Mylonite Zone Research Articles

Variscan basement tectonics and Alpine shear zones in the external Balkanides: Structural data from the Vezhen Massif, Central Stara Planina Mts., Bulgaria

The Alpine Balkanides in Bulgaria are developed over Variscan orogenic fragments, which are variously affected by late-stage thick-skinned thrust tectonics. Often, these fragments hold both Variscan and Alpine structural records. А key area to shed light on important aspects of the tectonic evolution of the external parts of the Alpine Balkanide orogen is the Vezhen Massif in the Central Stara Planina Mountains. Although it is built exclusively of Paleozoic crystalline rocks, its structure was long considered as an example of intense mid Eocene to early Oligocene (Late Alpine) shortening. Detailed fieldwork in the area shows a more complicated tectonic framework and provides evidence of a polyphase structural evolution, involving an important stage of Variscan metamorphism and deformation and two stages of north-vergent Alpine shortening. The Stargel-Boluvanya Tectonic Zone that affects the western Vezhen Massif is one of the most important Variscan structures in the Balkan fold-and-thrust belt. Structural data indicate that the metamorphic basement records initial top-to-north thrusting, followed by further shortening, which led to folding and localized strike-slip deformation. The Variscan syn-metamorphic fabric is cross-cut by several post-kinematic igneous bodies of late Carboniferous to Permian ages. The tectonic history continues with a development of a network of greenschist facies north-vergent mylonitic zones for which an Early Alpine (post-late Permian but pre-Late Cretaceous) age is assumed. Late Alpine north-vergent thrusting is evident only on the eastern and northern flanks of the Vezhen Massif. In a broad structural context, the documented record in the basement rocks of the Vezhen Massif is comparable with those of the external massifs of the Alps and Iberia.

Mеsozoic Tectonothermal Evolution of the Zagan Metamorphic Core Complex in Western Transbaikalia: 40Ar/39Ar and FTA Dating

Abstract A model of tectonothermal evolution of the Zagan metamorphic core complex (MCC) based on the new data from 40Ar/39Ar dating of amphibole, mica, and apatite fission-track dating is discussed. A relationship with the long-range impact of processes from the collision of the North China (Amurian–North China) block with the Siberian continent in the Mesozoic era is proposed. The Zagan MСС was formed in the Cretaceous period on the southern flank of a high mountain uplift of Western Transbaikalia, composed of late Paleozoic granitoids of the Angara–Vitim batholith. According to 40Ar/39Ar dating of amphiboles and micas from the mylonite zone, the active development time of the Zagan MCC corresponds to the early Cretaceous epoch (131, 114 Ma). The tectonic exposure of the core from about 15 km to the depths of about 10 km occurred at a rate of tectonic erosion of 0.4–0.3 mm/year as a result of post-collisional extension of the Mongol–Okhotsk orogen. Apatite fission-track dating shows that further exhumation and cooling of the rocks to about 3 km occurred in the lower-upper Cretaceous epoch (112, 87 Ma). The erosional denudation rate was about 0.3 mm/year.MCC- metamorphic core complexes, AFT- apatite fission-track

Fluvial deposits of the Patlanoaya Group in southern Mexico: the stratigraphic record of an Early Mississippian supradetachment basin formed along the margin of Gondwana during the closure of the Rheic Ocean

ABSTRACT The Rheic Ocean closure stratigraphic record in southern Mexico is largely exposed in the Paleozoic Acatlán Complex. One of the most distinctive characteristics of the Acatlán Complex is the presence of high-pressure rocks, interpreted as remnants of the Rheic Ocean and the adjacent continental margin of Gondwana, which were subducted and then exhumed to the surface. Despite its importance in reconstructing the dynamics of the westernmost Rheic Ocean closure, the timing and mechanisms of exhumation of these high-pressure rocks remain debated. Some authors have tentatively interpreted the late Paleozoic, fluvial to marine succession of the Patlanoaya Group in southern Mexico as the sedimentary response to the exhumation of high-pressure rocks of the Acatlán Complex. Therefore, these sedimentary rocks may be particularly helpful in understanding the timing and evolution of high-pressure rocks exhumation during the Rheic Ocean closure. In this work, the tectono-sedimentary evolution of the continental deposits of the Patlanoaya Group was reconstructed, integrating sedimentologic, petrographic, and U-Pb detrital geochronological data to test the different scenarios proposed for the exhumation of the Acatlán Complex high-pressure rocks. Our data suggest that the lower Patlanoaya Group was deposited in an Early Mississippian basin with a two-stage evolution: a phase characterized by debris flow deposits derived from small-scale reliefs composed of low-grade metamorphic rocks and a subsequent phase marked by an NNE-trending fluvial system deposits sourced from low-metamorphic grade rocks of a mylonitic zone. Therefore, the lower Patlanoaya Group deposition was controlled by a major NNE-trending mylonitic shear zone associated with the development of a supradetachment basin linked to the progressive exhumation of high-pressure rocks during an Early Mississippian major extensional event. This work highlights the importance of sedimentary deposits in reconstructing the late Paleozoic history of the Acatlán Complex during the Rheic Ocean closure and the final stage of the Pangea assembly.

THE ZABAIKALIAN PARAMETRIC BOREHOLE: A FIRST EVIDENCE ABOUT DEEP STRUCTURE OF THE BORSHCHOVOCHNY METAMORPHIC CORE COMPLEX

The Zabaikalian parametric borehole drilled out a metamorphic core complex on the reference geological and geophysical profile within the Central Asian fold belt, south of the Mongol-Okhotsk suture zone. In the section of the borehole drilled to a depth of 2600 m, a set of geophysical, geological and laboratory-analytical studies identified three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (weakly metamorphosed rocks) is represented by serpentinite melange, which is separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate, which is part of the crystalline basement complex, includes rocks of granite-schist and granite-plagiogneiss subcomplexes. The granite-schist subcomplex is composed of mafic and intermediate schist with amphibolites and quartzites. Biotite granite is widespread in the section; it forms layered veins with signs of synkinematic formations. According to preliminary estimates, the age of biotite granite is about 130 Ma. The schist is characterized by mylonite structure at intervals of interbedding with biotite granite veins. In accordance with data of geophysical surveys in the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors on the seismic reflection-CDP section, which can be traced to a depth of at least 10 km. The granite-plagiogneiss subcomplex is represented by plagiogneiss, leucogranites, and gneissic granites. In the Zabaikalian parametric borehole, a detailed section of the orshchovochny metamorphic core complex, fully characterized by core samples, of the was obtained, which should be considered as a reference for studying these structures. The investigation of this section will answer later many questions about the formation and evolution of the Central Asian fold belt.

Switching from the top-NW backthrusting to top-SE post-orogenic deformation in the Aegean domain: Insights from the calcite microfabric and siliciclastic rocks of the Amorgos Unit (SE Cyclades, Greece).

Detailed microstructural analysis was conducted in the imbricated Amorgos stratigraphy to decipher the complex deformation history of the calcitic mylonitic zones related to different tectonic events. Four major thrust zones were developed during syn-metamorphic tectonic burial by top-NW general shear deformation. Dynamic recrystallization is the dominant deformation mechanism, evolving from the SGR to the BLG field, characterizing a downward increase in strain from the highest Potamos to the lower Paradesia thrusts under low-T conditions. Mesoscopic kinematics within these thrusts reveal top-NW shearing. However, in microscale, both top-NW and top-SE kinematics are observed. In most cases, this top-SE reversal resulted from strain partitioning due to a strong pure shear component. However, the Mouros and Richti shear zones present dominant top-SE kinematics in all scales. Additionally, overprinting relationships within the metaflysch of the Mouros shear zone suggest that the top-SE kinematics of this zone post-dates the former top-NW ones. Therefore, we suggest that a switch from the top-NW general shear deformation during the syn-orogenic compressional phase to a late top-SE extensional deformation occurred in the early Miocene, resulting in the exhumation of the Amorgos Unit below 200 °C, via the Santorini Detachment System.

Two metasomatic events recorded by noble gas characteristics in the Finero mantle wedge: Extreme fractionation (He, Ar) and seawater penetration into the mantle deformation zone

Here, we report that the Finero phlogopite peridotite massif, southern Italy, a sliver of the former subcontinental lithospheric mantle (SCLM), experienced two stages of fluid/melt percolation, which had different origins and scales in the mantle wedge based on noble gas analysis and the deformation structure. In the field investigation, we found a mylonite zone several tens of meters wide in the massif, inferred to have been caused by deformation in the mantle. Systematic differences in noble gas compositions between samples from inside and outside the mylonite zone reflect two stages of fluid/melt infiltration. In the first event, the fluid/melt with a noble gas mixture of seawater, radiogenic, and mantle components penetrated the Finero SCLM. It likely occurred simultaneously with the famous metasomatism that crystallized phlogopite and amphibole throughout the massif. This first event produced a 4He/40Ar ratio much higher than those of the convecting mantle and crustal components, probably during metasomatism by hydrous melt derived from the slab in the mantle wedge. The second event was probably synchronous with the development of mylonitization and locally supplied seawater-rich components. The 40Ar/36Ar ratios tend to be atmospheric as mylonitization progresses. Our results provide two insights: (i) the first demonstration of the formation of extremely high He/Ar ratios during mantle metasomatism of natural samples and (ii) differences in the deformation structure affecting the noble gas composition and distribution in the SCLM. Regarding (i), the elemental fractionation can be attributed to two factors: one is the difference in the solubility between noble gas elements in particular minerals in the slab, and the other is the difference in the diffusion coefficients of He and Ar. Our results imply that the recycling of volatiles within subduction zones is significantly influenced by the thermal structures of slabs. This is because these thermal structures control the stability of hydrous minerals on the slabs, temperature conditions, and potentially the efficiency of diffusion in mantle wedges.

The Trans-Baikal Parametric Borehole: First Evidence of the Deep Structure of the Borshchovochnyi Metamorphic Core Complex

The Trans-Baikal parametric borehole drilled out a metamorphic core complex in the reference geological and geophysical profile within the Central Asian fold belt, south of the Mongol–Okhotsk suture zone. In the section of the borehole drilled to a depth of 2600 m, a set of geophysical, geological, and laboratory–analytical studies made it possible to identify three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (slightly metamorphosed rocks) is represented by a serpentinite mélange separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate as a part of the crystalline basement complex comprises rocks of granite–schist and granite–plagiogneiss subcomplexes. The granite–schist subcomplex is composed of mafic and intermediate schist with amphibolites and quartzites. Biotite granite is widespread in the section as layered veins with synkinematic signs. According to the preliminary estimates, the age of biotite granite is about 130 Ma. Schist is characterized by a mylonite structure at intervals of interbedding with biotite granite veins. In accordance with the geophysical survey data on the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors in the seismic RSS–CDP section, which can be traced to a depth of at least 10 km. The granite–plagiogneiss subcomplex is represented by plagiogneiss, leucogranite, and gneissic granite. In the Trans-Baikal parametric borehole, a detailed section of the Borshchovochnyi metamorphic core complex, fully characterized by core samples, was obtained. It should be considered as a reference for studying these structures. The investigation of this section will make it possible later to answer many questions about the formation and evolution of the Central Asian fold belt.

TECTONOTHERMAL EVOLUTION OF THE ZAGAN METAMORPHIC CORE COMPLEX IN TRANSBAIKALIA AS A RESULT OF THE CRETACEOUS – PALEOCENE MONGOL-OKHOTSK POST-COLLISIONAL OROGEN DESTRUCTION

Thermochronological reconstructions of the Zagan metamorphic core complex were carried out using samples from the central part of the core, mylonite zone detachment and lower nappe with U/Pb zircon dating, 40Ar/39Ar amphibole and mica dating, and apatite fission-track dating. In the tectonothermal evolution of the metamorphic core, there was distinguished an active phase (tectonic denudation) of the dome structure formation during the Early Cretaceous (131–114 Ma), which continued in the Late Cretaceous – Paleocene (111–54 Ma) in passive phase (erosive denudation). During an active phase, there was initiated a large-amplitude gently dipping normal fault (detachment), which was accompanied by tilting (sliding of rocks along subparallel listric faults). As a result, about 7 km thick rock strata underwent denudation over 17 Ma at a rate of about 0.4 mm/year. In passive phase, about 6 km thick rock strata were eroded over 57 Ma, with a denudation rate of about 0.1 mm/year. Thus, the Zagan metamorphic core complex was tectonically exposed from the mid-crust to depths of about 9 km in the Early Cretaceous as a result of post-collisional collapse of the Mongol-Okhotsk orogen. Further cooling of the rocks in the metamorphic core to depths of about 3 km occurred in the Late Cretaceous – Pliocene as a result of destruction of more than 6 km high mountains.

Early Neoproterozoic tectonics in the Marwar Crustal Block, NW India, the relevance of the Phulad Shear Zone, and implications for Rodinia reconstruction

Abstract Palaeomagnetic studies in the Malani Igneous Suite (ca. 770–750 Ma) of the Marwar Crustal Block, NW India, provide essential constraints on palaeogeographic reconstructions of the Rodinia Supercontinent. The Malani Igneous Suite is intrusive into megacrystic granite and granitegneissic enclaves of the Marwar Crustal Block. A crustalscale NE–SW ductile transpressional shear zone with a southeasterly dip known as the Phulad Shear Zone (820–810 Ma) separates this Marwar Crustal Block from the rocks farther east. The preshearing tectonic evolution of the Marwar Crustal Block is poorly understood. Three phases of ductile deformations (D1, D2, and D3, with associated fabrics S1, S2, and S3) were identified in the Marwar Crustal Block. The D1 deformation is restricted to enclave gneisses. The megacrystic granite was emplaced syntectonically during D2 deformation. The S2 magmatic foliation (NNW–SSE) in the megacrystic granite is oblique to the Phulad Shear Zone. The D3 deformation in the megacrystic granite is synchronous with the Phulad Shear Zone mylonites. Another porphyritic granite (Phulad granite, ca. 820 Ma) emplaced syntectonically during D3 deformation along and across the Phulad Shear Zone. U-Pb zircon dates in the Marwar Crustal Block document ca. 890 Ma and ca. 860 Ma magmatic dates. U-Pb-Th monazite dates in the Marwar Crustal Block show a strong peak at ca. 820 Ma. By integrating critical field relations, deformational patterns, and geochronology, we argue that the Marwar Crustal Block shows distinct geological evolution prior to its accretion with the remaining parts of India. We propose that the accretion of the Marwar Crustal Block must be younger than ca. 860 Ma and culminated during ca. 820–810 Ma to form the Greater India landmass along the Phulad Shear Zone.

Lithium isotopic and fluid mobile trace element systematics of the Bay of Islands altered forearc upper to lower ophiolitic crust

Bulk rock Li isotope and select fluid mobile element traverses through the crustal section of the Late Cambrian Bay of Islands Ophiolite Complex (BOIC) in the Taconic Humber Arm Allochthon in the Newfoundland Appalachians are presented to examine the extent and conditions of alteration at different pseudo-stratigraphic levels in the ancient forearc oceanic crust that was obducted during the Middle Ordovician.N-MORB normalized enrichment factors (EFs) calculated for Li and fluid mobile elements (FMEs) show variable extents of enrichments or depletions from shallow to deep oceanic crustal sections in the BOIC that are consistent with a broad range of alteration-related δ7Li values from −2.97 to +20.69 ‰ (vs. unaltered MORB range of +1.5 to +6.1 ‰ with a mean of +3.4 ± 1.4 ‰). EFs and fluid-rock reaction modeling for Li contents and δ7Li values indicate alteration dominantly controlled enrichments and depletions of Li isotopic and FMEs systematics through all crustal levels of the BOIC ophiolite. Alteration fluids are dominated by modern seawater-like δ7Li composition rather than the recently proposed Cambrian seawater that is based on ancient carbonate shell compositions.BOIC basaltic samples proximal to the York Harbor Mine (YHM), similar to the classic mafic Cu-Zn Cyprus-type volcanic massive sulfide (VMS) deposits, show light δ7Li values of −2.97 to +1.59 ‰, but highly enriched Li contents (14.0–40.7 μg/g). Modeling results indicate that these YHM basalts were altered by metal-rich magmatic fluids, in part, derived from deeper plagiogranite intrusions near the base of the sheeted dikes that are possibly admixed with seawater-derived hydrothermal fluids below the VMS deposit. Magmatic volatile exsolution may be associated with the crystallization of directly underlying subvolcanic silicic intrusions, resulting in over-pressuring and fracturing of the carapace that may assist in localizing the high temperature (400–200 °C) upflow zones of the seafloor hydrothermal system.The BOIC layered gabbroic rocks also show evidence of variable extent of alteration as reflected by both Li content (EF both >1 and < 1) and δ7Li composition (+0.79 to +9.47 ‰), likely indicating high-temperature amphibolite (800–600 °C within the ductile deformation field) to lower temperature greenschist facies paragenesis, with down-temperature complex retrograde petrogenetic lineages. Mylonites and ductile shear zones commonly show heavier δ7Li values between +5.47 and + 18.83 ‰. We suggest that mylonite zones and coarser-grained high-strain zones allowed near-axis fluid penetration at high to lower temperatures that extended to the basal gabbroic sections in the crust during ongoing igneous activity. Shear zones were associated with rheologic weakening and initial permeable pathways for hydrothermal fluids. Shallow on-axis and deep off-axis hydrothermal circulation likely occurred at a relatively fast-spreading rate, which may account for the significant shallow to deep crustal heat extraction as predicted by the multiple sill model of lithospheric cooling.

Dating mylonitic overprinting of ancient rocks

Deformation in shear zones is difficult to date because mylonites can preserve partially reset pre-existing isotopic signatures. However, mylonites can be key structural elements in terrane recognition, so an accurate estimation of their age is important. Here we determine the in situ Rb–Sr isotopic composition of mica from major NE-SW trending mylonitic zones in the Archean Akia Terrane of Greenland and complement this information with inverse thermal history modelling. Rb–Sr isochrons indicate a dominant age of radiogenic-Sr accumulation in biotite of around 1750 million years (Ma) ago. Yet, magmatic titanite is unreset yielding a U–Pb age of around 2970 Ma. These constraints require that biotite Rb–Sr directly dates mylonitic fabric generation. The 1750 Ma mylonites, associated with the Proterozoic Nagssugtoqidian Orogeny, overprint Archean crust widely regarded as preserving evidence of early Earth horizontal tectonics.

Development of the Median Tectonic Line-related shear zone, southwest Japan: An analysis of strain localization processes

The development of a mylonite zone along the Median Tectonic Line (MTL), southwest Japan, which developed in the Cretaceous granitic rocks, was examined to understand strain localization during cooling. The mylonites are E-W trending, north-dipping, and represent the hanging wall of the MTL. The mylonite zone includes both protomylonite and mylonite, which were derived from two distinct protoliths: tonalite (protomylonite) and granite (mylonite) and occur at distances of c. 0–300 m and 300–700 m from the MTL, respectively. We used quartz microstructures and crystallographic preferred orientations (CPOs) and two-feldspar thermometry to infer the spatiotemporal development of deformation conditions in the MTL mylonite zone. The mylonitic granite is sub-divided into southern and northern mylonite zones, which exhibit A- and B-type quartz microstructure formed by subgrain rotation and grain boundary migration recrystallization, respectively. In both the mylonitic granite and protomylonitic tonalite, quartz c-axis CPOs primarily display a moderate-temperature Y-maximum pattern, as well as a less common low-temperature type-I crossed-girdle pattern. In the northern zone, quartz deformation initially occurred at moderate temperatures of 435 °C–490 °C, forming mylonitic granite with B-type quartz microstructure. As temperature decreased to 355 °C–435 °C, the strain localized into the southern zone to form A-type quartz microstructure. The protomylonitic tonalite formed under similar temperature conditions. At temperatures below 355 °C, ductile deformation ceased in the majority of the granite and tonalite, with only a narrow zone of ultramylonite up to 50 m wide, deforming adjacent to the MTL at conditions approximating the brittle–ductile transition in quartz (c. 300 °C). Strain localization did not occur within a single shear zone, but in several deforming strands that formed in distinct protoliths, one of which developed into the narrow ultramylonite zone reworked as the brittle MTL, likely controlled by cooling.

Forsterit-serpentinový mramor s klinohumitem a spinelem u Chotěboře (moldanubikum; Český masiv); příklad retrográdní metamorfózy dolomitického mramoru

A body of dolomite marble near Chotěboř was found in migmatized biotite paragneiss of the Moldanubicum directly on the border with migmatites/orthogneisses of the Kutná Hora Crystalline Complex. The area underwent significant tectonic reworking of rocks along the Přibyslav mylonite zone. Therefore, only small relics of early prograde HT/LP metamorphism at T &gt; 650 - 700 °C were preserved in this marble. They are represented by the assemblage dolomite+calcite+forsterite+Zn-spinel+clinohumite ±Ba-phlogopite and rare baddeleyite. This mineral assemblage underwent strong low-temperature retrograde metamorphism (&lt; 400 - 300 °C) under the conditions of high H2O activity (XCO2 &lt; 0.10 - 0.15), which was manifested by almost complete serpentinization of forsterite. Partial alteration of clinohumite produced serpentine with numerous inclusions of a TiO2-mineral and rare sphalerite. The examined serpentine marble (ophicalcite) is lithologically and mineralogically similar to the dolomite marbles found in the Strážek Moldanubicum.

Structural and Metamorphic History of the Leech River Shear Zone, Vancouver Island, British Columbia

AbstractThe Leech River Shear Zone (LRSZ) on southern Vancouver Island separates the metasedimentary schists of the Leech River Complex (LRC) from the accreted oceanic plateau of the Siletz‐Crescent terrane. The juxtaposition of these rock units suggests a possible origin as a subduction plate boundary but tectonic context of the LRSZ has yet to be fully established. We present field, microstructural, petrological, and geochronological observations that constrain the structural and metamorphic history of LRSZ. The mylonite zone of the LRSZ straddles the lithologic contact between the schists of the LRC and basalts of the Siletz‐Crescent terrane. Foliation orientations, a steeply plunging stretching lineation, and kinematic indicators all suggest reverse‐sinistral motion. Compositions of garnet in the schist and amphibole in the metabasalt record synkinematic growth at temperature and pressure conditions of 550°C–570°C and 450–490 MPa. These metamorphic conditions require elevated geotherms that are consistent with plate models that position the Kula‐Farallon Ridge and Yellowstone Hotspot in the region in the Eocene (∼50 Ma). Detrital zircon U‐Pb age distributions for the Leech River Schist have Paleocene maximum depositional ages and are similar to the Upper Nanaimo Group that unconformably overlies the Wrangellia terrane. Our ages support early Paleocene deposition of the schist in a subduction trench/slope environment followed by underthrusting and underplating. These results establish the exhumed mylonite zone of the LRSZ as a paleo‐plate interface that was active during Eocene subduction.

Shear zone evolution and the path of earthquake rupture

Abstract. Crustal shear zones generate earthquakes, which are at present unpredictable, but advances in mechanistic understanding of the earthquake cycle offer hope for future advances in earthquake forecasting. Studies of fault zone architecture have the potential to reveal the controls on fault rupture, locking, and reloading that control the temporal and spatial patterns of earthquakes. The Pofadder Shear Zone exposed in the Orange River in South Africa is an ancient, exhumed, paleoseismogenic continental transform which preserves the architecture of the earthquake source near the base of the seismogenic zone. To investigate the controls on earthquake rupture geometries in the seismogenic crust, we produced a high-resolution geologic map of the shear zone core mylonite zone. The core consists of ∼ 1–200 cm, pinch-and-swell layers of mylonites of variable mineralogic composition, reflecting the diversity of regional rock types which were dragged into the shear zone. Our map displays centimetric layers of a unique black ultramylonite along some mylonite interfaces, locally adding to thick composite layers suggesting reactivation or bifurcation. We present a set of criteria for identifying recrystallised pseudotachylytes (preserved earthquake frictional melts) and show that the black ultramylonite is a recrystallised pseudotachylyte, with its distribution representing a map of ancient earthquake rupture surfaces. Pseudotachylytes are most abundant on interfaces between the strongest wall rocks. We find that the geometry of lithologic interfaces which hosted earthquakes differs from interfaces lacking pseudotachylyte at wavelengths of ≳ 10 m. We argue that the pinch-and-swell structure of the mylonitic layering, enhanced by viscosity contrasts between layers of different mineralogy, is expected to generate spatially heterogeneous stress during viscous creep in the shear zone, which dictated the path of earthquake ruptures. The condition of rheologically layered materials causing heterogeneous stresses should be reasonably expected in any major shear zone, is enhanced by creep, and represents the pre-seismic background conditions through which earthquakes nucleate and propagate. This has implications for patterns of earthquake recurrence and explains why some potential geologic surfaces are favored for earthquake rupture over others.

Constraints on the evolution of the eastern margin of the Hearne craton: New data from the southwestern Rottenstone Domain, Trans-Hudson Orogen, Saskatchewan

The Rottenstone Domain (RD) is a complex lithotectonic supracrustal belt of the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan, Canada, located between the Archean Hearne craton and the juvenile Paleoproterozoic Reindeer Zone (RZ). Its evolution and crustal affinity have been under considerable debate in the past few decades, with most interpretations considering it to be part of RZ, but some hinting that certain elements might have affinities to the Archean hinterland. On the basis of bedrock mapping, U-Pb, Sm-Nd and lithogeochemical data, the rocks of the southwestern RD were subdivided into several lithodemic entities, which were used to reconstruct a plate tectonic framework for the western part of the Trans-Hudson Orogen during Orosirian time.A U-Pb zircon crystallization age of 2518 ± 6 Ma and depleted mantle model age (TDM) of 2.8 Ga for an augen granite confirmed that the western part of RD is part of the Hearne craton. A newly discovered structural inlier at the eastern boundary of RD consists of a central unit of 2045 ± 9 Ma augen granite, with TDM ages between 2.8 and 2.6 Ga, that was intruded into gneissic granite with a TDM of 3.3 Ga. The augen granite was interpreted as representing a rift-related granite, emplaced into Archean rocks of the Hearne craton. Two lithotectonic assemblages, one dominated by feldspathic quartzite and psammite the other by migmatitic psammite-psammopelite, are in structural contact with the granitic inliers and have model ages of 2.6–2.4 Ga. One of the feldspathic quartzite samples contains detrital zircon age modes at 1910 and 1860 Ma, and lesser Archean modes. Both assemblages were likely deposited in a forearc basin to the 1.865–1.850 Ga Wathaman continental arc.Recognition of Archean components and metasedimentary assemblages with older model ages than those in the RZ to the east, suggest that this southern part of RD is more closely affiliated with the Hearne craton. The eastern edge of RD coincides with a mylonite zone (Howard Lake shear zone), which likely represents a fundamental, although multiply reactivated, structural break separating Archean granitoids and structurally overlying metasedimentary rocks from rocks of the more juvenile Reindeer Zone.

The role of gravitational body forces in the development of metamorphic core complexes

Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands.

Deep hydration and lithospheric thinning at oceanic transform plate boundaries.

Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. Here we present ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending down to ~60 km depth below sea level. We performed 3D thermal modelling that suggests the anomaly is likely to be due to extensive serpentinization down to ~16 km depth, overlying a hydrated, shear mylonite zone down to 32 km depth. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere-asthenosphere boundary, causing significant thinning of the lithosphere at the transform fault. The presence of a thinned lithosphere at transform faults could explain observations of volcanism, thickened crust and intra-transform spreading centres at transform faults. It also suggests that migration and mixing of water-induced melt with the high temperature melt may occur beneath the ridge axis.

Modern style nappe stacking in the Paleoproterozoic lower crust: An example from the snowbird tectonic zone, Canadian Shield

The Intra Tantato shear zone (ITSZ), which occurs within the East Athabasca mylonite triangle in the southern portion of the ∼1.9 Ga Snowbird tectonic zone in northern Saskatchewan, Canada, is a low to high angle, curvi-planar, kilometer-thick mylonite zone hosted within granulite-facies gneiss that records a complex history with both thrust- and normal-sense motion. New field mapping, microstructural analysis, and quartz crystallographic preferred orientations (CPO) detail a composite history for the ITSZ that includes top-to-the-ENE thrusting of the hanging wall over the footwall that is overprinted by normal-sense top-to-the-WSW reactivation. Quartz CPOs further indicate constrictional deformation related to both events and quartz c-axis opening angles define deformation temperatures of ∼750 ± 50 °C for the thrusting event and ∼550 ± 50 °C for the later, normal-sense reactivation.Monazite and garnet geochronology on rocks within and around the ITSZ indicate it may have nucleated along a pre-existing 2.6–2.5 Ga sub-horizontal fabric. This appears to have occurred immediately after the ∼1.920 Ga metamorphic peak of the hanging wall and during the ∼1.905 Ga metamorphic peak of the footwall. Monazite ages from small matrix grains in interstitial positions in specimens that record reactivation kinematics indicate overprinting occurred at ∼1.860–1.830 Ga, contemporaneous with the earliest recorded activity along the nearby Grease River Shear Zone.Overall, this tectonic evolution can be attributed to a crustal thickening process that occurred in the Athabasca mylonite triangle during the Snowbird orogeny at ∼1.9 Ga and during the post-collisional exhumation processes. Moreover, these results show that the lower continental crust can actively accommodate shortening during large-scale collisional events with tectonic units piling up via nappe-stacking processes similarly to what is typically observed in the middle crust.

Petrogenesis of the East Hoerba harzburgites, SW Tibet: Implications for melt stagnation in the lithospheric mantle of Neo-Tethys

In this study we present petrographic and compositional data for harzburgites from the eastern parts of the Hoerba Massif of the Yarlung-Zangbo Suture Zone (YZSZ), SW Tibet. These harzburgites are cross cut by semi-ductile mylonitic zones. The investigated rock samples contain aggregates of unstrained clinopyroxene with tapered edges. They also contain olivine with a high forsterite [Fo# = Mg/(Mg + Fe) × 100] content (89.3–91.8), orthopyroxene with a low Al 2 O 3 content (2.0–5.6 wt%), and clinopyroxene porphyroclasts with Cr 2 O 3 contents between 0.2 and 1.0 wt% and low lanthanide contents. Chromian spinel (Cr-spinel) in the East Hoerba harzburgites has low Cr# [100 × Cr/(Cr + Al)] and high Mg# [100 × Mg/(Mg + Fe)] values (23.7–39.2 and 62.2–76.3, respectively). These harzburgites have chondrite-normalized rare earth element (REE) patterns characterized by enrichments in the light (L-)REE and positive slopes from the middle to the heavy REE. Semi-quantitative geochemical modeling demonstrates that the bulk-rock compositions of the East Hoerba harzburgites differ from those of typical abyssal and forearc peridotites. Clinopyroxene trace element modeling suggests that the East Hoerba harzburgites initially underwent melting below a mature spreading center, followed by melting in the mantle wedge above a Cretaceous Neo-Tethyan subduction system. Enrichment of the harzburgite samples in LREE is interpreted as due to post-melting entrapment of small volumes of a LREE-rich melt in the lithospheric mantle. We suggest that the East Hoerba harzburgites are sub-oceanic peridotites that underwent melt depletion events and melt stagnation before their exhumation at the Neo-Tethyan seafloor. • The East Hoerba Massif is composed of variably depleted harzburgites. • They were formed from mantle depletion events and post-melting magma stagnation. • They were generated beneath a transitional arc–back-arc setting. • They do not represent fragments of old sub-continental lithospheric mantle.