Axis Fabrics Research Articles

Tectonics of the Papua‐Woodlark Region

AbstractThe Papua‐Woodlark region exemplifies many plate tectonic processes, from active continental breakup to double plunging subduction zones. We present a synthesis of geological and geophysical data from the region and interpret these data to characterize the present plate boundaries. A subducted slab shown to underlie the Trobriand forearc is associated with the Papuan volcanic arc and the diapiric emplacement of felsic gneissic domes in the Woodlark rift from previously subducted continental materials in the mantle wedge. In contrast, the Suckling‐Dayman metamorphic core complex formed in the footwall of the Mai'iu extensional detachment, augmented by calc‐alkaline and high‐K magmatic intrusion and uplift. Utilizing Global Positioning System relative velocities, transform fault azimuths, ridge axis azimuths, and two fault slip directions, we derive a plate kinematic model for the Australia, Woodlark, Trobriand, and Solomon Sea plates to describe the neotectonics and provide insight to the past tectonics. We present two cases—with and without subduction at the Trobriand Trough—and favor the former based on our characterization of the plate boundaries. Our model estimates current Trobriand subduction rates of 4.5–4.7 mm/yr with obliquity increasing westwards, and full spreading rates, increasing eastwards along the Woodlark Basin spreading center, of 20–40 mm/yr. Based on seafloor morphology, we further estimate the time of the change in the pole of the Woodlark Basin opening at 450 Ka followed by a decrease in spreading rate and complete reorientation of the spreading axis fabric at 200 Ka.

Initiation Timing of the Jiamusi–Yitong Fault Zone in NE China

The NE–striking Jiamusi–Yitong fault zone (JYFZ) is the most important branch in the northern segment of the Tancheng–Lujiang fault zone. The precise shearing time of its large–scale sinistral strike–slip has yet to determined and must be constrained. Detailed field investigations and comprehensive analyses show that strike–slip faults or ductile shear belts exist as origination structures along the western region of Yitong Graben. The strike of the shear belts trend to the NE–SW with steep mylonitic foliation. The zircon U–Pb dating result for the granite was 264.1±1 Ma in the ductile shear belt of the JYFZ. The microstructural observation (rotated feldspar porphyroclasts, S–C fabrics, and quartz c–axis fabrics, etc.) demonstrated the sinistral shearing of the ductile shear zones. Moreover, the recrystallized quartz types show a transitional stage of the subgrain rotation toward the recrystallization of the grain boundary migration (SR–GBM). Therefore, we suggest that the metamorphic grade of the shear zone in the ductile shear zones should have reached high greenschist facies conditions, and the deformation temperatures should approximately 450–500°C, which is obviously higher than the blocking temperature of muscovite (300–400°C). Hence, the 40Ar /39Ar isochron age of muscovite from ductile shear zones should be a cooling age (162.7±1 Ma). We infer that the sinistral strike–slipping event at the JYFZ occurred in the late Jurassic period, and it was further inferred from the ages of the main geological events in this region that the second sinistral strike–slip age of the Tancheng–Lujiang fault zone occurred during the period of tectonic movements in the Circum–Pacific tectonic domain. This discovery also indicates the age of the Tancheng–Lujiang fault zone that stretches to northeastern China. The initiation of the JYFZ in the late Jurassic is related to the speed and direction of oblique subduction of the west Pacific Plate under the Eurasian continent and is responsible for collision during the Jurassic period.

Two-dimensional and three-dimensional inherent fabric in cross-anisotropic granular soils

This paper characterizes fabric anisotropy of granular soils by using both two-dimensional (2D) and three-dimensional (3D) images and evaluates the accuracy and effectiveness of using 2D images to characterize fabric anisotropy in 3D soils. The X-ray computed tomography (X-ray CT) is used to reconstruct the 3D volumetric images of five air-pluviated sand specimens. Then, six slices are obtained by vertically cutting the 3D volumetric image in an angle increment of 30°. The 3D and 2D images are analyzed to determine long axis fabric, contact normal fabric, and branch vector fabric. The results show that 2D images produce satisfactory predictions for long axis fabric and branch vector fabric. The 2D images produce satisfactory predictions for contact normal fabric for rounded to well-rounded sands, but underestimate or contain large uncertainties to predict contact normal fabric for very angular to sub-rounded soils. This study shows that particle shapes affect the inherent fabric anisotropy in sands. Elongated sands tend to have stronger degrees of long axis and branch vector fabric anisotropy. Angular sands tend to have stronger degrees of contact normal fabric anisotropy.

Structural analysis and deformation characteristics of the Yingba metamorphic core complex, northwestern margin of the North China craton, NE Asia

The presence of the Yingba (Yinggete–Bagemaode) metamorphic core complex (MCC) is confirmed near the Sino-Mongolian border in China. We report its structural evolution and the rheological features of ductile shear zones within this complex. Three deformations (Ds, Dm, and Db) since the Late Jurassic are identified. Ds is characterized by ductile structures that resulted from early NW-oriented, low-angle, extensional ductile shearing. Dm is associated with partial melting and magmatic diapirism, which accelerated the formation of the dome-like geometry of the Yingba MCC. Synchronously with or slightly subsequently to Ds and Dm, the Yingba MCC was subjected to brittle, extensional faulting (Db), which was accompanied by the exhumation of the lower crust and the formation of supracrustal basins. The ductile shearing (Ds) developed under greenschist-to amphibolite-facies metamorphic conditions (400–650 °C), as indicated by microstructures in quartz and feldspar, quartz [c] axis fabrics, and two-feldspar geothermometry. The mean kinematic vorticity estimates of 48–62% show a pure shear-preferred flow during Ds. The Yingba MCC provides an excellent sample that recorded an intermediate to high temperature shearing, which also implies the widely extensional regime in northeastern Asia at that time.

Mid-Miocene initiation of orogen-parallel extension, NW Nepal Himalaya

Geologic field mapping surveys integrated with structural, thermochronological, and geochronological analyses confirm the existence of an orogen-parallel strike-slip−dominated shear zone in the upper Karnali valley of northwestern Nepal. This shear zone obliquely cuts through the upper Greater Himalayan Sequence and is characterized by a S-dipping, high-strain foliation and intensely developed ESE-WNW−trending, shallow-plunging mineral elongation lineation. Monazite grains within the Greater Himalayan Sequence are deformed and transposed parallel to the orogen-parallel shear zone and ESE-WNW elongation lineations. In situ U-Th/Pb monazite geochronology constrains metamorphism between 19 and 15 Ma, which is consistent with the timing of Neohimalayan metamorphism and S-directed extrusion of the Greater Himalayan Sequence across the Himalaya, and it is therefore interpreted to have preceded orogen-parallel strike-slip deformation. Mineral deformation mechanisms and quartz c -axis patterns of orogen-parallel fabrics record a rapid increase in temperature of deformation from ∼350 °C along upper levels of the shear zone to greater than 630 °C at ∼2.5 km depth structurally below the shear zone. Symmetric quartz c -axis fabrics further suggest deformation included a significant component of pure shear. The 40 Ar/ 39 Ar thermochronology of foliation-defining muscovite indicates that orogen-parallel shearing was active in the area between ca. 13 and 10 Ma while temperatures cooled through the muscovite closure temperature for argon. By integrating these data with the current understanding of tectonic processes in the Himalaya, we interpret a transition from S-directed extrusion of the Greater Himalayan Sequence to orogen-parallel extension between ca. 15 and 13 Ma in the upper Karnali valley. Integration of our findings with chronological constraints from other migmatite-cored domes supports the growing recognition of a Himalayan-wide mid-Miocene initiation of orogen-parallel extension.

Microscale strain partitioning? Differential quartz crystallographic fabric development in Phyllite, Hindu Kush, Northwestern Pakistan

Abstract. Spatially referenced quartz c axis fabrics demonstrate the preservation of multiple, distinct fabrics in a specimen collected from northwestern Pakistan. The overall fabric yielded by the specimen is dominated by a single population of quartz grains, while the fabric signatures of two other unique, spatially distinct populations are overwhelmed. It is these minor fabrics, however, that provide information on temperature of deformation (403 ± 50 °C), differential stress (8.6 + 2.6/−1.5 MPa to 15.0 +3.8/−2.5 MPa), strain rate (10−16 s−1 to 10−15 s−1), and strain partitioning recorded by the specimen.

An experimental study of effect of pre‐existing fabric on deformation of foliated mylonite at high temperature and pressure

We performed deformation experiments on a foliated mylonite under high temperature and pressure conditions in this study. To investigate the effect of pre‐existing fabric on the rheology of rocks, our samples were drilled from natural mylonite with the cylinder axis parallel to the foliation (PAR) and perpendicular to the foliation (PER). We performed 25 tests on seven PAR samples and 21 tests on seven PER samples at temperatures ranging from 600 to 890 °C, confining pressures ranging from 800 to 1400 MPa, and steady‐state strain rates of 1 × 10−4, 1 × 10−5 and 2.5 × 10−6 s−1. In the temperatures of 600–700 °C, the deformation is accommodated by semi‐brittle flow, with the average stress exponent being 6–7 assuming power law flow; in the temperature range of 800–890 °C, deformation is mainly by plastic flow, with an average stress exponent of n = 3 and activation energies of Q = 354 ± 52 kJ/mol (PER and PAR samples). The experimental results show that the strengths of PER samples are higher than those of PAR samples. Deformation microstructures have been studied by optical and electron microscopy. The original foliation of PER samples is destroyed by deformation and replaced by a new foliation, but the deformation of PAR samples followed the original foliation. Electron backscatter diffraction (EBSD) measurements show a strong lattice preferred orientation (LPO) of the quartz c axis fabrics of the starting samples and deformed PER and PAR samples. However, the c axis fabric of quartz in experimentally deformed PER and PAR samples varied with temperature and strain rate is different from that seen in the starting mylonite sample. The initial quartz c axis fabric of the starting mylonite sample has been transformed into a new fabric during experimental deformation. Dehydration melting of biotite and hornblende occurred in both PER and PAR samples at temperatures of 800–890 °C. Copyright © 2014 John Wiley & Sons, Ltd.

Deformation characteristics and formation mechanism of the Yunmengshan metamorphic core complex

The Yunmengshan metamorphic core complex in the middle part of the Yanshan Fold and Thrust Belt records crust extension processes of the eastern North China Craton during its peak destruction. Development of the metamorphic core complex was controlled by the generally NNE-striking Dashuiyu Shear Zone. The shear zone dips SE and becomes shallower NE-wards, leading to exposures of a ductile shear zone in the southern and middle parts and brittle faults in the northern part. Exposure structures, microstructures, and quartz C- axis fabrics indicate that the ductile shear zone belongs to an extensional shear zone with a top-to-the-SE shear sense. Deformation temperatures of 300-520 ℃ suggest a midcrustal origin for the ductile shear zone. A ductile deformation belt in the footwall of the shear zone is only as wide as 1-3 km, indicating no widespread mid-crustal ductile flow in the region during the deformation. Zircon U-Pb dating of dykes and plutons as well as hornblende and biotite 40 Ar/ 39 Ar dating demonstrate that the metamorphic core complex originated at 135 Ma and experienced intense shearing of the Dashuiyu Shear Zone, development of the supradetachment basins, and synkinematic intrusion during 135-125 Ma. The metamorphic core complex was subjected to rapid exhumation during 125-114 Ma when the Dashuiyu Shear Zone suffered continuous activity and passive doming. The shear zone and its hanging wall were cut or replaced by a series of brittle faults when they were uplifted to a brittle regime, showing that exhumation took place in continuous extensional activities. The metamorphic core complex turned into slow exhumation in an extensional regime in the following latest Early Cretaceous. The evolution history suggests that the Yunmengshan metamorphic core complex was developed by the rolling-hinge model, a common formation mechanism for intraplate metamorphic core complexes in the North China Craton, under the continuous NW-SE extension during the Early Cretaceous (135-100 Ma).

Antigorite‐induced seismic anisotropy and implications for deformation in subduction zones and the Tibetan Plateau

AbstractThe present study, which is a follow‐up of the Journal of Geophysical Research paper by Ji et al. (2013a), provides a new calibration for both seismic and fabric properties of antigorite serpentinites. Comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic‐preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high T serpentinite, which is essentially controlled by the antigorite c axis fabric, is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock. Extrapolation of the experimental data with both pressure and temperature suggests that Vp anisotropy decreases but shear wave splitting (ΔVs) and Vp/Vs increase with increasing pressure in either cold or hot subduction zones. For a cold, steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening or trench‐parallel movements, forming trench‐parallel seismic anisotropy. For a hot, shallowly subducting slab, however, antigorite is most likely deformed by simple shear or transpression. Trench‐normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30°. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneity in Vp, Vs and attenuation, shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks, hydrated zones of subducting lithospheric mantle, and the crust‐mantle boundary if the temperature is below 700°C in the region of interest.

Evolution and time of formation of the Hohhot metamorphic core complex, North China: new structural and geochronologic evidence

The Hohhot metamorphic core complex (MCC) is one of the typical MCCs in the North China craton. Its fault systems consist of the master Hohhot detachment zone (HHDZ) on the southern flank of the Daqing Shan, and the lowermost and uppermost northern detachment zones on the northern flank. Ductile deformation temperatures of three zones were estimated as 500 ± 50°C, 650 ± 50°C, and 400 ± 50°C, respectively, by analysis of microstructures of minerals and quartz [c] crystallographic axis fabrics using electron backscattered diffraction. These measurements suggest that previous 40Ar/39Ar ages could not represent the time of the high-temperature (>500°C) ductile deformations. Therefore, we used U-Pb zircon ages of mylonitized and non-mylonitized granitic intrusions to constrain the timing of the early high-temperature shearing. Strongly mylonitized granites and weakly mylonitized granites in the lowermost northern detachment zone yielded zircon U-Pb ages of 148 ± 1 and 140 ± 1 million years respectively. A syn-kinematic pluton in the lower plate of the MCC gave a U-Pb age of 142 ± 1 million years. These allow us to speculate on the possibility that SE-directed, early tangential, high-temperature ductile shearing probably was initiated during ca. 148–140 Ma (or ca. 142 Ma) at depth, with the thrust events occurring at shallow levels. A strongly mylonitized granitic dike and a non-mylonitized pluton in the master HHDZ yielded ages of 142 ± 1 and 132 ± 2 million years respectively. A non-mylonitized pluton intrusive into the uppermost northern detachment zone was dated at 131 ± 1 Ma. All these suggest that major extensional ductile shearing along the detachment zones took place during ca. 140–132 Ma. Using these new U-Pb ages, combined with previously published 40Ar/39Ar cooling ages that range from 127 Ma to 119 million years for the master HHDZ and supradetachment basins, we discuss and derive the time of formation process of the MCC. This is one of only a few cases of detailed study of timing for the development of an MCC from earlier deep-level shearing to later thermal uplifting (doming).

Kinematic evolution of the Ama Drime detachment: Insights into orogen-parallel extension and exhumation of the Ama Drime Massif, Tibet–Nepal

The Ama Drime Massif is a north–south trending antiformal structure located on the southern margin of the Tibetan Plateau that is bound by the Ama Drime and Nyönno Ri detachments on the western and eastern sides, respectively. Detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the Ama Drime detachment to document spatial and temporal patterns of deformation. Deformation temperatures estimated from quartz and feldspar microstructures, quartz [ c] axis fabrics, and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between ∼400 and 650 °C. Micro- and macro-kinematic indicators suggest west-directed displacement dominated over this temperature range. Mean kinematic vorticity estimates record early pure shear dominated flow (49–66% pure shear) overprinted by later simple shear (1–57% pure shear), high-strain (36–50% shortening and 57–99% down-dip extension) dominated flow during the later increments of ductile deformation. Exhumation of the massif was accommodated by at least ∼21–42 km of displacement on the Ama Drime detachment. Samples from the Nyönno Ri detachment were exhumed from similar depths. We propose that exhumation on the Nyönno Ri detachment during initiation of orogen-parallel extension (11–13 Ma) resulted in a west-dipping structural weakness in the footwall that reactivated as the Ama Drime detachment.

Kinematics of the Greater Himalayan sequence, Dhaulagiri Himal: implications for the structural framework of central Nepal

Abstract: In the Dhaulagiri region of central Nepal quartz-rich specimens sampled from the Greater Himalayan sequence yield well-defined quartz c -axis fabrics with a dominant top-to-the-SW sense of shear. These fabrics reflect pervasive crystal-plastic deformation extending more than 8 km structurally below previously mapped locations of the Main Central thrust fault. Quartz c -axis fabric opening angles suggest deformation temperatures of c . 500 ± 50 °C within the lower portion of the Greater Himalayan sequence and up to c . 670 ± 50 °C within the migmatitic upper portion. These temperatures generally plot within error of garnet–biotite temperature estimates for metamorphic assemblages and are interpreted to reflect Tertiary deformation during extrusion of the mid-crust. The quartz c -axis data, and a new, detailed tectonostratigraphic map, constrain the position of the Ramgarh thrust in the Dhaulagiri region to be coincident with the Main Central thrust at the base of the pervasively deformed exhumed mid-crustal core. Mean kinematic vorticity numbers ( W m ) measured in specimens sampled from the lower portion of the Greater Himalayan sequence range between 0.49 and 0.80 ( c . 66–41% pure shear) with an average value of 0.67 ( c . 53% pure shear). These data indicate that extrusion of the mid-crust was facilitated by a significant component of pure shear strain.

Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet

This paper presents quantitative data on strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab. The data were collected from the Tibetan side of the Everest Massif where two low-angle normal faults bound the upper surface of the Greater Himalayan Slab, the earlier and structurally lower Lhotse Detachment and the later and structurally higher Qomolangma Detachment. Greenschist- to sillimanite-grade quartz-rich metasedimentary rocks exposed in the Rongbuk to North Col region of the Everest Massif are characterized by cross-girdle quartz c -axis fabrics indicating approximate plane strain conditions. Fabric opening angles progressively increase with depth beneath the overlying Lhotse Detachment, and indicate progressively rising deformation temperatures of 525–625 ± 50 °C at depths of 300–600 m beneath the detachment. Deformation temperatures of c . 450 °C are indicated by fabric opening angles in epidote amphibolite-facies mylonites located closer to the overlying detachment. A top down-to-the-north (normal) shear sense is indicated by the asymmetry of microstructures and c -axis fabrics, but the degree of asymmetry is low at distances greater than 400 m beneath the detachment, and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine, indicating that deformation occurred at close to peak metamorphic temperatures. These ‘quenched’ fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies. Mean kinematic vorticity numbers ( W m ) were independently calculated by three analytical methods. Calculated W m values range between 0.67 and 0.98, and indicate that although a simple shear component is generally dominant, particularly in greenschist-facies mylonites located between the Lhotse and overlying Qomolangma detachments, there is also a major component of pure shear in samples located at 400–600 m beneath the Lhotse Detachment (pure and simple shear make equal contributions at W k =0.71). Our integrated strain and vorticity data indicate a shortening of 10–30% perpendicular to the upper surface of the Greater Himalayan Slab and confirm that the upper surface of the slab is a ‘stretching fault’ with estimated down-dip stretches of 10–40% (assuming plane strain deformation) measured parallel to the flow plane–transport direction.

A renewed argument for crystal size control of ice sheet strain rates

At present, it is generally believed that crystal size has no direct influence on strain rate in the ice sheets and that the fraction of strain rate enhancement there which is not ascribable to c axis fabric is due to impurity content. Here we challenge this view because it is not consistent with recent results from analyses of deformation at Meserve Glacier and instead ascribe residual enhancement in the ice sheets to variations in crystal size. We resurrect the idea that variations of crystal size can be an important part of the total shear enhancement in the ice sheets, though agree with Paterson that this effect is generally dominated by variations of crystal fabric. We propose that the enhanced shear strain rate of ice age ice in southern Greenland, as inferred from tilt of the Dye 3 borehole, can be explained as a result of combined fabric variations and crystal size variations, with these two ice properties accounting for roughly 70% and 30% of the average enhancement, respectively. Permitting a grain size dependence of ice viscosity also resolves the quandary concerning closure and tilt of the Agassiz Ice Cap borehole.

Physical and structural properties of the Greenland Ice Sheet Project 2 ice core: A review

Substantial data sets have been collected on the relaxation characteristics, density, grain size, c axis fabrics, and ultrasonic velocities of the Greenland Ice Sheet Project 2 (GISP2) core to its contact with bedrock at 3053.4 m. Changes in all these properties paralleled closely those found in cores from Byrd Station, Antarctica, and Dye 3, Greenland. Density increased progressively with depth to a maximum of 0.921 Mg/m3 at about 1400 m, at which depth the ice became bubble free. Below about 2000 m, in situ densities began to decrease in response to increasing ice sheet temperatures. Since drilling, much of the ice core has undergone significant volume expansion (relaxation) due to microcracking and the exsolving of enclathratized gases, especially in the brittle ice zone between 650 and 1400 m. Grain size increased linearly to about 1000 m, thereafter remaining fairly constant until the Younger Dryas event at 1678 m where a twofold to threefold decrease in grain size occurred. These grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical, including a small increase in c axis concentration across the Younger Dryas/Holocene boundary. Increased dust levels in the Wisconsin ice have contributed to the maintenance of a fine‐grained texture which, with its strong vertical c axis fabric, persisted to nearly 3000 m. However, beginning at about 2800 m, layers of coarse‐grained ice intermixed with the much finer‐grained matrix ice are observed. Below 3000 m the ice became very coarse grained. This change, attributed to annealing recrystallization at elevated temperatures in the ice sheet, was accompanied by a dispersed or ring‐like redistribution of the c axes about the vertical. Ultrasonic measurements of vertical and horizontal P wave velocities made at 10‐m intervals along the entire length of the GISP2 core fully confirmed the results of the crystallo‐optical observations. A return to fine‐grained ice coincided with the first appearance of brown, silty ice 13 m above bedrock. Bedrock material consisted of 48 cm of till, including boulders and cobbles, overlying gray biotite granite comprising the true bedrock. There is evidence that disturbed structure in the GISP2 cores begins little more than 70% of the way through the ice sheet. This disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10% of the ice sheet.

Isothermal flow of an anisotropic ice sheet in the vicinity of an ice divide

Simulations of glacier flow are commonly based on the assumption that ice has an isotropic viscosity. Here we examine the plane flow of ice in the special region of an ice divide using a constitutive relation for an anisotropic, incompressible viscous body that is orthotropic and transversally isotropic. Ice is assumed to be isotropic at the ice sheet surface, with the continuous development of a vertical single maximum c axis fabric with increasing depth. We consider the theoretical case of an isothermal ice sheet over a horizontal bedrock, with no slip at the ice‐bedrock interface. The ice sheet surface elevation is imposed, and the flow corresponding to the steady state is calculated, using a two‐dimensional finite difference model based on the resolution of a pressure‐Poisson equation. In this model, all components of the stress and strain rate tensor are calculated. The main conclusion is that for a fixed surface elevation, the general flow pattern accelerates when the anisotropic behavior of the ice is taken into account due to the greater fluidity with respect to shear stress. The downward motion of the ice is faster, despite a higher resistance to vertical deformation. As a result, the dominance of shear strain rate in the flow of polar ice is stronger in the anisotropic case than in the isotropic case. The shear stresses are slightly relaxed, while the longitudinal stresses are significantly increased in the anisotropic case.

The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina

The Red River Fault zone (RRF) is the major geological discontinuity that separates South China from Indochina. Today it corresponds to a great right-lateral fault, following for over 900 km the edges of four narrow (< 20 km wide) high-grade gneiss ranges that together form the Ailao Shan-Red River (ASRR) metamorphic belt: the Day Nui Con Voi in Vietnam, and the Ailao, Diancang and Xuelong Shan in Yunnan. The Ailao Shan, the longest of those ranges, is fringed to the south by a strip of low-grade schists that contain ultramafic bodies. The ASRR belt has thus commonly been viewed as a suture. A detailed study of the Ailao and Diancang Shan shows that the gneiss cores of the ranges are composed of strongly foliated and lineated mylonitic gneisses. The foliation is usually steep and the lineation nearly horizontal, both being almost parallel to the local trend of the gneissic cores. Numerous shear criteria, including asymmetric tails on porphyroclasts, C-S or C′-S structures, rolling structures, asymmetric foliation boudinage and asymmetric quartz 〈c〉 axis fabrics, indicate that the gneisses have undergone intense, progressive left-lateral shear. P-T studies show that left-lateral strain occurred under amphibolite-facies conditions (3–7 kb and 550–780°C). In both ranges high-temperature shear was coeval with emplacement of leucocratic melts. Such deformed melts yield U Pb ages between 22.4 and 26.3 Ma in the Ailao Shan and between 22.4 and 24.2 Ma in the Diancang Shan, implying shear in the Lower Miocene. The mylonites in either range rapidly cooled to ≈ 300°C between 22 and 17 Ma, before the end of left-lateral motion. The similarity of deformation kinematics, P-T conditions, and crystallization ages in the aligned Ailao and Diancang Shan metamorphic cores, indicate that they represent two segments of the same Tertiary shear zone, the Ailao Shan-Red River (ASRR) shear zone. Our results thus confirm the idea that the ASRR belt was the site of major left-lateral motion, as Indochina was extruded toward the SE as a result of the India-Asia collision. The absence of metamorphic rocks within the 80 km long “Midu gap” between the gneissic cores of the two ranges results from sinistral dismemberment of the shear zone by large-scale boudinage followed by uplift and dextral offset of parts of that zone along the Quaternary Red River Fault. Additional field evidence suggests that the Xuelong Shan in northern Yunnan and the Day Nui Con Voi in Vietnam are the northward and southward extensions, respectively, of the ASRR shear zone, which therefore reaches a length of nearly 1000 km. Surface balance restoration of amphibolite boudins trails indicates layer parallel extension of more than 800% at places where strain can be measured, suggesting shear strains on the order of 30, compatible with a minimum offset of 300 km along the ASRR zone. Various geological markers have been sinistrally offset 500–1150 km by the shear zone. The seafloor-spreading kinematics in the South China Sea are consistent with that sea having formed as a pull apart basin at the southeast end of the ASRR zone, which yields a minimum left-lateral offset of 540 km on that zone. Comparison of Cretaceous magnetic poles for Indochina and South China suggests up to 1200 ± 500 km of left-lateral motion between them. Such concurrent evidence implies a Tertiary finite offset on the order of 700 ± 200 km on the ASRR zone, to which several tens of kilometers of post-Miocene right-lateral offset should probably be added. These results significantly improve our quantitative understanding of the finite deformation of Asia under the thrust of the Indian collision. While being consistent with a two-stage extrusion model, they demonstrate that the great geological discontinuity that separates Indochina from China results from Cenozoic strike-slip strain rather than more ancient suturing. Furthermore, they suggest that this narrow zone acted like a continental transform plate boundary in the Oligo-Miocene, governing much of the motion and tectonics of adjacent regions. 700 and 200 km of left-lateral offset on the ASRR shear zone and Wang Chao fault zone, respectively, would imply that the extrusion of Indochina alone accounted for 10–25% of the total shortening of the Asian continent. The geological youth and degree of exhumation of the ASRR zone make it a worldwide reference model for large-scale, high-temperature, strike-slip shear in the middle and lower crust. It is fair to say that this zone is to continental strike-slip faults what the Himalayas are to mountain ranges.

Mapping c-axis fabrics to study physical processes in ice

AbstractMapping the spatial distribution of c-axis orientations in ice thin sections is not much more difficult than preparing c-axis scatter plots but can reveal additional information about processes responsible for the observed fabric and texture of the ice. Distributions of angles between c axes of neighboring grains from the Byrd Station (West Antarctica) ice core suggest that polygonization causes average grain-size to stabilize below 400 m depth.

A three-stage structural evolution of the Quadrilátero Ferriífero: consequences for the Neoproterozoic age and the formation of gold concentrations of the Ouro Preto area, Minas Gerais, Brazil

Abstract The Quadrilatero Ferrifero occurs in the southern part of the Sao Francisco Craton (Brazil), where three major Proterozoic supracrustal units (the “Rio das Velhas”, “Minas” and “Itacolomi” Supergroups) are exposed together with Archaean to Palaeoproterozoic granito-gneissic basement. Microtectonic and kinematic analyses have been undertaken in the area close to Ouro Preto in order to precise the tectonic evolution and to distinguish the tectonic features related to the Transamazonian and Brasiliano orogenies (respectively ∼ 2000 and ∼600 Ma ). Three main tectonic events have been recognized. The earlier event, De, is clearly expressed within basement rocks of the Bacao Complex and surrounding supracrustal units. A stretching lineation (Le lineation), roughly oriented N—S, can be observed. Numerous shear criteria such as drag folds and asymmetric pressure shadows indicate a top-to-the-south sense of movement along the southern border of the Bacao Complex. This event was developed within amphibolite facies conditions, fact confirmed by quartz 〈c〉 axis fabrics. This earlier deformation is related to plutonic activity and reactivation of the Archaean basement expressed by the occurrence of scarce leucocratic granitoid rocks intruded within the Bacao gneisses. It is here attributed to the Transamazonian orogeny in agreement with recent UPb dating ( ∼ 2000 Ma ). The large-scale folds (i.e. Mariana anticline, Dom Bosco syncline) which define the dominant structural geometry of the Quadrilatero Ferrifero were formed during this event. The second tectonic event, Dm, has been defined as a major thrusting towards the WNW related to the Brasiliano cycle. A well-expressed stretching and mineral lineation, oriented E—W to NW—SE (Lm lineation) and associated NW-verging shear criteria characterize this event. Occurrence of muscovite, biotite and more scarcely sillimanite indicate that deformation developed in upper greenschist/lower amphibolite facies. Quartz 〈c〉 axis fabrics confirm the sense of shearing but the large scattering of several fabrics traduces a weak intensity of strain. All supracrustal and gneiss formations were affected by the thrusting which is developed on a folded surface previously formed during the De event. The third event, Dm′, is characterized by hangingwall down movement of the nappe pile in response to relaxation of the Brasiliano compressional Dm forces. This event, developed in greenschist facies, is marked by the occurrence of metre-scale drag-folds, sometimes associated to the development of a new lineation Lm′. Gold mineralization, which occurs along the Main Mineralized Contact (located between the “Minas” and “Rio das Velhas” Supergroups), is found within late veins which comprise large recrystallized quartz, carbonate, sulphide, chlorite and tourmaline. Microscopic analysis and veins geometry allow us to conclude that the veins were emplaced mostly after the ductile Dm deformation. It is proposed that the veins were formed by filling of open cavities created during Dm′, tectonics. This study highlights that the northwest-verging thrusting, which is the main tectonic event of the Quadrilatero Ferrifero, is only relevant of the Brasiliano orogeny. A model of structural evolution which involves plutonic activity at ∼ 2000 Ma within the Archaean Bacao Complex, followed by Neoproterozoic thrust tectonics (Brasiliano) is proposed and discussed. The age of gold concentration, assumed to be achieved during the relaxation that occurred in response to the thrusting event, is re-interpreted and related to the late stage of the Brasiliano tectonics.

Shear-wave detection of asymmetric c-axis fabrics in the GISP2 ice core, Greenland

Abstractc-axis fabrics of the GISP2 ice core from central Greenland have been measured rapidly and accurately in the field, using both compressional and shear waves generated by an inexpensive, commercially available, “idiot-proof” device. Compressional-wave data were collected at 10 m intervals for the upper 2250 m of the ice sheet and show progressive clustering of c axes toward the vertical with increasing depth but no large steps at climatic boundaries in the core. The degree of clustering measured by ultrasound agrees closely with that measured using traditional optical techniques but the ultrasound technique is easier and faster than optical methods. A slight asymmetry in the c-axis clustering is revealed by the shear-wave data and increases with increasing depth, indicating that deformation is not symmetric about the vertical at the site.