Alleghanian tectono-thermal evolution of the dextral transcurrent Hylas zone, Virginia Piedmont, U.S.A.

A naturally constrained stress profile through the middle crust in an extensional terrane

Derivation of gold by oxidative metamorphism of a deep ductile shear zone: Part 1. Conceptual model

Two kinds of crystalline thrust sheets form in the internides of mountain chains. Type C megathrust sheets are internally brittle slabs of intact crust (composite basement) that detach within the thermally softened ductile-brittle transition (DBT) and, once formed, behave as thin-skinned thrust sheets. Type F thrust sheets are fold-related lobe-shaped thrust sheets that form below or within the DBT by attenuation of the common limb between antiforms and synforms in passive- or flexural-flow folding; transport is controlled by ductile flow. Type C megathrust sheets form by continent-continent or arc-continent collision accompanying A-subduction; Type F sheets form via A- or B-subduction below the DBT. Both result in crustal thickening. Individual Type C megathrusts are very strong (compared to large foreland sheets), with maximum size and displacement attained where crystalline thrusts ramp into weak zones in platform sedimentary rocks. Here crustal thickness may be duplicated, but α (basal detachment) and β (surface slope) angles remain constant (and near zero) because of slab geometry. Coefficient of internal friction along the base of nascent Type C megathrust sheets would be low (<0.3), while the sheet itself would be very strong and coherent, with a high coefficient of internal friction (≥0.85). Foreland thrusts are driven ahead of Type C sheets as crystalline and foreland thrusts merge into the Coulomb wedge of the foreland. Behaviour modes also merge here, because, once formed, Type C sheets commonly ramp onto the platform and propagate along the basal detachment of the deforming platform wedge. Thin platform assemblages on continental promontories restrict the size and displacement of Type C and foreland sheets. Crustal duplexes form in ramp zones as thrusts can no longer propagate along the DBT. Duplexes of platform sedimentary rocks (± basement) may also form beneath the crystalline sheet and arch the sheet above. Late macro- and meso-scale structures (isolated domes, out of sequence thrusts, open folds, some crenellations, and ductile shears) related to thrust emplacement may form within the sheet. Thrust-related meso- and microfabrics are mostly concentrated in or near the fault zone. Character of fault rocks varies with location, rate of motion on the fault, availability of fluid, and ambient T-P conditions in the fault zone.

Research Article| July 01, 1992 Influence of the state of stress on the brittle-ductile transition in granitic rock: Evidence from fault steps in the Sierra Nevada, California Roland Bürgmann; Roland Bürgmann 1Department of Geology, Stanford University, Stanford, California 94305-2115 Search for other works by this author on: GSW Google Scholar David D. Pollard David D. Pollard 1Department of Geology, Stanford University, Stanford, California 94305-2115 Search for other works by this author on: GSW Google Scholar Author and Article Information Roland Bürgmann 1Department of Geology, Stanford University, Stanford, California 94305-2115 David D. Pollard 1Department of Geology, Stanford University, Stanford, California 94305-2115 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (7): 645–648. https://doi.org/10.1130/0091-7613(1992)020<0645:IOTSOS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Roland Bürgmann, David D. Pollard; Influence of the state of stress on the brittle-ductile transition in granitic rock: Evidence from fault steps in the Sierra Nevada, California. Geology 1992;; 20 (7): 645–648. doi: https://doi.org/10.1130/0091-7613(1992)020<0645:IOTSOS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Left-lateral strike-slip faults in the Lake Edison granodiorite (central Sierra Nevada, California) are composed of en echelon segments. Relative displacements across the faults apparently are transferred between segments by ductile shearing at right steps, and by extensional fracturing at left steps. The granodiorite within right steps displays mylonitic foliation, and thin sections show textures in quartz associated with dislocation glide, recovery processes, and dynamic recrystallization, whereas textures in feldspar are related to fracturing. Only centimetres outside the right steps, the rock fabric is approximately isotropic and deformation is accommodated by mineralized opening- mode fractures. The stress field calculated for the right-step geometry, when a boundary element model is used, shows an increase in mean compressive stress of up to 25 MPa within the step relative to that outside. This difference in stress apparently produced the contrasting behaviors of the granitic rock. Experimentally derived power-law flow laws do not predict these behaviors. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Late Cretaceous garnet-bearing two-mica Ireteba Pluton, located in the southern Basin and Range, was tilted and exhumed due to extension in the Colorado River Extensional Corridor in the middle Miocene. Previous mapping shows that the pluton's western part developed brittle deformation while the eastern part developed ductile deformation. The transition is marked as a Miocene Brittle-Ductile Transition (BDT), located at ~8.5 km depth relative to the Miocene paleo-surface marked by coeval overlying volcanics and knowledge of regional crustal tilting. We use outcrop observations and microstructures to define the spatial range, strain characteristics, and deformational temperature across this BDT. From structurally shallow to deep levels, we observe the following changes. At paleodepths above 7-8 km, we observe fractures in the pluton mostly free of ductile deformation except for local discrete ductile shear zones and remnant magmatic fabrics. Quartz and feldspar show cold-moderate-temperature dynamic recrystallization (~300-450°C). At paleodepths of ~8-9 km, we observe more pervasive narrow zones with strongly developed ductile lineation and foliation and mm-scale to 10 cm- scale ductile shear zones with intermittent weakly deformed zones. Samples across a ~3 m shear zone at this depth document a decrease in feldspar porphyroclast sizes from 2.5 mm to 0.6 mm, and quartz and feldspar show recrystallization mechanisms indicative of hotter temperatures (~450 - >550°C). At paleodepths >10 km, we observe more strongly lineated, diffuse ductile fabrics and shear zones, with hotter deformation textures (~>550°C), including pervasive myrmekitic textures. Based on our observations, the deformation pattern in the Ireteba pluton overall shows a temperature-increasing trend toward depth, and local strain heterogeneity exists. We suggest that the BDT can be defined by a ~1-2-km-wide zone characterized by the reduction of feldspar porphyroclast sizes and more pervasive narrow shear zones, and no sharp boundaries of the BDT are observed.

Studies of crustal faulting and rock friction invariably assume the effective normal stress that determines fault shear resistance during frictional sliding is the applied normal stress minus the pore pressure. Here we propose an expression for the effective stress coefficient αf at temperatures and stresses near the brittle‐ductile transition (BDT) that depends on the percentage of solid‐solid contact area across the fault. αf varies with depth and is only near 1 when the yield strength of asperity contacts greatly exceeds the applied normal stress. For a vertical strike‐slip quartz fault zone at hydrostatic pore pressure and assuming 1 mm and 1 km shear zone widths for friction and ductile shear, respectively, the BDT is at ~13 km. αf near 1 is restricted to depths where the shear zone is narrow. Below the BDT αf = 0 is due to a dramatically decreased strain rate. Under these circumstances friction cannot be reactivated below the BDT by increasing the pore pressure alone and requires localization. If pore pressure increases and the fault localizes back to 1 mm, then brittle behavior can occur to a depth of around 35 km. The interdependencies among effective stress, contact‐scale strain rate, and pore pressure allow estimates of the conditions necessary for deep low‐frequency seismicity seen on the San Andreas near Parkfield and in some subduction zones. Among the implications are that shear in the region separating shallow earthquakes and deep low‐frequency seismicity is distributed and that the deeper zone involves both elevated pore fluid pressure and localization.

Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

Brittle–ductile transition, shear failure and leakage in shales and mudrocks

Deformation history of the Hengshan–Wutai–Fuping Complexes: Implications for the evolution of the Trans-North China Orogen

Strike-slip faults are widely developed throughout the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic collages in the world, and may have played a key role in its evolution. Recent studies have shown that a large number of Late Paleozoic–Early Mesozoic ductile shear zones developed along the southern CAOB. This study reports the discovery of a NW–SE striking, approximately 500 km long and up to 2 km wide regional ductile shear zone in the southern Alxa Block, the Southern Alxa Ductile Shear Zone (SADSZ), which is located in the central part of the southern CAOB. The nearly vertical mylonitic foliation and subhorizontal stretching lineation indicate that the SADSZ is a ductile strike-slip shear zone, and various kinematic indicators indicate dextral shearing. The zircon U-Pb ages and the 40Ar/39Ar plateau ages of the muscovite and biotite indicate that the dextral ductile shearing was active during Middle Permian to Middle Triassic (ca. 269–240 Ma). The least horizontal displacement of the SADSZ is constrained between ca. 40 and 50 km. The aeromagnetic data shows that the SADSZ is in structural continuity with the coeval shear zones in the central and northern Alxa Block, and these connected shear zones form a ductile strike-slip duplex in the central part of the southern CAOB. The ductile strike-slip duplex in the Alxa Block, including the SADSZ, connected the dextral ductile shear zones in the western and eastern parts of the southern CAOB to form a 3000 km long E-W trending dextral shear zone, which developed along the southern CAOB during Late Paleozoic to Early Mesozoic. This large-scale dextral shear zone was caused by the eastward migration of the orogenic collages and blocks of the CAOB and indicates a transition from convergence to transcurrent setting of the southern CAOB during Late Paleozoic to Early Mesozoic.

We present an analytic formulation to model creep events at the transition between brittle behavior in the crust and viscous behavior in ductile shear zones. We assume that creep events at the brittle ductile transition (BDT) are triggered by slip on optimally oriented fractures or network of fractures filled with weak ductile material. These events are expressed as transient flow in ductile shear zones likely aided by the release of crustal fluids. We show that the creep in the shear zone can be modeled as the motion of a forced damped oscillator composed of a brittle viscoelastic crust, a ductile shear zone and a creeping zone of fractures at the BDT. The time scale of the events varies between seconds to thousands of years depending on the viscous, elastic and brittle‐plastic properties of the fractured BDT, the shear zone and the crust. The nature of the events depends on the aspect ratio, γ of the shear zone thickness, Hw to the length of the fractured zone, w. We find that thick shear zones with small fractures at the BDT are stiff and generate creep oscillations. Thin shear zones with well‐connected fractures over a large width have very small stiffness and are well lubricated. They generate slow creep events or steady creep event. The former are similar to transient slip events and the latter to creep at the far field tectonic rates. The viscosity of the shear zone, ηw enhances lubrication if it is small and stiffness if it is large.

Strain localization at brittle-ductile transition depths during Miocene magmatism and exhumation in the southern Basin and Range

Diancangshan metamorphic massif is one of the four metamorphic massifs developed along the Ailaoshan-Red River strike-slip fault zone, Yunnan, China. It has experienced multi-stage metamorphism and deformation, especially since the late Oligocene it widely suffered high-temperature ductile shear deformation and exhumation of the metamorphic rocks from the deep crust to the shallow surface. Based on the previous research and geological field work, this paper presents a detailed study on deformation and metamorphism, and exhumation of deep metamorphic rocks within the Diancangshan metamorphic massif, especially focusing on the low-temperature overprinted retrogression metamorphism and deformation of mylonitic rocks. With the combinated experimental techniques of optical microscope, electron backscatter diffraction attachmented on field-emission scanning electron microscopy and cathodoluminescence, our contribution reports the microstructure, lattice preferred orientations of the deformed minerals, and the changes of mineral composition phases of the superposition low-temperature retrograde mylonites. All these results indicate that: (1) Diancangshan deep metamorphic rock has experienced early high-temperature left-lateral shear deformation and late extension with rapid exhumation, the low-temperature retrogression metamorphism and deformation overprinted the high-temperature metamorphism, and the high-temperature microstructure and texture are in part or entirely altered by subsequent low-temperature shearing; (2) the superposition of low-temperature deformation-metamorphism occurs at the ductile-brittle transition; and (3) the fluid is quite active during the syn-tectonic shearing overprinted low-temperature deformation and metamorphism. The dynamic recrystallization and/or fractures to micro-fractures result in the strongly fine-grained of the main minerals, and present strain localization in micro-domians, such as micro-shear zones in the mylonites. It is often accompanied by the decrease of rock strength and finally influences the rheology of the whole rock during further deformation and exhumation of the Diancangshan massif.

Neoproterozoic to Early Paleozoic polyorogenic deformation in the southeastern margin of the Yangtze Block: Constraints from structural analysis and 40Ar/39Ar geochronology

Experimental study of the brittle–ductile transition in hot cutting of SG iron specimens