a vertical axis, i.e., the northeast corner of the Indian Plate since 41Ma. The newly discovered NE-trending Sudian, Yingjiang, and Lianghe strike-slip shear zones are subordinate ductile faults accommodating the initially rapid clockwise rotation of the Tengchong Terrane. The detachments caused mid-crustal decoupling and faster SW-ward extrusion below the sedimentary cover, whereas the strike-slip shear zones accommodated extrusion and clockwise rotation of the Tengchong Terrane around the proto-Eastern Himalayan syntaxis since the late Eocene.

Empirical paths of poles to planes (eppps) constrain the kinematics of geological shear zones

Ductile shear zones are tabular bodies of deformed rocks bound by less deformed wall rocks. This work introduces a simple empirical approach to analysing the 3D kinematics of shear zones. The orientations of pre-shear planar markers distorted across natural shear zones by local strains are systematically measured and plotted as poles on lower hemisphere equal area projections that constrain smooth empiricalpaths ofpoles toplanes (eppps). Such eppps recording local strain gradients are used to fix a reference frame to the plane of greatest shear in any homogeneous bulk strain. Assuming that space can be taken as a proxy for time, the curvatures of pre-shear planar markers across shear zones are interpreted as the records of the 3D bulk strain histories of shear zones. The sig- or zig-moidal symmetries of sheared markers record different amounts of the same general strain within the same overall movement pattern (i.e. in a constant flow field) whatever its geometry or history. In effect eppps represent the strain memories of shear zones with successively inward readings recording successively younger shearing. In planes other than the bulk XY, great circle eppps indicate simple shear while hyperbolic eppps indicate pure shear. Eppps for suites of shear zones in Proterozoic gneisses in Sweden exhibit the parabolic shapes indicative of pure rather than simple shear.

Paleoarchean partial convective overturn in the Singhbhum Craton, Eastern India

Analyses of fold superposition structures and kinematics of shear zones along a 120km long E–W corridor across the Singhbhum Craton (Eastern India) are combined with U–Pb–Th chemical ages of texturally constrained monazites in supracrustals, gneisses and granitoids to reconstruct the Paleoarchean tectonic history of the western part of the Craton. Metamorphic sub-greenschist facies shallow marine to platformal sediments of the Iron Ore Group (IOG) and the accordantly deformed Jagnnathpur meta-lavas were accreted onto a basement composed of >3.4Ga amphibolite facies para-amphibolites/quartzites/muscovite schists (Older Metamorphic Group, OMG) and anatectic Older Metamorphic Tonalite Gneisses (OMTG). Crustal shortening was followed by emplacement-ascent of the Singhbhum granitoid pluton, SGP (3.2–3.3Ga) synchronous with sinking of supracrustal screens along N/NNE-trending steep-dipping sinistral–normal ductile shear zones. Deformation microstructures indicate the bulk of the pluton responded to exhumation-strain by low-T (≤500°C) ductile flow of quartz and K-feldspar, with plagioclase being largely unaffected. But granitoids along the western margin of pluton emplaced concurrent with westward-retreating exhumation of the thickened crust was deformed at T≥650°C. Since all granitoids in the pluton share the same N/NNE-trending tectonic fabric, microstructural features imply phased emplacement of the pluton during exhumation strain. The transition from crustal shortening to exhumation interrupted by phased granitoid emplacement between 3.2 and 3.4Ga is explained by a two-stage partial convective overturn model involving structures typical of supracrustal-down, granitoid-up sense of movement. The tectonic restoration in the western part of the Singhbhum Craton is new and complements growing evidence that accretion and convective overturn may have operated during the Paleoarchean.

Himalayan-type escape tectonics along the Superior Boundary Zone in Manitoba, Canada

The Superior Boundary Zone separates the Neoarchean Superior Province to the southeast from the Paleoproterozoic Reindeer Zone of the Trans-Hudson Orogen to the northwest. In northern Manitoba, late convergence along the zone in the Paleoproterozoic has previously been interpreted in terms of a promontory model, where the Superior Province indents the Reindeer Zone towards the northwest. We present a modified promontory model that includes a WNW transport direction of the Superior Province and lateral escape of a segment of the Superior Province (the Split Lake Block) along the northern margin of the promontory. The model is based on the kinematics of an array of shear zones along the Superior Boundary Zone that are interpreted as having been active concurrently. Tectonic escape of the Split Lake Block occurred in a manner similar to the extrusion and rotation of the Indochina block along the eastern margin of the Himalayas, but at a much smaller scale. Our model explains shear zone kinematics, but also the curvilinear nature of some of the shear zones and a high component of along-strike stretch along one of the shear zones.

Deformation partitioning during transpressional emplacement of a ‘mantle extrusion wedge': the Ronda peridotites, western Betic Cordillera, Spain

Abstract: Composite tectonic emplacement of the world's largest outcropping body of subcontinental mantle rocks, the Ronda peridotites, occurred during Early Miocene continental subduction and as a result of oblique plate convergence. Partitioning of transpressional deformation between coeval orogen-parallel wrenching and orogen-perpendicular, pure thrusting components is recorded by shear zone kinematics and dynamothermal metamorphism in the footwall to the ultramafic rocks. Left-lateral shear, characterizing the deeper, high-pressure (eclogitic) portions of the continental subduction system, propagated through the mantle into the overlying continental crust of the overriding plate, whereas top-to-the-foreland frontal thrusting dominated at the leading edge of the hot peridotite body. Top-to-the-hinterland shear along the upper contact of the peridotites is consistent with the kinematics expected for an extrusion wedge consisting of subcontinental mantle rocks. On the other hand, coeval strike-parallel extension and thinning of the crustal rocks overlying the peridotites confirms that, similarly to other Alpine–Mediterranean examples, partitioned transpression resulted in the development of a complex deformation pattern, with kinematically linked shear zones aiding exhumation. The proposed model provides a new, coherent scenario for syn-convergence peridotite emplacement and exhumation in the western Betic Cordillera.

Fabric Development in a Middle Devonian Intraoceanic Subduction Regime: The Careón Ophiolite (Northwest Spain)

A Middle Devonian suprasubduction zone ophiolite, the Careón Unit (northwest Spain), displays amphibolite‐facies ductile deformation fabrics related to the onset of the Rheic Ocean closure. Two different fabrics, an early high‐T foliation and a subsequent lower‐T foliation, each of which characterized by distinct deformation mechanisms, have been identified in two distinct crustal‐scale shear zones of the same ophiolitic thrust sheet. Combined quantitative texture analysis by electron backscattered diffraction and time‐of‐flight neutron diffraction, were carried out on the shear zones and correlated with micro‐ and macrostructural data. The results indicate that the regional lineation and shear zone kinematics (east‐west, top‐to‐the‐east) represent fabrics developed essentially during the intraoceanic subduction of the Rheic Ocean, and their orientation may be considered a reference vector for convergence models in this part of the Variscan belt.

P–T conditions and kinematics of shear zones from the southern Adelaide Fold‐Thrust Belt, South Australia: Insights into the dynamics of a deeply eroded orogenic wedge

Mechanisms are examined for the development of shear zones in the external portion of the southern Adelaide Fold‐Thrust Belt, based on geometric and kinematic analyses, balanced cross‐sections, microstructure, pressure, and temperature of deformation estimates. Kinematic indicators reveal the zones were overthrust towards the west‐northwest. Shear‐zone quartz‐grain microstructures show mainly intracrystalline deformation and dynamic recrystallisation where there is greater grain dislocation. These features become less well developed in the shear zones closer to the hinterland, than in those on the foreland side. Primary fluid inclusions in deformed quartz veins reveal that average temperatures were in excess of 358°C and indicate an average pressure of about 300 MPa. Microprobe analyses of coexisting biotite‐muscovite‐quartz‐feldspar reveal an average peak metamorphic pressure of 250 ± 100 MPa, compatible with the estimated pressure of 300 MPa from primary fluid inclusions. Modified fluid inclusions, however, suggest an average entrapment pressure of less than 150 MPa, which suggests that they suffered overpressuring during exhumation, implying that the amount of shear‐zone exhumation decreases towards the foreland. Based on this information, an orogenic wedge model is the preferred mechanism for shear‐zone development. The earliest generated and propagated shear zones in the hinterland were buried at greater depth and remained at higher temperatures and pressures before being exhumed. Later shear zones generated closer to the foreland propagated from shallower depths, implying that they experienced lower pressures and temperatures before uplift.

Dome emplacement and formation of kilometre-scale synclines in a granite–greenstone terrain (Quadrilátero Ferrı́fero, southeastern Brazil)

The Quadrilátero Ferrı́fero is a portion of the Brazilian Precambrian shield containing a granite–greenstone terrain characterized by granite–gneiss domes encircled by kilometre-scale synclines. Integration of porphyroblast-matrix microtextures, c-axis fabrics analysis and strain analysis with outcrop-scale structural relationships indicates that the main phase of domes and synclines evolution occurred during the westerly directed 0.8–0.6 Ga Pan-African-Brasiliano regional shortening. This tectonic event has produced the single pervasive foliation observed in the folded supracrustal sequences of the Quadrilátero Ferrı́fero. Dip-slip ductile shear zones developed at the dome–supracrustal interface are synchronous with dome evolution during the Pan-African-Brasiliano deformation, as indicated by contemporaneous growth of aureole porphyroblasts in the shear planes. These shear zones facilitated steeply directed downward flow of country rock material around the granite–gneiss domes. The marked density contrast between granite–gneiss material and the iron-rich supracrustals provided a gravitationally favourable environment for downward wallrock flow. Variations in shear zone kinematics away from the domes, combined with kilometer-scale viscous drag, contributed to produce kilometre-scale synclines in the supracrustal rocks constrained between upward moving domes.

Sigmoidal wall-rock fragments: application to the origin, geometry and kinematics of en échelon vein arrays

Wall-rock fragments found within en échelon vein arrays may provide valuable information on the origin of vein systems, shear zone kinematics, and vein geometry. These fragments are sigmoidal and comprise intact rock between adjacent veins or fragments enclosed within individual veins. We present field examples of sigmoidal rock fragments within en échelon veins that grew as opening-mode fractures in response to an applied simple shear. By virtue of their unique shape and alignment of pre-existing fabric, we demonstrate how small wall-rock fragments may be used as kinematic indicators as well as to predict geometries of individual veins and entire vein arrays. Such a tool may be especially useful in mining exploration where the precise boundaries of large veins are unknown, or in stockworks where the rock is composed primarily of vein material.

Numerical simulations of granular shear zones using the distinct element method: 1. Shear zone kinematics and the micromechanics of localization

Two‐dimensional numerical simulations were conducted using the distinct element method (DEM) to examine the influences of particle size distribution (PSD) and interparticle friction μp on the nature of deformation in granular fault gouge. Particle fracture was not allowed in this implementation but points in PSD space were examined by constructing assemblages of particles with self‐similar size distributions defined by the two‐dimensional power law exponent D. For these numerical “experiments,” D ranged from 0.81 to 2.60, where D=1.60 represents the two‐dimensional equivalent of a characteristic PSD to which cataclastically deforming gouge is thought to evolve. Experiments presented here used μp values of 0.10 and 0.50 and were conducted using normal stress σn on the shear zone walls of 70 MPa. Shear strain within these simulated assemblages was accommodated by intermittent displacement along discrete slip surfaces, alternating between broadly distributed deformation along multiple slip planes and highly localized deformation along a single, sharply defined, subhorizontal zone of slip. Slip planes corresponded in orientation and sense of shear to shear structures observed in natural gouge zones, specifically Riedel and Y shears; the oblique Riedel shears showed more extreme orientations than typical, but their geometries were consistent with those predicted for low‐strength Coulomb materials. The character of deformation in the shear zone varied with PSD due to changes in the relative importance of interparticle slip and rolling as deformation mechanisms. A high degree of frictional coupling between large rolling particles in low D (coarse‐grained) assemblages resulted in wide zones of slip and broadly distributed deformation. In higher D assemblages (D &gt;= 1.60), small rolling particles self‐organized into columns that separated large rolling particles, causing a reduction in frictional resistance within the deforming assemblage. This unusual particle configuration appears to depend on a critical abundance of small particles achieved at D ≈ 1.60 and may enable strain localization in both real and simulated granular assemblages.

Brasiliano crustal extension and emplacement fabrics of the mangerite-charnockite pluton of Umarizal, North-east Brazil

Shear zone kinematics and anisotropy of magnetic susceptibility (AMS) data have been used in a Brasiliano/Pan-African fayalite-clinopyroxene (±orthopyroxene) syenite of Umarizal (NE Brazil) in order to reveal its emplacement mechanisms. The pluton is located at the intersection of two major shear zones, a NE-trending transcurrent Portalegre shear zone (PSZ) and a NW- to E-trending Frutuoso Gomes shear zone (FGSZ). The FGSZ dips gently NE in its NW-trending branch and N in the E-trending branch. Asymmetric porphyroclasts and S C′ shear planes observed in mylonitic gneisses along the FGSZ point to a top to the East (extensional) movement. Hysteresis and low-field susceptibility measurements on mangerites and charnockites indicate that magnetic susceptibility is principally carried by multidomain magnetite. In the core of the pluton, the magnetic anisotropy is weaker ( P<1.05) and characterized by neutral ( L= F) to rod-shaped ( L> F) AMS ellipsoids. The magnetic anisotropy increases south-westward ( P>1.10) approaching the FGSZ and the AMS ellipsoid becomes dominantly disc shaped ( F> L). Magnetic lineations plunge gently to NW in the western part of the pluton and to ENE on the East. The absence of steep lineations, except around a large xenolith in the center of the pluton, exclude the presence of magmatic feeding zones at the actual level of exposure. Foliations are roughly parallel to the pluton contact, but most of them dip moderately towards North-east. In the SW border the magnetic foliation is parallel to the mylonitic foliation displayed in the FGSZ. Although emplaced within migmatized host rocks, the kinematics of the shear zones and the magmatic fabric pattern rule out a buoyancy-driven diapiric emplacement of the pluton. Syn-magmatic shear deformation associated with EW crustal stretching accounts for the fabrics of emplacement recorded in the Umarizal pluton.

Synkinematic(?) intrusion of the “anorogenic” 1425 Ma Beer Bottle Pass pluton, southern Nevada

The tectonic setting of a transcontinental belt of 1500–1300 Ma granitic intrusions that extends from southern California to Labrador is controversial; however, the granites are conventionally considered to be “anorogenic.” Detailed field, microstructural, and geochronologic data from the 1425 Ma Beer Bottle Pass pluton, southern Nevada, indicate that major mylonite zones recording dextral‐contractile strains were probably active during and/or shortly after pluton emplacement and suggest that the anorogenic interpretation for this pluton requires reevaluation. Mylonite zones up to 100 m thick strike northeast, dip moderately northwest, and contain a consistent west plunging elongation lineation. Mylonites occur along 15% of the exposed granite‐wall rock contact and extend into both the pluton and the wall rock. Mesoscopic and microscopic kinematic indicators record an oblique, dextral/reverse (pluton side down) movement sense. Synkinematic mineral assemblages of hornblende and biotite and dynamic recrystallization of feldspars suggest that deformation occurred minimally under amphibolite facies conditions. A K/Ar biotite date of 1399±32 Ma, obtained from a sample of mylonitic granite, suggests that deformation took place during or soon following pluton crystallization. We reject forcible emplacement of the Beer Bottle Pass pluton as a mechanism for formation of the mylonite zones because (1) rocks near the granite‐wall rock contact are largely unstrained, (2) the mylonite zones conform only locally to the pluton‐wall rock contact, (3) mylonite zones strike at high angles to, and truncate, the intrusive pluton‐wall rock contact, (4) the pluton‐side‐down shear sense is more compatible with a uniform sense simple shear zone than a forcibly intruding pluton, and (5) fabrics indicative of noncoaxial deformation dominate over flattening fabrics. We suggest that the Beer Bottle Pass pluton is fundamentally synkinematic with respect to either (1) a local, contractile deformational event or (2) regional strains produced by distant plate tectonic processes operative between 1500 and 1300 Ma as suggested by Nyman and Karlstrom (1994). The timing and kinematics of shear zones associated with other circa 1400 Ma plutons in the western United States (e.g., Graubard and Mattinson, 1990; Nyman et al., 1994; Kirby et al., this issue) support the latter interpretation. Our kinematic data are more consistent with a contractile or transpressive tectonic setting at circa 1400 Ma (Nyman and Karlstrom, 1994; Nyman et al, 1994) than with models involving regional extension (e.g., Windley, 1993).

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.