Amphibolite Facies Mylonites Research Articles

The Borborema Strike-Slip Shear Zone System (NE Brazil): Large-Scale Intracontinental Strain Localization in a Heterogeneous Plate

Abstract Large-scale strike-slip faults are fundamental tectonic elements of the continental lithosphere. They constitute plate boundaries (continental transforms), separate terranes with contrasting geological histories within accretionary orogens, or accommodate heterogeneous deformation in intracontinental settings. In ancient orogens, where deeper levels of the crust are exposed, these faults are expressed as shear zones materialized by up to tens of km-wide mylonitic belts. The Borborema shear zone system in northeastern Brazil is one of the largest and best-exposed intracontinental strike-slip shear zone systems in the world, cropping out over 250,000 km2. Here, we review its main geophysical, structural, petrologic, and geochronologic characteristics and discuss the factors controlling its development. This complex continental scale shear zone system is composed of a set of NE- to NNE-trending dextral shear zones from which there are two major E-trending dextral shear zones with horse-tail terminations into the transpressional belt branch, as well as several smaller E-trending dextral and NE-trending dextral and sinistral shear zones. The major shear zones are marked by extensive linear or curvilinear magnetic gradients, implying their continuation at depth. The major shear zones are materialized by migmatite to amphibolite-facies mylonites, but the entire system shows evidence of late deformation at lower temperatures. The system developed during the late stages of the Neoproterozoic Brasiliano (Pan-African) orogeny (mainly from 590 to 560 Ma), postdating by more than 20 Ma the main stage of contractional deformation. Localization of strike-slip shearing in this intraplate setting was controlled by rheological contrasts between blocks with distinct Paleoproterozoic histories, the presence of preorogenic Neoproterozoic rifts, the craton geometry, and zones of enhanced magmatic activity, highlighting the importance of rheological heterogeneity in controlling shear zone nucleation and evolution.

Microstructures, deformation mechanisms and seismic properties of a Palaeoproterozoic shear zone: The Mertz shear zone, East-Antarctica

The Mertz shear zone (MSZ) is a lithospheric scale structure that recorded mid-crustal deformation during the 1.7Ga orogeny. We performed a microstructural and crystallographic preferred orientation (CPO) study of samples from both mylonites and tectonic boudins that constitute relics of the Terre Adélie Craton (TAC). The deformation is highly accommodated in the MSZ by anastomosed shear bands, which become more scattered elsewhere in the TAC. Most of the MSZ amphibolite-facies mylonites display similar CPO, thermal conditions, intensity of deformation and dominant shear strain. Preserved granulite-facies boudins show both coaxial and non-coaxial strains related to the previous 2.45Ga event. This former deformation is more penetrative and less localized and shows a deformation gradient, later affected by a major phase of recrystallization during retrogression at 2.42Ga. Both MSZ samples and granulite-facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted CPO of minerals (quartz, feldspar, biotite, amphibole and orthopyroxene). In particular, we highlight the development of an “uncommon” CPO in orthopyroxene from weakly deformed samples characterized by (010)-planes oriented parallel to the foliation plane, [001]-axes parallel to the stretching lineation and clustering of [100]-axes near the Y structural direction.Lastly, we computed the seismic properties of the amphibolite and granulite facies rocks in the MSZ area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the MSZ. Our results reveal that (i) the low content of amphibole and biotite in the rock formations of the TAC, and (ii) the interactions between the CPO of the different mineralogical phases, generate a seismically isotropic crust. Thus, the seismic anisotropy recorded by the seismic stations of the TAC, including the MSZ, must be due to mantle rather than crustal structures.

Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from 40Ar/39Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton

The Liaonan metamorphic core complex formed during crustal extension in the Liaodong Peninsula, eastern North China craton, and consists of the Jinzhou detachment fault, Proterozoic–Paleozoic sedimentary rocks in its upper plate, and exhumed high-grade Archean metamorphic rocks and Early Cretaceous granitic plutons in the lower plate. Exhumation of its footwall from mid-crustal levels is evidenced in the detachment fault zone by the temporal transition from amphibolite facies mylonitization at depth, through retrograde chloritic shearing and brecciation, to brittle faulting during final uplift. The footwall mylonite zone is 2.5–3.5 km thick and includes Early Cretaceous (128–118 Ma) granitic rocks, together with older metamorphic rocks. The 40Ar/39Ar ages of muscovite, hornblende, biotite, and K-feldspar from the mylonitic rocks record that the core complex cooled between ca. 120 and 107 Ma, from the time of initial crystallization of zircons (122–118 Ma) at 700–800 °C in syntectonic leucocratic dikes and granitic rocks, to closure of argon diffusion in hornblende, micas, and K-feldspar at ∼500 to ∼200 °C. Throughout the eastern North China craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of pull-apart basins, and regional alkaline igneous activity, reflects regional extensional tectonics in the Early Cretaceous. This accompanied lithospheric thinning, possibly resulting from the rollback of the subducted Pacific plate along the eastern Asian margin during the Early Cretaceous.

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.

Extensional detachment faulting and core-complex formation in the southern Barberton granite–greenstone terrain, South Africa: evidence for a 3.2 Ga orogenic collapse

Abstract The Barberton greenstone belt in South Africa is an Early- to Mid-Archaean, very low-grade metamorphic supracrustal belt that is bordered in the south by a mid- to lower crustal gneiss terrain. Detailed mapping of the contacts between the supracrustal and gneiss domains along the southern margin of the greenstone belt shows that the supracrustal rocks are separated from the high-grade metamorphic gneiss terrain by an extensional detachment that is situated at and close to the base of the belt. The extensional detachment is approximately 1-km wide and its location corresponds with the heterogeneous, melange-like rocks of the Theespruit Formation. Within the detachment zone, two main strain regimes can be distinguished. Amphibolite-facies rocks at and below the granite–greenstone contacts are characterized by rodded gneisses and strongly lineated amphibolite-facies mylonites. The bulk constrictional deformation at these lower structural levels records, in a subhorizontal orientation, the vertical shortening and horizontal, NE–SW directed stretching of the mid-crustal rocks. The prolate, coaxial fabrics are overprinted by greenschist-facies mylonites at higher structural levels that cut progressively deeper into the underlying high-grade basement rocks. These mylonites have developed during non-coaxial strain and kinematic indicators consistently point to a top-to-the-NE sense of movement of the greenstone sequence with respect to the lower structural levels. This relationship between bulk coaxial NE–SW stretching of mid-crustal basement rocks and non-coaxial, top-to-the-NE shearing along retrograde mylonites at upper crustal levels is consistent with an extensional orogenic collapse of the belt and the concomittant exhumation of deeper crustal levels. The exhumation was initiated under amphibolite-facies conditions at depths of approximately 18 km. The extensional collapse is coeval with or shortly follows the main D2 collisional event in the Barberton greenstone belt at ca. 3230–3220 Ma. Voluminous plutonism at ca. 3225 Ma along the northern margin of the belt is possibly related to the orogenic collapse and associated decompression melting of lower crustal rocks. The extensional collapse coincides with the onset of the coarse-clastic Moodies Group sedimentation which suggests that the small, isolated Moodies basins formed as supradetachment basins in the collapsing hanging wall of the detachment. The steepening of lithologies and fabrics to their present-day vertical attitudes is ascribed to a late-stage solid-state diapiric component of the exhumed hot and buoyant basement gneisses that underlie the relatively cool and dense mafic and ultramafic supracrustal succession.

The influence of grain boundary fluids on the microstructure of quartz-feldspar mylonites

Quartz-rich domains in pre-Alpine, water-deficient, amphibolite facies (510–580 °C, 250–450 MPa), pegmatite mylonites from Mont Mary (MM), western Alps, preserve a fine dynamically recrystallized grain size, without significant annealing, despite the high synkinematic temperatures and subsequent static greenschist facies Alpine overprint. The microstructure is dramatically different from more typical water-rich amphibolite facies mylonites, such as from the Simplon Fault Zone in the central Alps, where the recrystallized grain size is on the millimetre-scale. The difference reflects the dominant strain-induced recrystallization mechanism: (1) progressive subgrain rotation and grain boundary bulging for the dry MM examples; and (2) fast grain boundary migration for the wet Simplon examples. The grain boundary microstructure imaged with SEM is also very different, with most grain boundaries in the dry MM samples lacking porosity, whereas grain boundaries in the wet samples are decorated by a multitude of pores. Quartz grain boundaries from both wet and dry samples are locally coated by thin (100’s of nanometres), possibly amorphous, silica films. Despite the differences in microstructure, the crystallographic preferred orientations (CPOs) of quartz-rich domains from both areas are very similar. Water-deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism(s) but apparently have little influence on the intracrystalline slip systems, as reflected in the CPOs (strong c-axis Y maxima). In MM mylonites, both K-feldspar and plagioclase (An 33-38) dynamically recrystallize, consistent with the inferred metamorphic conditions. Under water-deficient conditions, mid- to lower-crustal rocks can deform heterogeneously under transitional ductile-brittle conditions at high differential stress (for MM ca. 300–500 MPa, as estimated for dry Mohr–Coulomb failure) and preserve this high-stress microstructure, because of the low mobility of dry grain boundaries.

Microstructural evolution of the Seridó Belt, NE-Brazil: the effect of two tectonic events on development of c-axis preferred orientation in quartz

The polyphase evolution of the Seridó Belt (NE-Brazil) includes D 1 crust formation at 2.3–2.1 Ga, D 2 thrust tectonics at 1.9 Ga and crustal reworking by D 3 strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D 2 and D 3 events were used to constrain the tectono-thermal evolution of the belt. D 2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600–650 °C) through grain boundary migration, subgrain rotation and operation of quartz 〈 c〉-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from 〈 c〉-prism to positive and negative 〈 a〉-rhombs. During D 3, enhancement of ductility by dissipation of heat that came from syn-D 3 granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D 3 shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from 〈 a〉-prism to 〈 a〉-basal slip indicates a thermal path from 600 to 350 °C. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Seridó Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths.

Low-temperature thermochronological record of exhumation of the Bitterroot metamorphic core complex, northern Cordilleran Orogen

The Bitterroot metamorphic core complex is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the North American Cordilleran Orogen, in the northern Idaho batholith region. Extension was accommodated mainly on the Bitterroot mylonite zone, a 500–1500-m-thick shear zone that deforms granitic intrusive rocks as young as 53–52 Ma, as well as older high-grade metamorphic rocks and plutons. Exhumation of the core complex, in Eocene time, is marked in the shear zone by the transition from amphibolite-facies mylonitization, to greenschist-facies mylonitization, chloritic brecciation, to brittle faulting that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 53 –30 Ma based on U–Pb, Ar–Ar, and fission-track data. Apatite and zircon fission-track data record the lower-temperature part of the exhumation history and help define when the shear zone became inactive, as well as the transition from rapid, core complex-style extension to slower basin-and-range-style extension. They indicate that the western part of the complex was exhumed to within 1–2 km of the surface by ∼48–45 Ma, while the eastern part of the complex was still at amphibolite-facies conditions and that the eastern part of the complex was not exhumed below ∼60 °C until after 30 Ma. Younger apatite fission-track ages (≤26 Ma) on the eastern range front of the Bitterroot Mountains suggest that the present topographic expression of the mylonite front was due to Miocene high-angle faulting and widening of the Bitterroot Valley.

Relationships between crustal partial melting, plutonism, orogeny, and exhumation: Idaho–Bitterroot batholith

The northern Idaho–Bitterroot batholith region is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the Cordilleran orogen. The relative timing of metamorphism, partial melting, intrusion, and deformation in this area may provide an analogue for magmatic and deformation processes active at mid-crustal depths in modern orogenic belts. Crustal thickening in this part of the Cordilleran began before 100 Ma, but the early stages of this process are poorly constrained. U–Pb zircon dates indicate that high-grade metamorphism was coincident with intrusion of syntectonic quartz diorite plutons at ca. 75–80 Ma, in the Bitterroot metamorphic core complex, where the deepest crustal levels are exposed. Metamorphic conditions reached ∼0.65–0.75 GPa and ∼600–750 °C in the northeastern part of the core complex. Upper amphibolite facies conditions caused widespread partial melting in quartzofeldspathic gneiss facilitated by muscovite breakdown in underlying semi-pelitic schist. New and previously published U–Pb zircon ages indicate that major partial melting of the lower and middle crust occurred between ca. 65 and 53 Ma, leading to the intrusion of the voluminous “main-phase” granitic plutons as thick (3–4 km) sills. Intrusion was accompanied by renewed upper amphibolite facies metamorphism and partial melting forming migmatites at ∼0.65 GPa pressure. The youngest mid-crustal granitic intrusions are about the same age as initial collapse of the orogen and extension at ca. 52–50 Ma. Extension was accommodated mainly on the Bitterroot mylonite zone that deforms the younger intrusions as well as the older high-grade rocks. Therefore, large-scale partial melting of the middle and lower crust followed crustal thickening by as much as 15–35 Ma, but pre-dated extension and exhumation by only 1–3 Ma. Collapse in this sector of the Cordilleran orogen appears to have been focussed where partial melting and plutonism were most intense and long-lived. Exhumation is revealed by the transition from amphibolite facies mylonitization, to greenschist-facies shearing, to brittle faulting, to inactivity of the shear zone that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 51–38 Ma, based on U–Pb and Ar–Ar results. Alkali-feldspar granites were emplaced during the onset of exhumation, but were intruded only into the shallowest Eocene crustal levels. Their generation may have been linked to decompression of the lithospheric column during crustal thinning.

Litogeochemistry of paleoproterozoic orthogranulites from the Rio Preto (MG) - Vassouras (RJ) region, Central Ribeira Belt, southeastern Brazil

The granulites of the Juiz de Fora Complex (JFC) integrate the pre-1.8 Ga basement of the Intermediate Thrust Sheet of the Ribeira Belt's central segment. This tectonic domain is characterized by intense tectonic imbrication of the JFC granulites with Meso- to Neoproterozoic metasedimentary units. In the Rio Preto (MG)-Vassouras (RJ) region, the JFC comprises three lithological associations: orthogranulites, orthogneisses and paragranulites which crop out as several thrust sheets formed by intense shortening during the Brasiliano Orogeny. The main contacts are characterized by the transformation of the granulites into amphibolite facies mylonites. The geochemistry of the orthogranulites leads to the identification of two tectonic associations. One comprises two calc-alkaline series compositionally similar to rocks of modern magmatic arcs, while the other contains two groups of basic rocks, respectively the tholeiitic series and alkaline to transitional basalts, both of intraplate extensional tectonic setting. The relationships between these distinct tectonic environments need further geochronological background, as the Brasiliano shortening obliterated the original paleogeography of this important Paleoproterozoic segment of the Ribeira belt.

The Anita Shear Zone: A major, middle Cretaceous tectonic boundary in northwestern Fiordland

Abstract A 3‐4 km wide shear zone, here named the Anita Shear Zone, runs parallel to the west coast of northern Fiordland and is marked by amphibolite facies mylonites and intensely deformed gneisses. This shear zone marks a tectonic boundary between the metasediments and meta‐granites of the Tuhua Sequence and the gneisses of the Arthur River Complex and Western Fiordland Orthogneiss. Detailed structural analysis indicates that the Anita Shear Zone has been folded and reoriented into its current steep northeast trend by late folds associated with steeply dipping retrograde shear zones. Removing the effects of the overprinting folds reveals that the Anita Shear Zone was originally subhorizontal or shallowly dipping. The Anita Shear Zone is very similar in many aspects to a middle Cretaceous shear zone in Doubtful Sound which has formed along the same lithological boundary. Both shear zones separate the Tuhua Sequence from the Western Fiordland Orthogneiss, indicating that there is no intrusive contact bet...

Orogen-parallel tangential motion in the Late Devonian – Early Carboniferous southern Appalachians internides

In the internal southern Appalachians, orogen-parallel lineation in flat-lying amphibolite facies mylonites occurs over more than 700 km, from North Carolina to Alabama, along the Inner Piedmont western boundary. In several areas distributed along this zone, evidence of orogen-parallel tangential movement is consistently associated with the lineation, suggesting the amphibolite facies mylonites mark an orogen-scale movement zone, probably a décollement zone that formed in the middle crust.Amphibolite facies mylonites occur in the Clairmont Formation north of Atlanta, Georgia. Abundant mesoscopic and microscopic kinematic indicators together with a dominant southwest-trending lineation on a subhorizontal mylonitic foliation reveal a ductile southwestward movement of the Inner Piedmont on subhorizontal shear planes during the mylonitization. Synkinematic hornblende and biotite crystals extracted from a specimen of amphibolite suggest minimum K–Ar cooling ages of, respectively, 349 and 327 Ma. A minimum age of 350 Ma is thus suggested for the main activity of the western Piedmont décollement zone.Orogen-parallel tangential movement along the western Piedmont décollement zone is coeval with orogen-parallel strike-slip faulting in the high-angle Ocmulgee fault in central Piedmont. These faults shape a shear zone system that probably formed at the boundary of the North American craton, in relation with a transpressional regime of deformation.

Petrofabric, P-wave anisotropy and seismic reflectivity of high-grade tectonites

Ji, S., Salisbury, M.H. and Hanmer, S., 1993. Petrofabric, P-wave anisotropy and seismic reflectivity of high-grade tectonites. Tectonophysics, 222: 195–226. In order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones, we have undertaken experimental measurements of P-wave velocities up to 600 MPa for 20 granulite and upper amphibolite facies mylonites from the Snowbird tectonic zone (Canada). The rocks, whose composition ranges from ultramafic to felsic, have experienced extremely large ductile strain. It is found that the amphibolitic mylonites exhibit significant V p anisotropy (6–13% at 600 MPa), whereas the pyroxene-bearing granulite-facies mylonites and the quartz or feldspathic (tonalitic, granitic and diatexitic) mylonites are quasi-isotropic. In order to constrain interpretation of the measured V p properties, microstructural analyses have been systematically performed in the samples studied. It is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low. The amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong LPO ([001]∥lineation, and (100)∥foliation). The anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic: V p ( X) > V p ( Y) > V p ( Z), where X is parallel to stretching lineation, Y is normal to the lineation in the foliation plane and Z is normal to the foliation plane. But the anisotropy pattern in the mica-rich mylonites is transversely isotropic: V p ( X) = V p ( Y) > V p ( Z). If sufficiently large volumes of the crust display such patterns, this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques. The study demonstrates that while seismic reflectivity is strongly lithology-controlled, fabric-induced anisotropy can both enhance and decrease seismic reflectivity. Hence, the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones, but may indicate changes in composition or metamorphic grade.

Shear-wave velocities, anisotropy and splitting in high-grade mylonites

Shear-wave velocities have been measured for 10 high-grade, granulite- and upper amphibolite-facies mylonites from the Snowbird tectonic zone (Canada) at various pressures up to 600 MPa in order to explore the kinematic significance of shear-wave splitting in the lower crust. At 600 MPa, V s anisotropy ranges from 11% in amphibolitic mylonites to 2% in granulite-facies pyroxene-bearing and tonalitic mylonites. Hornblende-bearing granitic mylonites display intermediate V s anisotropy (6–7%). V s anisotropy is lower than V p anisotropy in hornblende-rich mylonites, whereas V s anisotropy is higher than V p anisotropy in feldspar-rich mafic, tonalitic, granitic and diatexitic mylonites. Shear-wave splitting is pronounced in rocks displaying strong S-wave anisotropy. In most of the high-grade mylonites studied, the polarization direction of the first arrival, V s1 , corresponds to the extension lineation ( X) when propagation is parallel to Y (perpendicular to lineation and parallel to foliation), or to the Y-direction when propagation is parallel to X. For all other propagation-directions, the polarization direction of V s1 is oblique to the X-, Y- and Z-directions. Therefore, it may be difficult to infer the structural implications of shear-wave splitting in lower continental crustal shear zones if one does not know (1) the orientation of the propagation and fast vibration-directions with respect to in-situ lineation and foliation, and (2) the lattice preferred orientation of in-situ rock-forming minerals.

Western Old Woman Mountains shear zone: Evidence for late ductile extension in the Cordilleran orogenic belt

Research Article| September 01, 1991 Western Old Woman Mountains shear zone: Evidence for late ductile extension in the Cordilleran orogenic belt Brian S. Carl; Brian S. Carl 1Vanderbilt University, Nashville, Tennessee 37235 Search for other works by this author on: GSW Google Scholar Calvin F. Miller Calvin F. Miller 1Vanderbilt University, Nashville, Tennessee 37235 Search for other works by this author on: GSW Google Scholar Author and Article Information Brian S. Carl 1Vanderbilt University, Nashville, Tennessee 37235 Calvin F. Miller 1Vanderbilt University, Nashville, Tennessee 37235 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (9): 893–896. https://doi.org/10.1130/0091-7613(1991)019<0893:WOWMSZ>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 Brian S. Carl, Calvin F. Miller; Western Old Woman Mountains shear zone: Evidence for late ductile extension in the Cordilleran orogenic belt. Geology 1991;; 19 (9): 893–896. doi: https://doi.org/10.1130/0091-7613(1991)019<0893:WOWMSZ>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 Rocks within the 1-km-thick Western Old Woman Mountains shear zone (WSZ) contain ductilely deformed quartz and ductilely and brittlely deformed feldspars, indicating greenschist to lower amphibolite facies mylonitization. Foliation measured in 73 Ma mid-crustal granitoids within the zone generally dips west-southwest, and sense-of-shear indicators demonstrate top-to-the-west sense of movement parallel to gently southwest plunging lineation. The top of the shear zone is not exposed, but 10 km to the west unmetamorphosed Late Cretaceous-age upper crust is present. The WSZ is thus most simply interpreted as a normal-sense low-angle shear zone. Timing of deformation is constrained to the interval 73 to ca. 65 Ma by the age of deformed granitoids and 40Ar/39Ar chronology. This interval coincides with a period of rapid cooling of rocks now exposed in the Old Woman Mountains that probably resulted from 4 to 8 km of unroofing. We suggest that movement along the WSZ is responsible for at least some of this unroofing The proposed history involves tectonic denudation along a low-angle ductile shear zone and is similar to that demonstrated for Tertiary Cordilleran metamorphic core complexes. 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 Cossato-Mergozzo-Brissago Line and the Pogallo Line (Southern Alps, Northern Italy) and their relationships with the late-Hercynian magmatic and metamorphic events

The Hercynian basement of the Southern Alps west of Lago di Lugano consists of two units: the Ivrea-Verbano, which is well known for its lower crust and upper mantle rocks and as the seat of a marked gravimetric positive anomaly, and the Serie dei Laghi, an intermediate to upper crustal metasedimentary and metagranitic complex. The contact between them is a subvertical tectonic discontinuity, the Cossato-Mergozzo-Brissago Line (CMB) and Pogallo Line, recently the subject of considerable interest as the possible analogue of a low-angle normal fault (later tilted to the present upright position), such as those increasingly revealed by the deep reflection seismics in thinned continental crust. Late Hercynian igneous activity mainly took place along this lineament. The contact is however very complex. The Cossato-lower Valsesia segment is poorly exposed and has been reactivated by a younger fault (Cremosina Line). In the Valsesia-Val d'Ossola (Toce River) segment the CMB Line is expressed as a strong structural and lithological contrast as well as by a narrow belt of mylonites; in the Val Cannobina-Lago Maggiore sector the mylonites are distributed over a considerable thickness mostly within the Serie dei Laghi. They are connected with large-scale ductile folding on nearly vertical axes (“Schlingenbau”). The CMB Line is dissected by several orthogonal and oblique subvertical faults (Grottaccio, Quarna, Val Lessa, Val Pogallo faults). The presence of appinitic calcalkaline dykes and stocks of about 285 Ma, intruded along particular segments of the CMB isolated by those faults, suggests that the attitude of the main tectonic boundary was upright at the time of intrusion. The intrusion apparently occurred in a transtensional environment and induced a temperature increase in the country rocks causing a low pressure metamorphic overprint in the Val Cannobina-Lago Maggiore segment and dehydration melting in a belt 2–3 km wide between Valsesia and Val d'Ossola. The Pogallo Line cuts across the CMB Line with a sinistral horizontal slip of about 11 km and is also characterized by amphibolite facies mylonites. The absence of mylonitic appinites and consistent geologic evidence point to formation of the fault before 285 Ma. A significant Mesozoic displacement along both faults is therefore excluded, even though post-Permian reactivations are demonstrated by the presence of Rb-Sr and K-Ar mica mixed ages along the CMB Line. A new evolutionary model can be proposed to account for most of the geological, petrological and geophysical data: Early Paleozoic continental accretion, subduction and magmatic arc followed by a more external, probably oblique, Hercynian reactivation inducing important transcurrent movements along CMB and Pogallo before the Early Permian magmatic phase.

Evidence for deformation‐induced K‐feldspar replacement by myrmekite

ABSTRACT Several examples of deformation‐induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths, namely a muscovite‐poor S‐C mylonite and a single foliation, muscovite‐poor mylonitic gneiss. Back‐scattered SEM and conventional optical microscopy show that in both rock types, syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of K‐feldspar grains that faced the local inferred incremental shortening direction for the mylonite. Myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction.The replacement of K‐feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains. We suggest that the ambient temperature, pressure and chemical activities were such that the replacement reaction was favoured, but the addition of extra strain energy along the high‐pressure sides of the grains localized the reaction at these sites. This energy could arise from elastic strain, or strain associated with tangled dislocations or twin boundaries. The relative roles of stress and strain energy concentrations in driving the replacement reaction are not known, but both were probably important.

Detachment zones of Cordilleran metamorphic core complexes: thermal, fluid and metasomatic regimes

Metamorphic core complexes in Arizona are characterized by extensional tectonics, in which listrically faulted and unmetamorphosed volcanic rocks of an upper plate are tectonically superimposed on high-grade rocks of a lower plate along low angle detachment faults. The detachment faults are marked by microbreccias and chloritic breccias which developed by hydraulic fracturing and metasomatism that overprinted subjacent amphibolite facies mylonites. Oxygen and hydrogen isotope data indicate that the mylonites formed at ∼500 °C under conditions of low water/rock ratio. Chloritic breccias developed at 300–350 °C during incursion of hydrothermal fluids along the detachment fault. Hydrothermal alteration involved massive additions of Fe, Mn, and Mg, with concomitant depletions of K and Na, reflecting the hydrolysis of K-feldspar and plagioclase to chlorite.

Fluid flow, metasomatism and amphibole deformation in an imbricated ophiolite, North Cascades, Washington

Metasomatic tremolite-rich mylonites are widespread in imbricate thrust slices of ultramafic rocks of the ophiolitic Ingalls Complex in Washington State. Protoliths for these amphibolite-facies mylonites were peridotite and serpentinite. Abundant syntectonic tremolite veins in the ultramafites record narrowly channelized flow of infiltrating fluids, whereas metasomatic mylonite zones record more pervasive flow. Fluids were probably released mainly by prograde devolatization reactions within serpentinite and mafic ophiolitic rocks that experienced earlier hydrothermal metamorphism. Olivine apparently deformed by dislocation creep in the mylonites. In the tremolite-rich rocks, locally preserved amphibole porphyroclasts deformed mainly by microfracturing. Acicular tremolites, which dominate the mylonites, form syntectonic overgrowths on porphyroclasts and probably record diffusive mass transfer which may have accompanied cataclasis. Acicular tremolites subsequently were folded and define both post-crystalline crenulations and polygonal arcs. Fluid flow, deformation and metamorphism were apparently complexly interrelated in the imbricate zone. Thrusts juxtaposed contrasting rock types that were sources and sinks for fluids, and shear zones focused fluid flow. Metamorphism probably facilitated deformation through the release of fluids during dehydration reactions. High fluid pressure may have led to hydraulic fracturing and may have controlled strain softening in the tremolitic mylonite zones as it favored microcracking and diffusive mass transfer over dislocation creep. Infiltrating metasomatic fluids probably play an important role in the evolution of shear zones in many ultramafic bodies during medium-grade metamorphism.