Mylonite Zone Research Articles

Structural evolution along the Susa Shear Zone: the role of a first-order shear zone in the exhumation of meta-ophiolite units (Western Alps)

In the Western Alps, different shear zones acting at different depths have been investigated for explaining multistage exhumation of (U)HP units, and several exhumation models have been proposed for explaining present-day stacking of different tectonometamorphic units. This study aims to reconstruct the tectonic evolution of the Susa Shear Zone (SSZ), a polyphasic first-order shear zone, outcropping in the Susa Valley. The SSZ consists of a thick mylonitic zone, along which units characterized by different Alpine metamorphic P–T peaks are coupled. In the study area, the footwall of the SSZ mostly consists of oceanic units (i.e., Internal Piedmont Zone), which record eclogitic conditions, whereas the hanging wall consists of oceanic units (i.e., External Piedmont Zone), which record blueschist-facies conditions. These tectonic units were deformed during subduction- and exhumation-related Alpine history, throughout four main regional deformation phases (from D1 to D4), and were coupled along the SSZ, wherein two shearing events have been distinguished (T1 and T2). T1 occurred during early exhumation and was characterized by “apparent reverse” Top-to-E kinematics, whereas T2 occurred during late exhumation and was characterized by Top-to-W kinematics. Detailed fieldwork and structural analysis allowed us to describe the main features of the different deformation stages and define the deformation relative timing. As final result, we propose a four-step geodynamic model, focused on the different stages developed along the SSZ, from pre-T1 to syn-T2, showing the geometrical relationships between the tectonic units involved in the exhumation. The model aims at explaining the role of the SSZ in the axial sector of the Western Alps.

Using apatite to resolve the age and protoliths of mid-crustal shear zones: A case study from the Taxaquara Shear Zone, SE Brazil

Shear zones accommodate strain and facilitate migration of hydrothermal fluid and magma through the crust. Unravelling the deformation history of shear zones requires correspondence between the closure temperature of mineral geochronometers and the temperature of deformation. Here, we adopt apatite U–Pb-trace element analysis as a tool for dating deformation and tracing the protoliths of mid-crustal shear zones through a case study of the Taxaquara Shear Zone (TSZ), a major transpressional shear zone in the southern Ribeira Belt of SE Brazil. Apatite from mylonites in the TSZ yield U–Pb ages of 558–536 Ma, considering uncertiainties, which slightly overlap with 40Ar/39Ar ages of 538 ± 2 Ma from muscovite in the lower limit. The closure temperature of apatite is estimated at 500–460 °C, which is slightly higher than that estimated for syn-kinematic muscovite (445–420 °C). Apatite from shear zone mylonites has Sr/Y and LREE systematics typical of apatite from S- and I-type granitoids, suggesting the adjacent and undeformed Pilar do Sul and Piedade granites are the likely protoliths of the mylonites. This interpretation is supported by new U–Pb ages of ca. 605 Ma from pre-kinematic zircon and titanite from mylonites, which corresponds closely with new U–Pb apatite ages and previously published U–Pb monazites ages from the Pilar do Sul Granite. We suggest the U–Pb system of apatite in the TSZ was reset via volume diffusion during rapid cooling given that it preserves the igneous geochemical signatures. Moreover, this interpretation is consistent with the lower apatite closure temperature (500–460 °C) relatively to the temperature of deformation (530–480 °C). The revised ~560–535 Ma age for the TSZ demonstrates that it post-dates the collisional phase of the Ribeira Belt (620–595 Ma and 595–565 Ma), indicating protracted strain accommodation during the Brasiliano–Pan African orogeny, and supports correlation with the 600–550 Ma and 570–550 Ma transpressional Dom Feliciano and Kaoko Belts. This study demonstrates that apatite is a powerful tool for unravelling the history of mid-crustal shear zones as it is stable in a wide range of lithotypes, has trace element compositions that are sensitive to the environment of formation, and Pb closure temperatures typical of mid-crust conditions. U–Pb-trace element analysis of apatite provides a robust means to date shear zones that can be complimentary to, or independent of, more traditional 40Ar/39Ar analysis of mica or amphibole.

Rheological evolution of a pseudotachylyte-bearing deep crustal shear zone in the western Canadian shield

Crustal-scale continental strike-slip fault systems are typically thought to be thermally segregated into an upper seismogenic expression and a lower zone of ductile deformation likely extending into the lower crust. The ability of the lower crust to sustain stress is fundamental to understanding the mechanical behavior of the lithosphere, yet significant debate exists regarding the strength of the ductile crust. The Cora Lake shear zone in the western Churchill Province of the Canadian Shield is a 4–7 km wide zone of granulite-to upper amphibolite-facies mylonite which hosts synkinematic pseudotachylyte, suggesting a locally strong lower crust. Kilometer-scale mapping suggests that the shear zone localized at a major lithological and terrane boundary, with the pseudotachylyte network centered within the ca. 1 km-wide more intensely deformed, finer-grained ultramylonitic core. In this contribution, we use optical petrography and electron-backscatter diffraction to describe microstructure across a transect of the Cora Lake shear zone. By combining these observations with grain size piezometry, experimental flow laws, and regional pressure-temperature paths, we conclude that stress amplification from a relatively common type of km-scale lithological heterogeneity and cooling history is the most reasonable explanation for the deep crustal pseudotachylyte.

Magmatic and tectonic fabrics in the Upper Jurassic La Hoya Pluton, North Patagonian Batholith (~43°S) as a record of the early stages of the Andean deformation

The magmatic record of the North Patagonian Batholith starts in Middle Jurassic times and rapidly grew in Early Cretaceous times, where it experienced its main building phase. Slightly after, in the Late Cretaceous, the North Patagonian fold and thrust belt experienced its first contractional deformation pulse. The La Hoya Pluton is an Upper Jurassic shallow intrusive body emplaced in the North Patagonian Andes in the proximities of the city of Esquel in Argentina. Its excellent exposures, which bear magmatic and high- and low-temperature solid-state deformation structures, are studied in this work through field, microstructural, and AMS studies. The initial shape of the pluton could be envisaged as having a geometry compatible with coalescent laccoliths, given by the presence of subhorizontal magmatic to solid-state foliations with associated E-W or N–S trending magmatic to solid-state lineations. These magmatic structures, which bear a mild solid-state overprint, were probably formed during progressive pluton cooling. Moreover, the nature of these structures suggests that the pluton cooled fast at shallow crustal levels, indicating that they are probably related to its emplacement processes that started, at least, in Late Jurassic times. Contrastingly, NW-SE and NE-SW trending high-angle subvertical fractures and low-temperature mylonitic zones with steeply plunging lineations, which are compatible with an E-W shortening direction, cut the previously formed subhorizontal magmatic structures. These compressive structures found in the La Hoya Pluton were probably developed during the Late Cretaceous main stage of contractional deformation. The contractional deformation is constrained to have happened before the intrusion of undeformed basaltic dikes crosscutting the mylonitized areas of the La Hoya Pluton. These dikes have an 40Ar-39Ar age in plagioclase of 42.15 ± 0.40 Ma and geochemical features typical of volcanic arcs above a subduction zone. Their geochemical signature is sensitive to the process of arc resumption after the Paleogene waning of arc activity, as it is transitional between the alkaline-like signature of Pilcaniyeu Belt (~57.8–42; Paleocene-Eocene) and the predominantly slab-derived magmas of El Maitén Belt (~37–19; Late Eocene-Early Miocene). These dikes probably formed part of the early stages of the El Maitén Belt.

Structural and tectonothermal evolution of the ultrahigh-temperature Bakhuis Granulite Belt, Guiana Shield, Surinam: Palaeoproterozoic to recent

We constrain the multistage tectonic evolution of the Palaeoproterozoic UHT metamorphic (P ​= ​0.9–1.0 ​GPa, T >1000 ​°C, t ​= ​2088–2031 ​Ma) Bakhuis Granulite Belt (BGB) in Surinam on the Guiana Shield, using large- to small-scale structures, Al-in-hornblende thermobarometry and published fluid inclusion and zircon geochronological data. The BGB forms a narrow, NE–SW striking belt between two formerly connected, ~E–W oriented granite-greenstone belts, formed between converging Amazonian and West African continental masses prior to collision and Transamazonian orogeny. Inherited detrital zircon in BGB metasediments conforms agewise to Birimian zircon of West Africa and suggests derivation from the subsequently subducted African passive margin. Ultrahigh-temperature metamorphism may have followed slab break-off and asthenospheric heat advection. Peak metamorphic structures result from layer-parallel shearing and folding, reflecting initial transtensional exhumation of the subducted African margin after slab break-off. A second HT event involves intrusion, at ca. 0.49 ​GPa, of charnockites and metagabbros at 1993–1984 ​Ma and a layered anorthosite at 1980 ​Ma, after the BGB had already cooled to <400 ​°C. The event is related to northward subduction under the greenstone belts, along a new active margin to their south. A pronounced syntaxial bend in the new margin points northward towards the BGB and is likely the result of indentation by an anticlinorial flexural bulge of the subducting plate. Tearing of the subducting oceanic plate along this bulge explains why the charnockites are restricted to the BGB. The BGB subsequently experienced doming under an extensional detachment exposed in its southwestern border zone. Exhumation was focused in the BGB as a result of the flexural bulge in the subducting plate and localised heating of the overriding plate by charnockite magmatism. The present, straight NE–SW long-side boundaries of the BGB are superimposed mylonite zones, overprinted by pseudotachylites, previously dated at ca. 1200 Ma and 950 ​Ma, respectively. The 1200 ​Ma mylonites reflect transpressional popping-up of the BGB, caused by EW-directed intraplate principal compressive stresses from Grenvillian collision preserved under the eastern Andes. Further exhumation of the BGB involved the 950 ​Ma pseudotachylite decorated faulting, and Phanerozoic faulting along reactivated Meso- and Neoproterozoic lineaments.

Sources and Effects of Fluids in Continental Retrograde Shear Zones: Insights from the Kuckaus Mylonite Zone, Namibia

Midcrustal rocks in retrograde metamorphic settings are typically H2O-undersaturated and fluid-absent and have low permeability. Exhumed continental retrograde faults, nonetheless, show evidence for the operation of fluid-mediated weakening mechanisms during deformation at midcrustal conditions. To explore the origin and effects of fluids in retrograde faults, we study the Kuckaus Mylonite Zone (KMZ), an exhumed crustal-scale, strike-slip shear zone in the southern Namibian Namaqua Metamorphic Complex. The KMZ deformed quartzofeldspathic migmatised gneisses at midcrustal retrograde conditions (450-480°C, 270-420 MPa) in the Mesoproterozoic, 40 Ma after granulite facies peak metamorphism at 825°C and 550 MPa. The mylonites contain fully hydrated retrograde mineral assemblages, predominantly adjacent to anastomosing high-strain zones, providing evidence of local H2O saturation and fluid presence during deformation. Whole rock and quartz veinδ18O values suggest that at least some of the fluids were meteoric in origin. The rocks across the shear zone retain the effect of different protoliths, implying little effect of fluid-rock interaction on whole rock major element chemistry. Together with a general scarcity of quartz veins, this suggests that fluid/rock ratios remained low in the KMZ. However, even small amounts of H2O allowed reaction weakening and diffusion-precipitation, followed by growth and alignment of phyllosilicates. In the ultramylonites, a fine grain size in the presence of fluids allowed for grain size sensitive creep. We conclude that the influx of even small volumes of fluids into retrograde shear zones can induce drastic weakening by facilitating grain size sensitive creep and retrograde reactions. In retrograde settings, these reactions consume fluids, and therefore elevated fluid pressures will only be possible after considerable weakening has already occurred. Our findings imply that the range of seismic styles recently documented at active retrograde transform faults may not require high fluid pressures but could also arise from other local weakening mechanisms.

Structural evolution of the eastern segment of the Irtysh Shear Zone: Implications for the collision between the East Junggar Terrane and the Chinese Altai Orogen (northwestern China)

The tectonic evolution of the Central Asian Orogenic Belt (CAOB) involved multiple episodes of arc accretions/collisions, but the reconstruction of these tectonic processes remains relatively poorly constrained. Evidence for a collision between the intra-oceanic island arc system of the East Junggar Terrane and the active margin of the Siberian Craton (Chinese Altai Orogen) is recorded in the eastern segment of the Irtysh Shear Zone (northwestern China). Field observations from the Qinghe area show that this segment of the Irtysh Shear Zone consists of four NW–SE sinistral mylonitic zones, and domains bounded by these mylonitic zones show variable fold patterns. In the northern part of the Irtysh Shear Zone (southern Chinese Altai Orogen), three generations of structures (DCA1–DCA3) are recognized. The earliest generation of foliation (SCA1) is only recognized locally and is transposed to the orientation of the dominant DCA2 foliation (SCA2). The latter is associated with a shallowly plunging stretching lineation, which is sub-parallel to the NW–SE hinge of DCA3 folds. These third-generation folds (DCA3) show a steeply-dipping axial plane trending NW–SE. In the southern part of the Irtysh Shear Zone (northern East Junggar Terrane), the structural pattern is simpler and only involves a single generation of penetrative foliation (SEJ1). New U–Pb detrital zircon and 40Ar/39Ar data provide constraints on the timing of collision and deformation. These results suggest that the collision occurred after the Early Carboniferous, with the timing of sinistral shearing constrained to the Early–Middle Permian. The combination of sinistral shearing and NW–SE DCA3 folds likely represents an episode of transpressional deformation, which was driven by oblique collision between the East Junggar Terrane and the Chinese Altai Orogen during the Early–Middle Permian. The occurrence of original sub-horizontal SCA2 foliation and associated orogen-parallel stretching lineation might indicate that following the initial collision (after the Early Carboniferous), the southern Chinese Altai Orogen was subjected to orogen-parallel extension. In a larger-scale context, sinistral kinematics along the Irtysh Shear Zone in northwestern China and northeastern Kazakhstan, together with coeval dextral strike-slip deformation farther south, might reflect an eastward escape of orogenic materials, possibly in response to the Permian convergence of the Siberian, Baltic, and Tarim cratons.

The Main Central Thrust zone along the Alaknanda and Dhauli Ganga valleys (Garhwal Himalaya, NW India): Insights into an inverted metamorphic sequence

New tectono-metamorphic data along the Alaknanda-Dhauli Ganga valleys in the Garhwal Himalaya (NW India) are presented focusing on sheared rocks of the Main Central Thrust zone (MCTz), one of the first order regional structures in the Himalaya. In regard of large hot orogens (LHOs), the MCTz corresponds to the base of the low-viscosity infrastructure. Our new findings support that the MCTz is a c. 4 km thick mylonitic zone, affecting different lithologies and encompassing a possible (previously defined) protolith boundary. Four selected mylonitic samples, at different structural position along the MCTz, were studied with a multi-disciplinary approach in detail, supporting that an inverted metamorphic field gradient, with temperature (T) and pressure (P) ranging from c. 500 up to c. 700 °C and c. 0.50–0.60 GPa up to >1.0 GPa, was present. This inverted P-T array is also discernable by the distribution of Al-rich minerals (e.g. chloritoid, staurolite, kyanite) in the intercalation of sheared quartzite. In situ U-(Th)-Pb monazite geochronology indicates that the high-T shearing in the upper part of the MCTz mylonites developed from c. 20 Ma on after a stage of prograde metamorphism that lasted until c. 23 Ma.The integration of our data with available structural, metamorphic and geochronological information along the MCTz profile, in the study area, sheds light on the development of the associated inverted metamorphic sequence (IMS). These data carry also empirical finite patterns to be tested against theoretical models investigating the possible roles of viscous heating and “tectonic overpressure” in the development of the IMS in the Himalaya, and possibly in LHOs in general. Both phenomena, if have been active at all, are not obvious in regard of the current state of geothermobarometric methods. On the contrary, a model for the tectonic assembly of the IMS is favored. The anomalous apparent P and T inverted field gradients are interpreted to be the result of the differential and diachronous transport along a shear zone experiencing heterogeneous vertical thinning linked to the non-coaxial flow.

On the initiation mechanism of the Daguangbao landslide triggered by the 2008 Wenchuan (Ms 7.9) earthquake

The Daguangbao landslide, with a volume of 1.16 × 109 m3 and an area of 7.2 km2, is the largest landslide triggered by the 2008 Wenchuan earthquake in China. To examine the initiation mechanism of this landslide, a detailed field investigation and a series of saturated triaxial tests were carried out. The field investigation revealed that the Daguangbao landslide initiated along a mylonite zone, the soil of which was mainly sand. Abundant groundwater was observed in the tunnel and the mylonite zone soils had high water contents. In addition, the experimental results showed that the soils from the mylonite zone have a high liquefaction potential. The pore water pressures in the soils under cyclic triaxial tests increased rapidly, which could have caused the soils failure. Therefore, the mechanism by which pore water pressure within the mylonite zone accumulates under seismic loading, which promotes a continuous decrease in the shear strength of soils, ultimately led to the rapid initiation of the Daguangbao landslide.

Tracking progressive deformation of an orogenic wedge through two successive internal thrusts: Insights from structure, deformation profile, strain, and vorticity of the Main Central thrust (MCT) and the Pelling-Munsiari thrust (PT), Sikkim Himalayan fold thrust belt

We compare and contrast the structural evolution of two successive internal thrusts, the Main Central thrust (MCT) and the Pelling-Munsiari thrust (PT), from the crystalline core of the Sikkim Himalaya in the context of progressive deformation involving a footwall Lesser Himalayan duplex; the PT is its roof thrust. The mylonitic foliations in both the shear zones are overprinted by successive cleavages. The youngest cleavage from the PT zone continues within the hangingwall MCT sheet that records one additional cleavage possibly indicating time-transgressive cleavage development. The cleavages become steeper while their intensity decreases structurally higher up within individual thrust sheets. The Rs-values and angular shear strain decrease from the mylonite zones to structurally higher up within the sheets. Both the shear zones are Type II with decelerating strain paths. Rs-θ′ relationships and microstructures indicate thrust-parallel stretch is greater than thrust-perpendicular component. Both the shear zones record strain partitioning with lesser competent mylonite domains recording higher Rxz, and a greater proportion of simple-shear than the protomylonite domains. The shear zones recorded the growth of the duplex during progressive deformation that contributed to pure-shear dominated general-shear, higher flattening strain and greater translation on its roof thrust, the PT, than the overlying MCT.

Late Ottawan orogenic collapse of the Adirondacks in the Grenville province of New York State (USA): Integrated petrologic, geochronologic, and structural analysis of the Diana Complex in the southern Carthage-Colton mylonite zone

Abstract Crustal-scale shear zones can be highly important but complicated orogenic structures, therefore they must be studied in detail along their entire length. The Carthage-Colton mylonite zone (CCMZ) is one such shear zone in the northwestern Adirondacks of northern New York State (USA), part of the Mesoproterozoic Grenville province. The southern CCMZ is contained within the Diana Complex, and geochemistry and U-Pb zircon geochronology demonstrate that the Diana Complex is expansive and collectively crystallized at 1164.3 ± 6.2 Ma. Major ductile structures within the CCMZ and Diana Complex include a northwest-dipping penetrative regional mylonitic foliation with north-trending lineation that bisects a conjugate set of mesoscale ductile shear zones. These ductile structures formed from the same 1060–1050 Ma pure shear transitioning to a top-to-the-SSE shearing event at ∼700 °C. Other important structures include a ductile fault and breccia zones. The ductile fault formed immediately following the major ductile structures, while the breccia zones may have formed at ca. 945 Ma in greenschist facies conditions. Two models can explain the studied structures and other regional observations. Model 1 postulates that the CCMZ is an Ottawan orogeny (1090–1035 Ma) thrust, which was later reactivated locally as a tectonic collapse structure. Model 2, the preferred model, postulates that the CCMZ initially formed as a subhorizontal mid-crustal mylonite zone during collapse of the Ottawan orogen. With continued collapse, a metamorphic core complex formed and the CCMZ was rotated into is current orientation and overprinted with other structures.

Transpressive Structures in the Ghadir Shear Belt, Eastern Desert, Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian‐Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the Arabian‐Nubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km‐long, NW‐oriented brittle‐ductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike‐slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along‐strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure‐shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple‐shear dominated. These domains are comparable to splay‐dominated and thrust‐dominated strike‐slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike‐slip and thrust‐sense shear zones. The earlier fabric (S1), is locally recognized in low strain areas and SW‐ward thrusts. S2 is associated with a shallowly plunging stretching lineation (L2), and defines ∼NW‐SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ∼NNW–SSE tight‐minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub‐parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear‐dominant deformation in fold zones. Dextral ENE‐striking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW‐ to NNW‐striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss‐cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike‐slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strike‐slip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian‐Nubian Shield during agglutination of East and West Gondwana.

40Ar/39Ar dating of cataclastic K-feldspar: A new approach for establishing the chronology of brittle deformation

Constraining the timing of brittle deformation, such as in high-angle normal or strike-slip faulting, is technically challenging. In this study we present a new approach of age determinations from cataclastic K-feldspars of brittle faults developed in rocks of the Tan–Lu fault zone and from Louzidian and Liaonan metamorphic core complexes in NE China. The attempt is based on field investigations, microstructural observations, back-scattered electron (BSE) imaging, electron-microprobe analyses, X-ray diffraction, scanning electron microscope (SEM) results, and 40Ar/39Ar step-heating of K-feldspar separated from brittle fault zones of cataclastic mylonite, granite and gneiss. SEM and BSE images of individual K-feldspar crystals reveal distinct grain deformation features, including grain-scale faults (fractures and microfractures) that occur parallel or perpendicular to each other. The 40Ar/39Ar dating results provide well-defined plateau spectra for the cataclastic K-feldspar grains in the Tan–Lu fault zone (74.5 ± 1.3 Ma), and of brecciated gneisses and mylonites that formed at depth in the Louzidian and Liaonan metamorphic core complexes (~120–129 Ma and 116 ± 2 Ma). These data suggest complete resetting of the Ar isotope system of the cataclastic K-feldspars, allowing to constrain the timing of their deformation.

The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile

The Paposo fault zone is a major brittle-ductile strand of the Atacama fault system (AFS), which records sinistral shear associated with Cretaceous oblique subduction beneath northern Chile. New detailed geologic mapping, macro- and microstructural data, and zircon geo/thermochronology reveal insight into the structural evolution of the southern portion of the Paposo fault. The core of the Paposo fault is defined by a ~50 m-thick zone of illite-rich gouge that dips steeply ESE and juxtaposes fractured but unfoliated Early Jurassic tonalite west of the fault against mylonitic Late Jurassic to Early Cretaceous granitoids east of the fault. The mylonite zone east of the fault is ~0.7–1 km thick and includes a ~100–500 m-thick band of hydrothermally-altered ultramylonite derived from Latest Jurassic (146.5 ± 1.5 Ma) to Early Cretaceous (138.5 ± 1.6 Ma) granodiorite. West of this ultramylonite zone, a ~150–350 m-thick zone of younger tonalite (136.0 ± 1.8 Ma) parallels the Paposo fault and grades from protomylonite to mylonite <10 m from the gouge zone on its western margin. Mylonitic foliations dip steeply to moderately SE with lineations that typically plunge ~10–30° SW. Most SE-dipping mylonitic fabrics record oblique sinistral-reverse shear that is consistent with the overall pattern of small-scale brittle faults and S-C-C’ fabrics in the Paposo fault gouge zone. However, in several parts of the hydrothermally-altered mylonite zone, symmetric microstructures and S > L tectonite fabrics most likely record a significant component of zone-normal flattening. The upper age limit of deformation is constrained to be younger than the Late Jurassic/Early Cretaceous granodiorite. Hydrothermal alteration and development of high-strain zones in the Late Jurassic/Early Cretaceous granodiorite are locally associated with mafic dikes that do not cut the younger protomylonitic tonalite, indicating that most of the hydrothermal alteration and mylonitic strain occurred between ~139 Ma and 136 Ma. The Paposo fault gouge zone formed between 150 °C and 200 °C based on clay mineralogy and the illite Kübler index. The timing of gouge formation most likely overlaps with cooling below ~180–190 °C recorded by a zircon (U-Th)/He date of 116.6 ± 6.2 Ma from the Late Jurassic/Early Cretaceous granodiorite. Together these data constrain brittle and ductile deformation to the Early Cretaceous, similar to the age of deformation along other segments of the AFS and coeval with co-spatial arc magmatism. Regionally, the Paposo segment of the AFS is arcuate, trending NNW-SSE in the northern end and NNE-SSW in the southern end. Previous studies of fault strands along the northern portion of the Paposo segment document sinistral transtension, whereas oblique sinistral-reverse shear and local coaxial flattening record sinistral transpression along the southern portion of the Paposo fault. We propose that transtension and transpression along the AFS are controlled by the arcuate geometry, and both are compatible with sinistral simple shear along the N-S-trending magmatic arc.

Geochronology of the southern margin of the Bighorn Batholith, Wyoming

The Bighorn Mountains in north-central Wyoming reveal one of the largest exposures of 2800 Ma to 3000 Ma rocks in Laurentia. The northern part of the crystalline core is composed of the composite Bighorn batholith, whereas the central and southern areas of the range expose older gneiss complexes as well as minor supracrustal rocks. We provide new high-resolution LA-ICPMS U-Pb data on zircons sampled from eleven samples of tonalite, granodiorite, mylonite, and migmatite from the southern margin of the Bighorn batholith in the headwaters of the north fork of Paint Rock Creek. These rocks range from strongly foliated to massive and are difficult to subdivide into mappable units in the field because of their lithologic and structural similarities. Several cross-cutting mylonite zones (&lt;10-meter-wide) that trend ~N70°E-N80E and dip steeply are present in the study area. Three distinct age populations are evident: ~2930-2940 Ma, 2905-2915 Ma, and 2880-2890 Ma. Several samples contain xenocrystic zircons &gt;3000 Ma, ranging to 3500 Ma, which indicates assimilation of older crust. Each of the three age populations reported here are older than the previously reported age of ~2850 Ma age for the northern Bighorn Batholith but within the 2890 Ma, 2940 Ma, and 2950 Ma age groupings previously reported for the southern gneiss terrane. Three conclusions can be drawn from these data. First, the Bighorn batholith, at least along the southern margin, contains phases at least 80 million years older than the northern phase of the body, and emplacement was protracted and occurred over a ~100 Ma period. Second, episodes of both intrusion and shearing took place in this area as the various phases of the Bighorn batholith were emplaced. Finally, the existence of inherited zircons within the Bighorn batholith in the age range of ~3.0 Ga to 3.5 Ga indicates that the Bighorn batholith intruded through older crust. This older crust is perhaps a northern extension of Sacawee block in the northern Granite Mountains of central Wyoming, which may underlie the Bighorn Mountains.

Geological interpretation of the 2008 seismic reflection, refraction and magnetotelluric data from the northern Eyre Peninsula, Gawler Craton

A deep seismic reflection profile and coincident refraction and magnetotelluric data were acquired across the northern Eyre Peninsula, Gawler Craton in 2008 to enhance the prospectivity of this region for uranium and geothermal energy, by establishing the crustal architecture and regional geodynamic framework. The seismic line crossed several tectonic domain boundaries, including the Kalinjala Mylonite Zone, which is a key structure in the southern and central Gawler Craton. In the vicinity of the seismic section, this mylonite zone dips moderately to the east and appears to be a major crustal-scale fault which cuts through to the Moho; it separates middle to lower crust of differing seismic reflectivity. The changes in seismic character correspond with the boundary between the Olympic Fe-oxide Cu- Au-U Province (host of Olympic Dam and Prominent Hill deposits) in the east, and the Central Gawler Au Province in the west, and also are suggested to limit the extent of the South Australian Heat Flow Anomaly.

Evolution of a rapidly slipping, active low-angle normal fault, Suckling-Dayman metamorphic core complex, SE Papua New Guinea

Evolution of a rapidly slipping, active low-angle normal fault, Suckling-Dayman metamorphic core complex, SE Papua New Guinea

Determination of strength variation in jointed anisotropic rocks behavior using UCS and Brazilian Tensile test

Abstract The objective of this study is to determine the variations of strength on anisotropic rock using Uniaxial Compressive Strength (UCS) and Brazilian Tensile Strength (BTS) tests. The variability and distribution of UCS and BTS values were statistically estimated using measured strength of rock. The strength obtained was later to correlate with the quality of rock mass zones. The location of study is at the Ulu Jelai Hydroelectric Dam project, Cameroon Highland, the NW corner of Pahang on the upper reaches of the Sg. Bertam downstream of the existing Sultan Abu Bakar Dam and east of the township of Ringlet. The study is essentially required due to the exposure of sheared bedrock in the mylonite zones beneath the proposed dam. Six (6) boreholes were drilled at different depths along the dam foundation and 23 cores were collected and tested using the UCS and BTS tests. Simple regression analysis was conducted and a wide range of data on uniaxial compression strength and tensile strength of the rock were tabulated. The correlation between UCS and BTS obtained in this experiment is moderately correlated with R2 = 0.8066. Based on the UCS and BTS results, the overall range of UCS was in between 0.9 and 96.0 MPa and tensile strength range is between 0.5 and 14.31 MPa. For further correlation, UCS and Young’s Modulus were studied to shows good reliability (R2 = 0.818). The distribution of UCS data of each borehole is affected by the anisotropic rock behaviour that controls the quality of rock mass zoning.

Weak and Slow, Strong and Fast: How Shear Zones Evolve in a Dry Continental Crust (Musgrave Ranges, Central Australia)

AbstractThe strike‐slip Davenport Shear Zone in Central Australia developed during the Petermann Orogeny (~550 Ma) in an intracontinental lower crustal setting under dry subeclogite facies conditions (~650 °C, 1.2 GPa). This approximately 5‐km‐wide mylonite zone encloses several large low‐strain domains, allowing a detailed study of the initiation of shear zones and their progressive development. Quartzo‐feldspathic gneisses and granitoids contain compositional layers, such as quartz‐rich pegmatites, mafic bands, and dykes, which should preferentially localize viscous deformation if favorably orientated. This is not observed, except for long, continuous, and fine‐grained dolerite dykes. Instead, many shear zones, typically a few millimeters to centimeters in width but extending for tens of meters, commonly exploited pseudotachylytes and are sometimes parallel to a network of little overprinted fractures. The recrystallized mineral assemblage in the sheared pseudotachylyte is similar to that in the host gneiss, without associated hydration due to fluid‐rock interaction. Lack of localization in quartz‐rich, coarser‐grained (typically &gt;50 μm) rocks compared to mafic dykes, precursor fractures, and pseudotachylytes implies that localization in the dry lower crust preferentially occurs along elongate, planar fine‐grained layers. Transient high stress repeatedly initiated fractures, providing finer‐grained, weaker, planar precursors that localized subsequent ductile shear zones. This intimate interplay between brittle and ductile deformation suggests a local source for lower crustal earthquakes, rather than downward migration of earthquakes from the shallower, usually more seismogenic part of the crust.

Timing of strain partitioning and magmatism in the Scottish Scandian collision, evidence from the high Ba–Sr Orkney granite complex

The Orkney granite complex dominates the outcropping basement on Orkney, Scotland. It comprises a grey and a pink variably foliated granite, and structurally younger pegmatites and aplites. Based on geochemical characteristics the granites are assigned to the Scottish high Ba–Sr granites. The granites are deformed by synmagmatic extensional east–west-trending mylonite zones. These are locally overprinted by similarly oriented extensional phyllonites and, in one case, by similarly oriented extensional faults. The grey and the pink granites are dated by zircon U–Pb CA-ID-TIMS to 431.93 ± 0.46 and 430.26 ± 0.92 Ma, respectively. An aplite cutting mylonitic granite and cut by phyllonite is dated to 428.50 ± 0.31 Ma. We interpret the shear zones to record north–south extension during emplacement and cooling of the granites, likely at a shallow crustal depth (4–12 km). The extension is best explained by a subsidiary pull-apart structure related to displacement on the Great Glen Fault. In this case, the Orkney granite complex dates transcurrent faulting to 432–429 Ma, coeval with the 431–429 Ma Moine Thrust. This indicates that strain partitioning and high Ba–Sr magmatism across the Scottish Highlands was an immediate response to attempted subduction of Avalonia beneath Laurentia during the Scandian collision. Supplementary material: Geochronological and geochemical data (Tables 1 and 2) as spreadsheets are available at https://doi.org/10.6084/m9.figshare.c.4304387