Ductile Deformation Features Research Articles

Asymmetric Brittle Deformation at the Pāpaku Fault, Hikurangi Subduction Margin, NZ, IODP Expedition 375

AbstractQuantifying fault damage zones provides a window into stress distribution and rheology around faults. International Ocean Discovery Program (IODP) Expeditions 372/375 drilled an active thrust splay fault within the Hikurangi subduction margin. The fault, which is hosted in Pleistocene clastic sediments, is surrounded by brittle fractures and faults as well as ductile deformation features. We find that fracture density in the damage zone enveloping the fault is asymmetric, with the hanging wall showing greater overall fracture density and at greater distances from the fault than the footwall. Furthermore, the peak in fracture density occurs within an area of mesoscale folding and localized slip in the hanging wall rather than adjacent to the main fault zone. We attribute the asymmetry in damage to disparate deformation histories between the hanging wall and footwall, greater ductile deformation within the footwall, and/or dynamic stress asymmetry around a propagating rupture. Damage asymmetry is common at shallow depths in subduction zones and influences the mechanical and hydrological properties of the fault, such as channelized fluid flow and fault stability. Finally, we demonstrate that subduction zone faults show similar damage‐displacement scaling as continental faults.

A Turbulent Magmatic Density Current and the Origin of the Anastomosing UG-1 Chromitites at Dwars River in the Bushveld Complex

AbstractThe origin of the enigmatic UG-1 chromitites at Dwars River in the eastern limb of the Bushveld Complex has been vigorously debated. The UG-1 chromitites form an anastomosing network of multiple layers that are hosted in poikilitic anorthosites and their formation has previously been explained by depositional, erosional, and intrusive processes. We propose that the UG-1 chromitites formed in response to the emplacement of a turbulent magmatic density current into the developing chamber. We use theoretical constraints in fluid mechanics to describe the evolution of the current and in this context, we provide an explanation for the enigmatic igneous features that are preserved at Dwars River. The current was emplaced as an instantaneous single pulse (fixed volume) of dense plagioclase-charged magma (i.e. a plagioclase slurry) that turbulently propagated along the chamber floor. Settling of the initial cargo of entrained plagioclase laths resulted in the formation of a thick sequence of feldspathic mush. The remaining melt turbulently outruns on the chamber floor ahead of the feldspathic mush. This led to the density current becoming stratified into two layers: (1) a basal granular layer, and (2) an upper melt-rich layer. Both layers evolved in a viscous-dominated regime from a Newtonian to a non-Newtonian behaviour (i.e. a power law fluid). In the current’s propagation stage, the resident magma in the chamber was entrained and efficiently admixed into the melt-dominated upper layer, producing a superheated chromite-saturated hybrid melt. Chromite precipitation from this hybrid melt led to the formation of a series of chromite-laden slurries that flowed in the turbulent current and were split (‘bifurcated’) as they passed around rising buoyant plagioclase diapirs that were developing in the back of the current. This led to the anastomosing and bifurcating nature of the UG-1 chromitites. The chromite slurries merged in the tail of the current and eventually back-injected into the deposited plagioclase cumulates on the chamber floor—forming the main ∼2-m thick UG-1 chromitite layer. The cooling and crystallization of the propagating upper layer of the current led to an increase in its viscosity that slowed its velocity until it eventually stagnated on the chamber floor. The mechanical sorting of crystals in plume structures became dominant at the stagnation stage. We argue that spatial variations in crystal packing controlled the heterogeneous distribution of brittle and ductile deformation features that are observed at Dwars River. In situ crystallization of trapped pore melts led to the nucleation of large orthopyroxene and plagioclase oikocrysts that cemented the anorthosite and the chromitite layers, respectively. The emplacement of magmatic density currents and intra-chamber magma mixing may be pertinent to the development of stratiform chromitites in incrementally constructed magma chambers.

The evolutionary characteristics and mechanisms of coal chemical structure in micro deformed domains under sub-high temperatures and high pressures

To understand the physic-chemical evolutionary characteristics and mechanisms of coal chemical structure, sub-high temperature and high pressure coal deformation experiments were conducted by using the self-developed TRTP-2000 experimental system. The micro deformed features and the macromolecular structural characteristics of experimental samples were detected by various in-situ methods. With the decrease of strain rate, the ductile deformation features in samples gradually become distinct. Brittle deformation lead to the breakage of oxygen functional groups and side chains through the coupling of mechanochemistry and frictional thermal energy. Ductile deformation with a higher strain rate resulted in a total increase in aromatic carbon, a relative decrease in the disorder degree of the molecular structure, and a reduction in secondary structure defects respectively. While the excessive strain energy accumulation under ductile deformation at a lower strain rate can lead to the dislocation and slip of the aromatic layer in coal molecular structure and an increase in secondary structure defects. Besides, the alteration of coal molecular structure may increase the ultra micropore volume within coal that is critical for the retention of CBM.

Postshock Thermally Induced Transformations in Experimentally Shocked Magnetite

AbstractWe studied the effect of 973 K heating in argon atmosphere on the magnetic and structural properties of a magnetite‐bearing ore, which was previously exposed to laboratory shock waves between 5 and 30 GPa. For this purpose magnetic properties were studied using temperature‐dependent magnetic susceptibility, magnetic hysteresis and low‐temperature saturation isothermal remanent magnetization. Structural properties of magnetite were analyzed using X‐ray diffraction, high‐resolution scanning electron microscopy and synchrotron‐assisted X‐ray absorption spectroscopy. The shock‐induced changes include magnetic domain size reduction due to brittle and ductile deformation features and an increase in Verwey transition temperature due to lattice distortion. After heating, the crystal lattice is relaxed and apparent crystallite size is increased suggesting a recovery of lattice defects documented by a mosaic recrystallization texture. The structural changes correlate with modifications in magnetic domain state recorded by temperature‐dependent magnetic susceptibility, hysteresis properties and low‐temperature saturation isothermal remanent magnetization. These alterations in both, magnetic and structural properties of magnetite can be used to assess impact‐related magnetic anomalies in impact structures with a high temperature overprint.

Paleomagnetism and magnetic fabrics of Mio-Pliocene hypabyssal rocks of the Combia event, Colombia: tectonic implications

Mio-Pliocene hypabyssal rocks of the Combia event in the Amaga basin (NW Andes-Colombia), contain a deformational record of the activity of the Cauca-Romeral fault system, and the interaction of terranes within the Choco and northern Andean blocks. Previous paleomagnetic studies interpreted coherent counterclockwise rotations and noncoherent modes of rotation about horizontal axes for the Combia intrusives. However, rotations were determined from in-situ paleomagnetic directions and the existing data set is small. In order to better understand the deformational features of these rocks, we collected new paleomagnetic, structural, petrographic and magnetic fabric data from well exposed hypabyssal rocks of the Combia event. The magnetizations of these rocks are controlled by a low-coercivity ferromagnetic phase. Samples respond well to alternatingfield demagnetization isolating a magnetization component of moderate coercivity. These rocks do not have ductile deformation features. Anisotropy of magnetic susceptibility and morphotectonic analysis indicate that rotation about horizontal axes is consistently to the south-east, suggesting the need to apply a structural correction to the paleomagnetic data. The relationships between magnetic foliations and host-rock bedding planes indicate tectonic activity initiated before ~10 Ma. We present a mean paleomagnetic direction (declination D = 342.8°, inclination I = 12.1°, 95% confidence interval α95 = 12.5°, precision parameter k = 8.6, number of specimens n = 18) that incorporates structural corrections. The dispersion S = 27° of site means cannot be explained by secular variation alone, but it indicates a counterclockwise rotation of 14.8° ± 12.7° relative to stable South America. Paleomagnetic data within a block bounded by the Sabanalarga and Cascajosa faults forms a more coherent data set (D = 336.5°, I = 17.4°, α95 = 11.7°, k = 12.5, n = 14), which differs from sites west of the Sabanalarga fault and shows a rotation about a vertical axis of 20.2° ± 10.7°. Deformation in the Amaga basin may be tentatively explained by the obduction of the Canas Gordas terrane over the northwestern margin of the northern Andean block. However, it can also be related to the local effects of the Cauca-Romeral fault system.

Geochemistry and thermodynamics of an earthquake: A case study of pseudotachylites within mylonitic granitoid

Pseudotachylites are melts produced by frictional heating during seismic slip. Understanding their origin and their influence on slip behavior is critical to understanding the physics of earthquakes. To provide insight into this topic, we conducted a case study in the proto-mylonitic to mylonitic Asbestos Mountain granitoid in the eastern Peninsular Ranges batholith (California), which records both ductile (mylonites) and brittle deformation features (pseudotachylites and ultracataclasites). U–Pb chronology and Zr thermometry of titanite porphyroblasts in the mylonites indicate that mylonitization of the plutons occurred at near solidus conditions (∼750°C) over a 10 Ma interval from 89 to 78 Ma. Mylonitization resulted in recrystallization of quartz, plagioclase and biotite, with the biotite concentrated into biotite-rich foliation planes. Subsequent brittle deformation is superimposed on the ductile fabrics. Micro-XRF elemental mapping and in situ LA-ICP-MS analyses on these brittle deformation products show that the pseudotachylites are more mafic (lower Si, but higher Fe) and K-rich than the host mylonite, while the ultracataclasites are intermediate between the host and the pseudotachylites. Inverse mass balance calculations show that both brittle deformation products are depleted in quartz but enriched in biotite, with the pseudotachylites showing the most significant enrichment in biotite, indicating preferential involvement of biotite during brittle deformation. We suggest that biotite-rich layers generated during ductile deformation may have been the preferred locus of subsequent brittle deformation, presumably because such layers represent zones of weakness. Frictional heating associated with slip along such planes results in melting, which causes a decrease in viscosity, in turn leading to further strain localization. During the short time span of an earthquake, frictional melting appears to be a disequilibrium process, in which the minerals are melted in order of their melting points, from biotite (∼800°C) to plagioclase (∼1400°C) and finally to quartz (∼1700°C), rather than by equilibrium melting, which results in silicic eutectoid melts at lower temperatures (∼650°C). Thus, with progressive slip, melt composition should evolve from mafic to felsic, eventually approaching the bulk composition of the host rock. The mafic composition of the pseudotachylites thus indicates that they formed between the melting point of biotite and plagioclase (800–1400°C). Our chemical and modeling analyses on the pseudotachylites suggest that the chemical composition of pseudotachylites can potentially be used to constrain the thermodynamic conditions in the shear zone as well as earthquake source mechanics.

Repeated advance and retreat of the East Antarctic Ice Sheet on the continental shelf during the early Pliocene warm period

Diatom analysis of a sediment core recovered at IODP Site U1358 on the continental shelf off the Adélie Coast indicated that the lower section of the core contained an assemblage dating back to the Thalassiosira innura Zone of the lower Pliocene that ranges from 4.2 to 5.12Ma. Based on lithological descriptions at both a macro- and micro-scale of this early Pliocene part of the core, four facies were interpreted from the diamictons representing the progressive advance and retreat of the grounding line over the site. Facies 1a and 1b contain a distinct directional signal from the orientation of the a-axis of clasts with several phases of fabric development along with both brittle and ductile deformation features that are common in sediments that have been subglacially deformed. Facies 1c and 1d are finely laminated and were deposited in open marine conditions. The four facies within the depositional model provide for the first time direct evidence for ice advancing across the shelf adjacent to the Wilkes Subglacial Basin on at least four occasions separated by three periods of open marine conditions indicating retreat of grounded ice inland of the site during a warmer than present early Pliocene. The times of open marine conditions are correlated with previous findings from the neighbouring rise sites that also indicated an oscillating ice margin. This has significant implications because firstly it suggests a dynamic East Antarctic Ice Sheet (EAIS) that is probably far more sensitive to climatic and oceanic forcing even during relatively short time periods than had previously been thought. Secondly it suggests that proxies used to interpret the advance and retreat of the grounding line from the rise can be linked with direct evidence of grounding line migration from the shelf. It also has important implications for the future behaviour and sensitivity of the EAIS under present continuing warming conditions. Together with results from the rise, this paper provides a crucial ice extent target for a new ice sheet model of this region during the Pliocene.

Fatigue and cyclic deformation behavior of brazed steel joints

To investigate the fatigue assessment of brazed steel joints, stress controlled fatigue tests were conducted with specimens of the steel AISI CA 6-NM (1.4313) and with its brazed joints. Brazing was performed in a shielding gas furnace under H2 atmosphere with Au 18wt% Ni as filler metal. Experiments were performed at a load ratio of R=0.1 with different specimen geometries to compare their fatigue behavior and to investigate the failure mechanism.The results of the experiments—based on a lifetime oriented approach—show the existence of two different regimes depending on the number of cycles to fracture (Nf). For Nf<104 the maximum tolerable loads for all specimens approach the ultimate tensile strength of the substrate material, whereas for Nf>104 the substrate material provides the highest strength, followed by the brazed round specimens and by the brazed T-joint specimens. Investigations on the failure mechanisms revealed that for brazed specimens, fatigue and residual fracture occurred always in the interface of the brazing zone. The crack path is characterized by interfacial jumps, accompanied by ductile deformation features. The analysis of the strain evolution during the cyclic loading experiments shows that the cyclic deformation behavior is significantly influenced by cyclic creep. Furthermore, the experiments show that brazed round specimen exhibit higher strains at similar loading amplitudes, compared to the substrate material. These new findings were also confirmed by FE-calculations, showing an inhomogeneous distribution of local stresses and strains in the proximity of the braze layer. The archived results show the complex interactions of a braze layer on the cyclic deformation behavio—compared to its bulk material—and lead to a better understanding of the fatigue assessment of brazed steel joints.

Glacitectonic deformation in the Chuos Formation of northern Namibia: implications for Neoproterozoic ice dynamics

Abstract The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously interpreted as the product of glaciomarine deposition, glacially related mass movement, or rift-related sediment remobilisation in a non-glacial environment. These interpretations have wide ranging implications for the extent of ice cover during the supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of soft-sediment deformation structures on the macro- and micro-scale support glacitectonic derivation in response to overriding ice from the south/south-east. Overall, the upward increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) and pervasive glacitectonic lamination support subglacial deformation under high and sustained porewater pressures. In contrast, soft-sediment structures indicative of mass movements, including flow noses, tile structures, and basal shear zones, are not present. The close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary subglacial deformation during oscillation of the ice margin. These structures thus reveal evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key palaeoclimatic significance within ancient sedimentary successions.

The application of remote sensing and structural analysis in groundwater exploration in basement terrains, Darfur region, Western Sudan

Darfur region is one of the most vulnerable areas in Sudan that suffer from shortage in water supply. The objective of the current study is to utilize remote sensing techniques combined with the structural analysis to recognize the most potential fracture zones for groundwater occurrences in the hard rock terrains of Darfur region. The old ductile deformation features in Darfur region delineated from Landsat imageries are used for the structural analysis to determine and classify the fractures in the hard rock terrains of the region. Based on the structural analysis conducted in this study, Darfur region was divided into two domains—the western domain of the pre-Pan-African age that is affected by the two deformational phases (D1 and D2), and the eastern domain represented by the basement related to the Pan-African orogeny. The most potential fractures in the western domain are in the NW–SE and NE–SW trends that classified as extensional and release open fractures for the deformations D1 and D2. In the eastern domain, the main potential fractures for groundwater occurrence are in E–W and N–S directions that are classified as extensional and release open fractures of the deformation D3. From the results of the structural analysis, the main potential fracture systems in Darfur region trend are NW–SE, NE–SW, and E–W directions. The intersections of these fracture systems are the most promising targets for drilling, with consideration of the topography, the rates of recharge, and the underlying geology. The geophysical data and boreholes information in Zalingei and north of El Geneina areas in West Darfur confirm the results obtained from remote sensing data and structural analysis, in which the NW–SE, NE–SW, and E–W fractures trends are the most potential fractures in Darfur region.

New age data from the Dzirula massif, Georgia: Implications for the evolution of the Caucasian Variscides

The Caucasus mountain belt has a complex tectono-thermal history that is as yet poorly understood due to the scarcity of reliable geochronological information. Here, we report new monazite and zircon data from the Dzirula massif in the southern Caucasus, which permit development of a model for Variscan LP-HT (low pressure–high temperature) metamorphism in this region. Data are presented for two key lithologies of the Dzirula massif: 1) a group of variably deformed granitoid (mainly granodioritic-tonalitic) rocks and 2) metapelitic LP-HT cordierite-biotite-sillimanite migmatites and paragneisses. Electron-microprobe Th-U-total Pb monazite dating demonstrates that the LP-HT metamorphism in the Dzirula massif is Variscan, and occurred around 330 Ma. LA-ICP-MS zircon dating reveals that the granitoids include two different magmatic series. An older, mainly tonalitic, series is of Lower Cambrian age. Younger intrusions, including gabbros and diorites, but mainly tonalites and granodiorites, are Variscan in age. The older series show ductile deformation features and, along with the metapelites, experienced Variscan high-grade metamorphism that resulted in penetrative blastic recrystallization and anatexis. The younger series is generally undeformed, but often shows a magmatic foliation, and variable alteration under greenschist facies conditions. It is suggested that the Variscan intrusions facilitated the regional LP-HT metamorphic event at 330 to 340 Ma. Age and petrographic data from the Greater Caucasus imply a similar evolution as observed in the Dzirula massif. The Dzirula massif can thus be used as a proxy to model the evolution of the Caucasian Variscides.

Sulfide-Bearing Veinlets Throughout the Stratiform Mineralization of the Central African Copperbelt: Temporal and Genetic Implications

The Central African Copperbelt of Zambia and Democratic Republic of Congo, along with its broad time equivalents in Botswana and Namibia (the Kalahari Copperbelt), is the world’s largest sediment-hosted, stratiform copper province. The economically dominant stratiform copper ± cobalt or silver mineralization, accepted by many as at least partly early diagenetic in origin, is ubiquitously accompanied by sulfide-bearing quartz-carbonate veins and veinlets. Traditionally, this veining is considered to be the product of either metamorphic lateral secretion or discrete late-diagenetic to synorogenic mineralization events. This view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies, a situation that is taken to imply contemporaneity of the two styles. Consequently, during the mineralization, the siltstone, sandstone, or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing, implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis. This conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes. Both the disseminated and veinlet mineralization styles lack ductile deformation features in many Congolese deposits, but are variably metamorphosed and deformed in some deposits in Zambia, Botswana, and Namibia. In conjunction, these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism (~765–<715 Ma), most probably spanning peak Damara-Lufilian metamorphism and ductile deformation (~530 Ma) and continuing in places until at least ~500 Ma. Thus, basin inversion, contractional tectonism, and associated uplift and exhumation, and not the earlier extension and rifting, seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion.

Late tectonic and metamorphic evolution of the Piedmont accretionary wedge (Queyras Schistes lustrés, western Alps): Evidences for tilting during Alpine collision

This paper addresses the contribution of extension in the upper crust during syncollisional exhumation of subducted material. We present structural data based on the analysis of ductile, brittle-ductile, and brittle deformation features in metapelitic schists and meta-basalts from the Schistes lustres nappe stack in the Piedmont zone of the western Alps. To link the structures with synkinematic pressure and temperature estimates, geothermobarometry was performed using fluid inclusion investigation in quartz together with temperature estimations based on metamorphic chlorite composition. Samples used for geothermobarometry were collected taking into account the structural framework within veins associated with extensional structures observed at the regional scale. The early generation of veins associated with boudinage occurred at 415–345 °C and 6–2.5 kbar, and subsequent incipient normal faulting occurred at temperatures of 315–235 °C and pressures <1.7 kbar. The transition from ductile to brittle extension occurred between ca. 27 Ma and ca. 10 Ma. These results indicate a continuous extension in the upper crust during exhumation and continuous deformation associated with regional-scale tilting of the exhumed Schistes lustres tectonic pile. The extensional tectonic regime in greenschistfacies conditions remains active today in most of the internal Alpine zones as the Alpine collision continues.

Thermal maturity assessment of the Upper Triassic to Lower Jurassic Nampo Group, mid‐west Korea: Reconstruction of thermal history

Abstract Thermal maturity of the Upper Triassic to Lower Jurassic Nampo Group, a sediment‐fill of the Chungnam Basin located in the central western part of South Korea, was assessed by illite crystallinity measurement and sandstone microtexture analysis. The Nampo Group consists of a fluvio‐lacustrine deposit bearing meta‐anthracitic coals and was over‐thrusted by the basement rocks. Sandstones are characterized by down sequence increasing illite crystallinity, from anchizone to epizone, which is strongly suggestive of burial heating. Deep‐burial diagenesis and deformation are evidenced by well‐developed pressure solution textures, whose intensity tends to increase down sequence, and by ductile deformation in the lowermost strata. On the basis of the result of illite crystallinity measurement, the maximum paleo‐temperature and total burial depth of the Nampo Group are estimated to be ca 340°C and 10 km, respectively; these conditions are in good agreement with the observed ductile deformation features. The absence of strata younger than the Nampo Group in and around the Chungnam Basin suggests that deep burial of the Nampo Group was caused by tectonic crustal loading. The tectonic overload was because of basement over‐thrusting that occurred during the Jurassic Daebo orogeny, which is closely related to the orthogonal subduction of the Izanagi Plate beneath the East Asian continent. Subsequent hydrothermal alteration disturbed the thermal maturity pattern, resulting in anomalously high illite crystallinity and meta‐anthracitization.

Caracteres structuraux et dynamiques d'un bassin en transtension; la plate-forme tunisienne a l'Ypresien terminal

Abstract In Tunisia, Ypresian carbonate deposits occur on a platform preserving vestige of an inherited fault network. The fault network delineates blocks of different sizes accounting for the platform morphology and in turn for variation in thickness of the Ypresian sequence. The nummulitic limestone carapace of the Ypresian sequence is fractured by faults and joints of various orientations which are systematically sealed by marly beds of early Lutetian age. This indicates that the fault network was reactivated during late Ypresian. Geometric and kinematic study indicates that this strike slip late reactivation is accomodated by normal fault. These structures originated in soft sediment undergoing diagenesis. Depending on bed competency, both ductile and brittle deformation features were recognized in the fault. This superficial tectonic event represents, a recent reactivation of ancient fractures cartographically located on the boundaries of late Ypresian megablocks. Kinematic study of the deformation within and along the boundaries of one of these blocks, (Ousseltia block), indicates a late Ypresian, early Lutetian strike-slip distensive faulting dynamic. Stress-field orientation indicates a rapid re-orientation in time from a predominantly NW-SE extensional tectonic to a NE-SW extensional event. Stratigraphic dating of that tectonical crisis coincides with a turbulent period of relative motions between Europe and Africa.

New Mexico middle-crustal cross sections: 1.65-Ga macroscopic geometry, 1.4-Ga thermal structure, and continued problems in understanding crustal evolution

Recent detailed work in key regions along two north–south transects in northern New Mexico highlights continued controversy about Proterozoic tectonic evolution. Ductile deformation features (folds, ductile thrusts, and associated foliations and lineations) are grouped into three deformation generations. D 1 includes crytic bedding-parallel foliation and fold nappes. D 2 involves north-verging, km-scale inclined folds, the main shortening foliation, and D 2 , structures that further attenuate or reactivate F 2 folds. D 3 involves east–west-trending open folds and domes and associated crenulation cleavage. Although others can dominate locally, S 2 is the dominant regional foliation that could possibly be imaged seismically. Map relationships around ca. 1.65- and ca. 1.42-Ga plutons and porphyroblast-matrix studies of dated minerals show that D 3 occurred at ca. 1.42. The age of D 2 is more uncertain and could be 1.65 or 1.42 Ga. Metamorphic studies also indicate multiple metamorphic events, M 1 –M 3 , that may relate to the deformational events. New geochronology indicates that most metamorphic minerals grew (or were reset) at ca. 1.47–1.35 Ga. U-Pb dates on metamorphic zircon, monazite, titanite, staurolite, garnet, and tourmaline suggest regional metamorphism to 550–700° C at 1.47–1.42 Ga. Metamorphic aureoles are present around plutons, but the highest grades of metamorphism are in areas with no exposed 1.42-Ga plutons. Metamorphism is interpreted to record a regional mantle-driven thermal event, the latter parts of which correspond to a time of pluton emplacement. 40 Ar/ 39 Ar dates record post–1.42-Ga cooling: the highest grade rocks yield the youngest cooling ages, indicating slow cooling and gradual unroofing of the 1.42-Ga thermal profile following 1.42-Ga metamorphism. Our preferred model is that macroscopic geometries (D 1 –D 2 ) were established by 1.65 Ga, and that regional amphibolite-grade metamorphism and associated D 3 deformation at 1.47–1.42 Ga produced localized high-strain domains and fabric reactivation at exposed levels. At deeper levels, structures and assemblages may increasingly record 1.42-Ga reactivation.

Heterogeneity of the middle crust: Implications for strength of continental lithosphere

Paleoproterozoic rocks in the southwestern United States record tectonism at depths of 10–20 km during 1.75–1.65 Ga amalgamation of arcs to North America. These exposures of middle crust are characterized by great heterogeneity of composition, structure, and metamorphic grade. The crust is dominated by subvertical fabric and is laterally segmented into 10-km-scale blocks by subvertical shear zones. Metamorphic temperature gradients of 200 °C occur within <10 km length scales at various 10–20 km depths. Hottest areas are near granitoid plutons (10 km depths) or dike swarms (20 km depths) and are dominated by subhorizontal foliations. Lowest temperature blocks (depths <10 km) show brittle-ductile thrust belt geometries, and all levels show a range of brittle, semi-brittle, and ductile deformation features. Our synthesis suggests that the synorogenic middle crust exhibits a distinctive heterogeneity of tectonism influenced strongly by advective heating and resulting strength variations. Segmentation of the crust into tectonic blocks is interpreted to be related to the 10 km/10 m.y. space and time scales of plutonism and melt transfer in the middle crust. By analogy to experimental data we hypothesize that the middle crust was a spatially and temporally complex strength beam during deformation, with vertical fabric recording strain hardening during whole lithosphere failure. Heterogeneity and a dominance of vertical fabric are consistent with a general lack of middle crustal reflectivity and may have influenced differential uplift during subsequent tectonism.

Black Mountains crustal section, Death Valley extended terrain, California

Recent reconstructions of Cenozoic extension in the southern Great Basin juxtapose the Panamint Range above the Black Mountains and indicate major tectonic denudation of the Black Mountains during Tertiary extension. Structural and geologic characteristics of the Black Mountains and consideration of the regional geology of surrounding ranges suggest that the Black Mountains block may represent a 10 to 30 km preextensional crustal section that has been uplifted along a major detachment zone. Geobarometry and ductile deformation features in a synrift, intermediate to mafic batholith and metamorphism of Eocambrian sedimentary rock on the western side of the range appear to support this hypothesis. These data suggest that the Black Mountains block is one of the youngest and perhaps among the deepest exposed examples of Cordilleran metamorphic core complexes.

Microstructural features of deformed rocks across the brittle-ductile transition

Based on the investigation of fault rocks from several major fault zones in China, this paper deals with the microstructural features of deformed rocks across the brittle-ductile transition and three types of coexistence of different deformation microstructures are summarized. These are: (i) Coexistence of different deformation microstructures of same mineral, for example, feldspar shows both brittle and ductile deformation features; (ii) Coexistence of different deformation microstructures of different minerals, for example, in same mylonite specimen quartz shows completely ductile deformation features while feldspar shows brittle deformation features; (iii) Coexistence of different deformation microstructures of same mineral at different scale, for example, feldspar shows ductile flow features without fractures under the optical microscope while quartz shows microfractures and “microcrashed zones” under TEM. The authors consider that these coexistence of different deformation microstructures should be helpful to study and understand the process of rock deformation across the brittle-ductile transition.

Nucleation and growth of retrograde shear zones: an example from the Needle Mountains, Colorado, U.S.A.

Brittle and ductile deformation features are heterogeneously developed in late Proterozoic retrograde, normal-slip shear zones that cross-cut amphibolites and quartzo-feldspathic gneisses in the Needle Mountains. Replacement of amphibolite-grade minerals by greenschist facies assemblages is spatially related to transgranular fractures that cross-cut the older gneissosity. Steeply dipping, chlorite-lined slip surfaces are associated with chlorite + quartz ± muscovite-filled dilational jogs and narrow breccia zones. Arrays of slip surfaces are transitional into similarly oriented phyllonite zones, within which the foliation locally overprints early cataclastic zones. The nucleation of these shear zones under greenschist facies conditions involved fluid infiltration along early transgranular fractures and hydration of Fe-Mg-bearing minerals. Initial displacement occurred by slip along these fractures and local cataclasite development. Fluids were episodically drawn into dilatant sites along these slip surfaces. A transition to plastic deformation resulted directly from the metamorphic breakdown of feldspars and amphiboles to phyllosilicates, which allowed strain localization into narrow phyllonite zones. Thus, dilatancy produced by early brittle deformation appears to be the main process by which fluids infiltrate into developing greenschist-grade, retrograde shear zones. Reaction-softening via hydration reactions causes a brittle-to-plastic transition and results in strain localization.